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, and to impart kinetic stabilisation of otherwise highly reactive species, notably so when the metal centre is a main-group element. Additionally, when applied to early transition metals and lanthanides, such ligands have led an impressive range of catalysts; since by controlling 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. For example, since its first synthesis<DeVries, 1960 #473> and application,<King, 1967 #443> 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. <Schumann, 1991 #55; Jutzi, 1989 #2; Fagan, 1981 #457; Marks, 1982 #458; Spirlet, 1992 #161> 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 polymerisation<Tilley, 1980 #150; Burns, 1987 #225> 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.<Robbins, 1982 #177; Gassman, 1983 #262; Mach, 1987 #39; Gassman, 1988 #263; Gassman, 1991 #264> 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.<Robbins, 1982 #177> 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.<Mach, 1987 #39> 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.<Gassman, 1992 #265; Gassman, 1991 #264>
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. -C5H4Me<Zinnen, 1980 #243; Brennan, 1986 #292> and -C5H4But.<Hani, 1985 #102; Shen, 1990 #142> (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,<Ziegler, 1925 #428; Zhang, 1982 #498> -C5(CH2Ph)5,<Hirsch, 1978 #497; Chambers, 1986 #64> and -C5Me4(CF3)<Gassman, 1992 #265> 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. <DeVries, 1960 #473; Kohl, 1983 #34; Fendrick, 1984 #165; Fendrick, 1988 #208> 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 = Li <Evans, 1992 #394> or K <Jutzi, 1987 #492>) (linear and zigzag polymeric chains respectively). (ii) The preparation of Zr derivatives <Thiele, 1993 #526> and their application as dehydropolymerisation catalysts.<Imori, 1991 #175> (iii) The isolation of [Yb(h -C5H4SiMe3)2(thf)2], one of the first X-ray crystallographically characterised LnII metallocenes;<Lappert, 1980 #200> the isolation and structural characterisation [Sm{h -C5H3-1,3-(SiMe3)2}2(thf)], <Hitchcock, 1991 #488> [Ln{h -C5H3-1,3-(SiMe3)2}2] (Ln = Eu or Yb),<Hitchcock, 1992 #141> [Th{h -C5H3-1,3-(SiMe3)2}3],<Blake, 1986 #172> 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).<Engelhardt, 1988 #157; AAConstantine12, #493; Andersen, 1987 #227; Gardiner, 1991 #132>
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(SiMe2But)]. The extreme steric demand of this cyclopentadienyl ligand is also illustrated in the molecular structure of its Fe metallocene derivative, [Fe(h -C5Ph5)2], for which a large R(F) factor (0.159) was obtained, indicating a significant amount of disorder throughout the crystal lattice, resulting from the unfavourably close proximity of the ten Ph groups.<Schumann, 1994 #71> Furthermore, it was not possible to obtain [Fe(h -C5Ph5)2] by conventional means such as metathesis, as used in the preparation of the Sn analogue. This has been attributed to the essentially reduced nucleophilicity of the -C5Ph5 ligand (due to its extreme steric bulk) with regards to ligand substitution at metal centres.<Powell, 1983 #436> [Fe(h -C5Ph5)2] was prepared in ca. 30 % yield by heating Fe(h -C5Ph5)(CO)2Br in boiling xylene for two days! The novel electronic and steric properties that the -C5Ph5 ligand confers on metals is further illustrated, for example, in the case of Ni: (i) Diamagnetic Ni(h -C5Ph5)2 contrasts with paramagnetic [Ni(h -C5H5)2].<Wilkinson, 1954 #429; Barnett, 1974 #430; Schott, 1973 #431> (ii) The dimeric compound [{Ni(h -C5Ph5)(m -Br)}2] decomposes at temperatures > 200 ° C, the -C5Me5 analogue decomposes at temperatures > 10 ° C, whilst the -C5H5 analogue is unknown.<Klaui, 1986 #432>
The main limitation of metallocene derivatives of this bulky cyclopentadienyl ligand is their extremely low solubility in almost all common organic solvents, e.g. crystals of [Sn(h-C5Ph5)2] were grown only from hot 1-methylnaphthalene. Improved solubility may be obtained by modification of the phenyl groups of C5HPh5 and its derivatives but none of the resulting compounds, e.g. E{h -C5(C6H4R)5}2 [E = Sn or Pb; R = C(OEt)2Me] are crystalline.<Lowack, 1994 #45> For this reason and in contrast to the -C5Me5 ligand, few examples of compounds containing the -C5Ph5 ligand have been structurally characterised.<Ban, 1973 #439; Hoberg, 1977 #441; Powell, 1983 #436; Chambers, 1986 #64; Behrens, 1986 #440; Janiak, 1989 #434; Field, 1989 #435; Slocum, 1990 #437; Adams, 1990 #438; Thewalt, 1991 #433; Field, 1991 #442; Aroney, 1993 #344> Furthermore, the -C5Ph5 ligand is not particularly attractive for use in the design of catalytically active compounds, since it confers such a sterically crowded coordination environment about its metal centre, limiting its access and reducing its reactivity (Section 1.2.2). However, for this reason, ligand substitution reactions that proceed via a dissociative mechanism will display enhanced rates upon replacement of a -C5H5 ligand for a -C5Ph5 ligand.<Adams, 1990 #438>
A number of related, -C5HPh4, metallocene derivatives have been reported, with the ligand prepared via a single-step, high yield route (82 %). The lower symmetry of these O2-stable metallocenes, E(h-C5HPh4)2 (E = Ge, Sn or Pb),<Schumann, 1988 #85> enhances their solubilities, but X-ray crystallographic data for these compounds is as yet unavailable, presumably due to similar problems with non-crystallinity as found for the -C5Ph5 ligand and its metallocene-derivatives. The molecular structure of [Fe(h-C5HPh4)2], however, has been determined and in this more ordered structure, the two C5HPh4 rings are orientated 180 ° with respect to each other. The Fe-centroid distance (1.695 ≈) is closer to that predicted for optimal Fe-cyclopentadienyl bonding.<Castellani, 1986 #353>
The heteroleptic metallocenes, [M(h -C5Ph5)(h -C5H5)2] (M = Sn<Heeg, 1988 #26; Janiak, 1988 #90> or Fe<Aroney, 1993 #344>) are more soluble than their homoleptic -C5Ph5 metallocene analogues, with recrystallisation possible from toluene and chlorobenzene solutions respectively: As expected, the respective molecular structures are that of bent and parallel metallocenes.
Interestingly, the earliest examples of transition metal -C5Ph5 derivatives were not prepared via the -C5Ph5 anion, rather, they involved inter alia the bromide, C5Ph5Br, e.g. in the preparation of M(h -C5Ph5)(CO)2Br (M = Fe<McVey, 1965 #477; Field, 1989 #435> or Ru<Adams, 1990 #438>), or the radical, × C5Ph5, e.g. in the preparation of Ni(h -C5Ph5)2.<Schott, 1973 #431> The structurally characterised Pd compound, [{Pd(h -C5Ph5)2}2{m -(PhCº CPh)}], was in fact prepared from the reaction of PhCº CPh with Pd3(O2CMe)6 in MeOH.<Ban, 1973 #439>
The introduction of five CH2Ph groups onto the cyclopentadienyl ring results in an extremely bulky cyclopentadienyl ligand, but due to the flexibility of the CH2Ph groups arising from the -CH2- moiety, a ligand that is not as sterically demanding as the -C5Ph5 ligand. An examination of the molecular structures of [E{h -C5(CH2Ph)5}2] (E = Ge, Sn or Pb), for example, reveals an isostructural series of bent metallocene structures.<Schumann, 1985 #124; Schumann, 1986 #63>
The enhanced electronic properties of SiMe3-substituted cyclopentadienyl systems with respect to -C5H5, imparted through silylation, aswell as those of solubility and lipophilicity, are illustrated by the cyclopentadienyl chemistry of TlI. The tris-silyl-substituted cyclopentadienyl TlI compound Tl{h -C5H2-1,2,4-(SiMe3)3},<Jutzi, 1985 #405> possess reasonable solubilities in common aprotic solvents and displays nJ(205(203)Tl-1H) (n = 2 or 4) and 3J(205(203)Tl-13C) couplings in its 1H and 13C{1H} NMR spectra respectively whereas Tl(h -C5H5) forms a polymeric chain structure in the solid state, exhibits low solubilities in common aprotic solventsand no 2J(205(203)Tl-1H) couplings were observed in the 1H NMR spectrum.<Frasson, 1963 #417; Berar, 1975 #418; Fritz, 1971 #419> The silyl substituents then, by virtue of their ability to delocalise negative charge (through p back-acceptance into the energetically low-lying 3d orbitals of the Si atoms) can significantly alter the nature of the metal-ring bonding in these p -systems;
With regards to FeII metallocene derivatives with silyl-substituents, several compounds have been reported and structurally characterised, e.g. [Fe{h -C5H4(SiMe2SiMe3)}],<Hirotsu, 1968 #348> [Fe{h -C5H4(SiPh3)}(h -C5H5)],<Palmer, 1994 #341> [Fe{h -C5H4(SiMe3)}2],<Foucher, 1995 #126> [Fe{h -C5H3-1,3-(SiMe3)2}2]<Okuda, 1989 #349> and Fe{h -C5H4(SiMe2Cl)}2,<Zechel, 1995 #15; Kan, 1961 #338>