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

Complexation reactions of Fe-II and Fe-III salts with the tridentate ligands 2-(diphenylphosphanyl)benzaldehyde benzoylhydrazone [(H)PNO] and N-[2-(diphenylphosphanyl)benzylidenel-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 Fe-II/Fe-III paramagnetic complex [Fe(kappa(3)-(H)PNO}2]Cl[FeCl4], whereas the reaction between FeC12 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{kappa(3)- (H)PNO}(2)][OTf](2). A diamagnetic bis-chelate ferrous complex [Fe(kappa(3)- (PNO)211 was instead obtained from the reaction of FeC12 with a twofold excess of (H)PNO in the presence of NaOMe. In the case of the PNN ligand, the 1:1 reaction with FeC12 gave the neutral paramagnetic trigonal-bipyramidal complex [Fe(kappa(3)-PNN)Cl-2]. The X-ray structures of [Fe{kappa(3)-(H)PNO}(2)]CI[FeCl4], [Fe-2{kappa(3)-(H)PNO}(2)Cl-6] and [Fe(kappa(3)-PNN)Cl-2] 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