Influence of Electronic Spin and Spin–Orbit Coupling on Decoherence in Mononuclear Transition Metal Complexes

Enabling the rational synthesis of molecular candidates for quantum information processing requires design principles that minimize electron spin decoherence. Here we report a systematic investigation of decoherence via the synthesis of two series of paramagnetic co­or­din­ation complexes. These complexes, [M­(C₂O₄)₃]^3– (M = Ru, Cr, Fe) and [M­(CN)₆]^3– (M = Fe, Ru, Os), were prepared and interrogated by pulsed electron paramagnetic resonance (EPR) spectroscopy to assess quantitatively the influence of the magnitude of spin (<i>S</i> = ¹/₂, ³/₂, ⁵/₂) and spin–orbit coupling (ζ = 464, 880, 3100 cm^–1) on quantum decoherence. Coherence times (<i>T</i>₂) were collected via Hahn echo experiments and revealed a small dependence on the two variables studied, demonstrating that the magnitudes of spin and spin–orbit coupling are not the primary drivers of electron spin decoherence. On the basis of these conclusions, a proof-of-concept molecule, [Ru­(C₂O₄)₃]³⁻, was selected for further study. The two parameters establishing the viability of a qubit are a long coherence time, <i>T</i>₂, and the presence of Rabi oscillations. The complex [Ru­(C₂O₄)₃]^3– exhibits both a coherence time of <i>T</i>₂ = 3.4 μs and the rarely observed Rabi oscillations. These two features establish [Ru­(C₂O₄)₃]^3– as a molecular qubit candidate and mark the viability of coordination complexes as qubit platforms. Our results illustrate that the design of qubit candidates can be achieved with a wide range of paramagnetic ions and spin states while preserving a long-lived coherence