Synthesis and Structures of Cationic Aluminum and Gallium Amidinate Complexes

Abstract

Aluminum and gallium amidinate complexes, {RC(NR‘) 2 }MMe 2 (R, R‘ = alkyl; M = Al, Ga), react with the “cationic activators” [Ph 3 C][B(C 6 F 5 ) 4 ] and B(C 6 F 5 ) 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 }AlMe 2 (R‘ = i Pr, 1a; R‘ = Cy, 3a ) with 0.5 equiv of [Ph 3 C][B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 yields {MeC(NR‘) 2 } 2 Al 2 Me 3 + (R‘ = i Pr, 2a +; R‘ = Cy, 4a + ) as the B(C 6 F 5 ) 4 - or MeB(C 6 F 5 ) 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-η 1,η 1 and μ-η 1,η 2 amidinate bridges. NMR studies show that 2a + undergoes two dynamic processes in solution: (i) a μ-η 1,η 1 /μ-η 1,η 2 amidinate exchange and (ii) Me exchange between the two metal centers. The reaction of {MeC(N i Pr) 2 }GaMe 2 ( 1b ) with 0.5 equiv of B(C 6 F 5 ) 3 yields {MeC(N i Pr) 2 } 2 Ga 2 Me 3 + ( 2b + ), whose structure and dynamic properties are similar to those of 2a + . The reaction of the bulkier t Bu-substituted amidinate complexes { t BuC(N i Pr) 2 }MMe 2 (M = Al, 6a; M = Ga, 6b ) with 0.5 equiv of [Ph 3 C][B(C 6 F 5 ) 4 ] yields { t BuC(N i Pr) 2 }MMe 2 ·{ t BuC(N i Pr) 2 }MMe + (M = Al, 7a +; M = Ga, 7b + ) as the B(C 6 F 5 ) 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-η 1,η 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(C 6 F 5 ) 3 to yield [ 7a ][MeB(C 6 F 5 ) 3 ] and [ 7b ][MeB(C 6 F 5 ) 3 ], respectively, which decompose by C 6 F 5 - transfer to yield { t BuC(N i Pr) 2 }M(Me)(C 6 F 5 ) (M = Al, 9a; M = Ga, 9b ) and boron species. The “super-bulky” amidinate complexes { t BuC(N t Bu) 2 }MMe 2 (M = Al, 12a; M = Ga, 12b ) react with 1 equiv of [Ph 3 C][B(C 6 F 5 ) 4 ] to yield { t BuC(N t Bu) 2 }MMe + (M = Al, 13a +; M = Ga, 13b + ) as the B(C 6 F 5 ) 4 - salts. The salts [ 13a ][B(C 6 F 5 ) 4 ] and [ 13b ][B(C 6 F 5 ) 4 ] are thermally unstable and could not be isolated. However, the NMR data for 13a + and 13b + in C 6 D 5 Cl 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.