Oxamidato complexes. Part 6. Complex formation between copper(II) and N,N′-bis[2-(dimethylamino)ethyl]oxamide (H2dmaeoxd). Preparation, crystal structure, and magnetic properties of [Cu2(dmaeoxd)(NCO)2(H2O)2]

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Abstract
The protonation and deprotonation of N,N′-bis[2-(dimethylamino)ethyl]oxamide (H2dmaeoxd, H2L) and its complex formation with CuII have been investigated by potentiometry in aqueous solution at 25 °C and 0.1 mol dm–3 NaNO3. The proton association constants βj corresponding to the equilibria L2–+jH+⇌ HjL(j-2)+ were found to be log β2= 24.0(2), log β3= 32.63(1) and log β4= 40.46(1). The values of the stability constants βpqr concerning the equilibria pCu2++qL2–+rH+⇌[CupLqHr][2(p–q)+r]+ were log β212= 34.11(8), log β211= 29.52(8), log β210= 24.74(2), log β320= 44.87(5), log β430= 64.09(7) and log β110= 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 P21/a and Z= 2, with a= 11.589(3), b= 12.456(1), c= 6.771(2)Å, and β= 102.39(2)°. Least-squares refinement gave a final R(R′) factor of 0.038 (0.045) for 2249 unique reflections having I 3σ(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)Å. 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 trasmit electronic effects are discussed and compared to those of related N,N′-dialkyloxamidate ligands.

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