Oxovanadium(IV)-amide binding. Synthetic, structural, and physical studies of {N-[2-(4-oxopent-2-en-2-ylamino)phenyl]pyridine-2-carboxamido}oxovanadium(IV) and {N-[2-(4-phenyl-4-oxobut-2-en-2-ylamino)phenyl]pyridine-2-carboxamido}oxovanadium(IV)