THEORETICAL INVESTIGATION OF KETENE BONDING MODES IN BIS(PHOSPHINE) PALLADIUM KETENE COMPLEXES

Abstract

Theoretical studies were carried out on a series of bis(phosphine) palladium ketene complexes (PR₃)₂Pd(CH₂=C=O), and on the related CH₂=C=O and Pd(PR₃)₂ molecular fragments in order to investigate the electronic structure and the bonding of the ketene ligand to the metal fragment in these complexes. An analysis of the frontier MOs has been performed in order to understand the interactions between the ketene and the metal fragments. The calculated results have shown that the η²-(C,C) mode is preferred over the η²-(C,O) mode by 10–15 kcal/mol in bis(phosphine) palladium ketene complexes. The basicity and bulkiness of the phosphine ligands PR₃ have little effect on the bonding mode in (PR₃)₂Pd(CH₂=C=O) complexes. The most stable structure was calculated to be the η²-(C,C) square planar geometry with the CH₂ group of ketene out of the molecular plane. Comparison and discussion between the two bonding modes were also presented in this paper.     

Nickel(II) Phosphane- and N-heterocyclic Carbene Complexes with Tridentate,Dianionic [S,N,O]2– Ligands

Square-planar dinuclear nickel(II) complexes containing [S,N,O]^2– ligands formed by condensation of acetyl acetone or benzoyl acetone with cysteamine were prepared. These dimeric nickel(II) compounds undergo bridge-cleavage reactions with the cage phosphine PTA (PTA = 1,3,5-triaza-7-phosphaadamantane) or in situ generated N-heterocyclic carbenes. The resulting diamagnetic, square planar Ni^II complexes were characterized by spectroscopic methods and X-ray diffraction.     

Palladium(I)-bridging allyl dimers for the catalytic functionalization of CO2

In general, the chemistry of both η(1)-allyl and η(3)-allyl Pd complexes is extremely well understood; η(1)-allyls are nucleophilic and react with electrophiles, whereas η(3)-allyls are electrophilic and react with nucleophiles. In contrast, relatively little is known about the chemistry of metal complexes with bridging allyl ligands. In this work, we describe a more efficient synthetic methodology for the preparation of Pd(I)-bridging allyl dimers and report the first studies of their stoichiometric reactivity. Furthermore, we show that these compounds can activate CO(2) and that an N-heterocyclic carbene-supported dimer is one of the most active and stable catalysts reported to date for the carboxylation of allylstannanes and allylboranes with CO(2).     

Biological and Catalytic Applications of Pd(II)-Indenyl Complexes Bearing Phosphine and N-Heterocyclic Carbene Ligands

A general synthetic entryway into novel cationic Pd(II) indenyl complexes bearing one alkyl/aryl phosphine and one N-heterocyclic carbene is reported. All metal complexes have been exhaustively characterized by spectroscopic and structural analyses, highlighting that the indenyl fragment has an hapticity intermediate between η³ and η⁵. Most of the target complexes are stable in solid state and in solution for a long time. Two different applications of these organopalladium compounds are proposed. Firstly, they have been tested as antiproliferative agents towards three different ovarian cancer cell lines, showing a cytotoxicity significantly higher than that of cisplatin, with a clear dependence on the nature of the coordinated phosphine. Moreover, the similar cytotoxicity towards cisplatin-sensitive and cisplatin-resistant cell lines suggests that these new palladium derivatives act with a different mechanism of action with respect to classical platinum-based drugs. Finally, the water-soluble palladium complexes bearing 1,3,5-triaza-7-phosphaadamantane (PTA) have demonstrated interesting catalytic performances in Suzuki–Miyaura coupling in aqueous media, being, inter alia, readily and efficiently recyclable.     

Synthesis of a New Type of Amphilic and Water—soluble Tertiary Phosphine Ligands Substituted by an Ethoxylated Phosphonic Acid Chain and Their Palladium Complexes

The highly water-soluble phosphine ligands Na2O3PCH2CH2NH(CH2CH2O)nCH2CH2N-(CH2PPh)22(n=1,2,3) were prepared by a new and simple route under mild conditions in good yield; the palladium (Ⅱ） complexes of the ligands 3a-c with 2:1 or 4:1 -PPh2 to Pd^2 molar ratio were also prepared and characterized.     

Benign synthesis of quinolinecarboxamide ligands,H2bqbenzo and H2bqb and their Pd(II) complexes: X-ray crystal structure,electrochemical and antibacterial studies

Two carboxamide ligands, H₂bqbenzo {3,4-bis(2-quinolinecarboxamido)benzophenone} and H₂bqb {N,N′-bis[(2-quinolinecarboxamide)-1,2-benzene]}, have been prepared using tetrabutylammonium bromide as an environmentally benign reaction medium. Two new Pd(II) complexes, [Pd^II(bqbenzo)] (1) and [Pd^II(bqb)] (2), have been synthesized, characterized, and their structures determined by single crystal X-ray diffraction. The di-anionic ligands, bqbenzo^2? and bqb^2?, are coordinated via two N_amide atoms and the nitrogens of the two quinoline rings, with Pd?N_amide < Pd–N_quinoline bond lengths. The geometry around palladium(II) in both complexes is distorted square planar. The electrochemical behaviors of the ligands and their Pd(II) complexes have been investigated by cyclic voltammetry in DMF. An irreversible Pd^II/I reduction is observed at ?1.06 V for 1 and at ?1.177 V for 2, indicating the influence of the R substituent on the central phenyl ring of carboxamide ligands on the Pd^II/I reduction potential. The ligands and palladium complexes were also screened for in vitro antibacterial activity. The Pd(II) complexes show strong biological activity against S.typhi and E.coli as Gram ?ve and B.cereus and S.aureus as Gram +ve bacteria comparable to the antibiotic penicillin. The antibacterial results also reveal that coordination of Pd(II) significantly improves the activity.     

Abnormal N-heterocyclic Carbene Based Ni(II) π-allyl Complex towards Molecular Oxygen Activation

Abnormal N-heterocyclic carbene (aNHC) based Ni(II) π-allyl complexes ( 3 and 4 ) were synthesized starting from a Ni(0) precursor. These complexes were characterized by NMR spectroscopy, single-crystal X-ray crystallography ( 4 ) and elemental analysis data. The underlying mechanism for the formation of Ni(II) η³-allyl complexes from a Ni(0) precursor on treatment with a free abnormal N-heterocyclic carbene in absence of any external additive or oxidant was unraveled. Later, complex 3 was exposed to O₂ gas under ambient pressure resulting in molecular oxygen activation to form a μ-hydroxo bridged Ni(II) dimer.     

The insertion of the dioxides of carbon and sulphur into the palladium-carbon bond

Bis ( η¹, η²-allyl) palladium phosphine complexes react with carbon dioxide and sulphur dioxide by insertion into the palladium-carbon σ-bond to give η³-allylpalladium-carboxylate and -S-sulphinate complexes.     

Dinuclear macrocyclic palladium complexes having pincer coordinating groups and their catalytic properties in Mizoroki-Heck reactions

An improved synthetic method of palladium(II) dinuclear macrocyclic complexes have been described. Each of the two isomers of the complexes [Pd₂LBr₂] has a macrocyclic ligand L in which two 2,6-bis(diaminomethyl)phenyl units coordinate to the Pd(II) centers with N, C, N donor atoms. Substitution of the bromo ligands of one of the isomer of the complexes with acetonitrile ligands affords a new dinuclear complex. Catalytic activities of these complexes were studied for the Mizoroki-Heck type reactions of iodobenzene and styrene. High turnover number up to 30,000 was achieved using one of the isomer of the complexes.     

Unmasking the Action of Phosphinous Acid Ligands in Nitrile Hydration Reactions Catalyzed by Arene–Ruthenium(II) Complexes

The catalytic hydration of benzonitrile and acetonitrile has been studied by employing different arene–ruthenium(II) complexes with phosphinous (PR₂OH) and phosphorous acid (P(OR)₂OH) ligands as catalysts. Marked differences in activity were found, depending on the nature of both the P‐donor and η⁶‐coordinated arene ligand. Faster transformations were always observed with the phosphinous acids. DFT computations unveiled the intriguing mechanism of acetonitrile hydration catalyzed by these arene–ruthenium(II) complexes. The process starts with attack on the nitrile carbon atom of the hydroxyl group of the P‐donor ligand instead of on a solvent water molecule, as previously suggested. The experimental results presented herein for acetonitrile and benzonitrile hydration catalyzed by different arene–ruthenium(II) complexes could be rationalized in terms of such a mechanism.     

Kinetics and Mechanism of the Racemization of Aryl Allenes Catalyzed by Cationic Gold(I) Phosphine Complexes

The kinetics of the racemization of aromatic 1,3‐disubstituted allenes catalyzed by gold phosphine complexes has been investigated. The rate of gold‐catalyzed allene racemization displayed first‐order dependence on allene, and catalyst concentration and kinetic analysis of gold‐catalyzed allene racemization as a function of allene and phosphine electron‐donor ability established the accumulation of electron density on the phosphine atom and the depletion of electron density on the terminal allenyl carbon atoms in the rate‐limiting transition state for racemization. These and other observations were in accord with a mechanism for allene racemization involving rapid and reversible inter‐ and intramolecular allene exchange followed by turnover‐limiting, unimolecular conversion of a chiral gold η²‐allene complex to an achiral η¹‐allylic cation intermediate through a bent and twisted η¹‐allene transition state. With respect to proper ligand selection, these studies reveal that both electron‐poor phosphine ligands and polar solvents facilitate racemization.     

Alkyl,Silyl, and Phosphane Ligands—Classical Ligands in Nonclassical Bonding Modes

Alkyl, silyl, and phosphane ligands are amongst the most familiar and ubiquitous ligands in organometallic and coordination chemistry. The C, Si, and P donor atoms of these ligands are sp³‐hybridized and the ligands are related to each other by the isolobal analogy: (CR₃)−(SiR₃)−PR₃. Herein, we demonstrate that although a number of unusual observations concerning the reactivity and bonding of these ligands appears unrelated at first sight, they in fact provide offer an exiting and consistent picture that may form the basis for new paradigms. The characterization of stable complexes in which alkyl, silyl, and phosphane ligands behave as symmetrical bridges confirms that there is no inherent thermodynamic instability associated with these bonding situations, and, in fact, reactivity studies suggest that these ligands should be able to bridge between metal centers in reaction intermediates or transition states.     

Metalated 1, 3‐Azaphospholes: η1‐(1H‐1, 3‐Benzazaphosphole‐P)M(CO)5 and μ2‐[(1, 3‐Benzazaphospholide‐P)(cyclopentadienide)nickel] Complexes

1H‐1, 3‐Benzazaphospholes react with M(CO)₅(THF) (M = Cr, Mo, W) to give thermally and relatively air stable η¹‐(1H‐1, 3‐Benzazaphosphole‐P)M(CO)₅ complexes. The ¹H‐ and ¹³C‐NMR‐data are in accordance with the preservation of the phosphaaromatic π‐system of the ligand. The strong upfield ³¹P coordination shift, particularly of the Mo and W complexes, forms a contrast to the downfield‐shifts of phosphine‐M(CO)₅ complexes and classifies benzazaphospholes as weak donor but efficient acceptor ligands. Nickelocene reacts as organometallic species with metalation of the NH‐function. The resulting ambident 1, 3‐benzazaphospholide anions prefer a μ₂‐coordination of the η⁵‐CpNi‐fragment at phosphorus to coordination at nitrogen or a η³‐heteroallyl‐η⁵‐CpNi‐semisandwich structure. This is shown by characteristic NMR data and the crystal structure analysis of a η⁵‐CpNi‐benzazaphospholide. The latter is a P‐bridging dimer with a planar Ni₂P₂ ring and trans‐configuration of the two planar heterocyclic phosphido ligands arranged perpendicular to the four‐membered ring.     

A Sterically Constrained Tricyclic PC3 Phosphine: Coordination Behavior and Insertion of Chalcogen Atoms into P−C Bonds

A tricyclic phosphine has been generated that has a rigid molecular backbone with the P atoms exclusively bound to C(sp²) atoms as well as a very large Tolman angle and buried volume. It is an interesting new ligand in coordination chemistry (Au, Pd complexes) and shows unusual insertion reactions into its endocyclic P?C bonds facilitated by its inherent molecular strain.     

Effective Modular Assembling of Novel Ligands Starting from (Thio)Phosphorylated Anilines

Abstract

A series of novel oligodentate ligands has been designed by the modular assembly of ortho-(thio)phosphorylated anilines with a range of commercially available carbonyl compounds. The new ligands readily form complexes with a series of metal ions (Pd(II), Cu(II), Ni(II), and Re(I)) both in neutral and deprotonated forms, providing OO, SN, ONO, SNO, SNS, SNN, and ONN donor sets. The structure and composition of the resulting complexes depend on the nature and mutual disposition of the donating groups as well as on the reaction conditions.     

Pd(II) complexes of novel phosphine ligands: Synthesis,characterization, and catalytic activities on Heck reaction

Novel phosphine oxides, (((3-methylpyridin-2-yl)amino)methyl)diphenylphosphine oxide (1) and diphenyl((pyrazin-2-ylamino)methyl)phosphine oxide (2), were synthesized and characterized. Phosphines ligands (3 and 4) were obtained by the reduction of 1 and 2 with AlH₃, monitored by ³¹P NMR spectroscopy. Pd(II) complexes of 3 and 4 were synthesized and characterized (5 and 6). The catalytic activity of 5 and 6 was tested on the reaction of styrene with both activated and deactivated aryl bromides in air. The results of the catalytic experiments were discussed through DFT calculations.     

Molybdenum Tricarbonyl Complexes Supported by Linear PNP Ligands: Influence of P- and N-Substituents on Relative Stability,Stereoisomerism and on the Activation of Small Molecules

Series of linear tridentate PN^PhP^R-ligands (R=Me, Et, Pln, Ph, Cyp, ⁱPr, Cy, ^tBu) and molybdenum tricarbonyl complexes [Mo(CO)₃PN^PhP^R] (R=Ph, Et, Cyp, ⁱPr, Cy,) were synthesized and characterized using NMR-, IR-, and Raman spectroscopy as well as X-ray crystallography. The influence of the different phosphine donor groups of the PN^PhP^R ligands on the bonding and activation of CO ligands is investigated. Importantly, all complexes are found to adopt a fac geometry, both in solution and in the solid state. This is in contrast to analogous complexes supported by PN^HP ligands. DFT calculations reveal that the phenyl ring at the central amine function is the cause of the preferred geometry, hindering isomerization to a mer geometry.     

Tris(tetramethylguanidinyl)phosphine: The Simplest Non-ionic Phosphorus Superbase and Strongly Donating Phosphine Ligand

We report the synthesis and properties of the much sought-after tris(1,1,3,3-tetramethylguanidinyl) phosphine P(tmg)₃, a crystalline, superbasic phosphine accessible through a short and scalable procedure from the cheap and commercially available bulk chemicals 1,1,3,3-tetramethylguanidine, tris(dimethylamino)-phosphine and phosphorus trichloride. The new phosphine exhibits exceptional electron donor properties and readily forms transition metal complexes with gold(I), palladium(II) and rhodium(I) precursors. The formation of zwitterionic Lewis base adducts with carbon dioxide and sulfur dioxide was explored. In addition, the complete series of phosphine chalcogenides was prepared from the reaction of P(tmg)₃ with N₂O and the elemental chalcogens.     

Design of pincer complexes based on (methylsulfanyl)acetic/propionic acid amides with ancillary S‐ and N‐donors as potential catalysts and cytotoxic agents

Pincer complexes featuring readily tunable tridentate ligand frameworks comprise one of the most actively studied classes of organometallic and metal–organic compounds and find extensive use in catalysis, organic synthesis, materials science, and other fields of chemistry and allied disciplines. Currently growing attention is devoted to non‐classical ligand scaffolds, such as functionalized carboxamides, which offer multiple options for directed structural modifications. In this study, the reactions of (methylsulfanyl)acetyl and propanoyl chlorides with 2‐(aminomethyl)pyridine, 2‐(2‐aminoethyl)pyridine, 8‐aminoquinoline and 2‐(diphenylthiophosphoryl)aniline afford a series of new pincer‐type ligands based on functionalized carboxamides. The ligands obtained readily undergo direct cyclopalladation under the action of PdCl₂(NCPh)₂ in dichloromethane at room temperature, resulting in Pd(II) pincer complexes with N,N,S‐ and S,N,S‐donor sets. Importantly, some of the cyclopalladated derivatives can also be produced efficiently under solvent‐free conditions according to the approach recently developed by our group. The complexes obtained have been tested for cytotoxicity against several human cancer cell lines and catalytic activity in the model Suzuki reaction. The results have been compared to those for the related Pd(II) pincer complexes to define the main structure–activity relationships and to outline the most promising structures for further investigations.     

Platinum(II), palladium(II), nickel(II), and gold(I) complexes of the “electrospray-friendly” thiolate ligands 4-SC5H4N− and 4-SC6H4OMe−

The series of platinum(II), palladium(II), and nickel(II) complexes [ML₂(dppe)] [M = Ni, Pd, Pt; L = 4–SC₅H₄N or 4–SC₆H₄OMe; dppe = Ph₂PCH₂CH₂PPh₂] containing pyridine-4-thiolate or 4-methoxybenzenethiolate ligands, together with the corresponding gold(I) complexes [AuL(PPh₃)], were prepared and their electrospray ionization mass spectrometric behavior compared with that of the thiophenolate complexes [M(SPh)₂(dppe)] (M = Ni, Pd, Pt) and [Au(SPh)(PPh₃)]. While the pyridine-4-thiolate complexes yielded protonated ions of the type [M + H]⁺ and [M + 2H]²⁺ ions in the Ni, Pd, and Pt complexes, an [M + H]⁺ ion was only observed for the platinum derivative of 4-methoxybenzenethiolate. Other ions, which dominated the spectra of the thiophenolate complexes, were formed by thiolate loss and aggregate formation. The X-ray crystal structure of [Pt(SC₆H₄OMe–4)₂(dppe)] is also reported.