The reaction of trimethylaluminum with the hexameric tert-butylalumoxane, [( t Bu)Al-(µ 3 -O)] 6 , has been investigated. Reaction of [( t Bu)Al(µ 3 -O)] 6 with 1 equiv of AlMe 3 results in the formation of two isomers (A and B) of the hybrid tert-butylmethylalumoxane, [Al 7 (µ 3 -O) 6 -( t Bu) 6 Me 3 ]. The structures of compounds A and B, as determined by NMR spectroscopy and mass spectrometry, consist of [Al 6 (µ 3 -O) 6 ( t Bu) 5 Me] alumoxane cages, formed via tert-butyl/ methyl exchange, in which one of the edges of the Al 6 O 6 cage is complexed to the ( t Bu)-AlMe 2 formed during alkyl exchange. The difference between the isomers results from the geometric relationship of the cage Al-Me group and the opened edge. The activity of [Al 7 -(µ 3 -O) 6 ( t Bu) 6 Me 3 ], for the [Me 2 C(Cp)(Flu)]ZrBz 2 -catalyzed polymerization of 1,5-hexadiene, is significantly increased in comparison to [( t Bu)Al(µ 3 -O)] 6 . The effect of additional equivalents of AlMe 3 on the cocatalytic activity of [Al 7 (µ 3 -O) 6 ( t Bu) 6 Me 3 ] suggests that a maximum activity is obtained at a [( t Bu)Al(µ 3 -O)] 6 to AlMe 3 ratio of 1:6. Under conditions of equal Al:Zr ratio the [( t Bu)Al(µ 3 -O)] 6 (AlMe 3 ) 6 system has a higher activity than a representative sample of commercial methylalumoxane (MAO). 1 H NMR suggests that the reaction of [( t Bu)Al-(µ 3 -O)] 6 with 6 equiv of AlMe 3 yields ( t Bu)AlMe 2 as the only tert-butyl-containing species and a proposed AlMe 3 "free" form of MAO. Whereas the activity of [Al 7 (µ 3 -O) 6 ( t Bu) 6 Me 3 ] and MAO shows slight inhibition by the addition of Al( i Bu) 3 , the activity of the [( t Bu)Al(µ 3 -O)] 6 / (AlMe 3 ) 6 system is unaffected. The reaction of Cp 2 Zr(CD 3 ) 2 with [Al 7 (µ 3 -O) 6 ( t Bu) 6 Me 3 ] demonstrates that methyl exchange does not occur between a metallocene and the alkyls of the alumoxane cage, but does occur with the complexed ( t Bu)AlMe 2 .

To develop an understanding of the factors that control the strength of the Lewis acidbase interactions in five-coordinate compounds of aluminum, dimeric dialkylaluminum compounds [R 2 Al{µ-O(CH 2 ) n ER′ x }] 2 (n ) 2, 3; ER′ x ) OR′, SR′, NR′ 2 ) have been prepared from AlR 3 and the appropriate substituted alcohol, thiol, or amine: [R The molecular structures of compounds 1, 2, 6, 7, 12, 13, 16, 18-22, and [Me 2 Al(µ-OCH 2 -CH 2 NMe 2 )] 2 have been determined by X-ray crystallography. The solution structures have been probed by 13 C NMR spectroscopy using the alkoxide derivatives, [R 2 Al(µ-O n Bu)] 2 , R ) t Bu (8), i Bu (9), and Et (10), as a comparison, which show that compounds 1-7 and 11-23 exist as an equilibrium mixture between the four-coordinate isomer and its five-coordinate isomer, where K eq ) [4-coord]/ [5-coord]. Factors that control the extent of this equilibrium and hence the coordination about aluminum include the steric bulk of the substituents at aluminum (R) and the Lewis base donor (R′), the basicity of the neutral donor group (ER′ x ), and the chelate ring size (as determined by n). The intramolecular bond dissociation energies (BDEs) of the Lewis base donor (ER′ x ), as determined by variable-temperature NMR spectroscopy, for compounds 1, 12, 15, 16, [Me 2 Al(µ-OCH 2 CH 2 OMe)] 2 , and [R 2 Al(µ-OCH 2 -CH 2 NMe 2 )] 2 (2.3-13.2 kJ‚mol -1 ) are significantly lower than observed for their fourcoordinate analogues, R 3 Al(ER′ x+1 ) (63.8-125.5 kJ‚mol -1 ). In addition, the relative donor ability (thioether > ether > amine) is the opposite of that expected. The similarity in BDE between compound 1 and 11 suggests that no cooperative effects are present. Ab initio calculations on the four-and five-coordinate isomers of the model compound [H 2 Al(µ-OCH 2 -CH 2 OH)] 2 indicate that the surprisingly weak fifth coordination ligation in [R 2 Al{µ-O(CH 2 ) n -ER′ x }] 2 is a consequence of significant steric hindrance between the aluminum alkyl groups (R) and the Lewis base donor group. A quantitative measure of the thermodynamic destabilization that occurs upon substitution of H with Me or t Bu is proposed on the basis of the derivation of the Lennard-Jones (12,6) potential from solution thermodynamic and X-ray structural data.

No abstract

Reaction of Al(Bu t ) 3 with between 1 and 2 equivalents of HOCH 2 CH 2 CH 2 NMe 2 allows for the isolation of the Lewis acid-base complex, (Bu t ) 3 Al[O(H)CH 2 CH 2 CH 2 NMe 2 ] 1, which undergoes alkane elimination above 45 ЊC to yield [(Bu t ) 2 Al(µ-OCH 2 CH 2 CH 2 NMe 2 )] 2 2. Compound 2 is also formed directly when 2 equivalents of Al(Bu t ) 3 react with 1 equivalent of HOCH 2 CH 2 CH 2 NMe 2 . The molecular structure of 1 shows an Al᎐O bond distance comparable to that found in the bridging alkoxide compounds 2 and [(Bu t ) 2 Al(µ-OPr n )] 2 3, suggesting that the Al᎐O ؒ ؒ ؒ H unit may be considered analogous to a bridging alkoxide unit, Al(µ-OR)Al, as a consequence of a significant contribution from the zwitterionic alkoxide Ϫ /ammonium ϩ form made possible by a strong intraligand hydrogen bond. The kinetics of the conversion of 1 into 2 have been studied. A large activation energy and positive deuterium isotope effect are consistent with breaking of the hydrogen bond during the transition state. The reaction of (Bu t ) 3 Al(NMe 3 ) 4 with ethanol yields [(Bu t ) 2 Al(µ-OEt)] 2 5. The reaction of Al(Bu t ) 3 with HN(Me)(CH 2 ) n NMe 2 (n = 3 or 2) yields the stable Lewis acid-base adducts (Bu t ) 3 Al[NH(Me)CH 2 CH 2 CH 2 NMe 2 ]