Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
Coordination chemistry and reactivity of early transition
metal complexes with the amine phenolate ligands
Thesis for the Degree of “Doctor of Philosophy”
By
Stanislav Groysman
Submitted to the Senate of Tel Aviv University
September 2005
Coordination chemistry and reactivity of early transition
metal complexes with the amine phenolate ligands
Thesis for the Degree of “Doctor of Philosophy”
By
Stanislav Groysman
Submitted to the Senate of Tel Aviv University
September 2005
This work was carried out under the supervision of Professor Moshe Kol and
Professor Israel Goldberg.
I am deeply grateful to my teacher and mentor, Professor Moshe Kol (Shiko),
for guidance, support, brilliant ideas, and never-ending inspiration.
I wish to thank Professor Israel Goldberg for his guidance in the beautiful field of X-
ray crystallography.
I wish to thank my group members, Dr. Edit Tshuva, Dr. Dalia Gut-Regev, Adi Yeori,
Shimrit Gendler, Sharon Segal, Ekaterina (Katy) Sergeeva, and Sheba D. Bergman,
for their support. Especially, I wish to express my deep appreciation to Mrs. Dvora
Reshef for the everyday help, and for the help in olefin polymerization project.
I wish to thank Professor Zeev Goldschmidt and Dr. Elisheva Genizi from Bar Ilan
university, and Dr. Michael Shuster from Carmel Olefins Ltd, for our fruitful
collaboration.
This work is dedicated to my parents, Alexander and Galina, and to my wife,
Marianna.
Table of contents
1 List of publications
3 Abstract
15 Introduction
15 I-1. Alkoxo (phenolate) ligand vs. Cp ligand
16 I-2. From a simple alkoxo to a bulky phenolate ligand
17
I-3. Multidentate ligands: Enhanced stabilization and easier design of
metal centers
19 I-4. Multidentate phenolate ligands
25
I-5. Synthetic pathways to early transition metal complexes bearing
phenolate ligands
26
I-6. Reactivity of early transition metal complexes supported by
multidentate phenolate ligands
27 I-6.1. Olefin polymerization by well-defined Group IV metal complexes
32
I-6.2. Stabilization of metal-carbon double bonds and olefin metathesis
reactivity
34 I-6.3. Phenolate ligands in bioinorganic chemistry
37 I-7. Amine phenolate ligands
38 I-7.1. The amine tris(phenolate) ligands
39 I-7.2. The amine bis(phenolate) ligands with the additional, sidearm, donor
41 I-7.3. The amine mono(phenolate) ligands with two sidearm donors
43 Discussion
43 D-1. Ligand synthesis
45 D-2. Complex synthesis
47 D-3. Coordination chemistry of the amine phenolate metal complexes
54
D-4. Reactivity of the early transition metal complexes with amine
phenolate ligands
54 D-4.1. Olefin polymerization
60 D-4.2. H-abstraction reactions
63 D-4.3. Modeling the active site of haloperoxidase
67 References
72 Appendix: Symbols and abbreviations
Publications
תקציר א
List of publications
1
List of publications presented in this work
1. S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldschmidt Group IV complexes of an Amine Bis(phenolate) Ligand Featuring a THF Sidearm Donor: From Highly Active to Living Polymerization Catalysts of 1-Hexene Inorg. Chim. Acta 2003 345, 137.
2. S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldschmidt From THF to Furan: Activity Tuning and Mechanistic Insight via Sidearm Donor Replacement in Group IV Amine Bis(phenolate) Polymerization Catalysts
Organometallics 2003, 22, 3013. 3. S. Groysman, E. Y. Tshuva, D. Reshef, S. Gendler, I. Goldberg,M. Kol, Z.
Goldschmidt, M. Shuster and G. Lidor High-Molecular Weight Atactic Polypropylene Prepared by Zirconium Complexes of an Amine Bis(phenolate) Ligand Isr. J. Chem. 2002, 42, 373.
4. S. Groysman, E. Tshuva, I. Goldberg, M. Kol, Z. Goldschmidt, and M. Shuster Diverse Structure-Activity Trends in the Amine Bis(phenolate) Titanium Polymerization Catalysts Organometallics 2004, 23, 5291.
5. S. Groysman, S. Segal, M. Shamis, I. Goldberg, M. Kol, Z.
Goldschmidt and E. Hayut-Salant Tantalum(V) Complexes of an Amine Triphenolate Ligand: a Dramatic Difference in Reactivity Between the Two Labile Positions J. Chem. Soc. Dalton Trans. 2002, 3425.
6. S. Groysman, S. Segal, I. Goldberg, M. Kol and Z. Goldschmidt Ta(V) Complexes of a Bulky Amine Tris(phenolate) Ligand: Steric Inhibition vs. Chelate Effect
Inorg. Chem. Commun. 2004, 7, 938. 7. S. Groysman, I. Goldberg, M. Kol and Z. Goldschmidt
Pentabenzyltantalum: Straightforward Synthesis, X-ray Structure and Application in the Synthesis of [O2N]TaBn3-Type and [O3N]TaBn2-Type complexes Organometallics 2003, 22, 3793.
8. S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldschmidt
Tribenzyl Tantalum(V) Complexes of Amine Bis(phenolate) Ligands: Investigation of α-Abstraction vs. Ligand Backbone β-Abstraction Paths Organometallics 2004, 23, 1880.
9. S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldshmidt Exploring Routes to Tantalum(V) Alkylidene Complexes Supported by
List of publications
2
Amine Tris(Phenolate) Ligands Adv. Synth. Cat 2005, 347, 409.
10. S. Groysman, I. Goldberg, M. Kol and Z. Goldschmidt Vanadium(III) and Vanadium(V) Amine Tris(phenolate) Complexes Inorg. Chem. 2005, 44, 5073. 11. S. Groysman, E. Sergeeva, I. Goldberg and M. Kol Group IV Complexes of a Tetradentate Amine Mono(phenolate) Ligand: A Second Sidearm Donor Stabilizes Cationic Species Inorg. Chem. 2005, 44, 8188. 12. S. Groysman, E. Sergeeva, I. Goldberg and M. Kol Salophan complexes with Group IV Metals Eur. J. Inorg. Chem. 2005, 2480.
Abstract
3
Abstract
Living polymerization of 1-hexene by titanium amine bis(phenolate) complexes
This work is fully described in articles 1 and 2.
Ti complexes of the amine bis(phenolate) ligands having bulky t-Bu groups in the
ortho positions of the phenolate rings, and possessing a sidearm donor, have all shown a
tendency to catalyze a living polymerization, to a certain degree. Aiming at the most living
catalyst of this system, we postulated that a strong sidearm oxygen donor obtained via a THF
substituent, would induce a pronounced living polymerization activity of the derived catalyst.
TiO
R
t-Bu
t-Bu
O
R
Nt-Bu
Ot-Bu
R = CH2Ph, Me
TiO
R
t-Bu
t-Bu
O
R
Nt-Bu
Ot-Bu
R = CH2Ph
Figure A-1. Dialkyl Ti complexes bearing the amine bis(phenolate) THF and furan ligands.
Dialkyl Ti-THF complexes (Figure A-1) were activated toward 1-hexene by the
reaction with B(C6F5)3. This polymerization system showed an unprecedented behavior: The
polymerization was living for almost a week (at RT), and the resulting polymer had a very
high Mw (around 106), and a very narrow MWD (PDI = 1.09) (Figure A-2). In addition, this
catalytic system enabled the copolymerization of two α-olefins, 1-hexene, and 1-octene at
RT, forming a di-block copolymer of narrow MWD. To further support our hypothesis that
the pre-condition for the living polymerization in the amine bis(phenolate) catalysts was the
dative strength of the oxygen sidearm donor, we prepared the dibenzyl amine bis(phenolate)
Ti complex, having a weak oxygen donor: furan. The structure of the analogous Zr complex
indicated that the furan oxygen was bound to the metal, and that the bonding was indeed
Abstract
4
weak. As anticipated, the Ti-furan/B(C6F5)3 system showed a different character of the
polymerization catalysis, when compared with Ti-THF: the polymerization in this system was
ca. 10-fold faster, but was not “truly” living, as the MWD was narrow only in the first few
hours of the reaction. Further investigation of the THF-Ti and furan-Ti catalysts enabled a
mechanistic insight into the reactivity of the amine bis(phenolate) catalysts, supporting a
mononuclear nature of the latter, and displaying no polymeryl transfer.
Figure A-2. “Immortal” polymerization of 1-hexene by THF-Ti: dependence of molecular weight (Mw) on time.
The numbers indicate the polydispersity index (PDI) values of the polymer samples.
Highly active zirconium catalysts for 1-hexene and propylene polymerization
This work is fully described in articles 1, 2 and 3.
In analogy to the Ti species, the dibenzyl amine bis(phenolate) Zr complexes,
providing the most convenient form of the pre-catalyst, were prepared by a toluene-
elimination reaction of tetrabenzylzirconium with the ligand precursor. The activation of the
THF-Zr complex with a borane co-catalyst gave rise to a highly efficient 1-hexene
polymerization catalyst, whose activity (21,000 gpol mmolcat-1 h-1) is defined as “very high”.
The resulting polymer was of relatively high Mw, having a PDI of around 2, indicating a
“single-site” species. Similarly, the furan-Zr catalyst was highly active in 1-hexene
polymerization. In contrast to the THF-Zr, furan-Zr led to a low-Mw polymer.
1.12
1.061.06
1.051.07
1.061.11
1.09
0100000200000300000400000500000600000700000800000900000
0 2 4 6 8Polym. Time (days)
Mol
ecul
ar W
eigh
t
Abstract
5
Next, we investigated the reactivity of the Zr-based catalysts in the polymerization of
the industrially challenging monomer, propylene. Two types of the pre-catalyst were
employed: The first having two benzyl groups at the labile positions, and the second having
two chloro groups. Both pre-catalysts, upon activation with MAO, led to a very efficient
polymerization of propylene (Scheme A-1). The optimized activity of this system was as high
as 13,000 gpol mmolcat-1 h-1. The resulting polypropylene was uncommon, combining
complete atacticity and relatively high molecular weight, thereby displaying elastomeric
properties.
Menn
Atactic polypropylene
Mw = 8,000 - 400,000250 - 1,100 equiv of MAO Me
Zr pre-catalyst
Scheme A-1. Propylene polymerization by the amine bis(phenolate) Zr catalyst.
Influence of the phenolate substituents on the reactivity of titanium catalysts
This work is fully described in article 4.
Aiming at developing of new activity modes, and discovering new structure activity
relationships, we prepared a series of Ti-based catalysts, possessing ortho (para) substituents
of varying steric and electronic nature (Scheme A-2). As the reactivity of these systems may
depend on the nature of the sidearm donor, two such series were prepared: One possessing a
NMe2 donor, and the other possessing an OMe donor. For the NMe2 series, the gradual
reduction of steric hindrance led to a considerable increase of the catalyst activity: The
catalyst having ortho Me groups had an activity of 1,400 vs. 30 for the catalyst having t-Bu
groups. An additional increase in activity (up to 8,000) was obtained using the chloro-
substituted ligands. Surprisingly, the reduction of the steric bulk, and use of electron-
withdrawing groups had no effect on the activity of catalysts featuring the OMe sidearm
Abstract
6
donor. The 13C NMR characterization of the polymer samples shed light on the origin of
these diverse trends for the two series: All catalysts of the NMe2 series produced a relatively
regioregular poly(1-hexene), whereas the reduction of the steric bulk in the OMe series led to
a substantial increase in the degree of regioerrors. The latter are known to decrease
polymerization rate significantly.
TiO
PhH2C
t-Bu
t-Bu
D
CH2Ph
Nt-Bu
Ot-Bu
t-Bu vs. Me
D = NMe2, OMe
TiO
PhH2C
Me
Me
D
CH2Ph
NMe
OMeTi
O
PhH2C
Me
D
CH2Ph
N
OMe
TiO
PhH2C
Cl
Cl
D
CH2Ph
NCl
OCl
Me
Me Me vs. Cl
D = NMe2, OMeD = NMe2, OMeD = NMe2
Me vs. H
Scheme A-2. Dibenzyl amine bis(phenolate) Ti(IV) complexes, displaying varying steric and electronic
influence at the metal center
The most efficient catalyst of these series led to an unusual polymer: Poly(1-hexene)
of an extremely high molecular weight (Mw = 4,300,000). In contrast to the “regular” poly(1-
hexene), which is a sticky oily substance, this polymer is in a rubbery state, showing the
properties of an elastomer.
Coordination chemistry of Ta(V) complexes with amine tris(phenolate) ligands
This work is fully described in articles 5 and 6
Wishing to extend the chemistry of the amine phenolate ligands beyond group IV
metals, we turned to investigate their chemistry with the group V metal, tantalum (V). The
Abstract
7
amine tris(phenolate) ligand, bearing Me groups at the ortho, para positions of the phenolate
rings, was chosen as a first ligand in this study. Two metal precursors, Ta(OEt)5 or
Ta(NMe2)5, reacted successfully with a ligand precursor, leading to a well-defined rigid
octahedral complexes of Cs-symmetry (Scheme A-3). Subsequently, these complexes were
treated with excess of Me3SiCl. Surprisingly, only one of the “labile” monodentate ligands
was exchanged. The solid-state structure determination revealed that the remaining OEt
(NMe2) group is trans to the central amine donor. The differentiation between the “labile”
positions was so strong, that the remaining OEt group did not undergo a metathesis reaction
at all, even when the reaction was carried out for weeks, or when stronger reagents (HCl, for
example) were employed. The desired dichloro complex could be obtained only via the
replacement of the more basic dimethylamido group, after prolonged reaction times.
TaO
X
Me
Me
O
X
NMe
OMe
Me
Me
HO
N
OHMe
Me
Me
Me
HO
Me
Me
TaX5
X = OEt, NMe2
TaO
Cl
Me
Me
O
X
NMe
OMe
Me
Me
xs. Me3SiCl
xs. Me3SiClTa
O
Cl
Me
Me
O
Cl
NMe
OMe
Me
Me
X = OEt, NMe2
4 days, X = NMe2
TaCl5
Scheme A-3. Coordination chemistry of Ta(V) in the amine tris(phenolate) environment
In contrast to the non-bulky ligand, the reaction of the sterically-crowded amine
tris(phenolate) ligand with Ta(NMe2)5 proceeded slowly, via a relatively stable dinuclear
intermediate, whose nature was established by means of X-ray diffraction. When formed, the
bis(dimethylamido) complex of the sterically-crowded ligand did not lead to a clean
Abstract
8
metathesis reaction with any chlorinating agent, while the analogous di(ethoxo) complex
allowed a clean substitution of a single group.
Synthesis, structure, and reactivity of pentabenzyltantalum
This work is fully described in article 7.
Pentabenzyltantalum was prepared by Schrock in 1976 by a two-step reaction
sequence: a reaction between tantalum pentachloride and dibenzyl zinc to produce
TaCl2(CH2Ph)3, which was further reacted with dibenzylmagnesium. We were able to
develop a more direct procedure, by preparation of pentabenzyltantalum via the single-step
reaction of TaCl5 with PhCH2MgCl. As the X-ray structure of this fundamental compound
was never reported, we solved its crystal structure as well (Figure A-3). Subsequently, we
evaluated the potential of Ta(CH2Ph)5 as a precursor to Ta(V) organometallic complexes with
the amine phenolate ligands. Two representative ligands of the amine bis(phenolate) and the
amine tris(phenolate) types were reacted with Ta(CH2Ph)5. Both ligands reacted smoothly,
leading to the expected tribenzyl, and dibenzyl complexes, respectively, in high yields.
Figure A-3. Molecular structure of pentabenzyltantalum.
Abstract
9
Investigation of α-abstraction vs. ligand backbone β-abstraction paths in the tribenzyl
tantalum(V) complexes of amine bis(phenolate) ligands
This work is fully described in article 8.
A variety of tribenzyl amine bis(phenolate) Ta(V) complexes could be prepared
smoothly using the pentabenzyltantalum route. All these complexes displayed a similar
propensity: They are hexacoordinate, and the fourth, sidearm donor is a “dormant” donor in
these species. We postulated that such a “dormant” donor may cause various organometallic
transformations in the parent tribenzyl species, and thus we carried out a thorough SAR study
of the H-abstraction reactions in this system as a function of various structural modifications.
TaO
PhH2C
R
R
D
CHPh
NR
ORTaO
PhH2C
R
R
D
CH2Ph
NR
OR
α-Hβ-H
C
Ta
N
PhH2C O
O CH2Ph
Ph HH
DHH
R
R
α
β
Scheme A-4. Possible H-abstraction routes for the tribenzyl complexes
When subjected to thermolysis, these species underwent two well-defined reactions:
Abstraction of an �-proton (from one of the benzyl groups), forming the metal-carbon double
bond (benzylidene), or abstraction of a β-proton (from the benzyl position in the amine
bis(phenolate) ligand), forming a new metal-carbon single bond (Scheme A-4). The choice of
the abstraction pathway was found to depend on two structural parameters: The presence of
the “dormant” donor, and the steric bulk at the ortho positions of the phenolate rings. The
presence of the sidearm donor, accompanied by small (H, Cl) ortho substituents led to
smooth �-H abstraction. When the sidearm donor was present, and the ortho substituents were
Abstract
10
bulkier, the main reaction was facile β−Η abstraction. In one particular case, i.e. t-Bu
phenolate substituents, the β−Η activation took place even at RT. In these cases, the sidearm
donor was bound to the metal following the H-abstraction and toluene elimination. When the
donor was absent (an [ONO]-type ligand), the reaction took the β−Η abstraction pathway
exclusively, and proceeded much slower. All the H-abstraction reactions were of first order in
the reactant, indicating an intramolecular mechanism.
Exploring routes to tantalum(V) alkylidene complexes supported by amine tris(phenolate)
ligands.
This work is fully described in article 9.
At the next step, an alkylidene functionality supported by amine tris(phenolate)
ligands was pursued. On the way to the desired alkylidene species we employed two
fundamental organometallic species, tris(neopentyl) mono(neopentylidene) tantalum(V)
(Ta(CH2t-Bu)3(CHt-Bu)), and pentabenzyltantalum, as starting materials. The reaction of the
amine tris(phenolate) ligand precursor with Ta(CH2t-Bu)3(CHt-Bu) proceeded via the O-H
addition to the metal-carbon double bond, forming a phenoxo-tetraneopentyl complex, one of
the most sterically congested organometallic species. Further reactivity of this complex was
less well-defined, as it decomposed into unidentified species, not containing an alkylidene
function.
The pentabenzyltantalum route led to the octahedral dibenzyl complexes with a
variety of amine tris(phenolate) ligand precursors. Unlike the tribenzyl amine bis(phenolate)
complexes, the dibenzyl complexes were found to be remarkably stable toward thermolysis,
decomposing only at 120 ºC after prolonged reaction times. Instead of forming the
mononuclear terminal alkylidene functionality, the reaction unexpectedly led to the rare
dinuclear µ-benzylidene complex, in which both Ta(V) centers were of octahedral geometry
(Figure A-4).
Abstract
11
Figure A-4. Molecular structure of µ-benzylidene di-Ta(V) complex.
Vanadium(III) and Vanadium(V) amine tris(phenolate) complexes
This work is fully described in article 10.
Following the investigation of the chemistry of the heavier group V member,
tantalum, we turned to the lighter group V member, vanadium. The chemistry of this metal
was studied especially in light of its bioinorganic relevance, as vanadium participates in the
active site of several enzymes. Three amine tris(phenolate) ligands featuring different steric
and electronic influence were employed in this study (Scheme A-5). V(III) complexes of
these ligands were obtained by reaction between the ligand precursors and VCl3(THF)3, in the
presence of Et3N. All the resulting complexes were of almost perfect TBP geometry, in which
the three phenolate oxygens lied in the equatorial positions, one axial positions was occupied
by the central nitrogen, and the other by an additional ligand: THF. The V(III) complexes of
the “electron-rich” ligands, having ortho, para Me and t-Bu phenolate substituents, were
found to undergo smooth and clean oxidation to V(V) oxo species, when exposed to air.
Alternatively, the V(V) complexes could be prepared directly, starting from VO(OPr)3. The
crystallographic and multinuclear NMR studies of these complexes supported their structure
Abstract
12
to be TBP, similar to the V(III) species, with the sole difference being replacement of the
axial neutral THF ligand by the dianionic oxo ligand. This structure is noteworthy: It mimics,
to a great extent, the structure of the (resting) active site in V-dependent haloperoxidase.
Scheme A-5. Coordination chemistry of V(III) and V(V) amine tris(phenolate) complexes
The reaction of the “electron-poor” ortho, para dichloro substituted ligand with
VO(OPr)3 resulted in the formation of V(V) species having a different structure. The X-ray
solution disclosed an octahedral amine tris(phenolate) complex, carrying an aqua ligand, in
addition to the oxo ligand. In addition to the octahedral species, the products mixture
contained a small fraction of a TBP complex. Thus, it appears that the amine tris(phenolate)
complexes are able to switch between the TBP and octahedral geometries. In addition, the
V(V) amine tris(phenolate) species have shown an oxygen-transfer reactivity, being able to
catalyze, albeit slowly, oxygen transfer from a peroxide to styrene and stilbene.
Group IV complexes of a tetradentate amine mono(phenolate) ligand: a second extra donor
stabilizes cationic species
This work is fully described in (submitted) article 11.
VO
OO
NR
R
R
RR
R
O
VO
L
Cl
Cl
ClO
O
NCl
O
Cl
Cl
N
OHHO
OH
R
R
R
R
R
RVCl3(THF)3
VO
OO
NR
R
R
RR
R
O
R =
R = Me, t-Bu, Cl
Me, t-BuairVO(OPr)3
R = Cl
VO(OPr)3R = Me, t-Bu
- L, ∆
+ L
Abstract
13
The amine mono(phenolate) ligand precursor was prepared by a single-step Mannich
condensation between a secondary amine, a substituted phenol, and formaldehyde. Upon
reaction with suitable Group IV metal precursors, this ligand precursor led to the highly
flexible tris(alkoxo) complexes with Ti(IV) and Zr(IV), and the tribenzyl complex with
Zr(IV) metal. At the next step, the removal of one of the alkoxo (alkyl) groups by means of
Lewis or Bronsted acid (B(C6F5)3 or [PhNMe2H][B(C6F5)4]) led to a fast and clean formation
of cationic complexes, in which the “extra” donor was bound to the metal (Scheme A-6). The
cationic alkoxo complexes display a rigid C1-symmetry, while the analogous alkyl species are
presumably of Cs-symmetry. The species demonstrate a remarkable stability: The cationic
alkoxo species are stable both in solution and in solid state for weeks, while the cationic alkyl
species are stable for days.
MO
O OR
OR
OR
Nt-Bu
t-BuO
Me
Me
OH
N
OMe
MeO
t-Bu
t-Bu
M(OR)4 MO
O O
OR
Nt-Bu
t-Bu
Me Me
OR
+
B(C6F5)4 -
[PhNMe2H]
[B(C6F5)4]
Scheme A-6. Synthesis and structure of the neutral tris(alkoxo) and cationic bis(alkoxo) Group IV metal
complexes with the amine mono( phenolate) ligand. M = Ti(IV), Zr(IV), R = Ot-Bu, Oi-Pr.
Salophan complexes of group IV metals
This work is fully described in article 12.
Sequential diamine bis(phenolate) Salophan ligands are similar to Salan ligands, with
the only difference between them being the nature of the link between the two nitrogen
donors: Flexible ethylene (diamine) bridge in Salan, and rigid ortho-phenylene (diamine)
bridge in Salophan. Early transition metal complexes of Salan ligands are in the focus of
Abstract
14
intensive investigation by several research groups; in contrast, no early transition metal
complexes of Salophan ligands have been reported thus far, with the exception of a single Mo
dioxo complex. We synthesized three new Salophan ligand precursors, featuring ortho Me,
ortho, para di-chloro, and ortho, para di-t-Bu phenolate substituents, in addition to the
previously reported prototypical Salophan ligand, bearing only hydrogen substituents at the
phenolate rings. The ligand pecursors were prepared by a simple sequence of condensation
and reduction. All the ligand precursors were reacted with Ti(Oi-Pr)4 and Zr(Ot-Bu)4
(Scheme A-8). We found that the unsubstituted Salophan led to a complex product mixture
for both metal precursors, whereas the ortho-Me substituted ligand led to a clean complex
only with zirconium. Other, namely ortho, para di-chloro, and ortho, para di-t-Bu substituted
ligands, gave a well-defined coordination chemistry with both metals. NMR analysis
indicated that all the Salophan ligands act as dianionic ligands, forming hexa-coordinate
complexes of C2-symmetry. X-ray structure analysis of these species revealed that the
wrapping mode of the ligands was fac-fac, and the orientation of the labile groups was cis
(Scheme A-7).
Scheme A-7. Wrapping mode of the Salophan ligands around Group IV metals
N N
OHR1
R2
HOR1
R2
H H
O
N
MN
OOR
R1
R1
R2
R2RO
H
H
M(OR)4M=Zr: R=Ot-Bu, R1=Me, R2=H
M=Zr: R=Ot-Bu, R1=R2=Cl
M=Ti: R=Oi-Pr, R1=R2=Cl
M=Ti: R=Oi-Pr, R1=R2=t-Bu
M=Ti: R=Oi-Pr, R1=R2=t-Bu
Introduction
15
Introduction
This thesis is devoted to the study of early transition metal complexes with amine
phenolate ligands, and presents their coordination chemistry and reactivity. In the
introduction I will describe various ligand systems with emphasis on oxygen-donor ligands,
their characteristic chemistry, and reactivity in the fields relevant to this work.
I-1. Alkoxo (phenolate) ligand vs. Cp ligand
In recent years we are witnessing a huge interest in the development of new ligand
systems that may be used as an alternative to the cyclopentadienyl (Cp) ligand in the
chemistry of early transition metals. Ligands based on the alkoxo- and amido donors have
attracted the most considerable attention, as these “hard” multi-electron donors successfully
stabilize high oxidation states of the early transition metals, that are defined as “hard acids”.1
The alkoxo (phenolate) ligands are especially attractive: The ligand precursors are
sufficiently acidic, bind the “oxophilic” early transition metals firmly, and may be viewed as
isoelectronic with the cyclopentadienyl ligand.2,3
Several important differences between cyclopentadienyl ligands and alkoxo ligands
should be taken into account. The alkoxo ligand is less electron-donating than the Cp ligand,
thus the alkoxo complexes are less electron-saturated.2-4 A simple alkoxo or phenoxo ligand
is less bulky than the cyclopentadienyl ligand. As a result, the use of a simple alkoxo
(phenoxo) ligand normally leads to complex product mixtures.2-4 An additional obstacle in
the chemistry of the alkoxo (phenoxo) ligands is their ability to bridge metal atoms.4 An
aggregation of metal complexes causes ligands redistribution, and provides low-energy
pathways for the decomposition of coordinatively unsaturated metal complexes.
Introduction
16
I-2. From a simple alkoxo to a bulky phenolate ligand.
Two general strategies have been developed to overcome the obstacles encountered in
the use of simple alkoxo or phenolate ligands: Employing bulky monodentate ligands, or
designing multidentate ligands. The use of bulky ligands allows accommodation of only a
limited number of ligands around the metal center, which could lead to the desired structure
of a metal complex, and to prevent aggregation. Throughout the last two decades, the motif of
bulky monodentate alkoxo, siloxo, and phenoxo ligands has been carefully investigated.2,3,5,6
The steric bulk of a ligand may be approximated using the “cone angle” concept.1,7 For the
Cp ligand, the cone angle was estimated to be 136º.7 Wolczanski and coworkers have
introduced a number of bulky alkoxo and siloxo ligands, with cone angles close to this value
(Figure I-1).2 The substantial steric bulk of the ligand allowed a clean metathesis reaction,
leading to the alkoxo/siloxo complexes of desired structures. As a result of the coordinative
unsaturation at the metal center, some of these complexes exhibited an unusual reactivity,
including carbon monoxide cleavage, oxidative addition of various substrates, and formation
of metal-metal bonded complexes.2
Figure I-1. Cone angles of the Cp, Ot-Bu, and “tritox” ligands
Bulky phenolates were subject of even more thorough investigation, especially by the
groups of Rothwell and Schrock.3,5,6 The phenolate may be viewed as a more "user-friendly"
ligand than simple alkoxide, as it is more acidic and its properties are more easily controlled
M
O
Ct-Bu
t-But-Bu
M
M
O
CMe MeMe
136 o < 90 o 125 o
Introduction
17
by the utilization of the appropriate substituents. As phenolate is a planar rather than a cone-
shaped ligand, the “cone angle” term is less appropriate for this ligand. The steric bulk of the
phenolate is normally transferred via its ortho substituents that point toward the metal; the
bulky ortho groups prevent aggregation of the metal centers and lead to the well-defined
constitution of a metal complex. For example, for 2,6 di-t-Bu phenolate ligands, complexes
containing only two such ligands (in axial positions), in addition to three alkyl ligands, were
typically formed for tantalum (V) (Figure I-2).5
CH3
Ta
CH3H3C
O O
t-Bu
t-Bu
t-Bu
t-Bu
Figure I-2. A well-defined mononuclear complex of Ta(V) carrying a 2,6 di-t-Bu phenolate ligand.
I-3. Multidentate ligands: Enhanced stabilization and easier design of metal centers.
Exploitation of the chelate effect provides a second possible strategy for the
stabilization of metal-alkoxide complexes. In general, multidentate ligands bind the metal
firmly due to the chelate effect,1 and, not less importantly, may enable precise control of the
structure and reactivity at the metal center. The following non-alkoxo ligand system, relevant
to the studies presented in this work, illustrates this trend. The triamidoamine [N3N] ligand
system, investigated initially by Verkade,8 was introduced to the early transition metals realm
by Schrock.9 These ligands were shown to bind to metals in a tetradentate, almost uniformly
C3v-symmetrical manner, creating a 3-fold-symmetric axial “pocket” (Figure I-3). In that
pocket, 3 orbitals are available for bonding with additional ligand(s): two π orbitals and one σ
orbital. In addition to the electronic consideration, this pocket was influenced sterically by
bulky substituents at the amido nitrogens. All this led to extremely rich chemistry of the
Introduction
18
metal-ligand multiple bonds. The formation of metal-ligand multiple bonds in this position
was frequently unavoidable: For instance, the reaction of various [N3N]TaCl2 complexes with
any Grignard reagent bigger than methyl led uniformly to the formation of the tantalum
alkylidene (M=CR2) function, and tungsten alkyl complexes [N3N]WR readily lost
dihydrogen at RT to form metal alkylidyne (M≡CR) complexes. In addition, the unique
ability of this ligand system to stabilize multiple bonds allowed to fulfill one of the “Holy
Grails” of modern inorganic chemistry: catalytic dinitrogen activation to ammonia via a 14-
step cycle including the states of imide and nitride, 10 of the intermediate species being
isolated and characterized.10
N
NN
R R
NR
z
dz2 dxz
N
N NTa
H R'R R
NR
N
NNW
R'R R
NR
N
NNMo
NR R
NR
N
NN
Mo
PR R
NR
Figure I-3. Orbitals available for the axial ligand(s) in a 3-fold symmetrical pocket (only one orbital of the
degenerate set (dxz, dyz) is shown); examples of multiply bonded ligands in the [N3N] metal complexes
Introduction
19
I-4. Multidenatate phenolate ligands.
Several multidentate phenolate-based ligand systems became widely known and
extensively used in recent years.4 Simple methylene- or ethylene-bridged bis(phenolate)
ligands have been investigated by several groups. These ligands, providing only two donors,
forming an 8-membered chelate ring (or a 9-membered chelate ring), and being relatively
flexible, do not impose a rigid geometry at the metal. Thus, the resulting early transition
metal complexes may be tetra-, penta-, or hexacoordinate, depending on the reaction
conditions (Scheme I-1).11 The use of a mononuclear metal precursor for these ligands
generally leads to a formation of a mononuclear product. In contrast, for dinuclear metal
precursors (Mo2(NMe2)6 and W2(NMe2)6), the ligand can bind either to a single metal or to
bridge between the metal centers (Scheme I-2).12
OHR1
R2
OHR1
R2
TiCl4
OR1
R2
OR1
R2
Ti
Cl Cl
+ THF- THF
OR1
R2
OR1
R2
TiCl
ClTHF
ZrCl4THF2
OR1
R2
OR1
R2
ZrCl Cl
THFTHF
Scheme I-1. Coordination behavior of methylene-bridged bis(phenolate) ligand with Group IV metals.
Introduction
20
OR1
R2
O R1
R2
Mo2(NMe2)6
OR1
R2
OR1
R2
Mo Mo
OR1
R2
OR1
R2
R2N
NR2 +Mo
MoO
R1
R2
OR1
R2
NR2
R2N
OH
R1
R2
OH
R1
R2
Scheme I-2. Coordination behavior of the bis(phenolate) ligand with Mo(VI) dinuclear metal centers.
Another example of a relatively simple chelate ligand system is the α, α’-bridged
bis(naphtholate) (BINOL) ligand.13 This ligand leads mostly to tetra- or penta-coordinate
complexes; in the case of bulky ortho substituents, the formation of bis(homoleptic)
complexes is diminished to a large extent (Scheme I-3). The most unique feature of this
ligand is its intrinsic chirality; thus, BINOL provides a chiral backbone in the resulting metal
species. For the Group IV metals possessing identical monodentate ligands (X), the resulting
tetrahedral complexes are C2-symmetrical, and the labile positions are homotopic.14
OHOH
OO
MXX
MX4
R
R
R
R
Scheme I-3. A BINOL ligand precursor, and the resulting tetrahedral M(IV) complex.
Sequential methylene-bridged tris(phenolate) ligands were investigated by Kawaguchi
and coworkers.15 As in the respective bis(phenolate) ligands, the phenolate rings in the
tris(phenolate) ligands are connected at the ortho positions through methylene linkers. These
Introduction
21
ligands demonstrated even more flexible behavior than the corresponding bis(phenolates):
Several conformations for the coordination of this type of ligands to the metal center were
reported. For the early transition metals (Ti(IV), Ta(V), Nb(V)), these ligands normally form
dinuclear complexes. The nearby metal centers in these complexes could be bridged by
additional monodentate (hydrido) or bidentate (DME) ligands, or by the phenolate oxygens
themselves (Figure I-4).
Figure I-4. A dinuclear di-Ti complex with the “open-chain” tris(phenolate) ligand.
The ligands presented in the previous paragraphs are all sequential (“open-chain”)
phenolate-only ligands. A very important family of multidentate phenolate ligands is the
family of cyclic poly(phenolate) ligands, termed calixarenes (“bowl-shaped”) after Gutsche.16
This general term includes all cyclic poly(phenolates), connected by methylene bridge; most
of the inorganic and organometallic chemistry was performed with the ligand containing four
phenolate units (calix[4]arene).17 This ligand may adopt various conformations, with the
“cone” conformation being the most common for the early transition metal complexes
(Scheme I-4). The basic calix[4]arene ligand is tetraanionic, which generally results in the
formation of dinuclear complexes for the Group IV metals, and may also lead to the dinuclear
species with Group V metals (Scheme I-4). For Group V metals, formation of a mononuclear
comlex may be achieved by using a Cp group as an additional, fifth ligand. In contrast, this
O
O
Ot-Bu
Ti
O
O
O
t-Bu
Ti
Cl
Cl
Introduction
22
tetraanionic tetradentate ligand normally forms a mononuclear well-defined species with
Group VI (M(VI)) metal centers. A variety of di- and trianionic calix[4]arene derivatives
could be synthesized by a selective Williamson alkylation at the phenolate oxygens. For the
corresponding dianionic ligands, mononuclear Group IV complexes are obtained.
Scheme I-4. Top: coordination chemistry of the tetraanionic calix[4]arene toward early transition metals.
Bottom: A mononuclear Zr(IV) complexes of a dianionic calix[4]arene.
The chemistry of chalcogen-bridged bis(phenolate) and bis(naphtolate) ligands
([OEO], E = S, Te) with Group IV metals has been studied mainly by Kakugo and coworkers,
and by Okuda and coworkers.18 These ligands are dianionic and tridentate, binding to the
OO O O
HH H H
t-Bu t-Bu t-Bu t-Bu
Ti(NMe2)4
OO O O
t-Bu t-Bu t-Bu t-Bu
OOOO
t-But-But-But-Bu
TiTi OOOO
t-But-But-But-Bu
WO
W(O)Cl4calix[4]arene
CpTaCl4
OOOO
t-But-But-But-Bu
Ta
OO O O
HMe H Me
t-Bu t-Bu t-Bu t-Bu
n-BuLi
ZrCl4O
O O OMe Me
t-Bu t-Bu t-Bu t-Bu
ZrCl Cl
OO O O
Me Me
t-Bu t-Bu t-Bu t-Bu
Zr
PhH2C CH2Ph
PhCH2MgCl
Introduction
23
metal through the phenolate oxygens and the central neutral donor. Due to the 5-membered
chelate rings, these ligands tend to adopt a more rigid coordination at the metal. The
coordination mode of these ligands is facial (fac) in octahedral geometry, which leads to the
cis disposition of the phenolate oxygens. For potentially bridging labile ligands (i.e., OR, Cl),
the resulting metal derivatives are dimeric, possessing nearly octahedral environment
(Scheme I-5). In contrast, the organometallic complexes of these ligands are monomeric and
pentacoordinate. Further studies on Mo(VI), W(VI), and Sm(III) metal complexes with this
type of ligand have all indicated a facial binding of the [OEO] ligand to the metal center.19
S
O Ti
O
R
RCl
Cl Cl
S
OTi
O
R
R
Cl
OH OH
RRTiCl4
MeLi
O O
RR
Ti
S
S
Me Me
S
O Ti
O
R
RTHF
Cl Cl
THF
Scheme I-5. Coordination chemistry of the [OSO] ligands at Ti(IV) metal centers.
Possibly, the most appreciated veteran in the field of multidentate phenolate ligands is
the “sequential” tetradentate di(imine) bis(phenolate) ligand, also known as SALEN ligand
(Scheme I-6).20 Complexes of nearly all early transition metals with this ligand have been
prepared and characterized. This ligand, being dianionic and tetradentate, leads to well-
defined penta- or hexacoordinate mononuclear complexes, generally avoiding the formation
Introduction
24
of the bis(homoleptic) or bridging species. In octahedral geometry, this rigid ligand normally
imposes a “mer-mer” planar disposition of its four donor atoms, leaving the two remaining
positions mutually trans (Scheme I-6).
NN
HOOHRR
NN
OO
RR
M
X
X
MX4
Scheme I-6. SALEN ligand precursor, and the resulting metal (M(IV)) complex.
Reducing the C=N double bonds in the SALEN ligand leads to the di(amine)
bis(phenolate) [ONNO]-type SALAN ligand precursor (Scheme I-7). In analogy to the Group
IV SALEN metal complexes, SALAN metal complexes are also well-defined mononuclear
species of octahedral geometry. However, these saturated ligands demonstrate completely
different binding mode: The fac-fac coordination of the [ONN] fragments, resulting in cis
disposition of the monodentate ligands. Thus, the resulting metal species are C2-symmetrical
and chiral-at-metal.21 A similar binding mode has been reported for the di(thio)
bis(phenolate) [OSSO] ligands.22
NN
OHR
R
HO
R
RMX4
O
N
M
N
OX
R
R
R
RX
Scheme I-7. Wrapping mode of the SALAN ligands around Group IV metal centers
Introduction
25
“Breaking” the SALEN ligand into two parts leads to bidentate monoanionic
phenoxy-imine ligands. Group IV metal complexes bind two such ligands, in addition to two
monodentate ligands (chloro or alkyl groups). Similar to the di(amine) bis(phenolate) ligand
complexes, and in contrast to the SALEN metal complexes, the phenoxy-imine ligands are
oriented such that the overall symmetry is C2, the two phenolate oxygens mutually trans, and
the monodentate ligands are cis to one another (Scheme I-8).23
Scheme I-8. Phenoxy-imine ligands, and their Group IV complexes. The resulting catalysts are termed “FI”
catalysts after phenoxy-imine ligands.
The recent findings in the field of multidentate phenolate-based ligands were recently
reviewed.4 It should be noted, however, that this field remains relatively unexplored, in
comparison with the polydentate amido ligands,24 and major effort is invested today in the
design and exploration of new chelate phenolate-based ligands.
I-5. Synthetic pathways to early transition metal complexes bearing phenolate ligands
The salt metathesis route between the ligand salt (Li, Na, etc.) and the metal chloride
provides the most commonly used method for the preparation of early transition metal
complexes with various sorts of ligands (Scheme I-9). However, the chemistry of the
phenolate-based ligands with early transition metals is rather unique, as the phenol function
in the ligand precursor is substantially more acidic than most other functions (such as simple
MX4
R'N
OH2 MN
O
R'N
O
R'X X
R
R
R
Introduction
26
alkohol, amine, or alkane). This may be employed to help avoiding the rather notorious salt
metathesis route normally required for the amido or cyclopentadienyl ligands. Thus,
homoleptic metal alkoxides or amides may be employed as the metal precursors in alcohol or
amine elimination reaction, respectively (Scheme I-10). One of the goals of the current work
was to show that the homoleptic metal alkyl precursors, such as tetrabenzyltitanium or
pentabenzyltantalum, could be used as starting materials of choice in the chemistry of
phenolate ligands.
ArOH + BuLiMClx ArOMClx-1 + LiClArOLi
Scheme I-9. The salt metathesis route to early transition metal complexes with the phenolate ligands
ArOH + M(OR)x ArOM(OR)x-1 + ROH
ArOH + M(NR2)x ArOM(NR2)x-1 + R2NH
ArOH + MRx ArOMRx-1 + RH
Scheme I-10. The route of the alcohol, amine, and alkane elimination reactions, respectively, to early transition
complexes with phenolate ligands
I-6. Reactivity of early transition metal complexes supported by multidentate phenolate
ligands
In the last decade, phenolate-based chelating ligands became widely abundant in the
frontier fields of inorganic chemistry and catalysis. Among other fields, early transition metal
complexes of multidentate phenolates show an extraordinary reactivity in the fields of
enantioselective transformations,25 olefin polymerization, lactide and caprolactone
Introduction
27
polymerization,26 olefin metathesis, and bioinorganic chemistry. In the following sub-
paragraphs, a description of the fields relevant to this work is presented.
I-6.1. Olefin polymerization by well-defined Group IV metal complexes
One of the research fields most significantly influenced by the chelating phenolate-
based ligands is the field of α-olefin polymerization catalysts.27 The field of α-olefin
polymerization emerged in 1954, when Ziegler discovered that a combination of a simple
early transition metal complex (TiCl4), and a main group compound (AlEt3) catalyzes a
polymerization of ethylene to polyethylene under mild conditions.28 Soon after, Natta and
coworkers reported the stereoregular (isotactic) polymerization of propylene by the same
catalyst, producing a crystalline polymer of high melting point (Scheme I-11).29 This catalytic
system was heterogeneous, and therefore ill-defined. As a result, this multi-site system
produced polymers mixtures of broad molecular weight distribution and different properties,
and impeded a thorough understanding of the polymerization mechanism. Soon, it became
understood that the preparation of the well-defined polymers demands the preparation of
well-defined (“single-site”) catalysts.
Me
TiCl4
AlR3Me
n
x +
Mey
isotactic polypropylene: crystalline, high-melting
atactic polypropylene:amorphous, oily
Scheme I-11. Propylene polymerization by the heterogeneous system TiCl4/AlR3
The first homogeneous models were constructed from the Group IV bent
metallocenes (CpMX2) and alkyl aluminum compounds as activators (“co-catalysts”)
Introduction
28
(Scheme I-12). In contrast to the heterogeneous systems, these catalytic systems showed low
activity in the polymerization of ethylene, and negligible activity toward propylene and
higher alpha-olefins.30 Two major discoveries revolutionized this field. First, the reactivity of
the metallocene-based catalysts was significantly improved by the invention of a new co-
catalyst: Methylalumoxane (MAO), a partially hydrolyzed Me3Al having an oligomeric
form.31 Second, the invention of chiral (C2-symmetric) ansa-metallocenes has led the way to
the desired stereoregular (isotactic) polypropylene.32 Consequently, Cs-symmetric
metallocenes having enantiotopic sites led to the syndiotactic polypropylene,33 that was
inaccessible via the heterogeneous group IV Ziegler-Natta catalysts.
Scheme I-12. Top: Propylene polymerization by C2v-symmetric metallocenes. Middle: Propylene
polymerization by C2-symmetric ansa-metallocenes. Bottom: Propylene polymerization by Cs-symmetric ansa-
metallocenes.
ZrClCl
AlR3 Men
Me
n
Low activity, low Mw, atactic polypropylene
ZrCl Cl
ZrCl Cl
Me
n
Me
n
High activity, high Mw, isotactic polypropylene
High activity, high Mw, syndiotactic polypropylene
MAO
Al(Me)-O n(MAO)
Men
Me n/2Me
Introduction
29
One should not underestimate the contribution of metallocene catalysts to the field of
α-olefin polymerization, as these catalysts shed light on polymerization mechanism, and led
to novel polymer types.34 However, the versatility of a single ligand family in determining of
catalyst structure (metal geometry, its electron and steric saturation) is limited by definition.
In addition, the synthesis of various functionalized Cp ligands, and their metal complexes
may be quite cumbersome, and almost no metallocenes found their way to the polymer
industry for various reasons. Thus, in the middle of the 1990’s, a search for new ligand
systems for polymerization catalysts has begun.
At first, the amido-based ligands attracted most of the attention, as the derived
catalysts were shown to lead to “living polymerization”.24a Several criteria have been
proposed for living polymerization, among them are negligible termination (or chain transfer)
and fast initiation relative to propagation.35 Living polymerization can be of great importance,
as it leads to well-defined polymers of high-Mw and narrow molecular weights distribution. In
addition, living polymerization allows the preparation of block-copolymers. Prior to these
reports, no living polymerization catalysts, operating at ambient conditions, had been
reported in the field of Ziegler-Natta polymerization. In 1996, McConville and coworkers
have reported that a Ti-based catalyst, carrying a diamido ligand, polymerizes 1-hexene in a
living fashion (Scheme I-13).36 The living character of polymerization was indicated by a
linear rise in the molecular weight of the resulting polymer vs. time and vs. monomer
consumed, and by a narrow molecular weight distribution (MWD, or PDI). Since then, the
amido-based polymerization catalysts were thoroughly studied, especially in relation to living
polymerization.24a,37, 38
Introduction
30
Scheme I-13. Living polymerization of 1-hexene by diamido-based Ti catalyst
The great promise of the phenolate-based polymerization was demonstrated in the
work of Schaverien and coworkers in 1995 describing highly isotactic polymerization of 1-
hexene using BINOL Ti(IV) and Zr(IV) dibenzyl complexes (see Scheme I-3).14 It should be
noted, however, that the major interest in the phenolate-based catalysts has arisen only
recently, with the development of four families of polymerization catalysts.39 The first family
is the family of Ti catalysts based on the [OSO] ligands (Scheme I-5) that have led to the
active polymerization of ethylene and styrene.18 The second family is the family of “FI”
catalysts (Scheme I-6).23 All the complexes of this family are C2-symmetric, and the steric
and electronic parameters of this system may be easily controlled by the phenolate
substituents, and the imine substituents. As a result of fine structural modifications, some of
these catalysts exhibited unusual reactivities. For example, the FI-Zr catalysts, possessing a
bulky group at the ortho position of the phenolate rings were reported to be the most active
ethylene polymerization catalysts ever reported. The sterically-hindered and the electron-
deficient FI-Ti catalysts, possessing bulky groups at the ortho positions, and the meta-
difluorophenyl groups bound to the nitrogen donors, led to the living and syndiotactic
polymerization of propylene (Scheme I-14).
TiN
N
Me
Me
i-Pri-Pr
i-Pr i-Pr
B(C6F5)3Bu
nBu
nPDI < 1.1
Living poly(1-hexene)
Introduction
31
Me
n
Living polymerization yielding
syndiotactic polypropylene
MAOMe n/2Me
TiN
O
t-Bu
RN
O
t-Bu
RCl Cl
FF
R =
Scheme I-14. Propylene polymerization by FI-Ti catalyst
The third family consists of the amine bis(phenolate) metal complexes discussed in
the present work. Finally, the fourth family is the family of the di(amine) bis(phenolate)
ligands.21,40 The labile positions, at which the polymerization reaction takes place, are
homotopic in this catalyst, due to the rigid C2-symmetric structure of the complex. Thus, in
analogy to the C2-symmetric metallocenes, an isotactic polymerization of various α-olefins
was observed.21a, 21b, 40 In addition, a living and isotactic polymerization of an α-olefin (1-
hexene) was achieved by the Zr catalyst of this family (Scheme I-15).21a The precondition for
the living polymerization in this system is the presence of bulky groups in ortho positions,
which are presumed to retard termination.
Scheme I-15. 1-hexene polymerization by di(amine) bis(phenolate) Group IV complex
nBu
nBu
Living and isotactic poly(1-hexene)
B(C6F5)3
O
N
ZrN
OCH2Ph
t-Bu
t-Bu
t-Bu
t-BuPhH2C
Me
Me
Introduction
32
Given these findings, the potential of the phenolate-based multidentate ligands has
been widely recognized by multiple research groups. Thus, at the present time, novel
phenolate-based olefin polymerization catalysts are designed, and their reactivity is
investigated by many research groups worldwide.
I-6.2. Stabilization of metal-carbon double bonds and olefin metathesis reactivity
The first metal-carbon double bond of the ”alkylidene” type was reported by Schrock
in 1974.41 In the course of investigation of Ta(V) homoleptic alkyl complexes, Schrock
discovered that the compound formed by the reaction between Ta(CH2t-Bu)3Cl2 and two
equivalents of t-BuCH2Li was actually Ta(=CHt-Bu)(CH2t-Bu)3, instead of Ta(CH2t-Bu)5
(Scheme I-16). Following the discovery that the alkylidene function mediates olefin
metathesis,42 metal-carbon multiple bonds became a subject of principal interest in
organometallic chemistry.43
Ta(CH2t-Bu)3Cl2 Tat-BuH2C
t-BuH2C
CH
CH2t-Bu
CH2t-Bu
t-Bu H
2 NpLi
- 2 LiCl
- NpHTa
CH2t-Bu
C(H)t-Bu
t-BuH2C
t-BuH2C
Scheme I-16. The α-hydrogen abstraction reaction, leading to the formation of an alkylidene complex
The major route that leads to the alkylidene function is the α-hydrogen abstraction
reaction, taking place in a dialkyl precursor (Scheme I-16).43 If the alkyl group carries β-
hydrogens, β-H elimination may compete with the α-hydrogen abstraction reaction. The α-
hydrogen abstraction reaction normally goes via a monometallic mechanism, being therefore
of first order in the reactant.44
Introduction
33
TaO
O
THF
R R
R
R
O
R
RCt-Bu
H
Mo
N
CHRO
O
i-Pr i-PrR
R RR
Figure I-5. Alkylidene complexes, supported by monodentate phenolate ligands.
Monodentate phenolate-based ligands were extensively used to support the alkylidene
functionality, and to study its structure and reactivity (Figure I-5).3,5 However, only recently
multidentate phenolate ligands have been introduced into this field. Following the highly
efficient Mo(VI) metathesis catalysts, supported by two monodentate alkoxo or phenolate
ligands, Schrock and coworkers prepared Mo(VI) alkylidene complexes, supported by a
single bis(phenolate) or BINOL ligands.45 The resulting species are mainly tetracoordinate,
containing an imido function in addition to the alkylidene and the phenolate oxygens
(Scheme I-17). Most significantly, these species are chiral, as a result of the ligand backbone,
and may be prepared in an enantiomerically pure form. These species have been found to be
highly effective stereoselective catalysts for various asymmetric olefin metathesis reactions.
For example, enantiomerically-pure small rings could be prepared from achiral precursors in
high yields, and with excellent enatiomeric excess (Scheme I-17).46
Introduction
34
Mo
N
CHROO
i-Pr i-Prt-Bu
t-Bu
Me
Me Me
MeO O
Me H
Me
Me2 mol %
99% ee, 93% yield
Scheme I-17. An asymmetric RCM reaction catalyzed by the Schrock-Hoveyda catalyst.
I-6.3. Phenolate ligands in bioinorganic chemistry
The field of bioinorganic chemistry is rapidly growing in the last years. In general,
this field is concerned with the active sites of metallo-enzymes, and with simple model
compounds, that mimic the structure and reactivity of the active site of the enzyme.47 The
metallo-enzymes that contain oxophilic early transition metals (mostly V, and Mo to a lesser
extent) may exhibit an oxygen-rich environment at the active site. For V, two enzyme
families are known today. The enzyme of the first family, V-dependent nitrogenase, reduces
dinitrogen into ammonia; the active site of this enzyme contains V ligated mainly by sulfur
donors.48,49 The second family of V-dependent enzymes is V-dependent haloperoxidases.48,49
These enzymes catalyze the oxidation of halide to hypohalous acid (using peroxide as the O
atom donor), which, in turn, halogenates organic substrates (Scheme I-18, top). In addition,
these enzymes catalyze the asymmetric O atom transfer to sulfides, forming optically active
sulfoxides (Scheme I-18, bottom).
Introduction
35
Scheme I-18. Reactivity of V-dependent haloperoxidase. Top: Halogenation of organic substrates (X = Cl, Br).
Bottom: Oxidation of sulfides to sulfoxides.
Considerable structural data has been gained about the V-dependent
haloperoxidases.50 The (resting) active site of all the enzymes of this family contains a V(V)
metal center in a trigonal bipyramidal geometry, coordinated to four oxygen atoms, and to
one nitrogen atom (Figure I-6). During the catalytic cycle, the metal retains its oxidation
state. The proposed catalytic cycle involves a peroxo (side-on) intermediate, in which the
activation of the O atom takes place. At the final step, before the release of the oxidant, the
metal center is of hexacoordinate octahedral geometry.51 In the hexacoordinate geometry, V
binds five oxygen ligands, and one nitrogen ligand (imine) in one of the axial positions.
X- + H2O2 + H+ "V" HOX + H2O
HOX + RH RX + H2O
RS
R+ H2O2 R
SR
+ H2O
O"V"
Introduction
36
Figure I-6. Proposed catalytic cycle for V-dependent haloperoxidase
Appropriate structural models for the active site of V-dependent haloperoxidase are
V(V) complexes, having NO4 donors set (where N is a neutral donor) preferably in a trigonal
bipyramidal geometry.48,49 It is worth noting that trigonal bipyramidal geometry is rare for
pentacoordinate V complexes; usually, a distorted square pyramidal geometry is observed.48c
In addition, the metal center should be able to switch between the penta- and hexacoordinate
geometries. The number of compounds strictly matching these conditions is very limited.
However, many complexes that come close to these conditions might be viewed as “good”
structural models. Most of these models are based on the bi- or tridentate imine-phenolate
ligands (Figure I-7).48,49 Some of these complexes have been found to be functional models
as well, being able to conduct oxidation of organic and inorganic substrates, such as olefins
(to epoxides) and sulfides (to sulfoxides).51 This field is of high interest to the inorganic
V
OHN(His404)
O
OOH
N(His496)
V
OHN(His404)
O
OOH
N(His496)
H -OOH
Cl-
-2 H2OVO
O
O
N(His496)O
Cl-
N(His404)
H+, H2O
HOCl
VO
O
O
N(His496)OH
N(His404)
H
ClO H
H2O2
Introduction
37
community nowadays, as it may open the way to efficient oxidation catalysts, operating
under ambient conditions, and using environment-friendly oxidants, such as hydrogen
peroxide or molecular oxygen.
Figure I-7. Selected structural models of the active site of V-dependent haloperoxidase
I-7. Amine phenolate ligands
The present work describes the structural chemistry and reactivity of early transition
metal complexes of the divergent tetradentate amine-phenolate ligands (with a minor
exception of sequential tetradentate Salophan ligands). This ligand family includes three sub-
groups: trianionic amine tris(phenolate) ligands; dianionic amine bis(phenolate) ligands,
having an additional donor on a sidearm; and monoanionic amine mono(phenolate) ligands,
carrying two such donors (Figure I-8). In contrast to many other ligand precursors, the
ligands of this family can be easily prepared from commercially available starting materials
in a single-step Mannich condensation. All these ligands may be viewed as “super-chelating”
ligands: they provide four donor atoms, and, as divergent ligands, do not require a specific
wrapping of the ligand precursor for the binding to the metal. In addition, these ligands may
enable a precise control of the geometry at the metal center, by restricting the number of
possible isomers in both penta- and hexacoordinate geometries.
NO
Vt-Bu
Me
Me
NO t-Bu
Me
Me
O
NN
OOMe
t-Bu t-Bu
MeV
O
O N
O O
V
OH2
OH2O
Br
Introduction
38
Figure I-8. The family of the amine phenolate ligands
I-7.1. The amine tris(phenolate) ligands
The amine tris(phenolate)s are old organic compounds but very young ligands.52 Even
though they are ideally suitable for stabilization of high-oxidation state early transition
metals, and despite their structural similarity with the well-known tripodal ligands, such as
triethanol amine,8 or the triamido amine,8,9 their potential has not been evaluated until the end
of 1990’s. The first transition metal (Fe(III)) complex of an amine tris(phenolate) ligand was
reported only in 1998.53a Soon after, the amine tris(phenolate) complexes of main group
elements (Ga(III), In(III), Si(IV), P(V)) were reported.53b,54 The amine tris(phenolate) ligands
were introduced into the chemistry of early transition metals by our group in 2001.55 A year
later, the amine tris(phenolate) Ti complexes were shown to lead to the active lactide
polymerization catalysts.56 Since then, these ligands draw an ever-increasing attention as a
rigid and versatile platform for the early transition metals.4
According to their coordination chemistry with the early transition (Ti(IV)), and the
main group metals, these ligands are indeed “tripodal”, or “atrane” ligands,8 as they lead to a
well-defined mononuclear TBP complexes, in which the phenolate oxygens occupy the
equatorial positions, and the amine occupies one of the axial positions, while the second axial
position is occupied by a fifth (monodentate) ligand (Figure I-9).55,56 In this respect, a
N
OH HOR R
R
HO
N
OH HO
D
R R
N
OHD
D
Amine tris(phenolate):Tetradentate trianionicno sidearm donors
Amine bis(phenolate):Tetradentate dianionicone sidearm donor
Amine mono(phenolate):Tetradentate monoanionictwo sidearm donors
R
Introduction
39
comparison between the reactivity of these tripodal oxygen-donor ligands and the reactivity
of tridodal nitrogen-donor ligands (triamidoamine) is revealing. Aiming at an easier
preparation of well-defined mononuclear complexes, the amine tris(phenolate) ligands should
be more useful than the triethanolamine ligands,8 according to the considerations presented
earlier, i.e., they are more acidic, and allow a precise steric control via their ortho
substituents. In addition, these ligands may possibly switch between hexacoordinate and
penta-coordinate metal geometries, thus enabling their participation in various catalytic
applications.
Figure I-9. A tripodal amine tris(phenolate) Ti(IV) complex
I-7.2. The amine bis(phenolate) ligands with the additional “sidearm” donor
The dianionic tetradentate amine bis(phenolate) ligands, carrying a sidearm donor,
were first reported in 1988 for Mo(VI).57 Our group has begun to explore the chemistry of
these ligands with Group (IV) metals in 1999, aiming at the preparation of α-olefin
polymerization catalysts. The complexes were prepared using per(alkoxo) (Ti(Oi-Pr)4) and
per(benzyl) (Zr(CH2Ph)4 and Hf(CH2Ph)4) metal precursors (Scheme I-19).58,59 For Ti, the
preparation of dialkyl complexes was accomplished by the reaction of the dialkoxo complex
with Me3SiCl, and further alkylation with the corresponding Grignard reagent.58b,58c In all
cases, the amine bis(phenolate) ligands have led to well-defined mononuclear complexes; in
most of the cases the sidearm donor was bound to the metal, accomplishing an octahedral
geometry at the metal center. The resulting complexes exhibited an almost uniform structure
TiO
OO
NR
R
R
RR
R
X
Introduction
40
at the metal center, in which the phenolate oxygens were mutually trans, and the labile
positions were cis (as required for an α-olefin polymerization pre-catalyst), being consistent
therefore with an overall Cs-symmetrical structure.
MO
X
t-Bu
t-Bu
D
X
Nt-Bu
Ot-Bu
N
OH HOt-But-Bu
t-Bu t-Bu
D
MX4
M = Ti, Zr, Hf
X = Oi-Pr, CH2Ph
D = NMe2, OMe, py, SMe, NEt2
- 2 HX
Scheme I-19. Synthesis and structure of the amine bis(phenolate) Group IV metal complexes
In combination with the co-catalyst (B(C6F5)3), the amine bis(phenolate) Group IV
metal complexes have led to active 1-hexene polymerization catalysts. Keeping in mind a
uniform structure of the pre-catalyst, a thorough structure-activity relationship study was
carried out. In general, the amine bis(phenolate) ligand precursor possesses several “degrees
of freedom”: The nature of the phenolate substituents, the type of the sidearm donor, and the
bridge between the central amine and the sidearm donor. At the first step of this study, most
of the ligands under investigation had bulky (t-Bu) substituents in the ortho positions, in
order to minimize the formation of the bis(homoleptic) complexes for the large metal ions (Zr
and Hf). The additional degree of freedom is the nature of the metal ion. Overall, the Zr
amine bis(phenolate) complexes exhibited the highest activity (standing among the highest
activities ever reported for 1-hexene polymerization),59a-c Hf complexes were somewhat less
active,59c and Ti complexes possessed much lower activity, displaying, however, a living
character of the polymerization in several cases.58b, 58d The presence of the sidearm donor was
found to be the most crucial parameter for high activity in Zr or for living polymerization in
the Ti series. As for the nature of sidearm donor, the non-bulky hard donors (NMe2 and OMe)
Introduction
41
led to the most active Zr catalysts, presenting activities of 21,000 and 50,000 gpol mmolcat-1 h-
1, respectively (Scheme I-20).59c For Ti, the OMe sidearm donor led to an unprecedented
living polymerization of 1-hexene for 31 h.58d Based on these results, the amine
bis(phenolate)-supported metal complexes seem to be one of the most promising non-
metallocene olefin polymerization catalysts today. Thus, further studies, aiming at the
discovery of highly active and living polymerization/block copolymerization catalysts, and
new activity modes, are being carried extensively.
Scheme I-20. Reactivity of Ti and Zr amine bis(phenolate) pre-catalysts in 1-hexene polymerization
I-7.3. The amine mono(phenolate) ligands with two sidearm donors
Viewing the tetradentate amine tris(phenolate) ligands and the amine bis(phenolate)
ligands as a single family, the amine mono(phenolate) ligands, carrying two sidearm donors
may be regarded as the “missing” relative in this family. These ligands possess an
“unnecessary”, at first sight, sidearm donor for Group IV M(IV) metals in octahedral
geometry. However, such a donor may probably be of high importance in cationic species, in
which one of the labile ligands is removed, and may possibly lead to dicationic
polymerization catalysts. This monoanionic ligand may successfully stabilize M(III)
complexes of octahedral geometry, possessing two additional labile groups. Furthermore, this
MO
PhH2C
t-Bu
t-Bu
D
CH2Ph
Nt-Bu
Ot-Bu
Bu
B(C6F5)3
Bun
for M = Zr: highly active polymerization catalysts;
for M = Ti: living polymerization catalysts
D = NMe2, OMen
Introduction
42
“dormant” donor may trigger the α-elimination reaction to form the metal-ligand multiple
bonds in the high-oxidation state early transition metals. Until now, no early transition metal
complexes of such ligands have been reported.60
Discussion
43
Discussion
This work encompasses coordination chemistry and reactivity of early transition metal
complexes with a variety of amine phenolate ligands. In this section various parameters that
bring this work together are discussed, starting from ligand and complex synthesis, and
concluding with the reactivity at the pre-designed metal sites. In addition, this section
highlights novelties brought to the realm of inorganic/organometallic chemistry and catalysis
by complexes of the amine phenolate ligands.
D-1. Ligand synthesis
The amine bis(phenolate) ligands and the amine tris(phenolate) ligands presented
herein have been prepared by the group of Prof. Z. Goldschmidt of Bar Ilan university. The
symmetrical amine bis(phenolate) ligands have been prepared by a single-step Mannich
condensation between the corresponding phenol, primary amine and formaldehyde (Scheme
D-1). The ligands were obtained as crystalline solids after recrystallization. The amine
tris(phenolate) ligands were synthesized by a straightforward reaction between
hexamethylene tetraamine and the corresponding phenol. Overall, this route is very practical,
leading to a large variety of ligand precursors in high yields. The amine mono(phenolate)
ligand was prepared in our laboratory following a similar preparation.
Discussion
44
R
R
OH+ CH2O D NH2+
MeOHR
RN
OH HOR
R
D
N N
N
N
R
R
OH+
R
RN
OH HOR
RMeOH
HO
R
R
R
R
OH+ CH2O D
NH+MeOH
R
RN
OHD
D
2
Scheme D-1. Synthesis of various amine phenolate ligands via Mannich condensation
Sequential (Salophan) ligands were synthesized by a different route, including a
condensation between the substituted 2-hydroxy benzaldehydes and ortho-phenylene
diamine, and subsequent reduction with sodium borohydride (Scheme D-2). For the majority
of the ligands, a useful one-pot synthesis was developed, followed by a simple work-up,
leading to the pure Salophan ligand precursors without a need for chromatography or
recrystallization.
R
R
OH
H
OH2N
H2N+
N
N
OH
OH
R
R
R
R
HN
HN
OH
OH
R
R
R
R
NaBH4MeOH
MeOH
Scheme D-2. Synthesis of Salophan ligands
Discussion
45
D-2. Complex synthesis
As described in the Introduction section, the phenolate ligand precursors are
sufficiently acidic, thus not requiring the notorious salt metathesis route for preparation of the
metal complexes. A variety of metal precursors, purely inorganic or organometallic, were
available for this purpose. The most convenient precursors are the commercially available
homoleptic alkoxide complexes, such as Ti(Oi-Pr)4, and Ta(OEt)5, or the related VO(OPr)3
(Scheme D-3). As the amine phenolate ligands are “super-chelate” ligands, and the alkoxide
function is more basic than the phenolate function, the reactions normally proceeded to full
conversion, and the yields were quantitative. The reactivity of the homoleptic amides, such as
Zr(NMe2)4, or Ta(NMe2)5, towards the amine phenolate ligand precursors resembled that of
the metal alkoxides to a large extent.
Ta(OEt)5 + LigH3 LigTa(OEt)2 + 3 HOEt
V(=O)(OPr)3 + LigH3 LigV(=O) + 3 HOPr
Scheme D-3. Samples for alcohol-elimination reactions between the amine tris(phenolate) ligand precursors and
metal-alkoxide precursors.
The major achievement of this work from a synthetic point of view is the pursuit of
the “alkane elimination” reactions between the homoleptic organometallic metal precursors
and the amine phenolate ligand precursors. These routes are advantageous for the preparation
of an organometallic complex. Some homoleptic organometallic species e.g., Zr(CH2Ph)4,
and Hf(CH2Ph)4 are known for their stability, and thus have been widely used as metal
precursors in “toluene elimination” reactions with acidic ligand precursors. However, the
remaining member of the Group IV triad, Ti(CH2Ph)4, is much less stable, and therefore was
seldom used as a starting material. Previously, an alternative route was developed in our
Discussion
46
group, including the reaction of the ligand precursor with Ti(Oi-Pr)4, followed by
chlorination with TMSCl, and finally an alkylation with PhCH2MgCl (Scheme D-4).58 In the
present research, we demonstrated that Ti(CH2Ph)4 is stable enough, and highly useful for
preparation of titanium benzyl complexes with amine phenolate ligands.
Ti(Oi-Pr)4 + LigH2 LigTi(Oi-Pr)2
Ti(CH2Ph)4 + LigH2
Me3SiCl LigTiCl2PhCH2MgCl LigTi(CH2Ph)2
LigTi(CH2Ph)2
Scheme D-4. Top: A previous synthesis of amine bis(phenolate) Ti(IV) dibenzyl complexes, consisting of the
three steps; Bottom: A single-step synthesis of amine bis(phenolate) Ti(IV) dibenzyl complexes
Until recently, the chemistry of “toluene elimination” reactions had been confined to
the tetrabenzyl Group IV metal complexes only. In the course of our study of Ta(V)
chemistry with the amine phenolate ligands, we attempted the use of the Group V (Ta(V))
pentabenzyl complex as a starting material in that reaction. First, we developed a simplified
preparative route for pentabenzyltantalum, and solved its crystal structure, which had not
been reported previously. Thereafter, we reacted it with a variety of amine bis- and amine
tris(phenolate) ligands (Scheme D-5). This work proved that pentabenzyltantalum is the
“precursor of choice” in this chemistry, as it led to the desired organometallic species in high
yields in a single step.
Discussion
47
Scheme D-5. Structure, and toluene-elimination reactions of pentabenzyltantalum with the amine phenolate
ligand precursors
An additional synthetic route that we employed in this work relies on the reaction
between a metal chloride precursor and the ligand precursor in the presence of a mild base:
Et3N. This route was found to be particularly useful when the utilization of metal alkoxide or
metal alkyl precursors was not possible, as in the case of V(III) complexes. In this route,
triethylamine serves to absorb HCl, forming the EtN·HCl salt that is insoluble in common
organic solvents such as ether or THF. Thus, the desired products are obtained by a rather
simple filtration-recrystallization sequence.
D-3. Coordination chemistry of the amine phenolate early transition metal complexes
One of the most pronounced advantages of the amine phenolate ligands is their well-
defined coordination chemistry. Two isomers are feasible in octahedral geometry for the
amine bis(phenolate) complexes of the LigMX2 type (Figure D-1). In the first isomer, the
phenolate oxygens are trans to each other, leading to an approximate Cs-symmetry of the
metal complexes. In the second isomer, the phenolate oxygens are cis to each other, and the
resulting symmetry is C1. Throughout this research, we have synthesized dozens of such
complexes, bearing alkoxo, amido, benzyl or methyl monodentate ligands. All of these
LigH2LigTa(CH2Ph)3- 2 CH3Ph
LigH3LigTa(CH2Ph)2
- 3 CH3Ph
Discussion
48
complexes exhibited a trans disposition of the phenolate oxygens. However, Mountford and
coworkers have recently demonstrated the viability of the C1-symmetric zirconium
complexes as well, in which the oxygens are in a cis disposition.61
MO
R
R1
R1
D
R
NR1
OR1
M = Ti(IV), Zr(IV), Hf(IV)
R = Oi-Pr, NMe2, Me, CH2Ph, Cl
R1 = t-Bu, Me, Cl, Br
D = NMe2, OMe, OEt, THF, furan, py, SMe
MO
R
R1
O
R
NR1
D
R1
R1M = Zr(IV)
R = Cl
R1 = t-Bu
D = py
Cs-symmetry C1-symmetry
N
OH HOR1
R1
R1
R1
D
MO
R
R1
R1
R
R
NR1
OR1
M = Ta(V)
R = CH2Ph
R1 = t-Bu, Me, Cl, Br
D = NMe2, OMe, Me
Cs-symmetry
D
Figure D-1. Possible wrapping modes of the tetradentate amine bis(phenolate) ligands around a Group IV
(M(IV)), and Group V (Ta(V)) metals in octahedral geometry
The second structural parameter under investigation in the chemistry of the amine
bis(phenolate) ligands was binding of the fourth, sidearm, donor. Previously, it was shown
that, because of steric pressure, the sidearm donor might remain uncoordinated to the Group
Discussion
49
IV metal, forming a pentacoordinate complex.59b Thus, the binding of a sidearm donor should
not be taken for granted a priori. In this work, we employed several types of sidearm donors:
THF, open-chain ether (-OMe), non-bulky amine (-NMe2), and furan. All these, including the
weak aromatic donor (furan), were found to bind to the Group IV metal center, thus leading
to octahedral geometry at the metal.
For a Group V metal complex (Ta(V)) not bearing multiply bonded ligands, the
tetradentate dianionic amine bis(phenolate) ligand is expected to bind in a tridentate fashion
(i.e. the sidearm donor remaining unbound) yielding octahedral geometry at the metal center
(Figure D-1). This was supported by numerous X-ray structure determinations. However, this
“dormant” donor may coordinate, or even trigger the removal of one of the benzyl groups in a
LigTa(CH2Ph)3-type complex, which will be demonstrated in the following section.
The coordination chemistry of the amine tris(phenolate) complexes was found to be
well-defined as well. Octahedral LigMX2 complexes of M(V) metal centers may exist as a
single geometrical isomer (for identical X’s, see Figure D-2). However, these ligands may
probably lead to pentacoordinate metal centers, in the case of metal centers in lower than
M(V) oxidation state (V(III), Ti(IV)), or for the metal centers in M(V) oxidation state bearing
a multiply bound ligand (oxo (=O), alkylidene (=CHR)).
Discussion
50
MO
X
R1
R1
O
X
NR1
OR1
R1
R1
MO
OO
NR1
R1
R1
R1 R1
R1
L
M = Ta(V)
X = OEt, NMe2, Cl, CH2Ph, µ-CHPh
R1 = Me, t-Bu, Cl
M = V(III),
L = THF
R1 = Me, t-Bu, Cl
MO
OO
NR1
R1
R1
R1 R1
R1
X
MO
OO
NR1
R1
R1
R1 R1
R1
R
M = Ti(IV)
L = Oi-Pr
R1 = Me, t-Bu, Cl
M = V(V)R = OR1 = Me, t-Bu
N
OH HOR1
R1
R1
R1
HO
R1
R1
C3-symmetry C3-symmetry
Cs-symmetryC3-symmetry
Figure D-2. Coordination modes for early transition metals in the amine tris(phenolate) environment
All the Ta(V) complexes with the amine tris(phenolate) ligands were found to be
octahedral. As the amine tris(phenolate) is a tripodal ligand, possessing a central donor, and
three equivalent “arms”, its chemistry may be correlated with the chemistry of other tripodal
ligands, and in particular triamidoamine. The coordination chemistry of the amine
tris(phenolate) ligands with Ta(V) stands in sharp contrast to the behavior of the
triamidoamine ligands: The triamidoamine ligands did not lead to octahedral complexes
when the metal center was hexa-coordinate, giving instead a C3-symmetrical pocket.9 For one
specific monodentate ligand (benzyl), the amine tris(phenolate) Ta(V) complexes were C3v-
Discussion
51
symmetric on the NMR timescale down to 203 K; however, this may probably be explained
by a dynamic process recently found in the hexacoordinate amine tris(phenolate) Ti(IV)
complexes.62 Furthermore, the amine tris(phenolate) ligands do not show the tendency found
for the triamidoamine ligands, as they do not lead to TBP Ta(V) complexes with the axial
position occupied by a multiply-bonded ligand. Even when the remaining (“monodentate”)
ligand was nominally “double-bonded” (alkylidene), the complex was of octahedral
geometry, and the alkylidene ligand was found in a rare bridging (µ-alkylidene) mode. This
difference in coordination chemistries between two ligand systems may stem from the
presence of an additional π orbital on the phenolate oxygen, that destabilizes the orbitals
configuration in the “axial pocket” found for the triamidoamine ligands (Figure I-3).
In contrast, vanadium complexes of the amine tris(phenolate) ligands normally
featured a penta-coordinate TBP geometry. For V(III), the remaining axial position was
occupied by a neutral ligand (THF). For V(V), THF was replaced by a doubly-bonded ligand,
the oxo group. This structure provides the first evidence for viability of the “triamidoamine-
type” binding of our tripodal ligand to the metal: i.e., the structure in which the phenolate
oxygens occupy equivalent equatorial positions, and the multiply bonded ligand is found in
the axial position.
We found that an exchange between these two geometries could take place if an
electron-deficient amine tris(phenolate) ligand was bound to V(V) (Scheme D-6). This
complex was found to support both geometries at RT, coordinating an additional neutral
molecule at the sixth coordination site. As expected, the equilibrium between the
pentacoordinate and the hexacoordinate geometries was temperature-dependent, with the
TBP form prevailing at elevated temperatures, and the octahedral form prevailing at RT. The
preference for a hexacoordinate complex in this case is attributed to the overall electron-
deficiency at