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), ps(OEP)(CO)(1-MeIm), and Fe(TPP )(iPrNC)(1-MeIm), where OEP -2,3,7,8,12, 13,17,18-octaethylporphyrinate, TP P = 5,10,15,20-tetraphenylporphyrinate, and 1-MeIm = 1-methylimidazole, usi ng single-crystal X-ray diffraction, solid-state nuclear magnetic resonance (NMR), and density functional theory (DFT) methods. Unlike the situation f ound with the Fe-,.Ru-, Os(TPP)(CO)(1-MeIm) analogues, which have ruffled p orphyrins, all four systems here have essentially planar porphyrin rings, a nd a rule is developed that successfully predicts the presence or absence o f ring distortion in a broad range of metalloporphyrins. In each of the thr ee CO complexes, the M-C-O bond is close to linear and untilted, but with t he iPrNC adduct, there are noticeable ligand distortions supporting the ide a that RNC groups (but not CO) maybe distorted in metalloproteins. Solid-st ate C-13, N-15 O-17 NMR shifts and shift 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 t he model system, and the effects which might contribute to this difference are explored theoretically. Unlike CO, electrostatic field effects are unli kely to make a major contribution to protein shielding. Neither are Fe-C-N tilt-bend distortions, although a bend at nitrogen is energetically feasibl e and also gives a large deshielding, as seen with proteins.