Vinylidene complexes from carbynes

Experimental Procedure 3.1.4. Preparation of a Molybdenum Vinylidene Complex from a Carbyne Complex Tetrabutylanunoniuih Cyano(ethoxycarbonyl) vinylidene (dicarbonyl) hydro-tris(3,5-dimethyl-1 -pyrazblyl)borato molybdenum [526] [37] pp 151 and 188... 

Table 1.4 Some vinylidene complexes, L M=C=CRR, obtained from carbynes.

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

Several hundred examples of vinylidene complexes have been prepared. Vinylidene complexes have been prepared by rearrangement of alkyne complexes, additions of acid or base to acetylide complexes, by deprotonation of carbyne complexes, by dehydration of acyl complexes, and by ot-hydrogen shifts from vinyl complexes. Syntheses from alkjme and from acetylide complexes are most common. A complex of a terminal alkyne and a transition metal can exist as an alkyne complex or as a vinylidene complex. Although the free vinylidene is much higher in energy than the free alkyne, the vinylidene complex is often more stable tlnan the alkyne complex. Vinylidene complexes are most often obtained with late transition metals because this tautomer possesses less repulsion between the filled (i-orbitals of the metal and the filled ir-orbitals of the ligand. 

The basicity at the 3-carbon is illustrated by the reactions in Equations 13.26 and 13.27. Reaction of the octahedral rhenium vinylidene in Equation 13.26 with HBF generates the cationic carbyne complex from addition of a proton to the basic 3-carbon. Because low-valent metals are often basic, addition of a proton to the vinylidene 3-carbon is likely to occur by a multi-step process initiated by protonation of the metal center. This initial protonation at the metal center would then be followed by migration of the proton to the 3-carbon. The reaction of acid with the iridium vinylidene in Equation 13.27 illustrates this mechanism. In this case, protonation first generates an iridium-hydride complex. The hydride in this complex tlien migrates to the p-carbon to generate an alkylidyne complex. ... 

The reluctance of the carbyne carbon to react with nucleophiles is revealed by the reaction with LiEt3BH (see Scheme 6). Here the most electrophilic site is not the carbyne carbon but the ipara position of the aryl ring in the carbyne substituent Both ruthenium and osmium five coordinate, cationic, carbyne complexes undergo this reaction. The structure of a representative example, the osmium compound derived from the p-tolyl carbyne complex, has been determined by X-ray crystallography [16]. The unusual vinylidene complex reacts with HCl to produce a substituted benzyl derivative. The reaction may proceed through the intermediate a-vinyl complex depicted in Scheme 6 although there is also the possibility that the vinylidene compound is in equilibrium with the carbene tautomer as shown below. 

A series of general headings can be applied to the synthetic routes used for generating vinylidene complexes (i) reactions of alkynes with labile and/or coordinatively unsaturated species, (ii) reactions with alkynes in the presence of a halide abstracting agent, (iii) formation from alkynyl complexes, and (iv) formation from carbyne complexes. 

Following the synthesis of metal carbyne complexes, the first metalladiyne derivative was prepared by treatment of W =C(OEt)C=CPh (CO>5 with BX3 (X = C1, Br, I) (pentane, -45°C) to give rranj-W(=CC=CPh)(X)(CO)4 (334 Scheme 77) in good yields (30-60%). Subsequent reactions with NHMea give W sCCH=CPh(NMc2) (X)(CO)4 by addition to the C=C triple bond, the structure of which indicates a contribution from the vinylidene resonance form. ... 

P. Gonzalez-Herrero, B. Weberndorfer, K. Ilg, J. Wolf, and H. Werner, The Sensitive Balance Between Five-Coordinate Carbene Ruthenium Complexes and Six-Coordinate Carbyne Ruthenium Complexes Formed from Ruthenium Vinylidene Precursors, Organometallics 20, 3672-3685 (2001). 

Dr. M.T. Duarte (Institute Superior Tecnico) for the X-ray diffraction analysis of one of the isocyanide complexes, Dr.R. Henderson (Nitrogen Fixation Laboratory, Univ. Sussex) for stopped-flow kinetic studies. Dr. E.G. Bakalbassis and Prof. C.A. Tsipis (Aristotle Univ., Thessaloniki)for the extended Huckel MO calculations, as well as, from our laboratory, Dr. M.F.N.N. Carvalho (some isocyanide and aminocarbyne complexes, and stopped-flow studies). Lie. M.A.N.D.A. Lemos (electrochemical studies). Lie. S.P.R. Almeida and Lie. M.F.C. Guedes da Silva (some vinylidene and carbyne complexes, and electrochemical studies). 

Tungsten carbyne complexes are synthesised from alkynide complexes by double addition of electrophiles. Thus addition of FS03Me or Et30 to [W(CCBu )C0)5] leads to the vinylidene W(=C=CButR)(C0)5 (R = Me or Et). Addition of CF3S03H/Me4NI to this vinylidene or directly to the alkynide allows the high yield isolation of t ran -WI (CO ) [ C (CHBu R ) ]. ... 

Formation from Alkenyl, Vinylidene or Carbyne Complexes... 

The carbene complexes [Os(P Bu2Me)2 =G(OR)Me HGl] form rapidly at low temperatures upon addition of vinyl ethers to [Os(P Bu2Me)2H3Gl], via the intermediacy of 77 -alkene complexes. While the carbene complexes generally decompose upon warming to form a mixture of products, changing the phosphine to P Pt3 allows the clean formation of the carbyne complex [Os(P Pr3)2(=GMe)HGl] (R = Ph) from H2G=GH(OPh), but the vinylidene [Os(P Pr3)2(=G=GH2)HGl] with H2G=GH(OEt). This difference in reactivity arises from the better stability of PhO compared to EtO as a free nucleophile, and the Bronsted basicity of the ethoxide anion that allows it to deprotonate a Os=GMe to afford the vinylidene product. ... 

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