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.