Optimization of the efficiency of oxidation catalysts based on iron bispidine complexes

Abstract

High-valent bispidine iron-oxo complexes are among the most efficient nonheme iron oxidation catalysts. Here, we report the synthesis and structural analysis of two derivatives of the known pentadentate bispidine ligand L 1 {L1 = 2,4-pyridyl-7-(pyridine-2-ylmethyl)-3,7- diazabicyclo[3.3.1]nonane}: L2 and L3 {L2 = 2,4-pyridyl-7-[1-(pyridine-2-yl)ethyl]-3,7-diazabicyclo[3.3.1]nonane; L 3 = 2,4-pyridyl-7-[phenyl(pyridine-2-yl)methyl]-3,7-diazabicyclo[3.3. 1]nonane}, and of their FeII complexes. The yield of the catalytic epoxidation of cyclooctene and styrene with iodosylbenzene as oxidant increases from the L1- to the L2- to the L3-based catalyst (e.g., the yield of styrene oxide, with MeCN as solvent, under anaerobic conditions, is 40%, 90%, 96%, respectively), and this is correlated to the FeIII/II reduction potentials {[Fe(L1)(NCMe)] n+ (1.01 V), [Fe(L2)(NCMe)]n+ (1.13 V), and [Fe(L3)(NCMe)]n+ (1.19 V), in MeCN, vs. SCE}. Although this correlation is not unexpected, the interpretation is not entirely trivial, and this is discussed in detail. The rigidity of the bispidine ligands and their preference for relatively large metal ions (low oxidation states, high spin multiplicities) is believed to be responsible for the efficiency of the bispidine-based catalyst systems, and the present results show possible approaches to further improve the performance of these catalysts. Both the redox potential and the efficiency of the catalytic oxidation by a biomimetic bispidine-based iron precatalyst are optimized by increasing the ligand rigidity. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.