14014-06-3Relevant articles and documents
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Breuer
, p. 2236 (1935)
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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
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.
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.
