Four
mononuclear cobalt(II) complexes with pseudo tetrahedral geometry
were isolated with different counteranions; their structure solution
reveals the molecular formula as [Co(L1)4]X2 [where L1 = thiourea (NH2CSNH2) and X = NO3 (1), Br (2), and
I (3)] and [Co(L1)4](SiF6) (4). The detailed analysis of direct-current (dc)
magnetic data reveals a zero-field splitting (ZFS; D) with m
S = ±3/2 as the ground levels (D < 0) for the four complexes.
The magnitude of the ZFS parameter is larger, in absolute value, for 1 (D = −61.7 cm–1) than the other three complexes (−5.4, −5.1, and −12.2
cm–1 for 2–4, respectively).
The sign of D for 1, 2,
and 4 was unambiguously determined by X-band electron
paramagnetic resonance (EPR) spectroscopy of the diluted samples (10%)
at 5 K. For 3, the sign of D was naturally
endorsed from the frequency-dependent out-of-phase signal (χM″) observed in the absence of an external dc magnetic
field and confirmed by high-frequency EPR (70–600 GHz) experiments
performed on a representative pure polycrystalline 3,
which gave a quantitative D value of −5.10(7)
cm–1. Further, the drastic changes in the spin Hamiltonian
parameters and their related relaxation dynamics phenomena (of 2–4 compared to 1) were rationalized
using ab initio complete-active-space self-consistent field/n-electron
valence perturbation theory calculations. Calculations disclose that
the anion-induced structural distortion observed in 2–4 leads to a nonfavorable overlap between the
π orbital of cobalt(II) and the π* orbital of the sulfur
atom that reduces the overall |D| value in these
complexes compared to 1. The present study demonstrates
that not only the first but also the second coordination sphere significantly
influences the magnitude of the ZFS parameters. Particularly, a reduction
of D of up to ∼90% occurs (in 2–4 compared to 1) upon a simple
variation of the counteranions and offers a viable approach to modulate
ZFS in transition-metal-containing single-molecule magnets.