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 [Fe-2(mu-O2CArTol)(4)(L)(2)] (L = NH2(CH2)(2)SBn (1); NH2(CH2)(3)SMe (2); NH2(CH2)(3)CCH (3)), where (-O2CArTol) is 2,6-di(p-tolyl)benzoate, and [Fe-2(t-O2CArXyl)(2)(O2CArXyl)(2)(L)(2)] (L = NH2(CH2)(3)SMC (4); NH2(CH2)(3)CCH (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 I reacts with dioxygen to afford PhCHO in similar to 30% yield, attributed to oxidative dealkylation of the pendant benzyl group. Compound 3 decomposes to form (FeFeIII)-Fe-II and (FeFeIV)-Fe-III 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)(3)CCH)(2)] (6), was isolated from the reaction mixture following oxidation. (c) 2005 Elsevier Inc. All rights reserved.