Synthesis, structural, magnetic, DFT calculations and CShM studies of three new pentanuclear Mn(II) clusters

The ditopic ligand PyPzOAPz (N-[(Z)-amino(pyrazin-2-yl)methylidene]-5-methyl-1-(pyridin-2-yl)-1H-pyrazole-3-carbohydrazonic acid) was synthesized by in situ condensation of methyl imino pyrazine-2-carboxylate with 5-methyl-1-(2-pyridyl) pyrazole-3-carbohydrazide. In this work we have also used two of our earlier ligands PzCAP (5-methyl-N-[(1E)-1-(pyridin-2-yl) ethylidene]-1H-pyrazole-3-carbohydrazonic acid) (Dalton Trans., 2009, 8215) and PzOAP (N-[(Z)-amino(pyridin-2-yl) methylidene]-5-methyl-1H-pyrazole-3-carbohydrazonic acid) (Dalton Trans., 2007, 1229). These ligands PzCAP, PzOAP and PyPzOAPz were made to react with Mn(ClO4)(2)center dot 6H(2)O to produce three pentanuclear Mn(II) clusters [Mn-5(PzCAP)(6)](ClO4)(4) (1), [Mn-5(PzOAP)(6)](ClO4)(4) (2) and [Mn-5(PyPzOAPz)(6)](ClO4)(4) (3). These complexes have been characterized by X-ray structural analyses and variable temperature magnetic susceptibility measurements. All complexes have a pentanuclear core with trigonal bipyramidal arrangement of Mn(II) atoms, where, the axial metal centers have a N3O3 chromophore and the equatorial centers have N4O2 with an octahedral arrangement. These Mn-5(II) clusters 1, 2 and 3 show the presence of antiferromagnetic coupling within the pentanuclear manganese(II) core (J = -2.95, -3.19 and -3.00 cm(-1) respectively). Density functional theory calculations and continuous shape measurement (CShM) studies have been performed on these complexes to provide a qualitative theoretical interpretation of the antiferromagnetic behaviour shown by them. The pentanuclear Mn(II) cluster (1) on reaction with Cu(NO3)(2)center dot 6H(2)O in 1 : 1 mole proportion in CH3OH:H2O (60 : 40) forms a homoleptic [2 x 2] tetranuclear Cu-4(II) grid [Cu-4(PzCAP)(4)(NO3)(2)](NO3)(2)center dot 8H(2)O (4). The same Cu-4(II) grid is also obtained from a direct reaction between the ditopic ligand PzCAP with Cu(NO3)(2)center dot 6H(2)O in 1 : 1 mole proportion. This conversion of a cluster to a grid is a novel observation.