BASED ON REVIEW BY SAMUEL DAGOME AND DAVID A. ATWOOD

Organo aluminum cations: Synthesis and Reactivity studies and their applications

Siddharth Shankar Tripathi

ABOUT ALUMINIUM....

• Predicted by Lavoiser,Antoine (1787)• Isolated by Oerdted,Hans Chrstian(1825)• Named by Davy, Humphrey (1807)

• Atomic Num=13 ; St. at. weight=26.9815 ; group-13 ; P-block element ; Valence Config=2,8,3 ; Face centred cubic lattice

General oxidaton states=3,2,1 Post transition element sometmes behaves as metalloid. Isotopes - Al(26) found in traces ; HL=7.17X10^5 years

Al(27) 100% ; stable.

General discussion of Al cations

• Group 13 cations have been known for a long time since 1930s but only lately well defined cationic compounds of aluminium were prepared in 1984 in combination with crown ethers and alkyl-aluminium dichlorides to produce solution species that form liquid clathrates.

• Discovery of Salen class of ligands allowed straightforward formation of a wide variety of well defined group 13 cations which opened the way to new possibilities in research.

general structure of salen based Al cations

*This class of compounds has received such importance due to the beneficial lewis acidity of such cations versus their neutral analogues. [ Lewis-acdity type catalysis]

***Although the chemistry of cationic organoaluminium complexes is not well developed ,the reports in the literature indicate that is a rich area for future development .***

FOOD FOR THOUGHT ???

**Two-Coordinate Trivalent Cations.

Metallocenium Cations. Aluminocenium Cations.

Discovery-Scnockel first reported the synthesis and structure of a well defined two- coordinate aluminocenium cation Cp*

2Al+ (4+, Cp*

) C5Me5 as a Cp*AlCl3- salt by disproportionation of Cp*

3Al in the presence of AlCl3, thereby showing that two-coordinate organoaluminum species could be readily obtained.

The synthesis of Cp*2Al+ as a bis-pentabenzylcyclopendienide Li salt, which was obtained as a side product of the reaction between Li[C5Bz5] and the Al(I) compound Cp*Al . The synthesis of the alumenocenium salt [Cp*2Al][AlCl4], through the reaction of Al2Cl6 with Cp*2AlCl, has been reported.

SCHEME FOR PRODUCTION

CONTINUED

CATIONS

Different cations reported :-

1. Aluminocenium cation Cp2Al+

PREP: is accessible as a salt MeB(C6F5)3 by reaction of the neutral alkyl precursor Cp2AlMe with B(C6F5)3.

STABILITY: The Lewis acidic Cp2Al+ cation exhibits limited stability in solution at room temperature.

USE: Found to be an effective initiator of the cationic polymerization of isobutene at low temperature, affording polymers with molecular weights ranging from 3 × 105 to 1.8 × 106, with low polydispersities.

CATIONS

scheme for accessing aluminocenium cation Cp2Al+ (5+)

CATIONS

Dialkyl Aluminium Cations.

Prep: Treatment of AlEt3 with a carborane trityl

salt [Ph3C][CB11H6X6] (X=Cl, Br) to obtain the required ion as a tight pair in benzene soln.

CATIONS

DIARYL ALUMINIUM CATIONSPREP: Prepared by the ionization of the hydrido aluminum

derivative (2,6-Mes2C6H3)2AlH with [Ph3C][B(C6F5)4].

PS: UNLIKE the alkyl salt the solution of aryl salts have no interaction between the cation and the

anion ionization of the hydrido aluminum derivative (2,6-Mes2C6H3)2AlH with [Ph3C][B(C6F5)4]

The solid-state structure of cation exhibits a quasi-two-coordinate metal centre.

Al cation is more reactive than its Ga analogue.

Oligomerizes 1-octene but doesnt react with ethylene.

The features represent more lewis acidity of Al than any other member of the group.

**THREE COORDINATE Al CATIONS

DISCOVERY-

In the late 1990s, Jordan first reported the synthesis of stable three co-ordinated Al cations of the type {LX}AlR+ (R=alkyl) where LX- is a bulky mono anionic pie-delocalized N,N-bidendate ligand of the type aminotroponiminate or beta-diketiminate.

Solid-state structure of[27][B(C6F5)4]

CONTINUED >>

It is noteworthy that the cation LX=NacNac- and R=Me could be generated as a MeB(C6F5)4- salt by reaction of {NacNac}AlMe2 with B(C6F5)3.

UNUASUAL THINGS HAPPEN...

• Stability of salt species [MeB(C6F5)3] .

• Observation of a Me- rather than C6F5- transfer from

MeB(C6F5)3- to {Nac-Nac}AlMe+.

• Such type of alkyl cations are unstable and decompose by irrev C6F5

- transfer.

Note: Crystallisation of [B(C6F5)4] allowed first X-ray crystallography detemination of a quasi-3 co-ordinate Al cation.

CONTINUED >>

{R'C(NR)2}AlMe+ cation: This is 3 co-ordinate Al cations supported by a pie-

delocalised bulky amidinate ligand where [R=R'=Bu; R=iPr,R'=2,6-dimesitylphenyl] .

PREP:

Reactn of {R′C(NR)2}AlMe2 with [Ph3C][B(C6F5)4].

CONTINUED >>

STABILITY: This type of cation is unstable in solution and

can only be observed in NMR. This may be partially ascribed to the nature of the ancilliary

ligand; in the amidinate Al cation, the more strained four membered Al-metallacycle may partially rationalize the lack of stability.

NOTE: The nature of the counterion associated with the

electrophilic {LX}AlR+ Al cation as well as the nature of the Al-R+ moiety may be crucial for the stability of such cations.

FACTORS AFFECTING STABILITY

It has been found that the B(C6F5)4- anion seems

to be suitable to yield stable {LX}AlR+ cations, while the more reactive RB(C6F5)3 anion may irreversibly react with it through a C6F5

- ligand transfer.

As for the nature of the Al-R moiety, its steric properties may significantly influence the stability of the formed cations.

Al alkyl cations ( Et,Pr,iBu) are stable as B(C6F5)4-

salts while the coordinate cations {iPr-ATI}AlR2 derivatives (R=Bz,Cy) readily decompose.

MORE OF THEM....

Bidentate N-Imidoylamidine Cations: Three co-ordinate Al methyl and hydrido cations supported by a

disymmetric amidoimine ligand derived from a sterically bulky N-imidoylamidine were recently synthesised.

PREP:

The Al methyl cation was produced by a classical Me- abstaction reaction, the Al hydrido cation was produced by the

flollowing scheme :- (using the appropriate neutral N-imidoylamidine, [Ph3C][B(C6F5)4], and 2 equiv of the

amine Al adduct AlH3-NMe2Et )

CONTINUED >>

STABILITY: These cations are quite robust and stable.

These exist as fully dissociated B(C6F5)- salt in solution.

MONODENTATE LIGANDS ONLY

The use of bulky bidentate ligands has clearly been used to a great extent for generation of statble 3 co-ordinate group 13 cations , cationic Al species of the type LAlX2

+, has also cropped up.

As a result a series of 3 co-ordinated dipiperidinoamino aluminum cations (tmp)2AlL+ (tmp=2,2,6,6-tetramethylpiperidino) as AlX 4- salts have been reported.

PREP:

The salts have been prepared through halide abstraction at the corresponding neutral precursors

(tmp)2Al(X)L by AlX3 (X=Br, I)

STABILITY: It has been found that sterically demanding monodentate X-type

ligands such as tmp yield stable low-coordinate Al cations. Attempts to prepare cations as BPh4

- , B(C6F5)4-, or Otos- salts

through halide abstraction led to either decomposition products or the formation of molecular adducts.

ITS REACTIVITY ( The {LX}AlR+cation)

The reaction of the cation towards most substrates generally yields the formation of much more stable four-coordinated Al cations.

The comparison of its reactivity to its natural precursor can provide deep insight and can form a fundamental and apllied form of view to gain insight on the influence of cationic charges on these systems.

REACTIVITY TOWARDS >>

• LEWIS BASES:

The reactivity of {LX}AlR+ cation towards lewis bases is dominated by its lewis acidic character;

Thus these 3 coordinated cations readliy form robust four-coordinated cationic adducts {LX}Al(R)(L)+ in the presence of common lewis bases (L) such as THF,tertiary amines and PMe3.

AND REACTIONS CONTINUE >>

Even with unsaturated substrates the reactivity is dominated by lewis acidity unlike the neutral cations.

KETONES:

When acetone is added to e {iPr-ATI}AlR+ (R=Et, Pr, iBu) cations, the corresponding cationic Al-ketone Lewis base adducts {iPr-ATI}Al(R) (O=CMe2)+ readily form.

But slowly the beta H2 transfer to an isopropoxide Aldication.

SOME MORE

• PROPYLENE OXIDE: The three-coordinate {iPrATI}AliBu+

cation is a very efficient initiator of propylene oxide (500 equiv of PO, 240 t.o./h) to yield atactic poly(propylene oxide).

This is a polymerisation reaction where a cationic lewis acid mechanism has been proposed to be the most likely one.

{ predicted from the fact that 6 coordinate Al cations polymerise PO in a cationic manner.}

REACTIONS >>

• TOWARDS ALKYNES: The cations {iPr-ATI}AlR+ (R=Et, iBu)

were found to catalytically dimerize tBuC′CH to the head-to-tail dimer 2-tBu-5,5-dimethyl-1-hexen-3-yne (RT, 4 t.o./h, >90% selectivity).

The mechanism followed was an

insertion/σ-bond metathesis mechanism accd to which the the cationic vinyl Al cation generated by an initial beta-hydrogen reaction between {ATI}AlEt+ and tBuC′CH, reacts with additional tBuC′CH by σ-bond metathesis to yield the alkynyl complex {ATI}Al-(C′CtBu)+ and tBuC′CH;

HOW IT TURNS OUT TO BE SO

Here the Al alkynyl cation is predicted to be the actual catalyst of the dimerization preocess.

This reactivity of Al alkynyl cation with tBuC′CH thus represents the first example of an Al-C insertion reactivity toward unsaturated substrates of a {LX}MR+ type species.

** FOUR COORDINATED CATIONS

• INTRODUCTION:

These are the most common cations as they incorporate an electronically saturated metal center, adopting its preferred tetrahedral geometry.

It is due to this fact that a number of cations of this form have been reported.

To ease the understanding and presentation I shall here discuss according to six structural types (A-F) reflecting the principal coordination modes observed so far for these species.

STRUCTURES.

SYNTHESIS

• TYPE A CATIONS :

Type A cations are accessible mainly through R- abstraction or protonolysis method.

• TYPE B,C,D CATIONS: Many unusual schemes have been

reported,revealing new aspects of reactivity of Al chelates and affording interesting Al cationic species.

Eg: Unexpected formation of {L2}AlR2+ species was observed in

attempts to ionize {LX}AlR2 aluminium derivatives with [Ph3C][B(C6F5)4] in view of generating {LX}AlR+ cations.

CATIONS ADOPTING MODES A-D

REACTIVITY OF A-D CATIONS

• The potential catalytic applications of these Al cations remain to be studied and evaluated.

• The lack of interest in them is because of the fact that they incorporate an electronically saturated Al centre specially in the case of {L2}AlX2

+ (type B cations) with no liable ligand L susceptible to generate a vacant coordination site in catalytic conditions. Thus these derivatives are remarkably stable.

TYPE E-F Al CATIONS

• The cations of the type {LX}Al(X)(L)+ and {L2X}Al(X)+ (X=alkyl,

alkoxide) have been categorised under this.

• These species are of interest because of the fact that the potential lability of L renders such species a formal source of highly reactive, yet more stable, three coordinate {LX}AlX+ .

SYNTHESIS:

Almost all Type E {LX}Al(R)(L)+ Al cations were prepared through a R-/H- abstraction reaction by B(C6F5)3 or [Ph3C][B(C6F5)4] from {LX}AlR2 in the presence of a Lewis

base L.

SYNTHESIS .

Alternatively for E type cations may be prepared by protonolysis reaction between {LX}AlR2 and [HNMe2Ph][B(C6F5)4] may also be used.

For type F cations barring some these were also prepared by the “R-/ H- abstraction” ionization pathway but in the absence of an external Lewis base because the ancillary ligand LL′X- incorporates an extra Lewis base L′ that stabilizes the generated cationic center.

A CHEAPER METHOD ??

Four-coordinate Al cationic alkyl and aryl species may also be accessible in a straightforward manner through chloride abstraction as recently demonstrated by the preparation of cations from reaction of a neutral precursor {LL′X}Al(Me)(Cl) with NaBPh4 or AlCl3 .

LIMITATIONS: the solution structure of these species may be complicated by ligand exchange reaction between the Al cation and the counterion BPh4

-, yielding a mixture of cations.

LESSON: the use of an inert counterion is often a key requirement for the generation of well-defined fourcoordinate Al alkyl cations.

STABILITY AND THEIR WORTH.

The Type E or Type F cations are remarkably stable when created as Me(BC6F5)3

- or B(C6F5)4- salts. More reactive

counterions such as AlCl4may even be used without loss of stability.

• Potential sources of three-coordinate Al cations.• Have drawn a lot of attention due to application in

polymerization catalysis of various substrates.• Extensive usage in converting MMA to PMMA. • Atactic polyactic acis has been synthesised due to

polymerisation of (D,L)-lactide initiated by Al complex in the presence of propylene oxide.

• Have been implicated in a B(C6F5)3-catalyzed hydroalumination reaction of benzophenone and benzaldehyde.

FIVE COORDINATED CATIONS

Five coordinated cations remain somewhat rare and typically require the use of chelate ligands to provide the appropriate number of coordination sites.

Most of these cations have emerged as a result of studies of six-coordinate cations or from a fundamental exploration of any Al ligand combinations.

One of the first five-coordinate aluminum compounds reported employed diaza-18-crown-6 to create a compound of formula (AlEt)2(diaza-18-crown-6)2

+ with EtAlCl3- as the counteranions.

While isolating intermediates in a reaction to produce salen based SalenAlBr cation, an adventitious product was isolated containing possibly the first example of five-coordinate Salen Al cation. The compound, [Salen(tBu)Al(MeOH){µ-O(MeO)2P(O)} Salen(tBu)Al{µO(MeO)2P(O)}Salen(tBu)Al]Br , may have formed by having Salen(tBu)AlBr dedimerize the known compound [Salen(tBu)Al{O(MeO)2P(O)}]2. These cations have been used to dealkylate phosphate esters.

Solid state structure of trinuclear Salen-Al cation in the salt species.

SUMMARY OF FIVE COORDNIATED

The exception of getting five coordinated cationic Al compunds from adventitious isolation during pursuit of other compounds was solved by a series of compounds supported by a ligand containing a triazacyclonane functionality.

This created an environment where abstraction of an Al or In alkyl group would lead to either mononuclear or dinuclear five-coordinate cations. A key observation was made in this study that dimeric compounds containing an Al2O2 four-membered ring would not undergo alkyl abstraction by either B(C6F5)3 or [Ph3C][B(C6F5)4].

THUS A DISCRETE CLASS OF FIVE COORDINATED Al CATIONS

COULD NOW BE PREPARED IN HIGH YIELDS.

SIX COORDINATE {LASTLY :) }

The SIX COORDINATE cations also come in variants based on different coordination modes.

PREPARATION : TYPE A >> Prepared by base-assisted heterogeneous cleavage of the [AlCl3]2 dimer. This produces cations of the formula AlCl2(L)4

+ as AlCl4- salts with L = thf, thf

(with EtAlCl3- as the anion), pyridine, NH3 etc.

TYPE B >> AlX2(L)4+ cations reported including the following: (i) an Al dihydride

compound, AlH2(thf)4+, with the anion{Cp3Yb}2(Na)- produced through autoionization of

aluminum hydride ; (ii)the Aldifluoro cations AlF2(py)4+ (as a Cl - salt) and AlF2(py)4

+· 2pyH+·3Cl- (py )pyridine), which may be formed through a F/Cl exchange reaction between AlCl3·3py and Me3SiF in pyridine.

TYPE C >> Salt elimination reactions with two pyrazolylborate ligands (tris(3,5-dimethyl-1pyrazolyl)hydridoborate) can create six-coordinate Aluminum cations of formula AlL2

+·GaCl4- .

TYPE D >> Supposed to be derived from crystallisation of four-coordinate cation from THF.

EXAMPLE

PREP METHODS...

TYPE E >> Majority of work that has occured has resulted in E cations with the Salen ligands. The cation precursors are formed formed by combination of the SalenH2 ligand with the dialkyl Al halide to yield the SalenAlX species. In the case of Al, the addition of base (H2O, thf, etc.) produces the cation [SalenAl(base)2]+. The use of tetradentate dianionic ligands creates the type E class of cations.

TYPE F >> The only example of a type F cation, was prepared by dissolution of the bimetallic (SalpenN3H{AlMeCl}2) in thf followed by redistribution which formed SalpenN3HAl(thf)+ ·AlMe2Cl2-.

Scheme depicting preparation of the type F cation.

THE ALUMINIUM DICATION

The unique beta-diketiminate-supported starting material, LAl(OTf)2 (L=HC-{CMeN(C6F5)2}2-) ,

was used to prepare a six-coordinate cation [LAl(terpy)][OTf] and the first example of a chelated dicationic aluminum compound LAl(tren)2

+·2OTf- .

The solid state structure of the mononuclear Al dication

APPLICATIONS OF THESE GUYS

[SalenAl(MeOH)2][BPh4] polymerized propylene oxide.

Salen(tBu)Al(MeOH)2 was found to oligomerize propylene oxide to produce low Mw oligomers with Mn=427 and PDI=1.5

Binuclear boron halide chelate compounds can dealkylate a wide range of phosphates at ambient temperature.

It was also shown that mononuclear Schiff base aluminum compounds such as Salen(tBu)AlBr could dealkylate organophosphate esters under mild conditions.

The regioselective ring-expansion carbonylation of aziridines to form lactams was achieved with the catalyst, [Salophen(tBu)Al(thf)2][Co(CO)4] (Salophen=(N,N′-ophenylene-bis(3,5-di-tert-butylsalicylideneimine)).

SUMMARY

Six-coordinate aluminum cations (and related neutral derivatives) supported by Salen and porphyrin ligands have now become widely used for catalytic applications including propylene oxide polymerization and a wide range of carbonylation reactions.

These applications rely on the stable environment provided by the ligands and the Lewis acidity of the cations. In the carbonylation reactions, added Lewis basic ligands are needed to mitigate the cationic charge on the aluminum atom.

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