Acceptor-Substituted Cyclopentadienyl Compounds

Abstract

Efficient methodologies for the synthesis of acceptor-substituted perfunctionalized cyclopentadienyl (Cp) compounds were investigated. A facile multigram, one-pot synthesis of [FeC10(HgO2CC3H7)10] from ferrocene and Hg(O2CC3H7)2 is reported. In the corresponding compound, the Hg-C bonds are inert towards oxygen, moisture and even strong Brønsted acids like trifluoroacetic acid and [C5F5NH][SbF6]. Instead, protonation of the carboxylic groups is observed yielding [FeC10(HgO2CCF3)10] and [FeC10Hg10(C5F5N)n][SbF6]10. In the compound [FeC10Hg10(C5F5N)n][SbF6]10, the labile C5F5N ligands are readily displaced by MeCN or tetrahydrothiophene (THT) to afford rare examples of organometallic decacations [FeC10(HgL)10][SbF6]10 (L = MeCN, THT). Electrochemical investigations on the (soluble) permercurated compounds reveal increasing redox potentials of the corresponding Fe(II)/Fe(III) redox couples with increasing Lewis acidity of the Hg-sites. The isolation of the oxidized forms was realized by reaction with [NO]+ or [NO2]+ salts or MoF6. Furthermore, the first crystallographic characterization of permetalated aromatic compounds [FeC10(HgX)10] (X = Cl, O2CCF3, O2CCCl3), [FeC10(HgTHT)10][SbF6]10 and [FeC10(HgMeCN)10][SbF6]10[MoF6] is presented. Complete halodemercuration is observed in the reaction of [FeC10(HgO2CC3H7)10] with K[Br3] followed by halogenation with FeBr3 and elemental Br2. An oxidation potential of E1/2 = 1.1V renders the corresponding ferrocenium cation as potent oxidizing agent. The isolation is realized by reaction of [FeC10Br10] with AsF5. Further functionalization of [FeC10Br10] is achieved by metalation with elemental Mg or by lithium-halogen-exchange with tBuLi. Quenching experiments with dimethylsilylchloride (DMSCl) yielded polysilylated compounds. Full functionalization was achieved after multiple metalation-silylation sequences. The resulting product [FeC10DMS10] displays the first example of a persilylated metallocene. A series of polysilylated derivatives [FeC10DMSnH10-n] (n = 7, 8, 9, 10) is analyzed by CV, single-crystal XRD, NMR and UV/VIS spectroscopy to evaluate the effect of silylation on the electronic properties of metallocenes. The obtained data are supported by quantum-chemical calculations. In the context of perhalogenated Cp compounds, the reaction of C5X6 (X = Cl, Br) with AsF5 and SbF5 is investigated. Here, the formation of unprecedented [2+2]-cycloaddition products of two Cp cations [C5X5]+ is observed. The obtained dications [C10X10]2+ are analyzed by XRD and NMR spectroscopy. DFT calculations reveal that the dimerzation to [2+2]- instead of [2+4]-products is thermodynamically preferred due to the formation of two allylic p-electron systems. Furthermore, in cooperation with the group of Prof. Dr. Schulz the electrochemical properties of [C5(C6F5)5]+ are presented.