Anion influence on the structure and magnetic properties of a series of multidimensional pyrimidine-2-carboxylato-bridged copper(II) complexes

Seven new polynuclear copper(II) complexes of formula [Cu(mu-pymca)2] (1) (pymca(-) = pyrimidine-2-carboxylato), [Cu(mu-pymca)Br] (2), [Cu(mu-pymca)CI] (3), [Cu(mu-pymca)(SCN)(H2O)]center dot 4H(2)O (4), [Cu(mu-pymca)N-3] (5), [Cu-2(mu(1,5)-dca)(2)(pymca)(2)] (6) (dca = dicyanamide), and K{[mu-Au(CN)(2)](2)[(Cu(NH3)(2))(2)(mu-pymca)]}[Au(CN)(2)](2) (7) have been synthesized by reactions of K-pymca with copper(II) ions in the presence of different counteranions. Compound 1 is a linear neutral chain with a carboxylato bridging ligand in a syn-anti coordination mode, whereas complexes 2 and 3 consist of cationic linear chains with cis and trans bis(chelating) pymca bridging ligands. Complex 4 adopts a helical pymca-bridged chain structure. In complex 5, zigzag pymca-bridged chains are connected by double end-on azide bridging ligands to afford a unique honeycomb layer structure. Complex 6 is a centrosymmetric dinuclear system with double mu(1,5)-dicyanamide bridging ligands and pymca end-cap ligands. Complex 7 is made of pymca-bridged dinuclear [Cu(NH3)(2)(mu-pymca)Cu(NH3)(2)](3+) units connected by [Au(CN)(2)](-) anions to four other dinuclear units, giving rise to cationic (4,4) rectangular nets, which are linked by aurophilic interactions to afford a singular 3D network. Variable-temperature magnetic susceptibility measurements show that complex 1 exhibits a very weak antiferromagnetic coupling through the syn-anti (equatorial-axial) carboxylate bridge (J = -0.57 cm(-1)), whereas complexes 2-4 and 7 exhibit weak to strong antiferromagnetic couplings through the bis(chelating) pymca bridging ligand (J = - 17.5-276.1 cm(-1)). Quantum Monte Carlo methods have been used to analyze the experimental magnetic data for 5, leading to an antiferromagnetic coupling (J = -34 cm(-1)) through the pymca ligand and to a ferromagnetic coupling (J = 71 cm(-1)) through the azide bridging ligands. Complex 6 exhibits a very weak antiferromagnetic coupling through the dicyanamide bridging ligands (J = -5.1 cm(-1)). The magnitudes of the magnetic couplings in complexes 2-5 have been explained on the basis of the overlapping between magnetic orbitals and DFT theoretical calculations.