Experimental and Computational Evidence for the Participation of Nonclassical Dihydrogen Species in Proton Transfer Processes on Ru–Arene Complexes with Uncoordinated N Centers. Efficient Catalytic Deuterium Labeling of H₂ with CD₃OD

Abstract

The protonation with Brønsted acids HB (HBF 4 and CF 3 CO 2 H) of [RuH 2 (arene)(PPh 2 Het)] derivatives (PPh 2 Het = dpim, Het = 2-N-methylimidazolyl; PPh 2 Het = PPh 2 py, Het = 2-pyridyl) that contain uncoordinated N atoms has been analyzed experimentally by NMR spectroscopy and through computational studies. Initially, at low temperature, the uncoordinated N atoms of the phosphine are protonated and a proton−hydride exchange is observed by 1 H NMR spectroscopy. The proton transfer leads to the corresponding cationic trihydride intermediates, which exhibit a dual character of classical and nonclassical hydrides, with the nonclassical species being more stable, especially when Het = 2-pyridyl. In fact, the release of H 2 and the formation of the respective monohydride complexes [RuH(arene)(κ 2 -N,P-PPh 2 Het)]B was observed at room temperature. The participation of the uncoordinated N center in the proton transfer process in the stabilization of RuH(H 2 ) + with respect to RuH 3 + species and in the hydride−dihydrogen exchange (cis effect) are discussed. Calculations on the complex [RuH 3 (p-cymene)(PPh 3 )] + have also been carried out for the sake of comparison. A dual character was also found, but in this case the classical species is more stable. H/D exchange of the hydride ligands of the dihydride complexes, using CD 3 OD as the deuterium source, has been studied. The very rapid deuterium labeling of H 2 , catalyzed by the aforementioned dihydrides, has been observed. The known compound [RuH 2 (p-cymene)(PPh 3 )] is also active in this labeling process, and the possible mechanism for both the H/D exchange and the deuterium labeling of H 2 is discussed in light of theoretical studies.

Dihydrogen Complexation

“…Dihydrogen complexes and dihydrogen bonded intermediates are also increasingly invoked as catalytic intermediates. − …”

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

“…36,37 The presence of all species depicted in Scheme 9 was confirmed experimentally and theoretically for several (arene)-RuH n (κ 1 -P-R 2 PHet) m systems (arene = Cp*, p-cymene; n = 2, 3; m = 1, 2) bearing phosphines with noncoordinated nitrogen centers (Het = pyridine, quinoline, 2-N-methylimidazolyl, etc.). 28,220,221 The protonation occurs initially at the heterocycle N atom, yielding compounds stabilized by hydrogen (NH + •••X) or dihydrogen (NH + •••HRu) bonding. Such adducts are stable only at low temperatures, above which proton transfer from the NH group to the hydride ligand proceeds, eventually resulting in H 2 evolution and the formation of thermally stable [(arene)RuH n−1 (κ 2 -P,N-R 2 PHet) m ] + species.…”

Versatile Rh- and Ir-Based Catalysts for CO2Hydrogenation, Formic Acid Dehydrogenation, and Transfer Hydrogenation of Quinolines

“…The value of the bite angle is 78.70(9)°, similar to I . The bond lengths within the cationic Ru-centered component are in the expected range − and these include the Ru–C p ‑cymene average distance of 2.19 Å. When compared with I the bonds involving the Ru atom are similar except the Ru–NH 2 distance that is shorter for 1BF 4 (2.128(2) and 2.134 for I ) .…”