Reductive Elimination Of Benzene Research Articles

Multiple Aromatic C-H Bond Activations by an Unsaturated Dirhenium Carbonyl Complex.

Reactions of Re2(CO)8(μ-C6H5)(μ-H), 1, with naphthalene and anthracene have yielded the first multiply-CH activated arene products through the reductive elimination of benzene from 1 and multiple oxidative-additions of the dirhenium octacarbonyl grouping to these polycyclic aromatic compounds under very mild conditions. In addition, compound 1 was found to react with itself to yield the bis-Re2-metalated C6H4 bridged compound Re2(CO)8(μ-H)(μ-1,μ-3-C6H4)Re2(CO)8(μ-H), 3. Reaction of 1 with naphthalene yielded two doubly CH activated isomers, Re2(CO)8(μ-H)(μ-η2-1,2-μ-η2-3,4-C10H6)Re2(CO)8(μ-H), 4, 41% yield, and Re2(CO)8(μ-H)(μ-η2-1,2-μ-η2-5,6-C10H6)Re2(CO)8(μ-H), 5, via the mono CH activated intermediate Re2(CO)8(μ-η2-C10H7)(μ-H), 2. Compound 4 contains two Re2(CO)8(μ-H) groups on one C6 ring formed by CH activations at the 2- and 4-positions. Compound 5 contains two Re2(CO)8(μ-H) groups; one formed by CH activation at the 2-position on one C6 ring and the other formed by CH activation at the 6-position (or centrosymmetrically related 2'-position) on the second C6 ring. The Re2(CO)8(μ-H) groups are coordinated to the C6 rings by binuclear σ + π coordination to two adjacent carbon atoms in the rings. Compound 1 reacts with anthracene to yield the mono-CH activated compound Re2(CO)8(μ-η2-1,2-C14H9)(μ-H), 6, and two doubly CH activated compounds, Re2(CO)8(μ-H)(μ-η2-1,2-μ-η2-3,4-C14H8)Re2(CO)8(μ-H), 7, and Re2(CO)8(μ-H)(μ-η2-1,2-μ-η2-5,6-C14H8)Re2(CO)8(μ-H), 8. Compounds 7 and 8 are isomers that are structurally similar to 4 and 5. Compounds 7 and 8 can also be obtained in good yields from the reaction of 6 with 1. In the presence of a 5/1 ratio of 1/anthracene, a small amount (5% yield) of the tetra-substituted anthracene product [Re2(CO)8(μ-H)]4(μ-η2-1,2-μ-η2-3,4-μ-η2-5,6-μ-η2-7,8-C14H6), 9, was formed. Compound 9 contains four σ + π coordinated Re2(CO)8(μ-H) groups formed by oxidative additions of the CH bonds of anthracene to the Re2(CO)8 groups at the 2, 4, 6, and 8 positions of the three ring system. Molecular orbital calculations have been performed for all new compounds in order to develop an understanding of the bonding of the ring systems to the Re2(CO)8(μ-H) groups. All new compounds were characterized by single-crystal X-ray diffraction analyses.

Synthesis, Structure, and Reactivity of Hydridoiridium Complexes Bearing a Pincer‐Type PSiP Ligand

A series of iridium tetrahydride complexes [Ir(H)(4)(PSiP-R)] bearing a tridentate pincer-type bis(phosphino)silyl ligand ([{2-(R(2)P)C(6)H(4)}(2)MeSi](-), PSiP-R, R=Cy, iPr, or tBu) were synthesized by the reduction of [IrCl(H)(PSiP-R)] with Me(4)N·BH(4) under argon. The same reaction under a nitrogen atmosphere afforded a rare example of thermally stable iridium(III)-dinitrogen complexes, [Ir(H)(2)(N(2))(PSiP-R)]. Two isomeric dinitrogen complexes were produced, in which the PSiP ligand coordinated to the iridium center in meridional and facial orientations, respectively. Attempted substitution of the dinitrogen ligand in [Ir(H)(2)(N(2))(PSiP-Cy)] with PMe(3) required heating at 150 °C to give the expected [Ir(H)(2)(PMe(3))(PSiP-Cy)] and a trigonal bipyramidal iridium(I)-dinitrogen complex, [Ir(N(2))(PMe(3))(PSiP-Cy)]. The reaction of [Ir(H)(4)(PSiP-Cy)] with three equivalents of 2-norbornene (nbe) in benzene afforded [Ir(I)(nbe)(PSiP-Cy)] in a high yield, while a similar reaction of [Ir(H)(4)(PSiP-R)] with an excess of 3,3-dimethylbutene (tbe) in benzene gave the C-H bond activation product, [Ir(III)(H)(Ph)(PSiP-R)], in high yield. The oxidative addition of benzene is reversible; heating [Ir(III)(H)(Ph)(PSiP-Cy)] in the presence of PPh(3) in benzene resulted in reductive elimination of benzene, coordination of PPh(3), and activation of the C-H bond of one aromatic ring in PPh(3). [Ir(III)(H)(Ph)(PSiP-R)] catalyzed a direct borylation reaction of the benzene C-H bond with bis(pinacolato)diboron. Molecular structures of most of the new complexes in this study were determined by a single-crystal X-ray analysis.

C−H and C−CN Bond Activation of Acetonitrile and Succinonitrile by [Tp′Rh(PR3)

The [Tp′Rh(PR3)] fragments, where Tp′ = tris(3,5-dimethylpyrazolyl)borate, R3 = Me3 (a) or PhMe2 (b), were shown to be applicable for the activation of the C−H and C−CN bonds of acetonitrile. The photoirradiation of Tp′Rh(PR3)H2 (1a,b) in acetonitrile at room temperature afforded the complexes Tp′Rh(PR3)(CH2CN)H (3a,b), by the selective oxidative addition of the primary C−H bond of acetonitrile to the [Tp′Rh(PR3)] fragment generated by the dehydrogenation of 1. The thermal reactions of Tp′Rh(PMe3)(Ph)H (2a) or Tp′Rh(κ2-C6H4-2-PMe2)H (2b) in acetonitrile at 80 °C also resulted in the formation of 3a and 3b, respectively, by the oxidative addition of the C−H bond to the [Tp′Rh(PR3)] fragment, generated by the reductive elimination of benzene (from 2a) or intramolecular reductive elimination of the PPhMe2 ligand (from 2b). Heating the isolated C−H bond activated complexes 3a,b in acetonitrile at higher temperature (100 °C) for longer times (≈5 days) resulted in the formation of the corresponding C−CN bond ac...

Synthesis, characterization, and C–H/C–C cleavage reactions of two rhodium–trispyrazolylborate dihydrides

Treatment of Tp′Rh(PMe 3)Cl 2 and Tp′Rh(CNCH 2CMe 3)Cl 2 with Cp 2ZrH 2 produces Tp′Rh(PMe 3)H 2 and Tp′Rh(CNCH 2CMe 3)H 2, respectively, in excellent yield. Photolysis of benzene solutions of each dihydride complex generates hydrogen and the fragment [Tp′Rh(L)] which inserts into the solvent C–H bond. The phosphine dihydride has also been shown to be a catalyst for the hydrogenation of biphenylene, showing a capability to cleave C–C bonds. Reductive elimination of benzene from Tp′Rh(PMe 3)PhH is nearly 250 times slower than from Cp*Rh(PMe 3)PhH.

Improvements in the Hydrogenation of Nitrile Rubber Using Wilkinson's Catalyst

Abstract RhCl(PPh3)3 is an efficient catalyst precursor for the selective hydrogenation of C=C in acrylonitrile butadiene rubber(“nitrile rubber”, NBR). The established technology for the process using RhCl(PPh3)3 is to carry out reaction in the presence of a large excess of triphenylphosphine (PPh3), with monochlorobenzene (MCB) as solvent. In parallel with the hydrogenation of unsaturation in the rubber there is a side reaction involving the MCB, which produces benzene. This likely occurs via oxidative addition of the C-Cl bond in the monochlorobenzene to a Rh intermediate in the catalytic cycle for hydrogenation, followed by reductive elimination of benzene in conjunction with H2 addition to the Rh centre. This leads to formation of less active Rh intermediates which lead to rapid deterioration of catalytic activity in the absence of excess PPh3. It was postulated that some of the PPh3 in solution acts as a base that “mops up” excess HCl formed as a by product of the catalytic cycle. Supporting evidence comes from a novel improvement of the hydrogenation process, where the deactivation of catalyst, can be offset by the presence of bases, such as amines and metal oxides (as an alternative to adding a large excess of PPh3). This modification can improve catalyst activity with respect to levels of Rh used, or could be used to minimize the level of added co-catalysts needed to maintain useful rates of hydrogenation.

Stereoselective Decarbonylation of Methanol to Form a Stable Iridium(III) trans-Dihydride Complex

Thermolysis of the known cationic iridium complex [(PNP)Ir(C6H5)H]PF6 (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) in methanol results in reductive elimination of benzene and stereoselective methanol decarbonylation to form the new iridium(III) trans-dihydride carbonyl complex [(PNP)Ir(CO)H2]PF6 (trans-2). The iridium(III) dihydride complex [(PNP)Ir(H)2]PF6 (3) is formed as a minor product. cis-[(PNP)Ir(CO)H2]PF6 (cis-2) has been independently prepared by the reaction of dihydride 3 with CO and undergoes reductive elimination of hydrogen at room temperature to form the iridium(I) carbonyl [(PNP)Ir(CO)]PF6 (4), demonstrating that cis-2 is not an intermediate in the decarbonylation reaction.

The mechanism of protonolysis of phenylplatinum(II) bonds in complexes with phenyl trans to nitrogen or carbon donors

Addition of acids of the form HX (X = Cl, BF4, CF3SO3, CF3CO2) to complexes [PtPh2(NN)] (NN = bu2bpy = 4,4'-di-tert-butyl-2,2'-bipyridine) and [PtPh(NCN)] (NCN = 2,6-C6H3(CH2NMe2)2) at –78 °C gave the corresponding phenyl(hydrido)platinum(IV) complexes [PtX(H)Ph2(NN)] and [PtX(H)Ph(NCN)], which decomposed by reductive elimination of benzene at about –20 °C to give the platinum(II) complexes [PtXPh(NN)] and [PtX(NCN)]. Further addition of HCl to [PtClPh(NN)] at low temperature gave [PtHCl2Ph(NN)], which decomposed above –10 °C, to give benzene and [PtCl2(NN)]. The reaction of DBF4 in the presence of excess CD3OD with [PtPh2(NN)] led to formation of C6H5D and C6H4D2 but with [PtPh(NCN)] no multiple deuterium incorporation was observed in the product benzene. The mechanisms of these reactions are discussed. Key words: platinum, phenyl, benzene, protonolysis.

Phosphido-phosphinidene transformations. Ru 3(CO) 6-[P(Ph)C(Ph)C(Ph)](μ-PPh 2) 2: A novel six vertex nido-pentagonal bipyramidal structure via reductive elimination and acetylene insertion at a triruthenium centre

The complex Ru 3(CO) 6[P(Ph)C(Ph)C(Ph)](μ-PPh 2) 2 has been obtained by treating (μ-H)Ru 3(CO) 7(μ-PPh 2) 3 with C 2Ph 2 in aliphatic hydrocarbons and its structure has been determined by an X-ray diffraction study. The alkyne inserts into a RuP bond and there is reductive elimination of benzene, formed by coupling of a phenyl group of the μ-PPh 2 ligand with the cluster μ-hydride.

Carbon-hydrogen oxidative addition and reductive elimination reactions in a dinuclear iridium complex

Thermolysis of the hydridoallyliridium complex ({eta}{sup 5}-C{sub 5}Me{sub 5})({eta}{sup 3}-C{sub 3}H{sub 5})(H)Ir at relatively high concentrations of starting material in benzene leads to the dinuclear benzene C-H oxidative addition product ({eta}{sup 5}-C{sub 5}Me{sub 5})(C{sub 6}H{sub 5})Ir({eta}{sup 1},{eta}{sup 3}-C{sub 3}H{sub 4})({mu}-H)-Ir({eta}{sup 5}-C{sub 5}Me{sub 5}) (1). Heating this material causes reductive elimination of benzene from the dinuclear complex 1 in a clean first-order process. The lack of dependence of the rate of this process on the concentration of entering ligand demonstrates than an intermediate is formed in the reductive elimination; this species is proposed to be the coordinatively unsaturated dinuclear complex ({eta}{sup 5}-C{sub 5}Me{sub 5})Ir({eta}{sup 1},{eta}{sup 3}-C{sub 3}H{sub 4})Ir({eta}{sup 5}-C{sub 5}Me{sub 5}) (13). Transient intermediate 13 reacts with dative ligands such as phosphines, ethylene, and tert-butyl isocyanide to give direct trapping products ({eta}{sup 5}-C{sub 5}Me{sub 5})(L)Ir({eta}{sup 1},{eta}{sup 3}-C{sub 3}H{sub 4})Ir({eta}{sup 5}-C{sub 5}Me{sub 5}) (2-6), formed by coordination of the ligand to one of the iridium centers. Reaction of 13 with reagents having activated X-H bonds, such as benzene-d{sub 6}, H{sub 2}, and acetonitrile, leads to intermolecular oxidative addition products having structures analogous to that of 1. Comparative inter- and intramolecular kinetic isotope effect studies have provided evidence that in the reversible reactionmore » of 13 with benzene, an additional {eta}{sup 2}-arene intermediate is not required to explain the results. Thermolysis of 2 and 4, the PMe{sub 3{sup {minus}}} and ethylene-trapped products of 13, leads to three new dinuclear complexes (7, 8, and 9). These materials are formed by intramolecular insertion into the dative ligand C-H bonds, followed by rearrangement. 30 refs., 6 figs., 2 tabs.« less

ChemInform Abstract: Crystal Structure of (Ni2Ru3(η‐C5H5)2(CO)9(μ5‐PPh)), an Open Square‐Pyramidal Cluster Obtained via Condensation of Coordinatively Unsaturated (Ru3(μ‐H)(CO)9(μ‐PPh2)) with (Ni2(η‐C5H5)(CO)2).

AbstractThe pentanuclear title cluster (I) is obtained via "condensation" of the title compounds in refluxing hexane with reductive elimination of benzene.

Notes. Crystal structure of [Ni2Ru3(η-C5H5)2(CO)9(µ5-PPh)], an open square-pyramidal cluster obtained via condensation of co-ordinatively unsaturated [Ru3(µ-H)(CO)9(µ-PPh2)] with [Ni2(η-C5H5)2(CO)2

The novel pentanuclear cluster [Ni2Ru3(η-C5H5)2(CO)9(µ5-PPh)](2) has been obtained via‘condensation’ of [Ni2(η-C5H5)2(CO)2] with the 46-electron complex [Ru3H(CO)9(µ-PPh2)](1) and reductive elimination of benzene. Its structure has been determined by X-ray diffraction methods. Crystals are monoclinic, space group P21/a with a= 21.122(6), b= 12.656(2), c= 21.149(8)Å, β= 99.53(2)°, and Z= 8. The structure has been solved from diffractometer data by direct and Fourier methods and refined by full-matrix least squares to R= 0.056 for 4 318 observed reflections. Two crystallographically independent, but almost identical, pentanuclear clusters are present in which the metals are in an open square-pyramidal arrangement with the phosphorus from the PPh ligand interacting with all of them.