Peter Pickel

Halogen exchange and expulsion: ligand stabilized dihalogen silicon dications

The first ligand stabilized SiCl22+ dications were synthesized using N-methylimidazole as co-ordinating ligand. The compounds SiCl4, SiBr2Cl2, and SiH2Cl2 form six-co-ordinated dicationic compounds of almost octahedral symmetry with similar structures which were investigated by single crystal X-ray analysis and density functional calculations. The structures exhibit particularly short dative Si–N bonds of about 1.90 A. Complexes crystallized from the same solvent are isostructural. A different solvent, though, leads to geometrical variations. It was also discovered that the halogen exchange process among mixed silicon tetrahalides occurs under much milder conditions than previously thought and proceeds with considerable speed even without a catalyst.

Neue dikationische Silicium-Komplexe mit N-Methylimidazol/New Dicationic Silicon Complexes with N-Methylimidazole

The reactions of the dibromosilanes Et2SiBr2, Ph2SiBr2, EtHSiBr2, PhHSiBr2 and MeHSiBr2 with N-methylimidazole (NMI) led to white powders stable at room temperature (with Et2SiBr2 an 1:2 adduct, with EtHSiBr2 and Ph2SiBr2 1:3 adducts, and with PhHSiBr2 and MeHSiBr2 1:4 adducts). The compounds crystallize from solutions in chloroforme and acetonitrile. The X-ray single crystal investigations show in all five cases dicationic complexes.

Two Cyclotetrasiloxanes at 143K

The crystal structures of octaphenylcyclotetrasiloxane [C 48 H 40 O 4 Si 4 , (I)] and 2,4,6,8-tetrakis(tetramethylene)cyclotetrasiloxane [a tetraspiro compound: cyclotetrasiloxane-2,4,6,8-tetraspiro-tetrakis (cyclopentane), C 16 H 32 O 4 Si 4 , (II)] were determined at 143 K. Compound (I) is compared with the room-temperature structure and the structure of the monoclinic polymorph, both reported previously. Whereas the siloxane ring of (I) is nearly planar, the siloxane ring of (II) exhibits a chair conformation. The angles between the siloxane ring and the various phenyl substituents of (I) vary considerably and are on average smaller in the triclinic form than in the monoclinic form. Greatly differing Si-O-Si angles, as found in the monoclinic form (153 and 167°), appear only in one of the molecules in the asymmetric unit of (I) [153.4(1) and 163.7(1)°]; the Si-O-Si angles of the second molecule differ by only 3.0°. The Si-O-Si angles of (II) display a difference of 5.2°. The silacyclopentane rings of (II) exhibit different conformations.

4-Methylpyridinium Pentachloro(4-methylpyridine-N)titanate(IV) Acetonitrile Solvate at 143K

The structure of the title compound, (C 6 H 8 N)[TiCl 5 -(C 6 H 7 N)].C 2 H 3 N, consists of discrete complexes in which the Ti atom has a distorted pseudo-octahedral coordination with a 4-methylpyridinium cation as counterion. The 4-methylpyridine ligand and one chloro ligand occupy axial positions with the other four chloro ligands in the equatorial plane. The acetonitrile solvent molecule occupies a cavity in the structure.