Synthesis and Properties of New Dimeric η2-Diyne Complexes of Cobalt Linked through an Azobenzene Ligand

Abstract

The reaction between 4,4′-diiodoazobenzene (0) with excess trimethylsilylacetylene or dec-1-yne in the presence of catalytic amounts of PdCl2(PPh3)2 and CuI, Sonogashira coupling conditions, gave rise to the formation of 4,4′-bis(trimethylsilylethynyl)azobenzene (1) and 4,4′-bis(dec-1-ynyl)azobenzene (2), in good yield, in addition to icosa-9,11-diyne (3) as byproduct in the last case. The analogous reactions from 0 and the alkynyl-cobalt complexes Co2(CO)4(μ-L-L)(μ2-η2-SiMe3C2)](C≡CH) (L-L = dppm, dmpm) afforded 12–14. Complexes 4–7 and 15 have been obtained by direct reaction between Co2(CO)8 and the organic ligands 1–3. In order to increase the stabilization of Co–Co bonds, diphenylphosphinomethane-substituted alkyne-carbonyl complexes 8–11 were prepared by substitution reaction of carbonyl ligands in the presence of Me3NO at the Co2(CO)6 units from 4–7. All products were characterized by analytical and spectroscopic data (IR, 1H, 13C, and 31P NMR, and MS). Crystals of 9 suitable for single-crystal X-ray diffraction were grown, and the molecular structure of this compound is discussed. In this paper we report a comparative electrochemical study of these complexes by means of cyclic and square-wave voltammetry techniques, the trans–cis photoisomerization study of azobenzene compounds, and the thermotropic liquid-crystalline behavior.