Publisher Summary Organometallic chemistry encountered rapid expansion experienced by the synthesis, spectroscopy, structural chemistry, theory, and reactivity of compounds characterized by terminal carbene (methylene, A) and carbyne (methylidyne, B) functionalities. The chemistry of μ-methylene complexes did not develop as extensively as that of its mononuclear counterparts, the latter being characterized by terminal carbene ligands. As in the bonding of carbenes with single atoms metal, the methylene group has a filled orbital (a 1 ) that can act as a sigma donor to the system. Symmetrical methylene (alkylidene) bridges represent the predominant geometry. Stability against thermolysis and photolysis is one of the striking properties of dimetallacyclopropanes, especially of those involving carbonyl and cyclopentadienyl ligands on the metals. There is so far not a single exception to the rule that a μ-methylene complex is at least as stable as its μ-carbonyl counterpart. Substitution reactions at the methylene bridge have been observed, albeit the yields were quite low suggesting that major side reactions had occurred. The molecular regimes in discrete di- and polynuclear complexes are mostly coordinatively saturated, whereas metal surfaces, especially if they are relatively flat and close-packed, have no coordinatively saturated surface atoms, even in the presence of chemisorbed species. There is an array of well-established analogues of the μ-methylene complexes in which the heavier congeners of carbon adopt bridging positions.

The Methylene Bridge

The present article reviews the development of well-defined transition metal compounds which catalyze the ring-opening polymerization of strained cyclic olefins by the olefin metathesis reaction (ROMP). Several titana- and tantalacyclobutane compounds and tungsten- and molybdenum carbenes polymerize norbornene and norbornene derivatives by a “living” mechanisms, thus producing polymers of controlled molecular weights with narrow molecular weight distributions. Some of these systems have been successfully applied to the synthesis of block and graft copolymers. Studies on Ru-based catalysts resulted in the development of olefin metathesis polymerizations in aqueous solutions.

The development of well-defined catalysts for ring-opening olefin metathesis polymerizations (ROMP)

Publisher Summary This chapter focuses on the chemical design surfaces for active solid catalysts that can be regarded as a promising and scientific way to prepare well-defined active surfaces with excellent catalytic properties as well as to elucidate reaction mechanisms, including dynamic changes of active sites. These are new generation of catalysts resulting from scientific development at the boundary between homogeneous and heterogeneous chemistry. The use of such chemically and structurally controllable surface systems, rather than the more conventional supported catalysts with heterogeneous metal centers, can lead to novel information on the origins of catalysis. Once the essential processes of activity, selectivity, and specificity have been identified, it should be possible to incorporate this knowledge into the rational synthesis of practical catalysts that are deliberately tailored for specific reaction chemistries. Chemical surface design has strategic advantages in the synthesis of catalysts with novel and desired surface structures and compositions, and in the elucidation of heterogeneous catalysis on a molecular level.

Chemical Design Surfaces for Active Solid Catalysts

Possible role of hydrido-metal complexes in metathesis, isomerization, dimerization, and polymerization of alkenes

In this paper views regarding the nature of the transition state in olefin metathesis are discussed. Earlier concepts that assumed a pairwise scrambling mechanism have been recently challenged. Metathesis studies involving acyclic as well as cyclic olefins have been reported which strongly suggest a chain process that may involve a carbene-to-metallocycle interconversion as the pathway responsible for transalkylidenation. Catalytic systems that display a low apparent metathesis activity on terminal olefins display a unique specificity in cross reactions with internal olefins. Critical experiments indicate that terminal olefins rapidly undergo regenerative metathesis preferentially. Results suggest that in the transient metallocycle, the most stringent steric requirements occur at the β-carbon, although the origin of this steric demand is not obvious.

E. I. Muetterties

Papers

Mechanism for the olefin metathesis reaction