Kevin A. Mead

Bio: Kevin A. Mead is an academic researcher from University of Bristol. The author has contributed to research in topics: Carbyne & Carbene. The author has an hindex of 12, co-authored 31 publications receiving 348 citations.

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

Abstract: 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.

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]

Abstract: 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.

Heteronuclear metal complexes with bridging methoxymetylidyne ligands: X-ray crystal structures of [AuRu3(µ2-COMe)(CO)10(PPh3)] and [Fe3Pt(µ3-H)(µ3-COMe)(CO)10(PPh3)]

Abstract: Heteronuclear cluster compounds can be prepared from reactions between [M3(µ-H)(µ-COMe)(CO)10](M = Fe or Ru) and [ AuMePPh3], or between the tri-iron compound and [Pt(C2H4)2(PPh3)]; the structures of [AuRu3(µ2-COMe)(CO)10(PPh3)] and [Fe3Pt(µ3-H)(µ3-COMe)(CO)10(PPh3)] have been established by X-ray diffraction.

Organic chemistry of dinuclear metal centres. Part 14. Synthesis, X-Ray structure, and reactivity of the ruthenium–ruthenium double-bonded complex [Ru2(μ-CO)(μ-C2Ph2)(η-C5H5)2]

Abstract: Ultraviolet irradiation of the metallacycle [Ru2(CO)(μ-CO){μ-C(O)C2Ph2}(η-C5H5)2] (1) in tetrahydrofuran (thf) gives the complex [Ru2(μ-CO)(μ-C2Ph2)(η-C5H5)2] (2), shown by X-ray diffraction to have a ruthenium–ruthenium double bond [RuRu 2.505(1) A] bridged transversely by a diphenylacetylene ligand. The loss of two molecules of CO in forming (2) is reversible; under 100 atm of CO at 50 °C complex (2) is converted into (1) in 60% yield. Treatment of unsaturated complex (2) with diazoalkanes RCHN2 (R = H, Me, or CO2Et) results in the corresponding uptake of two alkylidene units to form [Ru2(CO)(μ-CHR){η-C(Ph)C(Ph)CHR}(η-C5H5)2], existing as isomers for R = Me or CO2Et due to differing orientations of the μ-CHR substituent. The structure of [Ru2(CO)(μ-CH2){μ-C(Ph)C(Ph)CH2}(η-C5H5)2] (3) has been established by X-ray diffraction, revealing that one methylene co-ordinates to the dinuclear metal centre while the other links with the alkyne. There are non-bonding C–C distances of 3.07 A between the two μ-carbons of the complex, but only 2.78 A separating the μ-CH2 carbon and the CH2 carbon of the C(Ph)C(Ph)CH2 ligand. On thermolysis the latter two carbons link, accompanied by other processes, to afford [Ru2(CO)(μ-CO){μ-C(Ph)C(Ph)CHMe}(η-C5H5)2] (5). A co-product of the reaction of diazoethane with (2) is the di-μ-vinyl complex [Ru2(CO) (μ-CHCH2){μ-C(Ph)CHPh}(η-C5H5)2] (8). X-ray diffraction reveals that the two β-carbons of the vinyl groups are 2.99 A apart and it is these rather than the two μ(α) carbons (3.06 A apart) which link on thermolysis, affording complex (5) once more. Thermolysis of [Ru2(CO)(μ-CHCO2Et){μ-C(Ph)C(Ph)CH(CO2Et)}(η-C5H5)2] does not effect carbon–carbon bond formation. Instead, CO is ejected and its site occupied by an oxygen of a carboethoxy group in the complex [Ru2(μ-CHCO2Et){μ-C(Ph)C(Ph)CHC(O)OEt}(η-C5H5)2]. Treatment of complex (1) with BH3·thf or LiMe–HBF4–NaBH4 converts the metallacyclic ketone group into CH2 or CHMe respectively, yielding [Ru2(CO)(μ-CO){μ-C(Ph)C(Ph)CHR}(η-C5H5)2] (R = H or Me). The nature of the processes observed on thermolysis of complexes (3) and (8) suggests the importance of least-motion effects in determining the course of carbon–carbon bond formation at a dinuclear metal centre.

Replacement of hydrido-ligands in triruthenium complexes by triphenylphosphinegold groups. Crystal structures of [AuRu3(µ-COMe)(CO)10(PPh3)], [AuRu3(µ-H)2(µ3-COMe)(CO)9(PPh3)], and [Au3Ru3(µ3-COMe)(CO)9(PPh3)3]

Abstract: The compound [AuMe(PPh3)] reacts under mild conditions (diethyl ether, ambient temperatures) with the compounds [M3(µ-H)(µ-COMe)(CO)10] and [Ru3(µ-H)3(µ3-COMe)(CO)9] to give the complexes [AuM3(µ-COMe)(CO)10(PPh3)][M = Fe (1) or Ru (2)], [AuRu3(µ-H)2(µ3-COMe)(CO)9(PPh3)](3), [Au2Ru3(µ-H)(µ3-COMe)(CO)9(PPh3)2](4), and [Au3Ru3(µ3-COMe)(CO)9(PPh3)3](5). Spectroscopic properties of the new species are reported and discussed, and the structures of (2), (3), and (5) have been established by X-ray diffraction studies. The structure of [AuRu3(µ-COMe)(CO)10(PPh3)](2) can be regarded as a molecule of [Ru3(µ-H)(µ-COMe)(CO)10] in which the bridging hydrido-ligand is replaced by a bridging AuPPh3 group, thus producing a ‘butterfly’ metal atom core (interplanar angle 117°) with the gold atom occupying a ‘wing-tip’ site. The COMe ligand bridges the body of the butterfly on the convex side. The Au–Ru bonds [2.760(2) and 2.762(2)A] are ca. 0.1 A shorter than the non-bridged Ru–Ru bonds [2.845(2) and 2.839(3)A] but the bridged Ru–Ru bond is significantly longer at 2.879(2)A. Crystals of (2) are triclinic, space group P, and the asymmetric unit comprises two molecules of complex. The structure has been refined to R 0.075 for 4 868 intensities measured to 20 = 40° at 220 K. In [AuRu3(µ-H)2(µ3-COMe)(CO)9(PPh3)](3) the carbyne ligand triply bridges an equilateral triangle [Ru–Ru 2.865(2)–2.879(2)A] of ruthenium atoms, while on the opposite side of the triangle there are two edge-bridging hydrido-ligands and one edge-bridging AuPPh3 group. Each ruthenium atom carries three terminal carbonyl ligands, giving octahedral co-ordination if the Ru–Ru bonds are ignored. The molecule has approximate Cs symmetry, not required crystallographically. The structure is triclinic, space group P, and has been refined to R 0.042 for 3 247 intensities measured to 2θ= 45° at 293 K. The complex [Au3Ru3(µ3-COMe)(CO)9(PPh3)3](5) crystallises with half a molecule of CH2Cl2 per molecule of (5) incorporated into the crystals. Again the carbyne ligand triply bridges a near-equilateral triangle of Ru atoms [Ru–Ru 2.895(3)–2.929(2)A], but on the opposite side of this triangle one gold atom is co-ordinated to form a tetrahedron [Au–Ru 2.818(2), 2.825(2), and 2.987(2)A]. The two faces of this tetrahedron adjacent to the long Au–Ru bond are each further triply bridged by AuPPh3 ligands. The two Au–Au distances in this bicapped tetrahedral structure are 2.930(1) and 3.010(1)A; the difference between these probably arises from the packing of the bulky triphenylphosphine ligands. Crystals of (5) are monoclinic, space group P21/n, and the structure has been refined to R 0.050 for 4 279 intensities measured to 2θ= 45° at 293 K.

Vinylidene and Propadienylidene (Allenylidene) Metal Complexes

Abstract: 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.

Four-electron alkyne ligands in molybdenum(II) and tungsten(II) complexes

Abstract: Publisher Summary This chapter explores that the term “four-electron donor,” which is used to describe the alkyne ligand in circumstances where alkyne π ⊥ donation supplements classic metal–olefin bonding. The utility of this scheme lies in its simplicity, and with some reluctance is relied on the “four-electron donor” terminology to suggest global properties of metal alkyne monomers. The general implications and specific hazards characterizing these descriptors are typical of broad classification schemes in chemistry—they are often conceptually helpful but seldom specifically correct. Criteria for recognizing four-electron alkyne donation encompass stoichiometry, structure, spectra, and reactivity. The chapter reviews that the chemistry that has been developed for molybdenum (II) and tungsten (II) alkyne monomers encompasses syntheses, structures, spectra, molecular orbital descriptions, and reactions. The Mo (II) and W (II) complexes addressed in the chapter are not unique in terms of alkyne π ⊥ donation. Related alkyne chemistry is appearing for d4 metals other than molybdenum and tungsten, as well as for d 2 complexes in general. The chapter also examines that chromium alkyne chemistry and reflects the importance of π ⊥ donation, but the stoichiometries differ from those of heavier Group VI monomers.

The Methylene Bridge

Abstract: 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.