Vanadium-salen and -salan complexes: Characterization and application in oxygen-transfer reactions

Abstract

Salen complexes are a versatile and standard system in oxidation catalysis. Their reduced derivatives, called salan, share their versatility but are still widely unexplored. We report the synthesis of a group of new vanadium-salen and -salan complexes, their characterization and application in the oxidation of simple organic molecules with H 2 O 2 . The ligands are derived from pyridoxal and chiral diamines (1,2-diaminocyclohexane and 1,2-diphenylethylenediamine) and were easily obtained in high yields. The V IV complexes were prepared and characterized in the solid state (Fourier transform infrared, FTIR, and magnetic properties) and in solution by spectroscopic techniques: UV-vis, circular dichroism (CD), electron paramagnetic resonance (EPR), and 51 V NMR, which provide information on the coordination geometry. Single crystals suitable for X-ray diffraction studies were obtained from solutions containing the V IV -pyr(S,S-chan) complex: [V V O{pyr(S,Schen)}] 2 (μ-O) 2 ؒ2(CH 3 ) 2 NCHO, where the ligand is the "half" Schiff base formed by pyridoxal and 1S,2S-diaminocyclohexane. The dinuclear species shows a OV V (μ-O) 2 V V O unit with tridentate ligands and two μ-oxo bridges. The V IV complexes of the salan-type ligands oxidize in organic solvents to a V V species, and the process was studied by spectroscopic techniques. The complexes were tested as catalysts in the oxidation of styrene, cyclohexene, and cumene with H 2 O 2 as oxidant. Overall, the V-salan complexes show higher activity than the parent V-salen complexes and are an alternative ligand system for oxidation catalysis.

Salophen and salen oxo vanadium complexes as catalysts of sulfides oxidation with H2O2: Mechanistic insights

“…The variation of the coordination sphere was designed in order to highlight correlation between the electronic distribution on the metal center with the reactivity in oxidation reactions, similar approach has been recently used with vanadium containing strongly related salen and salan complexes [24].…”

Thirty years through vanadium chemistry

“…Taking into account their much higher intrinsic stability and much lower susceptibility toward hydrolysis, several publications were released either with characterization of metal complexes [153,[160][161][162][163][164] or using these very versatile ligands to prepare suitable complexes to prepare metal-based catalytic systems [163,[165][166][167][168][169][170][171][172].…”

“…My group has been mostly involved in the development of homogeneous and supported vanadium systems, mostly for catalytic epoxidations [165,167] and oxidations, namely of alkanes and alkenes [155,138,166,222,[253][254][255][256][257], of sulfides (sulfoxidations) [155,165,169,258,259,260], brominations [223,260,261], oxidative desulfurization [262], oxidative aminations [264] and hydroaminations [264]. Some of the processes involve homogeneous systems, others have addressed complexes supported in polystyrene [138,253,254,263] or encapsulated in zeolites [155,256].…”

Pyridoxal Based Fluorescent Chemosensor for Detection of Copper(II) in Solution With Moderate Selectivity and Live Cell Imaging

“…Pyridoxal moiety imparts a physiological feasibility as it is the biologically active form of vitamin B 6 , pyridoxal 5-phosphate (PLP) and a versatile enzyme cofactor responsible for amino acid metabolism in organisms ranging from bacteria to human . Several metal complexes of Schiff bases derived from pyridoxal and amino acids/polyamines have been reported [51][52][53][54][55][56][57][58][59][60][61][62]. Recently, a new Schiff base was generated by condensing equimolar pyridoxal with 2-aminoethyl pyridine and its vanadium and copper complexes were prepared [63,64].…”