In the “Devil’s Triangle” of catalysis there is a close inter-relationship between reactivity, selectivity and lifetime. Often, new catalysts are designed based on the promise of increased selectivity and reactivity while chemists cross their fingers and hope for longevity. By now it is well known that Grubbs metathesis catalysts are extremely active, chemoselective and robust, even in the presence of air, water and polar functional groups. However, as the saying goes, all good things must end. With the end in metathesis activity comes the natural question, “why?” While several research groups have investigated the products and mechanisms of catalyst decomposition, a new report out of the Fogg group in Organometallics1 brings an increased focus on the formation of Ru-CO complexes that are devoid of metathesis active carbenes. This post highlights Fogg et al.’s recent account as well as other relevant chemistry.
Heating Ru(II) and Ru(III) compounds in the presence of primary alcohols has long been known to form Ru-CO complexes.2 Specifically, heating the common Ru precursor [(COD)Ru(Cl)2]n in ethanol or RuCl3•(H2O)x in methanol or 2-methoxyethanol with PCy3 yields (PCy3)2Ru(H)(CO)(Cl), 1, a well-known olefin isomerization catalyst.3 An improved synthetic preparation has since been reported.4 In the context of metathesis catalysts (Figure 1), complexes 1 and 2 were observed upon washing Grubbs 1st and 2nd generation catalysts (PCy3)2Ru(CHPh)(Cl)2, 3, and (H2IMes)(PCy3)Ru(CHPh)(Cl)2, 4 with methanol.5 Complex 1 was also formed by thermal decomposition of (PCy3)2Ru(CHOEt)(Cl)2. Dinger and Mol then studied the mechanism for decomposition of 3 to 1 in the presence of alcohol,3b proposing an interesting mechanism involving double-protonation of the benzylidene of 3 or 4 by methanol to liberate toluene accompanied by decarbonylation of formaldehyde. Consistent with the proposal of chloride for methoxide substitution at Ru, added bases greatly accelerated the rate of reaction. Strangely, 1 was also a product of heating 3 with triethylamine/water or oxygen alone. In the presence of oxygen alone, the source of carbonyl was proposed to be the alpha-carbon of the benzylidene fragment, which is oxidized to form an intermediate benzoyl moiety. In the presence of Et3N/water, the mechanism of hydridocarbonyl formation is unclear. Taken together, however, the observations in these accounts certainly implicate 1 as a thermodynamic sink in this motif.
Figure 1. (top) Decomposition of 1st and 2nd generation Grubbs catalyst by methanol and (bottom) proposed mechanism.
A similar compound was observed by Ozerov et al.6 during the attempted synthesis of (PNP)Ru(H2)(H). (PNP)Ru(CO)(H) was formed in the presence of secondary alcohols, which typically are inert to decarbonylation unless forcing conditions are employed. The authors proposed and supported a mechanism involving [Ru] insertion into the C-C bond in acetone (a by-product of dihydrogen hydride formation) followed by loss of 2 equivalents of methane. Another highlight of this chemistry is that carbonate is a viable source of the carbonyl ligand. Like the bis-phosphine complexes, this is clearly another motif in which Ru-CO formation is favored. DFT calculations elucidated a ~40 kcal/mol preference for the carbonyl ligand compared to H2!
In Fogg’s latest work,1 1st and 2nd generation Grubbs catalysts were treated directly with excess methoxide. This leads to new Ru(OMe)(H)(CO)x complexes extremely rapidly at room temperature. Decomposition of 3 leads to both the mono- and dicarbonyl complexes, as shown in Figure 2. Formation of the dicarbonyl liberates an equivalent of H2 which binds to the monocarbonyl adduct. The corresponding 2nd generation monocarbonyl decomposition product is resistant to a second decarbonylation event and is the sole product of the reaction of 4 with excess methoxide.
Figure 2. Decomposition of Grubbs catalysts to novel methoxyhydride complexes by methoxide.
The summation of this work indicates that, under many conditions, formation of Ru(H)(CO) species is a result of facile decomposition pathways, presumably due to the large thermodynamic driving force of Ru-CO bond formation. Primary alcohols, especially in the presence of base, induce decomposition by creating nucleophilic alkoxide ions that undergo decarbonylation at the Ru metal. Though these experiments focus on widely used 3 and 4, their mechanisms warn of a general pathway affecting the diverse suite of L2Ru(Cl2)(CHR) catalysts during their syntheses, handling, and applications.
1 Beach, N. J.; Lummiss, J. A. M.; Bates, J. M.; Fogg, D. E. Organometallics 2012, ASAP.
2 a) Moers, F. G.; Langhout, J. P. Recl. Trav. Chim. Pays-Bas 1972, 91, 591-600. b) Yi, C. S.; Lee, D. W.; Chen, Y. Organometallics 1999, 18, 2043-2045.
3 a) Dinger, M. B.; Mol, J. C. Eur. J. Inorg. Chem. 2003, 2827-2833. b) Dinger, M. B.; Mol, J. C. Organometallics 2003, 22, 1089-1095.
4 Beach, N. J.; Dharmasena, U. L.; Drouin, S. D.; Fogg, D. E. Adv. Synth. Catal. 2008, 350, 773-777.
5 Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.; Scholl, M.; Choi, T. L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 2546-2558.
6 Celenligil-Cetin, R.; Watson, L. A.; Guo, C.; Foxman, B. M.; Ozerov, O. V. Organometallics 2005, 24, 186-189.