Dividing the Spoils: Role of Pyrazine Ligands and Perchlorate Counterions in the Magnetic Properties of Bis(pyrazine)diperchloratecopper(II), [Cu(pz)₂](ClO₄)₂

Abstract

A complete first-principles bottom-up computational study of the magnetic properties of [Cu(pz)2](ClO4)2 is presented. A remarkable agreement is observed in the whole range of temperatures between simulated and experimental magnetic susceptibility data. Interestingly, the simulated heat capacity values show an anomaly close to the Néel temperature of 4.21 K associated with a transition from a two-dimensional (2D) antiferromagnet to a three-dimensional (3D) ordered state. The antiferromagnetic behavior of [Cu(pz)2](ClO4)2 is due to a 2D magnetic topology owing to two antiferromagnetic J(AB) interactions through pyrazine ligands. Although presenting a very similar molecular arrangement, the numerical values of the two magnetically significant J(AB) couplings differ by 25% (-10.2 vs -7.3 cm(-1)). This difference can be ascribed to three main contributions: (i) the central pyrazine ring shearing-like distortion, (ii) the effect of the orientation of the perchlorate counterions, and (iii) a hitherto unrecognized skeleton-counterion cooperation arising from different hydrogen bonding contributions in the two most significant J(AB) couplings. The impact of the orientation of the perchlorate counterions is disclosed by comparison to J(AB) studies using structurally similar ligands but with different electronegativity (namely, BF4(-), BCl4(-), and BBr4(-)). Pyrazine ligands and perchlorate counterions prove to be noninnocent.

Control of the third dimension in copper-based square-lattice antiferromagnets

“…We note that the disparity in the two exchange strengths for 4 is smaller than that found in Ref. 51 . This is probably because the authors of that work used a 10 K, rather than a 100 K, structure; the disparity develops on cooling.…”

“…More recent theoretical studies have made use of density-functional theory (DFT) and, in particular, compound 4 has been investigated using first-principles calculations by Vela et al 51 . According to their calculations a disparity of about 30% (21.0 K vs 29.6 K) is present at 15 K between the exchange strengths through the two crystallographically distinct pyrazine molecules in this material 52 .…”

“…By contrast, using the 165 K structure their calculations show only a very small difference in these exchange energies. Vela et al 51 rule out the tilting of the pyrazines as a contributory factor in the low-temperature disparity and instead attribute it to a combination of three effects, which are, in increasing order of importance: (i) hydrogen bonding between the O atoms of the perchlorates and the H atoms of the pyrazines; (ii) a shear-like distortion of the pyrazine rings; and (iii) the orientation of the ClO − 4 ligands. The authors of Ref.…”

“…The authors of Ref. 51 suggest that the role of ClO − 4 molecules is to increase the spin-density along the primary exchange pathway leading to an enhanced interaction strength. By extension, for the other samples considered here, where the ClO − 4 counterions occupy voids between the layers, this enhancement is not expected to occur, which could explain the large disparity in the in-plane exchange energies.…”

Quasi-two-dimensionalS=12magnetism ofCu[C6H2(<

“…4 Despite numerous experimental studies, microscopic aspects of Cu 2+ magnets with organic components are relatively less developed. 5,6 Many of these systems are easy to understand empirically, because organic molecules provide only a few linkages between the spin- …”