Unexpected bonding ambiguity of the open/closed-shell R2PO ligand family: the case of phosphinoyl vs. phosphoryl if bound to transition metals in their low oxidation state.

DFT pre-exploration of a broad set of heterocyclic compounds, focusing on 1,3,2-dihetero-phospholanes/phospholidines and open-shell R2P-O iron complexes, reveals a square pyramidal coordination geometry of the resulting iron(+I) centre, in stark contrast to the trigonal bipyramidal structure of the precursor iron(0) complexes. The latter clearly indicates an intramolecular redox process, initiated by a homolytic N-O bond cleavage transforming a phosphanoxyl ligand into a phosphoryl ligand, but not into a stable phosphinoyl ligand. The reaction cascade and the influence of co-ligands on the spin population distribution were studied in detail. Experimentally, the consequence(s) of the homolytic N-O bond cleavage of the first isolated aminoxyl-phosphane iron complex, bearing the 1,3-dimethyl-1,3,2-diazaphospholidine ligand scaffold, was investigated as a case in point using NMR, EPR and Mössbauer spectroscopy. Co-ligand effects were also addressed by DFT analysis.