Sterically demanding, cyclopentadienyl-based ligands have played an important role in organometallic chemistry for almost four decades. Primarily, such ligands serve to prevent the oligomerisation or polymerisation of electronically and coordinatively unsaturated species,1,2,3,4 and to impart kinetic stabilisation of otherwise highly reactive species, notably so when the metal centre is a main-group element.5,6 Additionally, when applied to early transition metals and lanthanides, such ligands have led an impressive range of catalysts; since by controling the steric saturation about the metal centre, the selectivity of the catalytic processes may be influenced, and furthermore by tailoring the cyclopentadienyl substituents the Lewis acidity and/or electron deficiency at the metal centre may be adjusted.7 For example, since its first synthesis8 and application,9 the -C5Me5 ligand has become possibly the most ubiquitous cyclopentadienyl ligand employed in organometallic chemistry. Its preferred use to that of -C5H5 is a result of its more electron-rich nature, increased steric bulk (which confers extended shielding of the coordination sphere about the central metal atom), increased stability, solubility and often crystallinity that it confers upon its metal derivatives and as such, has allowed the isolation and characterisation of a wealth of interesting p -complexes whose -C5H5 analogues are often too unstable to be isolated under ordinary conditions.10,11,12,13 The replacement of the five H substituents for five Me substituents results in changes in the structural properties which often take the form of reduced nuclearities, due to the increased steric demand of the ligand blocking oligomerisation or polymerisation14,15 and thus increased solubilities (due in part to the increased lipophilicity imparted by the five Me groups). The electronic effects of the -C5Me5 ligand imparted by the five Me groups have been studied extensively, particularly in the case of transition metal-based organometallic compounds. It is well understood that the Me groups of the -C5Me5 ligand are very electron donating (relative to H) and serve to increase electron density in the valence region and at the metal centre.16,17,18,19,20 For example the greater propensity for the metallocenes, [M(h-C5Me5)2] (M = V, Cr, Fe, Co or Ni) to undergo oxidation in comparison to their unsubstituted analogues,16 or the enhanced resistance to reduction of Ti(h-C5H5-nMen)Cl3 (n = 0, 1, 3, 4 or 5) with increasing degree of methylation of the cyclopentadienyl ligands.18 Such electronic changes are a direct result of through-bond electron donation, rather than as a result of steric interactions of the Me groups resulting in major structural changes.21,20
With such advantages of cyclopentadienyl substitution, numerous other alkyl-substituted cyclopentadienyl derivatives have been reported. These generally possess simple substitution patterns and are prepared by: (i) Metallation of C5H6, treatment with RCl etc. and subsequent metallation again to afford the cyclopentadienyl anion, e.g. -C5H4Me22,23 and -C5H4But.24,25 (ii) Metallation and subsequent treatment with RCl of the corresponding parent metallocene, M(h-C5H5)2 to afford, e.g. M(h-C5H4R)2. Many of the more highly substituted systems, however, -C5Ph5,26,27 -C5(CH2Ph)5,28,29 and -C5Me4(CF3)21 for example, require multi-step syntheses, often with many low yielding steps which may result from difficulties encountered with separations, extractions and distillations, as in the syntheses of -C5Me5 and -C5HMe4, for example.8,30,31,32 Increasing silylation of the cyclopentadienyl system results in advantageous properties similar to those observed on moving to the -C5Me5 system from the parent -C5H5 system, i.e. increased kinetic stability, lipophilicity, volatility, electron density in the valence region, and reduced nuclearities.
The lipophilic and steric properties of SiMe3 substitution may be illustrated by considering: (i) The isolation of the solvent-free derivatives, [M(h-C5H4SiMe3)]n (M = Li33 or K34) (linear and zigzag polymeric chains respectively). (ii) The preparation of Zr derivatives35 and their application as dehydropolymerisation catalysts.36 (iii) The isolation of [Yb(h-C5H4SiMe3)2(thf)2], one of the first X-ray crystallographically characterised LnII metallocenes;37 the isolation and structural characterisation [Sm{h-C5H3-1,3-(SiMe3)2}2(thf)],38 [Ln{h-C5H3-1,3-(SiMe3)2}2] (Ln = Eu or Yb),39 [Th{h-C5H3-1,3-(SiMe3)2}3].40 The enhanced lipophilic properties of such ligand system are apparent in the crystallographic characterisation of isomorphous [M{h-C5H3-1,3-(SiMe3)2}2(thf)] (M = Ca or Sr).41,42,43,44
With these features in mind, we have turned our attention to the synthesis of a mixed alkyl- and silyl-substituted cyclopentadienyl system, -Cps [Cps = C5Me4(SiR2R')]. With regards to FeII metallocene derivatives with silyl-substituents, several compounds have been reported and structurally characterised, e.g. [Fe{h-C5H4(SiMe2SiMe3)}],45 [Fe{h-C5H4(SiPh3)}(h-C5H5)],46 [Fe{h-C5H4(SiMe3)}2],47 [Fe{h-C5H3-1,3-(SiMe3)2}2]48 and Fe{h-C5H4(SiMe2Cl)}2.49,50