Observation of Redox-Induced Electron Transfer and Spin Crossover for Dinuclear Cobalt and Iron Complexes with the 2,5-Di-<i>tert</i>-butyl-3,6-dihydroxy-1,4-benzoquinonate Bridging Ligand

Dinuclear [(TPyA)MII(DBQ2−)MII(TPyA)](BF4)2 [TPyA = tris(2-pyridylmethyl)amine; DBQ2− = 2,5-di-tert-butyl-3,6-dihydroxy-1,4-benzoquinonate; M = Co (12+), Fe (22+), Ni (32+)] complexes have been prepared by the reaction of M2+, TPyA, H2DBQ, and triethylamine in MeOH solution. Their monooxidized form [(TPyA)MIII(DBQ•3−)MIII(TPyA)]3+ [Co = (13+), Fe (23+)] has been synthesized by using ferrocenium tetrafluoroborate, and the dioxidized form of 12+, [(TPyA)CoIII(DBQ2−)CoIII(TPyA)]4+ (14+), has been obtained by using thianthrinium tetrafluoroborate. These dinuclear compounds were characterized by X-ray crystallography, electrochemistry, magnetism, and EPR spectroscopy. Valence ambiguous 13+ forms via redox-induced electron transfer, whereby the one-electron oxidation of the [CoII(DBQ2−)CoII]2+ core forms [CoIII(DBQ•3−)CoIII]3+, and it also exhibits spin crossover behavior to the core [CoIII(DBQ2−)CoII]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 DBQ2−/•3−. Due to the interdimer offset face-to-face π−π and/or herringbone interactions, 12+, 13+, and 22+ show extended 1-D and/or 2-D supramolecular structures. The existence of DBQ•3− in 13+ is confirmed from both solid-state magnetic and solution EPR data. Co- and Ni-based 12+ and 32+ show weak antiferromagnetic interactions [12+: g = 2.44, J/kB = −3.20 K (−2.22 cm−1); 32+: g = 2.13, J/kB = −3.22 K (−2.24 cm−1), H = −2JS1•S2 for 12+ and 32+], while Fe-based 22+ exhibits strong spin crossover behavior above room temperature. 12+ has three reversible one-electron transfer waves at E1/2 (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 [CoIIDBQ2−CoII]2+ reduction to [CoIIDBQ•3−CoII]+, and oxidation to [CoIIIDBQ•3−CoIII]3+ and [CoIIIDBQ2−CoIII]4+, respectively. The mechanism of the multielectron transfer oxidation from [CoIIDBQ2−CoII]2+ to [CoIIIDBQ•3−CoIII]3+ is unknown; the energy of stabilization for oxidizing the CoII centers in the presence of DBQ•3−, relative to oxidizing the CoII centers in the presence of DBQ2− is computed to be 1.45 eV. 22+ 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 (22+/23+ and 23+/24+), and a one-electron reduction (22+/2+). 22+ has the [FehsII(DBQ2−)FehsII]2+ electronic structure that becomes [FehsIII(DBQ•3−)FehsIII]3+ upon oxidation. The latter undergoes spin crossover above room temperature to populate the [FehsIII(DBQ2−)FehsII]3+ excited state.