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(d_π)–O(p_π)–Cr(d_π) bonding within near-linear CrOCr units. An upper limit of 0 on pK_a 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-O^2– 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 b_2g(highest occupied) and b_1u(lowest unoccupied) levels by virtue of π interactions with the diimine ligands.