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(μ-O 2CAr Tol) 4(L) 2] (L = NH 2(CH 2) 2SBn ( 1); NH 2(CH 2) 3SMe ( 2); NH 2(CH 2) 3CCH ( 3)), where −O 2CAr Tol is 2,6-di( p-tolyl)benzoate, and [Fe 2(μ-O 2CAr Xyl) 2(O 2CAr Xyl) 2(L) 2] (L = NH 2(CH 2) 3SMe ( 4); NH 2(CH 2) 3CCH ( 5)), where −O 2CAr Xyl 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 −O 2CAr Tol carboxylate form tetrabridged structures, but those containing the more sterically demanding −O 2CAr Xyl 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 Fe IIFe III and Fe IIIFe IV 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 −O 2CAr Tol carboxylate was replaced by the −O 2CAr Xyl ligand in 5, different behavior was observed. The six-coordinate iron(III) complex with one bidentate and two monodentate carboxylate ligands, [Fe(O 2CAr Xyl) 3(NH 2(CH 2) 3CCH) 2] ( 6), was isolated from the reaction mixture following oxidation.