High-Valent Metal Oxo Species in Water Oxidation
A major focus of research on development of new OER catalysts is based on molecular design of the catalytic centers, and extraction of structure-function relationships from structurally well-defined catalysts. There is ongoing speculation on the role of high oxidation state metal centers in the mechanisms of water splitting, in both synthetic catalysts and in nature, making such species the target of many synthetic and spectroscopic studies. Attempts to generate and observe a Co(IV) oxo complex utilize a dianionic, square planar cobalt(II) complex that reacts with O2 in the presence of acetonitrile to give a cyanomethylcobalt(III) complex formed by C–H bond cleavage. Kinetic isotope experiments reveal a moderate KIE value of ca. 3.5 using either O2 or PhIO, which is consistent with involvement of a cobalt(IV) oxo species.1
Recent Projects
In nature, the water-splitting active site positions several manganese centers in close proximity, to allow cooperative behavior in storing redox equivalents, coupling oxygen ligands at neighboring metal sites, etc. One aspect of our research in this area focuses on synthesis and study of multi-metallic systems that feature aspects of this cooperativity. For example, a dinucleating, naphthyridine DPFN ligand was employed to assemble a diamond-shaped Co2(μ-OH)2(OH2)2 core, since this structural unit has been postulated as a key component in cobalt-based OER catalysts.2 The cubane complex Mn4O4{O2P(OtBu)2}6, which derives structural inspiration from the photosystem II oxygen-evolving complex, was synthesized and fully characterized. Core oxygen atoms within this complex are transferred upon reaction with an O-atom acceptor (PEt3), to give the butterfly complex [Mn4O2{O2P(OtBu)2}6(OPEt3)2]. The cubane structure is restored by reaction of the latter complex with the O-atom donor PhIO.3
A detailed investigation of the chemistry of the cobalt oxo cubane Co4O4(pyr)4(OAc)4 revealed that its isolable oxidation product spontaneously splits water, and this allowed a detailed mechanistic investigation of oxygen evolution. This is the first experimentally determined mechanism for water splitting with a molecular, first-row metal catalyst system, and it shows that the cubane must be oxidized twice to produce oxygen.4 Subsequent studies have shown that the ligands of this cobalt cluster can be systematically modified to tune the redox properties, and corresponding data provide free energy relationships that allow accurate predictions of redox potentials for designing new cubanes and evaluating potential reactive intermediates.5
Ongoing research explores the synthetic tuning of such oxo clusters to optimize catalytic efficiency and improve stability. A further goal is to utilize synthetic oxo clusters of this type to address long-standing mechanistic questions concerning the OEC in nature, as well as heterogeneous oxide catalysts. Toward this goal, manganese has been doped into the cobalt oxo cubane, and spectroscopic studies with the Britt group at UC Davis have revealed the electronic structure of these heterometallic systems. In a collaboration with the Borovik group at UC Irvine, the cobalt oxo cubane has been incorporated into a protein in an effort to exploit secondary interactions that may improve catalytic efficiency. Mutagenesis of the streptavidin protein places a redox-active tyrosine residue in hydrogen-bonding contact with the cubane, thereby enabling multi-electron PCET.6
References
1. "Efficient C–H bond activations via O2 cleavage by a dianionic Cobalt(II) complex." A. I. Nguyen, R. G. Hadt, E. I. Solomon and T. D. Tilley, Chem. Sci., 2014, 5, 2874-2878. DOI: 10.1039/C4SC00108G
2. "A molecular structural analog of proposed dinuclear active sites in cobalt-based water oxidation catalysts." T. C. Davenport, H. S. Ahn, M. S. Ziegler and T. D. Tilley, Chem. Comm., 2014, 50, 6326-6329. DOI: 10.1039/C3CC46865H
3. "Oxygen-Atom Transfer Chemistry and Thermolytic Properties of a Di-tert-butylphosphate-Ligated Mn4O4 Cubane." K. M. Van Allsburg, E. Anzenberg, W. Drisdell, J. Yano and T. D. Tilley, Chem. Eur. J. 2015, 21, 4646-4654. DOI: 10.1002/chem.201406114
4. "Mechanistic Investigation of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation." A. I. Nguyen, M. S. Ziegler, P. Oña-Burgos, M. Sturzbecher-Hohne, W. Kim, D. E. Bellone and T. D. Tilley, J. Am. Chem. Soc. 2015, 137, 12865-12872. DOI: 10.1021/jacs.5b08396
5. "Synthetic control and empirical prediction of redox potentials for Co4O4 cubanes over a 1.4 V range: implications for catalyst design and evaluation of high-valent intermediates in water oxidation." A. I. Nguyen, J. Wang, D. S. Levine, M. S. Ziegler and T. D. Tilley, Chem. Sci. 2017, 8, 4274-4284. DOI: 10.1039/c7sc00627f
6. "Artificial Metalloproteins Containing Co4O4 Active Sites." L. Olshansky, R. Huerta-Lavorie, A. I. Nguyen, J. Vallapurackal, A. Furst, T. D. Tilley and A. S. Borovik, J. Am. Chem. Soc. 2018, 140, xxxx. DOI: 10.1021/jacs.7b13052