Synthesis, Spectroscopic Characterization, and Determination of the Solution Association Energy of the Dimer [Co{N(SiMe₃)₂}₂]₂: Magnetic Studies of Low-coordinate Co(II) Silylamides [Co{N(SiMe₃)₂}₂L] (L = PMe₃, Pyridine, and THF) and Related Species That Reveal Evidence of Very Large Zero-field Splittings


The synthesis, magnetic, and spectroscopic characteristics of the synthetically useful dimeric cobalt(II) silylamide complex [Co{N(SiMe 3)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 (ΔG reacn) 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 J ex 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 B, 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.




National Science Foundation (U.S.)


Financial support by National Science Foundation (U.S.), Grant Nos. CHE-1263760 and DBIO-722538; National Science Foundation (U.S.) Graduate Research Fellowship Program (Grant No. DGE-1148897)

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Publication Date

01 Oct 2013