Solid-state NMR, crystallographic and density functional theory investigation of Fe-CO and Fe-CO analogue metalloporphyrins and metalloproteins

We have synthesized and characterized the following four metalloporphyrins: Fe(OEP)(CO)(1-MeIm), Ru(OEP)(CO)(1-MeIm), Os(OEP)(CO)(1- MeIm), and Fe(TPP)(iPrNC)(1-MeIm), where OEP = 2,3,7,8,12,13,17,18- octaethylporphyrinate, TPP = 5,10,15,20-tetraphenylporphyrinate, and 1-MeIm = 1-methylimidazole, using single-crystal X-ray diffraction, solid-state nuclear magnetic resonance (NMR), and density functional theory (DFT) methods. Unlike the situation found with the Fe-, Ru-, Os(TPP)(CO)(1-MeIm) analogues, which have ruffled porphyrins, all four systems here have essentially planar porphyrin rings, and a rule is developed that successfully predicts the presence or absence of ring distortion in a broad range of metalloporphyrins. In each of the three CO complexes, the M-C-O bond is close to linear and untilted, but with the iPrNC adduct, there are noticeable ligand distortions supporting the idea that RNC groups (but not CO) may be distorted in metalloproteins. Solid-state ¹³C, ¹⁵N, and ¹⁷NMR shifts and shifts and tensors determined experimentally are in generally good agreement with those computed via DFT. For isocyanide binding to proteins, the experimental shifts are more deshielded than in the model system, and the effects which might contribute to this difference are explored theoretically. Unlike CO, electrostatic field effects are unlikely to make a major contribution to protein shielding. Neither are Fe-C-N tilt-bend distortions, although a bend at nitrogen is energetically feasible and also gives a large deshielding, as seen with proteins.