Crystallographic identification of a ‘stepped-cubane’ structure for the Cu4O4 core in [Cu4L2(bipy)4(µ3-OH)2][ClO4]4(HL = 5-hydroxy-6-methylpyridine-3,4-dimethanol, bipy = 2,2′-bipyridine)

Abstract

The structure of [Cu₄L₂(bipy)₄(µ₃-OH)₂][ClO₄]₄ containing a Vitamin B₆ ligand, pyridoxine (5-hydroxy-6-methylpyridine-3,4-dimethanol, HL), and 2,2′-bipyridine (bipy) has been determined by single-crystal X-ray analysis. This is the first report on a copper(II) complex having a ‘stepped-cubane’ structure. The compound crystallizes in the triclinic space group P(Z= 1) with a= 11.015(3), b= 11.902(1), c= 13.142(2)Å, α= 105.07(1), β= 102.22(1) and γ= 99.12(1)°; R= 0.054). The co-ordination geometry around each copper is trigonally distorted square pyramidal. Two of the basal sites are occupied by bipyridyl nitrogens in a bidentate fashion. The remaining basal positions for Cu(1) are filled by a phenolic oxygen and a 4-hydroxymethyl oxygen of the L moiety, whereas for Cu(2) they are occupied by two µ₃-OH oxygens. The axial sites are occupied by a µ₃-OH oxygen and the 4-hydroxymethyl oxygen of the same pyridoxine for Cu(1) and Cu(2), respectively. Both the bridging nature of the 4-hydroxymethyl oxygen of the L moiety and the unsymmetrical bridging nature of the µ₃-OH groups with axial–equatorial bridging are novel features. The structure is discussed in relation to stepped-cubane structures reported in the literature. A comparative study is also made with µ₃-hydroxo-bridged copper(II) complexes. Both the plasticity effect of Cu^II and the stacking interactions between the various rings appear to be important in stabilizing this unusual structure.