Lattice-Engineered Micromodulation of Intramolecular Electron-Transfer Rates in Trinuclear Mixed-Valence Iron Acetate Complexes

The factors influencing the rate of intramolecular electron transfer in the solid state have been investigated for the oxo-centered, mixed-valence complexes [Fe₃O(O₂CCH₃)₆(4-Me-py)₃]S, where S is either CHC1₃(1), CH₃CC1₃ (2), CH₃CHC1₂(3), or C₆H₆(4), and [Fe₃O(O₂CCH₃)₆(py)₃]S, where S is either CHC1₃(5) or C₅H₅N (6). Complex 1 crystallizes in the rhombohedral space group R32 with three molecules in a unit cell with dimensions a = 18.759 (6) Å, and c = 10.373 (3) A at 298 K. The final discrepancy factors are R = 0.0323 and R_w = 0.0363 for 1166 reflections with F_○ > 5σ(F_○). The bond distances about each iron atom are Fe-O(oxide) = 1.912 (1) Å, Fe-N = 2.221 (4) Å and an average Fe-O(acetate) distance of 2.080 Å. Along the c-axis of complex 1, Fe₃O complexes and CHC1₃ solvate molecules occupy alternating sites of 32 symmetry. The CHCl₃ solvate molecule is disordered about the three C₂ axes which are perpendicular to the C₃ axis. This disorder was modeled with the three Cl atoms in a plane, above and below which the C-H moiety was disordered with the C-H vector along the C₃ axis. Examination of the parameters (thermal and distances) resulting from this fit clearly indicates that the C-H vector of the CHC1₃ solvate is not just sitting on the C₃ axis but is also jumping to positions off the C₃ axis. Complex 5 also crystallizes in the R32 space group with Z = 3, a = 17.819 (6) Å, and c = 10.488 (3) Å at 298 K [R= 0.0327 and R_w = 0.0377 for 1373 reflections with F_○ > 5σ(F_○)]. The CHC1₃ solvate molecules in 5 are disordered the same way as those in 1. Substantiation for the onset of dynamic disorder of the CHC1₃ solvate molecules comes from heat capacity data measured for complex 5 between 14 and 300 K. A phase transition with two peaks closely centered at 207.14 and 208.19 K was found. The total transition enthalpy and entropy gain for the phase transition are ΔH = (5107 ± 44)J mol^-1 and AS = (28.10 ± 0.44) JK^-1 mol^-1. The entropy gain first appears at ~100 K as the temperature is increased from 14 K. The experimental ΔS can be accounted for by a combination of Fe₃O complexes converting from valence trapped in one vibronic state to dynamically converting between all four vibronic states (ΔS = Rln4) and each of the CHCl₃ molecules going from static to dynamically jumping between eight positions (ΔS = Rln 8). The total ΔS = Rln 32 (= 28.82 JK^-1 mol^-1) agrees with the experimental entropy gain. Variable temperature ⁵⁷Fe Mössbauer data are presented for all six complexes. In general, a valence-trapped spectrum with Fe^II and Fe^III doublets is seen at low temperatures. As the temperature is increased, a third doublet characteristic of an undistorted (delocalized) complex appears. Further increase in temperature changes the spectrum at a higher temperature to only a single doublet for delocalized complexes. The two temperatures for these occurrences are seen to be ~160 and ~208 K for complex 5; the latter temperature agrees with the culmination temperature seen in the C_p data for the phase transition of complex 5. The Mössbauer spectrum of complex 1 changes fairly abruptly with the third doublet first seen at ~81 K and the complete conversion to Mossbauer detrapped at ~90-95 K. This agrees with the DTA data for this complex which shows a peak at 95 K. It is interesting that complex 3 with its less symmetric CH₃CHC1₂ solvate molecule becomes valence detrapped on the Mössbauer time scale at 45° higher temperature than for the CH₃CC1₃ solvate 2. A magnetically oriented sample of 20 small crystals of CDC1₃ solvate 5 in a wax block was employed with solid-state ²H NMR spectroscopy to probe the dynamics of the chloroform solvate molecule. With the magnetic field directed down the c-axis the quadrupole splitting of the single ²H NMR doublet remained relatively constant (196–204 kHz) in the range 295–208 K and then decreased below the phase transition temperature at 208 K to become eventually 144 kHz at 110 K. Above 208 K the CDC1₃ solvate molecule is jumping between eight positions, four with the C-D vector pointed up and four down, where for the four up positions one C-D vector position is along the c-axis and the other three are at an angle of 24.7° relative to the c-axis. At 110 K the C-D vector is static and takes only one position at an angle of 31.5° from the c-axis. The nature of the phase transitions seen for each of the 1–6 complexes is discussed. Intermolecular pyridine-pyridine ligand overlaps appear to be important in determining where the phase transition from a valence-trapped to a valence-detrapped state occurs. The solvate molecules, one above and one below each Fe₃O complex, also likely affect the rate of intramolecular electron transfer in each Fe₃O complex as the solvate molecules go from static to dynamic. The experimental results in this paper are compared to the predictions of a theoretical model based on a molecular field calculation.