Electronic Structure of Strained Silicon- and Sulfur-Bridged [1]Ferrocenophanes and an Analogous Dicarbon-Bridged [2]Ferrocenophane:  An Investigation by Photoelectron Spectroscopy and Density-Functional Theory

Abstract

He I and He II photoelectron (PE) spectra of [Fe(η-C5H4)2SiMe2] (1), [Fe(η-C5H3Me)2SiMe2] (2), [Fe(η-C5H4)(η-C5Me4)SiMe2] (3), [Fe(η-C5Me4)2SiMe2] (4), [Fe(η-C5H4)2C2H4] (5), and [Fe(η-C5H4)2S] (6) have been measured and assigned. The d bands of 1−5 show less structure than that of ferrocene, consistent with a loss of degeneracy of the e2 orbitals on bending. Compound 6, which has the largest inter-ring angle of the series, shows two separate d bands. The trend in the first ionization energy closely parallels the variation in oxidation potential. Density functional calculations on 1, 5, and 6 give geometries in good agreement with the structures found from X-ray diffraction. Ionization energies calculated were also in excellent agreement with experiment. Good agreement was also found between the calculated d−d transitions and the position of the first spin-allowed band in the optical spectra of 1 and 6. Estimates of strain energy in bending ferrocene and octamethylferrocene were obtained, and octamethylferrocene was shown to be significantly more difficult to bend. Compounds 1 and 6 were both shown to have a low-lying empty orbital, partially located on the ipso carbon, which is a possible site for nucleophilic attack in the polymerization process undergone by these compounds.

Strained Metallocenophanes and Related Organometallic Rings Containing π‐Hydrocarbon Ligands and Transition‐Metal Centers

“…1.2 eV in Figure 2 a). [17] Indeed, thermal ROP at 180-200 8C to afford an insoluble polyferrocenylborane has been reported. [15b, 18] Ian Manners, born in London in 1961, received his PhD from the University of Bristol in 1985 in transition-metal chemistry (with N. G. Connelly) and conducted postdoctoral work on main-group chemistry with P. Paetzold (RWTH Aachen, Germany), and on polymeric materials with H. R. Allcock (Penn State, USA).…”

“…[13,[86][87][88] Trends such as decreasing ring tilt and FeÀCp distances are associated with strengthened FeÀCp bonding as the Cp ligands become increasingly electron-rich on substitution of Cp hydrogen atoms for electron-donating groups. [17] Other effects, such as increased stability towards air and moisture, are observed when sterically bulky substituents are introduced, such as the tBu groups in 25 and 26. [13,88] Interestingly, in 24 the trimethylsilyl substituents are forced to stack on top of each other and the steric repulsions between these large groups presumably contribute to the large tilt angle (26.38, compare a = 20.88 for 9 (R 1 = R 2 = Me).…”

“…This increase in energy has been verified experimentally by measuring the enthalpy associated with ring-opening selected [1]ferrocenophanes by using differential scanning calorimetry (theory: ca. À100 kJ mol À1 ) [17] are observed for the highly tilted sulfur-bridged [1]ferrocenophane 33 (a = 31.058), whereas values of À70 to À80 kJ mol À1 are typical for moderately tilted sila [1]ferrocenophanes (9, a = 19-228). Less tilted [2]ferrocenophanes (38, a = 14.8-18.58) polymerize thermally with small negative values of DH ROP (À10 to À20 kJ mol À1 ).…”

“…[129] DFT calculations closely predict the geometries found from single-crystal X-ray diffraction experiments. [17] …”

“…[167] Braunschweig and co-workers have reported the introduction of boron bridges to form the first bora [1]chromarenophanes 81-83 and dibora[2]chromarenophane 93 from the reaction of 1,1'-dilithiobis(benzene)chromium and the corresponding aminodihaloboranes [Eq. (17) and Figure 11]. …”