Magnetostructural Studies on Tetranuclear Manganese [Mn2IIIMn2II] Complexes of 9-Hydroxyphenalenone with Weak π•••π Interactions

Two mixed-valent tetranuclear MnII/III complexes of formula [Mn4(L)4(OAc)2(OMe)4]center dot 2H2O (1) and [Mn4(L)4(OAc)2(OMe)4] (2), in which HL = 9-hydroxyphenalenone, were synthesized by a solvothermal method and magnetostructurally characterized. Complex 1 crystallizes in the monoclinic space group P21/c with a = 12.69 angstrom, b = 15.51 angstrom, c = 15.40 angstrom, beta = 114.35 degrees and Z = 2. Complex 2 crystallizes in the triclinic space group P$\bar {1}$ with a = 8.74 angstrom, b = 12.06 angstrom, c = 13.89 angstrom, a = 114.23 degrees beta = 102.16 degrees, gamma = 98.54 degrees and Z = 1. Both 1 and 2 are products of the same reaction and have the same molecular formula except for the solvent of crystallization. X-ray crystallographic studies show that 1 and 2 have six coordinated MnII and MnIII ions and almost the same tetranuclear core, which resembles a rhomboidal butterfly motif except that the MnIII ions are at the wing-tip and MnII ions in the body positions. The MnII/III ions that form the metal core in 1 and 2 have two mu 3-Oalkoxo and four mu 2-Oalkoxo bridges along with two carboxylate anions that bridge the MnII and MnIII centres situated at the body and the wing-tip positions. Complex 1 shows intermolecular pi center dot center dot center dot pi and C-H center dot center dot center dot pi interactions. Detailed direct current (DC) magnetic susceptibility studies in the temperature range 2300 K suggest that 1 and 2 are antiferromagnetically coupled. The exchange coupling constants and g values were estimated by theoretical fitting of the magnetic susceptibility data. We find that it is difficult to determine a model with three different coupling constants (J1, J2, J3) unambiguously from the magnetic susceptibility data. Nevertheless, the bonding parameters of the metal core for 1 and 2 suggest a simplification to two exchange coupling constants (Jbb and Jwb) with a reasonably good fit. In addition, the theoretical fitting of the magnetic data when correlated to the structure shows good agreement for complexes with similar cores in the literature and it appears that the exchange coupling constants correlate with the solid angle formed by the mu 3-O(alk)oxo bonded to the MnII/III ions.