Heme Carbonyls:  Environmental Effects on ν_C-O and Fe−C/C−O Bond Length Correlations

Abstract

The synthesis and characterization of four low-spin (carbonyl)iron(II) tetraphenylporphyrinates, [Fe (TPP)(CO)(L)], where L = 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated) and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of ν C-O in toluene solution (1969-72 cm −1 ) but a large range of CO stretching frequencies in the solid-state cm −1 ). The large solid-state variation results from CO interactions in the solid-state as shown by an examination of the crystal structures of the four complexes. The high precision of the four structures obtained allows us to make a number of structural and spectroscopic correlations that describe the Fe-C-O and N Im -Fe-CO units. The values of ν C-O and the Fe-C and C-O bond distances are strongly correlated and provide a structural as well as a spectroscopic correlation of the π back-bonding model. The interactions of CO described are closely related to the large range of CO stretching frequencies observed in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO).

New heme–dioxygen and carbon monoxide adducts using pyridyl or imidazolyl tailed porphyrins

“…Somewhat surprisingly, while one might expect that a stronger axial trans donor (e.g., imidazolyl > pyridyl > THF) would lead to a measurably reduced the C-O stretching frequency, the data here shows that does not seem to be the case. However, this finding has precedent; a lack of correlation of ν (C-O) with base strength is observed for a number of other cases [28, 29, 43]. …”

“…CO favors strongly binding to five-coordinated ferrous species to form stable six-coordinate heme-Fe II carbonyl complexes. Determination of either ν(C-O) or Fe-(CO) stretching vibrations can provide insights into the heme-iron environment electronic structure [23, 24, 28, 29]. Laser photoejection of CO and the study of the kinetics of CO-rebinding or “flash and trap” experiments to probe the fast reactions between O 2 and heme Fe II complexes also provides a great deal of fundamental information about the active-site nature and/or the O 2 -binding process [25, 30–33].…”

New Insights on the Electronic and Molecular Structure of Cyanide-Ligated Iron(III) Porphyrinates

“…Similar pairs of saddled and ruffled cores are seen in a series of [Fe(TPP)(CO)(L)] complexes. [58] Prior to the current study, the only mixed-ligand cyano complexes that had been structurally characterized were two derivatives with a pyridine ligand trans to the cyanide, [Fe(TPP)(CN) (Py)] [14] and [Fe(OEP)(CN)(Py)]. [15] In those two derivatives, the Fe-N(Py) bond distance was relatively long at 2.075(3) Å A or 2.087(3) Å, respectively.…”

Comparison of Cyanide and Carbon Monoxide as Ligands in Iron(II) Porphyrinates

“…Comparable 100 K structures of (f) [Fe(OEP)(CO)],[20] (g) [Fe(OEP)(CO) 2 ][20] and (h) [Fe(TPP)(CO)(1-MeIm)][24] are also given. As shown in the boxed key (upper left) a number of structural parameters are given for each structure.…”

“…[24] The large variation in ν CO in the six-coordinate mixed ligand species reflects the sensitivity of the vibration to its immediate environment. [24] We were thus surprised that the cyano derivative with ion pair interactions to the terminal cyanide nitrogen atoms had no effect on the CN frequency.…”