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 Phosphine

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• 作者：Partha Basu ; Brian W. Kail ; Charles G. Young
• 刊名：Inorganic Chemistry
• 出版年：2010
• 出版时间：June 7, 2010
• 年：2010
• 卷：49
• 期：11
• 页码：4895-4900
• 全文大小：714K
• 年卷期：v.49,no.11(June 7, 2010)
• ISSN：1520-510X

文摘

The oxygen atom transfer reactivity of the dioxo-Mo(VI) complex, Tp^iPrMoO₂(OPh) (Tp^iPr = hydrotris(3-isopropylpyrazol-1-yl)borate), with a range of tertiary phosphines (PMe₃, PMe₂Ph, PEt₃, PBuⁿ₃, PEt₂Ph, PEtPh₂, and PMePh₂) 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, namely, Tp^iPrMoO₂(OPh) + PR₃ → Tp^iPrMoO(OPh)(OPR₃). The calculated free energy of activation for the formation of the OPMe₃ intermediate (Chem. Eur. J. 2006, 12, 7501) is in excellent agreement with the experimental ΔG value reported here. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide by acetonitrile, Tp^iPrMoO(OPh)(OPR₃) + MeCN → Tp^iPrMoO(OPh)(MeCN) + OPR₃, 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^iPrMoO(OPh)(OPMe₃) (ΔH = 56.3 kJ mol⁻¹; ΔS = −125.9 J mol⁻¹ K⁻¹; ΔG = 93.8 kJ mol⁻¹) and Tp^iPrMoO(OPh)(OPEtPh₂) (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^iPrMoO(OPh)(OPEt₃) (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^iPrMoO(OPh)(OPMe₃)]⁺, 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.