Element−Element Addition to Alkynes Catalyzed by the Group 10 Metals

Publisher Summary The rapid development of the chemistry of transition-metal complexes containing terminal carbene (A) or carbyne (B) ligands has been followed more recently by much research centered on bridged methylene compounds (C). As often occurs in new and developing areas of chemistry, some confusion about the nomenclature of these complexes has arisen. Protonation or alkylation of several ethynyl–metal derivatives gives the corresponding vinylidene complexes in high yield. Several vinylidene complexes of the group VI metals have been obtained by heating σ–chlorovinyl derivatives with tertiary phosphines, phosphites, arsines, or stibines. Several complexes containing μ -C=CHR ligands have been obtained directly from 1-alkynes and two equivalents (or excess) of an appropriate precurso. A general route to complexes containing propadienylidene ligands is by loss of water or alcohols from suitable carbene or vinylidene precursors, or of oxo or alkoxy functions from ynolate anions. The majority of the chemistry of vinylidene and propadienylidene complexes is concerned with their synthesis and reactions. Vinylidene is one of the best π-acceptors known and is exceeded only by SO 2 and CS. The vinylidene ligand occupies an important place in the sequence of reactions linking a variety of well-known η 1 -carbon-bonded ligands. Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces.

Vinylidene and Propadienylidene (Allenylidene) Metal Complexes

The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds

Phosphorus ylides are not only classical reagents in organic chemistry, but also play an increasingly important role as novel components in organometallic compounds. These metallic “ylide complexes” are either synthesized from “preformed ylides” and coordination compounds by addition or substitution, on the building block principle, or they are formed, in sometimes complicated reactions, from phosphanes, metal complexes, and C1 substrates in the coordination sphere of the metals. The resulting metal-carbon bonds are greatly modified in their properties by the immediate presence of the phosphonium center and often belong to the most stable of M-C structural units. The metal can come from any group of the periodic table, including the lanthanoids and actinoids. Numerous preparative and structural studies are gradually enabling us to gain an overall picture of the scope of this area of research.

Phosphorus Ylides in the Coordination Sphere of Transition Metals: An Inventory

Publisher Summary Organometallic chemistry encountered rapid expansion experienced by the synthesis, spectroscopy, structural chemistry, theory, and reactivity of compounds characterized by terminal carbene (methylene, A) and carbyne (methylidyne, B) functionalities. The chemistry of μ-methylene complexes did not develop as extensively as that of its mononuclear counterparts, the latter being characterized by terminal carbene ligands. As in the bonding of carbenes with single atoms metal, the methylene group has a filled orbital (a 1 ) that can act as a sigma donor to the system. Symmetrical methylene (alkylidene) bridges represent the predominant geometry. Stability against thermolysis and photolysis is one of the striking properties of dimetallacyclopropanes, especially of those involving carbonyl and cyclopentadienyl ligands on the metals. There is so far not a single exception to the rule that a μ-methylene complex is at least as stable as its μ-carbonyl counterpart. Substitution reactions at the methylene bridge have been observed, albeit the yields were quite low suggesting that major side reactions had occurred. The molecular regimes in discrete di- and polynuclear complexes are mostly coordinatively saturated, whereas metal surfaces, especially if they are relatively flat and close-packed, have no coordinatively saturated surface atoms, even in the presence of chemisorbed species. There is an array of well-established analogues of the μ-methylene complexes in which the heavier congeners of carbon adopt bridging positions.

The Methylene Bridge

Under u.v. radiation a variety of alkynes (HC2H, MeC2Me, PhC2Ph, MeO2CC2CO2Me, MeC2H, PhC2H, and PhC2Me) reacts with [Fe2(CO)4(η-C5H5)2] to form complexes [Fe2(CO)(µ-CO){µ-σ : η3-C(O)C2R2}(η-C5H5)2] in 10–90% yields. Only PhC2Ph produces an analogous complex with [Ru2(CO)4(η-C5H5)2], but [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2Ph2}(η-C5H5)2] undergoes alkyne exchange on heating in toluene with HC2H, MeC2Me, MeC2H, PhC2H, or PhC2Me to afford the appropriate [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2R2}(η-C5H5)2] in near quantitative yield. The linking of alkyne and CO to produce a dimetallacyclopentenone ring was established through an X-ray diffraction study of the title compound. Crystals are orthorhombic, space group Pbca, with Z= 8 in a unit cell of dimensions a= 14.797,(3), b= 17.805(8), and c= 16.739(8)A. The structure was solved by heavy-atom methods and refined by least squares to R 0.033 for 3 726 diffractometer-measured reflection intensities. The molecule contains a dimetallacyclopentenone ring in which the ethylenic bond is η2-bound to ruthenium, so that the bridging C(O)C(Ph)C(Ph) ligand is σ-co-ordinated to one ruthenium and η3-co-ordinated to the other. Compounds [M2(CO)(µ-CO){µ-σ : η3-C(O)C(R′)C(R2)}(η-C5H5)2](M = Fe or Ru) in which R1≠ R2 exist as isomers as a result of linking of either end of the alkyne with CO. Steric factors appear to determine the relative stability of the isomers when one of R is H, but electronic factors are influential when neither is H. The dimetallacyclopentenones are fluxional, undergoing synchronous carbonyl ‘insertion’ into, and elimination from, the dimetallacycle. Free energies of activation appear dependent upon the size of the ‘alkyne’ substituents. In boiling toluene, carbonyl elimination from the dimetallacycle in [Fe2(CO)(µ-CO){µ-σ : η3-C(O)C2(CO2Me)2}(η-C5H5)2] becomes irreversible, and the dimetallacyclobutene complex [Fe2(CO)2(µ-CO){µ-C2(CO2Me)2}(η-C5H5)2] is formed quantitatively as cis and trans isomers. Only cis-[Ru2(CO)2(µ-CO){µ-C2(CO2Me)2}(η-C5H5)2] is generated when [Ru2(CO)(µ-CO)(µ-σ : η3-C(O)C2Ph2}(η-C5H5)2] is heated with MeO2CC2CO2Me. The ease with which carbon–carbon bond-making and -breaking occurs at the di-iron and diruthenium centres is recognised.

Organic chemistry of dinuclear metal centres. Part 1. Combination of alkynes with carbon monoxide at di-iron and diruthenium centres: crystal structure of [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2Ph2}(η-C5H5)2]

Protonation of the complexes [M2(CO)(µ-CO){µ-C(O)C2R2}(η-C5H5)2][M = Fe or Ru; R2= H2, Ph2, H(Me), or H(Ph)] with HBF4·OEt2 results in rapid carbon–carbon bond cleavage and formation of the µ-vinyl cations [M2(CO)2(µ-CO){µ-C(R)C(H)R}(η-C5H5)2]+, containing a cis arrangement of R groups. Addition of HBF4·OEt2 to [Ru2(CO)(µ-CO){µ-C(O)C2Me2}(η-C5H5)2] produces the cation [Ru2(CO)(µ-CO)({µ-C(H)(O)C2Me2}(η-C5H5)2]+, which isomerises slowly to [Ru2(CO)2(µ-CO){µ-C(Me)C(H)Me}(η-C5H5)2]+. The µ-vinyl cations exist in solution as isomers with cis and trans orientations of terminal ligands, shown by variable-temperature n.m.r. to interconvert; cis isomers additionally display a fluxional oscillation of the µ-vinyl ligand. Treatment of the cations with Na BH4 yields µ-carbene complexes [M2(CO)2(µ-CO){µ-C(R)CH2R}(η-C5H5)2] in good yield, also as cis and trans isomers which interconvert in solution. These result from hydride addition to the β-carbon of the µ-vinyl, but addition to the α-carbon is apparent in the low-yield co-formation of the terminal ethylene complex [Ru2(CO)(µ-CO)2(C2H4)(η-C5H5)2] from the cation [Ru2(CO)2(µ-CO)(µ-CHCH2)(η-C5H5)2]+. Addition of sodium tetrahydroborate to [Ru2(CO)(µ-CO){µ-C(H)(O)C2Me2}(η-C5H5)2]+ provides the complex [Ru2(CO)(µ-CO){µ-C(Me)C(Me)CH2}(η-C5H5)2], completing a conversion of metallacyclic CO to CH2. Regeneration of the µ-vinyl cation [M2(CO)2(µ-CO)(µ-CHCH2)(η-C5H5)2]+ is achieved by treatment of [M2(CO)2(µ-CO){µ-C(H)Me}(η-C5H5)2] with [CPh3][BF4].

Organic chemistry of dinuclear metal centres. Part 3. µ-Carbene complexes of iron and ruthenium from alkynes viaµ-vinyl cations

Bridging methylcarbenc complexes [M2(CO)2-(µ-CO)(µ-CHMe)(η-C5H5)2](M = Fe or Ru) react with alkynes RC2′R1(R = R1= H, Me, CO2Me; R = Me, R1= H) under u.v. irradiation to produce complexes [M2(CO)(µ-CO)(µ-η1,η3-CRCR1CHMe)(η-C5H5)2], shown through an X-ray diffraction study of [Fe2(CO)(µ-CO)-{µ-η1,η3-C(CO2Me)C(CO2Me)CHMe}(η-C5H5)2] to arise from linking of the µ-carbene and alkyne.

Combination of alkynes with µ-carbenes at a dimetal centre, and X-ray structure of [Fe2(CO)(µ-CO){µ-η1,η3-C(CO2Me)C(CO2Me)CHMe}(η-C5H5)2]: implications for metathesis and alkyne polymerisation

Upon treatment with methyl-lithium followed by HBF4·OEt2 a carbon monoxide ligand of the µ-alkylidene complex [Ru2(CO)2(µ-CO)(µ-CMe2)(η-C5H5)2](1) is converted into µ-ethylidyne, giving [Ru2(CO)2(µ-CMe)(µ-CMe2)(η-C5H5)2]+(2). This is deprotonated readily by water to form the µ-vinylidene complex [Ru2(CO)2(µ-CCH2)(µ-CMe2)(η-C5H5)2](3), which quantitatively regenerates (2) with HBF4·OEt2. Addition of NaBH4 to (2) results in hydride attack on µ-CMe to yield the di-µ-alkylidene complex [Ru2(CO)2(µ-CHMe)(µ-CMe2)(η-C5H5)2](4) as cis and trans isomers. The structure of the trans isomer has been established by X-ray diffraction. Crystals are triclinic, space group P, with Z= 2 in a unit cell for which a= 8.474(2), b= 7.802(3), c= 12.989(5)A, α= 99.42(3), β= 96.96(3), and γ= 107.73(3)°. The structure was solved by heavy-atom methods and refined to R 0.026 (R′ 0.031) for 4 092 independent intensities. A ruthenium–ruthenium single bond of 2.701(1)A is symmetrically bridged by ethylidene [mean Ru–C 2.079(3)] and isopropylidene [mean Ru–C 2.107(3)A] ligands to form an approximately planar Ru2C2 ring with a non-bonding Me2C··CHMe distance of 3.20 A. Upon thermolysis the alkylidenes link to evolve Me2CCHMe, Me2CHCHCH2, and Et(Me)CCH2. The absence of C4 and C6 hydrocarbons indicates that the alkylidene coupling occurs intramolecularly, and the electronic and stereochemical requirements of this process are discussed. Unlike mono-µ-alkylidene complexes, [Ru2(CO)2(µ-CO)(µ-CR2)(η-C5H5)2], the cis and trans forms. of (4) do not interconvert thermally below 145 °C, but u.v. irradiation effects a slow trans to cis isomerisation. U.v. irradiation of (4) in the presence of dimethyl acetylenedicarboxylate promotes ethylidene–alkyne linking to form [Ru2(CO)(µ-CMe2){µ-C(CO2Me)C(CO2Me)CHMe}(η-C5H5)2], but with ethyne both of the alkylidenes are lost and the ruthenium–ruthenium double-bonded complex [Ru2(µ-CO)(µ-C2H2)(η-C5H5)2] is produced.

Organic chemistry of dinuclear metal centres. Part 12. Synthesis, X-ray crystal structure, and reactivity of the di-µ-alkylidene complex [Ru2(CO)2(µ-CHMe)(µ-CMe2)(η-C5H5)2]: alkylidene linking

The µ-carbene complexes [Ru2(CO)2(µ-CO)(µ-CMe2)(η-C5H5)2] and [Ru2(CO)2(µ-CO)(η-CH2)(η-C5H5)2] undergo double insertion with ethyne and dimethyl acetylenedicarboxylate, respectively, to yield the title compounds; these complexes have been shown by X-ray diffraction to contain five-carbon chains of differing stereochemistry, attributed to the different steric demands of the carbene substituents.

Stereochemical control of alkyne oligomerisation at a diruthenium centre: X-ray structures of [Ru2(CO)(µ-CO)(µ-C4H4CMe2)(η-C5H5)2] and [Ru2(µ-CO){µ-C4(CO2Me)4CH2}(η-C5H5)2]

Andrew F. Dyke

Papers

Organic chemistry of dinuclear metal centres. Part 1. Combination of alkynes with carbon monoxide at di-iron and diruthenium centres: crystal structure of [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2Ph2}(η-C5H5)2]

Organic chemistry of dinuclear metal centres. Part 3. µ-Carbene complexes of iron and ruthenium from alkynes viaµ-vinyl cations

Combination of alkynes with µ-carbenes at a dimetal centre, and X-ray structure of [Fe2(CO)(µ-CO){µ-η1,η3-C(CO2Me)C(CO2Me)CHMe}(η-C5H5)2]: implications for metathesis and alkyne polymerisation

Organic chemistry of dinuclear metal centres. Part 12. Synthesis, X-ray crystal structure, and reactivity of the di-µ-alkylidene complex [Ru2(CO)2(µ-CHMe)(µ-CMe2)(η-C5H5)2]: alkylidene linking

Stereochemical control of alkyne oligomerisation at a diruthenium centre: X-ray structures of [Ru2(CO)(µ-CO)(µ-C4H4CMe2)(η-C5H5)2] and [Ru2(µ-CO){µ-C4(CO2Me)4CH2}(η-C5H5)2]