Evidence for π 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-Me₃C₆H₂ and 2,4,6-Prⁱ₃C₆H₂ has allowed the structural characterization of two monomeric boron-sulfur compounds (2,4,6-Me₃C₆H₂)₂B(SPh) 1 and (2,4,6-Prⁱ₃C₆H₂)B(SPh)₂ 2. The crystal structures of 1 and 2 show a close alignment between the boron and sulfur p orbitals that is consistent with a π interaction. Furthermore, the B-S distances in 1 [1.790(6)] and 2 [1.801(6) Å] are slightly shorter than the sum of the covalent radii (with allowance made for ionic effects) of boron and sulfur. Variable-temperature ¹H and ¹³C NMR data for 2 indicate an average barrier to rotation around the B-S bond of ca. 12 kcal mol^-1. This value is significantly 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 π bond over the three atom BS₂ π system which has a similar electronic arrangement to that of an allyl anion. Crystal data at 130 K: 1, triclinic, space group P1, a = 7.851(5), b = 11.685(6), c = 13.096(7) Å, α = 63.52(2), β = 73.93(2), γ = 74.54(2)°, Z = 2, R = 0.073; 2, monoclinic, space group P2₁/c, a = 18.373(3), b = 12.713(2), c = 10.844(2) Å, β = 99.96(1)°, Z = 4, R = 0.072.