N‐Heterocyclic carbenes (NHCs) have drawn considerable interest in the field of nanomaterials chemistry as highly stabilizing ligands enabling the formation of strong and covalent carbon–metal bonds. ...Applied to gold nanoparticles synthesis, the most common strategy consists of the reduction of a preformed NHC‐AuI complex with a large excess of a reducing agent that makes the particle size difficult to control. In this paper, we report the straightforward synthesis of NHC‐coated gold nanoparticles (NHC‐AuNPs) by treating a commercially available gold(I) precursor with an easy‐to‐synthesize NHC‐BH3 reagent. The latter acts as both the reducing agent and the source of surface ligands operating under mild conditions. Mechanistic studies including NMR spectroscopy and mass spectrometry demonstrate that the reduction of gold(I) generates NHC‐BH2Cl as a by‐product. This strategy gives efficient control over the nucleation and growth of gold particles by varying the NHC‐borane/gold(I) ratio, allowing unparalleled particle size variation over the range of 4.9±0.9 to 10.0±2.7 nm. Our strategy also allows an unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as‐prepared NHC‐AuNPs exhibit narrow size distributions without the need for extensive purification or size‐selectivity techniques, and are stable over months.
NHC‐boranes are demonstrated to be efficient dual reagents, as both reducing agent and source of surface ligands, for the synthesis of NHC‐coated gold nanoparticles. This strategy allows the synthesis of spherical and monodisperse AuNPs of variable sizes by adjusting the gold to NHC‐borane ratio. The NHC‐boranes can also be exploited in a seeded growth process, a strategy never before reported with NHC stabilized gold nanoparticles. The protocol is simple, operates under mild conditions and avoids the use of extensive purification methods.
Small‐molecule catalysts as mimics of biological systems illustrate the chemists’ attempts at emulating the tantalizing abilities displayed by nature's metalloenzymes. Among these innate behaviors, ...spin multistate reactivity is used by biological systems as it offers thermodynamic leverage towards challenging chemical reactivity but this concept is difficult to translate into the realm of synthetic organometallic catalysis. Here, we report a rare example of molecular spin catalysis involving multistate reactivity in a small‐molecule biomimetic copper catalyst applied to aziridination. This behavior is supported by spin state flexibility enabled by the redox‐active ligand.
Putting a (molecular) spin on copper catalysis: Metalloenzymes routinely perform multielectronic transformations using multistate reactivity and redox cofactors but this behavior is difficult to emulate in synthetic systems. We report a rare example of molecular spin catalysis in the context of copper‐catalyzed aziridination. The molecular spin fluxionality enabled by the redox‐active ligands is central to this behavior.
Coupling reactions are staples in the synthetic world and their efficiency relies on well‐defined, mostly bis‐electronic, elementary catalytic steps. An area of great interest currently lies in the ...taming of radical species and their efficient introduction in catalytic cycles. Among these species bearing radical character, redox‐active ligands hold much potential and can be used to sustain synthetically relevant couplings by introducing ligand‐based electronic contribution. This minireview aims at presenting the current state of this promising field.
Ligand, camera, action! The advent of innovative (catalytic) pathways relying on ligand‐based redox events is reviewed in the context of coupling reactions. This cooperative approach between ligand and metal can offer attractive alternatives to the classic two‐electron catalytic cycles and foster new reactivities.
Flavins and their alloxazine isomers are key chemical scaffolds for bioinspired electron transfer strategies. Their properties can be fine‐tuned by functional groups, which must be introduced at an ...early stage of the synthesis as their aromatic ring is inert towards post‐functionalization. We show that the introduction of a remote metal‐binding redox site on alloxazine and flavin activates their aromatic ring towards direct C−H functionalization. Mechanistic studies are consistent with a synthetic sequence involving ground‐state single electron transfer (SET) with an electrophilic source followed by radical‐radical coupling. This unprecedented reactivity opens new opportunities in molecular editing of flavins by direct aromatic post‐functionalization and the utility of the method is demonstrated with the site‐selective C6 functionalization of alloxazine and flavin with a CF3 group, Br or Cl, that can be further elaborated into OH and aryl for chemical diversification.
Selective functionalization of flavin analogues is desirable for property tuning but requires de novo synthesis. A synthetic strategy involving the introduction of a remote redox site followed by copper complexation turns an alloxazine and a flavin into redox‐active structures and activates their aromatic ring for site‐selective aromatic C−H functionalization. This unprecedented mechanism opens new vistas in the chemistry of flavins and analogues.
Metalloenzymes are nature's own catalysts and offer as such endless inspirational source for the chemists seeking selectivity in transformations. Metalloenzymes involved in oxidoreduction processes ...have specific subunits dedicated to electron and proton transfer, and these so‐called redox cofactors perform highly orchestrated redox events. This minireview offers a perspective on the development of biomimetic and bioinspired innovative approaches interfacing redox cofactors engineering with metal‐based catalysis, nanochemistry, light‐activation, supramolecular chemistry and artificial metalloenzymes to devise and build new synthetic systems using nature's finest electron transfer tools.
Cofactors are key factors: This Minireview offers a perspective on the development of biomimetic and bioinspired strategies interfacing redox cofactors engineering with topics such as metal‐based catalysis, nanochemistry, light‐activation, supramolecular chemistry and artificial metalloenzymes to devise and build new synthetic systems using nature's finest electron transfer tools.
Copper catalysis finds applications in various synthetic fields by utilizing the ability of copper to sustain mono- and bielectronic elementary steps. Further to the development of well-defined ...copper complexes with classical ligands such as phosphines and N-heterocyclic carbenes, a new and fast-expanding area of research is exploring the possibility of a complementing metal-centered reactivity with electronic participation by the coordination sphere. To achieve this electronic flexibility, redox-active ligands can be used to engage in a fruitful "electronic dialogue" with the metal center, and provide additional venues for electron transfer. This review aims to present the latest results in the area of copper-based cooperative catalysis with redox-active ligands.
The reactivity of a stable copper(II) complex bearing fully oxidized iminobenzoquinone redox ligands towards nucleophiles is described. In sharp contrast with its genuine low‐valent counterpart ...bearing reduced ligands, this complex performs high‐yielding C−N bond formations. Mechanistic studies suggest that this behavior could stem from a mechanism akin to reductive elimination occurring at the metal center but facilitated by the ligand: it is proposed that a masked high oxidation state of the metal can be stabilized as a lower copper(II) oxidation state by the redox ligands without forfeiting its ability to behave as a high‐valent copper(III) center. These observations are substantiated by a combination of advanced EPR spectroscopy techniques with DFT studies. This work sheds light on the potential of redox ligands as promoters of unusual reactivities at metal centers and illustrates the concept of masked high‐valent metallic species.
CuIII in hiding: A stable copper(II) complex bearing fully oxidized iminobenzoquinone redox ligands reacts as a copper(III) species and performs high‐yielding C−N bond formation. Mechanistic studies suggest that this behavior could stem from a mechanism akin to reductive elimination occurring at the metal center but facilitated by the ligand.
Mechanisms combining organic radicals and metallic intermediates hold strong potential in homogeneous catalysis. Such activation modes require careful optimization of two interconnected processes: ...one for the generation of radicals and one for their productive integration towards the final product. We report that a bioinspired polymetallic nickel complex can combine ligand‐ and metal‐centered reactivities to perform fast hydrosilylation of alkenes under mild conditions through an unusual dual radical‐ and metal‐based mechanism. This earth‐abundant polymetallic complex incorporating a catechol‐alloxazine motif as redox‐active ligand operates at low catalyst loading (0.25 mol%) and generates silyl radicals and a nickel‐hydride intermediate through a hydrogen atom transfer (HAT) step. Evidence of an isomerization sequence enabling terminal hydrosilylation of internal alkenes points towards the involvement of the nickel‐hydride species in chain walking. This single catalyst promotes a hybrid pathway by combining synergistically ligand and metal participation in both inner‐ and outer‐ sphere processes.
A dual metal and radical reactivity is enabled by a polymetallic nickel complex merging alloxazine and catechol redox‐active subunits. This is the first catalytic application of an alloxazine 3d metal complex and this complex promotes silyl radical generation and chain walking of internal alkenes.
Biological systems provide attractive reactivity blueprints for the design of challenging chemical transformations. Emulating the operating mode of natural systems may however not be so easy and ...direct translation of structural observations does not always afford the anticipated efficiency. Metalloenzymes rely on earth-abundant metals to perform an incredibly wide range of chemical transformations. To do so, enzymes in general have evolved tools and tricks to enable control of such reactivity. The underlying concepts related to these tools are usually well-known to enzymologists and bio(inorganic) chemists but may be a little less familiar to organometallic chemists. So far, the field of bioinspired catalysis has greatly focused on the coordination sphere and electronic effects for the design of functional enzyme models but might benefit from a paradigm shift related to recent findings in biological systems. The goal of this review is to bring these fields closer together as this could likely result in the development of a new generation of highly efficient bioinspired systems. This contribution covers the fields of redox-active ligands, entatic state reactivity, energy conservation through electron bifurcation, and quantum tunneling for C-H activation.
This review provides insights on how enzymatic reactivity tricks such as redox-active ligands, entatic state reactivity, electron bifurcation, and quantum tunneling can benefit chemists in the design of bioinspired catalytic systems.
Tandem CH activation/arylation between unactivated arenes and aryl halides catalyzed by iron complexes that bear redox‐active non‐innocent bisiminopyridine ligands is reported. Similar reactions ...catalyzed by first‐row transition metals have been shown to involve substrate‐based aryl radicals, whereas our catalytic system likely involves ligand‐centered radicals. Preliminary mechanistic investigations based on spectroscopic and reactivity studies, in conjunction with DFT calculations, led us to propose that the reaction could proceed through an inner‐sphere CH activation pathway, which is rarely observed in the case of iron complexes. This bielectronic noble‐metal‐like behavior could be sustained by the redox‐active non‐innocent bisiminopyridine ligands.
A radical choice! A low‐valent iron complex with non‐innocent bisiminopyridine ligands performs CH activation/arylation of unactivated aryl compounds (see figure). The reaction likely involves ligand‐based radicals, whereas previously reported iron‐based systems imply substrate‐based radicals.
PDF
