Manifestation of stereoelectronic effects on the calculated carbon-hydrogen bond lengths and one-bond 1J(C-H) NMR coupling constants. Relative acceptor ability of the carbonyl (C=O), thiocarbonyl (C=S), and methylidene (C=CH2) groups toward C-H donor bonds

Theoretical examination [B3LYP/6-31G(d,p), PP/IGLO-III//B3LYP/6-31G(d,p), and NBO methods] of six-membered cyclohexane 1 and carbonyl-, thiocarbonyl-, or methylidene-containing derivatives 2-27 afforded precise structural (in particular, C-H bond distances) and spectroscopic (specifically, one-bond (1)J(C)(-)(H) NMR coupling constants) data that show the consequences of stereoelectronic hyperconjugative effects in these systems. Major observations include the following. (1) sigma(C)(-)(H)(ax)() -->(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() (Y = O, S, or CH(2)) hyperconjugation leads to a shortening (strengthening) of the equatorial C-H bonds adjacent to the pi group. This effect is reflected in smaller (1)J(C)(-)(H)(ax)() coupling constants relative to (1)J(C)(-)(H)(eq)(). (2) Comparison of the structural and spectroscopic consequences of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) hyperconjugation in cyclohexanone 2, thiocyclohexanone 3, and methylenecyclohexane 4 suggests a relative order of acceptor orbital ability C=S > C=O > C=CH(2), which is in line with available pK(a) data. (3) Analysis of the structural and spectroscopic data gathered for heterocyclic derivatives 5-12 reveals some additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y), pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)(), n(X) --> sigma(C)(-)(H)(ax)(), n(beta)(O) --> sigma(C)(-)(H)(eq)(), and sigma(S)(-)(C) --> sigma(C)(-)(H)(eq)() stereoelectronic effects that is, nevertheless, attenuated by saturation effects. (4) Modulation of the C=Y acceptor character of the exocyclic pigroup by conjugation with alpha-heteroatoms O, N, and S in lactones, lactams, and methylidenic analogues 13-24 results in decreased sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugation. (5) Additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugative effects is also apparent in 1,3-dicarbonyl derivative 25 (C=Y equal to C=O), 1,3-dithiocarbonyl derivative 26 (C=Y equal to C=S), and 1,3-dimethylidenic analogue 27 (C=Y equal to C=CH(2)).     

Ruthenium complexes with tridentate PNX (X = O, S) donor ligands

The ligands, PhPNXMe (1), PhPNXPh (2), and PhPNSMe (3), (PhPNX = 2-Ph2P-C6H4CH[double bond, length as m-dash]NC6H4X-2; X = O, S) have been prepared. A range of new ruthenium complexes were synthesised using these and related ligands, namely: [{RuCl(PhPNO)}2Cl] (4), [Ru(PhPNO)2] (5), [RuCl(PhPNXR)(PPh3)]BPh4 [X = O, R = Me (6); X = O, R = Ph (7); X = S, R = Me (8)], [{RuCl(PhPNX'R)}2Cl]X [X' = O, R = Me, X = Cl(-) (9); X' = S, R = Me, X = BPh4(-) or PF6(-) (10)], and [RuCl(PhPNO-eta 6C6H5)]BPh4 (11). The catalytic activity of these complexes with respect to the hydrosilyation of acetophenone and the hydrogenation of styrene has been investigated, giving an insight into the requirements for an active complex in these reactions.     

Reaction systems related to dissimilatory nitrate reductase: nitrate reduction mediated by bis(dithiolene)tungsten complexes

Kinetics of the oxygen atom transfer reactions [M(IV)(QC6H2-2,4,6-Pr(i)3)(S2C2Me2)2]1- + XO --> [M(VI)O(QC6H2-2,4,6-Pr(i)3)(S2C2Me2)2]1- + X in acetonitrile with substrates XO = NO3- and (CH2)4SO have been determined. The reactants are bis(dithiolene) complexes with M = Mo, W and sterically encumbered axial ligands with Q = O, S to stabilize mononuclear square pyramidal structures. The complex [MoIV(SC6H2-2,4,6-Pr(i)3)(S2C2Me2)2]1- is an analogue of the active site of dissimilatory nitrate reductase which in the reduced state contains a molybdenum atom bound by two pyranopterindithiolene ligands and a cysteinate residue. Nitrate reduction was studied with tungsten complexes because of unfavorable stability properties of the molybdenum complexes. Product nitrite was detected by a colorimetric method. All reactions with both substrates are second-order with associative transition states (deltaS approximately -20 eu). Variation of atoms M and Q, together with data from prior work, allows certain kinetics comparisons to be made. Among them, k2W/k2Mo = 25 for (CH2)4SO reduction (Q = S), an expression of the kinetic metal effect. Further, k2S/k2O = 28 and approximately 10(4) for nitrate and (CH2)4SO reduction, respectively, effects attributed to relatively more steric congestion in achieving the transition state with hindered phenolate vs thiolate ligands. The effect is more pronounced with the larger substrate. These results demonstrate the feasibility of tungsten-mediated nitrate reduction by direct atom transfer using molecules with both axial thiolate and phenolate ligands. Complexes of the type [M(IV)(OR)(S2C2Me2)2] are capable of reducing biological N-oxide, S-oxide, and nitrate substrates and thus constitute functional analogue reaction systems of enzymic transformations.     

Imidomolybdenum(IV) porphyrin complexes: synthesis, characterization, and intermetal imido transfer reactivity

The imido(meso-tetra-p-tolylporphyrinato)molybdenum(IV) complexes, (TTP)Mo=NR, where R = C6H5 (1a), p-CH3C6H4 (1b), 2,4,6-(CH3)3C6H2 (1c), and 2,6-(i-Pr)2C6H4 (1d), can be prepared by the reaction of (TTP)MoCl2 with 2 equiv of LiNHR in toluene. Upon treatment of the imido complexes with pyridine derivatives, NC5H4-p-X (X = CH3, CH(CH3)2, C[triple bond]N), new six-coordinate complexes, (TTP)Mo=NR.NC5H4-p-X, were observed. The reaction between the molybdenum imido complexes, (TTP)Mo=NC6H5 or (TTP)Mo=NC6H4CH3, and (TTP)Ti(eta2-PhC[triple bond]CPh) resulted in complete imido group transfer and two-electron redox of the metal centers to give (TTP)Mo(eta2-PhC[triple bond]CPh) and (TTP)Ti=NC6H5 or (TTP)Ti=NC6H4CH3.     

Comparative kinetics and mechanism of oxygen and sulfur atom transfer reactions mediated by bis(dithiolene) complexes of molybdenum and tungsten

Although the kinetics and mechanism of metal-mediated oxygen atom (oxo) transfer reactions have been examined in some detail, sulfur atom (sulfido) transfer reactions have not been similarly scrutinized. The reactions [M(IV)(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-) + Ph(3)AsQ --> [M(VI)Q(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-) + Ph(3)As (M = Mo, W; Q = O, S) with variable substituent X' have been investigated in acetonitrile in order to determine the relative rates of oxo versus sulfido transfer at constant structure (square pyramidal) of the atom acceptor and of atom transfer at constant structure of the atom donor and metal variability of the atom acceptor. All reactions exhibit second-order kinetics and entropies of activation (-25 to -45 eu) consistent with an associative transition state. At parity of atom acceptor, k(2)(S) (0.25-0.75 M(-1)s(-1)) > k(2)(O) (0.023-0.060 M(-1)s(-1)) with M = Mo and k(2)(S) (4.1-66.7 M(-1)s(-1)) > k(2)(O) (1.8-9.8 M(-1)s(-1)) with M = W. At constant atom donor and X', k(2)(W) > k(2)(Mo) with reactivity ratios k(2)(W)/k(2)(Mo) = 78-184 (Q = O) and 16-89 (Q = S). Rate constants refer to 298 K. At constant M and Q, rates increase in the order X' = Me less, similar OMe < H < Br < COMe < CN; increasing electron-withdrawing propensity accelerates reaction rates. The probable transition state involves significant Ph(3)AsQ...M bond-making (X' rate trend) and concomitant As-Q bond weakening (bond energy order As-O > As-S). Orders of oxo and sulfido donor ability of substrates and complexes are deduced on the basis of qualitative reactivity properties determined here and elsewhere. This work complements previous studies of the reaction systems [M(IV)(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-)/XO where the substrates are N-oxides and S-oxides and k(2)(W) > k(2)(Mo) at constant substrate also applies. The reaction order of substrates is Me(3)NO > (CH(2))(4)SO > Ph(3)AsS > Ph(3)AsO. This research provides the first quantitative information of metal-mediated sulfido transfer.     

Proton transfer to nickel-thiolate complexes. 2. Rate-limiting intramolecular proton transfer in the reactions of [Ni(SC(6)H(4)R-4)(PhP[CH(2)CH(2)PPh(2)](2))](+) (R = NO(2), Cl, H, Me, or MeO)

The protonation of [Ni(SC(6)H(4)R-4)(triphos)](+) (triphos = PhP[CH(2)CH(2)PPh(2)](2); R = NO(2), Cl, H, Me, or MeO) by [lutH](+) (lut = 2,6-dimethylpyridine) to form [Ni(S(H)C(6)H(4)R-4)(triphos)](2+) is an equilibrium reaction in MeCN. Kinetic studies, using stopped-flow spectrophotometry, reveal that the reactions occur by a two-step mechanism. Initially, [lutH](+) rapidly binds to the complex (K(2)(R)) in an interaction which probably involves hydrogen-bonding of the acid to the sulfur. Subsequent intramolecular proton transfer from [lutH](+) to sulfur (k(3)(R)) is slow because of both electronic and steric factors. The X-ray crystal structures of [Ni(SC(6)H(4)R-4)(triphos)](+) (R = NO(2), H, Me, or MeO) show that all are best described as square-planar complexes, with the phenyl substituents of the triphos ligand presenting an appreciable barrier to the approach of the sterically demanding [lutH](+) to the sulfur. The kinetic characteristics of the intramolecular proton transfer from [lutH](+) to sulfur have been investigated. The rate of intramolecular proton transfer exhibits a nonlinear dependence on Hammett sigma(+), with both electron-releasing and electron-withdrawing 4-R-substituents on the coordinated thiolate facilitating the rate of proton transfer (NO(2) > Cl > H > Me < MeO). The rate constants for intramolecular proton transfer correlate well with the calculated electron density of the sulfur. The temperature dependence of the rate of the intramolecular proton transfer reactions shows that deltaH() is small but increases as the 4-R-substituent becomes more electron-withdrawing [deltaH = 4.1 (MeO), 6.9 (Me), 11.4 kcal mol(-)(1) (NO(2))], while DeltaS() becomes progressively less negative [deltaS = -50.1 (MeO), -41.2 (Me), -16.4 (NO(2)) cal K(-)(1) mol(-)(1)]. Studies with [lutD](+) show that the rate of intramolecular proton transfer varies with the 4-R-substituent [(k(3)(NO)2)(H)/(k(3)(NO)2)(D) = 0.39; (k(3)(Cl))(H)/(k(3)(Cl))(D) = 0.88; (k(3)(Me))(H)/(k(3)(Me))(D) = 1.3; (k(3)(MeO))(H)/(k(3)(MeO))(D) = 1.2].     

Kinetic isotope effects in cycloreversion of rhenium (V) diolates

Cycloreversion of 4-methoxystyrene from the corresponding Tp'Re(O)(diolato) complex (Tp' = hydrido-tris-(3,5-dimethylpyrazolyl)borate) was measured competitively for various isotopomers at 103 degrees C. Primary ((12)C/(13)C) and secondary ((1)H/(2)H) kinetic isotope effects were determined. The primary KIEs were k(12C)/k(13C) = 1.041 +/- 0.005 at the alpha position and 1.013 +/- 0.006 at the beta position. Secondary KIEs were k(H)/k(D) = 1.076 +/- 0.005 at the alpha position and 1.017 +/- 0.005 at the beta position. Computational modeling (B3LYP/LACVP+) located a transition state for concerted cycloreversion of styrene from TpRe(O)(OCH(2)CHPh) exhibiting dramatically different C-O bond lengths. A Hammett study on cycloreversions of substituted styrenes from a series of Tp'Re(O)(diolato) showed dichotomous behavior for electron donors and electron-withdrawing groups as substituents: rho = -0.65 for electron donors, but rho = +1.13 for electron-withdrawing groups. The data are considered in light of various mechanistic proposals. While the extrusion of 4-methoxystyrene is concluded to be a highly asynchronous concerted reaction, the Hammett study reflects a likelihood that multiple reaction mechanisms are involved.     

Synthesis and structures of bis(dithiolene)-tungsten(IV) complexes related to the active sites of tungstoenzymes

Recent protein crystallographic results on tungsten enzymes and primary sequence relationships between certain molybdenum and tungsten enzymes provoke interest in the generalized bis(dithiolene) complexes [WIV(QR)(S2C2R'2)2]1- and [WVIO(QR)(S2C2R'2)2]1- (Q = O, S, Se) as minimal representations of enzyme sites. The existence and stability of W(IV) complexes have been explored by synthesis. Reaction of [W(CO)2(S2C2Me2)2] (1) with PhO- results in complete CO substitution to give [W(OPh)(S2C2Me2)2]1- (2). Reaction of 1 with PhQ- affords the monocarbonyls [W(CO)(QPh)(S2C2Me2)2]1- (Q = S (3), Se (5)). The use of sterically demanding 2,4,6-Pri3C6H2Q- also yields monocarbonyls, [W(CO)(QC6H2-2,4,6-Pri3)(S2C2Me2)2]1- (Q = S (4), Se (6)). The X-ray structures of square pyramidal 2 and trigonal prismatic 3-6 (with unidentate ligands cis) are described. The tendency to substitute one or both carbonyl ligands in 1 in the formation of [MIV(QAr)(S2C2Me2)2]1- and [MIV(CO)(QAr)(SeC2Me2)2]1- with M = Mo and W is related to the M-Q bond length and ligand steric demands. The results demonstrate a stronger binding of CO by W(IV) than Mo(IV), a behavior previously demonstrated by thermodynamic and kinetic features of zerovalent carbonyl complexes. Complexes 3-6 can be reversibly reduced to W(III) at approximately -1.5 V versus SCE. On the basis of the potential for 2(-2.07 V), monocarbonyl ligation stabilizes W(III) by approximately 500 mV. This work is part of a parallel investigation of the chemistry of bis(dithiolene)-molybdenum (Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263) and -tungsten complexes related to enzyme active sites.     

Coordinatively unsaturated W(IV)-bis(pyridine) complexes, a reactive source of W(IV)

Addition of 2 equiv of a sigma-donor ligand (L = pyridine, 4-picoline, or quinoline) to complexes of the type [W(NPh)(eta(4)-arene)(o-(Me3SiN)2C6H4)] (arene = CH3CH2C6H5 (3), CH3CH2CH2C6H5 (4)) gave the W(IV)L2 compounds, [W(NPh)(o-(Me3SiN)2C6H4)(C5H5N)2] (5), [W(NPh)(o-(Me3SiN)2C6H4)(p-C6H7N)2] (6), and [W(NPh)(o-(Me3SiN)2C6H4)(C9H7N)2] (7). Synthesis of compounds 5 and 6 by Na degrees reduction of [W(NPh)(o-(Me3SiN)2C6H4)Cl2] in the presence of 3 equiv of L (L = 5, pyridine or 6, 4-picoline) is also presented. Compounds 5, 6, and 7 display hindered rotation of the donor ligands about the W-N bonds, resulting from a steric interaction with the Me3Si groups of the diamide ligand. The coordinative unsaturation of 5 and 6 has also been explored. Compounds 5 and 6 readily react with either CO and PMe3 to generated the six coordinate complexes [W(NPh)(o-(Me3SiN)2C6H4)(C5H5N)2(CO)] (8a), [W(NPh)(o-(Me3SiN)2C6H4)(C6H7N)2(CO)] (8b), [W(NPh)(o-(Me3SiN)2C6H4)(C5H5N)(PMe3)2] (10a), and [W(NPh)(o-(Me3SiN)2C6H4)(C6H7N)(PMe3)2] (10b), respectively.     

Synthesis of the pivalamidate-bridged pentanuclear platinum(II,III) linear complexes with Pt...Pt interactions

Pentanuclear linear chain Pt(II,III) complexes [[Pt2(NH3)2X2((CH3)3CCONH)2(CH2COCH3)]2[PtX'4]].nCH3COCH3 (X = X' = Cl, n = 2 (1a), X = Cl, X' = Br, n = 1 (1b), X = Br, X' = Cl, n = 2 (1c), X = X' = Br, n = 1 (1d)) composed of a monomeric Pt(II) complex sandwiched by two amidate-bridged Pt dimers were synthesized from the reaction of the acetonyl dinuclear Pt(III) complexes having equatorial halide ligands [Pt2(NH3)2X2((CH3)3CCONH)2(CH2COCH3)]X' ' (X = Cl (2a), Br (2b), X' ' = NO3-, CH3C6H4SO3-, BF4-, PF6-, ClO4-), with K2[PtX'4] (X' = Cl, Br). The X-ray structures of 1a-1d show that the complexes have metal-metal bonded linear Pt5 structures, and the oxidation state of the metals is approximately Pt(III)-Pt(III)...Pt(II)...Pt(III)-Pt(III). The Pt...Pt interactions between the dimer units and the monomer are due to the induced Pt(II)-Pt(IV) polarization of the Pt(III) dimeric unit caused by the electron withdrawal of the equatorial halide ligands. The density functional theory calculation clearly shows that the Pt...Pt interactions between the dimers and the monomer are made by the electron transfer from the monomer to the dimers. The pentanuclear complexes have flexible Pt backbones with the Pt chain adopting either arch or sigmoid structures depending on the crystal packing.     

Synthesis and P-P cleavage reactions of [P(X)X']2; X-ray structures of [Co[P(X)X'](CO)3] and P4[P(X)X']2[X = N(SiMe3)2, X'= NPri2

Treatment of P(X)(X')Cl with KC8 gave the crystalline diphosphine [P(X)X']2 (1) which dissociated reversibly into the phosphinyl radical *P(X)X' (2), a plausible intermediate in the reaction of with [Cr(CO)6], [Co(NO)(CO)3] or P4, yielding [Cr[P(X)X']2(CO)3] (3), [Co[P(X)X'](CO)3] (4), or 1,4-P4[P(X)X']2 (5); the P(X)X' substituent is pyramidal at P in but planar in [X = N(SiMe3)2, X'= NPri2].     

Kinetics and mechanism of rhenium-catalyzed oxygen atom transfer from pyridine N-oxides to phosphines

The oxygen atom transfer (OAT) reaction cited does not occur on its own in >10 h. Oxorhenium(V) compounds having the formula MeReO(dithiolate)PZ(3) catalyze the reaction; the catalyst most studied was MeReO(mtp)PPh(3), 1, where mtpH(2) = 2-(mercaptomethyl)thiophenol. The mechanism was studied by multiple techniques. Kinetics (initial-rate and full-time-course methods) established this rate law: v = k(c)[1][PyO](2)[PPh(3)](-1). Here and elsewhere PyO symbolizes the general case XC(5)H(4)NO and PicO that with X = 4-Me. For 4-picoline, k(c) = (1.50 +/- 0.05) x 10(4) L mol(-1) s(-1) in benzene at 25.0 degrees C; the inverse phosphine dependence signals the need for the removal of phosphine from the coordination sphere of rhenium prior to the rate-controlling step (RCS). The actual entry of PPh(3) into the cycle occurs in a fast step later in the catalytic cycle, after the RCS; its relative rate constants (k(4)) were evaluated with pairwise combinations of phosphines. Substituent effects were studied in three ways: for (YC(6)H(4))(3)P, a Hammett correlation of k(c) against 3sigma gives the reaction constant rho(c)(P) = +1.03, consistent with phosphine predissociation; for PyO rho(c)(N) = -3.84. It is so highly negative because PyO enters in three steps, each of which is improved by a better Lewis base or nucleophile, and again for (YC(6)H(4))(3)P as regards the k(4) step, rho(4) = -0.70, reflecting its role as a nucleophile in attacking a postulated dioxorhenium(VII) intermediate. The RCS is represented by the breaking of the covalent N-O bond within another intermediate inferred from the kinetics, [MeReO(mtp)(OPy)(2)], to yield the dioxorhenium(VII) species [MeRe(O)(2)(mtp)(OPy)]. A close analogue, [MeRe(O)(2)(mtp)Pic], was identified by (1)H NMR spectroscopy at 240 K in toluene-d(8). The role of the "second" PyO in the rate law and reaction scheme is attributed to its providing nucleophilic assistance to the RCS. Addition of an exogenous nucleophile (tetrabutylammonium bromide, Py, or Pic) caused an accelerating effect. When Pic was used, the rate law took on the new form v = k(NA)[1][PicO][Pic][PPh(3)](-1); k(NA) = 2.6 x 10(2) L mol(-1) s(-1) at 25.0 degrees C in benzene. The ratio k(c)/k(NA) is 58, consistent with the Lewis basicities and nucleophilicities of PicO and Pic.     

Dinuclear palladium-azido complexes containing thiophene derivatives: reactivity toward organic isocyanides and isothiocyanates

Cyclopalladated tetranuclear Pd(II) complexes, [Pd2(micro-Cl)2(Y)]2 (Y = L1 or L2; H2L1 = di(2-pyridyl)-2,2'-bithiophene; H2L2 = 5,5'-di(2-pyridyl)-2,2':5',2'-terthiophene), containing two pyridyl-alpha, alpha'-disubstituted derivatives of thiophene were prepared. Treating these products with PR3 and subsequently with NaN3 produced the dinuclear Pd-azido complexes [(PR3)2(N3)Pd-Y-Pd(N3)(PR3)2] (Y = L1 or L2) or a cyclometallated complex [(PR3)(N3)Pd-Y'-Pd(N3)(PR3)] (Y' = C,N-L2). Reactions of these Pd-azido complexes with CN-Ar (Ar = 2,6-Me(2)C(6)H(3), 2,6-i-Pr(2)C(6)H(3)) or R-NCS (R = i-Pr, Et, allyl) led to the complexes containing end-on carbodiimido groups [(PMe3)2(N[double bond]C[double bond]N-Ar)Pd-Y-Pd(N[double bond]C[double bond]N-Ar)(PMe3)2] or S-coordinated tetrazole-thiolato groups {(PMe3)2[CN4(R)]S-Pd-Y-Pd-S[CN4)(R)](PMe3)2}. Interestingly, when treated with elemental sulfur, the carbodiimido complexes transformed into the cyclometallated derivatives, [(PMe3)(N[double bond]C[double bond]N-Ar)Pd-Y'-Pd(N[double bond]C[double bond]N-Ar)(PMe3)] (Y' = C,N-L1, C,N-L2). We also report the preparation of linear, thienylene-bridged dinuclear Pd complexes [L2(N3)Pd-X(or X')-Pd(N3)L2] (L = PMe3 or PMe2Ph; H2X = 2,2'-bithiophene or H2X' = 2,2':5',2'-terthiophene) and their reactivity toward organic isocyanide and isothiocyanates.     

Terpyridine Zn(II), Ru(III), and Ir(III) complexes: the relevant role of the nature of the metal ion and of the ancillary ligands on the second-order nonlinear response of terpyridines carrying electron donor or electron acceptor groups

Coordination of 4'-(C6H4-p-X)-2,2':6',2'-terpyridines [X = NO2, NBu2, (E)-CH=CH-C6H4-p-NBu2, (E,E)-(CH=CH)2-C6H4-p-NMe2] to Zn(II), Ru(III), and Ir(III) metal centers induces a significant enhancement of the absolute value of the second-order nonlinear optical (NLO) response of the terpyridine, measured by means of both electric field induced second harmonic generation and solvatochromic methods. By varying the nature of the metal center, the enhanced second-order NLO response shifts from positive to negative. Such a shift is controlled by electronic charge-transfer transitions, such as metal-to-ligand or ligand-to-metal transitions, in addition to the intraligand charge transfer. The enhancement generated by coordination is also controlled by the chelation effect and by fine-tuning of the ancillary ligands.     

Transition metal carbene chemistry. 4. Nucleophilic attachment of DABCO to fischer carbene complexes in MeCN

Rate constants for the attachment of DABCO (1,4-diazabicyclo[2.2.2]octane) to Fischer carbene complexes of the type (CO)(5)Cr=C(XR)C(6)H(4)Z (X = O and S) in dry MeCN at 25 degrees C are reported. Hammett rho values are 2.18 +/- 0.13 and 0.89 +/- 0.07 for DABCO reactions with (CO)(5)Cr=C(OMe)C(6)H(4)Z (10-Cr-Z) and (CO)(5)Cr=C(SMe)C(6)H(4)Z (11-Cr-Z), respectively. The rho values for the reaction of 10-Cr-Z and 11-Cr-Z with CH(CN)(2)(-) in 50% MeCN-50% H(2)O (v/v) are comparable to the present reactions. The reaction of DABCO with 10-Cr-Z is more closely related to the reaction of (n-Bu)(3)P with (CO)(5)W=C(OMe)C(6)H(5)-Z (23) which also provided a rho value 2.22. The much higher rho values and hence much higher reactivity of methoxy carbene complexes over the corresponding thiomethyl derivatives fit a pattern observed previously for alkoxide ion, OH(-), amine, and thiolate ion nucleophiles, and a rational explanation comes from the consideration of the substituent effects not only on the transition state but also on the reactant. A major difference between 10-Cr-Z and 11-Cr-Z is that the pi-donor effect of the methoxy group is much stronger than that of thiomethyl group. This leads to a substantial contribution of the zwitterionic form to the structure of 10-Cr-Z with much more localized positive charge on the methoxy group than the negative charge on the (CO)(5)Cr moiety. This leads to overall destabilization by an electron-withdrawing phenyl substituents resulting an increase in reactivity. The ethoxycarbene complexes are somewhat less reactive than their methoxy counterparts due to the somewhat more ground state stabilization through its stronger pi donor effect and partly due to steric crowding exerted by the slightly larger ethoxy group in the transition state. Higher k(1)(W)/k(1)(Cr) ratios for (thiomethyl)carbene complexes than methoxy or ethoxycarbene complexes are related to the intrinsic rate constant which is higher for ((thiomethyl)carbene)tungsten complexes than the corresponding Cr ones resulting in an enhanced ratio.     

Kinetics of the reactions between [S(2)MoS(2)Cu(SC(6)H(4)R-4)](2-)(R = MeO, H, Cl or NO(2)) and CN(-): substitution mechanism at a 3-coordinate Cu(I) site

The kinetics of the reaction between [S(2)MoS(2)Cu(SC(6)H(4)R-4)](2-)(R = MeO, H, Cl or NO(2)) and CN(-) to form [S(2)MoS(2)CuCN](2-) have been studied in MeCN using stopped-flow spectrophotometry. In all cases, the rate law is of the form, Rate ={k+k(2)(R)[CN(-)]}[S(2)MoS(2)Cu(SC(6)H(4)R-4)(2-)]. It is proposed that both k and k correspond to associative substitution mechanisms. The k pathway involves attack by CN(-) at the copper site followed by dissociation of the thiolate. The k pathway involves attack of the solvent (MeCN) at the copper site, followed by dissociation of the thiolate to form [S(2)MoS(2)Cu(NCMe)](-). Subsequent rapid substitution of the coordinated solvent by cyanide produces [S(2)MoS(2)CuCN](2-). The evidence that both the k and k pathways involve associative mechanisms are: (i) the 4-R-substituent on the thiolate ligand has a similar effect on both k and k, with electron-withdrawing 4-R-substituents facilitating substitution; (ii) both the k and k pathways are associated with similar activation parameters (for k(1)(H): DeltaH++ = 5.5 +/- 0.5 kcal mol(-1), DeltaS++ = -23.9 +/- 2.0 cal deg(-1) mol(-1); for k(2)(H): DeltaH++ = 2.3 +/- 0.5 kcal mol(-1), DeltaS++ = - 23.9 +/- 2.0 cal deg(-1) mol(-1)) and (iii) addition of C(6)H(5)S(-) results in a similar increase in both k and k.     

The reduction of tris-dithiolene complexes of molybdenum(VI) and tungsten(VI) by hydroxide ion: kinetics and mechanism

The kinetic study of the spontaneous reduction of some neutral tris-dithiolene complexes [ML3] of molybdenum(VI) and tungsten(VI), (L = S2C6H4(2-), S2C6H3CH3(2-) and S2C2(CH3)2(2-); M = Mo or W) by tetrabutylammonium hydroxide in tetrahydrofuran-water solutions demonstrates that OH- is an effective reductant. Their reduction is fast, clean and quantitative. Depending upon both the molar ratio in which the reagents are mixed and the amount of water present, one- or two-electron reductions of these tris-dithiolene complexes were observed. If Bu4NOH is present in low concentration or/and at high concentrations of water, the total transformation of the neutral M(VI) complex into the monoanionic M(V) complex is the only observed process. Stopped-flow kinetic data for this reaction are consistent with the rate law: -d[ML3]/dt = d[ML3-]/dt = k[ML3][Bu4NOH]. The proposed mechanism involves nucleophilic attack of OH- to form a mono-anionic seven-coordinate intermediate [ML3OH]-, which interacts with another molecule of [ML3] to generate the monoanionic complex [ML3]- transfering the oxygen from coordinated OH- to water. Hydrogen peroxide was identified as the reaction product. The molybdenum complexes are more difficult to reduce than their corresponding tungsten complexes, and the values of k obtained for the molybdenum and tungsten series of complexes increase as the ene-1,2-dithiolate ligand becomes more electron-withdrawing (S2C6H4(2-) > S2C6H3CH3(2-) > S2C2(CH3)2(2-)). This investigation constitutes the only well-established interaction between hydroxide ion and a tris(dithiolene) complex, and supports a highly covalent bonding interaction between the metal and the hydroxide ion that modulates electron transfer reactions within these complexes.     

Oxidation of symmetric disulfides with hydrogen peroxide catalyzed by Methyltrioxorhenium(VII)

Organic disulfides with both alkyl and aryl substituents are oxidized by hydrogen peroxide when CH(3)ReO(3) (MTO) is used as a catalyst. The first step of the reaction is complete usually in about an hour, at which point the thiosulfinate, RS(O)SR, can be detected in nearly quantitative yield. The thiosulfinate is then converted, also by MTO-catalyzed oxidation under these conditions, to the thiosulfonate and, over long periods, to sulfonic acids, RSO(3)H. In the absence of excess peroxide, RS(O)SR (R = p-tolyl), underwent disproportionation to RS(O)(2)SR and RSSR. Kinetics studies of the first oxidation reaction established that two peroxorhenium compounds are the active forms of the catalyst, CH(3)ReO(2)(eta(2)-O(2)) (A) and CH(3)ReO(eta(2)-O(2))(2).(OH(2)) (B). Their reactivities are similar; typical rate constants (L mol(-)(1) s(-)(1), 25 degrees C, aqueous acetonitrile) are k(A) = 22, k(B) = 150 (Bu(2)S(2)) and k(A) = 1.4, k(B) = 11 (Tol(2)S(2)). An analysis of the data for (p-XC(6)H(4))(2)S(2) by a plot of log k(B) against the Hammett sigma constant gave rho = -1.89, supporting a mechanism in which the electron-rich sulfur attacks a peroxo oxygen of intermediates A and B.     

Effective monkey saddle points and berry and lever mechanisms in the topomerization of SF(4) and related tetracoordinated AX(4) species

The topomerization mechanisms of the SF(4) and SCl(2)F(2) sulfuranes, as well as their higher (SeF(4), TeF(4)) and isoelectronic analogues PF(4)(-), AsF(4)(-), SbF(4)(-), SbCl(4)(-), ClF(4)(+), BrF(4)(+), BrCl(2)F(2)(+), and IF(4)(+)), have been computed at B3LYP/6-31+G and at B3LYP/6-311+G. All species have trigonal bipyramidal (TBP) C(2)(v)() ground states. In such four-coordinated molecules, Berry rotation exchanges both axial with two equatorial ligands simultaneously while the alternative "lever" mechanism exchanges only one axial ligand with one equatorial ligand. While the barrier for the lever exchange in SF(4) (18.8 kcal mol(-1)) is much higher than that for the Berry process (8.1 kcal mol(-1)), both mechanisms are needed for complete ligand exchange. The F(ax)F(ax) and F(eq)F(eq) isomers of SF(2)Cl(2) have nearly the same energy and readily interconvert by BPR with a barrier of 7.6 kcal mol(-1). The enantiomerization of the F(ax)F(eq) chiral isomer can occur by either the Berry process (transition state barrier 8.3 kcal mol(-1)) or the "lever" mechanism via either of two C(s)() transition states, based on the TBP geometry: Cl(ax) <--> Cl(eq) or F(ax) <--> F(eq) exchanges with barriers of 6.3 and 15.7 kcal mol(-1), respectively. Full scrambling of all ligand sites is possible only by inclusion of the lever mechanism. Planar, "tetrahedral", and triplet forms are much higher in energy. The TBP C(3)(v) structures of AX(4) either have two imaginary frequencies (NIMAG = 2) for the X = F, Cl species or are minima (NIMAG = 0) for the X = Br, I compounds. These "effective monkey saddle points" have degenerate modes with two small frequencies, imaginary or real. Although a strictly defined "monkey saddle" (with degenerate frequencies exactly zero) is not allowed, the flat C(3)(v) symmetry region serves as a "transition state" for trifurcation of the pathways. The BPR mechanism also is preferred over the alternative lever process in the topomerization of the selenurane SeF(4) (barriers 5.9 vs. 12.1 kcal mol(-1)), the tellurane TeF(4) (2.1 vs. 6.4), and the interhalogen cations ClF(4)(+) (2.5 vs 14.8), BrF(4)(+) (4.7 vs. 11.3), BrF(2)Cl(2)(+) (14.6 vs. 17.4), and IF(4)(+) (1.4 vs. 6.0), as well as for the series PF(4)(-) (7.0 vs. 9.0), AsF(4)(-) (9.3 vs. 17.2), and SbF(4)(-) (3.8 vs. 5.3 kcal mol(-1)), all computed at B3LYP/6-311+G with the inclusion of quasirelativistic pseudopotentials for Te, I, and Sb. The heavier halogens increasingly favor the lever process, where the barrier (2.6 kcal mol(-1)) pertaining to the effective monkey saddle point (C(3)(v) minimum for SbCl(4)(-)) is less than that for the Berry process (8.2 kcal mol(-1)).     

Iron(III) [bond] Salen complexes as enzyme models: mechanistic study of oxo(salen)iron complexes oxygenation of organic sulfides

The oxidation of a series of para-substituted phenyl methyl sulfides was carried out with several oxo(salen)iron (salen = N,N'-bis(salicylidine)ethylenediaminato) complexes in acetonitrile. The oxo complex [O=Fe(IV)(salen)](*+), generated from an iron(III) [bond] salen complex and iodosylbenzene, effectively oxidizes the organic sulfides to the corresponding sulfoxides. The formation of [O [double bond] Fe(IV)(salen)](*+) as the active oxidant is supported by resonance Raman studies. The kinetic data indicate that the reaction is first-order in the oxidant and fractional-order with respect to sulfide. The observed saturation kinetics of the reaction and spectral data indicate that the substrate binds to the oxidant before the rate-controlling step. The rate constant (k) values for the product formation step determined using Michaelis-Menten kinetics correlate well with Hammett sigma constants, giving reaction constant (rho) values in the range of -0.65 to -1.54 for different oxo(salen)iron complexes. The log k values observed in the oxidation of each aryl methyl sulfide by substituted oxo(salen)iron complexes also correlate with Hammett sigma constants, giving positive rho values. The substituent effect, UV-vis absorption, and EPR spectral studies indicate oxygen atom transfer from the oxidant to the substrate in the rate-determining step.