Thermodynamic and kinetic investigation of oxygen atom transfer to phosphines, metal phosphido and vanadium(III) complexes: Mechanistic differences and effect of axial base coordination on metal chalcogen bonds

As we try to find ways to oxidize atmospheric nitrogen, it becomes important to us to understand oxygen atom transfer (OAT) reactions. We began with understanding OAT to different types of tri substituted phosphines (PR3, R=alkyl, aryl). We used mesityl nitrile oxide (MesCNO) as OAT reagent. We explored thermodynamic, kinetic, mechanistic pathways of OAT and figured out the differences in P=O bond strength as well as the effect of different substitutions (R groups in PR3) on the rate of OAT. With the help of computational analyses, we found that the is initial interaction between phosphorous and carbon of MesCNO. We wanted to explore the C=O bond strengths in NHC carbenes as they are electronically similar to phosphines by performing OAT from MesCNO. Instead of OAT, we made NHC-MesCNO adducts where the carbine carbon and the carbon of CNO in MesCNO were connected. This further solidified our theory of previously proposed phosphine-MesCNO intermediate through R3P˙˙˙˙C(Mes)NO interaction. These adducts showed higher rate of OAT compared to MesCNO itself. That encouraged us to explored the chemistry of these adducts. Not only by MesCNO, we also explored adducts formed between MesCNO and several isocyanates (RNCO, R=Ph, adamentyl, 2,6-di (trifluoro methyl) phenyl). Isocyanate adducts showed higher dipolemoment compared to MesCNO adducts. We, then moved on from OAT to non-metal such as phosphines to early transition metal complex such as vanadium tris anilide. In this instance we used several OAT reagents containing N-O bonds with different bond strengths. They showed significant differences in mechanism in OAT. Unfortunately, VO bond strength too great to transfer the oxygen from vanadium. We used another vanadium complex, V[(Me3SiNCH 2CH2)3N] with axial amine ligand and observed that VO bond strength decreases due to the presence of the axial amine ligand. Adducts formed between Ad-NC (Ad=adamantyl) and two vanadium complexes mentioned previously showed the same type of bond weakening due to the axial ligand. We tried to react vanadium tris anilide with molecular oxygen as it seemed thermodynamically favored to make vanadium oxide due to strong VO bond and expected to form high energy oxygen atom that could be used as effective OAT reagent. To our surprise they made peroxy vanadium complex. This complex was rather interesting because it has the potential to do further OAT. In this work we have explored significant aspects of OAT in depth which exposes valuable insights in catalytic and organometallic chemistry.