DOI: 10.1002/cptc.201900194 Articles 1 2 3 Efficient Cyclization of the Norbornadiene-Quadricyclane 4 5 Interconversion Mediated by a Magnetic 6 7 [Fe3O4À CoSalphen] Nanoparticle Catalyst 8 + [a] + [a] [a] 9 Tobias Luchs , Patrick Lorenz , and Andreas Hirsch* 10 11 12 We report a novel, inexpensive and effective process for the nanoparticles [Fe3O4À CoSalphen] combine a high surface area 13 repeatable photoisomerization of norbornadiene (NBD) to its of catalytically active molecules with straightforward separation 14 metastable isomer quadricyclane (QC), followed by catalytically by the action of an external magnetic field. In combination with 15 induced strain energy release via back-conversion of QC to the promising interconversion couple NBD1-QC1, which fea- 16 NBD. By utilization of a quasi-homogeneous catalyst based on tures outstanding stability (t1/2 =450 days at room temperature) 17 magnetic core-shell nanoparticles, tedious purification steps are and a high energy storage potential (88.34 kJ/mol), the nano- 18 avoided. The core of this material is comprised of Fe3O4 and a particle catalyst [Fe3O4À CoSalphen] shows great potential for 19 catalytically active cobalt(II) complex is anchored on the particle technical applications in molecular solar thermal (MOST) 20 surface as a self-assembled monolayer (SAM). These core-shell energy-storage systems. 21 22 23 1. Introduction by about 89 kJ/mol more stable than QC in case of the parent 24 unfunctionalized, liquid molecule.[7] This together with the low 25 Within one hour, the sun provides more energy to the Earth‘s molecular weight enables high storage capacities. 26 surface than humans consume in a whole year.[1] However, Due to the low quantum yield and lack of absorbance in 27 scooping this enormous potential is not trivial, therefore, highly the visible region of the solar spectrum a triplet photo- 28 efficient technologies are mandatory. Next to common photo- sensitizer, such as acetophenone, is needed to bring about the 29 voltaic devices, molecular solar thermal (MOST) energy storage photoisomerization from neat NBD to QC.[8] This can be 30 and release systems such as organometallic (fulvalene)diruthe- circumvented by designing so called “push-pull” NBD deriva- 31 nium compounds,[2] azobenzenes[3] or dihydroazulene/vinylhepta- tives (Figure 1), they show much higher quantum yields and 32 fulvene couples[4] recently attracted increasing attention.[5] In these provide a better overlap with the solar spectrum. Recently, 33 devices the energy is harvested and transformed into a storable considerable progress has been made in this field.[9] 34 form on a molecular level. Afterwards, the stored energy can be As stated above the main advantage of MOST systems is that 35 released as thermal energy on demand.[2] The big advantage of the stored energy can be released on demand. Therefore, an 36 these systems is that they circumvent the problems associated efficient pathway to isomerize QC back to NBD is needed. 37 with the discontinuous energy production of solar based devices. Although, thermal initiation is possible it is unfavorable due to 38 While the energy produced by common photovoltaics is only energetic reasons. From a practical point of view a catalytically 39 available while the sun is shining, MOST systems are not restricted induced back-reaction is desirable. Various methods have been 40 to this limitation. The valence isomerization of norbornadiene investigated in the past; most approaches employ the catalysis of 41 (NBD) to the metastable and energy rich quadricyclane (QC) unsaturated coordination transition metal complexes,[6] but also [10] 42 (Figure 1) is regarded a very promising concept candidate for this the use of Ag(I) or metal oxides such as MoO3, WO3 or V2O5 has 43 purpose.[6] been reported.[11] In many cases the back-reaction proceeds via an 44 In QC the energy is stored as strain energy due to the oxidized form of QC. It has been shown that this intermediate can 45 introduction of three- and four-membered rings. Overall NBD is also be generated directly via electrochemistry.[12] Only a few of 46 the known catalysts fulfil the crucial requirements such as absence 47 of side reactions, high turnover frequency and long-term stability. [a] T. Luchs,+ P. Lorenz,+ Prof. Dr. A. Hirsch 48 Department of Chemistry and Pharmacy Very promising results were obtained with square planar 49 Friedrich-Alexander-Universität Erlangen-Nürnberg complexes of Co(II).[13–15] For more detailed information we refer to 50 Nikolaus-Fiebiger-Straße 10, 91058 Erlangen (Germany) a review from Chernoivanov and co-workers.[6] A drawback of E-mail: [email protected] 51 homogeneous catalysts is the tedious removal of the catalyst after [+] These authors contributed equally to this work. 52 Supporting information for this article is available on the WWW under the reaction. In most cases a purification step must be 53 https://doi.org/10.1002/cptc.201900194 implemented which hampers cyclability of the energy storage 54 © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. and release process. Recently, it has been shown that it is possible 55 This is an open access article under the terms of the Creative Commons to deposit catalytically active cobalt phthalocyanine on charcoal Attribution Non-Commercial License, which permits use, distribution and 56 reproduction in any medium, provided the original work is properly cited to avoid this problem, but significant leaching of the catalyst was 57 and is not used for commercial purposes. also detected.[16] However, beside the purification aspect, also the ChemPhotoChem 2020, 4, 52–58 52 © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA Wiley VCH Donnerstag, 30.01.2020 2001 - closed* / 145760 [S. 52/58] 1 Articles 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Figure 1. Left: The parent NBD-QC MOST energy storage and release system. The energy is stored as strain energy by the photochemical conversion of NBD to QC and can be released on demand with a suitable catalyst. Right: “Push–pull” substituted NBD derivative. By introducing electron-donating and electron- 16 withdrawing groups into one of the double bonds of NBD, the photochemical properties can be significantly improved. 17 18 À 1 19 activity of the immobilized catalyst has to be considered. Depend- energy storage capacity (ΔHstorage =88.34 kJmol ). NBD1 was 20 ing on the solid support material, the accessibility of the active quantitatively converted to its high energy isomer QC1 upon 21 catalyst sites can be hindered. For a technical application of a irradiation at 310 nm, followed by treatment with the magnetic 22 MOST system, the development of simple, robust, inexpensive core-shell nanoparticles [Fe3O4À CoSalphen]. This led to quanti- 23 and effective methods for each step in the reversible isomer- tative back-conversion to the stable NBD1 derivative. After the 24 ization of the NBD-QC system along with facile purification is catalytic back-conversion the catalyst could easily be removed 25 required. Due to their high surface to volume ratio in comparison by the action of an outside magnet and further cycles of 26 to the bulk material,[17] the tunability of their surface properties[18] photochemical isomerization to QC1 and catalytic back-con- 27 and their facile synthesis, metal oxide nanoparticles are an version to NBD1 could be performed without deterioration of 28 excellent platform for this task. the nanoparticle catalyst or the energy storage material NBD1- 29 The core of these nanoparticles provides intrinsic properties QC1. We further investigated the catalytic potential of [19] [20] 30 such as paramagnetism and photocatalytic activity, while [Fe3O4À CoSalphen] together with a triplet photosensitizer and 31 the interaction with the environment can be modified by the parent NBD to show the promising potential of this 32 depositing self-assembled monolayers (SAMs) of active mole- magnetic core-shell nanoparticle catalyst in MOST applications, 33 cules on the oxide surface. The resulting core-shell materials due to the facile separation by the action of an external 34 combine the intrinsic properties of the core with the tunable magnetic field. 35 properties of the shell. These include dispersibility,[21] 36 biocompatibility[22] or supramolecular aggregation towards 37 defined superstructures.[23] In recent years magnetic core-shell 2. Results and Discussion 38 nanoparticles have attracted a lot of interest as a solid support 39 for a series of applications including water remediation,[24] 2.1. Synthesis and Characterization of a CoSalphen Catalyst [25] 40 biosensing and catalytic processes. In contrast to conven- ([Fe3O4À CoSalphen]) Immobilized on Magnetic Iron Oxide NPs 41 tional solid support materials, magnetic core-shell nanoparticles 42 combine a high surface area with facile separation by For the preparation of [Fe3O4À CoSalphen], commercially available 2 43 application of a magnetic field, thus rendering filtration or Fe3O4 nanoparticles with a specific surface area of 100 m /g 44 centrifugation as separation methods unnecessary. (determined by BET analysis) were treated with the carboxylic acid 45 Herein we report an inexpensive, effective and repeatable bearing cobalt Salphen complex (CoSalphen) in isopropanol 46 process for the light induced photoisomerization of NBD to QC (Figure 2). To ensure a high degree of surface functionalization 47 and the catalytic back-conversion of QC to NBD without the the nanoparticles were stirred with an