A pentanuclear iron catalyst designed for water oxidation

Abstract

Although the oxidation of water is efficiently catalysed by the oxygen-evolving complex in photosystem II (refs 1 and 2), it remains one of the main bottlenecks when aiming for synthetic chemical fuel production powered by sunlight or electricity. Consequently, the development of active and stable water oxidation catalysts is crucial, with heterogeneous systems considered more suitable for practical use and their homogeneous counterparts more suitable for targeted, molecular-level design guided by mechanistic understanding. Research into the mechanism of water oxidation has resulted in a range of synthetic molecular catalysts, yet there remains much interest in systems that use abundant, inexpensive and environmentally benign metals such as iron (the most abundant transition metal in the Earth's crust and found in natural and synthetic oxidation catalysts). Water oxidation catalysts based on mononuclear iron complexes have been explored, but they often deactivate rapidly and exhibit relatively low activities. Here we report a pentanuclear iron complex that efficiently and robustly catalyses water oxidation with a turnover frequency of 1,900 per second, which is about three orders of magnitude larger than that of other iron-based catalysts. Electrochemical analysis confirms the redox flexibility of the system, characterized by six different oxidation states between Fe(II)5 and Fe(III)5; the Fe(III)5 state is active for oxidizing water. Quantum chemistry calculations indicate that the presence of adjacent active sites facilitates O-O bond formation with a reaction barrier of less than ten kilocalories per mole. Although the need for a high overpotential and the inability to operate in water-rich solutions limit the practicality of the present system, our findings clearly indicate that efficient water oxidation catalysts based on iron complexes can be created by ensuring that the system has redox flexibility and contains adjacent water-activation sites.

Iron Catalysis in Organic Synthesis: A Critical Assessment of What It Takes To Make This Base Metal a Multitasking Champion

“…6,13−15 It is reasonable to assume that base metal catalysis in general and iron catalysis in particular will eventually challenge the enduring dominance of the more “noble” metals 16 over the field of homogeneous catalysis. A look into timely monographs shows that some of the scarcest, most expensive, and often physiologically suspect transition metals currently hold a surprisingly big stake (Pd, Ru, Rh, Ir, Os, Pt, Au, etc.…”

Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

“…4 Molecular transition metal complexes constitute an excellent platform to examine these factors since significant information based on an arsenal of spectroscopic, electrochemical and analytical techniques can be used together with the valuable complementary information provided by computational studies. 5,6,7,8,9,10,11,12,13,14 The best water oxidation catalysts reported today are based on seven coordinated Ru complexes containing dianionic ligands such as [2,2'-bipyridine]-6,6'-dicarboxylato (bda 2-) 15,16,17 and [2,2':6',2''-terpyridine]-6,6''-dicarboxylato (tda 2-) (see Figure 1 for drawn structures of these ligands). Particularly impressive is the seven coordinate complex [Ru IV (tda--N 3 O)(py)2(O) eq ], 4 IV (O), (the superscript in roman numbers indicates the formal oxidation state of Ru; py is pyridine; the "eq" superscript means equatorial) that is capable of oxidizing water to dioxygen at maximum turnover frequencies (TOFMAX) of 7,700 s -1 and 50,000 s -1 at pH = 7.0 and pH = 10.0 respectively.…”

Topological insights in polynuclear Ni/Na coordination clusters derived from a schiff base ligand

“…These entities are of great interest for their aesthetically beautiful structures [2][3][4], unexpected transformations [5][6][7] and potential applications in magnetism [8][9][10], luminescence [11][12][13][14][15], catalysis [16][17][18], etc. Paramagnetic transition metal CCs are of intense interest and have attracted a vast amount of attention since the discovery that some CCs behave as single-molecule magnets (SMMs) [19][20][21].…”