Transfer hydrogenation with abnormal dicarbene rhodium(iii) complexes containing ancillary and modular poly-pyridine ligands

Abstract

Treatment of an abnormal dicarbene ligated rhodium(iii) dimer with 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen) or 2,2':6',2''-terpyridine (terpy) results in coordination of the N-donor ligands and concomitant cleavage of the dimeric structure. Depending on the denticity of the pyridyl ligand, this situation retains one (L = terpy) or two (L = bipy, phen) flexible sites for substrate coordination. In the case of the bipy complexes, modification of the electron density at Rh, without directly affecting the steric environment about the metal centre, was achieved by the incorporation of electron-donating or electron-withdrawing substituents on the bipy backbone. The dicarbene pyridyl complexes were active in transfer hydrogenation catalysis of benzophenone at 0.15 mol% catalyst loading in a iPrOH/KOH mixture. The catalysts displayed a strong characteristic colour change (yellow to purple) after activation which allowed for visual monitoring of the status of the reaction. The colour probe and the robustness of the active catalysts proved useful for catalyst recycling. The catalytic activity sustained over five consecutive substrate batch additions and gave a maximum overall turnover number of 3100.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

“…The dimeric structure of the rhodium(dicarbene) complexes 290 is readily cleaved in the presence of donor groups, such as diphosphines and diimines, resulting in the formation of dicarbene complexes with diphosphine ligands 291 274 or with diimine ligands 292−294 (Scheme 116). 275 Metalation of a pyridinium−imidazolium system reveals remarkably diverse reactivity patterns of the imidazolium ligand site, including C−C bond-making and -breaking processes, C−N bond cleavage, and C−H bond activation. 276 For example, rhodation of the pyridinium-functionalized 2methylimidazolium salt 295 with [Rh(COD)Cl] 2 yields the 2imidazolylidene rhodium complex 296 as a result of imidazolium C(sp 2 )−C(sp 3 ) bond activation (Scheme 117).…”

“…Replacement of the two MeCN ligands in 404 by substituted bipyridine or phenanthroline ligands (292a, 293) does not enhance catalytic activity (TOF 50 300 h −1 ), though complex 292a undergoes a reversible color change from yellow to purple upon formation of the catalytically active species and hence provides a diagnostic probe for the catalytic on/off state of the complexes (Scheme 162). 331 Introduction of diphosphine ligands yields complex cis/trans-291, which is a considerably more active catalyst that reaches TOFs of 1000 h −1 and up to 4000 TONs. 274 Optimization of the catalytic conditions allowed transfer hydrogenation to be run with only 1 mol % base instead of the often used 10 mol %.…”

N-Heterocyclic Carbenes (NHCs): An Introduction

“…They need careful isolation and storage. The consequential unusual firmness of NHC-metal complexes has been explored in many demanding protocols such as coupling reactions [55,56], polymerization [57,58], transfer hydrogenation [59][60][61], photocatalysis [62,63], and many other [64][65][66][67][68][69][70].…”

Cobalt Complexes as Efficient Cooperative Catalysts for Transfer Hydrogenation

“…The saturated alcohols, obtained as the TH products, have widespread applications in the chemical, pharmaceutical and agrochemical industries [17–20] . Although considerable progress has been made in the field of TH; the use of precious metals such as rhodium, iridium, palladium and ruthenium are the major limiting factors [3,7,16,21–27] . Therefore, the contemporary research is focussed on developing the catalysts which are based on earth‐abundant and non‐precious metals [28] .…”