Structure and reactivity of neutral and cationic trans - N, N′-dibenzylcyclam zirconium alkyl complexes

摘要

Reactions of (Bn_2Cyclam)ZrCl_2 (1) (where Bn 2Cyclam = trans-N,N′(PhCH_2)_2Cyclam) with appropriate alkylating reagents produced (Bn_2Cyclam)ZrR_2 (R = Me (2), CH_2Ph (3), ~nBu (4), CCPh (5)). Activation of the ortho C-H bond of the two macrocycle benzyl substituents in complexes 2, 3, and 4 was thermally induced, leading to the formation of a bis(ortho-metalated) complex, ((C_6H_4CH_2)_2Cyclam)Zr (6). This reaction proceeds along with RH elimination and converts the original dianionic tetracoordinated cyclam in a tetraanionic hexacoordinated ligand where two new Zr-C_(Ph) bonds complete the metal sphere. Treatment of 6 with one equivalent of HC≡CPh led to ((C_6H_4CH _2)BnCyclam)Zr(CCPh) (7) via protonation of one Zr-CPh bond by phenylacethylene. Further reaction of 7 with an excess of HC≡CPh led to 5. The reactions of 2 and 3 with B(C_6F_5)_3 are strongly solvent dependent. Solvent-stabilized cationic species of formula [(Bn_2Cyclam)ZrR(Solv)][RB(C_6F_5)_3] were obtained from reactions of 2 in CD_2Cl_2 (10) or d _8-THF (12) and from 3 in d_8-THF (9). The reactions of 3 in CD_2Cl_2 or d_8-toluene gave [(Bn 2Cyclam)Zr(η~2-CH_2Ph)][ PhCH _2B(C_6F_5)_3] (8). Finally, the reaction of 2 in d_8-toluene led to [(Bn_2Cyclam)Zr(C _6F_5){CH_2B(C_6F_5) _2}] (14); the precursor of the latter is the zwitterionic [(Bn _2Cyclam)Zr(CH_3){CH_3B(C_6F _5)_3}] (11), which then undergoes methane elimination and C_6F_5 migration from boron to zirconium. The mechanism of this reaction was studied by DFT and revealed that (i) methane elimination is assisted by one alpha-agostic C-H bond and (ii) migration of the C _6F_5 ring is supported by one bridging fluorine bond between the zirconium and one of the C_6F_5 rings that remains bonded to boron.