Computational Mechanistic Studies on Reactions of Transition Metal Complexes with Noninnocent Pincer Ligands: Aromatization–Dearomatization or Not

Abstract

The development of green chemistry has attracted the chemists' attentions in recent years. Among them, Milstein and coworkers have discovered a new mode of metal-ligand cooperation in complexes, where an aromatization-dearomatization process of the pyridine or acridine-based PNP and PNN "pincer" ligands appears to be a key element. These complexes were reported to lead to unusual X-H (X=H, C, O, N, and B) activation reactions and to environmentally benign catalysis involving dehydrogenative coupling reactions and hydrogenation reactions, representing an important development in green chemistry. This review provides a summary of theoretical studies on the mechanisms of the reactions mediated by transition metal complexes with noninnocent pincer ligands synthesized by Milstein and coworkers. The aromatization-dearomatization process of the pyridine or acridine-based PNP and PNN "pincer" ligands were found to play important roles in some reactions, while other reactions do not involve the aromatization-dearomatization process. For some reactions, several research groups proposed different mechanisms to explain the same reaction. Thus, to compare these mechanisms, we recalculate their rate-determining steps by using the functionals that are calibrated to produce results close to those from coupled cluster calculations. Moreover, the understanding of the reaction mechanisms can help researchers to improve the current reactions and design new reactions.

Low‐Pressure Hydrogenation of Nitriles to Primary Amines Catalyzed by Ruthenium Pincer Complexes. Scope and mechanism

“…In line with the above NMR experiments, and with the literature regarding hydrogenation catalysis by pyridine‐based PNP‐Ru complexes a plausible mechanism of the ruthenium catalyzed hydrogenation of nitriles to primary amines is postulated in Scheme . Initially, complex 2 binds with the nitrile trans to the Ru−H bond to yield intermediate 7 .…”

“…In the presence of H 2 , 7 forms the trans ‐dihydride complex 8 with concomitant aromatization of the pincer ligand and liberation of free nitrile. The saturated complex 8, which is an intermediate in the catalytic cycle, transfers to the nitrile a hydride ligand and a proton from the pincer arm, by an outer sphere mechanism involving metal‐ligand cooperation by aromatization–dearomatization, possibly via transition state 9 (or via stepwise hydride and proton transfer), to give the de‐aromatized imine complex 10 . Imine release from 10 followed by addition of H 2 regenerates the dihydride complex 8 .…”

Cobalt Pincer Complexes in Catalytic C–H Borylation: The Pincer Ligand Flips Rather Than Dearomatizes

“…56–58 Pincer ligand benzylic deprotonations and hydrogen abstractions in related Co(I) complexes suggest that the metal-ligand cooperation in the Rh complexes may involve deprotonation at the benzylic site, 55 consistent with calculations on the Ru mechanism. 56–58 To confirm the conclusions obtained with model systems, we recalculated this pathway by using complex 2 with the full ligand ( iPr PNP). Considering that the pyridine ligand and other atoms use different basis sets in the optimizations (see SI1 in Computational Details), we employed another basis set BS4 (LANL2DZ for Co and 6–31G* for the others) to optimize the species involved in this pathway.…”

“…Furthermore, related PNP pincer ligated metal complexes synthesized by Milstein and coworkers are well known to show a mode of metal-ligand cooperation based on the ligand’s aromatization/dearomatization. 56–58 This gives rise to an interesting question that whether the trans -( iPr PNP)CoH 2 (BPin) complex shows similar reactivity or not during its catalytic cycle. Building on our previous mechanistic studies of the borylation with transition metal complexes.…”

Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

“…The involvement of metal-ligand bifunctional transformations in noble metal catalysis has been extensively studied and heavily debated. 27,[170][171][172][173] Mechanistic studies of the Fe-PNP-catalysed chemoselective reduction of aldehydes in the presence of ketones were conducted by the authors of the original reports as well as by independent groups, 33,34,167 but were not able to find a clear consensus on the nature of reduction selectivity and, importantly, the active species in catalysis.…”