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.

Alternating Ferro/Antiferromagnetic Copper(II) Chain Containing an Unprecedented Triple Formato/Hydroxido/Sulfato Bridge

“…The pyrimidyl rings of the bipyrimidine molecule are coplanar (mean deviation 0.0085 Å), which is in agreement with the observed values in related compounds where the bpym acts as bis­(bidentate) bridging ligand. − …”

“…Despite the relatively large Cu···Cu separation (ca. 5.4–5.9 Å), these moderate/strong AF interactions are observed in those systems where both N atoms of the bpym ligand occupy equatorial positions − (eq–eq cases in Table ) and are attributed to the good overlap between the magnetic orbitals (built up essentially from the d x 2 – y 2 Cu­(II) orbitals) and the delocalized orbitals of the bpym bridges …”

“…5.4− 5.9 Å), these moderate/strong AF interactions are observed in those systems where both N atoms of the bpym ligand occupy equatorial positions 35−40 (eq−eq cases in Table 4) and are attributed to the good overlap between the magnetic orbitals (built up essentially from the d x 2 −y 2 Cu(II) orbitals) and the delocalized orbitals of the bpym bridges. 40 Several attempts have been performed to analyze the more relevant structural features in order to explore the existence of magneto-structural correlations in [Cu(bpym)Cu] systems, but as far as we know, no satisfactory correlation has been obtained. In fact, from the structural and magnetic data reported in Table 4 no straightforward correlation between the J coupling constant and any structural parameters can be observed.…”

“…In this kind of system, special attention has been addressed to coordination architectures that include multiatomic bridging ligands (MABLs). Thus, MABLs such as oxalate − azide − or 2,2′-bipyrimidine (bpym) − have been extensively used in magneto-structural studies because of their remarkable ability in mediating magnetic interactions between paramagnetic centers as well as the variety of dimensionalities (from both structural and magnetic points of view) and topologies that they can originate when adopting different coordination modes. Despite the variety of bpym-including ternary systems characterized, those that include carboxylate groups are relatively uncommon.…”

Synthesis, crystal structure and Hirshfeld surface analysis of the [Cu2(3,5-dimethyl-1H-pyrazole)4(ClO4)2] complex

“…It is characterized by high basicity (due to the inductive effect of the methyl groups) in comparison with simple pyrazole, and has been intensively studied in coordination to different transition metals during several decades. Despite the fact that the coordination behavior of DIMPA towards Cu II (ClO 4 ) 2 is represented by 10 entries in Cambridge Crystallographic Database [4][5][6][7][8][9][10], unfortunately there is no crystallochemical information about analogous Cu(I) perchlorate compounds. To analyze the coordination abilities of Cu(I) as a "soft acid" towards DIMPA (possessing border-line basicity) in the presence of perchlorate anions, we undertook the synthesis and structural characterization of the new complex compound [Cu 2 (DIMPA) 4 (ClO 4 ) 2 ] (1).…”