Ferrocenyl-Imidazolylidene Ligand for Redox-Switchable Gold-Based Catalysis. A Detailed Study on the Redox-Switching Abilities of the Ligand

Bi- and tridentate coordination behaviour of a novel bis(phosphinimino)methanide ligand

Herein, we report the synthesis of a novel ferrocenyl-functionalized bis(phosphinimino)methane ligand (CH2 (PPh2 NFc)2 ). Deprotonation of CH2 (PPh2 NFc)2 with KN(SiMe3 )2 gave the dimeric species [K{CH(PPh2 NFc)2 }]2 , which was further reacted with ECl2 (E=Ge, Sn) to yield the tetrylene compounds [{CH(PPh2 NFc)2 }ECl]. The ligand and the resulting tetrylenes were examined for their electrochemical properties with the aid of cyclic voltammetry. Furthermore, the reaction of the tetrylenes [{CH(PPh2 NFc)2 }ECl] with [AuC6 F5 (tht)] resulted in the bimetallic complexes [{(AuC6 F5 )CH(PPh2 NFc)2 }ECl] with an unusual Au coordination on the ligand backbone.

Recent advances utilized in artificial switchable catalysis

Developing “green” catalytic systems with desirable performance such as solubility, recyclability, and switchability is a great challenge. However, inspired by nature, the studies on synthesis and activity of artificial switchable metal catalysts and organocatalysts have become an intense, fervid, and challenging field of research. The peculiarity of these catalysts is that they can be generally triggered in the “on” or “off” states by several external stimuli such as light, heat, solvents, pH change, coordination events or ion influxes, redox processes, mechanical forces, or other changes in reaction conditions. A large number of review articles are available in these areas. However, most efforts are currently focused on the invention of new types of switchable catalysts with different forms of stimuli–response units incorporated within their architectures in order to achieve control over the catalytic activity and regio-, chemo- and stereocontrol of various chemical reactions. Thus, in this review, we begin with a brief introduction to switchable catalysts, followed by discussion of types of stimuli and the influence factors on their activities in the field of biomedical engineering, and catalysis as well as related catalytic mechanisms summarized and discussed. The emphasis is on the recent advances utilized in artificial switchable catalysis.

Catalytic systems based on the use of stimuli–responsive materials can be switched from an “on” active state to an “off” inactive state. Consequently, switchable catalysis, both chemical and biological, has played a pivotal role in this ‘greening’ of the pharmaceutical industry.

Synthesis of disubstituted furans catalysed by [(AuCl)2(μ-bis(phosphino)metallocene)] and Na[BArF24]

Two new gold compounds with bis(phosphino)metallocene ligands were prepared and characterized by NMR and X-ray crystallography. These and related compounds are catalysts in formation of disubstituted furans from terminal alkynes and pyridine-N-oxide.

Multistimuli-Responsive [3]Dioxaphosphaferrocenophanes with Orthogonal Switches

Novel multistimuli‐responsive phosphine ligands comprising a redox‐active [3]dioxaphosphaferrocenophane backbone and a P‐bound imidazolin‐2‐ylidenamino entity that allows switching by protonation are reported. Investigation of the corresponding metal complexes and their redox behaviour are reported and show the sensitivity of the system towards protonation and metal coordination. The experimental findings are supported by DFT calculations. Protonation and oxidation events are applied in Rh‐catalysed hydrosilylations and demonstrate a remarkable influence on reactivity and/or selectivity.

Redox-Switchable Cycloisomerization of Alkynoic Acids with Napthalenediimide-Derived N-Heterocyclic Carbene Complexes

Two naphthalene‐diimide (NDI) bis‐imidazolium salts have been used as N‐heterocyclic carbene (NHC) precursors for the preparation of NDI‐functionalized complexes of rhodium and iridium of general formula [MCl(NDI‐NHC)(COD)] (M=Rh, Ir; NDI‐NHC=NDI‐functionalized NHC ligand). Comparison of the IR spectra of the complexes [IrCl(NDI‐NHC)(CO)₂] and their related one‐ and two‐electron reduced forms, reveal that each one‐electron reduction produces a decrease of the average ν(CO) of 9–10 cm⁻¹, indicating a significant enhancement of the electron‐richness of the metal. The [MCl(NDI‐NHC)(COD)] complexes were tested in the catalytic cycloisomerization of alkynoic acids. The one‐electron reduced forms showed greatly enhanced activities. For the cyclization of 5‐hexynoic acid, the two‐electron reduction of the ligand produced further enhancement of the catalytic activity, therefore showing that the catalyst can switch between three redox species with three distinct catalytic activities.

Gold Catalysis and Furans: A Powerful Match for Synthetic Connections

This review summarizes the advances made on the synthesis and functionalization of furans via gold catalysis during the period between 2016 and 2020. A separate section is dedicated to the tandem gold-catalyzed synthesis and functionalization of furans.

1 Introduction

2 Gold-Catalyzed Synthesis of Furans

2.1 Cycloisomerizations of Alkynyl and Cumulenyl Alcohols

2.2 Cycloisomerizations of Alkynyl and Allenyl Ketones

2.3 Reactions with External Oxidants

2.4 Miscellaneous

3 Gold-Catalyzed Functionalization of Furans

3.1 Cycloadditions

3.2 Furan Ring Decorations

3.3 Reactions Involving Furan Ring Opening

4 Gold-Catalyzed Tandem Synthesis and Functionalization of Furans­

4.1 Cycloisomerizations Followed by Gold-Catalyzed Cycloaddition

4.2 Cycloisomerizations to a Gold 1,3- or 1,4-Dipole and Intermolecular Annulation

4.3 Cycloisomerizations to a Gold Carbene and Intermolecular Trapping

5 Conclusion

Triple the fun: tris(ferrocenyl)arene-based gold(i) complexes for redox-switchable catalysis

The modular syntheses of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes and their homotrinuclear gold(i) complexes are reported. Choosing the arene core allows fine-tuning of the exact oxidation potentials and thus tailoring of the electrochemical response. The tris[chloridogold(i)] complexes were investigated in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, showing cooperative behaviour vs. a mononuclear chloridogold(i) complex. Adding one, two, or three equivalents of 1,1′-diacetylferrocenium[tetrakis(perfluoro-tert-butoxy)aluminate] as an oxidant during the catalytic reaction (in situ) resulted in a distinct, stepwise influence on the resulting catalytic rates. Isolation of the oxidised species is possible, and using them as (pre-)catalysts (ex situ oxidation) confirmed the activity trend. Proving the intactness of the P–Au–Cl motif during oxidation, the tri-oxidised benzene-based complex has been structurally characterised.

Trinuclear gold(i) complexes of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes with four accessible oxidation states catalyse the ring-closing isomerisation of N-(2-propyn-1-yl)benzamide with different rates depending on their redox state.

A mechanistic study of microstructure modulation in olefin polymerizations using a redox-active Ni(ii) α-diimine catalyst

Density functional theory and experimental evidence provide insight into the mechanism of polyolefin microstructure modulation using redox-active Ni(ii) α-diimine catalysts.

Recent developments in the synthesis and applications of chiral ferrocene ligands and organocatalysts in asymmetric catalysis

This review highlights recent advances in the synthesis and application of ferrocene-containing chiral ligands and organocatalysts in asymmetric catalysis.

Coordination Booster-Catalyst Assembly: Remote Osmium Outperforming Ruthenium in Boosting Catalytic Activity

Presented herein is a set of bimetallic and trimetallic "coordination booster-catalyst" assemblies in which the coordination complexes [Ru^II (terpy)₂ ] and [Os^II (terpy)₂ ] acted as boosters for enhancement of the catalytic activity of [Ru^II (NHC)(para-cymene)]-based catalytic site. The boosters accelerated the oxidative loss of para-cymene from the catalytic site to generate the active catalyst during the oxidation of alkenes and alkynes into corresponding aldehydes, ketones and diketones. It was found that the boosting efficiency of the [Os^II (terpy)₂ ] units was considerably higher than its congener [Ru^II (terpy)₂ ] unit in these assemblies. Mechanistic studies were conducted to understand this unique improvement.

Zirconium complexes supported by a ferrocene-based ligand as redox switches for hydroamination reactions

The synthesis of (thiolfan*)Zr(NEt2)2 (thiolfan* = 1,1'-bis(2,4-di-tert-butyl-6-thiophenoxy)ferrocene) and its catalytic activity for intramolecular hydroamination are reported. In situ oxidation and reduction of the metal complex results in reactivity towards different substrates. The reduced form of (thiolfan*)Zr(NEt2)2 catalyzes hydroamination reactions of primary aminoalkenes, whereas the oxidized form catalyzes hydroamination reactions of secondary aminoalkenes.

Redox-Switchable Ring-Opening Polymerization with Ferrocene Derivatives

Switchable catalysts incorporate stimuli-responsive features and allow synthetic tasks that are difficult or impossible to accomplish in other ways. They mimic biological processes in that they can provide both spatial and temporal control, unlike most reactions promoted by human-made catalysts that usually occur according to carefully optimized conditions. In the area of switchable catalysis, redox-switchable ring-opening polymerization (ROP) has attracted much attention, emerging as a powerful strategy for the development of environmentally friendly biodegradable copolymers, especially those containing blocks with complementary properties. Controlling the sequence and regularity of each copolymeric building block can affect the material properties significantly since they are directly related to the respective microstructures. Such control can be exerted with a well-designed redox-switchable catalyst by timing the oxidation and reduction events. In highly selective systems, one form of the catalyst reacts with a monomer until the redox state of the catalyst is altered, at which point the altered state of the catalyst reacts with another monomer. The reaction time may be varied from one cycle to another to generate various designer multiblock copolymers. The first instance of redox-mediated ROP was described by N. Long and co-workers in 2006. This example, as well as many early reported redox-switchable catalysts, could only achieve an on/off switch of activity toward a single monomer or substrate. However, our efforts brought on a general strategy for designing redox-switchable metal complexes that can catalyze different reactions in two oxidation states. In recent years, our contributions to this research field led to the synthesis of several multiblock copolymers prepared from biorenewable resources. This Account provides an overview of reported redox-switchable polymerization catalysts that allow for complementary reactivity in different oxidation states and highlights the potential of this strategy in preparing biodegradable materials. First, we define the field of redox-switchable catalysis and illustrate the design and significance of our ferrocene-chelating ligands, in which the oxidation state of iron in ferrocene can control the reactivity of the resulting metal complexes remotely. Next, we illustrate recent advances in the synthesis of new biodegradable copolymers including (1) how to tune the activity of the ROP catalysts by exploring various metal centers and ferrocene-based ligand combinations; (2) how to synthesize new multiblock copolymers of cyclic esters, epoxides, and carbonates by redox-switchable ROP; and (3) how to understand the mechanism of these reactions by discussing both experimental and theoretical investigations. By the application and development of redox-switchable strategies, various novel materials and reactions can be expected in the future.

Switching the Electron-Donating Ability of Phosphines through Proton-Responsive Imidazolin-2-ylidenamino Substituents

Stimuli-responsive ancillary ligands are valuable tools to control the activity and selectivity of transition-metal catalysts. The synthesis and characterization of a series of metal complexes containing phosphines with proton-responsive imidazolin-2-ylidenamino substituents are reported. Determination of the ligand-donor properties revealed that protonation of each substituent increases the Tolman electronic parameter (TEP) of the phosphine by 22 cm^-1 , hence allowing for switching of the electron-donor power of phosphine 2 within an unprecedented range (ΔTEP=43.4 cm^-1 ).

Gold(ii) in redox-switchable gold(i) catalysis

Gold(ii) species catalyse the cyclisation of N(2-propyn-1-yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline without halide abstraction while the saturated gold(i) complex is inactive. Redox-switching between gold(ii) and gold(i) turns catalytic turnover on and off.

Redox-responsive phosphonite gold complexes in hydroamination catalysis

Very high activities were observed in the redox-induced hydroamination of alkynes by employing a redox-active gold(i) complex featuring an electron-deficient, terphenyl-substituted phosphonite-based ligand. The hydroamination proceeds roughly two-fold faster with the in situ oxidized catalysts than with their reduced form.

Redox- and light-switchable N-heterocyclic carbenes: a “soup-to-nuts” course on contemporary structure–activity relationships

This Feature Article offers in-depth, design-to-application discussions of redox-switchable N-heterocyclic carbenes that have been field tested.

Homo- and Heteropolymetallic Complexes of the Hybrid, Ambidentate N-Heterocyclic Carbene Ligand IMes-acac

The anionic 5-acetylimidazol-2-ylidene-4-olate 1 ^-, named as "IMes-acac", is composed of fused diaminocarbene and acetylacetonato units in the same IMes-based imidazolyl ring. The bifunctional compound 1 ^- is shown to act as an effective, ditopic bridging ligand for transition metal centers. Several new complexes supported by this ligand were prepared, including the complex [RuCl(p-Cym)(κ² O,O-1·H)](BF₄) (2), which can be regarded as a metallated imidazolium salt, the homobimetallic complex [((COD)Rh)(RhCl(COD))(μ-1κ² O,O:2κ¹ C-1)] (4), the heterobimetallic complexes [((p-Cym)ClRu)(RhCl(COD))(μ-1κ² O,O:2κ¹ C-1)] (3), [((p-Cym)ClRu)(RhCl(CO)₂)(μ-1κ² O,O:2κ¹ C-1)] (5), [((p-Cym)ClRu)(Cu(IPr))(μ-1κ² O,O:2κ¹ C-1)] (9), the anionic homoleptic Cu(I) complexes [Cu(κ¹ C-1)₂]K ([10]K) and [Cu(κ¹ C-1)₂](NEt₄) ([10](NEt₄)), and the heterotrimetallic complex [((p-Cym)RuCl)₂(Cu)(μ-1κ² O,O:3κ¹ C-1)(μ-2κ² O,O:3κ¹ C-1)](PF₆) (11). Preliminary studies on the possible preparation of supramolecular metallopolymers and electrochemical studies on the series of complexes are also reported.

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

The chemistry of carbeniophosphines and phosphoniocarbenes, which have general structures derived formally from the three-component "carbene/phosphine/positive charge" association, is presented. These two complementary classes of carbon-phosphorus-based ligands, defined by the presence of an inverted cationic coordinating structure (C+ ∼P: vs. P+ ∼C:) have the common purpose of positioning a positive charge in the vicinity of the metal center. Through selected examples, the synthetic methods, coordination properties, and general reactivity of both cationic species is described. Particular emphasis is placed on the influence of the positive charge on the respective chemical behavior of the two classes of compound.

Synthesis and activation potential of an open shell diphosphine

A paramagnetic W^III alkyne complex bearing free terminal diphenylphosphino groups at the side-on coordinated alkyne was synthesized using a stepwise template strategy. This moderately stable metal supported open shell diphosphine shows an unprecedented spontaneous splitting of nitric oxide providing a W^II-η²-C₂{P(NH₂)Ph₂}{P(O)Ph₂}⁺ complex featuring an amino phosphonium and a phosphine oxide substituent.