Proof of large positive zero-field splitting in a Ru25+ paddlewheel

We present the synthesis, as well as the structural and magnetic characterization, of [Ru-2(D(3,5Cl(2)Ph)F)(4)Cl(0.5H(2)O)]center dot C6H14 (D(3,5-Cl2Ph)F = N,/N-di(3,5-dichlorophenyl)formamidinate), a Ru-2(5+) compound having a B-4(2u) ground state derived from a delta(2)pi(4)delta(2)pi*(2)delta* 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 (chi T) 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 chi T and M(H) plots yielded D/lk(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(perpendicular to) = 2.182, g(perpendicular to) = 1.970, and D = 79.8 cm(-1). The totality of the results demands D >> 0.