DOI: 10.1002/chem.201402163

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& Silicon Chemistry

The Pentamethylcyclopentadienylsilicon(II) Cation: Synthesis, Characterization, and Reactivity Peter Jutzi*[a]

Chem. Eur. J. 2014, 20, 9192 – 9207

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Minireview Abstract: In the flourishing chemistry of divalent silicon, pcomplex formation between silicon and the pentamethylcyclopentadienyl (Cp*) group is one of the successful strategies for thermodynamic and/or kinetic stabilization. Here, the diverse reactivity of the [Cp*Si] + ion is described. Its chemistry is characterized by the addition of anionic and neutral nucleophiles and by the easy hapticity change and the leaving-group character of the Cp* group. Several novel

1. Introduction For more than a century, stable molecular silicon compounds have been regarded as species in which the silicon atom uses all four of its valence electrons in chemical bonding. This feature is the basis for many classes of compounds isolable under ordinary conditions of temperature and pressure. In contrast, molecular compounds, in which two of the four valence electrons remain non-bonded in a so-called “lone-pair” (I in Figure 1, named “silylenes” in analogy to “carbenes”) were clas-

Figure 1. Neutral and cationic monomeric silicon(II) species.

sified as highly reactive species, a long way from being isolable under normal conditions. This classification is based mainly on the properties of the parent species and of derivatives with small organic or inorganic substituents R. The silylene H2SiD cannot be isolated in pure form and is characterized only in rare-gas matrices at low temperatures, in dilute solution or diluted in the gas phase;[1] several derivatives R2SiD were discussed as highly reactive intermediates in many chemical transformations, but could never be isolated.[2, 3] Cationic species of the type [RSiD] + (II in Figure 1, named “silyliumylidenes”) possess only three valence electrons with two of them remaining as a lone-pair. They were regarded as even more elusive, in agreement with the properties of the parent species [HSiD] + and of derivatives like [ClSiD] + : these species can only be generated in the gas-phase under high-vacuum conditions.[4, 5] It is interesting to note that the cation [HSiD] + is detected in the solar spectrum as well as in interstellar space.[6] Of course, the di-cationic species [SiD]2 + (III in Figure 1) possesses only a lonepair of electrons and represents the most elusive species. The presence of one vacant orbital in species R2SiD, of two vacant orbitals in cations [RSiD] + and of three vacant orbitals in the dication [Si]2 + is a reason for a pronounced Lewis acid character.

sandwich and half-sandwich p-complexes of divalent silicon were synthesized, and a novel access to the class of cyclotrisilenes was found. A reversible adduct formation is the basis for the catalytic activity of the [Cp*Si] + ion in the specific oligoether degradation. Homo- or heterolytic Cp*Si bond cleavage allows the use of the cation as a source of silicon atoms in silicon cluster synthesis and in nanoparticle formation.

The formation of donor–acceptor complexes of the type [R2SiD(donor)], [RSiD(donor)2] + or [SiD(donor)3]2 + was not regarded as a possible strategy for the isolation of stable adducts. During the last few decades, the classes of molecular silicon compounds stable under ordinary conditions of temperature and pressure have experienced an important extension. This is due to the considerable progress made in the chemistry of divalent silicon. Using the concept of kinetic and/or thermodynamic stabilization, several novel classes of compounds have been discovered; the most important ones are presented with their typical structural framework in Scheme 1.[7–29] Their thermodynamic stabilization is realized 1) by electron delocalization from a substituent to the central silicon atom, 2) by the use of chelating or p-bonded substituents and/or 3) by the complexation with strong s-donor ligands L. In the neutral compounds, both di-coordinate (silylenes) and higher coordinate silicon atoms (silicon(II) species) are present. In the ionic compounds containing cationic silicon units, only higher coordinate silicon atoms are found. Historically, p-complex formation was the first successful tool used to stabilize a neutral monomeric silicon(II) compound: the complex decamethylsilicocene (Me5C5)2SiD (Cp*2SiD, F in Scheme 1) possessing a divalent and ten-coordinate silicon atom was published in 1986.[18] The diverse reactivity of this compound has been reviewed, including the interesting topic of small-molecule activation. The p-complex can be regarded as the resting state of a true silylene with s-bonded cyclopentadienyl substituents.[30] The chemistry of several other classes of neutral SiII compounds is meanwhile also well documented.[31–40] Comprehensive reviews were published in 2009[34] and in 2013.[38] Only recently, the rich chemistry of N-heterocyclic silylenes and silicon(II) species (C, L, and M in Scheme 1) has been extensively reviewed, especially in connection with the metal-free activation of small molecules and with the application of such molecules as ligands in catalytically relevant transition-metal complexes.[35–39] After the fundamental discovery in 2008, that an N-heterocyclic carbene (NHC) can stabilize a naked Si2 unit in the adduct L!SiD = SiD L (L = NHC),[43] such types of excellent s-donors have found application also for the stabilization of monomeric R2SiD compounds. Highly reactive species of the type Hal2SiD or Hal(R)SiD (Hal = halogen) were successfully stabilized in form of their NHC adducts (Q and R in Scheme 1).[23–27] It is worth mentioning that complexes of type Q are easily prepared and !

[a] Prof. em. Dr. P. Jutzi Faculty of Chemistry University of Bielefeld 33615 Bielefeld (Germany) E-mail: [email protected] Chem. Eur. J. 2014, 20, 9192 – 9207

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Scheme 1. Classes of compounds containing monomeric silicon(II) units.

seem best suited for further substitution reactions and thus for the synthesis of novel silicon(II) compounds. In the class of compounds containing a cationic [RSiD] + unit, p-complexation once more turned out to be a successful tool: the first stable compound in this series, published in 2004, was the half-sandwich complex [(Me5C5)SiD] + [B(C6F5)4] (H in Scheme 1), possessing a five-coordinate silicon atom stabilized in a three-dimensional aromatic system.[15] A higher coordinate [RSiD] + ion can be stabilized also by a chelating bulky b-diketiminato substituent, as shown by more recent experiments. This strategy led to a two-dimensional aromatic ring system containning a sterically protected two-coordinate silicon atom (N in Scheme 1).[20] The chemistry of this interesting cation is so far rather unexplored. A further strategy uses adduct formation with strong neutral s-donor molecules. Rather surprisingly, even highly reactive [HalSiD] + ions can be stabilized in this manner, as documented by the very recent synthesis of salts containing cationic adducts with NHCs of the type [(NHC)2 ! SiDHal] + (Hal = I,[26] Cl;[28] S and T in Scheme 1, respectively). Interestingly, not only NHC molecules, but also a chelating bis(iminophosphorane) ligand can stabilize a [HalSiD] + cation, as shown by the isolation of a compound containing the [ClSiD] + species (U in Scheme 1).[29] Rather unexpectedly, NHC molecules can stabilize even the elusive [SiD]2 + unit, as documentted by the very recently described dication of the type [(NHC)3 ! SiD]2 + (V in Scheme 1).[26] Similar to adducts of type Q, adducts of type S, T, U, and V should also be well suited for a further application in synthesis. A three-coordinate silicon(II) cation, Chem. Eur. J. 2014, 20, 9192 – 9207

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stabilized both by a chelating amidinato ligand (as present in species of type L) and by a neutral donor molecule (as present in species R) was described only very recently.[40] The fascinating progress in the field of low-valent silicon compounds is not restricted to monomeric species of divalent silicon. Following the concepts of kinetic and/or thermodynamic stabilization with suitable substituents and of complex formation with strong s-donor molecules, several other novel classes of compounds containing di-, mono-, and even zerovalent silicon have become accessible during the last decade. These important classes of compounds are not within the scope of this review and are only briefly mentioned in references.[41–44] In the class of compounds containing a higher coordinate silicon(II) cation, only the p-complex [(Me5C5)SiD] + [B(C6F5)4] (1, cation G with R = Methyl in Scheme 1) so far has been investigated in more detail. The many aspects of its chemistry including a description of structure and bonding are presented in this article.

2. [(Me5C5)Si] + [B(C6F5)4] : Synthesis, Structure, and Bonding 2.1. Synthesis The half-sandwich cations [(Me5C5)El] + (El = element) of the heavier congeners germanium, tin, and lead can be easily prepared by the reaction of the corresponding sandwich complex (Me5C5)2El with the protic species HBF4·Et2O and HO3SCF3 re-

Dr. Peter Jutzi was active for three decades as Full Professor in the Faculty of Chemistry at the University of Bielefeld; since 2008 he holds the position of Professor Emeritus. His research interests during the last decade are in the fields a) synthetic organosilicon chemistry, b) nanoparticle synthesis and mechanistic studies, and c) dynamic covalent chemistry with organometallic compounds.

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Minireview spectively, with corresponding elimination of pentamethylcyclopentadiene.[45] In contrast, the reaction of decamethylsilicocene (Me5C5)2Si (2) with HO3SCF3 in a 1:1 ratio results in the formation of the oxidative addition product;[46] with a 1:2 ratio, a protonation takes place to give the unique [(Me5C5)2SiH] + ion.[47] In the reaction with HBF4·Et2O, elimination of BF3 and Me5C5H and fluoride transfer leads to the reactive silylene Me5C5(F)Si, which dimerizes to the fluoro-bridged species Me5C5Si(F,F)SiC5Me5, which finally forms the stable cyclotetrasilane [{Me5C5(F)Si}4].[48] In view of these experimental findings, it is not surprising that it took several years to find the right conditions for the stabilization of the [(Me5C5)Si] + ion (G in Scheme 1). It was first detected in the 1H NMR spectrum after reacting the silicocene 2 with the protic species [H(OEt2)2] + [B(C6F5)4] ; the latter was prepared only in the year 2000.[49] The reaction was performed at 70 8C by mixing the reagents within the spectrometer, using a special “two-chamber” NMR tube.[50] In the reaction mixture, the cation was stable for several days even at room temperature. Under normal laboratory conditions, the protonated pentamethylcyclopentadiene in form of the salt [Me5C5H2] + [B(C6F5)4)] turned out to be the protonating reagent of choice, leading exclusively to the salt [(Me5C5)Si] + [B(C6F5)4)] (1) and to two equivalents of pentamethylcyclopentadiene [Eq. (1)].[15] Compound 1 was obtained as colorless needles, which were thermally stable but extremely air- and moisture sensitive.

The stabilization by a weakly coordinating anion (WCA) is essential. Later on, salts containing the Krossing-type[51] WCA {Al[OC(CF3)3]4} or the Reed-type[52] WCA [CHB11H5Cl6] were prepared.[53] Meanwhile, some modifications in the synthetic procedure of 1 and 2 have been proposed to improve the yield of these compounds.[54] Based on the reactivity and the polar nature of 1, a restricted solubility in organic solvents is observed. The compound is soluble in dichloromethane and is stable in this solvent for several weeks; in dimethoxyethane, a slow [Cp*Si] + -catalyzed decomposition of the solvent takes place within several days (see Section 3.5.); in tetrahydrofuran, a fast polymerization of the solvent is initiated by the [Cp*Si] + cation. 2.2. Structure and Bonding Structural information about the salt 1 is based on an X-ray crystal structure analysis, NMR spectroscopic data, and on theoretical calculations.[15, 55] A representation of the molecular structure in the solid-state is given in Figure 2. The structure of the cation deviates only to a small extent from an ideal pentagonal-pyramidal arrangement. The distance from the Si atom to the center of the pentamethylcyclopentadienyl (Cp*) ring is 1.76 ; the SiC(Cp*) distances are in the range from 2.14 to Chem. Eur. J. 2014, 20, 9192 – 9207

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Figure 2. The molecular structure of [(Me5C5)Si] + [B(C6F5)4)] (1), H atoms omitted; space-filling model of the [(Me5C5)Si] + ion.

2.16 . These values are substantially smaller than those observed in the D5d isomer of the silicocene Cp*2Si[14] (F in Scheme 1) (SiCp* ring center 2.11 , SiC(Cp*) 2.42 ). The CC distances in the Cp* unit are in the range of 1.43–1.44 . The carbon atoms of the five methyl groups are positioned in the plane of the C5 ring, comparable to the situation in the D5d isomer of Cp*2Si.[14] Only rather weak cation–anion contacts are concluded from the long SiF distances. As documented also by the space-filling model of the [(Me5C5)Si] + ion in Figure 1, the silicon atom possesses a rather open coordination hemisphere; thus, steric protection cannot play a dominant role in the stabilization of this species. The 1H and 13C NMR data indicate the presence of a pentagonnal-pyramidal cation also in solution; only one signal for the methyl groups and one for the ring carbon atoms is observed. Interestingly, the 29Si NMR signal appears in the very high-field region at d = 400.2 ppm, expected in case of threedimensional aromaticity[56] in the SiCp* unit. Quantum chemical calculations at various levels of theory have been performed for the [(Me5C5)Si] + ion.[15, 55] The degree of stabilization on the way from [HSi] + to [RSi] + species has been calculated for different substituents R (R = H3Si, H2P, F, H3C, Cl, HCC, HS, H5C6, H2N, OH, h5-H5C5, h5-Me5C5) on the basis of isodesmic reactions ([HSi] + + RSiH3 !SiH4 + [RSi] + + DE), resulting in the best stabilization for the h5-Me5C5 group (DE = 104 Kcal mol1). For compounds of the composition (Me5C5)El (El = Group 13 element, Group 14 element cation, Group 15 element dication, isolobal fragments) three orbitals (a1, e1 symmetry) are expected for the pentagonal-pyramidal C5El framework and one orbital (a1 symmetry) for the so-called “lone-pair” of electrons in the exo-position.[57, 58] For the [(Me5C5)Si] + ion, Figure 3 displays the molecular orbitals (MOs) spanning the highest-occupied and the lowest unoccupied MOs in the energy range from 23.338 to 1.765 eV, as calculated on the HF level. It is worth noting that the “lone-pair” at silicon does not represent the HOMO, but the HOMO1. The HOMO is doubly degenerate and represents the p-interaction of the Cp* group with the perpendicularly oriented p-orbitals at silicon. The corresponding antibonding combinations represent the doubly degenerate LUMO. The strongest interaction is present in the

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Minireview 3. Reactivity of the [(p-Me5C5)Si] + Cation 3.1. Introductory remarks The chemistry of the neutral sandwich compound (Me5C5)2Si (2) is dominated by the fact that this species can react like a nucleophilic silylene, in most cases caused by an h5/3/2–h1 rearrangement of the two Me5C5 ligands and followed by transfer from the + 2 to the + 4 formal oxidation state at silicon. This reactivity pattern has been used to perform several types of transformations with organic, organometallic, and inorganic substrates, including the activation of small molecules.[30] In contrast, the chemistry of the half-sandwich cation [(Me5C5)Si] + in compound 1 is dominated by its strong electrophilic character, by preservation of the formal + 2 oxidation state at silicon, and by the easy hapticity change and the leaving-group character of the Me5C5 group. In the following, the corresponding reaction types are described in more detail.

Figure 3. Frontier molecular orbitals of the [(Me5C5)Si] + ion.

orbital of a1 symmetry. The large HOMO–LUMO energy gap is typical of a closed-shell situation. The calculated natural charge at silicon (0.96,[15] 0.99[55]) indicates strong electron donation from a formal Cp* anion to a formal Si2 + cation and mainly covalent bonding between these fragments. To get additional insights into the nature of the electronic interaction in the Cp*Si fragment, Bader’s quantum theory of atoms in molecules (QTAIM) has been employed.[59] The fivemembered carbon ring possesses a ring critical point (RCP with 1(r) = 0.0498 au) at its center, which contributes to a cage critical point (CCP with 1(r) = 0.0481 au), a prerequisite for the observation of three-dimensional aromaticity. For a qualitative chemical understanding, the bonding in the [(Me5C5)Si] + cation can be represented by the structures a, b, and c (see Figure 4).

3.2. p-Complexes of the type (p-Me5C5)SiR 3.2.1. Novel sandwich complexes (p-Me5C5)(p-R5C5)Si The reaction of the [(Me5C5)Si] + ion in 1 with reactants other than pentamethyl-substituted cyclopentadienyl anions leads to novel silicocene derivatives. This reaction was used to check the steric and electronic requirements for a sufficient stabilization of this type of p-complex (see Scheme 1). Lithium penta(isopropyl)cyclopentadienide[61] reacts with 1 to give almost quantitatively the sandwich complex pentamethylcyclopentadienyl(pentaisopropylcyclopentadienyl)silicon (3) as a moderately air-sensitive compound.[62] The molecular structure of 3 (one enantiomer) is presented in Figure 5 together with a space-filling model. The parallel cyclopentadienyl ring planes are arranged in a staggered conformation in order

Figure 4. Representations of the bonding of the [(Me5C5)Si] + ion.

Representation a is based on the cluster formalism and the respective electron-counting rules.[60] The parent species [(C5H5)Si] + is regarded as a nido-cluster derived from the boron species [B6H6]4 by isoelectronic and isolobal substitution (BH = CH + ; BH = Si). Electron delocalization in the cluster framework results in three-dimensional aromaticity.[56] The valencebond description in b explains already at a first glance the electronic situation at silicon (sp-hybridization, octet rule) and the ability of the Cp* group to simultaneously engage in s and p electron donation; this triple-bond notation so far has not found access to the literature, also not for the description of comparable isoelectronic fragments. The description in c is the preferred one for p-Cp*–metal complexes, of course with less information about the concrete bonding situation. Chem. Eur. J. 2014, 20, 9192 – 9207

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Figure 5. Molecular structure (H atoms omitted) and a space-filling model of 3 (one enantiomer).

to minimize the nonbonding contacts between methyl and isopropyl groups. The chirality is caused by the isopropyl groups oriented either clockwise or counterclockwise around the Cp ring. Due to pronounced steric constraints, the Cp(centroid)Si distances (2.18 and 2.19 ) are greater than those in the D5d conformer of decamethylsilicocene (2; 2.11 ). NMR investigations confirm a rigid structure in solution; no interconversion of enantiomers is observed. In the sandwich complex 3, the silicon(II) center is very effectively protected by the

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Minireview two Cp ligands; a haptotropic shift of at least one of these ligands is regarded to be impossible. As a consequence, the chemistry of this compound is characterized by an electrophilic attack at the peripheral p-system of one of the Cp ligands (see Section 3.8.). Lithium 1,3-bis(trimethylsilyl)cyclopentadienide[63] reacts with 1 to exclusively give the complex pentamethylcyclopentadienyl[1,3-bis(trimethylsilyl)cyclopentadienyl]silicon (4), whereas partial ligand redistribution is observed in the reaction of lithium 1,2,4-trimethylcyclopentadienide[64] leading to the silicocenes 5, 6, and 2 (see Scheme 2).[62] The reaction with lithium cyclopentadienide was performed in a special “two chamber” NMR tube[50] showing that the p-complex cyclopentadienyl(pentamethylcyclopentadienyl)silicon (7) is quantitatively formed at 50 8C, but starts to decompose already at 30 8C.[62]

For the stabilization of the first aryl-substituted silicon(II) compound, the extremely bulky terphenyl substituent 2,6Tip2H3C6 (Ar*)[65–68] was used. Reaction of 1 with the lithium salt LiAr*[71] led to the formation of the p-complex (Me5C5)(2,6Tip2H3C6)Si (8) as yellow, air- and moisture-sensitive crystals [Eq. (2)].[72]

In Figure 6, the molecular structure of 8 is presented from two perspectives. It shows an h3-bonded Cp* ligand and a sbonded terphenyl substituent with the central phenyl ring in a conformation, which does not allow p-backbonding to the silicon atom. The steric requirements lead to a deviation from linearity of the Si-C1-C4(phenyl ring plane) vector and to

Figure 6. Molecular structure of 8 (H atoms omitted) and another perspective (framework only).

Scheme 2. Novel sandwich complexes 3–7 starting from 1.

Generally, the central atom in sandwich and half-sandwich complexes of main-group elements experiences a pronounced high-field NMR shift, which can be used as a diagnostic probe.[45] As shown in Scheme 2, the 29Si NMR shift in complex 3 appears at the very end of the so far experimentally observed high-field region. Substitution of alkyl groups in the Cp system by trimethylsilyl groups or by hydrogen atoms in the complexes 4–7 causes an appreciable downfield shift, although the signals still appear in the high-field region.

a rather long SiC(phenyl) bond (1.97 ). According to DFT calculations, the overall charge that resides on the Cp*Si fragment is + 0.469, indicating a strong contribution of a polar resonance structure. The “lone-pair” of electrons at silicon is located in an orbital of mainly s-character. In the h3-Cp*Si unit, the SiC distances are shorter (2.10–2.28 ) than those in the h3Cp*Si unit of the bent conformation of decamethylsilicocene (h3-Cp*)(h2-Cp*)Si (2, 2.32–2.42 ). NMR data of 8 are in accord with a fluxional structure in solution. The 29Si chemical shift of d = 51.6 ppm is observed about 450 ppm downfield from the shift of 1 and indicates the loss of ring current effects present only in the latter.

3.2.2. The aryl-substituted silicon(II) complex (p-Me5C5)Si(2,6Tip2H3C6)

3.2.3. The amino-substituted silicon(II) complex (p-Me5C5)Si N(SiMe3)2 and its “dimer”

Mainly very recent experiments have shown that extremely bulky aryl groups play an important rule for the kinetic stabilization of novel bonding situations in several classes of lowvalent Group 14 elements.[34, 42e, 65–68] Interestingly, aryl-substituted monomeric silicon(II) compounds of the type Ar2Si or Ar(R)Si, isolable under ordinary conditions, have been unknown so far. The sterically congested aryl(alkyl)silylene (Tip)(1,2,3-tri-tert-butylcyclopropenyl)Si (Tip = 2,4,6-iPr3H2C6) is stable in solution up to 200 K, but could not be isolated.[69] The kinetically better stabilized diarylsilylene (Tb)(Mes)Si (Tb = 2,4,6[CH(SiMe3)2]3H2C6 ; Mes = 2,4,6-Me3H2C6) was generated in solution, but could not be isolated.[70]

In a landmark work, it was shown as early as 1977 that bis(trimethylsilyl)amino substituents can stabilize monomeric compounds of divalent germanium, tin, and lead of the type E[N(SiMe3)2]2 (E = Ge, Sn, Pb).[73] In the p-complex chemistry of germanium it was shown already in 1986, that the combination of a Cp* ligand with a bis(trimethylsilyl)amino substituent allows the isolation of the monomeric complex h2-Cp*GeN(SiMe3)2.[74] In the p-complex chemistry of silicon, the application of the Cp* together with the bis(trimethylsilyl)amino group creates a unique situation: A monomeric p-Cp* complex is present in solution, and a “dimerization” to a disilene with trans-positioned and now s-bonded Cp* substituents takes

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Minireview place on crystallization. This phase-dependent process is fully reversible.[75] Reaction of 1 with lithium bis(trimethylsilyl)amide led to the p-complex Cp*SiN(SiMe3)2 (9) in the form of a colorless liquid. Upon attempts to crystallize this species from organic solvents, the deep yellow disilene 10 crystallized from the colorless solution [Eq. (3)]. A colorless solution containing 9 was again obtained after treatment of crystalline 10 with an organic solvent. The transformation process could be repeated several times.[75] Later it was found, that the disilene 10 is accessible also by the reduction of dichloro(pentamethylcyclopentadienyl)silane with potassium bis(trimethylsilyl)amide [Eq. (4)].[76] Figure 8. Molecular structure of 10 (H atoms omitted).

3.2.4. The imino-substituted silicon(II) complex (p-Me5C5)Si N=CR2

The p-complex 9, which is extremely difficult to handle, is characterized by spectroscopic data, by the formation of a cycloaddition product, and by calculations. The calculations reveal the presence of a h2-bonded Cp* ligand and of a silylamino group in a conformation that prevents electron backdonation to the central silicon atom. The calculated structure of 9 is presented in Figure 7. NMR investigations reveal a fluxional structure in solution; the 29Si NMR signal is observed at d = 10.1 ppm. The molecular structure of the disilene 10 obtained by X-ray crystallography is given in Figure 8. It features a short Si=Si bond of 2.168  and s-bonded cyclopentadienyl and amino substituents in a trans-position to the Si=Si double bond. The reversible phase-dependent transformation is mainly caused by the different steric and stereoelectronic effects of the Cp* group exerted in the p-complex 9 and in the disilene 10. The term “molecular Jack in the box” is appropriate to describe the phase-dependent phenomenon. Quite different equilibrium situations in the silylene/disilene system have been described only very recently.[8, 27]

Figure 7. MOLDEN plot of 9 (H atoms omitted). Chem. Eur. J. 2014, 20, 9192 – 9207

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In transition-metal chemistry, the imidazolin-2-iminato group acts as a 2s- and either as a 2p- or 4p-electron donor resulting in some multiple-bond character between the nitrogen and the metal atom.[77–79] In the context with this flexible bonding situation, the behavior of this group in the p-complex chemistry of divalent silicon became interesting. Reaction of 1 with the lithium reagent LiL {L = bis[2,6-di(isopropyl)phenyl]imidazolin-2-iminate}[80] led to the formation of the complex (p-Me5C5)Si(L) (11) as colorless crystals [Eq. (5)]. Alternatively, complex 11 was prepared in low yield by reduction of the silicon(IV) compound (Me5C5)Si(Br)2(L) [Eq. (6)].[81]

The molecular structure of 11 obtained by X-ray crystallography is presented in Figure 9. It shows an h2-bonded Cp* ligand and a s-bonded iminato substituent in a conformation that allows some p-donation from the nitrogen to the silicon atom, in accord with the comparatively short SiN bond (1.692(5) ). However, the WBI (Wiberg Bond Index) value (0.798) still indicates a single-bond character. From several conceivable resonance structures, the imino–silicon(II) structure given in Equation (5) is calculated to be the dominant contribution. Calculations further show that the HOMO in 11 is represented mainly by the lone-pair orbital at silicon. In valence bond terms, the bonding at silicon is best described by the assumption of a sp2-type hybridization (comparable to the situation in 13, see Figure 11, below). NMR data of 11 are in accord with a fluxional structure in solution. The 29Si NMR signal at d = 43.8 ppm appears upfield-

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Figure 9. Molecular structure of 11 (H atoms omitted) and another view (framework only).

shifted relative to the signal of the amino-substituted complex 9 (d = 10.1), implying comparatively better electron donation to the silicon atom in 11. In the p-complex chemistry of divalent silicon, only very few examples are known so far showing the formation of a donor– acceptor compound with the silicon lone-pair as donor.[30] In contrast, many such examples are known in the chemistry of N-heterocyclic silicon(II) compounds; they turned out to be important novel ligands in catalytically relevant transition-metal complexes.[35, 37] In this context, the reaction of 11 with the Lewis acid tris(pentafluorophenyl)borane is of special importance. Hereby, the stable 1:1 adduct 12 is formed in high yield [Eq. (7)].[81] Further donor–acceptor complexes with 11 as donor and with other acceptor units from the main-group or from the transition-metal chemistry are conceivable.

The molecular structure of the adduct 12 was determined by X-ray crystallography. Surprisingly, the h2-coordination of the Cp* group has changed to a h1-mode, and the silicon atom remains three-coordinate in a planar geometry. The flexible coordination mode of the Cp* group is a prerequisite for the observed changes in the coordination sphere of the silicon atom. Rather short N=Si and N=C distances as well as the trend in direction of a linear C-N-Si unit support the importance of the 1-sila-2-azaallene resonance structure as presented in Equation (7). In the 29Si NMR spectrum of 12, a significant downfield shift (d = 114.5 ppm) relative to that of 11 (d = 43.8 ppm) indicates a decreased electron density at silicon upon coordination of the boron compound. Theoretical calculations support the experimental findings.[81] 3.2.5. The metallo-substituted silicon(II) complex (p-Me5C5)Si Fe(CO)2(C5Me5) Several classes of compounds containing transition-metal–silicon bonds are known in the literature, including species with single, double, triple, and electron-deficient bonds, with the silChem. Eur. J. 2014, 20, 9192 – 9207

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icon atom in different formal oxidation states.[82] It is worth noting that compounds in which a transition-metal fragment serves as a substituent of a divalent silicon atom (metallosilicon(II) compounds) have remained almost unknown so far. The ferrio-substituted p-complex (Me5C5)SiFe(CO)2(C5Me5) (13) is the first representative of this class.[82] Reaction of 1 with the salt Na[Fe(CO)2Cp*] led to the formation of 13 as air-sensitive and thermolabile brown crystals. In the solid state, complex 13 is stable at 30 8C for some weeks; at room temperature it rearranges to the silicon(IV) insertion product 14 [Eq. (8)].[82] For comparison, the analogous germanium compound p-Cp*GeFe(CO)2Cp* has been described as thermostable, red crystals.[83] The appealing transformation of 13 by loss of CO into the triple bond species Cp*SiFe(CO)Cp* was unsuccessful so far.[84]

The molecular structure of 13 determined by X-ray crystallography (see Figure 10) shows two p-bonded Cp* groups, the one h3-bonded to silicon, the other h5-bonded to iron. The fluxional structure of the h3-Cp*Si unit in solution is documented by NMR spectroscopy. As expected, the FeSi bond in the silicon(II) compound 13 is longer (2.3677(6) ) than that in the silicon(IV) compound H3SiFe(CO)2Cp* (2.287(2) ).[85] According to theoretical calculations, the electron density of the FeSi bond in 13 is mainly located at iron (74.2 %) and less at silicon (25.8 %). Following the population analysis, a charge of + 0.216 is remaining at the Cp*Si fragment (for comparison: + 0.99 in [Cp*Si] + [55]).

Figure 10. Molecular structure of 13 (H atoms omitted).

The geometry at the silicon atom in 13 is described on the basis of sp2 hybridization, with one orbital representing the lone-pair of electrons and the other two engaged in s-bonding to the Fe atom and to the Cp* unit. The remaining vacant porbital interacts mainly with a filled p-orbital of the Cp* fragment and to a lesser extent with a filled d-orbital at iron (see Figure 11). The resulting 3-center-4-electron bond leads to a small HOMO–LUMO gap, which is the reason for the color

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Minireview and for the contribution of paramagnetic terms to the diamagnetic ground-state. As a further consequence, the 29Si NMR signal of 13 Figure 11. Orbital interactions is observed at very low field (d = + in the Fe-Si-Cp* unit of 13. 316.7 ppm). A comparable bonding situation has been described for m-phosphinidene transition-metal complexes, connected with the observation of low-field 31P NMR signals.[86]

3.2.6. Competition between the Cp* and a b-diketiminato group in silicon(II) chemistry As already pointed out in the Introduction (Scheme 1), chelating substituents play a very important role in the chemistry of low-valent silicon. In this class of compounds, the application of sterically encumbered b-diketiminato substituents, also described with the term “NacNac”, has been very successful.[19, 20, 35–38] We have been interested to answer the question, which one of three possible coordination modes is the preferred one in a silicon(II) compound bearing both a Cp* and a NacNac group: a bonding situation 1) with a p-Cp* and a mono-coordinate NacNac group, or 2) with a chelating NacNac and a s-Cp* group, or 3) with a p-Cp*and a chelating NacNac group. Theoretical calculations performed at the RIBP86/TZVP level feature as the preferred structure the situation as described in point 2 (structure a in Scheme 3).

Scheme 3. Reaction of 1 with LiNacNac.

The result of the chemical experiment is somewhat surprising and can only indirectly confirm the calculations. Reaction of 1 with the lithium compound Li[HC(CMeNAryl)2]·DME (Aryl = 2,6-iPr2H3C6 ; DME = dimethoxyethane) (LiNacNac)[87] led to the formation of the tricyclic silicon(IV) compound 14, characterized by X-ray crystallography and NMR spectroscopy.[88] The proposed reaction sequence is described in Scheme 3: The silicon(II) compound a (the calculated structure) is formed as an intermediate, which rearranges by reductive cleavage of the (Ar)NC bond of the NacNac group; a further CC bond formation and a proton-transfer reaction finally lead to compound 14. Chem. Eur. J. 2014, 20, 9192 – 9207

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An important message is obtained from the above described experiment: in the chemistry of divalent silicon, both a cyclopentadienyl and a NacNac group can be involved in unwanted reactions, such as cycloadditions (Cp group) or redox processes (NacNac group). 3.2.7 The p-bonding in slipped half-sandwich complexes Concerning the p-bonding between a Cp fragment and a main-group element, some high-level theoretical calculations have been performed, so that this interaction is quite well understood.[89–92] Also the differences between the p-bonding to a main-group element and to a transition-metal have been clearly worked out. Transition-metals typically form a rather strong, covalent h5- and, in only few cases, h3-bonds. In contrast, main-group elements show “a bewildering variety of bonding arrangements”[91] from symmetric h5 to various highly fluxional slipped hx (x = 3,2,1) structures ranging from mainly covalent to mainly ionic.[58, 91] Herein, we will concentrate on the discussion of the p-Cp* Si interaction in the novel silicon(II) complexes 8, 9, 11, and 13 and for comparison in the bent conformer of decamethylsilicocene (2). These interactions are characterized by flat electron densities and small density gradients,[92] which lead to highly fluxional structures. In solution, this situation is documented by averaged 1H and 13C NMR signals, even at low temperatures (60 8C). In the solid state, X-ray crystallographic data show distortions typical of a h2- or a h3-coordination. For the gaseous state, mainly theoretical calculations inform about the distortions and the activation parameters for rearrangement processses. Table 1 collects important structural parameters from crystallographic studies and from calculations. These data give information about 1) the planarity of the C5 unit in the Cp* system, 2) the SiC(Cp*) distances in the p-bond (h2 or h3), 3) the CC distances in the C5 perimeter, and 4) the deviation of the MeC(Cp) bonds from the Cp ring plane (angle a). The collected data are interpreted as follows: 1) It is important to note that the C5 perimeter of the Cp system in the silicon(II) complexes 2, 8, 9, 11, and 13 remains planar within given error limits; evidently, the p-coordination is not strong enough to induce pronounced deviations from planarity. This situation is different from the h3-Cp transition-metal interaction in transition-metal complexes like [(h3-Cp)(h5-Cp)W(CO)2] (b = 160.38)[93] and [(2,2’bipy)(h3-Cp)(h5-Cp)V] (b = 171.78; bipy = bipyridine).[94] As portrayed in Figure 12, the slip distortion leads to a p-allyltype interaction, which causes a strong deviation from planarity, as expressed by the angle b between the planes C1C2-C3 and C1-C3-C4-C5. In the transition-metal complexes, the C4C5 distances in the folded C5 perimeter are characteristic of a noncoordinated C=C double bond (1.35 and 1.37  in the tungsten and vanadium complexes, respectively), the C1C5 and the C3C4 distances (1.45 and 1.42 ) correspond to typical CC s-bonds, and the CC distances in the allyl units (1.42 and 1.41 ) are the result of h3-Cp coordination.

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Minireview Table 1. Structural parameters (distances in ; angles in 8) of the p-complexes 2, 8, 9, 11 and 13 from X-ray crystal structure data and from theoretical calculations (to analyze the h2, 3Cp*Si interaction in slipped half-sandwich complexes).

SiC1 a-C1Me SiC2 a-C2Me SiC3 a-C3Me C1C2 C2C3 C3C4 C4C5 C5C1 C5 ring

8

13

2b [a]

9x [b]

11

2b [a]

2.2821(19) 5.9 2.0965(19) 21.8 2.2678(19) 7.9 1.432(3) 1.437(3) 1.438(3) 1.374(3) 1.434(3) planar

2.4349(19) 1.4 2.1362(17) 14.1 2.210(2) 7.2 1.436(3) 1.434(3) 1.424(3) 1.396(3) 1.409(3) planar

2.417(7) 0.1 2.323(7) 7.1 2.375(7) 3.3 1.39(1) 1.397(9) 1.41(1) 1.38(1) 1.40(1) planar

2.275 16 2.275 16 – – 1.448 1.447 1.406 1.427 1.432 planar

2.124(6) 7.2 2.218(6) 11.4 – – 1.4343(90) 1.4157(98) 1.3824(90) 1.3784(90) 1.4339(98) planar

2.324(8) 6.3 2.347(7) 4.0 – – 1.42(1) 1.41(1) 1.37(1) 1.37(1) 1.40(1) planar

[a] Bent isomer. [b] Calculations B3LYP/TZVP.

The above conclusions can be summarized as follows: The h2 or h3 interaction in Cp*Si complexes is much “softer” than that in a Cp*–transition-metal system.[91] The energy difference between a h2- and a h3-complex and also the activation energy for haptotropic shifts is rather small. Bearing in mind the flat potential energy surface in such complexes, an overinterpretation of experimental and theoretical data must be avoided.

Figure 12. Graphical presentation of two modes of h3-Cp coordination (M: transition metal).

2) In the h3-Cp*Si complexes 8 and 13, the central distance (SiC2) is shorter and the peripheral distances (SiC1, Si C3) are longer relative to the SiC distances in the h5[Cp*Si] + ion (2.14–2.16 ). In the bent isomer of decamethylsilicocene (2b), all three SiC distances in the h3-Cp* unit are comparably longer due to the weaker p-interaction of the individual Cp* ligand. In the h2-Cp*Si complexes 9 and 11, the SiC distances are in the range (11) or a little longer (9) than those found in the h5-[Cp*Si] + ion; in the h2-Cp* unit of the sandwich 2b, the distances are as expected somewhat longer than those in 9 and 11. 3) The CC distances in the C5 ring reflect the asymmetric Si coordination: they differ with the respective coordination mode. Short CC distances (C4C5 in 8, 13 and 2b ; C3-C4C5 in 9, 11, 2b) are observed for the C atoms of the noncoordinated p-system. The shortest CC distance is observed in 8 (1.374(3) ); this value is close to that observed for noncoordinated double-bond units (ca. 1.35 ) in transition-metal complexes. The other CC distances do not allow a clear distinction between p-coordinated CC and formally s-bonded CC systems; in contrast, clear distinctions are given in comparable transition-metal complexes.[93, 94] 4) A deviation from the C5 ring plane is observed for those methyl substituents, the carbon atoms of which are involved in p-bonding (angle a in Table 1). This deviation is explained by the tendency of the involved C atom to form a hybrid orbital with a higher p-orbital contribution. Similar effects have been discussed for comparable transitionmetal complexes.[95] Chem. Eur. J. 2014, 20, 9192 – 9207

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3.3. A novel route to cyclotrisilenes In the chemistry of the [Cp*Si] + ion, the silicon analogue of the vinylcarbene–cyclopropene cyclization[96] is implicated for the first time. Reaction of 1 with the lithium disilenide Li[RSi=SiR2] (R = 2,4,6-iPr3H2C6, Tip) led to the formation of the cyclotrisilene 15, the first member in this class of compounds, which contains only carbon-based substituents (Scheme 4).[97] The formation of 15 is explained by an intramolecular attack of the silicon lone-pair at the b-Si atom of the Si=Si double bond in the transient species 15 a, followed by a cyclic rearrangement of electrons in the Si3 fragment and a change in the Cp* coordination mode from p to s. Compound 15 is characterized by NMR spectroscopy and by an X-ray crystal structure analysis of

Scheme 4. Synthesis of the cyclotrisilene 15 and reversible formation of the adduct 16.

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Minireview the adduct 16 with a N-heterocyclic carbene (NHC). The adduct formation turned out to be reversible in solution.[97] Interestingly to note, an equilibrium between a stable, fully Tipsubstituted cyclotrisilene and an isolable NHC adduct of the corresponding disilenyl silylene was reported only very recently. An adduct comparable with 16 (Tip instead of Cp*) was discussed as an intermediate.[100] In the context with structure, bonding, and reactivity of multiple bond systems of the heavier Group 14 elements, the adduct formation and the reversibility of this process is worth mentioning. Very recent investigations of double and triple bond species of germanium, tin, and lead have shown the importance of zwitterionic and diradical contributions to the ground-state structures (see Scheme 5), which illustrate both the weakness of the element–element multiple bonds and the preference of trans-bent geometries. Even more important, such contributions are the prerequisite for quite unexpected and novel reactivity patterns.[66–68, 98]

wards h3-bonding. The averaged SiC distance (2.176 ) is longer than that in the ether-free cation (2.147 ). The structural parameters of the cation in adduct 17 calculated at the ab initio MP2/TZVPP level are nearly identical to those found experimentally. The weak Figure 13. Molecular structure SiO bonding is based on purely of the cation in 17. electrostatic, van der Waals-type interactions. The two SiO interactions cause a reduction of the covalent character of the Cp*Si bond. As a result, the positive charge at silicon (+ 1.11 in 17, + 0.96 in 1) and the negative charge at the Cp* fragment (0.21 in 17, + 0.04 in 1) is slightly increased.[55] Similarly, from a solution of 1 and the cyclic oligoether [12]crown-4 in CH2Cl2/hexane, the crystalline and highly airand moisture-sensitive 1:1 complex 18 could be isolated [Eq. (10)].[102]

Scheme 5. Resonance structures of multiple bond systems of Group 14 elements.

In silicon chemistry, such polar contributions are less pronounced. The first experimentally observed example in a triple bond species was presented only recently by the formation of an adduct between the disilyne RSiSiR (R = SiiPr[CH(SiMe3)2]2 and the NHC 1,3,4,5-tetramethylimidazol-2-ylidene.[99] The reversible processes described for cyclotrisilenes represent the first examples with compounds containing a Si=Si double bond.[97, 100] Reversible coordination is a characteristic phenomenon in transition-metal chemistry, but becomes increasingly important also in the chemistry of low-valent main-group elements in the context with catalytic activity.[67] 3.4. The oligoether adducts [(h5-C5Me5)Si(dme)] + and [(h5C5Me5)Si([12]crown-4)] + A reversible coordination to the [Cp*Si] + ion was observed in some reactions with oligoethers. From reactions performed in dimethoxyethane (DME) it was suspected that this solvent might not only coordinate to, but further interact with the cation.[101] In this context, the system [Cp*Si] + /oligoether was investigated in more detail. From a solution of 1 and DME in [D2]dichloromethane, the crystalline, highly air-sensitive and thermolabile DME complex 17 could be isolated. A brief treatment of 17 in vacuo resulted in the reverse reaction [Eq. (9)].[102] Despite the instability of the adduct 17, its structure could be ascertained by X-ray crystallography (Figure 13). The DME molecule coordinates rather weakly in an asymmetric bidentate fashion to the Cp*Si unit (SiO distances 2.82 and 2.90 ), and the Cp* ligand shows only a small deviation from h5- toChem. Eur. J. 2014, 20, 9192 – 9207

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An X-ray crystal structure analysis (Figure 14) showed that the four oxygen atoms of the crown ether molecule are weakly coordinated to the silicon atom with an averaged SiO distance of 2.94 . The Cp*Si fragment shows only small deviations from h5-bonding and longer SiC distances (2.191 ) compared to those present in 1 (2.147 ). The half-sandwich-like crown ether coordination to a silicon atom is observed here for the first time. The Figure 14. Molecular structure cation in 18 can be described by of the cation in 18. a bent-sandwich structure, composed of an h5-bonded Cp* group and a h4-bonded [12]crown4 molecule as ligands and a silicon atom as the center of the sandwich molecule. In the context with crown ether coordination to divalent Group 14 elements, it has to be mentioned that only recently several surprisingly stable complexes with divalent germanium or tin have been isolated and structurally characterized.[103–106] 3.5. The catalyst [(p-Me5C5)Si] + It was a surprising observation that the complex [(h5-Cp*)Si(dme)] + [B(C6F5)] (17) was decomposed selectively in CD2Cl2 within some days, with quantitative transformation of the DME molecule into dimethyl ether and 1,4-dioxane. From the reac-

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Minireview tion mixture, the complex [(h5-Cp*)Si] + [B(C6F5)] (1) could be recovered [Eq. (11)]. As expected from these observations, the transformation of DME could also be performed in the presence of only catalytic amounts of 1. The catalytic process worked also in neat DME, and a high conversion rate was observed even with a low catalyst loading (ca. 0.5 mol %).[102]

Scheme 7. [Cp*Si] + -catalyzed reactions.

A plausible pathway for the catalyzed process is described in Scheme 6. Two DME molecules are expected to be weakly coordinated by their oxygen atoms to the silicon center of the cationic catalyst. The resulting complex [Cp*Si(dme)2] + [B(C6F5)4] (19) could neither be isolated nor observed by NMR spectroscopy. Calculations at the SCS-MP2/TZVPP level of theory[102] feature four rather weak SiO contacts in the range

The splitting of CO bonds in oligo- and polyethers is a key step in organic synthesis; a controlled catalytic degradation has not been reported so far in the literature. Some unique features of the [Cp*Si] + cation seem to be responsible for its function as catalyst: 1) The open coordination sphere of the half-sandwich structure enables up to four weak and stereochemically nonrigid interactions with oxygen atoms from oligoether units; 2) such interactions are strong and selective enough to induce rearrangement reactions in the ether framework; and 3) the flexible Cp*Si bonding can adjust to the Si O bonding and is regarded as a prerequisite for the catalytic function. The search for further transformations catalyzed by the [Cp*Si] + ion is promising.

3.6. One-electron reduction to the thermolabile compound (Me5C5)SiSi(C5Me5)

Scheme 6. Catalytic cycle in the transformation of DME.

of 2.90–3.03  and only a small deviation from an h5-Cp*Si bonding. The coordination of the first DME molecule is exothermic by 15 kcal mol1, and that of the second DME molecule is exothermic by 8 kcal mol1. The transformation of two DME molecules into two molecules of dimethyl ether and one molecule of 1,4-dioxane is slightly endothermic by 2.4 kcal mol1. The weak SiO contacts induce a rearrangement of s-bond and lone-pair electrons in the framework of the two DME molecules under formation of dimethyl ether and of diglyme, the latter still coordinated to the silicon atom. In a second step, the diglyme molecule is degraded in a similar fashion to dimethyl ether and 1,4-dioxane, and the catalytic cycle can start again. By the intermediate formation and transformation of diglyme, the proposed catalytic cycle indicates that other oligo(ethylene glycol) diethers might also be degraded. Indeed, further investigations have confirmed the generalization of this process. Representative examples are collected in Scheme 7. Interestingly, cyclic ethylene glycol diethers can also be degraded, as demonstrated by the transformation of the crown ether [12]crown-4, albeit in a rather slow reaction. Chem. Eur. J. 2014, 20, 9192 – 9207

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Experiments to synthesize the silicon(I) complex (Me5C5)Si Si(C5Me5) (20) by one-electron reduction of (1) have been unsuccessful so far. The wanted species easily disproportionates to decamethylsilicocene (2) and to elemental silicon. Thus, reaction of 1 with the electron-transfer reagents sodium naphthalenide[107] or sodium dicarbonyl(pentamethylcyclopentadienyl)ferrate[82] performed at 60 8C in DME as solvent resulted in the formation of 2 and a yellow-brown colloidal DME solution containing dispersed silicon nanoparticles [Eq. (12)]. Similarly, the reduction of 1,1,2,2-tetrachloro-1,2-bis(pentamethylcyclopentadienyl)disilane led to the formation of 2 and silicon [Eq. (13)].[108] Bearing in mind the quite different substituent (R) dependent structures of species with the composition RSiSiR,[42, 109]

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Minireview and with the aim to better understand the easy disproportionation of the compound (Me5C5)Si-Si(C5Me5) (20) into 2 and elemental silicon, a theoretical calculation of the structure of 20 was performed.[110] Very interestingly, the calculated structure comprises a quite novel coordination mode of a p-bonded cyclopentadienyl group, as presented in Figure 15: Three neighboring carbon atoms of the Cp* unit are involved in p-bonding to two silicon atoms from a s-Cp*SiSi unit. As a consequence

bottom-up synthesis of molecularly defined silicon clusters is still in its infancy.[44]

Figure 15. Calculated structure of Cp*SiSiCp*; H atoms omitted; distances in : Si1C1 = 1.953, Si1Si2 = 2.287, Si1C2 = 1.944, Si1C3 = 2.175, Si2 C3 = 2.237, Si2C4 = 2.012.

of p-coordination, the methyl substituents at the three carbon atoms involved in p-bonding deviate from the plane of the C5 ring. In the p-bonded Cp*Si2 unit, three of the four SiC distances are longer and one distance is shorter than the SiC distance in the s-Cp*Si unit. On the basis of the calculated structure of 20, the easy elimination of a silicon atom can be explained in a rational way: only a small rotation of the Cp* groups is necessary to extrude a silicon atom (Si2 in Figure 15) and to build the thermodynamically stable sandwich complex Cp*2Si. More detailed investigations of the silicon extrusion have not been performed so far.

The Cp*Si bond in 15 can be split also heterolytically by a nucleophilic substitution process, as indicated by the formation of the known silicon cluster Si5Tip6 (22) in the reaction with the lithium disilenide Li[RSi=SiR2] (R = Tip) [Eq. (16)].[97] It is evident that this reaction type also enables the synthesis of novel silicon cluster molecules.

3.7. [(p-Me5C5)Si] + : A stoichiometric source of silicon in molecular cluster compounds Due to the fact that the pentamethylcyclopentadienyl radical as well as the pentamethylcyclopentadienide anion are rather stable entities, the Cp*Si bond in silicon(IV) and in silicon(II) compounds can easily be split either homo- or heterolytically.[58, 111] In derivatives of the [Cp*Si] + ion, both reaction types have been described. They offer a novel access to the class of molecular silicon clusters. The Cp*Si bond in the cyclotrisilene 15 can be split homolytically either in a one-electron reduction process or by thermal treatment to give a silicon radical intermediate, which rearranges to the already known silicon cluster Si6R6 21 (R = Tip)[97] featuring a novel type of aromaticity [Eq. (14)]. In the overall reaction sequence, the [Cp*Si] + ion formally acts as a source of silicon. From this observation a more general synthetic strategy can be envisaged. Following the reaction sequence described in Equation (15), Cp*Si bond splitting to an intermediate silicon radical opens a new entry to the class of neutral silicon cluster molecules containing substituent-free silicon atoms. The Chem. Eur. J. 2014, 20, 9192 – 9207

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3.8 The novel half-sandwich silicon(II) cation [(p-iPr5C5)Si] + Protonation of the mixed substituted silicocene 3 (prepared from 1) with the proton source H(OEt2)2 + Al[OC(CF3)3]4 [113] led quantitatively to the air-sensitive, but thermally stable salt 23 containing the [penta(isopropyl)cyclopentadienyl]silicon(II) cation [Eq. (17)]:[62]

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Minireview pentadienyl complexes with divalent silicon as the central atom are promising species to be used as novel types of catalysts and as entry points for the flourishing chemistry of lowand zerovalent silicon.

Acknowledgements

Figure 16. Molecular structure (H atoms omitted) and space filling model of the [(iPr5C5)Si] + ion in 23.

Compound 23 was characterized by NMR spectroscopy and by an X-ray crystal structure analysis (Figure 16). NMR studies show the equivalence of all five isopropyl groups. Simular to the situation in the silicocene 3, the methyl groups of the isopropyl units are chemically non-equivalent due to their fixed exo- or endo-orientation. The pronounced shielding of the silicon atom—compared to the situation in the [Cp*Si] + ion—is nicely documented by the space-filling models presented in Figure 16 and in Figure 2. Comparable to the situation in the [Cp*Si] + ion, the 29Si chemical shift is observed at the highfield end of the ppm scale (d = 397.4 ppm). So far, further studies with this compound containing the second representative in the class of (cyclopentadienyl)silicon(II) cations have not been performed. For steric reasons, novel pathways in the reaction with small molecules and in single-electron reduction processes can be envisaged.

The work presented from our group was supported by the Deutsche Forschungsgemeinschaft (DFG) in the project Ju 67/ 41. This support is gratefully acknowledged. The invaluable experimental contributions of several co-workers (their names are listed in the references) are highly appreciated, especially those of Dr. K. Leszczyn´ska, who worked as a single combatant for some years. I thank Dr. M. J. Cowley, University of Edinburgh (UK), for helpful comments and for improving the English. Keywords: catalysis · cyclopentadienyl ligands · silicon · silicon(II) cation · nucleophilic addition · pi interactions

4.0. Summary and Outlook During the last few decades, p-complex formation has turned out to be an important tool in the chemistry of divalent silicon. After the investigation of the neutral sandwich complex Cp*2Si, the cationic half-sandwich species in the ionic compound [Cp*Si] + [B(C6F5)4)] (1) has recently been at the foreground of interest. The chemistry of the [Cp*Si] + ion is dominated by its strong electrophilicity and by the easy hapticity change and the leaving-group character of the Cp* ligand. The reaction with anionic nucleophiles leads to novel sandwich and slipped half-sandwich complexes containing silicon–element bonds with elements from the p- and d-block. The reaction with neutral oxygen-donor molecules leads reversibly to cationic complexes under preservation of the h5-bonding mode of the Cp* ligand. Interestingly, the [Cp*Si] + ion acts as a “metal-free” catalyst in the previously unknown specific degradation of oligoethers. A p–s haptotropic shift of the Cp* group in a transient silicon(II) compound has opened a novel synthetic route to the class of cyclotrisilenes. Homo- or heterolytic cleavage of the Cp*Si bond is the basis to use the [Cp*Si] + ion as a source of silicon particles and even as a formal source of a silicon atom. Protonation of the sandwich Cp*(iPr5C5)Si and subsequent Cp*H elimination has led to the half-sandwich cation [(iPr5C5)Si] + , the second representative in this class of silicon(II) complexes. In this review, the presumably most important reaction types of the [Cp*Si] + ion have been described. It is expected that in the near future further important examples based on the given reactivity patterns will be presented. Cationic cycloChem. Eur. J. 2014, 20, 9192 – 9207

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