OXAMIDATO COMPLEXES .2. COPPER(II) AND NICKEL(II) COMPLEXES WITH OXAMIDE-N,N'-DIACETIC ACID - SOLUTION STUDY, SYNTHESIS, CRYSTAL-STRUCTURES, AND MAGNETIC-PROPERTIES

The complex formation between Cu(II) and Ni(II) and oxamide-N,N'-diacetic acid (H4glyox, H4L) has been studied by potentiometry in aqueous solution at 25-degrees-C in 0.1 mol dm-3 NaNO3. The two first acidity constants of H4L were pK(al) = 2.86 (1) and pK(a2) = 3.64 (1). The logarithms of the equilibrium constants for its Cu(II) and Ni(II) complexes according to the equations M2+ + H2L2- <--> M(H2L) and 2M2+ + H2L2- <--> M2L + 2H+ were 2.62 (1) and -2.18 (1) for Cu(II) and 1.93 (1) and -11.09 (1) for Ni(II), respectively. Two new compounds of formulas {[Cu(H2glyox)(H2O)3].4H2O}n (1) and [Ni2(glyox)(H2O)6].4H2O (2) have been synthesized, and their crystal structures have been determined by X-ray crystallographic methods. The compounds crystallize in the monoclinic system, space groups P2(1)/m and P2(1)/c, respectively, with cell dimensions a = 6.0070 (4) angstrom, b = 22.329 (1) angstrom, c = 5.8015 (3) angstrom, beta = 105.784 (5)-degrees, and Z = 2 for 1 and a = 6.9885 (7) angstrom, b = 9.0530 (6) angstrom, c = 14.250 (1) angstrom, beta = 99.655 (7)-degrees, and Z = 2 for 2. The structure of 1 was solved by direct methods and that of 2 by the heavy-atom method. Least-squares refinements converged at R = 0.026 in both cases. The structure of 1 consists of infinite chains in which the oxamidato ligand, deprotonated only at the carboxylate groups, is bis-monodentate. The copper atom is situated at a mirror plane in a distorted square-pyramidal environment. The copper-copper intrachain separation is 13.321 angstrom. The structure of 2 consists of neutral binuclear units in which the oxamidato ligand is fully deprotonated and acts in a bis-tridentate fashion. Nickel coordination is distorted octahedral. The nickel-nickel intramolecular separation is 5.368 angstrom. The magnetic properties of 1 and 2 have been investigated in the 4.2-300 K temperature range. A Curie law is observed for 1, whereas a relatively strong antiferromagnetic coupling is observed for 2, evidenced by a rounded maximum in the susceptibility curve at approximately 41 K (J = -25 cm-1 and g = 2.25, where J is the exchange parameter in the isotropic Hamiltonian H = -JS(A).S(B)). These magnetic behaviors are easily understood in the frame of a localized-orbital model of exchange interactions. We discuss the striking influence of acid-base properties upon the coordination ability of the ligand and the very different physical properties which can result therefrom.