Influence of a Counteranion on the Zero-Field Splitting of Tetrahedral Cobalt(II) Thiourea Complexes.

Four mononuclear cobalt(II) complexes with pseudo tetrahedral geometry were isolated with different counteranions; their structure solution reveals the molecular formula as [Co(L)]X [where L = thiourea (NHCSNH) and X = NO (), Br (), and I ()] and Co(L) (). The detailed analysis of direct-current (dc) magnetic data reveals a zero-field splitting (ZFS; ) with = ±/ as the ground levels ( < 0) for the four complexes. The magnitude of the ZFS parameter is larger, in absolute value, for ( = -61.7 cm) than the other three complexes (-5.4, -5.1, and -12.2 cm for -, respectively). The sign of for , , and was unambiguously determined by X-band electron paramagnetic resonance (EPR) spectroscopy of the diluted samples (10%) at 5 K. For , the sign of was naturally endorsed from the frequency-dependent out-of-phase signal (χ″) 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 , which gave a quantitative value of -5.10(7) cm. Further, the drastic changes in the spin Hamiltonian parameters and their related relaxation dynamics phenomena (of - compared to ) 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 - leads to a nonfavorable overlap between the π orbital of cobalt(II) and the π* orbital of the sulfur atom that reduces the overall || value in these complexes compared to . 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 of up to ∼90% occurs (in - compared to ) upon a simple variation of the counteranions and offers a viable approach to modulate ZFS in transition-metal-containing single-molecule magnets.