Kinetics of oxygen-atom transfer reactions involving molybdenum dithiolene complexes (original) (raw)
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Journal of The American Chemical Society, 1994
The complexes, [Bu4Nl2[Mov102(mnt)2] ( l ) , [B~4N]~[Mo~~O(mnt)2] (2), and [Ph3PNPPh3] [EtdN] [MoV-OCl(mnt)z] ( 3 ) (mnt2-= 1,2-dicyanoethylenedithiolate) have been synthesized as possible models for active sites of sulfite oxidase which is proposed to contain molybdenum cofactor with dithiolene coordination around molybdenum. The structure of the [Bu$]+ salt of complex anion of 1 has been determined by X-ray crystallography. The compound crystallizes in space group P21/c, with a = 14.200(3) A, b = 19.402(4) -4, c = 18.967(3) A, / 3 = 95.48(1)', and Z = 4. [MovIO2(mnt)2]2-is a distorted octahedron with the oxo groups cis to each other and trans to the dithiolene sulfur atoms. The complexes 1-3 have been characterized by IR, UV-visible, 13C NMR, and negative ion FAB mass spectra. Complex 1 shows a quasireversible reduction and proton coupled electron transfer reaction. Complex 2 undergoes an one-electron reversible oxidation; but on the coulometric time scale it disproportionates to a tris dithiolene complex, [MoIv(mnt)3]2-and Moo3. Complex 2 in the presence of Cl-is oxidized irreversibly with the appearance of a new quasireversible couple corresponding to the electrochemical detection of [M0~OCl(mnt)2]~-/ [M0~~OCl(mnt)2]3-. The EPR parameters of 3 and [MovO(mnt)211-are reported. The 35,37C1 superhyperfine splitting of the chloro complex 3 is shown in relevance to Mo-Cl interaction in native sulfite oxidase. Complex 1 oxidizes HS03-to HS04-with the formation of 2 and without forming the biologically irrelevant fi-oxo Mo(V) dimer. This reaction follows enzymatic substrate saturation kinetics with apparent K M (Michaelis-Menten constant) = O.OlO(*O.OOl) M and k2 (kob at substrate saturation concentration and is proportional to V,,,) = 0.87(*0.04) s-1 in MeCN/H20 (2) Rajagopalan, K. V. In Molybdenum and Molybdenum Containing Enzymes; Coughlan, M., Ed.; Pergamon Press: Oxford, 1980. 0002-7863/94/1516-9061$04.50/0 terminal oxo group appears to be present in the Mo(V) and Mo(1V) stateskvbof the enzyme. The hydroxo ligand of the Mo(V) species is suggested to be cis to the terminal oxo group as shown by 1 H superhyperfine splitting.4 No oxomolybdoenzyme has yet
Journal of The American Chemical Society, 1996
The complex [Et 4 N] 2 [W VI O 2 (mnt) 2 ] (1), [Et 4 N] 2 [W IV O(mnt) 2 ] (2), and [Et 4 N] 2 [W VI O(S 2 )(mnt) 2 ] (3) (mnt 2-) 1,2-dicyanoethylenedithiolate) have been synthesized as possible models for the tungsten cofactor of inactive red tungsten protein (RTP) and the active aldehyde ferredoxin oxidoreductase (AOR) of the hyperthermophilic archaeon Pyrococcus furiosus. The [Ph 4 P] + salt of the complex anion of 1‚2H 2 O crystallizes in space group Pbcn, with a ) 20.526(3) Å, b ) 15.791(3) Å, c ) 17.641(3) Å, and Z ) 4. The W VI O 2 S 4 core of [Ph 4 P] 2 [W VI O 2 (mnt) 2 ]‚2H 2 O has distorted octahedral geometry with cis dioxo groups. 2 crystallizes in space group P2 1 2 1 2, with a ) 14.78(3) Å, b
Journal of the American Chemical Society, 1988
before, most recently in those of the respective tetramethylammonium salts. 15 Crystal structure determinations of the remaining complexes of the system, with 5 to 8 mol of hydrogen fluoride per mol of drogen fluorides), too. Anions H4F5and, in one instance each, HsN.4HF, H5F6-and H7Fsare already established species in the solid ~t a t e .~J~ Supplementary Material Available: Listing of anisotropic thermal parameters for the non-hydrogen atoms (2 pages); structure factor tables (8 pages). Ordering information is given on any current masthead page. pyridine, most probably these as pyridinium ply(hy-32001-55-1; C5H,N.2HF, 87979-78-0; C5H5N.3HF, 79162-49-5; C5-(15) Mootz, D.; Boenigk, D. Abstract: The kinetics and mechanism of the oxygen atom transfer reactions Mo02(L-NS2) + (RF)3P -MoO(L-NS2)(DMF) + (RF)3P0 (1) and MoO(L-NS2)(DMF) + XO -L Md2(L-NS2) + X, with X = (RF)2S0 (2) and 3-fluoropyridine N-oxide (3), have been investigated in DMF solutions (L-NS2 = 2,6-bis(2,2-diphenyl-2-mercaptoethyl)pyridine(2-), RF = p-C,H4F). The following rate constants (297.5 K) and activation parameters were obtained: reaction 1, k2 = 9.7 (4) X M-I s-l, AH* = 11.7 (6) kcal/mol, AS* = -28.4 (1.6) eu; reaction 2, kl = 14.0 (7) X IO4 s-', AH* = 22.1 (1.3) kcal/mol, AS* = 2.6 (1.6) eu; reaction 3, kl = 16.0 (8) X lo4 s-l, AH* = 23.4 (1.4) kcal/mol, AS* = 7.2 (2.0) eu. Reactions 2 and 3 exhibit saturation kinetics, under which the rate-determining step is intramolecular atom transfer. Mechanisms and transition states are proposed. The activation parameters are the first measured for oxo transfer from substrate; the small activation entropies suggest a transition state structurally similar to the complex MoO(L-NS2)(XO) formed in a labile equilibrium prior to oxo transfer to Mo. Coupling of reaction 1 with reaction 2 or 3 affords the catalytic reaction 4, ( R F )~P + XO -(RF),PO +
Inorganic chemistry, 2010
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, viz. 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 ΔG ‡ value reported here. The second step of the reaction, i.e., 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 ) (ΔH ‡ = 56.3 kJ mol −1 ; ΔS ‡ = −125.9 J mol −1 K −1 ; ΔG ‡ = 93.8 kJ mol −1 ) and Tp iPr 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 iPr 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 iPr 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.