Steric, Electronic, and Secondary Effects on the Coordination Chemistry of Ionic Phosphine Ligands and the Catalytic Behavior of Their Metal Complexes

Modular Synthesis of Phosphino Hydrazones and Their Use as Ligands in a Palladium-Catalysed Cu-Free Sonogashira Cross-Coupling Reaction

Phosphino hydrazones represent a versatile class of nitrogen-containing phosphine ligands. Herein, we report a modular synthesis of phosphino hydrazone ligands by hydrazone condensation reaction of three different aryl hydrazines with 3-(diphenylphosphino)propanal (PCHO). Complexation reactions of these phosphino hydrazone ligands with palladium(II) and platinum(II) were investigated and the catalytic activity of the palladium(II) complexes was explored in a Cu-free Sonogashira cross-coupling reaction achieving yields up to 96 %. Additionally it was shown that the catalytically active species is homogeneous.

Synthesis of Hybrid Ligands with Nitrile and Cage Phosphane Donor Groups and their Applications in Gold-Mediated Reactions

Altering the donor properties of phosphane ligands through substituent variation is an established tool in coordination chemistry and catalysis. This contribution describes the synthesis of two new hybrid donors (L) combining 1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphaadamantane-8-yl (PCg) and nitrile donor groups at different molecular scaffolds, viz. ferrocene-1,1'-diyl (fc) and 1,2-phenylene. These ligands were used to prepare dimeric Au(I) complexes [Au2 (μ(P,N)-L)2 ][SbF6 ]2 , which were evaluated as silver-free, preformed catalysts in Au-mediated cycloisomerization of (Z)-3-methylpent-2-en-4-yn-1-ol to 2,3-dimethylfuran. The catalyst featuring the ferrocene-based ligand, viz., [Au2 (μ(P,N)-CgPfcCN)2 ][SbF6 ]2 , showed the best catalytic performance at low catalyst loading (0.5 or 0.15 mol.%), which exceeded that of its diphenylphosphanyl analog [Au2 (μ(P,N)-Ph2 PfcCN)2 ][SbF6 ]2 studied earlier and the prototypical Au(I) precatalyst [Au(PPh3 )(MeCN)][SbF6 ].

Exploring steric and electronic parameters of biaryl phosphacycles

Steric and electronic parameters of the newly developed biaryl phosphacycles derived from the phobane[3.3.1] (Phob) and phosphatrioxa-adamantane (Cg) moieties were quantified from various experimental and theoretical methods.

A comprehensive review of caged phosphines: synthesis, catalytic applications, and future perspectives

Caged phosphines are versatile ligands due to their rigid backbones, exhibiting a range of catalytic activities, as depicted through the given pictorial representation.

Sulfonated Phosphine Ligands in the Ruthenium Catalyzed Biphasic Hydrogenation of Unsaturated Hydrocarbons

Abstract

The hydrogenation of monocyclic aromatic hydrocarbons by ruthenium catalysts has been investigated in the presence of sulfonated monophosphine ligands with different stereoelectronic properties under biphasic conditions. The effects of (i) the number of sulfonate groups, (ii) the steric demand of the ligand and (iii) the σ-donor ability of the phosphorus atom have been investigated in the catalytic reactions. The TEM analysis of the catalytically active systems revealed that in situ formed soluble ruthenium nanoparticles (RuNPs) are responsible for the catalytic turnover. The Ru/sulfonated phosphine system with the properly tuned ligand exhibited high activity (8800 h−1) and selectivity in the hydrogenation of benzene to cyclohexane. Furthermore, the aqueous phase containing RuNPs could be recycled four times without a considerable loss of activity.

Graphic Abstract

Mechanistic details of metal-free cyclization reaction of organophosphorus oxide with alkynes mediated by 2,6-lutidine and Tf2 O

Theoretical investigations have elucidated the mechanism of metal-free electrophilic phosphinative cyclization of alkynes reaction reported by Miura and coworkers. Two competitive mechanisms I and II were explored without or with 2,6-lutidine. Both of I and II involve transformation of P(V) to P(III), electrophilic addition, ring opening and cyclization/cyclization, hydrogen-transfer, and oxidation. The rate-determining step of mechanism I and competitive less-step II is electrophilic [2 + 1] cycloaddition and electrophilic addition via single CP bond formation with activation barrier of 13.5 and 10.6 kcal/mol, respectively. Our calculation results suggested that the cumulative effect of the isomer of 2,6-lutidine and Tf₂ O as well as TfO^- affects the title reaction to some extent, and simultaneously activates key reaction sites and reverses the polarities of them via the formation of abundant noncovalent interactions to decrease activation barriers of TSs. In addition, the effects of two series substituents on reactivity of phosphine oxide were investigated. Therefore, our study will serve as useful guidance for more efficient metal-free synthesis of organophosphorus compounds mediated by pyridine reagents.

Tailoring Cu+ for Ga3+ Cation Exchange in Cu2-xS and CuInS2 Nanocrystals by Controlling the Ga Precursor Chemistry

Nanoscale cation exchange (CE) has resulted in colloidal nanomaterials that are unattainable by direct synthesis methods. Aliovalent CE is complex and synthetically challenging because the exchange of an unequal number of host and guest cations is required to maintain charge balance. An approach to control aliovalent CE reactions is the use of a single reactant to both supply the guest cation and extract the host cation. Here, we study the application of GaCl₃-L complexes [L = trioctylphosphine (TOP), triphenylphosphite (TPP), diphenylphosphine (DPP)] as reactants in the exchange of Cu⁺ for Ga³⁺ in Cu_2-xS nanocrystals. We find that noncomplexed GaCl₃ etches the nanocrystals by S^2- extraction, whereas GaCl₃-TOP is unreactive. Successful exchange of Cu⁺ for Ga³⁺ is only possible when GaCl₃ is complexed with either TPP or DPP. This is attributed to the pivotal role of the Cu_2-xS-GaCl₃-L activated complex that forms at the surface of the nanocrystal at the onset of the CE reaction, which must be such that simultaneous Ga³⁺ insertion and Cu⁺ extraction can occur. This requisite is only met if GaCl₃ is bound to a phosphine ligand, with a moderate bond strength, to allow facile dissociation of the complex at the nanocrystal surface. The general validity of this mechanism is demonstrated by using GaCl₃-DPP to convert CuInS₂ into (Cu,Ga,In)S₂ nanocrystals, which increases the photoluminescence quantum yield 10-fold, while blue-shifting the photoluminescence into the NIR biological window. This highlights the general applicability of the mechanistic insights provided by our work.

Effect of Ancillary Ligand in Cyclometalated Ru(II)-NHC-Catalyzed Transfer Hydrogenation of Unsaturated Compounds

In an effort to develop efficient Ru(II)-NHC-based catalyst considering their stereoelectronic effect for hydride-transfer reaction, we found that the ancillary NHC ligand can play a significant role in its catalytic performance. This effect is demonstrated by comparing the activity of two different types of orthometalated precatalysts of general formula [( p-cymene)(NHC)Ru^II(X)] (NHC = an imidazolylidene-based ImNHC, compound 2a-c, or a mesoionic triazolylidene-based tzNHC, compound 4) in transfer hydrogenation of carbonyl substrates. The electron-rich precatalyst, 2c, containing p-OMe-substituted NHC ligand performed significantly better than both unsubstituted complex 2a and p-CF₃ substituted electron-poor complex 2b in ketone reduction. Whereas bulky mesoionic triazolylidene ligand containing complex 4 was found to be superior catalyst for aldehyde reduction and the precatalyst 2a is more suitable for the selective transfer hydrogenation of a wide range of aromatic aldimines to amines. To the best of our knowledge, this is the first systematic study on the effect of stereoelectronic tuning of ancillary orthometalated NHC ligand in Ru(II)-catalyzed transfer hydrogenations of various types of unsaturated compounds with broad substrate scope.

Oligo(ethylene glycol) Length Effect of Water-Soluble Ru-Based Olefin Metathesis Catalysts on Reactivity and Removability

A study of reaction kinetics and removal efficiency of a family of ruthenium (Ru)-based olefin metathesis catalysts containing ethylene-glycol-oligomer-tethered N-heterocyclic carbene (NHC) ligands has been carried out, with a focus on variation of ethylene glycol oligomer length. The length of ethylene glycol oligomer was precisely defined by sequential addition of repeating units. Due to the dual solubility of ethylene glycol oligomer, the produced catalyst was highly soluble in both aqueous and organic solvents (dichloromethane). In aqueous solution, the polarity increase with longer ethylene glycol oligomers enhanced the reactivity in homogeneous solution. The length of ethylene glycol oligomer did not significantly affect olefin metathesis rate in organic solution. Yet the removal efficiency of catalyst strongly relies on the length of ethylene glycol oligomer. A longer ethylene glycol oligomer demonstrated better catalyst removal efficiency. The tested catalyst removal method was aqueous extraction from organic solution using its higher water solubility property compared to its lower organic solvent (dichloromethane) solubility property. The results obtained from the aqueous extraction catalyst removal method demonstrated similar and/or better removal rates compared to previously reported host-guest catalyst removal methods.

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

The chemistry of carbeniophosphines and phosphoniocarbenes, which have general structures derived formally from the three-component "carbene/phosphine/positive charge" association, is presented. These two complementary classes of carbon-phosphorus-based ligands, defined by the presence of an inverted cationic coordinating structure (C+ ∼P: vs. P+ ∼C:) have the common purpose of positioning a positive charge in the vicinity of the metal center. Through selected examples, the synthetic methods, coordination properties, and general reactivity of both cationic species is described. Particular emphasis is placed on the influence of the positive charge on the respective chemical behavior of the two classes of compound.

Small "Yaw" Angles, Large "Bite" Angles and an Electron-Rich Metal: Revealing a Stereoelectronic Synergy To Enhance Hydride-Transfer Activity

Cyclometalated complexes are an important class of (pre)catalysts in many reactions including hydride transfer. The ring size of such complexes could therefore be a relevant aspect to consider while modulating their catalytic activity. However, any correlation between the cyclometalating ring size and the catalytic activity should be drawn by careful assessment of the pertinent geometrical parameters, and overall electronic effects thereof. In this study, we investigated the vital role of key stereoelectronic functions of two classes of iridacyclic complexes-five-membered and six-membered cycles-in manupulating the catalytic efficiency in a model hydride-transfer reaction. Our investigation revealed that there exists an interesting multidimensional synergy among all the relevant stereoelectronic factors-yaw angle, bite angle, and the electronic properties of both the ligand and the metal center-that governs the hydride donor ability (hydricity) of the complexes during catalysis. Thus the six-membered chelate complexes with small yaw and large bite angles, strong donor ligand, and electron-rich metal were found to be better catalysts than their five-membered analogues. A frontier molecular orbital analysis supported the significant role of the above stereoelectronic synergistic effect associated with the chelate ring to control the hydride donor ability of the complexes.

Transition Metal Complexes Coordinated by Water Soluble Phosphane Ligands: How Cyclodextrins Can Alter the Coordination Sphere?

The behaviour of platinum(II) and palladium(0) complexes coordinated by various hydrosoluble monodentate phosphane ligands has been investigated by ³¹P{¹H} NMR spectroscopy in the presence of randomly methylated β-cyclodextrin (RAME-β-CD). This molecular receptor can have no impact on the organometallic complexes, induce the formation of phosphane low-coordinated complexes or form coordination second sphere species. These three behaviours are under thermodynamic control and are governed not only by the affinity of RAME-β-CD for the phosphane but also by the phosphane stereoelectronic properties. When observed, the low-coordinated complexes may be formed either via a preliminary decoordination of the phosphane followed by a complexation of the free ligand by the CD or via the generation of organometallic species complexed by CD which then lead to expulsion of ligands to decrease their internal steric hindrance.

A convenient access to N-phosphonio-substituted NHC metal complexes [M = Ag(i), Rh(i), Pd(ii)]

NHC Pd(ii) complexes featuring a propyl bridge were ionized by the introduction of a phosphonium moiety, which may coordinate to the metal center or remain pendant.

Formic acid as a hydrogen storage material – development of homogeneous catalysts for selective hydrogen release

Liquid energy: formic acid is an ideal candidate for catalytic release and storage of hydrogen.

Synthesis and reactivity of α-cationic phosphines: the effect of imidazolinium and amidinium substituents

Mono- and dicationic phosphines have been synthesized through the reaction of chloroimidazolinium or chloroamidinium salts with secondary or primary phosphines respectively. The resulting ligands, which depict a significantly reduced donor ability compared with their neutral analogues, have been used to design Pt(II) and Au(I) complexes that effectively catalyse the hydroarylation of alkynes.

Synthetic and structural study of the coordination chemistry of a peri-backbone-supported phosphino-phosphonium salt

Coordination chemistry of an acenaphthene peri-backbone-supported phosphino-phosphonium chloride (1) was investigated, revealing three distinct modes of reactivity. The reaction of 1 with Mo(CO)4(nor) gives the Mo(0) complex [(1)Mo(CO)4Cl] (2), in which the ligand 1 exhibits monodentate coordination through the phosphine donor and the P-P bond is retained. PtCl2(cod) reacts with the chloride and triflate salts of 1 to form a mononuclear complex [(1Cl)PtCl2] (3) and a binuclear complex [((1Cl)PtCl)2][2TfO] (4), respectively. In both of these complexes, the platinum center adds across the P-P bond, and subsequent chloride transfer to the phosphenium center results in phosphine-chlorophosphine bidentate coordination. [((1)PdCl)2] (5) was isolated from the reaction of 1 and Pd2(dba)3 (dba = dibenzylideneacetone). Oxidative addition to palladium(0) results in a heteroleptic phosphine bridging phosphide coordination to the Pd(II) center. In addition, reaction of 1 with BH3·SMe2 leads to the bis(borane) adduct of the corresponding mixed tertiary/secondary phosphine (6), with BH3 acting as both a reducing agent and a Lewis acid. The new compounds were fully characterized, including X-ray diffraction. The ligand properties of 1 and related bonding issues are discussed with help of DFT computations.

α-Cationic phosphines: synthesis and applications

In coordination chemistry, typical ancillary ligands are anionic or neutral species. Cationic ones are exceptions and, when used, the positively charged groups are normally attached to the periphery and not close to the donating atom. However, this concept article highlights a series of recent experimental, as well as theoretical results, suggesting that the utility in catalysis of cationic phosphines with no spacer between the phosphorus atom and the positively charged group(s) has been largely overlooked. In fact, a growing number of studies indicate that, because of their specific architecture, these cationic ligands depict excellent π-acceptor character that can exceed that of phosphites or polyfluorinated phosphines. This property has been used to increase the Lewis acidity of the metals they coordinate. Specifically, new extreme π-acid catalysts, mainly based on Pt(II) and Au(I) , have been recently prepared and their superior performance demonstrated along several mechanistically distinct transformations. In this concept article the current state of the art is critically assessed and possible future directions of the topic discussed.

Recent development in functionalized ionic liquids as reaction media and promoters

The recent development in the use of functionalized ionic liquids as both reaction media and promoters is reviewed. Ionic liquids designed with special functionality can enhance the reaction rate, selectivity, and productivity in various chemical reactions. In this context, some important chemical reactions involving C–C, C–O, C–N, and C–S bond formation in functionalized ionic liquids are discussed. These functionalized ionic liquids were found to provide distinct advantages over conventional organic solvents in these catalytic reactions.