CAAC Ligands Research Articles

Anionic Olefin Metathesis Catalysts Enable Modification of Unprotected Biomolecules in Water.

Stability problems have limited the uptake of cationic olefin metathesis catalysts in chemical biology. Described herein are anionic catalysts that improve water-solubility, robustness, and compatibility with biomolecules such as DNA. A sulfonate tag is installed on the cyclic (alkyl)(amino) carbene (CAAC) ligand platform, chosen for resistance to degradation by nucleophiles, base, water, and β-elimination. Hoveyda-Grubbs catalysts bearing the sulfonated CAAC ligands deliver record productivity in metathesis of unprotected carbohydrates and nucleosides at neutral pH. Decomposed catalyst has negligible impact on metathesis selectivity, whereas N-heterocyclic carbene (NHC) catalysts degrade rapidly in water and cause extensive C=C migration.

Synthesis and Application of Robust Spiro [Fluorene-9] CAAC Ruthenium Alkylidene Complexes for the "One-Pot" Conversion of Allyl Acetate to Butane-1,4-diol.

A series of a novel CAAC ligands featuring a spiro-fluorene group have been synthesized and complexed with ruthenium alkylidenes, yielding the corresponding Hoveyda-type derivatives as a new family of olefin metathesis catalysts. The novel complexes have been characterized by XRD, HRMS and NMR measurements. The synthetised complexes were tested in catalysis and showed good activity in olefin metathesis, as demonstrated on diethyl diallylmalonate and allyl acetate substrates. The unique backbone in the ligand with the large, yet inflexible condensed system renders interesting properties to the catalyst, exemplified by the good catalytic performance and improved Z-selectivity. In addition, the complex can also serve as a hydrogenation catalyst in a consecutive (one-pot) reaction. The latter reaction can convert allyl acetate to butane-1,4-diol, a valuable chemical intermediate for biodegradable polybutylene succinate (PBS).

Tailoring Carbene-Metal-Amides for Thermally Activated Delayed Fluorescence: A Computationally Guided Study on the Effect of Cyclic (Alkyl)(amino)carbenes.

Gold-centered carbene-metal-amides (CMAs) containing cyclic (alkyl)(amino)carbenes (CAACs) are promising emitters for thermally activated delayed fluorescence (TADF). Aiming at the design and optimization of new TADF emitters, we report a density functional theory study of over 60 CMAs with various CAAC ligands, systematically evaluating computed parameters in relation to photoluminescence properties. The CMA structures were primarily selected based on experimental synthesis prospects. We demonstrate that TADF efficiency of the CMA materials originates from a compromise between oscillator strength coefficients and exchange energy (ΔEST). The latter is governed by the overlap of HOMO and LUMO orbitals, where HOMO is localized on the amide and LUMO over the Au-carbene bond. The S0 ground and excited T1 states of the CMAs adopt approximately coplanar geometry of carbene and amide ligands, but rotate perpendicular in the excited S1 states, resulting in degeneracy or near-degeneracy of S1 and T1, accompanied by a decrease in the S1-S0 oscillator strength from its maximum at coplanar geometries to near zero at rotated geometries. Based on the computations, promising new TADF emitters are proposed and synthesized. Bright CMA complex (Et2CAAC)Au(carbazolide) is obtained and fully characterized in order to demonstrate that excellent stability and high radiative rates up to 106 s-1 can be obtained for the gold-CMA complexes with small CAAC-carbene ligands.

A tunable family of CAAC-ruthenium olefin metathesis catalysts modularly derived from a large-scale produced ibuprofen intermediate.

A series of tunable CAAC-based ruthenium benzylidene complexes with increased lipophilicity derived from a ketone being a large-scale produced key substrate for a popular nonsteroidal anti-inflammatory drug-ibuprofen was obtained and tested in various olefin metathesis transformations. As a group, these catalysts exhibited higher activity than their known analogues containing a smaller and less lipophilic phenyl substituent on the α-carbon atom, but in individual reactions, the size of the N-aryl moiety was revealed as a decisive factor. For example, in the cross-metathesis of methyl oleate with ethylene (ethenolysis)-a reaction with growing industrial potential-the best results were obtained when the N-aryl contained an isopropyl or tert-butyl substituent in the ortho position. At the same time, in the RCM, CM, and self-CM transformations involving larger olefinic substrates, the catalysts with smaller aryl-bearing CAAC ligands, where methyl and ethyl groups occupy ortho, ortho' positions performed better. This offers a great deal of tunability and allows for selection of the best catalyst for a given reaction while keeping the general structure (and manufacturing method) of the ibuprofen-intermediate derived CAAC ligand the same.

Isolation of Homo-/Mixed-Valence Ag12 , Ag29 , and Ag8 Clusters Stabilized by Cyclic Alkyl(amino) Carbene-Anchored Monoanionic Phosphorus Ligand.

Silver clusters are attractive candidates for their promising optical properties, and biomedical activities. Herein, we report on the first syntheses and isolation of three homo-/mixed-valence silver nanoclusters (NCs) with Ag12 Cl3 , Ag29 , and Ag8 cores [((cAAC)P)6 Ag12 Cl3 ](OTf)3 (1), [((cAAC)P)6 Ag29 ] (2), and [((cAAC)P)4 Ag8 ] (3) having three-/twofold symmetry, employing cyclic alkyl(amino) carbene (cAAC)-supported phosphinidenide (cAACP- ) as the π-accepting stabilizing ligand. The average diameters of Ag NCs 1, and 2 are approximately 1.6 to 2 nm. The redox non-innocent mono-atomic phosphorus anions (P- ) anchored with cAAC ligands are generated in situ by the reaction of AgOTf with a boryl-phosphaalkene (cAAC)P-B(Ni Pr2 )2 through cleavage of the P-B bond with the help of a triflate anion (OTf- ) as a weak nucleophile. Equivalent number of the (cAAC)P- anions generated in situ are oxidized to produce the corresponding bis-phosphinidene (cAAC)2 P2 leading to the generation of Ag0 ions in solution for the formation of the unprecedented mixed-valence Ag NC 2. Complex 3 is achieved by treating potassium phosphinidenide cAACPK with AgNTf2 . The ligand field and the steric hindrance of the (cAAC)P units play crucial roles in stabilizing complexes 1-3, further providing a three- (1, 2)/two- (3) fold stand. The Ag12 Cl3 NC (1) with a tricationic core [AgI 12 Cl3 ] was found to be diamagnetic, and fluorescent, emitting green light at 563 nm when excited at 400 nm. In contrast, the neutral Ag29 (2) and Ag8 (3) clusters were found to be paramagnetic, and NMR silent showing characteristic EPR signals for Ag0 at room temperature.

Steric and Electronic Properties of Indole‐Derived CAAC Ligands

AbstractThe electronic properties of two indole‐derived CAAC ligands, featuring N−Et (II) or N−H (IV) substituents, have been investigated by DFT calculations. The calculations revealed frontier orbitals (HOMO and LUMO) with energies closely related to those of the conventional N‐DIPP substituted CAAC I. The selenium adduct of the N−H substituted CAAC 1 has been prepared and characterized by X‐ray diffraction methods and 77Se NMR spectroscopy revealing π‐acceptor properties of the CAAC similar to classical CAACs of type I. While the electronic properties of the indole‐derived CAACs II and IV are similar to those of classical CAACs, their % Vbur is significantly smaller due to the smaller N‐substituents falling in the range of % Vbur for sterically less demanding N‐heterocyclic carbenes.

Cyclic (Alkyl)(amino)carbenes: Synthesis of Iminium Precursors and Structural Properties.

Using readily available preallylated aldehydes, we report a simple and divergent synthesis of cyclic (alkyl)(amino)carbene (CAAC) iminium precursors. Using a combination of crystallographic data and steric maps, we further elaborate on the specific steric properties of CAAC ligands with respect to state-of-the-art phosphine and carbene ligands.

Estimations of Fe0/-1-N2 interaction energies of iron(0)-dicarbene and its reduced analogue by EDA-NOCV analyses: crucial steps in dinitrogen activation under mild conditions.

Metal complexes containing low valence iron atoms are often experimentally observed to bind with the dinitrogen (N2) molecule. This phenomenon has attracted the attention of industrialists, chemists and bio-chemists since these N2-bonded iron complexes can produce ammonia under suitable chemical or electrochemical conditions. The higher binding affinity of the Fe-atom towards N2 is a bit ‘mysterious’ compared to that of the other first row transition metal atoms. Fine powders of α-Fe0 are even part of industrial ammonia production (Haber–Bosch process) which operates at high temperature and high pressure. Herein, we report the EDA-NOCV analyses of the previously reported dinitrogen-bonded neutral molecular complex (cAACR)2Fe0–N2 (1) and mono-anionic complex (cAACR)2Fe−1–N2 (2) to give deeper insight of the Fe–N2 interacting orbitals and corresponding pairwise intrinsic interaction energies (cAACR = cyclic alkyl(amino) carbene; R = Dipp or Me). The Fe0 atom of 1 prefers to accept electron densities from N2via σ-donation while the comparatively electron rich Fe−1 centre of 2 donates electron densities to N2via π-backdonation. However, major stability due to the formation of an Fe–N2 bond arises due to Fe → N2 π-backdonation in both 1 and 2. The cAACR ligands act as a charge reservoir around the Fe centre. The electron densities drift away from cAAC ligands during the binding of N2 molecules mostly via π-backdonation. EDA-NOCV analysis suggests that N2 is a stronger π-acceptor rather than a σ-donor.

CAAC-IPr*: easily accessible, highly sterically-hindered cyclic (alkyl)(amino)carbenes.

IPr* (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) has emerged as a powerful highly hindered and sterically-flexible ligand platform for transition-metal catalysis. CAACs (CAAC = cyclic (al-kyl)(amino)carbenes) have gained major attention as strongly electron-rich carbon analogues of NHCs (NHC = N-heterocyclic carbene) with broad applications in both industry and academia. Herein, we report a merger of CAAC ligands with highly-hindered IPr*. The efficient synthesis, electronic characterization and application in model Cu-catalyzed hydroboration of alkynes is described. The ligands are strongly electron-rich, bulky and flexible around the N-Ar wingtip. The availability of various IPr* and CAAC templates offers a significant potential to expand the existing arsenal of NHC ligands to electron-rich bulky architectures with critical applications in metal stabilization and catalysis.

CAACs as efficient ancillary ligands for the synthesis of robust catalysts

CAACs have emerged as an attractive class of ancillary ligands, forming strong bonds with transition metals as well as main-group elements and stabilise both low and high oxidation states. These strongly nucleophilic and electrophilic carbenes have a small HOMO−LUMO energy gap compared to N-heterocyclic carbenes (NHCs). The robust metal complexes formed with CAAC ligands have been utilised in different areas such as catalysis, small-molecule activation, medicinal chemistry, and bioinorganic chemistry. This review describes recent developments in the synthesis of various types of CAAC ligands, their binding ability to transition metals and applications of CAAC−based metal complexes in homogeneous catalysis via hydrogenation, hydroboration, coupling reactions, olefin metathesis and asymmetric transformations.

Cyclic (Alkyl)(amino)carbene Ligands Enable Cu‐Catalyzed Markovnikov Protoboration and Protosilylation of Terminal Alkynes: A Versatile Portal to Functionalized Alkenes**

AbstractRegioselective hydrofunctionalization of alkynes represents a straightforward route to access alkenyl boronate and silane building blocks. In previously reported catalytic systems, high selectivity is achieved with a limited scope of substrates and/or reagents, with general solutions lacking. Herein, we describe a selective copper‐catalyzed Markovnikov hydrofunctionalization of terminal alkynes that is facilitated by strongly donating cyclic (alkyl)(amino)carbene (CAAC) ligands. Using this method, both alkyl‐ and aryl‐substituted alkynes are coupled with a variety of boryl and silyl reagents with high α‐selectivity. The reaction is scalable, and the products are versatile intermediates that can participate in various downstream transformations. Preliminary mechanistic experiments shed light on the role of CAAC ligands in this process.

Solid-State Isolation of Cyclic Alkyl(Amino) Carbene (cAAC)-Supported Structurally Diverse Alkali Metal-Phosphinidenides.

Cyclic alkyl(amino) carbene (cAAC)-supported, structurally diverse alkali metal-phosphinidenides 2-5 of general formula ((cAAC)P-M)n (THF)x [2: M=K, n=2, x=4; 3: M=K, n=6, x=2; 4: M=K, n=4, x=4; 5: M=Na, n=3, x=1] have been synthesized by the reduction of cAAC-stabilized chloro-phosphinidene cAAC=P-Cl (1) utilizing metallic K or KC8 and Na-naphthalenide as reducing agents. Complexes 2-5 have been structurally characterized in solid state by NMR studies and single crystal X-ray diffraction. The proposed mechanism for the electron transfer process has been well-supported by cyclic voltammetry (CV) studies and Density Functional Theory (DFT) calculations. The solid state oligomerization process has been observed to be largely dependent on the ionic radii of alkali metal ions, steric bulk of cAAC ligands and solvation/de-solvation/recombination of the dimeric unit [(cAAC)P-M(THF)x ]2 .

Two‐Coordinate Cu(I) and Au(I) Complexes Supported by BICAAC and CAAC Ligands

AbstractTwo‐coordinate Cu(I) and Au(I) complexes supported by bicyclic (alkyl)(amino)carbene, [BICAAC‐CuCl] (1), [BICAAC‐CuI] (2), [(BICAAC)2Cu]+[PF6]– (3) and [(BICAAC)2Au]+[AuCl2]– (6) have been synthesized. The reaction of cyclic (alkyl)(amino)carbene, CAACcy with CuCl afforded [CAACcy‐CuCl] (4) and its further reaction with KPF6 gave [(CAACcy)2Cu]+[PF6]– (5). Complexes 1–6 have been characterized by multinuclear NMR, IR and UV‐Vis., spectroscopic method and high‐resolution mass spectrometry (HRMS). Single crystal X‐ray structure of heteroleptic complexes 1 and 4 and homoleptic complexes [(BICAAC)2Cu]+[CuI2]– (2’), 3 and 6 have also been determined. The crystal structure of these complexes confirmed linear two‐coordinate geometry around the metal centers. In the solid‐ state, complexes 1, 2’, 4 and 6 displayed C−H⋯M (M=Cu, Au) and weak non‐covalent C−H⋯X (X=Cl, I) and C−H⋯H−C interactions. Computational calculations correlate well to the experimentally observed geometry and help elucidate the absorption characteristics type of transitions and the frontier orbitals involved in them.

Synthesis and Activity of Six-Membered Cyclic Alkyl Amino Carbene–Ruthenium Olefin Metathesis Catalysts

Ru–cyclic alkyl amino carbene (Ru–CAAC) olefin metathesis catalysts perform extraordinarily in metathesis macrocyclization and ethenolysis, but previous studies have been limited to the use of five-membered CAAC (CAAC-5) ligands. In this work, we synthesized a different group of ruthenium catalysts with more σ-donating and π-accepting six-membered CAAC (CAAC-6) ligands, and their metathesis activity was probed through initiation studies, ring-closing metathesis (RCM), cross-metathesis, and ethenolysis. These catalysts display higher initiation rates than analogous Ru–CAAC-5 complexes but demonstrate lower activity in RCM and ethenolysis.

Different Donor-Acceptor Interactions of Carbene Ligands in Heteroleptic Divalent Group 14 Compounds, LEL' (E=C-Sn; L=N-Heterocyclic Carbene; L'=Cyclic Alkyl(Amino) Carbene).

The electronic structure and reactivity of heteroleptic divalent group 14 compounds, 1E (E=C-Sn) with NHC and cAAC ligands have been studied at the BP86/TZ2P level of theory and compared with homoleptic group 14 compounds. The EDA-NOCV (energy decomposition analysis-natural orbitals for chemical valence) analysis indicates that the interaction between the two carbene ligands and the central C-atom in 1C can be best represented as one 3c-2e electron sharing σ-bond and one 3c-2e donor-acceptor σ-bond. There exists an electron sharing interaction between the π-type orbital on the central C-atom and the C-N π* orbital of cAAC and a π-back-donation from the σ-type lone pair on the central C-atom to the π*-MO of NHC. This bonding description is equivalent to the localized bonding representation, where the central C-atom forms two electron sharing bonds and two donor-acceptor bonds with cAAC and NHC ligands. However, the bonding between the carbene ligands and the heavier group 14 element can be best represented as two 2c-2e donor-acceptor σ-bonds and a π-back-donation from group 14 element to C-N π* orbital of cAAC. This bonding description is well supported by the geometrical and Natural Bond Orbital (NBO) analyses. Hence, 1C can be best described as a carbene and the heavier analogues can be best described as tetrylones. However, the high first (287.6-274.3 kcal mol-1 ) and second proton affinities (162.0-158.5 kcal mol-1 ) suggest that 1E (E=C-Sn) behave as tetrylones.

Silicon-fluorine chemistry: from the preparation of SiF2 to C-F bond activation using silylenes and its heavier congeners.

This feature article is intended to provide a background to the history of the isolation of silicon(ii) fluorides and the different synthetic methodologies used to generate them. Although first detected in the 1970s, the chemistry of silicon(ii) fluorides has not encountered serious research efforts for a rather long period of time. This is somewhat surprising given the fact that the chemistry of compounds with divalent silicon has undergone a renaissance during last three decades. Recently, the interest in silicon(ii) fluorides have been resparked with tremendous progress being achieved in this area, in particular, with respect to their synthesis and structural characterisation. The successful isolation of cyclic alkyl amino carbene (cAAC) stabilized silicon difluoride has completed the classic progression of SiF2, from a transient intermediate to spectroscopically detected molecule to a stable compound. The related germanium(ii), tin(ii), and lead(ii) fluoride chemistry will also be discussed. Apart from the isolation of tetrel(ii) fluorides, the use of compounds with low valent group 14 elements for the selective activation and functionalisation of C-F bonds has witnessed some remarkable advances, which will also be summarized in this feature article.

Synthesis and Reactivity of Cyclic (Alkyl)(Amino)Carbene Stabilized Nickel Carbonyl Complexes

Cyclic (alkyl)(amino)carbenes cAACcy (1b) and cAACmenthyl (1c) react with [Ni(CO)4] to give the 18 VE complexes [Ni(CO)3(cAACcy)] (2b) and [Ni(CO)3(cAACmenthyl)] (2c). With these in hand, the donor-strength and the steric profile of the respective cAAC ligands were evaluated. CAACcy and cAACmenthyl possess similar overall-donating properties (Tolman electronic parameter (TEP) = 2046 (1b) and 2042 (1c)) as common NHCs, though they are also known to be better π-acceptors. 3,3-Diamino-2-aryloxyacrylimidamide 3b, arising from the reaction of cAACcy (1b) with released CO molecules, was obtained as side-product of CO substitution reactions at nickel carbonyls. In contrast to cAACmenthyl (%Vbur = 42), the sterically less encumbered cAACcy (%Vbur = 38) undergoes a subsequent CO substitution at [Ni(CO)3(cAACCy)] (2b) to afford the 16 VE complex [Ni(CO)(cAACcy)2] (4b). Treatment of both [Ni(CO)3(cAACmethyl)] (2a) and [Ni(CO)(cAACmethyl)2] (4a) with allyl bromides led to the formation of cAAC-stabilized allyl nickel...

NMR analysis of an Fe(i)-carbene complex with strong magnetic anisotropy.

A tricoordinated FeI complex with two cyclic-alkyl(amino) carbene (cAAC) and one chlorido ligand, (cAAC)2FeCl (1), is studied by means of 1H NMR spectroscopy and DFT calculations. Due to the cAAC ligands, which can take significant amounts of spin density from the metal center, and due to the magnetic anisotropy of the FeI ion (P. P. Samuel et al., J. Am. Chem. Soc., 2014, 136, 11964-11971), compound 1 is a rare example of a paramagnetic d-block compound which is expected to have significant contributions from both contact and pseudocontact terms to the hyperfine NMR shift. Compound 1 is fluxional, which makes the analysis of its 1H NMR spectrum more difficult but allows a preliminary assignment from EXSY spectra. Then, a software-aided approach enabled a satisfactory signal assignment of all protons which are distanced from the FeI center and carbene cyclic core, and thereby the extraction of the axial and rhombic components of the magnetic susceptibility anisotropy tensor (Δχ). Components of Δχ enable the calculation of zero-field spitting D and E parameters from solution NMR measurements of 1, and these parameters are compared to previously reported experimental and theoretical values.

Cyclic (Alkyl)(Amino)Carbene Complexes of Rhodium and Nickel and Their Steric and Electronic Parameters.

N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) are of great interest, as their electronic and steric properties provide a unique class of ligands and organocatalysts. Herein, substitution reactions involving novel carbonyl complexes of rhodium and nickel were studied to provide a deeper understanding of the fundamental electronic factors characterizing CAAC(methyl) , which were compared with the large array of data available for NHC and sterically more demanding CAAC ligands.

Highly Selective Hydrogenation of Aromatic Ketones and Phenols Enabled by Cyclic (Amino)(alkyl)carbene Rhodium Complexes

Air-stable Rh complexes ligated by strongly σ-donating cyclic (amino)(alkyl)carbenes (CAACs) show unique catalytic activity for the selective hydrogenation of aromatic ketones and phenols by reducing the aryl groups. The use of CAAC ligands is essential for achieving high selectivity and conversion. This method is characterized by its good compatibility with unsaturated ketones, esters, carboxylic acids, amides, and amino acids and is scalable without detriment to its efficiency.