Metal ion substitution and aldehyde exchange for Cu4III aggregates from two types of piperazine-based Schiff base ligands: Synthesis, X-ray structures, magnetic studies and theoretical validation

Two new tetranuclear Cu(II)complexes [Cu-4(L2)(2)(mu(2)-OH)(2)(H2O)(2)]center dot(ClO4)(2)center dot DMF center dot 2H(2)O (1) and [Cu-4(L3)2(mu(2)-OH)(2)(H2O)(2)]center dot(ClO4)(2)center dot DMF center dot 2H(2)O (2) have been synthesized containing piperazine based Schiff base ligands H(2)L2 (asymmetric) and H(2)L3 (symmetric). H(2)L2 contains one o-vanillin and a salicylaldehyde unit while H(2)L3 has two o-vanillin units. An interesting synthetic methodology involving transition metal ion induced aldehyde exchange reaction between two Schiff base ligands has been explored for the synthesis of H(2)L2 and H(2)L3. While the latter can be obtained by direct condensation of the respective amine and aldehyde, the former can only be synthesized by this procedure developed. Both 1 and 2 contain a piperazine ring bridging two Cu(II )centers in chair conformation in the rare axial-equatorial mode of coordination. Two piperazine bridged Cu-2 moieties are further connected via two hydroxido and two phenoxido bridges. Of the two O-phenoxido-Cu bonds one is short (basal) and the other is long (apical) while the hydroxido O bridges the two Cu-II centers symmetrically in basal-basal manner giving rise to a unique [Cu2O2] moiety where, of the four bonds linking the metals, three are basal (short) and one axial (long). Magnetic studies reveal weak antiferromagnetic interactions across the two piperazine bridges with an average exchange constant value of -4.007 cm(-1) and -4.507 cm(-1) for 1 and 2 respectively. The antiferromagnetic exchanges across the phenoxido-hydroxido bridges were found to be stronger with the average value being - 43.036 cm(-1) for 1 and - 38.031 cm(-1) for 2. Computational studies further support the experimental findings and the spin density plots in the HS configuration reveal that the exchange across the piperazine bridge takes place via through space mechanism. The spin density analysis in the HS and BS configurations also reveal that the exchange interaction is propagated mainly through the hydroxido bridge. The unsymmetrical nature of H(2)L2 leads to greater spin density at the hydroxido bridge in 1 leading to slightly stronger exchange interaction.