Complexes of borane and N-heterocyclic carbenes: a new class of radical hydrogen atom donor

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator.     

Hydrogenation Catalysts Based on Polynuclear Palladium Complexes with Organophosphorus Ligands

A new procedure is proposed for the preparation of hydrogenation catalysts. This procedure includes the synthesis of cyclic tetranuclear palladium complexes with bridging diphenylphosphide ligands followed by a reaction with Pd(CH₃COO)₂ in the presence of hydrogen to form nanosized particles. In the test catalysts, the ensembles of palladium atoms (or palladium hydrides) immobilized on supramolecular structures formed by the association of phosphinidene and phosphide complexes of palladium are responsible for the catalytic activity.     

The first stable mononuclear silyl palladium hydrides

The reaction of tertiary silanes with the low valent palladium complex [(mu-dcpe)Pd]2 affords equilibrium mixtures with mononuclear silyl palladium hydrides. These complexes have been characterized by NMR spectroscopy and, in one case, by X-ray crystallography. The silyl palladium hydride complexes rapidly interchange silicon and hydride coordination environments in solution which give rise to extraordinary temperature-dependent kinetic isotope effects for the fluxional process. An intermediate 2eta-Si-H complex is proposed for the interchange.     

有机金属配合物控制的活性自由基聚合研究进展

活性自由基聚合(Living radical polymerization,LRP)是高效可控地制备结构新颖的高分子材料的重要方法,近年来逐渐成为高分子合成领域的研究热点.本文综述了一类重要的活性自由基聚合方法,即有机金属配合物控制的自由基聚合(Organometallic mediated radical polymerization,OMRP)的最新研究进展,介绍了钛、钒、铬、钼、铁、锇、钴、铑、钯和铜等有机金属配合物为催化剂控制的OMRP,并简述了OMRP方法在光致LRP、嵌段共聚物的合成、与其他LRP方法联用及聚合物端基修饰和转化等方面的拓展.此外,本文还对有待进一步深入探索的领域和问题提出了建议和展望.     

Electron exchange involving a sulfur-stabilized ruthenium radical cation

Half-sandwich Ru(II) amine, thiol, and thiolate complexes were prepared and characterized by X-ray crystallography. The thiol and amine complexes react slowly with acetonitrile to give free thiol or amine and the acetonitrile complex. With the thiol complex, the reaction is dissociative. The thiolate complex has been oxidized to its Ru(III) radical cation and the solution EPR spectrum of that radical cation recorded. Cobaltocene reduces the thiol complex to the thiolate complex. The 1H and 31P NMR signals of the thiolate complex in acetonitrile become very broad whenever the thiolate and thiol complexes are present simultaneously. The line broadening is primarily due to electron exchange between the thiolate complex and its radical cation; the latter is generated by an unfavorable redox equilibrium between the thiol and thiolate complexes. Pyramidal inversion of sulfur in the thiol complex is fast at room temperature but slow at lower temperatures; major and minor conformers of the thiol complex were observed by 31P NMR at -98 degrees C in CD2Cl2.     

Use of styrene radical cations as probes for the complexation dynamics of charged guests with alpha- and beta-cyclodextrins

The complexation dynamics of radical cations with cyclodextrins (CD) was studied using photophysical techniques. Radical cations of 4-vinylanisole and trans-anethole were formed within alpha- and beta-CD cavities by two-photon photolysis of the respective styrene precursors. Exit of the radical cations from alpha-CD complexes with 1: 1 and 1:2 (guest: CD) stoichiometries and beta-CD complexes with 1:1 stoichiometries occurred with lifetimes shorter than 100 ns. Most of the radical cations formed escape from the CD cavities, but a small portion do react with alpha-CD when this host is present in high concentrations.     

Addition of Bromotrichloromethane and Tetrachloromethane to cis-Cyclooctene,Cyclohexene, and Norbornadiene in the Presence of Palladium(II) Complexes

Addition of bromotrichloromethane and tetrachloromethane to alkenes in the presence of palladium(II) complexes begins with single-electron transfer from the palladium atom to polyhalomethane molecule, followed by fragmentation of the radical anion thus formed and generation of trichloromethyl radical. Halogen transfer to a carbon-centered radical arising from addition of trichloromethyl radical at the double bond occurs both from the polyhalomethane molecule and from halide palladium complex.     

Measurement of the hydride donor abilities of [HM(diphosphine)2]+ complexes (M = Ni, Pt) by heterolytic activation of hydrogen

[M(diphosphine)2]2+ complexes (where M = Ni and Pt) react with hydrogen in the presence of bases to form the corresponding hydrides, [HM(diphosphine)2]+. In seven cases, equilibria have been observed from which the hydride donor ability (DeltaGdegrees(H-)) of the hydrides can be calculated. For six of these complexes, the DeltaGdegrees(H-) values calculated using heterolytic activation of hydrogen are compared with those based on thermodynamic cycles using pK(a) measurements and electrochemical half-wave potentials. The agreement between these two methods is good (within 1 kcal/mol). The reactivity of the various [M(diphosphine)2]2+ complexes toward hydrogen parallels their measured hydride acceptor abilities.     

Matrix infrared spectra and density functional calculations of transition metal hydrides and dihydrogen complexes

Metal hydrides are of considerable importance in chemical synthesis as intermediates in catalytic hydrogenation reactions. Transition metal atoms react with dihydrogen to produce metal dihydrides or dihydrogen complexes and these may be trapped in solid matrix samples for infrared spectroscopic study. The MH(2) or M(H(2)) molecules so formed react further to form higher MH(4), (H(2))MH(2), or M(H(2))(2), and MH(6), (H(2))(2)MH(2), or M(H(2))(3) hydrides or complexes depending on the metal. In this critical review these transition metal and dihydrogen reaction products are surveyed for Groups 3 though 12 and the contrasting behaviour in Groups 6 and 10 is discussed. Minimum energy structures and vibrational frequencies predicted by Density Functional Theory agree with the experimental results, strongly supporting the identification of novel binary transition metal hydride species, which the matrix-isolation method is well-suited to investigate. 104 references are cited.     

Mixed-metal hydride complexes containing ZrHA1 bridges. synthesis and relation to transition-metal-catalyzed reactions of aluminum hydrides

Dialkylaluminum hydrides and Cp₂ZrX₂ (Cp = π⁵-C₅H₅; X = Cl, H, alkyl) were found to react by aggregation to give mixed ZrAl trihydride complexes containing ZrHAl bridges. Reactions of these mixed-metal hydrides are discussed and structural parallels between them and known aluminum hydride complexes are drawn.     

Exchange of organic radicals with organo-cobalt complexes formed in the living radical polymerization of vinyl acetate

Exchange of organic radicals between solution and organo-cobalt complexes is experimentally observed and the reaction pathway is probed through DFT calculations. Cyanoisopropyl radicals from AIBN (2,2'-azobisisobutyronitrile) enter solutions of cobalt(II) tetramesityl porphyrin ((TMP)Co(II)*, 1) and vinyl acetate (VAc) in benzene and react to produce transient hydride (TMP)Co-H and radicals (*CH(OAc)CH2C(CH3)2CN (R1*)) that proceed on to form organo-cobalt complexes (TMP)Co-CH(OAc)CH3 (4, Co-R2) and (TMP)Co-CH(OAc)CH2C(CH3)2CN (3, Co-R1), respectively. Rate constants for cyanoisopropyl radical addition with vinyl acetate and hydrogen atom transfer to (TMP)Co(II)* are reported through kinetic studies for the formation and transformation of organo-cobalt species in this system. Rate constants for near-degenerate exchanges of radicals in solution with organo-cobalt complexes are deduced from (1)H NMR studies and kinetic modeling. DFT computations revealed formation of an unsymmetrical adduct of (TMP)Co-CH(OAc)CH3 (4) with *CH(OAc)CH3 (R2*) and support an associative pathway for radical interchange through a three-centered three-electron transition state [R...Co...R]. Associative radical interchange of the latent radical groups in organo-cobalt porphyrin complexes with freely diffusing radicals in solution that is observed in this system provides a pathway for mediation of living radical polymerization of vinyl acetate.     

Palladium-catalyzed addition of arylboronic acids to aldehydes

[reaction: see text] Arylboronic acids react with aldehydes in the presence of a base and a catalytic amount of a palladium(0) complex with chloroform, affording the corresponding secondary alcohols in good yields. General palladium complexes have no catalytic activity without chloroform. Chloroform is essential for this reaction, and palladium complex that was prepared from Pd(PPh3)4 with CHCl3 showed good catalytic acitivity as well.     

Titanium‐mediated living radical styrene polymerizations. V. Cp2TiCl‐catalyzed initiation by epoxide radical ring opening: Effect of solvents and additives

The effects of solvents, additives, ligands, and solvent in situ drying agents as well as catalyst and initiator concentrations have been investigated in the Cp₂TiCl‐catalyzed radical polymerization of styrene initiated by epoxide radical ring opening. On the basis of the solubilization of Cp₂Ti(III)Cl and the polydispersity of the resulting polymer, the solvents rank as follows: dioxane ≥ tetrahydrofuran > diethylene glycol dimethyl ether > methoxybenzene > diphenyl ether ≥ bulk > toluene ? pyridine > dimethylformamide > 1‐methyl‐2‐pyrrolidinone > dimethylacetamide > ethylene carbonate, acetonitrile, and trioxane. Alkoxide additives such as aluminum triisopropoxide and titanium(IV) isopropoxide are involved in alkoxide ligand exchange with the epoxide‐derived titanium alkoxide and lead to broad molecular weight distributions, whereas similarly to strongly coordinating solvents, ligands such as bipyridyl block the titanium active site and prevent the polymerization. By contrast, softer ligands such as triphenylphosphine improve the polymerization in less polar solvents such as toluene. Although mixed hydrides such as lithium tri‐tert‐butoxyaluminum hydride, sodium borohydride, and lithium aluminum hydride react with bis(cyclopentadienyl)titanium dichloride to form mixed titanium hydride species ineffective in polymerization control, simple hydrides such as lithium hydride, sodium hydride, and especially calcium hydride are particularly effective as in situ trace water scavengers in this polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2015–2026, 2006     

Xenon difluoride induced aryl iodide reductive elimination: a simple access to difluoropalladium(II) complexes

Palladium(II) aryliodo complexes bearing chelating diphosphine ligands react with XeF2, giving iodoarene and rare palladium(II) difluoro complexes. The reaction is general with regard to the aryl group, with even C6F5-I undergoing facile reductive elimination from a Pd center.     

Kinetics and mechanism of palladium(II) acetate reduction by hydrogen on the surface of a carbon support

The kinetics of the reduction of Pd(II) compounds by dihydrogen on the surface of a carbon support has been investigated for palladium acetate as an example. A kinetic model has been constructed for this reaction. An autocatalytic mechanism is suggested, in which the key role is played by Pd(0) compounds and their hydrides. The reaction occurring on the support surface is compared with the same reaction in solutions of palladium phosphine acetate complexes, where a similar mechanism is observed. One of the most important features of the surface reaction is the relatively slow reduction of the Pd(I) compounds to Pd(0). This makes it possible to obtain materials with a high Pd(I) content of 5% and above.     

Photochemical oxidative addition of B-H bonds at ruthenium and rhodium

Metal phosphine hydrides of type RuP(4)H(2) and RhP(3)H(3) react photochemically with HB(pin)(pin = pinacolate) to form metal boryl hydride complexes via 16-electron intermediates generated by H(2) loss; the second order rate constants for reaction of the intermediates with HB(pin) are even larger than those for reaction with Et(3)SiH.     

Cyanides,Isocyanides, and Hydrides of Zn,Cd and Hg from Metal Atom and HCN Reactions: Matrix Infrared Spectra and Electronic Structure Calculations

Zinc and cadmium atoms from laser ablation of the metals and mercury atoms ablated from a dental amalgam target react with HCN in excess argon during deposition at 5 K to form the MCN and MNC molecules and CN radicals. UV irradiation decreases the higher energy ZnNC isomer in favor of the lower energy ZnCN product. Cadmium and mercury atoms produce analogous MCN primary molecules. Laser ablation of metals also produces plume radiation which initiates H-atom detachment from HCN. The freed H atom can add to CN radical to produce the HNC isomer. The argon matrix also traps the higher energy but more intensely absorbing isocyanide molecules. Further reactions with H atoms generate HMCN and HMNC hydrides, which can be observed by virtue of their C−N stretches and intense M−H stretches. Computational modeling of IR spectra and relative energies guides the identification of reaction products by providing generally reliable frequency differences within the Zn, Cd and Hg family of products, and estimating isotopic shifts using to ¹³C and ¹⁵N isotopic substitution for comparison with experimental data.     

Investigation of the aqueous transmetalation of pi-allylpalladium with indium salt: the use of the Pd(OAc)2-TPPTS catalyst

pi-Allylpalladium complexes could be generated in water by the palladium(0) water soluble catalyst prepared in situ from palladium acetate and TPPTS. These complexes were transmetalated with indium to react with benzaldehyde. The aqueous solution of Pd(0)(TPPTS)(n) could be reused without deterioration of the catalyst in the first and second recycling. The system proved to be efficient with primary and secondary allylic substrates. The stereochemical outcome of the allylation through umpolung of allylpalladium, was also studied using models with a restraint conformation.     

Sustainable radical reduction through catalyzed hydrogen atom transfer reactions (CHAT-reactions)

A system with coupled catalytic cycles is described that allows radical reduction by catalyzed hydrogen atom transfer (CHAT) from transition metal hydrides. These intermediates are generated through H₂ activation. Radical generation is carried out by titanocene catalyzed electron transfer to epoxides. The reaction provides a novel entry into the atom-economical reduction of radicals that has long been considered as a critical issue for the industrial application of radical chemistry.     

Polymeric fullerene hydrides. Birch reduction of [60] fullerene polymers from solution-based photopolymerization and free radical polymerization reactions

Fullerene polymers represent a new class of carbon materials for potential hydrogen storage applications. Poly[60]fullerene polymers were obtained by covalently linking [60]fullerene molecules in photochemical reactions. [60]Fullerene polymers were also prepared in free radical reactions of [60]fullerene with radical initiator benzoyl peroxide. The polymeric [60]fullerene materials were hydrogenated under Birch reduction conditions. The hydrides, which contain ≈3.5% (wt/wt) of hydrogen, were characterized by use of gel permeation chromatography, NMR, FT-IR, and elemental analysis. The results are compared with those of monomeric [60]fullerene hydrides.