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Quantitation of the ligand effect in oxo-transfer reactions of dioxo-Mo(VI) trispyrazolyl borate complexes.

ABSTRACT: The oxygen atom transfer reactivity (OAT) of dioxo-Mo(VI) complexes of hydrotrispyrazolyl borate (hydrotris(3,5-dimethylpyrazolyl)borate, Tp(Me2); hydrotris(3-isopropylpyrazol-1-yl)borate, Tp(iPr)) with tertiary phosphines (PMe(3), PMe(2)Ph, PEt(3), PEt(2)Ph, PBu(n)(3), PMePh(2), or PEtPh(2)) has been investigated. In acetonitrile, these reactions proceed via the formation of a phosphoryl intermediate complex that undergoes a solvolysis reaction. We report the synthesis and characterization of several phosphoryl complexes. The rates of formation of phosphoryl complexes and their solvation were determined by spectrophotometry. The rates of the reactions and the properties of the phosphoryl species were investigated using the Quantitative Analysis of Ligand Effect (QALE) methodology. The results show that, at least in this system, the first step of the reaction is controlled primarily by the steric factor, and in the second step, both electronic and steric factors are important. We also analyzed the effect of ligands on the reaction rate i.e., Tp(Me2)vs. Tp(iPr).

SUBMITTER: Basu P

PROVIDER: S-EPMC3566299 | biostudies-literature | 2013 Mar

REPOSITORIES: biostudies-literature

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Quantitation of the ligand effect in oxo-transfer reactions of dioxo-Mo(VI) trispyrazolyl borate complexes.

Basu Partha P Kail Brian W BW Adams Andrew K AK Nemykin Victor N VN

Dalton transactions (Cambridge, England : 2003) 20121204 9

The oxygen atom transfer reactivity (OAT) of dioxo-Mo(VI) complexes of hydrotrispyrazolyl borate (hydrotris(3,5-dimethylpyrazolyl)borate, Tp(Me2); hydrotris(3-isopropylpyrazol-1-yl)borate, Tp(iPr)) with tertiary phosphines (PMe(3), PMe(2)Ph, PEt(3), PEt(2)Ph, PBu(n)(3), PMePh(2), or PEtPh(2)) has been investigated. In acetonitrile, these reactions proceed via the formation of a phosphoryl intermediate complex that undergoes a solvolysis reaction. We report the synthesis and characterization of s ...[more]

PMID: 23212540

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Project description:The oxygen atom transfer reactivity of the dioxo-Mo(VI) complex, Tp(iPr)MoO(2)(OPh) (Tp(iPr) = 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, namely, Tp(iPr)MoO(2)(OPh) + PR(3) --> Tp(iPr)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 DeltaG() value reported here. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide by acetonitrile, Tp(iPr)MoO(OPh)(OPR(3)) + MeCN --> Tp(iPr)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(iPr)MoO(OPh)(OPMe(3)) (DeltaH(++) = 56.3 kJ mol(-1); DeltaS(++) = -125.9 J mol(-1) K(-1); DeltaG(++) = 93.8 kJ mol(-1)) and Tp(iPr)MoO(OPh)(OPEtPh(2)) (DeltaH(++) = 66.5 kJ mol(-1); DeltaS(++) = -67.6 J mol(-1) K(-1); DeltaG(++) = 86.7 kJ mol(-1)) by acetonitrile are indicative of I(a) mechanisms. In contrast, the corresponding parameters for the solvolysis reaction of Tp(iPr)MoO(OPh)(OPEt(3)) (DeltaH(++) = 95.8 kJ mol(-1); DeltaS(++) = 26.0 J mol(-1) K(-1); DeltaG(++) = 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(iPr)MoO(OPh)(OPMe(3))](+), by acetonitrile was calculated to be 1.9 x 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.

Dataset Information

Quantitation of the ligand effect in oxo-transfer reactions of dioxo-Mo(VI) trispyrazolyl borate complexes.

Publications

Quantitation of the ligand effect in oxo-transfer reactions of dioxo-Mo(VI) trispyrazolyl borate complexes.

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