EVIDENCE FOR PI-BONDING IN THE BORON THIOLATE COMPOUNDS (2,4,6-ME3C6H2)2B(SPH) AND (2,4,6-PRI3C6H2)B(SPH)2

The use of the bulky aryl substituent groups 2,4,6-Me3CH2 and 2,4,6-Pr i3C6H2 has allowed the structural characterization of two monomeric bo ron-sulfur compounds (2,4,6-Me3C6H2)2B(SPh) 1 and (2,4,6-Pr13C6H2)B(SP h)2 2. The crystal structures of 1 and 2 show a close alignment betwee n the boron and sulfur p orbitals that is consistent with a pi interac tion. Furthermore, the B-S distances in 1 [1.790(6)] and 2 [1.801(6) a ngstrom] are slightly shorter than the sum of the covalent radii (with allowance made for ionic effects) of boron and sulfur. Variable-tempe rature H-1 and C-13 NMR data for 2 indicate an average barrier to rota tion around the B-S bond of ca. 1 2 kcal mol-1. This value is signific antly less than the 18.4 kcal mol-1 reported earlier for 1. The lower rotational barrier observed in 2 is consistent with the delocalization of the B-S pi bond over the three atom BS2 pi system which has a simi lar electronic arrangement to that of an allyl anion. Crystal data at 1 30 K: 1, triclinic, space group P1BAR, a = 7.851 (5), b = 11.685(6), c = 13.096(7) angstrom, alpha = 63.52(2), beta = 73.93(2), gamma = 74 .54(2)degrees, Z = 2, R = 0.073; 2, monoclinic, space group P2(1)/c, a = 18.373(3), b = 12.713(2), c = 10.844(2) angstrom, beta = 99.96(1)de grees, Z = 4, R = 0.072.