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Title: Mononuclear ruthenium polypyridine complexes that catalyze water oxidation

Abstract

Over the past decade, significant advances have been made in the development of molecular water oxidation catalysts (WOCs) in the context of developing a system that would accomplish artificial photosynthesis. Mononuclear ruthenium complexes with polypyridine ligands have drawn considerable attention in this regard, due to their high catalytic activity and relatively simple structure. In this perspective review, we will discuss mononuclear Ru polypyridine WOCs by organizing them into four groups according to their ligand environments. Each group will be discussed with regard to three fundamental questions: first, how does the catalyst initiate O–O bond formation? Second, which step in the catalytic cycle is rate-determining? Third, how efficient is the catalyst according to the specific descriptors such as turnover frequency? All discussion is based on the high-valent ruthenium intermediates that are proposed in the catalytic cycle according to experimental observation and theoretical simulation. Two fundamental mechanisms are set forth. An acid–base mechanism that involves the attack of a water molecule on the oxo of a high valent Ru=O species to form the O–O bond. Subsequent steps lead to dissociation of O 2 and rehydration of the metal center. A second mechanism involves the formation of a Ru–O˙ radical species, two ofmore » which then couple to form a Ru–O–O–Ru species that can release O 2 afterwards. The acid–base mechanism appears to be more common and mechanistic differences could result from variation directly related to polypyridine ligand structures. Thus, understanding how electronic, steric, and conformational properties can effect catalyst performance will lead to the rational design of more effective WOCs with not only ruthenium but also other transition metals.« less

Authors:
 [1];  [2]
  1. Univ. of Houston, Houston, TX (United States). Dept. of Chemistry; Friedrich-Alexander Univ. of Erlangen–Nuremberg (FAU), Erlangen (Germany). Dept. of Chemistry and Pharmacy and Inorganic Chemistry
  2. Univ. of Houston, Houston, TX (United States). Dept. of Chemistry
Publication Date:
Research Org.:
Univ. of Houston, Houston, TX (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); The Welch Foundation, Houston, TX (United States)
OSTI Identifier:
1434583
Grant/Contract Number:
FG02-07ER15888; E-621
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Chemical Science
Additional Journal Information:
Journal Volume: 7; Journal Issue: 11; Journal ID: ISSN 2041-6520
Publisher:
Royal Society of Chemistry
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 14 SOLAR ENERGY

Citation Formats

Tong, Lianpeng, and Thummel, Randolph P. Mononuclear ruthenium polypyridine complexes that catalyze water oxidation. United States: N. p., 2016. Web. doi:10.1039/c6sc02766k.
Tong, Lianpeng, & Thummel, Randolph P. Mononuclear ruthenium polypyridine complexes that catalyze water oxidation. United States. doi:10.1039/c6sc02766k.
Tong, Lianpeng, and Thummel, Randolph P. Fri . "Mononuclear ruthenium polypyridine complexes that catalyze water oxidation". United States. doi:10.1039/c6sc02766k. https://www.osti.gov/servlets/purl/1434583.
@article{osti_1434583,
title = {Mononuclear ruthenium polypyridine complexes that catalyze water oxidation},
author = {Tong, Lianpeng and Thummel, Randolph P.},
abstractNote = {Over the past decade, significant advances have been made in the development of molecular water oxidation catalysts (WOCs) in the context of developing a system that would accomplish artificial photosynthesis. Mononuclear ruthenium complexes with polypyridine ligands have drawn considerable attention in this regard, due to their high catalytic activity and relatively simple structure. In this perspective review, we will discuss mononuclear Ru polypyridine WOCs by organizing them into four groups according to their ligand environments. Each group will be discussed with regard to three fundamental questions: first, how does the catalyst initiate O–O bond formation? Second, which step in the catalytic cycle is rate-determining? Third, how efficient is the catalyst according to the specific descriptors such as turnover frequency? All discussion is based on the high-valent ruthenium intermediates that are proposed in the catalytic cycle according to experimental observation and theoretical simulation. Two fundamental mechanisms are set forth. An acid–base mechanism that involves the attack of a water molecule on the oxo of a high valent Ru=O species to form the O–O bond. Subsequent steps lead to dissociation of O2 and rehydration of the metal center. A second mechanism involves the formation of a Ru–O˙ radical species, two of which then couple to form a Ru–O–O–Ru species that can release O2 afterwards. The acid–base mechanism appears to be more common and mechanistic differences could result from variation directly related to polypyridine ligand structures. Thus, understanding how electronic, steric, and conformational properties can effect catalyst performance will lead to the rational design of more effective WOCs with not only ruthenium but also other transition metals.},
doi = {10.1039/c6sc02766k},
journal = {Chemical Science},
number = 11,
volume = 7,
place = {United States},
year = {Fri Aug 05 00:00:00 EDT 2016},
month = {Fri Aug 05 00:00:00 EDT 2016}
}

Journal Article:
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  • Bis(diimine) complexes of ruthenium(II) have been prepared. The chelates used are 6,6'-dimethyl-2,2'-bipyridine (6,6'-dmbp) or 2,9-dimethyl-1,10-phenanthroline (2,9-dmp). Due to the steric hindrance created by the methyl groups ..cap alpha.. to the nitrogen atoms, the bis chelate complexes synthesized are all cis, with respect to the two remaining coordination sites, and cannot be photoisomerized to their trans isomers, in contrast with the equivalent complexes containing unsubstituted diimines. In addition, condensation of the mononulcear species to hydroxo- or oxo-bridged species of higher nuclearity is strictly prevented. The complexes have been characterized and studied by spectroscopic methods (UV-visible, IR, and /sup 1/H and /supmore » 13/C NMR). Their electrochemical behavior has been investigated in relation to the oxidation of water to molecular oxygen. The most significant results are the following: dinuclear species like (bpy)/sub 2/(H/sub 2/O)RuORu(H/sub 2/O)(bpy)/sub 2//sup 4 +/ are required for catalytic generation of O/sub 2/ from water, in agreement with previously reported data. On the other hand, the complexes presently reported display no activity toward oxidation of water. 40 references, 4 figures, 6 tables.« less
  • A potential new route to production of hydrogen is being probed at the University of North Carolina, Chapel Hill, where chemists are tinkering with organic ruthenium complexes to catalyze water splitting reactions driven by sunlight. They claim the process is the first direct solar cleavage of water that does not depend on either photovoltaic mechanisms or microorganisms. The process works by photoinduced electron transfer.
  • The spectroscopic and electrochemical properties of a series of polynuclear complexes containing cyano-bridged ruthenium polypyridine units are consistent with a valence-localized model. In all the Ru(II) forms, the lowest metal-to-ligand charge-transfer (MLCT) states are localized on N-bonded moieties. The photophysical properties at 298 and 77 K indicate that efficient intercomponent energy-transfer processes from C-bonded to N-bonded chromophoric units take place. The mixed-valence forms of these complexes show low-energy absorption bands which are assigned to metal-to-metal charge-transfer transitions involving C-bonded Ru(II) and N-bonded Ru(III) centers. The resonance Raman spectrum of [NC-Ru[sup II](bpy)[sub 2]-CN-Ru[sup III](bpy)[sub 2]-CN][sup 2+] under near-resonance conditions with themore » Ru(II) [yields] Ru(III) charge-transfer band shows enhancement of the bridging cyanide stretching as expected for this type of electronic transition. In the infrared spectra the number of cyanide stretching bands supports the valence-localized model. For the representative [NC-Ru[sup II](bpy)[sub 2]-CN-Ru[sup III](bpy)[sub 2]-CN][sup 2+] complex, three CN stretches (one bridging, two terminal) are observed. Time-resolved infrared measurements for the MLCT excited state of [NC-Ru[sup II](bpy)[sub 2]-CN-Ru[sup II](bpy)[sub 2]-CN][sup +] are reported. The excited-state IR spectrum shows features similar to those of the chemically prepared mixed-valence dimer, [NC-Ru[sup II](bpy)[sub 2]-CN-Ru[sup III](bpy)[sub 2]-CN][sup 2+], strongly suggesting that valence delocalization is not significant in the excited state. The electronic factors affecting the frequency of the bridging cyanide are analyzed by examining the behavior of the mixed-valence ions [M-NC-M[prime](bpy)[sub 2]-CN-M][sup 6+/4+] and [NC-M[prime](bpy)[sub 2]-CN-M][sup 3+/2+] (M = [Ru(NH[sub 3])[sub 5]][sup 2+/3+]; M[prime] = Ru[sup II], Os[sup II], Re[sup I]).« less
  • From analysis of three methods for synthesis of symmetrical dinuclear ruthenium (II) complexes [(LL){sub 2}XRu(BL)RuX(LL){sub 2}](BF{sub 4}{sub 2} (LL=2,2{prime}-bipyridyl, 1,10-phenanthroline; X=Cl, NO{sub 2}) with bridging ligands [BL=pyrazine (pyz), 4,4{prime}-bpy), and trans-1,2-bis (diphenylphosphino) ethylene (dppv)] the optimal way of obtaining these compounds was established, which involves destruction of the nitroso group coordinated in the mononuclear fragments cis-[Ru(LL){sub 2}(NO)X]{sup 2+} by azide ion, followed by introduction of a bridging ligand into a labile position of [Ru(LL){sub 2}(Solv)X]{sup +} under mild conditions. Dinuclear complexes are stable in acetonitrile and methanol solutions in darkness. The reactivity of these compounds in the dark is limitedmore » to replacement of the Cl ligands at elevated temperatures. Under the action of visible light photodissociation of the dimers with BL=pyz and 4,4{prime}-bpy into the fragments [Ru(LL){sub 2}(BL)X]{sup +} and [Ru(LL){sub 2}(Solv)X]{sup +} occurs, whereas the compounds with BL = dppv are photostable. Irradiation of mononuclear [Ru(LL);{sub 2}(BL)X]+ complexes results in photosolvation of the terminal ligands BL = pyz and 4,4{prime}-bpy. The quantum yields of these processes in acetone and methanol were measured. The nature of the bridging ligand is the main factor that determines the photochemical activity of the dimers. 27 refs., 2 figs., 3 tabs.« less
  • Application of time-resolved infrared spectroscopy has had an important impact on transition metal photochemistry. The emphasis has been on metal carbonyl and metal cyano complexes because the oscillator strengths of {anti v}(CO) and {anti v}(CN) are high, and tunable lasers are available in the relevant spectral region. Until recently, time-resolved infrared spectroscopy using Fourier transform interferometry has been limited to a time resolution of {ge}5 {mu}s. However, application of step-scan FT-IR has greatly expanded the time window. It is now possible to acquire spectra with high resolution and sensitivity on a time scale of tens of nanoseconds over the entiremore » mid-IR region. In this communication, the authors report the application of step-scan FT-IR absorbance difference time-resolved spectroscopy ({sup 2}FT-IR {Delta}A TRS) with spectra acquired on the 10 ns time scale to the study of electronic structure in the metal-to-ligand charge transfer (MLCT) excited states of two related complexes of ruthenium(II) containing only the ligands 2,2{prime}-bipyridine (bpy), 4-(carboxyethyl)-4{prime}-methyl-2,2{prime}-bipyridine (4-COOEt-4{prime}-CH{sub 3}bpy) and 4,4{prime}-(dicarboxyethyl)-2,2{prime}-bipyridine (4,4{prime}-(COOEt){sub 2}bpy). Comparison of the relative vibrational energies of the MLCT states leads to specific and significant conclusions regarding the distribution of electron density in these states.« less