Crystal Structure and Electron Configuration of {MNO}⁸ Heme Complexes

Area of Science:

• Inorganic Chemistry
• Coordination Chemistry
• Materials Science

Background:

• Decades of research exist on nitrosyl (NO) heme complexes and their reduced form, nitroxyl (NO-) derivatives.
• Disagreements persist regarding the electronic configuration of nitroxyl complexes, with most evidence from theoretical calculations.

Purpose of the Study:

• To present new polymorphic forms of nitrosyl cobalt(II) and iron complexes with a halogenated porphyrin ligand.
• To experimentally investigate the electronic configuration of these {MNO}8 (M = Fe, Co) complexes.
• To resolve ambiguities in the electronic structure of nitroxyl complexes.

Main Methods:

• X-ray crystallography to determine crystal structures of new polymorphic forms.
• X-ray Absorption Near Edge Structure (XANES) spectroscopy.
• Electron Paramagnetic Resonance (EPR) spectroscopy.
• Temperature-dependent Mössbauer spectroscopy for {FeNO}8 complex.

Main Results:

• Two new polymorphic forms of [CoCp2][Fe(TFPPBr8)(NO)] and two of nitrosyl cobalt(II) complexes were synthesized and characterized.
• EXANES, EPR, and Mössbauer studies provided insights into the electronic structures of {FeNO}7 and {CoNO}8 complexes.
• Mössbauer and crystallographic data for [CoCp2][Fe(TFPPBr8)(NO)] suggest an electronic configuration intermediate between low-spin Fe(II)-NO- and Fe(I)-NO•.

Conclusions:

• The electronic configuration of [Fe(TFPPBr8)(NO)]- is best described as an electronic intermediate.
• This study provides crucial experimental data to resolve the long-standing debate on nitroxyl complex electronic configurations.
• The findings contribute to a deeper understanding of metal-nitrosyl bonding and electronic states.