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Rhenium(I) Bis(η6‐arene) Complexes: Highly Stable and Versatile Scaffolds for Catalysis


Hernández-Valdés, Daniel. Rhenium(I) Bis(η6‐arene) Complexes: Highly Stable and Versatile Scaffolds for Catalysis. 2020, University of Zurich, Faculty of Science.

Abstract

For more than 50 years the chemistry of [Re(η6‐arene)2]+ complexes has remained mostly undeveloped. In the last few years, novel synthetic and analytical methods revealed these complexes to be building blocks of great interest for organometallic chemistry. The structural similarity of Re(I) bis‐arenes to ferrocene forces us to compare the two. Similar to ferrocene, [Re(η6‐arene)2]+ complexes show remarkable stability at high temperatures, in air and under light irradiation. No decomposition is seen under these conditions even in the presence of coordinating solvent. They are extremely difficult to reduce and harder to oxidize than ferrocene. Structural flexibility is also observed from the free rotation around the metal axis and the cationic nature makes them more water soluble than ferrocene.
[Re(η6‐arene)2]+ complexes are usually synthesized via the Fischer‐Hafner reaction starting from K[ReO4] using a reducing agent and AlCl3. The high reactivity of AlCl3 limits the synthesis to some alkylated aromatic hydrocarbons but is generally incompatible with functionalized arenes. Here, we present an extension of the direct syntheses of [Re(η6‐C6H5‐R)2]+ with substituted anilines such as Nmethylaniline, N,N‐dimethylaniline and triphenylamine. Besides, the synthesis of [99(m)Tc(C6H5N(CH3)2)2]+ analog was also achieved as a proof for applicability in technetium chemistry. Unfortunately, direct synthesis of sophisticated bisarene complexes remains restricted. Therefore, further derivatizations are needed in general to introduce active functionalities into the basic [Re(η6‐C6H6)2]+ framework. New precursor complexes such as [Re(η6‐C6H5R)(η6‐C6H6)]+ and [Re(η6‐C6H5R)2]+ (R= Cl, COH, B(OH)2) were introduced and their reactivity was evaluated. Using these new building blocks, additional conjugations of potential ligands to one or both arenes were performed. Several polypyridyl ligands were successfully introduced into the bis‐arene scaffold. Co(II) complexes of several [Re‐tetrapyridyl] and [Re‐terpyridyl] ligand models were synthesized and the catalytic activity towards water reduction was extensively studied. For the case of the [Re‐tetrapyridyl(Co)] complex, its activity compares well with the parent catalyst in turn‐over numbers (TON) and frequencies (TOF).
The synthetic versatility offered by [Re(η6‐arene)2]+ sandwich complexes opens many unexpected possibilities. We have synthesized the first planar chiral ortho‐cycloruthenated rhenium bis‐arene compounds known in literature. These cycloruthenanted compounds were obtained as two diastereomeric pairs of enantiomers that presented spontaneous dimerization in solution. The dimerization phenomenon was intensively investigated by FT‐IR, NMR, kinetics as well as DFT calculations. The presence of two carbonyl ligands in the complexes raises the question about their use in CO2 reduction catalysis. Cyclic voltammetry showed the ability to reduce CO2, but high onset potentials limited the applications in photocatalysis.
Following the idea of using complexes of the type {Ru(Ligand)(CO)2} as potential CO2 reduction catalysts, we introduced one or two catalytic subunits {Ru(dmbpy)(CO)2Cl2} in the [Re(η6‐C6H6)2]+
scaffold. High efficiency and selectivity for the photo‐ and electrocatalytic reduction of CO2 to CO were observed. TONs and TOFs were found to be comparable or higher than for the unconjugated catalyst. Cooperativity effects were observed for the complexes presenting two catalytic subunits. Spectroelectrochemistry experiments and DFT calculations showed that the synergistic communication between the two {Ru(dmbpy)(CO)2} subunits is responsible for the enhancement in catalytic performance. In addition, CPE experiments showed stable catalytic performance over a long period and high faradaic efficiencies.

Abstract

For more than 50 years the chemistry of [Re(η6‐arene)2]+ complexes has remained mostly undeveloped. In the last few years, novel synthetic and analytical methods revealed these complexes to be building blocks of great interest for organometallic chemistry. The structural similarity of Re(I) bis‐arenes to ferrocene forces us to compare the two. Similar to ferrocene, [Re(η6‐arene)2]+ complexes show remarkable stability at high temperatures, in air and under light irradiation. No decomposition is seen under these conditions even in the presence of coordinating solvent. They are extremely difficult to reduce and harder to oxidize than ferrocene. Structural flexibility is also observed from the free rotation around the metal axis and the cationic nature makes them more water soluble than ferrocene.
[Re(η6‐arene)2]+ complexes are usually synthesized via the Fischer‐Hafner reaction starting from K[ReO4] using a reducing agent and AlCl3. The high reactivity of AlCl3 limits the synthesis to some alkylated aromatic hydrocarbons but is generally incompatible with functionalized arenes. Here, we present an extension of the direct syntheses of [Re(η6‐C6H5‐R)2]+ with substituted anilines such as Nmethylaniline, N,N‐dimethylaniline and triphenylamine. Besides, the synthesis of [99(m)Tc(C6H5N(CH3)2)2]+ analog was also achieved as a proof for applicability in technetium chemistry. Unfortunately, direct synthesis of sophisticated bisarene complexes remains restricted. Therefore, further derivatizations are needed in general to introduce active functionalities into the basic [Re(η6‐C6H6)2]+ framework. New precursor complexes such as [Re(η6‐C6H5R)(η6‐C6H6)]+ and [Re(η6‐C6H5R)2]+ (R= Cl, COH, B(OH)2) were introduced and their reactivity was evaluated. Using these new building blocks, additional conjugations of potential ligands to one or both arenes were performed. Several polypyridyl ligands were successfully introduced into the bis‐arene scaffold. Co(II) complexes of several [Re‐tetrapyridyl] and [Re‐terpyridyl] ligand models were synthesized and the catalytic activity towards water reduction was extensively studied. For the case of the [Re‐tetrapyridyl(Co)] complex, its activity compares well with the parent catalyst in turn‐over numbers (TON) and frequencies (TOF).
The synthetic versatility offered by [Re(η6‐arene)2]+ sandwich complexes opens many unexpected possibilities. We have synthesized the first planar chiral ortho‐cycloruthenated rhenium bis‐arene compounds known in literature. These cycloruthenanted compounds were obtained as two diastereomeric pairs of enantiomers that presented spontaneous dimerization in solution. The dimerization phenomenon was intensively investigated by FT‐IR, NMR, kinetics as well as DFT calculations. The presence of two carbonyl ligands in the complexes raises the question about their use in CO2 reduction catalysis. Cyclic voltammetry showed the ability to reduce CO2, but high onset potentials limited the applications in photocatalysis.
Following the idea of using complexes of the type {Ru(Ligand)(CO)2} as potential CO2 reduction catalysts, we introduced one or two catalytic subunits {Ru(dmbpy)(CO)2Cl2} in the [Re(η6‐C6H6)2]+
scaffold. High efficiency and selectivity for the photo‐ and electrocatalytic reduction of CO2 to CO were observed. TONs and TOFs were found to be comparable or higher than for the unconjugated catalyst. Cooperativity effects were observed for the complexes presenting two catalytic subunits. Spectroelectrochemistry experiments and DFT calculations showed that the synergistic communication between the two {Ru(dmbpy)(CO)2} subunits is responsible for the enhancement in catalytic performance. In addition, CPE experiments showed stable catalytic performance over a long period and high faradaic efficiencies.

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Item Type:Dissertation (monographical)
Referees:Alberto Roger, Holland Jason P, Abram Ulrich
Communities & Collections:07 Faculty of Science > Department of Chemistry
UZH Dissertations
Dewey Decimal Classification:540 Chemistry
Language:English
Place of Publication:Zürich
Date:2020
Deposited On:05 Feb 2021 16:40
Last Modified:05 Feb 2021 16:52
OA Status:Closed

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