Ligands In Homogeneous Catalysis Research Articles

Monofluorophos-Metal Complexes: Ripe for Future Discoveries in Homogeneous Catalysis.

The discovery that cyclic (ArO)2PF can support Rh-catalysts for hydroformylation with significant advantages in tuning regioselectivity transformed the study of metal complexes of monofluorophos ligands from one of primarily academic interest to one with potentially important applications in catalysis. In this review, the syntheses of monofluorophosphites, (RO)2PF, and monofluorophosphines, R2PF, are discussed and the factors that control the kinetic stability of these ligands to hydrolysis and disproportionation are set out. A survey of the coordination chemistry of these two classes of monofluorophos ligands with d-block metals is presented, emphasising the bonding of the fluorophos to d-block metals, predominantly in low oxidation states. The application of monofluorophos ligands in homogeneous catalysis (especially hydroformylation and hydrocyanation) is discussed, and it is argued that there is great potential for monofluorophos complexes in future catalytic applications.

Wingtip‐Flexible N‐Heterocyclic Carbenes: Unsymmetrical Connection between IMes and IPr

AbstractIMes (IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene) and IPr (IPr=1,3‐ bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) represent by far the most frequently used N‐heterocyclic carbene ligands in homogeneous catalysis, however, despite numerous advantages, these ligands are limited by the lack of steric flexibility of catalytic pockets. We report a new class of unique unsymmetrical N‐heterocyclic carbene ligands that are characterized by freely‐rotatable N‐aromatic wingtips in the imidazol‐2‐ylidene architecture. The combination of rotatable N−CH2Ar bond with conformationally‐fixed N−Ar linkage results in a highly modular ligand topology, entering the range of geometries inaccessible to IMes and IPr. These ligands are highly reactive in Cu(I)‐catalyzed β‐hydroboration, an archetypal borylcupration process that has had a transformative impact on the synthesis of boron‐containing compounds. The most reactive Cu(I)‐NHC in this class has been commercialized in collaboration with MilliporeSigma to enable broad access of the synthetic chemistry community. The ligands gradually cover %Vbur geometries ranging from 37.3 % to 52.7 %, with the latter representing the largest %Vbur described for an IPr analogue, while retaining full flexibility of N‐wingtip. Considering the modular access to novel geometrical space in N‐heterocyclic carbene catalysis, we anticipate that this concept will enable new opportunities in organic synthesis, drug discovery and stabilization of reactive metal centers.

Sulfonium Cation in the Service of π-Acid Catalysis.

While still rare, cationic ligands offer much promise as tunable electron-withdrawing ligands for π-acid catalysis. Recently, we introduced pincer-type sulfonium cations into the list of available strongly π-acidic ancillary ligands. However, the M-S bond in sulfonium complexes of these ligands was found highly labile, precluding their catalytic applications. Herein we demonstrate that this obstacle can be overcome by increasing the rigidity of the sulfonium pincer scaffold. X-ray analyses confirm that despite bearing a formal positive charge, the sulfur atom of this newly designed sulfonium ligand maintains its coordination to the Pt(II)-center, while DFT calculations indicate that by doing so it strongly enhances the electrophilic character of the metal. Kinetic studies carried out on three model cycloisomerization reactions prove that such a tris-cationic sulfonium-Pt(II) complex is highly reactive, compared to its thioether-based analogue. This proof-of-concept study presents the first example of employing sulfonium-based ligands in homogeneous catalysis.

Sulfonium Cation in the Service of π‐Acid Catalysis**

AbstractWhile still rare, cationic ligands offer much promise as tunable electron‐withdrawing ligands for π‐acid catalysis. Recently, we introduced pincer‐type sulfonium cations into the list of available strongly π‐acidic ancillary ligands. However, the M−S bond in sulfonium complexes of these ligands was found highly labile, precluding their catalytic applications. Herein we demonstrate that this obstacle can be overcome by increasing the rigidity of the sulfonium pincer scaffold. X‐ray analyses confirm that despite bearing a formal positive charge, the sulfur atom of this newly designed sulfonium ligand maintains its coordination to the Pt(II)‐center, while DFT calculations indicate that by doing so it strongly enhances the electrophilic character of the metal. Kinetic studies carried out on three model cycloisomerization reactions prove that such a tris‐cationic sulfonium‐Pt(II) complex is highly reactive, compared to its thioether‐based analogue. This proof‐of‐concept study presents the first example of employing sulfonium‐based ligands in homogeneous catalysis.

Oxidation of 1,2-diarylalkynes to 1,2-diketones on acidic mesoporous ZSM-5 supported palladium catalyst

1,2-diketones are important compounds in natural products and bioactive molecules. Current synthesis methods are involved the use of various environmentally hazardous additives and expensive organic ligands in homogeneous catalysis, which limited their practical application. Acidic mesoporous ZSM-5 zeolite supported palladium (Pd/HMZSM-5) was found to be able to transform 1,2-diarylalkynes to 1,2-diketones without any additives. The CC bond of 1,2-diarylalkyne coordinated with PdII sites to form a metal-π complex that was easily nucleophilic attacked by DMSO to form final product. The acidic sites on the catalyst can promote the adsorption and coordination of 1,2-diarylalkyne with PdII and improve the reaction activity.

From a Fluorenyl Substituted Ylide-Functionalized Phosphine to a Neutral Phosphide via P-C Bond Cleavage.

Bulky ylide-substituted phosphines have recently found application as potent ligands in homogeneous catalysis. The attempted synthesis of the ylide-substituted fluorenylphosphine YPh P(Cy)Flu [YPh =Cy3 P(Ph)C; Flu=9-methylfluorenyl] now resulted in the unexpected elimination of 9-methylenefluorene during the deprotonation step of the intermediary α-phosphino phosphonium salt to yield the secondary ylide-substituted phosphine YPh P(Cy)H. This phosphine underwent formal H2 elimination under basic conditions to form a cyclic phosphonium ylide with a P-C-P-C four-membered ring via deprotonation of one cyclohexyl group of the PCy3 moiety. Upon coordination to transition metals the secondary ylidylphosphine forms a neutral phosphide ligand by shift of the proton into the ylide-backbone and formation of zwitterionic metal complexes.

Acidic hierarchical porous ZSM-5 assembled palladium catalyst: A green substitute to transform primary amides to nitriles

Metal catalyzed dehydration of primary amides is an attractive route for the synthesis of nitriles, but this transformation is often promoted by various environmentally hazardous additives or organic ligands in homogeneous catalysis, which limits its industrial application. The acidic mesoporous ZSM-5 assembled palladium (Pd/ZSM-5-H) was found to be able to systemically convert primary amides to nitriles in the absence of any additives with excellent applicability. The highly dispersive Pd(II) species grafted on ZSM-5-H via the strong binding interaction between AlO4 tetrahedron and Pd species (Pd/ZSM-5-H) drive the significantly higher reaction activity (100%) than Pd/Silicalite-1-H (30%) and Pd(OAc)2 (20%). Furthermore, the active centers of Pd/ZSM-5-H in the catalytic process were attributed to [PdOH]+ by the acidity characterization and theoretical calculations, and the primary amides to nitriles catalyzed by [PdOH]+ in Pd/ZSM-5-H is thermodynamically and kinetically favorable with –OH group in [PdOH]+ as an agent of proton transfer.

Ruthenium(II) Complexes of Heteroditopic N-Heterocyclic Carbene Ligands: Efficient Catalysts for C-N Bond Formation via a Hydrogen-Borrowing Strategy under Solvent-Free Conditions.

Both imidazol-2-ylidene (ImNHC) and 1,2,3-triazol-5-ylidene (tzNHC) have evolved to be elite groups of N-heterocyclic carbene (NHC) ligands for homogeneous catalysis. To develop efficient ruthenium(II)-based catalysts incorporating these ligands for C-N bond-forming reactions via hydrogen-borrowing methodology, we utilized chelating ligands integrated with ImNHC and mesoionic tzNHC donors connected via a CH2 spacer with a diverse triazole backbone. The synthesized ruthenium(II) complexes 3 are found to be highly efficient for C-N bond formation across a wide range of primary amine and alcohol substrates under solvent-free conditions, and among all of the complexes studied here, catalyst 3a with a mesityl substituent displayed maximum activity. To our delight, catalyst 3a is also effective for the selective mono-N-methylation of various anilines utilizing methanol as a coupling partner, known to be relatively more difficult than other alcohols. Furthermore, complex 3a also delivers various substituted quinolines successfully via the reaction of 2-aminobenzyl alcohol with several secondary alcohols. Importantly, catalyst 3a exhibited the highest activity among the reported ruthenium(II) complexes for both the N-benzylation of aniline [achieving a turnover number (TON) of 50000] and the realization of quinoline 8a by reacting 2-aminobenzyl alcohol with 2-phenylethanol (attaining a TON of 30000).

Heteroditopic Ru(II)– and Ir(III)–NHC Complexes with Pendant 1,2,3-Triazole/Triazolylidene Groups: Stereoelectronic Impact on Transfer Hydrogenation of Unsaturated Compounds

Imidazol-2-ylidene (ImNHC) and 1,2,3-traizol-5-ylidene (tzNHC) have been established as important classes of carbene ligands in homogeneous catalysis. To develop Ru(II)/Ir(III) complexes based on t...

Applications of α-Cationic Phosphines as Ancillary Ligands in Homogeneous Catalysis

The unique architecture of α-cationic phosphanes, in which one or more positively charged groups are directly attached to the phosphorus center dramatically affects both the physicochemical propert...

Are Phosphines Viable Ligands for Pd-Catalyzed Aerobic Oxidation Reactions? Contrasting Insights from a Survey of Six Reactions

Phosphines are the broadest and most important class of ligands in homogeneous catalysis, but they are typically avoided in Pd-catalyzed aerobic oxidation reactions because of their susceptibility to oxidative degradation. Recent empirical reaction-development efforts have led to a growing number of Pd/phosphine catalyst systems for aerobic oxidative coupling reactions, but few of these studies have assessed the fate of the phosphine ligand. Here, we assess six different oxidative coupling reactions, including the homocoupling of boronic acids, amino- and alkoxycarbonylation reactions, intramolecular C–H annulation, and enantioselective Fujiwara–Moritani C–C coupling. The fate and role of the phosphine, analyzed by 31P NMR spectroscopy throughout the reaction time course in each case, varies in different reactions. In one case, the phosphine has an inhibitory effect and leads to lower selectivity relative to ligand-free conditions. In other cases, the phosphine ligands have a beneficial effect on the reac...

Mono- and Digold(I) Complexes with Mesoionic Carbenes: Structural Characterization and Use in Catalytic Silver-Free Oxazoline Formation

Triazolylidenes are a prominent class of mesoionic carbenes that have found use as supporting ligands in homogeneous catalysis in recent years. We present here the syntheses of three new mononuclear gold(I) chlorido and two new dinuclear gold(I) chlorido complexes. The ligands in the aforementioned complexes are derived from either the corresponding monotriazolium or the bitriazolium salts. All complexes have been characterized by 1H and 13C{1H} NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies. Structural characterization delivers a delocalized bonding situation within the triazolylidene ligands and a linear coordination at the gold(I) centers. The gold(I) centers in all cases are bound to one triazolylidene-C donor and a chlorido ligand. Additionally, for the digold(I) complexes large Au–Au distances were observed, ruling out the existence of aurophilic interactions in these digold complexes in the solid state. All of the gold(I) complexes were tested as (pre)catalysts fo...

Expanding the Scope of Chelating Triazolylidenes: Mesoionic Carbenes from the 1,5-"Click"-Regioisomer and Catalytic Synthesis of Secondary Amines from Nitroarenes.

Chelating 1,2,3-triazolylidenes have been established as privileged ligands in homogeneous catalysis. We present herein a new approach towards chelating 1,2,3-triazolylidene ligands based on the 1,5-regioisomer of the corresponding triazole, which can be obtained through simple click chemistry. The new ligands are compared to their 1,4-regioisomeric counterparts through coordination to the ruthenium p-cymene fragment. The complexes are characterized structurally and spectroscopically and are employed as (pre)catalysts in the reductive condensation of nitroarenes and primary alcohols to yield secondary amines. The activity of chelating mesoionic carbene ligands obtained from the two different regioisomers of the triazoles are compared and contrasted in catalysis. The performance of the ruthenium complexes with mesoionic carbenes could be improved through the choice of the employed base and reaction conditions, giving rise to the most effective systems thus far. The results presented here prove the utility of chelating mesoionic carbenes as an extremely potent class of ligands for the synthesis of secondary amines from nitroarenes.

Development of some important nitrogen donor ligands for transition metal homogeneous catalysis

This review provides a broad overview of the literature related to the importance of ligands in homogeneous catalysis. In particular, it describes the types of nitrogen donor ligands that have typically been used for homogeneous catalysis. We surveyed the important transition metal homogenous catalysts explicitly from 2011 up to early 2014 and summarize their comparative catalytic activities. Generally, the main factors observed are the ligand structure, electron donor property and steric bulk which can affect the catalytic activity. Electron count and inductive effect can also influence the efficiency of homogeneous catalyst.

Pyridine and related ligands in transition metal homogeneous catalysis

This review provides a broad overview of the literature related to the importance of pyridine and related ligands in homogeneous catalysis. In particular, it describes the various ways by which this ligand can stabilised the metal within a complex for homogeneous catalysis. We surveyed the important transition metal homogenous catalysts containing pyridine and related ligand acting as backbone for other ligands in homogeneous catalytic reactions explicitly from 2011 up to early 2014 and summarized their comparative catalytic activities.

Oxidative degradation of the organometallic iron(II) complex [Fe{bis[3-(pyridin-2-yl)-1H-imidazol-1-yl]methane}(MeCN)(PMe3)](PF6)2: structure of the ligand decomposition product trapped via coordination to iron(II).

Iron is of interest as a catalyst because of its established use in the Haber-Bosch process and because of its high abundance and low toxicity. Nitrogen-heterocyclic carbenes (NHC) are important ligands in homogeneous catalysis and iron-NHC complexes have attracted increasing attention in recent years but still face problems in terms of stability under oxidative conditions. The structure of the iron(II) complex [1,1'-bis(pyridin-2-yl)-2,2-bi(1H-imidazole)-κN(3)][3,3'-bis(pyridin-2-yl-κN)-1,1'-methanediylbi(1H-imidazol-2-yl-κC(2))](trimethylphosphane-κP)iron(II) bis(hexafluoridophosphate), [Fe(C17H14N6)(C16H12N6)(C3H9P)](PF6)2, features coordination by an organic decomposition product of a tetradentate NHC ligand in an axial position. The decomposition product, a C-C-coupled biimidazole, is trapped by coordination to still-intact iron(II) complexes. Insights into the structural features of the organic decomposition products might help to improve the stability of oxidation catalysts under harsh conditions.

Ru(II), Os(II), and Ir(III) complexes with chelating pyridyl-mesoionic carbene ligands: structural characterization and applications in transfer hydrogenation catalysis.

Chelating ligands with one pyridine donor and one mesoionic carbene donor are fast establishing themselves as privileged ligands in homogeneous catalysis. The synthesis of several new Ir(III)-Cp*- and Os(II)-Cym complexes (Cp* = pentamethylcyclopentadienyl, Cym = p-cymene=4-isopropyl-toluene) derived from chelating pyridyltriazolylidenes where the additional pyridine donor was incorporated via the azide part of the triazole is presented. Furthermore, different 4-substituted phenylacetylene building blocks have been used to introduce electronic fine-tuning in the ligands. The ligands thus can be generally described as 4-(4-R-phenyl)-3-methyl-1-(pyridin-2-yl)-1H-1,2,3-triazol-5-ylidene (with R being H (L(1)), Me (L(2)), OMe (L(3)), CN (L(4)), CF3 (L(5)), Br (L(6)) or NO2 (L(7))). The corresponding complexes (Ir-1 to Ir-7 and Os-1 to Os-7) were characterized by standard spectroscopic methods, and the expected three-legged, piano-stool type coordination was unambiguously confirmed by X-ray diffraction analysis of selected compounds. Together with Ru(II) analogues previously reported by us, a total of 21 complexes were tested as (pre)catalysts for the transfer hydrogenation of carbonyl groups, showing a remarkable reactivity even at very low catalyst loadings. The electronic effects of the ligands as well as different substrates were investigated. Some mechanistic elucidations are also presented.

ChemInform Abstract: Synthesis of Homochiral Mono‐ and Bis‐phosphine Ligands for Homogeneous Catalysis

AbstractReview: 22 refs.

Synthesis and Reactivity of Heteroditopic Dicarbene Rhodium(I) and Iridium(I) Complexes Bearing Chelating 1,2,3-Triazolylidene–Imidazolylidene Ligands

1,2,3-Triazol-5-ylidenes (tzNHC) have become a popular class of NHC ligands in homogeneous catalysis. Herein, we introduce chelate monovalent Rh- and Ir(cod) complexes bearing bidentate ligands that combine this tzNHC and an Arduengo-type NHC motif. The reactivity of these complexes with H2 and CO gas has been investigated, leading to an interesting octahedral [Ir(tzNHC-CH2-NHC)(CO)2(H)2]OTf complex and [M(tzNHC-CH2-NHC)(CO)2]OTf complexes. The carbonyl stretching frequencies of the latter indicate that the ligand has stronger electron-donating properties than classic di-NHC ligands. The square planar rhodium and iridium NHC-tzNHC complexes have been applied in transfer hydrogenation employing isopropyl alcohol as the hydrogen donor, in which they show moderate activity (Ir > Rh) toward a range of ketones as well as for an aldehyde, an imine, and a diene. The new dicarbene complexes proved to be more active for this reaction than the analogues in which the triazolyl moiety coordinates through a nitrogen donor.

Vibrational spectra of a ferrocenyl phosphine derivative chemisorbed on 3-aminopropylsilyl modified silica gel

Samples of silica gel dried at different temperatures, silica gel modified with 3-aminopropylsilyl (APS) and silica gel modified with APS and further with a ferrocenyl phosphine derivative were investigated by DRIFT, transmission FTIR and MicroRaman spectroscopy. The reaction between 3-aminopropyltrimethoxysilane (APTMS) and silica gel was mainly identified by the diminishing or vanishing intensity of the stretching band of the free OH groups in the silica gel. Further chemical reaction of the APS groups with a ferrocenyl phosphine derivative (suitable as ligand in homogeneous catalysis) was identified in the IR spectra by the appearance of the CN stretching band of the formed Schiff base, and diminishing intensity of the δ(NH2) modes. According to the IR spectra the reaction of the ferrocenyl phosphine derivative with the APS-modified silica gel is almost quantitative. From the recorded IR and Raman spectra, conclusions concerning the substitution of APTMS methoxy groups during the chemisorption on silica gel were derived. Through deconvolution of the complex Raman band in the siloxy stretching region of the APS-modified silica gel, the newly formed siloxy bond was identified.