OXAMIDATO COMPLEXES .6. COMPLEX-FORMATION BETWEEN COPPER(II) AND N,N'-BIS[2-(DIMETHYLAMINO)ETHYL]OXAMIDE (H-2DMAEOXD) - PREPARATION, CRYSTAL-STRUCTURE, AND MAGNETIC-PROPERTIES OF [CU2(DMAEOXD)(NCO)2(H2O)2]

The protonation and deprotonation of N,N'-bis[2-(dimethylamino)ethyl]oxamide (H-2dmaeoxd, H-2L) and its complex formation with Cu(II) have been investigated by potentiometry in aqueous solution at 25-degrees-C and 0.1 mol dm-3 NaNO3. The proton association constants beta(j) corresponding to the equilibria L2- + jH+ half arrow right over half arrow left H(j)L(j-2)+ were found to be log beta2 = 24.0(2), log beta3 = 32.63(1) and log beta4 = 40.46(1). The values of the stability constants beta(pqr) concerning the equilibria pCu2+ + qL2- + rH+ half arrow right over half arrow left [Cu(p)L(q)H(r)][2(p-q)+r]+ were log beta212 = 34.11 (8), log beta211 = 29.52(8), log beta210 = 24.74(2), log beta320 = 44.87(5), log beta430 = 64.09(7) and log beta110 = 16.92(2). The compound of formula [Cu2(dmaeoxd)(NCO)2(H2O)2] has been synthesised and its crystal structure determined by single-crystal X-ray diffraction methods at room temperature. It crystallizes in the monoclinic system, space group P2(1)/a and Z = 2, with a = 11.589(3), b = 12.456(1), c = 6.771 (2) angstrom, and beta = 102.39(2)degrees. Least-squares refinement gave a final R (R') factor of 0.038 (0.045) for 2249 unique reflections having I greater-than-or-equal-to 3sigma(I). The structure consists of neutral centrosymmetric oxamidato-bridged dinuclear copper(II) units. The co-ordination geometry around each copper(II) ion is distorted square pyramidal: the equatorial plane comprises the oxygen and nitrogen atoms of the amide, the nitrogen atom of the amine group and the nitrogen atom of the cyanate ligand, whereas the apical position is filled by a water molecule. The intramolecular copper-copper separation is 5.306(1) angstrom. Variable-temperature magnetic susceptibility measurements (50-300 K) of this complex reveal a strong antiferromagnetic coupling through the oxamidate, the relevant parameters being g = 2.12 and J (singlet-triplet energy gap) = -560 cm-1. The chelating ability of dmaeoxd2- and its ability to transmit electronic effects are discussed and compared to those of related N,N'-dialkyloxamidate ligands.