Hydrogen Transfer Activation via Stabilization of Coordinatively Vacant Sites: Tuning Long-Range π-System Electronic Interaction between Ru(0) and NHC Pendants

Abstract

The insertion of a pyridine substituent on the lateral chain of N-heterocyclic carbene ligands enhances the catalytic activity of [Ru­(CO)2(cyclopentadienone)­(NHC)] complexes, precursors of Shvo-type catalysts, toward the hydrogenation of 4-fluoroacetophenone in refluxing 2-propanol as hydrogen donor. DFT calculations evidence the role of pyridine in the donor/acceptor properties and complexes reactivity both in the case of imidazolylidene and triazolylidene ligands. Although the NHC-pyridine derived complexes perform somewhat worse than the forerunner Shvo dimer, the slower evolution of the reaction steps is compatible with the FT-ATR spectroscopy time scale and allows to identify in situ intermediates, invisible when using the original Shvo catalyst due to its higher reaction speed. Thus, mechanistic insight has been disclosed by synergic contribution of FT-ATR in situ experiments and DFT calculations performed on the slowest precatalyst (1N) chosen as a model. The proposed reaction mechanism has been based on both energy profile and experimentally identified intermediate.

Synthesis and Reactivity of Poly(propyleneimine) Dendrimers Functionalized with Cyclopentadienone N-Heterocyclic-Carbene Ruthenium(0) Complexes

“…3g1 pyridine 3 0 20 3g1-5 did not show any catalytic activity in the absence of additives, as already observed in the case of the mononuclear congeners [34]. Addition of 1 equivalent of CAN, with respect to Ru loading, promotes CO release [34,35] and leads to the pre-catalyst activation [58] resulting in some catalytic activity. Nevertheless, only 15-17% conversion could be reached after 24 h for all the five generations, which allow us to discard any positive dendritic effect.…”

“…A similar outcome was found by adding 10 eq. of pyridine for each ruthenium(0) center to 3g1 (entry 6 of Table 2), confirming the activation effect of the latter additive stated for pyridyl functionalized NHC ligands [58]. Although the pre-catalyst activation could be achieved also by pyridine, this was not used to test the following generations of dendrimer due to the high amount of additive required compared to CAN and the limited enhancement in the conversion (20% vs 17%).…”

The competition between dehydrogenation and dehydration reactions for primary and secondary alcohols over gallia: unravelling the effects of molecular and electronic structure via a two-pronged theoretical/experimental approach

“…With short alcohols possibly being produced from renewable feedstocks, processes able to convert them into more useful chemicals or to exploit them as useful reactants are uninterruptedly sought. As a few examples, we mention their upgrade to higher mass alcohols via Guerbet-type reactions, 1,2 the production of short chain olefins [3][4][5][6][7][8][9] or dienes, 1 and their use as reducing agents via MPV-type processes, [10][11][12][13] or for the direct alkylation of activated aromatic compounds. [14][15][16][17][18][19][20][21][22] When heterogeneously catalysed, a few of the processes mentioned require the in situ production of carbonylic compounds (i.e.…”

Advances in Catalytic Routes for the Homogeneous Green Conversion of the Bio‐Based Platform 5‐Hydroxymethylfurfural

“…Ru-hydride-containing Shvo catalyst 2 exploits the cyclopentadienone ligand as the basic site for the selective bifunctional hydrogenation of polar double bonds [90][91][92][93][94][95][96][97] and proved very useful also in HMF selective reduction of BHMF. [98] Noteworthy, the complex 2 is not the main actor of catalysis but behaves as catalyst precursor, leading to active species that participate in the HMF conversion mechanism.…”