Sterically and Electronically Flexible Pyridylidene Amine Dinitrogen Ligands at Palladium: Hemilabile cis/trans Coordination and Application in Dehydrogenation Catalysis

Ligand design is crucial for the development of new catalysts and materials with new properties. Herein, the synthesis and unique hemilabile coordination properties of new bis-pyridylidene amine (bis-PYE) ligands to palladium, and preliminary catalytic activity of these complexes in formic acid dehydrogenation are described. The synthetic pathway to form cationic complexes [Pd(bis-PYE)Cl(L)]X with a cis-coordinated N,N-bidentate bis-PYE ligand is flexible and provides access to a diversity of Pd^II complexes with different ancillary ligands (L=pyridine, DMAP, PPh₃, Cl, P(OMe)₃). The ¹H NMR chemical shift of the trans-positioned PYE N−CH₃ unit is identified as a convenient and diagnostic handle to probe the donor properties of these ancillary ligands and demonstrates the electronic flexibility of the PYE ligand sites. In the presence of a base, the originally cis-coordinated bis-PYE ligand adopts a N,N,N-tridentate coordination mode with the two PYE units in mutual trans position. This cis–trans isomerization is reverted in presence of an acid, demonstrating a unique structural and steric flexibility of the bis-PYE ligand at palladium in addition to its electronic adaptability. The palladium complexes are active in formic acid dehydrogenation to H₂ and CO₂. The catalytic performance is directly dependent on the ligand bonding mode, the nature of the ancillary ligand, the counteranion, and additives. The most active system features a bidentate bis-PYE ligand, PPh₃ as ancillary ligand and accomplishes turnover frequencies up to 525 h⁻¹ in the first hour and turnover numbers of nearly 1000, which is the highest activity reported for palladium-based catalysts to date.     

Palladium-Catalyzed Hydrosilylation of Unactivated Alkenes and Conjugated Dienes with Tertiary Silanes Controlled by Hemilabile Hybrid P,O Ligand

Novel P,O-type ligands, N-disulfonyl bicyclic bridgehead phosphorus triamides, were synthesized and utilized in Pd-catalyzed hydrosilylation involving tertiary silanes, unactivated alkenes, and conjugated dienes. The ligand displayed a remarkable level of reactivity for alkene hydrosilylation with tertiary silanes and its use resulted in a significant improvement in the regioselectivity of diene hydrosilylation towards 1,2-hydrosilylation. X-ray crystallographic analysis confirmed the bidentate nature of the ligand, with coordination of phosphorus and oxygen. Control experiments provided evidence for the formation of Pd⁰ species and the reversibility of Pd−H insertion in the reaction mechanism. Density functional theory (DFT) computations supported the importance of the hemilabile P,O ligand in stabilizing both the rate-determining transition state of Pd−H insertion and the transition state of reductive elimination that determines the regioselectivity.     

Stereospecificity and Self-Selectivity in the Generation of a Chiral Molecular Tetrahedron by Metal-Assisted Self-Assembly

The chiral bidentate ligand (S,S)-1 reacts stereospecifically with Ga³⁺ to generate a [Ga₄(L)₆]¹²⁻ molecular tetrahedron although similar ligands generate [Ga₂(L)₃]⁶⁻ complexes. The assembly of this complex is self-selective as a racemic mixture of the ligand sorts by chirality to generate an enantiomeric pair of homochiral complexes.     

A novel,C2-symmetric,chiral bis-cyclosulfinamide-olefin tridentate ligand in Rh-catalyzed asymmetric 1,4-additions

A C₂-symmetric, chiral bis-cyclosulfinamide-olefin ligand composed of two 1-oxo-2,3-dihydro-1,2-benzisothiazole moieties with rigid skeletons and a conformationally flexible butenylene chain is disclosed for the first time. HRMS and ¹H NMR analyses verify that the in situ-generated complex of the ligand and [Rh(C₂H₄)₂Cl]₂ possesses a rhodium (I) center coordinated to the tridentate ligand via two sulfinyl moieties and a CdbndC bond. The chiral ligand provided extremely high enantioselectivity (up to >99%ee) in the Rh-catalyzed asymmetric 1,4-additions of arylboronic acids to cyclohexenone and cyclopentenone. The tridentate ligand gave much higher enantioselectivity than the analogous chiral bidentate ligands.     

Absolute Asymmetric Synthesis: Protected Substrate Oxidation

Three new conglomerates incorporating bidentate sulfide ligands coordinated by Ru^II centers have been prepared. Total spontaneous resolution by slow crystallization gives highly enantioenriched crystal batches, which are used in enantioselective oxidation of the sulfide ligands to give chiral sulfoxide complexes with >98 % ee. All relevant stereoisomers have been characterized by single‐crystal X‐ray diffraction, CD spectroscopy, and chiral HPLC. If the ligand range can be extended to monodentate sulfides, a large‐scale and recyclable process for enantioselective oxidation of sulfides can be designed.     

L-Shaped Heterobidentate Imidazo[1,5-a]pyridin-3-ylidene (N,C)-Ligands for Oxidant-Free AuI/AuIII Catalysis

In the last decade, major advances have been made in homogeneous gold catalysis. However, Au^I/Au^III catalytic cycle remains much less explored due to the reluctance of Au^I to undergo oxidative addition and the stability of the Au^III intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N-heterocyclic carbene) ligands through the development of a new class of L-shaped heterobidentate ImPy (ImPy=imidazo[1,5-a]pyridin-3-ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ-donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for Au^I/Au^III redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.     

Configurational assignment of vic-amino alcohols from their circular dichroism spectra with dirhodium tetracetate as an auxiliary chromophore

The circular dichroism spectra of a variety of vic-amino alcohols in the presence of dirhodium tetraacetate as an auxiliary chromophore were measured in ethanol as the solvent. The method was tested with several model compounds, representing both acyclic and cyclic amino alcohols, including biologically important adrenergic drugs as well as amino sugars. The study demonstrated that the sign of the Cotton effects is determined by the preferred helicity of the O–C–C–N unit in the chiral complex formed in situ. The combined analysis of the CD, UV–vis, ¹H and ¹³C NMR indicated predisposition to form chiral complexes by initial coordination of the amino alcohol at the axial coordination site followed by migration to the equatorial position. Finally, after migration of the ligand to an equatorial position(s) a bridging or a chelating complex is formed. Hence, vic-amino alcohols in ethanol act as bidentate ligands in the end.     

Complementing Pyridine‐2,6‐bis(oxazoline) with Cyclometalated N‐Heterocyclic Carbene for Asymmetric Ruthenium Catalysis

A strategy for expanding the utility of chiral pyridine‐2,6‐bis(oxazoline) (pybox) ligands for asymmetric transition metal catalysis is introduced by adding a bidentate ligand to modulate the electronic properties and asymmetric induction. Specifically, a ruthenium(II) pybox fragment is combined with a cyclometalated N‐heterocyclic carbene (NHC) ligand to generate catalysts for enantioselective transition metal nitrenoid chemistry, including ring contraction to chiral 2H‐azirines (up to 97 % ee with 2000 TON) and enantioselective C(sp³)?H aminations (up to 97 % ee with 50 TON).     

Supramolecular Self Assembly of [Cu(CN)4]3− Ions with Cationic {Ph3Sn+} Units in the Presence of Neutral Bidentate Ligands

The supramolecular interplay of the Ph ₃ Sn⁺ unit and the [Cu(CN) ₄ ]^3? ion with either 4,4′-bipyridine (bpy), trans-1,2-bis(4-pyridyl)ethene (tbpe), 1,2-bis(4-pyridyl)ethane (bpe), pyrazine (pyz), or methylpyrazine (mepyz) as bidentate ligands in presence of H ₂ O has been investigated for the first time. The products obtained have the general formula [(Ph ₃ Sn) ₃ Cu(CN)₄·L·XH₂O], where L is a bidentate ligand and X = 0–2. H ₂ O molecules are usually coordinated to tin atoms and are involved in two significant O─H─N hydrogen bonds, wherein the nitrogen atoms belong either to the bidentate ligand or the M-coordinated cyanide ligands. The structures of these supramolecular coordination polymers were investigated by elemental analysis, X-ray powder diffraction, and IR, mass, and NMR spectra.     

Enantioselective Oxidative Gold Catalysis Enabled by a Designed Chiral P,N‐Bidentate Ligand

A newly developed P,N‐bidentate ligand enables enantioselective intramolecular cyclopropanation by a reactive α‐oxo gold carbene intermediate generated in situ. The ligand design is based on our previously proposed structure (with a well‐organized triscoordinated gold center) of the carbene intermediate in the presence of a P,N‐bidentate ligand. A C₂‐symmetric piperidine ring was incorporated in the ligand as the nitrogen‐containing moiety. A range of racemic transformations of α‐oxo gold carbene intermediates have been developed recently, and this new class of chiral ligands could enable their modification for asymmetric synthesis, as demonstrated in this study.     

Coordination diversity in palladium(II)-picolinamide ligand complexes: structural and quantum chemical studies

The separation of different metal ions can be successfully accomplished by using picolinamide-based ligands. We herein report the first X-ray structure of picolinamide-based ligands of the type C₅H₄NCONR₂ (where R=ⁱC₃H₇ (L1) and ⁱC₄H₉ (L2)) and C₅H₄NCONHR (R=^tC₄H₉ (L3)) with palladium(II) ion. We have synthesized and characterized the structures of two palladium complexes, [PdCl₂(L1)₂] (1) and [PdCl₂L3] (3). In 1, ligand L1 forms a 2?:?1 complex with palladium(II) chloride, whereas in 3, the ligand L3 forms a 1?:?1 complex. Further, in 1, the ligand L1 acts as a monodentate ligand and is bound only through pyridyl-N atom, whereas in 3, the ligand L3 acts as a bidentate chelating ligand and is bound through both the pyridyl-N and amido-O atoms to the Pd(II) center. Electronic structure calculations are carried out to understand the experimental coordination diversity in the Pd complexes. Our calculations clearly suggest that a combination of steric hindrance of the ligand and the electronic effect of metal ions may modulate the coordination preferences.     

Photochemical reactions of <Emphasis Type="Italic">cis</Emphasis>-[(η<Superscript>4</Superscript>-NBD)M(CO)<Subscript>4</Subscript>] (NBD = norbornadiene; M = Cr,Mo) olefin complex with ligand,containing S and N donor atoms

New complexes cis-[M(CO)₄-DABRd] (M = Cr(I), Mo(II) and fac-[M(CO)₃-SAT] (M = Cr(III), Mo(IV)) have been synthesized by the photochemical reactions of cis-[(η⁴-NBD)M(CO)₄] (NBD is norbornadiene; M=Cr, Mo) with 5-(4-dimethylaminobenzylidene) rhodanine (DABRd) and salicylidene-3-amino-1,2,4-triazole (SAT) ligands and characterized by elemental analysis, FT-IR and ¹H NMR spectroscopy, and mass spectrometry. The spectroscopic studies show that the DABRd ligand acts as a bidentate ligand coordinating via both NH-(S)C=S sulfur donor atoms in I and II and SAT ligand behaves as a tridentate ligand coordinating via its all imine nitrogen-C=N-donor atoms in III and IV to the metal center. The article was submitted by the authors in English.     

New Functionalised Phosphorus (III) Ligands

Abstract

Whereas the number of functionalised tertiary phosphines, and to a lesser extent phosphites, has flourished in recent years, related systems bearing one (or more) P-N bonds are poorly developed. Here we describe a facile synthetic route to some new, potentially hemilabile, P,O-hybrids. These ligands are of particular interest for use in catalysis[1] and also as suitable precursors for further functionalisation.     

Functional ligands and complexes for new structures, homogeneous catalysts and nanomaterials

In this account, we focus on results from our laboratory to illustrate recent developments in various fields of organometallic chemistry. Studies on hemilabile P,N donor ligands and on the ion-pair behaviour of cationic Pd(II) complexes have led to the full characterization of complexes with η¹-allyl ligands. This still rare bonding mode for the allyl ligand in palladium chemistry allows facile insertion of CO into the Pd-C σ-bond, in contrast to the situation in related η³-allyl Pd(II) complexes. In order to develop new homogeneous catalysts for the selective dimerization and oligomerization of ethylene, a range of Ni(II) complexes have been prepared with new chelating P,N ligands where P represents a phosphine, phosphinite or phosphonite donor group and N a pyridine or oxazoline moiety. Finally, we shall examine bottom-up approaches to the formation of new nanomaterials of magnetic or catalytic interest by covalent anchoring of metal complexes and clusters into mesoporous materials using functional phosphine or alkyne ligands containing an alkoxysilyl group.     

Chiral Bifunctional Phosphine‐Carboxylate Ligands for Palladium(0)‐Catalyzed Enantioselective C−H Arylation

Previous enantioselective Pd⁰‐catalyzed C?H activation reactions proceeding via the concerted metalation‐deprotonation mechanism employed either a chiral ancillary ligand, a chiral base, or a bimolecular mixture thereof. This study describes the development of new chiral bifunctional ligands based on a binaphthyl scaffold which incorporates both a phosphine and a carboxylic acid moiety. The optimal ligand provided high yields and enantioselectivities for a desymmetrizing C(sp²)?H arylation leading to 5,6‐dihydrophenanthridines, whereas the corresponding monofunctional ligands showed low enantioselectivities. The bifunctional system proved applicable to a range of substituted dihydrophenanthridines, and allowed the parallel kinetic resolution of racemic substrates.     

Strategies for the synthesis of chiral, bidentate bis(perfluoroorganyl)phosphane ligands

Chiral, bidentate phosphane ligands, so-called PP ligands, are most frequently synthesized by reacting chiral ditosylates with diarylphosphanide ions.To use the P(CF₃)₂⁻ ion in nucleophilic substitution reactions, it is necessary to reduce the negative hyperconjugation, which is associated with a C-F activation. For this reason we synthesized different bis(trifluoromethyl)phosphanido complexes of mercury, silver and tungsten and investigated their use in nucleophilic substitution reactions. The most reactive compound, resulting from this study so far, is the pentacarbonylbis(trifluoromethyl)phosphanidotungstenate, [W{P(CF₃)₂}(CO)₅]⁻, which exhibits nearly the same bonding situation for the P(CF₃)₂ unit as in the free P(CF₃)₂⁻ ion. For use in synthesis of bis(trifluoromethyl)phosphane derivatives, Lewis acids are desirable, which stabilize the P(CF₃)₂⁻ ion by an intermediary formation of a donor acceptor adduct and can be split off after the synthesis of bis(trifluoromethyl)phosphane derivatives, as could be achieved using the extremely weak Lewis acids, CS₂ and acetone. These results could be established in the synthesis of the first example of an chiral, bidentate bis(trifluoromethyl)phosphane derivative.To synthesize a chiral, bidentate bis(pentafluorophenyl)phosphane derivative, a different synthetic strategy is necessary that does not involve the P(C₆F₅)₂⁻ ion, which decomposes even at low temperature. The implementation of functional P(CN)₂⁻ ions leads to the synthesis of functional, chiral bidentate dicyanophosphane derivatives which finally can be transformed into the corresponding bis(pentafluorophenyl)phosphane derivatives.     

A family of simple amide-derived air-stable P,O-ligands for Suzuki cross-coupling of unactivated aryl chlorides

A family of air-stable amide-derived phosphine (Aphos) ligands engineered on simple N,N-dialkyl aryl amide scaffolds has been designed and prepared by one-pot synthesis from the amides in high yields. The Aphos ligands have been used, in analogous to their atropisomeric variations, as hemilabile bidentate P,O-ligands in various Pd-catalyzed C-N and C-C bond forming reactions. We present here our results on the highly efficient Suzuki cross-coupling reactions of unactivated and/or sterically hindered aryl chlorides with arylboronic acids and a relationship of Aphos structures with catalytic efficacy.     

Asymmetric construction of quaternary carbon stereocenter by Pd-hemilabile ligand-catalyzed allylic substitution

The catalyst comprised [PdCl(η³-C₃H₅)]₂ and a simple chiral hemilabile ferrocene ligand, 1′,2-bis(diphenylphosphinoethyl)ferrocenyl alcohol, provides synthetically acceptable results for an enantioselective Pd-catalyzed allylic alkylation of cyclohexanone derivatives bearing an electron-withdrawing group at the α-position to form the quaternary carbon with up to 90% enantioselectivity under mild reaction conditions.     

Inducing Axial Chirality in a Supramolecular Catalyst

A new type of ligand, which is able to form axially chiral, supramolecular complexes was designed using DFT calculations. Two chiral monomers, each featuring a covalently bound chiral auxiliary, form a bidentate phosphine ligand with a twisted, hydrogen‐bonded backbone upon coordination to a transition metal center which results in two diastereomeric, tropos complexes. The ratio of the diastereomers in solution is very temperature‐ and solvent‐dependent. Rhodium and platinum complexes were analyzed through a combination of NMR studies, ESI‐MS measurements, as well as UV‐VIS and circular dichroism spectroscopy. The chiral self‐organized ligands were evaluated in the rhodium‐catalyzed asymmetric hydrogenation of α‐dehydrogenated amino acids and resulted in good conversion and high enantioselectivity. This research opens the way for new ligand designs based on stereocontrol of supramolecular assemblies through stereodirecting chiral centers.     

Tetra(μ3-hydroxo) bridged copper(II) tetranuclear cubane complexes: synthesis,crystal structure,and DNA binding studies

The tridentate Schiff base H₂L was synthesized by the condensation of equimolar amount of 1-amino-2-propanol and salicylaldehyde. The reaction of H₂L with an equimolar amount of Cu(CH₃COO)₂·H₂O in methanol leads to the formation of the tetranuclear Cu₄L₄, 1. However, reaction of equimolar amount of H₂L, copper(II) acetate, and 2,4,6-trimethylaniline in methanol forms a mixture of products which includes a discrete mononuclear complex Cu(L′)₂, 2m (where HL′ is a bidentate ligand), in addition to the tetranuclear Cu₄L₄ species, 2c. In both tetranuclear cubane species, the tridentate H₂L is both a chelating and a bridging ligand, after deprotonation of the enolic and the phenolic OH. The copper(II) centers are five-coordinate with a [N, O₄] donor set from the ligands. The coordination geometry about each copper is distorted square pyramidal with one nitrogen and two oxygen from one ligand and two oxygen from adjacent ligands in the next unit of the cubane. In mononuclear 2m, the ligand is bidentate and the coordination geometry around copper(II) is square planar. The absorption spectra strongly suggest that tetranuclear 1 interacts with CT-DNA.