Magnetic and relaxation properties of vanadium(iv) complexes: an integrated ¹H relaxometric, EPR and computational study

We report a detailed study of the magnetic and relaxation properties of a series of oxovanadium(iv) complexes comprising the aqua ion [VO(H₂O)₅]²⁺ and [VO(ox)₂]²⁻ (ox = oxalate), [VO(nta)]⁻ (nta = nitrilotriacetate), [VO(dtpa)]³⁻ (dtpa = diethylenetriaminepentaacetate) and [VO(acac)₂] (acac = acetylacetonato) in solution. The complexes were characterized using continuous wave (X-band) and pulsed (Q-band) EPR measurements and ¹H nuclear magnetic relaxation dispersion (NMRD) studies in the 0.01-120 MHz ¹H Larmor frequency range. The ⁵¹V A-tensor parameters obtained from the analysis of EPR spectra are in good agreement with those obtained using theoretical calculations at the DFT and coupled-cluster levels (DLPNO-CCSD), while g-tensors were obtained with CASSCF/NEVPT2 calculations. EPR measurements reveal significant differences in the electronic T^e₁ and T^e_m relaxation times, with [VO(acac)₂] showing a markedly different behaviour due to the trans coordination geometry. The NMRD profiles measured at different temperatures have contributions from both the outer- and inner-sphere mechanisms, with the latter showing contributions from the dipolar and scalar mechanisms. The rotational correlation times (τ_R) obtained from the fitting of NMRD and EPR data are in good mutual agreement. The scalar mechanism depends on the hyperfine coupling constants of the coordinated water molecule ^Ha_iso, which were obtained from the fitting of the NMRD profiles and DFT calculations. Finally, the analysis of the data provided information on the exchange rate of coordinated water molecules, which display mean residence times of ∼7-17 μs at 298 K.