Clamshell structure of Sc(biphenyl) from high resolution photoelectron spectroscopy

Sc(biphenyl) is produced in a laser vaporization molecular beam source and characterized by pulsed field ionization-zero electron kinetic energy (ZEKE) photoelectron spectroscopy and density functional theory. The theory predicts two low-energy isomers: a clamshell structure with Sc binding to both phenyl rings and a half-sandwich structure with Sc binding to single ring. The joint experimental and theoretical study shows that the Sc(biphenyl) complex prefers the clamshell structure with a twelve-fold binding mode. From the free ligand to the clamshell structure of the complex, phenyl rings first rotate to become coplanar, then bend toward the Sc atom, and finally are clamped in place by Sc binding.     

Replacement of N-heterocyclic carbenes by N-heterocyclic silylenes at a Pd(0) center: Experiment and theory

Via NMR-spectroscopy the relative reactivity of N-heterocyclic silylenes (NHSi) and carbenes (NHC) was studied. Reaction of sterically crowded bis-N-heterocyclic Pd(0) carbene complexes with free N-heterocyclic silylenes led to complete displacement of the N-heterocyclic carbene, which is unexpected knowing that usually a silylene is a weaker bound ligand compared to a carbene. High-level DFT calculations on a small model system and the experimentally used complexes confirm the experimental findings and indicate that steric interactions play an important role in the substitution reaction.     

Effect of the eluent composition in gel chromatography of99mTc (Sn)-methylene diphosphonate

Gel chromatography of^99mTc(Sn)-MDP on Sephadex has been performed by using the eluents of different composition and pH. Samples with constant ligand/reductant ratio were eluted by pure physiological saline as well as by saline which contained either the ligand only or both the ligand and the reductant. Their concentrations in the eluent were the same as those used in the preparation of the labeled complex.     

Sandwich double-decker lanthanide(III) "intracavity" complexes based on clamshell-type phthalocyanine ligands: synthesis, spectral, electrochemical, and spectroelectrochemical investigations

Phthalocyanine compounds of novel type based on a bridged bis-ligand, denoted "intracavity" complexes, have been prepared. Complexation of clamshell ligand 1,1'-[benzene-1,2-diylbis(methanediyloxy)]bis[9(10),16(17),23(24)-tri-tert-butylphthalocyanine] ((clam,tBu)Pc(2)H(4), 1) with lanthanide(III) salts [Ln(acac)(3)]?n?H(2)O (Ln = Eu, Dy, Lu; acetylacetonate) led to formation of double-deckers (clam,tBu)Pc(2)Ln (2?a-c). Formation of high molecular weight oligophthalocyanine complexes was demonstrated as well. The presence of an intramolecular covalent bridge affecting the relative arrangement of macrocycles was shown to result in specific physicochemical properties. A combination of UV/Vis/NIR and NMR spectroscopy, MALDI-TOF mass-spectrometry, cyclic voltammetry, and spectroelectrochemistry provided unambiguous characterization of the freshly prepared bis-phthalocyanines, and also revealed intrinsic peculiarities in the structure-property relationship, which were supported by theoretical calculations. Unexpected NMR activity of the paramagnetic dysprosium complex 2?b in the neutral π-radical form was observed and examined as well.     

Carbon monoxide-promoted carbene insertion into the aryl substituent of an N-heterocyclic carbene ligand: Buchner reaction in a ruthenium carbene complex

In the presence of carbon monoxide, ruthenium carbenes give a net insertion/ring expansion (Buchner reaction) into one of the aromatic rings of the N-heterocyclic carbene ligand. In alkene metathesis applications, the N-heterocyclic carbene ligand is both robust and typically inert to reactions with the metal-bound carbene. This unique reaction is completely regioselective. The complexes obtained through ring expansion were fully characterized in the solid state using X-ray crystallography and in solution using NMR and IR spectroscopy.     

A new approach to internal Lewis pairs featuring a phosphenium acid and a pyridine base

A bis(imino)acenaphthene (BIAN) ligand containing a pendant Lewis base has been used as a new framework to support a N-heterocyclic phosphenium cation (NHP). Reactivity studies demonstrate the ability of the ligand to act as a Lewis base, while the phosphorus centre provides a Lewis acidic site, giving new opportunities in NHP chemistry.     

Preparation,structure and catalytic properties of a binuclear Pd(0) complex with bridging silylene ligands

In contrast to the N-heterocyclic carbene (NHC) 1, the homologous N-heterocyclic silylene (NHS) 4 acts as a bridging ligand to Pd(0), giving rise to the dinuclear complex 5 which is catalytically active in Suzuki reactions.     

Ruthenium(II)-NHC-catalyzed (NHC = perhydrobenzimidazol-2-ylidene) alkylation of amines using the hydrogen borrowing methodology under solvent-free conditions

Transition Metal Chemistry - New ruthenium(II) complexes with N-heterocyclic carbene ligand were synthesized by transmetalation reactions between silver(I) N-heterocyclic carbene complexes and...     

Luminescent square-planar platinum(II) complexes with tridentate 3-bis(2-pyridylimino)isoindoline and monodentate N-heterocyclic ligands

A series of platinum(II) complexes with 1,3-bis(2-pyridylimino)isoindoline (BPI) derivatives were prepared by substitution of the coordinated Cl in the precursor complex Pt(BPI)Cl with a N-heterocyclic ligand such as pyridine, phthalazine or phenanthridine. These complexes display orange to red luminescence in fluid dichloromethane solutions and in the solid states at room temperature. The photophysical properties were tuned by introducing electron-withdrawing -NO(2) or electron-donating -NH(2) to the BPI ligand. The DFT computational studies suggest that the emission in the N-heterocyclic ligand substituted platinum(II) complexes originates mainly from the (3)[π→π*(BPI)] (3)IL triplet excited state, mixed with some (3)[dπ(Pt)→π*(BPI)] (3)MLCT character. Compared with the precursor Pt(BPI)Cl, both the low-energy absorption and the emission in the N-heterocyclic ligand substituted platinum(II) complexes exhibits a distinct blue-shift due to an obviously enhanced contribution from the (3)IL state and a reduced (3)MLCT character.     

Designing the "search pathway" in the development of a new class of highly efficient stereoselective hydrosilylation catalysts

The direct coupling of oxazolines and N-heterocyclic carbenes leads to chelating C,N ancillary ligands for asymmetric catalysis that combine both an "anchor" unit and a stereodirecting element. Reacting various N-substituted imidazoles with 2-bromo-4(S)-tert-butyl- and 2-bromo-4(S)-isopropyloxazoline gave the imidazolium precursors of the stereodirecting ancillary ligands. A library of ten different ligand precursors was obtained by using this simple procedure (65-97 % yield). These protioligands were metalated in a subsequent step by reaction with [{Rh(mu-OtBu)(nbd)}2] (nbd=norbornadiene), generated in situ from KOtBu and [{RhCl(nbd)}2] giving the corresponding N-heterocyclic carbene complexes [RhBr(nbd)(oxazolinyl-carbene)] 4 a-j in good yields. X-ray diffraction studies of two of the rhodium complexes, 4 d and 4 j, established a distorted square-pyramidal coordination geometry with the bromo ligand occupying the apical position. The rhodium-carbene bond length was found to be 2.070(4) A (4 d) and 2.012(3) A (4 j). Complexes 4 a-j were treated with AgBF4 in dichloromethane, giving the active cationic square-planar catalysts for the hydrosilylation of ketones. As a reference reaction for the catalyst optimisation, the hydrosilylation of acetophenone with diphenylsilane was studied and the system optimised with respect to the counterion (BF(4) (-)), solvent (THF) and the silane reducing agent (diphenylsilane). The reaction product (1-phenylethanol) was obtained with the highest enantiomeric excess (ee) by carrying out the reaction at -60 degrees C, whilst the enantioselectivity drops upon going both to lower and higher temperatures. The observation that the temperature dependence of the ee values goes through a maximum indicated a change in the rate-determining step as the temperature is varied. The determination of the initial reaction rate in the hydrosilylation of acetophenone upon varying the catalyst (4 d) and substrate concentrations at -55 degrees C established a rate law for the initial conversion which is first-order in both substrates as well as the catalyst (Vi = k[4][PhCOMe][Ph2SiH2]). The catalytic system derived from complex 4 d was found to afford high yields and good enantioselectivities in the reduction of various aryl alkyl ketones (acetophenone: 92 % isolated yield and 90 % ee, 2-naphtyl methyl ketone: 99 % yield, 91 % ee). The selectivity for the reduction of prochiral dialkyl ketones is comparable or even superior to the best previously reported for prochiral nonaromatic ketones; whereas cyclopropyl methyl ketone is hydrosilylated with an enantioselectivity of 81 % ee, the increase of the steric demand of one of the alkyl groups leads to improved ee's, reaching 95 % ee in the case of tert-butyl methyl ketone. Linear chain n-alkyl methyl ketones, which are particularly challenging substrates, are reduced in good asymmetric induction, such as 2-octanone (79 % ee) and even 2-butanone (65 % ee).     

Comparative experimental and EXAFS studies in the Mizoroki-Heck reaction with heteroatom-functionalised N-heterocyclic carbene palladium catalysts

A study on the Mizoroki-Heck coupling of selected aryl bromides with acrylates catalysed by a series of Pd complexes of bidentate pyridyl-, picolyl-, diphenylphosphinoethyl- and diphenylphosphinomethyl-functionalised N-heterocyclic carbene (NHC) is reported. The observed activity is dependent on the type of solvent and base used and the nature of the "classical" donors of the mixed-donor bidentate ligand and its bite angle. A mechanistic model is presented for the pyridine-functionalised NHC complexes based on an in situ EXAFS study under dilute catalyst conditions (2 mM Pd). The model involves pre-dissociation of the pyridine functionality and oxidative addition of ArBr in the early stages of the reaction, as well as formation of monomeric and dimeric Pd species at the time of substrate conversion.     

Conjugate reduction of alpha,beta-unsaturated carbonyl compounds catalyzed by a copper carbene complex

[reaction: see text] An N-heterocyclic carbene copper chloride (NHC-CuCl) complex (2) has been prepared and used to catalyze the conjugate reduction of alpha,beta-unsaturated carbonyl compounds. The combination of catalytic amounts of 2 and NaOt-Bu with poly(methylhydrosiloxane) (PMHS) as the stoichiometric reductant generates an active catalyst for the 1,4-reduction of tri- and tetrasubstituted alpha,beta-unsaturated esters and cyclic enones. The active catalytic species can also be generated in situ from 1,3-bis(2,6-di-isopropylphenyl)-imidazolium chloride (1) CuCl(2).2H(2)O in the presence of NaOt-Bu and PMHS.     

Catalytic and coordination facets of single-site non-metallocene organometallic catalysts with N-heterocyclic scaffolds employed in olefin polymerization

This review discusses the principles underlying mononucleating N-heterocyclic ligand design, selectivity of metal centers, preparation of organometallic catalysts with a N-heterocyclic backbone, and their catalytic activity in olefin oligo/polymerization. A vast number of N-heterocyclic organometallic compounds have been applied for the polymerization on account of their modest cost, low toxicity, and the large availability of transition metals in stable and variable oxidation states, which makes them versatile precursors for these reactions. The main points of focus in this review are the key advances made over more the past 25 years in the design and development of non-metallocene single-site organometallic catalysts bearing different N-heterocyclic scaffolds as a backbone. These catalysts are applied as precursors for the transformation of ethylene, higher α-olefins, and cyclic olefins into oligo/polymers. Emphasis is placed on the architecture of ligand peripheries for tuning the formed polymer properties and the consequences on product formation of different alkyl or aryl substituents directly attached to the metal center in a N-heterocyclic ligand system.     

Intramolecular "hydroiminiumation and -amidiniumation" of alkenes: a convenient, flexible, and scalable route to cyclic iminium and imidazolinium salts

Addition of a stoichiometric amount of HCl to alkenylaldimines, -formamidines, and -amidines results in the protonation of the sp2-nitrogen atom. The resulting alkenylaldiminium, -formamidinium, and -amidinium salts can be isolated and fully characterized, including single-crystal X-ray diffraction studies. Heating solutions of these salts induces ring closure cleanly and regioselectively via formal "exo" addition of the nitrogen-hydrogen bond to the pendent carbon-carbon double bond, affording the corresponding cyclic aldiminium, dihydroisoquinolinium, and imidazolinium salts. Of special interest, novel 4,4-disubstituted imidazolinium salts are accessible via this synthetic route. Similarly, addition of phosgene to alkenyl ureas and alkenyl amides, followed by gentle heating, cleanly affords C-chloro imidazolinium, and cyclic C-chloro iminium salts, respectively. Treatment of the latter with tetrakis(triphenylphosphine)palladium allows for the preparation of the first transition-metal complex bearing a cyclic arylaminocarbene as ligand. Deuterium labeling experiments suggest that the mechanism of the hydroiminiumation and -amidiniumation reactions involves an intramolecular proton transfer to the double bond in the rate-determining step. This novel synthetic methodology gives access to a variety of N-heterocyclic carbene (NHC) and cyclic alkyl- and arylaminocarbene (CAAC) precursors.     

Diamidonaphthalene-supported pnictogenium cations: synthesis of an N-heterocyclic stibenium cation by a novel protonation route

A 1,8-bis(alkylamido)naphthalene framework has been applied to the construction of N-heterocyclic arsenium and stibenium cations; a novel synthetic route, involving protonation of an ancillary amido ligand, was used to generate the base-stabilized stibenium cation.     

Extending the series: synthesis and characterization of a dicationic N-heterocyclic selenium carbene analogue

The room-temperature reaction between the Dipp2DAB ligand, SnCl2, and SeCl4 results in the quantitative formation of a dicationic N-heterocyclic "carbenoid". This represents the first example of a chalcogenium dication that mimics the ubiquitous Arduengo-type carbenes; however, the electronic structure is significantly different.     

Probing the potential of N-heterocyclic carbenes in molecular electronics: redox-active metal centers interlinked by a rigid ditopic carbene ligand

Bimetallic homonuclear iron(II) and ruthenium(II) N-heterocyclic carbene complexes have been synthesized and crystallographically analyzed. As a spacer ligand for interconnecting the two redox-active metal centers, a ditopic carbene ligand has been used that comprises two carbene sites annelated to benzene. Detailed electrochemical and spectroelectrochemical analyses of the bimetallic systems revealed that despite the potentially pi-delocalized nature of the ditopic ligand, the iron centers are only moderately coupled. In the ruthenium complexes, the intermetallic interactions are very weak and the centers are electrochemically nearly independent. A model is proposed for rationalizing these observations which is based on (I) relatively weak charge delocalization in the spacer ligand and (II) on electrostatic factors governing the metal-carbene bond.     

Highly convenient amine-free sonogashira coupling in air in a polar mixed aqueous medium by trans- and cis-[(NHC)2PdX2] (X=Cl, Br) complexes of N/O-functionalized N-heterocyclic carbenes

Two new trans- and cis-[(NHC)(2)PdX(2)] (X=Cl, Br) complexes of N/O-functionalized N-heterocyclic carbenes employed in a highly convenient amine-free Sonogashira cross-coupling reaction in air in a polar mixed aqueous medium are reported. Specifically, the trans-[{1-benzyl-3-(3,3-dimethyl-2-oxobutyl)imidazol-2-ylidene}(2)PdBr(2)] (3) and cis-[{1-benzyl-3-(N-tert-butylacetamido)imidazol-2-ylidene}(2)PdCl(2)] (4) complexes effectively catalyzed the Sonogashira cross-coupling reaction of aryl iodides with substituted acetylenes in air in a mixed solvent (DMF/H(2)O, 3:1 v/v) under amine-free conditions. Interestingly, these trans- and cis-[(NHC)(2)PdX(2)] (X=Cl, Br) complexes, with two N-heterocyclic carbene ligands, exhibited superior activity compared with the now popular PEPPSI (pyridine enhanced precatalyst preparation, stabilization and initiation)-themed analogues, trans-[(NHC)Pd(pyridine)X(2)] (X=Cl, Br), 3 a and 4 a, with one N-heterocyclic carbene ligand and a "throw away" pyridine ligand in a trans disposition to each other. The higher activities of 3 and 4 compared with PEPPSI analogues 3 a and 4 a are attributed to more-electron-rich metal centers, as revealed by DFT studies, in the former complexes and is in concurrence with a more electron-rich metal center being effective in facilitating the oxidative addition of aryl halide, often a rate-determining step in palladium-mediated cross-coupling reactions. Complexes 3 and 4 were prepared from the corresponding silver analogues by transmetalation with [(cod)PdCl(2)], whereas the corresponding PEPPSI analogues 3 a and 4 a were obtained directly from the imidazolium halide salts by reaction with PdCl(2) in pyridine in the presence of K(2)CO(3) as base.     

What difference one double bond makes: Electronic structure of saturated and unsaturated n-heterocyclic carbene ligands in Grubbs 2nd generation-type catalysts

N-heterocyclic carbene (NHC) ligands are a versatile and useful class of ligands that have enjoyed much success over the past few decades in organometallic chemistry. This fact is exemplified most convincingly in Grubbs 2nd generation olefin metathesis catalysts. We explore the electronic impact of the NHC-ligand by decoupling electronic and steric effects through simplified model N-heterocyclic carbenes. Saturated and unsaturated N-heterocyclic carbene ligands give rise to fundamentally different frontier orbitals in these catalysts, suggesting a need to classify them as two electronically distinct ligand classes.     

Function of an N-heterocyclic carbene ligand based on concept of chiral mimetic

Function of a new N-heterocyclic carbene ligand based on the concept of a chiral mimetic is described. With (4R,5R)-4,5-diphenyl-1,3-dialkyl-4,5-dihydro-3H-imidazol-1-ium tetrafluoroborates as N-heterocyclic carbene precursors, Rh-catalyzed enantioselective arylation (up to 27% ee) of aromatic aldehydes with arylboronic acids and Pd-catalyzed enantioselective intramolecular alpha-arylation (up to 66% ee) of N-(2-bromoaryl)-N-alkyl-2-arylpropanamides are investigated.