Investigating Pathways of Molecular H 2 Exchange in (μ-H) 2 Os 3 (CO) 10 Silvio Aime,* Walter Dastru ` , Roberto Gobetto, Francesca Reineri, Andrea Russo, and Alessandra Viale Dipartimento di Chimica I.F.M., Universita ` di Torino, V. P. Giuria 7, 10125 Torino, Italy Received January 18, 2001 Summary: The occurrence of two pathways responsible for the reaction of (μ-H) 2 Os 3 (CO) 10 with molecular hydrogen has been elucidated by comparing the results obtained from H 2 /D 2 isotopic exchange experiments and para-H 2 effects observed in the cluster’s 1 H NMR spec- trum. The most efficient exchange process is based on an associative mechanism that leads to the formation of the elusive (η-*H 2 )(H)(μ-H)Os 3 (CO) 10 species (*H 2 is either a D 2 or a para-H 2 molecule), which undergoes *H-H elimination. The other pathway (whose efficiency increases with temperature) is based on the dissociation of H 2 from (μ-H) 2 Os 3 (CO) 10 to form the highly unsatur- ated “Os 3 (CO) 10 ” moiety, which promptly adds a *H 2 molecule, yielding the asymmetrical *H(μ-*H)Os 3 (CO) 10 S intermediate (where S is a stabilizing solvent molecule), which releases the S molecule to re-form (μ-*H) 2 Os 3 - (CO) 10 . Introduction A better understanding of the molecular hydrogen activation and of the hydrogen atoms’ mobility in transition metal complexes represents a key step in the mechanism interpretation in many catalytic reactions. A number of approaches have been exploited for the investigation of the intimate mechanisms in catalytic hydrogenations. In recent years interesting new insights have been provided by the use of para-hydrogen. In fact, it has been shown that para-H 2 allows the detection of hydrogenation reaction intermediates even when they are present at very low concentrations, thanks to the extraordinary enhancement of the NMR signals due to the para hydrogen induced polarization (PHIP effect). 1-9 Recently we reported that by reacting (μ-H) 2 Os 3 (CO) 10 with D 2 the formation of the three isotopomers (μ- H) 2 Os 3 (CO) 10 ,(μ-H)(μ-D)Os 3 (CO) 10 , and (μ-D) 2 Os 3 (CO) 10 occurs. 10 A mechanism involving the reversible oxidative addition and reductive elimination of molecular hydro- gen on (μ-H) 2 Os 3 (CO) 10 via the formation of a fluxional tetra(hydride, deuteride) intermediate has been pro- posed; however, such an intermediate has not been detected. Moreover, some years ago, Po ¨e et al. reported that (μ-H) 2 Os 3 (CO) 10 may release molecular hydrogen to form the highly reactive “Os 3 (CO) 10 ” intermediate, possibly stabilized by a solvent molecule. 11 We deemed it of interest to gain more insights into the pathways leading to association/dissociation of molecular hydrogen in (μ-H) 2 Os 3 (CO) 10 by comparing the results from H 2 /D 2 exchange with the effects in the 1 H NMR spectra obtained by reacting the cluster with para-H 2 . Results and Discussion It has been reported 12,13 that a para-H 2 effect can be detected as a strongly enhanced signal also in com- pounds containing two equivalent protons. The enhance- ment is observed if such species are generated in hydrogenation reactions via asymmetrical intermediates whose CSA (chemical shift anisotropy) and/or CSA/DD (interference term between the chemical shift anisotropy and the dipolar interactions) contributions to the relax- ation are significantly different for the two added protons. The hydride resonance of (μ-H) 2 Os 3 (CO) 10 has already been observed as an enhanced emission signal when the compound was produced by reacting Os 3 - (CO) 10 (NCCH 3 ) 2 and para-H 2 , via the intermediate H(μ- H)Os 3 (CO) 10 (NCCH 3 ) species. 12 Furthermore, a strong emission signal was observed for free molecular hydro- gen in the case of the reversible interaction of para-H 2 with H(μ-H)Ru 3 (μ-CO)(CO) 10 . 13 On the basis of this knowledge and by carefully considering the mechanism previously proposed for the H 2 exchange reaction on (μ-H) 2 Os 3 (CO) 10 (Scheme 1), some para-H 2 effects on the resonances of both the cluster and molecular H 2 are expected in their reaction. In fact, statistically all of the three possible pathways reported in Scheme 1 could occur. An enhanced signal for (μ-H) 2 Os 3 (CO) 10 is expected when both the hydrogen atoms of the para-H 2 molecule remain on the cluster (path i), while when they are both eliminated as * Corresponding author. 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(11) Po ¨e, A. J.; Sampson, C. N.; Smith, R. T.; Zheng, Y. J. Am. Chem. Soc. 1993, 115, 3181. (12) Aime, S.; Canet, D.; Gobetto, R. J. Am. Chem. Soc. 1998, 120, 6770. (13) Aime, S.; Canet, D.; Dastru ` , W.; Gobetto, R.; Viale, A. J. Phys. Chem. A 1999, 103, 9702. 2924 Organometallics 2001, 20, 2924-2927 10.1021/om0100378 CCC: $20.00 © 2001 American Chemical Society Publication on Web 06/01/2001