First stable reduced form of [Co5]+10: fine tuning of linear pentacobalt(ii) complexes containing delocalized metal–metal bonds through ligand modification

Abstract

Pentacobalt EMACs [Co₅(μ₅-dpzpda)₄X₂] (X = Cl⁻ (1), NCS⁻ (2)) with fine-tuning of the supporting ligand based on the tripyridyldiamine ligand, N,N′-di(pyrazin-2-yl)pyridine-2,6-diamine (H₂dpzpda), and their reduced form (Ph₄P)[Co₅(μ₅-dpzpda)₄X₂] (X = Cl⁻ (3), NCS⁻ (4)) were first synthesized and structurally characterized. The structures of 1–4 showed direct Co–Co bonds with Co–Co distances in the range 2.2385(7)–2.2888(15) Å, and valence electrons delocalized through the whole metal chain with distances of longer than 9.06 Å. The distances of the inner Co–Co bonds and the Co–axial ligands became longer after reduction, whereas no significant change was observed in the distances of the outer Co–Co bonds and Co–N (supporting ligand) bonds, which was consistent with the MO analysis. Electrochemical studies on both 1 and 2 showed one reversible oxidation and one reversible reduction at E_1/2 = +0.82 and −0.05 V for 1, and at E_1/2 = +0.89 and +0.02 V for 2, respectively. The redox reactions of the thiocyanate complex 4 happened at higher potentials than the chloride complex 3. A magnetism study of 1–4 revealed anomalous magnetic behaviour similar to that of heptacobalt EMACs, and a deviation from the Curie–Weiss law was observed. The χ_MT value at 300 K is 0.84 and 1.16 emu K mol⁻¹ for 1 and 2, respectively, suggesting spin-equilibrium or a spin-admixture between doublet and quartet states arising from the Boltzmann distribution over different energy levels. Similar results were obtained for 3 and 4, showing intermediate χ_MT values between a diamagnetic and a triplet state of 0.15–0.96 emu K mol⁻¹ in the temperature range 5–300 K. The structural and magnetic results were interpreted through an EHMO study.