Synthesis, crystal structure and magnetic properties of polymeric tetrakis(pyrazolyl)borate copper(II) complexes

Abstract

Polynuclear copper(II) complexes [Cu₂{μ-B(pz)₄}(μ-ox)(NO₃)(H₂O)].2H₂O (1.2H₂O), [Cu₂{μ-B(pz)4}(μ-ox)(H₂O)₂](ClO₄).2H₂O (2.2H₂O) and [Cu2{μ-B(pz)4}(μ-NO₃)(NO₃)₂(H₂O)] (3) are prepared and structurally characterized by single crystal X-ray crystallography (ox, oxalate ion C₂O₄2−). Complexes 1 and 2 have oxalate and tetrakis(pyrazolyl)borate bridging ligands to form one-dimensional (1D) chains. The coordinating atoms of the oxalate and tetrakis(pyrazolyl)borate in 1 and 2 occupy the basal positions giving CuN₂O₃ square pyramidal geometry. The alternate copper centers in 1 are bonded to nitrate and aqua axial ligands. Complex 2 has each copper bonded to one axial aqua ligand. In the 1D polymeric structure of complex 3, the copper(II) centers are linked through nitrate and tetrakis(pyrazolyl)borate ligands. Each copper atom has a distorted octahedral CuN₂O₄ coordination geometry with two long Cu-O bonds. The electronic spectra of the complexes 1-3 display a broad d-d band within 676-721 nm in DMF. Variable temperature magnetic susceptibility measurements in the range 20 to 300 K show strong antiferromagnetic behavior of the complexes 1 and 2 giving respective magnetic moment values of 1.49 and 1.52 mB (per copper) at 298 K. The coplanar orbital topology involving the {Cu2(μ-ox)}²⁺ moiety in 1 and 2 facilitates strong antiferromagnetic coupling between two {d_x²-y² } magnetic orbitals (J: −320 cm⁻¹ 1; −353 cm⁻¹ 2). Complex 3 is essentially paramagnetic giving meff value of 1.93 mB (per copper) in the temperature range 20 to 300 K. Such a magnetic behavior arises due to orthogonality of the orbital overlap arising from the axial-equatorial bridging mode of the nitrate ligand. In 1-3, the tetrakis(pyrazolyl)borate bridging ligand is apparently ineffective in mediating magnetic exchange interaction due to non-availability of orbitals for exchange interactions. The complexes show the formation of different supramolecular net-work structures due to hydrogen bonding interactions.