Synthesis, crystal structure and magnetic properties of two oxalato-bridged dimetallic trinuclear complexes combined with a polar cation

Two isostructural heterometallic trinuclear oxalato-bridged complexes of formula C-4[MCr2(ox)(6)(H2O)(2)]center dot nH(2)O (C+ = 4-aminopyridinium; ox(2-) = oxalate dianion; M2+ = Mn2+, n = 3, 1; M2+ = Co2+, n = 3.25, 2) have been synthesized by using direct self-assembly methods combining C-3[Cr(ox)(3)] and the chloride salts of the corresponding metal ion. The crystal structures of both compounds have been resolved by single-crystal X-ray diffraction. They crystallize in the C2/c space group [a = 11.5113(15) angstrom, b = 20.250(3) angstrom, c = 21.810(4) angstrom, beta = 100.447(10)degrees, V = 5161.6(3) angstrom(3), and Z = 4 for 1, and a = 11.4334(16) angstrom, b = 20.243(2) angstrom, c = 21.805(3) angstrom, beta = 101.113(9)degrees, V = 4951.9(11) angstrom(3), and Z = 4 for 2]. The structures of 1 and 2 consist of discrete linear [MCr2(ox)(6)](4-) bimetallic trinuclear units, pyridinium cations and crystallization water molecules. The linear trinuclear unit is built from a central trans-diaquametal(II), linked to two Cr(ox)(3)](3-) entities by oxalate bridges. One of the oxalate ions is coordinated to the central metal ion whereas the other two oxalate ligands are non-bridging. In the crystal, intermolecular hydrogen bonds involving oxalate ligands, water molecules and pyridinium cations, build a complex three-dimensional network. Variable-temperature magnetic susceptibility measurements for 1 and 2 indicate a weak ferromagnetic interaction (J = +1.16 and +2.62/+2.70 cm(-1) for 1 and 2, respectively) between the two terminal Cr-III (S-Cr = 3/2) and the central high-spin Mn-II (S-Mn = 5/2) and Co-II (S-Co = 3/2) ions. The nature and the amplitude of the exchange interaction are rationalized using DFT calculations and orbital interpretations.