Aqua palladium complexes: synthesis, properties and applications

An N-heterocyclic carbene-based pincer system of palladium and its versatile reactivity under oxidizing conditions

NHC-based pincers (NHC = N-heterocyclic carbene) have been broadly employed as supporting platforms, and their palladium complexes have found many synthetic applications. However, previous studies mainly focused on the NHC pincers of palladium featuring an oxidation number of +II. In contrast, oxidation of these well-defined Pd(II) species and the study of their fundamental high-valent Pd chemistry remain largely undeveloped. In addition, from a perspective of PdII/PdIV catalysis, the reactivity and degradation of NHC pincers in catalytically relevant reactions have not been well understood. In this work, a series of Pd(II) complexes supported by a well-known NHC^Aryl^NHC pincer platform have been prepared. Their reactivity towards various oxidizing reagents, including halogen surrogates, electrophilic fluorine reagents, and alkyl/aryl halides, has been examined. In some cases, ambient-characterizable high-valent Pd NHCs, which have been scarcely reported, were obtained. The carbenes incorporated into the pincer framework proved to be effective spectator donors. In contrast, the central aryl moiety exhibits versatile reactivity and collapse pathways, allowing it to function either as a spectator or a non-innocent actor.

Control over Anion Coordination on Pd(II), Cu(I), and Ag(I) with Regioisomeric Phosphine-Carboxylate Ligands

The coordination of anionic donors is involved at various stages of catalytic cycles in transition-metal catalysis, but control over the spatial positioning of anions around a metal center is a challenge in coordination chemistry. Here we show that regioisomeric phosphine-carboxylate ligands provide spatial anion control on palladium(II) centers by favoring either κ2, cis-κ1, or trans-κ1 coordination of the carboxylate donor. Additionally, the palladium(II) carboxylates, which contain a methyl donor, upon protonation, deliver metal-alkyl complexes that feature a coordinated carboxylic acid. Such complexes can be considered as models for the minima that follow the concerted metalation-deprotonation transition state for C-H activation. The predictability of the coordination modes is further demonstrated on silver(I) and copper(I) centers, for which less common structures of mononuclear and dinuclear complexes can be obtained by using spatial anion control. Our results demonstrate the potential for spatial control over carboxylate anions in coordination chemistry.

Unraveling the Hydration Shell Structure and Dynamics of Group 10 Aqua Ions

We present ab initio simulations based on subsystem DFT of group 10 aqua ions accurately compared against experimental data on hydration structure. Our simulations provide insights into the molecular structures and dynamics of hydration shells, offering recalibrated interpretations of experimental results. We observe a soft, but distinct second hydration shell in Palladium (Pd) due to a balance between thermal fluctuations, metal-water interactions, and hydrogen bonding. Nickel (Ni) and platinum (Pt) exhibit more rigid hydration shells. Notably, our simulations align with experimental findings for Pd, showing axial hydration marked by a broad peak at about 3 Å in the Pd-O radial distribution function, revising the previously sharp "mesoshell" prediction. We introduce the "hydrogen bond dome" concept to describe a resilient network of hydrogen-bonded water molecules around the metal, which plays a critical role in the axial hydration dynamics.

Zwitterionic Aqua Palladacycles with Noncovalent Interactions for meta-Selective Suzuki Coupling of 3,4-Dichlorophenol and 3,4-Dichlorobenzyl Alcohol in Water

The site-selective reaction of substrates with multiple reactive sites has been a focus of the current synthetic chemistry. The use of attractive noncovalent interactions between the catalyst and substrate is emerging as a versatile approach to address site-selectivity challenges. Herein, we designed and synthesized a series of palladacycles, to control meta-selective Suzuki coupling of 3,4-dichlorophenol and 3,4-dichlorobenzyl alcohol. Noncovalent interactions directed zwitterionic aqua palladacycles catalyzed meta-selective Suzuki couplings of 3,4-dichloroarenes bearing hydroxyl in water have been developed. Experiments and density functional theory (DFT) calculations demonstrated that the electrostatic interactions play a critical role in meta-selective coupling of 3,4-dichlorophenol, while meta-selective coupling of 3,4-dichlorobenzyl alcohol arises due to the hydrogen-bonding interactions.

Hydrogen-bond-acceptor ligands enable distal C(sp3)-H arylation of free alcohols

The functionalization of C-H bonds in organic molecules is one of the most direct approaches for chemical synthesis. Recent advances in catalysis have allowed native chemical groups such as carboxylic acids, ketones and amines to control and direct C(sp3)-H activation1-4. However, alcohols, among the most common functionalities in organic chemistry5, have remained intractable because of their low affinity for late transition-metal catalysts6,7. Here we describe ligands that enable alcohol-directed arylation of δ-C(sp3)-H bonds. We use charge balance and a secondary-coordination-sphere hydrogen-bonding interaction-evidenced by structure-activity relationship studies, computational modelling and crystallographic data-to stabilize L-type hydroxyl coordination to palladium, thereby facilitating the assembly of the key C-H cleavage transition state. In contrast to previous studies in C-H activation, in which secondary interactions were used to control selectivity in the context of established reactivity8-13, this report demonstrates the feasibility of using secondary interactions to enable challenging, previously unknown reactivity by enhancing substrate-catalyst affinity.

Ligand-Centered Triplet Diradical Supported by a Binuclear Palladium(II) Dipyrrindione

Oligopyrroles form a versatile class of redox-active ligands and electron reservoirs. Although the stabilization of radicals within oligopyrrolic π systems is more common for macrocyclic ligands, bidentate dipyrrindiones are emerging as compact platforms for one-electron redox chemistry in transition-metal complexes. We report the synthesis of a bis(aqua) palladium(II) dipyrrindione complex and its deprotonation-driven dimerization to form a hydroxo-bridged binuclear complex in the presence of water or triethylamine. Electrochemical, spectroelectrochemical, and computational analyses of the binuclear complex indicate the accessibility of two quasi-reversible ligand-centered reduction processes. The product of a two-electron chemical reduction by cobaltocene was isolated and characterized. In the solid state, this cobaltocenium salt features a folded dianionic complex that maintains the hydroxo bridges between the divalent palladium centers. X-band and Q-band EPR spectroscopic experiments and DFT computational analysis allow assignment of the dianionic species as a diradical with spin density almost entirely located on the two dipyrrindione ligands. As established from the EPR temperature dependence, the associated exchange coupling is weak and antiferromagnetic (J ≈ −2.5 K), which results in a predominantly triplet state at the temperatures at which the measurements have been performed.

The coordination and redox chemistry of the dipyrrindione scaffold, which is found in several heme metabolites, is investigated in heteroleptic palladium(II) complexes. The bis(aqua) complex undergoes a deprotonation-driven dimerization to form a hydroxo-bridged binuclear species. Crystallographic, electrochemical, and spectroscopic data, as well as computational analysis, demonstrate that a two-electron reduction of the binuclear complex leads to a diradical dianion with spin density delocalized over the two dipyrrindione ligands.

Hydrogen-Bonding-Assisted Cationic Aqua Palladium(II) Complex Enables Highly Efficient Asymmetric Reactions in Water

Metal-bound water molecules have recently been recognized as a new facet of soft Lewis acid catalysis. Herein, a chiral palladium aqua complex was constructed that enables carbon-hydrogen bonds of indoles to be functionalized efficiently. We embraced a chiral 2,2'-bipyridine as both ligand and hydrogen-bond donor to configure a robust, yet highly Lewis acidic, chiral aqua complex in water. Whereas the enantioselectivity could not be controlled in organic solvents or under solvent-free conditions, the use of aqueous environments allowed the σ-indolylpalladium intermediates to react efficiently in a highly enantioselective manner. This work thus describes a potentially powerful new approach to the transformation of organometallic intermediates in a highly enantioselective manner under mild reaction conditions.

Synthesis of Well-Defined High-Valent Palladium Complexes by Oxidation of Their Palladium(II) Precursors

The last decade has witnessed the rapid development of high-valent Pd-involved organic transformations. This has also led to a steadily growing number of publications concerning the preparation of isolable and characterizable palladium(III) and palladium(IV) complexes. A variety of one-electron and two-electron oxidants have been employed to give access to high-oxidation-state Pd compounds. Undoubtedly, the study of these stoichiometric reactions has great implications for relevant Pd-mediated catalysis. In this minireview, the focus is on the synthetic approaches to structurally determined Pd^III/IV complexes starting from their Pd^II precursors, and the advances in this research area from early 2010 to late 2019 will be highlighted. Chemical oxidations exploiting various oxidizing agents including 1) hypervalent iodine reagents; 2) halogens; 3) electrophilic fluorination reagents; 4) alkyl/aryl halides; 5) ferrocenium salts; 6) peroxides/O₂ ; 7) sulfonyl chlorides; and 8) others are covered. A "greener" electrooxidation manner has also been reviewed.

Effects of Ligand Environment in Zr(IV) Assisted Peptide Hydrolysis

In this DFT study, activities of 11 different N₂O₄, N₂O₃, and NO₂ core containing Zr(IV) complexes, 4,13-diaza-18-crown-6 (I'_N2O4), 1,4,10-trioxa-7,13-diazacyclopentadecane (I'_N2O3), and 2-(2-methoxy)ethanol (I'_NO2), respectively, and their analogues in peptide hydrolysis have been investigated. Based on the experimental information, these molecules were created by altering protonation states (singly protonated, doubly protonated, or doubly deprotonated) and number of their ligands. The energetics of the I'_N2O4, and I'_NO2 and their analogues predicted that both stepwise and concerted mechanisms occurred either with similar barriers, or the latter was more favorable than the former. They also showed that the doubly deprotonated form hydrolyzed the peptide bond with substantially lower barriers than the barriers for other protonation states. For NO₂ core possessing complexes, Zr-(NO₂)(OH^H)(H₂O/OH)_n for n = 1-3, the hydroxyl group containing molecules were found to be more reactive than their water ligand possessing counterparts. The barriers for these complexes reduced with an increase in the coordination number (6-8) of the Zr(IV) ion. Among all 11 molecules, the NO₂ core possessing and two hydroxyl group containing I'_DNO2-2H complex was found to be the most reactive complex with a barrier of 28.9 kcal/mol. Furthermore, barriers of 27.5, 28.9, and 32.0 kcal/mol for hydrolysis of Gly-Glu (negative), Gly-Gly (neutral), and Gly-Lys (positive) substrates, respectively, by this complex were in agreement with experiments. The activities of these complexes were explained in terms of basicity of their ligand environment and nucleophilicity of the Zr(IV) center using metal-ligand distances, charge on the metal ion, and the metal-nucleophile distance as parameters. These results provide a deeper understanding of the functioning of these complexes and will help design Zr(IV)-based synthetic metallopeptidases.

Efficient access to a designed phosphapalladacycle catalyst via enantioselective catalytic asymmetric hydrophosphination

A chiral phosphine auxiliary was generated with excellent ee via catalytic asymmetric hydrophosphination of 3-(naphthalen-1-ylmethylene)pentane-2,4-dione. The subsequent metal complexation of the monophosphine yielded two different coordination complexes depending on the reaction conditions. The ortho-palladation of both coordination complexes resulted in the formation of a single dimeric phosphapalladacycle complex that could be further converted to the monomeric bisacetonitrile derivative. Moreover, the palladium complex exhibits interesting oxophilicity as the stable bisaquo derivative could be isolated and characterized crystallographically. The catalytic potential of the phosphapalladacycle was also demonstrated.

Polymer [Pd(CH2SO2C6H4Me)2]n, a precursor to remarkably stable Pd organometallics

Stable (arylsulfonyl)alkylpalladium complexes obtained from the polymer [Pd(CH₂SO₂C₆H₄Me)₂]_n undergo cis-to-trans isomerization in order to produce systems with stronger hydrogen bonds.

Aqueous Suzuki Coupling Reactions of Basic Nitrogen-Containing Substrates in the Absence of Added Base and Ligand: Observation of High Yields under Acidic Conditions

A series of aqueous heterogeneous Suzuki coupling reactions of substrates containing basic nitrogen centers with phenylboronic acid in the absence of added base and ligand is presented. High yields of products were obtained by employing aryl bromides containing aliphatic 1°, 2°, and 3° amine substituents, and good to high yields were obtained by employing a variety of substituted bromopyridines. In the former series, the pH of the aqueous phase changed from basic to acidic during the course of the reaction, while in the latter series the aqueous phase was on the acidic side of the pH scale throughout the entire course of reaction. A mechanistic interpretation for these observations, which generally preserves the oxo palladium catalytic cycle widely accepted in the literature, is presented.

Synthesis, structural and theoretical studies of dithiodiglycolamide compounds of palladium(II)

The reaction of palladium(ii) halide with dithiodiglycolamide ligands yielded compounds of the type [PdX2L] (where X = Cl, L = (CH2SCH2CON(i)Pr2)2 (1); L = (CH2SCH2CON(i)Bu2)2 (2); L = (CH2SCH2CONBu2)2 (3); L = C7H6(SCH2CON(i)Bu2)2 (4); X = Br, L = (CH2SCH2CON(i)Bu2)2 (5); X = I, L = (CH2SCH2CON(i)Bu2)2 (6)), whereas palladium(ii) nitrate yielded compounds of the type [PdL2](NO3)2 (where L = (CH2SCH2CON(i)Pr2)2 (7); L = (CH2SCH2CON(i)Bu2)2 (8)). All compounds were characterized by using IR, (1)H NMR spectral techniques and CHN analyses. The structures of compounds 4, 5 and 7 have been determined by using X-ray diffraction methods. The structures show that the ligands bond through the thioether group to the metal centre in all compounds. They show further that the palladium(ii) ion is surrounded by four atoms (two halogens and two thio groups in 4 and 5 and four thio groups in 7) in a square planar arrangement. The dithiodiglycolamide ligand acts as a bidentate chelating ligand and bonds through both the thioether groups to the metal centre, leaving the carbamoyl groups uncoordinated. Theoretical studies reveal that the 1 : 2 compound is energetically more stable and nicely correlates with the IR carbamoyl stretching frequencies as compared to the 1 : 1 compound in which the ligand acts as a tetradentate ligand.

Theoretical insights into the functioning of metallopeptidases and their synthetic analogues

CONSPECTUS: The selective hydrolysis of a peptide or amide bond (-(O═)C-NH-) by a synthetic metallopeptidase is required in a wide range of biological, biotechnological, and industrial applications. In nature, highly specialized enzymes known as proteases and peptidases are used to accomplish this daunting task. Currently, many peptide bond cleaving enzymes and synthetic reagents have been utilized to achieve efficient peptide hydrolysis. However, they possess some serious limitations. To overcome these inadequacies, a variety of metal complexes have been developed that mimic the activities of natural enzymes (metallopeptidases). However, in comparison to metallopeptidases, the hydrolytic reactions facilitated by their existing synthetic analogues are considerably slower and occur with lower catalytic turnover. This could be due to the following reasons: (1) they lack chemical properties of amino acid residues found within enzyme active sites; (2) they contain a higher metal coordination number compared with naturally occurring enzymes; and (3) they do not have access to second coordination shell residues that provide substantial rate enhancements in enzymes. Additionally, the critical structural and mechanistic information required for the development of the next generation of synthetic metallopeptidases cannot be readily obtained through existing experimental techniques. This is because most experimental techniques cannot follow the individual chemical steps in the catalytic cycle due to the fast rate of enzymes. They are also limited by the fact that the diamagnetic d(10) Zn(II) center is silent to electronic, electron spin resonance, and (67)Zn NMR spectroscopies. Therefore, we have employed molecular dynamics (MD), quantum mechanics (QM), and hybrid quantum mechanics/molecular mechanics (QM/MM) techniques to derive this information. In particular, the role of the metal ions, ligands, and microenvironment in the functioning of mono- and binuclear metal center containing enzymes such as insulin degrading enzyme (IDE) and bovine lens leucine aminopeptidase (BILAP), respectively, and their synthetic analogues have been investigated. Our results suggested that in the functioning of IDE, the chemical nature of the peptide bond played a role in the energetics of the reaction and the peptide bond cleavage occurred in the rate-limiting step of the mechanism. In the cocatalytic mechanism used by BILAP, one metal center polarized the scissile peptide bond through the formation of a bond between the metal and the carbonyl group of the substrate, while the second metal center delivered the hydroxyl nucleophile. The Zn(N3) [Zn(His, His, His)] core of matrix metalloproteinase was better than the Zn(N2O) [Zn(His, His, Glu)] core of IDE for peptide hydrolysis. Due to the synergistic interaction between the two metal centers, the binuclear metal center containing Pd2(μ-OH)([18]aneN6)](4+) complex was found to be ∼100 times faster than the mononuclear [Pd(H2O)4](2+) complex. A successful small-molecule synthetic analogue of a mononuclear metallopeptidase must contain a metal with a strong Lewis acidity capable of reducing the pKa of its water ligand to less than 7. Ideally, the metal center should include three ligands with low basicity. The steric effects or strain exerted by the microenvironment could be used to weaken the metal-ligand interactions and increase the activity of the metallopeptidase.

Palladium(0)-mediated C–H bond activation of N-(naphthyl)salicylaldimine and related ligands: utilization of the resulting organopalladium complexes in catalytic C–C and C–N coupling reactions

Aldimine ligands undergo palladium(0) mediated C–H bond activation to afford cyclopalladated complexes, which efficiently catalyze C–C and C–N coupling.

Phosphinocyclodextrins as confining units for catalytic metal centres. Applications to carbon-carbon bond forming reactions

The capacity of two cavity-shaped ligands, HUGPHOS-1 and HUGPHOS-2, to generate exclusively singly phosphorus-ligated complexes, in which the cyclodextrin cavity tightly wraps around the metal centre, was explored with a number of late transition metal cations. Both cyclodextrin-derived ligands were assessed in palladium-catalysed Mizoroki-Heck coupling reactions between aryl bromides and styrene on one hand, and the rhodium-catalysed asymmetric hydroformylation of styrene on the other hand. The inability of both chiral ligands to form standard bis(phosphine) complexes under catalytic conditions was established by high-pressure NMR studies and shown to have a deep impact on the two carbon-carbon bond forming reactions both in terms of activity and selectivity. For example, when used as ligands in the rhodium-catalysed hydroformylation of styrene, they lead to both high isoselectivity and high enantioselectivity. In the study dealing with the Mizoroki-Heck reactions, comparative tests were carried out with WIDEPHOS, a diphosphine analogue of HUGPHOS-2.

Synthesis and spectroscopic characterization of 3,4-difluoroacetophenone-thiosemicarbazone and its palladium(II) complex: evaluation of antimicrobial and antitumour activity

A new cis-palladium(II)diaqua(3,4-difluoroacetophenonethiosemicarbazone complex (Pd(II) complex) is synthesized using 3,4-difluoroacetophenonethiosemicarbazone(L). The L and its Pd(II) complex are characterized and confirmed by elemental analyses, electrochemical analyses, FT-IR, FT-Raman, UV-Vis, HRMS and LC-MS techniques. Ligand L is further characterized by (1)H, (13)C and (19)F NMR spectroscopy. The crystal structure of L is unambiguously characterized by single X-ray crystallography. The ligand (L) belongs to monoclinic system with P2(1)/C space group and the unit cell parameters are a(Å)=9.1144(7), b(Å)=13.7928(7), c(Å)=8.4174(5), α(°)=90, β(°)=100.715, γ(°)=90 and volume V(A(3))=1039.73(11). The Raman bands observed for the L and its Pd(II) complex are in good agreement with the FT-IR spectral data. The Pd(II) complex is found to be highly efficient in inhibiting the growth of human pathogens like Salmonella typhimurium and Klebsiella pneumonia with MIC value 10.0μg/mL whose inhibition zones are almost comparable with the standard antibiotic. The synthesized compounds have shown antiproliferative activity against the human breast cancer cell lines MDA-MB231 by intermitting the regular pathway of ribonucleotidereductase.

Role of π-acceptor effects in controlling the lability of novel monofunctional Pt(II) and Pd(II) complexes: crystal structure of [Pt(tripyridinedimethane)Cl]Cl

The kinetics and mechanism of substitution reactions of novel monofunctional [Pt(tpdm)Cl](+) and [Pd(tpdm)Cl](+) complexes (where tpdm = tripyridinedimethane) and their aqua analogues with thiourea (tu), L-methionine (L-met), glutathione (GSH), and guanosine-5'-monophosphate (5'-GMP) were studied in 0.1 M NaClO(4) at pH = 2.5 (in the presence of 10 mM NaCl for reactions of the chlorido complexes). The reactivity of the investigated nucleophiles follows the order tu > l-met > GSH > 5'-GMP. The reported rate constants showed the higher reactivity of the Pd(II) complexes as well as the higher reactivity of the aqua complex than the corresponding chlorido complex. The negative values reported for the activation entropy as well as the activation volume confirmed an associative substitution mode. In addition, the molecular and crystal structure of [Pt(tpdm)Cl]Cl was determined by X-ray crystallography. The compound crystallizes in a monoclinic space group C2/c with two independent molecules of the complex and unit cell dimensions of a = 38.303(2) Å, b = 9.2555(5) Å, c = 27.586(2) Å, β = 133.573(1)°, and V = 7058.3(8) Å(3). The cationic complex [Pt(tpdm)Cl](+) exhibits square-planar coordination around the Pt(II) center. The lability of the [Pt(tpdm)Cl](+) complex is orders of magnitude lower than that of [Pt(terpyridine)Cl](+). Quantum chemical calculations were performed on the [Pt(tpdm)Cl](+) and [Pt(terpyridine)Cl](+) complexes and their reactions with thiourea. Theoretical computations for the corresponding Ni(II) complexes clearly demonstrated that π-back-bonding properties of the terpyridine chelate can account for acceleration of the nucleophilic substitution process as compared to the tpdm chelate, where introduction of two methylene groups prevents such an effective π-back bonding.

Mechanism of peptide hydrolysis by co-catalytic metal centers containing leucine aminopeptidase enzyme: a DFT approach

In this density functional theory study, reaction mechanisms of a co-catalytic binuclear metal center (Zn1-Zn2) containing enzyme leucine aminopeptidase for two different metal bridging nucleophiles (H(2)O and -OH) have been investigated. In addition, the effects of the substrate (L-leucine-p-nitroanilide → L-leucyl-p-anisidine) and metal (Zn1 → Mg and Zn2 → Co, i.e., Mg1-Zn2 and Mg1-Co2 variants) substitutions on the energetics of the mechanism have been investigated. The general acid/base mechanism utilizing a bicarbonate ion followed by this enzyme is divided into two steps: (1) the formation of the gem-diolate intermediate, and (2) the cleavage of the peptide bond. With the computed barrier of 17.8 kcal/mol, the mechanism utilizing a hydroxyl nucleophile was found to be in excellent agreement with the experimentally measured barrier of 18.7 kcal/mol. The rate-limiting step for reaction with L-leucine-p-nitroanilide is the cleavage of the peptide bond with a barrier of 17.8 kcal/mol. However, for L-leucyl-p-anisidine all steps of the mechanism were found to occur with similar barriers (18.0-19.0 kcal/mol). For the metallovariants, cleavage of the peptide bond occurs in the rate-limiting step with barriers of 17.8, 18.0, and 24.2 kcal/mol for the Zn1-Zn2, Mg1-Zn2, and Mg1-Co2 enzymes, respectively. The nature of the metal ion was found to affect only the creation of the gem-diolate intermediate, and after that all three enzymes follow essentially the same energetics. The results reported in this study have elucidated specific roles of both metal centers, the nucleophile, indirect ligands, and substrates in the catalytic functioning of this important class of binuclear metallopeptidases.