The coupling of electron- and proton-transfer steps provides a general concept to control the driving force of redox reactions. N splitting of a molybdenum dinitrogen complex into nitrides coupled to a reaction with Brønsted acid is reported. Remarkably, our spectroscopic, kinetic, and computational mechanistic analysis attributes N-N bond cleavage to protonation in the periphery of an amide pincer ligands rather than the {Mo-N -Mo} core. The strong effect on electronic structure and ultimately the thermochemistry and kinetic barrier of N-N bond cleavage is an unusual case of a proton-coupled metal-to-ligand charge transfer process, highlighting the use of proton-responsive ligands for nitrogen fixation.

Keywords: Osmium / Hydrido ligand / Proton transfer / Hydrogen bonds / Density functional calculationsThe interaction between Cp*OsH(dppe) (1) and a series of proton donors (HA) of increasing strength [indole, CFH 2 CH 2 OH (MFE), CF 3 CH 2 OH (TFE), (CF 3 ) 2 CHOH (HFIP), p-nitrophenol, and HBF 4 ·Et 2 O] has been investigated experimentally by variable temperature IR and NMR spectroscopy in solvents with different coordinating abilities (alkanes, dichloromethane and their mixtures) and computationally at the DFT/B3PW91 level using different models. Both the IR and NMR spectroscopic data for the interaction with weak proton donors conform to the criteria of M-H···H-A bond formation. Theoretical calculations, however, indicate an asymmetric bifurcated interaction with a significant contribution from the metal atom, which is greater than that previously found for the corresponding Fe system. The basicity factor of 1 (E j = 1.47) is greater than those of the Ru (1.39) and Fe (1.35) congeners, in agreement with previous studies

In recent years, metal complexes, in which both the metal center and the ligand play active and cooperative roles, have emerged as very promising catalysts, capable of activating and forming chemical bonds through nonoxidative pathways (i.e., alternative to the conventional oxidative addition/reductive elimination sequence). The key mechanistic steps usually include a reversible switching between the coordination modes of the ligands that are bound to the catalytically active metal center (Scheme 1).For example, metal-amide/metal-amine interconversion, originally described by Noyori and Ikariya, [1] led to the discovery of very efficient asymmetric hydrogenation catalysts. [2] Aromatization/dearomatization in heteroaromatic PNP and PNN pincer ligands made nonoxidative activation of HÀH, [3] CÀH, [4] and NÀH [5] bonds possible and a family of hydrogenation/dehydrogenation catalysts was established by Milstein. [6] Nonaromatic switchable PNP pincer systems have been proven to facilitate heterolytic bond cleavage and catalyze transfer [7] and acceptorless dehydrogenation/hydrogenation reactions. [8] So far, ligand-metal cooperation relies on a dynamic interplay between carbometalated and a-or b-H eliminated species in electron rich CA C H T U N G T R E N N U N G (sp 3 )-metalated pincer complexes, [9] as well as on keto-enol [10] or lactam-lactim tautomerism. [11] These systems established a basis for the design of mild catalysts for nonoxidative (de)hydrogenation processes [12] and chemoselective H/D exchange. [13] The examples are impressive, however, they provide only a hint of the real synthetic potential concealed in ligandmetal cooperation. Therefore, we strongly believe that further extension of this concept to new reaction schemes will eventually lead to the discovery of conceptually novel catalysis.Herein, we wish to report on the development of Ir III -and Rh III -based catalysts that are capable of promoting olefin hydroformylation through the unprecedented metal-ligand cooperating mechanism. In our previous work, we developed the Ir III PCA C H T U N G T R E N N U N G (sp 3 )P pincer complex 1, which forms molecular hydrogen by an intramolecular interaction that involves both the metal center (hydride) and a polar O-based ligand sidearm (Scheme 2). [14] The catalyst showed very high activity in the acceptorless dehydrogenation of primary and secondary alcohols. We also observed that the release of molecular hydrogen is efficiently intercepted by the presence of unsaturated substrates.Based on this observation, we hypothesized that the hydroformylation of double bonds may proceed through a reversed metal-ligand cooperating mechanism (Scheme 3). The hypothetical catalytic cycle is likely to include the following elementary steps: i) heterolytic nonoxidative H 2 activation by the species a through alkoxide ring-opening to form the M À H complex b; ii) migratory insertion of an alkene into the Ir À H bond, resulting in the formation of c; iii) carbonylation of c, followed by migratory insertion of CO into th...

The coupling of electron‐ and proton‐transfer steps provides a general concept to control the driving force of redox reactions. N2 splitting of a molybdenum dinitrogen complex into nitrides coupled to a reaction with Brønsted acid is reported. Remarkably, our spectroscopic, kinetic, and computational mechanistic analysis attributes N−N bond cleavage to protonation in the periphery of an amide pincer ligands rather than the {Mo−N2−Mo} core. The strong effect on electronic structure and ultimately the thermochemistry and kinetic barrier of N−N bond cleavage is an unusual case of a proton‐coupled metal‐to‐ligand charge transfer process, highlighting the use of proton‐responsive ligands for nitrogen fixation.

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