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Synthetic strategies to incorporate Ru-terpyridyl water oxidation catalysts into MOFs: direct synthesis vs. post-synthetic approach.

ABSTRACT: Incorporating molecular catalysts into metal-organic frameworks (MOFs) is a promising strategy for improving their catalytic longevity and recyclability. In this article, we investigate and compare synthetic routes for the incorporation of the potent water oxidation catalyst Ru(tda)(pyCO2H)2 (tda = 2,2':6',2''-terpyridine-6,6''-dicarboxylic acid, pyCO2H = iso-nicotinic acid) as a structural linker into a Zr-based UiO-type MOF. The task is challenging with this particular metallo-linker because of the equatorial dangling carboxylates that can potentially compete for Zr-coordination, as well as free rotation of the pyCO2H groups around the HO2CpyRupyCO2H axis. As a consequence, all attempts to synthesize a MOF with the metallo-linker directly under solvothermal conditions led to amorphous materials with the Ru(tda)(pyCO2H)2 linker coordinating to the Zr nodes in ill-defined ways, resulting in multiple waves in the cyclic voltammograms of the solvothermally obtained materials. On the other hand, an indirect post-synthetic approach in which the Ru(tda)(pyCO2H)2 linker is introduced into a preformed edba-MOF (edba = ethyne dibenzoic acid) of UiO topology results in the formation of the desired material. Interestingly, two distinctly different morphologies of the parent edba-MOF have been discovered, and the impact that the morphological difference has on linker incorporation is investigated.

SUBMITTER: Liseev T

PROVIDER: S-EPMC7116355 | biostudies-literature | 2020 Oct

REPOSITORIES: biostudies-literature

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Synthetic strategies to incorporate Ru-terpyridyl water oxidation catalysts into MOFs: direct synthesis <i>vs.</i> post-synthetic approach.

Liseev Timofey T Howe Andrew A Hoque Md Asmaul MA Gimbert-Suriñach Carolina C Llobet Antoni A Ott Sascha S

Dalton transactions (Cambridge, England : 2003) 20201001 39

Incorporating molecular catalysts into metal-organic frameworks (MOFs) is a promising strategy for improving their catalytic longevity and recyclability. In this article, we investigate and compare synthetic routes for the incorporation of the potent water oxidation catalyst Ru(tda)(pyCO2H)2 (tda = 2,2':6',2''-terpyridine-6,6''-dicarboxylic acid, pyCO2H = iso-nicotinic acid) as a structural linker into a Zr-based UiO-type MOF. The task is challenging with this particular metallo-linker because o ...[more]

PMID: 32996947

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Project description:Cobalt polyoxometalates (Co-POMs) have emerged as promising water oxidation catalysts (WOCs), with the added advantage of their molecular nature despite being metal oxide fragments. In comparison with metal oxides, that do not offer well-defined active surfaces, POMs have a controlled, discrete structure that allows for precise correlations between experiment and computational analyses. Thus, beyond highly active WOCs, POMs are also model systems to gain deeper mechanistic understanding on the oxygen evolution reaction (OER). The tetracobalt Weakley sandwich [CoII 4(H2O)2(B-α-PW9O34)2]10- (Co4-WS) has been one of the most extensively studied. We have compared its activity with that of the iron analog [FeIII 4(H2O)2(B-α-PW9O34)2]6- (Fe4-WS) looking for the electronic effects determining their activity. Furthermore, the effect of POM nuclearity was also investigated by comparison with the iron- and cobalt-monosubstituted Keggin clusters. Electrocatalytic experiments employing solid state electrodes containing the POMs and the corresponding computational calculations demonstrate that CoII-POMs display better WOC activity than the FeIII derivatives. Moreover, the activity of POMs is less influenced by their nuclearity, thus Weakley sandwich moieties show slightly improved WOC characteristics than Keggin clusters. In good agreement with the experimental data, computational methods, including pK a values, confirm that the resting state for Fe-POMs in neutral media corresponds to the S1 (FeIII-OH) species. Overall, the proposed reaction mechanism for Fe4-WS is analogous to that found for Co4-WS, despite their electronic differences. The potential limiting step is a proton-coupled electron transfer event yielding the active S2 (FeIV[double bond, length as m-dash]O) species, which receives a water nucleophilic attack to form the O-O bond. The latter has activation energies slightly higher than those computed for the Co-POMs, in good agreement with experimental observations. These results provide new insights for the accurate understanding of the structure-reactivity relationships of polyoxometalates in particular, and or metal oxides in general, which are of utmost importance for the development of new bottom-up synthetic approaches to design efficient, robust and non-expensive earth-abundant water oxidation catalysts.

Dataset Information

Synthetic strategies to incorporate Ru-terpyridyl water oxidation catalysts into MOFs: direct synthesis vs. post-synthetic approach.

Publications

Synthetic strategies to incorporate Ru-terpyridyl water oxidation catalysts into MOFs: direct synthesis <i>vs.</i> post-synthetic approach.

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