Anion Influence on the Structure and Magnetic Properties of a Series of Multidimensional Pyrimidine-2-carboxylato-Bridged Copper(II) Complexes

Abstract

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)Cl] (3), [Cu(mu-pymca)(SCN)(H2O)] x 4 H2O (4), [Cu(mu-pymca)N3] (5), [Cu2(mu1,5-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.

Theoretical Studies of Some Polymeric-Cu(II)-Pyrimidine-Carboxylate Ligands: Crystal Parameters and J Exchange Coupling

“…In the a direction bridging is through the carboxylate group, while in the c direction bridging is through the bromine ions. Suarez-Varela et al [4] argued that the exchange coupling between the copper centers in the c axis is weak. This is due to the long Cu-Br distance.…”

“…2 for our compounds are quite reasonable, because this equation includes not only bond distances but also all separations between atom pairs, including those atoms which are far apart. J Coupling and g parameters: The magnetic susceptibility measurements of compounds 1 and 2 at different temperatures indicate their antiferromagnetic nature [4,5]. Since the calculation of the J coupling parameters using ORCA software involves two centers at a time, we chose isolated dimers for our compounds.…”

“…These are given in Tables 3 and 4. According to Suarez-Varela et al [4] the experimental magnetic susceptibility data were fit to eqn. 3 and the parameters J, g and ρ were extracted from the fit.…”

“…4 to determined the parameter J, g and ρ from the experimental suscepilities at different temperature [4]. These were reproduced from eqn.…”

“…The pyrimidine complexes under theoretical investigation here are compounds (1) and (2). Compound 1 is [Cu(pymca)2Br]n, where pymca stands for pyrimidine-2-carboxylate [4]. The crystal belongs to the orthorhombic space group Pmc21 and consists of cationic linear chains with cis-and trans-bis(pymca) bridging ligands; the coordination being through pyrimidine nitrogens and carboxylate groups.…”