Tris(t-butyl)terpyridine-copper(ii) complexes and ligand field effects

Abstract

A series of [Cu(L₃′)(NCS)(L/X)]^+/0 complexes (L₃′ = 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine; L = CH₃OH; X = NCS, [NCAuCN]) has been prepared and characterized with X-ray crystallography, UV-vis and IR spectroscopies, and DFT calculations. The structure of starting material [Cu(L₃′)(NO₃)₂], 1b, was determined as well as the structures of the five-coordinate copper isothiocyanate complexes [Cu(L₃′)(NCS)(MeOH)](PF₆), 2b, [Cu(L₃′)(NCAuCN)(NCS)], 4, and two pseudo-polymorphs of bis-isothiocyanate [Cu(L₃′)(NCS)₂], 3a and 3b·CH₃CN. The square pyramidal structures of 3a and 3b differ markedly from the more trigonal bipyramidal [Cu(terpy)(NCS)₂]. Electronic structure calculations suggest that the σ-donor ability of the polyimine ligand controls the metal center geometry. The IR spectroscopic analysis shows that the ν(CN) stretching frequencies do not correlate predictably with structural parameters, such as the Cu–N–CS bond angle, demonstrating the oversimplification of the linear Cu–NC–S vs. bent Cu–NCS modelling of isothiocyanate bonding.