Metal-Metal Cooperativity in Chemical Transformations
Multinuclear metal centers play an important role in catalytic systems, including metalloenzymes and the active sites of heterogeneous catalysts. In cases involving dinuclear centers, many metalloenzymes are known for which interaction of the metal centers is thought to play a crucial role in the catalytic function. Indeed, some enzymes containing multinuclear active sites, such as the tetramanganese center in the Oxygen Evolving Complex of Photosystem II, have been proposed to utilize only two manganese centers in bond-making and bond-breaking events of the catalytic cycle. Dinuclear metal complexes are expected to have several advantages over mononuclear complexes including cooperative activation of substrates, lower oxidation or reduction potentials required to store multiple redox equivalents on the complex, enhanced catalysis via multielectron processes, and electronic interactions between the metal centers potentially beneficial to catalysis. This project is designed to provide conceptual underpinnings for the use of cooperativity effects in the design of bimetallic catalysts. This work features strategically designed dinucleating ligands, for which the structures can be tailored to manipulate properties.
Recent Projects
On the basis of preliminary design considerations and molecular mechanics calculations, it seemed that naphthyridine-based ligands would be well suited for assembly of two first-row metal complexes bridged by small-atom donors such as oxygen.1 An early investigation of a fluorinated version of this ligand (DPFN) provided a dicobalt complex that structurally resembles the proposed active site in cobalt oxide electrocatalysts for water oxidation. This study also revealed key insights into the coordination chemistry of such bimetallic cores.2
The DPFN ligand, and a dimethyl analogue (DPEN), were found to bind two Cu(I) atoms at a close distance, and DFT calculations indicate that frontier, σ-type orbitals on each metal center overlap in an angular geometry, resulting in an acceptor orbital that is positioned between the metal atoms. Thus, the copper atoms intimately cooperate to bind ligands, and in the case of acetonitrile this gives a rare example of 3-center 2-electron bonding in which both electrons are donated by the bridging L-type ligand.3
These bimetallic complexes offer opportunities to explore cooperative effects for closely positioned metals. For example, dicopper units are of recent interest as they appear to mediate the conversion of methane to methanol in zeolites and in nature. Interestingly, [(DPFN)CuICuI(μ-NCMe)]2+ is a strong "bimetallic electrophile" that abstracts aryl groups from BPh4-, B[3,5-(CF3)2C6H3]4- and B(C6F5)4-, with the aryl group adopting a bridging position in the bimetallic complex. The resulting dicopper complexes are readily oxidized to give rare examples of formally CuII organometallic compounds, in mixed-valence CuICuII species.4
The dicopper bridging position is also featured in the binding and activations of substrates in a demonstrated catalytic cycle for the azide-alkyne "click" coupling reaction. Our study of this system provided the first conclusive evidence for a bimetallic mechanism for this coupling. The availability of mixed-valence complexes in this system allowed us to determine that this catalysis is associated with CuICuI, and not CuICuII.5
References
1. "Dinucleating Naphthyridine-Based Ligand for Assembly of Bridged Dicopper(I) Centers: Three-Center Two-Electron Bonding Involving an Acetonitrile Donor." T. C. Davenport and T. D. Tilley Angew. Chem. Int. Ed. 2011, 50, 12205-12208. DOI: 10.1002/anie.201106081
2. "Dinuclear First-Row Transition Metal Complexes with a Naphthyridine-Based Dinucleating Ligand." T. C. Davenport and T. D. Tilley, Dalton Trans. 2015, 44, 12244–12255. DOI: 10.1039/C4DT02727B
3. "Molecular cobalt electrocatalyst for proton reduction at low overpotential." H. S. Ahn, T. C. Davenport and T. D. Tilley, Chem. Comm. 2014, 50, 3834-3847. DOI: 10.1039/C3CC49682A
4. "Aryl Group Transfer from Tetraarylborato Anions to an Electrophilic Dicopper(I) Center and a Mixed-Valence μ-Aryl Dicopper(I,II) Complex." M. S. Ziegler, K. V. Lakshmi and T. D. Tilley, J. Am. Chem. Soc. 2016, 138, 6484–6491. DOI: 10.1021/jacs.6b00802
5. "Dicopper Cu(I)Cu(I) and Cu(I)Cu(II) Complexes in Copper-Catalyzed Azide-Alkyne Cycloaddition." M. S. Ziegler, K. V. Lakshmi and T. D. Tilley, J. Am. Chem. Soc. 2017, 139, 5378–5386. DOI: 10.1021/jacs.6b13261