Electron transfer. 30. Chromium(III)-bound pyrazine radicals

Electron Transfer. 130. Reductions with Indium(I)1

“…The absence of appreciable acceleration may then be taken as evidence that the more rapid reductions of complexes II and III proceed mainly through the gem -diol forms. Moreover, the very slow reactions of the carbonyl-substituted complexes VI and VII (both of which are reduced rapidly by Cr II ),9b are in accord with the conclusion that reductive attack on the CO function, which is prominent in reductions by Cr 2+ and substantial in reductions by Eu 2+ ,9b does not contribute significantly to the mechanistic picture for In + .…”

“…The situation with respect to N-donor bridges remains clouded. The azido, pyrazine, and 2,6-pyridinedicarboxylato derivatives of (NH 3 ) 5 Co III are reduced by Cr 2+ unusually rapidly ( k Cr > 10 5 M -1 s -1 in each case), 9a,, all three through precursors featuring N−Cr bridging. With In + the rate for the N 3 -bound oxidant is consistent with inner-sphere attack (Table ), but bridging is at most marginal for the pyrazine complex and nondetectable for that of the 2,6-diacid (Table ).…”

“…Since these complexes, like their parent acids, are largely converted by aquation to gem -diols ( VIII ) (5), we may again be seeing a reflection of chelation in the precursor complex ( IX ), as is thought to occur with reductions of these carbonyl-substituted derivatives by Ti(III) . Alternatively, however, it may be asked whether the principal path for these reactions instead uses the carbonyl form of the oxidants, as is the case for reductions by Cr(II) 22 and Eu(II) 9a. It is recognized that these strongly reducing dipositive centers may operate by initial localized 1e reduction of the carbonyl function (the radical-cation mechanism), and the very negative formal potential estimated by Latimer 6a for the In(I,II) couple (−0.40 V), although highly approximate, suggests that this mechanistic variation might apply to In(I) as well.

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“…Indium powder (150 mesh), anhydrous acetonitrile, and anhydrous silver trifluoromethanesulfonate (silver triflate) were Aldrich products. Cobalt(III) complexes not available from previous studies 8 were prepared by literature methods …”

Thermal intramolecular cobalt(III)-iron(II) and cobalt(III)-titanium(III) electron-transfer reactions involving outer-sphere and inner-sphere precursor complexes

“…From the data in Table I it follows clearly that for the Co(III)/Fe(II) couple both parameters-the oxidizing and reducing ability of the involved metal ions-affect the rate of intramolecular ET: the 1:1 complexes of 1 and 2 with [Fe-(OH2)6]2+, respectively, do not undergo thermal ET, and in the case of the respective 2:1 complexes (4 and 5) only 4 produces Fe(III) and Co(II).26 Substitution of NH3 groups at the Co(III) centers by a cyclic secondary amine reduces the oxidizing power of Co(III) substantially and decreases thereby the overall free energy change of the intramolecular reaction.…”