1,2-Addition alkylidyne species

Molybdenum(VI) compounds containing Mo—C bonds are a recent addition to the literature. These compounds include both oxo and non-oxo species. The latter have an alkylidyne (alternatively formulated as an alkylcarbido) group which is a bonding equivalent to the oxo group. 

The addition of Lewis base substrate molecules to solutions containing alkylidyne-metal species prior to treatment with acid introduces a new dimension in the protonation reactivity of these complexes. This was... 

The tungsten alkylidyne complex (t-BuO)3W(C-t-Bu) [130-132] is capable of catalyzing alkyne metathesis polymerization of cyclic alkynes. In addition, it can be used in acyclic diene metathesis (ADIMET) [133, 134]. The synthesis of the corresponding Mo-compound Mo(CCH2SiMe3)(OAd)3 (Ad=adamantyl) has been reported by C.C. Cummins et al. [135]. Though the active species is not known yet, a mixture of Mo(CO)6 and 4-chlorophenol is nowadays used for purposes of convenience [136]. 

The (f7 -cyclopentadienyl)(/ 3-alkylidyne)trinickel clusters, (NiCp)3C-R, are among the best studied nickel clusters. The three nickel atoms together with the cyclo-pentadienyl rings form a system of 45 valence electrons (VE) in which each nickel atom possesses 17 VE. Therefore this system tends to achieve the additional three electrons required from an alkylidyne ligand to form a stable cluster of 48 VE in which all nickel atoms fulfill the 18 electron rule. The exeptions are the classic Fischer-Palm 49-electron Ni3Cp3(/i3-CO)2 cluster and the paramagnetic 53-electron Ni3Cp3( 3-S)2 species.f ... 

Mass spectroscopy appears to be a very useful method of identification of rj -cyclopentadienyl)(/r3-alkylidyne)trinickel clusters. The isotope pattern of trinickel species is very characteristic and fragmentation is often identical or very similar. The most characteristic fragments which appear in addition to the parent ions are as follows m/e 304 (Ni3Cp2)+, 246 (Ni2Cp2)+, 188 (NiCp2)+ and 123 (NiCp)+. 

Alternative mechanisms could be the elimination of neopentane, followed by a direct, o-bond metathesis [25] (Scheme 2.10b), or the addition of propane into the alkylidyne moiety [86-88] (Scheme 2.10c). The resulting alkylidene with the pendant alkyl group should then generate the key hydride intermediate via a P-H transfer as the propagative species. 

The first well-defined, stable d°-alkylidene species having catalytic activity in alkene metathesis was Ta(=CHtBu)(OtBu)2Cl(PMe3) (5) [16]. However, research efforts moved rapidly to Mo and/or W alkylidenes, first in the form of 0X0 complexes (6) [17, 26], and then with the more easily accessible, imido bisalkoxy alkylidene complexes of general formula M(=NR )(=CHR )(OR )2 (7) [18, 19, 27], as well as the isoelectronic Re-alkylidyne-alkylidene complexes Re(=CR )(=CHR )(OR )2 (8) [28]. In 2001, it was shown that the silica-supported, Re-alkylidyne-alkylidene complex (=SiO)Re(=CtBu)(=CHtBu)(CH2tBu) (12) was unexpectedly very active [21,29]. According to computational studies, which will be described later in this chapter, the remarkable reactivity of this silica-supported catalyst arises from the different nature of the X and Y ligands (referred to as dissymmetric complexes) [7]. In addition, new molecular precursors based on bis-alkyl and bis-pyrrolyl M(=NR )(=CHR )(X)2 complexes became available [30]. Consequently, from 2005, considerable research effort has been devoted toward generating silica-supported (14 to 16 in Scheme 6.4) [23, 31, 32] and molecular (17 and 18 in Scheme 6.4) [22, 33, 34] Mo- and W-imido complexes... 

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