Impact of ligand chlorination and counterion tuning on high-field spin relaxation in a series of V(iv) complexes

Abstract

Methods of controlling spin coherence by molecular design are essential to efforts to develop molecular qubits for quantum information and sensing applications. In this manuscript, we perform the first studies of how arrangements of ^35/37Cl nuclear spins in the ligand shell and counterion selection affect the coherent spin dynamics of V(IV) complexes at a high magnetic field. We prepared eight derivatives of the vanadium triscatecholate complex with varying arrangements of ^35/37Cl substitution on the catechol backbone and R₃NH⁺ counterions (R = Et, n-Bu, n-Hex) and investigated these species via structural and spectroscopic methods. Hahn-echo pulsed electron paramagnetic resonance (EPR) experiments at high-frequency (120 GHz) and field (ca. 4.4 T) were used to extract the phase-memory relaxation time (T_m) and spin-lattice relaxation (T₁) times of the series of complexes. We found T_m values ranging from 4.8 to 1.1 μs in the temperature range of 5 to 40 K, varying by approximately 20% as a function of substitutional pattern. In-depth analysis of the results herein and comparison with related studies of brominated analogues disproves multiple hypothesized mechanisms for T_m control. Ultimately, we propose that more specific properties of the halogen atoms, e.g. the chemical shift, V⋯Cl hyperfine coupling, and quadrupolar coupling, could be contributing to the V(IV) T_m time.