Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2015 Aug 1;6(8):4954-4972.
doi: 10.1039/c5sc01414j. Epub 2015 Jun 9.

Bioinspired design of redox-active ligands for multielectron catalysis: effects of positioning pyrazine reservoirs on cobalt for electro- and photocatalytic generation of hydrogen from water

Jonah W Jurss  1   2   3 , Rony S Khnayzer  4   5 , Julien A Panetier  1   6 , Karim A El Roz  4 , Eva M Nichols  1   3 , Martin Head-Gordon  1   3 , Jeffrey R Long  1   6 , Felix N Castellano  4 , Christopher J Chang  1   7   3   8
Affiliations

Bioinspired design of redox-active ligands for multielectron catalysis: effects of positioning pyrazine reservoirs on cobalt for electro- and photocatalytic generation of hydrogen from water

Jonah W Jurss et al. Chem Sci. .

Abstract

Mononuclear metalloenzymes in nature can function in cooperation with precisely positioned redox-active organic cofactors in order to carry out multielectron catalysis. Inspired by the finely tuned redox management of these bioinorganic systems, we present the design, synthesis, and experimental and theoretical characterization of a homologous series of cobalt complexes bearing redox-active pyrazines. These donor moieties are locked into key positions within a pentadentate ligand scaffold in order to evaluate the effects of positioning redox non-innocent ligands on hydrogen evolution catalysis. Both metal- and ligand-centered redox features are observed in organic as well as aqueous solutions over a range of pH values, and comparison with analogs bearing redox-inactive zinc(ii) allows for assignments of ligand-based redox events. Varying the geometric placement of redox non-innocent pyrazine donors on isostructural pentadentate ligand platforms results in marked effects on observed cobalt-catalyzed proton reduction activity. Electrocatalytic hydrogen evolution from weak acids in acetonitrile solution, under diffusion-limited conditions, reveals that the pyrazine donor of axial isomer 1-Co behaves as an unproductive electron sink, resulting in high overpotentials for proton reduction, whereas the equatorial pyrazine isomer complex 2-Co is significantly more active for hydrogen generation at lower voltages. Addition of a second equatorial pyrazine in complex 3-Co further minimizes overpotentials required for catalysis. The equatorial derivative 2-Co is also superior to its axial 1-Co congener for electrocatalytic and visible-light photocatalytic hydrogen generation in biologically relevant, neutral pH aqueous media. Density functional theory calculations (B3LYP-D2) indicate that the first reduction of catalyst isomers 1-Co, 2-Co, and 3-Co is largely metal-centered while the second reduction occurs at pyrazine. Taken together, the data establish that proper positioning of non-innocent pyrazine ligands on a single cobalt center is indeed critical for promoting efficient hydrogen catalysis in aqueous media, akin to optimally positioned redox-active cofactors in metalloenzymes. In a broader sense, these findings highlight the significance of electronic structure considerations in the design of effective electron-hole reservoirs for multielectron transformations.

PubMed Disclaimer

Figures

Fig. 1
Fig. 1. (A) Mononuclear metalloenzyme active sites with optimally positioned redox non-innocent organic cofactors. (B) A homologous series of molecular cobalt catalysts for water reduction to H2 containing pyrazine donors in a PY5Me2-type framework.
Scheme 1
Scheme 1. Synthesis of pentadentate PY5Me2-type ligands (1–3) containing pyrazine(s) at key positions in the framework.
Fig. 2
Fig. 2. Crystal structures of cations in the following salts: 1-Co, [(ax-PY4PZMe2)Co(OH2)](OTf)2; 2-Co, [(eq-PY4PZMe2)Co(OH2)](OTf)2; 3-Co, [(PY3PZ2Me2)Co(OH2)](OTf)2; 1-Zn, [(ax-PY4PZMe2)Zn(OH2)](OTf)2; 2-Zn, [(eq-PY4PZMe2)Zn(OH2)](OTf)2; 3-Zn, [(PY3PZ2Me2)Zn(OH2)](OTf)2. Thermal ellipsoids are drawn at the 70% probability level. Hydrogen atoms have been omitted for clarity.
Fig. 3
Fig. 3. Cyclic voltammetry of catalysts (1 mM) in acetonitrile with various concentrations of chloroacetic acid (up to 20 equivalents, 20 mM). A, B, and C. Catalysts 1-Co, 2-Co, and 3-Co, respectively. The CV in black in these three graphs is the indicated catalyst in the absence of acid. D. Comparison of CVs for each catalyst, 1-Co (red), 2-Co (blue), and 3-Co (black), with 20 eq. of chloroacetic acid. Scan rate = 100 mV s–1; 3 mm dia. glassy carbon.
Fig. 4
Fig. 4. Aqueous cyclic voltammetry of catalysts 1-Co (red), 2-Co (blue), and 3-Co (black) in 1 M pH 7 KPBS at glassy carbon electrode. Catalyst concentration is 1 mM and scan rate = 100 mV s–1. Background is shown as a dotted gray line.
Fig. 5
Fig. 5. Cyclic voltammograms as a function of pH and plots of the first reductive peak potential (E p,c(1)) vs. pH for 1-Co, 2-Co, and 3-Co. Conditions: 0.9 mM catalyst, 0.03 M buffer, 0.1 M KNO3, 100 mV s–1 scan rate, glassy carbon electrode (3 mm dia). Note: CV at pH 7 is omitted for clarity in series of CVs for 1-Co. Legend: pH 3 (black), pH 4 (red), pH 5 (green), pH 6 (blue), pH 7 (cyan), pH 8 (magenta).
Fig. 6
Fig. 6. RDEV of the series of cobalt catalysts. Panels: axial isomer 1-Co, [(ax-PY4PZMe2)Co(OH2)]2+; equatorial isomer 2-Co, [(eq-PY4PZMe2)Co(OH2)]2+; and catalyst 3-Co, [(PY3PZ2Me2)Co(OH2)]2+. Top: rotation rates of 100–3600 rpm; bottom: apparent n app based on 1e– Co(iii/ii) couple at 400 rpm. Conditions: 0.3 mM catalyst, 0.1 M pH 7 KPBS, 0.1 M KNO3, 25 mV s–1 scan rate, glassy carbon electrode (3 mm dia).
Fig. 7
Fig. 7. A. Comparison of n app vs. applied potential for catalysts 1-Co (red), 2-Co (blue), and 3-Co (black) based on steady-state voltammograms in Fig. 6 (bottom). The vertical dashed line denotes the thermodynamic potential for water reduction at pH 7. B. Steady-state voltammogram of catalyst 1-Co at 100 rpm, 25 mV s–1.
Fig. 8
Fig. 8. Photocatalytic H2 production of 1-Co, 2-Co, and 3-Co with [Ru(bpy)3]2+ and ascorbic acid under 452 ± 10 nm (540 mW). H2 evolution and TON vs. time measured over 8 h of photocatalysis. Conditions: 2.0 × 10–5 M Co(ii) catalyst, 3.3 × 10–4 M [Ru(bpy)3]2+, and 0.3 M H2A/HA– at pH 5.5.
Fig. 9
Fig. 9. Isosurface (0.07 au) plots of the canonical highest occupied molecular orbitals (HOMOs, top) and β-spin localized orbitals (bottom) for 1′-Co + e–, 2′-Co + e– and 3′-Co + e– (S = 1). The Löwdin population analyses are given for cobalt.
See this image and copyright information in PMC

References

    1. Meyer T. J. Acc. Chem. Res. 1989;22:163.
    2. Alstrum-Acevedo J. H., Brennaman M. K., Meyer T. J. Inorg. Chem. 2005;44:6802. - PubMed
    3. Lewis N. S., Nocera D. G. Proc. Natl. Acad. Sci. U. S. A. 2006;103:15729. - PMC - PubMed
    4. Hambourger M., Moore G. F., Kramer D. M., Gust D., Moore A. L., Moore T. A. Chem. Soc. Rev. 2009;38:25. - PubMed
    5. Gray H. B. Nat. Chem. 2009;1:7. - PubMed
    6. Cook T. R., Dogutan D. K., Reece S. Y., Surendranath Y., Teets T. S., Nocera D. G. Chem. Rev. 2010;110:6474. - PubMed
    7. Liu C., Dasgupta N. P., Yang P. Chem. Mater. 2014;26:415.
    1. Esswein A. J., Nocera D. G. Chem. Rev. 2007;107:4022. - PubMed
    2. Artero V., Chavarot- Kerlidou M., Fontecave M. Angew. Chem., Int. Ed. 2011;50:7238. - PubMed
    3. Du P., Eisenberg R. Energy Environ. Sci. 2012;5:6012.
    4. Wang M., Chen L., Sun L. Energy Environ. Sci. 2012;5:6763.
    5. Thoi V. S., Sun Y., Long J. R., Chang C. J. Chem. Soc. Rev. 2013;42:2388. - PubMed
    1. Bhugun I., Lexa D., Saveant J.-M. J. Am. Chem. Soc. 1996;118:3982.
    2. Sun L., Akermark B., Ott S. Coord. Chem. Rev. 2005;249:1653.
    3. Appel A. M., DuBois D. L., DuBois M. R. J. Am. Chem. Soc. 2005;127:12717. - PubMed
    4. Tye J. W., Lee J., Wang H.-W., Mejia-Rodriguez R., Reibenspies J. H., Hall M. B., Darensbourg M. Y. Inorg. Chem. 2005;44:5550. - PubMed
    5. Appel A. M., Lee S.-J., Franz J. A., DuBois D. L., DuBois M. R. J. Am. Chem. Soc. 2009;131:5224. - PubMed
    6. Gloaguen F., Rauchfuss T. B. Chem. Soc. Rev. 2009;38:100. - PMC - PubMed
    7. Tard C., Pickett C. J. Chem. Rev. 2009;109:2245. - PubMed
    8. DuBois M. R., DuBois D. L. Chem. Soc. Rev. 2009;38:62.
    9. Jacques P.-A., Artero V., Pecaut J., Fontecave M. Proc. Natl. Acad. Sci. U. S. A. 2009;106:20627. - PMC - PubMed
    10. Helm M. L., Stewart M. P., Bullock R. M., DuBois M. R., DuBois D. L. Science. 2011;333:863. - PubMed
    11. Rose M. J., Gray H. B., Winkler J. R. J. Am. Chem. Soc. 2012;134:8310. - PubMed
    1. Fisher B. J., Eisenberg R. J. Am. Chem. Soc. 1980;102:7361.
    1. Houlding V., Geiger T., Kölle U., Grätzel M. J. Chem. Soc., Chem. Commun. 1982:681.