Thiocyanato and azido oxo-bridged chromium(III) dimers with aromatic diimine ligands

Abstract

The spectroscopic, magnetic and electrochemical characteristics of four [{Cr(N–N)2L}2O]2+ dimers are reported, where N–N = 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) and L–= NCS– or N–3. Electronic spectra are dominated by strong, diimine ligand-centred π–π* transitions, red-shifted on co-ordination to the chromium(III) centres. Strong antiferromagnetic coupling in [{Cr(bipy)2(NCS)}2O][ClO4]2·H2O (J=–247 cm–1) and [{Cr(phen)2(NCS)}2O][ClO4]2·H2O (J=–271 cm–1) indicates a diamagnetic ground state arising from Cr(dπ)–O(pπ)–Cr(dπ) bonding within near-linear CrOCr units. An upper limit of 0 on pKa for both [{Cr(N–N)2(NCS)}2OH]3+ complexes reveals substantial bipy- and phen-promoted CrOCr π-bonding strength enhancements relative to the previously characterized [{Cr(tpma)(NCS)}2O]2+ dimer [tpma = tris(2-pyridylmethyl)amine]. Oxidative and reductive cyclic voltammograms of the µ-O2– dimers have been determined in an effort to identify the highest occupied and lowest unoccupied molecular orbitals. Both magnetic and electrochemical findings suggest the splitting of formally non-bonding b2g(highest occupied) and b1u(lowest unoccupied) levels by virtue of π interactions with the diimine ligands.