Ligand Field Design Enables Quantum Manipulation of Spins in Ni2+ Complexes

Creating the next generation of quantum systems requires control and tunability, which are key features of molecules. One class of systems with promise for the subclass of quantum sensing applications which require water solubility are d8 Ni2+ ions in octahedral symmetry. Yet, most Ni2+complexes feature large zero-field splitting, precluding manipulation by commercial microwave sources due to the relatively large spin-orbit coupling constant of Ni2+ (630 cm-1). Since low lying excited states also influence axial zero-field splitting, D, a combination of strong field ligands and rigidly held octahedral symmetry can ameliorate these challenges. Towards these ends, we performed a theoretical and computational analysis of the electronic and magnetic structure of a candidate molecular qubit, focusing on the impact of ligand field strength on D. Based on those results, we synthesized 1, [Ni(ttcn)2](BF4)2 (ttcn = 1,4,7-trithiacyclononane), which we computationally predict will have a small D (Dcalc = +1.15 cm–1). High-field high-frequency electron paramagnetic resonance (EPR) data yield spin Hamiltonian parameters: gx = 2.1018(15), gx = 2.1079(15), gx = 2.0964(14), D = +0.555(8) cm–1 and E = +0.072(5) cm–1, which confirm the expected weak zero-field splitting. Dilution of 1 in the diamagnetic Zn analogue, [Ni0.01Zn0.99(ttcn)2](BF4)2 (1') led to a slight increase in D to ~0.9 cm–1. The design criteria in minimizing D in 1 via combined computational and experimental methods demonstrates a path forward for EPR and optical addressability of a general class of S = 1 spins.

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