Iron complexes of new hydrophobic derivatives of tris(2-pyridylmethyl)amine: synthesis, characterization, and catalysis of alkane oxygenation by H2O2

Several new iron(II) triflate complexes of 6-py-substituted derivatives of tris(2-pyridylmethyl)amine (TPA); [Fe(3–6)(CH3CN) x ](CF3SO3)2 (3 = tris-(6-methoxymethyl-2-pyridylmethyl)amine, x = 0; 4 = tris-(6-octoxymethyl-2-pyridylmethyl)amine, x = 0; 5 = 6-(N′-hexylureido-2-pyridylmethyl)-bis(2-pyridylmethyl)amine, x = 1; and 6 = bis-(6-(N′-hexylureido)-2-pyridylmethyl)-2-pyridylmethylamine) have been synthesized to probe the introduction of a hydrophobic peralkyl chain (4–6) and hydrogen-bonding (from urea N–H) (4 and 5) on the selective catalysis of alkane oxygenation by H2O2 in CH3CN. For [Fe(6)](CF3SO3)2, hexadentate coordination of iron involving the two urea carbonyls was confirmed by X-ray structural analysis. Despite this, [Fe(6)](CF3SO3)2 exhibited the highest A/K ratio (3.3), yet seen for catalysis of H2O2 oxygenation of cyclohexane by an iron(II) complex of a bis(6-py)- or tris(6-py)-substituted TPA derivative. This has been tentatively attributed to a degree of O–O cleavage assistance within high spin seven-coordinate [Fe(6)OOH]2+ via urea N–H protonation of the departing "OH" leading to participation from oxo-iron(IV) or (V) intermediates. The lack of similar hydrogen-bonding assistance results in [Fe(6)OOBut]2+ being one of the most long-lived high spin peroxoiron(III) complexes yet synthesized (t 1/2 = 1.3 h at 298 K).