STRUCTURES OF THE TETRAFLUOROBORATE SALTS OF TRIPHENYLCARBENIUM IONS WITH ONE AND WITH 2 4-METHOXY SUBSTITUENTS - STABLE CARBOCATIONS

p-Methoxyphenyldiphenylcarbenium tetrafluoroborate (1): [C20H17O][BF4], M(r) = 360.2, monoclinic, P2(1)/n, a = 7.962 (1), b = 15.463 (2), c = 14.567 (1) angstrom, beta = 95.54 (1)-degrees, V = 1785.1 angstrom3, Z = 4, D(x) = 1.340 g cm-3, lambda(Mo K-alpha) = 0.71073 angstrom, mu = 1.0 cm-1, F(000) = 744, T = 295 K, R = 0.063 for 2356 unique observed reflections. Di(p-methoxyphenyl)phenylcarbenium tetrafluoroborate (2): [C21H19O2][BF4], M(r) = 390.2, monoclinic, P2(1)/n, a = 7.942 (2), b = 16.773 (4), c = 14.447 (2) angstrom, beta = 90.54 (2)-degrees, V = 1924.4 angstrom3, Z = 4, D(x) = 1.347 g cm-3, lambda(Cu K-alpha) = 1.54184 angstrom, mu = 9.2 cm-1, F(000) = 808, T = 295 K, R = 0.114 for 1259 unique observed reflections. Both cations have a propeller structure, with the aryl rings twisted out of the plane of the carbenium-ion centre. The angle of twist is smaller for the 4-methoxyphenyl groups [26.6 (2), 27.8 (5) and 29.6 (5)-degrees] than for the phenyl groups [31.3 (2), 38.6 (2) and 37.3 (5)-degrees] as a consequence of greater conjugation.