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Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst

Johnson, Samantha I. and Gray, Harry B. and Blakemore, James D. and Goddard, William A., III (2017) Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst. Inorganic Chemistry, 56 (18). pp. 11375-11386. ISSN 0020-1669. doi:10.1021/acs.inorgchem.7b01698.

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Recent work has shown that Cp*Rh(bpy) [Cp* = pentamethylcyclopentadienyl, bpy = 2,2′- bipyridine] undergoes endo protonation at the [Cp*] ligand in the presence of weak acid (Et_3NH^+; pK_a = 18.8 in MeCN). Upon exposure to stronger acid (e.g., DMFH+; pK_a = 6.1), hydrogen is evolved with unity yield. Here, we study the mechanisms by which this catalyst evolves dihydrogen using density functional theory (M06) with polarizable continuum solvation. The calculations show that the complex can be protonated by weak acid first at the metal center with a barrier of 3.2 kcal/mol; this proton then migrates to the ring to form the detected intermediate, a rhodium(I) compound bearing endo η^4-Cp*H. Stronger acid is required to evolve hydrogen, which calculations show happens via a concerted mechanism. The acid approaches and protonates the metal, while the second proton simultaneously migrates from the ring with a barrier of ∼12 kcal/mol. Under strongly acidic conditions, we find that hydrogen evolution can proceed through a traditional metal–hydride species; protonation of the initial hydride to form an H–H bond occurs before migration of the hydride (in the form of a proton) to the [Cp*] ring (i.e., H–H bond formation is faster than hydride–proton tautomerization). This work demonstrates the role of acid strength in accessing different mechanisms of hydrogen evolution. Calculations also predict that modification of the bpy ligand by a variety of functional groups does not affect the preference for [Cp*] protonation, although the driving force for protonation changes. However, we predict that exchange of bpy for a bidentate phosphine ligand will stabilize a rhodium(III) hydride, reversing the preference for bound [Cp*H] found in all computed bpy derivatives and offering an appealing alternative ligand platform for future experimental and computational mechanistic studies of H_2 evolution.

Item Type:Article
Related URLs:
URLURL TypeDescription Information
Gray, Harry B.0000-0002-7937-7876
Blakemore, James D.0000-0003-4172-7460
Goddard, William A., III0000-0003-0097-5716
Additional Information:© 2017 American Chemical Society. Received: July 18, 2017; Published: September 1, 2017. The authors thank Dr. Jay Winkler, Dr. Robert Nielsen, Dr. Sijia Dong, Dr. Davide Lionetti, Yufeng Huang, and Sydney Corona for numerous helpful discussions. This work was supported by the US National Science Foundation through the CCI Solar Fuels Program (CHE-1305124) and the Resnick Sustainability Institute at Caltech (fellowship to S.I.J.). J.D.B. was supported during preparation of this manuscript by an award from the State of Kansas through the University of Kansas New Faculty General Research Fund. The authors declare no competing financial interest.
Group:Resnick Sustainability Institute
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Resnick Sustainability InstituteUNSPECIFIED
University of KansasUNSPECIFIED
Issue or Number:18
Record Number:CaltechAUTHORS:20170905-141449913
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Official Citation:Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst Samantha I. Johnson, Harry B. Gray, James D. Blakemore, and William A. Goddard, III Inorganic Chemistry 2017 56 (18), 11375-11386 DOI: 10.1021/acs.inorgchem.7b01698
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:81148
Deposited By: Tony Diaz
Deposited On:05 Sep 2017 22:09
Last Modified:15 Nov 2021 19:41

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