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 str ongly influenced by the steric properties of the amidinate ligand. The reac tion 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 M eB(C6F5)(3)-salts. X-ray crystallographic analyses establish that 2a(+) and 4a(+) are double-amidinate-bridged dinuclear cations, in which the two met al centers are linked by mu-eta(1),eta(1) and mu-eta(1),eta(2) amidinate br idges. NMR studies show that 2a(+) undergoes two dynamic processes in solut ion: (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)Ga2 Me3+ (2b(+)), whose structure and dynamic properties are similar to those o f 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 t hermally unstable. An X-ray crystallographic analysis establishes that 7b() is a single-amidinate-bridged dinuclear cation, in which the two metal ce nters 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 ri gid in solution. 6a and 6b also react with B(C6F5)(3) to yield [7a][MeB(C6F 5)(3)] and [7b][MeB(C6F5)(3)], respectively, which decompose by C6F5- trans fer- 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 th ermally unstable and could not be isolated. However, the NMR data for 13a() and 13b(+) in C6D5Cl are consistent with base-free, three-coordinate stru ctures 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 amidi nate species.