Iron-Porphyrin NO Complexes with Covalently Attached N-Donor Ligands: Formation of a Stable Six-Coordinate Species in Solution

A series of substituted tetraphenylporphyrin type macrocycles (TMP or To-F₂PP) with covalently attached N-donor ligands (pyridine or imidazole linker) have been synthesized. Linkers with varying chain lengths and designs have been applied to systematically investigate the effect of chain length and rigidity on the binding affinity of the linker to the corresponding Fe(II)−NO heme complexes. The binding of the linker is monitored in solution using a variety of spectroscopic methods including UV−vis absorption, EPR, and IR spectroscopy. Both the N−O stretching frequency and the imidazole ¹⁴N hyperfine coupling constants show a good correlation with the Fe−(N-donor) bond strength in these systems. The complexes with covalently attached pyridyl and alkyl imidazole ligands only exhibit weak interactions of the linker with iron(II). However, the stable six-coordinate complex [Fe(To-F₂PP-BzIM)(NO)] (4) is obtained when a rigid benzyl linker is applied. This complex exhibits typical properties of six-coordinate ferrous heme-nitrosyls in which an N-donor ligand is bound trans to NO, including the Soret band at 427 nm and the typical nine line ¹⁴N hyperfine splitting in the EPR spectrum. A crystal structure has been obtained for the corresponding zinc complex. Here, we report the first systematic study on the requirements for the formation of stable six-coordinate ferrous heme nitrosyl complexes in solution at room temperature in the absence of excess axial N-donor ligand.