Exploration of Vanadium(IV)-Based Single-Ion Magnet Properties in Diphosphonate-Supported Mixed-Valent Polyoxovanadates
Pragyansh Singh, Sören Schlittenhardt, Dewendra Thakre, Saroj Kumar Kushvaha, Sunil Kumar, Harsha S. Karnamkkott, Mario Ruben, Masooma Ibrahim, Abhishek Banerjee, Kartik Chandra Mondal
Abstract
Due to their potential future applications in high-density data storage devices, single-ion magnets (SIMs) have become one of the most exciting classes of materials for research at present. Vanadium complexes, with their unique multiple oxidation states and coordination geometries, are excellent candidates for investigating such properties. In the present study, we have explored the SIM properties of two mixed-valent organo-polyoxovanadyl complexes, viz. (NH4)4[H6(VIVO2)(VV2O5)2{O3P-C(O)(CH2-2-C5NH4)-PO3}2]·9H2O (1) and (NH4)4[H6(VIVO2)(VV2O5)2{O3P-C(O)(CH2-3-C5NH4)-PO3}2]·8H2O (2), each of which contains four vanadium(V) atoms and one vanadium(IV) atom. One unpaired electron on the central Kramers vanadium(IV) (S = 1/2) atom gives the molecule its magnetic moment, which is responsible for the reversal of its magnetization/spin at low temperatures. As such, the investigation of these complexes has involved a combination of experimental techniques, including superconducting quantum interference device (SQUID) magnetometry, electron paramagnetic resonance (EPR) spectroscopy, and a computational technique that used the CASSCF-based wave function theory and included relativistic effects by considering NEVPT2 for more accurate results. AC magnetic susceptibility measurements have revealed the single ion magnet (SIM) behaviors of both the complexes under the application of an external DC magnetic field, which were characterized by maxima in the plots of the “out-of-phase” magnetic susceptibility against the AC frequency (χ″ vs ν) at different temperatures. The spin relaxation time (τ) has been determined to be in the range of 2–10 K. From the fitting of the plot of relaxation time (τ) versus temperature to different models, we have tried to understand the type of slow relaxation process present in the system under a particular applied DC magnetic field. Finally, the ab initio method, viz. the CASSCF-based computational methods, has been employed to justify/rationalize and correlate the experimental results.