Participation of (η3-Allyl)ruthenium(II) Complexes in C−C Bond Formation and C−C Bond Cleavage. A Theoretical Study

Abstract

Coupling reactions of formaldehyde with RuBr(η3-C3H5)(CO)3 (R1), [Ru(η3-C3H5)(HCHO)(CO)3]+ (I2), and RuBr(η3-C3H5)(HCHO)(CO)2 (I3) were theoretically investigated with ab initio MP2-MP4(SDQ), CCSD(T), and DFT(B3LYP) methods. In R1, the coupling reaction takes place through two transition states, as follows: coordination of formaldehyde with the ruthenium(II) center occurs through the first transition state, to afford an (η1-allyl)ruthenium(II) formaldehyde complex, RuBr(η1-C3H5)(HCHO)(CO)3, as an intermediate, and then C−C bond formation between the η1-allyl ligand and formaldehyde takes place through the second transition state, to afford The activation energy (Ea) is 19.9 (12.0) kcal/mol for the first transition state and 12.5 (5.4) kcal/mol for the second transition state, where the values given without parentheses are Ea values calculated with the MP4(SDQ) method and the values in parentheses are those calculated with the DFT method. In I2 and I3, the coupling reaction proceeds through one transition state, to afford and with considerably larger Ea values of 50.7 (30.6) and 34.8 (32.0) kcal/mol, respectively. Even in I2, however, the allyl−aldehyde coupling reaction easily occurs through two transition states like that of R1, when one more formaldehyde molecule coordinates with the ruthenium(II) center; the Ea value is 10.5 (4.6) kcal/mol for the first transition state and 13.6 (6.7) kcal/mol for the second transition state. In this case, one formaldehyde molecule plays the role of a spectator ligand and the second formaldehyde undergoes a coupling reaction with the allyl ligand. From these results, it should be concluded that the allyl−aldehyde coupling reaction proceeds easily in the coordinatively saturated (η3-allyl)ruthenium(II) complex and that the coordinatively unsaturated (η3-allyl)ruthenium(II) complex becomes reactive when two molecules of formaldehyde coordinate with the ruthenium(II) center. IRC calculation of the allyl−aldehyde coupling reaction of R1 clearly shows that the C−C bond formation between the η1-allyl ligand and formaldehyde occurs after the second transition state concomitantly with the bond alternation in the η1-allyl ligand. Electron redistribution in the reaction indicates that the allyl−aldehyde coupling reaction is understood in terms of electrophilic attack of formaldehyde to the allyl ligand. Reverse C−C bond cleavage proceeds with a moderate Ea value of 16.6 (13.5) kcal/mol in to afford [Ru(η3-C3H5)(HCHO)(CO)3]+, with a similar Ea value of 19.6 (11.1) kcal/mol in to afford RuBr(η3-C3H5)(CO)3 + HCHO, and with a considerably large Ea value of 27.0 (26.8) kcal/mol in to afford RuBr(η3-C3H5)(HCHO)(CO)2. is the best for this C−C bond cleavage. This is because the coordinatively unsaturated complex with electron-accepting ligands yields a stable (η3-allyl)ruthenium(II) complex in which the η3-allyl ligand is strongly electron donating and needs two coordination sites. Though the C−C bond cleavage of occurs with a moderate Ea value, this reaction is much less exothermic than that of