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Norbornadiene-Functionalized Triazatriangulenium and Trioxatriangulenium Platforms

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Norbornadiene-Functionalized Triazatriangulenium and Trioxatriangulenium Platforms Norbornadiene-functionalized triazatriangulenium and trioxatriangulenium platforms Roland Löw1, Talina Rusch2, Tobias Moje1, Fynn Röhricht1, Olaf M. Magnussen*2 and Rainer Herges*1 Full Research Paper Open Access Address: Beilstein J. Org. Chem. 2019, 15, 1815–1821. 1Otto Diels Institute for Organic Chemistry, University of Kiel, doi:10.3762/bjoc.15.175 Otto-Hahn-Platz 4, 24118 Kiel, Germany and 2Institute for Experimental and Applied Physics, University of Kiel, Leibnizstraße Received: 11 April 2019 19, 24098 Kiel, Germany Accepted: 17 July 2019 Published: 30 July 2019 Email: Olaf M. Magnussen* - [email protected]; This article is part of the thematic issue "Molecular switches". Rainer Herges* - [email protected] Guest Editor: W. Szymanski * Corresponding author © 2019 Löw et al.; licensee Beilstein-Institut. Keywords: License and terms: see end of document. [2 + 2] cycloaddition; [2 + 2] cycloreversion; norbornadiene; photochemical isomerization; quadricyclane; self-assembled monolayers; TATA platform; thermal isomerization; TOTA platform Abstract Triazatriangulenium (TATA) and trioxatriangulenium (TOTA) ions are particularly suited systems to mount functional molecules onto atomically flat surfaces such as Au(111). The TATA and TOTA units serve as platforms that absorb onto the surface and form ordered monolayers, while the functional groups are protruding upright and freestanding from the central carbon atoms. Azoben- zene derivatized TATA’s are known to exhibit extremely fast cis→trans isomerization on metal surfaces, via a peculiar non-adia- batic singlet→triplet→singlet mechanism. We now prepared norbornadienes (NBD) and quadricyclanes (QC) attached to TATA and TOTA platforms which can be used to check if these accelerated rates and the spin change mechanism also apply to [2 + 2] cycloreversions (QC→NBD). Introduction Recently, we discovered that the thermochemically forbidden erate the rate of isomerization by three orders of magnitude cis–trans isomerization of azobenzenes can be efficiently cata- [2-4]. To keep the reacting azobenzene molecule at a distance lysed by a very peculiar mechanism on bulk gold [1]. In hetero- of about 14 Å from the surface, it is not directly absorbed onto geneous catalysis, the surface is chemically involved in bond the surface but mounted on a carefully designed molecular making and breaking. In contrast to this conventional surface framework. This approach was named the platform approach catalysis the new mechanism does not involve direct contact [5]. Three molecular units are combined in a modular way to with the surface. Electronic coupling via a conjugated π-system achieve a controlled adsorption on the surface: the platform, a to the conducting band of a bulk metal is sufficient to accel- spacer and the functional molecule. Triazatriangulenium 1815 Beilstein J. Org. Chem. 2019, 15, 1815–1821. (TATA) and trioxatriangulenium (TOTA) units are used as mo- lecular platforms. They adsorb on the surface and form ordered monolayers. A linear, π-conjugated spacer (e.g., an ethynyl unit) is attached to the central carbon atom and the functional molecule is mounted on top. This architecture allows investigat- ing molecules on surfaces under controlled conditions. The size of the platforms determines the intermolecular distances and enforces an upright orientation of the free standing functional groups [6]. The length and the nature of the spacer is used to tune the distance and electronic coupling of the functional system on top [1]. Figure 1: Structures of the norbornadiene platform 1a and the quadri- Preliminary experiments proved that the electronic coupling to cyclane platform 1b (for geometry coordinates and further details see Supporting Information File 1). The norbornadiene isomerizes upon ir- the surface is the decisive parameter controlling the cis–trans radiation with 385 nm to the quadricyclane 1b. Back-isomerization to rate acceleration of azobenzenes and not the length of the spac- the norbornadiene 1a is achieved upon irradiation with 311 nm or ther- mochemically with a half-life of 31 days. er [1]. A full conjugation path from the azobenzene on top through the ethynyl spacer and the platform to a bulk gold sur- face shortens the half-life of the metastable cis-isomer from cyclane 1b decays into the ground state of the norbornadiene 1a days to seconds even though the azo N=N group is 11 bonds [9], which is a precondition for an efficient QC→NBD isomeri- and 14 Å away from the surface. A singlet→triplet→singlet zation via our postulated non-adiabatic singlet→triplet→singlet pathway was suggested to explain the dramatic rate accelera- mechanism. tion. Results and Discussion To obtain further insight into this unusual mechanism and to If our proposed spin change mechanism, mediated by the con- explore the scope and limitation of the general concept, we are ducting electrons in bulk gold were correct, interruption of the aiming at the extension from simple cis→trans isomerization to conjugation path and decoupling from the surface should other thermochemically forbidden reactions such as [2 + 2] restore the properties of the system (half-life of the metastable cycloreversions. Moreover, a deeper understanding of the non- quadricyclane) in solution. According to this line of thought, we adiabatic, catalytic process and successful application of the synthesized molecule 2 with a 2-methylphenyl group inserted concept to the QC→NBD isomerization could contribute to the into the spacer unit (see below Scheme 2). The methyl group elucidation of the mechanisms of bulk metal catalysis and open prevents a planar arrangement of the phenyl group and the new ways to design new catalytic systems. double bond of the NBD unit and thus lowers the conjugation. Towards this end, and following the “platform concept”, we de- We also synthesized the corresponding system directly signed a cyano-substituted norbornadiene, which is functionali- connected to a TOTA platform 3. Functionalized TOTA mole- zed with an acetylene spacer on a TATA platform to investi- cules are more stable than the corresponding TATA systems gate an eventual “spin-catalysed” [2 + 2] cycloreversion on bulk and usually can be sublimed without decomposition, which is a gold of quadricyclane 1b to norbornadiene 1a (Figure 1). necessary precondition for ultra-high vacuum STM investiga- tions. The cycloreversion of most quadricyclane systems proceeds smoothly in solution upon irradiation in the presence of triplet The 3-bromo-2-cyano-substituted norbornadiene 4 was synthe- sensitizers [7]. If 1b is adsorbed on a gold surface the bulk gold sized as described in the literature (Scheme 1) [10-12]. could take the role of a triplet sensitizer, mediate the spin change (which otherwise is forbidden) and accelerate the 4 was converted to 5 with trimethylsilylacetylene (72%) in a cycloreversion. Substitution in the 2 and 3 positions shifts the Sonogashira cross-coupling reaction. The triazatriangulenium bathochromic absorption to 375 nm which is in agreement with ion 6 was synthesized according to a procedure of Laursen and similar systems [8]. Furthermore, the cyano and ethynyl groups Krebs [13]. The platform 6 was functionalized with norborna- provide a complete conjugation path across the double bond of diene 5 by deprotection of the acetylene with potassium hydrox- norbornadiene to the metal. Additionally, it is known that elec- ide and in situ formation of the C–C bond between the acety- tron-withdrawing groups in 2 or 3 position change the triplet lene and the central C atom of the platform 6 to yield the energy hypersurface in such a way that a triplet excited quadri- norbornadiene-substituted platform 1 (62%). 1816 Beilstein J. Org. Chem. 2019, 15, 1815–1821. Scheme 1: Syntheses of the norbornadiene TATA platform 1 and TOTA platform 3. a) TMS-acetylene, Pd(PPh3)4, Cu(I)I, Et3N, toluene, N2, 60 °C, F 1.3 h; b) KOH, THF, N2, reflux, 5 h; c) sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBAr 4), DCM, rt, 2 h; d) n-BuLi, THF, N2, −78 °C to rt, 20 h. The TOTA cation with the tetrakis(3,5-bis(trifluoromethyl)phe- TOTA platform 8 to obtain the norbornadiene-functionalized + F − nyl)borate anion (TOTA [BAr 4] , 8) was obtained by ion TOTA platform 3 (22%, Scheme 1). The synthesis of the corre- + − exchange of the TOTA tetrafluoroborate 7 (TOTA BF4 ) to sponding TATA platform including an additional 2-methyl- achieve a high solubility in organic solvents [14]. 3-Bromo-2- phenyl group (2) was obtained in a convergent synthesis cyanonorbornadiene (4) was subjected to halogen–metal (Scheme 2). Boronic ester 9 was synthesized as described in the exchange with n-BuLi and coupled with the central atom of the literature [15]. In a Suzuki cross-coupling reaction norborna- Scheme 2: Synthesis of methylphenylnorbornadiene platform 2. a) Pd(PPh3)4, Na2CO3, toluene, EtOH, H2O, N2, reflux, 12 h; b) KOH, THF, N2, reflux, 1 h. 1817 Beilstein J. Org. Chem. 2019, 15, 1815–1821. diene 4 was coupled with 9 to the extended norbornadiene 10 1a isomerizes quantitatively to quadricyclane 1b by irradiation (38%), which was attached to the TATA platform 6 to yield the with 385 nm. Upon irradiation of 1b under nitrogen with extended norbornadiene platform 2 (44%). 311 nm, the cycloreversion yields 28% norbornadiene (Table 1). The photophysical properties and the switching behaviour of 1, 2 and 3 were determined in solution (THF). The UV–vis spec- The efficiency of the cycloreversion is higher under air (52%), tra of the norbornadienes (NBD, black, 1a, 2a, 3a) and quadri- however, slow decomposition was observed (cleavage of the cylanes (QC, red, 1b, 2b, 3b) and the spectra of the QCs after TATA+ platform). Obviously, in the presence of oxygen, the irradiation with 311 nm for 1b and 2b or 254 nm for 3b (blue) photochemical cycloreversion proceeds via a triplet radical are shown in Figure 2. The bathochromic absorption maximum mechanism. This agrees with observations described in the liter- of norbornadiene 1a is at 375 nm (as compared to <300 nm in ature [16].
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