Determinants of the FeXO (X = C, N, O) Vibrational Frequencies in Heme Adducts from Experiment and Density Functional Theory

Vibrational spectra of CO, NO, and O 2 adducts of heme proteins contain information on interactions of the heme and its bound ligands with the surrounding protein matrix that may help in elucidating the mechanism of small-molecule activation. Whereas the heme−CO system is well studied and a framework exists for the interpretation of such interactions, heme−NO and −O 2 complexes have not been systematically investigated. Here we examine resonance Raman spectra of all three classes of adducts, combining literature values with new data for Fe II NO porphyrins having both electron-donating and electron-withdrawing substituents. Negative linear correlations are observed for all three adducts between their Fe−XO and X−O stretching frequencies. The slopes of these correlation lines are −0.4 for five-coordinate FeCO and FeNO porphyrins and −0.8 for five-coordinate FeO 2 adducts. Thus, Fe−NO and Fe−O 2 bonds are equally or even more sensitive than Fe−CO bonds to electronic influences that affect metal-to-ligand π back-bonding. However, the responses of the NO and O 2 adducts to trans ligand binding are very different from those for CO complexes. Ligands trans to CO displace the plot to steeper slopes and lower Fe−CO frequencies, reflecting competition of the ligand lone pairs for the σ acceptor orbital, d z 2 . However, no displacement of the line is observed for six-coordinate FeNO and FeO 2 adducts, but only a shift to higher positions on the line, indicating greater back-bonding. We infer that trans ligand competition for the d z 2 orbital is not as effective for NO and O 2 as for CO, reflecting the lower energy of the N and O orbitals relative to that of the C orbitals. These results are discussed with the aid of a simple bonding model involving FeXO valence isomers. To examine this model, we applied density functional theory to five- and six-coordinate XO adducts of Fe(II) porphine. Geometries were in good agreement with experiment, as were vibrational frequencies for CO adducts. However, DFT overestimated the Fe−NO bond extension on binding a trans ligand and predicted a decrease in the Fe−NO stretching freqency, whereas an increase is observed. The predicted frequency change was likewise in the wrong direction for Fe−O stretching in six- vs five-coordinate FeO 2 adducts. The results suggest that DFT captures the essential features of back-bonding, but not of the σ competition with the trans ligand, in the cases of NO and O 2 .