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 f...