14014-06-3Relevant articles and documents
N-doping of organic electronic materials using air-stable organometallics: A mechanistic study of reduction by dimeric sandwich compounds
Guo, Song,Mohapatra, Swagat K.,Romanov, Alexander,Timofeeva, Tatiana V.,Hardcastle, Kenneth I.,Yesudas, Kada,Risko, Chad,Bredas, Jean-Luc,Marder, Seth R.,Barlow, Stephen
, p. 14760 - 14772 (2012)
Several 19-electron sandwich compounds are known to exist as "2×18-electron" dimers. Recently it has been shown that, despite their air stability in the solid state, some of these dimers act as powerful reductants when co-deposited from either the gas phase or from solution and that this behavior can be useful in n-doping materials for organic electronics, including compounds with moderate electron affinities, such as 6,13-bis[tri(isopropyl)silylethynyl]pentacene (3). This paper addresses the mechanisms by which the dimers of 1,2,3,4,5-pentamethylrhodocene (1 b 2), (pentamethylcyclopentadienyl)(1,3,5-trialkylbenzene)ruthenium (alkyl=Me, 2 a2; alkyl=Et, 2 b2), and (pentamethylcyclopentadienyl)(benzene)iron (2 c2) react with 3 in solution. Vis/NIR and NMR spectroscopy, and X-ray crystallography indicate that the products of these solution reactions are 3.- salts of the monomeric sandwich cations. Vis/NIR kinetic studies for the Group 8 dimers are consistent with a mechanism whereby an endergonic electron transfer from the dimer to 3 is followed by rapid cleavage of the dimer cation. NMR crossover experiments with partially deuterated derivatives suggest that the C-C bond in the 1 b2 dimer is much more readily broken than that in 2 a 2; consistent with this observation, Vis/NIR kinetic measurements suggest that the solution reduction of 3 by 1 b2 can occur by both the mechanism established for the Group 8 species and by a mechanism in which an endergonic dissociation of the dimer is followed by rapid electron transfer from monomeric 1 b to 3. Doped up: Air-stable dimers of pentamethylrhodocene and pentamethylcyclopentadienyl arene ruthenium and iron can be used to n-dope acceptors such as bis[tri(isopropyl)silylethynyl] pentacene. NMR crossover experiments and variable-temperature Vis/NIR kinetic measurements indicate that, depending on the reaction conditions and the choice of dimer and acceptor, this doping can take place by two different mechanisms (see scheme). Copyright
Isotope quantum effects in water around the freezing point
Hart, R. T.,Mei, Q.,Benmore, C. J.,Neuefeind, J. C.,Turner, J. F. C.,et al.
, p. 1 - 8 (2008/10/09)
We have measured the difference in electronic structure factors between liquid H2 O and D2 O at temperatures of 268 and 273 K with high energy x-ray diffraction. These are compared to our previously published data measured from 279 to 318 K. We find that the total structural isotope effect increases by a factor of 3.5 over the entire range, as the temperature is decreased. Structural isochoric temperature differential and isothermal density differential functions have been used to compare these datato a thermodynamic model based upon a simple offset in the state functi on. The model works well in describing the magnitude of the structural differences above ～310 K, but fails at lower temperatures. The experimental results are discussed in light of several quantum molecular dynamics simulations and are in good qualitative agreement with recent temperature dependent, rotationally quantized rigid molecule simulations.
New insights on the mechanism of palladium-catalyzed hydrolysis of sodium borohydride from11B NMR measurements
Guella,Zanchetta,Patton,Miotello
, p. 17024 - 17033 (2008/10/09)
To gain insight on the mechanistic aspects of the palladium-catalyzed hydrolysis of NaBH4 in alkaline media, the kinetics of the reaction has been investigated by 11B NMR (nuclear magnetic resonance) measurements taken at different times during the reaction course. Working with BH4- concentration in the range 0.05-0.1 M and with a [substrate]/[catalyst] molar ratio of 0.03-0.11, hydrolysis has been found to follow a first-order kinetic dependence from concentration of both the substrate and the catalyst (Pd/C 10 wt %). We followed the reaction of NaBH4 and its perdeuterated analogue NaBD4 in H2O, in D 2O and H2O/D2O mixtures. When the process was carried out in D2O, deuterium incorporation in BH4 - afforded BH4-nDn- (n = 1, 2, 3, 4) species, and a competition between hydrolysis and hydrogen/deuterium exchange processes was observed. By fitting the kinetics NMR data by nonlinear least-squares regression techniques, the rate constants of the elementary steps involved in the palladium-catalyzed borohydride hydrolysis have been evaluated. Such a regression analysis was performed on a reaction scheme wherein the starting reactant BH4- is allowed both to reversibly exchange hydrogen with deuterium atoms of D2O and to irreversibly hydrolyze into borohydroxy species B(OD)4-. In contrast to acid-catalyzed hydrolysis of sodium borohydride, our results indicate that in the palladium-catalyzed process the rate constants of the exchange processes are higher than those of the corresponding hydrolysis reactions.
The structure of Co(η-C5H5)2+ and NMe4+ intercalates of MnPS3: An X-ray, neutron-diffraction, and solid-state NMR study
Evans,O'Hare,Clement
, p. 4595 - 4606 (2007/10/02)
This paper reports a series of structural studies on the intercalation compounds of MnPS3, with the aim of trying to understand the source of the magnetic and NLO properties they exhibit. Both neutron and X-ray powder experiments and single crystal studies have been performed on Mn1-xPS3{G}2x(H2O)y for G = Co(η-C5H5)2+ [x = 0.34, y = 0.3] and N(CH3)4+ [x = 0.32, y = 0.9]. These investigations have suggested that the individual host layers of these intercalates contain an ordered arrangement of metal vacancies, giving rise to large superlattices in the ab plane of the crystal; it is this vacancy arrangement which is believed to give rise to the changes observed in the magnetic properties of these systems on intercalation. In the case of the Co(η-C5H5)2+ intercalate, the degenerate ways of layer stacking lead to a considerably disordered overall structure such that the 3-dimensional diffraction of this intercalate can be described by a considerably smaller overall unit cell than the pristine host lattice. These layer shifts also lead to a 3-layer repeat structure and a change in symmetry of the host lattice from CZ/m to R3m. The overall diffraction pattern can then be described by a cell of dimensions a = b = 3.532(3) A?, c = 35.57(2) A?, γ= 120°. 2H solid-state NMR experiments have been used to investigate the orientational preferences and dynamic properties of the cobaltocene guest molecules; these have shown a static arrangement of guest molecules at room temperature lying with their principal molecular axes parallel to the host lattice layers. As the temperature is increased the molecules begin to rotate around their C2 axis. The tetramethylammonium intercalate appears to be considerably more ordered than the cobaltocenium intercalate, suggesting the existence of a guest superlattice in addition to the metal vacancy superlattice. The overall structure is then best explained by a hexagonal metal vacancy lattice of dimension a = 10.6 A?, and a guest lattice of dimensions 18.3 × 6.1 A?. The combination of these two cells leads to an overall hexagonal cell of dimensions a = b = 36.6 A?. This suggests that the overall structure of these materials can only be fully explained by adopting a unit cell with the formula Mn60P72S216{Guest}12.
Metal ion - biomolecule interactions. Part 16. C(2)-H isotopic exchange in Co(III)-coordinated imidazoles
Buncel, Erwin,Yang, Fan,Moir, Robert Y.,Onyido, Ikenna
, p. 772 - 780 (2007/10/03)
Transition-metal-bound imidazoles are suitable models for evaluating the roles of metal ions in biomolecules having the imidazole moiety and similar heterocyclic residues as part of their structure.Such studies provide useful insights into metal-biomolecule interactions in biological systems, especially when the lability of the metal-ligand bond is substantially reduced, such that the identity of the metal-ligand complex is preserved during the course of the reaction under investigation.The present paper reports on a kinetic study of tritium exchange from the C(2) position of the imidazole moiety in the substitution-inert complex cations -imidazole>3+ (1) and -imidazole>3+ (2).Rate-pH profiles have been determined in aqueous solution at 60 deg C.Both substrates are believed to react through rate-determining attack of hydroxide ion (kM+ pathway) at C(2)-T.Dissection of the kinetic data reveals an additional pathway for 1 consequent upon deprotonation of its pyrrole-like N-H(T) to yield 3, which is then attacked by hydroxide at C(2) (kM pathway).The ratio kM+/kM=103 that is obtained is in accord with the expected reduced reactivity of 3.Comparison of the present data with those reported for a variety of heterocyclic substrates shows that the order of reactivity, protonated >> metal ion coordinated >>neutral form of substrates, prevails.The superiority of the proton over metal ions in catalyzing isotopic hydrogen exchange is attributed to its larger gorund state acidifying effect coupled with the greater transition state stabilization in affords, relative to metal ions.The exchange reaction of 3 via the kM pathway is the first example of a rective anionic species in which the negative charge is located α to the exchanging C-H.Key words: tritium exchange, cobalt (III)-coordinated imidazoles.
Gas phase kinetics of the reactions of Na0 with H2, D2, H2O, and D2O
Ager, Joel W.,Howard, Carleton J.
, p. 921 - 925 (2007/10/02)
The gas phase reactions of the NaO radical with H2, D2, H2O, and D2O were studied in a flow tube reactor at room temperature.The reaction of NaO with H2 has two exothermic product channels, NaOH + H and Na + H2O.Both channels were observed and the Na formation channel produces some Na in the 32P state.The rate constants for the abstraction channel for H2 and D2 reactants are (2.6+/-1.0) X 10-11 and ( 1.1 X 0.4) X 10-11 cm3 molecule-1 s-1 at 296+/-2 K.The reaction of NaO with H2O was shown to be second order and the products are assumed to be NaOH and OH.The rate constants for H2O and D2O reactants are (2.2+/-0.4) X 10-10 and ( 1.2+/-0.2) X 10-10 cm3 molecule-1 s-1 at 298+/-1 K.The measured NaO + H2O rate constant is compared to the predicted collision rate constant from a model based on the large attractive dipole-dipole force between NaO and H2O.The role of these reactions in mesospheric Na chemistry is briefly discussed.
