Ligand Effects on the Structure and Magnetic Properties of Alternating Copper(II) Chains with 2,2′-Bipyrimidine- and Polymethyl-Substituted Pyrazolates as Bridging Ligands

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 (dx2–y2) (1 and 2) or d­(z2) (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.