OXAMIDATO COMPLEXES .3. A STUDY OF THE FORMATION OF HOMOPOLYMETALLIC AND HETEROPOLYMETALLIC CU(II)-H2APOX-M(II) COMPLEXES (H2APOX = N,N'-BIS(3-AMINOPROPYL)OXAMIDE - M(II) = MN, CO, NI, CU, ZN, CD) - SYNTHESIS AND CRYSTAL-STRUCTURE OF [CU2(APOX)(NCO)2]

Potentiometric studies on the quaternary system Cu2+-M2+-apox2--H+ (M = Mn, Co, Ni, Zn, Cd; apox2- = dianion of N,N'-bis(3-aminopropyl)oxamide, C8H18N4O2 (L2-)) 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+ <--> HjL(j-2)+ (beta(j)) were found to be log beta(2) = 24.0 (2), log beta(3) = 34.15 (2), and log beta(4) = 43.60 (2). The values of the stability constants related to the equilibria pCu2+ + qL2- <--> [Cu(p)L(q)]2(p-q)+ (beta(pq)) were log beta(11) = 20.33 (2), log beta(21) = 28.15 (2), log beta(32) = 51.68 (4), and log beta(43) = 74.35 (7), and those concerning the equilibria pM2+ + qCu2+ + rL2- <--> [M(p)Cu(q)L(r)]2(p+p-r)+ (beta(pqr)) were log beta(111) = 23.86 (2) (Mn), 24.31 (2) (Co), 24.86 (2) (Ni), 24.87 (2) (Zn), and 23.80 (2) (Cd); log beta(122) = 46.79 (3) (Mn), 47.77 (3) (Co), 48.70 (4) (Ni), 48.81 (4) (Zn), and 46.90 (3) (Cd); and log beta(133) = 69.58 (3) (Mn), 70.66 (4) (Co), 72.30 (5) (Ni), 72.04 (5) (Zn), and 69.60 (3) (Cd). Since the ligand L2- can exhibit both cis and trans conformations in its metal complexes, we have carried out an ab initio calculation to determine the energy involved in such conformational change using (CH3NCO)2(2-) as a model system. The trans conformation is predicted to lie 2.25 kcal below the cis one. Both the sign and magnitude of this result are in good agreement with our experimental data, and this result accounts for the anomalies observed in the values of the stepwise stability constants. The molecular structure of the complex [Cu2L(NCO)2] has been determined by single-crystal X-ray analysis. The compound crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.237 (3) angstrom, b = 7.715 (3) angstrom, c = 7.161 (3) angstrom, beta = 94.02 (1)-degrees, and Z = 2. Refinement of the atomic parameters by least-squares techniques gave a final R factor of 0.036 (R(w) = 0.039) for 1101 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 cyanato groups. The former acts as a bis-terdentate bridge whereas the latter is bound through nitrogen in an end-on fashion. The copper environment can be 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 the nitrogen atom of the cyanato ligand whereas the apical position is filled by a nitrogen atom of another cyanato group. The values of the stability constants of the system Cu2+-M2+-L2- are compared and discussed in light of the available structural data.