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)benzylidene]‐2‐(2‐pyridyl)ethanamine (PNN) was carried out. For (H)PNO, reaction of a stoichiometric amount of anhydrous FeCl 2 in the presence of a primary alcohol such as MeOH, EtOH or n BuOH gave the mixed‐valence Fe II /Fe III paramagnetic complex [Fe{κ 3 ‐(H)PNO} 2 ]Cl[FeCl 4 ], whereas the reaction between FeCl 2 and a twofold excess of (H)PNO in the presence of AgOTf (OTf = triflate, OSO 2 CF 3 ) led to the isolation of the bis‐chelate paramagnetic ferrous complex [Fe{κ 3 ‐(H)PNO} 2 ][OTf] 2 . A diamagnetic bis‐chelate ferrous complex [Fe{κ 3 ‐(PNO) 2 }] was instead obtained from the reaction of FeCl 2 with a twofold excess of (H)PNO in the presence of NaOMe. In the case of the PNN ligand, the 1:1 reaction with FeCl 2 gave the neutral paramagnetic trigonal‐bipyramidal complex [Fe(κ 3 ‐PNN)Cl 2 ]. The X‐ray structures of [Fe{κ 3 ‐(H)PNO} 2 ]Cl[FeCl 4 ], [Fe 2 {κ 3 ‐(H)PNO} 2 Cl 6 ] and [Fe(κ 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)