P-Methylene complexes

Another synthetic approach to p.-methylene complexes originates from the very simple idea that there might be an analogy with olefin cyclo-propanation (146) in organometallic chemistry if one had starting materials at hand that contain metal-metal double bonds (147). This type... 

The ability of a p-carbene to react with an unsaturated hydrocarbon and form an enlarged dimetallocycle encourages speculation over their role in such processes as alkene metathesis and Fischer-Tropsch synthesis. In Scheme 6 a possible mechanism for metathesis initiated by a p-carbene is presented, owing much to other workers (T7,22). Reactions of p-carbenes with alkenes are under investigation in our laboratory. Recently Pettit has observed that the p-methylene complex [Fe2(C0)8(p-CH2)] generates propene when subjected to a pressure of ethene and has also suggested the intermediacy of a three-carbon dimetallocycle (23). 

In addition to complexes of terminal alkylidenes, many complexes containing alkylidenes bridging two metal atoms have been made these are commonly known as p-methylene complexes. A wide range of complexes containing other metal-carbon bonds are known such as the carbyne complexes (for example, Cr = CPh(CO)5) which contain metal-carbon triple bonds. While such species may well be involved in some catalytic reactions, their importance in industrially catalyzed processes still has to be evaluated. 

By the reaction of Co2(CO)6(dppm) (dppm = bis-diphenylphosphanomethane, Ph2PCH2PPh2) with diazomethane or ethyl diazoacetate stable p-methylene complexes 17,18,19, and 20 were prepared (reactions 8.35-8.38). On the basis of NMR spectra obtained at different temperatures, an equilibrium transformation of p-methylene into terminal methylene in complex 17 (reaction 8.39) [72] was shown. 

M. R. Collier, M. F. Lappert, and M. M. Truelock, p-Methylene Transition Metal Binuclear Compounds Complexes with Me3Si- and Related Ligands, J. Organomet. Chem. 25, C36-C38 (1970). 

In addition to complexes containing carbenes bonded to a single metal atom (M=CRR ), complexes are also known where the carbene ligand bridges two metal atoms (M-CRR -M, p-methylenes) similarly there is an extensive chemistry of metal-carbynes (M=CR, with a formal triple bond between the metal and the ligand) and p-methylidynes where the ligand is again... 

METHYLENATION Alkyidimesitylboranes. p,-Chlorobis(cyclopentadienyl)(dimethyl-aluminum)-p.-methylene-titanium. Methylene bromide-Zinc-Titanium(IV) chloride. Organomolybdenum reagents. Titanocene methylene-Zn halide complex. N,N,P-Tri-methyl-P-phenylphosphinothioic amide. Trimethylstannylmethyllithium. 

A related sequence, involving the action of Li[BHEt3] on [Ru2(CO)4-(ti -C5H5)2], produces the previously described methylene complex [Ru2(CO)2(p--CO)( x-CH2)(t) -C5H5)2] in excellent yield. 

---