Problematic ArF-Alkynyl Coupling with Fluorinated Aryls. From Partial Success with Alkynyl Stannanes to Efficient Solutions via Mechanistic Understanding of the Hidden Complexity.

The synthesis of aryl-alkynyl compounds is usually achieved via Sonogashira catalysis, but this is inefficient for fluorinated aryls. An alternative method reported by Shirakawa and Hiyama, using alkynylstannanes and hemilabile PN ligands, works apparently fine for conventional aryls, but it is also poor for fluorinated aryls. The revision of the unusual literature cycle reveals the existence and nature of unreported byproducts and uncovers coexisting cycles and other aspects that explain the reasons for the conflict. This knowledge provides a full understanding of the real complexity of these aryl/alkynylstannane systems and the deviations of their evolution from that of a classic Stille process, providing the clues to design several very efficient alternatives for the catalytic synthesis of the desired ArF-alkynyl compounds in almost quantitative yield. The same protocols are also very efficient for the catalytic synthesis of alkynyl-alkynyl' hetero- and homocoupling.

Expanding the Concepts Trans Influence and Back-Donation: Hybrid and Side Donations in [Cp*MIII(L)XY] (M = Rh, Ir) Complexes with CO, CN-, and CNR Ligands. A Window to Cis Influence.

Analysis of the bonding contributions in molecules [MIIICp*(L)XY] (M = Rh, Ir; Cp* = C5Me5; L = CO, CN-, CNR) has uncovered a rich variety of types of interaction that seem to have escaped detection so far, in spite of the continuous popularity of cyclopentadienyl transition-metal complexes since the 1970s. At variance with the M-C≡O bond in square-planar systems, which shows typical metal-to-CO π-back-donation, the nonorthogonal arrangement of the Cp* plane and Rh-C≡O fragment and the pseudooctahedral geometry lead to the observation of many direct lateral donations from other ligands that do not involve the metal orbitals, and we name side donations, for instance, Cp* → π*(CO), Cl → π*(CO), and F → π*(CO). Hybrid donations partially involving the metal, M-Caryl → π*(CO), are also observed. The summation of multiple contributions other than back-donation can easily account for about 20% of the electron donation to the π*(C≡O) orbitals.

Ranking Ligands by Their Ability to Ease (C6F5)2NiIIL → Ni0L + (C6F5)2 Coupling versus Hydrolysis: Outstanding Activity of PEWO Ligands.

The NiII literature complex cis-[Ni(C6F5)2(THF)2] is a synthon of cis-Ni(C6F5)2 that allows us to establish a protocol to measure and compare the ligand effect on the NiII → Ni0 reductive elimination step (coupling), often critical in catalytic processes. Several ligands of different types were submitted to this Ni-meter comparison: bipyridines, chelating diphosphines, monodentate phosphines, PR2(biaryl) phosphines, and PEWO ligands (phosphines with one potentially chelate electron-withdrawing olefin). Extremely different C6F5-C6F5 coupling rates, ranging from totally inactive (producing stable complexes at room temperature) to those inducing almost instantaneous coupling at 25 °C, were found for the different ligands tested. The PR2(biaryl) ligands, very efficient for coupling in Pd, are slow and inefficient in Ni, and the reason for this difference is examined. In contrast, PEWO type ligands are amazingly efficient and provide the lowest coupling barriers ever observed for NiII complexes; they yield up to 96% C6F5-C6F5 coupling in 5 min at 25 °C (the rest is C6F5H) and 100% coupling with no hydrolysis in 8 h at -22 to -53 °C.

Fluorinated vs Nonfluorinated PR2(biaryl) Ligands and Their [AuCl(L)] Complexes: Synthesis, X-ray Structures, and Computational Study of Weak Interactions. Bond, No Bond, and Beyond.

Six fluorinated PR2(biaryl) phosphines, Ln, with R = Ph, Cy and biaryl = C6H4-C6F5, C6F4-C6H5, C6F4-C6F5, have been prepared. Their [AuCl(Ln)] complexes and H congeners with PhJohnPhos or CyJohnPhos have been studied in order to examine the interactions that bring the distal aryl close to the Au-Cl bond region. X-ray, DFT structure optimization, QTAIM, and NCI methods allow for some understanding of the forces involved. The "no bond" noncovalent distal-aryl/Au-Cl weak interactions are produced at forced short distances achieved under intramolecular structural ligand pressure. Enhanced vdW distal-aryl/Au interactions at "no bond" distances shorter than the sum of Au and C vdW radii and weaker distal-aryl/Cl interactions at "no bond" distances beyond the sum of Cl and C vdW radii counterbalance the unfavorable structural distortion of the free ligand, providing some extra stability of the molecule on the order of 2-10 kcal mol-1. The F substituents in the distal aryl induce shorter aryl distances to the Au-Cl zone, pointing overall to stronger π-aryl polarization as being mainly responsible for the NCIs with gold. The interactions in the C···Cl zone, where the distances are larger than the sum of vdW radii, contribute only about 5%, according to energy estimations using NBOs.

Striking Increase in Hole Mobility upon Metal Coordination to Triphenylene Schiff Base Semiconducting Multicolumnar Mesophases.

This paper reports the synthesis, liquid-crystal behavior, and charge-transport properties in the mesophase of triphenylene Schiff bases and their copper(II), nickel(II), and oxovanadium(IV) complexes. The thermal and electronic properties of the Schiff bases are modulated by coordination to the corresponding metal moieties, which have the ability to self-assemble into linear structures and help the alignment of the triphenylene columns. This produces two kinds of electronically nonconnected columnar regions, one purely organic and one more inorganic. The most remarkable effect is a striking charge mobility enhancement in the metal-containing mesophases, due to the contribution of the more inorganic columns: in comparison to values of hole mobility along the columnar stacking for the purely organic columnar mesophases, on the order of 10-7 cm2 V-1 s-1, these values jump to 1-10 cm2 V-1 s-1 in these hybrid inorganic/organic columnar materials.

RhI Ar/AuI Ar' Transmetalation: A Case of Group Exchange Pivoting on the Formation of M-M' Bonds through Oxidative Insertion.

By combining kinetic experiments, theoretical calculations, and microkinetic modeling, we show that Pf/Rf (C6 F5 /C6 Cl2 F3 ) exchange between [AuPf(AsPh3 )] and trans-[RhRf(CO)(AsPh3 )2 ] does not occur by typical concerted Pf/Rf transmetalation via electron-deficient double bridges. Instead, it involves asymmetric oxidative insertion of the RhI complex into the (Ph3 As)Au-Pf bond to produce a [(Ph3 As)Au-RhPfRf(CO)(AsPh3 )2 ] intermediate, followed by isomerization and reductive elimination of [AuRf(AsPh3 )]. Interesting differences were found between the LAu-Ar asymmetric oxidative insertion and the classical oxidative addition process of H2 to Vaska complexes.

Comparing Protonolysis and Transmetalation Reactions: Microcalorimetric Studies of C-AuI Bonds in [AuRL] Complexes.

The protonolysis of C-Au bonds in [AuRL] organometallic complexes has been studied by calorimetry for 12 R groups. The experimental data have been combined with density functional theory calculations to obtain bond dissociation energy (BDE) values. The C-Au BDE values show a good correlation with the corresponding isolobal C-H BDE values. The heat released in the protonolysis of [AuRL] has also been measured for R = Ph and L = P(OPh)3, PPh3, PMe3, PCy3, and IPr, and these values strongly depend on the trans influence of L because of the mutual destabilization of the L-Au and Au-C bonds. The enthalpies of the transmetalation reaction [AuR(PPh3)] + SnIBu3 → [AuI(PPh3)] + SnRBu3 for seven R groups have been measured and compared with those of the corresponding [AuR(PPh3)] protonolysis.

Triphenylene-Imidazolium Salts and Their NHC Metal Complexes, Materials with Segregated Multicolumnar Mesophases.

Two imidazolium salts containing one or two pentadodecyloxytriphenylene units linked through a hexyloxy chain and Br-, [AuBr mCl4- m]-, or [PtBr mCl4- m]2- ( m = 0-3) as counterion have been prepared. Reaction of the imidazolium bromides with M2O (M = Cu, Ag), or carbene transmetalation from the silver product, leads to N-heterocyclic carbene complexes [MX(NHC)] (M = Cu, X = Br; M = Au, X = Cl, C≡CPh), [Ag(NHC)2][AgBr2], and [PtCl2(NHC)2], with NHC bearing one or two triphenylene fragments. Except for the gold derivatives and one Cu complex, the rest of them behave as liquid crystals organized in columnar mesophases (rectangular c2 mm or p2mg or hexagonal p6mm symmetries) with melting points in the range 30 to 60 °C and clearing points in the range 57-112 °C. The mesophase structures were determined by small-angle X-ray scattering. Structural studies and models point to nanosegregation of triphenylene columns and imidazolium/metal carbene moieties, separated by alkoxy chains, leading to multicolumnar systems. The compounds display emission spectra related to the triphenylene core in solution, in the mesophase, in the isotropic liquid, and in the solid state.

In Situ Generation of ArCu from CuF2 Makes Coupling of Bulky Aryl Silanes Feasible and Highly Efficient.

A bimetallic system of Pd/CuF2, catalytic in Pd and stoichiometric in Cu, is very efficient and selective for the coupling of fairly hindered aryl silanes with aryl, anisyl, phenylaldehyde, p-cyanophenyl, p-nitrophenyl, or pyridyl iodides of conventional size. The reaction involves the activation of the silane by Cu(II), followed by disproportionation and transmetalation from the Cu(I)(aryl) to Pd(II), upon which coupling takes place. Cu(III) formed during disproportionation is reduced to Cu(I)(aryl) by excess aryl silane, so that the CuF2 system is fully converted into Cu(I)(aryl) and used in the coupling. Moreover, no extra source of fluoride is needed. Interesting size selectivity towards coupling is found in competitive reactions of hindered aryl silanes. Easily accessible [PdCl2 (IDM)(AsPh3)] (IDM = 1,3-dimethylimidazol-2-ylidene) is by far the best catalyst, and the isolated products are essentially free from As or Pd (<1 ppm). The mechanistic aspects of the process have been experimentally examined and discussed.

Promoting Difficult Carbon-Carbon Couplings: Which Ligand Does Best?

A Pd complex, cis-[Pd(C6 F5 )2 (THF)2 ] (1), is proposed as a useful touchstone for direct and simple experimental measurement of the relative ability of ancillary ligands to induce C-C coupling. Interestingly, 1 is also a good alternative to other precatalysts used to produce Pd0 L. Complex 1 ranks the coupling ability of some popular ligands in the order Pt Bu3 >o-TolPEWO-F≈tBuXPhos>P(C6 F5 )3 ≈PhPEWO-F>P(o-Tol)3 ≈THF≈tBuBrettPhos≫Xantphos≈PhPEWO-H≫PPh3 according to their initial coupling rates, whereas their efficiency, depending on competitive hydrolysis, is ranked tBuXPhos≈Pt Bu3 ≈o-TolPEWO-F>PhPEWO-F>P(C6 F5 )3 ≫tBuBrettPhos>THF≈P(o-Tol)3 >Xantphos>PhPEWO-H≫PPh3 . This "meter" also detects some other possible virtues or complications of ligands such as tBuXPhos or tBuBrettPhos.

Diamine-catalyzed addition of ZnEt2 to PhC(O)CF3 : two mechanisms and autocatalytic asymmetric enhancement.

NMR spectroscopic studies of the catalytic addition reaction of ZnEt2 to PhC(O)CF3 in the presence of three very efficient catalysts [TMEDA, tBuBOX, and L; where L is a chiral diamine synthesized from optically pure (R,R)-1,2-diphenylethylenediamine and (S)-2,2'-bis-(bromomethyl)-1,1'-binaphthalene] reveal large differences in their behavior. For the ligands TMEDA and tBuBOX, the catalysis shows no unusual features and proceeds via [(NN)Zn(Et){OC(CF3 )(Et)Ph}]. For NNL, the observation of autocatalytic asymmetric enhancement during the catalysis, and unusual inverse concentration dependence on the reaction rate, indicate the participation of an additional novel catalytic cycle that goes through a dinuclear intermediate containing one ZnEt2 and one ZnEt fragment connected by NN and OR bridges. Interestingly, the (19) F NMR signals of the main product of the reaction ([Zn(Et){OC*(CF3 )(Et)Ph}]2 ) allowed us to assess the enantioselectivity of the processes in situ without the assistance of chiral chromatography.

H-Bonded adducts of [2,4,6-{(C10H21O)3C6H2NH}3C3N3] with [LnM{PPh2(C6H4CO2H)}] displaying columnar mesophases at room temperature.

Displacement of a labile ligand from appropriate precursor complexes by 2- or 4-PPh2C6H4COOH yields neutral gold(I) and gold(III) [AuXn(PPh2C6H4COOH)] (n = 1, X = Cl; n = 3, X = C6F5), cationic gold(I) [Au(PPh2C6H4COOH)2](CF3SO3), and neutral chromium(0) [Cr(CO)5(PPh2C6H4COOH)] metallo-organic acids. [AuCl(4-PPh2C6H4COOH)], [Au(C6F5)3(4-PPh2C6H4COOH)], and [Cr(CO)5(2-PPh2C6H4COOH)] have dimeric structures with typical carboxylic H-bond bridges, whereas [Au(C6F5)3(2-PPh2C6H4COOH)] gives a monomeric species with the carboxylic acid H bonded to cocrystallized solvent molecules. All gold-containing acids are emissive at 77 K in the range 404-520 nm and some of them also at 298 K with emission maxima from 441 to 485 nm. Reaction of these acid metal complexes with the triazine mesogen 2,4,6-{(C10H21O)3C6H2NH}3C3N3 affords some new hydrogen-bonded gold(I) and chromium(0) supramolecular adducts, but the related gold(III) complexes do not form adducts. The 4-diphenylphosphinobenzoic adducts display a columnar hexagonal mesophase (Colhex) at room temperature, with a random one-dimensional stacking of the pseudo-discoid triazine-metallo-organic adducts into columns, where the metallo-phosphinoacid fragments act as the fourth branch of the trifold triazine core. The 2-diphenylphosphinobenzoic mixtures do not display mesophases, as they appear in the X-ray studies as mixtures of the triazine and the metallo-phosphinoacid complex. The aggregates are luminescent at 77 K, with emission maxima in the range 419-455 nm.

Alignment of palladium complexes into columnar liquid crystals driven by peripheral triphenylene substituents.

A new triphenylene-imine (ImH) and its ortho-palladated complexes (µ-X)2[Pd2Im2] (X = CH3COO(-), Cl(-), Br(-)), (µ-Cl)(µ-SCnH2n+1)[Pd2Im2] (n = 6, 12), [PdIm(acac)] [PdIm Cl(CNC6H4OC12H25)], [PdImCl(CNC6H3(OC12H25)2)], and [PdImCl(CNC6H2(OC12H25)3)] have been prepared. The free imine ligand is not a liquid crystal, but most ortho-metalated dinuclear palladium complexes and the mononuclear trialkoxyphenyl isocyanide derivative display columnar mesophases at temperatures close to ambient. For the dimeric complexes the mesophase obtained is always columnar rectangular (Colr), with an uncommon structure: the dimeric triphenylene-Pd complex-triphenylene molecules give rise to a triple-column stacking consisting of two columns of stacked triphenylene groups connected to a central column formed by stacking of two ortho-palladated dimeric moieties. For the trialkoxyphenyl isocyanide derivative a related polymer-like arrangement of columns alternating stacking of triphenylenes with stacking of two ortho-palladated dimeric moieties is found. The mesophase structure is columnar oblique (Colob). The free imine and all palladium complexes exhibit fluorescence at room temperature in dichloromethane solution, associated with the triphenylene core.

Room-temperature columnar mesophases in triazine-gold thiolate metal-organic supramolecular aggregates.

Supramolecular mono- and dinuclear liquid-crystalline gold(I) aggregates have been synthesized by means of hydrogen bond interactions of 2,4,6-triarylamino-1,3,5-triazine with thiolate moities of gold metalloacids [Au(PR3)(SC6H4COOH)] or [µ-(binap){Au(SC6H4COOH)}2], in 1:1 and 2:1 molar ratio, respectively. All of the supramolecular aggregates display a stable columnar hexagonal mesophase (Col(h)) at room-temperature. The supramolecular arrangement within the columns consists of the one-dimensional stacking of triazine units, with the core of the attached metallic thioacid fragments acting as the fourth branch. The phosphine-containing moieties of the metallic thioacid protrude out in the aliphatic continuum. These complexes do not show metallophilic interactions, but this strategy appears very promising for the future design of room-temperature LC mesophases containing interacting metallic fragments.

[Pd(Fmes)2(tmeda)]: a case of intermittent C-H···F-C hydrogen-bond interaction in solution.

The X-ray structure of the title compound [Pd(Fmes)2 (tmeda)] (Fmes=2,4,6-tris(trifluoromethyl)phenyl; tmeda=N,N,N',N'-tetramethylethylenediamine) shows the existence of uncommon CH⋅⋅⋅FC hydrogen-bond interactions between methyl groups of the TMEDA ligand and ortho-CF3 groups of the Fmes ligand. The (19) F NMR spectra in CD2 Cl2 at very low temperature (157 K) detect restricted rotation for the two ortho-CF3 groups involved in hydrogen bonding, which might suggest that the hydrogen bond is responsible for this hindrance to rotation. However, a theoretical study of the hydrogen-bond energy shows that it is too weak (about 7 kJ mol(-1) ) to account for the rotational barrier observed (ΔH(≠) =26.8 kJ mol(-1) ), and it is the steric hindrance associated with the puckering of the TMEDA ligand that should be held responsible for most of the rotational barrier. At higher temperatures the rotation becomes fast, which requires that the hydrogen bond is continuously being split up and restored and exists only intermittently, following the pulse of the conformational changes of TMEDA.

Intimate relationship between C-I reductive elimination, aryl scrambling and isomerization processes in Au(III) complexes.

19F NMR monitoring shows that heating trans-[AuIIIRf2I2]- solutions (Rf = C6F3Cl2-3,5) leads to formation of cis-[AuRf2I2]-, [AuRf3I]- and [AuRfI3]-via kinetic competition between isomerization and Rf/I scrambling. The system evolution is driven by the easy Rf-I reductive elimination from [AuRfI3]- (forming also [AuI2]-), which is faster than any of the Rf-Rf couplings from the coexisting species, hindering the commonly desired and thermodynamically preferred C-C coupling. A kinetic model where I- dissociation triggers both isomerization and transmetalation steps is proposed, which fits well the experimental data. DFT calculations support that the lower bond strength of AuIII-I compared to other halides produces a pathway switch that makes C-I coupling kinetically preferred. Consequently, it is better avoided in reactions looking for C-C coupling.

Solid state luminescence of phosphine-EWO ligands with fluorinated chalcone skeletons and their PdX2 complexes: metal-promoted phosphorescence enhancement.

Complexes trans-[PdX2L2] (X = Cl and Br), where L is 1-(PR2),2-(CHCH-C(O)Ph)-C6F4 (R = Ph, Cy, and iPr), display phosphorescent emission in the solid state, whereas due to their substantially lower lifetimes, the free ligands exhibit fluorescent behaviour. Alternatively, structurally identical derivatives with halide replaced by CN- or Pd replaced by Pt are non-emissive. DFT calculations explain this diverse behaviour, showing that the hybridization of orbitals of the MX2 moiety with those of the chalcone fragment of ligands is significant only for the LUMO of the emissive compounds. In other words, in our complexes, only MLMCT processes (LM = Metal-perturbed Ligand-centered orbital) lead to observable luminescence.

Bimetallic catalysis using transition and Group†11 metals: an emerging tool for C-C coupling and other reactions.

Bimetallic catalysis refers to homogeneous processes in which either two transition metals (TM), or one TM and one Group 11 (G11) element (occasionally Hg also), cooperate in a synthetic process (often a C-C coupling) and their actions are connected by a transmetalation step. This is an emerging research area that differs from the isolated or tandem applications of the now classic processes (Stille, Negishi, Suzuki, Hiyama, Heck). Most of the reactions used so far combine Pd with a second metal, often Cu or Au, but syntheses involving very different TM couples (e.g., Cr/Ni in the catalyzed vinylation of aldehydes) have also been developed. Further development of the topic will soon demand a good knowledge of the mechanisms involved in bimetallic catalysis, but this knowledge is very limited for catalytic processes. However, there is much information available, dispersed in the literature, coming from basic research on exchange reactions occurring out of any catalytic cycle, in polynuclear complexes. These are essentially the same processes expected to operate in the heart of the catalytic process. This Review gathers together these two usually isolated topics in order to stimulate synergy between the bimetallic research coming from more basic organometallic studies and the more synthetic organic approaches to this chemistry.

Detection and reactivity of a palladium alkoxycarbene.

A characterful carbene: The high electrophilicity of a genuine palladium alkoxycarbene, obtained by transmetalation, is evident in its reactivity. Nucleophilic attack on two electrophilic centers (red) is observed. Alkyl abstraction and addition to the carbene carbon by different nucleophiles occur. This palladium(II) alkoxycarbene also undergoes comproportionation with palladium(0) to give an unprecedented palladium(I) dimeric carbene (see scheme).

Strong metallophilic interactions in the palladium arylation by gold aryls.

It's the second step that counts: arylation of Pd by Au takes place through transition states and intermediates featuring strong Au⋅⋅⋅Pd metallophilic interactions. However, the aryl transfer from [AuArL] to [PdArClL(2)] is thermodynamically disfavored and will not occur unless an irreversible Ar-Ar coupling from [PdAr(2)L(2)] follows.