Macrocycle- and metal-centered reduction of cobalt trithiadodecaazahexaphyrin (Hhp). Metal-to-ligand charge transfer in {(Co I2CoIIO)(Hhp˙4−)}2−

Abstract

Reduction of free-base trithiadodecaazahexaphyrin, H₃Hhp, by potassium graphite in the presence of cobalt(II) acetylacetonate and cryptand[2.2.2] produces {Cryptand[2.2.2](K⁺)}₂{(Co^I₂Co^IIO)²⁺(Hhp˙⁴⁻)}²⁻·2C₆H₄Cl₂ (1). Co atoms in Co₃O have square-planar geometry with short Co–N and Co–O bonds of 1.852–1.898(3) Å length. A nearly planar shape of Hhp macrocycles in 1 with equal N_meso–C bonds indicates the aromatic character of Hhp˙⁴⁻. Magnetic measurements show the presence of two weakly interacting paramagnetic Hhp˙⁴⁻ and Co^II centers with the S = 1/2 spin states below 100 K providing an effective magnetic moment of 2.50μ_B and separate EPR signals characteristic of these species. Magnetic moment reversibly increases above 100 K reaching a value of 3.68μ_B at 300 K. We attributed this increase to a population of the excited state which is separated from the ground state by 490 K energy gap. This is also accompanied by growth of a new broad EPR signal at g = 2.11–2.03 above 100 K (the estimated gap is 497 K). According to the calculations, the excited state can be populated due to electron transfer from Co^I to Hhp˙⁴⁻ accompanied by the formation of {(Co^ICo^II₂O)³⁺(Hhp⁵⁻)}²⁻ with a triplet Hhp⁵⁻ macrocycle. As a result, the S = 1/2 + 1/2 + 1 spin system is formed in the excited state. Coupling between Co^II ions and triplet Hhp⁵⁻ can provide one broad EPR signal with a g-factor value that is intermediate between those of individual species. Salt 1 shows a broad absorption band at 1770 nm (5500 cm⁻¹), which is attributed to a symmetry-allowed low-energy d–π transition at the absorption of the light quantum.