Synthesis and oxidation of carboxylate-bridged diiron(II) complexes with substrates tethered to primary alkyl amine ligands

The synthesis and crystallographic characterization of a series of diiron(II) complexes with sterically hindered terphenyl carboxylate ligands and alkyl amine donors are presented. The compounds [Fe2(μ-O2CArTol)4(L)2] (L = NH2(CH2)2SBn (1); NH2(CH2)3SMe (2); NH2(CH2)3CCH (3)), where −O2CArTol is 2,6-di(p-tolyl)benzoate, and [Fe2(μ-O2CArXyl)2(O2CArXyl)2(L)2] (L = NH2(CH2)3SMe (4); NH2(CH2)3CCH (5)), where −O2CArXyl is 2,6-di(3,5-dimethylphenyl)benzoate, were prepared as small molecule mimics of the catalytic sites of carboxylate-bridged non-heme diiron enzymes. The compounds with the −O2CArTol carboxylate form tetrabridged structures, but those containing the more sterically demanding −O2CArXyl ligand have only two bridging ligands. The ancillary nitrogen ligands in these carboxylate-rich complexes incorporate potential substrates for the reactive metal centers. Their oxygenation chemistry was studied by product analysis of the organic fragments following decomposition. Compound 1 reacts with dioxygen to afford PhCHO in ∼30% yield, attributed to oxidative dealkylation of the pendant benzyl group. Compound 3 decomposes to form FeIIFeIII and FeIIIFeIV mixed-valence species by established bimolecular pathways upon exposure to dioxygen at low temperatures. Upon decomposition, the alkyne-substituted amine ligand was recovered quantitatively. When the −O2CArTol carboxylate was replaced by the −O2CArXyl ligand in 5, different behavior was observed. The six-coordinate iron(III) complex with one bidentate and two monodentate carboxylate ligands, [Fe(O2CArXyl)3(NH2(CH2)3CCH)2] (6), was isolated from the reaction mixture following oxidation.