Synthesis, Spectroscopic Characterization, and Determination of the Solution Association Energy of the Dimer [Co{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>]<sub>2</sub>: Magnetic Studies of Low-Coordinate Co(II) Silylamides [Co{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>L] (L = PMe<sub>3</sub>, Pyridine, and THF) and Related Species That Reveal Evidence of Very Large Zero-Field Splittings
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The synthesis, magnetic, and spectroscopic characteristics of the synthetically useful dimeric cobalt(II) silylamide complex [Co{N(SiMe3)2}2]2 (1) and several of its Lewis base complexes have been investigated. Variable-temperature nuclear magnetic resonance (NMR) spectroscopy of 1 showed that it exists in a monomer–dimer equilibrium in benzene solution and has an association energy (ΔGreacn) of −0.30(20) kcal mol–1 at 300 K. Magnetic data for the polycrystalline, red-brown [Co{N(SiMe3)2}2]2 (1) showed that it displays strong antiferromagnetic exchange coupling, expressed as −2JexS1S2, between the two S = 3/2 cobalt(II) centers with a Jex value of −215(5) cm–1, which is consistent with its bridged dimeric structure in the solid state. The electronic spectrum of 1 in solution is reported for the first time, and it is shown that earlier reports of the melting point, synthesis, electronic spectrum, and magnetic studies of the monomer “Co{N(SiMe3)2}2” are consistent with those of the bright green-colored tetrahydrofuran (THF) complex [Co{N(SiMe3)2}2(THF)] (4). Treatment of 1 with various Lewis bases yielded monomeric three-coordinated species[Co{N(SiMe3)2}2(PMe3)] (2), and [Co{N(SiMe3)2}2(THF)] (4), as well as the previously reported [Co{N(SiMe3)2}2(py)] (3)and the four-coordinated species [Co{N(SiMe3)2}2(py)2] (5) in good yields. The paramagnetic complexes 2–4 were characterized by electronic and 1H NMR spectroscopy, and by X-ray crystallography in the case of 2 and 4. Magnetic studies of 2–5 and of the known three-coordinated cobalt(II) species [Na(12-crown-4)2][Co{N(SiMe3)2}3] (6) showed that they have considerably larger χMT products and, hence, magnetic moments, than the spin-only values of 1.875 emu K mol–1 and 3.87 μB, which is indicative of a significant zero-field splitting and g-tensor anisotropy resulting from the pseudo-trigonal crystal field. A fit of χMT for 2–6 yields a large g-tensor anisotropy, large negative D-values (between −62 cm–1 and −82 cm–1), and E-values between ±10 cm–1 and ±21 cm–1.
