Breaking an electronically preferred symmetry by steric effects in a series of [Ir(biph)X(QR3)2] compounds (X=Cl or I, Q=P or As)

Abstract

A hybrid quantum mechanical/molecular mechanics IMOMM (B3LYP:MM3) method has been applied to a series of five-coordinate 16-electron ML₅ Ir^III compounds having a relatively flat potential for a distortion from Y to T geometry and for which crystal structures have been obtained. In this series of type [Ir(biph)X(QPh₃)₂] (biph=biphenyl-2.2′-diyl; Q=P, X=Cl, 2a; Q=As, X=Cl, 2b; Q=P, X=I, 2c), the halide is found to lie in the (biph)Ir plane but off the C₂ axis of the {Ir(biph)Q₂} fragment by a variable angular distortion φ. While φ=0 is preferred electronically for [Ir(C₄H₄)Cl(PH₃)₂], the steric bulk of the real systems 2a–2c leads to φ taking experimental values of 8.2–17.2°. The observed deviation of the halide from the C₂ axis is shown by IMOMM to be the result of a direct interaction of the phenyl substituents of the axial ligands with the equatorial ligands and not to an electronic effect. The crystal structures for 2b and 2c have been determined.