Magneto−Structural Correlations in 2D and 3D Extended Structures of Manganese(II)−Malonate Systems

Two polymeric malonato-bridged manganese(II) complexes of formula [Mn(mal)(H₂O)₂]_n (1) and [Mn₂(mal)₂(4,4‘-bipy)(H₂O)₂]_n (2) have been synthesized and characterized (mal = malonate dianion; 4,4‘-bipy = 4,4‘-bipyridine). The crystal structure of complex 1 was already known. Complex 2 crystallizes in monoclinic space group P2₁/n, Z = 2, with unit cell parameters of a = 7.2974(10) Å, b = 18.7715(10) Å, c = 7.514(3) Å, and β = 91.743(12)°. The structure determination reveals that the complex [Mn₂(mal)₂(4,4‘-bipy)(H₂O)₂]_n (2) is a 3D network being composed of Mn−malonate sheets which are pillared by bidentate 4,4‘-bipy spacer forming small voids. The Mn−Mn distances through Mn−μ-(O3−C8−O4)−Mn, Mn−μ-(O1−C6−O2)−Mn, and Mn−μ-4,4‘-bipy−Mn bridges are 5.561, 5.410, and 11.723 Å, respectively. The magnetic behaviors of complexes 1 and 2 in the temperature range 300−2 K are very close, corresponding to a weak antiferromagnetic coupling. The magnetic pathways of complex 1 are through two Mn−O−C−O−Mn with anti-anti conformation and two Mn−O−C−O−Mn with syn-anti conformations and in complex 2 through all Mn−O−C−O−Mn with syn-anti conformations. Both syn-anti and anti-anti conformations create weak antiferromagnetic coupling, and the susceptibility data are fitted by the expansion series of Lines and the Curély formula for an S = 5/2 antiferromagnetic quadratic layer, based on the exchange Hamiltonian H = −Σ_nnJS_iS_j. The best fit is given by the superexchange parameters J = −0.32 cm^-1 and g = 2.00 for complex 1 and J = −0.14 cm^-1, J_inter = −0.031 cm^-1, and g = 2.00 for complex 2. Finally, in both the complexes there is a magnetic pathway Mn−O−C−C−C−O−Mn, and this pathway through the three carbon atoms of the malonato-bridging ligand could be considered negligible.