The pentamethylcyclopentadienylsilicon(II) cation: synthesis, characterization, and reactivity.

In the flourishing chemistry of divalent silicon, π-complex 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 sandwich and half-sandwich π-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 Cp*Si+ cation as a stoichiometric source of silicon.

The Cp*Si(+) cation acts as a stoichiometric source of silicon in the reaction with the disilenide Tip(2)Si=Si(Tip)Li (Tip = 2,4,6-(i)Pr(3)C(6)H(2)) affording known neutral unsaturated silicon clusters. It thereby provides a conceptually different approach to this novel class of compounds. The proposed mechanism involves a Cp*-substituted cyclotrisilene in which Cp*(-) acts as a leaving group upon single electron reduction or in a nucleophilic substitution step.

From nanoscale liquid spheres to anisotropic crystalline particles of tin: decomposition of decamethylstannocene in organic solvents.

Routes are presented for synthesizing nano- and mesostructured ß-tin particles in the form of monocrystalline spheres, cubes, and bars, as well as polycrystalline rods and needles, by the decomposition of decamethylstannocene in organic solvents under various conditions. The formation of the observed shapes is based on the presence of liquidlike and of partly crystalline droplets. These particle stages allow structure-determining processes such as entire coalescence, oriented superficial coalescence or superficial induced crystallization. Entire coalescence and oriented superficial coalescence take place in the absence of surfactants; the superficially induced crystallization occurs in the presence of ionic additives. The observed tin morphologies depend on the competition between droplet growth and crystallization behavior. The different tin particles are investigated by electron microscopy (SEM, TEM, HRTEM), selected area electron diffraction (SAED), and differential scanning calorimetry (DSC).

Reversible transformation of a stable monomeric silicon(II) compound into a stable disilene by phase transfer: experimental and theoretical studies of the system {[(Me3Si)2N](Me5C5)Si}n with n = 1,2.

The salt (eta(5)-pentamethylcyclopentadienyl)silicon(II) tetrakis(pentafluorophenyl)borate (5) reacts at -78 degrees C with lithium bis(trimethylsilyl)amide in dimethoxyethane (DME) as solvent to give quantitatively the compound [bis(trimethylsilyl)amino][pentamethylcyclopentadienyl]silicon(II) 6A in the form of a colorless viscous oil. The reaction performed at -40 degrees C leads to the silicon(IV) compound 7, the formal oxidative addition product of 6A with DME. Cycloaddition is observed in the reaction of 6A with 2,3-dimethylbutadiene to give the silicon(IV) compound 8. Upon attempts to crystallize 6A from organic solvents such as hexane, THF, or toluene, the deep yellow compound trans-1,2-bis[bis(trimethylsilyl)amino]-1,2-bis(pentamethylcyclopentadienyl)disilene (6B), the formal dimer of 6A, crystallizes from the colorless solution, but only after several days or even weeks. Upon attempts to dissolve the disilene 6B in the described organic solvents, a colorless solution is obtained after prolonged vigorous shaking or ultrasound treatment. From this solution, pure 6A can be recovered after solvent evaporation. This transformation process can be repeated several times. In a mass spectroscopic investigation of 6B, Si=Si bond cleavage is observed to give the molecular ion with the composition of 6A as the fragment with the highest mass. The X-ray crystal structure analysis of the disilene 6B supports a molecule with a short Si=Si bond (2.168 A) with efficiently packed, rigid sigma-bonded cyclopentadienyl substituents and silylamino groups. The conformation of the latter does not allow electron donation to the central silicon atom. Theoretical calculations at the density functional level (RI-BP86 and B3LYP, TZVP basis set) confirm the structure of 6B and reveal for silylene 6A the presence of an eta(2)-bonded cyclopentadienyl ligand and of a silylamino group in a conformation that prevents electron back-donation. Further theoretical calculations for the silicon(II) compound 6A, the disilene 6B, and the two species 11 and 11* derived from 6A (which derive from Si=Si bond cleavage) support the experimental findings. The reversible phase-dependent transformation between 6A and 6B is caused by (a) different stereoelectronic and steric effects exerted by the pentamethylcyclopentadienyl group in 6A and 6B, (b) some energy storage in the solid state structure of 6B (molecular jack in the box), (c) a small energy difference between 6A and 6B, (d) a low activation barrier for the equilibration process, and (e) the gain in entropy upon monomer formation.

[2,6-(Trip)2H3C6](Cp*)Si: a stable monomeric arylsilicon(II) compound.

Si takes a rest: A bulky sigma-bound terphenyl substituent and a pi-bound Cp* ligand enable the isolation and full characterization of the first aryl-substituted, monomeric silicon(II) compound 1, which can be regarded as the "resting state" of a true silylene containing a sigma-bound Cp* group. The conformation of the aryl group prevents aryl-Si pi back-bonding.

Reliable stabilization and functionalization of nanoparticles through tridentate thiolate ligands.

Novel amphiphilic trithiolates possess excellent properties for gold nanoparticle (AuNP) stabilization and functionalization and cannot be replaced by exchange reactions.

Novel pi-complexes of divalent silicon: mixed substituted neutral sandwich compounds and the half-sandwich cation (iPr5C5)Si+.

The pentamethylcyclopentadienylsilicon(II) cation, Me5C5Si+, opens up access to novel silicocene derivatives; the penta-iso-propylcyclopentadienylsilicon(II) cation, iPr5C5Si+, is obtained by reaction of the mixed silicocene (iPr5C5)(Me5C5)Si with H(OEt2)2+ Al[OC(CF3)3]4-.

The trinuclear gallium-bridged Ferrocenophane [{Fe(eta5-C5H4)2}3Ga2]: synthesis, bonding, structure, and coordination chemistry.

The trinuclear ferrocenophane [{Fe(eta(5)-C(5)H(4))(3)}(2)Ga(2)] (3) featuring two sp(2)-hybridized gallium atoms in bridging positions between three ferrocene-1,1'-diyl units represents a novel type of ferrocene derivative. Compound 3 is obtained by thermal treatment of 1,1'-bis(dimethylgallyl)ferrocene (1) in nondonor solvents or in diethyl ether as solvent and subsequent thermal decomplexation. The [1.1]ferrocenophane [{Fe(eta(5)-C(5)H(4))(2)}(2){GaMe}(2)] (2) is an intermediate in the formation of 3. The reaction of 3 with an excess of trimethylgallium leads back to 1 and proves the reversibility of the multistep reaction sequence. Theoretical calculations reveal a carousel-type D(3h) structure for 3. The compound can best be described as being composed of three only weakly interacting ferrocenediyl units covalently connected by gallium atoms without any pi-bond contribution in the Ga--C bonds. Owing to steric constraints 3 cannot be reduced to the dianion 3(2-), which would feature a Ga--Ga bond. Compound 3 represents a stereochemically rigid difunctional Lewis acid allowing the formation of the adducts 3 a-3 d possessing linear donor-aceptor-aceptor-donor arrangements. Crystal structure data for 3 a-3 d show a symmetry-reduced chiral ferrocenophane core (D(3h)-->D(3)). A polymeric rodlike structure is observed for 3 b and 3 d caused by pi-stacking effects (3 b) or by a difunctional donor-acceptor interaction (3 d). In solution, the chirality of the adducts is lost by rapid interconversion of the enantiomers. A cyclic voltammogram of 3 b in pyridine reveals three quasi-reversible oxidation steps at -356, -154, and 8 mV, indicating only weak electron delocalization in the cationic species. The redox potentials of the pyridine adduct 3 b are compared with those of other pyridine-stabilized gallyl-sustituted ferrocene derivatives and with ferrocene itself.

Novel rhodium and ruthenium carbonyl cluster complexes with face- and edge-bridging GaCp* ligands. Synthesis and structural characterization of the Rh6(CO)12(mu3-GaCp*)4 and Ru6(eta6-C)(mu2-CO)(CO)13(mu3-GaCp*)2(mu2-GaCp*) clusters.

Reactions of hexanuclear carbonyl clusters of rhodium Rh(6)(CO)(16) and ruthenium Ru(6)(eta(6)-C)(micro(2)-CO)(CO)(16) with GaCp*(Cp*= C(5)Me(5)) in the mild conditions result in substitution of CO ligands and formation of the Rh(6)(CO)(12)(micro(3)-GaCp*)(4) and the Ru(6)(eta(6)-C)(micro(2)-CO)(CO)(13)(micro(3)-GaCp*)(2)(micro(2)-GaCp*) cluster derivatives.

New magnetic nanoparticles for biotechnology.

Paramagnetic carriers, which are linked to antibodies enable highly specific biological cell separations. With the colloidal synthesis of superparamagnetic Co and FeCo nanocrystals with superior magnetic moments the question about their potential to replace magnetite as the magnetically responsive component of magnetic beads is addressed. Starting from a magnetic analysis of the corresponding magnetophoretic mobility of Co and FeCo based alloys their synthesis and resulting microstructural and magnetic properties as function of the underlying particle size distribution are discussed in detail. The stability of the oleic acid ligand of Co nanocrystals has been investigated. The oxidation kinetics were quantified using magnetic measurements. As a result, this ligand system provides sufficient protection against oxidation. Furthermore, the kinetics of the synthesis of Fe(50)Co(50) nanoparticles has been monitored employing Fourier transform infra red (FT-IR) spectroscopy and is modeled using a consecutive decomposition and growth model. This model predicts the experimentally realized FeCo nanoparticle composition as a function of the particle size fairly well. High-resolution transmission electron microscopy (HRTEM) was performed to uncover the resulting microstructure and composition on a nanometer scale.

The (Me5C5)Si+ cation: a stable derivative of HSi+.

The reaction of decamethylsilicocene, (Me5C5)2Si, with the proton-transfer reagent Me5C5H2+B(C6F5)4- produces the salt (Me5C5)Si+ B(C6F5)4(2), which can be isolated as a colorless solid that is stable in the absence of air and moisture. The crystal structure reveals the presence of a cationic pi complex with an eta5-pentamethylcyclopentadienyl ligand bound to a bare silicon center. The 29Si nuclear magnetic resonance at very high field (delta = - 400.2 parts per million) is typical of a pi complex of divalent silicon. The (eta5-Me5C5)Si+ cation in 2 can be regarded as the "resting state" of a silyliumylidene-type (eta1-Me5C5)Si+ cation. The availability of 2 opens new synthetic avenues in organosilicon chemistry. For example, 2 reacted with lithium bis(trimethylsilyl)amide to give the disilene E-[(eta1-Me5C5)[N(SiMe3)2]Si]2(3).

[{Fe(C5 H4 )2 }3 {Ga(C5 H5 N)}2 ]: A Trinuclear Gallium-Bridged Ferrocenophane with a Carousel Structure.

A molecular carousel, that is the appearance of the structure of the title compound [1⋅2 py], the first complex of a new structure type prepared by a highly selective condensation reaction from the equally new complex [Fe(C5 H4 GaMe2 )2 ]. The first gallium bridged [m.m]ferrocenophane [{Fe(C5 H4 )2 }2 {GaMe(C5 H5 N)} 2 ] is formed as an intermediate; this compound can also be prepared in a planned synthesis and can be converted into [1⋅2 py] by warming. The complex [1⋅2 py] as well as [1⋅2 Et2 O] and [1⋅2 DMSO], which were formed by donor exchange, offers various interesting properties, for example, [1⋅2 DMSO] can be reversibly oxidized to the mono-, di-, and trication.

Stabilization of Organosilicenium Ions by means of Intramolecular Coordination of O, S, or P Ligands.

For the intramolecular stabilization of silicenium ions (R3Si+), O, S, and P donors as well as the known nitrogen-containing systems (as in 1 a) are suitable. The silyl cations in 1 a-d show a trigonal-bipyramidal structure; dynamic processes can be proved by NMR spectroscopy for 1 c, d. Calculations on model compounds document substantial differences in the bonding relationships and support the structural findings. Furthermore, preliminary experiments with 1 b-d indicate significant differences in reactivity.