The effects of an interannular bridge on the electronic structure of ferrocenium cations

Abstract

Minor perturbations caused by the interannular trimethylene bridges in a series of ferrocenium cations have pronounced effects on the electronic structure. The X-ray structure of 1,1′:3,3′-bis(propane-1,3-diyl)ferrocenium triiodide has been determined at 298 K: monoclinic, C2/c, a = 11.110(2) A ̊ , b = 9.372(2) A ̊ , c = 18.003(2) A ̊ , β = 107.294(9)°, Z = 4, D calc = 2.401 g cm −1, R F = 0.033, and R WF = 0.040. Compound 1,1′:2,2′-bis(propane-1,3-diyl)ferrocenium triiodide crystallizes in orthorhombic space group Pnma with four molecules in a unit cell with dimensions a = 8.699(1) A ̊ , b = 13.461(2) A ̊ , c = 15.747(3) A ̊ ; R F = 0.041, R WF = 0.042. The compound 1,1′:2,2′:4,4′-tri(propane-1,3-diyl)ferrocenium triiodide crystallizes in orthorhombic space group Cmca with sixteen molecules in a unit cell with dimensions a = 18.456(4) A ̊ , b = 33.401(7) A ̊ , and c = 13.470(5) A ̊ . The final discrepancy factors are R F = 0.077, and R WF = 0.090. An unusually large quadrupole splitting ( ΔE Q = 0.936 mm s −1) is seen in the Mössbauer spectrum of 1,1′:2,2′-bis(propane-1,3-diyl)ferrocenium triiodide at 300 K. We suggest that the larger ΔE Q value is mainly a result of the metal non-bonding orbitals (d x 2− y 2 , d xy ) starting to interact with the ligand π orbitals as the Cp rings are bent back. The magnetic susceptibility measurements indicate that the electronic ground state for the interannular trimethylene bridge ferrocenium cations is 2 E 2g. The visible absorption spectra of the various ferrocenium cations show a low-energy ligand-to-metal 2 E 2g→ 2 E 1u charge-transfer transition. In the case of 1,1′:2,2′:4,4′-tri(propane-1,3-diyl)ferrocenium triiodide, a vibrational structure was observed for this charge-transfer transition at room temperature.