Influence of Charge and Coordination Number on Bond Dissociation Energies, Distances, and Vibrational Frequencies for the Phosphorus-Phosphorus Bond

摘要

We report a comprehensive and systematic experimental and computational assessment of the P-P bond in prototypical molecules that represent a rare series of known compounds. The data presented complement the existing solid-state structural data and previous computational studies to provide a thorough thermodynamic and electronic understanding of the P-P bond. Comparison of homolytic and heterolytic bond dissociation for tricoordinate-tricoordinate, tricoordinate-tetracoordinate, and tetracoordinate- tetracoordinate P-P bonds in frameworks 1-6 provides fundamental insights into covalent bonding. For all types of P-P bond discussed, homolytic dissociation is favored over heterolytic dissociation, although the distinction is small for 2~(1+) and 6~(1+). The presence of a single cationic charge in a molecule substantially strengthens the P-P bond (relative to analogous neutral frameworks) such that it is comparable with the C-C bond in alkanes. Nevertheless, P-P distances are remarkably independent of molecular charge or coordination number, and trends in values of d(PC) and ν_(symm)(PC) imply that a molecular cationic charge is distributed over the alkyl substituents. In the gas phase, the diphosphonium dication 3~(2+) has similar energy to two [PMe_3]~+ radical cations, so that it is the lattice enthalpy of 3[OTf]_2 in the solid-state that enables isolation, highlighting that values from gas-phase calculations are poor guides for synthetic planning for ionic compounds. There are no relationships or correlations between bond lengths, strengths, and vibrational frequencies.