NMR Study On The Interactions Between Diperoxovanadates And Pyridine-like Ligands

Thirty years, for the interactions between peroxovanadates and organic ligandshave attracted great interest in both chemistry and biology. In this paper, multinuclear(¹H,¹³C and⁵¹V), magnetic resonance （COSY, HSQC and HMBC）, and variabletemperature NMR, together with theoretical calculations were used to study structuresof all species in interactions between [OV（O₂）₂（D₂O）]/[OV（O₂）₂（HOD）]（abbr. bpV）and pyridine-like ligands. The coordination mechanisms and rules were explored. Themain results are summarized as follows:1. To understand how the monodentate/bidentate pyridine-like ligands affect onthe reaction equilibrium, the interactions between bpV and monodentate/bidentatepyridine-like ligands in solution were explored using multinuclear (¹H,¹³C and⁵¹V)magnetic resonance （COSY, HSQC, HMBC） and variable temperature NMR in0.15mol/L NaCl ionic medium for mimicking the physiological condition. The reactivityamong these ligands is2,2’-bipyridine>4,4’-dicarboxylate-2,2’-bipyridine> pyridine> isonicotinate. The competitive coordination results in the formation ofsix-coordinated （pyridine or isonicotinate） or seven-coordinated （2,2’-bipyridine or4,4’-dicarboxylate-2,2’-bipyridine） species [OV（O₂）₂LL’]^n-（LL′=monodentate/bidentate pyridine-like ligands, n=1,2,3）. The experimental resultsindicated that the coordinating ability of bidentate ligands is stronger than that ofmonodentate ligands, and the coordinating ability of unsubstituted ligands is strongerthan that of corresponding carboxylate substituted ligands.2. To understand the substitution effects of3-methyl-pyridine derivatives on thereactions. A series of3-methyl-pyridine derivatives and bpV in solution wereexplored using multinuclear (¹H,¹³C and⁵¹V) magnetic resonance （COSY and HSQC）NMR in0.15mol/L NaCl ionic medium for mimicking the physiological conditions.The reactivity among these3-methyl-pyridine derivatives is3-picoline> nicotinate> nicotinamide> ethyl nicotinate. The competitive coordination results in the formationof new six-coordinated species [OV（O₂）₂L]^n-（L=3-methyl-pyridine derivatives, n=1,2）. The results of density functional calculations provide a reasonable explanationon the reactivity of the3-methyl-pyridine derivatives. Solvation effects play animportant role in these reactions.3. To understand the effects of4-（pyridin-2-yl）pyrimidine （abbr. pprd）substitution on reaction equilibrium, the interactions between a series of pprd-likeligands and bpV were explored using multinuclear (¹H,¹³C and⁵¹V) magneticresonance （COSY and HSQC） and variable temperature NMR in0.15mol/L NaClionic medium that mimicks the physiological condition. The reactivity among thesepprd-like ligands is4-（pyridin-2-yl）pyrimidine≈4-（pyridin-2-yl）pyrimidin-2-amine>2-methyl-4-（pyridin-2-yl）pyrimidine>2-ethyl-4-（pyridin-2-yl）pyrimidine>4-（6-methylpyridin-2-yl）pyrimidine. Competitive coordination results in theformation of new hepta-coordinated compounds [OV（O₂）₂L]（L=pprd-like ligands）.When the ligand is4-（pyridin-2-yl）pyrimidine, a pair of isomers （Isomer A and B, theformer is main product） have been observed, which is attributed to different types ofcoordination between vanadium atom and ligands in the species [OV（O₂）₂（pprd）].The single-crystal X-ray diffraction results indicate that NH4[OV（O₂）₂（pprd）]·2H₂O incrystal state has the same coordinated ways as Isomer A. However, in the othercrystal NH₄[OV（O₂）₂（2-NH₂-pprd）]·3H₂O, the coordinated way of organic ligand isthe same as its solution structure and hydrogen bonds play an important role in itscoordination procession. These experimental results indicate that when the aromaticring has one substituted group, both the steric effect （for alkyl） and the hydrogen bondforces （for amido） affect the reaction equilibrium; when the aromatic ring has twosubstituted groups, only the steric effect affects the reaction.