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 CHCl3 (1), CH3CCl3 (2), CH3CHCl2 (3), or C6H6 (4), and [Fe3O(O2CCH3)6(py)3]S, where S is either CHCl3 (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 Rw = 0.0363 for 1166 reflections with Fo > 5σ(Fo). 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 CHCl3 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 CHCl3 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 Rw = 0.0377 for 1373 reflections with Fo > 5σ(Fo)]. The CHCl3 solvate molecules in 5 are disordered the same way as those in 1. Substantiation for the onset of dynamic disorder of the CHCl3 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 ΔS = (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 = Rln 4) 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 FeII and FeIII 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 Cp 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 Mössbauer 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 CH3CHCl2 solvate molecule becomes valence detrapped on the Mössbauer time scale at 45° higher temperature than for the CH3CCl3 solvate 2. A magnetically oriented sample of 20 small crystals of CDCl3 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 CDCl3 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.
|Number of pages||14|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1989 Dec 1|
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