The Influence of the Oxygen Atom Acceptor on the Reaction Coordinate and Mechanism of Oxygen Atom Transfer From the Dioxo-Mo(VI) Complex, TpiPrMoO2(OPh), to Tertiary Phosphines

• Published in: Inorganic chemistry 2010-06, Vol.49 (11), p.4895-4900
• Main Authors: Basu, Partha, Kail, Brian W., Young, Charles G.
• Format: Article
• Language: eng ; jpn
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic902500h

The oxygen atom transfer reactivity of the dioxo-Mo(VI) complex, Tp i Pr MoO 2 (OPh) (Tp i Pr = hydrotris(3-isopropylpyrazol-1-yl)borate), with a range of tertiary phosphines (PMe 3 , PMe 2 Ph, PEt 3 , PBu n 3 , PEt 2 Ph, PEtPh 2 and PMePh 2 ) has been investigated. The first step in all the reactions follows a second-order rate law indicative of an associative transition state, consistent with nucleophilic attack by the phosphine on an oxo ligand, viz. Tp i Pr MoO 2 (OPh) + PR 3 → Tp i Pr MoO(OPh)(OPR 3 ). The calculated free energy of activation for the formation of the OPMe 3 intermediate ( Chem. Eur. J . 2006, 12 , 7501) is in excellent agreement with the experimental ΔG ‡ value reported here. The second step of the reaction, i.e., the exchange of the coordinated phosphine oxide by acetonitrile, Tp i Pr MoO(OPh)(OPR 3 ) + MeCN → Tp i Pr MoO(OPh)(MeCN) + OPR 3 , is first-order in starting complex in acetonitrile. The reaction occurs via a dissociative interchange (I d ) or associative interchange (I a ) mechanism, depending on the nature of the phosphine oxide. The activation parameters for the solvolysis of Tp i Pr MoO(OPh)(OPMe 3 ) (ΔH ‡ = 56.3 kJ mol −1 ; ΔS ‡ = −125.9 J mol −1 K −1 ; ΔG ‡ = 93.8 kJ mol −1 ) and Tp i Pr MoO(OPh)(OPEtPh 2 ) (ΔH ‡ = 66.5 kJ mol −1 ; ΔS ‡ = −67.6 J mol −1 K −1 ; ΔG ‡ = 86.7 kJ mol −1 ) by acetonitrile are indicative of I a mechanisms. In contrast, the corresponding parameters for the solvolysis reaction of Tp i Pr MoO(OPh)(OPEt 3 ) (ΔH ‡ = 95.8 kJ mol −1 ; ΔS ‡ = 26.0 J mol −1 K −1 ; ΔG ‡ = 88.1 kJ mol −1 ) and the remaining complexes by the same solvent are indicative of an I d mechanism. The equilibrium constant for the solvolysis of the oxo-Mo(V) phosphoryl complex, [Tp i Pr MoO(OPh)(OPMe 3 )] + , by acetonitrile was calculated to be 1.9 × 10 −6 . The oxo-Mo(V) phosphoryl complex is more stable than the acetonitrile analogue, whereas the oxo-Mo(IV) acetonitrile complex is more stable than the phosphoryl analogue. The higher stability of the Mo(V) phosphoryl complex may explain the phosphate inhibition of sulfite oxidase.