Structure and Reactivity of Neutral and Cationic trans-N,N′-Dibenzylcyclam Zirconium Alkyl Complexes

Reactions of (Bn(2)Cyclam)ZrCl2 (1) (where Bn(2)Cyclam = trans-N,N'(PhCH2)(2)Cyclam) with appropriate alkylating reagents produced (Bn(2)Cyclam)ZrR2 (R = Me (2), CH2Ph (3), Bu-n (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, ((C6H4CH2)(2)Cyclam)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 equivalent to CPh led to ((C6H4CH2)BnCyclam)Zr(CCPh) (7) via protonation of one Zr-C-Ph bond by phenylacethylene. Further reaction of 7 with an excess of HC equivalent to CPh led to 5. The reactions of 2 and 3 with B(C6F5)(3) are strongly solvent dependent. Solvent-stabilized cationic species of formula [(Bn(2)Cyclam)ZrR(Solv)]-[RB(C6F5)(3)] were obtained from reactions of 2 in CD2Cl2 (10) or d(8)-THF (12) and from 3 in d(8)-THF (9). The reactions of 3 in CD2Cl2 or d(8)-toluene gave [(Bn(2)Cyclam)Zr(eta(2)-CH2Ph)][PhCH2B(C6F5)(3)] (8). Finally, the reaction of 2 in d(8)-toluene led to [(Bn(2)Cyclam)Zr(C6F5){CH2B(C6F5)(2)}] (14); the precursor of the latter is the zwitterionic [(Bn(2)Cyclam)Zr(CH3){CH3B(C6F5)(3)}] (11), which then undergoes methane elimination and C6F5 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 C6F5 ring is supported by one bridging fluorine bond between the zirconium and one of the C6F5 rings that remains bonded to boron.