LEWIS BASICITY OF SILATRANES AND THE MOLECULAR-STRUCTURES OF ETOSI(OCH2CH2)3N, ME2O+SI(OCH2CH2)3N, AND CF3CO2HETOSI(OCH2CH2)3N

The silatranyl group is shown to be sufficiently electron releasing in alkoxysilatranes (ROSi(OCH2CH2)3N) to allow isolation of the hydrogen-bonded adduct CF3C(O)OHEtOSi(OCH2CH2)3N (7), the protonated cation HEtO+Si(OCH2CH2)3N (13(BF4)) and the alkylated cations R2O+Si(OCH2CH2)3N (R = Me, 6(BF4); R = Et, 12(BF4)) of which 7 and 6(BF4) and also the alkoxysilatrane EtOSi(OCH2CH2)3N (5) have been structured by X-ray means. The SiN(ax) bond length in 6(BF4) (1.965 (5) angstrom) is the shortest reported for a silatrane, and the O(H)O distance in 7 (2.489 angstrom) is the shortest recorded for an unsymmetrical hydrogen bond. The SiO(ax) distance increases by a total of 0.17 angstrom in the order 5 < 7 < 6 (BF4). The greatest downfield shifts of a H-1 or C-13 resonance in a variety of alkoxysilatranes in hydrogen bonding, protonating, or alkylating environments are observed to occur at the O(ax)R group. That electrophilic attack can also occur at O(eq) in solutions of these compounds is suggested by broadening of the OCH2CH2N protons of PhSi(OCH2CH2)3N in the presence of Me3OBF4. Measurement of phenol v(OH) shifts reveals the basicity order (Me3Si)20 < Si(OR)4 < alkoxysilatranes less-than-or-equal-to Me3SiOMe < Et2O, which places the electron-releasing ability of the silatranyl group ahead of (RO)3Si but below an Et group. Parameters for the X-ray crystallography determined structures are as follows: 5, monoclinic (P2(1)/n), Z = 8, a = 10.956 (4) A, b = 11.187 (2) angstrom, c = 17.638 (8) angstrom, beta = 95.84 (4) degrees; 6(BF4), monoclinic (P2(1)c), Z = 4, a = 8.976 (2) angstrom, b = 11.517 (1) angstrom, c = 12.387 (2) angstrom, beta = 91.943 (8) degrees; 7, orthorhombic (Pbca), Z = 8, a = 12.655 (4) angstrom, b = 11.446 (4) angstrom, c = 19.489 (3) angstrom.