Photoswitchable Metal-Mediated Catalysis: Remotely Tuned Alkene and Alkyne Hydroborations

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.

Dithienylethene-Based Photoswitchable Phosphines for the Palladium-Catalyzed Stille Coupling Reaction

Homogeneous transition metal catalysis is a constantly developing field in chemical sciences. A growing interest in this area is photoswitchable catalysis, which pursues in situ modulation of catalyst activity through noninvasive light irradiation. Phosphorus ligands are excellent targets to accomplish this goal by introducing photoswitchable moieties; however, only a limited number of examples have been reported so far. In this work, we have developed a series of palladium complexes capable of catalyzing the Stille coupling reaction that contain photoisomerizable phosphine ligands based on dithienylethene switches. Incorporation of electron-withdrawing substituents into these dithienylethene moieties allows variation of the electron density on the phosphorus atom of the ligands upon light irradiation, which in turn leads to a modulation of the catalytic properties of the formed complexes and their activity in a model Stille coupling reaction. These results are supported by theoretical computations, which show that the energy barriers for the rate-determining steps of the catalytic cycle decrease when the photoswitchable phosphine ligands are converted to their closed state.

Photoswitchable electron-rich phosphines: using light to modulate the electron-donating ability of phosphines

The synthesis and properties of photoswitchable electron-rich phosphines containing N-heterocyclic imines equipped with a photochromic dithienylethene unit are reported. Heteronuclear NMR spectroscopy and UV/vis studies reveal that the imine substituents undergo reversible electrocyclic ring-closing and ring-opening reactions upon exposure to UV and visible light, respectively. The photoisomerization alters the electron-donating ability of the phosphines by up to ΔTEP = 8 cm-1.

Light-controlled pKa value of chiral Brønsted acid catalysts in enantioselective aza-Friedel-Crafts reaction

Bis(dithienylethene)-based BINOL-derived phosphoric acid (DTE-BPA) has been developed as a light-controlled chiral organocatalyst for the first time. The photoinduced modulation of the reactivity and selectivity via the open/close isomerization of the DTE scaffold led to superior light-controlled ability in the enantioselective aza-Friedel-Crafts reaction of aldimines with indoles. DFT studies showed that photoisomerization is accompanied by a shift of 1.1 pKa units between the open and closed isomers.

N^N vs. N^E (E = S or Se) coordination behavior of imino-phosphanamidinate chalcogenide ligands towards aluminum alkyls: efficient hydroboration catalysis of nitriles, alkynes, and alkenes

The synthesis, characterization, and catalytic application of six aluminum alkyl complexes supported by various imino-phosphanamidinate chalcogenide ligands are described. Six different unsymmetrical imino-phosphanamidinate chalcogenide ligands [NHI^RP(Ph)(E)NH-Dipp] [R = 2,6-diisopropylphenyl (Dipp), E = S (2a-H), Se (2b-H); R = mesityl (Mes), E = S (3a-H), Se (3b-H); R = tert-butyl (^tBu), E = S (4a-H), Se (4b-H)] were prepared by the oxidation of respective imino-phosphanamide ligands (1a, 1b and 1c) with elemental chalcogen atoms (S and Se). The aluminum complexes with imino-phosphanamidinate chalcogenide ligands with the general formulae [κ²_NN-{NHI^RP(Ph)(E)N-Dipp}AlMe₂] [R = Dipp, E = S (5a), Se (5b); R = Mes, E = S (6a), Se (6b)] or [κ²_NE-{NHI^RP(Ph)(E)N-Dipp}AlMe₂] [R = ^tBu, E = S (7a), Se (7b)] were synthesized in good yields from the reaction of the suitable protic ligands (2a,b-H-4a,b-H) and trimethylaluminum in a 1 : 1 molar ratio in toluene at room temperature. All the protic ligands and aluminum complexes were well characterized by multi-nuclear NMR spectroscopy, and the solid-state structures of 2a,b-H-4a,b-H, 5a,b-6a,b and 7b are established by single crystal X-ray diffraction analysis. The aluminum complexes 5a,b-7a,b were tested as catalysts for the hydroboration of nitriles, alkynes, and alkenes under mild conditions. The catalytic hydroboration reactions of nitriles, alkynes, and alkenes were accomplished with complex 5b at a mild temperature under solvent-free conditions to afford a high yield of the corresponding N,N-diborylamines, vinylboranes and alkyl boronate esters, respectively.

How Robust Is the Reversible Steric Shielding Strategy for Photoswitchable Organocatalysts?

A highly appealing strategy to modulate a catalyst's activity and/or selectivity in a dynamic and noninvasive way is to incorporate a photoresponsive unit into a catalytically competent molecule. However, the description of the photoinduced conformational or structural changes that alter the catalyst's intrinsic reactivity is often reduced to a handful of intuitive static representations, which can struggle to capture the complexity of flexible organocatalysts. Here, we show how a comprehensive exploration of the free energy landscape of N-alkylated azobenzene-tethered piperidine catalysts is essential to unravel the conformational characteristics of each configurational state and explain the experimentally observed reactivity trends. Mapping the catalysts' conformational space highlights the existence of false ON or OFF states that lower their switching ability. Our findings expose the challenges associated with the realization of a reversible steric shielding for the photocontrol of Brønsted basicity of piperidine photoswitchable organocatalysts.

β-Cyclodextrin Supramolecular Recognition of bis-Cationic Dithienylethenes

The supramolecular interactions in water between β-cyclodextrin and the open and closed photochromic forms of two bis-cationic dithienylethenes, characterized by different electronic properties, were investigated aiming at underlying the key aspects of the recognition process. The dithienylethene equipped with the cyclopentenyl unit showed a difference in binding free energies to the β-cyclodextrin between the open and closed photochromic forms of about 1 kJ/mol. Conversely, the dithienylethene equipped with the perfluorinated cyclopentenyl unit not only was a better guest but showed a three times higher difference in the binding of free energies between the open and closed isomers.

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 pKa 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 .

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.

Azopyridine-based chiral oxazolines with rare-earth metals for photoswitchable catalysis

An azopyridine-based oxazoline was developed for utilizing azo group coordination and isomerization as a photoswitchable ligand. The ligand coordinated to rare-earth metal (RE) catalyst underwent efficient E/Z photoisomerization, suggesting tri- and bidentate coordination switching. The photoisomerization of the ligand enabled modulation of the enantioselectivity of an RE-catalyzed aminal forming reaction.

A photoresponsive palladium complex of an azopyridyl-triazole ligand: light-controlled solubility drives catalytic activity in the Suzuki coupling reaction

Herein, the design and synthesis of a click-derived Pd-complex merged with a photoswitchable azobenzene unit is presented. While in the trans-form of the switch the complex showed limited solubility, the photogenerated cis-form rendered the molecule soluble in polar solvents. This light-controllable solubility was exploited to affect the catalytic activity in the Suzuki coupling reaction. The effect of the substrate and catalyst concentration and light intensity on the proceeding and outcome of the reaction was studied. Dehalogenation of the aryl iodide starting material was found to be a major side reaction; however, its occurrence was dependent on the applied light intensity.

Synthesis, Photophysical, Photochromic, and Photomodulated Resistive Memory Studies of Dithienylethene-Containing Copper(I) Diimine Complexes

A series of dithienylethene-containing copper(I) diimine complexes have been synthesized and structurally characterized. Systematic studies on their photophysics, electrochemistry, and photochromism have been carried out. The photoinduced color changes of the copper(I) complexes have been achieved by photoexcitation into the metal-to-ligand charge-transfer (MLCT) absorption bands, indicating the photosensitization of light-induced cyclization by the ³MLCT excited state. In addition, by an increase in either the steric bulkiness around the copper(I) center or the structural rigidity of the complexes, the quantum efficiencies of photoluminescence and photocyclization can be effectively enhanced because of suppression of the flattening distortion of the complexes at the MLCT excited state. Furthermore, one of the complexes has been employed as an active component in the fabrication of solution-processed resistive memory devices. Notable lowering of the switching threshold voltage of the binary memory devices has been realized through photocyclization of the dithienylethene-containing copper(I) system.

Azobenzene Isomerization-Induced Photomodulation of Electronic Properties of N-Heterocyclic Carbenes

Azobenzene-based protonated N-heterocyclic carbenes (NHCs), N,N'-bis(azobenzene)imidazolium chlorides (IAz-X⋅HCl; X=OMe, Br, H) were successfully synthesized and switching abilities of the attached azobenzene units along with the concomitant photoinduced generation of geometric isomers were studied. Upon irradiation with 365 nm UV light, a p-methoxy-azobenzene derivative get transformed from all-trans isomer to nearly all-cis isomer at the photostationary state. The extent of photomodulation of electronic properties in the NHC ring of the p-methoxy-azobenzene derivative is determined through the Tolman Electronic Parameter (TEP). Iridium complex, [(IAz-OMe)IrCl(CO)₂ ] was synthesized and infrared spectra were recorded in dichloromethane solution as film in NaCl crystals and by drop-casting in an ATR crystal. Comparison in averaged carbonyl stretching frequency between all-trans isomer ( ν ˜ t t av ) and all-cis isomer ( ν ˜ c c a v ) indicates a significant decrement of Δ^tt-cc ν ˜ _av =2.7 cm^-1 (film) and 3.8 cm^-1 (ATR). Therefore, moderate to excellent enhancement of electron density (Δ^tt-cc TEP=2.3 cm^-1 [film] and 3.2 cm^-1 [ATR]) at the C-2 position of the NHC is achieved through trans→cis isomerization of the remotely placed azobenzene units. This fine phototuning of electron-donating ability at the catalytic center would pave the way to control NHC/NHC-metal catalyzed organic transformations through external stimuli.

In Situ Switching of Site-Selectivity with Light in the Acetylation of Sugars with Azopeptide Catalysts

We present a novel concept for the in situ control of site-selectivity of catalytic acetylations of partially protected sugars using light as external stimulus and oligopeptide catalysts equipped with an azobenzene moiety. The isomerizable azobenzene-peptide backbone defines the size and shape of the catalytic pocket, while the π-methyl-l-histidine (Pmh) moiety transfers the electrophile. Photoisomerization of the E- to the Z-azobenzene catalyst (monitored via NMR) with an LED (λ = 365 nm) drastically changes the chemical environment around the catalytically active Pmh moiety, so that the light-induced change in the catalyst shape alters site-selectivity. As a proof of principle, we employed (4,6-O-benzylidene)methyl-α-d-pyranosides, which provide a change in regioselectivity from 2:1 (E) to 1:5 (Z) for the monoacetylated products at room temperature. The validity of this new catalyst-design concept is further demonstrated with the regioselective acetylation of the natural product quercetin. In situ irradiation NMR spectroscopy was used to quantify photostationary states under continuous irradiation with UV light.

Non-Oxido-Vanadium(IV) Metalloradical Complexes with Bidentate 1,2-Dithienylethene Ligands: Observation of Reversible Cyclization of the Ligand Scaffold in Solution

Derivatives of 1,2‐dithienylethene (DTE) have superb photochromic properties due to an efficient reversible photocyclization reaction of their hexatriene structure and, thus, have application potential in materials for optoelectronics and (multi‐responsive) molecular switches. Transition‐metal complexes bearing switchable DTE motifs commonly incorporate their coordination site rather distant from the hexatriene system. In this work the redox active ligand 1,2‐bis(2,5‐dimethylthiophen‐3‐yl)ethane‐1,2‐dione is described, which reacts with [V(TMEDA)₂Cl₂] to give a rare non‐oxido vanadium(IV) species 3(M,M/P,P). This blue complex has two bidentate en‐diolato ligands which chelate the V^IV center and give rise to two five‐membered metallacycles with the adjacent hexatriene DTE backbone bearing axial chirality. Upon irradiation with UVA light or prolonged heating in solution, the blue compound 3(M,M/P,P) converts into the purple atropisomer 4(para,M/para,P). Both complexes were isolated and structurally characterized by single‐crystal X‐ray diffraction analysis (using lab source and synchrotron radiation). The antiparallel configuration (M or P helicity) present in both 3(M,M/P,P) and 4(para,M/para,P) is a prerequisite for (reversible) 6π cyclization reactions. A CW EPR spectroscopic study reveals the metalloradical character for 3(M,M/P,P) and 4(para,M/para,P) and indicates dynamic reversible cyclization of the DTE backbone in complex 3(M,M/P,P) at ambient temperature in solution.

Photoswitchable catalysis using organometallic complexes

Photoswitchable catalysis using organometallic complexes: a ligand design perspective.

Hydroboration of internal alkynes catalyzed by FeH(CO)(NO)(PPh3)2: a case of boron-source controlled regioselectivity

The Fe–H complex FeH(CO)(NO)(Ph₃P)₂ catalyzes the stereoselective, regiodivergent hydroboration of internal alkynes using either pinacolborane (HBpin) or bis(pinacolato)diboron (B₂pin₂) as a boron source.

Design, synthesis and study of a photochromic α,ω-diene: toward new classes of photoswitchable polymers

A 4,5-dithienylimidazolium salt outfitted with pendant styrenyl groups was synthesized and studied. The salt was found to undergo reversible electrocyclization upon UV irradiation; subsequent exposure to visible light reversed the reaction. Acyclic diene metathesis (ADMET) polymerization of the salt afforded a novel fluorescent polyelectrolyte.

Photoswitchable catalysis based on the isomerisation of double bonds

Photoswitchable catalysis is a young but rapidly evolving field that offers great potential for non-invasive dynamic control of both activity and selectivity in catalysis. This Feature Article summarises the key developments accomplished over the past years through the incorporation of photoswitchable double bonds into the structure of catalytically competent molecules.

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.

Earth-Abundant Transition Metal Catalysts for Alkene Hydrosilylation and Hydroboration: Opportunities and Assessments

The addition of a silicon-hydrogen or a boron-hydrogen bond across a carbon-carbon multiple bonds is a well-established method for the introduction of versatile silane and borane functional groups to base hydrocarbon feedstocks. Transition metal catalysis, historically with precious second- and third- row transition metals, has been used to broaden the scope of the hydrofunctionalization reaction, improve reaction rate and enhance selectivity. The anti-Markovnikov selectivity of platinum-catalyzed hydrosilylation of alkenes, for example, is an enabling synthetic technology in the multibillion-dollar silicones industry. Increased emphasis on sustainable catalytic methods and more economic processes has shifted focus to catalysis with more earth-abundant transition metals such as iron, cobalt and nickel. This review describes contemporary approaches and offers a contextual analysis of catalytic alkene hydrosilylation and hydroboration reactions using first-row transition metals. Emphasis is placed on defining advances in the field, what constitutes catalyst cost, safety, and important design features to enable precious metal-like reactivity, as well as new chemistry that is unique to first-row transition metals.

Modulation of the carboxamidine redox potential through photoinduced spiropyran or fulgimide isomerisation

Carboxamidines functionalized with either a spiropyran or fulgimide photoswitch were prepared on multigram scales. The thermal, electrochemical, and photochemical ring isomerizations of these compounds were studied and the results compared with related systems. The photochemical isomerisations were found to be reversible and could be followed by 1H NMR and UV-vis spectroscopy. The spiropyran/merocyanine couple was thermally active and an activation enthalpy of 116 kJ mol-1 was measured for ring-opening. These measurements yielded an enthalpy difference of 25 kJ mol-1 between the open and closed states which is consistent with DFT calculations. DFT calculations predicted a charge transfer to the carboxamidine group upon ring closure in the fulgimide and a charge transfer from the carboxamidine group upon switching the spiropyran to the merocyanine form. This was confirmed experimentally by monitoring the change in the oxidation potential assigned to the carboxamidine group. The potential of these molecules to therefore act as a new class of photoresponsive ligands that can modulate the ligand field of a complex is discussed.

Hydroboration of alkynes and nitriles using an α-diimine cobalt hydride catalyst

Addition of NaEt₃BH to (^Ph2PPrDI)CoCl₂ affords the corresponding monohydride, (^Ph2PPrDI)CoH. X-ray diffraction and DFT calculations indicate that this compound possesses a radical monoanion α-DI chelate and a Co(ii) centre. Notably, (^Ph2PPrDI)CoH catalyzes the hydroboration of alkynes and dihydroboration of nitriles under mild conditions.

Bimetallic gold(i) complexes of photoswitchable phosphines: synthesis and uses in cooperative catalysis

The first photoswitchable bimetallic gold catalysts based on an azobenzene backbone have been synthesized and their catalytic properties have been investigated.

A complementary pair of enantioselective switchable organocatalysts

A pair of enantioselective switchable bifunctional catalysts are shown to promote a range of conjugate addition reactions in up to 95 : 5 e.r. and 95% conversion. Each catalyst can be switched OFF using conditions that switch the other catalyst ON. Catalyst ON : OFF ratios of up to 98 : 2 and 1 : 99 were achieved, with a ratio of reaction rates of up to 16 : 1 between the ON and OFF states, maintained over complete ON-OFF-ON and OFF-ON-OFF cycles. However, simultaneous operation of the catalyst pair in the same reaction vessel, which in principle could allow product handedness to be switched by simple E-Z isomerisation of the catalyst pair, was unsuccessful. In this first generation complementary pair of enantioselective switchable organocatalysts, the OFF state of one catalyst inhibits the ON state of the other.

Smart N-Heterocyclic Carbene Ligands in Catalysis

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term "smart" is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

Redox-Active N-Heterocyclic Germylenes and Stannylenes with a Ferrocene-1,1'-diyl Backbone

We describe ferrocene-based N-heterocyclic germylenes and stannylenes of the type [Fe{(η⁵ -C₅ H₄ )NR}₂ E:] (1 RE; E=Ge, Sn; R=neopentyl (Np), mesityl (Mes), trimethylsilyl (TMS)), which constitute the first examples of redox-functionalised N-heterocyclic tetrylenes (NHTs). These compounds are thermally stable and were structurally characterised by means of X-ray diffraction studies, except for the neopentyl-substituted stannylene 1 NpSn, the decomposition of which afforded the aminoiminoferrocene [fc(NHCH₂ tBu)(N=CHtBu)] (2) and the spiro tin(IV) compound (1 Np)₂ Sn (3). DFT calculations show that the HOMO of the NHTs of our study is localised on the ferrocenylene backbone. A one-electron oxidation process affords ions of the type 1 RE^+. . In contrast to the NHC system 1 RC, the localised ferrocenium-type nature of the oxidised form does not compromise the fundamental tetrylene character of 1 RE^+. .

Bifunctional Molecular Photoswitches Based on Overcrowded Alkenes for Dynamic Control of Catalytic Activity in Michael Addition Reactions

The emerging field of artificial photoswitchable catalysis has recently shown striking examples of functional light-responsive systems allowing for dynamic control of activity and selectivity in organocatalysis and metal-catalysed transformations. While our group has already disclosed systems featuring first generation molecular motors as the switchable central core, a design based on second generation molecular motors is lacking. Here, the syntheses of two bifunctionalised molecular switches based on a photoresponsive tetrasubstituted alkene core are reported. They feature a thiourea substituent as hydrogen-donor moiety in the upper half and a basic dimethylamine group in the lower half. This combination of functional groups offers the possibility for application of these molecules in photoswitchable catalytic processes. The light-responsive central cores were synthesized by a Barton-Kellogg coupling of the prefunctionalized upper and lower halves. Derivatization using Buchwald-Hartwig amination and subsequent introduction of the thiourea substituent afforded the target compounds. Control of catalytic activity in the Michael addition reaction between (E)-3-bromo-β-nitrostyrene and 2,4-pentanedione is achieved upon irradiation of stable-(E) and stable-(Z) isomers of the bifunctional catalyst 1. Both isomers display a decrease in catalytic activity upon irradiation to the metastable state, providing systems with the potential to be applied as ON/OFF catalytic photoswitches.

Controlling ligand binding for tunable and switchable catalysis: cation-modulated hemilability in pincer-crown ether ligands

The development of cation-responsive “pincer-crown ether” complexes featuring tunable hemilability is reviewed in the context of switchable and tunable catalysis.