X-Ray Structures and Dynamic Solution Behaviours of Five-Coordinate Nickel(II) Complexes Containing Di- and Tritertiary Arsine Ligands

Abstract

A comparative study of solid-state structures and dynamic solution behaviour of two homoleptic five-coordinate NiII complexes containing the potentially tridentate ligands bis(2-(dimethylarsino)phenyl)methylarsine (1) and bis(2-(dimethylarsino)phenyl)phenylarsine (2) and the bidentate ligand 1,2-bis(dimethylarsino)benzene (3) is reported. [Ni(1)2](ClO4)2 (4) crystallises in the monoclinic space group P21/n and adopts a pseudo-square-pyramidal structure. One of the ligands is bidentate with an –AsMe2 group dangling at 3.748 (5) Å from the central Ni. In solution the dangling –AsMe2 is found to rapidly exchange with the coordinated –AsMe2 group trans to it at lower temperatures and with all the coordinated –AsMe2 groups at higher temperatures. [Ni(2)2](PF6)2 (5) crystallizes as 5·C4H10O in the monoclinic space group P21/c and adopts a pseudo-trigonal-bipyramidal geometry. One of the ligands is bidentate and an –AsMe2 group dangles at 4.137 (8) Å from the central Ni, bisecting an As–Ni–As angle of the trigonal plane. Compound 5 shows fluxional behaviour similar to 4 but the barriers to exchange have higher energies. [Ni(3)2X](ClO4) [X = Br (6), Cl (7)] undergo rapid intermolecular axial halide exchange in solution. Compound 6 crystallises in the monoclinic space group Cc and has a distorted square-pyramidal structure consisting of pseudo-chains of cations with potentially bridging Br donors held at ~3.8 Å from the Ni of an adjacent cation by steric interaction with Me groups. The X-ray structure of [Ni(3)2Br2] (8), which crystallises in the monoclinic space group P21/n, is also reported. The complex has a tetragonally distorted octahedral structure, with long Ni–Br bonds, and approximates the proposed transition state for the rapid halide exchange process observed for 6.