Facile Ru−H₂ Heterolytic Activation and Intramolecular Proton Transfer Assisted by Basic N-Centers in the Ligands

Abstract

The use of the phosphine PPh2py instead of PPh3 in complexes of the type [Cp*RuH(P)2] enormously alters the kinetic control of the proton-transfer reactions over this compound and its chemical behavior. The reaction at low temperature of [Cp*RuH(PPh2py)2], 2, with HBF4 gives as products the classical dihydride trans-[Cp*RuH2(PPh2py)2](BF4), 3 (1 equiv of HBF4) or the dihydrogen-bonded complex [Cp*RuHH(PPh2pyH)(PPh2py)](BF4)2, 4 (2 equiv of HBF4). These complexes exhibit very accessible intramolecular processes of proton transfer, and finally, a slow release of H2 takes place at room temperature. Derivatives 2 and 3 are active catalysts for the deuterium labeling of H2 using methanol-d4 as an isotopic source. This demonstrates that the release of hydrogen is reversible, that the heterolytic activation of H2 is an easy process, and that acid species participate in the intramolecular proton-transfer processes. These observations are supported by reaction-coordinate calculations at the DFT/B3LYP level that show the existence of a low-energy reaction path that easily transforms the classical trans dihydride complex into the nonclassical cis dihydrogen compound in a reversible way, through the involvement of hydrogen- and dihydrogen-bonded intermediates and the essential participation of the pyridine centers. The different energy minima of this reaction profile are very accessible through low-energy transition states, all of which have been located.

On the peculiarities of hydrogen bonding and proton transfer equilibria for organic vs organometallic bases

“…The equilibrium shifts toward the latter product with lowering pH (5.9 → 3.6). 31 In the gas phase, the dihydrido compound trans-[CpRu(H) 2 The low temperature proton transfer of the related Cp*RuH(PPh 2 py) 2 hydride was shown 33 to yield the classical dihydride as well as the DHB species RuH⋅HN. According to DFT calculations, the cause of this behavior is the facile protonation of the pyridine nitrogen atoms, which (after possible rotations around Ru-P and P-C(py) bonds) results in structures with pyH + ⋅⋅⋅py hydrogen bonds.…”

“…These two types of hydrogen bonding, M⋅HA and M-H⋅HA, are unique for transition metal (hydride) complexes and the terms "non-classical" or "unconventional" hydrogen bonding have been coined to address these interactions. 33 Thus, the dichotomy between different sites of proton donor attacks, classical and non-classical, could arise for transition metal complexes leading to coinciding or different sites of hydrogen bonding and protonation, like in organic bases. The following does not give a comprehensive literature survey for this phenomenon but will provide some examples from the authors' laboratory and from other published works.…”

Supramolecular catalysis. Part 1: non-covalent interactions as a tool for building and modifying homogeneous catalysts

“…For example, the o-pyridyl-diphenylphosphine ligand facilitates the heterolytic cleavage of H2 through hydrogen interactions and subsequent proton transfer. 366 Grotjahn and co-workers found that 121 367,368 and 122 365 (Fig. 34, a) are very efficient catalysts for the anti-Markovnikov hydration of alkynes providing aldehydes with high selectivity and activity.…”

The AZARYPHOS Family of Ligands for Ambifunctional Catalysis: Syntheses and Use in Ruthenium‐Catalyzed anti‐Markovnikov Hydration of Terminal Alkynes

“… Metal‐bound pyridylphosphanes involved in (presumed) ambifunctional interactions with catalysis substrates: a) selectivity‐ and rate‐enhancement in alkyne methoxycarbonylation, according to Drent5 and Matteoli;7 b) hydrogen bridges to acetylene CH bonds;10a c) a reaction intermediate of catalytic anti‐Markovnikov alkyne hydration;10b d) heterolytic splitting of H 2 at ruthenium;13 e) a presumed catalytic intermediate in a H 2 /D 2 exchange reaction;13b f) acceleration of ruthenium‐catalyzed nitrile hydration through inner‐sphere general base activation of water 14…”

“…Catalytic H 2 /D 2 ‐exchange reactions may benefit from ambifunctional activation of the heterolytic split of coordinated molecular hydrogen by a pyridylphosphane ligand (Scheme d) 13. It was proposed that the resulting hydride complex stores a bridging proton between two pyridyl groups (Scheme e) 13b. Finally, the acceleration of ruthenium‐catalyzed hydration of nitriles by 2‐diphenylphosphinopyridine has been ascribed to general base activation of water by a pyridine lone pair 14.…”