A series of bis(benzimidazole)-based cobalt(II) dichloride complexes containing a range of different central donors has been synthesized and characterized. The nature of the central donor affects the binding of the ligand to the cobalt centre and determines the coordination geometry of the metal complexes. All complexes have been shown to catalyse the polymerization of butadiene, in combination with MAO as the co-catalyst, to give cis-1,4-polybutadiene with high selectivity. The nature of the central donor has a marked influence on the polymerization activity of the catalysts, but does not affect the polymer microstructure. The addition of PPh(3) generally increases the polymerization activity of these cobalt catalysts and results in predominantly (60-70%) 1,2-vinyl-polybutadiene.

was found to be an efficient precatalyst, with styrene as an initiating species, for the alkene metathesis of various styrenes into symmetrical and unsymmetrical stilbene derivatives and for the ring-closing metathesis reaction of a sterically hindered olefin leading to a tetrasubstituted cycloolefin.Keywords: alkene metathesis; N-heterocyclic carbenes; ring-closing metathesis; ruthenium; stilbenes; styrenes Stilbene derivatives, in particular hydroxy-or alkoxysubtituted ones are gaining importance due to their biological activities for the prevention of cancer or cardiovascular diseases.[1] They are also an important family of building blocks for the design of conjugated materials with optical properties.[2] Wittig or WittigHorner reactions [3] constitute a priviledged approach for the synthesis of stilbene derivatives but they generate stoichiometric amounts of phosphine oxides hence requiring fastidious purification. Catalytic reactions such as Heck or Suzuki couplings can be used as more efficient ways to synthesize stilbene derivatives.[4] More recently catalytic olefin metathesis [5] appeared as a tool of choice for the efficient and stereoselective synthesis of stilbene derivatives thanks to the emergence of very active catalysts for cross-metathesis.[6] In particular, the ruthenium-based second generation Grubbs catalyst has been shown to efficiently promote the self-and cross-metathesis of styrene derivatives. [7] Other complexes based on molybdenum [8] or osmium [9] have also been described as efficient catalysts for this transformation.We recently reported the intramolecular vinyl C À H bond activation of an N-vinyl-N-heterocyclic carbeneruthenium(II) intermediate leading to a bidentate NHC-alkenyl ruthenium complex (Scheme 1). [10] In an attempt to explore the intermolecular version of this transformation, styrene was reacted with the readily available ruthenium complex[11] [IMes = 1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]. This reaction resulted in the stereoselective formation of E-stilbene in very good yield, thus suggesting a self-metathesis reaction. Here we describe several aspects of RuCl 2 A C H T U N G T R E N N U N G (p-cymene)-A C H T U N G T R E N N U N G (IMes) as catalyst precursor for alkene metathesis reactions. In the absence of an alkene metathesis carbene initiator, complex 1 acts as a pre-catalyst for the metathesis transformation of various styrene derivatives into the corresponding stilbenes including unsymmetrical ones. We show the dual ring-closing metathesis (RCM) vs. cycloisomerisation activity of 1 and that, in the presence of styrene as an activator, complex 1 is effective for the RCM of a sterically hindered diene.

The first terminal rhenium phosphinidene complex, [Re(CO) 5 (η 1 -PN i Pr 2 )][AlCl 4 ], has been synthesized by chloride abstraction from [Re(CO) 5 {P(Cl)-(N i Pr 2 )}]. The electrophilic character of the terminal phosphinidene ligand is demonstrated by phosphine addition at the unsaturated phosphorus center and by novel reactions with azobenzene, PhNdNPh, which generate, via C-H activation and P-N and P-C bond formation, coordinated benzodiazaphosphole ligands. The cations [Re(CO) 5 P(PPh 3 )N i Pr 2 ] + and [Re(CO) 5 -{P(PhNNHC 6 H 4 )N i Pr 2 }] + have been crystallographically characterized as their AlCl 4salts. The corresponding late-metal terminal phosphinidene complexes [Co(CO) 3 -(PR 3 )(η 1 -PN i Pr 2 )][AlCl 4 ] (R ) Ph, Et) also afford coordinated benzodiazaphospholes via reaction with PhNd NPh.