Water Induces a Structural Conversion and Accelerates the Oxygenation of Carboxylate-Bridged Non-Heme Diiron Enzyme Synthetic Analogues

Recently, we reported the synthesis of a carboxylate-rich non-heme diiron enzyme model compound [Fe₂(μ-O₂CAr^Tol)₄(4-CNPy)₂] (1), where ^-O₂CAr^Tol is 2,6-di-p-tolylbenzoate and 4-CNPy is 4-cyanopyridine (Yoon, S.; Lippard, S. J. J. Am. Chem. Soc. 2005, 127, 8386−8397). A metal-to-ligand charge-transfer band in the visible region of the optical absorption spectrum involving the nitrogen-donor ligand endowed this complex with a distinctive red color that facilitated analysis of its chemistry. Following this strategy, we prepared and characterized two related isomeric complexes, windmill (3) and paddlewheel (4) species having the formula [Fe₂(O₂CAr^Tol)₄(4-AcPy)₂], where 4-AcPy is 4-acetylpyridine. In anhydrous solvents, 1 and 4 adopt paddlewheel structures, but upon the addition of water, they convert to aquated forms, windmill structures having the composition [Fe₂(μ-O₂CAr^Tol)₂(O₂CAr^Tol)₂(4-RPy)₂(H₂O)₂]. This conversion is favored at low temperature and was studied by NMR spectroscopy. A kinetic analysis of the aquation reaction was undertaken by stopped-flow measurements between 198 and 223 K for both 1 and 4, which revealed a first-order dependence on both the diiron compound and water. The oxygenation rates for the water-containing complexes are much faster than those for the corresponding anhydrous complexes, being 20-fold faster for 4 and 10-fold more rapid for 1. The presence or absence of water had little effect on the activation enthalpies, suggesting that the loss of water may not be necessary prior to dioxygen binding in the transition state.