3.06 - Aluminum Organometallics

3.06
Aluminum Organometallics
A Mitra and D A Atwood,University of Kentucky,Lexington,KY,USA
ª2007Elsevier Ltd.All rights reserved.
3.06.1Introduction266 3.06.2Alkylaluminum Reactions266 3.06.2.1With Oxygen266 3.06.2.2Insertion of CO and Allenes into Al–C Bonds267 3.06.2.3With Water268 3.06.2.4With Alcohols269 3.06.2.5With Transition Metals for Polymerization270
3.06.2.5.1Ionic compounds271
3.06.2.5.2Neutral alkyl-bridged compounds271
3.06.2.5.3Chain-transfer reactions271
3.06.2.5.4Generalizations for cation-versus alkyl-bridged compound formation271 3.06.3Low-Coordinate Cations272 3.06.4Five-Coordinate Aluminum Alkyls272 3.06.
4.1General Considerations272 3.06.4.2Five-coordinate Salen Aluminum Alkyls274 3.06.4.3Five-coordinate Aluminum Compounds with Salen Ligands274 3.06.4.4Derivatization of Five-coordinate Aluminum Compounds274 3.06.4.5Five-coordinate Cationic Aluminum Alkyls274 3.06.4.6Catalysis275 3.06.5Hydroalumination Reaction275 3.06.6Multiple Bonding277 3.06.7Fluoride Compounds278 3.06.8Lewis Acid Catalysis278 3.06.8.1Oxidation of Alcohols278 3.06.8.2Olefin Polymerization279 3.06.8.3Aluminum-mediated1,2-Alkyl Migration279 3.06.8.4Reduction of Alcohols with LiAlH4279 3.06.8.5Acrylate Polylmerization Catalysts280 3.06.9New Developments280 3.06.9.1Intermetallic Compounds280 3.06.9.2 3-O-bridged Compounds280 3.06.9.3Cyclopentadienyl Compounds281 3.06.9.4Methylaluminum Cyanide281 3.06.9.5Lithium Aluminum Amides282 3.06.9.6Aluminum Complexes of Phenyl Hydrazones and Amidines282 3.06.9.7Tris(pyrazolyl)borate Compounds282 3.06.10Future Directions283 References283
265
266Aluminum Organometallics
3.06.1Introduction
Aluminum is the third most abundant element in the Earth’s crust and so applications involving this element are expected to be economical.Moreover,aluminum has unique properties such as its strong bonding to oxygen,three oxidation states(I,II and III),the ability to form cationic and anionic compounds,and the Lewis acidity of its organocompounds.For these and other reasons,organoaluminum compounds have broad utility in areas such as synthesis,catalysis,and materials science.Synthetic methods for new compounds and new areas of application are continually being C(1982)and COMC(1995)extensively covered the work on organoaluminum compounds prior to1993.1,2The more recent edition,written by Eisch,covers in great detail the various reactions, particularly the organic reactions,that employ organoaluminum reagents.Since then,the interest in this field has expanded rapidly and excellent book chapters and reviews have been devoted to this area.A listing of the various areas covered by these reviews is included in the following table:
Areas of study References
Low-valent(Al(I))aluminum complexes3,4
Organoaluminum halides5–8
Alanes9
Organoaluminum cluster compounds and compounds with low-valent aluminum3,4,10
Hydrolysis of organoaluminum compounds11
Coordination chemistry of aluminum at transition metals12
Multiple bonds involving aluminum13,14
Group13donor–acceptor bonds involving aluminum15
Catalysis with organoaluminum compounds16–21
Aluminum compounds with Schiff bases and -diketiminates22,23 Alumoxanes24and hydroalumination reactions25–27continue to enjoy extensive research attention,whereas group 13/15compounds in MOCVD28as well as organometallic chalcogen compounds of aluminum29are now mature fields.Reviews specifically directed to structural30and thermodynamic31data on organoaluminum compounds have been published.A new area of great concern in aluminum chemistry is the potential biological activity of the element.
An excellent coverage of this emerging subject with respect to Alzheimer’s disease is given.32There are many reviews and book chapters that cover the organometallic chemistry of aluminum in general.33–37
Given the enormous body of literature focused on compounds containing an Al–C bond,this chapter will focus on new compounds that can now be viewed as separate classes of Al–C bonded compounds,and the new important results that have been achieved for the pre-existing compound classes.For example,1993marked the beginning of an understanding of how alkylaluminum compounds combine with oxygen and water(with particular relevance to the use of MAO in olefin polymerization).Charged compounds,both low coordinate and chelated,were discovered and fully developed in the past decade.This time period also saw the first multiply bonded group13compounds.As always,aluminum research created stunning new compounds that are difficult to classify.These will be briefly mentioned,but could be regarded as the vanguard for totally new classes of compounds.
3.06.2Alkylaluminum Reactions
3.06.2.1With Oxygen
Aluminum compounds such as AlMe3are well known to be highly air and moisture sensitive due to the strength of the Al–O bond(,350kJ molÀ1)versus the Al–C bond(,255kJ molÀ1).Complete oxidation of1ml of AlMe3in ambient air can produce a fireball3ft in diameter.Recent studies under controlled conditions have produced a better understanding of the reactivity of aluminum alkyls.
In a matrix-isolation reaction of AlMe3with pure O2,it was demonstrated that the first reaction product is the insertion of oxygen into one of the Al–C bonds to produce[Me2AlOMe],which,in turn,becomes either dimeric or trimeric.The reaction is thought to proceed by interaction of O2with the[AlMe3]2dimer to give this product (Figure1(a)).This would help rationalize the breaking of a strong O–O bond.There was no evidence for the presence of peroxo compounds.38However,depending on the conditions,there may be multiple possibilities for the insertion
of O 2into group 13–carbon bonds.For example,in the combination of GaMe 3with O 2,a peroxide is produced,Me 2Ga(OOMe).39Indeed,there is a great deal of evidence for the presence of peroxide group 13compounds.The combination of M(t Bu)3(M ¼Ga,40In 41)with O 2resulted in the formation of the oxygen-bridged dimers,
[(t Bu)2M(OO t Bu)]2.In contrast,the same reaction with the aluminum derivative led to the formation of a dimeric alkoxide.42
Thus,peroxide compounds of aluminum could not be obtained with traditional organometallic reagents and those possessing bulky groups (for which insertion chemistry should be favored).The first structural characterization of an Al–OOR bond occurred when exploring the oxygen insertion into monomeric (t Bu)2Al(methylsalicylate).When conducted at low temperature,this reaction produced a compound with the typical alkoxide linkages,but also with a peroxo group (Figure 1(b)).43At higher temperatures the reaction led to a mixture of products that was difficult to characterize.The peroxo O–O distance of 1.38(2)˚A is slightly shorter than other main group or transition metal peroxide compounds.The Al–O distance for this group is 1.725(7)˚A.
Over several hours under ambient conditions,the compound slowly decomposes to the full alkoxide derivative,indicating the unstable nature of the peroxo species.Subsequent work has shown that the steric effects of the group 13substituents may dictate the outcome of the reaction.In studying the O 2-insertion chemistry of [R 2Al(pyrazole)]2(R ¼Me,Et,t Bu),it was found that the bulky t Bu derivative did not insert oxygen under any conditions.44The crystal structure of this compound revealed that the Al 2N 4six-membered ring was essentially planar.The C–Al–C angles were distorted from ideal Td geometry at 122 as were the N–Al–N angles at 100 .Similar bonding parameters would be expected for the Me and Et derivatives,which insert a single oxygen atom to give a compound having the putative structure shown in Figure 1(c).A key difference between the t Bu starting material and the Me and Et derivatives is that the latter are known to invert their configurations in solutions,potentially opening the aluminum to insertion by oxygen.Based on the matrix-isolation work described above,it would be useful to consider here the possibility of a bridging O 2group spanning the two Al–C bonds.Noting that the O 2bond length will be shorter than the N–N distance in the pyrazole ligand,it appears that there might be enough space for this type of interaction to take place in these compounds.However,the sequence of events culminating in a bis-alkoxide is clearly much more complicated.
3.06.2.2Insertion of CO and Allenes into Al–C Bonds
Matrix-isolation methods were used to identify the first CO adduct of a trialkyl aluminum.The matrix conditions allowed for the stabilization of a monomeric AlMe 3unit,which formed the Lewis acid–base adduct,Me 3Al–CO.45The dimer,[AlMe 3]2,did not form the CO adduct.Subsequent work utilized t Bu 3Al,which is known to be monomeric at room temperature.Moreover,t Bu 3Al was known to insert ethylene,thus giving rise to the expectation that CO would also form an insertion product.This was easily achieved by bubbling CO into a flask containing t Bu 3Al dissolved
in
Aluminum Organometallics 267
hexane.46The original colorless solution became bright yellow once the gas was introduced,and X-ray quality crystals were subsequently obtained in high yield (62%).The spectroscopic data and structure of the compound (Figure 2(a))confirmed the insertion of a CO molecule into one of the Al–t Bu bonds forming a dimeric acyl compound.In the structure,the aluminum atoms are in a distorted tetrahedral geometry with an Al–O distance of 1.866(2)˚A and C–O distance of 1.252(3)˚A.
These distances are similar to those found in ketones coordinated to four-coordinate aluminum.The compound is the first structurally characterized example of a CO insertion into an Al–C bond.
In two separate reactions,a new carbon–carbon double bond was created by the insertion of a heteroallene into the Al–C–Al bonds of a spirocyclic dimethylaluminum bis(iminophosphorano)methandiide compound (Figure 2(b)).47In both compounds,the C–C bond is shorter than a single bond but longer than a typical double bond,indicating delocalization within the interesting bicyclic ring systems.
3.06.2.3With Water
The controlled hydrolysis of aluminum alkyls yields industrially important oligomeric compounds of the formula (RAlO)n .These oligomers belong to a general class of compounds called alumoxanes,species which contain at least one bridging oxo group between two aluminum centers.The first structural characterization of alkylalumoxanes
[(R 2Al)2O]n and (RAlO)n was obtained after hydrolysis of tri-tert -butylaluminum.48The low temperature (À78 C)hydrolysis of Al(t Bu)3in pentane resulted in the formation of the trimeric hydroxide [(t Bu)2Al( -OH)]3(Figure 3(a))as the major product (20–55%yield)with the liberation of isobutene.A single 1H NMR resonance at ¼2.02ppm and a sharp O–H stretch at 3,584cm À1in the infrared spectrum substantiated the presence of hydroxyl groups.The 17O NMR spectrum showed a singlet at ¼1ppm,which was downfield from that observed for the gallium analog ( ¼À17ppm).The 27Al NMR exhibited a broad resonance at ¼139ppm for the four-coordinate aluminum.The crystal structure
of
Figure 2Insertion of CO and allenes into Al–C
bonds.
268Aluminum Organometallics
Aluminum Organometallics269 the compound revealed a planar hexagonal Al3O3ring.The ring was distorted,with the average intraring angle at oxygen
(142 )being significantly larger than at aluminum(98 ).The angle at oxygen was significantly larger than125.8 as
found in the gas-phase electron-diffraction determination of[Me2Al( -OMe)]3or127.8 as calculated for the model compound[H2Al( -OH)]3.This was explained by the presence of significant intramolecular inter-tert-butyl repulsion
that caused an increase in the intraring AlÁÁÁAl distance compared to the other two compounds.Since the Al–O distance
as well as OÁÁÁO distance remained invariant,the Al–O–Al angle must increase to accommodate the interligand steric interaction.The low-temperature hydrated salt hydrolysis of Al(t Bu)3with Al2(SO4)3?18H2O,followed by thermolysis, yielded the tetrameric alumoxane[(t Bu)2Al{( -OAl)(t Bu)2}]2(Figure3(c))as the major product and the octameric alumoxane[(t Bu)Al( 3-O)]8as the minor product.The absence of hydroxyl groups in[(t Bu)2Al{( -OAl)(t Bu)2}]2was confirmed by1H NMR and IR.A four-coordinate aluminum was indicated by a broad resonance at ¼142ppm. However,a resonance for a three-coordinate aluminum center was not observed in the27Al NMR at room temperature,
but it was clearly visible after warming the sample to80 C.
The hydrated salt hydrolysis of Al(t Bu)3in toluene using Al2(SO4)3?14H2O resulted in the formation of the dimeric hydroxide[(t Bu)2Al( -OH)]2as the major product.49This compound could also be prepared by the addition of water
to a refluxing toluene solution of Al(t Bu)3.The1H NMR showed a singlet at ¼1.12ppm and IR showed a stretch at
3697cmÀ1,thus confirming the presence of the hydroxyl group.The27Al NMR had a broad resonance at132ppm for
the four-coordinate aluminum.
An earlier attempt to insert an oxygen atom into the bonds of the dialane,R2Al–AlR2(R¼CH(SiMe3)2),led to the formation,instead,of a10–20%yield of the oligomeric hydroxide derivative,[R2Al(OH)]n,due to the presence of adventitious water.50In a subsequent study,the reaction was carried out in DMSO with1equiv.of water.51This produced[R2Al(OH)]3in91%yield(Figure3(b)).The IR showed a sharp absorption at3,700cmÀ1for the OH group
and the methane protons and carbon atoms exhibited significant high-field shifts in the1H and13C NMR spectra at
¼À0.88and1.4ppm,respectively.The hydroxy proton is shifted to low field at ¼2.71ppm by comparison to the trimeric compound[(t Bu)2Al( -OH)]3and the dimeric compound[(t Bu)2Al( -OH)]2discussed previously.In an attempt to explore the reactivity of the hydroxy group,an equimolar amount of isobutyllithium was added with
N,N9,N99-trimethyl triazinane,leading to an84%yield of the lithium-bridged compound with elimination of one
AlR3group(Figure3(d)).Thus,the expected deprotonation reaction did not occur.Surprisingly,the acidity of the OH
group is apparently reduced by bonding to aluminum.The Al–O bonds to the bridging OH group are,1.89˚A,in the
range found for other aluminum compounds with bridging hydroxide.In contrast,the Al–O bonds to the O–Li group are significantly shortened to,1.76˚A.The Al2LiO3heterocycle has a distorted boat conformation.The endocyclic angles
of the heterocycle are larger at the oxygen atoms(135.9(3)–143.7(2) )than those at the metal centers(97.1(1)–108.8(4) ).
3.06.2.4With Alcohols
A series of dimethyl-and dichloroaluminum compounds have been prepared in combination with alkoxylalcohols.52
The compounds can be generally represented by the formula,[X2Al( -OCHR1(CH2)n(CR2R3)OR4)]2,with X¼Me
or Cl and R1–R4being various combinations of H,Me,Et,Pr,and t Bu.Six compounds were characterized by X-ray crystallography,and all of the compounds were dimeric in the solid state with coordination environments around the aluminum atoms approximating trigonal bipyramidal(tbp).One representative compound(shown in Figure4(a)) contained a four-membered ring with Al–O distances of1.805(2)and1.905(3)˚A.The Al–ether oxygen bond distance
was2.388˚A,signifying weaker secondary bonding.This weaker bonding resulted in a solution-state equilibrium between four-and five-coordinate species(Figure4(a)and4(b)).However,this equilibrium was not observed in the
Cl2Al analogues,due to the increased Lewis acidity of the aluminum atom.The combination of an unsaturated
long-chain alcohol,such as10-undecen-1-ol(CH2T CH(CH2)8CH2OH),with AlEt3and MAO produced the com-pounds,[R2AlOR1]2and[RAl(OR1)2]2(where R¼Me,Et and R1¼10-undecen).53In the presence of excess alcohol,
the tetrametallic‘‘Mitsubishi’’äcompounds Al4R6(OR1)6(Figure4(c))form.The compounds consist of a central aluminum atom(with a27Al NMR resonance at either ¼8.21or7.3ppm)bonded to six oxygen atoms.The Mitsubishiäcompounds could also be prepared from the reaction of an aluminum trialkyl with aluminum trialkoxide.
The compounds[Al{( -OEt)2AlR2}3](R¼Me,Et,i Bu)(Figure4(d))were characterized structurally and spectro-scopically.54The compounds are approximately D3-symmetric with a planar Al2O2ring.The Al–O distances are
longer around the central six-coordinate aluminum(av.1.9˚A)than for the terminal four-coordinate aluminums
(av.1.8˚A).The27Al NMR had peaks around ¼10ppm for the central six-coordinate aluminum and around
¼150ppm for the terminal four-coordinate aluminum.These compounds are precursors for the preparation of
Al2O3nanoparticles.55In addition,they could be further derivatized by alkane-elimination reaction.
The combination of t Bu 3Al with 1,2-di(hydroxymethyl)benzene produces the binuclear complex
[t Bu 4Al 2(OCH 2C 6H 4CH 2OH)2](Figure 4(e))in quantitative yield.56The OH protons in the compounds are involved in intramolecular hydrogen bonding.Their chemical shifts in the 1H NMR appeared at ¼16.35ppm.The methylene units are equivalent and appear as singlets at 4.61ppm.This compound reacts with excess of t Bu 3Al to give the trimetallic compound [t Bu 5Al 3(OCH 2C 6H 4CH 2O)2](Figure 4(f)).The methylene protons now become a doublet of doublet AB system,with chemical shifts at ¼4.73and 4.58ppm due to the hindered rotation introduced by the presence of the third aluminum.
3.06.2.5With Transition Metals for Polymerization
Transition metal compounds formed with alkylaluminum reagents,or activated by these reagents,are important in olefin oligomerization and polymerization.The exact nature of the compounds formed in these combinations is still uncertain,although there is a growing consensus that a range of compounds may be forming in an equilibrium mixture.Evidence for two general types of compounds has been obtained.The first are ionic compounds such as
[Me 3Ti(solvent)]þ[AlMe 4]À(Figure 5(a)),which was spectroscopically characterized in the mixture of Me 4Ti and AlMe 3.57The second type of compound is a neutral alkyl-bridged compound,formed,for example,in the combination of ZrR 4with AlR 3(with R ¼CH 2Ph)(Figure 5(b)).58Another compound in this second group is the methylene-bridged compound Cp 2Ti( -CH 3)( -CH 2)AlMe 2,formed from the reaction of Cp 2TiMe 2with AlMe 3(Figure 5(c)).59This compound is a derivative of the Tebbe reagent,a similar derivative with a bridging chloride in the place of the methyl group,which acts as a carbene in subsequent reactions.60Despite the evidence for these two categories,it is still likely that the alkyl-bridged species separate to form a cationic catalytic transition metal species and an aluminate anion.The ease of formation of this cation,and its stability,will affect the resulting oligomerization and
polymerization
Figure 4Examples of alkoxy compounds formed from the reaction between aluminum alkyls and
alcohols.
270Aluminum Organometallics
Aluminum Organometallics271 reactions.It should be noted that the cationic nature of the boron analogs,for example,[Cp2ZrMe]þ[MeB(C6F5)3]À,are
well established.The following subsections will provide examples of the combination of metal and alkyl groups that appear to form in these two types of compounds.Details of the activities of these systems and the nature of the oligomers and polymers formed will not be given here,as they are covered in great detail in the various articles as well as
the many reviews that have appeared on this subject(for details,see References).
3.06.2.5.1Ionic compounds
Ni(sacsac)P(n Bu3)Cl(sacsac¼pentane-2,4-dithionate)was activated by AlEt2Cl to form a catalytically active species
for the oligomerization of ethene and propene.61This study is noteworthy in that it uses in situ UV–VIS spectroscopy
to monitor the course of the polymerization.In this reaction,the aluminum reagent serves both to activate the transition metal and to scavenge any moisture present.
A comprehensive study of15zirconecene monomethyl monochloro compounds and their exchange of chloride for
methyl with AlMe3indicates that the exchange process is a function of the electron deficiency at the metal.62With
low electron densities,the zirconium is more likely to exchange chloride for methyl.The study revealed low electron densities at the metal for the indenyl compounds by comparison to the cyclopentadienyl compounds.The study also revealed that steric effects are minor compared to the electronic effects of the ligands on the zirconium.
A recent new discovery is the fact that the hydrolysis of branched -alkyl-substituted aluminoxanes are,in some cases,as effective as co-catalysts in olefin polymerization as MAO.63,64For example,when combined with the the metallocenes,Cp*2ZrCl2,the hydrolysis products(Al/H2O¼2)of R3Al(R¼i Bu and i Oct)produced akylated ion
pairs with high polymerization activities.65The same combinations with Cp2ZrCl2did not produce active catalysts,a
result interpreted as due to the inhibition of -hydride elimination in the substituted metallocene derivatives.
A catalyst system formed from the reaction of Cp*2Zr(NMe2)2and AlMe3showed no olefin-polymerization activity
due to the formation of the stable but inactive heterodinuclear cation,[Cp*2Zr( -Me)2AlMe2]þ.In contrast,bulky
AlR3(R¼Et,i Bu)co-catalyzed systems were highly active.66In particular,the Cp*2Zr(NMe2)2combination with
Al(i Bu)3showed higher catalytic activity and higher molecular weight polymers than when using MAO as the aluminum co-catalyst.
3.06.2.5.2Neutral alkyl-bridged compounds
The combination of TiMe4,Cp2ZrMe2,Hf(CH2SiMe3)4with AlMe3was investigated using conductivity measure-
ments and1H NMR spectroscopy.67The conductivity of the CH2Cl2solutions of the mixtures containing Hf and Zr
did not differ substantially from the conductivity of the reactants,so it is likely that alkyl-bridged species may be formed.This was confirmed spectroscopically in the Hf–Al couple.
3.06.2.5.3Chain-transfer reactions
Chain transfer was observed to occur for Al(i Bu)3in olefin-polymerization systems with CpTiCl368and LZrCl2
(L¼rac-dimethylsilylenebis(indenyl)).69There was no effect of increase in Al(i Bu)3concentration on the molecular
weight of the polymer produced for the Zr system,but for the Ti system,there was a sharp decrease in molecular weight.The use of AlMe3instead of Al(i Bu)3led to decrease in both catalytic activity and polymer molecular weight
with increasing concentration.
3.06.2.5.4Generalizations for cation-versus alkyl-bridged compound formation
The use of chlorinated aluminum alkyls,such as AlEt2Cl,favors the formation of cation–anion pairs through the increased Lewis acidity created on the aluminum atom due to the presence of the chloride.The use of
non-halogenated aluminum alkyls leads to bridged structures that are more difficult to separate into ions. Moreover,trialkylaluminum and MAO systems undergo rapid alkyl exchange.70Adding further complexity to
these systems,there is evidence that the chloroaluminum compounds form dimeric chloride-bridged aluminum compounds,which are much more stable than the analogous alkyl-bridged aluminum dimers.62
The use of bulky trialkylaluminum reagents,such as Al(i Bu)3,with various group4metallocenes led to olefin-polymerization catalysts that rivaled those formed with MAO as the co-catalyst.
3.06.3Low-Coordinate Cations
One of the more important recent developments in organometallic aluminum chemistry has been the formation and isolation of low-coordinate compounds,and,in particular,cations.These were first prepared in reactions of various aluminum reagents with crown ethers to form the inclusion compounds known as‘‘liquid clathrates.’’71,72Most of the evidence supports the presence of ion pairs as the basis of the solvent inclusion effect.Indeed,the compound [AlMe2-18-crown-6]þ[AlMe2Cl2]Àwas isolated from one such system(the cation is shown in Figure6(a)).73This was the first time the Me2Alþunit had been structurally characterized.
Subsequently,the base-free cation Cp2Alþ(see Figure6(b))was characterized,and,importantly,used in the polymerization of isobutylene.74The compound was made by combining Cp2AlMe with B(C6F5)3in dichloro-methane.The27Al NMR of the compound had a single sharp peak at ¼À126.4ppm,which was in agreement with the calculated value.75
The discovery that cationic organometallic aluminum compounds could polymerize olefins led to the search for other ligand–AlMeþsystems that might be as catalytically active as metallocene pounds incorporating amidinate ligands were among the first to be found,76but were generally base stabilized(Figure6(c)).The fact that base-free systems were not isolated with these ligands was attributed to steric unsaturation.For example,the N–Al–N bond angles were69 .77The troponiminate ligands offered more steric protection,but their use still led to the isolation of alkyl-bridged and base-stabilized compounds,when0.5equiv.of the alkyl-abstraction reagent,[Ph3C][B(C6F5)4],was employed.78However,with an equimolar amount of the reagent,the base-free cations could be obtained as liquid clathrates in high yields(Figure6(d)).The compounds were found to be active in the polymerization of ethylene and methylmethacrylate.79
Another structure of a base-free three-coordinate aluminum alkyl cation was obtained when using -diketiminate ligands(Figure6(e)).80In the structure,the N2AlC plane is nearly planar.The Al–C bond length is1.905(2)˚A.The N–Al–N angle is102 and the N–Al–C angles are,126 .There were long secondary AlþÁÁÁF contacts in the structure. Schiff base ligands were used to prepare cations of the form[LAlMe]þafter combining the chloride-containing starting material with1equiv.of either AlCl3or NaBPh4(Figure6(f)).81Exposure of the cation to dry O2at0 C produced cationic alkoxy cations.These latter compounds were efficient catalysts for the polymerization of "-caprolactone.After1h,the conversion was95%,and the resulting polymers had a PDI of1.24–1.30.
3.06.4Five-Coordinate Aluminum Alkyls
3.06.
4.1General Considerations
Most five-coordinate aluminum alkyls are prepared by alkane elimination.The first structurally characterized five-coordinate simple aluminum alkyl chelate compound was reported in1993.82The compound
AlMe[MeO2CC6H4-o-O]2was formed by the reaction of Me3Al with2equiv.of MeO2CC6H4-o-OH.The
commonly
272Aluminum Organometallics
used ligands to support five-coordinate aluminum alkyls are Schiff bases,22substituted pyrroles,83multidentate phenolates,84,85amidophosphines,86acac,87and ketiminates.88Some representative examples of five-coordinate alu-minum alkyls with different types of ligands are shown in Figures 7(a)–7(g).Each of these compounds was structurally characterized by X-ray crystallography.The aluminum atoms in these compounds are in geometries that are not completely tbp or square planar (sqp).A quantitative measure has been proposed to describe the distortion from perfectly sqp or tbp geometry in five-coordinate compounds.The amount of this distortion is expressed by a value ‘‘ .’’89A perfectly sqp geometry has a value equal to zero,whereas a perfectly tbp geometry has a value equal to 1.This value may be important in determining the accessibility of a sixth coordination site.90
Five-coordinate aluminum alkyls have been part of a previous review with emphasis on crystallographic data.30Examples of chelated five-coordinate aluminum alkyls include mononuclear,dinuclear,91and trinuclear 56compounds.Some are mixed metal compounds with another group 13metal,for example,gallium.92Trimetallic compounds shown in Figure 7(g)were prepared by the reaction of 3equiv.of R 3Al (R ¼Me,Et,i Bu)with 2equiv.of 2,29-di(hydroxymethyl)biphenyl.The X-ray crystal structures were determined for R ¼Me and R ¼i Bu.The structure consists of two four-membered and two nine-membered rings.The central aluminum is in a tbp geometry.These compounds are the first alkylaluminum diolates with a tbp geometry at the central atom.
A unique example of a compound with five Al–C bonds is ( 5-Cp *)Al(I ).The gas-phase electron-diffraction study of ( 5-Cp *)Al(I )shows the compound to be monomeric with a C 5v -symmetry and Al–C bond distances of 2.39˚A.
93The effect of trans -influences on the stability of monomeric five-coordinate aluminum alkyls have been dis-cussed.94Studies have also focused on the equilibrium between four-coordinate and five-coordinate isomers in compounds of the type [R 2Al{ -O(CH 2)n ER 1x }]2(n ¼2,3;ER 1x ¼OR 1,SR 1,NR 12)(shown in Figure 7(h)for n ¼2).Factors that control this equilibrium and hence the coordination around the aluminum include the steric bulk of the substitutent at the aluminum (R),and the Lewis base donor R 1,the basicity of the neutral donor group ER 1x ,and the chelate ring size (as determined by n ).
95
Figure 7Examples of five-coordinate aluminum alkyls with different ligands.
Aluminum Organometallics 273。