Hydrides terminal complexes

V. Formation of Hydride-Vinylidene Complexes by Addition of Terminal... 

FORMATION OF HYDRIDE-VINYLIDENE COMPLEXES BY ADDITION OF TERMINAL ALKYNES TO OsHCI(CO)(P Pr3)2... 

The conditions under which cobalt hydrocarbonyl was reacted with olefin were also found to affect the distribution of products and the extent of isomerization of excess olefin (62, 73, 147). At low temperatures (0° C) under carbon monoxide (1 atm) very little isomerization of excess 1-pentene occurred and the main product was the terminal aldehyde. Under nitrogen or under carbon monoxide at 25° C, extensive olefin isomerization occurred and the branched aldehyde was mainly produced. The olefin isomerization is most satisfactorily accounted for by an equilibrium between alkylcobalt and olefin-hydride cobalt complexes [Eqs. (9) and (10)]. The carbon monoxide inhibition is most easily explained if the isomerization proceeds via the tricarbonyls rather than tetracarbonyls. This also explains why ethylcobalt tetracarbonyl is not in equilibrium with hydrocarbonyl and ethylene under conditions where the isomerization is rapid (62, 73). 

In another example, the cyclometalated iridium complex [Ir(ppy)2(4-vinylpyridine)Cl] has been attached via hydrosilation see Hydrosilation) to hydride-terminated poly(dimethylsiloxane) to produce a luminescent material. Evaluation of this material as a luminescent oxygen sensor revealed significantly improved sensitivity over dispersions of the original vinyl pyridine complex in poly(dimethylsiloxane). The luminescent material was blended with polystyrene to give a new sensor that exhibited increased sensitivity and maintained short response times to rapid changes in air pressure. 

Ruthenium hydride pincer complex [Ru(PNP)(H)2(H2)j [PNP=l,3-bis(di-terf-butyl-phosphinomethyl)pyridine] and its borane analog [Ru(PNP)(H)2(HBpin)] (HBpin=pinacolborane) catalyze the hydroboration of terminal alkynes to give selectively Z-vinylboronates in high yields (Scheme 32) [146]. Mechanistic studies... 

Scheme 32 Hydroboration of terminal alkynes to Z-vinylboronates using a ruthenium hydride pincer complex as catalyst...

There is little evidence to distinguish between A, B or C. X-ray studies of a number of diene iron tricarbonyl complexes have been made.f A comparison of the observed bond distances between the terminal carbons of diene systems and the adjacent carbons of a substituent group (1-44-1 49 A) with the calculated distances for an s/ hydridized terminal carbon (1-48 A) and an sp hybridized carbon atom (1-52-1-54 A) slightly favours structures A and C [16]. The C -H coupling constants of butadiene iron tricarbonyl are more consistent with sp hybridized carbons than with hybridization [16a]. The data also suggests that there is a small rotation of the CH2 hydrogens out of the C4-plane. 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

The proposed mechanism of H2 evolution by a model of [FeFeJ-hydrogenases based upon DFT calculations [204-206] and a hybrid quanmm mechanical and molecular mechanical (QM/MM) investigation is summarized in Scheme 63 [207]. Complex I is converted into II by both protonation and reduction. Migration of the proton on the N atom to the Fe center in II produces the hydride complex III, and then protonation affords IV. In the next step, two pathways are conceivable. One is that the molecular hydrogen complex VI is synthesized by proton transfer and subsequent reduction (Path a). The other proposed by De Gioia, Ryde, and coworkers [207] is that the reduction of IV affords VI via the terminal hydride complex V (Path b). Dehydrogenation from VI regenerates I. 

The hydrogenation activity of the isolated hydrides 3 and 6 towards cyclooctene or 1-octene was much lower than the Wilkinson s complex, [RhCKPPhj) ], under the same conditions [2] furthermore, isomerisation of the terminal to internal alkenes competed with the hydrogenation reaction. The reduced activity may be related to the high stability of the Rh(III) hydrides, while displacement of a coordinated NHC by alkene may lead to decomposition and Rh metal formation. 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

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