Molecular squares of Ni^II and Cu^II: ferromagnetic exchange interaction mediated by syn–anti carboxylate-bridging

The synthesis of four discrete tetranuclear complexes {[Ni^II(L²)][ClO₄]}₄·MeCN (1), {[Cu^II(L¹)(O₃SCF₃)]}₄·H₂O (2), {[Cu^II(L³)(OClO₃)]}₄·MeCN (3) and {[Cu^II(L⁴)][ClO₄]}₄·3MeCN·4H₂O (4), supported by a closely similar group of carboxylate–appended (2–pyridyl)alkylamine ligands [L¹(−): 3-[N-methyl-{2-(pyridin-2-yl)ethyl}amino]propionate; L²(−): 3-[(2-(pyridin-2-yl)ethyl){2-(pyridin-2-yl)methyl}amino]propionate; L³(−): 3-[N-isopropyl-{2-(pyridin-2-yl)methyl}amino]propionate and L⁴(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 Ni^IIN₃O₂ coordination and each Cu^II centre in 2–4 has distorted square-pyramidal Cu^IIN₂O₃ 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.