A survey of the crystal structures containing simple {M4O4} cubane units is reported. It shows that the average M-M distance in these complexes is relatively constant for a given metal ion M. The structures are all distorted from the idealised cube to a T(d) structure, and most show a further distortion which, however, usually maintains some elements of symmetry. A system for classifying the different types of ligand in these complexes is proposed. Two new cubanes of cobalt(II) and nickel(II) with the ligand (R,R)-bis-1,2-(1-methylbenzimidazol-2-yl)ethane-1,2-diol, (R,R)- or its enantiomer have been isolated and the crystal structure of the cobalt(II) complex confirms the cubane structure. Electronic, CD and (1)H NMR spectra and magnetic susceptibility data are reported. The magnetic data for these and other compounds in the literature are discussed in terms of the structural parameters.

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The ligand 1,2-bis(benzimidazol-2-yl)-1,2-ethanediol (H2bzimed, 1) and its N-methylated analogue (H2mbzimed, 2) form a variety of polynuclear complexes with copper(II), all of which contain a planar Cu2O2 lozenge as a central element and in which the bridging oxygen belongs to an alkoxo group of the ligand. Syntheses are reported for dinuclear [Cu2(Hmbzimed)2](ClO4)2 x 1.5H2O, Cu(2)2(2), and the tetranuclear species [Cu4(Hbzimed)4(ClO4)2](NO3)2 x 4H2O, Cu(4)1(4), [Cu4(Hmbzimed)2(mbzimed)Cl2](ClO4)2 x 2H2O x C2H5OH, Cu(4)2(3), and rac-[Cu4(H2bzimed)4(bzimed)(ClO4)2](ClO4)4 x 1.5H2O x 3.5C2H5OH, Cu(4)1(5). Crystal structures are reported for the tetranuclear species. Cu(4)1(4) shows a cubane structure, Cu(4)2(3) a stepped cubane structure, and rac-Cu(4)1(5) a novel structure in which one doubly deprotonated ligand lies between the two Cu2O2 units. Magnetic susceptibility measurements indicate that all complexes show antiferromagnetic coupling in the solid state. Studies in solution (ESI-MS, CD, NMR) show that Cu(2)2(2) and Cu(4)2(3) persist in solution but that Cu(4)1(4) dissociates partially and rac-Cu(4)1(5) completely. The six coordination modes of the ligands are discussed together with the effect of the N-methylation on the ligand conformation.