Flexibility is Key: Synthesis of a Tripyridylamine (TPA) Congener with a Phosphorus Apical Donor and Coordination to Cobalt(II).

Tripyridylamine (TPA), a tetradentate ligand that forms 5-membered chelate rings upon metal coordination, has demonstrated significant utility in synthetic inorganic chemistry. An analogue with a phosphorus apical donor is a desirable target for tuning electronic structure and enhancing reactivity. However, this congener has been synthetically elusive. Prior attempts have resulted in tridentate coordination to transition metal ions due to a lack of ligand flexibility. Herein, we report the successful synthesis of tris(2-pyridylmethyl)proazaphosphatrane (TPAP), a more accommodating tripyridyl ligand containing an apical phosphorus donor. The TPAP ligand forms 6-membered chelate rings upon coordination and binds in the desired tetradentate fashion to a Co(II) ion. Structural studies elucidate the importance of ligand flexibility in tripodal ligands featuring phosphorus donors. Cyclic voltammetry, UV-vis, and solution magnetic susceptibility experiments of Co(TPAP)(CH3CN) are also reported and compared to Co(TPA)(CH3CN). Notably, magnetic susceptibility measurements of Co(TPAP)(CH3CN) indicate a low spin electronic configuration, in contrast to Co(TPA)(CH3CN), which is high spin.