Modular Pincer-type Pyridylidene Amide Ruthenium(II) Complexes for Efficient Transfer Hydrogenation Catalysis

Abstract

A set of bench-stable ruthenium complexes with new N,N,N-tridentate coordinating pincer-type pyridyl-bis(pyridylideneamide) ligands was synthesized in excellent yields, with the pyridylidene amide in meta or in para position ( m-PYA and p-PYA, respectively). While complex [Ru( p-PYA)(MeCN)] is catalytically silent in transfer hydrogenation, its meta isomer [Ru( m-PYA)(MeCN)] shows considerable activity with turnover frequencies at 50% conversion TOF = 100 h. Spectroscopic, electrochemical, and crystallographic analyses suggest considerably stronger donor properties of the zwitterionic m-PYA ligand compared to the partially π-acidic p-PYA analogue, imparted by valence isomerization. Further catalyst optimization was achieved by exchanging the ancillary MeCN ligands with imines (4-picoline), amines (ethylenediamine), and phosphines (PPh, dppm, dppe). The most active catalyst was comprised of the m-PYA pincer ligand and PPh, complex [Ru( m-PYA)(PPh)(MeCN)], which reached a TOF of 430 h under aerobic conditions and up to 4000 h in the absence of oxygen. The presence of oxygen reversibly deactivates the catalytically active species, which compromises activity, but not longevity of the catalyst. Ligand exchange kinetic studies by NMR spectroscopy indicate that the strong trans effect of the phosphine is critical for high catalyst activity. Diaryl, aryl-alkyl, and dialkyl ketones were hydrogenated with high conversion, and α,β-unsaturated ketones produced selectively the saturated ketone as the only product due to exclusive C═C bond hydrogenation, a distinctly different selectivity from most other transfer hydrogenation catalysts.

Hydrogen Transfer Reactions of Carbonyls, Alkynes, and Alkenes with Noble Metals in the Presence of Alcohols/Ethers and Amines as Hydrogen Donors

“…The remarkable success of transfer hydrogenation reactions by the N,N,N-tridentate coordinating pincer-type ligand family was further proven by Albrecht et al [88]. Pyridylidene amides (PYAs) were the choice of usage, because the stabilization of different metal electronic configurations can be applied without the chemical transformation of the ligand skeleton (Scheme 19) [88].…”

“…Pyridylidene amides (PYAs) were the choice of usage, because the stabilization of different metal electronic configurations can be applied without the chemical transformation of the ligand skeleton (Scheme 19) [88]. The combination of the pincer motif and the PYA donor system leads to ruthenium complexes with excellent activity and selectivity (Scheme 19) [88].…”

Fast Transfer Hydrogenation(TH) in Aerobic Condition and Oxidation of Alcohols with N‐Methylmorpholine‐N‐oxide Catalyzed by Ru(II) Ligated with Chalcogenated Pyridines and PPh3

“…Some earlier reported Ru catalysts for fast TH are: [Ru(m-PYA) (MeCN) 3 ] 2 + [m-PYA = pyridylbis(m-pyridylideneamide)], [16] Ru(L) (PPh 3 )Cl 2 , [17] [L = 6-(2,2'-bipyridin-6-ylmethyl)-pyridin-2-ol/6'-(pyridin-2-ylmethyl)-2,2'-bipyridin-6-ol and 6'-((6-hydroxy pyridin-2-yl)methyl)-2,2'-bipyridin-6-ol/6,6'-(pyridine-2,6-diylbis (methylene)dipyridin-2-ol], (p-cymene)Ru(L)Cl 2 , [18] [L = 1,3-bis(2methylbenzyl)imidazolidin-2-ylidene, 1,3-bis(4-methyl-benzyl) imidazolidin-2-ylidene, 1,3-bis (4-ethylbenzyl)imidazoli din-2ylidene, 1,3-bis(4-i-proplbenzyl)imidazolidin-2-ylidene, 1,3-bis(4diethylaminobenzyl)imidazolid-2-ylidene, 1,3-bis(3,4-dimethoxybenzyl)imidazolidin-2-ylidene)], ruthenium complexes [19] of hydroxycyclopentadienyl and NHC's, complexes [21] Reaction conditions: catalyst 1-4(0.1 mol%); reactant (1 mmol); KOH (0.4 mmol); 15 mL of 2-propanol as solvent; bath temperature 80°C; reaction time 15 min; [b] yield%: protonation of aminophosphonium ligands in the presence of the dihydrogen complex RuH(H 2 )-(Cl)( [22] [L = 3-methyl-1-(2-picolyl)imidazol-2-ylidene, 3isopropyl-1-(2-picolyl)imidazol-2-ylidene, 3-phenyl-1-(2-picolyl) imidazol-2-ylidene, 3-mesityl-1-(2-picolyl)imidazol-2-ylidene, 3methyl-1-(2-picolyl)benzoimidazol-2-ylidene, and 3-methyl-1-(2picolyl)-4,5-dichloroimidazol-2-ylidene],…”

Highly Efficient Transfer Hydrogenation Catalysis with Tailored Pyridylidene Amide Pincer Ruthenium Complexes

“…[28][29][30] As ac onsequence of these uniquea nd flexible donor properties, PYAl igandsh ave been successfully appliedi navariety of catalytic transformations with appealing performances. [28][29][30][31][32][33][34][35][36] PYAs are ap articularly attractive sub-class of donor-flexible ligandsb ecause of their easy accessibility and their synthetic versatility,w hich provides ample opportunitiesf or ligand modifications. PYAv ariation has focused predominantly on modulation of the amide substituent R, including the incorporation of chelating groups, [24, 28-30, 35, 37] as wella st he positioning of the pyridinium site.…”

“…PYAv ariation has focused predominantly on modulation of the amide substituent R, including the incorporation of chelating groups, [24, 28-30, 35, 37] as wella st he positioning of the pyridinium site. [26,31,34,35] Remarkably,h owever,t he synthetic versatility of pyridines has not been explored so far, even thoughs uch modifications are bound to have ad irect impact on the donor properties of the PYAl igand.H ere, we have introduceds ubstituents on the PYAh eterocycle in meta-PYAp incer-type ligands and demonstrate that their electronic influenced irectly affects the catalytic properties of the coordinated ruthenium center.W eh ave exploited this methodology to develop highlya ctive transfer hydrogenation catalysts, which reach turnoverf requencies of more than 200 000 h À1 and operate at unusually low catalyst loadings in the ppm range, thus providing ac atalytic system that offersa na ttractive alternative to other high-performance catalysts.…”