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 Ueff (16.1(2), 17.1(3), and 19.1(7) cm–1) and relaxation times τ0 (3.5(3) × 10–7, 9.3(8) × 10–8, and 3.0(8)
× 10–7 s) are almost the same, despite distinct
differences in the ligand properties. In contrast, the isostructural
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 57Fe Mößbauer spectra of the
three high-spin iron complexes exhibit a doublet at 3 K with small
and similar values of the isomer shifts (δ), ranging between
0.57 and 0.59 mm/s, as well as an unusual small quadrupole splitting
(ΔEQ = 0.60 mm/s) in [Li­(15-crown-5)]­[Fe­{N­(SiMe3)2}3].