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# (M = Mn, Fe, Co) (Ar' = C6H3-2,6-(C6H3-2,6-iPr2)2, Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2).

The synthesis and characterization of the monomeric aryl transition metal amido complexes Ar'MN(H)Ar(#) (Ar' = C(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr(2))(2), Ar(#) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Me(3))(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-eta(6)-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.