Synthesis of Nitridorhenium(V) Complexes with PNP Ligands and the Thermodynamics of Adduct Formation with Lewis Acids

Nitridorhenium(V) PNP methyl and phenyl complexes were synthesized from their respective nitridorhenium(V) PNP dichloro precursors. Classical Lewis acid-base adducts of these complexes were isolated with the perfluorinated Lewis acids B(C₆F₅)₃ and Al(C₆F₅)₃, and diphenylzinc. In contrast to previously reported adducts with oxorhenium complexes, coordination of the Lewis acid to the terminal nitrogen atom does not result in a significant lengthening of the Re≡N bond, and multiple bond character is maintained. Further, an examination of the thermodynamics of adduct formation reveals that in contrast to oxorhenium complexes where adduct formation is irreversible, adduct formation with nitridorhenium complexes is reversible with K_eq values from 0.3 to 8.8. An examination of the influence of ligands in the basal plane of the square pyramidal nitridorhenium complexes as well as the influence of substituents on the phosphorus atoms on the PNP ligands was made by using steric maps of the complexes. These studies reveal that bulky ligands in the basal plane or on the phosphorus atoms in the PNP ligand have a significant effect on adduct formation with Lewis acids. However, when steric effects are similar, the electronic impact of the ligands are most influential in altering the basicity of the terminal nitrogen atom.