ELECTRON-TRANSFER KINETICS OF COPPER(II I) MACROCYCLIC TETRATHIAETHERCOMPLEXES - INFLUENCE OF RING SIZE UPON GATED BEHAVIOR/

The values of the electron self-exchange rate constants, k(11(ex)), fo r the copper(II/I) complexes formed with the cyclic tetrathiaethers [1 3]aneS(4) and [15]aneS(4) have been determined using H-1-NMR line-broa dening measurements in D2O at several different temperatures to yield the following results for 25 degrees C, corrected to mu = 0.1 M (NO3-) : for Cu-II/I([13]aneS(4)), k(11(ex)) = 3.2 x 10(5) M(-1) s(-1), Delta H-double dagger = 10 +/- 1 kJ mol(-1), Delta S-double dagger = -106 /- 7 J K-1 mol(-1); for Cu-II/I([15]aneS(4)), k(11(ex)) = 1.2 x 10(4) M(-1) s(-1), Delta H-double dagger = 21 +/- 1 kJ mol(-1), Delta S-doub le dagger = -97 +/- 7 J K-1 mol(-1). The cross-reaction rate constants have also been determined in aqueous solution by means of stopped-flo w methods for these complexes reacting with a series of selected oxida nts and reductants: Co-II(Me(4)[14]tetraeneN(4))(H2O)(2), Ru-II(NH3)(4 )bpy, Ru-III(NH3)(5)py, RU(II)(NH3)(6), COIII(Me(4)[14]tetraeneN(4))(H 2O)(2), Ru-III(NH3)(4)bpy, Ni-III([14]aneN(4))(H2O)2, Ru-III(NH3)(2)(b py)(2), and Fe-III(4,7-Me(2)phen)(3). The self-exchange rate constants calculated by applying the Marcus relationship to the rate constants for reactions involving Cu(II)L reduction (k(11(Red))) are within expe rimental error of each other and agree with the k(11(ex)) values deter mined by NMR. However, as in earlier studies, the self-exchange rate c onstant values calculated from Cu(I)L oxidation reactions (k(11(Ox))) are generally smaller, except for very slow cross reactions. This patt ern of behavior is in agreement with our previously proposed dual-path way square scheme in which conformational change and the electron-tran sfer step occur in a sequential, rather than a concerted, manner. For the Cu-II/I([13]aneS(4)) system; a lower limit of k(RP) greater than o r equal to 200 s(-1) is estimated for the rate constant representing t he conformational change from the ground state Cu(I) species to the me tastable intermediate which precedes the electron transfer step via th e preferred pathway. For the Cu-II/I([15]aneS(4)) system, a rough limi t of k(RP) less than or equal to 5 s(-1) is suggested by the data. Mor eover, for this latter system, the secondary pathway was found to be m ore competitive than is the case for either the Cu-II/I([13]aneS(4)) s ystem or the previously studied Cu-II/I([l4]aneS(4)) system, so that g ated behavior is anticipated to occur only within a very narrow set of conditions.