Iridium Dihydroxybipyridine Complexes Show That Ligand Deprotonation Dramatically Speeds Rates of Catalytic Water Oxidation

Author

Joseph DePasquale
Ismael Nieto
Lauren E. Reuther
Corey J. Herbst-Gervasoni
Jared J. Paul
Vadym Mochalin, Missouri University of Science and TechnologyFollow
Matthias Zeller
Christine M. Thomas
Anthony W. Addison
Elizabeth T. Papish

Abstract

We report highly active iridium precatalysts, [Cp*Ir(N,N)Cl]Cl (1-4), for water oxidation that are supported by recently designed dihydroxybipyridine (dhbp) ligands. These ligands can readily be deprotonated in situ to alter the electronic properties at the metal; thus, these catalyst precursors have switchable properties that are pH-dependent. The pK_a values in water of the iridium complexes are 4.6(1) and 4.4(2) with (N,N) = 6,6'-dhbp and 4,4'-dhbp, respectively, as measured by UV-vis spectroscopy. For homogeneous water oxidation catalysis, the sacrificial oxidant NaIO₄ was found to be superior (relative to CAN) and allowed for catalysis to occur at higher pH values. With NaIO₄ as the oxidant at pH 5.6, water oxidation occurred most rapidly with (N,N) = 4,4'-dhbp, and activity decreased in the order 4,4'-dhbp (3) > 6,6'-dhbp (2) » 4,4'-dimethoxybipyridine (4) > bipy (1). Furthermore, initial rate studies at pH 3-6 showed that the rate enhancement with dhbp complexes at high pH is due to ligand deprotonation rather than the pH alone accelerating water oxidation. Thus, the protic groups in dhbp improve the catalytic activity by tuning the complexes' electronic properties upon deprotonation. Mechanistic studies show that the rate law is first-order in an iridium precatalyst, and dynamic light scattering studies indicate that catalysis appears to be homogeneous. It appears that a higher pH facilitates oxidation of precatalysts 2 and 3 and their [B(Ar^F)₄]^- salt analogues 5 and 6. Both 2 and 5 were crystallographically characterized.

Recommended Citation

J. DePasquale and I. Nieto and L. E. Reuther and C. J. Herbst-Gervasoni and J. J. Paul and V. Mochalin and M. Zeller and C. M. Thomas and A. W. Addison and E. T. Papish, "Iridium Dihydroxybipyridine Complexes Show That Ligand Deprotonation Dramatically Speeds Rates of Catalytic Water Oxidation," Inorganic Chemistry, vol. 52, no. 16, pp. 9175 - 9183, American Chemical Society (ACS), Jan 2013.

The definitive version is available at https://doi.org/10.1021/ic302448d

Department(s)

Chemistry

International Standard Serial Number (ISSN)

0020-1669

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2013 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Jan 2013