Two tris-(3,5-disubstituted phen-yl)phosphines and their isostructural P^V oxides

The crystal structures of tris-(3,5-di-methyl-phen-yl)phosphine (C₂₄H₂₇P), (I), tris-(3,5-di-methyl-phen-yl)phosphine oxide (C₂₄H₂₇OP), (II), tris-(4-meth-oxy-3,5-di-methyl-phen-yl)phosphine (C₂₇H₃₃O₃P), (III), and tris-(4-meth-oxy-3,5-di-methyl-phen-yl)phosphine oxide (C₂₇H₃₃O₄P), (IV), are reported. The strucure of (III) has been described before [Romain et al. (2000 ▸). Organometallics, 19, 2047-2050], but it is rereported here on the basis of modern area-detector data and to facilitate comparison with the other structures reported here. Compounds (I) and (II) crystallize isostructurally in P2₁/c. Similarly, (III) and (IV) crystallize isostructurally in Pbca. The conformations of (I) and (II) in the solid state deviate strongly from helical, whereas those of (III) and (IV) are found to be closer to an ideal threefold rotational symmetry. The pyramidality indices, ∑(C-P-C), are 305.35 (16), 317.23 (15), 307.2 (4) and 318.67 (18)° for (I), (II), (III) and (IV), respectively. Each is found to be more pyramidal than Ph₃P or Ph₃PO. Hybrid DFT calculations incorporating terms for dispersion provide evidence that the causes of the increased pyramidality, despite the 3,5-dimethyl group substitution, include dispersion inter-actions. The calculated ∑(C-P-C) values are 304.8° for both (I) and (III) and 317.4° for both (II) and (IV), with no difference arising from the substitution at ring position 4.