Counterion influence on dynamic spin properties in a V(iv) complex

Abstract

Using transition metal ions for spin-based applications, such as electron paramagnetic resonance imaging (EPRI) or quantum computation, requires a clear understanding of how local chemistry influences spin properties. Herein we report a series of four ionic complexes to provide the first systematic study of one aspect of local chemistry on the V(IV) spin – the counterion. To do so, the four complexes (Et₃NH)₂[V(C₆H₄O₂)₃] (1), (n-Bu₃NH)₂[V(C₆H₄O₂)₃] (2), (n-Hex₃NH)₂[V(C₆H₄O₂)₃] (3), and (n-Oct₃NH)₂[V(C₆H₄O₂)₃] (4) were probed by EPR spectroscopy in solid state and solution. Room temperature, solution X-band (ca. 9.8 GHz) continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy revealed an increasing linewidth with larger cations, likely a counterion-controlled tumbling in solution via ion pairing. In the solid state, variable-temperature (5–180 K) X-band (ca. 9.4 GHz) pulsed EPR studies of 1–4 in o-terphenyl glass demonstrated no effect on spin–lattice relaxation times (T₁), indicating little role for the counterion on this parameter. However, the phase memory time (T_m) of 1 below 100 K is markedly smaller than those of 2–4. This result is counterintuitive, as 2–4 are relatively richer in ¹H nuclear spin, hence, expected to have shorter T_m. Thus, these data suggest an important role for counterion methyl groups on T_m, and moreover provide the first instance of a lengthening T_m with increasing nuclear spin quantity on a molecule.