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

A number of two-deck dicationic 3a,6a-diaza-1,4-diphosphapentalenes (DDP)(2)X-2 (X= halogen or complex ion) have been characterized. Interdeck distances P(1)center dot center dot center dot P(3) and P(2)center dot center dot center dot 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 P-31 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 angstrom) is energetically unfavorable, despite the absence of steric barriers. The total energy of the model dication as a function of the P center dot center dot center dot P separation has a minimum at the P-P distance of 2.85 angstrom. 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.