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.

Automated summary

A type of two-deck dicationic 3a,6a-diaza-1,4-diphosphapentalenes (DDP)(2)X-2 (X= halogen or complex ion) has been characterized. The interdeck distances P(1)center dot center dot center dot P(3) and P(2)center dot center dot center dot P(4) in the crystal structures exceed the sum of covalent radii but still fall within the sum of van der Waals radii. The P-31 NMR data show that phosphorus atoms are equivalent in solution, even at low temperatures (233 K). DFT calculations reveal that the formation of two equivalent P-P covalent bonds is energetically unfavorable, despite the absence of steric barriers.