Synthesis

Bis-silyl-substituted Fe^II metallocenes, e.g. [Fe{h -C₅H₄(SiMe₃)}₂],⁴⁷ are generally prepared by the reaction of Fe(h-C₅H₄Li.tmeda)₂ with two equivalents of SiR₃Cl (R = alkyl) [Scheme 1, ISIS]. The analogous reaction with SiR₂Cl₂ (at low temperature so as to avoid polycondensation reactions) does not afford the corresponding metallocene compounds, Fe{h -C₅H₄(SiR₂Cl)}₂, rather, the bridged [1]ferrocenophane species, Fe[h ,h¢ -{(C₅H₄)₂SiR₂}]. It is possible to prepare Fe{h -C₅H₄(SiMe₂Cl)}₂, however, by the chlorination of the corresponding aminosilyl-substituted compounds Fe[h -C₅H₄{SiMe₂(NR₂)}]₂ [NR₂ = NEt₂⁴⁹ or NC₅H₁₀ (1-piperidyl)].⁵⁰ Such compounds can be prepared by the reaction of Fe(h -C₅H₄Li.tmeda)₂ with two equivalents of an aminochlorodialkylsilane, e.g. SiMe₂(NR₂)Cl.[Scheme 2, ISIS]. However, to date, no SiR₂Cl-functionalised ferrocenes have been charaterised by single crystal X-ray diffraction.

As is the case for the analogous Group 14 metallocenes, it is possible to prepare 1 by the addition of a thf solution of two equivalents of LiC₅Me₄(SiMe₂Bu^t) to a slurry of anhydrous FeCl₂ in thf.[Scheme 3, ISIS] After stirring for four hours, a change from the pink colour of FeCl₂ in thf to pale orange resulted, whilst with stirring for a further twelve hours, the complete disappearance of the FeCl₂ slurry was accompanied by the formation of a deep orange solution and a small amount of white precipitate. Removal of the thf in vacuo, extraction with hexane and concentration of the hexane in vacuo afforded 1 as small orange needles in good yield (71 %).

Compound 1 was characterised, by ¹H, ¹³C{¹H} and ²⁹Si{¹H} solution state NMR, ¹³C{¹H} and ²⁹Si{¹H} CP MAS solid state NMR spectroscopies, single crystal X-ray diffraction,[Ortep Diagram] EI mass spectrometry and by microanalysis.

This reaction between LiC₅Me₄(SiMe₂Bu^t) and FeCl₂ is slow, yielding 1 only after stirring for ca. sixteen hours. An alternative method by which 1 was obtained in two hours rather than sixteen hours involved the rapid reaction of FeCl₂ with two equivalents of LiC₅Me₄(SiMe₂Cl) [generated in situ by the reaction of C₅HMe₄(SiMe₂Cl) with one equivalent of LiBu^t at low temperature]. This gives rise to the intermediate generation of Fe{h -C₅Me₄(SiMe₂Cl)}₂, which undergoes subsequent and rapid alkylation by LiBu^t of the two Si atoms. Recrystallisation of 1 from a concentrated hexane-toluene solution afforded large orange needles which could be handled in air for more than ten minutes without visible signs of decomposition.

While it proved difficult to isolate and structurally characterise the intermediate Fe{h -C₅Me₄(SiMe₂Cl)}₂, via the reaction of FeCl₂ with LiC₅Me₄(SiMe₂Cl), it was anticipated that an increase in the steric bulk about the Si centre might reduce the rate of attack by LiBu^t. Thus, the preparation of Si(h ¹-C₅HMe₄)Ph₂Cl, its Li salt and subsequent, in situ metathetical exchange with FeCl₂, in a manner analogous to that for the attempted preparation of Fe{h -C₅Me₄(SiMe₂Cl)}₂, was carried out.

The addition of SiPh₂Cl₂ to a thick white slurry of LiC₅HMe₄ (one equivalent) in thf with stirring for several hours resulted in the formation of a pale yellow, clear solution and a fine white precipitate. The analogous reaction using SiMe₂Cl₂ on the other hand proceeded with the formation of a clear solution immediately upon addition of a stoichiometric amount of the silane. Following removal of the thf in vacuo, extraction with hexane and removal of the hexane in vacuo, Si(h ¹-C₅HMe₄)Ph₂Cl was obtained as a pale yellow, viscous liquid in very high yield (97 %).

Subsequently the addition of one equivalent of LiBu^t, added dropwise with stirring to a solution of Si(h ¹-C₅HMe₄)Ph₂Cl in thf, cooled to -78 ° C using a dry-ice/acetone slush-bath, results in the formation of an intensely yellow solution. After stirring at low temperature for two hours, the addition of anhydrous FeCl₂ (one half molar equivalent) slurried in thf, also at -78 ° C, to the solution resulted in a slow colour change to deep orange/red. The solution was warmed to ambient temperature, stirred for a further two hours and the thf removed in vacuo. The residue was washed with hexane, extracted with hot toluene (which was subsequently removed in vacuo, [Fe{h -C₅Me₄(SiPh₂Cl)}₂] 2 was obtained as an O₂- and H₂O-sensitive orange powder (70 % yield).[Scheme 4, ISIS] Recrystallisation from a concentrated hot toluene solution afforded deep orange/red needles.

Compound 2 was characterised by ¹H and ¹³C{¹H} solution state NMR spectroscopies, single crystal X-ray diffraction, EI mass spectrometry and by microanalysis.

In a manner analogous to 1, 3 was prepared by the addition of a thf solution of two equivalents of LiC₅Me₄(SiMe₃) to a slurry of anhydrous FeCl₂ in thf.[Scheme 5, ISIS] While stirring for three hours, the complete disappearance of the FeCl₂ slurry was accompanied by the formation of a deep orange solution. Removal of the thf in vacuo, extraction with hexane and concentration of the hexane in vacuo afforded 3 as small orange cubes in high yield (80 %).

Compound 3 was characterised, by ¹H, ¹³C{¹H} and ²⁹Si{¹H} solution state NMR state, ¹³C{¹H} and ²⁹Si{¹H} CP MAS solid state NMR spectroscopies, single crystal X-ray diffraction,[Ortep Diagram] EI mass spectrometry and by microanalysis.

In contrast to the low temperature, in situ synthesis of 2, 4 is prepared by the addition of a thf solution of two equivalents of LiC₅Me₄(SiPh₂Me) to a slurry of anhydrous FeCl₂ in thf at room temperature.[Scheme 6, ISIS] After stirring for 5 hours, the thf was removed in vacuo and the product extracted with hot hexane. Removal of the hexane in vacuo and recrystallisation from toluene afforded 4 as orange cubes in high yield (83 %).

Compound 4 was characterised, by ¹H, ¹³C{¹H} and ²⁹Si{¹H} solution state NMR state NMR spectroscopies, single crystal X-ray diffraction, EI mass spectrometry and by microanalysis.

Molecular Structures

Examination of the molecular structures of 1-4 reveals monomeric structures typical of substituted Fe^II metallocenes such as [Fe(h -C₅Me₅)₂]. For compounds 1 and 3, the cyclopentadienyl rings are equidistant, planar, staggered and exactly parallel.

The molecular structures of 1 and 3 both possess a mirror plane defined by atoms C(5), Si and C(1). As is the case with the analogous metallocenes, [M{h-C₅Me₄(SiMe₂Bu^t)}₂] (M = Li, Mg, Ge^II, Sn^II, Pb^II and Hg), the two Si atoms are orientated trans with respect to one another such that the Bu^t group of 1 or one Me group of 3 is directed away from the Fe centre and the remaining two Me groups are directed towards the Fe centres and the meta Me groups of the C₅Me₄ ring below. The thermal ellipsoids, for 1 suggest some librational disorder of the ligands within the cyclopentadienyl ring planes and thus the geometric parameters for 1 are not particularly accurate (final residuals, R(F) = 0.108 and wR(F²) = 0.258); an uncommonly large R(F) factor (0.159) was also obtained for the similarly sterically crowded [Fe(h -C₅Ph₅)₂].

The molecular structures of 2 and 4 reveals them both to possess near-eclipsed cyclopentadienyl rings: From an eclipsed configuration of the cyclopentadienyl rings (and the ipso carbons), the two SiPh₂R groups are separated by ca. 90 ° rotation of the about the cyclopentadienyl centroid-Fe-cyclopentadienyl centroid axes. With this arrangement of the two rings and the steric bulk of the SiPh₂R substituents on one side of the metallocene, there is a concomitant deviation from their parallel orientation as the rings are tilted back by ca. 3 °. For 2 and 4 the SiPh₂R substituents are orientated such that the Si-Cl or Si-Me bonds point out in an equatorial direction, away from the metal centre, with one Ph group from each SiPh₂R substituent occupying an equatorial position, lying in a plane near-parallel to the other. The two other Ph groups occupy axial positions, each pointing away from the metal centres. That the two C₅Me₄(SiPh₂R) rings are not rotated by 180 ° as for 1 and 3, suggests that the molecular structure of 2 and 4 might be influenced by a, presumably weak, p -p interaction of the two equatorial Ph rings (cf. [Fe{h -(PC₄H₂-2,5-Ph₂)}₂],); given such a weak interaction, the role of crystal-packing forces cannot be discounted. For 2 the two intramolecular Fe^-Cl distances are 4.378 and 4.460 Å [Cl(1) and Cl(2) respectively] and thus it is unlikely that there is any interaction between the Fe centre and Cl(1) or Cl(2).

For all four metallocenes the Si-C(1) bonds are significantly bent out of the plane of the ring for ( ca. 13 - 17 ° ), reflecting the close proximity of the two C₅Me₄ rings due close inter-ring separations, were observed for all four metallocenes. The Fe-cyclopentadienyl centroid distances for 1- 4 are comparably long with respect to other highly substituted bis-cyclopentadienyl Fe^II systems. The average cyclopentadienyl ring C-C bond length and C-C-C angle are unexceptional and similar to those measured for [Fe(h-C₅H5)2] [1.400(2)Å] and [Fe(h -C₅Me₅)₂] [1.419(2)Å]. [Table 1]

Due to the predominant use of d-orbitals in the bonding of monomeric, divalent bis-cyclopentadienyl transition metal metallocenes, deviations from a parallel configuration of the two cyclopentadienyl rings (greater than ca. 5 ° ) are rarely observed. As can be seen from Table 1, the metal-centroid distances for Fe^II metallocenes generally increase with increasing steric bulk of the cyclopentadienyl ligands. Thus for example, [Fe(h -C₅Ph₅)₂] and [Fe(h -C₅HPh₄)₂] exhibit longer Fe centroid distances than 1 - 4 (for which no significant changes are observed).