Tipping the balance between twist and chair ligand conformers in multinuclear ethylzinc cyclophosphazenates

Abstract

Hexaanionic cyclophosphazenate ligands [(RN)₆P₃N₃]⁶⁻ provide versatile platforms for the assembly of multinuclear metal arrays due to their multiple coordination sites and highly flexible ligand core structure. This work investigates the impact of incrementally increasing the steric demand of the ligand periphery on the coordination behavior of ethylzinc arrays. It shows that the increased congestion around the ligand sites is alleviated by progressive condensation with the elimination of diethylzinc. The condensation is accompanied by a switch in ligand conformation: the phosphazene ring of the methyl derivative adopts the chair conformer in the dimeric complex (EtZn)₁₂[(RN)₆P₃N₃]₂, while the more sterically demanding isobutyl derivative exhibits twist conformers in both the monomeric complex (EtZn)₆[(iBuN)₆P₃N₃] and the condensed dimer (EtZn)₈Zn₂[(iBuN)₆P₃N₃]₂. Ethyl and propyl derivatives mark a tipping point: the dimeric complex (EtZn)₁₂[(RN)₆P₃N₃]₂ exhibits the chair conformation and is observed in solution and within the stabilising environment of a co-crystal. However, when crystallising in pure form, it transforms with the loss of one equivalent of Et₂Zn into the collapsed dimer (EtZn)₁₀Zn[(RN)₆P₃N₃]₂ which features the twist conformer.