Observation of redox-induced electron transfer and spin crossover for dinuclear cobalt and iron complexes with the 2,5-di-tert-butyl-3,6-dihydroxy-1,4-benzoquinonate bridging ligand.

Dinuclear (TPyA)M(II)(DBQ(2-))M(II)(TPyA)(2) [TPyA = tris(2-pyridylmethyl)amine; DBQ(2-) = 2,5-di-tert-butyl-3,6-dihydroxy-1,4-benzoquinonate; M = Co (1(2+)), Fe (2(2+)), Ni (3(2+))] complexes have been prepared by the reaction of M(2+), TPyA, H(2)DBQ, and triethylamine in MeOH solution. Their monooxidized form (TPyA)M(III)(DBQ(*3-))M(III)(TPyA) [Co = (1(3+)), Fe (2(3+))] has been synthesized by using ferrocenium tetrafluoroborate, and the dioxidized form of 1(2+), (TPyA)Co(III)(DBQ(2-))Co(III)(TPyA) (1(4+)), has been obtained by using thianthrinium tetrafluoroborate. These dinuclear compounds were characterized by X-ray crystallography, electrochemistry, magnetism, and EPR spectroscopy. Valence ambiguous 1(3+) forms via redox-induced electron transfer, whereby the one-electron oxidation of the Co(II)(DBQ(2-))Co(II) core forms Co(III)(DBQ(*3-))Co(III), and it also exhibits spin crossover behavior to the core Co(III)(DBQ(2-))Co(II) above room temperature. The M ions in 1 and 2 have a distorted octahedral geometry by coordination with four nitrogens of a TPyA, two oxygens of a DBQ(2-/*3-). Due to the interdimer offset face-to-face pi-pi and/or herringbone interactions, 1(2+), 1(3+), and 2(2+) show extended 1-D and/or 2-D supramolecular structures. The existence of DBQ(*3-) in 1(3+) is confirmed from both solid-state magnetic and solution EPR data. Co- and Ni-based 1(2+) and 3(2+) show weak antiferromagnetic interactions [1(2+): g = 2.44, J/k(B) = -3.20 K (-2.22 cm(-1)); 3(2+): g = 2.13, J/k(B) = -3.22 K (-2.24 cm(-1)), H = -2JS(1)*S(2) for 1(2+) and 3(2+)], while Fe-based 2(2+) exhibits strong spin crossover behavior above room temperature. 1(2+) has three reversible one-electron transfer waves at E(1/2) (vs SCE in MeCN) = -1.121, 0.007, and 0.329 V, and a fourth wave at -1.741 V that exhibits a slight chemical irreversibility. The first three correspond to Co(II)DBQ(2-)Co(II) reduction to Co(II)DBQ(*3-)Co(II), and oxidation to Co(III)DBQ(*3-)Co(III) and Co(III)DBQ(2-)Co(III), respectively. The mechanism of the multielectron transfer oxidation from Co(II)DBQ(2-)Co(II) to Co(III)DBQ(*3-)Co(III) is unknown; the energy of stabilization for oxidizing the Co(II) centers in the presence of DBQ(*3-), relative to oxidizing the Co(II) centers in the presence of DBQ(2-) is computed to be 1.45 eV. 2(2+) also has three reversible one-electron transfer waves at 0.802, 0.281, and -1.007 V that correspond to two successive one-electron oxidations (2(2+)/2(3+) and 2(3+)/2(4+)), and a one-electron reduction (2(2+)/2(+)). 2(2+) has the Fe(hs)(II)(DBQ(2-))Fe(hs)(II) electronic structure that becomes Fe(hs)(III)(DBQ(*3-))Fe(hs)(III) upon oxidation. The latter undergoes spin crossover above room temperature to populate the Fe(hs)(III)(DBQ(2-))Fe(hs)(II) excited state.