Browsing by Subject "Charge exchange."
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Item Chemistry, electrochemistry and electron transfer induced reactions of cobalt complexes with fluorinated ligands.(2008-03-03T17:20:55Z) Gunawardhana, Kihanduwage N.; Gipson, Stephen L. (Stephen Lloyd); Chemistry and Biochemistry.; Baylor University. Dept. of Chemistry and Biochemistry.The chemical or electrochemical reduction of the trifluoroacetyl complex CF3COCo(CO)3PPh3 involves a single electron transfer yielding trifluoromethyl radical and an anionic cobalt carbonyl complex. The mechanism is proposed to involve electron transfer followed by initial dissociation of either a carbonyl or phosphine ligand from the 19-electron [CF3COCo(CO)3PPh3 ]- anion. The resulting 17-electron intermediate undergoes subsequent one-electron reductive elimination of trifluoromethyl radical by homolytic cleavage of the carbon-carbon bond of the trifluoroacetyl group. The CF3· radical can be trapped by either benzophenone anion, forming the anion of [a]-(trifluoromethyl)benzhydrol, or Bu3SnH, yielding CF3H. The final organometallic product is an 18-electron anion, either [Co(CO)4]- or [Co(CO)3(PPh3)]-, depending upon which ligand is initially lost. The chemical or electrochemical reduction CF3Co(CO)3PPh3 is a two-electron process involving heterolytic cobalt-carbon bond cleavage to yield trifluoromethyl anion and cobalt carbonyl anions. The trifluoromethyl anion rapidly decomposes to fluoride and difluorocarbene. This carbene may dimerize to form C2F4. The unstable fluoro carbene can also be trapped by cyclohexene. The mechanism proposed for the reduction of C6F5Co(CO)3PPh3 involves a homolytic cobalt-carbon bond cleavage to form C6F5[bullet] radical. The resultant C6F5[bullet] radical abstracts hydrogen or deuterium from the solvent or trace amounts of water to produce C6F5H or C6F5D. With an excess of reducing agent this C6F5[bullet] radical can be further reduced to C6F5- anion before forming pentafluorobenzene by protonation. The inorganic fragment, the 18-electron [Co(CO)3PPh3]- anion, may participate in a ligand exchange reaction to form [Co(CO)4]-. In addition, interesting reactivity was observed between C6F5Co(CO)3PPh3 and tin hydrides, deuterides and chlorides without any reducing agents. We have demonstrated that ligand replacement reactions can be used for the synthesis of new cobalt-NHC complexes with fluorinated alkyl, acyl and aryl ligands. In addition, the X-ray crystal structure of CF3COCo(CO)3PPh3 was obtained to compare the bond lengths and bond angles with other related compounds. An unusual Co-C(acyl) bond length was observed for CF3COCo(CO)3PPh3. Considering the bond lengths of other alkyl and acyl complexes, it can generally be argued that the position of the alkyl/acyl equilibrium varies with the Co-C(alkyl/acyl) bond length.Item Electrochemistry and electron transfer induced substitution reactions of methylcyclopentadienylmolybdenum tricarbonyl complexes and electrospray ionization mass spectrometry and x-ray crystallographic characterization of related molybdenum complexes.(Oxford, UK : Elsevier, 2007) Munisamy, Thiruvengadam.; Gipson, Stephen L. (Stephen Lloyd); Chemistry and Biochemistry.; Baylor University. Dept. of Chemistry and Biochemistry.The complexes Cp'Mo(CO)3X (X = Cl, Br, I; Cp' = C5H4CH3) and [Cp'Mo(CO)3(L)]+ (L = CH3CN, CH3COCH3) were synthesized and their electrochemistry and electron transfer induced substitution reactions were studied. Electrochemical studies of Cp'Mo(CO)3X showed that it is reduced via a DISP-type mechanism. The mechanism was confirmed both chemically and electrochemically. Attempts to perform electron transfer induced substitution reactions in the presence of 2eligands formed [Cp'Mo(CO)3]- as the major product, in addition to Cp'Mo(CO)2(L)X, which was formed in greater amounts when the reducing agent was added in aliquots. [Cp'Mo(CO)3]- is proposed to form via the disproportionation pathway while Cp'Mo(CO)2(L)X is formed via a self-exchange substitution pathway. The disproportionation reaction occurs because of the large formation constants of the 19e- [Cp'Mo(CO)3X]- intermediates. The large formation constants of the 19e-[Cp'Mo(CO)3X]- complexes also prevent the electron transfer chain reaction pathway which has been observed for the isoelectronic CpFe(CO)2X (Cp = C5H5) complexes. The self-exchange substitution reaction occurs between the [Cp'Mo(CO)3]- formed from the disproportionation reaction and Cp'Mo(CO)3X and L. 31P NMR was used to confirm the reaction mechanism. The self-exchange substitution reaction is inhibited at low temperature and under a CO atmosphere. Complexes of the type [Cp'Mo(CO)3(L)]+ (L = CH3CN, CH3COCH3) showed an ECE-type reduction mechanism when studied using cyclic voltammetry and the electron transfer induced substitution formed [Cp'Mo(CO)3(PPh3)]+ and [Cp'Mo(CO)2(PPh3)2]+ as major products via an electron transfer chain pathway. These results confirm that cyclopentadienylmolybdenum carbonyl complexes can undergo an electron transfer chain reaction like the isoelectronic CpFe(CO)2X when unhindered by factors such as large formation constants. Electrospray mass spectrometry was used to characterize the complexes [Cp'Mo(CO)3(CH3CN)]PF6 and [{Cp'Mo(CO)3}2(μ-I)]BPh4. The mass spectra showed the molecular ion peaks in addition to fragment ion peaks for [M-nCO]+. Finally, X-ray crystal structures of cis-Cp'Mo(CO)2(PPh3)I, [{Cp'Mo(CO)3}2(μ-I)]BPh4, [Cp'Mo(CO)3(CH3CN)]BF4, [Cp'Mo(CO)3(C5H5N)]BPh4 and cis-[Cp'Mo(CO)2(C5H5N)2]BPh4 were obtained and their bond lengths and bond angles were found to be in good agreement with those in related molybdenum complexes.