Distorted Cubane [Mn₄O₃Cl]⁶⁺ Complexes with Arenecarboxylate Ligation: Crystallographic, Magnetochemical, and Spectroscopic Characterization

Synthetic procedures are described that allow access to Mn₄O₃Cl₄(O₂CR)₃(py)₃ (R = aryl) complexes, complementing previous work with R = alkyl. Carboxylate exchange of the R = Me complex (2) with the appropriate arenecarboxylic acid leads to preparation of the R = 3,5-Cl₂-Ph (3), Ph (4), 4-F-Ph (5), and 3,5-F₂-Ph (6) complexes. The crystal structure of 3 shows the [Mn₄O₃Cl]⁶⁺ core to be essentially superimposable on that of 2. Crystal data for 3 at −171 °C are as follows: hexagonal, R3̄, a = 19.056(4) Å, c = 28.271(6) Å, V = 8888.25 ų, Z = 6, R (R_w) = 0.0417 (0.0384) employing 1982 unique data with F > 3.0σ(F). Variable-temperature magnetic susceptibility data are presented for 3 at 10.0 kG in the 5–320 K range. The μ_eff/molecule value rises from 9.1 μ_B at room temperature to a maximum of 9.72 μ_B at 60.0 K and then decreases to 8.95 μ_B at 5.01 K. Fitting of the data to the appropriate theoretical expression gave the following fitting parameters: J₃₄ = −27.1 cm⁻¹, J₃₃ = + 11.1 cm⁻¹, and g = 1.95. These are similar to those for the R = Me complex (2) reported previously and similarly yield a well-isolated S = 9/2 ground state. This was confirmed by variable-field magnetization studies which verified an S = 9/2 ground state experiencing zero-field splitting (D = 0.50 cm⁻¹). The results of ¹H and ²H NMR studies on 4–6 are presented, together with those for the 4-picoline (7) and 3,5-lutidine (8) derivatives. The observed spectra are qualitatively interpreted vis-à-vis the spin delocalization mechanisms that are operative. It is concluded that contact shifts via π-spin delocalization and dipolar shifts are both contributors to the pyridine ¹H and ²H chemical shifts. The greater solubility of the R = aryl derivatives (except 3 and 6) compared to R = alkyl derivatives has allowed better-resolved toluene glass EPR spectra to be obtained for complex 5 than was previously possible for the R = alkyl complexes.