A New Class of Two‐Deck Dicationic Heteropentalenes Existing by the Balance between Interdeck Aromaticity and Covalent P−P Bonding

Abstract

A number of two‐deck dicationic 3a,6a‐diaza‐1,4‐diphosphapentalenes (DDP)2X2 (X=halogen or complex ion) have been characterized. Interdeck distances P(1)⋅⋅⋅P(3) and P(2)⋅⋅⋅P(4) in crystal structures, as a rule, significantly exceed the sum of covalent radii, however they are still within the sum of van der Waals radii. The 31P NMR data indicate that phosphorus atoms are equivalent in solution even at low temperatures (233 K). Based on DFT calculations, the formation of two equivalent P−P covalent bonds (<2.4 Å) is energetically unfavorable, despite the absence of steric barriers. The total energy of the model dication as a function of the P⋅⋅⋅P separation has a minimum at the P−P distance of 2.85 Å. This distance corresponds to the highest aromaticity index in the interdeck space (NICS(1.0)=−20.72). The energy of interdeck interactions is estimated at 10–15 kcal/mol.