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

Abstract

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 (<b>1</b>), Br (<b>2</b>), and I (<b>3</b>)] and [Co(L₁)₄](SiF₆) (<b>4</b>). The detailed analysis of direct-current (dc) magnetic data reveals a zero-field splitting (ZFS; <i>D</i>) with <i>m</i>_S = ±³/₂ as the ground levels (<i>D</i> < 0) for the four complexes. The magnitude of the ZFS parameter is larger, in absolute value, for <b>1</b> (<i>D</i> = -61.7 cm^-1) than the other three complexes (-5.4, -5.1, and -12.2 cm^-1 for <b>2</b>-<b>4</b>, respectively). The sign of <i>D</i> for <b>1</b>, <b>2</b>, and <b>4</b> was unambiguously determined by X-band electron paramagnetic resonance (EPR) spectroscopy of the diluted samples (10%) at 5 K. For <b>3</b>, the sign of <i>D</i> was naturally endorsed from the frequency-dependent out-of-phase signal (χ_M″) 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 <b>3</b>, which gave a quantitative <i>D</i> value of -5.10(7) cm^-1. Further, the drastic changes in the spin Hamiltonian parameters and their related relaxation dynamics phenomena (of <b>2</b>-<b>4</b> compared to <b>1</b>) 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 <b>2</b>-<b>4</b> leads to a nonfavorable overlap between the π orbital of cobalt(II) and the π* orbital of the sulfur atom that reduces the overall |<i>D</i>| value in these complexes compared to <b>1</b>. 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 <i>D</i> of up to ∼90% occurs (in <b>2</b>-<b>4</b> compared to <b>1</b>) upon a simple variation of the counteranions and offers a viable approach to modulate ZFS in transition-metal-containing single-molecule magnets.