Preparation, structure, and magnetochemistry of hexanuclear manganese oxide complexes: chemically and thermally induced aggregation of aquahexakis(benzoato)oxobis(pyridine)trimanganese forming products containing the [Mn6O2]10+ core

Abstract

A variety of synthetic procedures are described that convert the trinuclear complex Mn30(02CPh)6(py)2(H20) (1) into the hexanuclear complexes [Mn602(02CPh)lo(py)2(MecN)2].2MeCN (2) and [Mn602(02CPh)io(py)4].Et20 (3). With two exceptions, the procedures involve treatment of 1 with phenolic molecules (phenol, p-cresol, tyrosine, 2,2'-biphenol, 8-hydroxyquinoline) or the mononuclear Mn' complexes [Mn(bi~hen)~(biphenH)]~and [Mn(Br4biphen)2(02CPh)]2in MeCN and lead to high (50-75%) yields of complex 2. It is proposed that the mechanism involves reduction of the [Mn30l6+ unit of 1 to a [Mn30I5+ species, which spontaneously aggregates to 2 containing the [Mn602]10t core. This proposal is supported by the formation of 2 when 1 is reduced with the outer-sphere reductant sodium acenaphthylenide. Complex 3 is obtained in 34% yield when a PhCN solution of 1 is refluxed for 10 min. Complex 2 crystallizes in triclinic space group PT with, at -140 OC, a = 15.389 (4) A, b = 19.513 (6) A, c = 14.433 (4) A, a = 91.84 (1)O, (3 = 94.08 ( 1 ) O , y = 87.85 (l)', and Z = 2. The structure was solved and refined by using 5999 unique reflections with F > 3.0u(F). Complex 3 crystallizes in triclinic space group P i with, at -160 C, a = 24.394 (16) A, (3 = 19.876 (11) A, c = 19.245 (12) A, CY = 89.71 (3)O, (3 = 105.43 (3)O, y = 79.89 (3)O, and Z = 2. The structure was solved and refined by using 6000 unique reflections with F > 3.0u(F). Final values of discrepancy indices R (R , ) for 2 and 3 are 7.82 (7.17) and 7.51% (7.63%), respectively. Complex 2 contains a [Mn602],io+ core that can be conveniently described as two edge-sharing Mn4 tetrahedra at the center of each of which is a p4-02ion. Peripheral ligation to the octahedrally coordinated Mn centers is provided by ten bridging benzoate, two terminal py, and two terminal MeCN groups. The complex is mixed valence (Mn4Mn11'2), and the Mn' centers are assigned as the two central metal ions bridged by two 02ions. Complex 3 contains two independent [M~I ,O , ] '~ complexes in the asymmetric unit, each of which is essentially identical with that in complex 2, except that the terminal ligands are all py. The results of solid-state magnetic susceptibility studies on complex 2 in the temperature range 2.95-300 K are described. With use of the idealized symmetry of two edge-sharing tetrahedra (D2h), the derivation by the Kambe vector-coupling method of a theoretical model to account for the intramolecular exchange interactions is described. Least-squares fitting of the susceptibility versus temperature data to the model yields the parameters J, = -42.0 cm-I, J2 = -0.8 cm-I, J3 = -2.4 cm-l, and g = 1.90, where the J values refer to the Mn'-Mn', Mn-Mn', and Mn-Mn interactions, respectively. These exchange parameters give an ST = 0 ground state with two degenerate ST = 1 states at 4 cm-I and degenerate ST = 0, 1, 2 states at 9 cm-' higher in energy. The magnitudes and signs of the exchange parameters are compared with those reported for other oxo-bridged Mn complexes. Complexes 2 and 3 join a small but growing number of high-nuclearity Mn aggregates, and the prospective and potential procedures that could be employed for the synthesis of higher nuclearity Mn aggregates that might display molecular ferromagnetism are discussed.