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

ABSTRACT We report a new class of heterovalent, homometallic ring complexes, with {Ti IV 7 Ti III } cores: [py‐CH 2 NH 2 Et][Ti 7 TiO 8 (O 2 CR) 16 ] (R = t Bu and cyclopropyl). The compounds are made from [Ti(O i Pr) 4 ] reacted with carboxylic acid and py‐CH 2 NHEt. We also show that {Ti IV 7 Ti 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 7 Ti III } rings reveal Robin and Day Class II mixed‐valent behavior at room temperature, while EPR spectroscopy is consistent with a localized 3d 1 ( s = 1/2) configuration, with C 2 local symmetry, at low temperatures. We compare the EPR behavior of these new rings with the isostructural heterometallic {Cr III 7 Ni II } rings [py‐CH 2 NH 2 Et][Cr 7 NiF 8 (O 2 CR) 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 ( T 1 ) for {Ti IV 7 Ti III } are over two orders of magnitude longer at low temperatures than for the {Cr 7 Ni} equivalents, and that phase memory time constants ( T m ) are maintained to much higher temperatures.