Pyridazine-bridged copper(I) complexes of bis-bidentate ligands: tetranuclear [2x2] grid versus dinuclear side-by-side architectures as a function of ligand substituents

Eight bis-bidentate Schiff-base ligands, derived from 3,6-diformylpyridazine and substituted amino-benzenes, have been prepared. A variety of electron donating/withdrawing and/or sterically demanding/undemanding substituents were employed. Two ligands and five of the six pure copper(I) complexes have been structurally characterised. The sterically unhindered ligand derived from 3,5-difluoroaniline, L-m,L-m-F, was almost completely flat whereas the very sterically hindered ligand derived from trimethylaniline, L-o,L-o,L-p-Me, was severely twisted. The only dinuclear side-by-side complex obtained, [Cu-2(I)(Lo-Ph)(2)](PF6)(2), was of the ligand derived from 2-aminobiphenyl. All five of the other complexes are believed to be [2x2] tetranuclear grid complexes, and this was unequivocally shown to be the case for four of these complexes, [Cu-4(I)(Lp-Me)(4)](PF6)(4), [Cu-4(I)(L-o,L-p-Me)(4)](PF6)(4), [Cu-4(I)(L-m,L-m-F)(4)](PF6)(4) and [Cu-4(I)(L-m,L-m-Cl)(4)](PF6)(4). In all cases the copper(I) centres are substantially distorted from tetrahedral, with the most severe distortion present in the side-by-side complex. In the absence of any special effects, tetracopper(I) [2x2] grid architectures are observed to be the favored outcome for 1:1 reactions of these bis-bidentate ligands with copper(I) ions. Only when the aromaticity of the ligand was extended by employing a phenyl substituent on the phenyl rings, Lo-Ph, did a dicopper(I) side-by-side architecture result. Cyclic voltammetry in acetone revealed that the free ligands did not undergo reduction until potentials below -0.8 V, whereas between three and four reversible one electron reductions were observed, between +0.16 and -0.71 vs. AgCl/Ag, for the tetranuclear copper(I) [2x2] grid complexes. The redox potentials observed for these complexes are highly dependent on the nature of the ligand phenyl ring substituent(s). The side-by-side complex had one irreversible reduction process, E-pc ca.-0.5 V.