Reversible Photolysis of Nitrosobenzene cis-Dimer Monitored In Situ

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Reversible Photolysis of Nitrosobenzene cis-Dimer Monitored In Situ by Single Crystal Photocrystallography Philip P. Rodenbough, Durga Prasad Karothu, Tamara Gjorgjieva, Patrick Commins, Hideyuki Hara, and Pance Naumov Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00031 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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Crystal Growth & Design

Reversible Photolysis of Nitrosobenzene cis-Dimer Monitored In Situ by Single Crystal Photocrystallography Philip P. Rodenbough,a Durga Prasad Karothu,a Tamara Gjorgjieva,a Patrick Commins,a Hideyuki Hara,b and Panče Naumov*, a a

New York University Abu Dhabi, PO Box 129 188, Abu Dhabi, United Arab Emirates Bruker Biospin K. K., 3-9, Moriya, Kanagawa, Yokohama, Kanagawa 221-0022, Japan

b

In memory of Philip Coppens, the pioneer of chemical photocrystallography

Supporting Information: PDF, MOV, CIF ABSTRACT: A single crystal of the cis-dimer of nitrosobenzene was directly observed by photocrystallography to transition to a pair of monomers and reversibly re-dimerize. The remarkable displacement of the nitrogen atoms within the crystal—moving a total distance of 2.97(5) Å for the two atoms—suggests that the breadth of solid-state photochemical reaction systems susceptible to X-ray diffraction studies need not be limited to those with very small atomic displacements. Photocrystallography (X-ray photodiffraction),1–3 is an analytical branch of solid-state photochemistry4 in which light-induced changes are observed in crystals or powders with X-ray diffraction. In the steady-state version of the method, crystals are typically analyzed by single crystal X-ray diffraction (SC XRD) at low temperatures, exposed to light of appropriate energy, and analyzed again to observe the changes induced. The method, pioneered by the late Philip Coppens5–8 and others, has produced significant results in the past. Prominent examples include (but are not limited to) observation of nitrosyl,9,10 nitro/nitrito,11 sulfur dioxide12 and other13 linkage isomers, photochromic reactions,14 triplet carbene,15 as well as various other reaction pathways16 and unstable species.17,18 Although the requirement that the physical transformation or chemical reaction occurs up to sufficient yield with retention of crystal integrity (single-crystal-to-single-crystal transformation19) can be restrictive to the processes that can be analyzed, provided this obstacle is overcome, photocrystallography offers an unparalleled opportunity to observe the progression of certain chemical reactions within a crystal with atomic-scale resolution. In the results described herein, we report the reversible photolysis of nitrosobenzene cis-dimer as directly observed by photocrystallography (Scheme 1). Many aromatic nitroso compounds are known to dimerize in the solid state,20,21 and their reversible photolysis to a pair of monomers has been previously established by infrared spectroscopy.22 The photolysis of a trans-dimer to a pair of monomers of parabromonitrosobenzene was previously studied by photocrystallography,23 however, the atomic displacements were modest and the reverse reaction was not directly observed in a single crystal. In this study, a crystal of the more thermodynamically stable cis isomer of nitrosobenzene was obtained,20 and both the monomerization and re-dimerization reactions were monitored within a single crystal for the first time. The crystal was enriched with the monomer by UV irradiation at low temperature and the monomeric state was directly observed. The monomer was re-converted back to the dimer thermally, by warming at room temperature, and no trace of the monomer was found.

Scheme 1. (a) General structure for the nitroso compounds considered here, shown in the monomeric state. (b) cis-dimer state. (c) trans-dimer state. (d) The photocrystallographically observed photolysis and re-dimerization of the cis-dimer of nitrosobenzene 1.

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A series of the nitrosobenzenes 1‒4 in Scheme 1 was procured or prepared by standard literature methods20,24 and they were screened extensively for polymorphs by slow evaporation from a variety of solvents and solvent mixtures (for details, see Tables S1‒S3 in the Supporting Information, SI). Despite the rigorous screening of the various nitrosobenzenes, only samples matching the previously reported unit cell values25,26 were found. Since the unsubstituted nitrosobenzene 1 gave the best crystals in the cis isomer, it was selected for further photocrystallographic study.

Figure 1. VT FTIR studies of nitrosobenzene (1) powder in KBr pellet. (a) Changes in the IR spectrum with increasing time of exposure to UV irradiation showing progressive transformation to the monomeric state. The figure shows the spectrum of a single sample after successive treatments with UV irradiation. UV power was increased gradually over the course of the experiment and exposure time was typically about 2 minutes per treatment. (b) Warming of nitrosobenzene recovers the dimeric state in a reversible fashion. For this experiment, the sample was cooled to 80 K, the spectrum was recorded, and the sample was exposed to UV irradiation until no further changes were observed in the spectrum. Then the sample was warmed to 240 K, and the cycle was repeated. The spectra show photolysis of the dimer under UV irradiation at 80 K and complete recovery of the dimer upon warming to 240 K.

Figure 2. VT optical microscopy of a single crystal of nitrosobenzene (1). (a) A crystal before irradiation at 80 K. (b) The crystal after brief (approximately 30 s at low power) UV irradiation at 80 K. (c) The irradiated crystal after warming to 240 K, where partial recovery of crystal integrity is apparent. Scale bar is 250 µm.

The crystals of the cis-dimer of 1 were first studied by variable temperature (VT) IR spectroscopy (Figure 1). Consistent with previous reports,22 large changes in the IR spectrum (KBr pellet) occurred under UV irradiation at 80 K, indicating photolysis (monomerization) from the dimeric state (SI Figure S1). Warming the sample above 215 K resulted in complete recovery of the dimeric state (SI Figure S2), and successive cycles between the photoinduced monomer state and the thermally recovered dimeric state were demonstrated (Figure 1b). To understand these changes in a single crystal state, single crystals were observed by VT optical microscopy (Figure 2, SI Movie S1). UV irradiation under ambient conditions resulted in rapid crystal sublimation and/or possibly decomposition). When exposed to UV irradiation at 80 K, the crystals appeared to significantly deteriorate in quality. Upon warming, partial recovery of the crystallinity was observed. These observations indicate that the macroscopic integrity of the crystal was partially recovered by warming. A single crystal of high quality was selected and analyzed by SC XRD at 100 K (for crystallographic and refinement details, see SI Table S4). The crystal structure of the cis-dimer (orthorhombic system, space group Pbcn) conformed to previous reports (Figure 3a,b).25,26 While still being held at 100 K, the crystal was subjected to 20 minutes of irradiation with a broadband mercury UV lamp (main maxima at λ = 360, 420 and 445 nm). Subsequent SC XRD analysis at 100 K showed minor changes in the unit cell dimensions and significant changes in electron density (Figure 3e). The structure of the irradiated crystal was determined to comprise 91.4% the original dimer and 8.6% the corresponding monomer species (Figure 3b; additional analysis is provided in the contour plot of residual electron density in SI Figure S3). The crystal was then removed from the cold nitrogen stream, held at room temperature for 5 minutes, and then returned to the cold stream. SC XRD analysis showed that the excess residual electron density had disappeared (Figure 3f), that the molecules had re-


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Crystal Growth & Design turned completely to their original dimeric state, and that there was no resolvable amount of the monomer (Figure 3c). The crystal packing was identical with that before the irradiation (SI Figure S4). Subsequent studies with different crystals of nitrosobenzene saw monomer shares increase to 12.3%, 17.4%, and 19.1%, but these cases were not studied in detail and were not reverted completely to the dimeric state (SI Figure S5). In some cases, prolonged irradiation caused complete and irreversible powderization of the crystal. It is noteworthy that the initial crystal (with 8.6% yield) was able to completely revert to the dimeric state upon warming, whereas many photoproducts obtained in the crystalline state are prone to cracking, deterioration, or powderization upon warming.27 It should also be noted, however, that the nitrosobenzene crystal after irradiation had a decreased crystal quality, as indicated by increased residual factors and consistent with the microscopy experiments. This is to be expected for a reacting crystal; the crystal lattice is strained but retains its unit cell. In line with the optical microscopy observations (Figure 2), much of the crystal quality was recovered after the warming process. The distance traversed by the nitrogen atoms during the monomerization in this crystal is especially remarkable. In the previously reported case of photolysis of para-bromonitrosobenzene trans-dimer,23 the nitrogen—nitrogen distance increases from 1.30(5) Å to 2.30(5) Å, a 176 % change. In the nitrosobenzene cis-dimer reported here, the nitrogen—nitrogen distance more than triples, from 1.32(5) Å to 4.125(5) Å, a 313% change. One nitrogen atom moved 1.27(5) Å from its original (dimer) location and the other moved 1.70(5) Å in the opposite direction. Many photocrystallographic experiments report modest atomic displacements, and total interatomic contractions of less than 0.9 Å are typical.28–30 It is generally agreed that the reactions that have best chance of being observed in situ in single crystals are reactions that take place with small atomic displacements and within crystallographic cavities of sufficient space,31,32 and that the accumulation of photoproducts without destruction of the crystal is often very difficult.33 These results, with highly migratory nitrogen atoms, challenge that paradigm and suggest that bolder reactions may be observable with photocrystallographic methods.

Figure 3. Photocrystallographic evidence of a reversible monomerization of nitrosobenzene dimer. (a) ORTEP diagram of the initial dimeric state in a crystal before irradiation. (b) ORTEP diagram of UV-irradiated state with 8.6% population of monomer. The distance between twe two nitrogen atoms of the product is 2.97(5) Å. (c) ORTEP diagram of warmed and re-cooled state which had revereted back to the dimeric state. (d) Electron density difference Fourier map for initial dimer. (e) Electron density difference Fourier map for UV-irradiated state when refined solely as a dimer. Evidence of migrated nitrogen atoms is indicated by red splotches to either side of the blue nitrogen atoms that correspond to areas of significant electron density where a significant share of the nitrogen atoms have migrated. (f) Electron density difference Fourier map of warmed and re-cooled state which had reverted back to the dimeric state. All non-hydrogen atoms were refined with anisotropic displacement parameters, and in the ORTEPs are shown at 50% probability.

In order to try to better understand the mechanism of this monomerization and re-dimerization, EPR studies were also conducted (SI Figure S6). In a solution of benzene at room temperature, a radical was detected that resembled a nitroxide radical34 and the signal increased upon UV irradiation (SI Figure S6a). The experiments were also performed at 100 K on crystals of nitrosobenzene and found a spectrum very similar to the solution studies, however the increased intensity was much less significant, presumably caused by decreased absorption in the solid (SI Figure S6b). These results are unexpected and do not readily conform to the established closed-shell reaction in Scheme 1d. One possible explanation is that there are small amounts of the oxidized NO radical that exist in the solid state that remain undetected by SC XRD, but such a hypothesis would need further evidence for confirmation. The EPR results are suggestive that this seemingly simple monomer-dimer system may yet hold further surprises.



These overall results are significant technologically because they represent a positive development in the potential candidacy of aromatic C-nitroso compounds for use as on-off switches in molecular electronics,20 and they are significant methodologically because the large changes in atomic positions suggest an increased breadth of systems that may be suitable for future photocrystallographic studies.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Further experimental details (PDF) Movie of a crystal irradiated at ambient conditions (MOV) Crystallographic files (CIF)

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Tel: +971-(0)2-628-4572.

Author Contributions The manuscript was written with contributions from all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Dr. Liang Li of the New York University Abu Dhabi (NYUAD) Core Technology Platform is acknowledged for technical support with SC XRD experiments. This research was partially carried out using the Core Technology Platform resources at NYUAD.

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For Table of Contents Use Only Reversible Photolysis of Nitrosobenzene cis-Dimer Monitored In Situ by Single Crystal Photocrystallography Philip P. Rodenbough, Durga Prasad Karothu, Tamara Gjorgjieva, Patrick Commins, Hideyuki Hara, and Panče Naumov

Nitrosobenzene exists as a cis-dimer in the solid state, and its single crystals are found here to persist structurally through photolysis and re-dimerization. Infrared spectra confirm the reversible cycle of monomerization and dimerization, and photocrystallographic analysis details the remarkable long-distance migration of nitrogen atoms under UV irradiation.


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Reversible Photolysis of Nitrosobenzene cis-Dimer Monitored In Situ

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