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

Abstract

Poly nuclear copper(II) complexes [CU₂{μ-B(pz)₄}(μ-ox)(NO₃)(H₂O)].2H₂O (<b>1</b>. 2H₂O), [CU₂{μ-B(pz)₄}(μ-ox) (H₂O)₂](ClO₄).2H₂O (<b>2</b>.2H₂O) and [CU₂{μ-B (Pz)₄}(μNO₃)(NO₃)₂(H₂O)] (<b>3</b>) are prepared and structurally characterized by single crystal X-ray crystallography (ox, oxalate ion C₂O₄^2-). Complexes <b>1</b> and <b>2</b> 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 <b>1</b> and <b>2</b> occupy the basal positions giving CuN₂O₃ square pyramidal geometry. The alternate copper centers in <b>1</b> are bonded to nitrate and aqua axial ligands. Complex <b>2</b> has each copper bonded to one axial aqua ligand. In the 1D polymeric structure of complex <b>3</b>, 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 <b>1-3</b> display a broad <i>d-d </i>band within 676-721 nm in DMF. Variable temperature magnetic susceptibility measurements in the range 20 to 300 K show strong anti ferromagnetic behavior of the complexes 1 and 2 giving respective magnetic moment values of 1.49 and 1.52 <i>μ</i>_B (per copper) at 298 K. The coplanar orbital topology involving the {Cu₂(μ-ox)}²⁺ mo iety in <b>1</b> and <b>2</b> facilitates strong anti ferromagnetic coupling between two{<i>d</i>_x2-y2} magnetic orbitals (<i>J</i>: -320 cm^-1 <b>1</b>; -353 cm^-1 <b>2</b>). Complex <b>3</b> is essentially paramagnetic giving <i>μ</i>_eff value of 1.93<i> μ</i>_B (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 <b>1-3</b>, 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.