Methylidyne complexes

Protonation of 12 yields a compound best described as a face-protonated methylidyne complex, the tungsten-carbon bond length lying in the range observed for a triple bond (28). Protonation of the osmium compound 13 yields a true carbene complex, which for R = Ph has been characterized by X-ray crystallography (see Sections IV and VI). 

Examples of O-alkylation have also been demonstrated for triruthenium and triosmium anionic clusters, as well as the tetrairon cluster Fe CO)] - (31-33). This reaction has considerable promise as an entry into many different methylidyne complexes through the replacement of the OR" group, eq. 20 (38). 

Methylidine chloride, 6 249 Methylidyne complex, 26 949 Methyliminodiacetic acid (MIDA), 19 265 Methyl iodide, 14 376 Methyl ionone, 3 232 a-iso-Methylionone, 24 562, 566 a-n-Methylionone, 24 565 p-iso-Methylionone, 24 566 P-n-Methylionone, 24 565 Methyl isoamyl ketone (MIAK),... 

A major breakthrough in the synthesis of transition metal methylene and methylidyne complexes has been achieved by Stone and his group it originates from the simple idea that M=C double bonds in Fischer-type carbenes and M=C triple bonds in carbyne systems should add to low valent metal complexes as do C=C and C=C linkages, respec-... 

The rapid development of the chemistry of transition metal complexes containing terminal carbene (A) or carbyne (B) ligands (7) has been followed more recently by much research centered on bridged methylene compounds (C) (2). The importance of /t-methylidyne complexes, whether in recently established binuclear examples (D), the well-known trinuclear derivatives (E), or the unusual complexes (F), has also become apparent. All are based on one-carbon (C,) fragments, and considerable interest is centered on their possible significance as models for intermediates in surface-catalyzed reactions between carbon monoxide and hydrogen (Fischer -Tropsch reactions) and related processes. These topics have been extensively ... 

There are two well-characterized examples of a naked carbon atom bound by a triple bond to a metal center (Fig 14.3.8). The molybdenum carbide anion [CMo N(R)Ar 3]- (R = C(CD3)2(CH3), Ar = C6H3Me2-3,5), an isoelectronic analog of NMo N(R)Ar 3, can be prepared in a multistep procedure via deprotonation of the d° methylidyne complex HCMo N(R)Ar 3. The Mo=C distance of 171.3(9) pm is at the low end of the known range for molybdenum-carbon multiple bonds. In the diamagnetic, air-stable terminal ruthenium carbide complex Ru(=C )C12(LL/)(L = L = PCy3, or L = PCy3 and L = l,3-dimesityl-4,5-dihydroimidazol-2-ylidene), the measured Ru-C distance of 165.0(2) pm is consistent with the existence of a very short Ru=C triple bond. 

Deprotonation (LiBu, Li Bu, LDA) of the methylidyne complex Tp (OC)2W=CH (7 Scheme 3) affords orange Tp (OC)2W=CLi (8) which was not isolated but could be characterized by its broad 13C signal at 8 5 56.96 The v(CO) spectrum indicates the negative charge is mainly in the carbon s orbital. The pAa of the methylidyne is estimated at 28.7 (vs. 2-benzylpyridine), it being less acidic than HC=CPh or HC=CBu (pAa 23.2, 25.5, both vs. NH2Cy). Nucleophilic properties of the carbide anion are shown in reactions with electrophiles such as Mel, SiMe3(OTf), I2, and acyl compounds. 

As found with other complexes discussed in this chapter, the prevalence of even-numbered carbon chains is a result of the use of Cn (n = even) synthons in their construction. A few examples of odd-numbered chains, particularly when end-capped by trinuclear clusters which form i3-methylidyne complexes, are known. The following discussion will consider complexes in order of increasing n. 

A1.4 Metal-Carbene, -Methylene, -Carbyne and -Methylidyne Complexes... 

We have also prepared and studied alkynyl-substituted methylidyne complexes of the type tra/ts-W(C=CR)(=CH)(dmpe)2 (7) 16a-c). Deducing the nature of the frontier orbital M-CCR ir-interactions from the... 

The chemistry of the highly electrophilic cationic diiron bridging methylidyne complex [ Fe(CO)Cp 2(/x-CH)(/x-CO)] (75) has been probed extensively with an emphasis on C-C bond forming capabilities. For instance, alkenes react to give one of two possible products (i) alkyli-... 

Analysis of the ELF has also been used to understand the bonding in the uranium methylidyne complexes XjUCH (X=F, Cl, Br) [9]. The ELF of each of these complexes was characterized by a ring-shaped disynaptic basin lying perpendicular to the U-C bond, a defining feature of the ELF in the prototypical C-C triple bond of ethyne, leading the authors to assert the existence of an unprecedented U-C triple bond. 

Carbene complexes have also been shown to undergo a-hydrogen elimination, in this case to form alkylidyne complexes. This reaction is much less common than the reaction of alkyl complexes to form alkylidenes, but a few examples are well documented. Two examples of this transformation are shown in Equations 10.39 and 10.40. Schrock reported the first direct observation of this transformation (Equation 10.39). ° In this first example, an isolated carbene complex converted to an alkylidyne hydride complex upon abstraction of a chloride ligand with trimethylaluminum. In a second example, a double C-H activation process by two sequential a-hydrogen elimination reactions converts the starting tungsten-methyl complex in Equation 10.40 into a methylidyne complex. ... 

C (WsC) and tc (CCR) levels mix considerably is indicated by the fact that the transitions of the metallabutadiynes are significantly red shifted from those of nonconjugated methylidyne complexes, such as W(CH)(dmpe)2(w-Bu), and are quite sensitive to the nature of the alkynyl R group (Figure 1).3 Since interactions between 7C (W=C) and tc(CCR) would tend to counteract the shift due to those between tc (W=C) and 7C (CCR), this latter mixing may be greater than that indicated spectroscopically. 

ABSTRACT. The nature of the MsC bond has been probed by single-crystal X-ray diffraction and NMR, electronic, and Raman spectroscopic studies of methylidyne complexes of the type rra 5-W(sCH)L4X. 

Mayr has reported a convenient synthetic approach to the large-scale preparation of thermally stable aryl and alkyl methylidyne complexes of group 6 metals based on the abstraction of oxide from suitable metal carbonyl acylates [12]. This approach has been extended to provide alkylidyne complexes bearing alkynyl [13], ferrocene-diyl [14], cymantrenyl [15] and silyl [16] substituents (Scheme 3). 

In 1982, Casey and co-workers reported the first reactions that could be considered hydrocarbations because they involved the direct C-H bond addition across the C-C double bond of alkenes. They showed that the cationic bridging iron methylidyne complex undergoes this type of reaction with alkenes with anti-Markovnikov regioselectivity. No other hydrocarbation reactions had been reported until recently, when Kubiak and co-workers reported hydrocarbation reactions of a nickel carbene complex with alkenes. Thus, the dicationic aminocarbene complex 31 reacts with ethylene, resulting in a complete conversion to the ethylcarbene complex (Scheme 1). 

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