Qubit Control Limited by Spin–Lattice Relaxation in a Nuclear Spin-Free Iron(III) Complex

High-spin transition metal complexes are of interest as candidates for quantum information processing owing to the tunability of the pairs of MS levels for use as quantum bits (qubits). Thus, the design of high-spin systems that afford qubits with stable superposition states is of primary importance. Nuclear spins are a potent instigator of superposition instability; thus, we probed the Ph4P+ salt of the nuclear spin-free complex [Fe­(C5O5)3]3– (1) to see if long-lived superpositions were possible in such a system. Continuous-wave and pulsed electron paramagnetic resonance (EPR) spectroscopic measurements reveal a strong EPR transition at X-band that can be utilized as a qubit. However, at 5 K the coherent lifetime, T2, for this resonance is 721(3) ns and decreases rapidly with increasing temperature. Simultaneously, the spin–lattice relaxation time is extremely short, 11.33(1) μs, at 5 K, and also rapidly decreases with increasing temperature. The coincidence of these two temperature-dependent data sets suggests that T2 in 1 is strongly limited by the short T1. Importantly, these results highlight the need for new design parameters in pursuit of high-spin species with appreciable coherence times.