Cis Dimers Research Articles

Selective Dimerization of Terminal Alkynes Promoted by the Cationic Actinide Compound [(Et2N)3U][BPh4]. Formation of the Alkyne π-Complex [(Et2N)2U(C⋮CtBu)(η2-HC⋮CtBu)][BPh4

The cationic actinide complex [(Et2N)3U][BPh4] is an active catalytic precursor for the selective dimerization of terminal alkynes. The regioselectivity is mainly toward the geminal dimer, but for bulky alkyne substituents the unexpected cis dimer is also obtained. Mechanistic NMR studies show that a η2-π-complex is formed between a terminal alkyne and an uranium−acetylide intermediate. This latter complex has been characterized spectroscopically. A plausible mechanistic scenario is proposed for the dimerization process.

The vibrational spectroscopy of H2, N2, CO and NO adsorbed on the titanosilicate molecular sieve ETS-10

Dihydrogen, dinitrogen, carbon monoxide and nitric oxide have been adsorbed, at nominally liquid nitrogen temperature, on Na+- and K+-exchanged ETS-10. IR spectroscopy shows formation of M+···(H2), M+···(N2)n, M+···(CO)n and M+···(NO) (n=1, 2, . . . ; M+=Na, K) adducts prevalently involving alkali-metal cations located in the 12-membered channels. These adducts give main IR absorption bands in the range 4050–4150 cm-1 for H2, 2331–2333 cm-1 for N2, 2148–2176 cm-1 for CO, and 1820–1900 cm-1 for NO, which are assigned to the fundamental stretching mode of the diatomic molecules polarized by the electric field created by the metal ions. On Na-exchanged samples, the Na+···(N2) and Na+···(CO) species, formed at lowest dosage, evolve into Na+···(N2)n and Na+···(CO)n (n=2, 3) species upon increasing the gas phase pressure. This reversible '‘solvation’' process is not observed for K-exchanged samples. For adsorbed CO, the high intensity of the IR spectra allowed us to observe and assign overtone bands and combination modes. This result does not find a comparable precedent for CO adsorbed on other zeolites. From bands corresponding to the combination of metal–carbon with carbon–oxygen fundamental stretching modes, the wavenumber values corresponding to metal–carbon stretching vibrations in Na+···CO and K+···CO were deduced: 122 and 107 cm-1, respectively. Due to the strong tendency of NO towards dimerization, IR spectroscopy of adsorbed NO is complicated by the manifestations of cis and trans dimers interacting with alkali cations, and Na+···(NO) and K+···(NO) species could be distinctly observed only at the very initial stages of the adsorption process. Finally, as far as the H2/ETS-10 system is concerned, the most relevant result is that the ortho and para forms are clearly detected in the Na+-ETS-10 sample.

Solid-state photodimerization of 9-methylanthracene as studied by solid-state [formula omitted] NMR

The locus of the photodimerization reaction of 9-methylanthracene in the crystal was examined by high-resolution solid-state 13 C NMR techniques. Examination of the 13 C spectra of the products showed that only the trans dimer is formed by the solid state photodimerization, while both trans and cis dimers are formed by the photodimerization in benzene solution. The 1 H T 1 relaxation curves for the monomer and the dimer at various exposure times were separately observed via well-resolved 13 C peaks. The two T 1 curves exhibit characteristic features for relaxation of a weakly coupled two-spin system. By analyzing the T 1 curves, the spin diffusion rates between the monomer and the dimer and the fraction of the dimer were obtained for various exposure times. From the result, the maximum domain size of the minor component during the photodimerization process was estimated to be ca. 0.3 μm. The heterogeneous domain structure generated by the photodimerization indicates that the reaction takes place at defects of the crystal in the monomer.

Sterically induced cis isomerism in an Sb(III) imido dimer; synthesis, structure and solution dynamics of [(Me 2N)Sb(μ-NDipp)] 2 (Dipp = 2,6-Pr 2iC 6H 3)

The reaction of [Sb(NMe 2) 3] with the sterically bulky amine [DippNH 2] gives the unusual cis dimer [(Me 2N)Sb(μ-NDipp)] 2 1, containing a puckered Sb 2N 2 core. 1H NMR studies show that the cis conformation is retained in solution.

Structural stability of poly(1-propyne) dimers and trimers

The relative thermodynamic energies of the poly(1-propyne) dimers and trimers show that both the conjugation and steric forces play an important role on the structural stability of these molecular systems. The two 1,4-disubstituted butenyne cis and trans dimers are predicted to be practically the same in energy, whereas the 2,4-disubstituted butenyne dimer is of relatively lower thermodynamic stability. The trimers, cis-trans (structure 4) and trans-trans (structure 10), are calculated to be relatively stable, and thus it is more likely that the polymer formation is based on these two trimers with the tail-head-tail-head addition. © 1996 John Wiley & Sons, Inc.

Enhanced photoreactivity of acenaphthylene in water: a product and mechanism study

Acenaphthylene undergoes direct photolysis in dilute aqueous solutions at concentrations as low as 5 × 10 −7 M. The reactivity observed for acenaphthylene in water greatly exceeds that observed in organic solvents. The major products from direct photolysis in aerated aqueous solutions at concentrations above 5 × 10 −5 M are the cis and trans dimers of acenaphthylene. Triplet-sensitized dimerization in water produces a large excess of cis dimer (cis/trans = 14); however, direct photolysis initially produces a small excess of cis dimer (cis/trans = 1.5) and upon further conversion the ratio changes to favor the trans dimer (cis/trans = 0.6). The origin of the trans dimer is assigned to photolysis of crystalline acenaphthylene which is though to form as a coprecipitate with cis dimer during photolysis. The origin of the cis dimer is assigned to dimerization from the triplet manifold. The increased efficiency of acenaphthylene photolysis is attributed to a hydrophobic effect and partly to an enhanced efficiency of dimerization in crystalline acenaphthylene.

Infrared spectroscopy, infrared photoconversion, and ultraviolet photodissociation of NO dimers in neon and nitrogen matrices

The nitric oxide dimers isolated in neon or nitrogen matrices have been irradiated in the infrared by selected lines of a cw CO laser and in the UV at 193 nm by an ArF excimer laser. IR excitation of the antisymmetric mode of the cis dimer in neon induces a conversion to a special van der Waals dimer. At T<6 K UV irradiation dissociates the dimers mainly to NO monomers and, in smaller proportion, to NO2 in the neon matrix or to N2O in the nitrogen matrix. NO2 is formed exclusively from the cis dimer and N2O from the trans dimer. At higher temperature in nitrogen, the oxygen atoms produced in the N2O formation migrate and react with NO to form NO2. The vibrational spectroscopy in nitrogen reveals several new features. The symmetrical vibrational mode of planar trans-(NO)2, normally IR inactive, becomes observable at 5.8 K owing to the matrix strain. cis-(NO)2 is trapped in two sites and the energy difference between them changes abruptly around 14 K, suggesting a structural transformation in the neighborhood of the sites. Cooling at 5.8 K induces a reversible decrease of the cis dimer bands, correlated with the appearance of vdW dimer bands. In both matrices, the antisymmetric band of the cis dimer is broadened by a predissociation effect as in the gas phase.

Matrix isolation investigation of the vibrational spectroscopy and photochemistry of complexes of HCl and HBr with nitric oxide. ab initio study of the NO:HCl complex structure

The matrix isolation technique has been used to isolate and characterize the complexes two hydrogen halides (HCl and HBr) and the nitric oxide (monomer and cis dimer). Identification of the complexes was performed through the observation of the HX and (NO) x (x=1,2) submolecule stretching modes. Ab initio calculation shows that the most stable structure ClH:NO is linear with a 0.002 Å shortening of the NO bond distance and an increase of the HCl bond length with an identical amount. Photochemistry of the identified complexes was also studied. The photoproduct of both NO:HX and (NO) 2 HX complexes is XNO (X=Cl, Br) upon UV irradiation. In the case of the (NO) 2:HX complex the XNO molecule is complexed to HNO in the matrix cage.

Porphyrins 27. Synthesis of metal complexes of cis-1,2-bis(octaethyl-5-porphyrinyl)ethylene and their transformation into the trans isomers

The synthesis of the Cu, Ni, and Zn complexes of cis-1,2-bis(octaethyl-5-porphyrinyl)ethylene has been carried out and their transformation into the corresponding trans isomers has been investigated under various conditions. The crystal and molecular structure of the Ni complex of trans-1,2-bis(octaethyl-5-porphyrinyl)ethylene was determined by x-ray structural analysis. It was shown that the Zn and Ni complexes of the cis isomer were readily converted into the trans isomers in chloroform. A stepwise introduction of Ni atoms into the trans dimer was discovered on heating it with Ni(OAc)2 in DMF. The conversion of the Zn complex of the cis dimer into the trans dimer was facilitated on adding pyridine.

A valence bond study of the origin of the planarity of the C 2N 2O 2 units of cis and trans C-nitroso dimers

The results of some ab initio valence bond studies for cis, trans and perpendicular N-N dimers of HNO are reported, and used to provide a valence bond rationalization for the origin of the observed planarity, or near-planarity, of the C 2N 2O 2 units of cis and trans C-nitroso dimers. It is shown that as a consequence of some delocalization of the oxygen π electrons of the Lewis structures A and B: ▪ the planarity is associated with Pauling three-electron bond (N̈N̊ ⇔ N̊N̈) covalent-ionic resonance in the planar isomers, for example C⇔ D + mirror-image structures for the cis isomer: ▪ as well as with the presence of an N-N electron-pair π bond in planar A and B. The relative importance of these two types of contributions depends on the extent of oxygen π electron delocalization.

Modification of photochemical reactivity by zeolites: Cation controlled photodimerisation of acenaphthylene within faujasites

Photolyses of acenaphthylene included in M + Y zeolites gave the corresponding cis and trans dimers. The ratio of the dimers formed depends on the type of cation present in the supercage. Dimerization is believed to originate from the excited singlet state in the case of lower atomic weight cations and from the triplet state in the case of higher atomic weight cation exchanged zeolites. Dimerization provides insight into the distribution and migration of molecules within the zeolite internal structures.

ChemInform Abstract: IR Study of Physical Adsorption. Part 2. NO on Silica Aerosil Surfaces.

AbstractAt temp. slightly higher than 77 K nitric oxide is only adsorbed as cis dimers.

Study of the monomer‐dimer equilibrium of nitrosobenzene using multinuclear one‐ and two‐dimensional NMR techniques

AbstractThe monomer‐dimer equilibrium of nitrosobenzene in CDCI3 solution was studied in the temperature range ‐50 to 30 °C using 13C, 15N and 17O NMR spectroscopy. Decreasing temperature favours the dimeric azodioxy species, and below ca 10 °C both cis and trans dimers were detected in addition to the monomer. The relative abundance of the trans dimer is low (⩽6%) at all temperatures and this is the first report of its existence in solution. The monomer and dimer species were characterized by their 13C and 15N shifts, and exchange between them was measured quantitatively by 13C 2D‐EXSY spectra. Exchange occurred exclusively between the cis‐dimer and monomer and between the trans‐dimer and monomer, activation energies, ΔG≠(298.15 K) being 65.8±0.1 and 70.0±0.3 kJ mol−1, respectively.

Infrared study of physical adsorption. 2. Nitric oxide on silica aerosil surfaces

Nitric oxide is adsorbed at temperatures slightly higher than 77 K only as cis dimers. The overall adsorbed amount decreases upon dehydration temperature. Three kinds of adsorbed cis dimers are found, two of which (species A and B) H-bonded to a pair of silanols and one (species C) adsorbed nonspecifically via dispersion forces, as it occurs on methoxylated Aerosil. Interaction of (NO)/sub 2/) with only one silanol does not take place. Spectroscopic features concerning each species are obtained by computer simulation of IR spectra. Species A and B are thought to be H-bonded through the N and O ends of the (NO)/sub 2/ molecule, respectively.

31P and 195Pt NMR characteristics of new binuclear complexes of [Pt 2X 4](PR 3) 2] cis/trans isomers and of mononuclear analogs

The 31P and 195Pt chemical shifts are reported for the first time for complexes of binuclear platinum(II) of the type [Pt 2(μ-X) 2(X) 2(PR 3) 2]. These were identified as intermediate from the reaction of the [PtX 2COD)] complex with different tertiary phosphines (where X may be Cl or I and PR 3 may be PBz 3, PCy 3, PCyPh 2, PCy 2Ph or PPh 2C 6F 5). In addition, cis and trans-[PtX 2(PR 3) 2] were produced in the final step, and their 31P and 195Pt are also described (X = Cl or I; PR 3 = PBz 3, PCy 3, PPhCy 2, PPh 2Cy, PPh 2 iPr, PPh 2C 6F 5, PPh 3, P( m-tolyl) 3, P( p-tolyl) 3, PBu 3, PPhMe 2, PPh 2Me, (bis-1,2-Ph 2P)- C 6H 4, Ph 2PCH 2PPh 2 or Ph 2PCH 2CH 2PPh 2). The platinum-195 chemical shift is shown to be relatively sensitive to the nature of complex geometry. The unsymmetrical (unsym.) cis isomer absorbs at a lower frequency (upfield) than the symmetrical (sym.) trans isomer and in a somewhat higher platinum chemical shift than in the equivalent terminal complexes [PtX 2(PR 3) 2], The 1 J(Pt-P) couplings are consistent and related to the cone-angle data of the ligand and the 1 J value of sym. trans form is 1.26 times that for the unsym, cis form and shows some solvent dependency. Interestingly, two isomers of the unsym. cis dimer were identified, and then one of the isomers isomerized to the sym. trans dimer. The nature of the reaction intermediate and products was found to be very dependent on both the reactants and conditions.

Reaktive EC ( p-p)π-systeme : IX. Diels-alder-reaktionen des acyclischen arsaalkens F 3CAsCF 2

The Diels-Alder adducts 2–8: ▪ are prepared in a one-pot procedure by [2 + 4] cycloaddition of the perfluoro-2-arsapropene F 3CAsCF 2 ( 1) to the corresponding 1,3-dienes, producing 1 in situ by thermal (70–80°C) elimination of Me 3SnF from trimethylstannylbis(trifluoromethyl)arsane. The extreme reactivity of 1 is demonstrated by its reaction with thiophene. The observed formation of the cis and trans dimers (F 3CAsCF 2) 2 as by-products is due to partial cycloreversion of the Diels-Alder compounds.

Spectroscopic study of NO adsorption on magnesium oxide, nickel oxide and their solid solutions

The adsorption of NO on pure MgO at 77 K gives N2O2(cis) dimers on {100} faces and NO–2(nitrite), N2O2–2(hyponitrite) and N2O2(cis) dimers on edges and steps. The hyponitrite anions (which are the precursors of N2O gas formation) and the dimeric N2O2(cis) species are absent at room temperature, while the NO–2 species are formed in larger proportions. NO adsorption on dilute NiO–MgO solid solutions not only gives the species found on pure MgO but also surface Ni2+—NO nitrosyls; moreover, the presence of Ni2+ ions on the surface depresses the formation of N2O2–2(hyponitrite) anions and favours the formation of N2O in the gas phase. On pure NiO, surface Ni2+—NO nitrosyls are the main product of NO adsorption at both room temperature and 77 K. The measured coverage is, however, only half the theoretical one, because repulsive forces prevent the occupation of all surface Ni2+ sites. The stability of the Ni2+—NO adducts prevents the extensive formation of N2O2 dimers.

Reactive state selectivity in photodimerization through micellar counter-ion effects: photodimerization of acenaphthylenes

Photodimerization of acenaphthylene and 5,6-dichloroacenaphthylene solubilized in sodium dodecylsulphate (SDS), cetyltrimethylammonium chloride (CTAC), dodecyltrimethylammonium chloride (DTAC), cetyltrimethylammonium bromide (CTAB) and Triton X-100 micelles gives a mixture of cis and trans dimers. The magnitude of the cis:trans ratio is sensitive to the type of micelle used. In CTAB micelles the heavy atom effect of the bromide counter-ions leads to an increased triplet-derived trans dimer yield, whereas in micelles with light atom counter-ions (CTAC, DTAC and SDS) the singlet-derived cis dimer predominates.

ChemInform Abstract: REACTIONS USING MICELLAR SYSTEMS. IV. REGIOSELECTIVITY IN THE PHOTODIMERIZATIONS OF 2‐PYRIDONES IN MICELLES AND REVERSED MICELLES

AbstractDie Anteile der verschiedenen gebildeten [4 + 4]‐Dimeren (II)‐(V) bei der Photolyse des Pyridons (I) werden in verschiedenen micellaren und reversen micellaren Systemen bestimmt.

Synthesis and reactivity of some p-mesityl-substituted phosphorus compounds

Abstract : The preparation and characterization of a series of P-mesityl substituted phosphorus compounds containing the Si-N-P and/or Si-C-P linkages are described. The reaction of RPC12 (R = 2,4,60Me3C6H2) with one equivalent of (Me3Si)2NLi affords the thermally unstable chlorophosphine (Me3Si)2NP(R)Cl (compound 1) in nearly quantitative yield. Substitution reactions of compound 1 are used to prepare the stable derivatives (Me3Si)2NP(R)X (compound 2, X = Me; compound 3, X = H; compound 4, X = CH2SiMe3), while treatment with Me3SiN3 yields the unstable axidophosphine (Me3Si)2NP(R)N3 (compound 5). The methylphosphine compound reacts smoothly with CC14 via elimination of CHCl3 with a (1,3)-silyl shift to form the chlorophosphinimine Me3SiN=P(R) (CH2SiMe3)Cl (compound 6), which is readily converted to the P-Me analogue compound 7 by reaction with MeLi. Depending on the reaction stoichiometry, treatment of RPCl2 with Me3SiCH2MgCl gives either the chlorophosphine Me3SiCH2P(R)Cl (compound 8) as an unstable product from which compound 4 is produced by reaction with (Me3Si)2NLi, or the stable disubstituted phosphine (Me3SiCH2)2PR (compound 9). Decomposition of the azidophosphine compound 5 proceeds with elimination of nitrogen and formation of the dimeric forms of the di(imino)phosphorane RP double bond(NSiMe3)2 (compound 5 a). Either the cis (compound 10) or trans (compound 11) four-membered ring dimer can be isolated under appropriate conditions. Proton, C(13), P(31) NMR data are reported for the new compounds. (Author)