Delineating the Effects of Counterions on the Structural and Vibrational Properties of U(IV) Lindqvist Polyoxometalate Complexes

Molecular spin qubits based on molecules that feature accessible atomic clock transitions have demonstrated immense potential in quantum information science research, and exemplary in this regard is the holmium polyoxometalate, [Na9Ho(W5O18)2]•35H2O (HoW10). The coherence time of this molecule is limited by spin-phonon coupling driven decoherence processes, and one route to overcome this limitation is to increase the magnetic anisotropy of the metal included within the polyoxometalate (POM) complex. Herein we conducted a full investigation into the fundamental structural and vibrational properties of Lindqvist POMs that include uranium (IV), which also feature MJ = ± 4 ground states, similar to Ho(III) in HoW10. Based on recent results from our group that demonstrated the importance of the secondary lattice elements in tuning the distortion of the D4d symmetry in W10 POM complexes, we synthesized eight UW10 complexes with different alkali metal counterions and evaluated how the composition and packing of counterion species affected complex structural and vibrational properties Single crystal X-ray diffraction analysis on complexes 1-8 revealed changes in structural distortion parameters, i.e., skew angle, plane angle, and plane distance, as a function of differences in counterion configurations. Far-infrared and Raman spectra for 1-8 also demonstrated that vibrational mode frequencies [ν(WO5)2, ρ(UO8), ν/ρ(UO8), δ/ρ(UO4), POM deformation mode, δ(W-O-W/W=O/U-O-W), ν(W-O-W), and ν(U-O-W)] were sensitive to changes in counterion composition and packing. To more effectively compare different counterions configuration we developed counterion effective ionic radius (eIR) as a new structural parameter and comparisons between structural distortion parameters and counterion eIR strongly suggest modulation by the secondary lattice elements can affect structural and vibrational manifolds within POM complexes. Partial Least Squares (PLS) analysis was used to quantitatively evaluate correlations observed within this investigation, and PLS statistical models showed strong correlation between counterion eIR and both structural distortion parameters and vibrational mode frequencies. Overall, this investigation illustrates how to diversify the composition of lattice elements within UW10 complexes and confirms the integral role counterions play in modulating the structural and vibrational characteristics of Lindqvist POM complexes with f-elements.