Ligand field design enables quantum manipulation of spins in Ni2+ complexes

Abstract

Creating the next generation of quantum systems requires control and tunability, which are key features of molecules. To design these systems, one must consider the ground-state and excited-state manifolds. One class of systems with promise for 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 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 (D_calc = +1.15 cm⁻¹). High-field high-frequency electron paramagnetic resonance (EPR) data yield spin Hamiltonian parameters: g_x = 2.1018(15), g_x = 2.1079(15), g_x = 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, [Ni_0.01Zn_0.99(ttcn)₂](BF₄)₂ (1′) led to a slight increase in D to ∼0.9 cm⁻¹. The design criteria in minimizing D in 1via combined computational and experimental methods demonstrates a path forward for EPR and optical addressability of a general class of S = 1 spins.