Osmium complexes alkyne ligands

Cydoaddition reactions of allenylidene ligands with alkynes have also been described. Thus, heating a toluene solution of the neutral osmium complex 43 in the presence of dimethylacetylenedicarboxylate leads selectively to the allenyl-vinyli-dene 4S (Scheme 2.18) [31c[. The formal insertion of the alkyne into the Co =Cp... 

Triruthenium and triosmium complexes obtained from functionalized alkynes Complexes containing ligands with methoxycarbonyl (or acetato) groups Complexes obtained upon CO insertion into M-C bonds Hydration-dehydration reactions of alkynols promofed by silica or alumina Osmium derivafives obfained from oxygenafed ligands... 

The possibility of coordination of a two-electron ligand, in addition to arene, to the ruthenium or osmium atom provides a route to mixed metal or cluster compounds. Cocondensation of arene with ruthenium or osmium vapors has recently allowed access to new types of arene metal complexes and clusters. In addition, arene ruthenium and osmium appear to be useful and specific catalyst precursors, apart from classic hydrogenation, for carbon-hydrogen bond activation and activation of alkynes such compounds may become valuable reagents for organic syntheses. 

The addition of a phosphine group to the organic fragment has been studied in some detail in compounds with cluster-bound vinyl ligands. The zwitterionic adducts which are formed can then undergo nucleophilic addition reactions (411, 461, 462). A reaction of this type also occurs with amine-substituted alkynes coordinated to osmium and ruthenium complexes (117). 

Osmium carbyne (see Carbyne Complexes) or aUcylidyne complexes have a triple bond between the metal and the carbon atom of the ligand. Carbyne complexes are related to singlet carbenes. They are analogous to linear nitrosyl (see Nitrosyl Complexes) complexes and the osmium is usually in a lower oxidation state. Alkylidyne complexes are related to triplet carbenes and the bonding between the osmium and the carbon atom is similar to the C-C bond in an alkyne. 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

Alternative indirect methods have been used in the synthesis of (3,3-dimethylcyclopen-tyne)Cp2Zr(PMe3> [52] and (cyclohexyne)[CpMo(CO)2]2 [53]. The smallest cycloalkyne stabilized by complexation is cyclobutyne, incorporated by Adams into tri- and tetrametallic ruthenium and osmium clusters as a P3-ligand 30 using 1-Br and -SPh substituted cyclobutene precursors (Scheme 4-11) [54]. Liberation of these strained alkynes from their metallic bondage has not been reported but some have been shown to be reactive towards insertion of unsaturated substrates, as illustrated in Scheme 4-12 with the Zr-cyclohexyne derivative 31 [55]. 

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