Phosphine-NHC-Phosphonium Ylide Pincer Ligand: Complexation with Pd(II) and Unconventional P-Coordination of the Ylide Moiety.

An efficient synthesis of two pincer preligands [Ph2PCH(R)ImCH2CH2CH2PPh3]X2 (R = H, X = OTf; R = Ph, X = BF4) was developed. Subsequent reactions with PdCl2 and an excess of Cs2CO3 led to the formation of highly stable cationic ortho-metalated Pd(II) complexes [(P,C,C,C)Pd]X exhibiting phosphine, NHC, phosphonium ylide, and σ-aryl donor extremities. The protonation of one of the latter complexes with R = H affords the Pd(II) complex [(P,C,C)Pd(MeCN)](OTf)2 bearing an unprecedented nonsymmetrical NHC core pincer scaffold with a 5,6-chelating framework. The overall donor properties of this phosphine-NHC-phosphonium ylide ligand were estimated using the experimental νCN stretching frequency in the corresponding [(P,C,C)Pd(CNtBu](OTf)2 derivative and were shown to be competitive with the related bis(NHC)-phosphonium ylide and phenoxy-NHC-phosphonium ylide pincers. The presence of a phenyl substituent in the bridge between phosphine and NHC moieties in the ortho-metalated complex [(P,C,C,C)Pd](BF4) makes possible the deprotonation of this position using LDA to provide a persistent zwitterionic complex [(P,C,C,C)Pd] featuring a rare P-coordinated phosphonium ylide moiety in addition to a conventional C-coordinated one. The comparison of the 31P and 13C NMR data for these C- and P-bound phosphonium ylide fragments within the same molecule was performed for the first time, and the bonding situation in both cases was studied in detail by QTAIM and ELF topological analyses.

Coordination of Ethylamine on Small Silver Clusters: Structural and Topological (ELF, QTAIM) Analyses.

Amine ligands are expected to drive the organization of metallic centers as well as the chemical reactivity of silver clusters early growing during the very first steps of the synthesis of silver nanoparticles via an organometallic route. Density functional theory (DFT) computational studies have been performed to characterize the structure, the atomic charge distribution, and the planar two-dimensional (2D)/three-dimensional (3D) relative stability of small-size silver clusters (Agn, 2 ≤ n ≤ 7), with or without an ethylamine (EA) ligand coordinated to the Ag clusters. The transition from 2D to 3D structures is shifted from n = 7 to 6 in the presence of one EA coordinating ligand, and it is explained from the analysis of the Ag-N and Ag-Ag bond energies. For fully EA saturated silver clusters (Agn-EAn), the effect on the 2D/3D transition is even more pronounced with a shift between n = 4 and 5. Subsequent electron localization function (ELF) and quantum theory of atoms in molecules (QTAIM) topological analyses allow for the fine characterization of the dative Ag-N and metallic Ag-Ag bonds, both in nature and in strength. Electron transfer from ethylamine to the coordinated silver atoms induces an increase of the polarization of the metallic core.

Direct Access to Palladium(II) Complexes Based on Anionic C,C,C-Phosphonium Ylide Core Pincer Ligand.

The reaction of readily available imidazolium-phosphonium salt [MesIm(CH2)3PPh3](OTf)2 with PdCl2 in the presence of an excess of Cs2CO3 afforded selectively in one step the cationic Pd(II) complex [(C,C,C)Pd(NCMe)](OTf) exhibiting an LX2-type NHC-ylide-aryl C,C,C-pincer ligand via formal triple C-H bond activation. The replacement of labile MeCN in the latter by CNtBu and CO fragments allowed to estimate the overall electronic properties of this phosphonium ylide core pincer scaffold incorporating three different carbon-based donor ends by IR spectroscopy, cyclic voltammetry, and molecular orbital analysis, revealing its significantly higher electron-rich character compared to the structurally close NHC core pincer system with two phosphonium ylide extremities. The pincer complex [(C,C,C)Pd(CO)](OTf) represents a rare example of Pd(II) carbonyl species stable at room temperature and characterized by X-ray diffraction analysis. The treatment of isostructural cationic complexes [(C,C,C)Pd(NCMe)](OTf) and [(C,C,C)Pd(CO)](OTf) with (allyl)MgBr and nBuLi led to the formation of zwitterionic phosphonium organopalladates [(C,C,C)PdBr] and [(C,C,C)Pd(COnBu)], respectively.

Topological Analysis of Hydroxyquinoline Derivatives Interacting with Aluminum Cations or with an Al(111) Surface.

The reactivity of hydroxyquinoline derivatives (native molecules (Hq) and modified species (HqX, X = Br, SO3H, or SO3-)) is investigated either (i) with aluminum cations for the formation of chelates or (ii) with aluminum surfaces for their adsorption properties, in the framework of the dispersion-corrected Density Functional Theory (DFT-D). It is shown that the substituent X has no influence on the complexation to the aluminum cation of the deprotonated active form, i.e., the one exhibiting a phenolate moiety and referred to as q- for the native Hq and qXn- (n = 1 or 2) for its derivatives. The formation energies of the Alq3 and Al(qX)3 complexes, taking values of -60.87 ± 3.10 eV in vacuum and -24.30 ± 0.29 eV in water, are indicative of a strong chelating affinity of the q- and qXn- (n = 1 or 2) anions for the aluminum cations. ELF and QTAIM topological analyses on these complexes evidence that the bonding of the deprotonated species with the Al3+ ion is ionic with a very weak covalence degree. The para or ortho substituent X of the phenolate moiety of the qXn- (n = 1 or 2) derivatives modifies the electronic structure only locally and thus does not influence their O- or N-coordinating properties. The adsorption properties of the latter on an Al(111) surface have also been studied within periodic DFT-D calculations. The adsorbed species are strongly interacting with the Al(111) surface, as shown by the value of the adsorption energy of -3.69 ± 0.21 eV for the most stable geometries. Various adsorption modes of the q- and qXn- (n = 1 or 2) derivatives are characterized on the Al surface, depending on stabilizing or destabilizing interactions with the substituents X. On the basis of QTAIM descriptors, the bonding of the hydroxyquinoline species on the aluminum surface is characterized as ionic with a weak covalent character.

NHC Core Phosphonium Ylide-based Palladium(II) Pincer Complexes: The Second Ylide Extremity Makes the Difference.

The coordinating properties of N-heterocyclic carbene (NHC) (A), phenolate (B), and phosphonium ylide (C) moieties were investigated systematically through the preparation of a family of NHC, phosphonium ylide-based pincer ligands, where the third donor extremity can be either an NHC, a phenolate, or a phosphonium ylide. The overall donor character of such ligands [NHC(AaBbCc)] (a + b + c = 2) was analyzed by comparison of the molecular orbitals (energy and shape), oxidation potentials (Epox), and IR νCO and νCN stretching frequencies of their isostructural pincer Pd(II) complexes [NHC(AaBbCc)PdL][OTf] (L = NCCH3, CO, or CNtBu). The three categories of pincer complexes based on phosphonium ylides were easily obtained by acidic treatment of their highly stable ortho-metalated Pd(II) precursors prepared in a single step from readily available N-phosphonio-substituted imidazolium salts. Analysis of IR data indicated that NHC and phenolate ligands have a similar donor character but which remains lower than that of the phosphonium ylide. The impact on catalytic performance of the incorporation of a second strongly donating phosphonium ylide into the ligand architecture was illustrated in the Pd-catalyzed allylation of aldehydes.

Topological Analysis of Ag-Ag and Ag-N Interactions in Silver Amidinate Precursor Complexes of Silver Nanoparticles.

A series of silver amidinate complexes has been studied both experimentally and theoretically, in order to investigate the role of the precursor complex in the control of the synthesis of silver nanoparticles via an organometallic route. The replacement of the methyl substituent of the central carbon atom of the amidinate anion by a n-butyl group allows for the crystallization of the tetranuclear silver amidinate complex 3 instead of a mixture of di- and trinuclear silver amidinate complexes 1 and 2, as obtained with a methyl substituent. The relative stabilities and dissociation schemes of various isomeric arrangements of silver atoms in 3 are investigated at the computational DFT level of calculation, depending on the substituents of the amidinate ligand. The tetranuclear silver amidinate complex 4, exhibiting a diamondlike arrangement of the four silver atoms, is also considered. Ag-N bonds and argentophilic Ag-Ag interactions are finely characterized using ELF and QTAIM topological analyses and compared over the series of the related di-, tri-, and tetranuclear silver amidinate complexes 1-4. In contrast to the Ag-N dative bonds very similar over the series, argentophilic Ag-Ag interactions of various strengths and covalence degree are characterized for complexes 1-4. This gives insight into the role of the amidinate substituents on the nuclearity and intramolecular chemical bonding of the silver amidinate precursors, required for the synthesis of dedicated AgNPs with chemically well defined surfaces.

Classification of the Electronic Properties of Chelating Ligands in cis-[LL'Rh(CO)2] Complexes.

By analogy to the Tolman electronic parameter, a ligand electronic parameter, referred to as L2EP, is introduced here for estimating the donating ability of chelating ligands, featuring two coordinating extremities. It is based on the average of the computed infrared stretching frequencies of CO in a series of isostructural rhodium(I)-dicarbonyl complexes, that is linearly correlated to the number x of N-heterocyclic carbene coordinating ends (x = 0, 1, or 2). The L2EP values allow the design of an unified scale for the classification of the electron donation of chelating ligands, based on an ortho-phenylene bridge substituted by two coordinating extremities, which may have a different donating character. Strengths and limitations of the L2EP scale are illustrated for a large diversity of bidentate chelating ligands with coordinating ends ranging from extremely electron-rich phosphonium yldiides to extremely electron-poor amidiniophosphonites.

Towards Magnetic Carbo-meric Molecular Materials.

Numerous studies have underlined the putative diradical character of π-conjugated molecules that can be described by closed-shell Lewis structures, for instance, p-dimethylene p-n phenylenes, or long polyacenes. In the latter compounds, the only way to save the aromaticity of the six-membered rings is to give up the Lewis electron pairing in the singlet biradical ground state. The present work considers the possibility of doing the same by using the basic C2 units of carbo-meric architectures. A series of acyclic and cyclic carbo-meric architectures is studied by using UB3LYP DFT broken-symmetry calculations, including spin decontaminations and subsequent geometry optimization of the singlet diradical. The C2 units are shown to stabilize the singlet biradical by spin delocalization, two of them playing approximately the same role as one radical-insulating 1,4 phenylene moiety. The results are generalized to the investigation of open-shell polyradical singlet states of rigid hydrocarbon structures, the symmetry and rigidity of which can assist cooperativity and self spin polarization effect. Several synthesis targets with challenging magnetic/spin properties are suggested in the carbo-mer series.

"Carbo-aromaticity" and novel carbo-aromatic compounds.

While the concept of aromaticity is being more and more precisely delineated, the category of "aromatic compounds" is being more and more expanded. This is illustrated by an introductory highlight of the various types of "aromaticity" previously invoked, and by a focus on the recently proposed "aromatic character" of the "two-membered rings" of the acetylene and butatriene molecules. This serves as a general foundation for the definition of "carbo-aromaticity", the relevance of which is surveyed through recent results in the synthetic, physical, and theoretical chemistry of carbo-mers and in particular macrocyclic-polycyclic representatives constituting a natural family of "novel aromatic compounds". With respect to their parent molecules, carbo-mers are constitutionally defined as "carbon-enriched", and can also be functionally regarded as "π-electron-enriched". This is exemplified by recent experimental and theoretical results on functional, aromatic, rigid, σ,π-macrocyclic carbo-benzene archetypes of various substitution patterns, with emphasis on the quadrupolar pattern. For the purpose of comparison, several types of non-aromatic references of carbo-benzenes are then considered, i.e. freely rotating σ,π-acyclic carbo-n-butadienes and flexible σ-cyclic, π-acyclic carbo-cyclohexadienes, and to "pro-aromatic" congeners, i.e. rigid σ,π-macrocyclic carbo-quinoids. It is shown that functional carbo-mers are entering the field of "molecular materials" for properties such as linear or nonlinear optical properties (e.g. dichromism and two-photon absorption) and single molecule conductivity. Since total or partial carbo-mers of aromatic carbon-allotropes of infinite size such as graphene (graphynes and graphdiynes) and graphite ("graphitynes") have long been addressed at the theoretical or conceptual level, recent predictive advances on the electrical, optical and mechanical properties of such carbo-materials are surveyed. Very preliminary experimental results on a carbo-benzenoid fragment are finally disclosed.

Charge Effects in PCP Pincer Complexes of Ni(II) bearing Phosphinite and Imidazol(i)ophosphine Coordinating Jaws: From Synthesis to Catalysis through Bonding Analysis.

This contribution reports on a new family of Ni(II) pincer complexes featuring phosphinite and functional imidazolyl arms. The proligands (R) PIMC(H) OP(R') react at room temperature with Ni(II) precursors to give the corresponding complexes [((R) PIMCOP(R') )NiBr], where (R) PIMCOP(R) =κ(P) ,κ(C) ,κ(P) -{2-(R'2 PO),6-(R2 PC3 H2 N2 )C6 H3 }, R=iPr, R'=iPr (3 b, 84 %) or Ph (3 c, 45 %). Selective N-methylation of the imidazole imine moiety in 3 b by MeOTf (OTf=OSO2 CF3 ) gave the corresponding imidazoliophosphine [((iPr) PIMIOCOP(iPr) )NiBr][OTf], 4 b, in 89 % yield ((iPr) PIMIOCOP(iPr) =κ(P) ,κ(C) ,κ(P) -{2-(iPr2 PO),6-(iPr2 PC4 H5 N2 )C6 H3 }). Treating 4 b with NaOEt led to the NHC derivative [(NHCCOP(iPr) )NiBr], 5 b, in 47 % yield (NHCCOP(iPr) =κ(P) ,κ(C) ,κ(C) -{2-(iPr2 PO),6-(C4 H5 N2 )C6 H3 )}). The bromo derivatives 3-5 were then treated with AgOTf in acetonitrile to give the corresponding cationic species [((R) PIMCOP(R) )Ni(MeCN)][OTf] [R=Ph, 6 a (89 %) or iPr, 6 b (90 %)], [((R) PIMIOCOP(R) )Ni(MeCN)][OTf]2 [R=Ph, 7 a (79 %) or iPr, 7 b (88 %)], and [(NHCCOP(R) )Ni(MeCN)][OTf] [R=Ph, 8 a (85 %) or iPr, 8 b (84 %)]. All new complexes have been characterized by NMR and IR spectroscopy, whereas 3 b, 3 c, 5 b, 6 b, and 8 a were also subjected to X-ray diffraction studies. The acetonitrile adducts 6-8 were further studied by using various theoretical analysis tools. In the presence of excess nitrile and amine, the cationic acetonitrile adducts 6-8 catalyze hydroamination of nitriles to give unsymmetrical amidines with catalytic turnover numbers of up to 95.

Difluorenyl carbo-Benzenes: Synthesis, Electronic Structure, and Two-Photon Absorption Properties of Hydrocarbon Quadrupolar Chromophores.

The synthesis, crystal and electronic structures, and one- and two-photon absorption properties of two quadrupolar fluorenyl-substituted tetraphenyl carbo-benzenes are described. These all-hydrocarbon chromophores, differing in the nature of the linkers between the fluorenyl substituents and the carbo-benzene core (C-C bonds for 3 a, C-C=C-C expanders for 3 b), exhibit quasi-superimposable one-photon absorption (1PA) spectra but different two-photon absorption (2PA) cross-sections σ2PA. Z-scan measurements (under NIR femtosecond excitation) indeed showed that the C≡C expansion results in an approximately twofold increase in the σ2PA value, from 336 to 656 GM (1 GM = 10(-50) cm(4) s molecule(-1) photon(-1)) at λ = 800 nm. The first excited states of Au and Ag symmetry accounting for 1PA and 2PA, respectively, were calculated at the TDDFT level of theory and used for sum-over-state estimations of σ2PA(λi), in which λi = 2 hc/Ei, h is Planck's constant, c is the speed of light, and Ei is the energy of the 2PA-allowed transition. The calculated σ2PA values of 227 GM at 687 nm for 3 a and 349 GM at 708 nm for 3 b are in agreement with the Z-scan results.

The missing entry in the agostic-anagostic series: Rh(I)-η(1)-C interactions in P(CH)P pincer complexes.

The missing entry, namely, the "C-anagostic" or η(1)-C interaction, closing the agostic-anagostic series of metal-CH(aryl) interactions is found in a bis(amidiniophosphine) P(CH)P pincer rhodium complex. The three entries, namely, agostic η(2)-(C,H), anagostic (related to hydrogen bonding, thus recoined here as "H-anagostic"), and C-anagostic interactions, are unambiguously characterized by electron localization function (ELF) topological analysis. Other theoretical tools such as noncovalent interaction (NCI) analysis and multicenter electron delocalization indices (MCIs) support the ELF characterization. A η(2)-(C,H) agostic interaction is evidenced by a disynaptic V(C,H) or trisynaptic V(M,C,H) ELF basin with a significant quantum topological atoms in molecules (QTAIM) atomic contribution of the metal M and a large covariance (in absolute value) with the metal core basin C(M). The C-anagostic η(1)-C interaction is characterized by a disynaptic V(M,C) basin, a weak covariance (in absolute value) of V(C,H) and C(M) populations, and a negligible QTAIM atomic contribution of M to V(C,H). The relevance of these ELF signatures is evidenced in a selected series of related rhodium and osmium complexes.

Carbo-quinoids: stability and reversible redox-proaromatic character towards carbo-benzenes.

The carbo-mer of the para-quinodimethane core is stable within in a bis(9-fluorenylidene) derivative. Oxidation of this carbo-quinoid with MnO2 in the presence of SnCl2 and ethanol affords the corresponding p-bis(9-ethoxy-fluoren-9-yl)-carbo-benzene. The latter can be in turn converted back into the carbo-quinoid by reduction with SnCl2 , thus evidencing a chemical reversibility of the interconversion between a pro-aromatic carbo-quinoid and an aromatic carbo-benzene, and is reminiscent of the behavior of the benzoquinone/hydroquinone redox couple (in the red-ox opposite sense).

P(CH)P pincer rhodium(I) complexes: the key role of electron-poor imidazoliophosphine extremities.

The coordination chemistry of a potentially pincer-type dicationic meta-phenylene-bis(imidazoliophosphine) ligand 3 to neutral and cationic carbonylrhodium(I) centers has been investigated. Similarly to what was observed previously for its ortho-phenylene counterpart, 3 was found to bind to the RhCl(CO) fragment in a trans-chelating manner that makes possible a weak Rh-C(H) interaction, inferred from the nonbonding but relatively short Rh-C and Rh-H contacts observed in the solid state structure of the dicationic adduct (3)RhCl(CO) (5). Formation of the target PCP-type pincer complex could not be triggered despite multiple attempts to deprotonate the central C-H moiety in the initial dicationic adduct 5, or in the tricationic species [(3)Rh(CO)](+) (8) generated by abstraction of the chloride ion from 5. Complex 8 was identified on the basis of NMR and IR analyses as a Rh(I)-stabilized P(CH)P-intermediate en route to the anticipated classical PCP-type pincer complex. Analysis of the electronic structure of this intermediate computed at the density functional level of theory (DFT level) revealed a bonding overlap between a Rh d-orbital and π-orbitals of the m-phenylene ring. NBO analyses and calculated Wiberg indices confirm that this interaction comprises an η(1)-C-Rh bonding mode, with only secondary contributions from the geminal C and H atoms. Although the target PCP-type pincer complex could not be generated, treatment of the tricationic intermediate with methanol induced a P-CN(2) bond cleavage at both imidazoliophosphine moieties, resulting in the formation of a dicationic "open pincer" species, that is, a nonchelated bis((MeO)PPh(2))-stabilized aryl-Rhodium complex that is the κC-only analogue of the putative κP,κC,κP-PCP complex sought initially. Theoretical studies at the DFT level of experimental or putative species relevant to the final C-H activation process ruled out the oxidative addition pathway. Two alternative pathways are proposed to explain the formation of the "open pincer" complex, one based on an organometallic α-elimination step, the other based on an organic aromatization-driven ß-elimination process.

The fundamental chemical equation of aromaticity.

In the search for the chemical measure of molecular aromaticity, namely the energetic gain of a cyclic delocalized structure for just being cyclic, the numerous definitions of generic aromatic stabilisation energies proposed hitherto stand as approximates of the conceptual limit devised within the framework of spectral graph theory, i.e. the topological resonance energy (TRE). After a 36 year challenge, the TRE acyclic reference of any π-cyclic molecule, originally merely defined by an abstract matching polynomial, is now given a real chemical structure: the Möbius-twisted head-to-tail metathesis cyclo-dimer of the parent ring. The original treatment at the Hückel molecular orbital level of theory can now be extended to DFT or ab initio levels. The corresponding ring opening-closing-twisting chemical transformation provides the observable basis for the measure of aromaticity under either vertical or adiabatic conditions.

Carbene-stabilized phosphenium oxides and sulfides.

Carbene→chalcogenophosphenium adducts, which correspond to an intermolecular stabilization mode of the so far elusive, free oxo- and thiooxophosphenium species [R(2)P(+) = X] (X = O, S) by imidazolylidene (NHC) and diaminocyclopropenylidene (BAC) donors, have been isolated and fully characterized. The dative character of the R(2)C:→P(+)(X)Ph(2) bond was confirmed experimentally by nucleophilic displacement of the carbene donor with a chloride ion and by an exchange reaction of the NHC ligand of the NHC:→P(+)(O)Ph(2) adduct with an independently prepared BAC ligand, thereby giving the BAC:→P(+)(O)Ph(2) adduct. This dative character was further characterized by the DFT-calculated preference of carbene→chalcogenophosphenium systems for a heterolytic dissociation mode over a homolytic one.

On the P-coordinating limit of NHC-phosphenium cations toward Rh(I) centers.

Two types of imidazoliophosphane with additional electron-withdrawing substituents, such as alkoxy or imidazolio groups, are experimentally described and theoretically studied. Diethyl N,N'-2,4,6-methyl(phenyl)imidazoliophosphonite is shown to retain a P-coordinating ability toward a {RhCl(cod)} (cod = cycloocta-1,5-diene) center, thus competing with the cleavage of the labile C-P bond. Derivatives of N,N'-phenylene-bridged diimidazolylphenylphosphane were isolated in good yield. Whereas the dicationic phosphane proved to be inert in the presence of [{RhCl(cod)}(2)], the monocationic counterpart was shown to retain the P-coordinating ability toward a {RhCl(cod)} center, thus competing with the N-coordinating ability of the nonmethylated imidazolyl substituent. The ethyl phosphinite version of the dication, thus possessing an extremely electron-poor P(III) center, was also characterized. According to the difference between the calculated homolytic and heterolytic dissociation energies, the N(2)C⋅⋅⋅P bond of imidazoliophosphanes with aryl, amino, or alkoxy substituents on the P atom is shown to be of dative nature. The P-coordinating properties of imidazoliophosphanes with various combinations of phenyl or ethoxy substituents on the P atom and those of six diimidazolophosphane derivatives with zero, one, or two methylium substituents on the N atom, were analyzed by comparison of the corresponding HOMOs and LUMOs and by calculation of the IR C=O stretching frequencies of their [RhCl(CO)(2)] complexes. Comparison of the ν(CO)  values allows the family of the electron-poor Im(+) PRR' (Im = imidazolyl) potential ligands to be ranked in the following order versus (R,R'): P(OEt)(3)<(Ph,Ph)<(Ph,OEt)<(OEt,OEt)

From hexaoxy-[6]pericyclynes to carbo-cyclohexadienes, carbo-benzenes, and dihydro-carbo-benzenes: synthesis, structure, and chromophoric and redox properties.

When targeting the quadrupolar p-dianisyltetraphenyl-carbo-benzene by reductive treatment of a hexaoxy-[6]pericyclyne precursor 3 with SnCl(2)/HCl, a strict control of the conditions allowed for the isolation of three C(18)-macrocyclic products: the targeted aromatic carbo-benzene 1, a sub-reduced non-aromatic carbo-cyclohexadiene 4A, and an over-reduced aromatic dihydro-carbo-benzene 5A. Each of them was fully characterized by its absorption and NMR spectra, which were interpreted by comparison with calculated spectra from static structures optimized at the DFT level. According to the nucleus-independent chemical shift (NICS) value (NICS≈-13 ppm), the macrocyclic aromaticity of 5A is indicated to be equivalent to that of 1. This is confirmed by the strong NMR spectroscopic deshielding of the ortho-CH protons of the aryl substituents, but also by the strong shielding of the internal proton of the endocyclic trans-CH=CH double bond that results from the hydrogenation of one of the C≡C bonds of 3. Both the aromatics 1 and 5A exhibit a high crystallinity, revealed by SEM and TEM images, which allowed for a structural determination by using an X-ray microsource. A good agreement with calculated molecular structures was found, and columnar assemblies of the C(18) macrocycles were evidenced in the crystal packing. The non-aromatic carbo-cyclohexadiene 4A is shown to be an intermediate in the formation of 1 from 3. It exhibits a remarkable dichromism in solution, which is related to the occurrence of two intense bands in the visible region of its UV/Vis spectrum. These properties could be attributed to the dibutatrienylacetylene (DBA) unit that occurs in the three chromophores, but which is not involved in a macrocyclic π-delocalization in 4A only. A versatile redox behavior of the carbo-chromophores is evidenced by cyclic voltammetry and was analyzed by calculation of the ionization potential, electron affinity, and frontier molecular orbitals.

Towards the limit of atropochiral stability: H-MIOP, an N-heterocyclic carbene precursor and cationic analogue of the H-MOP ligand.

The configurational stability of biaryl motifs is addressed for the 1-naphthyl-N-benzimidazolyl motif substituted by a single diphenylphosphinyl group at the 2-position. The atropoenantiomers of the N-methylated cation H-MIOP, a less sterically locked analogue of the neutral H-MOP ligand, were resolved by enantiospecific cleavage of the N(2)C-P bond of the resolved enantiomers of BIMIONAP. The latter were obtained by enantiospecific N-methylation of the previously resolved enantiomers of neutral BIMINAP. PdCl(2) complexes of the P,C-chelating N-heterocyclic carbene (NHC)-phosphine ligands derived from (R)- and (S)-H-MIOP were prepared by two enantiospecific routes: by N(2)C-P bond cleavage from the (R)- and (S)-BIMIONAP-PdCl(2) complexes, or by simultaneous coordination of the P and C atoms of the in situ generated free NHC-phosphine. The enantiomerization pathways of H-MOP, H-MIOP, and corresponding NHC-phosphine have been investigated at the B3PW91/6-31G(d,p) level of theory. The calculated enantiomerization barriers of H-MOP and H-MIOP in acetonitrile are equal to 176.0 and 146.4 kJ mol(-1), respectively, and are mainly determined by the distortion of the naphthalene and/or benzimidazole motifs in the transition state. Beyond the stability of their optical rotation at room temperature, the respective calculated Oki's racemization temperatures of 334 and 225 °C allowed us to consider both ligands as configurationally stable.

Flexible diphosphine ligands with overall charges of 0, +1, and +2: critical role of the electrostatics in favoring trans over cis coordination.

The influence of the formal electrostatic interaction on the cis/trans coordination mode at a PdCl(2) center is investigated in a family of isostructural flexible diphosphine ligands Ph(2)P-X-C(6)H(4)-Y-PPh(2), where X and Y stand for neutral or cationic N,C-imidazolylene linkers. While the neutral and monocationic diphosphine spontaneously behave as classical cis-chelating ligands, only the dicationic diphosphine, where the electrostatic repulsion between the formal positive charges specifically takes place, is observed to behave as a trans-chelating ligand. The crucial role of electrostatics is analyzed on the basis of (31)P NMR data in solution and X-ray diffraction data in the crystal state. Comparative theoretical studies of the cis- and trans-chelated complexes, including EDA, static (31)P NMR, MESP, and AIM analyses, have been undertaken on the basis of DFT calculations in the gas phase or in the acetonitrile continuum. Whereas the cis-coordination mode is shown to be thermodynamically favored for the neutral ligand, the trans-coordination mode is found to be preferred for the dicationic homologue. The stereochemical preference is thus shown to be parallel to the expected effect of the formal electrostatic interaction. The results open perspectives for control of the cis- and trans-chelating behavior of flexible bidentate ligands by more or less reversible charge transfer at the periphery of the coordination sphere of a metallic center.