In this work, we report a single-site, heterogeneous Pd(II)–NHC-based oxidative C–H activation catalyst, designed on a self-supported coordination polymer platform. The system has been applied successfully for selective arene C–H monohalogenation reactions affording a good degree of efficiency and reusability. Control experiments suggested that the Ru(II)–terpyridine-based coordination network scaffolding the covalently attached Pd(II)–NHC catalytic centres was highly robust under oxidizing conditions and rendered superior activity than the homogeneous counterpart.

AbstractDisclosed here is a molecular switch which responds to acid-base stimuli and serves as a bi-state catalyst for two different reactions. The two states of the switch serve as a highly active and poorly active catalyst for two catalytic reactions (namely a hydrogenation and a dehydrogenative coupling) but in a complementary manner. The system was used in an assisted tandem catalysis set-up involving dehydrogenative coupling of an amine and then hydrogenation of the resulting imine product by switching between the respective states of the catalyst.

AbstractDisclosed here is a molecular switch which responds to acid-base stimuli and serves as a bi-state catalyst for two different reactions. The two states of the switch serve as a highly active and poorly active catalyst for two catalytic reactions (namely a hydrogenation and a dehydrogenative coupling) but in a complementary manner. The system was used in an assisted tandem catalysis set-up involving dehydrogenative coupling of an amine and then hydrogenation of the resulting imine product by switching between the respective states of the catalyst.

•A monodentate mono-NHC palladium(II) complex was developed as efficient arene CH activation-functionalization catalyst.•The monodentate mono-NHC catalyst was more efficient than a similar bidentate bis-NHC palladium(II) complex.•Kinetics and other mechanistic analyses provided the plausible reason for the observed higher activity of the catalyst.A simple and efficient CH activation catalyst was identified through a model structure-activity screening applied to a noncooperative, nonsymmetric bimetallic palladium(II)-N-heterocyclic carbene complex. Mechanistic studies based on kinetics and DOSY NMR spectroscopy provided the origin of the higher efficiency of the identified catalyst.Download high-res image (109KB)Download full-size image

Organic molecules containing imidazolium and pyridine backbone are of special interest in many branches of chemistry, and therefore, overcoming the scarcity of their catalytic functionalization protocol leading to structurally important derivatives is strongly desired. This study demonstrates an unprecedented bimodal C–H activation–functionalization catalysis on such organic molecules which contain imidazolium motif (potential N-heterocyclic carbene donor) and pyridine in chelating fashion and difficult to functionalize by trivial C–H activation strategy. The unique feature of this protocol is a flip-flop NHC-directed pyridine rollover and pyridine-directed NHC-rollover C–H activation within a stable “NHC–RhIII–pyridine” chelate platform followed by functionalization with internal alkynes to furnish structurally important annulated products. Electronic and steric factors play a key role in achieving such novel chemistry.Keywords: alkyne; N-heterocyclic carbene; pyridine; rhodium(III); rollover C−H activation

Imidazolium motif containing molecules are known to be of significant interest in diverse research areas, but there is a lack of functionalization protocols of these molecules. In a program to overcome this challenge, recently we developed a unique dual role of a latent imidazolium C–H bond which not only generated a metal–CNHC bond upon activation but also directed further aryl/heteroaryl C–H activation to furnish a library of potentially valuable products. Mechanistic investigation of any newly discovered catalytic reaction is at the heart of its future development for potential application in diverse fields. Motivated by this philosophy, we delineate in the present work the key mechanistic insights of this annulation reaction which unravel the crucial competition of two C–H bonds (imidazolium and aryl C–H) and two M–C bonds (M–CNHC and M–Caryl) in establishing the rate-limiting step and the alkyne-insertion regioselectivity in the reaction. Through careful isolation and X-ray structural characterization of the key seven-membered inserted intermediate along with a DFT rationale, the exclusive regioselectivity of the alkyne insertion to the M–CNHC bond was established. Kinetics studies were used to evaluate the rate-determining step of the reaction, which was found to be the initial nondirected imidazolium C–H activation step. These mechanistic insights should be useful in understanding similar C–H activation processes in general which are topical in the area of catalysis.Keywords: annulation; C−H activation; insertion; mechanism; N-heterocyclic carbene

A molecular coordination-switch controlled by acid–base input has been developed and utilized in switchable catalysis. The molecular switch consists of a hybrid pyridylidene–benzimidazole ligand bound to an IrIIICp* moiety wherein the benzimidazole functionality has been utilized for acid/base controlled reversible coordination, switching between an IrIII-benzimidazole species (form I; neutral imino-type N-coordination) and an IrIII-benzimidazolate species (form II; anionic amido-type N–Ir bonding). Owing to the distinctly different nature of the metal–ligand bonding, it has been demonstrated that while the form I is almost inactive (TOF 1 h–1) in catalytic hydrogenation of imine under ambient pressure and temperature, the form II is greater than an order of magnitude more efficient (TOF 15.8 h–1) in the same catalysis. Moreover, the catalysis could be switched OFF and ON efficiently for several cycles with the addition of acid and base, respectively. Spectroscopic studies and kinetics have been performed to understand the switching activity.Keywords: acid−base; hydrogenation; iridium(III); molecular switch; switchable catalysis

A remote RuII(terpy)2 unit incorporated in conjugation with the [NHC-RuII(para-cymene)] catalytic site, acts as a “coordination booster” for enhancing the catalytic efficiency to achieve excellent performance in selective oxidative scission of various carbon–carbon multiple bonds to the corresponding aldehydes, ketones and diketones. Generation of an active catalyst via oxidative loss of para-cymene from the precatalyst was found to be accelerated by the “coordination booster” through the electronic effect.

In parallel to the directing-group-assisted sp2 C–H bond activation–functionalization of aromatic backbones, a similar exercise with nonaromatic sp2 C–H bonds is also in high demand in synthetic chemistry despite several challenges pertinent to the latter process. In the presented protocol, N-heterocyclic carbene (NHC) motifs, appended to nonaromatic sp2 C–H bond-containing organic molecules, have been used for developing a rhodium(III)-catalyzed annulation reaction with internal alkynes to synthesize a class of imidazo[1,2-a]pyridinium architectures. Mechanistic studies highlight the directing role of the NHC ligand during the C–H activation process and intermediacy of the C–H-activated Rh-NHC metallacycle in the catalysis.

Selective and controlled oxidation of olefins to aldehydes is a commonly used important transformation in chemistry. However, chemists still use the dangerous and inconvenient ozonolysis method or the less selective, low-yielding Lemieux–Johnson protocol. In a program of developing effective catalysts for this important reaction, we disclose here that an ancillary ligand can play a dramatic role in the above catalytic phenomenon, depending on the design of the ligand precursor chosen. Proof-of-principle is demonstrated with the help of two newly designed [LnRuII-NHC] precatalysts (NHC = an imidazolydene-based NHC, Im-NHC, or a triazolydene-based NHC, Trz-NHC; Ln = para-cymene) for catalytic selective oxidation of olefins/alkynes to carbonyl compounds. With the electron-deficient Trz-NHC ligand, [(para-cymene)RuII(Im-Trz)]+ precatalyst was found to be an order of magnitude more efficient than the [(para-cymene)RuII(Im-NHC)]+ precatalyst.

Catalytic aromatic C–H bond activation and functionalization to useful molecules is highly important in chemical synthesis. This study introduces a novel use of the highly popular N-heterocyclic carbenes (NHC), for the first-time, in rhodium(III)-catalyzed cascade double aromatic C–H activation–annulation, within the backbone of readily available imidazolium substrates to synthesize a variety of nicely decorated polycyclic heteroaromatic molecules containing benzo[ij]imidazo[2,1,5-de]quinolizinium architectures which might be useful for developing new luminescent materials.Keywords: alkyne; annulation; C−H activation; N-heterocyclic carbene; rhodium(III)

The coordination of metalloligands to derive modified properties of the metal functionality is one of the interesting strategies practiced in materials chemistry and catalysis. In this work, a pendant terpyridine ligand has been utilized for templating a Cp*IrIII(NHC)-based (NHC = N-heterocyclic carbene) oxidation precatalyst to assess its modified oxidative behavior via electrochemical, UV-vis spectroscopic, and catalytic probes. These studies suggested that the coordination-template enhances the electron-deficiency at the IrIII redox center and affects the nature of the oxidized high-valent Ir-oxo species during chemical oxidation. Moreover, both the premodified and postmodified Cp*IrIII(NHC)-based complexes were found to be equally efficient in catalytic sp3 C–H oxidation reactions with NaIO4 as a mild sacrificial oxidant.

A new rhodium(III)-catalyzed protocol for C–H activation and functionalization of poorly reactive pyridine backbones with the aid of in-built N-heterocyclic carbene ligand has been presented. Mechanistic investigations including the isolation and single crystal X-ray structural characterization of the pre- and postfunctionalized rhodium(III)-intermediates suggested a plausible pyridine-coordination effect in the present NHC-directed cyclometalative annulation of the C(3)–H bond of the pyridine rings with internal alkynes. The strategy enables us to overcome the challenges of functionalizing robust metal–CNHC backbone concomitant with pyridine C–H activation under an oxidizing environment.

Bis-N-heterocyclic carbene (NHC)-chelated palladium(II) complexes have been synthesized, characterized fully including single-crystal X-ray structural analyses, and utilized for the first time toward catalytic oxidative C–H functionalization of arenes with PhI(OAc)2 and N-bromosuccinimide.

Disclosed herein is the unique conjugative role of N-heterocyclic carbene (NHC) ligands as a directing group in aromatic C–H activation, coupled with a facile NHC–alkenyl annulative reductive elimination which guided the RhIII-catalyzed intermolecular annulations of imidazolium salts and alkynes under ambient conditions leading to structurally important imidazo[1,2-a]quinolinium motifs.

Modulating the functionality of a synthetic transition metal complex by external stimuli is highly important for designing switchable systems. One prerequisite for achieving such dynamic activity is to generate molecular systems with in situ controllable electronic properties. To achieve dynamic control of the electronic properties, here we report the synthesis of two new N-heterocyclic carbene (NHC)–pyridyl IrIII/IrIII (3) and RuII/RuII (4) bimetallic complexes. These complexes include a latent stimuli-responsive labile site. This is utilized successfully for the on-demand, real-time modulation of the electronic properties of the systems in a reversible manner using external agents, as probed by spectroscopic and electrochemical techniques. These results display promising scope in the domain of transition metal–NHC chemistry, which can guide us in developing future smart organometallic systems.

The reaction of a series of pyridine-appended imidazolium salts with Pd(COD)Cl2 in the presence of KOtBu as base led to hydrolytic ring-opening followed by a new type of pincer tridentate (N,C,O) bonding of the in situ generated formamide-based new ligands toward the metal center. This unprecedented type of bonding resulted from a vinylic C–H activation by palladium stabilized with coordination from the pyridine nitrogen and aldehyde oxygen donors in a pincer fashion. The new stable, square-planar palladium(II) complexes (1–3) were characterized by 1D and 2D NMR spectroscopic and high-resolution mass spectrometric (HRMS) methods. The single-crystal X-ray structure of a representative complex (2) confirmed the above interesting bonding features.

Cyclometalative C–H activation within an N-heterocyclic carbene (NHC) ligand framework has been found to be wingtip-dictated under a competitive environment with two different N-substituents, namely, N-phenyl and N-pyridyl. It shows preferential C–H bond activation at one of the wingtip groups guided by the electronic nature of the C–H bond involved and the ring size of the metallacycle formed. Electrochemical analyses suggested that the resulting cyclometalated iridium(III) and ruthenium(II) complexes exhibit disparate electronic properties exerted by the different cyclometalating group (phenyl vs pyridyl). An implication of this electronic modulation in tunable catalytic activity was demonstrated in a model transfer hydrogenation reaction.

The coordination of metalloligands to derive modified properties of the metal functionality is one of the interesting strategies practiced in materials chemistry and catalysis. In this work, a pendant terpyridine ligand has been utilized for templating a Cp*IrIII(NHC)-based (NHC = N-heterocyclic carbene) oxidation precatalyst to assess its modified oxidative behavior via electrochemical, UV-vis spectroscopic, and catalytic probes. These studies suggested that the coordination-template enhances the electron-deficiency at the IrIII redox center and affects the nature of the oxidized high-valent Ir-oxo species during chemical oxidation. Moreover, both the premodified and postmodified Cp*IrIII(NHC)-based complexes were found to be equally efficient in catalytic sp3 C–H oxidation reactions with NaIO4 as a mild sacrificial oxidant.

Modulating the functionality of a synthetic transition metal complex by external stimuli is highly important for designing switchable systems. One prerequisite for achieving such dynamic activity is to generate molecular systems with in situ controllable electronic properties. To achieve dynamic control of the electronic properties, here we report the synthesis of two new N-heterocyclic carbene (NHC)–pyridyl IrIII/IrIII (3) and RuII/RuII (4) bimetallic complexes. These complexes include a latent stimuli-responsive labile site. This is utilized successfully for the on-demand, real-time modulation of the electronic properties of the systems in a reversible manner using external agents, as probed by spectroscopic and electrochemical techniques. These results display promising scope in the domain of transition metal–NHC chemistry, which can guide us in developing future smart organometallic systems.

Disclosed herein is the unique conjugative role of N-heterocyclic carbene (NHC) ligands as a directing group in aromatic C–H activation, coupled with a facile NHC–alkenyl annulative reductive elimination which guided the RhIII-catalyzed intermolecular annulations of imidazolium salts and alkynes under ambient conditions leading to structurally important imidazo[1,2-a]quinolinium motifs.

A remote RuII(terpy)2 unit incorporated in conjugation with the [NHC-RuII(para-cymene)] catalytic site, acts as a “coordination booster” for enhancing the catalytic efficiency to achieve excellent performance in selective oxidative scission of various carbon–carbon multiple bonds to the corresponding aldehydes, ketones and diketones. Generation of an active catalyst via oxidative loss of para-cymene from the precatalyst was found to be accelerated by the “coordination booster” through the electronic effect.

In this work, we report a single-site, heterogeneous Pd(II)–NHC-based oxidative C–H activation catalyst, designed on a self-supported coordination polymer platform. The system has been applied successfully for selective arene C–H monohalogenation reactions affording a good degree of efficiency and reusability. Control experiments suggested that the Ru(II)–terpyridine-based coordination network scaffolding the covalently attached Pd(II)–NHC catalytic centres was highly robust under oxidizing conditions and rendered superior activity than the homogeneous counterpart.