Co-ordinative interactions in chelated complexes of silicon. Part 5. Chirality of five-co-ordinate silicon compounds: crystal and molecular structures of 1-(chlorodimethylsilyl)- and 1-(dichloromethylsilyl)-1,2,3,4-tetrahydro-1,10-phenanthroline

Abstract

The crystal and molecular structures of 1-(chlorodimethylsilyl)-1,2,3,4-tetrahydro-1,10-phenanthroline, (C₁₂H₁₁N₂) SiMe₂Cl (1), and 1-(dichloromethylsilyl)-1,2,3,4-tetrahydro-1,10-phenanthroline, (C₁₂H₁₁N₂)SiMeCl₂(2), have been determined from diffractometer data. Both compounds crystallize isostructurally with orthorhombic symmetry (space group Pnam). For (1) : a= 1 857.3(6), b= 1 028.6(7), c= 723.3(5) pm, Z= 4; 7 258 observed reflections, R′= 0.053. For (2): a 1 853.4(4), b= 1.024.2(4), c= 720.3(1) pm, Z= 4; 3 658 observed reflections, R′= 0.033. The central Si has a trigonal-bipyramidal five-co-ordination. The molecular arrangement of ligands in the first co-ordination sphere of Si is achiral in (1) whereas Si occupies a centre of chirality in (2) and both enantiomeric forms are found in the same crystal. The co-ordinative Si ⋯ N bonds [202.8(7) pm in (1) and 202.7(4) pm in (2)] are extended 14.6%[for (1)] and 16.6%[for (2)] in comparison with the other intramolecular Si–N single bond [177.0(6) pm in (1) and 173.9(3) pm in (2)]. The topological arrangement around silicon is discussed with respect to stereochemistry of intermediates along the S_N2 pathway.