Ruthenium Vinylidene Complexes: Syntheses and Catalytic Properties

A first set of neutral 16-electron ruthenium vinylidene complexes 10 was easily prepared by Katayama and Ozawa (18) from common terminal alkynes and the arene ruthenium dimer 9, [RuCl₂(p-cymene)]₂, see Equation (i). Unfortunately, these complexes showed only moderate metathesis activity in processes such as RCM of unsubstituted α,ω-dienes and ROMP of highly strained norbornenes (19).

More effective neutral and cationic 16- and 18-electron ruthenium tridentate complexes, for example, 14, 15 and 16, were easily synthesised by van Koten and coworkers (21) by treating the ruthenium complex [RuCl₂(NN′N )(PPh₃)] (where NN′N is 2,6-bis[(dimethylamino)methyl]pyridine ligand) with 2 equivalents of Ag[BF₄], in CH₂Cl₂, in the presence of an excess of phenylacetylene (isolated yield 95%), see Equation (ii).

Significantly, a highly active and selective, coordinatively unsaturated, ruthenium 16-electron dicationic complex, 16a, was found to quantitatively promote ROMP of norbornene to polynorbornene under mild conditions, in the absence of any cocatalyst (Scheme I). The IR, ¹H and ¹³C{¹H} NMR spectra of the polynorbornene obtained under the above conditions indicated 90–95% trans C=C, in accordance with similar results reported previously (12).

Substantial progress was made by Louie and Grubbs through the synthesis of ruthenium vinylidene complexes coordinated with an imidazolylidene ligand (22). This class of ruthenium complexes, including complexes with formula 17 [IMes = 1,3-(2′,4′,6′-trimethylphenyl)imidazol-2-ylidene, R = Cy, R′ = tBu] and 18 (iPrIM = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, R′ = Ph) has been prepared directly from the bisphosphane ruthenium complex 10 (R = Cy) and free imidazoline carbenes or their salts (Scheme II).

Of these two complexes, 17 and 18, the ruthenium compound 17, possessing a mixed ligand system, displayed a substantial metathesis activity in RCM of diethyl diallylmalonate to substituted cycloolefin (Equation (iii)), although the reaction rate was slower than that with the parent bisimidazolylidene ruthenium carbene complex.

Detailed mechanistic investigations of the ruthenium-catalysed metathesis chemistry strongly indicated that increased ligand dissociation (that is of phosphane) is necessary to accelerate initiation and thereby enhance catalytic activity in this type of reaction. Thus, a phosphane-free coordinatively unsaturated ruthenium vinylidene complex 19 can be formed directly in situ from the commercial ruthenium dimer 9, N-heterocyclic carbene (IMes) as such or as its salts, and a terminal alkyne (Equation (iv)).

Indeed, the catalytic activity of complex 19 proved to be superior to that of the ruthenium complex 17, supporting the concept of a higher degree of unsaturation in the coordination sphere of the metal promoting catalysis. The pathway for generation of the true catalyst 21 from the catalyst precursor 19, by reaction with an olefin substrate, can be seen in Equation (v).

The particular catalytic behaviour of the pre-catalyst 19 in the RCM of diethyl diallylmalonate, metathesis homodimerisation CM of allyl benzene and ROMP of 1,5-cyclooctadiene is compared in Equations (vi), (vii) and (viii). It is worth noting that the solvent (hexane or tetrahydrofuran) plays an important role in the in situ generation of the ruthenium catalyst from these starting materials.

Another interesting array of ruthenium vinylidene complexes, 22, containing Schiff bases as chelating ligands, was prepared by Verpoort and coworkers (23–26) from ruthenium vinylidene complex 10 and various aromatic salicylaldimines, see Scheme III in which R = H or NO₂ and R′ = Ph, tBu or Me₃Si. This class of ruthenium complexes, easily accessible from [RuCl₂(p-cymene)]₂, 9, terminal akynes and salicylaldimine salts, showed good activity in olefin metathesis and enol ester synthesis due to the “one-arm” de-coordination ability of the bidentate Schiff base ligand creating unsaturation in the coordination sphere of the metal. These complexes, 22, have also been found to serve as excellent pre-catalysts in the RCM of α,ω-dienes and the ROMP of norbornene, substituted norbornene, cyclooctene and polycyclic olefins. Moreover, the related ruthenium vinylidene complexes that contain imidazolin-2-ylidene ligands displayed considerable stability, even for several days at high temperature (24).

Conclusions

Applying ruthenium vinylidene complexes in olefin metathesis reactions (RCM, CM, ROMP) seems to be a convenient alternative to the classical ruthenium bisphosphane catalysts largely employed in organic synthesis and polymer chemistry. Due to the particular steric and electronic environment provided by the ligands, some of the vinylidene ruthenium complexes exhibit remarkable activity and selectivity. They are readily accessible, only requiring commercially available starting materials for their synthesis. Importantly, when generated in situ, coordinatively unsaturated complexes displayed considerably high RCM activity. Combining vinylidene ligands with other specific ligands (such as imidazolylidene, Schiff bases, etc.), in the coordination sphere of the ruthenium core, allows further access to highly efficient ruthenium metathesis pre-catalysts.