Heterovalent, Homometallic Rings: Synthesis, Optical, and EPR Studies of {Ti^IV ₇Ti^III} Complexes

We report a new class of heterovalent, homometallic ring complexes, with {Ti^IV ₇Ti^III} cores: [py-CH₂NH₂Et][Ti₇TiO₈(O₂CR)₁₆] (R = ^tBu and cyclopropyl). The compounds are made from [Ti(OⁱPr)₄] reacted with carboxylic acid and py-CH₂NHEt. We also show that {Ti^IV ₇Ti^III} rings can be generated by photoreduction of {Ti^IV ₈} precursors in the presence of a suitable sacrificial electron donor, of relevance to photocatalysts. UV-vis/NIR spectroscopy of the {Ti^IV ₇Ti^III} rings reveal Robin and Day Class II mixed-valent behavior at room temperature, while EPR spectroscopy is consistent with a localized 3d¹ (s = 1/2) configuration, with C₂ local symmetry, at low temperatures. We compare the EPR behavior of these new rings with the isostructural heterometallic {Cr^III ₇Ni^II} rings [py-CH₂NH₂Et][Cr₇NiF₈(O₂CR)₁₆], which also have s = ½ ground states, due to strong intramolecular antiferromagnetic exchange interactions. We demonstrate that the electron spin lattice relaxation time constants (T₁) for {Ti^IV ₇Ti^III} are over two orders of magnitude longer at low temperatures than for the {Cr₇Ni} equivalents, and that phase memory time constants (T_m) are maintained to much higher temperatures.