HEPTA-TERT-BUTYLCYCLOTETRASILANE - A HIGHLY CROWDED CYCLOTETRASILANE

The reaction of di-tert-butyldichlorosilane with lithium produced hepta-tert-butylcyclotetrasilane (1) and trans-1,1,2,3,3,4-hexa-tert-butylcyclotetrasilane (2). The structures of 1 and 2 were determined by X-ray crystallography. Crystal data for 1: orthorhombic, Cmcm, a = 12.071(1) Angstrom, b = 17.964(2) Angstrom, c = 15.431(4) Angstrom, V = 3346.0(8) Angstrom(3), Z = 4, R = 0.067, R(W) = 0.051 for 1160 reflections. Crystal data for 2: tetragonal, P4(2)/m, a = 9.159(1) Angstrom, c = 18.292(1) Angstrom, V = 1534.6(2) Angstrom(3), Z = 2, R = 0.047, R(W) = 0.056 for 768 reflections. Compound 1 is a highly crowded molecule which contains unusually long Si-Si bonds (2.542(2) Angstrom) and a folded cyclotetrasilane ring with relatively small dihedral angles of 15.2 degrees and 16.1 degrees. In the UV spectrum of 1, absorption extends to the longer wavelength region than 2 and other alkyl-substituted cyclotetrasilanes so far reported. The oxidation potential of 1 (1.02 V vs. SCE) is significantly low compared with those of 2 and other less crowded cyclotetrasilanes.