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

Add time:08/14/2019    Source:sciencedirect.com

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) Å, b = 9.372(2) Å, c = 18.003(2) Å, β = 107.294(9)°, Z = 4, Dcalc = 2.401 g cm−1, RF = 0.033, and RWF = 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) Å, b = 13.461(2) Å, c = 15.747(3) Å; RF = 0.041, RWF = 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) Å, b = 33.401(7) Å, and c = 13.470(5) Å. The final discrepancy factors are RF = 0.077, and RWF = 0.090. An unusually large quadrupole splitting (ΔEQ = 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 ΔEQ value is mainly a result of the metal non-bonding orbitals (dx2−y2, dxy) 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 2E2g. The visible absorption spectra of the various ferrocenium cations show a low-energy ligand-to-metal 2E2g→2E1u 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.

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