Inorganic Chemistry, Vol.55, No.5, 2390-2401, 2016
Dicopper(II) Metallacyclophanes with N,N'-2,6-Pyridinebis(oxamate): Solution Study, Synthesis, Crystal Structures, and Magnetic Properties
The complexing ability of copper(II) in solution by the ligand N,N'-2,6-pyridinebis(oxamic acid) (H(4)mpyba, H4L) was determined through potentiometric and UV vis spectroscopy at 25 degrees C and 0.15 M NaCl. The logarithms of the equilibrium constants for its copper(II) complexes according to the eqs 2H(2)L + 2Cu reversible arrow [Cu-2(H2L)(2)], 2H(2)L + 2Cu reversible arrow [Cu-2(H2L) (HL)] + H, 2H(2)L + 2Cu reversible arrow [Cu-2(HL)(2)] + 2H, 2H(2)L + 2Cu reversible arrow [Cu-2(HL)(L)] + 3H, and 2H(2)L + 2Cu reversible arrow [Cu2L2] + 4H were 12.02(7), 8.04(5), 1.26(6), -7.51(6), and -16.36(6), respectively. The knowledge of the solution behavior has supported the synthesis of three new compounds bearing the common building block Cu2L24-. Their formulas are (Me4N)(4)[Cu-2(mpyba)(2)(H2O)(2)]center dot H2O (1), (Me4N)(4)[K2Na2Cu4(mpyba)(4)(H2O)(6.8)]center dot 1.6H(2)O (2), and [Na6Cu2(mpyba)(2)Cl-2(H2O)(8)]center dot 7H(2)O (3) (Me4N+ = tetramethylammonium cation). The [Cu-2(mpyba)(2)(H2O)(2)](4-) tetraanionic unit, which is present in 1, has a [3,3] metallacyclophane-type motif connected by two N-Cu-N bonds. In 2, a heterotrimetallic decanuclear nanocage is formed through front-to-front assembly of two [Cu-2(mpyba)(2)](4-) units, which also coordinate to potassium(I) and sodium(I) cations by means of carboxylate oxygens from oxamate. The structure of 3 consists of heterobimetallic layers of formula [Na6Cu2(mpyba)(2)Cl-2(H2O)(8)] and crystallization water molecules, which are interlinked by hydrogen bonds leading to a supramolecular three-dimensional network. The investigation of the magnetic properties of 1-3 in the temperature range 1.9-300 K shows the occurrence of ferromagnetic interactions between the dicopper(II) metallacyclophane unit [J = +6.85 (1), +7.40 (2), and +7.90 cm(-1) (3); H = -JS(Cu1).S-Cu2, where S-Cu1 = S-Cu2 = 1/2]. Theoretical calculations on 1-3 were carried to substantiate the nature and magnitude of the involved magnetic interactions and to support the occurrence of a spin polarization mechanism accounting for them.