Magneto-Structural Studies of Manganese(II) 2,4′-Benzophenone Dicarboxylate Based Coordination Polymers

The role of the N-donor auxiliary ligand in the formation of coordination polymers (CPs) has been examined for a series of Mn(II) aromatic dicarboxylate-based framework systems. The solvothermal reaction of L-shaped dicarboxylate with Mn(II) through a varying N-donor auxiliary ligand resulted in the crystallization of six new multinuclear Mn-based CPs: [Mn2(DMA)2(2,4 '-bpdc)2]center dot H2O MnCP1, [Mn(phen)(2,4 '- bpdc)] MnCP2, [Mn3(2,2 '-bipy)(DMA)2(2,4 '-bpdc)3] MnCP3, [Mn3(4,4 '-dimethyl-2,2 '- bipy)2(2,4 '-bpdc)2(HCOO)2] MnCP4, [Mn3(4,4 '-dimethyl-2,2 '-bipy)2(2,4 '- bpdc)2(CH3COO)2] MnCP5, and [Mn3(5,5 '-dimethyl-2,2 '-bipy)2(2,4 '-bpdc)2Cl2] MnCP6 (where 2,4 '-bpdc = benzophenone-2,4 '-dicarboxylate, phen = 1,10 phenanthroline, 2,2 '-bipy = 2,2 '-bipyridine, 4,4 '-dimethyl-2,2 '-bipy = 4,4 '-dimethyl-2,2 '-bipyridine, and 5,5 '-dimethyl-2,2 '-bipy = 5,5 '-dimethyl-2,2 '-bipyridine). The two 1D CPs, MnCP1 (square pyramidal) and MnCP2 (trigonal prism), respectively, were isolated from reactions in the absence and presence of an auxiliary ligand. The presence of phen led to anhydrous MnCP2 wherein C-H center dot center dot center dot O interactions stabilized the 1D chains. The crystal packing in the hydrated MnCP1 is facilitated due to carboxylate oxygens interacting strongly with disordered lattice water. The auxiliary ligand, 2,2 '-bipy and its derivatives in a suitable solvent led to the growth of four new solids built of a trimeric manganese cluster. MnCP3 is a layered solid while the methyl substituents on 2,2 '-bipy altered the manganese carboxylate extended coordination in a different manner: (i) 4,4 '-methyl substitution led to 1D (MnCP4) and 2D network (MnCP5); the bridging within the trimeric unit is also facilitated through in situ formed counter anions formate and acetate groups. (ii) 5,5 '-Methyl substitution led to a 1D chain (MnCP6) where the trimeric unit occurred through the interference by another counter anion (chloride). The paper provides a mechanistic pathway for the observed crystal packing in terms of supramolecular condensation of soluble 1:1 manganese dicarboxylate complex species through a combination of coordination and H-bonding interactions. It also highlights how the supramolecular assembly is influenced when another competing soluble manganese complex coprecipitates, thus providing a rationale to the observed composition and coordination framework. All the manganese carboxylates showed antiferromagnetic behavior. The magnetic behavior of the two structurally comparable solids, MnCP1 and MnCP2, were further supported by highlevel first principle-based computations.