Ligand effects on the structure and magnetic properties of alternating copper(II) chains with 2,2'-bipyrimidine- and polymethyl-substituted pyrazolates as bridging ligands.

Abstract

A novel series of heteroleptic copper(II) compounds of formulas {[Cu2(μ-H2O)(μ-pz)2(μ-bpm)(ClO4)(H2O)]ClO4·2H2O}n (1), {[Cu2(μ-H2O)(μ-3-Mepz)2(μ-bpm)](ClO4)2·2H2O}n (2), and {[Cu2(μ-OH)(μ-3,5-Me2pz)(μ-bpm)(H-3,5-Me2pz)2](ClO4)2}n (3) [bpm = 2,2'-bipyrimidine, Hpz = pyrazole, H-3-Mepz = 3-methylpyrazole, and H-3,5-Me2pz = 3,5-dimethylpyrazole] have been synthesized and structurally characterized by X-ray diffraction methods. The crystal structures of 1 and 2 consist of copper(II) chains with regular alternating bpm and bis(pyrazolate)(aqua) bridges, whereas that of 3 is made up of copper(II) chains with regular alternating bpm and (pyrazolate)(hydroxo) bridges. The copper centers are six- (1) or five-coordinate (2) in axially elongated, octahedral (1) or square-pyramidal (2) environments in 1 and 2, whereas they are five-coordinate in distorted trigonal-bipyramidal surroundings in 3. The values of the copper-copper separations across the bpm/pyrazolate bridges are 5.5442(7)/3.3131(6) (1), 5.538(1)/3.235(1) (2), and 5.7673(7)/3.3220(6) Å (3). The magnetic properties of 1-3 have been investigated in the temperature range of 25-300 K. The analysis of their magnetic susceptibility data through the isotropic Hamiltonian for an alternating antiferromagnetic copper(II) chain model [H = -J∑i=1-n/2 (S2i·S2i-1 + αS2i·S2i+1), with α = J'/J and Si = SCu = 1/2] reveals the presence of a strong to moderate antiferromagnetic coupling through the bis(pyrazolate)(aqua) [-J = 217 (1) and 215 cm(-1) (2)] and (pyrazolate)(hydroxo) bridges [-J = 153 cm(-1) (3)], respectively, whereas a strong to weak antiferromagnetic coupling occurs through the bis-bidentate bpm [-J' = 211 (1), 213 (2), and 44 cm(-1) (3)]. A simple orbital analysis of the magnetic exchange interaction within the bpm- and pyrazolate-bridged dicopper(II) fragments of 1-3 visualizes the σ-type pathways involving the (dx(2)-y(2)) (1 and 2) or d(z(2)) (3) magnetic orbitals on each metal ion, which account for the variation of the magnetic properties in these three novel examples of one-dimensional copper(II) compounds with regular alternating intrachain antiferromagnetic interactions.