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Research Interests:

Non-Innocent Ligands 

Ligands are considered innocent when they allow for the oxidation state of the central metal ion to be determined. A noninnocent ligand is one that by its very nature is redox-active but at a similar potential to the bound metal ion. The result is that the electronic structure (often upon oxidation/reduction) is difficult to described becuase the electron is delocalized over serval atoms. We have had a long standing interest in this phenomenon espeically as it applies to potentially interesting and novel (electro)catalysts.  To this end we use several electrochemical and spectroscopic techniques as well as hybride spectroelectrochemistry to study ligand noninnocence. 


Ferrocenylphosphine Ligands

1,1'-bis(diphenylphosphino)ferrocene ligand and derivatives such as alkyl substituted phosphine, represent a unique class of ligands because they contain a ferrocenyl backbone. This organometallic backbone plays an important roll in providing a flexible backbone.

Our research goals are in part concerned with studying the various ways in which dppf can stablize different coordination geometries, including the κ3 binding mode. We have found that this Fe→M interaction can be used to stablize an unpaired electron 

EPR (left) and UV-Vis-NIR and IR (right) Spectroelectrochemistry


Combining electrochemistry and spectroscopy allows us to study different electronic stuctures in situ. This allows for the study of short lived or quasi-stable states. We are also able to perform low temperature (-50 C) absorption and vibrational spectroscopy, which is especially useful for studying metal-carbonyls.


DFT Calculations showing the breaking of a M-M bond

Electronic Stuctures and Calculations

Density Functional Theory is used quite often by our group to help explain the electronic stuctures of our complexes. Our approach is that calcuations should complement the spectroscopy and vise versa. This is important because it allows us to have a higher level of trust in the calcuations and can help to explain the spectroscopy. 



K. M. Schäfer, L. Reinders, J. Fiedler, M. R. Ringenberg*, Twisting and Tilting 1,1'-Bis(dialkylphosphino)ferrocene with Low Valent Tricarbonylmaganese(I to -I)  Inorg. Chem. 2017, 56, 14688–14696. Link

Zhang, P.; Perfetti, M.; Kern, M.; Hallmen, P. P.; Ungur, L.; Lenz, S.; Ringenberg, M. R.; Frey, W.; Stoll, H.; Rauhut, G.; van Slageren, J., Exchange coupling and single molecule magnetism in redox-active tetraoxolene-bridged dilanthanide complexes. (Collaboration) Chem. Sci. 2018, 9, 1221-1230. Link

S. H. Schlindwein, M. R. Ringenberg*, M. Nieger,  D. Gudat  Synthesis and Spectroscopic Properties of "Charge-Inverted" Bis-benzenedithiolato-Complexes of Copper and Nickel, ZAAC, 2017, 643 1628-1634

M. H. Anjass, K. Kastner, F. Nägele, M. R. Ringenberg, J. F. Boas, J. Zhang, A. M. Bond, T. Jacob, C. Streb Stabilization of Low-Valent Iron(I) in a High-Valent Vanadium(V) Oxide Cluster, (Collaboration) Angew. Chem. Int. Ed. 2017, 56, 14749-14752  Link

M. R. Ringenberg*, F. Wittkamp, U.-P. Apfel, W. Kaim, Redox Induced Configurational Isomerization of Bisphosphine–Tricarbonyliron(I) Complexes and the Difference a Ferrocene Makes Inorg. Chem. 2017, 56, 7501-7511. Link

C. Sondermann, M. R. Ringenberg* Tuning the Overpotential of Electrocatalytically Active Cyclopentadienylnickel Complexes Containing 1,4-Diaza-1,3-butadienes (DAB) for Proton Reduction Dalton Trans. 2017, 46, 5143 - 5146 Link

M. R. Ringenberg*, M. Schwilk, F. Wittkamp, U.-P. Apfel, W. Kaim, Organometallic Fe-Fe Interactions: Beyond Common Metal-Metal Bonds and Inverse Mixed-Valent Charge Transfer. Chem. Eur. J. 2017, 1770-1774 Link

M. Heberle, S. Tschierlei, N. Rockstroh, M. R. Ringenberg, W. Frey, H. Junge, M. Beller, S. Lochbrunner, M. Karnahl, Chem. Eur. J. 2017, 23, 312-319. (Collaboration) Link

M. Ayerbe Garcia, W. Frey, M. R. Ringenberg, M. Schwilk, R. Peters, Chem. Commun. 2015, 51, 16806-16809. (Collaboration) Link


S. G. Keller, M. R. Ringenberg, D. Häussinger, T. R. Ward, “Evaluation of the Formate Dehydrogenase  Activity of Three-Legged Pianostool Complexes in Dilute Aqueous Solution” Eur. J. Inorg. Chem. 2014, 5860–5864. Link

C. Lo, M. R. Ringenberg, D. Gnandt, Y. Wilson, T. R. Ward, “Artificial metalloenzymes for olefin metathesis based on the biotin-(strept)avidin technology”, Chem. Commun., 2011,47, 12065-12067. Link

V.K.K. Praneeth; M. R. Ringenberg, T. R. Ward, “Redox-Active Ligands in Catalysis”, Angew. Chem. Int. Ed.,  2012, 51, 10228-10234. Link


B. Manor, M. R. Ringenberg, T. B. Rauchfuss, “Borane-Protected Cyanides as Surrogates of H-Bonded Cyanides in [FeFe]-Hydrogenase Active Site Models”, Inorg. Chem., 2014, 53, 7241-7247. Link

M. R. Ringenberg, T. B. Rauchfuss, “Protonation-Enhanced Lewis Acidity of Iridium Complexes Containing Noninnocent Amidophenolates”, Eur. J. Inorg. Chem., 2012, 3, 490-495. Link

M. R. Ringenberg, T. R. Ward, “Merging the best of two worlds: artificial metalloenzymes for enantioselective catalysis”, Chem. Commun., 2011,47, 8470-8476. Link

M. R. Ringenberg, D. L. Gray, T.B. Rauchfuss, “Oxidative Addition of a Diphosphine Anhydride to Iron(0) and Nickel(0): A Simple Approach to Installing Four Ligands”, Organometallics, 2011, 30, 2885–2888. Link

M. R. Ringenberg, M. J. Nilges, T. B. Rauchfuss, S. R. Wilson,  “Oxidation of Dihydrogen Using Iridium Complexes of Redox-active Ligands”, Organometallics, 2010, 29, 1956–1965. Link

M. R. Ringenberg, S. L. Kokatam, Z. M. Heiden, T. B. Rauchfuss, “Redox-Switched Oxidation of Dihydrogen Using a Non-Innocent Ligand”, J. Am. Chem. Soc. 2008, 130, 788-789. Link