Unusual MnIII/IV 4 Cubane and MnIII 16M4 (M = Ca, Sr) Looplike Clusters from the Use of Dimethylarsinic Acid

Three complexes are reported from the initial use of dimethylarsinic acid (Me2AsO2H) in MnIII/IV cluster chemistry, [Mn4O4­(O2AsMe2)6] (3; 2MnIII, 2MnIV), and [Mn16X4O8(O2CPh)16­(Me2AsO2)24] (X = Ca2+ (4) or Sr2+ (5); 16MnIII). They were obtained from reactions with [Mn12O12(O2CR)16(H2O)4] (R = Me, Ph) either without (3) or with (4 and 5) the addition of X2+ salts. Complex 3 contains a [Mn4O4]6+ cubane, whereas isostructural 4 and 5 contain a planar loop structure comprising four Mn4 asymmetric “butterfly” units linked by alternating anti,anti μ-O2AsMe2 and {X2(O2AsMe2)­(O2CPh)2} units. Variable-temperature magnetic susceptibility (χM) data were collected on dried microcrystalline samples of 3–5 in the 5.0–300 K range in a 0.1 T (1000 G) direct-current (dc) magnetic field. Data for 3 were fit to the appropriate Van Vleck equation (using the H = −2JŜ i·Ŝ j convention) for a cubane of virtual C 2v symmetry, giving J 33 = 0.0(1) cm–1, J 34 = −3.4(4) cm–1, J 44 = −9.8(2) cm–1, and g = 1.99(1), where the J ij subscripts refer to the oxidation states of the interacting Mn atoms. The ground state thus consists of two coupled MnIV and two essentially noninteracting MnIII. For 4 and 5, low-lying excited states from the high nuclearity and weak couplings prevent fits of dc magnetization data, but in-phase alternating current susceptibility χ′M T data down to 1.8 K indicate them to possess S = 4 ground states, if considered single Mn16 units. If instead they are treated as tetramers of weakly coupled Mn4 units, then each of the latter has an S = 2 ground state. Complexes 4 and 5 also exhibit very weak out-of-phase χ″ M signals characteristic of slow relaxation, and magnetization versus dc field scans on a single crystal of 4·15MeCN at T ≥ 0.04 K showed hysteresis loops but with unusual features suggesting the magnetization relaxation barrier consists of more than one contribution.