Dinitrogen activation promoted by medium-valent vanadium. Strategies for the preparation of complexes containing medium-valent vanadium supported by sulfur-containing ligands: A synthetic and theoretical study.

Abstract

$\rm \{\lbrack (TMS)NCH\sb2CH\sb2\rbrack\sb2N(TMS)\}\sb2V\sb2(\mu$-Cl)$\sb2$ was prepared by reaction of $\rm VCl\sb3(THF)\sb3$ with the corresponding amide, and then reacted with sulfur to yield what it is thought to be $\rm\{\lbrack (TMS)NCH\sb2CH\sb2\rbrack\sb2N(TMS)\}\sb2V\sb2(\mu$-S)$\sb2.$ X-ray crystallographic analysis was not possible due to the poor quality of the crystals. However reaction of $\rm\lbrack (Me\sb3Si)\sb2N\rbrack\sb2VCI(THF)$ with S$\sb8$ yielded crystals of $\rm\{\lbrack (Me\sb3Si\sb2N\rbrack\sb2V\}\sb2(\mu$-S)$\sb2\}.$ On the other hand reaction of $\rm VCl\sb2(TMEDA)\sb2$ with S$\sb8$ led to the formation of $\rm VCl\sb3(TMEDA)(THF)$ and $\rm V\sb3S\sb7Cl(TMEDA)\sb3$ which precipitated as a black powder. Reaction of $\rm V\sb3S\sb7Cl(TMEDA)\sb3$ with mercaptopyridine led to the formation of $\rm\lbrack V\sb3(\mu\sb3$-Cl)-($\mu$-$\rm S\sb2)\sb3\rbrack (Psy)\sb3Li(THF)\sb4$ meanwhile the reaction of $\rm Me\sb3Si\sb2NLi$ yielded a high valent tetrameric structure $\rm \{\lbrack (TMS)\sb2N\rbrack VS\sb3Li(THF)\sb2Li\}\sb4$ complex. Alternative sources of sulfur atoms such as diphenyl disulfide was also used. In that particular case the reaction yielded a dimer which the X-ray structure confirmed as $\rm (TMEDA)\sb2V\sb2Cl\sb2(\mu$-SPh)$\sb2(\mu$-Cl). Ab initio and extended Huckel calculations were carried out on several clusters in order to understand the electronic structures of the complexes and the interactions between the metal centers. Calculations on a vanadium-sulfur cubane cluster revealed the presence of an intricate "web" of molecular orbitals showing electronic communication between all four metallic centers.