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)]²⁺ are also reported and compared to [Co­(TPA)­(CH₃CN)]²⁺. Notably, magnetic susceptibility measurements of [Co­(TPAP)­(CH₃CN)]²⁺ indicate a low spin electronic configuration, in contrast to [Co­(TPA)­(CH₃CN)]²⁺, which is high spin.