Spectroscopic and Potentiometric Characterization of Oxovanadium(IV) Complexes Formed by 3-Hydroxy-4-Pyridinones. Rationalization of the Influence of Basicity and Electronic Structure of the Ligand on the Properties of V^IVO Species in Aqueous Solution

Abstract

Aqueous solution studies regarding the identification and characterization of complexes formed by the VIVO ion and 11 3-hydroxy-4-pyridinone derivatives have been performed using EPR and UV/vis spectroscopic techniques. For the three ligands (HL) adequately soluble in water (1-methyl-3-hydroxy-4-pyridinone, 1-methyl-2-ethyl-3-hydroxy-4-pyridinone, and 1,2-diethyl-3-hydroxy-4-pyridinone), potentiometric titrations were performed; the results are consistent with the formation of [V(IV)OL]+, [V(IV)OL2], [V(IV)OL2H(-1)]-, [(V(IV)O)2L2H(-2)], and [V(IV)L3]+ species. Bis chelated complexes are characterized by a cis-trans isomerism, the trans isomer being strongly favored with respect to the cis arrangement. Tris chelated non-oxo V(IV) species were prepared in CH3COOH; their spectroscopic features point to a d(z2) ground state and a geometry intermediate between an octahedron and a trigonal prism, related to the steric requirements of the substituent on the carbon atom in position 2 of the pyridinone ring. Four new solid derivatives, [V(IV)O(1,2-diethyl-3-hydroxy-4-pyridinonato)2], [V(IV)O(1-(p-tolyl)-2-ethyl-3-hydroxy-4-pyridinonato)2], [V(IV)O(1-(p-(n-butyl)phenyl)-2-ethyl-3-hydroxy-4-pyridinonato)2], and [V(IV)O(1-(p-(n-hexyl)phenyl)-2-ethyl-3-hydroxy-4-pyridinonato)2], were isolated and characterized; they exhibited a five-coordinate geometry close to square-pyramid. A criterion for establishing the degree of distortion toward the trigonal-bipyramid on the basis of the electronic absorption spectra is provided. Relationships between the pKa of the -OH group in position 3 of the ring and (i) log K of mono and bis chelated complexes, (ii) pK of the water molecule in cis-[V(IV)OL2(H2O)], (iii) log K of tris chelated species [V(IV)L3]+, and (iv) 51V hyperfine coupling constant (Az) have been established and discussed for a number of pyrone, pyridinone, and catechol ligands. The results are rationalized by assuming for pyridinones an electronic structure intermediate between that of pyrones and catechols. The relationships are valuable to the understanding of the behavior of VIVO species in aqueous solution.

Synthesis of Schiff Base Metal Complexes: A Concise Review

“…, were isolated and characterized by Garribba et al 73 Aqueous solution studies regarding the identification and characterization of complexes formed by the V(IV)O ion and 3-hydroxyl-4-pyridone derivatives have been performed using EPR and UV-Vis spectroscopic techniques. Garribba et al 74 have recently reported 2-pyrazinecarboxylic acid and three of its derivatives (5-methyl-2-pyrazinecarboxylic, 2,3-pyrazinedicarboxylic and 5-hydroxypyrazinecarboxylic acids) coordinate the V(IV)O ion forming VOL, VOL2 and (VO)2L2H2 species in acidic and neutral solutions.…”

A novel VIVO–pyrimidinone complex: synthesis, solution speciation and human serum protein binding

“…† For 3-hydroxy-4-pyridinone derivatives there is some consensus in the literature that the protonation of the HL species takes place mainly on the N atom of the ring and the pK a1 values obtained, in the range 3.5-3.8, are several orders of magnitude higher than those for 3-hydroxy-4-pyrones. 28,35 This may be explained considering that in the H 2 L + form of the compounds, the N atom is more efficient in delocalizing the positive charge in the ring than the O atom. Accordingly, for the H 2 L + form of the pyrimidinone mhcpe the most stable form (DFT-2 in Scheme 3) corresponds to the tautomeric form where both N-atoms are protonated.…”

“…Accordingly, for the H 2 L + form of the pyrimidinone mhcpe the most stable form (DFT-2 in Scheme 3) corresponds to the tautomeric form where both N-atoms are protonated. The tautomeric form 28,35 show that pyridinones are stronger bases than pyrones. This has been attributed to the delocalization of the partial negative charge to the O atom in position 4 of the ring.…”

“…This has been attributed to the delocalization of the partial negative charge to the O atom in position 4 of the ring. 28,35 For the HL species of mhcpe the most stable tautomer corresponds to the one characterized by X-ray diffraction. The tautomer with N(1) deprotonated and atom N(2) protonated is less stable by 4.6 kcal mol −1 (DFT-6), and the tautomer with both N atoms deprotonated is less favourable by 8.4 kcal mol −1 (DFT-5).…”

Preface: Recent Advances and Highlights of the 5th International Vanadium Symposium

“…Coordination complexes and some of their applications continue to be reported by many groups, and include salen-type complexes (e.g. 19,66,[171][172][173][174][175][176], pyrone, pyridinone and pyrimidinone complexes (177)(178)(179), several types of Schiff bases and related complexes (66, [180][181][182], picolinato (e.g. 183), quinoline derivatives (184) dipicolinato complexes (185), compounds involving ligands such as amino acids or derivatives (28,(186)(187)(188)(189)(190)(191)(192)(193)(194)(195)(196)(197), hydroxycarboxylates (198,199), but novel classes of coordination compounds and other new reports on previously known compounds are too many to name.…”