Selby A. R. Knox

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

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 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]

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.

Benzyne complexes of ruthenium: Models for dissociative chemisorption of benzene on a metal surface. Crystal structures of [Ru4(CO)10(μ-CO)(μ4-PR)(μ4-η4-C6H4)] (R = Ph and CH2NPh2), [Ru5(CO)13(μ4-PPh)(μ5-η6-C6H4)] and [Ru6(CO)12(μ4-PMe)2(μ3-η2-C6H4)2]

Abstract Heating a solution of [Ru 3 (CO) 11 (PPh 3 )] in toluene gives the μ 3 -, μ 4 - and μ 5 -benzyne complexes [Ru 3 (CO) 7 (μ-PPh 2 ) 2 (μ 3 -η 2 -C 6 H 4 )] ( 1a ) (33%), [Ru 4 (CO) 10 (μ-CO)(μ 4 -PPh)(μ 4 -η 4 -C 6 H 4 )] ( 2a ) (50%) and [Ru 5 (CO) 13 (μ 4 -PPh)(μ 5 -η 6 -C 6 H 4 )] ( 3a ) (7%), respectively. The structures of 2a and 3a have been established by X-ray diffraction, revealing that the benzyne in each case is bound to a square of ruthenium atoms by CRu σ-bonds to two adjacent rutheniums, and by η 2 -interactions to the other two atoms of the Ru 4 square. In 3a there is also η 2 -co-ordination to a third ruthenium atom, so that there is η 6 -co-ordination overall. The five metal atoms of 3a are arranged like a step site on a metal (111) surface and the complex can be viewed as a model for the aftermath of benzene CH activation on such a surface. The thermolysis of [Ru 3 (CO) 11 (PPh 2 CH 2 NPh 2 )] gives the μ 4 -PCH 2 NPh 2 analogue 2b of 2a , while from [Ru 3 (CO) 11 (PPh 2 Me)] the new cluster [Ru 6 (CO) 12 (μ 4 -PMe) 2 (μ 3 -η 2 -C 6 H 4 ) 2 ] ( 11 ) is isolated in 11% yield. The structures of 2b and of 11 have been determined by X-ray diffraction; 11 contains two μ 3 -benzyne ligands bound to triruthenium faces of an Ru 6 P 2 cluster. Thermolysis of the tritolylphosphine complexes [Ru 3 (CO) 11 (PAr 3 )](Ar = C 6 H 4 - m -Me or C 6 H 4 - p -Me) affords only analogues of 1 and 2 , containing in each case the μ-C 6 H 3 -4-Me ligand. Heating [Ru 3 (CO) 11 (AsPh 3 )] gives low yields of the arsenic analogues of 2 and 3 in addition to the major product [Ru 2 (CO) 6 (μ-AsPh 2 ) 2 ] (65%). The fluxional behaviour of complexes 1 and 2 , involving benzyne rotation on Ru 3 and Ru 4 centres, respectively, is discussed, and pathways for the formation of 1, 2 and 3 are proposed.

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

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.

The Coordination and Transformation of Arene Rings by Transition Metal Carbonyl Cluster Complexes

A survey of the synthetic pathways and of the reactivity and catalytic activity of transition metal carbonyl clusters substituted with benzyne (and, for comparison, benzene and diene) ligands is given. Cluster-surface analogies are helpful in gaining an understanding of the homogeneous and heterogeneous catalytic behaviour of the clusters.

Organic chemistry of dinuclear metal centres. Part 13. Synthesis, structure, and reactivity of [Ru2(CO)4(η5: η5′-C5H4CH2C5H4)]

Reaction of [Ru3(CO)12] with bis(cyclopentadienyl)methane in boiling toluene gives [Ru2(CO)4(η5:η5′-C5H4CH2C5H4)](1) in good yield. The molecular structure has been determined by X-ray diffraction. The structure was solved by heavy-atom methods and refined by least squares to give a final R 0.045 for 3 009 unique, observed diffractometer data. Crystals of (1) are orthorhombic, space group Pbca, with Z= 16 in a unit cell of dimensions a= 18.066(9), b= 14.321(6), and c= 22.156(10)A. The molecule consists of a staggered (OC)2Ru–Ru(CO)2 unit bridged by a bis(cyclopentadienylene)methane ligand which is η5-bound to each ruthenium atom. I.r. spectroscopy reveals that this structure dominates in solution, but that a low concentration of a carbonyl-bridged isomer is also present. Sequential treatment of complex (1) with LiMe, tetrafluoroboric acid, and NaBH4 affords the bis-µ-ethylidene complex [Ru2(CO)2(µ-CHMe)2(η5:η5′-C5H4CH2C5H4)](7) the structure of which has also been determined by X-ray diffraction, and solved and refined as above to a final R 0.025 for 1 813 data. Crystals of (7) are monoclinic, space group P21/n(non-standard setting of P21/c, no.14), with Z= 4 in a unit cell of dimensions a= 8.702(3), b= 12.653(4), c= 14.508(6)A, and β= 98.01(3)°. The molecule contains an (OC)Ru–Ru(CO) unit bridged by two ethylidene groups so as to give a highly folded Ru2(µ-C)2 core. The methyl groups of the ethylidenes are oriented anti with respect to a bridging bis(cyclopentadienylene)methane ligand bound in an η5 : η5′ fashion as for complex (1). Although the µ-C ⋯µ-C distance in complex (7) is relatively short (3.11 A), thermolysis induces alkylidene linking less efficiently than for related η-C5H5 complexes with a non-folded geometry, a difference attributed to the reduced flexibility in (7) arising from the coupling of the η-C5 rings. Reaction of complex (1) with Li[BHEt3]–water affords the µ-methylene complex [Ru2(CO)2(µ-CO)(µ-CH2)(η5:η5′-C5H4CH2C5H4)]. Photolysis of (1) in the presence of diphenylacetylene gives [Ru2(CO)2(µ-CO)(µ-σ: σ-C2Ph2)(η5: η5′-C5 H4CH2C5H4)], containing a ‘parallel’ two-electron alkyne ligand. The Ru–Ru bond of (1) is cleaved by iodine to yield [Ru2I2(CO)4(η5:η5′-C5H4CH2C5H4)], which is readily converted into[Ru2Me2(CO)4(η5: η5′- C5H4CH2C5H4)] on treatment with Li[CuMe2].

Phosphorus-carbon bond cleavage at a di-iron centre: synthesis of μ-phosphidomethyl complexes [Fe2(CO)6(μ-CH2PR2)(μ-PR2)] from [Fe2(CO)6(μ-R2PCH2PR2)]

Abstract Upon heating in toluene at reflux the di-iron heptacarbonyl complexes [Fe 2 (CO) 6 (μ-CO)(μ-R 2 PCH 2 PR 2 )] (RPh, Me, Et, i Pr, OEt) lose carbon monoxide, resulting in phosphorus-methylene bond cleavage to give the μ- phosphidomethyl complexes [Fe 2 (CO) 6 (μ-CH 2 PR 2 )(μ-PR 2 )]. The ability of the phenyl group to stabilise the μ- CH 2 PR 2 ligand is seen in the thermolyses of the diphosphine complex [Fe 2 (CO) 6 (μ-CO)(μ-Ph 2 PCH 2 PMe 2 )], which undergoes selective Me 2 PCH 2 bond cleavage to yield [Fe 2 (CO) 6 (μ-CH 2 PPh 2 )(μ-PMe 2 )], and of the bis-diphosphine complexes [Fe 2 (CO) 4 (μ-CO)(μ-R 2 PCH 2 PR 2 )(μ-Ph 2 PCH 2 PPh 2 )] (RPh, Me), which results only in Ph 2 PCH 2 bond cleavage to give [Fe 2 (CO) 4 (μ-R 2 PCH 2 PR 2 )(μ-CH 2 PPh 2 )(μ-PPh 2 )]. The ubiquity of μ-CH 2 PPh 2 is attributed to the existence of a zwitterionic form in which positive charge residing on phosphorus is dispersed into the phenyl rings. The complexes [Fe 2 (CO) 4 (μ-R 2 PCH 2 PR 2 )(μ-CH 2 PPh 2 )(μ-PPh 2 )] exist as mixtures of geometric isomers a and b , identified by 31 P NMR spectroscopy and an X-ray diffraction study on the major isomer a of [Fe 2 (CO) 4 (μ-Me 2 PCH 2 PMe 2 )(μCH 2 PPh 2 )(μ-PPh 2 )] as its dichloromethane solvate, which contains a cis arrangement of phosphido and phosphidomethyl ligands with the diphosphine lying trans to the latter. Methyl substitution in the diphosphine backbone suppresses phosphorus-methylene bond cleavage and results instead in ortho-metalation and phosphorus-phenyl bond cleavage. Thus, on heating [Fe 2 (CO) 6 (μ-CO){μ-Ph 2 PCH(Me)PPh 2 }] carbon monoxide and benzene are lost and [Fe 2 (CO) 6 {μ-PhPCH(Me)P(Ph)(C 6 H 4 - o )}] is formed, structurally characterised by X- ray diffraction. Substitution of two methyl groups into the diphosphine backbone favours ortho -metalation more strongly still and UV irradiation of the chelate complex [Fe(CO) 3 {η 2 -Ph 2 PC(Me 2 )PPh 2 }] in the presence of iron pentacarbonyl yields [Fe 2 (CO) 6 {μ-PhPC(Me 2 )P(Ph)(C 6 H 4 - o )}] directly. The structure of [Fe 2 (CO) 6 (μ-CO){μ-Ph 2 PCH(Me)PPh 2 }] as its hexane solvate was examined by X-ray diffraction, for comparison with that of [Fe 2 (CO) 6 (μ- CO){μ-Ph 2 PCH 2 PPh 2 }]. The structure analysis was not satisfactory but no significant differences between the molecular structures were observed. The suppression of backbone P-C cleavage by methyl substitution is attributed to the destabilisation of the zwitterionic form of μ-CR 2 PPh 2 , which has negative charge residing on the carbon.

The sequential linking of alkynes at dichromium and dimolybdenum centres; X-ray crystal structure of [Cr2(CO)(µ-C4Ph4)(η-C5H5)2]

In heptane at reflux, the alkynes RCCR (R = Ph, H, or CO2Me) react with [Cr2(CO)4(η-C5H5)2] to give complexes [Cr2(CO)(µ-C4R4)(η-C5H5)2]. An X-ray diffraction study on the product from PhCCPh shows that the crystals of [Cr2(CO)(µ-C4Ph4)(η-C5H5)2] when grown from dichloromethane–hexane incorporate ¼CH2Cl2 per molecule of complex and are orthorhombic, with Z= 8 in a unit cell of dimensions a= 19.569(4), b= 19.731(5), c= 16.637(2)A, and space group Pbcn(no. 60). The structure has been solved by heavy-atom methods from 3 166 data for which l 3.0σ(l), collected on a four-circle diffractometer, and refined to R 0.066. The axis of the molecule comprises a (η-C5H5)CrCr(η-C5H5) moiety which is non-linear, with the cyclopentadienyl rings in an unsymmetrical trans relationship to one another, and with a carbonyl ligand semi-bridging in a plane which is effectively a mirror plane for the whole molecule. Two PhCCPh molecules have joined to form a four-carbon chain, of which the two terminal atoms form a quasi-tetrahedral group with the two chromium atoms [CrCr 2.337(2), Cr–C 2.025(7) mean, C ⋯ C(non-bonded)ca. 2.7 A], and the two central atoms are π-bonded to that Cr atom which does not carry the carbonyl ligand. The two metal atoms lie on opposite sides of the plane through the four-carbon portion of the CrC4Ph4 ring. The alkynes RCCH (R = Ph or Me) react with [Cr2(CO)4(η-C5H5)2] to yield as major products the complexes [Cr2(CO)(η-C4H2R2)(η-C5H5)2] in which there is a head-to-tail arrangement of the Ph or Me groups in the C4Cr rings. However, with phenylacetylene the isomer containing the ring system [graphic omitted]Ph was also detected. The compounds [Mo2(CO)n(η-C5H5)2](n= 4 or 6) react with PhCCPh in octane at reflux to give [Mo2(CO)(µ-C4Ph4)(η-C5H5)2], and similarly [Mo2(CO)4(η-HC2H)(η-C5H5)2] affords [Mo2(HC2H)(PhC2Ph)2(η-C5H5)2], a ‘fly-over’ complex containing a six-carbon chain bridging a MoMo bond. Reaction of [Mo2(CO)4(η-HC2H)(η-C5H5)2] with RCCR (R = CO2Me) yields the complexes [Mo2(CO)2(µ-C6H2R4)(η-C5H5)2](two isomers) and [Mo2(η-C8H2R6)(η-C5H5)2](two isomers) in which C6 and C8 chains bridge Mo–Mo and MoMo bonds respectively. The structures of these species were deduced from 1H and 13C n.m.r. spectra. Reaction of [Mo2(CO)4(µ-RC2R)(η-C5H5)2] with RCCR (R = CO2Me) gives [Mo2(CO)2(µ-C6R6)(η-C5H5)2] and [Mo2(µ-C8R8)(η-C5H5)2](two isomers), characterised inter alia by their 1H and 13C n.m.r. spectra. Treatment of [Mo2(HC2H)(PhC2Ph)2(η-C5H5)2] with MeO2CCCCo2Me gives two isomers of compositon [Mo2(HC2H)(PhC2Ph)2(MeO2CC2CO2Me)(η-C5H5)2], thereby establishing that the complexes with four linearly linked alkynes are formed from the species with three such linked groups. Possible mechanisms for carbon-chain growth on the dichromium and dimolybdenum centres are discussed and are related to the changes in multiplicities of the metal–metal bonds.

A convenient entry into diruthenium chemistry

Abstract In boiling toluene, diphenylacetylene is readily displaced from the dimetallocycle [Ru 2 (CO)(μ-CO) {μ-C(O)C 2 Ph 2 } (η-C 5 H 5 ) 2 ] by a variety of reagents (P(OMe) 3 , SO 2 , R 2 CN 2 , Ph 2 PCH 2 ) to produce [Ru 2 (CO){P(OMe) 3 }(μ-CO) 2 - (η-C 5 H 5 ) 2 ] or [Ru 2 (CO) 2 (μ-CO)(μ-L)(η-C 5 H 5 ) 2 ] (L  SO 2 , CR 2 , CH 2 ) in high yield.

C-C bond formation at polynuclear metal centers

Studies on C-C bond formation between simple hydrocarbon species such as CH2, C=CH2, CH=CH2, CH2=CH2, CH2=C=CH2 and CH≡CH at a diruthenium center suggest that the process is promoted when the dimetal center can readily compensate for the two electrons “lost” in the formation of the new C-C bond. Thus, whereas μ-CH2 and ethene combine only under forcing conditions, the combination of μ-CH2 with allene or ethyne, which have additional π-electrons available for coordination, occurs readily at room temperature. Likewise, the availability of uncoordinated π-electrons in μ-C=CH2 allows vinylidene to link rapidly with ethene at room temperature. Alkyne complexes [Ru2(CO)(μ-RC≡CR)(η-C5H5)2] (R=CF3 or Ph) react only under vigorous conditions with additional alkyne to give [Ru2(CO)(μ-C4R4) (η-C5H5)2], but give these same species at room temperature in the presence of acid, shown to be due to the intermediacy of highly reactive 30-electron μ-vinyl cations. Thermally, alkyne linking proceedsvia three-alkyne species [Ru2(μ-C6R6)(η-C5H5)2] to a four-alkyne complex [Ru2(μ-C8R8)(η-C5H5)2], containing an unprecedented C8 ligand composed of a C6 ring with a C2 “tail.” Treatment of [Ru2(CO)(μ-RC≡CR)(η-C5H5)2] with unsaturated metal fragments gives trimetal complexes such as [Ru3(CO)5(μ3-CF3C≡CCF3) (η-C5H5)2]. The MeCN derivative of this species undergoes unusual linking processes on reaction with additional alkyne to giveinter alia [Ru3(CO)3(μ3-CCF3){μ3-C3(CF3)3}(η-C5H5)2], arising from alkyne cleavage, and [Ru3(CO)3{μ3-C4(CF3)2(CO2Me)2}(η-C5H5)2], a closo-pentagonal bipyramidal Ru3C4 cluster.