Proof of large positive zero-field splitting in a Ru₂ ⁵⁺ paddlewheel

We present the synthesis, as well as the structural and magnetic characterization, of [Ru₂(D(3,5-Cl₂Ph)F) ₄Cl(0.5H₂O)]·C₆H₁₄ (D(3,5-Cl₂Ph)F = N,N′-di(3,5-dichlorophenyl)formamidinate), a Ru₂⁵⁺ compound having a ⁴B_2u ground state derived from a σ²π⁴δ* ²δ* electron configuration. The persistence of this configuration from 27 to 300 K is shown by the invariance of the Ru-Ru distance. Orientation-dependent magnetic susceptibility (χT) and magnetization (M(H)) data are in accord with a spin quartet ground state with large magnetocrystalline anisotropy associated with a large axial zero-field splitting (D) parameter. Theoretical fits to χT and M(H) plots yielded D/k _B = +114 K, implying an S = ±1/2 Kramers doublet ground state at low temperature. Single-crystal and powder EPR data are consistent with this result, as the only observed transition is between the M_s = ±1/2 Zeeman levels. The g values are g_⊥ = 2.182, g _∥ = 1.970, and D = 79.8 cm^-1. The totality of the results demands D ≫ 0.