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 μ3-oxo trinuclear clusters [Fe3(μ3-O)(μ-O2CCH3)6(4-Phpy)3](ClO4) (1) and [Fe3(μ3-O)(μ-O2CAd)6(4-Mepy)3](NO3) (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 1H NMR. An equilibrium between the mixed pyridine clusters [Fe3(μ3-O)(μ-O2CAd)6(4-Mepy)3–x(4-Phpy)x](NO3) (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: kex298 = 6.5 ± 1.3 × 10–1 s–1, ΔH‡ = 89.47 ± 2 kJ mol–1, and ΔS‡ = +51.8 ± 6 J K–1 mol–1 for 1 and kex298 = 3.4 ± 0.5 × 10–1 s–1, ΔH‡ = 91.13 ± 2 kJ mol–1, and ΔS‡ = +51.9 ± 5 J 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 [FeIII2FeII(μ3-O)(μ-O2CCH3)6(4-Phpy)3] (3). In 3, the intramolecular FeIII/FeII 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-Phpy ligand exchange reactions on these two clusters, suggesting an outer-sphere mechanism: k2298 = 72.4 ± 1.0 × 103 M–1 s–1, ΔH‡ = 18.18 ± 0.3 kJ mol–1, and ΔS‡ = −90.88 ± 1.0 J K–1 mol–1. The [Fe3(μ3-O)(μ-O2CCH3)6(4-Phpy)3]+/0 electron self-exchange reaction is compared with the more than 3 orders of magnitude faster [Ru3(μ3-O)(μ-O2CCH3)6(py)3]+/0 self-exchange reaction (ΔΔGexptl‡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 ΔGexptl‡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 (6.1 kJ mol–1) which arises as a result of the fact that each encounter is not electron-transfer spin-allowed for the iron redox couple.