Slow Magnetic Relaxation in Trigonal-Planar Mononuclear Fe(II) and Co(II) Bis(trimethylsilyl)amido Complexes-A Comparative Study

Alternating current magnetic investigations on the trigonal-planar high-spin Co2+ complexes [Li(15-crown-5)] [Co{N(SiMe3)(2)}(3)], [Co{N(SiMe3)(2)}(2)(THF)] (THF = tetrahydrofuran), and [Co{N(SiMe3)(2)}(2)(PCy3)] (Cy = -C6H13 = cyclohexyl) reveal that all three complexes display slow magnetic relaxation at temperatures below 8 K under applied dc (direct current) fields. The parameters characteristic for their respective relaxation processes such as effective energy barriers U-eff (16.1 (2), 17.1(3), and 19.1(7) cm(-1)) and relaxation times tau(0) (3.5(3) x 10(-7), 9.3(8) x 10(-8), and 3.0(8) x 10(-7) s) are almost the same, despite distinct differences in the ligand properties. In contrast, the isostnictural high-spin Fe2+ complexes [Li(15-crown-5)] [Fe{N(SiMe3)(2)}(3)] and [Fe{N-(SiMe3)(2)}(2)(THF)] do not show slow relaxation of the magnetization under similar conditions, whereas the phosphine complex [Fe{N(SiMe3)(2)}(2)(PCy3)] does, as recently reported by Lin et al. (Lin, P.-H.; Smythe, N. C.; Gorelsky, S. I.; Maguire, S.; Henson, N. J.; Korobkov, I.; Scott, B. L.; Gordon, J. C.; Baker, R. T.; Murugesu, M. J. Am. Chem. Soc. 2011, 135, 15806.) Distinctly differing axial anisotropy D parameters were obtained from fits of the dc magnetic data for both sets of complexes. According to density functional theory (DFT) calculations, all complexes possess spatially nondegenerate ground states. Thus distinct spin-orbit coupling effects, as a main source of magnetic anisotropy, can only be generated by mixing with excited states. This is in line with significant contributions of excited determinants for some of the compounds in complete active space self-consistent field (CASSCF) calculations done for model complexes. Furthermore, the calculated energetic sequence of d orbitals for the cobalt compounds as well as for [Fe{N(SiMe3)(2)}(2)(PCy3)] differs significantly from the prediction by crystal field theory. Experimental and calculated (time,dependent DFT) optical spectra display characteristic d-d transitions in the visible to near-infrared region. Energies for lowest transitions range from 0.19 to 0.35 eV; whereas, for [Li(15-crown-5)][Fe{N(SiMe3)(2)}(3)] a higher value is found (0.66 eV). Zero-field Fe-57 Mossbauer spectra of the three high-spin iron complexes exhibit a doublet at 3 K with small and similar values of the isomer shifts (delta), ranging between 0.57 and 0.59 mm/s, as well as an unusual small quadrupole splitting (Delta E-Q = 0.60 mm/s) in [Li(15-crown-5)][Fe{N(SiMe3)(2)}(3)].