Probing the dynamic magnetic properties of [MVINX4]- complexes (M = Tc and Re, X = Cl and Br) in search of potential spin qubit candidates

Here we present the experimental and theoretical study of the square-pyramidal d¹ complexes NBu₄[MNX₄] (M = Re, Tc; X = Cl, Br), probing their potential as molecular spin qubits. Single-crystal X-ray diffraction confirms that all four adopt the tetragonal P4/n space group with well-isolated [MNX₄]⁻ units. Direct-current magnetic susceptibility is consistent with magnetically isolated S = ½ centres across the series, while alternating-current measurements reveal field-induced out-of-phase signals exclusively in NBu₄[ReNCl₄] (1), identifying it as the only possible spin qubit candidate in the family. Continuous-wave EPR at X-band and high-field frequencies established the g anisotropy and the large hyperfine coupling to the ¹⁸⁵,¹⁸⁷Re nuclei, while pulsed EPR at Q-band (35 GHz) probed the relaxation and coherence times of 1 from 5 to 25 K. Spin-phonon coupling calculations were performed to contrast the behaviour across 1–4, and are consistent with the slow relaxation observed only in 1.