Structurally different dinuclear copper(II) complexes with the same triazolopyrimidine bridging ligand

Four binuclear copper(II) compounds with the anionic form of the ligand 4,5-dihydro-5-oxo[1,2,4]triazolo[1,5-a]pyrimidine (5tpO(-)) have been isolated, their formulae being [Cu-2(5tPO)(4)(H2O)2](.)2H2O (1), [Cu-2(phen)(2)(5tpO)(2)(H2O)(2)]-(NO3)(2)(.)4H(2)O (2), [Cu-2(biim)(2)(5tpO)(2)(H2O)](ClO4)(2)(.)5.5H(2)O (3), and [Cu-2(CH3CO2)(2)(5tpO)(2)(H2O)(2)] (4) (phen = 1.10-phenanthroline, biim = bisimidazole). A related mononuclear complex, [Cu(phen)(2)(5HtpO)(2)](NO3)(2) (5), has also been prepared. The crystal structure of compounds 1-3 has been determined by X-ray diffraction, showing their binuclear nature with four (1) or two (2, 3) bridging 5tpO(-) moieties. The triazolopyrimidine ligand binds the copper atoms through N-3 and N-4 in compounds 1 and 3, whereas a novel binding mode through N-3 and the exocyclic oxygen atom has been found in 2. The study of the solution behaviour of 2 and 5 shows the important role of the solvent in the formation of the mono or binuclear complex. The reduction of the metal centres to Cu-1 has been observed by electrochemical methods, demonstrating that this ligand stabilizes this oxidation state. Magnetic susceptibility measurements and electronic paramagnetic resonance (EPR) spectra seem to indicate a dimeric nature also for compound 4. The antiferromagnetic interaction between the copper atoms inside the dimer is very strong for 4 (2J = -221.3 cm(-1)) and 1 (2J = -104.9 cm(-1)), fairly strong for 3 (2J = -21.6 cm(-1)), and very weak or non-existent for 2. The nature of the copper-copper interaction in compound 1 has been analyzed by using molecular orbital calculations.