A spectroscopic study on the coordination and solution structures of the interaction systems between biperoxidovanadate complexes and the pyrazolylpyridine-like ligands
In order to understand the substitution effects of pyrazolylpyridine (pzpy) on the coordination reaction equilibria, the interactions between a series of pzpy-like ligands and biperoxidovanadate ([OV(O2)2(D2O)]−/[OV(O2)2(HOD)]−, abbrv. bpV) have been explored using a combination of multinuclear (1H, 13C, and 51V) magnetic resonance, heteronuclear single quantum coherence (HSQC), and variable temperature NMR in a 0.15 mol L−1 NaCl D2O solution that mimics the physiological conditions. Both the direct NMR data and the equilibrium constants are reported for the first time. A series of new hepta-coordinated peroxidovanadate species [OV(O2)2L]− (L = pzpy-like chelating ligands) are formed due to several competitive coordination interactions. According to the equilibrium constants for products between bpV and the pzpy-like ligands, the relative affinity of the ligands is found to be pzpy > 2-Ester-pzpy ≈ 2-Me-pzpy ≈ 2-Amide-pzpy > 2-Et-pzpy. In the interaction system between bpV and pzpy, a pair of isomers (Isomers A and B) are observed in aqueous solution, which are attributed to different types of coordination modes between the metal center and the ligands, while the crystal structure of NH4[OV(O2)2(pzpy)]·6H2O (CCDC 898554) has the same coordination structure as Isomer A (the main product for pzpy). For the N-substituted ligands, however, Isomer A or B type complexes can also be observed in solution but the molar ratios of the isomer are reversed (i.e., Isomer B type is the main product). These results demonstrate that when the N atom in the pyrazole ring has a substitution group, hydrogen bonding (from the H atom in the pyrazole ring), the steric effect (from alkyl) and the solvation effect (from the ester or amide group) can jointly affect the coordination reaction equilibrium.