Iron(II) complexes of sterically bulky α-ketocarboxylates.: Structural models for α-ketoacid-dependent nonheme iron halogenases

Reaction of the sterically hindered alpha-ketocarboxylate 2,6-di(mesityl)benzoylformate (MesBF) with the iron(II) complexes LFeCl2 [L = N,N,N',N'-tetramethylpropylenediamine (Me(4)pda) or 6,6'-dimethyl-2,2'-bipyridine (dmby)] yielded LFe(CI)(MesBF) (1 or 2). X-ray crystal structures of these complexes showed that they closely model the active site structure of the nonheme iron halogenase enzyme SyrB2. A similar synthetic procedure using benzoylformate with L = dmby yielded (dmby)Fe[(O2CC(O)Ph)](2) (3) instead, demonstrating the need for the sterically hindered a-ketocarboxylate to assemble the halogenase model compounds. In order to make reactivity comparisons among the structurally related iron(II) complexes of benzoylformates of varying steric properties, the complexes [LFe(O2CC(O)Ar)](n) (4-6) were prepared, where L' = tris(pyridylmethyl)amine (tpa) and Ar = 2,6-dimesitylphenyl, 2,6-dip-tolylphenyl, or 2,4,6-trimethylphenyl, respectively. X-ray structures for the latter two cases (5 and 6) revealed dinuclear topologies (n = 2), but UV-vis and H-1 NMR spectroscopy indicated that all three complexes dissociated in varying degrees to monomers in CH2Cl2 Solution. Although compounds 1-6 were oxidized by O-2, oxidative decarboxylation of the alpha-ketocarboxylate ligand(s) only occurred for 3. These results indicate that the steric hindrance useful for structural modeling of the halogenase active site prohibits functional mimicry of the enzyme.