Magnetic Properties of Weakly Exchange-Coupled High Spin Co(II) Ions in Pseudooctahedral Coordination Evaluated by Single Crystal X‑Band EPR Spectroscopy and Magnetic Measurements

NICOLÁS I. NEUMAN; E. WINKLER; O. PEÑA; M.C.G. PASSEGGI; A.C. RIZZI; C.D. BRONDINO

INORGANIC CHEMISTRY

AMER CHEMICAL SOC

Lugar: Washington; Año: 2014 vol. 53 p. 2535 - 2544

0020-1669

We report single-crystal X-band EPR andmagnetic measurements of the coordination polymer catena-(trans-(μ2-fumarato)tetraaquacobalt(II)), 1, and the Co(II)-doped Zn(II) analogue, 2, in different Zn:Co ratios. 1 presentstwo magnetically inequivalent high spin S = 3/2 Co(II) ions perunit cell, named A and B, in a distorted octahedral environmentcoordinated to four water oxygen atoms and trans coordinated totwo carboxylic oxygen atoms from the fumarate anions, in whichthe Co(II) ions are linked by hydrogen bonds and fumaratemolecules. Magnetic susceptibility and magnetization measurementsof 1 indicate weak antiferromagnetic exchange interactionsbetween the S = 3/2 spins of the Co(II) ions in the crystal lattice. Oriented single crystal EPR experiments of 1 and 2 were usedto evaluate the molecular g-tensor and the different exchange coupling constants between the Co(II) ions, assuming an effectivespin S′= 1/2. Unexpectedly, the eigenvectors of the molecular g-tensor were not lying along any preferential bond direction,indicating that, in high spin Co(II) ions in roughly octahedral geometry with approximately axial EPR signals, the presence ofmolecular pseudo axes in the metal site does not determine preferential directions for the molecular g-tensor. The EPRexperiment and magnetic measurements, together with a theoretical analysis relating the coupling constants obtained from bothtechniques, allowed us to evaluate selectively the exchange coupling constant associated with hydrogen bonds that connectmagnetically inequivalent Co(II) ions (|JAB1/2| = 0.055(2) cm−1) and the exchange coupling constant associated with a fumaratebridge connecting equivalent Co(II) ions (|JAA1/2| ≈ 0.25 (1) cm−1), in good agreement with the average J3/2 value determined frommagnetic measurements.