η1-silolyl-FeCp(CO)2 complexes. Is there a way to sila-ferrocene?
Graphical abstract
Synthesis and structure of silyl-substituted η1-silolyl iron-complexes and their attempted transformation to metallocenes.
Introduction
One of the milestones in organometallic chemistry is the synthesis of ferrocene by Kealy and Pauson in 1951 and the determination of its structure by Wilkinson and Woodward in 1952. These achievements opened up the way to the use of metallocenes in applied and material science from polymerization catalysis to UV fluorescent materials [1]. Nowadays several metallocenes are known in which not just the central iron atom is replaced by other transition metals but also the ligands are changed to heterocyclopentadienyl analogues containing other main group elements such as P, As or Sb [2].
Interestingly, the number of known metallocenes possessing silicon or germanium containing heterocycles is small (Scheme 1). The first heavy metallocene analogue 1 was synthesized by Tilley et al. reacting Cp*[RuCl4] with in situ generated Li[Me4C4GeSi(SiMe3)3] [3], and the ferrocene analogue 2 was also published 9 years later [4]. The synthesis and NMR characterization of the first metallocene containing a silolyl ring (3) has been reported in 1994 [5]. Sekiguchi et al. synthesized metallocenes containing trisila- (4) and disilagermacyclopentadienyl (5) units using the appropriate complex source ([Cp*RuCl]4 or Cp*Fe(acac)) and the lithium salt of the heterocyclopentadienyl analogue [6], [7]. Hafnium (6, 7) and zirconium (8) complexes with a bent sandwich structure were also reported (Scheme 2) [8].
While the synthesis of the possible silaferrocene precursors (9–11) was also carried out, their transformation (by heating or UV irradiation) to the metallocenes was unsuccessful [9], [10], [11] (Scheme 3).
Recently, we studied the substituent effect on the aromaticity of the silolide anions, and reported that Me3Si-substituents on the 2,5 position of the silolyl ring increase the aromaticity of the heterocycle significantly according to NICS and ISEc values [12]. Since the stability of the ferrocene might be related to the aromaticity of the cyclopentadienyl moiety, we envisaged that using 2,5-trimethylsilyl-substituted silolyls the corresponding ferrocene analogues might be synthesized. Hereby, we report on two new η1-silolyl-FeCp(CO)2 complexes, and their attempted transformation to the metallocenes, completed with DFT calculations for a better understanding of the experimental results.
Section snippets
Results and discussion
We reported recently the synthesis of 12, via an amine – chloride exchange route developed originally by Tamao [13], [14]. The synthesis of 13 was performed in an analogous manner, using Et2NSiMeCl2 as starting material. 14 was synthesized by the reaction between 12 and tBuLi in THF at −50 °C. Interestingly the 1H and 13C NMR spectra of 13 and 14 show that the signals connected to the orto-and meta-hydrogens and carbons of the phenyl groups on the β carbons of the silolyl ring show splitting or
Calculations
The energy and Gibbs free energy values (at 293 K, atmospheric pressure) of the decomposition of the η1-complexes to metallocenes are given for model compounds in Table 1.
In the case of the carbon analogue the energy difference between the starting material and the products is positive, but the Gibbs free energy difference (at 298 K and atmospheric pressure) is negative due to the formation of two free carbon-monoxid molecules, in agreement with the result of Yates and co-workers [27]. The
Conclusion
The synthesis of the new silols (13, 14) and η1-silolyl-FeCp(CO)2 complexes (15, 16) were carried out. 15 and 16 are the first Fe–Si covalent bonded silolyl complexes reported in the literature, which are characterized by 1H, 13C, and 29Si NMR, IR spectroscopies, as well as with single crystal X-ray diffraction. Transformation of these complexes to a ferrocene analogue sandwich complexes were unsuccessful using UV irradiation, Me3NO or by simple heating. The reason for the increased stability
General
All reactions were performed in oven-dried glassware under dried nitrogen or argon using standard Schlenk techniques. Solvents were dried according to well-known procedures (THF, diethyl ether, dioxane, and C6D6 over sodium/benzophenone, hexane over LiAlH4.) and distilled freshly before use. Solids were dried in vacuum. CDCl3 was dried over activated molecular sieves. 1.6 M n-butyl lithium and 1.6 M tert-butyl lithium solution in hexane from Merck Ltd. were used directly. NMR spectra were
Acknowledgements
The authors thank the Institut für Anorganische Chemie, Karlsruher Institut für Technologie (KIT) for the measurements of NMR spectra, mass spectra and for elemental analysis. Financial support from the Hungarian Scientific Research Fund OTKA NN 113772, K-100801 and COST CM10302 (SIPS) is gratefully acknowledged.
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