The new hydridoruthenium complexes RuH-Cl(CO)(NHC)(PPh 3 ) (NHC ) IMes (4a), H 2 IMes (4b)), conveniently accessible from RuHCl(CO)(PPh 3 ) 3 , exhibit high activity for hydrogenation of unactivated internal olefins and isomerization of terminal olefins. The lability of the PPh 3 ligand is fundamental to the utility of 4: where this group is replaced by the relatively nonlabile PCy 3 ligand, the activating effect of the N-heterocyclic carbene is suppressed.N-Heterocyclic carbenes (NHCs) have emerged as an exceptionally versatile class of ligands for transitionmetal catalysis. Owing to their enhanced σ-donating ability and thermal stability, 1,2 NHC derivatives display higher reactivity than the corresponding phosphine complexes in many Ru-catalyzed olefin metathesis reactions 3-7 and Pd-catalyzed coupling reactions. 8 They can also offer advantages over the thermally sensitive Crabtree catalyst in the Ir-catalyzed H 2 -hydrogenation of olefins. 9,10 While good to excellent activity, typically at high catalyst loadings, has been reported in transfer hydrogenation (or in some cases, direct hydrogenation) via NHC complexes of rhodium, 11 iridium, 9-11 nickel, 12
Reaction of the Grubbs catalyst RuCl2(PCy3)2(CHPh) (1) with lithium 2-[(2,6-diisopropylphenyl)imino]pyrrolide·Et2O (LiNN′·Et2O) gives alkylidene complex 5, containing a chelating, σ-bound iminopyrrolato unit. The structure of 5 is confirmed by X-ray crystallography. Treatment of 5 with pyridine generates RuCl(NN′)(py)2(CHPh) (6) via displacement of PCy3. Complex 5 effects ring-closing metathesis in air, displaying high reactivity relative to 6.Key words: ruthenium, alkylidene, metathesis, pyrrolimine, iminopyrrolato.
Optimizes to S1 the reference adduct. Given its relative instability, we rejected this species as a possible intermediate.
RuR 3 P Cl CH 2 CO CH 3 Scheme 3. Structure of triplet configuration of S13.Agostic Phosphine Interactions. A ruthenium tricyclohexylphosphine complex stabilized by an agostic interaction with a cyclohexyl C-H bond has been reported by Arliguie et al. 4 In principle, such an interaction could stabilize the ethyl intermediate (Scheme 4). We attempted to optimize ethyl intermediate structures featuring this interaction using tricyclohexylphosphine. In all cases, the intermediate optimized to a structure in which ethylene deinserted to revert to the ethylene adduct. The agostic interaction through the cyclohexyl group is evidently not strong enough to stabilize this intermediate.
Reaction of RuCl2(PPh3)3 with LiNN' (NN' = 2-[(2,6-diisopropylphenyl)imino]pyrrolide) affords a single product, with the empirical formula RuCl[(2,6-iPr2C6H3)N=CHC4H3N](PPh3)2. We identify this species as a sigma-pyrrolato complex, [Ru(NN')(PPh3)2]2(mu-Cl)2 (3b), rather than mononuclear RuCl(NN')(PPh3)2 (3a), on the basis of detailed 1D and 2D NMR characterization in solution and in the solid state. Retention of the chelating, sigma-bound iminopyrrolato unit within 3b, despite the presence of labile (dative) chloride and PPh3 donors, indicates that the chelate effect is sufficient to inhibit sigma --> pi isomerization of 3b to a piano-stool, pi-pyrrolato structure. 2D COSY, SECSY, and J-resolved solid-state 31P NMR experiments confirm that the PPh3 ligands on each metal center are magnetically and crystallographically inequivalent, and 31P CP/MAS NMR experiments reveal the largest 99Ru-31P spin-spin coupling constant (1J(99Ru,31P) = 244 +/- 20 Hz) yet measured. Finally, 31P dipolar-chemical shift spectroscopy is applied to determine benchmark phosphorus chemical shift tensors for phosphine ligands in hexacoordinate ruthenium complexes.
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