Reductive elimination/oxidative addition of carbon-hydrogen bonds at Pt(IV)/Pt(II) centers: mechanistic studies of the solution thermolyses of Tp(Me2)Pt(CH3)2H

Reductive elimination of methane occurs upon solution thermolysis of kappa(3)-Tp(Me)2Pt(IV)(CH(3))(2)H (1, Tp(Me)2 = hydridotris(3,5-dimethylpyrazolyl)borate). The platinum product of this reaction is determined by the solvent. C-D bond activation occurs after methane elimination in benzene-d(6), to yield kappa(3)-Tp(Me)2Pt(IV)(CH(3))(C(6)D(5))D (2-d(6)), which undergoes a second reductive elimination/oxidative addition reaction to yield isotopically labeled methane and kappa(3)-Tp(Me)2Pt(IV)(C(6)D(5))(2)D (3-d(11)). In contrast, kappa(2)-Tp(Me)2Pt(II)(CH(3))(NCCD(3)) (4) was obtained in the presence of acetonitrile-d(3), after elimination of methane from 1. Reductive elimination of methane from these Pt(IV) complexes follows first-order kinetics, and the observed reaction rates are nearly independent of solvent. Virtually identical activation parameters (DeltaH(++)(obs) = 35.0 +/- 1.1 kcal/mol, DeltaS(++)(obs) = 13 +/- 3 eu) were measured for the reductive elimination of methane from 1 in both benzene-d(6) and toluene-d(8). A lower energy process (DeltaH(++)(scr) = 26 +/- 1 kcal/mol, DeltaS(++)(scr) = 1 +/- 4 eu) scrambles hydrogen atoms of 1 between the methyl and hydride positions, as confirmed by monitoring the equilibration of kappa(3)-Tp(Me)()2Pt(IV)(CH(3))(2)D (1-d(1)()) with its scrambled isotopomer, kappa(3)-Tp(Me)2Pt(IV)(CH(3))(CH(2)D)H (1-d(1'). The sigma-methane complex kappa(2)-Tp(Me)2Pt(II)(CH(3))(CH(4)) is proposed as a common intermediate in both the scrambling and reductive elimination processes. Kinetic results are consistent with rate-determining dissociative loss of methane from this intermediate to produce the coordinatively unsaturated intermediate [Tp(Me)2Pt(II)(CH(3))], which reacts rapidly with solvent. The difference in activation enthalpies for the H/D scrambling and C-H reductive elimination provides a lower limit for the binding enthalpy of methane to [Tp(Me)2Pt(II)(CH(3))] of 9 +/- 2 kcal/mol.     

Vinyl-vinyl coupling on late transition metals through C-C reductive elimination mechanism. A computational study

A detailed density functional study was performed for the vinyl-vinyl reductive elimination reaction from bis-sigma-vinyl complexes [M(CH=CH(2))(2)X(n)]. It was shown that the activity of these complexes decreases in the following order: Pd(IV), Pd(II) > Pt(IV), Pt(II), Rh(III) > Ir(III), Ru(II), Os(II). The effects of different ligands X were studied for both platinum and palladium complexes, which showed that activation barriers for C-C bond formation reaction decrease in the following order: X = Cl > Br, NH(3) > I > PH(3). Steric effects induced either by the ligands X or by substituents on the vinyl group were also examined. In addition, the major factors responsible for stereoselectivity control on the final product formation stage and possible involvement of asymmetric coupling pathways are reported. In all cases DeltaE, DeltaH, DeltaG, and DeltaG(aq) energy surfaces were calculated and analyzed. The solvent effect calculation shows that in a polar medium halogen complexes may undergo a reductive elimination reaction almost as easily as compounds with phosphine ligands.     

Oxidation of dimethyldi(2-pyridyl)borato Pt(II)Me(n) complexes, n = 1, 0, with H2O2: tandem B-to-Pt methyl group migration and formation of C-O bond

New dimethyldi(2-pyridyl)borato (dmdpb) platinum(II) complexes, (dmdpb)Pt(II)Me(SMe(2)) (1), (dmdpb)Pt(II)(L)(SMe(2))(+), L = MeOH (2), MeCN (3), supported by dimethylsulfide ligand and featuring one (1) or no hydrocarbyls at the metal (2, 3) were prepared and their oxidation with hydrogen peroxide was studied. Both complex 1 bearing the formal charge of +1 on the metal and the methanol complex 2 capable of losing the proton of the methanol ligand to form the methoxide derivative 4 charged similarly to 1, are reactive towards H(2)O(2). However, the cationic complex 3 with a formal charge of +2 on the metal does not react with H(2)O(2). The oxidation of the monomethyl platinum(II) complex 1 leads to the B-to-Pt methyl transfer and formation of a robust dimethyl Pt(IV) species 5 which does not undergo C-O reductive elimination up to 100 °C. By contrast, oxidation of 2 in methanol-d(4) leads to quantitative formation of dimethyl ether-d(3), CD(3)OCH(3). It was presumed that the latter reaction involves the B-to-Pt methyl transfer and formation of a highly reactive cationic monomethyl Pt(IV) species whose methyl group carbon atom can accept nucleophilic attack by the methanol-d(4) solvent to form dimethyl ether-d(3).     

Dimethylplatinum(II) complexes: computational insights into Pt-C bond protonolysis

A detailed density functional theory (DFT) study of the protonation and subsequent methane elimination reactions of dimethylplatinum(II) complexes in presence of triflic acid in various solvents has been undertaken to contribute to the debate concerning the mechanism of the electrophilic cleavage of the Pt-C bond in Pt(II) complexes. Both mechanisms of direct one-step proton attack at the Pt-C bond (S(E)2) and stepwise oxidative-addition on the central metal followed by reductive elimination (S(E)(ox)) have been explored for a series of dimethylplatinum(II) complexes changing the nature of the ancillary ligands and the solvent. Theoretical calculations show that the most likely mechanism cannot be predicted on the basis of spectator ligands donating properties only. A one-step protonolysis pathway is characteristic for complexes containing P based ligands, whereas for complexes containing N based and, in general, hard poor-donor ligands a common behavior cannot be indicated. Solvent nucleophilicity can influence the rate of the S(E)(ox) rate mechanism, whereas its steric hindrance can induce a change of the preferred mechanism. The hypothesis that five-coordinate methyl hydrido platinum(IV) intermediates might be formed along the S(E)(ox) pathway is not supported. Only six-coordinate Pt(IV) hydride complexes are calculated to be stable intermediates generated by direct protonation at the platinum center. Formation and experimental detection of six-coordinate Pt(IV) hydrides, nevertheless, cannot be considered a definite evidence that a S(E)(ox) mechanism is operative because such intermediates can be also generated by a hydrogen migration to Pt from the carbon atom of the σ-complex methane molecule formed by a S(E)2 attack. For all the examined complexes methane loss occurs by an associative mechanism. Both solvent and anion of the acid can assist methane displacement. Calculations have been also carried out to probe whether the preference for a concerted or a stepwise mechanism should be predicted on the basis of two proposed criteria: metal-complex charge distribution as a consequence of the Pt-C bond polarization and the nature of the highest occupied molecular orbital (HOMO).     

Competitive aryl-iodide vs aryl-aryl reductive elimination reactions in Pt(IV) complexes: experimental and theoretical studies

The platinum(IV) complex trans-(dmpe)Pt(IV)(Ar)2I2 (2, dmpe = 1,2-dimethylphosphinoethane, Ar = 4-FC6H4) rapidly reacts, upon moderate heating in solution under ambient light, via two distinct pathways: isomerization to the corresponding cis-isomer (3) and Ar-I reductive elimination to give (dmpe)Pt(II)(Ar)I (4). Complex 3 undergoes, upon prolonged heating at high temperatures, an exclusive Ar-Ar reductive elimination reaction to give (dmpe)Pt(II)I2. Experimental and DFT studies showed that the 2-to-3 isomerization proceeds via three pathways: photochemical or thermal phosphine chelate opening and a mechanism involving cleavage of the Pt-I bond. The isomerization reaction is significantly slowed down but not stopped in the absence of light or in the presence of an excess of tetra-n-butylammonium iodide. On the other hand, the Ar-I reductive elimination from 2 proceeds via the Pt(delta+)-I(delta-) ion pairlike transition state. Use of the rigid dmpe analogue 1,2-dimethylphosphinobenzene (dmpbz) as the ligand shuts down the chelate ring-opening isomerization pathway and enables faster Ar-I reductive elimination thus making the latter reaction the major reaction route for the dmpbz supported trans-diiodo Pt(IV) complex 8.     

Mechanistic information on the reductive elimination from cationic trimethylplatinum(IV) complexes to form carbon-carbon bonds

Cationic complexes of the type fac-[(L(2))Pt(IV)Me(3)(pyr-X)][OTf] (pyr-X = 4-substituted pyridines; L(2) = diphosphine, viz., dppe = bis(diphenylphosphino)ethane and dppbz = o-bis(diphenylphosphino)benzene; OTf = trifluoromethanesulfonate) undergo C-C reductive elimination reactions to form [L(2)Pt(II)Me(pyr-X)][OTf] and ethane. Detailed studies indicate that these reactions proceed by a two-step pathway, viz., initial reversible dissociation of the pyridine ligand from the cationic complex to generate a five-coordinate Pt(IV) intermediate, followed by irreversible concerted C-C bond formation. The reaction is inhibited by pyridine. The highly positive values for DeltaS()(obs) = +180 +/- 30 J K(-1) mol(-1), DeltaH(obs) = 160 +/- 10 kJ mol(-1), and DeltaV()(obs) = +16 +/- 1 cm(3) mol(-1) can be accounted for in terms of significant bond cleavage and/or partial reduction from Pt(IV) to Pt(II) in going from the ground to the transition state. These cationic complexes have provided the first opportunity to carry out detailed studies of C-C reductive elimination from cationic Pt(IV) complexes in a variety of solvents. The absence of a significant solvent effect for this reaction provides strong evidence that the C-C reductive coupling occurs from an unsaturated five-coordinate Pt(IV) intermediate rather than from a six-coordinate Pt(IV) solvento species.     

Formation of Ethers from Alcohols with Chloride Complexes of Platinum(II) as Catalysts

A new catalytic reaction of the etherification of alcohols in the system ROH-PtCl ₄ ²⁻ has been observed. At 70 °C in the presence of catalytic amounts of chloride complexes of platinum(II) methanol gave dimethyl ether. Methyl tert-butyl ether and di-tert-butyl ether were formed analogously from a mixture of methanol and tert-butanol. In the reaction with ethanol the products were diethyl ether and a π-ethylene complex of platinum(II). It is suggested that the step-wise mechanism includes the oxidative addition of the alcohol with the intermediate formation of an alkyl complex of platinum(IV), the decomposition of which by reductive elimination under the influence of a second molecule of alcohol or an alkoxide anion gives an ether and regenerates the catalyst, a chloride complex of platinum(II).__________Translated from Teoreticheskaya i Eksperimental’naya Khimiya, Vol. 41, No. 3, pp. 190–193, May–June, 2005.     

Mechanism and Origins of Diastereo- and Regioselectivities of Palladium-Catalyzed Remote Diborylative Cyclization of Dienes via Chain-Walking Strategy

Density functional theory calculations have been performed to investigate the palladium-catalyzed remote diborylative cyclization of dienes. The computations reveal that the reaction proceeds through a rarely explored Pd(II)/Pd(IV) catalytic cycle, and the formal σ-bond metathesis between the alkylpalladium intermediate and B₂pin₂ occurs via the pathway of the B−B oxidative addition/C−B reductive elimination involving the high-valent Pd(IV) species. The diastereoselectivity is determined by the migratory insertion into the Pd−C bond, which is mainly due to the combination of the torsional strain effect, steric repulsion and C−H—O hydrogen-bonding interaction. The steric hindrance around the reacting carbon group in the C−B reductive elimination turns out to be a key factor to provide the driving force of the chain walking of the Pd center to the terminal primary carbon position, enabling the experimentally observed remote regioselectivity.     

Reductive elimination of ethane from five-coordinate platinum(IV) alkyl complexes

Five-coordinate platinum(IV) alkyl complexes bearing sterically non-demanding pyridylpyrrolide ligands, (LX)PtMe(3) [LX = 3,5-di-tert-butyl-2-(2-pyridyl)pyrrolide (3a) and 3,5-diphenyl-2-(2-pyridyl)pyrrolide (3b)] have been prepared. An X-ray structure of 3a establishes that it is a five-coordinate Pt(IV) complex with a square-pyramidal geometry. Thermolysis of 3a or 3b in C(6)D(6) with ethylene results in reductive elimination of ethane (C(2)H(6)) and methane (CH(4) and CH(3)D) and the formation of cyclometalated platinum(II) ethylene complexes 4a or 4b, respectively. Results of kinetic investigations of the reaction of 3b are consistent with a mechanism of direct C-C reductive elimination from the five-coordinate Pt(IV) compound. Thermolysis of 3a in C(6)D(6) with no ethylene present forms a novel dinuclear complex (5-d(6)).     

Formation mechanism of 2-methyl-2-buten-1,4-diol and 2-methyl-3-buten-1,2-diol from 2-methyl-1,3-butadiene on a head-to-head pivalamidato-bridged cis-diammineplatinum(III) binuclear complex

Reactions of a pivalamidato-bridged head-to-head (HH) platinum(III) binuclear complex with 2-methyl-1,3-butadiene (isoprene) and p-styrenesulfonate and of an α-pyrrolidonato-bridged HH platinum(III) binuclear complex with p-styrenesulfonate were studied kinetically using UV-vis spectrophotometry and (1)H NMR spectroscopy, and detailed reaction mechanisms are proposed. Pt(III) binuclear complexes react with p-styrenesulfonate in four successive steps with mechanisms similar to that for an HH α-pyridonato-bridged Pt(III) binuclear complex with p-styrenesulfonate. In the case of isoprene, four steps were observed on the basis of UV-vis spectrophotometry. However, the reaction kinetics for steps 1 and 2 correspond to those for the previous reaction system, and those for steps 3 and 4 do not correspond to those for the previous system or to those observed by using (1)H NMR spectroscopy for the present isoprene system. By using UV-vis spectrophotometry, it was shown that isoprene preferentially π-coordinates to the Pt(N(2)O(2)) atom via the double bond adjacent to the methyl group in step 1. In step 2, a second isoprene molecule π-coordinates to the Pt(N(4)) atom, which is the rate-determining step, followed by nucleophilic attack of a water molecule on the π-coordinated isoprene on the Pt(N(2)O(2)) atom to form two isomeric σ-complexes. In the same step, π-coordinated isoprene on the Pt(N(4)) atom of the σ-complexes is released. This is different from the reaction of the Pt(III) binuclear complexes with other olefins. In step 3, reductive elimination of the σ-complexes occurs to form two diols and the HH pivalamidato-bridged Pt(II) binuclear complex. Finally, acid decomposition of the Pt(II) binuclear complex occurs to form monomers in step 4. From (1)H NMR spectroscopic observations, fast isomerization between σ-complexes and reductive elimination of the σ-complexes occurs in step 3, and isomerization from a 1,4-diol to a 1,2-diol occurs in step 4.     

Ethylenebis(triphenylphosphine)platinum as a probe for niobium-mediated diphosphorus chemistry

Ethylenebis(triphenylphosphine)platinum is used as a trap for the P2-containing molecule W(CO)5(P2), which is eliminated at room temperature from a niobium-complexed diphosphaazide ligand. The rate of W(CO)5(P2) elimination is unaffected by the presence of the platinum species. Attempts to generate and trap free P2 with the platinum ethylene complex were hindered by the direct reaction between the platinum starting material and the P2 generator, (Mes*NPP)Nb(N[Np]Ar)3. In this case, reductive cleavage of the P-P bond in the diphosphaazide ligand is induced by platinum coordination, resulting in the formation of a trimetallic system with two bridging, three-coordinate phosphorus atoms.     

Alkyl carbon-nitrogen reductive elimination from platinum(IV)-sulfonamide complexes

Platinum(IV) complexes containing monodentate sulfonamide ligands, fac-(dppbz)PtMe(3)(NHSO(2)R) (dppbz = o-bis(diphenylphosphino)benzene; R = p-C(6)H(4)(CH2)(3)CH(3) (1a), p-C(6)H(4)CH(3) (1b), CH(3) (1c)), have been synthesized and characterized, and their thermal reactivity has been explored. Compounds 1a-c undergo competitive C-N and C-C reductive elimination upon thermolysis to form N-methylsulfonamides and ethane, respectively. Selectivity for either C-N or C-C bond formation can be achieved by altering the reaction conditions. Good yields of the C-N-coupled products were observed when the thermolyses of 1a-c were conducted in benzene-d(6). In contrast, exclusive C-C reductive elimination occurred upon themolysis of 1a,b in nitrobenzene-d(5). When the thermolyses of 1a were performed in the presence of sulfonamide anion NHSO2R- in benzene-d(6), ethane elimination was completely inhibited and C-N reductive elimination products were formed in high yield. Mechanistic studies support a two-step reaction pathway involving initial dissociation of NHSO(2)R(-) from the platinum center, followed by nucleophilic attack of this anion on a methyl group of the resulting five-coordinate platinum(IV) cation to form MeNHSO(2)R and (dppbz)PtMe(2). C-C reductive elimination to form ethane occurs directly from the five-coordinate Pt(IV) cation.     

The unique chemistry of platina-beta-diketones

This review presents syntheses, structures and the reactivity of platina-beta-diketones [Pt2{(COR)2H}2(mu-Cl)2] (R = alkyl, omega-phenylalkyl), being the first electronically unsaturated (16 ve; ve-valence electrons) and kinetically labile metalla-beta-diketones. They were found to react with amines, yielding platina-beta-diketonates of platina-beta-diketones having Pt(4) zigzag chains analogous to platinum blue complexes. Reactions of platina-beta-diketones with monodentate and bidentate N-, P-, As-, O-, and S-donor ligands are described resulting in the formation of acyl(hydrido)platinum(IV) complexes, acyl(chloro)platinum(II) complexes, platinum complexes having enamine-amide type ligands, and of platinum(II) complexes with cyclic aminocarbene ligands, respectively. These reactions are discussed in terms of oxidative addition and reductive elimination reactions showing that platina-beta-diketones react as hydroxycarbene complexes whose OH groups are intramolecularly hydrogen-bridged to acyl ligands. Furthermore, the synthesis and structures of dinuclear platinum(II) complexes with bridging mu-acyl(hydroxycarbene) ligands are presented.     

Mechanistic information from low-temperature rapid-scan and NMR measurements on the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II) dimethyl complex

A detailed kinetic study of the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II) dimethyl complex was undertaken in dichloromethane over the temperature range of -90 to +10 degrees C by stopped-flow techniques. Time-resolved UV-vis monitoring of the reaction allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. The second-order rate constant for the protonation step was determined to be 15200 +/- 400 M(-1) s(-1) at -78 degrees C, and the corresponding activation parameters are DeltaH = 15.2 +/- 0.6 kJ mol(-1) and DeltaS = -85 +/- 3 J mol(-1) K(-1), which are in agreement with the addition of a proton that results in the formation of the platinum(IV) hydrido complex. The kinetics of the second, methane-releasing reaction step do not show an acid dependence, and the MeCN concentration also does not significantly affect the reaction rate. The activation parameters for the second reaction step were found to be DeltaH = 75 +/- 1 kJ mol(-1), DeltaS = +38 +/- 5 J mol(-1) K(-1), and DeltaV = +18 +/- 1 cm(3) mol(-1), strongly suggesting a dissociative character of the rate-determining step for the reductive elimination reaction. The spectroscopic and kinetic observations were correlated with NMR data and assisted the elucidation of the underlying reaction mechanism.     

Platinum complexes of diazo ligands. Studies of regioselective aromatic ring amination, oxidative halogen addition and reductive halogen elimination reactions

2-(Arylazo)pyridine ligands, L1a-1c react with the salt K2[PtCl4] to give the mononuclear complexes [PtCl2(L1)](1), which readily react with ArNH2 to yield the monochloro complexes of type [PtCl(L2)](HL2= 2-[(2-(arylamino)phenyl)azo]pyridine)(2) via regioselective ortho-amine fusion at the pendent aryl ring of coordinated L1. Oxidative addition of the electrophiles Y2(Y = Cl, Br, I) to the square-planar platinum(II) complex, has led to syntheses of the corresponding octahedral platinum(IV) complexes, [PtY3(L2)](3) in high yields. Ascorbate ion reductions of the platinum(IV) complexes, , resulted in reductive halogen elimination to revert to the platinum(II) complexes almost quantitatively. Isolation of products and X-ray structure determination of the representative complexes followed all these chemical reactions. In crystal packing, the compound [PtCl2(L1c)](1c) forms dimeric units with a Pt...Pt distance of 3.699(1) A. In contrast, the crystal packing of 2b revealed that the molecules are arranged in an antiparallel fashion to form a noncovalent 1D chain to accommodate pi(aryl)-pi(pyridyl) and Pt-pi(aryl) interactions. Notably, the oxidation of [Pt(II)Cl(L2a)](2a) by I2 produced a mixed halide complex [Pt(IV)ClI2(L2a)](5), which, in turn, is reduced by ascorbate ion to produce [Pt(II)I(L2a)] with the elimination of ClI. All the platinum(II) complexes are brown, the platinum(IV) complexes, on the other hand, are green. Low-energy visible range transitions in the complexes of the extended ligand [L2]- are ascribed to ligand basedpi-pi* transitions. Cyclic voltammetric behaviour of the complexes is reported.     

Studies of reductive elimination reactions to form carbon-oxygen bonds from Pt(IV) complexes.

The platinum(IV) complexes fac-L(2)PtMe(3)(OR) (L(2) = bis(diphenylphosphino)ethane, o-bis(diphenylphosphino)benzene, R = carboxyl, aryl; L = PMe(3), R = aryl) undergo reductive elimination reactions to form carbon-oxygen bonds and/or carbon-carbon bonds. The carbon-oxygen reductive elimination reaction produces either methyl esters or methyl aryl ethers (anisoles) and L(2)PtMe(2), while the carbon-carbon reductive elimination reaction affords ethane and L(2)PtMe(OR). Choice of reaction conditions allows the selection of either type of coupling over the other. A detailed mechanistic study of the reductive elimination reactions supports dissociation of the OR(-) ligand as the initial step for the C-O bond formation reaction. This is followed by a nucleophilic attack of OR(-) upon a methyl group bound to the Pt(IV) cation to produce the products MeOR and L(2)PtMe(2). C-C reductive elimination proceeds from L(2)PtMe(3)(OR) by initial L (L = PMe(3)) or OR(-) (L(2) = dppe, dppbz) dissociation, followed by C-C coupling from the resulting five-coordinate intermediate. Our studies demonstrate that both C-C and C-O reductive elimination reactions from Pt(IV) are more facile in polar solvents, in the presence of Lewis acids, and for OR(-) groups that contain electron withdrawing substituents.     

Mechanistic and computational studies of oxidatively-induced aryl-CF3 bond-formation at Pd: rational design of room temperature aryl trifluoromethylation

This article describes the rational design of first generation systems for oxidatively induced Aryl-CF(3) bond-forming reductive elimination from Pd(II). Treatment of (dtbpy)Pd(II)(Aryl)(CF(3)) (dtbpy = di-tert-butylbipyridine) with NFTPT (N-fluoro-1,3,5-trimethylpyridinium triflate) afforded the isolable Pd(IV) intermediate (dtbpy)Pd(IV)(Aryl)(CF(3))(F)(OTf). Thermolysis of this complex at 80 °C resulted in Aryl-CF(3) bond-formation. Detailed experimental and computational mechanistic studies have been conducted to gain insights into the key reductive elimination step. Reductive elimination from this Pd(IV) species proceeds via pre-equilibrium dissociation of TfO(-) followed by Aryl-CF(3) coupling. DFT calculations reveal that the transition state for Aryl-CF(3) bond formation involves the CF(3) acting as an electrophile with the Aryl ligand serving as a nucleophilic coupling partner. These mechanistic considerations along with DFT calculations have facilitated the design of a second generation system utilizing the tmeda (N,N,N',N'-tetramethylethylenediamine) ligand in place of dtbpy. The tmeda complexes undergo oxidative trifluoromethylation at room temperature.     

Carbon monoxide promoted reductive elimination of hydrogen from Tp' platinum complexes

Acid-assisted reductive elimination of hydrogen from Tp'PtH(3) and of methane and hydrogen from Tp'PtMeH(2) (Tp' = hydridotris(3,5-dimethylpyrazolyl)borate) is examined herein. Loss of H(2) is observed from solutions containing platinum(IV) complexes of the type Tp'Pt(R)(H)(2) (R = Me, H) upon protonation and addition of a ligand such as CO. Results of kinetic studies on reductive elimination of H(2) and formation of [kappa(2)-(HTp')Pt(R)(L)][BAr'(4)] products from intermediates derived from Tp'Pt(R)(H)(2) precursors are described. Elimination appears to occur from cationic 6-coordinate [kappa(2)-(HTp')Pt(R)(H)(2)(L)][BAr'(4)] species.     

Bis[N,N'-diisopropylbenzamidinato(-)]silicon(II): a silicon(II) compound with both a bidentate and a monodentate amidinato ligand

Well looked-after: reductive HCl elimination of the λ(6)-silicon(IV) complex 1 leads to the λ(3)-silicon(II) species 2, a novel type of donor-stabilized silylene. Reaction of 2 with [W(CO)(6)] and with I(2) yields the λ(5)-silicon(II) complex 3 and the λ(6)-silicon(IV) complex 4, respectively.     

Kinetics and mechanism for reduction of the anticancer prodrug trans,trans,trans-[PtCl2(OH)2(c-C6H11NH2)(NH3)] (JM335) by thiols

The reduction of the platinum(IV) prodrug trans,trans,trans-[PtCl2(OH)2(c-C6H11NH2)(NH3)] (JM335) by L-cysteine, DL-penicillamine, DL-homocysteine, N-acetyl-L-cysteine, 2-mercaptopropanoic acid, 2-mercaptosuccinic acid, and glutathione has been investigated at 25 degrees C in a 1.0 M aqueous perchlorate medium with 6.8 < or = pH < or = 11.2 using stopped-flow spectrophotometry. The stoichiometry of Pt(IV):thiol is 1:2, and the redox reactions follow the second-order rate law -d[Pt(IV)]/dt = k[Pt(IV)][RSH]tot, where k denotes the pH-dependent second-order rate constant and [RSH]tot the total concentration of thiol. The pH dependence of k is ascribed to parallel reductions of JM335 by the various protolytic species of the thiols, the relative contributions of which change with pH. Electron transfer from thiol (RSH) or thiolate (RS-) to JM335 is suggested to take place as a reductive elimination process through an attack by sulfur at one of the mutually trans chloride ligands, yielding trans-[Pt(OH)2(c-C6H11NH2)(NH3)] and RSSR as the reaction products, as confirmed by 1H NMR. Second-order rate constants for the reduction of JM335 by the various protolytic species of the thiols span more than 3 orders of magnitude. Reduction with RS- is approximately 30-2000 times faster than with RSH. The linear correlation log(kRS) = (0.52 +/- 0.06)-pKRSH--(2.8 +/- 0.5) is observed, where kRS denotes the second-order rate constant for reduction of JM335 by a particular thiolate RS- and KRSH is the acid dissociation constant for the corresponding thiol RSH. The slope of the linear correlation indicates that the reactivity of the various thiolate species is governed by their proton basicity, and no significant steric effects are observed. The half-life for reduction of JM335 by 6 mM glutathione (40-fold excess) at physiologically relevant conditions of 37 degrees C and pH 7.30 is 23 s. This implies that JM335, in clinical use, is likely to undergo in vivo reduction by intracellular reducing agents such as glutathione prior to binding to DNA. Reduction results in the immediate formation of a highly reactive platinum(II) species, i.e., the bishydroxo complex in rapid protolytic equilibrium with its aqua form.