Molecular squares of NiII and CuII: ferromagnetic exchange interaction mediated by syn-anti carboxylate-bridging

The synthesis of four discrete tetranuclear complexes {[Ni-II(L-2)][ClO4]}(4)center dot MeCN (1), {[Cu-II(L-1)(O3SCF3)]}(4)center dot H2O (2), {[Cu-II(L-3)(OClO3)]}(4)center dot MeCN (3) and {[Cu-II(L-4)][ClO4]}(4)center dot 3MeCN center dot 4H(2)O (4), supported by a closely similar group of carboxylate-appended (2-pyridyl) alkylamine ligands [L-1(-): 3-[N-methyl-{2-(pyridin-2-yl)ethyl}amino]propionate; L-2(-): 3-[(2-(pyridin-2-yl)ethyl){2-(pyridin-2-yl) methyl}amino]propionate; L-3(-): 3-[N-isopropyl-{2-(pyridin-2-yl)methyl}amino]propionate and L-4(2-): 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)] is described. Structural characterization reveals that each Ni-II centre in 1 has square-pyramidal (NiN3O2)-N-II coordination and each Cu-II centre in 2-4 has distorted square-pyramidal (CuN2O3)-N-II coordination, utilizing three N and one carboxylate O in 1 and two N and carboxylate O (one in 2 and 3; two in 4) of the ligand and the fifth/fourth coordination is provided by an oxygen atom belonging to the carboxylate group of an adjacent molecule. A CF3SO3- or a ClO4- ion provides an O coordination in 2 and 3, respectively. Temperature-dependent magnetic studies reveal the existence of ferromagnetic exchange interaction in each case, due to the presence of equatorial-equatorial syn-anti carboxylate bridge between M-II centres.