Carbyne Complexes Research Articles

Experimental and Theoretical Investigations of Catalytic Alkyne Cross-Metathesis with Imidazolin-2-iminato Tungsten Alkylidyne Complexes

The imidazolin-2-iminato tungsten alkylidyne complexes [Me3CC≡W(NImR)(OR′)2] (4a: R = tBu, R′ = CMe3; 4b: R = Dipp, R′ = CMe3; 5a: R = tBu, R′ = CMe(CF3)2; 5b: R = Dipp, R′ = CMe(CF3)2 have been prepared from [Me3CC≡W(OCMe3)3] (2) and [Me3CC≡W{OCMe(CF3)2}3(dme)] (3, dme = 1,2-dimethoxyethane) by reaction with the lithium reagents Li(NImtBu) and Li(NImDipp), generated with MeLi from 1,3-di-tert-butylimidazolin-2-imine (ImtBuNH) or 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (ImDippNH), respectively. Reaction of 3 with Li[N(tBu)Ar]·OEt2 (Ar = 3,5-dimethylphenyl) afforded the amido complex [Me3CC≡W{N(tBu)Ar}{OCMe(CF3)2}2] (6). Addition of Li[OCPh(CF3)2] to [Me3CC≡WCl3(dme)] (1) produced the dme-free complex [Me3CC≡W{OCPh(CF3)2}3] (7), which, upon treatment with Li(NImtBu), gave the alkylidene complex 8, presumably formed by activation and addition of an ortho-C−H bond across the W≡C bond in the intermediate alkylidyne complex. Treatment of 1 with Li[OC(CF3)3] led to the substitution of only two chloride...

Formation of carbyne complexes in reactions of laser-ablated Os atoms with halomethanes: characterization by C–H(X) and Os–H(X) stretching absorptions and computed structures

Reactions of laser-ablated Os atoms with halomethanes have been investigated. Small carbyne complexes are produced in reactions of Os atoms with fluoromethanes and identified through matrix infrared spectra and vibrational frequencies computed by density functional theory. The preference for the carbon-osmium triple bond is traced to the low energy of the Os carbyne products. The C-H and C-X stretching absorptions of the carbyne complexes are observed on the high frequency sides of the corresponding precursor bands, which result from the high s character in the C-H bond and interaction between the C-X and C-Os stretching modes, respectively. The calculated Os complex structures show a large variation with the ligands and electronic states, similar to the analogous Ru complex structures. The present report also compares previous Fe, Ru, and Os results and supports the general trend that the higher oxidation state complexes become more stable on going down the family group column.

Metal fragment isomerisation upon grafting a d2 ML4 perhydrocarbyl Os complex on a silica surface: origin and consequence

Grafting of Os bisalkylidene complexes, [Os(=CHtBu)(2)(CH(2)tBu)(2)], on a silica partially dehydroxylated at 700 degrees C selectively yields an alkylidyne complex [([triple bond]SiO)Os([triple bond]CtBu)(CH(2)tBu)(2)] according to mass balance analysis, IR and solid state NMR spectroscopies, but 70% of the silanols remains unreacted. Grafting corresponds to a replacement of one alkyl by a siloxy ligand and induces the isomerisation of the metal fragment from a bis-alkylidene to an alkyl alkylidyne. Molecular (B3PW91) and periodic DFT-calculations show that the bis-alkylidene is the energetically favoured isomer for the perhydrocarbyl complex, while the alkyl alkylidyne isomer is more stable when one of the alkyl ligands is replaced by a siloxy ligand. The change of the nature of the ligand is accompanied with a change of geometry: from a distorted tetrahedral structure for [Os(=CHtBu)(2)(CH(2)tBu)(2)] to a butterfly-geometry with apical siloxy and alkylidyne ligands for [([triple bond]SiO)Os([triple bond]CtBu)(CH(2)tBu)(2)]. Finally, DFT calculations show that grafting occurs via a sigma-bond metathesis between the silanol and a metal-alkyl bond and not through the typical addition of the silanol onto the alkylidene ligand.

Generation of α-Phosphinocarbene Complexes and Their Evolution: New Light on Relevant Isomerization Pathways

The η1-α-phosphinocarbene complex Cp(CO)2Mn═CR(PHMes) (R = Ph, Me) is generated by nucleophilic attack of MesPH2 at the electrophilic center of the cationic Mn−carbyne complex [Cp(CO)2Mn≡CR]+, followed by deprotonation. One of its identified isomerization pathways to η1-phosphaalkene derivatives is shown to be both base-catalyzed and stereoselective, and to involve a transient phosphidocarbene species engaged in an acid-catalyzed migratory CO-insertion step.

Facile Generation of Lithiocarbyne Complexes: [M(≡CLi)(CO)2{HB(pzMe2)3}] (M = Mo, W; pz = Pyrazol-1-yl)

The reactions of [M(≡CX)(CO)2{HB(pzMe2)3}] (M = Mo, W; X = Cl, Br; pz = pyrazol-1-yl) with nBuLi provide the lithiocarbyne complexes [M(≡CLi)(CO)2{HB(pzMe2)3}], which react with a range of electrophiles to provide variously functionalized carbyne and chalcocarbonyl complexes [Mo(≡CE)(CO)2{HB(pzMe2)3}] (E = H, Me, SiMe3, SPh, SePh, Se−, Te−, Fe(CO)2Cp).

Chemistry of Unsaturated Group 6 Metal Complexes with Bridging Hydroxy and Methoxycarbyne Ligands. 4. Carbonyl, Isocyanide, and Diphosphine Derivatives of the Complexes [M2(η5-C5H5)2(μ-COMe)(μ-PR2)2]BF4 (M = W, R = Ph; M = Mo, R = Et)

The unsaturated complexes [M2Cp2(μ-COMe)(μ-PR2)2]BF4 [M = W, R = Ph (1a); M = Mo, R = Et (1b); Cp = η5-C5H5] react with CO under pressure [ca. 4 atm (1a) or ca. 60 atm (1b)] to give the corresponding electron-precise dicarbonyl derivatives [M2Cp2(μ-COMe)(μ-PR2)2(CO)2]BF4, which display a parallel arrangement of the carbonyl ligands (W−W = 2.9020(5) Å for the ditungsten complex). The reactions with CNtBu take place rapidly at room temperature to give analogously the corresponding derivatives with parallel isocyanide ligands, cis-[M2Cp2(μ-COMe)(μ-PR2)2(CNtBu)2]BF4. At low temperature, however, a less stable isomer can be detected, which in the ditungsten system is identified as the isomer having a transoid arrangement of isocyanide groups, trans-[M2Cp2(μ-COMe)(μ-PR2)2(CNtBu)2]BF4, while for the dimolydenum system that isomer is identified as the ketenylimine complex [Mo2Cp2{μ-η1:η1-C(OMe)C(NtBu)}(μ-PEt2)2(CNtBu)]BF4, a product resulting from the C−C coupling between carbyne and isocyanide ligands; these products rearrange slowly at room temperature to yield the corresponding and more stable cis isomers. The reactions of compounds 1a,b with the diphosphine Me2PCH2PMe2 (dmpm) yield analogously addition derivatives of formula [M2Cp2(μ-COMe)(μ-PR2)2(μ-dmpm)]BF4, with the diphosphine ligand being placed preferentially either trans to the carbyne or trans to a diphenylphosphide bridge depending on the experimental conditions. The structure of the first isomer was confirmed through an X-ray study on the ditungsten complex (W−W = 2.917(1) Å). Diazoalkanes N2CRR′ (R = H, R′ = SiMe3; R = R′ = Ph) react with the ditungsten carbyne complex 1a without loss of dinitrogen nor coupling to the methoxycarbyne ligand to yield, after spontaneous demethylation of the latter, the neutral diazoalkane complexes [W2Cp2(μ-PPh2)2(κ1-N2CRR′)(CO)]. Carbonylation of the isocyanide complex cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CNtBu)2]BF4 does not induce any coupling process but just partial substitution to give the carbonyl isocyanide derivative cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CO)(CNtBu)]BF4, whereas its reduction with sodium amalgam induces an N−C(tBu) bond cleavage, yielding the cyanide derivative cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CN)(CNtBu)]. The latter complex can be methylated with [Me3O]BF4 to give the methylisocyanide derivative cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CNMe)(CNtBu)]BF4, and this reduction/methylation sequence can be repeated once more to give the bis(isocyanide) complex cis-[W2Cp2(μ-COMe)(μ-PPh2)2(CNMe)2]BF4 via the corresponding cyanide intermediate.

Infrared Spectra of Insertion, Methylidene, and Methylidyne Complexes in Reactions of Laser‐Ablated Ruthenium Atoms with Halomethanes and Methane

Reactions of laser-ablated Ru atoms with halomethanes and methane have been investigated. Small carbyne and carbene complexes or insertion products are produced in the reactions of halomethanes via C–X insertion and halogen migration, whereas only the insertion complex is formed in the reactions of methane. The results are consistent with computed product stabilities. The diagnostic C–H and C–X stretching absorptions of the carbyne complexes are observed on the blue side of the corresponding precursor bands. The high C–H and C–X stretching frequencies are explained with the higher s character in hybridization and combination with the C≡Ru stretching motion, respectively. The structures of the carbyne complexes are highly distorted and show a large variation with the ligands, and the extra electron pair on the Ru atom evidently plays an important role in the carbyne. In contrast to early transition-metal analogs, the Ru carbene complexes show no agostic distortion.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Methylidyne Complexes Prepared by Reactions of Laser-Ablated Os Atoms with Methane, Methyl Halides, and Ethane: Observation of the C—H and Os—H Stretching Absorptions

Simple Os carbyne complexes (HC≡OsH3, HC≡OsH2F, HC≡OsH2Cl, HC≡OsH2Br, and CH3C≡OsH3) are produced in reactions of laser-ablated Os atoms with small alkanes and methyl halides via oxidative C−H insertion and α-hydrogen migration. The diagnostic methylidyne C—H stretching absorptions of HC≡OsH3 and its halide derivatives are observed about 200 cm−1 above the precursor C—H stretching bands, and the frequencies are consistent with s character in hybridization in the C—H bond. The methylidyne complexes all have computed Cs structures, and particularly the HC≡OsH3 and CH3C≡OsH3 complexes have two equivalent shorter and one longer Os—H bonds, parallel to the case of Re in similar complexes. The strong unique Os—H stretching absorption at an exceptionally low frequency is traced to the stretching mode of the long Os—H bond and antibonding character in the doubly occupied HOMO.

Infrared Spectra of Methylidynes Formed in Reactions of Re Atoms with Methane, Methyl Halides, Methylene Halides, and Ethane: Methylidyne C−H Stretching Absorptions, Bond Lengths, and s Character

Rhenium carbyne complexes (HC identical with ReH 3, HC identical with ReH 2X, HC identical with ReHX 2, [X = F, Cl, and Br] and CH 3C identical with ReH 3) are produced by reactions of laser-ablated Re atoms with methane, methyl halides, methylene halides, and ethane via oxidative C-H(X) insertion and alpha-hydrogen migration in favor of the carbon-metal triple bond. The stabilities of the carbyne complexes relative to other possible products are predicted by DFT calculations. The diagnostic methylidyne C-H stretching absorptions of HC identical with ReH 3 and its mono- and dihalo derivatives are observed on the blue sides of the precursor C-H stretching bands, and the frequency decreases and the bond length increases in the order of H, F, Cl, and Br, following the decreasing s character in hybridization for the C-H bond. The dihalo methylidynes have higher C-H stretching frequencies and s characters than the monohalo species. The rhenium methylidynes have C s structures, and as a result the HC identical with ReH 3 and CH 3C identical with ReH 3 complexes have two equivalent shorter and one longer Re-H bonds, as compared to the tungsten methylidyne HC identical with WH 3 with three equivalent W-H bonds.

Preparation of Cyclophanes by Room-Temperature Ring-Closing Alkyne Metathesis with Imidazolin-2-iminato Tungsten Alkylidyne Complexes

Room-temperature ring-closing alkyne metathesis of 1,2-, 1,3-, and 1,4-bis(3-pentynyloxymethyl)benzenes has been investigated in the presence of catalytic amounts of an imidazolin-2-iminato tungsten alkylidyne complex. The m- and p-diynes selectively form the respective [10]metacyclophane or [10.10]paracyclophane, respectively, whereas a mixture of monomeric and dimeric cycloalkynes is obtained in the case of the o-diyne. DFT calculations reveal that the different selectivities can be attributed to the relative thermodynamic stability of the emerging cyclophanes.

Chemistry of Unsaturated Group 6 Metal Complexes with Bridging Hydroxy and Methoxycarbyne Ligands. 3. Formation and Cleavage of C−C and C−O Bonds in the Reactions of the Complexes [Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4(R = Me, Et)

The reactions of the 30-electron complexes [Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4 (Cp = η5-C5H5; R = Me, Et) with CO, CNtBu, or tetraethylpyrophosphite (tedip) lead to the new 32-electron complexes [Mo2Cp2{µ-η2:η2-C(OMe)C(OR)}(µ-PCy2)(CO)2]BF4, [Mo2Cp2{µ-η2:η2-C(OMe)C(OMe)}(µ-PCy2)(CNtBu)2]BF4, and [Mo2Cp2{µ-η2:η2-C(OMe)C(OMe)}(µ-PCy2)(µ-tedip)]BF4, respectively. These products contain a bonded dialkoxyacetylene molecule resulting from the coupling of two alkoxycarbyne ligands, induced by the addition of the donor molecules to the unsaturated dimetal center of the starting material, and this coupling can be fully reversed in the dicarbonyl product upon photochemical treatment. The structure of the diisocyanide and tedip complexes, both displaying quite short intermetallic lengths (ca. 2.66 Å), was confirmed crystallographically. The structure of the dicarbonyl derivative, which in solution exhibits cis and trans isomers, was optimized using DFT methods, which also led to the prediction of short intermetallic distances (2.75 Å). Demethylation of the dicarbonyl complexes leads to the new carboxycarbyne derivatives [Mo2Cp2{µ-C(CO2R)}(µ-PCy2)(CO)2] (R = Me, Et) as a result of the unexpected 1,2-shift of the methoxyl group occurring in the ketenyl intermediate (not detected) presumably formed first. This proposal is supported by DFT calculations, these predicting a moderate activation barrier (ΔG⧧298 = 78 kJmol−1) for the spontaneous transformation (ΔG298 = −51 kJ mol−1) of the ketenyl intermediate into the carbyne complex finally isolated. The bis(methoxycarbyne) complex is also reactive toward the 16-electron fragment Ru(CO)4, to give the 46-electron cluster [Mo2RuCp2(µ-COMe)2(µ-PCy2)(CO)4]BF4, which concentrates most of its electronic unsaturation at the dimolybdenum center (Mo−Mo = 2.686(1) Å), in spite of the rearrangement of the carbyne ligands, both of them now bridging one of the Mo−Ru edges, according to an X-ray diffraction study.

Preparation and Structures of Several Complexes Containing Carbon-Rich Ligands Attached to Polynuclear Metal Carbonyls

Several new gold-containing cluster complexes have been prepared from the reactions of gold alkynyl complexes, LnM-Cx-Au(PPh3), (x = 3, 4, 6) with Ru3(CO)10(NCMe)2. The bis-cluster complex 1,4-{AuRu3(CO)9(PPh3)(μ3-C2)}2C6H4 was obtained from Ru3(CO)10(NCMe)2 and 1,4-{(Ph3P)Au(C≡C)}2C6H4. The complexes Ru3(μ-H){μ3-C2C≡C[Ru(PP)Cp′]}(CO)9 [PP = (PPh3)2, Cp′ = Cp; PP = dppe, Cp′ = Cp*] were also obtained as minor by-products and synthesised independently from Ru(C≡CC≡CH)(PP)Cp′. A reaction between Co3{μ3-CC≡CC≡CAu(PPh3)}(μ-dppm)(CO)7 and Ru3(CO)12 afforded {(Ph3P)(OC)9AuRu3}C≡CC≡CC{Co3(μ-dppm)(CO)7} 7. Related complexes AuRu3{μ3-C2C≡[M(CO)2Tp]}(CO)9(PPh3) (M = Mo 8, W 9) were obtained from {Tp(OC)2M}≡CC≡C{Au(PPh3)}, while the mixed metal cluster complexes MoM2(C2Me)(CO)8Tp (M = Ru 13, Fe 14) were obtained from M(≡CC≡CSiMe3)(CO)2Tp (M = Mo, W) with Fe2(CO)9 and Ru3(CO)12, respectively. Reactions of the Mo carbyne complex with Co2(LL)(CO)6 [LL = (CO)2, μ-dppm] or nickelocene afforded complexes 15–17 in which Co2 and Ni2 fragments, respectively, had coordinated to the C≡C triple bond. XRD structural determinations of 7, 8, 14, 16 and {Tp(OC)2W}≡CC≡CC≡{Co3(μ-dppm)(CO)7} (18-W) are reported.

Alkane metathesis by a tungsten carbyne complex grafted on gamma alumina: Is there a direct chemical role of the support?

The activity in alkane metathesis of tungsten complexes grafted on gamma alumina raises many questions regarding the structure of the active site and the reaction pathways. We examine the case of a perhydrocarbyl tungsten complex on alumina from periodic density functional theory calculations. The quantum chemistry exploration of energy pathways suggests two possible mechanisms for alkane metathesis. The first mechanism occurs on the grafted complex only but is significantly activated. The second mechanism shows that hydration defects on the alumina surface can be active in alkane dehydrogenation toward alkenes, whereas the tungsten complex efficiently catalyzes the subsequent olefin metathesis step. The calculations thus underscore the role of the alumina support in the mechanism.

Terminal Carbido Complexes of Osmium: Synthesis, Structure, and Reactivity Comparison to the Ruthenium Analogues

The first terminal carbide complex of osmium, Os(⋮C)(PCy3)2Cl2 (4), was synthesized via S-atom abstraction from Os(CS)(PCy3)2Cl2 (4-S) by Ta(OSi-t-Bu3)3 (7). Compound 4 reacts with HO3SCF3 (HOTf) to form the first cationic osmium methylidyne complex [Os(⋮CH)(PCy3)2Cl2][OTf] (18). The analogous ruthenium complex [Ru(⋮CH)(H2IMes)(PCy3)Cl2][OTf] is not observed upon protonation of Ru(⋮C)(H2IMes)(PCy3)Cl2 (2) with HOTf. Substitution of the chloride ligands in 4 is surprisingly difficult compared to the case of 4-S, but Os(⋮C)(PCy3)2I2 can be obtained. Compound 4 also reacts with a variety of electrophiles to afford cationic five-coordinate and neutral six-coordinate carbyne complexes. In general, 4 reacts more readily with electrophiles than does 1, and cationic carbyne complexes formed from 4 are more resistant to degradation than are their Ru counterparts.

Synthesis of ruthenium phenylindenylidene, carbyne, allenylidene and vinylmethylidene complexes from (PPh 3) 3−4RuCl 2: A mechanistic and structural investigation

The reaction of (Ph 3P) 3RuCl 2 with 1,1-diphenyl-2-propyn-1-ol was investigated in various solvents. The reaction in thf under reflux is reported to produce the (PPh 3) 2Cl 2Ru(3-phenylindenylidene) complex ( 3) which has undergone rearrangement of the allenylidene C 3-spine. We have improved the reliability of the reported synthesis by adding acetyl chloride which converts the formed water of the reaction and thus increases the acidity of the reaction solution. Without the additive, we observed the exclusive formation of an intermediate of the transformation and identified it as dinuclear (PPh 3) 2ClRu(μ-Cl) 3(PPh 3) 2Ru C C CPh 2 complex ( 5). The reaction of (Ph 3P) 3−4RuCl 2 with 1,1-diphenyl-2-propyn-1-ol in CH 2Cl 2 or C 2H 4Cl 2 under reflux in the presence of excess conc. aqueous HCl afforded the new, neutral (PPh 3) 2Cl 3Ru C–CH CPh 2 carbyne complex ( 7), an HCl adduct of previously elusive (PPh 3) 2Cl 2Ru C C CPh 2 complex 6 in high yields. In contrast to the formation of complex 3, the reaction in a non-coordinating solvent did not afford the rearrangement of the allenylidene C 3-spine. Complex 7 was converted into complex 3 in thf under reflux under loss of a molecule HCl. Complex 7 was converted with triethylamine under loss of HCl to complex 6. Pentacoordinate complex 6 was crystallized in the presence of O-donor ligands (EtOH, MeOH and H 2O) to give hexacoordinate (PPh 3) 2Cl 2(ROH)Ru C C CPh 2 (R = H, CH 3, C 2H 5) complexes ( 9)–( 11) with the O-donor coordinating in trans-position to the allenylidene moiety. The reaction of complex 7 with 2 equiv. of 4-( N, N-dimethylamino)pyridine (DMAP) gave hexacoordinate (PPh 3) 2Cl 2(DMAP)Ru C C CPh 2complex ( 12) with one molecule DMAP also coordinating in trans-position to the allenylidene group. Methanol and acetic acid in the absence of strong bases afforded the Fischer-carbene complexes (PPh 3) 2Cl 2Ru C(OCH 3)–CH CPh 2 ( 14) and (PPh 3) 2Cl 2Ru C(OAc)–CH CPh 2 ( 15) where the nucleophile added to the α-carbon atom. The structures of complexes 5, 7, 9–11, 14, and 15 were solved via X-ray crystallography.

Oxidative Reactions of the MoIV Dialkyl Complex [{(3‐CF3C6H4NCH2CH2)2NMe}Mo(CH2SiMe3)2

The structure of [(CF3N2NMe)Mo(CH2SiMe3)2] (in which (CF3N2NMe)2- is [(3-CF3C6H4NCH2CH2)2NMe]2-) is approximately trigonal bipyramidal with one axial and one equatorial alkyl ligand. Heating of solutions of [(CF3N2NMe)Mo(CH2SiMe3)2] in [D6]benzene in the presence of five equivalents of 2-butyne led to diamagnetic [(CF3N2NMe)Mo(CHSiMe3)(eta(2)-MeC[triple bond]CMe)], whose structure is approximately square pyramidal with the alkyne occupying the axial site. Addition of one equivalent of cyclohexene sulfide to [(CF3N2NMe)Mo(CH2SiMe3)2] at room temperature produced the diamagnetic, dimeric molybdenum(IV) sulfido complex, [{(CF3N2NMe)MoS}2]. This complex is composed of two approximately trigonal bipyramidal centers, each containing one axial and one equatorial sulfur atom. Oxidation of [(CF3N2NMe)Mo(CH2SiMe3)2] with hexachloroethane resulted in formation of tetramethylsilane, HCl, and the sparingly soluble, red alkylidyne complex, [{(CF3N2NMe)Mo(CSiMe3)Cl}2]. This complex forms a dimer through bridging chlorides. The oxidation reactions of [(CF3N2NMe)Mo(CH2SiMe3)2] with 2-butyne, cyclohexene sulfide, or C2Cl6 are all proposed to proceed by alpha-hydrogen abstraction in the Mo(VI) species to yield (initially) the Mo=CHSiMe3 species and tetramethylsilane.

Effect of Different Bases and Phosphorus Ylide on the Selective Deprotonation of Phosphorus Ylide Adduct Cp*TaCl4(CH2PPh3)

The effect of different bases and phosphorus ylide on the selective deprotonation of phosphorus ylide adduct Cp*TaCl4(CH2PPh3) (4) was studied. The acidity of the methylene hydrogen atoms in ylide adduct complex 4, [Cp*TaCl4(CH2PPh3)], is weaker than those in the derivative complex [CpTaCl4(CH2PPh3)]. Under the same conditions complex 4 cannot transylidate. With the strong, but weakly coordinating base bis(trimethylsilyl)amide a double deprotonation of two methyl groups on the pentamethylcyclopentadienyl ring occurs, and the novel ylide adduct complex 6, with an η3-ally-η4-butadiene ligand, is obtained. Only when elimination of three chloride ligands is compensated by at least three coordinating ylide molecules can the strong base bis(trimethylsilyl)amide selectively deprotonate in a Ta−CH position, affording the expected carbyne complex 5.

Preparation of Tungsten Alkyl Alkylidene Alkylidyne Complexes and Kinetic Studies of Their Formation

An equilibrium mixture of alkyl alkylidyne W(CH2SiMe3)3(CSiMe3)(PMe3) (1a) and its bis(alkylidene) tautomer W(CH2SiMe3)2(=CHSiMe3)2(PMe3) (1b) has been found to undergo an alpha-hydrogen abstraction reaction in the presence of PMe3 to form alkyl alkylidene alkylidyne W(CH2SiMe3)(=CHSiMe3)(CSiMe3)(PMe3)2 (2). In the presence of PMe3, the formation of 2 follows first-order kinetics, and the observed rate constant was found to be independent of the concentration of PMe3. The activation parameters for the formation of 2 are Delta H = 28.3(1.7) kcal/mol and Delta S = 3(5) eu. In the presence of PMe2Ph, an equilibrium mixture of W(CH2SiMe3)3(CSiMe3)(PMe2Ph) (3a) and its bis(alkylidene) tautomer W(CH2SiMe3)2(=CHSiMe3)2(PMe2Ph) (3b) was similarly converted to W(CH2SiMe3)(=CHSiMe3)(CSiMe3)(PMe2Ph)2 (4). The observed rate of this reaction was also independent of the concentration of PMe2Ph. These observations suggest a pathway in which the tautomeric mixtures 1a,b and 3a,b undergo rate-determining, alpha-hydrogen abstraction, followed by phosphine coordination, resulting in the formation of the alkyl alkylidene alkylidyne complexes 2 and 4.

Redox induced reactions of transition metal π- and σ,π-complexes

In this review, redox-induced reactions of π- and σ,π-complexes leading to the selective formation (or cleavage) of C–H, C–C, and C–O bonds have been summarized. To illustrate the synthetic potential of such methodology, the following representative reactions studied in our group are discussed: (1) oxidatively induced hydrogen elimination from “open” cyclic diene and dienyl complexes resulting in formation of “closed” dienyl and arene complexes, respectively; (2) reductive activation of C–H bonds in diene, vinylidene, and carbyne complexes forming new multiple C–C bonds; (3) oxidative dehydrodimerization of vinylidene complexes into binuclear μ-divinylidene species; and (4) oxidatively induced addition of oxygen nucleophiles to μ-divinylidene complexes affording cyclic μ-dicarbene derivatives.

Operationally Unsaturated Pincer/Rhenium Complexes Form Metal Carbenes from Cycloalkenes and Metal Carbynes from Alkanes

Operationally unsaturated (i.e., 16/18-electron) (PNPR)Re(H)4, where PNPR is N(SiMe2CH2PR2)2, is reactive at 22 degrees C with cyclic olefins. The first observed products are generally (PNPR)Re(H)2(cycloalkylidene), with hydrogenated olefin as the product of hydrogen abstraction from the tetrahydride. The tetrahydride complex with R = tBu generally fails to react (too bulky), that with R = cyclohexyl suffers a (controllable) tendency to abstraction of 3H from one ring, forming an eta3-cyclohexenyl compound, and that with R = iPr generally gives the richest bimolecular reactivity. The cyclic monoolefins studied show distinct reactivity, C6 giving first the carbene and then coordinated cyclohexadiene, C5 giving carbene, then diene, and then eta5-C5H5, C8 giving carbene and then eta2-cyclooctyne, and C12 giving an eta3-allyl. Norbornene gives a pi-complex of the norbornene in thermal equilibrium with its carbene isomer; at 90 degrees C, hydrocarbon ligand Calpha-Cbeta bond cleavage occurs to give, for the first time, a carbyne complex from an internal olefin. Two compounds synthesized here have the formal composition "(PNPR)Re + olefin", and each of these is capable of dehydrogenating the methyl group of a variety of alkanes at 110 degrees C to form (PNP)ReH triple bond (CR).