The effect of porphyrin ruffling on the intrinsic binding of carbon monoxide in iron(II) hybrid basket-handle porphyrins by multinuclear NMR and FT-IR spectroscopy

Abstract

Carbonyl complexes of single-side hindered hemes, the ‘hybrid basket-handle’ porphyrins, have been investigated by ¹H, ¹³C, ¹⁵N, ⁵⁷Fe NMR and FT-IR spectroscopy in order to obtain information about the intrinsic properties of the Fe–C bond. The effect upon CO binding of severe distortion of the porphyrin skeleton that occurs in the more tightly packed structures, is discussed. Reduced ability of the iron atom to back-donate d-electrons to the π*-orbital of the CO ligand as a result of the distortion is suggested to be a mechanism for CO discrimination in these hemes. The ⁵⁷Fe NMR resonance for the more encumbered structures is shifted by several hundred ppm to lower frequency while the ¹³C and ¹⁵N chemical shifts are very little affected. These data indicate that severe electronic changes occur at the ligand binding-site in the more encumbered structures but they are mainly located at the iron atom. Differences, between the looser and the tighter models, in binding of nitrogen ligands opposite to the carbonyl are reported and provide evidence that the CO affinity is reduced in the tighter complexes. This provides a novel way of probing CO binding and is based on the fact that formation of axial bonds in carbonyl iron(II) porphyrins is an allosteric process.