Catalysts

One of our major research lines is the investigations of molecular H2 formation catalysts, in particular iron complexes modelled after the active site of FeFe-Hydrogenase enzymes. Our research aims at the characterization of these catalysts regarding structure and reactivity of catalytic intermediates by direct spectroscopic observation to provide mechanistic insight and aid the directed design of improved noble-metal free catalysts.

Molecular structures illustrating the electron and proton transfer steps of the catalytic cycle.
Fig. 1. Molecular structures of intermediates in catalytic H2 formation with an iron complex modelled after the active site of hydrogenase enzymes. The structures illustrate electron and proton transfer steps of the catalytic cycle and are shown together with a time-resolved infrared spectrum that provides information on the structure of the intermediates and the kinetics of their transformations.

Particular questions concern e.g. the kinetics of proton transfer steps and the preferred protonation sites in different metal oxidation states, elucidating the role of basic sites in the second coordination sphere as design elements for molecular redox catalysts.

Three-dimensional reaction scheme illustrating possible mechanistic routes for electron and proton transfer reactions.
Fig. 2. Three-dimensional reaction scheme illustrating possible mechanistic routes for electron and proton transfer reactions relevant for catalytic H2 formation with an iron complex modelled after the active site of hydrogenase enzymes. The scheme distinguishes different sequences of electron transfer and protonations of either the ligand or the metal sites of the catalyst, including proton shuttling between these sites.

For more information see e.g.:

  • Aster, A.; Wang, S.; Mirmohades, M.; Esmieu, C.; Berggren, G.; Hammarström, L.; Lomoth, R., Metal vs. ligand protonation and the alleged proton-shuttling role of the azadithiolate ligand in catalytic H2 formation with FeFe hydrogenase model complexes. Chem. Sci. 2019, 10, 5582–5588. DOI: 10.1039/c9sc00876d 
  • Wang, S. H.; Pullen, S.; Weippert, V.; Liu, T. F.; Ott, S.; Lomoth, R.; Hammarström, L., Direct Spectroscopic Detection of Key Intermediates and the Turnover Process in Catalytic H2 Formation by a Biomimetic Diiron Catalyst. Chem. Eur. J. 2019, 25 (47), 11135-11140. DOI: 10.1002/chem.201902100
  • Wang, S. H.; Aster, A.; Mirmohades, M.; Lomoth, R.; Hammarström, L., Structural and Kinetic Studies of Intermediates of a Biomimetic Diiron Proton-Reduction Catalyst. Inorg. Chem., 2018, 57, 768-776.  DOI: 10.1021/acs.inorgchem.7b02687

Photosensitizers

As photosensitizers for charge separation we are investigating iron complexes with N-heterocyclic carbene (NHC) ligands. These ligands are the first to effectively prevent the ultrafast deactivation of excited charge transfer (CT) states via metal centered excited states, a process that previously limited the lifetime of the CT states of iron complexes to the sub-picosecond time scale essentially excluding applications in photochemistry.

Evolution of molecular structures of iron carbene complexes and their excited state lifetimes from tens of picoseconds to several nanoseconds.
Fig. 1. Evolution of molecular structures of iron carbene complexes and their excited state lifetimes from tens of picoseconds to several nanoseconds. Potential energy diagrams illustrate the effect of the improved ligand design on the relative energies and geometries of the photochemically active charge-transfer states and the deleterious metal centered states for the cases of iron(II) and iron(III) complexes.

The NHC ligands enabled improvements in CT state lifetimes over several orders of magnitude for both Fe(II) and Fe(III) complexes, extending now to the nanosecond time scale. In particular, our discovery of long-lived, luminescent ligand-to-metal CT states (2LMCT) of Fe(III)NHC complexes led to a paradigm-changing approach to photoactive iron complexes as an alternative to the widely considered metal-to-ligand CT states (3MLCT).  Notably, it has been possibly to demonstrate first examples of electron transfer reactions of these excited states both on semiconductor surfaces and with molecular electron donors and acceptors.

Current work is focused on the detailed characterization of the excited state reactivity to evaluate the potential of these iron based photosensitizers for replacing complexes of scarce noble metals in applications based on their excited state electron transfer reactions or luminescence properties.

For more information see e.g.:

  • Kaul, N.; Lomoth, R. The Carbene Cannibal: Photoinduced Symmetry-Breaking Charge Separation in an Fe(III) N-Heterocyclic Carbene, J. Am. Chem. Soc. 2021, 143, 29, DOI: 10.1021/jacs.1c03770.
  • Kjær, K. S.; Kaul, N.; Prakash, O.; Chabera, P.; Rosemann, N. W.; Honarfar, A.; Gordivska, O.; Fredin, L. A.; Bergquist, K. E.; Häggström, L.; Ericsson, T.; Lindh, L.; Yartsev, A.; Styring, S.; Huang, P.; Uhlig, J.; Bendix, J.; Strand, D.; Sundström, V.; Persson, P.; Lomoth, R.; Wärnmark, K., Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime. Science, 2019, 363, 249-253. DOI: 10.1126/science.aau7160
  • Chabera, P.; Kjaer, K. S.; Prakash, O.; Honarfar, A.; Liu, Y. Z.; Fredin, L. A.; Harlang, T. C. B.; Lidin, S.; Uhlig, J.; Sundström, V.; Lomoth, R.; Persson, P.; Wärnmark, K., FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime. J. Phys. Chem. Lett. 2018, 9, 459-463. DOI: 10.1021/acs.jpclett.7b02962
  • Chabera, P.; Liu, Y. Z.; Prakash, O.; Thyrhaug, E.; El Nahhas, A.; Honarfar, A.; Essen, S.; Fredin, L. A.; Harlang, T. C. B.; Kjaer, K. S.; Handrup, K.; Ericson, F.; Tatsuno, H.; Morgan, K.; Schnadt, J.; Häggström, L.; Ericsson, T.; Sobkowiak, A.; Lidin, S.; Huang, P.; Styring, S.; Uhlig, J.; Bendix, J.; Lomoth, R.; Sundström, V.; Persson, P.; Wärnmark, K., A low-spin Fe(III) complex with 100-ps ligand-to-metal charge transfer photoluminescence. Nature, 2017, 543, 695-699. DOI: 10.1038/nature21430
Last modified: 2022-02-09
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