Platinum(II) Terpyridine Complex That Switches Its Photochemical

Oct 24, 2018 - Platinum(II) Terpyridine Complex That Switches Its Photochemical Reactivity in Response to Its Chromic Behavior in the Crystalline Stat...
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Platinum(II) Terpyridine Complex That Switches Its Photochemical Reactivity in Response to Its Chromic Behavior in the Crystalline State Kentaro Tashiro,*,† Hiroyoshi Ohtsu,‡ Masaki Kawano,‡ Tatsuhiro Kojima,§ and Tatsuhisa Kato|| †

Inorg. Chem. Downloaded from pubs.acs.org by REGIS UNIV on 10/24/18. For personal use only.

International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan § Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan || Institute for Liberal Arts and Sciences, Kyoto University, Yoshida-Nihonmatsu, Sakyo-ku, Kyoto 606-8501, Japan S Supporting Information *

ABSTRACT: A platinum(II) terpyridine complex having an enantiopure lactate anion afforded hydrated crystals Lor D-1hyd containing infinite chains of interacting Pt centers, while their dehydration induced crystal-to-crystal transformation into L- or D-1dehyd, respectively, exhibiting less significant Pt−Pt and/or ligand−ligand interactions. That transformation was accompanied by changes in the color as well as the photochemical reactivity of the crystals, where L-1dehyd showed higher reactivity than L1hyd in the presence of amines under visible-light irradiation.

Figure 1. (a) Molecular structures of platinum(II) terpyridine complexes L- and D-1 and (b) perspective view (40% thermal ellipsoids) of the molecular packing of L-1 with water molecules in its single crystal (L-1hyd). H atoms are omitted for clarity.

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rystalline assemblies of d8 and d10 metal complexes have attracted special attention because of their unique chromic behavior in response to alternation of their crystal-packing structures.1−5 Because they can visualize external environmental changes that cause alternation of the neighboring metal−metal and/or ligand−ligand orientations in their crystal lattices, their applications as sensing materials have been extensively explored.6−10 In sharp contrast, little has been experimentally surveyed on the relationship between the chromic behavior and chemical reactivity so far, while chromic phenomena in general, inevitably involving shifts of the highest occupied and/or lowest unoccupied molecular orbital levels, could significantly change the chemical reactivity of the corresponding materials. Here we report that a platinum(II) complex switches its activity toward its photochemical reduction in response to the hydration− dehydration-mediated chromic behavior of its crystals. To the best of our knowledge, this is the first report of different chemical reactivity on a group 10 or 11 metal complex tuned by its chromic behavior in the solid state. Being initially motivated by an idea of colorimetric chiral sensing by means of vapochromic responses of asymmetric crystals,11 we prepared the chiral crystal 1hyd. Instead of using chiral platinum(II) terpyridines,12−15 we made the crystal chiral through anion exchange of the achiral chloro(2,2′:6′,2″terpyridine)platinum(II) chloride with either an enantiopure L- or D-lactate anion (L- or D-1, respectively, in Figure 1a), followed by recrystallization from water/acetone. In the mother liquor, the crystals showed reddish orange color, while during © XXXX American Chemical Society

washing with acetone, they turned darkish yellow, which, however, went back to the starting color upon exposure of the crystals to air. X-ray crystallography on a single crystal obtained with an L-lactate anion revealed that the reddish-orange crystal contained three molecules of water with respect to one platinum(II) complex (L-1hyd; CCDC 1518419; Figure 1b). Those water molecules together with lactate CO2− moieties formed ladder-shaped hydrogen-bonding networks between columnarly stacked chloro(2,2′:6′,2″-terpyridine)platinum(II) complexes in the crystal. The Pt centers in a single column were equally spaced with a uniform Pt−Pt distance of 3.361(3) Å to afford an infinite pseudolinear chain of interacting metal centers with a Pt−Pt−Pt angle of 166.76(4)° along with the b axis, while the neighboring terpyridine complexes in each column adopted a head-to-tail orientation. We also confirmed that the crystal structure obtained with a D-lactate anion (D-1hyd; CCDC 1518420; Figure S1) was a mirror image of the structure with an L-lactate anion. While 1hyd showed vapochromic responses upon exposure to the vapors of several chemicals, we later noticed that the dehydration of 1hyd induced by these vapors played the main role for its chromic behavior.3,10,16 Thermogravimetry on L-1hyd Received: August 27, 2018

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DOI: 10.1021/acs.inorgchem.8b02400 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry revealed that heating of the crystals over 80 °C under a stream of dry air led to the loss of all of the water molecules from the crystal (Figure S2). Variable-temperature powder X-ray diffraction (PXRD) analysis on the crystals of L-1 demonstrated that the dehydration of L-1hyd was accompanied by a crystal-tocrystal transformation into another phase (L-1dehyd; Figure 2d).17 When dipped in acetone, the crystals also adopted the

different color of 1hyd and 1dehyd in mind, dehydrogenation of Hantzsch 1,4-dihydropyridine 2 (Figure 3), known to be

Figure 3. Proposed reaction mechanism of 1 and 2 upon visible-light photoirradiation and the appearance of their mixtures in (a and b) dehydrated acetone and (c and d) acetone/water (99:1 by volume) (a and c) before and (b and d) after photoirradiation for 1 h.

Figure 2. PXRD data of solid L-1: (a) allowed to stand at 300 K; (b) soaked in acetone/water (9:1 by volume) at 293 K; (c) soaked in acetone at 293 K; (d) heated at 383 K and then cooled to 100 K. Inset: Appearance of L-1 under the corresponding conditions.

photocatalyzed by transition-metal complexes under anaerobic conditions,22,23 was chosen as the first test reaction to compare the responses of 1hyd and 1dehyd upon photoirradiation. When a mixture of 2 (50 μmol) and suspended L-1dehyd (5 μmol) in degassed dehydrated acetone (3 mL) (Figure 3a) was irradiated with visible light (≥420 nm) under argon, the suspended crystals exhibited a marked color change within 10 min and turned to dark violet in 20 min (Figure S7). With continuous photoirradiation, the solution was also gradually colorized to a similar color, which reached a plateau after 50 min (Figures 3b and S7). The resulting suspension showed broad absorption bands centered at 544 and 781 nm (Figure S13), assignable to the absorptions of one-electron-reduced L-1 (L-1−) by referencing the spectral features of an electrochemically reduced chloro(2,2′:6′,2″-terpyridine)platinum(II) complex.24,25 Electronspin resonance (ESR) spectroscopy on the photoirradiated mixture also confirmed the production of L-1− (Figure S15a).24 The appearance of the reaction mixture was intact after termination of photoirradiation, but upon exposure of the mixture to air, its violet color vanished promptly, leaving a colorless solution with a darkish-yellow solid. 1H NMR spectroscopy on the supernatant of the resulting mixture in acetone-d6, after exposure to air, revealed that it is a mixture of 2 and its dehydrogenated product 3 (Figures 1 and S16a), while the precipitate dissolved in D2O showed only signals assignable to the platinum(II) terpyridine complex 1 (Figure S18b). From the integral ratio of the ethyl CH2 signals of 2 (δ = 4.10) and 3 (δ = 4.37), conversion of the dehydrogenation reaction of 2 after

dehydrated phase, as proven by its PXRD pattern (Figure 2c). Upon the addition of water (≥1% in volume) to the media, the dehydrated crystals quickly recovered their reddish orange color by returning to the hydrated form (Figure 2b), demonstrating that the dehydration/hydration-mediated crystal-to-crystal transformation between 1hyd and 1dehyd is reversible. Photoexcitation of L-1hyd under air or dipped in wet acetone afforded red emission (λmax = 687 nm; Figure S3a, red or green, respectively). Given that the packing structure of L-1 in the corresponding crystals allowed the metal−metal interaction between neighboring Pt centers (Figure 1b), the emission was assignable to that originating from the metal−metal-to-ligand charge-transfer (MMLCT) excited state.5,10,18 On the other hand, L-1dehyd dipped in acetone exhibited orange emission centered at 621 nm (Figure S3a, blue). The blue shifts observed in both of the emission and corresponding excitation profiles of L-1dehyd with respect to those of L-1hyd (Figure S3) indicated that the Pt−Pt and/or ligand−ligand interactions became less significant upon transformation from 1hyd to 1dehyd.10,18 A comparison of the electrochemical oxidation profiles of L-1hyd and L-1dehyd revealed that the energy level of the highestoccupied crystal orbital (HOCO) of 1dehyd was lower than that of the HOCO of 1hyd by more than 0.2 eV (Figure S5), again suggesting the smaller degree of Pt−Pt interaction in 1dehyd.5,10 Metal−metal interactions of similar platinum complexes were reported to effect their photocatalytic activity in solution-phase hydrogen production reactions.19−21 With also the obviously B

DOI: 10.1021/acs.inorgchem.8b02400 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

observed for 1 with DMAE, suggested that the catalytic activity of vapochromic crystals might also be controlled by gaseous substrates (vapochromic catalysts). Considering the possibility of such a substrate-triggered activation of the catalysts, photocatalytic applications of the crystalline forms of 1 and related compounds are currently ongoing.

photoirradiation for 1 h was evaluated to be 12% (=1.2 equiv with respect to L-1). By considering the background conversion (3%) from 2 to 3 in the absence of 1, it is most likely that the photochemical reduction of 1 occurred by conversion of a stoichiometric amount of 2 to 3. Of interest, a mixture of 2 (50 μmol) and suspended L-1hyd (5 μmol) in degassed acetone/ water (3 mL, 99:1 by volume; Figure 3c) was less photoresponsive (Figures 3d and S7) than the mixture of 2 and L1dehyd, where the former displayed conversion from 2 to 3 in 6% (=0.6 equiv with respect to L-1) after photoirradiation for 1 h (Figure S16b).26 Water did not retard the same reaction when a homogeneous mixture of L-1 and 2, dissolved in dimethyl-d6 sulfoxide, was subjected to photoirradiation (Figure S17). The primary process of the photocatalytic dehydrogenation of 2 to 3 was initially proposed to be H-atom abstraction by the photoexcited (terpyridine)platinum(II) complex,22 which, however, was later questioned by possible photoinduced electron transfer from 2 to the photoexcited platinum(II) complex, as suggested from transient absorption spectrometry.27,28 In order to gain further mechanistic insight into the observed photochemical reduction of 1, we replaced amine 2 with triethylamine (TEA), which serves as an electron donor but not as a H-atom donor.27 Photoirradiation on a mixture of TEA (50 μmol) and suspended L-1dehyd (5 μmol) in degassed dehydrated acetone (3 mL) under argon for 1.5 h caused the gradual color change of the suspended crystals to reddish purple (Figure S9), which was again due to the photoreduction of L-1, as confirmed by ESR spectroscopy (Figure S15b). This also suggested that the reduction of 1 occurred via photoinduced electron transfer from amines to photoexcited 1 (Figure 3), where TEA reacted less rapidly than 2, as is the case of the similar platinum(II) terpyridine complex reported.27 In sharp contrast to L-1dehyd, L-1hyd with TEA showed no detectable color change in the same time scale (Figure S10). The stronger photooxidizing capability of 1dehyd than 1hyd, as observed with amines, was rationalized by the lower HOCO level of the former than of the latter (Figure S5) as a result of the weaker Pt−Pt and/or ligand−ligand interactions in 1dehyd.5,10 As mentioned above and as is often the case for these types of metal complexes,29,30 the crystals of 1 exhibited vapochromic behavior. With the observed different photochemical reactivities of crystals L-1hyd and L-1dehyd as the forms of suspensions in mind, we further compared the reactivities of these crystals under amine vapor. Screening of volatile amines revealed that the vapor of highly water-miscible 2-(dimethylamino)ethanol (DMAE) effectively induced the vapochromic response of L-1hyd to afford another crystal (Figure S11b), which was assigned as L1dehyd from its emission profiles (Figure S4). On the other hand, if DMAE was wet with water (10% by volume), no color change of L-1hyd was visually detected (Figure S12b). Photoirradiation of L-1dehyd under amine vapor in anaerobic conditions caused the solid to turn dark green (Figure S11c), which turned back to red upon exposure to air (Figure S11d), suggesting the photoconversion of L-1dehyd into an oxygen-sensitive form of the complex. In contrast, no color change was observed upon the photoirradiation of L-1hyd under wet amine vapor in anaerobic conditions (Figure S12c). These results demonstrated that the vapochromic behavior of the crystalline assemblies of 1 could be coupled with the activation/deactivation of 1. In summary, we reported the switching phenomena in the photochemical reactivity of platinum(II) complex 1 coupled with the chromic behavior of its crystals. The chemical reactivity of metal complexes tuned via their vapochromic behavior, as

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02400. Synthesis, characterization, and photochemical reactions of 1 (PDF) Accession Codes

CCDC 1518419−1518420 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-29-860-4879. Fax: +81-29-860-4706. ORCID

Kentaro Tashiro: 0000-0001-7424-0830 Masaki Kawano: 0000-0001-9886-4226 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synchrotron radiation experiments were performed at NW2A of the Photon Factory-Advanced Ring, KEK, with its approval (Grant 2014G048) or at the BL02B2 beamlines of SPring-8, JASRI, with its approval (Grant 2016A1073).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b02400 Inorg. Chem. XXXX, XXX, XXX−XXX