NMR Study of Ligand Exchange and Electron Self-Exchange between Oxo-Centered Trinuclear Clusters [Fe3(μ3-O)(μ-O2CR)6(4-R´py)3]+/0

The syntheses, single crystal X-ray structures, and magnetic properties of the homometallic mu(3)-oxo trinuclear clusters Fe-3(mu(3)-O)(mu-O2CCH3)(6)(4-Phpy)(3) (1) and F(e)3(mu(3)-O)(mu-O(2)CAd)(6)(4-Mepy)(3) (2) are reported (Ad = adamantane). The persistence of the trinuclear structure within 1 and 2 in CD2Cl2 and C2D2Cl4 solutions in the temperature range 190-390 K is demonstrated by H-1 NMR. An equilibrium between the mixed pyridine dusters Fe-3(mu(3)-O)(mu-O(2)CAd)(6)(4-Mepy)(3-x)(4-Phpy)(x) (x = 0, 1, 2, 3) with a close to statistical distribution of these species is observed in CD2Cl2 solutions. Variable-temperature NMR line-broadening made it possible to quantify the coordinated/free 4-Rpy exchanges at the iron centers of 1 and 2:k(ex)(298) = 6.5 +/- 1.3 x 10(-1) s(-1), Delta H-double dagger = 89.47 +/- 2 kJ mol(-1), and Delta S-double dagger = +51.8 +/- 6 J K-1 mol(-1) for 1 and k(ex)(298) = 3.4 +/- 0.5 x 10(-1) s(-1) , Delta H-double dagger = 91.13 +/- 2 kJ mol(-1), and Delta S-double dagger = +51.9 +/- 53 K-1 mol(-1) for 2. A limiting D mechanism is assigned for these ligand exchange reactions on the basis of first-order rate laws and positive and large entropies of activation. The exchange rates are 4 orders of magnitude slower than those observed for the ligand exchange on the reduced heterovalent cluster [(Fe2FeII)-Fe-III(mu(3)-O)(mu-O2CCH3)(6)(4-Phpy)(3)] (3). In 3, the intramolecular Fe-III/Fe-II electron exchange is too fast to be observed. At low temperatures, the 1/3 intermolecular second-order electron self-exchange reaction is faster than the 4-Phv ligand exchange reactions on these two clusters, suggesting an outer-sphere mechanism: k(2)(298) = 72.4 +/- 1.0 x 10(3) M-1 s(-1), Delta H-double dagger = 18.18 +/- 0.3 kJ mol(-1), and Delta S-double dagger = -90.88 +/- 1.0 J K-1 mol(-1). The Fe-3(mu(3)-O)(mu-O2CCH3)(6)(4-Phpy)(3) electron self-exchange reaction is compared with the more than 3 orders of magnitude faster Ru-3(mu(3)-O)(mu-O2CCH3)(6)(py)(3) self-exchange reaction (Delta Delta G(exptl)(double dagger 298) = 18.2 kJ mol(-1)). The theoretical estimated self-exchange rate constants for both processes compare reasonably well with the experimental values. The equilibrium constant for the formation of the precursor to the electron-transfer and the free energy of activation contribution for the solvent reorganization to reach the electron transfer step are taken to be the same for both redox couples. The larger Delta G(exptl)(double dagger 298) for the 1/3 iron self-exchange is attributed to the larger (11.1 kJ mol(-1)) inner-sphere reorganization energy of the 1 and 3 iron clusters in addition to a supplementary energy (6.