Alkylidynes characterization

The alkene-W complexes used in this study are complexes 33, 166, and 168 (Schemes 5 and 31). The deprotonation of the W-alkene functionality has been carried out on 166, which has three methyl substituents at the C=C bond, in order to prevent any rearrangement of the 1-metallacyclopropene to the corresponding alkylidyne.22b The reaction of 166 with LiBu led to the 1-metallacyclopropene 169 (Scheme 32), which was isolated in quite good yield. The alkylidene 169, a very stable compound, was characterized both in solution and in the solid state.28 The h NMR spectrum shows a four-fold symmetry calixarene skeleton, while the 13C NMR spectrum contains a resonance at 271.0 ppm for the alkylidene carbon. 

Several alkyls and aryls with M=0 (or M=NR) groups have been characterized. A simple methyl, OsOMe4 is obtainable from 0s04. The dioxo alkyl, 0s02(CH2Bu )2,125 can be converted into alkylidene and alkylidyne compounds,... 

The reactions of alkylidenes (see Alkylidene) and alkylidynes (see Alkylidyne) of niobium and tantalum are typically characterized by nucleophilic alkylidene and alkylidyne ligands. Cp Ta(CH2SiMe3)2(=CHSiMe3) reacts with PhOH to give alkyl phenoxo Cp Ta(CH2SiMe3)3(OPh), and is hydrolyzed in the presence of a trace amount of H2O to give the 0x0 dialkyl derivative [Cp Ta(CH2SiMe3)20] . ... 

A rather significant result was reported by Hbhn and Werner the spectroscopic characterization of the first iridium alkylidyne complexes [Eq. (20)1 (61,62). Protonation of the iridium vinylidene complexes 17 was found to occur initially at the metal center to afford the vinylidene hydrido... 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

Parallel to the development of catalysts for olefin metathesis, the first alkyne metathesis catalysts were W and Mo metal oxides or carbonyls suspended on alumina or silica.65 The first homogeneous catalysts were developed by Mortreux and consisted of a mixture of Mo(CO)6 and substituted phenols.66 It was not until the work of Schrock and his collaborators, however, that a well-defined, isolable alkylidyne catalyst (38) was synthesized, characterized, and shown to catalyze alkyne metathesis.67 Later modifications on 38 included substituting the alkoxy groups with fluorinated analogs, and for the corresponding Mo alkylidynes (39), the fluorinated alkoxy groups are essential for catalytic activity.68... 

A tungsten complex which possesses all three types of metal-carbon bonds, single (alkyl), double (alkylidene), and triple bonds (alkylidyne) has been prepared as shown in eq. (3) and structurally characterized [15]. 

Detailed electrochemical characterization has been presented for a series of alkylidyne-capped triangular clusters of general formula MCo2(p3-CR) (CO)8(C5H5), (M = Mo, Cr, W R = Ph, Me), [50, 51]. As an example, Fig. 16 shows the molecular structure of MoCo2(C-Ph)(CO)8(C5Hs) [52]. 

Isolated, well-characterized alkylldene complexes of Ta, Nb, W, Mo, Re, Ir and Fe have been obtained by the a-hydride elimination reaction. In some cases these and other tdkylidene complexes have been synthesized by other reactions, such as hydrogenation of alkylidyne complexes, alkylldene transfer or protonation of alkoxy complexes. Discus-... 

Therefore, if the above assumptions are correct, one would expect that the reaction of NiCp2 with LiPh carried out in the presence of an alkene should also lead to the formation of trinickel cluster. Reactions of nickelocene with phenyllithium in the presence of 1-decene, 1-hexene or (2,4,4-trimethyl-1-pentene) were studied. The reactions of nickelocene with phenyllithium and alkene were carried out in THF at temperatures ranging from —60°C to room temperature. Products were separated by column chromatography on alumina using hexane/toluene mixture as eluents. In the case of 1-decene and 1-hexene trinickel clusters, (NiCp)3C(CH2)vCH3 7 and (NiCp)3C(CH2)4CH3 8, were isolated and fully characterized. For (2,4,4-trimethyl-1-pentene), only traces of corresponding trinickel cluster were obtained.These studies completely confirmed the particular role of coordinatively and electronically unsaturated species (NiCp)2 in these reactions and allowed a mechanism of the formation of (cyclopentadienyl)(alkylidyne)trinickel clusters to be proposed. 

The well-characterized four-coordinate Re(VII) alkylidene-alkylidyne complex, Re(=CHCMe3)(=CCMe3)[OCMe(Cp3)2]2 brings about the metathesis of cw-pent-2-ene, but its activity is significantly lower than that of related Mo or W catalysts (Toreki 1990, 1993). 

OLEFIN METATHESIS BY WELL-CHARACTERIZED RE(VII) ALKYLIDENE/ALKYLIDYNE COMPLEXES... 

In the last decade, impressive advances have been made in the synthesis of alkylidene and alkylidyne complexes that contain metals from groups 4 [5] and 5 [5i,j,m, 6], especially Ti and V, but - except for the ROMP of norbornene by vanadium complexes - group 4 and 5 metals have not shown wide-ranging activity for olefin metathesis. Well-characterized rhenium(VII) complexes are known to be active for metathesis, and many rhenium(Vll) alkylidene and alkylidyne complexes have been isolated [2a], but little attention has been paid to the syntheses of rhenium alkylidene or alkylidyne complexes in the last decade. Theoretical calculations have been carried out on M(X)(CHR )(Y)(Z) complexes... 

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