The molecular and crystal structures of the tris(dimethylamino)phosphoranes (Me2N)3P–X (X = BH3, CH2, NH or O)

Abstract

Single crystal X-ray diffraction studies and ab initio calculations up to the MP2/6-31G* level of theory were carried out for the isoelectronic compounds (Me₂N)₃PBH₃, (Me₂N)₃PNH and (Me₂N)₃PO. The calculations predict all three compounds to adopt C_s symmetry in the ground state. The alternative geometries with local or total C₃ symmetry also correspond to local minima on the potential hypersurface. In the solid state (Me₂N)₃PBH₃ and (Me₂N)₃PNH do not obey exact C_s symmetry, but are close to it, while (Me₂N)₃PO shows crystallographic mirror symmetry. The structural data are compared with earlier studies on (Me₂N)₃PCH₂ and the free phosphine P(NMe₂)₃, which both are also almost C_s symmetric. In the series (Me₂N)₃P–X (X = BH₃, CH₂, NH or O) the P–X bond lengths decrease, the P–N (pseudo) in plane distances are 1.683(1), 1.698(4), 1.677(1) and 1.659(1) Å, the (pseudo) out of plane P–N distances are (av. = average) 1.654(av.), 1.668(av.), 1.656(av.) and 1.649(1) Å. The (pseudo) in plane angles X–P–N are 114.9(1), 122.4(4), 120.2(1) and 118.7(1)°, while the (pseudo) out of plane angles X–P–N are 112.3(av.), 110.0(av.), 110.4(av.) and 110.0(av.)°. In all structures the NMe₂ groups in the (pseudo) plane of symmetry are steeply pyramidal at nitrogen, while the (pseudo) out of plane NMe₂ groups are markedly flattened. The other structural parameters are also discussed in comparison. Ab initio calculations (MP2/6-31G*) on the isoelectronic (Me₂N)₃PF⁺ predict this system to favour C₃ over C_s symmetry. In contrast, the parent system (H₂N)₃PF⁺ is predicted to prefer C_s symmetry.