Spin-State Crossover with Structural Changes in a Cobalt(II) Organometallic Species: Low-Coordinate, First Row, Heteroleptic Amido Transition Metal Aryls. Synthesis and Characterization of Ar′MN(H)Ar<sup>#</sup> (M = Mn, Fe, Co) (Ar′ = C<sub>6</sub>H<sub>3</sub>-2,6-(C<sub>6</sub>H<sub>3</sub>-2,6-<sup><i>i</i></sup>Pr<sub>2</sub>)<sub>2</sub>, Ar<sup>#</sup> = C<sub>6</sub>H<sub>3</sub>-2,6-(C<sub>6</sub>H<sub>2</sub>-2,4,6-Me<sub>3</sub>)<sub>2</sub>)
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The synthesis and characterization of the monomeric aryl transition metal amido complexes Ar′MN(H)Ar# (Ar′ = C6H3-2,6-(C6H3-2,6-iPr2)2, Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2, M = Mn (1), Fe (2), Co(3a, b)) are reported. The compounds were characterized by X-ray crystallography, electronic and infrared spectroscopy, and magnetic measurements. At about 90 K the complexes 1 and 2 possess quasi-two coordinate geometry with a weak, secondary, M---C interaction involving a flanking aryl ring from an amido group. In contrast, at the same temperature, their cobalt analogue 3a features a strong Co-η6-flanking ring interaction to give an effectively higher coordination geometry. Magnetic studies of 1−3a showed that 1 and 2 have high spin configurations, whereas the cobalt species 3a has a low-spin configuration (S = 1/2). However, 3a undergoes a spin crossover to a high spin (S = 3/2) state 3b near 229 K. An X-ray structural determination above the crossover temperature at 240 K showed that the low temperature structure of 3a had changed to 3b which involves a weak secondary M---C interaction analogous to those in 1 and 2. The complexes 1−3 are very rare examples of heteroleptic quasi-two coordinate open shell transition metal complexes.
