Synthesis and structures of cationic aluminum and gallium amidinate complexes

Article abstract of DOI:10.1021/ja992104j

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' = (i)Pr, 1a; R' = Cy, 3a) with 0.5 equiv of [Ph₃C][B(C₆F₅)₄] or B(C₆F₅)₃ yields {MeC(NR')₂}₂Al₂Me₃⁺ (R' = (i)Pr, 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-*h² 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(N(i)Pr)₂}GaMe₂ (1b) with 0.5 equiv of B(C₆F₅)₃ yields {MeC(N(i)Pr)₂}₂Ga₂Me₃⁺ (2b⁺), whose structure and dynamic properties are similar to those of 2a⁺. The reaction of the bulkier (i)Bu-substituted amidinate complexes {(t)BuC(N(i)Pr)₂}MMe₂ (M = Al, 6a; M = Ga, 6b) with 0.5 equiv of [Ph₃C][B(C₆F₅)₄] yields {(t)BuC(N(i)Pr)₂}MMe₂·{(t)BuC(N(i)Pr)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 {(t)BuC(N(i))Pr)₂}M(Me)(C₆F₅) (M = Al, 9a; M = Ga, 9b) and boron species. The 'super-bulky' amidinate complexes {(t)BuC(N(t)Bu)₂}MMe₂ (M = Al, 12a; M = Ga, 12b) react with 1 equiv of [Ph₃C][B(C₆F₅)₄] to yield {(t)BuC(N(i)Bu)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.