Synthesis, X-ray structures, and catalytic applications of palladium(II) complexes bearing N-heterocyclic iminocarbene ligands

Two new Pd(II) N-heterocyclic iminocarbene complexes (C-N)PdCl₂ that contain 5-membered chelate rings have been prepared by carbene transfer from a silver iminocarbene precursor to (COD)PdCl₂. The new Pd imonocarbene complexes, as well as two that have been previously reported (altogether three 5-membered and one 6-membered chelate ring complexes) have been evaluated as catalysts for the Suzuki-Miyaura coupling reaction. The complexes were found to be active in the reaction, but without exceptional catalytic performances. The 5-membered chelate ring complexes appeared to be more robust and remained active for a longer time than the 6-membered ring congener. The catalytic performance of the 5-membered chelate ring complexes appeared to be rather insensitive to the steric demands of the imine-N-aryl group. The X-ray structure of one of the Ag iminocarbene complexes reveals the κ¹(C) bonding of the iminocarbene moiety in a nearly linear Ag(I) complex; two monomeric units are associated through a weak Ag-Ag interaction. The X-ray structures of two new Pd iminocarbene complexes (C-N)PdCl₂ confirm the chelating κ²(C,N) nature of the iminocarbene moiety; in both complexes, the Pd-Cl distances trans to carbene-C are slightly longer than those trans to imine-N.     

Twelve-, fourteen- and sixteen-membered macrocyclic ligands and a study of the effect of ring size on ligand field strength

Complexes of Cr^III and Ni^II with a macrocyclic tetradendate 12-, 14- and 16-membered nitrogen donor ligand were prepared and characterized on the basis of elemental analysis, molar conductance, magnetic susceptibility measurements, i.r., electronic and e.p.r. spectra. The complexes are of the high-spin type. The effect of ring size on the Dq_(xy) value is also discussed and the field strength was found to have a maximum value for the 14-membered ring, when metal ion fits best into the cavity rather than when the metal ion is compressed by the macrocycle.     

Direct functionalization of labile alkoxyamines

Direct esterification of a labile alkoxyamine R¹R²NOR³, which was previously reported as unsuccessful, is achieved by a Mitsunobu reaction or a nucleophilic substitution. Ester derivatives are obtained under smooth conditions and easily purified. Macrocyclization attempts on ester derivatives were successful for five-membered ring lactones and unsuccessful for 13-membered ring lactones. Moreover, the success of the cyclization was dramatically dependent on the quality of the solution degassing. Poor degassing led to unexpected carbonate alkoxyamine.     

Ring and nitrogen inversion in a flexible highly symmetrical molecule: The conformations of 1,3,7,9,13,15,19,21-octaazapentacyclo[19,3,1,1,3,7,19,13,115,19]

The possible conformations of the title compound and their modes of interconversion via ring and nitrogen inversion processes are delineated. At high temperatures (> +80°) the ¹H NMR spectra are consistent with time averaged D_4th symmetry and rapid ring and nitrogen inversion. At lower temperatures (ca. ?10°) the time-averaged symmetry if D_2d and inversion of the 6-membered rings is frozen out, nitrogen inversion remaining rapid. The free energy of activation for the total inversion of all four 6-membered rings is 13·5 kcal mole ^?1, higher than in similar monocyclic systems. This higher energy is a reflection of the multiple ring inversion pathway required for total inversion of all the 6-membered rings.     

Nanowires: size evolution,reversibility, and one-atom contacts

Size-evolutionary patterns of mechanical and electronic transport properties in junctions are discussed, showing formation of nanowires upon narrowing, regardless of the initial dimensions of the junctions. Mechanical and conductance reversibility of such nanowires in elongation and compression cycles is studied. Ab-initio localspin- density functional based molecular dynamics simulations show formation of a stable “one-atom” contact in a sodium wire at 189 K, with the atomic structure near the apex of the nanowire correlated to that of two pentagonal pyramidal Na₇ clusters joined by the apex atom. The average electronic conductance through the one-atom contact, at 189 K, is estimated to be ～4.5 (2e ²/h).     

Two novel one-dimensional coordination polymers [Cu2(μ-Br)2(pcaede)2 · 2H2O] n and [Cu2(μ-Br)2(pcaede)2 · 0.5THF · 0.5H2O] n formed via self-assembly of 4-pyridine-carboxylic acid 1,2-ethanediyl ester (pcaede) with copper(I) bromide

Copper(I) bromide reacts with 4-pyridinecarboxylic acid 1,2-ethanediyl ester (pcaede) in wet Me₂CO/DMSO or THF/DMSO mixed solvents to give two novel coordination polymers [Cu₂(μ-Br)₂(pcaede)₂ · 2H₂O] _n (1) and [Cu₂(μ-Br)₂(pcaede)₂ · 0.5THF · 0.5H₂O] _n (2), respectively. X-ray diffraction analyses revealed that (1) and (2) consist of infinite one-dimensional Cu^I ion chains, in which the consecutive three Cu^I ions are bridged respectively by two Br atoms and two pcaede ligands to form a string of alternately arranged four-membered Cu₂Br₂ rings and 30-membered Cu₂(pcaede)₂ rings. In addition, while in each repeated structural unit of (1) there are water molecules as guests, respectively located in a 30-membered ring and between two 30-membered rings in adjacent Cu^I ion chains, in (2) the two kinds of guest molecules 0.5THF and 0.5H₂O lie respectively in the 30-membered ring and between the corresponding 30-membered rings in neighboring Cu^I ion chains. Structures of (1) and (2) were also characterized by i.r. spectroscopy, thermogravimetry and elemental analysis.     

Tragoponol, a dimeric dihydroisocoumarin from Tragopogon porrifolius L.

A phytochemical re-investigation of Tragopogon porrifolius L. (Asteraceae) yielded (7S,15S)-2,4,12-trihydroxy-7-(4-hydroxyphenyl)-10-methoxy-15-(4-methoxyphenyl)-7,8,15,16-tetrahydrodibenzo[c,i][1,7]dioxacyclododecine-5,13-dione, named tragoponol, a dimeric dihydroisocoumarin.The compound, which represents the first of its kind, is comprised of the open-chained forms of two different mono-methoxylated dihydroisocoumarin moieties, scorzocreticin and hongkongenin, which are connected via two ester bonds to form a macrolide with two lactone moieties featuring a 12-membered ring. The structure of the nearly symmetrical compound was established by HR mass spectrometry, CD measurements, and extensive 1D and 2D NMR experiments.     

The synthesis of and nuclear magnetic resonance strudies on bimetallic chromium(0), or molybdenum(0) carbonyl complexes containing bridging trans-Ph2PCHCHPPh2 ligands: Crystal structure of [Mo2(CO)8(μ-trans-PH2PCHCHPPh2)2]

The trans-PH₂PCH=CHPPh₂ (t-dppe)-bridged bimetallic 10-membered ring complexes [M₂(CO)₈(μ-t0dppe)₂] (M = Cr or MO) have been synthesized by treatment of the metal hexacarbonyl with one equivalent of t-dppe at elevated temperature. the fluxional process in the Mo(μ-t-dppe) Mo ring has been studied by variable temperature ¹³P-{¹H} NMR spectroscopy and is characterised by a change from an A₄ spin system at +20°C to an AA′BB′ system at −100°C. The bimetallic complex [Mo₂(CO)₆(μ-tdppe)₃] has been prepared and ¹³P-{¹H} NMR studies indicate that in solution all four phosphorus nuclei are equivalent, even at −115°C. Attempts to prepare related heterobimetallic complexes are also described. Crystals of [Mo₂(CO)₈(μ-trans-Ph₂PCH=CHPPh₂)₂] are monoclinic, space group P2₁/a, with a 2020.2(5), b 1423.1(5), c 2145.1(5) pm, β 113.95(2)°, and Z = 4; final R factor 0.0465 for 5668 observed reflections. The structure shows two [Mo(CO)₄] moieties linked by two trans-dppe bridges to give a 10-membered Mo₂P₄C₄ ring.     

Free-radical decarboxylation of carboxylic acids as a concerted abstraction and fragmentation reaction

Two variants of the reaction of radicals with the carboxyl group of carboxylic acids, namely, RO ₂ ^? + R _i COOH → ROOH + R _i CO ₂ ^? and RO _i ^? + R _j COOH → ROOH + R _i ^? + CO₂ are theoretically analyzed. It is demonstrated by the intersecting-parabolas method that if the reaction proceeded via the formation of an intermediate carboxyl radical, it would be much slower than is actually observed. Quantum-chemical calculations carried out by the density functional method using the nonempirical functional PBE have shown that the reactions of the methyl radical with the carboxyl group of acetic, butyric and vinylacetic acids include concerted H atom abstraction and C-C bond breaking. In the framework of the intersecting-parabolas model, an algorithm has been developed to calculate the activation energy and rate constant for X^? + R _i COOH → XH + CO₂ + R _i ^? reactions, where X = R^?, RO^?, HO^?, ArO^?, Ar₂N^? or H^?     

Estimation of O-H bond dissociation energies in alcohols and acids from kinetic data

The O-H bond dissociation energies (D _O-H) in five alcohols and six acids have been determined from experimental data (rate constants of radical reactions). The ratio of the rate constants of the reactions R¹O˙+RH→R¹OH+R˙ and R ⁱ O˙+RH→R ⁱ OH+R˙ and the intersecting parabolas method are used in the estimation procedure. The D _O-H values are used to calculate the activation energies and rate constants for hydrogen abstraction from 2-methylbutane, butene-1, and cumene by alkoxyl and carboxyl radicals. The geometric parameters of the transition state are calculated for these reactions.     

The electron impact induced fragmentation of aromatic aldoximes—III. On the loss of HCNO,OH⋅ and substituents and the role of substituent effects in the decomposition of cyclohexadiene type intermediate ions

The mechanisms for loss of HCNO, OH^˙ and the substituent X^˙ from aromatic aldosimes were elucidated with the aid of deuterium labelling, metastable ion characteristics and substituent effects. It is proposed that the loss of HCNO occurs through a cyclohexadiene type intermediate ion generated via a 6-membered ring hydroxyl hydrogen transer to the ortho position of the phenyl ring. This is followd by a second step which involves the trnsfer of a hydrogen atom from the ortho position to C-1. It is inferred from the corelation with the mesomeric effect (σ_R⁺) of substituents that this step is rate determining. Loss of OH^˙ and X^˙ proceed via the same cyclohexadiene type intermediate ion but, depending upon the substituent, other pathways are also followed.     

Enantioselective synthesis of 2-allyl and 2-(3-trimethylsilylpropargyl)-2-hydroxycyclohexanone using osmium-catalyzed asymmetric dihydroxylation

The catalytic asymmetric dihydroxylation of (1-cyclohexenyl) or (1-cyclopentenyl) acetonitrile 5 and 15 with AD-mix-β occurred with good enantiofacial selectivity (87 to 94.7% ee after recrystallization) giving (R,R)-diols in agreement with the mnemonic device. The 6-membered ring diol nitrile was easily transformed, via standard functional group manipulations, to 2-allyl and 2-(3-trimethylsilylprop-2-ynyl)-2-hydroxycyclohexanone in about 35% overall yield.     

8 and 16-Membered ring phosphorus compounds. Ring size dependence of the NMR parameters

The ¹H NMR spectral analysis of four 8-membered rings, 2-thioxo- (or 2-oxo-) 2-R-1,3,6,2- trithiaphocane, is reported. The stereochemistry of the 8-membered ring is discussed. The ³¹P NMR spectral parameters [δ³¹P, ¹J(PC)] obtained on several cyclic 1,3,2-dithiaphospha compounds of variable size (5, 6, 8, 12 and 16-membered rings) are discussed as a function of the ring size and of the geometry of the molecule.     

N-phosphonio formamidine derivatives: Synthesis,characterization, X-ray crystal structures,and deprotonation reactions

A simple and efficient method for the preparation of N-phosphonio formamidine derivatives of the general formula [R”₂N?C(H)=N?P(R’)R₂]⁺X^? is described. The data recorded in solution and the single crystal X-ray studies revealed that these compounds are best described by the combination of the two mesomeric N-phosphonio formamidine [R”₂N?C(H)=N?P(R’)R₂]⁺ and iminium phosphazene [R”₂N=C(H)?N=P(R’)R₂]⁺ forms. Formamidine phosphorus ylides ⁱPr₂N?C(H)=N?P(CH₂)R₂ were prepared after addition of ^tBuLi at –78 °C from the corresponding N-phosphonio compounds. [(PhCN)₂Pd(Cl)₂] was reacted with ⁱPr₂N?C(H)=N?P(CH₂)ⁱPr₂ to form the dimeric complex [(ⁱPr₂N?C(H)=N?P(CH₂)ⁱPr₂)Pd(Cl)(μ-Cl)]₂ which was structurally characterized by X-ray analysis. The deprotonation reactions conducted on [ⁱPr₂N?C(H)=N?PPh₃]⁺X^? occurred via an intramolecular rearrangement to give the cyanamide compound ⁱPr₂N?C≡N and PPh₃; transient formation of the amino-phosphazene-carbene ⁱPr₂N?C?N=PPh₃ was not observed.     

Synthesis of new molecular scaffolds: 3-aza-7,9-dioxa-bicyclo[4.2.1]nonane (8-exo BTKa) and 3-aza-8,10-dioxa-bicyclo[5.2.1]decane (9-exo BTKa) carboxylic acids

Two classes of enantiopure molecular scaffolds were prepared, whose lactam structure formally derives from the coupling between tartaric acid and β- or γ-ketoamines. We labelled these compounds as 8-exo and 9-exo BTKa, indicating the lactam size (8- and 9-membered ring, respectively). Starting from β- and γ-nitroketones, the synthesis involves the ketal formation by (R,R)-dimethyl tartrate. The subsequent amide bond formation occurs during the hydrogenation of the nitro group over Raney-Ni and no expected open chain amine was observed.     

Infrared band intensities: a comparative study of the transition moment matrix elements for the fundamental and overtones: Part II. Application to CO,HCl and OH

The various expressions considered in Part I for the transition moment matrix elements of fundamental and first two overtones are applied to carbon monoxide. The coefficients a_ij in the expressions R^io = Σa_ijp_j (where R^io is the transition moment integral for the O → i vibrational transition and p_j is the dipole moment derivative ?^jP/?XXX^j, XXX = (r — r_e)/r_e, r_e is equilibrium bond distance) are reported for i,j = 1, 2, 3. It is found that these coefficients do not vary by more than 5% when compared for the same i,j values in various expressions irrespective of the most exhaustive treatments used in deriving the original expressions. On the basis of the values of the coefficients obtained for CO, generalisations have been suggested on the effects of inclusion of mechanical and electrical anharmonicity to the intensities of fundamental and first two overtones. It is generally observed that the contribution of p'₁, is about 100 fold more than the contribution of p'₂, for R¹⁰. On the other hand the contributions of p'₁, and p, for R²⁰ and R³⁰ are of nearly equal magnitude but opposite in sign. The contribution of p'₁ to R¹⁰ is much higher than its contribution to R²⁰ and R²⁰. The various observations lead us to conclude that, whereas the effect of inclusion of mechanical anharmonicity on the intensity of the fundamental band is negligible, this effect is almost comparable to the effect of inclusion of electrical anharmonicity for the first two overtones. Simple forms of the a_ij expressions are applied to HC1 and OH to demonstrate the effect of variation of molecular constants on the a_ij values. On the basis of the observed trend in the values of these coefficients for CO, HCl and OH general remarks on the effects of hydrogen bonding on IR band intensities are given.     

Preparation and reaction of phosphorus peri-bridged naphthalenes and their adducts with Lewis acids

The reaction of dilithionaphthalene with R₂NPCl₂ (R = ⁱPr and Et) gave diisopropylamino-naphtho[1,8-bc]phosphete 3a and its diethyl analog 3b (phosphorus versions of single-atom peri-bridged naphthalene). The nature of the strained four-membered ring thus formed was examined by treating 3a with four types of electrophiles, BH₃, elemental sulfur, methyltriflate, and a metal fragment W(CO)₅, all of which gave the corresponding electrophile adducts with the strained ring retained. X-ray analysis was performed for 3b and all of the above adducts, showing that the trigonal plane of the amino group was commonly oriented almost perpendicular to the naphthalene plane, which was rationally understood on the basis of the favorable overlap between the lone pair orbital of the sp² nitrogen center and the σ¹ orbital(s) of the two P–C bonds. In addition, when the W(CO)₅ adduct, W(3a)(CO)₅, was treated with Pt(PPh₃)₄ and then with a CO gas, its four-membered ring was expanded to give a tungsten–platinum heterodinuclear complex having a five-membered platina–phospha heterocycle.     

The novel Cs4Nb6F8.5I3.5I6 octahedral niobium cluster fluoro-iodide: a step towards the Nb6F12 cluster core excision

The solid state synthesis of Cs₄Nb₆Fⁱ_8.5Iⁱ_3.5I^a₆ starting from Nb₆F₁₅ binary fluoride, as well as its crystal structure determined by X-ray single crystal diffraction, are presented in this work. This novel cluster compound is based on a Nb₆Iⁱ₃Fⁱ₆Lⁱ₃I^a₆ (L=F, I) discrete unit and crystallizes in the monoclinic system (space group C2/m; Z=4 ; a=10.4363(4) Å, b=18.1227(7) Å, c=19.5102(9) Å β=101.223(1)°, V=3619.5(3) Å³, R₁=0.057; wR₂=0.159). This halide is the first octahedral niobium cluster compound containing unshared terminal I^a ligands together with ordered μ₂-Iⁱ and μ₂-Fⁱ ligands on nine inner positions whilst the three last ones (Lⁱ) are slightly affected by a I/F random occupancy. The structural findings are discussed and compared with those of Nb₆F₁₅, Nb₆I₁₁, CsNb₆I₁₁ and the fluorochlorides and fluorobromides recently reported.     

Orientation‐Controlled Single‐Molecule Junctions

The conductivity of a single aromatic ring, perpendicular to its plane, is determined using a new strategy under ambient conditions and at room temperature by a combination of molecular assembly, scanning tunneling microscopy (STM) imaging, and STM break junction (STM‐BJ) techniques. The construction of such molecular junctions exploits the formation of highly ordered structures of flat‐oriented mesitylene molecules on Au(111) to enable direct tip/π contacts, a result that is not possible by conventional methods. The measured conductance of Au/π/Au junction is about 0.1 G_o , two orders of magnitude higher than the conductance of phenyl rings connected to the electrodes by standard anchoring groups. Our experiments suggest that long‐range ordered structures, which hold the aromatic ring in place and parallel to the surface, are essential to increase probability of the formation of orientation‐controlled molecular junctions.     

Supramolecular formation of large hydrogen-bonded rings in the structures of two amino acid derivatives

Two new amino acid derivatives N-(2-oxopyrrolidin-1-ylmethyl)-l-valine (PMV) and N,N-bis(2-oxopyrrolidin-1-ylmethyl)-β-alanine (PMA) were synthesized and their structures were determined by single crystal X-ray crystallography. The geometry and conformation of both molecular aggregates and their hydrogen bond networks are not similar. In the PMV crystal structure, PMV and the solvent water molecule are linked by O–H⋯O intermolecular hydrogen bonds resulting in two ring motifs R₁₂¹²(48) and R₄⁴(22). A three-dimensional supramolecular structure is formed by hydrogen bonds N–H⋯O between the layers. In the PMA crystal structure, each water molecule connects three PMA molecules through O–H⋯O intermolecular hydrogen bonds, and a ring motif R₄⁴(24) is formed in the structure. But there is no hydrogen bond interaction between the layers, in which van der Waals' interaction is involved only.