Cyclopentadienyl-, Indenyl- and Fluorenyl-Functionalized N ...broyo/paper4.pdf · Cyclopentadienyl-, Indenyl- and Fluorenyl-Functionalized N-Heterocyclic Carbene Metal Complexes:

MICROREVIEW

DOI: 10.1002/ejic.201100990

Cyclopentadienyl-, Indenyl- and Fluorenyl-Functionalized N-HeterocyclicCarbene Metal Complexes: Synthesis and Catalytic Applications

Beatriz Royo*[a] and Eduardo Peris[b]

Keywords: N-Heterocyclic carbenes / Cyclopentadienyl ligands / Half-sandwich complexes / Homogeneous catalysis

This microreview focuses on the preparation of metal com-plexes with N-heterocyclic carbene ligands linked to cyclo-pentadienyl (and related indenyl and fluorenyl) rings. Sincethe description of the first Ti and V complexes in 2006, thefield has grown to afford a large number of new complexes,in which the metals range from early to late transition metals.The ligands that are now available not only include a wide

Introduction

Since the first use of N-heterocyclic carbenes (NHCs) inthe design of homogeneous catalysts,[1] there has been anincreasing effort in the design of new NHC-containing li-gands with different topologies.[2] Now, we can find in theliterature, a large number of examples in which NHCs areincorporated into chelating,[3,4] pincer[5] and chiral[6] archi-tectures, but still, the search for new coordination modes

[a] Instituto de Tecnologia Química e Biologica da UniversidadeNova de Lisboa,Av. da República, EAN, 2780-157 Oeiras, PortugalFax: +351-214411277E-mail: [email protected]

[b] Departamento de Química Inorgánica y Orgánica,Universitat Jaume I,Avenida Vicente Sos Baynat s/n 12071 Castellón, Spain

Beatriz Royo graduated in Chemistry at the University of Alcalá (Madrid, Spain). She obtained her D.Phil from theUniversity of Sussex in 1992, under the supervision of Professor Michael F. Lappert. In 1993, she moved to the Universityof Alcalá as an Assistant Professor (1993–1997). Four years later, she joined the group of Prof. Carlos C. Romão at theresearch institute of Instituto de Tecnologia Química e Biológica (ITQB) of the University Nova of Lisbon in Portugal.In 2004, she became group leader of the Homogeneous Catalysis group at ITQB. The general research interest of hergroup is focused on the development of sustainable selective catalysts based on organometallic species.

Eduardo Peris graduated in Chemistry in 1988 in Valencia. He received his Ph.D. Degree in Chemistry (1991) at theUniversidad de Valencia, under the supervision of Prof. Pascual Lahuerta. In 1994, he joined Robert Crabtree’s group atYale University, where he stayed for two years working on a new research project involving the determination of hydrogenbonding to metal hydrides (dihydrogen bond). In October 1995, he moved to the Universitat Jaume I (Castellón-Spain)as an Assistant Professor (1995–1997), Lecturer (1997–2007) and finally Professor of Inorganic Chemistry. At theUniversitat Jaume I, he started a new research project related to the use of organometallic push–pull compounds withnonlinear optical properties. The current interest of his group is the design of N-heterocyclic-carbene-based compoundsfor homogeneous catalysis and biomedical applications.

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set of cyclopentadienyl-, indenyl- and fluorenyl-NHCs witha variety of tethers, but may even present chirality. Severalcatalytic applications of the new complexes are described,including borrowing hydrogen processes (transfer hydrogen-ation, β-alkylation of secondary alcohols, alkylation ofamines with primary alcohols), hydrosilylation of aldehydes,epoxidation and hydroformylation of olefins, among others.

and more effective NHC-containing catalysts is far fromhaving reached its climax. Polydentate NHC ligands inwhich the carbene is bound to a neutral or anionic donorby an organic linker are under constant development, be-cause they can offer stability and fine tuning of the stereo-electronic properties of the complex.[7]

Among some of the most active and versatile types ofNHC-based catalysts that can be found, “half-sandwich”complexes of the type “CpM(NHC)” (Cp is the cyclopen-tadienyl ligand) have provided very effective catalysts thatcover a large number of transition metals and a plethora ofcatalytic applications.[8] However, NHC-containing cyclo-pentadienyl (or related) groups are a class of ligands thathas remained elusive until the work published by Dano-poulos and co-workers in 2006,[9] in which two indenyl- andfluorenyl NHC-functionalized ligands were reported and

B. Royo, E. PerisMICROREVIEWcoordinated to TiIII and VIII. After this pioneering work,other authors have contributed to the field by synthesizingη5-cyclopentadienyl-NHC and its related indenyl- (Ind) andfluorenyl (Flu) ligands, as will be discussed in this manu-script.

The introduction of chelating NHC ligands to the coor-dination sphere of a metal catalyst have some interestingconsequences because it can increase its thermal stability[3,5]

and favour the rigidity required for the preparation of effec-tive asymmetric catalysts when a chiral centre is present.[6]

However, in the case of most half-sandwich “CpM(NHC)”complexes, the introduction of a chelating ligand may havean important catalytic drawback, since it may leave thecomplex with only one vacant coordination site (assumingan octahedral coordination sphere, three would be occupiedby the Cp ligand, and two by the bis-chelating ligand), thuslimiting its catalytic activity (Scheme 1, A). We clearly ob-served this effect in a series of reactions catalyzed by“Cp*Ir(NHC)” complexes.[10] An alternative way of prepar-ing chelating architectures is the preparation of suitableCp* ligands with pendant NHCs, thus affording systems inwhich the chelation does not consume any “extra” coordi-nation site (Scheme 1, B).

Scheme 1. Nonlinked (A) and linked (B) Cp-NHC systems.

In this microreview we aim to provide the reader withan overview of the works with respect to the preparation,reactivity and catalytic applications of complexes with N-heterocyclic carbene ligands linked to Cp, fluorenyl or in-denyl rings. We will pay special attention to the preparationof the ligands, since this has been the main bottleneck tojustify the elusiveness of this type of interesting systems. Allother aspects such as coordination procedures, structuralfeatures and catalytic applications will also be discussed.

Discussion

Synthesis of Cyclopentadienyl- and Its Annulated Indenyl-and Fluorenyl-Functionalized N-Heterocyclic CarbeneLigands

The synthesis of the first imidazolium salts with pendantindene or fluorine groups was disclosed by Danopoulos andco-workers in 2006.[9] The new imidazolium pro-ligandswere obtained by quaternization of β-bromoethylindene orβ-bromoethylfluorene with 2,6-diisopropylphenylimidazole(DiPP, 2,6-Pri2–C6H3) (Schemes 2 and 3). This syntheticroute was also suitable when other alkyl- and aryl-substi-

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tuted imidazoles and differently substituted β-ethyl- and γ-propylindenes and β-ethyl-fluorenes were used (Schemes 2and 3).[11–14]

Scheme 2. Synthesis of indenyl-functionalized imidazolium salts(1). Reagents and conditions: (i) for alkyl- or arylimidazole, diox-ane, reflux, 3–5 days (41–97% yield). Alternatively, for 1-methyl-and isopropyl-imidazoles, the reaction was carried out without sol-vent, room temperature, 48 h (60% yield).

Scheme 3. Synthesis of fluorenyl-functionalized imidazolium salts(2). Reagents and conditions: (i) alkyl- or arylimidazole, dioxane,reflux, 3–5 days (79–88% yield).

For the preparation of similar tetramethylcyclopen-tadienyl-azolium salts, the same route cannot be used be-cause of the lack of regioselectivity in the direct alkylationof 1,2,3,4-tetramethylcyclopentadiene,[15–17] so other alter-natives had to be proposed.

Two general methods were developed for the synthesis oftetramethylcyclopentadienyl-functionalized N-heterocycliccarbenes: (i) the one-pot synthesis route, starting from theeasily accessible 2-(2,3,4,5-tetramethylcylopentadienyl)-ethylamine, glyoxal and formaldehyde and subsequent treat-ment with iodomethane, afforded the corresponding imid-azolium salt 3 (Scheme 4),[18] and (ii) deprotonation at themethylene group of benzylimidazole with nBuLi and subse-quent reaction with 1,2,3,4-tetramethylfulvene followed bytreatment with methanol and iodomethane (Scheme 5).[19]

In this latter route, the Cp*-azolium salt (Cp* = η5-C5Me4)

Functionalized N-Heterocyclic Carbene Metal Complexes

was obtained as a mixture of tautomers that resulted fromthe different position of the double bonds in the cyclopen-tadienyl ring (Scheme 5).

Scheme 4. Synthesis of imidazolium salts (3). Reagents and condi-tions: (i) (NH4)2CO3, MeOH, room temperature, 18 h; (ii) MeI,acetone, room temperature, 24 h (66% yield).

Scheme 5. Synthesis of cyclopentadienyl-functionalized imid-azolium salts (4–8). Reagents and conditions: (i) BuLi, THF,–20 °C, 20 min; (ii) fulvene, room temperature, 1 h, treatment withMeOH and subsequent reaction with MeI, acetone, room tempera-ture, 12 h (60–85% yield).

The main advantage of using route (ii) is its versatility,which allows the introduction of different substituents onthe cyclopentadienyl ring and also at the linker, just bychoosing the appropriate fulvenes. By this procedure, thecorresponding unsubstituted- and sterically demanding tet-rabenzylcyclopentadienyl ligands were prepared.[20] An ad-ditional advantage of route (ii) is the introduction of astereogenic centre at the linker between Cp and the NHCfragments, which allows for the preparation of chiral li-gands, although they were always obtained as racemic mix-tures.

An enantiomerically pure cyclopentadienyl-NHC carb-ene ligand (9) was recently prepared by reaction of a chiralimidazole tosylate derivative[21] with LiCp, followed bytreatment with methyl iodide (Scheme 6).[22] This method

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is limited to unsubstituted cyclopentadienyl ligands sincetetraalkylcyclopentadienyls are subject to lack of regioselec-tivity in their direct alkylation. However, this route can beextended to the preparation of the corresponding enantio-merically pure indenyl-functionalized imidazolium salts (10,Scheme 6).[22]

Scheme 6. Synthesis of enantiomerically pure cyclopentadienyl-and indenyl-functionalized imidazolium salts (9 and 10, repec-tively). Reagents and conditions: (i) CpLi, THF, –20 °C, 16 h fol-lowed by reaction with MeI, acetone, room temperature, 16 h(64%); (ii) IndLi, THF, –20 °C, 16 h followed by reaction with MeI,acetone, room temperature, 16 h (57 % yield).

Cyclopentadienyl-, Indenyl- and Fluorenyl-FunctionalizedNHC Complexes of Early Transition Metals and Fe

Early transition metal complexes of indenyl- and fluor-enyl-functionalized NHC ligands were prepared by Dano-poulos and co-workers by reaction of the appropriate metalprecursors with the Ind- and Flu-NHC potassium salts.[9]

Deprotonation of the imidazolium proligands 1 and 2 byKN(SiMe3)2 was performed in two steps, the first equiva-lent of base attacked exclusively the imidazolium proton atC2 to produce the neutral Ind- and Flu-NHCs, and the ad-dition of a second equivalent of KN(SiMe3)2 led to the cor-responding potassium salts, which have been fully charac-terized.[9] The structure of (Flu-NHC)–K+ was determinedcrystallographically. It comprises polymeric “zig-zag”chains with potassium atoms and bridging fluorenyl units.[9]

(Ind-NHC)–K+ [or (Flu-NHC)–K+] salts have proven tobe especially effective for the preparation of early transitionmetal Ind-NHC complexes (and their related Flu-NHCs).It is interesting to note that high oxidation state, easily re-ducible groups 4 and 5 metal halides (e.g. TiCl4, VCl5, VCl4)were not suitable starting materials for this reaction becauseof competing reduction. The paramagnetic TiIII (11) andVIII (12) complexes were prepared from [TiCl3(THF)3] and[VCl3(THF)3], respectively. Aminolysis of [V(NMe2)4] bythe indenyl-imidazolium bromide salt was also ac-companied by reduction of the metal to VIII.[11] In contrast,the corresponding ZrIV complexes (15) could readily be pre-pared by direct reaction of the corresponding potassiumsalts with [ZrCl4(THF)2].[11] High oxidation TiIV complexes(13) were accessed by using [Ti(NtBu)Cl2(py)3] as metal

B. Royo, E. PerisMICROREVIEWprecursor. These reactions are summarized in Scheme 7. X-ray diffraction studies reveal that in these metal complexes,the ligand adopts a bidentate coordination mode.[11]

Scheme 7. Synthesis of group 4 and 5 metal complexes bearing in-denyl- and fluorenyl-functionalized NHCs (11–15). Reagents andconditions: (i) [TiCl3(THF)3], THF, addition at –78 °C, slow warm-ing to room temperature, 2 h (50% yield); (ii) [VCl3(THF)3] THF,addition at –78 °C, slow warming to room temperature, 2 h (40%yield); (iii) [TiCl2(NtBu)py3], THF, addition at –78 °C, slow warm-ing to room temperature, 1 h, (45% yield); (iv) [Ti(NMe2)2Cl2],benzene, 24 h, room temperature, 25% yield; (v) [ZrCl4(THF)2],THF, addition at –78 °C, slow warming to room temperature, over-night (66% yield).

Half-sandwich early transition metal complexes contain-ing a cyclopentadienyl ligand with a two-electron-donorpendant group have been scarcely studied.[23] In particular,those containing a phosphane-tethered group are veryrare.[24] Half-sandwich complexes of zirconium(IV) bearinga chelate phosphane function show decoordination of thependant phosphane fragment in coordinating solvents suchas THF.[16,17,25] In contrast, the N-heterocyclic pendantgroup in 11–15 are strongly coordinated to the metal centre.

To date, no catalytic studies have been performed withTi, Zr and V metal complexes bearing Ind- and Flu-NHCligands.

Preformed Ind- and Flu-functionalized carbenes can alsobe used for the preparation of Sc and Y complexes. Cuiand co-workers obtained the Sc (16, 20, 24) and Y (17, 21)complexes by alkanolysis of [M(CH2SiMe3)3(THF)2] (M =Sc, Y) with the neutral Ind- and Flu-NHCs [generated insitu by treatment of the Ind- and Flu-imidazolium saltswith Li(CH2SiMe3)].[26] A similar procedure was extendedto the synthesis of other lanthanide metals (18, 19, 22, 23,25, Scheme 8). The alkanolysis of [Y(CH2SiMe3)3(THF)2]with neutral Ind-NHC ligands carried out under different


conditions (by using ether instead of toluene as reactionsolvent and by allowing longer reaction times) afforded thedimeric yttrium complex 26.[11]

Scheme 8. Synthesis of indenyl- and fluorenyl-functionalized NHCcomplexes of rare earth metals (16–25).

The indenyl and fluorenyl-tethered NHC complexes 16–25 (each still bearing two trimethylsilylmethyl groups) dis-played isomeric solvent-free monomers adopting tetrahe-dral geometries. Their solvent-free nature indicates thestrong electron-donating character of the NHC ligand,since rare earth metal bis(alkyls) usually coordinate one ortwo solvent molecules.[27] The NHC ligand coordinates tothe metal in a η5/κ1 constrained geometry configurationmode, with the centroid of the five-membered cyclopen-tadienyl ring as the apex.

Complexes 16–25 were tested as catalysts in the polyme-rization of isopropene. Complexes 21–23, activated by alu-minium alkyls and [Ph3C][B(C6F5)4], initiated the livingpolymerization of isopropene with high activity, high 3,4-selectivity and medium syndioselectivity.[28] However, the Scanalogue (20) was inactive. Interestingly, the opposite phe-nomenon was observed in the copolymerization of ethylenewith norbornene. The in situ generated ternary catalyst sys-tems composed of the complexes (16, 19 or 20–25), AltBu3

and [Ph3C][B(C6F5)4] exhibited high catalytic activity in thecopolymerization of norbornene and ethylene. Their cata-lytic activity was significantly dependent on the metal andon the stereoelectronic properties of the ancillary ligand;the scandium complex 20 was the most active. The Lewisacid nature of Sc3+ along with the relatively weak electron-donor character of the fluorenyl group relative to its ana-logues containing Ind-NHC ligands explains its superiorcatalytic performance.[14]

The coordination chemistry of group 6 metals (Cr andMo) with NHC ligands tethered to cyclopentadienyl, in-denyl, and fluorenyl rings has also been investigated. Dano-polous reported the paramagnetic CrII complex 27 obtained


by aminolysis of [Cr(N(SiMe3)2)2(THF)2] with the Flu-imidazolium salt 2. The ligand in complex 27 is coordinatedin a monodentate bonding mode to yield a CrII-NHC com-plex with a dangling fluorene group (Scheme 9).[11]

Scheme 9. CrII-NHC complex 27 with monodendate NHC ligandscontaining dangling fluorenyl groups.

Interestingly, a molybdenum complex containing the ind-enyl fragment coordinated to the metal centre in a penta-hapto fashion with a dangling NHC group (29) was pre-pared by using the indenyl-functionalized triethylboraneadduct of NHC 28 (Scheme 10).[13] Yamaguchi and co-workers demonstrated that the protection of NHC by BEt3

can effectively control the coordination ability of the NHCligand, which allows the synthesis of bimetallic complexes.This concept was illustrated by the stepwise coordination

Scheme 10. Synthesis of molybdenum compounds 29–31 from anindenyl-functionalized triethylborane adduct of NHC.


of NHC to afford a chelate Mo complex (30) or a bimetalliccomplex (31), depending on the reaction conditions used.

Chelating molybdenum Cp-NHC complexes were pre-pared from the one-pot reaction of [MoCl(η3-C3H5)(CO)2-(NCMe)2] and the corresponding lithium NHC-cyclopen-tadienides (Scheme 11).[20] Complexes were fully charac-terized, and the crystal structure of [Mo(Cp*-NHC)(CO)2I]was determined by X-ray diffraction analysis and shows theligand coordinated in a bidentate coordination mode. Thechelating coordination of the ligand in complexes 32–34 re-sulted in an improvement of the stability of these species,relative to other known non-chelating-type [MoCp*(NHC)-(CO)2I] complexes.[29] The preparation of the Cp (32), Cp*(33), and CpBz (34) derivatives of the general formula[Mo(Cpx-NHC)(CO)2I] {Cpx = Cp (η5-C5H4), Cp* (η5-C5Me4), CpBz [η5-C5(CH2Ph)4]} provided a smooth changein the electronic properties of the complexes, which showeda clear influence on their catalytic outputs. The catalyticstudies on the epoxidation of cis-cyclooctene with tert-butylhydroperoxide (TBHP) as oxidant showed that the activityof catalysts 32–34 clearly depends on the nature of the Cpx-NHC ligand. Complex 34 was the most active catalyst, andafforded quantitative conversion after 20 h of reaction. Thisproves that the replacement of the Cp (or Cp*) by CpBz

clearly improves the catalytic performance of their metalcomplexes. These findings are in accord with the catalyticbehaviour of isoelectronic [MoCpBz(CO)3X] com-plexes.[30,31] The reaction proceeded smoothly toward epox-ide; without the detection of diols or any other by-products.In comparison with related complexes reported in the litera-ture [MoCp(CO)3X],[31] compound 34 is rather slow, but itsstability under oxidative conditions allows its activity forlonger times.

Scheme 11. Preparation of molybdenum compounds 32–34.

A series of piano-stool iron(II) complexes bearing Cp-NHC bidentate cyclopentadienyl-functionalized-NHCs ofthe general type [Fe(Cpx-NHC)(CO)I] {Cpx = Cp (35), Cp*(36), CpBz (37)} were obtained upon reaction with [Fe-(CO)4I2].[32] Their synthesis is depicted in Scheme 12. Com-plexes 35–37 are remarkably air- and moisture-stable com-pounds, and can be handled in air. The molecular structuresof 35 and 37 show that the cyclopentadienyl-NHC ligandchelates the iron centre in a four-legged piano-stool geome-try. The coordinatively unsaturated iron(II) paramagneticspecies [Fe(Cp*-NHC)Cl] (38) was prepared by using FeCl2as precursor material.[32]

B. Royo, E. PerisMICROREVIEW

Scheme 12. Coordination of cyclopentadienyl-functionalized NHCligands to iron (35–38). Reagents and conditions: (i) BuLi/THF–60 °C to room temperature; (ii) [Fe(CO)4I2] in THF, room tem-perature, 16 h (36–83 %); (iii) FeCl2 in THF, room temperature,16 h (36–67%).

The catalytic studies showed the potential of these ironcomplexes as catalysts in the reduction of carbonyl groupsthrough hydrogen transfer and hydrosilylation reactions.The iron complexes 35–38 displayed good activities in thecatalytic transfer hydrogenation of ketones by using 2-pro-panol as a hydrogen source. All compounds displayed sim-ilar activities. Interestingly, the different substitution on thecyclopentadienyl ring seems to not affect the catalytic per-formance in the hydrogen transfer reaction. Complex 38 ef-fectively catalyzed the hydrosilylation of aldehydes andshowed good tolerance for several functional groups (NO2,CF3, halogens, and esters).[32]

Late Transition Metals: Ruthenium, Rhodium, Iridum andNickel

Indenyl-NHC ligands were coordinated to ruthenium byWang and co-workers.[33] The authors proved that thermaltreatment of indenyl-functionalized imidazolium salts with[Ru3(CO)12] gave different products depending on the reac-tion conditions used.[33] The expected chelate monometalliccomplexes (39, Scheme 13) were prepared by direct reactionbetween the indenyl-imidazolium salt and the rutheniumsource. When instead of the imidazolium salt, the neutralindenyl-functionalized N-heterocyclic carbene was used,unexpected intramolecular C–H activated di- and trimetal-lic species were obtained (40 and 41, respectively).

Later, related cyclopentadienyl-NHC ruthenium com-plexes were prepared by a similar method by using the azol-ium salts and [Ru3(CO)12] to afford the expected chelatemonometallic species (42 and 43, Scheme 14) in highyield.[34] As previously suggested by Wang and co-workers,[33] the coordination of the ligand may be a conse-quence of the oxidative addition of the C–H bond of theimidazolium to Ru0, followed by reductive elimination ofhydrogen from Ru–H and a proton from the cyclopentadi-ene group, which subsequently coordinates to the metal as


Scheme 13. Synthesis of ruthenium complexes 39–41.

a cyclopentadienyl ligand. In this species, with the generalformula Ru(Cpx-NHC)(CO)I (Cpx = Cp*, CpBz), the pres-ence of the stereogenic centres at the aliphatic linker be-tween the NHC and Cpx rings and at the metal centre sug-gests that four stereoisomers (two diastereomers) are ex-pected,[35] but only one diastereomer was obtained. Theseruthenium complexes were characterized by spectroscopictechniques and X-ray diffraction methods.

Scheme 14. Coordination of tetramethyl- and tetrabenzylcyclopen-tadienyl-functionalized NHCs to ruthenium (42, 43).

The separation of the two enantiomers in 42 and 43 wasachieved by using a chiral amine, which, by coordination tothe metal, afforded the formation of two separable chiraldiastereomers, as shown in Scheme 15.[34]

Complexes 42 and 43 were tested in the catalytic isomer-ization of allylic alcohols. The more sterically crowded com-plex 43 showed very low activity, while 42 is very active inTHF and H2O. The isomerization of a O-deuterated allylicalcohol provides the corresponding ketone with the deute-rium at the α-carbon atom, in agreement with previouslyreported results.[36]


Scheme 15. Preparation of compounds 44 and 45.

Group 9 transition metals have also been coordinated toCp-, Ind- and Flu-NHC ligands. In 2008, the synthesis ofthe first tetramethylcyclopentadienyl-functionalized iridiumcomplex 46 (depicted in Scheme 16) was reported. Com-pound 46 was obtained from the reaction of the corre-sponding Cp*-imidazolium salt and [Ir(COD)Cl2]2 (COD= 1,5-cyclooctadiene) in the presence of Cs2CO3.[19]

Scheme 16. Coordination of tetramethylcyclopentadienyl-function-alized NHC to Ir (46). Reagents and conditions: (i) Cs2CO3 inNCMe at 50 °C, 4 h; (ii) KI, MeOH, reflux 16 h (68% yield).

Soon after compound 46 was described, some other teth-ered η5-Cp-NHC[18,22] and η5-Ind-NHC[37] iridium andrhodium complexes were reported. The general coordina-tion procedure consisted of the transmetallation from a pre-formed silver-NHC complex, which was generated and usedin situ. Scheme 17 displays the preparation of the η5-Cp-NHC complexes. The preparation of complexes 47 (Ir) and48 (Rh) is interesting because, together with the C–H acti-vation of the pentacyclic fragment, the second step of thereaction suggests the metal-mediated isomerization of theexocyclic double bond of the linker chain to form the finalη5-cyclopentadienyl part of the ligand.[18] As shown inScheme 17, the reaction to form the rhodium complex 48allowed the isolation of a reaction intermediate (49) inwhich the ligand is monocoordinated through the NHCfragment. Complex 49 was isolated as a mixture of two iso-mers, as a consequence of the restricted rotation of theNHC ligand about the Rh–C bond as a result of steric in-teraction with the cyclooctadiene ligand. This same effectwas observed for the related Ind-NHC and Flu-NHC com-plexes,[37] as will be discussed below.


Scheme 17. Preparation of compounds 47–52. Reagents and condi-tions: (i) Ag2O in 1,2-dichloroethane, 1 h under reflux; (ii)[MCl(cod)]2, reflux 1 h (41% yield for 49); (iii) catalytic amount ofAcOH, 16 h reflux (24% yield for 47 and 37% for 48); (iv) Ag2Oin 1,2-dichloroethane, 1 h under reflux, followed by addition of[MCl(cod)]2 and AcOH (catalytic amount), reflux 16 h; (v) KI,MeOH, reflux 16 h (24% yield for 50 and 51% for 51); (vi) LiOAc(excess) in MeOH at 60 °C, 16 h (quantitative conversion).

The coordination of 5 to [MCl(cod)]2 (M = Rh, Ir) wasalso performed by transmetallation of the in situ preformedAg-NHC complex, but needed further addition of aceticacid and KI to facilitate the reaction to the final MIII spe-cies 50 and 51 (Scheme 17). The reaction of the iridiumcomplex 50 with LiOAc led to the ortho-cyclometallation ofthe phenyl ring to form compound 52.[18]

The indenyl- and fluorenyl-NHC complexes of rhodiumand iridium were reported by Danopoulos and co-workersin 2009.[37] The authors were able to isolate a series of com-plexes in which the ligands adopt monodentate (53 and 55,Scheme 18), bidentate (57 and 58) or bridging modes (54).The authors observed that the fluorenyl or indenyl func-

Scheme 18. Rh complexes with monodentate NHC ligands con-taining a dangling indene group (53), η2-fluorenyl-fulvene (55) andbidentate Ind-NHC (57) and Ir compound 58.

B. Royo, E. PerisMICROREVIEWtionalities can adopt various hapticities (η1, η3 and η5). Me-tallation of the C–H bond of the linker was also observed.The coordination to Rh or Ir was performed either from thepreformed silver-NHC complex or from the correspondingpotassium compound (56). The molecular structures ofcomplexes 53–58 were confirmed by means of X-ray crys-tallography.

Although many of the Cp-NHC ligands described inSchemes 16 and 17 possess chiral centres at the tether, allwere used as racemic mixtures. The preparation of an enan-tiomerically pure Cp- and indenyl-NHC ligands remainedelusive until a new synthetic protocol based on the use ofthe recently reported chiral imidazole tosylate[21] was de-scribed.[22] Again, a series of complexes in which the ligandsadopted a chelate, monodentate and bridging forms wereobserved (Scheme 19). For the coordination of the ligands,it is observed that cyclopentadienyl-NHC ligands prefer thechelate coordination mode (59), and the indenyl system (60)prefers monodentate coordination through the NHC frag-ment. This observation is consistent with previous resultsshown for related indenyl-[37] and Cp-NHC[18,19] ligandsand their coordination to Rh and Ir. As previously ob-served for other related systems,[37] monodentate coordina-tion of the indenyl-NHC ligand afforded a mixture of twodiastereotopic rotamers (Scheme 18 only shows one ofthem, labelled 54), because of the restricted rotation of theligand about the NHC–Ir bond.[22] The enantiomericallypure nature of (R)-59 (M = Ir) was confirmed by its molec-ular structure determined by means of X-ray diffraction.

Scheme 19. Ir compounds 59–61.

The catalytic properties of some of the previous Ir andRh complexes were studied. The iridium compounds 46 and47 are excellent catalysts for a number of reactions involv-ing hydrogen-borrowing processes,[38] a fact that confirmsthe applicability of the tethered ligand. Specifically, bothcomplexes were very active in transfer hydrogenation, β-al-kylation of secondary alcohols with primary alcohols andamination of primary alcohols (Scheme 20).[18,19]

For all these processes, both 46 and 47 offer similar re-sults to those obtained by other related “IrCp*(NHC)” cat-alysts[39] and better than those obtained by [IrCp*Cl2]2 un-der the same reaction conditions.

The rhodium complex 57 was tested in the hydrofor-mylation of 1-octene, although extensive alkene isomeriza-tion competed with hydroformylation, and the isomerized


Scheme 20. Reactions involving hydrogen-borrowing processes:transfer hydrogenation, β-alkylation of secondary alcohols, and N-alkylation of amines.

alkenes were also hydroformylated. The methanol carb-onylation reaction was more successful, and 57 was a littleless active than the most-active standard Monsanto system,[RhI2(CO)2]–, but comparable to the electron-rich [Rh(Cp*-PEt2)(CO)] complex,[40] which suggests that reductive elimi-nation of acetyl iodide may be the rate-determining step ofthe process.[34]

A nickel complex bearing the Ind-NHC ligand was pre-pared by Shen and co-workers, and its catalytic perform-ance in the polymerization of styrene was investigated.[12]

The neutral Ind-NHC ligand (generated in situ by the reac-tion of 1 with nBuLi) was treated with [NiCl2(DME)](DME = dimethoxyethane) to obtain complex 62(Scheme 21). Complex 62 was significantly more air stablethan the related [NiInd(NHC)Cl] complexes, in which theNHC ligand is not linked to the indenyl fragment, whichdemonstrates that chelation enhances the stability of thecomplex. Complex 62, activated by NaBPh4, was catalyti-cally active in the polymerization of styrene and displayeda higher activity than related nonlinked indenyl systems.[12]

Scheme 21. Coordination of indenyl-functionalized NHC to Ni(62).

Conclusions

The results outlined above highlight significant prospectsfor the use of cyclopentadienyl-functionalized NHCs andtheir related indenyl- and fluorenyl-NHCs as ligands forearly and late transition metals.


The versatile coordination chemistry of these new ligandsto a variety of metals ranging from early to late transitionmetals and the interesting catalytic properties in reactionssuch as transfer hydrogenation, amination of alcohols withprimary amines, β-alkylation of secondary alcohols, hydro-silylation, isomerization of allylic alcohols, carbonylation ofmethanol, hydroformylation, epoxidation and polymeriza-tion have been demonstrated.

Although the number of complexes reported to date isstill relatively small, in some cases, it has been proven thatthe chelating coordination of the Cp-NHC ligand has im-portant implications that enhance both the stability of thecomplex and their catalytic performance.

The wide scope for electronic and steric tuning, the pos-sibility of hemilabile dynamic behaviour, chirality, and thesupport of homo- and heterobimetallic complexes are someof the very attractive features that most likely will be ex-ploited in the near future. In addition, the configurationalstability of chiral-at-metal complexes that has been demon-strated for one ruthenium complex can be taken as a prom-ising feature for interesting future developments.

Acknowledgments

We gratefully acknowledge financial support from FCT of Portu-gal, POCI 2010 and FEDER through the project PTDC/QUI-QUI/110349/2009 and the MICINN of Spain (CTQ2011-24055).

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Received: September 16, 2011Published Online: November 30, 2011

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