Magnetic and HFEPR Studies of Exchange Coupling in a Series of μ‑Cl Dicobalt Complexes Jia-Jia Liu, † Shang-Da Jiang,* ,† Petr Neugebauer, ‡ Joris van Slageren, ‡ Yanhua Lan, § Wolfgang Wernsdorfer, § Bing-Wu Wang, † and Song Gao* ,† † National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ Institut fü r Physikalische Chemie, Universitä t Stuttgart, Pfaffenwaldring 55, Stuttgart D-70569, Germany § Institut Né el-CNRS, 25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France * S Supporting Information ABSTRACT: We report three dinuclear cobalt(II) complexes, [Co(L)Cl 2 ] 2 (L = bpy, mbpy, and dmpbt), that are bridged solely by chloride ions. High-field electron paramagnetic resonance and magnetometric measurements were applied to investigate the magnetic intramolecular Co-Co interactions. Simulation results based on the multispin model reveal that the complexes are weakly ferromagnetically coupled and that the isotropic exchange coupling constants differ slightly for the three complexes. Moreover, the competing effects of zero- field splitting and magnetic coupling on the temperature-dependent magnetic susceptibility were analyzed. ■ INTRODUCTION Molecule-based magnetic materials (MMMs) are zero-dimen- sional magnetic systems that are excellently suited to investigating magnetostructural correlations. The results can be applied to one- or higher-dimensional materials, and the understanding gained can be exploited to develop MMMs with improved magnetic properties. 1 Previous studies have revealed that the magnetic properties of materials are largely determined by the magnetic anisotropy of the spin carriers and the magnetic interactions between them. 2 In addition, molecular engineering of magnetic couplings serves as the experimental foundation for the design and preparation of various magnetic materials and the investigation of magnetostructural correla- tions. In previous studies, spin carriers were usually connected by delocalized π ligands, 3 radicals, 4 carboxylates, 5 oxygen or hydroxyl groups, 6 and pseudohalide groups, e.g., azido anion, 7 thiocyanate, 8 and cyanide groups. 9 However, in-depth studies on complexes where the metal ions are exclusively bridged by chloride ions and of the magnetic coupling involved are limited. 10 Typically, the exchange coupling constants and the single ion zero-field splitting (ZFS) parameters of complexes based on ions such as Mn(III), Fe(II), Co(II), Ni(II), and Cu(II) are obtained by fitting the direct current (dc) magnetic susceptibility data only. 2c,4c,f,11 However, fitting solely χ M T and MH data leads to ambiguous results not only because of overparametrization but also because a bulk thermodynamic measurement is inherently insensitive to the energy spectrum of the complex. In contrast, magnetic resonance techniques give direct access to the energy levels and allow unambiguous determination of spin Hamiltonian parameters. In this context, high-field electron paramagnetic resonance (HFEPR), which can detect weak exchange interactions, plays an important role in the investigation of magnetostructural correlations. 3b,12 Thus, in 2007, the asymmetric interaction parameter D AB was first extracted from a powder sample of a dinuclear Ni(II) complex using a combination of magnetometric and HFEPR techniques. 13 In 2011, the exchange and Zeeman parameters of two dinuclear Co(II) compounds were determined by a combination of EPR and INS spectroscopies. 12e Furthermore, exchange couplings and single ion anisotropies in weakly coupled Mn III dimers were elucidated using HFEPR measure- ments performed on both single-crystal and powder samples. 3a The parameters derived from HFEPR simulations were successfully applied to fit the magnetic data. In another example of a dinuclear Co II complex bridged by a delocalized π system, 3b weak antiferromagnetic coupling and large positive axial ZFS parameters D were revealed by simulating the HFEPR spectra and fit quite well with magnetic data. Thus far, all reported examples for cobalt complexes have been based on delocalized π ligand bridges. In contrast, the origins of the nature and strength of the exchange interactions engendered by chloride bridging ligands remain largely unexplored. 14 Herein, we report three dinuclear cobalt(II) complexes, [Co(L)Cl 2 ] 2 (L = 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′- Received: October 6, 2016 Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.6b02368 Inorg. Chem. XXXX, XXX, XXX-XXX