NQNHC Ligand-Enabled Cu(I)-Catalyzed Double Hydroamination: A Regio- and Chemoselective Bicyclization of o-Amino 1,6-Diyne

In this work, we have developed an efficient method for the intramolecular double hydroamination of aniline by employing o-amino 1,6-diyne as a potential starting material. This protocol enables easy access to bioactive motif 3,4-dihydro-1H-[1,4]oxazino[4,3-a]indole through an intramolecular cascade bicyclization and concomitant isomerization pathway in one pot. This transformation has been effectively achieved by utilizing a stereoelectronically tuned, π-accepting NHC-supported copper(I) system. During ligand optimization trials, naphthoquinone-annulated N-heterocyclic carbene, Nq(IDipp) [1,3-bis(2,6-diisopropylphenyl)-4,5-naphthoquino-imidazolidene]-supported copper(I) complexes of the type Nq(IDipp)CuX (X = Cl or I) were synthesized and fully characterized using various spectroscopic techniques. For this conversion, NHC plays a crucial role in providing the optimum electron density around the metal center. It is a highly regio- and chemoselective transformation with a high atom economy and uses cheap, environmentally benign copper-based catalysts. Furthermore, a plausible mechanism has been proposed on the basis of experimental observations and literature support.

Azobenzene-Integrated NHC Ligands: A Versatile Platform for Visible-Light-Switchable Metal Catalysis

A new class of photoswitchable NHC ligands, named azImBA, has been developed by integrating azobenzene into a previously unreported imidazobenzoxazol-1-ylidene framework. These rigid photochromic carbenes enable precise control over confinement around a metal's coordination sphere. As a model system, gold(I) complexes of these NHCs exhibit efficient bidirectional E-Z isomerization under visible light, offering a versatile platform for reversibly photomodulating the reactivity of organogold species. Comprehensive kinetic studies of the protodeauration reaction reveal rate differences of up to 2 orders of magnitude between the E and Z isomers of the NHCs, resulting in a quasi-complete visible-light-gated ON/OFF switchable system. Such a high level of photomodulation efficiency is unprecedented for gold complexes, challenging the current state-of-the-art in photoswitchable organometallics. Thorough investigations into the ligand properties paired with structure-reactivity correlations underscored the unique ligand's steric features as a key factor for reactivity. This effective photocontrol strategy was further validated in gold(I) catalysis, enabling in situ photoswitching of catalytic activity in the intramolecular hydroalkoxylation and -amination of alkynes. Given the significance of these findings and its potential as a widely applicable, easily customizable photoswitchable ancillary ligand platform, azImBA is poised to stimulate the development of adaptive, multifunctional metal complexes.

Redox Active N-Heterocyclic Carbenes in Oxidative NHC Catalysis

An N-heterocyclic carbene (NHC) covalently linked to a quinone introduces a novel avenue for internal oxidations within oxidative NHC catalysis. The deployment of this hybrid NHC class promotes intramolecular electronic flow in the oxidation of the Breslow intermediate to acyl azolium. The use of the redox active NHC as a catalyst is facilitated by employing aerobic regeneration, yielding carboxylic esters with efficiencies of ≤99%, while generating water as the sole byproduct.

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.

A complete series of N-heterocyclic tetrylenes (Si-Pb) with a 1,1'-ferrocenediyl backbone enabled by 1,3,2-diazaborolyl N-substituents

The use of bulky 1,3,2-diazaborolyl N-substituents has allowed the synthesis of the complete series of ferrocene-based N-heterocyclic tetrylenes fc[(N{B})2E] (fc = 1,1'-ferrocenediyl, {B} = (HCNC6H3-2,6-iPr2)2B, E = Si-Pb). The silylene fc[(N{B})2Si] is inert towards NH3, CO2 or N2O under ambient conditions and thus significantly less reactive than the N-aryl homologue fc[(NC6H3-2,6-iPr2)2Si]. In accord with its higher reactivity, computational results indicate a more pronounced ambiphilicity of fc[(NC6H3-2,6-iPr2)2Si]. Our computational investigation of the model compound fc[(NBMe2)2Si] suggests that silylenes of this type may be superior to fc[(NC6H3-2,6-iPr2)2Si] in terms of ambiphilicity.

Supramolecular Control of the Oxidative Addition as a Way To Improve the Catalytic Efficiency of Pincer-Rhodium (I) Complexes

1 H NMR studies using a cationic complex with a pyridine-di-imidazolylidene pincer ligand of formula [Rh(CNC)(CO)]+ revealed that this compound showed high binding affinity with coronene in CH2 Cl2 . The interaction between coronene and the planar RhI complex is established by means of π-stacking interactions. This interaction has a strong impact on the electron-donating strength of the pincer CNC ligand, which is increased significantly, as demonstrated by the shifting of the ν(CO) stretching bands to lower frequencies. The addition of coronene increases the reaction rate of the nucleophilic attack of methyl iodide on the rhodium (I) pincer complex, and also has a positive effect on the performance of the complex as a catalyst in the cycloisomerization of 4-pentynoic acid. These findings highlight the importance of supramolecular interactions for tuning the reactivity and catalytic activity of square-planar metal complexes.

Boron-Centered Lewis Superacid through Redox-Active Ligands: Application in C-F and S-F Bond Activation

A series of redox-responsive ferrocenyl-substituted boranes and boronic esters were synthesized. Oxidation of the ferrocenyl ligand to the ferrocenium resulted in a drastic increase in the Lewis acidity beyond the strength of SbF₅ , which was investigated experimentally and computationally. The resulting highly Lewis acidic boron compounds were used for catalytic C-F and S-F bond activation.

Redox-Switchable Complexes Based on Nanographene-NHCs

A series of rhodium and iridium complexes with a N‐heterocyclic carbene (NHC) ligand decorated with a perylene‐diimide‐pyrene moiety are described. Electrochemical studies reveal that the complexes can undergo two successive one‐electron reduction events, associated to the reduction of the PDI moiety attached to the NHC ligand. The reduction of the ligand produces a significant increase on its electron‐donating character, as observed from the infrared spectroelectrochemical studies. The rhodium complex was tested in the [3+2] cycloaddition of diphenylcyclopropenone and methylphenylacetylene, where it displayed a redox‐switchable behavior. The neutral complex showed moderate activity, which was suppressed when the catalyst was reduced.

Reactions of Mesityl Azide with Ferrocene‐Based N‐Heterocyclic Germylenes, Stannylenes and Plumbylenes, Including PPh 2 ‐Functionalised Congeners

The reactivity of ferrocene‐based N‐heterocyclic tetrylenes [{Fe(η⁵−C₅H₄−NSitBuMe₂)₂}E] (E=Ge, Sn, Pb) towards mesityl azide (MesN₃) is compared with that of PPh₂‐functionalised congeners exhibiting two possible reaction sites, namely the E^II and P^III atom. For E=Ge and Sn the reaction occurs at the E^II atom, leading to the formation of N₂ and an E^IV=NMes unit. The germanimines are sufficiently stable for isolation. The stannanimines furnish follow‐up products, either by [2+3] cycloaddition with MesN₃ or, in the PPh₂‐substituted case, by NMes transfer from the Sn^IV to the P^III atom. Whereas [{Fe(η⁵−C₅H₄−NSitBuMe₂)₂}Pb] and other diaminoplumbylenes studied are inert even under forcing conditions, the PPh₂‐substituted congener forms an addition product with MesN₃, thus showing a behaviour similar to that of frustrated Lewis pairs. The germylenes of this study afford copper(I) complexes with CuCl, including the first structurally characterised linear dicoordinate halogenido complex [CuX(L)] with a heavier tetrylene ligand L.

Redox-Switchable Behavior of Transition-Metal Complexes Supported by Amino-Decorated N-Heterocyclic Carbenes

The coordination chemistry of the N-heterocyclic carbene ligand IMes^(NMe2)2, derived from the well-known IMes ligand by substitution of the carbenic heterocycle with two dimethylamino groups, was investigated with d⁶ [Mn(I), Fe(II)], d⁸ [Rh(I)], and d¹⁰ [Cu(I)] transition-metal centers. The redox behavior of the resulting organometallic complexes was studied through a combined experimental/theoretical study, involving electrochemistry, EPR spectroscopy, and DFT calculations. While the complexes [CuCl(IMes^(NMe2)2)], [RhCl(COD)(IMes^(NMe2)2)], and [FeCp(CO)₂ (IMes^(NMe2)2)](BF₄) exhibit two oxidation waves, the first oxidation wave is fully reversible but only for the first complex the second oxidation wave is reversible. The mono-oxidation event for these complexes occurs on the NHC ligand, with a spin density mainly located on the diaminoethylene NHC-backbone, and has a dramatic effect on the donating properties of the NHC ligand. Conversely, as the Mn(I) center in the complex [MnCp(CO)₂ ((IMes^(NMe2)2)] is easily oxidizable, the latter complex is first oxidized on the metal center to form the corresponding cationic Mn(II) complex, and the NHC ligand is oxidized in a second reversible oxidation wave.

A redox-switchable catalyst with an ‘unplugged’ redox tag

A series of bis-(propyl-imidazoliylidene)-napthalenediimide (NDI) complexes of rhodium and iridium showed effective redox-switching properties, despite the electronic disconnection between the NDI moiety and the metal centres.

Metal complexes bearing photochromic ligands: photocontrol of functions and processes

Metal complexes associated with photochromic molecules are attractive platforms to achieve smart light-switching materials with innovative and exciting properties due to specific optical, electronic, magnetic or catalytic features of metal complexes and by perturbing the excited-state properties of both components to generate new reactivity and photochemical properties. In this overview, we focus on selected achievements in key domains dealing with optical, redox, magnetic properties, as well as application in catalysis or supramolecular chemistry. We also try to point out scientific challenges that are still faced for future developments and applications.

Planar-chiral 1,1'-diaminoferrocenes

Planar-chiral homologues of 1,1'-diaminoferrocene, which bear a single additional substituent adjacent to one of the amino groups, are prepared as racemic mixtures in a few steps and in good yields from ferrocene. Various substituents relevant to steric shielding, coordination and further functionalisation are used, giving access to ferrocene-based planar-chiral diimines and diamines as well as stable N-heterocyclic carbenes and tetrylenes by transformations analogous to procedures established for 1,1'-diaminoferrocene.

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.

Photoswitchable Nitrogen Superbases: Using Light for Reversible Carbon Dioxide Capture

Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit. The resulting N-heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring-opened and ring-closed states is accompanied by substantial pK_a shifts of the NHIs by up to 8.7 units. Since only the ring-closed isomers are sufficiently basic to activate CO₂ via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light-controlled capture and release of CO₂ .

Half-sandwich manganese complexes Cp(CO)2Mn(NHC) as redox-active organometallic fragments

Oxidation of the half-sandwich Mn^I complexes Cp(CO)₂Mn(NHC) bearing dialkyl-, arylalkyl- and diarylsubstituted N-heterocyclic carbene ligands (NHC = IMe, IMeMes, IMes) affords the corresponding stable Mn^II radical cations [Cp(CO)₂Mn(NHC)](BF₄) isolated in 92-95% yield. Systematic X-ray diffraction studies of the series of Mn^I and Mn^II NHC complexes revealed the expected characteristic structural changes upon oxidation, namely the elongation of the Mn-CO and Mn-NHC bonds as well as the diminution of the OC-Mn-CO angle. ESR spectra of [Cp(CO)₂Mn(IMes)](BF₄) in frozen solution (CH₂Cl₂/toluene 1 : 1, 70 K) allowed the identification of two conformers for this complex and their structural assignment using DFT calculations. The stability of these NHC complexes in both metal oxidation states, moderate oxidation potentials and the ease of detection of Mn^II species by a variety of spectroscopic techniques (UV-Vis, IR, paramagnetic ¹H NMR, and ESR) make these compounds promising objects for applications as redox-active organometallic fragments.

Using Light to Modify the Selectivity of Transition Metal Catalysed Transformations

Light has a remarkable and often unique ability to promote chemical reactions. In combination with transition metal catalysis, it offers exciting opportunities to modify catalyst function in a non-invasive manner, most frequently being reported to switch on or accelerate reactions that do not occur in the dark. However, the ability to completely change reactivity or selectivity between two different reaction outcomes is considerably less common. In this Minireview we bring together examples of this concept and highlight their mechanistically distinct approaches. Our overview demonstrates how these non-natural, photo-switchable systems provide key fundamental mechanistic insights, enhancing our understanding and stimulating development of new catalytic activity, and how this might lead to tangible applications, impacting fields such as polymer chemistry.

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.

New frontiers: 1,4-diphosphinines and P-bridged bis(NHCs)

This review describes synthetic concepts and breakthroughs in 1,4-diphosphinine and related P-bridged bis(NHCs) chemistry, covering the last four decades, starting from monocyclic 1,4-dihydro-1,4-diphosphinines in the early 80s to the most recent and promising achievements of tricyclic 1,4-dihydro-1,4-diphosphinines and tricyclic 1,4-diphosphinines. Theoretical aspects are presented for 1,4-dihydro- and 1,4-diphosphinines considering HOMO LUMO situations as well as the degree of aromaticity. Moreover, fundamental characteristics of analytical data of these compounds are highlighted with special focus on substituent effects, structural aspects and trends of electrophilicity. The two P-centers and the heterocyclic rings of 1,4-dihydro- and 1,4-diphosphinines constitute a broad platform for substitution, reduction/oxidation, alkylation, complexation and cycloaddition reactions, i.e., a comprehensive compilation of reactivity aspects is presented. Furthermore, very recent developments in the synthesis and reactivity of tricyclic PV/V- and PIII/III-bridged bis(imidazole-2-ylidenes) will be discussed together with new perspectives derived from an antiaromatic middle ring. In total, our intention is to show new frontiers, i.e., new synthetic paths, thus creating novel opportunities for potential applications in molecular and materials chemistry.

Synthesis and properties of photoswitchable diphosphines and gold(I) complexes derived from azobenzenes

Diphosphines displaying azobenzene scaffolds and the corresponding bis-gold chloride complexes have been prepared and fully characterized by photophysical, spectroscopic and X-ray diffraction studies. DFT calculations provide complementary information on their electronic, structural and spectroscopic properties. Comparative investigations have been carried out on compounds featuring phosphorus functions in the meta- and para-positions, respectively, with respect to the azo functions, as well as on diphosphines with an ortho-tetrafluoro substituted azobenzene core. The effects of the substitution patterns on structural and spectroscopic properties are discussed.

The core of the matter - arene substitution determines the coordination and catalytic behaviour of tris(1-phosphanyl-1'-ferrocenylene)arene gold(I) complexes

Changing the aromatic core of C3-symmetric tris(ferrocenyl)arene-based tris-phosphanes has profound effects on their coordination behaviour towards gold(i). Depending on the arene (s-triazine, benzene, or trifluorobenzene), four different coordination modes can be distinguished and their preference has been rationalised using computational methods. The corresponding 1 : 1 ligand-to-metal complexes, studied by variable-temperature NMR spectroscopy, revealed fluctional behaviour in solution. Given the presence of up to three or six ferrocenylene spacers per complex, their electrochemistry was investigated. The redox-responsive nature of the complexes can be advantageously exploited in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, where the benzene-based 2 : 3 ligand-to-metal complex has been shown to display multiple activity states depending on the degree of (reversible) oxidation in a preliminary trial.

A Redox-Active Heterobimetallic N-Heterocyclic Carbene Based on a Bis(imino)pyrazine Ligand Scaffold

A new redox-active N-heterocyclic carbene (NHC) architecture is obtained using N-methylated pyrazinediimine iron complexes as precursors. The new species exhibit strong π-accepting/σ-donating properties and are able to ligate two metal centres simultaneously. The redox activity was demonstrated by the reversible chemical oxidation of a heterobimetallic Fe0 /RhI example, which affords an isolable ligand-based radical cation. The reversible redox process was then applied in the catalytic hydrosilylation of 4,4'-difluorobenzophenone, where the reaction rate could be reversibly controlled as a function of the catalyst oxidation state. The new NHC exhibits high electrophilicity and nucleophilicity, which was demonstrated in the reversible activation of alcohols and amines. The electronic structure of the resulting complexes was investigated through various spectroscopic and computational methods.

Influence of the Increasing Number of Organic Radicals on the Structural, Magnetic, and Electrochemical Properties of the Copper(II)-Dioxothiadiazole Family of Complexes

The preparation, structures, and electrochemical and magnetic properties supported by density functional theory (DFT) calculations of three new copper(II) compounds with [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide (td) and its radical anion (td·-) are reported: {[CuIICl(td)](μ-Cl)2[CuIICl(td)]} (1), which incorporates only neutral td ligands; [CuIICl(td·-)(td)]·2MeCN (2), which comprises one neutral td and one radical td·-; and PPN[CuIICl(td·-)2]·2DMA (3), where CuII ions are coordinated by two radical anions td·- (DMA, dimethylacetamide; PPN+, the bis(triphenylphosphine)iminium cation). All three compounds show interesting paramagnetic behavior with low-temperature features indicating significant antiferromagnetic coupling. The magnetic properties of 1 are dominated by CuII···CuII interactions (JCuCu) mediated through the Cl- bridges, while the magnetic properties of 2 and 3 are governed mainly by the td·-···td·- (Jtdtd) and CuII-td·- (JCutd) exchange interactions. The structure of 2 features only two major magnetic coupling pathways enabling the fitting of experimental data with Jtdtd = -36.0(5) cm-1 and JCutd = -12.6(2) cm-1 only. Compound 3 exhibits a complex network of magnetic contacts. Attempt to approximate its magnetic behavior using only a local magnetic contacts model resulted in Jtdtd = -5.6(1) cm-1 and two JCutd constants, -12.4(2) and -22.6(4) cm-1. The experimental fitting is critically compared with the results of broken symmetry density functional theory (BS DFT) calculations for inter- and intramolecular contacts. More consistent results were obtained with the M06 functional as opposed to popular B3LYP, which encountered problems reproducing some of the experimental intermolecular exchange interactions. Electrochemical measurements of 2 and 3 in MeCN showed three reversible nearly overlapping redox peaks appearing in a narrow potential range of -600 to -100 mV vs Fc/Fc+. Small differences between the redox events suggest that such compounds may be good candidates for new switchable materials, where the electron transfer between the metal and the ligand center is triggered by temperature, pressure, or light (valence tautomerism).

A tropylium annulated N-heterocyclic carbene

Derivatives of the cationic tropylium annulated imidazolylidene ITrop⁺ are obtained by hydride abstraction from related cycloheptatriene compounds. Spectroscopic, structural and theoretical data indicate that, as a cationic relative of benzimidazolylidenes, ITrop⁺ has highly reduced σ-donor and strong π-acceptor character.

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.

An Arylazopyrazole-Based N-Heterocyclic Carbene as a Photoswitch on Gold Surfaces: Light-Switchable Wettability, Work Function, and Conductance

A novel photoresponsive and fully conjugated N-heterocyclic carbene (NHC) has been synthesized that combines the excellent photophysical properties of arylazopyrazoles (AAPs) with an NHC that acts as a robust surface anchor (AAP-BIMe). The formation of self-assembled monolayers (SAMs) on gold was proven by ToF-SIMS and XPS, and the organic film displayed a very high stability at elevated temperatures. This stability was also reflected in a high desorption energy, which was determined by temperature-programmed SIMS measurements. E-/Z-AAP-BIMe@Au photoisomerization resulted in reversible alterations of the surface energy (i.e. wettability), the surface potential (i.e. work function), and the conductance (i.e. resistance). The effects could be explained by the difference in the dipole moment of the isomers. Furthermore, sequential application of a dummy ligand by microcontact printing and subsequent backfilling with AAP-BIMe allowed its patterning on gold. To the best of our knowledge, this is the first example of a photoswitchable NHC on a gold surface. These properties of AAP-BIMe@Au illustrate its suitability as a molecular switch for electronic devices.

Janus bis(NHCs) tuned by heteroatom-bridge oxidation states

Synthesis of the first tricyclic bis(carbenes) with facially opposed imidazole-2-ylidenes and two linking phosphorus centres in different oxidation states is presented using a modular, high-yield synthetic route. The formation of homo bimetallic coinage metal complexes provides a glimpse on their potential use.

Photoswitchable catalysis using organometallic complexes

Photoswitchable catalysis using organometallic complexes: a ligand design perspective.