Christian Burschka

The Donor‐Stabilized Silylene Bis[N,N′‐diisopropylbenzamidinato(−)]silicon(II): Synthesis, Electronic Structure, and Reactivity

A convenient and robust synthesis of bis[N,N-diisopropylbenzamidinato(-)]silicon(II) (1), a donor-stabilized silylene, has been developed (35u2005g scale). To get further information about the reactivity profile of 1, a series of oxidative addition reactions were studied. Treatment of 1 with PhSe-SePh (Se-Se bond activation), C6F6 (C-F activation), and CO2 (C=O activation/cycloaddition) yielded the neutral six-coordinate silicon(IV) complexes 10, 11, and 13, respectively. Treatment of 1 with N2O resulted in the formation of the dinuclear five-coordinate silicon(IV) complex 12 (oxidative addition/dimerization), which contains a four-membered Si2O2 ring. Compounds 10-13 were characterized by NMR spectroscopic studies in the solid state and in solution and by crystal structure analyses. Silylene 1 is three-coordinate in the solid state (from crystal structure analysis) and exists as the four-coordinate isomer 1 in benzene solution (from computational studies). Based on state-of-the-art relativistic DFT analyses, the four-coordinate species 1 was demonstrated to be the thermodynamically favored isomer in benzene solution (favored by ΔG = 6.6u2005kcal mol(-1) over the three-coordinate species 1). The reason for this was studied by bonding analyses of 1 and 1. Furthermore, the (29)Siu2005NMR chemical shifts of 1 and 1 were computed, and in the case of 1 it was analyzed how this NMR spectroscopic parameter is affected by solvation. These studies further supported the assumption that the silylene is four-coordinate in solution.

Novel neutral hexacoordinate silicon(IV) complexes with two bidentate monoanionic benzamidinato ligands

The novel neutral hexacoordinate bis(benzamidinato)silicon(iv) complexes 1-10 (SiN(4)X(2) skeletons; X = F, Cl, Br, C, N, S, Se) were synthesised and characterised by elemental analyses, single-crystal X-ray diffraction (except for 2) and NMR spectroscopy in the solid state and in solution. The dynamic behavior of 1 (SiN(4)Cl(2) skeleton) and 3 (SiN(4)F(2)) was additionally studied by variable-temperature NMR experiments. Compounds 1 and 2 (SiN(4)Br(2)) were obtained by reaction of SiCl(4) and SiBr(4), respectively, with two molar equivalents of the corresponding lithium amidinate. Compound 1 served as the starting material in the syntheses of 3-10, in which the two chloro ligands of 1 were substituted by two (pseudo)halogeno or one bidentate dianionic S,S, S,Se or Se,Se ligand. Compound 4 contains an SiN(4)C(2) skeleton and 5-7 contain an SiN(6) skeleton. With the preparation of 8 (SiN(4)S(2) skeleton), 9 (SiN(4)SSe) and 10 (SiN(4)Se(2)) it could be demonstrated that syntheses of hexacoordinate silicon(iv) complexes with soft chalcogeno ligand atoms are indeed feasible. Compounds 9 and 10 are the first hexacoordinate silicon(iv) complexes with Si-Se bonds.

Silicon Analogues of the RXR-Selective Retinoid Agonist SR11237 (BMS649): Chemistry and Biology

C/Si switch: Twofold sila‐substitution (C/Si exchange) in the RXR‐selective retinoids 4u2009a (SR11237) and 5u2009a leads to 4u2009b (disila‐SR11237) and 5u2009b, respectively. Chemistry and biology of the C/Si pairs are reported.

Novel Transition‐Metal (M=Cr, Mo, W, Fe) Carbonyl Complexes with Bis(guanidinato)silicon(II) Ligands

The donor-stabilized silylene 2 (the first bis(guanidinato)silicon(II) complex) reacts with the transition-metal carbonyl complexes [M(CO)6 ] (M=Cr, Mo, W) to form the respective silylene complexes 7-10. In the reactions with [M(CO)6 ] (M=Cr, Mo, W), the bis(guanidinato)silicon(II) complex 2 behaves totally different compared with the analogous bis(amidinato)silicon(II) complex 1, which reacts with [M(CO)6 ] as a nucleophile to replace only one of the six carbonyl groups. In contrast, the reaction of 2 leads to the novel spirocyclic compounds 7-9 that contain a four-membered SiN2 C ring and a five-membered MSiN2 C ring with a Muf8ffSi and Muf8ffN bond (nucleophilic substitution of two carbonyl groups). Compounds 7-10 were characterized by elemental analyses (C, H, N), crystal structure analyses, and NMR spectroscopic studies in the solid state and in solution.

Neutral Six-Coordinate and Cationic Five-Coordinate Silicon(IV) Complexes with Two Bidentate Monoanionic N,S-Pyridine-2-thiolato(−) Ligands

A series of neutral six-coordinate silicon(IV) complexes (4-11) with two bidentate monoanionic N,S-pyridine-2-thiolato ligands and two monodentate ligands R(1) and R(2) was synthesized (4, R(1) = R(2) = Cl; 5, R(1) = Ph, R(2) = Cl; 6, R(1) = Ph, R(2) = F; 7, R(1) = Ph, R(2) = Br; 8, R(1) = Ph, R(2) = N3; 9, R(1) = Ph, R(2) = NCO; 10, R(1) = Ph, R(2) = NCS; 11, R(1) = Me, R(2) = Cl). In addition, the related ionic compound 12 was synthesized, which contains a cationic five-coordinate silicon(IV) complex with two bidentate monoanionic N,S-pyridine-2-thiolato ligands and one phenyl group (counterion: I(-)). Compounds 4-12 were characterized by elemental analyses, NMR spectroscopic studies in the solid state and in solution, and crystal structure analyses (except 7). These structural investigations were performed with a special emphasis on the sophisticated stereochemistry of these compounds. These experimental investigations were complemented by computational studies, including bonding analyses based on relativistic density functional theory.

Neutral Pentacoordinate Halogeno‐ and Pseudohalogenosilicon(IV) Complexes with an SiSONCX Skeleton (X=F, Cl, Br, I, N, C): Synthesis and Structural Characterization in the Solid State and in Solution

A series of neutral pentacoordinate silicon(IV) complexes with an SiSONCX skeleton (X=F, Cl, Br, I, N, or C) was synthesized and structurally characterized by multinuclear solution-state and solid-state NMR spectroscopy and single-crystal X-ray diffraction. These compounds contain an identical tridentate dianionic S,N,O ligand, a monodentate (pseudo)halogeno ligand (F, Cl, Br, I, NCS, N(3), or CN), and a monodentate organyl ligand (methyl, phenyl, 4-(trifluoromethyl)phenyl, or pentafluorophenyl). For most of these compounds, a dynamic equilibrium between the pentacoordinate silicon(IV) complex and two isomeric tetracoordinate silicon species in solution was observed. Most surprisingly, comparison of two series of analogous compounds containing fluoro, chloro, bromo, or iodo ligands demonstrated that pentacoordination in these series of silicon(IV) complexes is favored in the rank order I approximately Br>Cl>F; i.e., increasing the softness of the halogeno ligand favors pentacoordination.

Neutral Penta‐ and Hexacoordinate Silicon(IV) Complexes Containing Two Bidentate Ligands Derived from the α‐Amino Acids (S)‐Alanine, (S)‐Phenylalanine, and (S)‐tert‐Leucine

The neutral hexacoordinate silicon(IV) complex 6 (SiO(2)N(4) skeleton) and the neutral pentacoordinate silicon(IV) complexes 7-11 (SiO(2)N(2)C skeletons) were synthesized from Si(NCO)(4) and RSi(NCO)(3) (R = Me, Ph), respectively. The compounds were structurally characterized by solid-state NMR spectroscopy (6-11), solution NMR spectroscopy (6 and 10), and single-crystal X-ray diffraction (8 and 11 were studied as the solvates 8 x CH(3)CN and 11 x C(5)H(12) x 0.5 CH(3)CN, respectively). The silicon(IV) complexes 6 (octahedral Si-coordination polyhedron) and 7-11 (trigonal-bipyramidal Si-coordination polyhedra) each contain two bidentate ligands derived from an alpha-amino acid: (S)-alanine, (S)-phenylalanine, or (S)-tert-leucine. The deprotonated amino acids act as monoanionic (6) or as mono- and dianionic ligands (7-11). The experimental investigations were complemented by computational studies of the stereoisomers of 6 and 7.

Neutral Pentacoordinate Silicon(IV) Complexes with Silicon–Chalcogen (S, Se, Te) Bonds

The neutral pentacoordinate silicon(IV) complexes 1 (SiS2ONC skeleton), 2 (SiSeSONC), 3 (SiTeSONC), 6/9 (SiSe2O2C), 7 (SiSe2S2C), and 8/10 (SiSe4C) were synthesized and structurally characterized by using single-crystal X-ray diffraction and multinuclear solid-state and solution-state (except for 6-9) NMR spectroscopy. With the synthesis of compounds 1-3 and 6-10, it has been demonstrated that pentacoordinate silicon compounds with soft chalcogen ligand atoms (S, Se, Te) can be stable in the solid state and in solution.

Silicon analogues of the retinoid agonists TTNPB and 3-methyl-TTNPB, disila-TTNPB and disila-3-methyl-TTNPB: chemistry and biology.

Twofold sila‐substitution (C/Si exchange) in the saturated ring of the tetrahydronaphthalene skeleton of the retinoid agonists TTNPB (1u2009a) and 3‐methyl‐TTNPB (2u2009a) leads to disila‐TTNPB (1u2009b) and disila‐3‐methyl‐TTNPB (2u2009b), respectively. The silicon compounds 1u2009b and 2u2009b were synthesized in multiple steps, and their identities were established by elemental analyses, multinuclear NMR experiments, and single‐crystal X‐ray diffraction studies. Like TTNPB (1u2009a) and 3‐methyl‐TTNPB (2u2009a), the analogous silicon‐based arotinoids 1u2009b and 2u2009b are strong pan‐RAR agonists and display the same strong differentiation and apoptosis‐inducing activity in NB4 promyelocytic leukemia cells as the parent carbon compounds. These results are in keeping with the nearly isomorphous structures of 1u2009a and 1u2009b bound to the complex of the RARβ ligand‐binding domain with the nuclear receptor (NR) box 2 peptide of the SRC‐1 coactivator. The contacts within the ligand‐binding pocket are identical except for helix H11, for which two turns are shifted in the disila‐TTNPB (1u2009b) complex. This study represents the first comprehensive structure–function analysis of a carbon/silicon switch in a signaling molecule and demonstrates that silicon analogues can have the same biological functionalities and conserved structures as their parent carbon compounds, and it illustrates at the same time that silicon analogues of biologically active compounds have the potential to induce alternative allosteric effects, as in the case of helix H11, which might allow for novel options in drug design.

Darstellung und Eigenschaften von Diphenylalkylthio- und Diphenylarylthio-gallanen: Kristallstruktur von Diphenylethylthiogallan

Abstract Triphenylgallane reacts with alkyl- and aryl-thiols, respectively, with formation of the corresponding diphenylalkyl- and diphenylaryl-thiogallanes. Spectra and some physical and chemical properties of the new compounds are given. The results of the X-ray structure determination of diphenylethylthiogallane are discussed.