Abstract
Azocarboxamides, a special class of azo ligands, display intriguing electronic properties due to their versatile binding modes and coordination flexibility. These properties may have significant implications for their use in homogeneous catalysis. In the present report, half-sandwich Ir–Cp* complexes of two different azocarboxamide ligands are presented. Different coordination motifs of the ligand were realized using base and chloride abstracting ligand to give N∧N-, N∧O-, and N∧C-chelated monomeric iridium complexes. For the azocarboxamide ligand having methoxy substituted at the phenyl ring, a mixture of N∧C-chelated mononuclear (Ir-5) and N∧N,N∧C-chelated dinuclear complexes (Ir-4) were obtained by activating the C–H bond of the aryl ring. No such C–H activation was observed for the ligand without the methoxy substituent. The molecular identity of the complexes was confirmed by spectroscopic analyses, while X-ray diffraction analyses further confirmed three-legged piano-stool structure of the complexes along with the above binding modes. All complexes were found to exhibit remarkable activity as precatalysts for the transfer hydrogenation of carbonyl groups in the presence of a base, even at low catalyst loading. Optimization of reaction conditions divulged superior catalytic activity of Ir-3 and Ir-4 complexes in transfer hydrogenation over the other catalysts. Investigation of the influence of binding modes on the catalytic activity along with wide range substrates, tolerance to functional groups, and mechanistic insights into the reaction pathway are also presented. These are the first examples of C–H activation in azocarboxamide ligands.
Highlights
Azocarboxamides, a special class of multifaceted redox-active azo chelating ligands have garnered considerable attention by virtue of its diverse coordination modes, and redox activity.[1−3] The azo group (−N N−) in the azocarboxamide ligand is surrounded by a carbonyl oxygen of the amide group, which can potentially be used as a donor atom along with the nitrogen atom of the azo group (N∧O-chelated ring, Chart 1)
In addition to the monometallic complexes, we report the first example of a bimetallic iridium complex with azocarboxamide ligand L2, in which two iridium centers are coordinated to the ligand in an unusual coordination mode
We have demonstrated that an N∧N- or N∧O- or N∧C-chelated half-sandwich iridium complex can be selectively synthesized from the reaction of an azocarboxamide ligand with [Cp*IrCl2]2 by changing the reaction conditions. 2-(4-Methoxyphenyl)-N-phenyldiazenecarboxamide ligand
Summary
Azocarboxamides, a special class of multifaceted redox-active azo chelating ligands have garnered considerable attention by virtue of its diverse coordination modes, and redox activity.[1−3] The azo group (−N N−) in the azocarboxamide ligand is surrounded by a carbonyl oxygen of the amide group, which can potentially be used as a donor atom along with the nitrogen atom of the azo group (N∧O-chelated ring, Chart 1).Chart 1. In the presence of a base, the azocarboxamide ligand can bind to a metal center via the deprotonated nitrogen atom of the amide together with the nitrogen atom of the azo group (N∧N-chelated ring).[1,3] Interestingly, it can provide an additional binding pocket to the metal center by activating the C−H bond from one of the phenyl rings. This depends on the functionalization of the phenyl ring and the electronic properties of the metal center and its ancillary ligands, leading to the formation of the stable five-membered cyclometalated complex (N∧C-chelated ring). While examples of the first two coordination modes are already available in the literature for ruthenium complexes,[1−3] the latter cyclometalated coordination mode is not reported for azocarboxamide ligands
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