Tuning the Catalytic Activity of a Pincer Complex of Rhodium(I) by Supramolecular and Redox Stimuli

We report the rhodium(I) complex [Rh(CNC−NDI)(CO)]⁺, in which CNC−NDI refers to a pincer-CNC ligand decorated with a naphthalenediimide moiety. Due to the presence of the planar CNC ligand and the naphthalenediimide moiety, the electronic nature of the complex can be modulated by means of supramolecular and redox stimuli, respectively. The metal complex shows a strong π–π-stacking interaction with coronene. This interaction has an impact on the electron-richness of the metal, as demonstrated by the shifting of the ν(CO) stretching band to a lower frequency. The addition of tetrabutylammonium fluoride facilitates the sequential one- and two-electron reduction of the NDI moiety of the ligand, thus resulting in a situation in which the ligand can increase its electron-donor strength in two levels. The nature of the interaction with the fluoride anion was studied computationally. The catalytic activity of the [Rh(CNC−NDI)(CO)]⁺ complex was tested in the cycloisomerization of alkynoic acids, where it is observed that the activity of the catalyst can be modulated between four levels of activity, which correspond to i) the use of the unmodified catalyst, ii) catalyst+coronene, iii) catalyst+2 equivalents of fluoride, and iv) catalyst+5 equivalents of fluoride.     

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes: Efficient and Simple Hydrogenation Catalysts

The use of bis(NHC) manganese(I) complexes 3 as catalysts for the hydrogenation of esters was investigated. For that purpose, a series of complexes has been synthesized via an improved two step procedure utilizing bis(NHC)-BEt₃ adducts. By applying complexes 3 with KHBEt₃ as additive, various aromatic and aliphatic esters were hydrogenated successfully at mild temperatures and low catalyst loadings, highlighting the efficiency of the novel catalytic system. The versatility of the developed catalytic system was further demonstrated by the hydrogenation of other substrate classes like ketones, nitriles, N-heteroarenes and alkenes. Mechanistic experiments and DFT calculations indicate an inner sphere mechanism with the loss of one CO ligand and reveal the role of BEt₃ as cocatalyst.     

Sterically Congested Bisphosphite Ligands for the Rhodium(I)-Catalyzed Hydrosilation of Ketones

Sterically congested bisphosphites were shown to be effective ligands for the Rh-catalyzed hydrosilation of ketones with diphenylsilane. The hydrosilation of 4-alkylcyclohexanones and ( m )-menthone led to a significant proportion of the less stable (axial) alcohol, which suggests that these reactions are under kinetic, rather than thermodynamic, control.     

Rhodium(I) PNN Pincer Complexes with Proton-responsive Ligands: Synthesis,Characterisation, and Catalytic Dehydrocoupling of Amine Boranes

Coordination of a pyridine-pyrazole-based PNN(H) ligand to Rh^I produces a family of neutral ( 1 ) and cationic ( 2Cl ) Rh^I complexes. Deprotonation of the parent Rh chloride complex with LiNiPr₂ results in formation of a dinuclear LiCl bridged species 3 bearing a pyrazolate fragment. Complexes 1 , 2Cl and 3 were tested as precatalyst for the dehydrocoupling of amine boranes. All complexes studied show activity for the formation of cyclic oligomers with N-methylcyclotriborazane as the main product. Base activation of the neutral Rh chloride complex 1 produces catalyst systems that are significantly more active than the parent system, suggesting that dehydrohalogenation of the Rh chloride precatalyst 1 is one of the key steps for catalyst formation.     

ATP Drives the Formation of a Catalytic Hydrazone through an Energy Ratchet Mechanism

An energy ratchet mechanism is exploited for the synthesis of a molecule. In the presence of adenosine triphosphate (ATP), hydrazone-bond formation between an aldehyde and hydrazide is accelerated and the composition at thermodynamic equilibrium is shifted towards the hydrazone. Enzymatic hydrolysis of ATP installs a kinetically stable state, at which hydrazone is present at a higher concentration compared to the composition at thermodynamic equilibrium in the presence of the degradation products of ATP. It is shown that the kinetic state has an enhanced catalytic activity in the hydrolysis of an RNA-model compound.     

An NHC–Silyl–NHC Pincer Ligand for the Oxidative Addition of C−H,N−H,and O−H Bonds to Cobalt(I) Complexes

N‐Heterocyclic carbene based pincer ligands bearing a central silyl donor, [CSiC]⁻, have been envisioned as a class of strongly σ‐donating ligands that can be used for synthesizing electron‐rich transition‐metal complexes for the activation of inert bonds. However, this type of pincer ligand and complexes thereof have remained elusive owing to their challenging synthesis. We herein describe the first synthesis of a CSiC pincer ligand scaffold through the coupling of a silyl–NHC chelate with a benzyl–NHC chelate induced by one‐electron oxidation in the coordination sphere of a cobalt complex. The monoanionic CSiC ligand stabilizes the Co^I dinitrogen complex [(CSiC)Co(N₂)] with an unusual coordination geometry and enables the challenging oxidative addition of E−H bonds (E=C, N, O) to Co^I to form Co^III complexes. The structure and reactivity of the cobalt(I) complex are ascribed to the unique electronic properties of the CSiC pincer ligand, which provides a strong trans effect and pronounced σ‐donation.     

Cover Picture: Supramolecular Self-Assembly as a Tool To Preserve the Electronic Purity of Perylene Diimide Chromophores (Angew. Chem. Int. Ed. 12/2023)

Organic semiconductors are promising for efficient, printable optoelectronics. However, strong excited-state quenching due to uncontrolled aggregation limits their use in devices. We report on the self-assembly of a supramolecular pseudo-cube formed from six perylene diimides (PDIs). The rigid, shape-persistent cage sets the distance and orientation of the PDIs and suppresses intramolecular rotations and vibrations, leading to non-aggregated, monomer-like properties in solution and the solid state, in contrast to the fast fluorescence quenching in the free ligand. The stabilized excited state and electronic purity in the cage enables the observation of delayed fluorescence due to a bright excited multimer, acting as excited-state reservoir in a rare case of benign inter-chromophore interactions in the cage. We show that self-assembly provides a powerful tool for retaining and controlling the electronic properties of chromophores, and to bring molecular electronics devices within reach.     

A Cooperative Rhodium/Secondary Phosphine Oxide [Rh/P(O)nBu2] Template for Catalytic Hydrodefluorination of Perfluoroarenes

The selective activation of C−F bonds under mild reaction conditions remains an ongoing challenge of bond activation. Here, we present a cooperative [Rh/P(O)nBu₂] template for catalytic hydrodefluorination (HDF) of perfluoroarenes. In addition to substrates presenting electron-withdrawing functional groups, the system showed an exceedingly rare tolerance for electron-donating functionalities and heterocycles. The high chemoselectivity of the catalyst and its readiness to be deployed at a preparative scale illustrate its practicality. Empirical mechanistic studies and a density functional theory (DFT) study have identified a rhodium(I) dihydride complex as a catalytically relevant species and the determining role of phosphine oxide as a cooperative fragment. Altogether, we demonstrate that molecular templates based on these design elements can be assembled to create catalysts with increased reactivity for challenging bond activations.     

Frontispiece: Controlled Self-assembly of Gold(I) Complexes by Multiple Kinetic Aggregation States with Nonlinear Optical and Waveguide Properties

Introduction of multiple kinetic aggregation states (Aggs) into the self-assembly pathway could bring complexity and flexibility to the self-assemblies, which is difficult to realize due to the delicate equilibria established among different Aggs bonded by weak noncovalent interactions. Here, we describe a series of chiral and achiral d¹⁰ Au^I bis(N-heterocyclic carbene, NHC) complexes, and the achiral complex could undergo self-assembly with multiple kinetic Aggs. Generation of multiple kinetic Aggs was realized by applying chiral or achiral seeds exhibiting large differences in elongation temperatures for their respective cooperative self-assembly processes. We further showed that the chiral Au^I self-assemblies having non-centrosymmetric packing forms exhibit nonlinear optical response of second harmonic generation (SHG), while the SHG signal is absent in the achiral analogue. The crystalline achiral Au^I self-assemblies could function as optical waveguides with strong emission polarization.     

Cationic Gold(I) Complexes: Highly Efficient Catalysts for the Addition of Alcohols to Alkynes

Gold complexes are almost unknown in homogeneous catalysis , and for a long time gold was even thought of as “catalytically dead”. This is clearly not the case for cationic phosphanegold(I ) complexes (see reaction below), which can catalyze the addition of alcohols to alkynes with turnover frequencies up to 1.5 s⁻¹ and total turnover numbers up to 10⁵! R=H, alkyl, Ph, CH₂OH.     

Study of incidence of DiPAMP ligand modification on the rhodium(I)-catalyzed asymmetric hydrogenation of α-acetamidostyrene

A series of P-stereogenic enantiopure 1,2-bis[(aryl)(phenyl)phosphino]ethane ligands was prepared through an extensive systematic incorporation of various substituents onto the P-o-anisyl rings of Knowles’ DiPAMP (DiPAMP = 1,2-bis[(o-anisyl)(phenyl)phosphino]ethane). The study of incidence of such modification on the Rh(I)-catalyzed hydrogenation of α-acetamidostyrene is reported revealing that substitution on position 3 is detrimental, while it is beneficial on position 5. Namely, a 2.5-fold increased catalyst activity coupled with a higher enantioselectivity (90% ee) was attained with the P-(2-MeO-3-naphthyl)-substituted ligand under mild conditions (1 bar H₂, rt in MeOH).     

Supramolecular Assembly: A Comparative Structural Study of the Incorporation of Bis(2-guanidinobenzimidazolo)nickel(II) into Supramolecular Arrays

The structural effects of incorporating a non-planar neutral metal complex, bis(2-guanidinobenzimidazolo)nickel(II), into three supramolecular arrays are described. The complex has a donor-acceptor-donor (DAD) hydrogen bonding motif on each ligand and this motif is used to link it to bis(biureto)nickelate(II) ions, or to 1,8-naphthalimide or phthalimide molecules, all of which incorporate a complementary acceptor-donor-acceptor (ADA) hydrogen bonding motif. The geometry about the metal ion as well as the nature of the network of hydrogen bonds formed have significant influences on the supramolecular structure adopted. An interesting combination of intramolecular hydrogen bonding and close ~ -stacking interactions also occur in each species.     

Synthesis, Characterization, and Catalytic Activity of Rh(I) Complexes with (S)-BINAPO, an Axially Chiral Inducer Capable of Hemilabile P,O-Heterobidentate Coordination

Summary.  The reaction of dinuclear rhodium(I) derivatives of the formula [Rh(DIOL)X]₂ with the axially chiral phosphinyl phosphane 2-(diphenylphosphinyl)-2′-(diphenylphosphanyl)-1,1′-binaphthalene ((S)-BINAPO, 1) leads to the formation of cationic complexes [(BINAPO)Rh(DIOL)]⁺ where the ligand (S)-BINAPO consistently displays a P,O-chelate coordination which is mantained even in solvents of fair polarity. The mononuclear rhodium(I) complexes (S)-2-diphenylphosphanyl-2′-diphenylphosphinyl-1,1′-binaphthalene-(1,5-cyclooctadiene) rhodium tetrafluoroborate (3b) and (S)-2-diphenylphosphanyl-2′-diphenylphosphinyl-1,1′-binaphthalene-(1,4-norbornadiene) rhodium tetrafluoroborate (3c) with 1,5-cyclooctadiene (COD) and 2,5-norbornadiene (NBD) as the diolefin were isolated and characterized. Both show a fluxional behaviour in solution which is due to the mobility of the diolefin rather than to a displacement-recombination of the oxygenated arm of the ligand. The mobility of the 1,4-norbornadiene ligand in 3c is extremely pronounced and the coordinated diolefin flexibility could be frozen only at about 200 K. These complexes are active but poorly stereoselective catalysts for the hydrogenation, hydroboration, and hydroformylation of alkenes. Received June 16, 2000. Accepted (revised) July 24, 2000     

Domino rhodium(I)-catalysed reactions for the efficient synthesis of substituted benzofurans and indoles

Rhodium(I) catalysts promote the transformation of o-alkynyl phenols and anilines to the corresponding benzo[b]furans and indoles. The reaction is postulated to proceed via a transient 3-rhodium heterocycle intermediate, which can be trapped with suitable electrophiles to give poly-substituted heterocycles. In the case of mono-substituted electron-withdrawn electrophiles, excellent yield and selectivity for conjugate addition versus Heck-Mizoroki reaction can be achieved. In the case of 2-alkynyl pyridine electrophiles, novel 2-(benzofuran-3-yl)vinylpyridines are formed.     

Ligand‐Controlled Formation of a Low‐Valent Pincer Rhodium(I)–Dioxygen Adduct Bearing a Very Short O?O Bond

Treatment of [{Me₂C₆H(CH₂P^tBu₂)₂}Rh(η¹‐N₂)] ( 1a ) with molecular oxygen (O₂) resulted in almost quantitative formation of the dioxygen adduct [{Me₂C₆H(CH₂P^tBu₂)₂}Rh(η²‐O₂)] ( 2a ). An X‐ray diffraction study of 2a revealed the shortest O? O bond reported for Rh? O₂ complexes, indicating the formation of a Rh^I? O₂ adduct, rather than a cyclic Rh^III η²‐peroxo complex. The coordination of the O₂ ligand in 2a was shown to be reversible. Treatment of 2a with CO gas yielded almost quantitatively the corresponding carbonyl complex [{Me₂C₆H(CH₂P^tBu₂)₂}Rh(CO)] ( 3a ). Surprisingly, treatment of the structurally very similar pincer complex [{C₆H₃(CH₂PⁱPr₂)₂)}Rh(η¹‐N₂)] ( 1b ) with O₂ led to partial decomposition, with no dioxygen adduct being observed.     

C1‐Symmetric Bicyclo[2.2.2]octa‐2,5‐diene (bod*) Ligands in Rhodium‐Catalyzed Asymmetric 1,4‐Addition of Arylboronic Acids to Enones and 1,2‐Addition to N‐[(4‐Nitrophenyl)sulfonyl]imines

A set of ten C₁‐symmetric chiral bicyclo[2.2.2]octa‐2,5‐dienes (bod*) 2 (Fig. 1) were tested as ligands in Rh‐catalyzed arylation reactions. The 1,4‐addition of arylboronic acids to cyclohex‐2‐en‐1‐one, cyclopent‐2‐en‐1‐one, and tert‐butyl cinnamate proceeded smoothly with excellent enantioselectivities (up to 99% ee; Tables 1–3). The challenging 1,2‐addition of triphenylboroxine to N‐[(4‐nitrophenyl)sulfonyl]imines yielded the product in high yield and in good enantioselectivity (up to 92% ee; Table 4). Generally, the use of C₁‐symmetric chiral bod* ligands bearing bulky substituents resulted in lower enantioselectivities, whereas several electron‐poor and electron‐rich bod* ligands gave higher enantioselectivities than the benchmark ligands reported in literature.     

Reversible Oxidative Addition of Zinc Hydride at a Gallium(I)-Centre: Labile Mono- and Bis(hydridogallyl)zinc Complexes

In the presence of TMEDA (N,N,N’,N’-tetramethylethylenediamine), partially deaggregated zinc dihydride as hydrocarbon suspensions react with the gallium(I) compound [(BDI)Ga] ( I , BDI={HC(C(CH₃)N(2,6-iPr₂-C₆H₃))₂}⁻) by formal oxidative addition of a Zn−H bond to the gallium(I) centre. Dissociation of the labile TMEDA ligand in the resulting complex [(BDI)Ga(H)−(H)Zn(tmeda)] ( 1 ) facilitates insertion of a second equiv. of I into the remaining Zn−H to form a thermally sensitive trinuclear species [{(BDI)Ga(H)}₂Zn] ( 2 ). Compound 1 exchanges with polymeric zinc dideuteride [ZnD₂]_n in the presence of TMEDA, and with compounds I and 2 via sequential and reversible ligand dissociation and gallium(I) insertion. Spectroscopic and computational studies demonstrate the reversibility of oxidative addition of each Zn−H bond to the gallium(I) centres.     

Conjugate phosphination of cyclic and acyclic acceptors using Rh(I)-phosphine or Rh(I)-carbene complexes. Probing the mechanism with chirality at the silicon atom or the phosphorus atom of the Si-P reagent

The Rh(I)-catalyzed conjugate phosphinyl transfer from an Si-P reagent to an electron-deficient acceptor requires individual protocols for cyclic and acyclic α,β-unsaturated carbonyls and carboxyls. While 1,4-addition to cyclic acceptors is catalyzed by a Rh(I)-phosphine complex, a Rh(I)-carbene complex is needed to promote conjugate phosphination of acyclic acceptors. General procedures for both systems are reported. Aside from monophosphine-derived Si-P reagents as phosphinide sources, dppe- as well as dppp-derived reagent having two Si-P units do also participate in this reaction. The mechanism of this Rh(I)-catalyzed activation of the Si-P reagent is still under debate. Control experiments with enantiopure silicon-stereogenic and racemic phosphorus-stereogenic Si-P reagents support a catalysis commencing with transmetalation rather than oxidative addition. Preparation and full characterization data of the Si-P compounds used in this investigation is included.