Experimental and Theoretical Studies on the Magnetic Properties of Manganese(II) Compounds with Mixed Isocyanate and Carboxylate Bridges

Two manganese(II) isocyanate complexes with different flexible zwitterionic dicarboxylate ligands, [Mn-2(bcpp)(NCO)(4)](n) (1; bcpp=1,3-bis(N-carboxylatomethyl-4-pyridinio)-propane) and [Mn-2(bcp)(NCO)(4)](n) (2; bcp=bis(N-carboxylatomethyl)-4,4'-bipyridinium, have been synthesized and characterized by X-ray crystallography and magnetic measurements. Both compounds consist of two-dimensional coordination layers in which uniform anionic chains with mixed (NCO)(2)(COO) triple bridges are cross-linked by flexible cationic 4,4'-trimethylenedipyridinium spacers. Magnetic studies revealed antiferromagnetic interactions through the triple bridges (J=-8.0 cm(-1) (1) and J=-8.6 cm(-1) (2)), which are stronger than those in the isoelectronic analogue (N-3)(2)(COO). To complement the experimental data, periodic and finite-cluster DFT and CASPT2 calculations were performed on the dimeric units of the (NCO)(2)(COO) and (N-3)(2)(COO) mixed-bridged systems to support the Heisenberg picture and stress the relative efficiency of the magnetic couplers. It was found that the isocyanate ligand plays a greater role in the conveyance of antiferromagnetic behavior than the azide counterpart, and that both pseudohalide bridges function cooperatively with the carboxylate group.