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)] (BF ₄) ₂ [TPyA = tris(2-pyridylmethyl)amine; DBQ ^2- = 2,5-di-fert-butyl-3,6-dihydroxy-1,4-benzoquinonate; M = Co (1 ²⁺), Fe (2 ²⁺), Ni (3 ³⁺)] complexes have been prepared by the reaction of M ²⁺, TPyA, H ₂DBQ, and triethylamine in MeOH solution. Their monooxidized form [(TPyA)M ^II(DBQ ^*3)M ^III(TPyA)] ³⁺ [Co = (1 ³⁺), Fe (2 ³⁺)] has been synthesized by using ferrocenium tetrafluo-roborate, and the dioxidized form of 1 ²⁺, [(TPyA)CoIII(DBQ ^2-)Co ^II(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 [CoIII(DBQ ^2-)Co ^II] ²⁺ core forms [Co ^III(DBQ ^•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-/•3-. Due to the interdimer offset face-to-face π-π 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 ³⁺ show weak antiferromagnetic interactions [1 ²⁺: g = 2.44, J/kB = -3.20 K (-2.22 cm ^-1); 3 ²⁺: g = 2.13, J/kB = -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 ^IIDBQ ^2-Co ^II] ²⁺ reduction to [Co ^IIDBQ ^*3-Co ^II] ⁺, and oxidation to [Co ^IIIDBCr ^•3-Co ^III3+ and [Co ^IIIDBQ ^2-Co ^III] ⁴⁺, respectively. The mechanism of the multielectron transfer oxidation from [Co ^IIDBQ ^2-Co ^II] ²⁺ to [Co ^IIIDBQ ^•3-Co ^III] ³⁺ 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] ²⁺ electronic structure that becomes [Fe _hs ^III(DBQ ^•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.