Electron-transfer kinetics of copper(II/I) macrocyclic tetrathiaether complexes. Influence of ring size upon gated behavior

Abstract

The values of the electron self-exchange rate constants, kl I (~~) , for the copper(II/I) complexes formed with the cyclic tetrathiaethers [ 131aneS4 and [ 1 51aneS4 have been determined using IH-NMR line-broadening measurements in D20 at several different temperatures to yield the following results for 25 OC, corrected to p = 0.1 M (NO3-): for CuIV1 ([13]aneS4), kll(ex) = 3.2 X 105 M-1 s-1, AH* = 10 f 1 kJ mol-', AS* = -106 f 7 J K-1 mol-1; for CuII/1- ([15]aneS,), kll(cx) = 1.2 X 104 M-1 s-1, AH* = 21 f 1 kJ mol-1, AS* = -97 f 7 J K-1 mol-'. The cross-reaction rate constants have also been determined in aqueous solution by means of stopped-flow methods for these complexes reacting with a series of selected oxidants and reductants: CoII(Med[ 141 tetraeneN4)(H20)2, Ru1I(NHs)4bpy, RuI1(NH3)5py, RuI1(NH3)6, CoI1l(Me4[ 141 tetraeneN4)( H20)2, Ru1I1(NHd4bpy, NilI1( [ 14]aneN4)(H20)2, Ru1lI(NH3)2(bpy)2, and FeI1I(4,7-Me2phen)3. The self-exchange rate constants calculated by applying the Marcus relationship to the rate constants for reactions involving CuIIL reduction (kl I ( R~) ) are within experimental error of each other and agree with the kl values determined by NMR. However, as in earlier studies, the self-exchange rate constant values calculated from CuIL oxidation reactions (kl '(ox)) are generally smaller, except for very slow cross reactions. This pattern of behavior is in agreement with our previously proposed dual-pathway square scheme in which conformational change and the electron-transfer step occur in a sequential, rather than a concerted, manner. For the Cul1/1([ 131aneS4) system, a lower limit of k~p 1 200 s-I is estimated for the rate constant representing the conformational change from the ground state Cu(1) species to the metastable intermediate which precedes the electron transfer step via the preferred pathway. For the CuII/I( [ 1 51aneS4) system, a rough limit of k~p I 5 s-I is suggested by the data. Moreover, for this latter system, the secondary pathway was found to be more competitive than is the case for either the CulI/I([l3]aneS4) system or the previously studied Cu1I/1( [ 14laneS4) system, so that gated behavior is anticipated to occur only within a very narrow set of conditions. IntroductionStudies on CuII/IL systems (L = coordinated ligand) have often

Electron Transfer by Copper Centers

“…A closely related ligand, designated as [15]aneNS 4 ( Figure 11), in which one of the sulfur donor atoms was replaced by an amine nitrogen donor atom, yielded k 11 ) (1-2) × 10 5 M -1 s -1 by NMR, and this value was shown to be consistent with k 11 values generated from both Cu II L reduction and Cu I L oxidation reactions. 151 The Cu II/I ([13]aneS 4 ) system, which was cited earlier with the macrocyclic polythiaether complexes, also yielded a large k 11 value of 3 × 10 5 M -1 s -1 as determined by NMR with corroborative k 11 values being generated from reduction reactions of Cu II L. 133 An examination of the crystal structures shows that this system is structurally similar to the Cu II/I ([15]aneS 5 ) system in that both Cu II ( [15]aneS 5 ) 169 and Cu II ( [13]aneS 4 ) 170 are square pyramidal, the latter having a water molecule occupying the apical site. Upon reduction of the former complex, one of the Cu-S bonds in the plane dissociates to generate a tetrahedral complex in which none of the donor atoms is required to change the orientation of its lone electron pair.…”

“…56,133,134 The electron-transfer kinetics for each of these five systems were measured with a total of four reductants and four oxidants, and independent k 11 values were also obtained from NMR line broadening measurements. 56,133,134 In all five systems, the calculated k 11(Red) values were in close agreement with the corresponding values obtained by NMR (Table 7). This observation was in sharp contrast to the original suggestion by Lee and Anson 39 that the k 11 value for Cu(II/I) systems, as determined directly without invoking eq 14, should be the geometric mean of k 11(Red) and k 11(Ox) (see section 4.3).…”

“…Of the five Cu(II/I) complex systems noted above, k RP values of 50, 120, and 50 s -1 were reported for [14]aneS 4 , syn- [14]aneS 4 -diol, and anti- [14]aneS 4 -diol, respectively, with limiting values of g200 and e5 s -1 for the complexes with [13]aneS 4 and [15]aneS 4 , respectively. 56,133,134 If all three terms in eq 20′′ contribute to the reaction kinetics (i.e., second-order kinetics via pathway A, first-order kinetics due to rate-limiting conformational change, and second-order kinetics via pathway B) as the counterreagent concentration is varied, curve fitting can be used to resolve the individual values of k 21(A) , k 21(B) , and k RP . 134 A prime example of such multifold behavior is illustrated in Figure 15 for the oxidation of Cu I (trans-cypt- [14]aneS 4 ) + by Ni III ( [14]aneN 4 ) 3+ for which the three limiting conditions overlap significantly as the oxidant concentration is varied.…”

Chelate ring sequence effects on thermodynamic, kinetic and electron-transfer properties of copper(ii/i) systems involving macrocyclic ligands with S4 and NS3 donor setsElectronic supplementary information (ESI) available: Tables S1–14: tabulations of experimental rate constants. See http://www.rsc.org/suppdata/dt/b3/b300602f/

“…Each of the cross-reaction rate constant values (k 12 and k 21 ) in Table 5 were used to generate values for the apparent electron self-exchange rate constant, k 11 , by means of the Marcus cross relationship: 59 where k 22 represents the self-exchange rate constant for the counter reagent; K 12 and K 21 represent the equilibrium constants for the overall reactions as calculated from the redox potentials; f 12 and f 21 represent non-linear terms calculated from As noted in the Introduction, our earlier studies on Cu II/I -( [14]aneS 4 -a) 1 and related systems, 2-7, 60 have shown that their electron-transfer kinetic behavior is consistent with a mechanistic scheme in which a major conformational change and the electron-transfer step occur sequentially rather than concertedly to yield at least two competing pathways, A and B, (Fig. 1).…”

Comparative study of donor atom effects on the thermodynamic and electron-transfer kinetic properties of copper(ii/i) complexes with sexadentate macrocyclic ligands. [CuII/I([18]aneS4N2)] and [CuII/I([18]aneS4O2)]

“…11, 20 Reduction of these complexes then involves the rupture of a basal Cu-S bond without the requirement of either donor atom inversion or the rupture of a solvent bond, and the electron-transfer kinetics tend to be relatively fast. 6,11 In fact, the copper system involving a substituted 12-membered macrocycle, oxathiane- [12]aneS 4 , has yielded the largest self-exchange rate constant yet recorded for an inorganic Cu(II/I) system. 11 Interestingly, expansion of the macrocyclic ring to 15-members incorporating five thioether sulfurs or four sulfur and one nitrogen donor atoms has also led to faster electron transfer.…”