STRUCTURAL STUDIES ON COMPOUNDS WITH SILICON OF HIGHER COORDINATION - MODELS FOR STUDY OF NUCLEOPHILIC-SUBSTITUTION ON SILICON CENTERS

Silicon compounds substituted by 2-(dimethylaminomethyl)phenyl groups [C6H4CH2N(CH3)2]nSiX4-n (X = Cl, H, organo group, n = 1-4) are available by the reaction of the appropriate chlorosilanes with 2-(dimethylaminomethyl)phenyllithium. A comparison of the Si-29 NMR spectroscopic shifts delta with the values obtained for corresponding phenyl silanes shows that there is higher coordination of the silicon atom by N --> Si contacts, except for the silanes 11 (X = Cl; n = 3) and 12 (n = 4). X-ray crystal structure analysis confirms pentacoordination of the silicon atom in the dichlorosilanes 4 (X = Cl; n = 2) and 5. For [C6H4CH2N(CH3)2]3SiH (10) heptacoordination of the silicon atom is found in the crystalline state [d(N - Si) = 301 pm]. The tetrasubstituted derivative [C6H4CH2N(CH3)2]4Si (12) has no N --> Si contacts in solution and in the solid state: steric congestion does not allow higher coordination. 10: colourless prisms, monoclinic, space group P2(1)/n; lattice constants: a = 889.0(3), b = 1677.5(8), c = 3471(1) pm; beta = 90.35(3)-degrees. 12: colourless prisms, pseudo tetragonal, space group I4(1)/a; lattice constants: a = b = 1699.7(4), c = 3398.9(17) pm.