Synthesis and structures of cationic aluminum and gallium amidinate complexes

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' = Pr-i, 1a; R' = Cy, 3a) with 0.5 equiv of [Ph3C][B(C6F5)(4)] or B(C6F5)(3) yields {MeC(NR')(2)}(2)Al2Me3+ (R' = Pr-i, 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 mu-eta(1),eta(1) and mu-eta(1),eta(2) amidinate bridges. NMR studies show that 2a(+) undergoes two dynamic processes in solution: (i) a mu-eta(1),eta(1)/mu-eta(1),eta(2) amidinate exchange and (ii) Me exchange between the two metal centers. The reaction of {MeC((NPr)-Pr-i)(2)}GaMe2 (1b) with 0.5 equiv of B(C6F5)(3) yields {MeC((NPr)-Pr-i)(2)}(2)Ga2Me3+ (2b(+)), whose structure and dynamic properties are similar to those of 2a(+). The reaction of the bulkier 'Bu-substituted amidinate complexes {'BuC((NPr2)-Pr-i)(2)}MMe2 (M = Al, 6a; M = Ga, 6b) with 0.5 equiv of [Ph3C][B(C6F5)(4)] yields {'BuC((NPr)-Pr-i)(2)}MMe2.{'BuC((NPr)-Pr-i)(2)}MMe2 (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 mu-eta(1),eta(2) 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 {'BuC((NPr)-Pr-i)(2)}M(Me)(C6F5) (M = Al, 9a; M = Ga, 9b) and boron species. The "super-bulky" amidinate complexes {'BuC((NBu)-Bu-i)(2)}MMe2 (M = Al, 12a; M = Ga, 12b) react with 1 equiv of [Ph3C][B(C6F5)(4)] to yield {'BuC((NBu)-Bu-i)(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.