Ligand driven geometry switch in octahedral and trigonal bipyramidal iron complexes containing (H)PNO and PNN ligands

Complexation reactions of FeII and FeIII salts with the tridentate ligands 2-(diphenylphosphanyl)benzaldehyde benzoylhydrazone [(H)PNO] and N-[2-(diphenylphosphanyl)benzylidene]-2-(2-pyridyl)ethanamine (PNN) was carried out. For (H)PNO, reaction of a stoichiometric amount of anhydrous FeCl2 in the presence of a primary alcohol such as MeOH, EtOH or nBuOH gave the mixed-valence FeII/FeIII paramagnetic complex [Fe{?3-(H)PNO}2]Cl[FeCl4], whereas the reaction between FeCl2 and a twofold excess of (H)PNO in the presence of AgOTf (OTf = triflate, OSO2CF3) led to the isolation of the bis-chelate paramagnetic ferrous complex [Fe{?3-(H)PNO}2][OTf]2. A diamagnetic bis-chelate ferrous complex [Fe{?3-(PNO)2}] was instead obtained from the reaction of FeCl2 with a twofold excess of (H)PNO in the presence of NaOMe. In the case of the PNN ligand, the 1:1 reaction with FeCl2 gave the neutral paramagnetic trigonal-bipyramidal complex [Fe(?3-PNN)Cl2]. The X-ray structures of [Fe{?3-(H)-PNO}2]Cl[FeCl4], [Fe2{?3-(H)PNO}2Cl6] and [Fe(?3-PNN)Cl2] are reported, together with CV and magnetic susceptibility measurements and an ab initio study aimed at rationalizing the different stoichiometries and structures observed with the two tridentate ligands.