Organic chemistry of dinitrogen and related ligands. Part 6. Crystal and molecular structure of [WBr(dppe)2{N2CN(Me)CH2CH2NMe}][PF6](dppe = Ph2PCH2CH2PPh2), a complex in which fluxional behaviour results from steric destabilisation of the ground state

Abstract

Diaminodiazomethane complexes [WBr(dppe)₂{NNC(NHR)₂}]⁺, which are obtained by reactions of the cationic dichlorodiazomethane complex [WBr(dppe)₂(NNCCl₂)]⁺ with primary amines, are rigid on the n.m.r. time-scale at 30 °C because of restricted rotation (ΔG^‡ > 57 kJ mol^–1) about the formal carbon–nitrogen double bond. In contrast, the cyclic analogues[WBr(dppe)₂{NN[graphic omitted]R}]⁺(R = Me or Et, n= 2 or 3) which are prepared by the action of di-secondary amines on the dichlorodiazomethane complex, are fluxional under the same conditions (ΔG^‡= 40 kJ mol^–1) with rapid rotation about the CN₂ bond. A single-crystal X-ray study of [WBr(dppe)₂{NN[graphic omitted]Me}][PF₆] reveals extreme steric strain due to interligand repulsions involving one of the N-methyl groups, implying that the much-reduced rotational barrier in this type of complex results from a greatly heightened ground-state energy. The complex crystallises in the triclinic space group P, with unit-cell parameters a= 12.546(3), b= 14.332(3), c= 17.174(3)Å, α= 74.07(1), β= 76.79(2), γ= 87.05(2)°, and Z= 2. The structure was solved by the heavy-atom method and refined to R= 0.047 for 5 502 independent observed reflections.