Reversible Addition of CO to Coordinatively Unsaturated High-Spin Iron(II) Complexes

Several new coordinatively unsaturated iron­(II) complexes of the types [Fe­(EN-iPr)­X2] (E = P, S, Se; X = Cl, Br) and [Fe­(ON-iPr)2X]­X containing bidentate EN ligands based on N-(2-pyridinyl)­aminophosphines as well as oxo, thio, and seleno derivatives thereof were prepared and characterized by NMR spectroscopy and X-ray crystallography. Mössbauer spectroscopy and magnetization studies confirmed their high-spin nature with magnetic moments very close to 4.9 μB, reflecting the expected four unpaired d-electrons in all these compounds. Stable low-spin carbonyl complexes of the types [Fe­(PN-iPr)2(CO)­X]­X (X = Cl, Br) and cis-CO,cis-Br-[Fe­(PN-iPr)­(CO)2X2] (X = Br) were obtained by reacting cis-Fe­(CO)4X2 with the stronger PN donor ligands, but not with the weaker EN donor ligands (E = O, S, Se). Furthermore, the reactivity of [Fe­(PN-iPr)­X2] toward CO was investigated by IR spectroscopy. Whereas at room temperature no reaction took place, at −50 °C [Fe­(PN-iPr)­X2] added readily CO to form, depending on the nature of X, the mono- and dicarbonyl complexes [Fe­(PN-iPr)­(X)2(CO)] (X = Cl) and [Fe­(PN-iPr)­(CO)2X2] (X = Cl, Br), respectively. In the case of X = Br, two isomeric dicarbonyl complexes, namely, cis-CO,trans-Br-[Fe­(PN-iPr)­(CO)2Br2] (major species) and cis-CO,cis-Br-[Fe­(PN-iPr)­(CO)2Br2] (minor species), are formed. The addition of CO to [Fe­(PN-iPr)­X2] was investigated in detail by means of DFT/B3LYP calculations. This study strongly supports the experimental findings that at low temperature two isomeric low-spin dicarbonyl complexes are formed. For kinetic reasons cis,trans-[Fe­(PN-iPr)­(CO)2Br2] releases CO at elevated temperature, re-forming [Fe­(PN-iPr)­Br2], while the corresponding cis,cis isomer is stable under these conditions.