| Abstract
| - 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 densityfunctional 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, anda rule is developed that successfully predicts the presence or absence of ring distortion in a broad range ofmetalloporphyrins. In each of the three CO complexes, the M−C−O bond is close to linear and untilted, butwith the iPrNC adduct, there are noticeable ligand distortions supporting the idea that RNC groups (but notCO) may be distorted in metalloproteins. Solid-state 13C, 15N, and 17O NMR shifts and shift tensors determinedexperimentally are in generally good agreement with those computed via DFT. For isocyanide binding toproteins, the experimental shifts are more deshielded than in the model system, and the effects which mightcontribute to this difference are explored theoretically. Unlike CO, electrostatic field effects are unlikely tomake a major contribution to protein shielding. Neither are Fe−C−N tilt−bend distortions, although a bendat nitrogen is energetically feasible and also gives a large deshielding, as seen with proteins.
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