Linear pentacobalt complexes: synthesis, structures, and physical properties of neutral and one-electron oxidation compounds

Abstract

A series of linear pentanuclear cobalt complexes, including both neutral and one-electron oxidized forms, have been synthesized. The one-electron oxidation products were prepared either by reaction with silver salts or by bulk electrolysis. In all of these complexes, the pentacobalt chain adopts a symmetrical arrangement and is helically wrapped by four tpda [the dianion of N,N′-bis(α-pyridyl)-2,6-diaminopyridine] ligands. Two sets of Co–Co bond distances are observed, in which the average internal (inner) bond is about 0.06 Å shorter than the external (outer) one. After one-electron oxidation, the average Co–Co and Co–N bond distances are not significantly different from those of the neutral analogues, whereas the Co–X (X = axial ligand) bond lengths exhibit a slight decrease in length. All of these complexes show two reversible redox couples at about +0.35 and +0.85 V (vs. Ag/AgCl). The first oxidation is a metal-centered reaction and the product has been structurally characterized. The second oxidation product is stable on the time scale of spectroelectrochemistry, but undergoes reduction to form the corresponding one-electron oxidation product under the crystallization conditions. The NMR results are consistent with the paramagnetism of both the neutral and oxidized complexes. The magnetic measurements indicate that the neutral and one-electron oxidized molecules have spin states of S = 1/2 and 1, respectively.