Cationic Ru carbene complexes with tBu PCH PtBu (dtbpm) as a chelating ligand, which are accessible by chloride abstraction from neutral precursors [(κ -dtbpm)Cl Ru=CHR] with trimethylsilyl triflate, are established as highly efficient ring opening metathesis polymerization catalysts (see scheme). Solv=solvent.

A new screening methodology, which combines in situ synthesis of complexes with an assay by electrospray ionization tandem mass spectrometry (ESI-MS), is introduced in order to investigate highly active, cationic ruthenium-carbene catalysts in ring-opening metathesis polymerization (ROMP). The parameter space, whic is defined by systematic variation of four structural features of the catalyst [[R2P(CH2),PR2-kappa2-P]XRu=CHR']+ (the halogen ligand, the diphosphane bite-angle, the steric bulk of the phosphane, and the carbene ligand) and the variation of the metathesis substrate, is mapped out. Chloride as the anionic ligand X, a small chelating angle (n = 1), and reduced steric demand of the substituents R (Cy versus tBu) lead to the most reactive complex in acyclic olefin metathesis, whereas variation of the carbene moiety CHR' has only a modest influence. The overall rate in the gas phase depends on the pi-complex preequilibrium and metallacyclobutane formation, which was found to be the rate-determining step. In ROMP reactions backbiting has a profound influence on the overall rate. Moreover, we were able to establish that the reactivity trends determined in the gas phase parallel solution-phase reactivity. The overall rate in solution is also determined by a favorable dimer/ monomer preequilibrium providing the active catalyst by facile dissociation of dicationic, dinuclear catalyst precursors.

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Complexes of the type [{(t-Bu) 2 P(CH 2 ) n P(t-Bu) 2 -κ 2 P}XRudCHR] 2 (Y) 2 have been prepared and characterized in solution as well as in the solid state. Parameters governing the ring opening metathesis polymerization (ROMP) activity of these highly active ruthenium catalysts in solution have been assessed. Especially, chloro-bridged dinuclear dicationic ruthenium carbenes with a bis(di-tert-butylphosphino)methane ligand and weakly coordinating or noncoordinating anions display extremely high ROMP activity. Comparison of the solution-phase reactivity reported here with high throughput screening results from gasphase experiments provide further mechanistic insight into the metathesis reaction with these compounds.