OXAMIDATO COMPLEXES .5. INFLUENCE OF THE STERIC CONSTRAINTS ON THE COMPLEX-FORMATION BETWEEN COPPER(II) AND N,N'(ALKYL-SUBSTITUTED)OXAMIDES - SYNTHESIS AND CRYSTAL-STRUCTURE OF [CU2(MAPOX)(N3)2]N (H2MAPOX = N,N'-BIS[3-(METHYLAMINO)PROPYL]OXAMIDE)

Potentiometric studies on the ternary systems Cu2+-mapox2--H+ and Cu2+-dmapox2--H+ where mapox2- and dmapox2- are the dianions of N,N'-bis[3-(methylamino)propyl]oxamide and N,N'-bis[3-(dimethylamino)propyl]oxamide, respectively, have been carried out at 25-degrees-C and 0.1 mol dm-3 NaNO3. The proton association constants corresponding to the equilibria L2- + jH+ beta(j) double-line arrow pointing left and right H(j)L(j-2)+ were found to be log beta-2 = 24.0 (2), log beta-3 = 34.31 (1), and log beta-4 = 44.06 (1) for L = mapox and log beta-2 = 24.0 (2), log beta-3 = 33.49 (1), and log beta-4 = 42.34 (1) for L = dmapox. The values of the stability constants related to the equilibria pCU2+ + qL2- + rH+ beta(pqr) double-line arrow pointing left and right [CupLqHr]2(p-q)+r]+ were log beta-212 = 36.70 (8), log beta-211 = 31.70 (8), log beta-210 = 26.41 (1), log beta-320 = 47.28 (2), log beta-430 = 67.53 (2), and log beta-110 = 17.09 (1) for L = mapox and log beta-212 = 34.64 (9), log beta-211, = 29.12 (5), log beta-210 = 23.24 (1), log beta-21-1 = 14.66 (1), and log beta-21-2 = 4.88 (1) for L = dmapox. The molecular structure of the complex [CU2(mapox)(N3)2]n has been determined by single-crystal X-ray analysis. The compound crystallizes in the monoclinic system, space group P2(1)/n, with a = 11.439 (3) angstrom, b = 10.522 (1) angstrom, c = 7.376 (1) angstrom, beta = 106.47 (4)-degrees, and Z = 2. Refinement of the atomic parameters by least squares gave a final R factor of 0.052 (R(w) = 0.059) for 1036 unique reflections having I greater-than-or-equal-to 3-sigma(I). The structure consists of neutral one-dimensional chains of copper(II) ions bridged alternatively by the oxamidato and azido groups. The former acts as a bis-terdentate ligand whereas the latter is bound through a nitrogen in an asymmetrical end-on fashion. Copper environment can be best described as square pyramidal: the equatorial plane is defined by the oxygen and nitrogen atoms of the amide, the nitrogen atom of the amine group, and a nitrogen atom of the azide ligand whereas the apical position is filled by a nitrogen atom of another azide group. The ligand dmapox presents only the trans conformation in its metal complexes, whereas the ligand mapox can adopt either the cis or trans conformations. The greater flexibility of mapox allows the formation of mono-, di-, tri-, and tetranuclear complexes, in contrast to the restriction to the dinuclearity imposed by the trans configuration in dmapox. The values of the stability constants of these systems and the different chelating capabilities of N,N'-bis(alkyl-substituted)oxamidate ligands are analyzed and discussed in the light of the available structural data and steric effects caused by the presence of methyl substituents on amine groups.