The Remarkable Axial Lability of Iron(III) Corrole Complexes

Flash photolysis of nitrosyl tris(aryl)corrolate complexes of iron(III), Fe(Ar₃C)(NO) (Ar₃C^3- = 5,10,15-tris(4-nitro-phenyl)corrolate (TNPC^3-), 5,10,15-tris(phenyl)corrolate (TPC^3-) or 5,10,15-tris(4-tolyl)corrolate (H₃TTC^3-)) leads to NO labilization. This is followed by the rapid reaction of NO with Fe^III(C) to regenerate the starting complex. The second-order rate constants for the back reactions (k_NO) were determined to be many orders of magnitude faster than the corresponding reactions of ferric porphyrin complexes and indeed are reminiscent of the very large values seen for those of the corresponding ferrous porphyrin analogues. These data are interpreted in terms of the strongly electron-donating character of the trianionic corrolate ligand and the likely triplet electronic configuration of the iron(III) complex. These reduce the affinity of the metal centers to Lewis bases to the extent that axial ligands bind very weakly or not at all. This property is illustrated by the nearly identical k_NO values (∼10⁹ M^-1 s^-1 at 295 K) recorded for the back reaction of Fe^III(TNPC) with NO after flash photolysis of Fe(TNPC)(NO) in toluene solution and in THF solution. Softer Lewis bases have a somewhat greater effect; for example, studies in 1:9 (v:v) acetonitrile:toluene and 1:9 pyridine:toluene gave k_NOvalues decreased ∼33% and ∼85%, respectively, but these both remain >10⁸ M^-1 s^-1. The potential roles of Lewis bases in controlling the dynamics of NO addition to Fe(TNPC) in toluene was investigated in greater detail by determining the rates as a function of pyridine concentration over a wide range (10^-4 to 2.5 M). These data suggest that, while a monopyridine complex, presumably Fe(TNPC)(py), is readily formed (K ≈ 10⁴ M), this species is about one-sixth as reactive as Fe(TNPC) itself. It appears that a much less reactive bis(pyridine) complex also is formed at high [py] but the equilibrium constant is quite small (<1 M^-1).