Enhanced Catalytic Activity of Iridium(III) Complexes by Facile Modification of C,N-Bidentate Chelating Pyridylideneamide Ligands

Abstract

A set of aryl-substituted pyridylideneamide (PYA) ligands with variable donor properties owing to a pronounced zwitterionic and a neutral diene-type resonance structure were used as electronically flexible ligands at a pentamethylcyclopentadienyl (Cp*) iridium center. The straightforward synthesis of this type of ligand allows for an easy incorporation of donor substituents such as methoxy groups in different positions of the phenyl ring of the C,N-bidentate chelating PYA. These modifications considerably enhance the catalytic activity of the coordinated iridium center toward the catalytic aerobic transfer hydrogenation of carbonyls and imines as well as the hydrosilylation of phenylacetylene. Moreover, these PYA iridium complexes catalyze the base-free transfer hydrogenation of aldehydes, and to a lesser extent also of ketones. Under standard transfer hydrogenation conditions including base, aldehydes are rapidly oxidized to carboxylic acids rather than reduced to the corresponding alcohol, as is observed under base-free conditions.

Optimization of Synthetically Versatile Pyridylidene Amide Ligands for Efficient Iridium‐Catalyzed Water Oxidation

“…These complexes are air-stable and were purified by standard silica columnc hromatography without any precautions. [19] The pyridyl C a ÀC b bonds are consistently shorter (average 1.35 )t han the C b ÀC g bonds (1.41 ), confirming substantial contributiono ft he neutrald iene-type resonance structure in the solid state, in agreement with previous studies (see Ta ble S1 in the Supporting Information). The molecular structures are unsurprising and bond lengthsa nd anglesa round the iridium center are identical within standard deviations for both complexes and similart or elatedc omplexes( Ta ble 1).…”

“…Cyclometalation was demonstrated for all the complexes by the loss of one proton signal in the aromatic region of the 1 HNMR spectrum.T he structures of complex 8b and 8c were unequivocally confirmed by single-crystal X-ray diffraction analysis ( Figure 3). [19,20,23] Electrochemical analysiso fc omplexes 8a-c by CV in CH 2 Cl 2 revealed af ully reversible oxidation at E 1/2 =+0.64 V( vs. SCE) for all complexes 8a-c,w hich is identical to the potentialm ea-Scheme1.Synthesis of IrCp*PYA-type complexes 8a-c with different alkyl chains. [19] The pyridyl C a ÀC b bonds are consistently shorter (average 1.35 )t han the C b ÀC g bonds (1.41 ), confirming substantial contributiono ft he neutrald iene-type resonance structure in the solid state, in agreement with previous studies (see Ta ble S1 in the Supporting Information).…”

“…Metalation with [Ir(Cp*)Cl 2 ] 2 afforded the cyclometalated complexes 2-5 ( Figure 2), whiche xhibited enhanced catalytic activity in transfer hydrogenation of ketones and imines, as well as in alkyne hydrosilylation. [19] The better catalytic performance relative to complex 1 was attributed to the highere lectron density at the active site and the ensuing stabilization of higher oxidation states during the catalytic cycle. In an effort to quantify the electronic impact of the different aryl-PYAl igands,w eh ave now performed electrochemical analysis.…”

“…Based on recent studies, which showedas ubstantial rate enhancement by simple alterations on keyp ositions of the ligands, [19,20] we have exploiteds uch modifications for enhancing the catalytic activity of complex 1 by modulating specifically 1) the donor properties of the Ph ligand, 2) the N-substituent on the PYAl igand, and3 )the donorp roperties of the PYA ligand. Based on recent studies, which showedas ubstantial rate enhancement by simple alterations on keyp ositions of the ligands, [19,20] we have exploiteds uch modifications for enhancing the catalytic activity of complex 1 by modulating specifically 1) the donor properties of the Ph ligand, 2) the N-substituent on the PYAl igand, and3 )the donorp roperties of the PYA ligand.…”

Cationic Protic Imidazolylidene NHC Complexes of Cp*IrCl+ and Cp*RhCl+ with a Pyridyl Tether Formed at Ambient Temperature

“…The rates of catalysis by the CpIr* species here are in the range of rates reported for a variety of Cp*Ir complexes, where temperatures of about 80 • C are used, often with co-catalytic base (recent examples: [25,26]). As a side note, to achieve faster rates, e.g., activity at ambient temperature, it would appear that a mono-dentate NHC is needed on Cp*Ir [27].…”