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

Abstract

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 (1)H 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).

Silylamide Complexes of Chromium(II), Manganese(II), and Cobalt(II) Bearing the Ligands N(SiHMe2)2 and N(SiPhMe2)2

“…61 In contrast, magnetic moments of 5.883(3) and 5.269(5) μ B for Co[N-(SiMe 3 ) 2 ] 2 (thf) and Co[N(SiMe 3 ) 2 ] 2 (py), respectively, were determined by SQUID (superconduction quantum interference device) measurements. 42 The bis(diphenylmethylsilyl)amide complex Co[N(SiMePh 2 ) 2 ] 2 exhibits a relatively small μ B -value of 4.42 μ B . 36 Conclusion.…”

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…For high-spin cobalt(II) complexes, [Co{N(SiMe 3 ) 2 } 2 L] with L = PR 3 , THF, pyridine, and N(SiMe 3 ) 2 , large and negative D values in the order of -50 to -80 cm -1 have been deduced from their magnetic behavior (19-21, Table 3) [42,45]. DFT calculations performed on these complexes confirmed that the ground states are non-degenerate and that the magnetic anisotropy is due to the second-order SOC contribution.…”

Synthesis, Structural, Spectroscopic, and Magnetic Characterization of Two-Coordinate Cobalt(II) Aryloxides with Bent or Linear Coordination

“…These were prepared by melting [Co{N(SiMe 3 ) 2 } 2 ] 2 with four equivalents of finely ground phenol under positive N 2 atmosphere until a color change was observed and eliminated HN(SiMe 3 ) 2 formed as a white vapor. The reaction was facilitated by the relatively low melting point of [Co{N-(SiMe 3 ) 2 } 2 ] 2 (89−90°C) 45 and the volatility of HN(SiMe 3 ) 2 . As soon as the brown silylamide melted, the slurry immediately became a dark red color.…”

“…The removal of HN(SiMe 3 ) 2 promoted completeness of the reaction, however, it was found that the use of excessive heat under reduced pressure could also result in the loss of [Co{N(SiMe 3 ) 2 } 2 ] 2 as a dark green vapor. 45 The crude red product was then extracted with hot toluene, concentrated, and left to stand undisturbed overnight at 25°C to yield dark red crystals that were suitable for X-ray analysis. X-ray crystallography showed that the product 2b had a cobalt(II) occupancy of 84.6(2)%; however, a second synthesis of the OAr iPr 4 derivative under similar conditions but where the resulting crude product was heated to ca.165°C at 5 × 10 −2 Torr for 2 h resulted in a dark red residue and colorless crystals which had sublimed on the upper walls of the flask.…”