PI and PIII Ions Supported by BZIMPY Ligands

Abstract

We report a class of compounds in which both P^III -X and P^I forms featuring the same ligand are stable and readily cycled with each other. A series of P^III -X (X=Cl, Br, I) dicationic triflate salts supported by benzyl- and allyl-substituted 2,6-bis(benzimidazole-2-yl)pyridine (BZIMPY) ligands is synthesized. Surprisingly, treatment of these with R₃ PO (R=Et, Oct) results in reduction to BZIMPY-ligated P^I monocationic triflate salts while treatment with Ph₃ P reduces but also substitutes the compound to produce Ph₃ P-BZIMPY-ligated P^I dicationic triflate salts. The mechanisms of these surprising reductions are probed experimentally and rationalized computationally. The P^III -X dications are shown to be strong Lewis acids both experimentally and computationally and to readily behave as X⁺ , PX, and P⁺ transfer agents in reactions with phosphines, NHCs, and diazabutadienes. The P^I mono- and dications are shown to be very effective P⁺ transfer agents when treated similarly. Oxidation from a monocationic P^I salt back to the dicationic P^III -X (X=Cl, Br) salt was achieved by treatment with N-chlorosuccinimide (NCS) or N-bromosuccinimide (NBS). Full characterization is reported using multinuclear nuclear magnetic resonance spectroscopy, elemental analysis, and single-crystal X-ray diffractometry where suitable crystals were isolated.

Keywords: 2,6-bis(benzimidazole-2-yl)pyridine (BZIMPY); phosphorus compounds; redox cycling; transfer agents.