PRONOUNCED EFFECTS OF INTERANNULAR TRIMETHYLENE BRIDGES ON THE INTRAMOLECULAR ELECTRON-TRANSFER RATE IN MIXED-VALENCE BIFERROCENIUM SALTS - VALENCE-TRAPPED ELECTRONIC-STRUCTURE WITH UNUSUAL STRUCTURAL AND MOSSBAUER CHARACTERISTICS

The X-ray structure determinations of two new constitutional isomers of mixed-valence biferrocenium salts and the effects of interannular trimethylene bridges on intramolecular electron-transfer rates in both solid and solution states are reported. The X-ray structure of 1',2:3',4:1''',2'':3''',4''-tetrakis(propane-1,3-diyl)-1,1''-biferrocene (17a) has been determined at 298 K: P1BAR, a = 9.144(1) angstrom, b = 15.051(2) angstrom, c = 17.844(4) angstrom, alpha = 106.39(3)-degrees, beta = 91.73(2)-degrees, gamma = 90.76(2)-degrees, Z = 4, D(c) = 1.496 g cm-3, R(F) = 0.040, and R(wF) = 0.046. The isomeric compound 17b crystallizes in the triclinic space group P1BAR with eight molecules in a unit cell with dimensions a = 9.209(1) angstrom, b = 24.132(2) angstrom, c = 24.501(4) angstrom, alpha = 116.808(9)-degrees, beta = 87.98(l)-degrees, gamma = 91.58(l)-degrees, D(c) = 1.451 g CM-3, R(F) = 0.038, and R(wF) = 0.041. The mixed-valence compound 1',2:3',4:1''',2'': 3''',4''-tetrakis(propane-1,3-diyl)-1,1''-biferrocenium triiodide (18a) crystallizes in the monoclinic space group P2(1)/c with a = 11.363(2) angstrom, b = 13.666(2) angstrom, c = 19.753(3) angstrom, beta = 104.793(12)-degrees, Z = 4, D(c) = 2.04 g cm-3, R(F) = 0.039, and R(wF) = 0.043. The isomeric mixed-valence compound 18b crystallizes in the monoclinic space group P2(1)/n with a = 11.578(1) angstrom, b = 12.108(2) angstrom, c = 21.162(3) angstrom, beta = 90.379-degrees, Z = 4, D(c) = 2.04 g CM-3, R(F) = 0.030, and R(wF) = 0.032. In common with most mixed-valence compounds, compounds 18a and 18b in CH2Cl2 have intervalence transition (IT) bands at 5050 and 4808 cm-1, respectively. The corresponding rate constants of intramolecular electron-transfer rates for 18a,b are 2.31 X 10(12) and 3.26 x 10(12) s-1, respectively. The Mossbauer results indicate that compounds 18a,b have valence-trapped electronic structures (electron-transfer rate less than approximately 10(7) S-1 in solid state). We believe that the most important factor controlling the difference of intramolecular electron-transfer rates between the solution state and the solid state is the degree of coplanarity between the two Cp rings in the fulvalenide bridge. We also demonstrate that the perturbations caused by the interannular trimethylene bridge have dramatic influences on the electronic structure in a series of model ferrocenium salts.