Steric and electronic evaluations of P(o-tol)R2, where R is phenyl or cyclohexyl: crystal structures of SeP(o-tol)R2.

Abstract

The crystal structures of SeP(o-tol)R2, where o-tol is ortho-tolyl (2-methylphenyl) and R is Ph (phenyl), namely (2-methylphenyl)diphenylphosphane selenide, C19H17PSe, or Cy (cyclohexyl), namely dicyclohexyl(2-methylphenyl)phosphane selenide, C19H29PSe, were determined to aid in the evaluation of the steric and electronic behaviour of these analogous phosphane compounds. The compounds crystallized in similar monoclinic crystal systems, but are differentiated in their unit cells by a doubling of the number of independent molecules for R= Cy (Z'= 2) and the choice of glide plane by convention. The preferred orientation for the o-tolyl substituent obtained from the X-ray structural analysis is gauche for R= Ph and anti for R= Cy (using the Se-P-Cipso-Cortho torsion angles as reference). Density functional theory (DFT) calculations showed both conformations to be equally probable and indicate that the preferred solid-state conformer is probably due to the minimization of repulsion energies, resulting in a packing arrangement primarily featuring weak C-H...Se interactions and additional C-H...π interactions in the R= Ph structure. A detailed electronic and steric analysis was conducted on both phosphanes using Se-P bond lengths, multinuclear NMR 1JSe-P coupling constants, theoretical topological evaluation and crystallographic and solid-angle calculations, and compared to selected literature examples. The results indicate that the use of the o-tolyl substituent increases both the electron-donating capability and the steric size, but is also dependent on whether the o-tolyl group adopts a gauche or anti conformation. The single-crystal geometrical data are unable to detect electronic differences between these two structures due to the somewhat large displacement parameters observed for the Se atom in the R= Cy structure.