PENTACOORDINATED MOLECULES .109. SYNTHESIS AND MOLECULAR-STRUCTURES OF PYRIDINE-CONTAINING LARGE-MEMBERED CYCLIC BIS(ALKOXY)SILANES

The new cyclic silanes [(C5H3N)(CH2O)(2)SiMe(2)](2) (1) and (C5H3N)(CH (2)CPh(2)O)(2)SiMe(2) (2) containing 16-membered and 10-membered rings , respectively, were prepared by the condensation reaction of Me(2)SiC l(2) with an appropriate pyridine diol in the presence of Et(3)N. X-ra y studies show that the dimeric formulation for 1 represents a tetraco ordinate cyclic silane, whereas 2 has a geometry halfway from a tetrah edron toward a trigonal bipyramid (TBP) as a result of Si-N-ax donor a ction. Si-29 and H-1 NMR indicate retention of the coordination geomet ry for 2 in solution that undergoes rapid Si-N cleavage and ring rearr angement. In comparison with other silanes containing five- and six-me mbered rings that exhibit nitrogen or oxygen coordination, the presenc e of larger rings, as in 2 and related silanes having sulfur coordinat ion, indicates that retention of donor action persists, thus largely r uling out ring size as a dominant factor controlling the possibility o f donor action at silicon. The dimeric silane 1 crystallizes in the tr iclinic space group P (1) over bar with a = 6.347(3) Angstrom, b = 12. 455(4) Angstrom, c = 14.289(5) Angstrom, alpha = 101.63(3)degrees, bet a = 102.99(3)degrees, gamma = 104.71(3)degrees, and Z = 2. The cyclic silane 2 crystallizes in the triclinic space group P (1) over bar with a = 9.733(4) Angstrom, b = 10.938(2) Angstrom, 14.312(3) Angstrom, al pha 89.03(2)degrees, beta = 74.59(3)degrees, gamma = 79.24(3)degrees, and Z = 2. The final conventional unweighted residuals are 0.040 (1) a nd 0.039 (2).