Control of molecular architecture by steric factors: mononuclear vs polynuclear manganese(III) compounds with tetradentate N2O2 donor Schiff bases

Three manganese(III) compounds, [Mn-III(vanoph)(DMF)(H2O)]ClO4 (1), [Mn-III(vanoph)(N-3)(H2O)]center dot 2H(2)O (2) and [Mn-III(saloph)(mu(1,3)-N-3)](n) (3), where H(2)vanoph = N,N'-(1,2-phenylene)-bis(3-methoxysalicylideneimine), H(2)saloph = N,N'-(1,2-phenylene)-bis(salicylideneamine) are tetradentate N2O2 ligands and DMF = N,N-dimethylformamide, have been prepared and characterised by elemental analysis, IR and UV-Vis spectroscopy and single-crystal X-ray diffraction studies. Compounds 1 and 2 are monomeric but compound 3 consists of a chain system with the repeating unit [Mn-III(saloph)(N-3)] bridged by mu-1,3 azide. Compound 1 crystallises in monoclinic space group P2(1)/n with cell dimensions of a = 11.1430(2), b = 16.3594(3), c = 15.4001(3) angstrom, beta = 108.417(1), Z = 4 whereas compounds 2 and 3 crystallise in orthorhombic space groups Pbca and Pna2(1), respectively, with cell dimensions of a = 16.069(3), b = 15.616(3), c = 18.099(4) angstrom, Z = 8 (for 2) and a = 18.760(9), b = 13.356(5), c = 6.616(3) angstrom, Z = 4 (for 3). In all the compounds, Mn(III) has a six-coordinated pseudo-octahedral geometry in which O(2), O(3), N(1) and N(2) atoms of the deprotonated di-Schiff base constitute the equatorial plane. In both compounds 1 and 2, water molecules are present in the fifth coordination sites in the apical positions. The sixth coordination sites are occupied by one O atom of a solvent DMF in compound 1 and an N atom of azide in compound 2. The coordinated water initiates hydrogen-bonded networks in both compounds 1 and 2 to form well-isolated supramolecular dimers. At room temperature the chi T-M values for the compounds 1 and 2 remain almost constant until 30 K. Below this temperature, the chi T-M values drastically drop to 0.72 cm(3) mol(-1) K for 1 and 0.52 cm(3) mol(-1) K for 2. The best fits were obtained with J = -0.92 cm(-1), vertical bar D vertical bar = 2.05 cm(-1), g = 2.0 and R = 8.1 x 10(-4) for 1 and J = -1.16 cm(-1), vertical bar D vertical bar = 2.05 cm(-1), g = 2.0 and R = 1.2 x 10(-3) for 2. However, in compound 3, two axial positions are occupied by the azide ions. The Mn center dot center dot center dot Mn repeating distance is 6.616 angstrom along the chain. Magnetic characterisation shows that the mu(1,3)-bridging azide ion mainly transmits an antiferromagnetic interaction (J = -6.36 cm(-1)) between Mn(III) ions. The presence of two methoxy groups increases the steric crowding in the H(2)vanoph moiety and thereby inhibits the formation of a polynuclear compound with this ligand.