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)](BF4)(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 M2+, TPyA, H(2)DBQ, and triethylamine in MeOH solution. Their monooxidized form [(TPyA)M-III(DBQ(center dot 3))M-III(TPyA)](3+) [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)](4+) (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](2+) core forms [Co-III(DBQ(center dot 3-))Co-III](3+), and it also exhibits spin crossover behavior to the core [Co-III(DBQ(2-))Co-II](3+) 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-/center dot 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(center dot 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)center dot 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)](2+) reduction to [Co(II)DBQ(center dot 3-)Co(II)](+), and oxidation to [Co(III)DBQ(center dot 3-)Co(III)](3+) and [Co(III)DBQ(2-)Co(III)](4+), respectively. The mechanism of the multielectron transfer oxidation from [Co(II)DBQ(2-)Co(II)](2+) to [Co(III)DBQ(center dot 3-)Co(III)](3+) is unknown; the energy of stabilization for oxidizing the Co-II centers in the presence of DBQ(center dot 3-), relative to oxidizing the Coll 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)](2+) eectronic structure that becomes [Fe-hs(III)(DBQ(center dot 3))Fe-hs(III)](3+) upon oxidation. The latter undergoes spin crossover above room temperature to populate the [Fe-hs(III)(DBQ(2-))Fe-hs(II)](3+) excited state.