Re carbyne

The oldest route to carbyne (CR) complexes involves a Lewis acid (A+) attack on Fischer-type carbenes (equation 15). In this way, several Re carbyne derivatives under the general formula [[Re](CR)]° + have been prepared. The leading examples are [ReCp(CO)2(CR)]+ (R = Ph, SiPhs) because the analogous alkyl carbynes (alkylidynes) are much less stable due to facile deprotonation as elegantly shown by the spontaneous near-quantitative proton loss shown in equation (16). ... 

This same method was used to prepare a number of highly luminescent, similar Re carbynes with P and N donors. ... 

Scheme 51. Reaction of triflic acid and methanol with TpRe(CO)(PMe3)( / -furan) at room temperature to form a Re carbyne complex. After 20 min of reaction time R = CH(OMe)2 while after 45 min...

Despite many publications on carbynes, their existence has not been universally accepted and the literature has been characterised by conflicting claims and counter claims [e.g., 27-29]. This is particularly tme of meteoritic carbynes. An interesting account of die nature of elemental carbon in interstellar dust (including diamond, graphite and carbynes) was given by Pillinger [30]. Reitmeijer [31] has re-interpreted carbyne diffraction data and has concluded that carbynes could be stratified or mixed layer carbons with variable heteroelement content (H,0,N) rather than a pure carbon allotrope. 

Dihalocarbene complexes are useful precursors to new carbenes by nucleophilic displacement of the chlorine substituents. This has been nicely illustrated for Fe(TPP)(=CCl2) by its reaction with two equivalents of Re(CO)5J to give the unusual /t-carbido complex Fe(TPP)=C=Re(CO)4Re(CO)5 which also contains a rhenium-rhenium bond. " The carbido carbon resonance was observed at 211.7 ppm in the C NMR spectrum. An X-ray crystal structure showed a very short Fe=C bond (1.605(13) A, shorter than comparable carbyne complexes) and a relatively long Re=C bond (1.957( 12) A) (Fig. 4, Table III). " ... 

The issue of the chemical nature of graphene edges, and thus of (re)active sites, was addressed head-on by Radovic and Bockrath [29]. Density functional theory was used to minimize problems that truly ab initio approaches such as Hartree-Fock are known to have, but careful comparison with experimental results in several seemingly unrelated fields was also performed. The key argument of relevance here is that, rather than being H-terminated, the sites that are most relevant for the chemical (re)activity of sp2 hybridized carbon materials are of carbene- and carbyne-type, as illustrated below for the zigzag (a) and armchair edges (b). 

Metallacyclopent-l-enes have been obtained by formal [2 + 3] cycloaddition of ethylene to Re(CtBu)(CHtBu)(OR)2 (a carbene/carbyne complex) [612]. Nucleophilic tungsten(II) complexes can react directly with cyclopentanone to yield cyclopentylidene complexes [613]. 

Reactions of this type proceed via metal carbyne complexes (equation 4). The most direct evidence is that such complexes (Mt = Mo, W, Re) can act as initiators for the... 

The rhenium carbyne complex Re(=CCMe3)(=NAr)(OR)2 is active for metathesis of internal acetylenes when OR is OCMe(CF3)2, but not when OR is OCMe2CF3, OCMe3 or OC6H3-i-Pr2-2,65 6. The complex W(=CMe)(Cl)(PMe3)4 undergoes stoichiometric metathesis with PhC=CPh but the product PhC=CMe remains coordinated to the metal centre759. 

Fig. 13. Progression of linearly re-conjugated all-carbon backbones from trans-polyacetylene (PA) through trans-poly(diacetylene) (PDA), trans-poly(triacetylene) (PTA), to carbyne.

Re imidoalkyl chemistry in Section 4.7 ended up in several important carbynes, for example, Re(CR)(CHR)L2Cl2 (R = t-Bu L = py) and Re(CR)(CHR)(OR)2, the latter being active in alkene metathesis." A remarkable preference for carbjmes over carbenes is seen in complex [Re(=C-t-Bu)(=CH-t-Bu)(CH2-t-Bu)2] (49), which is preferred to its tautomer Re(=CH-t-Bu)3(CH2-t-Bu) that would be an analogue to the utterly stable [ReMeOs]. ... 

A good approach for the synthesis of infinite polyyne or cumulene may be the polymerization of a monomeric compound. Hay [10] in 1969, Matsuda et al. [11] in 1984 and Kudryavtsev [12,13] have shown the possibility of synthesizing carbyne by oxidative dehydropolycondensation of acetylene. They found that carbyne powder was obtained by passing acetylene through an aqueous ammoniacal solution of a Cu(II) salt. The carbyne obtained by this method was nano-crystalline but it might be unstable under an oxygen atmosphere. Most recent re-investigation of this synthetic route by Cataldo and Capitani [14], who used solid state C-NMR, infrared and Raman spectroscopy and other analytical techniques, has revealed that the carbonaceous matter obtained is indeed rich in carbynoid structures but also consists of sp -and sp -carbon atoms. 

Properties red crystals. C H NMR (benzene- / ) 8(carbyne Q) = 304.6 ppm. Re N(2,6-C6H3-f Pr) (CCMe3) OCMe(CF3)2 2 reacts with 3-hexyne to give a rhenacyclobutadiene derivative. 2,2-Dimethyl-3-hexyne is lost rapidly from the initially formed a-/cr/-butyl-substituted metallacycles. 

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