Synthesis and Structures of Cationic Aluminum and Gallium Amidinate Complexes

Abstract

Aluminum and gallium amidinate complexes, {RC(NR‘)2}MMe2 (R, R‘ = alkyl; M = Al, Ga), react with the “cationic activators” [Ph3C][B(C6F5)4] and B(C6F5)3 to yield cationic Al and Ga alkyl species whose structures are strongly influenced by the steric properties of the amidinate ligand. The reaction of acetamidinate Al complexes {MeC(NR‘)2}AlMe2 (R‘ = iPr, 1a; R‘ = Cy, 3a) with 0.5 equiv of [Ph3C][B(C6F5)4] or B(C6F5)3 yields {MeC(NR‘)2}2Al2Me3+ (R‘ = iPr, 2a+; R‘ = Cy, 4a+) as the B(C6F5)4- or MeB(C6F5)3- salts. X-ray crystallographic analyses establish that 2a+ and 4a+ are double-amidinate-bridged dinuclear cations, in which the two metal centers are linked by μ-η1,η1 and μ-η1,η2 amidinate bridges. NMR studies show that 2a+ undergoes two dynamic processes in solution: (i) a μ-η1,η1/μ-η1,η2 amidinate exchange and (ii) Me exchange between the two metal centers. The reaction of {MeC(NiPr)2}GaMe2 (1b) with 0.5 equiv of B(C6F5)3 yields {MeC(NiPr)2}2Ga2Me3+ (2b+), whose structure and dynamic properties are similar to those of 2a+. The reaction of the bulkier tBu-substituted amidinate complexes {tBuC(NiPr)2}MMe2 (M = Al, 6a; M = Ga, 6b) with 0.5 equiv of [Ph3C][B(C6F5)4] yields {tBuC(NiPr)2}MMe2·{tBuC(NiPr)2}MMe+ (M = Al, 7a+; M = Ga, 7b+) as the B(C6F5)4- salts, the former of which is thermally unstable. An X-ray crystallographic analysis establishes that 7b+ is a single-amidinate-bridged dinuclear cation, in which the two metal centers are linked by a μ-η1,η2 amidinate bridge. NMR data establish that the structures of 7a+ and 7b+ are similar and both species are rigid in solution. 6a and 6b also react with B(C6F5)3 to yield [7a][MeB(C6F5)3] and [7b][MeB(C6F5)3], respectively, which decompose by C6F5- transfer to yield {tBuC(NiPr)2}M(Me)(C6F5) (M = Al, 9a; M = Ga, 9b) and boron species. The “super-bulky” amidinate complexes {tBuC(NtBu)2}MMe2 (M = Al, 12a; M = Ga, 12b) react with 1 equiv of [Ph3C][B(C6F5)4] to yield {tBuC(NtBu)2}MMe+ (M = Al, 13a+; M = Ga, 13b+) as the B(C6F5)4- salts. The salts [13a][B(C6F5)4] and [13b][B(C6F5)4] are thermally unstable and could not be isolated. However, the NMR data for 13a+ and 13b+ in C6D5Cl are consistent with base-free, three-coordinate structures or labile, four-coordinate solvated cations. These results provide a starting point for understanding the mechanism and reactivity trends in ethylene polymerization catalyzed by cationic Al amidinate species.