48175-68-4Relevant academic research and scientific papers
Hydrolysis of the nickel-carbon bond of organonickel tetramethylcyclam complexes
Ram,Espenson, James H.,Baka?, Andreja
, p. 4115 - 4118 (2008/10/08)
The cationic organometallic complexes (1R,4S,8R,11S)-RNi(tmc)+ (where tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) hydrolyze in alkaline, aqueous solutions to yield the alkane RH and HONi(tmc)+, the hydroxonickel(II) macrocycle. The rates of reaction are independent of pH (for 9 a few special cases, the hydrolysis rate constants are rather insensitive to the variation of R in a series of 20 compounds that includes alkyls (R = CH3, C2H5, n-C3H7, i-C3H7, sec-C4H9, CH2C6H5), cyclopropyl, and substituted alkyls (e.g., (CH2)nX, with X = Br, Cl, OH, and OTs; n = 3-6). These complexes, and also such bimetallic analogues as [Ni(tmc)(CH2)5Cr([15]aneN4)3+], have khyd in the range (0.8-2.7) × 10-2 s-1 at 25.0°C. The activation parameters also span a narrow range, with ΔH* ～ 16 kcal mol-1 and ΔS* ～ -14 cal mol-1 K-1. The solvent deuterium kinetic isotope effect determined for several of these complexes is appreciable, kH/kD = 2.2-3.5, suggesting appreciable proton transfer from water to the incipient hydrocarbon in the transition state. Nucleophilic assistance by OH-, OAc-, and NH3 was not observed. Exceptions to the general pattern were noted. The most striking cases are R = (CH2)3OH, where the enhanced rate of hydrolysis (khyd = 0.23 s-1) suggests assistance from the pendant OH group in a cyclic transition state and cycloalkyls (other than cyclopropyl), where the enhanced rates (e.g., cyclobutyl ～0.2 s-1 and cyclopentyl >2 s-1) suggest an acceleration by the strain of the nickel-carbon bond. In acidic solutions, the hydrolysis rate is enhanced because of direct protonolysis. The bimolecular reaction between C2H5Ni(tmc)+ and H3O+ has k = 5 × 105 M-1 s-1.
Free-radical pathways to alkyl complexes of a nickel tetraaza macrocycle
Ram,Bakac, Andreja,Espenson, James H.
, p. 3267 - 3272 (2008/10/08)
The cationic nickel(I) macrocycle (1R,4S,8R,11S)-(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)nickel(I) , abbreviated R,S,R,S-[Ni(tmc)]+, reacts in aqueous, alkaline solutions on the stopped-flow time scale with alkyl halides to form a new series of organonickel complexes. Kinetic data were obtained for a large number of alkyl halides. The trends in the rate constants are benzyl > allyl > secondary > primary > methyl > cyclopropyl, and RI > RBr > RCl. These trends suggest that carbon-centered free radicals R? are produced by a bimolecular reaction between Ni(tmc)+ and RX and are then captured by a second Ni(tmc)+. Further evidence for free-radical involvement comes from cyclization of the radical produced from 6-bromo-1-hexene, from the yields of products in those instances where dimerization of the free radical competes with its capture by Ni(tmc)+, and from the nonreactivity of alkyl tosylates. The organonickel complexes slowly hydrolyze in unimolecular processes to yield hydrocarbon and the nickel(II) complex R,S,R,S-[Ni(tmc)]2+. The organonickel complexes do not undergo unimolecular homolysis but react with Co(II) macrocycles with a 1:2 stoichiometry to form cobalt-carbon bonds. This reaction most likely occurs not by homolytic displacement but by electron transfer followed by radical capture.
Stabilization of the monovalent nickel complex with 1,4,8,11-tetraazacyclotetradecane in aqueous solutions by N- and C-methylation. An electrochemical and pulse radiolysis study
Jubran, Nusrallah,Ginzburg, Gregory,Cohen, Haim,Koresh, Yaacov,Meyerstein, Dan
, p. 251 - 258 (2008/10/08)
The divalent nickel complexes with 1,4,8,11-tetraazacyclotetradecane (L1), 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (L2), meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (L3), and 1,4,5,7,7,8,11,12,14,14-decamethyl-1,4,8,11-tetraazacyclotetradecane (L4) were reduced by reactions with eaq- and CO2-? and by electrochemical reactions in aqueous solutions. The redox potentials of the NiLi2+/NiLi+ couples are -1.58, -1.15, -1.42, and -0.98 V vs. SCE for i = 1, 2, 3, and 4, respectively. The UV absorption bands of NiLi+ are attributed to CTTS transitions. The kinetics of reduction of Co(NH3)63+, Ru(NH3)63+, O2, and N2O by NiLi+ are reported and discussed. The self-exchange rates of reaction between NiLi+ and NiLi2+ were calculated by using the Marcus cross relation. The EPR spectra of NiL2+ and NiL4+ are reported. The complexation of NiLi2+ by OH- was studied. The results are discussed in detail. NiL2+ and NiL4+ are suggested as new, powerful, easily attainable single-electron-reducing agents that can be used over a wide pH range in aqueous solutions.
