Paramagnetic π-Allyl complexes of palladium

Paramagnetic π-allyl complexes of palladium with phenoxyl or o-semiquinolate ligands have been prepared and their ESR spectra studied. The stability of the resulting free radicals, the influence of various substituens attached to the palladium atom on the pattern of distribution of spin density of the unpaired electron and the nature of dynamic processes occuring in paramagnetic allyl complexes are discussed.     

Cation‐Controlled Formation and Interconversion of the fac/fac and mer/mer Stereoisomers of a Triple‐Stranded Helicate

Two biscatecholester ligands with oligoether spacers were used to prepare dinuclear titanium(IV) triscatecholate based helicates. In the case of Li₄[( 1 / 2 )₃Ti₂], “classical” helicates with three internally bound Li⁺ ions and syn‐oriented ligands in the complex units (fac/fac isomer) were obtained. In the case of the sodium salt Na₄[( 2 )₃Ti₂], a different homochiral dinuclear triple‐stranded helicate with two internally bound Na⁺ ions was formed. The complex units are anti‐configured, and two of the ligand spacers are connecting internal with external positions of the helicate (mer/mer isomer). Removal of the sodium ions and addition of lithium ions leads to the switching from one topology to the other with an expanded helicate [( 2 )₃Ti₂]^4? as an intermediate. Switching back to the “non‐classical” helicate cannot be observed because severe structural rearrangements would be required.     

Palladium carbonyl complexes with anions of N-heterocyclic carboxylic and pyridine-2-sulfonic acids

Synthesis, spectral characteristics, and structure of palladium(I) carbonyl complexes containing anions of N-heterocyclic carboxylic acids and pyridine-2-sulfonic acid of the general formula [Pd(CO)(NHC-CO₂)]n/[Pd(CO)(NHC-SO₃)]n, where NHC is an N-heterocycle, were described. The resulting complexes can be attributed to a binuclear structure with bridging or terminal coordination of carbonyl ligands depending on the nature of the substituents in the heterocycle.

     

In Situ Photoisomerization of an Azobenzene-Based Triple Helicate with a Prolonged Thermal Relaxation Time

The phosphate-coordination triple helicates A₂L₃ (A=anion) with azobenzene-spaced bis-bis(urea) ligands (L) have proven to undergo a rare in situ photoisomerization (without disassembly of the structure) rather than the typically known, stepwise “disassembly-isomerization-reassembly” process. This is enabled by the structural self-adaptability of the “aniono” assembly arising from multiple relatively weak and flexible hydrogen bonds between the phosphate anion and bis(urea) units. Notably, the Z→E thermal relaxation rate of the isomerized azobenzene unit is significantly decreased (up to 20-fold) for the triple helicates compared to the free ligands. Moreover, the binding of chiral guest cations inside the cavity of the Z-isomerized triple helicate can induce optically pure diastereomers, thus demonstrating a new strategy for making light-activated chiroptical materials.     

A Bowl‐Shaped Circular Trinuclear Helicate Generated from a TiO4N2 Motif by a Multicomponent Self‐Assembly Approach

The synthesis of a bowl‐shaped trinuclear circular titanium‐based helicate is reported. The strategy allowing access to this neutral architecture is based on a multicomponent self‐assembly approach in which the ligands involved in the process are a bis‐biphenol strand and 2,2′‐ bipyrimidine. By reacting the bis‐biphenol ligand and 2,2′‐bipyrimidine with an equimolar amount [Ti(OiPr)₄], a bowl‐shaped architecture is obtained through the formation of 18 new coordination bonds. This aggregate built from three octahedral TiO₄N₂ nodes displays an unusually high stability in solution compared to related species. In addition, by modifying the stoichiometry of the initial components, two assemblies incorporating two titanium centers bridged by a 2,2′‐bipyrimidine ligand are obtained. The crystal structures of these species are reported.     

Dynamic Molecular Metamorphism Involving Palladium-Assisted Dimerization of π-Cation Radicals

A dynamic supramolecular approach is developed to promote the π-dimerization of viologen radicals at room temperature and in standard concentration ranges. The approach involves cis- or trans-protected palladium centers serving as inorganic hinges linking two functionalized viologens endowed with metal-ion coordinating properties. Based on detailed spectroscopic, electrochemical and computational data, we show that the one-electron electrochemical reduction of the viologen units in different dynamic metal/ligand mixtures leads to the formation of the same intramolecular π-dimer, regardless of the initial environment around the metallic precursor and of the relative ratio between metal and ligand initially introduced in solution. The large-scale electron-triggered reorganization of the building blocks introduced in solution thus involves drastic changes in the stoichiometry and stereochemistry of the palladium/viologen complexes proceeding in some cases through a palladium centered trans→cis isomerization of the coordinated ligands.     

Subcomponent Self‐Assembly of a 4 nm M4L6 Tetrahedron with ZnII Vertices and Perylene Bisimide Dye Edges

Formation of a tetrahedron with >4 nm perylene bisimide (PBI) dye edges and Zn^II vertices in a one‐pot 22 component self‐assembly reaction is reported. The luminescent polyhedron equilibrates to a Zn₂L₃ helicate and disassembles upon dilution. Insights into the subcomponent self‐assembly of extended PBI ligands help to refine design rules for constructing large photofunctional metallosupramolecular hosts.     

Synthesis,characterization and first application of chiral C2‐symmetric bis(phosphinite)–Pd(II) complexes as catalysts in asymmetric intermolecular Heck reactions

A series of new chiral C₂‐symmetric bis(phosphinite) ligands and their palladium(II) complexes have been synthesized and for the first time used as catalysts in the palladium‐catalysed asymmetric intermolecular Heck coupling reactions of 2,3‐dihydrofuran with iodobenzene or aryl triflate. Under optimized conditions, products were obtained with high conversions and moderate to good enantioselectivities. The new C₂‐symmetric bis(phosphinite) ligands and their palladium(II) complexes were characterized using multinuclear NMR and Fourier transform infrared spectroscopies and elemental analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

The Effect of the Spacer of Bis(biurea) Ligands on the Structure of A2L3‐type (A=anion) Phosphate Complexes

By tuning the length and rigidity of the spacer of bis(biurea) ligands L, three structural motifs of the A₂L₃ complexes (A represents anion, here orthophosphate PO₄^3?), namely helicate, mesocate, and mono‐bridged motif, have been assembled by coordination of the ligand to phosphate anion. Crystal structure analysis indicated that in the three complexes, each of the phosphate ions is coordinated by twelve hydrogen bonds from six surrounding urea groups. The anion coordination properties in solution have also been studied. The results further demonstrate the coordination behavior of phosphate ion, which shows strong tendency for coordination saturation and geometrical preference, thus allowing for the assembly of novel anion coordination‐based structures as in transition‐metal complexes.     

Novel (N‐heterocyclic carbene)Pd(pyridine)Br2 complexes for carbonylative Sonogashira coupling reactions: Catalytic efficiency and scope for arylalkynes,alkylalkynes and dialkynes

New N,N′‐substituted imidazolium salts and their corresponding dibromidopyridine–palladium(II) complexes were successfully synthesized and characterized. Reactions of palladium bromide with the newly synthesized N,N′‐substituted imidazolium bromides ( 2a and 2b ) in pyridine afforded the corresponding new N‐heterocyclic carbene pyridine palladium(II) complexes ( 3a and 3b ) in high yields. Their single‐crystal X‐ray structures show a distorted square planar geometry with the carbene and pyridine ligands in trans position. Both complexes show a high catalytic activity in carbonylative Sonogashira coupling reactions of aryl iodides and aryl diiodides with arylalkynes, alkylalkynes and dialkynes.     

New palladium (II) complexes containing phosphine‐nitrogen ligands and their use as catalysts in aminocarbonylation reaction

Aminocarbonylation of aryl halides, homogeneously catalysed by palladium, is an efficient method that can be employed for obtaining amides for pharmaceutical and synthetic applications. In this work, palladium (II) complexes containing P^N ligands were studied as catalysts in the aminocarbonylation of iodobenzene in the presence of diethylamine. Two types of systems were used: a palladium (II) complex formed in situ; and one prepared prior to the catalytic reaction. In general, the palladium complexes studied achieved high conversions in an average reaction time of less than 2 hr, which is less than that for the standard system (Pd (II)/PPh₃) used. The pre‐synthesized complexes were faster than their in situ counterparts, as the latter require an induction time to form the Pd/P^N species. The structure and electronic properties of the ligand P^N can influence both the activity and the selectivity of the reaction, stabilizing the acyl‐palladium intermediates formed in a better manner.     

Preparation and characterization of PEPPSI-palladium N-heterocyclic carbene complexes using benzimidazolium salts catalyzed Suzuki–Miyaura cross coupling reaction and their antitumor and antimicrobial activities

New palladium complexes were efficiently synthesized from the reaction of benzimidazolium salts 2a–e, potassium carbonate (K₂CO₃) and palladium chloride (PdCl₂) in pyridine (for 3a–e). The catalytic activity of these complexes in a catalytic system including palladium complexes and K₂CO₃ in DMF-H₂O was evaluated in Suzuki–Miyaura cross-coupling reactions of aryl bromides and chlorides with phenylboronic acid. Our novel complexes show excellent catalytic activities with high turnover numbers (TON) and high turnover frequencies (TOF) (e.g. for the Suzuki–Miyaura reaction: TON up to 370 and TOF up to 123.3?h^?1). Both benzimidazolium salts 2a–e and complexes 3 have been characterized using spectroscopic data and elemental analysis. The antimicrobial activity of the N-heterocyclic carbene palladium complexes 3a–e varies with the nature of the ligands. Also, the IC₅₀ values of both, complexes (3a–e) and benzimidazoles 2a–e, have been determined. In addition, the new palladium complexes were screened for their antitumor activity. Complexes 3e and 3d exhibited the highest antitumor effect with IC₅₀ values 6.85?μg/mL against MCF-7 and 10.75?μg/mL against T47D, respectively.     

Rigid pyridyl substituted NHC ligands,their Pd(0) complexes and their application in selective transfer semihydrogenation of alkynes

The synthesis of an air‐stable series of Pd⁰ complexes with unsymmetric bidentate N‐pyridine N‐heterocyclic carbene ligands has been described. The carbenes were generated by synthesis of the silver(I) complexes from the imidazolium salts, followed by transmetallation of the C‐N ligands to obtain the target electron‐rich zerovalent palladium compounds. The bidentate coordination behaviour of the ligands was confirmed by ¹H, ¹³C NMR and X‐ray spectroscopy. The complexes are active precatalysts for the selective transfer semihydrogenation of alkynes to Z‐alkenes, with selectivities up to 99%. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and Characterization of Bis(azido)bis(2‐chloropyridine)palladium(II), Bis(azido)bis(3‐chloropyridine)palladium(II), Bis(azido)bis(quinoline)palladium(II), and the Crystal Structure of Bis(azido)bis(quinoline)palladium(II)

Three new monomeric complexes of palladium(II) azide with 2‐chloropyridine ( 1 ), 3‐chloropyridine ( 2 ), and quinoline ( 3 ), have been synthesized by reaction of palladium nitrate and the respective Lewis‐base with sodium azide in a water/acetone mixture. All three compounds were characterized by IR, Raman, and multinuclear NMR spectroscopy. The composition of the complexes were confirmed by elemental analysis. The spectroscopic investigations confirm terminal azide ligands in trans position. Complex 3 was also characterized by crystallographic methods. Each palladium atom of 3 is surrounded in a distorted square planar fashion by 4 nitrogen atoms. The terminal azide ligands are in trans position.     

Synthesis,structure, and spectroscopy of two benzil-based α-diimine ligands and their palladium(II) complexes

Two α-diimine ligands were prepared in 60–70% yield via p-toluenesulfonic acid-catalyzed condensation reactions from benzil with 4-bromoaniline and with p-anisidine. Palladium(II) complexes were prepared from both ligands in 70–80% yield. X-ray structures were obtained for the ligand prepared from p-anisidine and its palladium(II) complex. A notable feature observed in the former was its unconjugated C–N double bonds, both in the (E)-configuration. The latter structure possessed two molecules of the metal complex in its unit cell, both of which have diimine cores with a degree of conjugation and a nonideal square-planar geometry around palladium caused by the small bite angles (79.61(3) and 79.15(3)°) of the diimine ligands. Solution-phase electronic absorption spectra of the ligands in chloroform have two bands from π→π ^? and n→π ^? transitions at 269–345?nm. Absorption spectra of the complexes in chloroform exhibited bands attributed to ligand-centered transitions that were red-shifted as compared to free ligands. Only the spectrum obtained from a chloroform solution of the palladium(II) complex with the diimine ligand prepared from p-anisidine featured a band at approximately 520?nm, which was assigned to a combination of d _π(Pd)→π ^? and n(Cl)→π ^? transitions.     

Piperazine‐ and DABCO‐bridged dinuclear N‐heterocyclic carbene palladium complexes: synthesis,structure and application to Hiyama coupling reaction

Four dinuclear N ‐heterocyclic carbene (NHC) palladium complexes were prepared by reaction of imidazolinium salts, PdCl₂ and bridging ligands (piperazine and DABCO) in one pot or by direct cleavage of the chloro‐bridged dimeric compounds [Pd(μ ‐Cl)(Cl)(NHC)]₂ with bridging ligands. All of the complexes were fully characterized using ¹H NMR, ¹³C NMR, high‐resolution mass and infrared spectroscopies, elemental analysis and single‐crystal X‐ray diffraction. The catalytic activities of the obtained palladium catalysts towards Hiyama coupling of aryl chlorides with phenyltrimethoxysilane were investigated and the results showed that the dinuclear palladium complexes were considerably active for the coupling reaction. Copyright © 2016 John Wiley & Sons, Ltd.     

A dynamic tricopper double helicate

The reaction between 8-aminoquinoline, 1,10-phenantholine-2,9-dicarbaldehyde, and copper(I) tetrafluoroborate gave a quantitative yield of a tricopper double helicate. The presence of dynamic covalent imine (C=N) bonds allowed this assembly to participate in two reactions not previously known in helicate chemistry: 1) It could be prepared through subcomponent substitution from a dicopper double helicate that contained aniline residues. An electron-poor aniline was quantitatively displaced; a more electron-rich aniline competed effectively with the aminoquinoline, setting up an equilibrium between dicopper and tricopper helicates that could be displaced towards the tricopper through the addition of further copper(I). 2) Both dicopper and tricopper helicates could be prepared simultaneously from a mixture of phenanthroline dialdehyde, aniline, and aminoquinoline, which contained all possible imine condensation products in equilibrium. Following the addition of copper(I), thermodynamic equilibration on both covalent and coordinative levels eliminated all partially-formed and mixed imine ligands from the mixture, leaving the helicates as exclusive products.     

Dinuclear PdII complexes bearing mixed NHC/Py/PPh3 donor set ligands: Catalytic applications and electrochemical investigations

Bimetallic palladium(II) complexes containing classical NHC donor ligands are becoming increasingly popular owing to their various catalytic applications. However, examples of the aforementioned complexes with mixed NHC/PPh₃ ligands are still rare. Bimetallic palladium(II) complexes possessing these mixed ligands are described starting from a C2-symmetric bis-imidazolium salt containing 4,4′-substituted central biphenyl ring. All the palladium(II) complexes have been tested as precatalysts in α-arylation of oxindole and Suzuki–Miyaura coupling reactions. The complex composed of mixed NHC/PPh₃ donor ligands shows superior catalytic activity compared with the corresponding PEPPSI type complexes when applied in α-arylation of oxindole. The dinuclear complexes display better activity compared with the mononuclear complexes. The preliminary electrochemical measurements show the facile oxidation of Pd^II in the presence of combined NHC/PPh₃ ligands compared with a combination of NHC/Py ligands.     

Übergangsmetallkomplexe mit Nitronylnitroxylradikalliganden

The stable free radicals, the isomers of 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide with the 2-substituentR (R=para-,meta-,ortho-pyridyl), have been prepared and used as ligands in copper(II), palladium(II) and platinum(II) complexes. The magnetic moments and the EPR spectra of the complexes and the free radicals have been investigated. Most of the complexes show a considerable intramolecular interaction between the radicalic groups of the ligands. No intramolecular interaction was found, however, between the transition metal ions and the unpaired electrons of the ligands. But by analysis of the EPR spectra in the solid state there was found in some cases an intermolecular interaction between the metal ion [copper(II)] and the unpaired electrons of the ligands.

     

A Remarkable Solvent Effect on the Nuclearity of Neutral Titanium(IV)‐Based Helicate Assemblies

The spontaneous self‐assembly of a neutral circular trinuclear Ti^IV‐based helicate is described through the reaction of titanium(IV) isopropoxide with a rationally designed tetraphenolic ligand. The trimeric ring helicate was obtained after diffusion of n‐pentane into a solution with dichloromethane. The circular helicate has been characterized by using single‐crystal X‐ray diffraction study, ¹³C CP‐MAS NMR and ¹H NMR DOSY solution spectroscopic, and positive electrospray ionization mass‐spectrometric analysis. These analytical data were compared with those obtained from a previously reported double‐stranded helicate that crystallizes in toluene. The trimeric ring was unstable in a pure solution with dichloromethane and transformed into the double‐stranded helicate. Thermodynamic analysis by means of the PACHA software revealed that formation of the double‐stranded helicates was characterized by ΔH(toluene)=?30 kJ mol^?1 and ΔS(toluene)=+357 J K^?1 mol^?1, whereas these values were ΔH(CH₂Cl₂)=?75 kJ mol^?1 and ΔS(CH₂Cl₂)=?37 J K^?1 mol^?1 for the ring helicate. The transformation of the ring helicate into the double‐stranded helicate was a strongly endothermic process characterized by ΔH(CH₂Cl₂)=+127 kJ mol^?1 and ΔH(n‐pentane)=+644 kJ mol^?1 associated with a large positive entropy change ΔS=+1115 J K^?1?mol^?1. Consequently, the instability of the ring helicate in pure dichloromethane was attributed to the rather high dielectric constant and dipole moment of dichloromethane relative to n‐pentane. Suggestions for increasing the stability of the ring helicate are given.