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 = −ΣnnJSiSj. 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, Jinter = −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.