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}_<i>n</i> (<b>1</b>), {[Cu₂­(μ-H₂O)­(μ-3-Mepz)₂­(μ-bpm)]­(ClO₄)₂·​2H₂O}_<i>n</i> (<b>2</b>), and {[Cu₂­(μ-OH)­(μ-3,5-Me₂pz)­(μ-bpm)­(H-3,5-Me₂pz)₂]­(ClO₄)₂}_<i>n</i> (<b>3</b>) [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 <b>1</b> and <b>2</b> consist of copper­(II) chains with regular alternating bpm and bis­(pyrazolate)­(aqua) bridges, whereas that of <b>3</b> is made up of copper­(II) chains with regular alternating bpm and (pyrazolate)­(hydroxo) bridges. The copper centers are six- (<b>1</b>) or five-coordinate (<b>2</b>) in axially elongated, octahedral (<b>1</b>) or square-pyramidal (<b>2</b>) environments in <b>1</b> and <b>2</b>, whereas they are five-coordinate in distorted trigonal-bipyramidal surroundings in <b>3</b>. The values of the copper–copper separations across the bpm/pyrazolate bridges are 5.5442(7)/3.3131(6) (<b>1</b>), 5.538(1)/3.235(1) (<b>2</b>), and 5.7673(7)/3.3220(6) Å (<b>3</b>). The magnetic properties of <b>1</b>–<b>3</b> 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 [<b>H</b> = −<i>J</i>∑_{<i>i</i>=1–<i>n</i>/2} (<b>S_2<i>i</i></b>·<b>S_{2<i>i</i>–1}</b> + α<b>S_2<i>i</i></b>·<b>S_2<i>i</i>+1</b>), with α = <i>J</i>′/<i>J</i> and <i>S</i>_<i>i</i> = <i>S</i>_Cu = 1/2] reveals the presence of a strong to moderate antiferromagnetic coupling through the bis­(pyrazolate)­(aqua) [−<i>J</i> = 217 (<b>1</b>) and 215 cm^–1 (<b>2</b>)] and (pyrazolate)­(hydroxo) bridges [−<i>J</i> = 153 cm^–1 (<b>3</b>)], respectively, whereas a strong to weak antiferromagnetic coupling occurs through the bis-bidentate bpm [−<i>J</i>′ = 211 (<b>1</b>), 213 (<b>2</b>), and 44 cm^–1 (<b>3</b>)]. A simple orbital analysis of the magnetic exchange interaction within the bpm- and pyrazolate-bridged dicopper­(II) fragments of <b>1</b>–<b>3</b> visualizes the σ-type pathways involving the (d<i>x</i>²–<i>y</i>²) (<b>1</b> and <b>2</b>) or d­(<i>z</i>²) (<b>3</b>) 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