Decomposition of a key intermediate in ruthenium-catalyzed olefin metathesis reactions

Dinuclear ruthenium complex, with a bridging carbide and a hydride ligand, and methyltricyclohexylphosphonium chloride result from thermal decomposition of olefin metathesis catalyst, (IMesH2)(PCy3)(Cl)2Ru=CH2. Involvement of dissociated phosphine in the decomposition is proposed. The dinuclear complex has catalytic olefin isomerization activity, which can be responsible for competing isomerization processes in certain olefin metathesis reactions.     

Molybdenum-molybdenum quadruple bonds: An asymmetrically substituted dimer containing a single acetate bridge. Crystal and molecular structure of [Mo2Cl3(μ-OAc)(PMe3)3]·PhMe

[Mo₂(OAc)₄] reacts with three or more equivalents of lithium chloride and PMe₃ in thf to give [Mo₂Cl₃(μ-OAc)(PMe₃)₃]0.75thf (1). The IR spectrum of the complex shows Mo---O and Mo---Cl stretches at 350 and 300 cm⁻¹ respectively and the ¹H and ¹³C NMR spectra suggest several species are present in solution. [Mo₂Cl₃(μ-OAc)(PMe₃)₃] converts slowly in thf to [Mo₂Cl₄(PMe₃)₄] and [Mo₂(OAc)₄]. The structure of [Mo₂Cl₃(μ-OAc) (PMe₃)₃]0.5C₆H₅Me (2) has been determined by single-crystal X-ray diffraction methods. Crystals of the toluene solvate are tetragonal with a = 20.726(2), c = 11.776(2) Å, space GROUP = I4cm. The structure was solved by Patterson and Fourier methods and refined to R of 0.035 for the 539 observed data. The molecule contains two metal centres each of which shows 5-fold coordination. The two molybdenum atoms are linked by an acetate bridge and a short Mo---Mo bond of 2.121(3) Å. Remaining coordination sites are occupied on Mo(1) by two Cl and one PMe₃ and on Mo(2) by one Cl and two PMe₃ groups.     

Bis(di-t-butylphosphino)methane complexes of rhodium: homogeneous alkyne hydrosilylation by catalyst-dependent alkyne insertion into Rh---Si or Rh---H bonds. Molecular structures of the dimer [(dtbpm)RhCl]2 and of the silyl complex (dtbpm) Rh[Si(OEt)3](PMe3)

The homogeneous, Rh-catalysed hydrosilylation of but-2-yne with triethoxysilane has been studied. All rhodium complexes employed as catalyst precursors contain ^tBu₂PCH₂P^tBu₂ (“dtbpm”) as a chelating ligand. The crystal and molecular structure of the dimer [(dtbpm)RhCl]₂ (10) has been determined by X-ray diffraction. Complex 10 is shown to be a sluggish catalyst in hydrosilylation reactions of hex-1-ene, whereas but-2-yne is hydrosilylated more rapidly. A much more efficient and highly selective catalyst is 10 with added PPh₃, equivalent to the use of monomeric (dtbpm)RhCl(PPh₃). (E)-2-Triethoxysilylbut-2-ene is formed exclusively and with high turnover numbers in this case. For both 10 and its PPh₃ derivative, the 14-electron fragment [(dtbpm)RhCl], formed by dissociation processes, is the most likely active intermediate in a Harrod-Chalk-type catalytic cycle. The PPh₃ dissociation equilibrium has been studied in detail for (dtbpm)RhCl(PPh₃) and its thermodynamic parameters have been determined. With rhodium alkyl complexes as catalyst precursors, a different type of alkyne hydrosilylation catalysis, involving direct alkyne insertion into the Rh---Si bond of an intermediate rhodium silyl complex, (dtbpm)Rh[Si(OEt)₃](PMe₃) (14), has been found. Complex 14 was synthesized independently from (dtbpm)RhMe(PMe₃) and characterized by X-ray diffraction. It is an equally active catalyst itself, yielding (E)-2-triethoxysilylbut-2-ene as the major product (90%) from but-2-yne and HSi(OEt)₃ (turnover number 1000 per 30 min). The insertion step of the alkyne into the Rh---Si bond of 14 and the formation of two stereoisomeric rhodium vinyl complexes were established independently for MeO₂CCCCO₂Me as a more reactive alkyne substrate. A catalytic cycle is proposed for this unprecedented hydrosilylation reaction. The synthesis of the ν³-benzyl complex (dtbpm)Rh(η³-CH₂C₆H₅) (23) is described. This compound allows an alternative, more efficient access to the new silyl complex (dtbpm)Rh[Si(OEt)₃](PMe₃).     

Carbene derivatives of areneruthenium(II) complexes in one step from terminal alkynes

The complexes [(η⁶-arene)Ru=C(OMe)CH₂R′)Cl(PR₃)]PF₆ (R′ = Ph; ARENE = Me₄C₆H₂, ⁱPr₃C₆H₃, Et₃C₆H₃; PR₃ = PMe₃, PPh₃, P(OMe)₃) have been made from RuCl₂(PR₃)(arene) precursors by activation at room temperature of phenylacetylene in methanol containing NaPF₆. The complex with R′ = ⁿBu, ARENE = Me₄C₆H₂, and PR₃ = PMe₃ is similarly formed from hex-1-yne but much more slowly, and a complex of the type [(p-cymene)Ru=C(OMe)CH₂R′)Cl(PR₃)]⁺PF₆⁻ could be obtained only when the phosphine was the bulky PPh₃ (10b). It has been shown that the steric hindrance by both arene and phosphine ligands contributes to the stabilization of the carbeneruthenium complexes.     

铝卟啉配合物催化二氧化碳与环氧丙烷共聚反应

铝卟啉是一类土壤环境友好的金属卟啉,尽管早在1978年Inoue就已经发现它可以催化CO₂和环氧丙烷的共聚反应,但是该催化体系一直面临催化活性低、聚合物相对分子质量低等难题。 本文通过改变铝卟啉催化剂配体中苯环上取代基的种类和位置,制备出中心金属电子环境差异化的铝卟啉,并以双三苯基膦氯化铵(PPNCl)为助催化剂,探讨其对CO₂与环氧丙烷的共聚反应的催化行为。 结果表明,当铝卟啉中苯环上2,4位同时被Cl^-取代后,在90 ℃和3 MPa压力下,转化频率(TOF)达到2672 h^-1。 当利用离去能力较强的对甲苯磺酸基团(OTs^-)作为铝卟啉的轴向配体,可以合成出数均相对分子质量达1.84×10⁵的脂肪族聚碳酸酯。     

The electrochemical reduction of CP2TiCl2 in the presence of trimethylphosphine

The electrochemistry of the complexes Cp₂TiCl_x(PMe₃)_2-x in anhydrous THF has been studied. The most stable complexes at the three oxidation states of titanium are Cp₂TiCl₂, Cp₂TiCl(PMe₃) and Cp₂Ti(PMe₃)₂, and each of these species is readily formed by electrolysis. It has also been demonstrated that oxidation/reduction of these species is followed by facile and rapid ligand exchange to form the preferred species in the new oxidation state provided a stoichiometric concentration of the required ligand is present. The consequences of this redox and ligand exchange chemistry for the synthetic reactions catalyzed by lower oxidation states of Ti are discussed. Finally, the voltammetry of a titanocycle is reported, and it is shown that the corresponding Ti^III metallocycle is stable.     

Multiple ligand transfer reaction between [CpRu(L)(AN)2][PF6] (L=AN, CO, P(OMe)3; AN=acetonitrile) and CpFe(CO)L′X (L′=CO, PMe3, PMe2Ph, PMePh2, PPh3, P(OPh)3; X=Cl, Br, I)

Treatment of ruthenium complexes [CpRu(AN)₃][PF₆] (1a) (AN=acetonitrile) with iron complexes CpFe(CO)₂X (2a–2c) (X=Cl, Br, I) and CpFe(CO)L′X (6a–6g) (L′=PMe₃, PMe₂Ph, PMePh₂, PPh₃, P(OPh)₃; X=Cl, Br, I) in refluxing CH₂Cl₂ for 3 h results in a triple ligand transfer reaction from iron to ruthenium to give stable ruthenium complexes CpRu(CO)₂X (3a–3c) (X=Cl, Br, I) and CpRu(CO)L′X (7a–7g) (L′=PMe₃, PMe₂Ph, PMePh₂, PPh₃, P(OPh)₃; X=Br, I), respectively. Similar reaction of [CpRu(L)(AN)₂][PF₆] (1b: L=CO, 1c: P(OMe)₃) causes double ligand transfer to yield complexes 3a–3c and 7a–7h. Halide on iron, CO on iron or ruthenium, and two acetonitrile ligands on ruthenium are essential for the present ligand transfer reaction. The dinuclear ruthenium complex 11a [CpRu(CO)(μ-I)]₂ was isolated from the reaction of 1a with 6a at 0°C. Complex 11a slowly decomposes in CH₂Cl₂ at room temperature to give 3a, and transforms into 7a by the reaction with PMe₃.     

Platinum-catalyzed selective hydration of hindered nitriles and nitriles with acid- or base-sensitive groups

Hindered tertiary nitriles can be hydrolyzed under neutral and mild conditions to the corresponding amides using platinum(II) catalysts with dimethylphosphine oxide or other secondary phosphine oxides (SPOs, phosphinous acids) as ligands. We have found that this procedure also works well for nitriles with acid- or base-sensitive groups, which is unprecedented in terms of yield and selectivity. The catalyst loading can be as low as 0.5 mol %. Amides are isolated as the only product in high yield, and no further hydrolysis to the corresponding acids takes place. Reactions are carried out at 80 degrees C but take place even at room temperature. When enantiopure secondary phosphine oxide ligands are used in the hydrolysis of racemic nitriles, no kinetic resolution is observed, presumably due to racemization of the ligand during the reaction.     

PCl3- and organometallic-free synthesis of tris(2-picolyl)phosphine oxide from elemental phosphorus and 2-(chloromethyl)pyridine hydrochloride

In the superbase KOH/H₂O/toluene/phase-transfer catalyst system, 2-picolyl chloride, generated in situ from 2-(chloromethyl)pyridine hydrochloride, reacts with elemental phosphorus at 65–95?°C for 3?h to afford tris(2-picolyl)phosphine oxide in 50% yield. Single crystal X-ray analysis of the latter revealed one polymorph form of this tertiary phosphine oxide.     

Platination of [3-X-7,8-Ph2-7,8-nido-C2B9H8] (X=Et, F): Synthesis and characterisation of slipped and 1,2→1,7 isomerised products

The reaction of the labelled carborane ligand [3-Et-7,8-Ph₂-7,8-nido-C₂B₉H₈]²⁻ with a source of {Pt(PMe₂Ph)₂}²⁺ affords non-isomerised 1,2-Ph₂-3,3-(PMe₂Ph)₂-6-Et-3,1,2-closo-PtC₂B₉H₈ (1). The analogous reaction between [3-F-7,8-Ph₂-7,8-nido-C₂B₉H₈]²⁻ and {Pt(PMe₂Ph)₂}²⁺ yields 1,8-Ph₂-2,2-(PMe₂Ph)₂-4-F-2,1,8-closo-PtC₂B₉H₈ (3). Compound 1 has a heavily slipped structure (Δ 0.72 Å), which to some degree obviates the need for C atom isomerisation. However, that it is a kinetic product of the reaction is evident from the fact that it reverts to isomerised 1,8-Ph₂-2,2-(PMe₂Ph)₂-4-Et-2,1,8-closo-PtC₂B₉H₈ (2) slowly at room temperature but more rapidly with gentle warming. The heteroatom and labelled-B atom positions in the isomerised compounds 2 and 3 may be explained most simply by the rotation of a CB₂ face of an intermediate based on the structure of 1. Compounds 1–3 were characterised by a combination of spectroscopic and crystallographic techniques.     

高稳定性无膦配体乙烯氢甲酰化催化体系研究

高活性、高稳定性的无膦配体多相氢甲酰化催化体系研究是催化化学领域的重要课题。本文以乙烯氢甲酰化这一反应为目标,发展出含有不同含氧官能团的活性炭为载体的负载纳米铑催化材料。其中,当以Rh/C-3这一材料为催化剂时,乙烯氢甲酰化反应的转化频率可以达到57889 mol/mol/h。该催化剂可以在固定床反应器上稳定运行2500小时保持活性稳定。表征发现,碳材料表面的内酯基团 (-CO2-)对催化材料的活性和稳定性具有重要的作用。这一研究对高活性、高稳定性的非膦配体多相氢甲酰化催化体系研究具有一定的启示。     

Asymmetric Hydrogenation of Isoxazolium Triflates with a Chiral Iridium Catalyst

The iridium catalyst [IrCl(cod)]₂–phosphine–I₂ (cod=1,5‐cyclooctadiene) selectively reduced isoxazolium triflates to isoxazolines or isoxazolidines in the presence of H₂. The iridium‐catalyzed hydrogenation proceeded in high‐to‐good enantioselectivity when an optically active phosphine–oxazoline ligand was used. The 3‐substituted 5‐arylisoxazolium salts were transformed into 4‐isoxazolines with up to 95:5 enantiomeric ratio (e.r.). Chiral cis‐isoxazolidines were obtained in up to 89:11 e.r., with no formation of their trans isomers, when the substrates had a primary alkyl substituent at the 5‐position. The mechanistic studies indicate that the hydridoiridium(III) species prefers to deliver its hydride to the C5 atom of the isoxazole ring. The hydride attack leads to the formation of the chiral isoxazolidine via a 3‐isoxazoline intermediate. Meanwhile, in the selective formation of 4‐isoxazolines, hydride attack at the C5 atom may be obstructed by steric hindrance from the 5‐aryl substituent.     

9-BBN activation. Synthesis, crystal structure and theoretical characterization of the ruthenium complex Ru[(μ-H)2BC8H14]2(PCy3)

Reaction of the bis(dihydrogen) ruthenium complex RuH₂(H₂)₂(PCy₃)₂ (1) with an excess of 9-borabicyclononane yields Ru[(μ-H)₂BC₈H₁₄]₂(PCy₃) (6) and the phosphine adduct PCy₃·HBC₈H₁₄. The new complex is characterized by NMR spectroscopy and X-ray diffraction. New X-ray data on 9-BBN dimer, from a measurement at 180 K, are also reported. DFT calculations (B3LYP) on Ru[(μ-H)₂BC₈H₁₄]₂(PMe₃) (7), the PMe₃ analogue of 6, confirm the ruthenium (II) formulation with two dihydroborate ligands. The data obtained using PH₃ or PMe₃ as models for PCy₃ in PR₃·HBC₈H₁₄ are also discussed.     

Chloride concentration and temperature effects on the hydration of Th(IV) ion: a molecular dynamics simulation

Molecular dynamics simulations have been performed to investigate the structural and dynamical properties of the second hydration shell of Th⁴⁺ ion at various chloride concentrations and temperatures. When the concentration increases (ca. 5 M), the hydration of Th⁴⁺ ion involves the displacement of the water molecules by Cl⁻ ligand and slightly decreases the total coordination number. The residence time of water molecules in the second hydration shell decreases as a function of increasing solution temperature.     

Iridium-catalyzed hydrogenation of N-heterocyclic compounds under mild conditions by an outer-sphere pathway

A new homogeneous iridium catalyst gives hydrogenation of quinolines under unprecedentedly mild conditions-as low as 1 atm of H(2) and 25 °C. We report air- and moisture-stable iridium(I) NHC catalyst precursors that are active for reduction of a wide variety of quinolines having functionalities at the 2-, 6-, and 8- positions. A combined experimental and theoretical study has elucidated the mechanism of this reaction. DFT studies on a model Ir complex show that a conventional inner-sphere mechanism is disfavored relative to an unusual stepwise outer-sphere mechanism involving sequential proton and hydride transfer. All intermediates in this proposed mechanism have been isolated or spectroscopically characterized, including two new iridium(III) hydrides and a notable cationic iridium(III) dihydrogen dihydride complex. DFT calculations on full systems establish the coordination geometry of these iridium hydrides, while stoichiometric and catalytic experiments with the isolated complexes provide evidence for the mechanistic proposal. The proposed mechanism explains why the catalytic reaction is slower for unhindered substrates and why small changes in the ligand set drastically alter catalyst activity.     

Acyl complexes of molybdenum: structural and reactivity studies

The following structural peculiarities of the agostic acyl structure ₂R) (R = H, SiMe₃) and some characteristic chemical reactivity of the M-η²-acyl and iminoacyl linkage are described. (i) A structural comparison of the bonding parameters within three agostic acetyl Mo complexes containing the dithioacid ligand, indicates that the agostic interaction strengthens upon increasing the electron-releasing properties of the S-chelating ligand. (ii) The acyl-xanthate complex Mo(C(O)Me)(S₂COR)(CO)(PMe₃)₂ undergoes loss of a sulfur atom from the coordinated xanthate and coupling with the acyl ligand to form complexes containing coordinated alkoxythiocarbonyl and monothioacetate ligands. The latter can be metathetically replaced by KS₂COR. (iii) Upon heating at 70°C η²-acyl-dicarbonyl bispirazolilborate complexes of molybdenum of the type Mo(H₂B(pz^*)₂)(η²-C(O)Me)(CO)₂(PMe₃) (pz^* = 3,5-dimethyl-pyrazol-1-yl) yield functionalized acyl ligands derived from the stereo- and regioselective intramolecular addition of one of the B---H bonds of the H₂B(pz^*)₂ group across the C=O moiety of the η²-acyl group. (iv) The η²-acyl-isocyanide complexes {Mo}(η²-C(O)R)(CNR′) ({Mo} = Mo(H₂B(pz^*)₂)(CO)(PMe₃)) undergo irreversible thermal isomerization to the corresponding η²-iminoacyl-carbonyl derivatives {MO}(η²-C(NR′)R)(CO). This isomerization reaction follows first-order kinetics.     

Selective Zinc-Catalyzed 1,2-hydroboration of N-heteroaromatics via a Non-Hydride Mechanism

A novel bidentate amine-imine ligand precursor LH has been synthesized. This compound was reacted with ZnMe₂ to generate the zinc methyl complex, LZnMe ( 4 ). The latter compound was fully characterized by NMR spectroscopy and single crystal X-ray diffraction. Compound 4 is a catalyst for the hydroboration and hydrosilylation of N-heterocycles, but with moderate catalytic activity. A more active catalyst, the zinc hydride complex LZnH ( 5 ) was synthesized by reacting the lithium salt LLi with ZnCl₂ followed by sequential reaction with tBuOK and PhMeSiH₂. Compound 5 catalyzes the selective 1,2-hydroboration of nitrogen heteroaromatics with decreased catalyst load and under mild conditions. Deuterium-labeling experiments and kinetic studies provided insight into the possible reaction mechanism. It is proposed that hydride transfer to the substrate proceeds directly from the reductant (borane) via a six-membered transition state facilitated by the catalyst, in which it plays an ambiphilic role, activating the substrate via coordination to the Lewis acidic zinc and enhancing the hydricity of the borane through coordination to the zinc hydride.     

Übergangsmetall-Carbin-Komplexe LXXXVII. Synthese neuer, thermisch stabiler, kationischer Diethylaminocarbin-Komplexes des wolframs

trans-I(CO)₂L₂WCNEt₂ complexes (L₂ = 2,2′-bipyridyl (2,2′-bipy); 1,10-phenanthroline (ophen)) react with PR₃ (R = Me, Et) and thus undergo substitution of the iodine ligand by the phosphine to yield the new, thermostable, cationic carbyne complexes, [(PR₃)(CO)₂L₂WCNEt₂]⁺ I⁻. The ionic character of the compounds has been established from electrical conductivity studies of their solutions. Spectroscopic investigations of the complexes, whose composition has been determined by elemental analysis, indicate that in this reaction the halogen ligand in the trans position has been displaced by the chelate ligand, while the phosphine ligand occupies a cis coordination site, relative to carbyne moiety.     

Novel intramolecular C-H bond activation in an iridium dppm complex

Reaction of TpIr(C(2)H(4))(2) (Tp = tris-pyrazolylborate) with various chelating phosphine ligands has been explored. Reaction with bis-diphenylphosphinoethane leads to complete displacement of the Tp ligand. With bis-diphenylphosphinomethane, an intramolecular proton transfer from the methylene bridge to the iridium center occurs to give an iridium hydride complex formally resulting from oxidative C-H bond activation. Reaction with 2,2-bis(diphenylphosphino)propane (dppip) affords an Ir(I) complex formulated as kappa(2)-TpIr(dppip). Protonation of this Ir(I) complex gives a six coordinate Ir(III) hydride species.     

Equilibrium constants and kinetics of carbon monoxide insertion in alkyl complexes of ruthenium(II)

The equilibrium constants of the reaction of cis, trans-[Ru(CO)₂(PMe₃)₂(CH₃)I] (Mc) with carbon monoxide to give cis, trans[Ru(CO)₂(PMe₃)₂ (COMe)i] (Ac) and trans, trans[Ru(CO)₂(PMe₃)₂(COMe)I] (At) were measured at various temperatures in toluene. The thermodynamic parameters are compared with those obtained for the isoelectronic complexes of iron, and the trend is discussed. The kinetics of the carbonylation reaction of Mc, as well as those of the inverse decarbonylation reaction of At were measured. The kinetics of the carbonylation of the new complex trans, trans-[Ru(CO)₂(PMe₃)₂(CH₃)I] (Mt) were also investigated. All the results afford further support to the previously proposed CO insertion mechanism occurring via methyl migration. The comparison of these kinetic results with those of isoelectronic complexes of iron indicates that ruthenium is more reactive than iron, which is reflected by its greater aptitude to act as catalyst in many processes.