Photoswitchable Arylazopyrazole-Based Ruthenium(II) Arene

Article pubs.acs.org/Organometallics

Photoswitchable Arylazopyrazole-Based Ruthenium(II) Arene Complexes Kesete Ghebreyessus* and Stefan M. Cooper, Jr. Department of Chemistry and Biochemistry, Hampton University, Hampton, Virginia 23668, United States S Supporting Information *

ABSTRACT: A new family of donor-functionalized photoswitchable arylazopyrazole-based ligands (3−5) was synthesized and characterized. The new ligands have been employed to prepare a series of novel photoswitchable half-sandwich ruthenium(II) cymene complexes of the type [(η6-p-cymene)Ru(L)Cl]+ (L = 1-(2-methylenepyridyl)-4-(phenyldiazenyl)-3,5-dimethyl-1Hpyrazole (6a), 1-(2-methylenepyridyl)-4-((4-bromophenyl)diazenyl)-3,5-dimethyl-1H-pyrazole (6b), 1-(2-benzothiazolyl)-3,5dimethyl-4-arylazopyrazole (7)). All of the complexes have been fully characterized by 1H NMR, 13C NMR, and UV−vis spectroscopy and elemental analyses. In addition, the structure of complex 6a was determined by X-ray crystallography. The UV−vis spectroscopic studies show that both the ligands and metal complexes exhibit excellent trans to cis photoisomerization of the arylazopyrazole moiety upon irradiation with 365 nm UV light. The cis isomer of the compounds can be switched back nearly quantitatively to the more stable trans form with 530 nm irradiation. Coordination of the metal ion has no significant influence on the photoswitching properties of the ligands. DFT and TD-DFT calculations were performed for geometry optimization of the ligands and to complement the experimental findings of the electronic transitions and absorption bands observed. The data obtained from these studies were in good agreement with the experimental results. These excellent photoswitchable properties make the new cationic Ru(II) azo compounds described in these studies interesting candidates for their potential application as photoswitchable systems in catalytic and medicinal chemistry.

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INTRODUCTION

More recently, Fuchter and co-workers have reported novel arylazopyrazole-based molecular photoswitches, wherein the switching properties of the azo moiety can be easily modulated by the substitution pattern on the pyrazole ring.20 In their studies, Fuchter and co-workers have shown that arylazopyrazoles undergo efficient photoinduced reversible trans to cis isomerization and exhibit superior properties in comparison to those of their azobenzene counterparts.20 These novel compounds also feature well-separated π−π* and n−π* absorption bands and possess outstanding half-lives for the cis isomer under ambient conditions. Isomerization of these photoresponsive units induces a conformational change around the NN double bond, similar to azobenzene.20 Since pyrazoles are widely used as ligands for a variety of metal complexes,21−32 these successful findings suggest that arylazo-

Photoswitchable molecules that exhibit substantial changes in structure or functional properties upon light irradiation show promise in a variety of applications, including molecular switches, switchable catalysis, bioimaging, and controlled drug release.1−6 In recent years, considerable efforts have been made in designing and preparing metal complexes that incorporate photoswitchable ligands in order to combine the inherent magnetic, electrochemical, catalytic, and biological properties of the metal complexes with the photoswitching ability of the photochrome.7−14 Several such efforts have employed azobenzene derivatives as photochromic ligands due to their efficient reversible photoinduced trans to cis isomerization, generating two isomers that display markedly different properties.7−14 However, photoswitching molecules based on analogous heteroarene azo compounds integrating both photochromic function and metal-binding units have been scarcely explored.15−19 © XXXX American Chemical Society

Received: June 29, 2017

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DOI: 10.1021/acs.organomet.7b00493 Organometallics XXXX, XXX, XXX−XXX

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Organometallics pyrazoles having additional donor groups can be favorable ligands for exploring the synthesis of hybrid metal−organic compounds with photoswitchable properties. To the best of our knowledge, metal complexes bearing photoswitchable arylazopyrazole ligands have not been reported to date. Half-sandwich ruthenium(II) arene complexes offer a versatile platform for the design and synthesis of novel compounds with potential applications in catalysis and medicinal and supramolecular chemistry. They have been widely used as catalysts in synthetic organic chemistry33−40 and are now being increasingly considered for the development of new chemical tools and metal-based drugs.41−45 Remarkably, ruthenium complexes are now commonly recognized as the second most promising type of new anticancer metal species apart from the platinum family of metallo-drugs.41−45 Varying the complex framework through modification of the arene (η6R-arene) and the other auxiliary ligands is crucial in tuning chemical reactivity and bioactivity.41−45 In this context, many different supporting auxiliary ligands have been used in combination with the Ru(II) arene scaffold to provide different reactivity profiles and bioactivities. Functionalization of the η6arene motif is possible, allowing modulation of the electronic and steric properties of the resulting complexes and conferring other interesting features for their use in catalysis and medicinal chemistry.48−51 However, the chemistry of ruthenium(II) arene complexes that possess photoswitchable ligands still remains largely unexplored.13,14 In this article, we present the synthesis and characterization of new photoswitchable ruthenium(II) arene complexes appended with functionalized arylazopyrazole (AAPz)-based ligands. AAPzs represent novel emerging molecular photoswitches with the pyrazole structural motif providing ample opportunities for ligand design and further modifications. These new complexes are designed with the intent to study if photoswitchable ruthenium(II) arene complexes can be prepared by coupling the arylazopyrazole groups and examine how the substitution pattern of the ligands affects their photochromic properties. In addition, the ruthenium(II) arene frameworks have been chosen as models for studying the coordination chemistry of the new ligands because of their many applications in metal-catalyzed organic transformations, potential therapeutic activity, and well-established chemical structures.40−45

Scheme 1. General Synthetic Route for the Photoswitchable Arylazopyrazole-Based Ligands 3−5

NMR, and UV−vis spectroscopy. The 1H NMR spectra of the ligands recorded in CDCl3 are presented in the Experimental Section. The position and intensity of the signals correspond to the reagents used in the synthesis. The most distinctive signals in the 1H NMR spectra of ligands 4a,b are those assigned to the CH proton adjacent to the pyridine nitrogen moiety, which appear as doublets at 8.57 ppm for 4a and 8.58 ppm for 4b. The methyl protons in the 3,5-positions of the pyrazole ring appear as singlets at 2.64 and 2.51 ppm (3), 2.57 and 2.54 ppm (4a), 2.56 and 2.52 ppm (4b), and 2.60 and 3.17 ppm (5), respectively. Other signals given in the Experimental Section are in agreement with the structures of the ligands. Synthesis of Ruthenium(II) Complexes. The three types of half-sandwich Ru(II) p-cymene complexes were readily prepared by following the synthetic route shown in Scheme 2. Reactions of the dimeric [(p-cymene)RuCl2]2 precursor with 2 equiv of the N,N-donor ligands 4a,b and 5 in dichloromethane gave the cationic half-sandwich mononuclear ruthenium(II) cymene complexes 6a,b and 7 as air-stable orange or brown solids in good yield. Isolated as their chloride salts, all of the complexes are soluble in common organic solvents such as acetone, chloroform, dichloromethane, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and methanol but insoluble in water. Similar reactions of the dimeric [(pcymene)RuCl2]2 precursor with ligand 3, however, resulted in a very unstable complex that decomposed easily. The 2ethylamine, 2-(2-methylenepyridine), and benzothiazole derivatives at the N-1 position of the pyrazole ring were mainly selected to vary the steric bulk of the ligands and to have a small library of ligands with varied features. The structures of the new Ru(II) cymene complexes 6a,b and 7 were confirmed by 1H NMR and 13C NMR spectroscopy and elemental analysis. As expected, the 1H NMR spectra of the complexes 6a,b and 7 in CDCl3 displayed well-defined signals that can be assigned

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RESULTS AND DISCUSSION Synthesis and Characterization. The route employed for the synthesis of the photoswitchable arylazopyrazole-based ligands 3−5 is outlined in Scheme 1. First, the 1,3-dione precursor compound 1 was prepared by diazo coupling of the 2,4-pentanedione with commercially available substituted anilines according to literature methods, and subsequent condensation reactions with hydrazine hydrate gave compound 2 in high yield.20 Treatment of compound 2 with (2chloroethyl)amine gave ligand 3 as an orange solid in 70% yield. 31,32 Similarly, reacting compound 2 with 2(chloromethyl)pyridine hydrochloride in the presence of excess KOH and TBAB as a phase-transfer catalyst afforded ligands 4a,b as sticky orange solids in good yield.31 After washing with hexane and diethyl ether several times and drying, the ligands become pure enough for the synthesis of the Ru(II) complexes. The new ligand 5 was easily prepared in high yield by condensation of the precursor 1 with 2-hydrazinobenzothiazole. All of the new ligands were fully characterized by 1H NMR, 13C B


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Organometallics

carbons. These chemical shifts are similar to those reported for similar ruthenium(II) arene compounds.33−49 X-ray Crystallographic Analysis. Single crystals suitable for X-ray crystallographic analysis were obtained by slow diffusion of petroleum ether into a CH2Cl2 solution of complex 6a. Although the resulting crystal quality was poor, giving rise to diffuse diffraction patterns, the molecular entity could be unambiguously determined. The selected bond angles and lengths are given in Figure 1, whereas crystallographic data and

Scheme 2. Synthetic Route for the Photoswitchable Aryazopyrazole-Containing Ruthenium(II) p-Cymene Complexes 6a,b and 7

Figure 1. ORTEP (50% probability) diagram of the half-sandwich Ru(II) p-cymene complex 6a. The anion is omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Ru1−N1 = 2.128(3), Ru2− N2 = 2.115(3), Ru1−Cl1 = 2.390(10), N4−N5 = 1.258(5); N1− Ru1−N2 = 84.60(12), N1−Ru1−Cl1 = 83.39, N2−Ru1−Cl1 = 86.98(9), N2−N3−C6 = 119.4(3), N1−C5−C6 = 117.2(3).

to the chelated arylazopyrazole-based ligands and the p-cymene moiety. Coordination of the N,N-chelating ligands to the Ru(II) center generated nonsymmetric complexes which induce significant modifications to the 1H NMR and 13C NMR signals of the p-cymene moiety in 6a,b. The 1H NMR spectra of the complexes 6a,b exhibit four broad doublets in the range 6.41−5.79 ppm (6a) and 6.33−5.78 ppm (6b), which are typical for the aromatic protons of the p-cymene derivative in Ru(II) arene systems.33−49 The proton resonance attributable to the methyl groups of the isopropyl derivative appear as two doublets at 1.32, 1.21 ppm (6a) and 1.30, 1.19 ppm (6b) and the methyl protons as a singlet at 2.86 ppm (6a) and 2.15 ppm (6b). The CH(CH3)2 proton appears as a septet at 2.93 ppm (6a) and 2.90 ppm (6b). The effect of the coordination of the pyrazole unit to the Ru(II) center and the difference in the proximity of the p-cymene moiety is also evident from the significant differences in the chemical shifts of the 3,5-methyl protons of the pyrazole ring. The methyl protons closer to the p-cymene moiety appeared at 2.84 ppm (6a) and 2.83 ppm (6b) in comparison to methyl protons away from the Ru(II) center that resonated at 2.19 ppm (6a) and 2.15 ppm (6b), respectively. A similar pattern was observed in the 1H and 13C NMR spectra of complex 7. In addition, the 1H NMR spectra of the Ru(II) complexes 6a,b contain doublet resonances at 8.93 and 8.89 ppm, respectively attributed to the CH proton adjacent to the pyridine nitrogen. These are shifted slightly downfield in comparison to the free ligands 4a,b, which resonated at 8.57 ppm (4a) and 8.58 ppm (4b), respectively. The signals of the methylene protons of the pyridine ring appear as a singlet at 6.00 ppm (6a) and 6.08 ppm (6b). Likewise, these resonances are shifted downfield relative to those of the free ligands, which appeared at 5.41 ppm (4a) and 5.40 ppm (4b), indicating coordination of the Ru(II) ion to the N,N-chelating arylazopyrazole ligands. The 13C NMR spectrum also exhibits appropriate signals. The methyl carbons of the p-cymene group display signals in the range of 15 and 22 ppm. The CH carbons of the isopropyl group appear around 29.9−30.6 ppm, while the cymene carbons appear in the range 83.8−106.1 ppm for the C−H

structural refinement parameters are gathered in Table S1 in the Supporting Information. The unit cell contains two slightly different Ru(II) p-cymene moieties. The 50% probability ORTEP view of only one of the complexes with atom numbering is shown in Figure 1. From Figure 1, it is clear that the complex adopts the commonly observed piano-stool geometry as reported in similar half-sandwich Ru(II) arene complexes,50 and the arylazopyrazole unit is presented in the most thermodynamically stable trans form. The molecular structure of complex 6a also clearly shows that the arylazopyrazole-based ligand 4a is coordinated in a bidentate fashion, forming a six-membered metallacycle with a bite angle of 118.3(3)°. The values of the Ru(II)−Nav and Ru(II)−Cl distances are on average 2.122(3) and 2.390(10) Å, respectively. Other selected bond angles and lengths are given in Figure 1. The Ru(II)−Cl bond length is found to be 2.390(10) Å, which is in agreement with other structurally characterized similar p-cymene Ru(II) complexes.50 The N−N (1.258(5) Å) distance observed is similar to the values observed for the free azo ligands, which conforms to the integrity of the azo functionality.20 UV−Vis Absorption Spectra of Ligands 3−5. The UV− vis spectra of the investigated ligands 3−5 exhibit essentially the same features. As exemplified in Figure 2a, the electronic spectra of the ligands 3−5 display intense bands in the 330− 346 nm range and weak bands at 435−450 nm in the visible region as common features. Under normal conditions, the free ligands 3−5 exist in the more thermodynamically stable trans conformation. The absorption spectra of the high-intensity bands of the trans isomer appear at 330 nm for 3, 337 nm for 4a, 342 nm for 4b, and 344 nm for 5 (Figure 2a). Similarly, the absorption maxima of the low-intensity bands appear at 436 nm for 3, 444 nm for 4a, 446 nm for 4b, and 450 nm for 5 (Figure 2a). The introduction of the bromo substituent to the arylazo ring in 4b resulted in a slight bathochromic shift of the C


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Organometallics

Figure 2. UV−vis absorption spectra of the trans isomers of (a) the arylazopyrazole-based ligands 3−5 and (b) Ru(II) complexes 6a,b and 7 measured in CHCl2 (2.0 × 10−5 M).

Figure 3. UV−vis spectral changes of compound 4a (2.0 × 10−5 M in CH2Cl2): (a) trans to cis isomerization after irradiation with 365 nm as a function of time; (b) cis to trans photoisomerization upon irradiation at 530 nm as a function of time.

photoisomerization of ligand 4a is discussed in this section. The changes in the UV−vis absorption spectra of ligand 4a in dichloromethane solution as a function of time are depicted in Figure 3. Upon irradiation with UV light (λ 365 nm) the trans form of ligand 4a was easily converted to the cis isomer, resulting in new absorption bands at 292 and 445 nm, marked with blue arrows in Figure 3a. This was also clearly shown by the characteristic changes in the absorbance of the band maxima, which involved a significant decrease in the π−π* transition of the arylazopyrazole moiety and an increase in the n−π* transition band (Figure 3a), which were indicative of the formation of the cis isomer. In addition, the intense band of the π−π* transition of the trans isomer was also blue-shifted and appeared at 292 nm upon exposure to UV light (λ 365 nm). On the other hand, the band due to the n−π* transition showed a slight red shift to 448 nm from 445 nm. The trans to cis isomerization of ligand 4a reached a photostationary state (PSS) within 2 min. Further irradiation of the sample solution beyond 2 min did not give any significant changes in the spectral features. For ligand 4a, the cis to trans ratio at the PSS was estimated to be 81:19 as determined by UV−vis spectroscopy using eq 1, as previously reported by Wada and others.52−54 This estimation was based on the assumption that before irradiation the high-intensity band at the absorption maximum wavelength (337 nm) of the trans form of 4a is due to the trans isomer only (i.e., no cis form is present).

absorption maxima in comparison to the unsubstituted 4a. There was also a slight red shift of the intense band in the benzothiazole-based ligand 5 (344 nm) in comparison to those of ligands 3 (330 nm) and 4a (337 nm). Analogous to reported literature studies on arylazopyrazoles, the intense absorption bands for the trans isomer were attributed to the π−π* transitions, whereas the broad, weak bands at around 435−450 nm were assigned to the n−π* transitions.20 UV−Vis Absorption Spectra of Complexes 6a,b and 7. The absorption spectra of the Ru(II) complexes 6a,b and 7 show absorption bands similar to those of the free ligands 4a,b and 5 (Figure 2b). Complexes 6a,b and 7 display a highintensity π−π* band at 331 nm (6a), 340 nm (6b), and 346 nm (7) (Figure 2b). These high-intensity absorption bands are well separated from the low-intensity n−π* bands observed at 434 nm (6a), 443 nm (6b), and 441 nm (7) (Figure 2b). Upon coordination to the Ru(II) center, the π−π* transition of ligand 5 exhibits a minor red shift, whereas the transitions for 4a,b show small blue shifts in comparison to the λmax values of the complexes 6a,b and 7. In all, the UV−vis absorption spectra of the Ru(II) complexes are quite similar to those of the free ligands, suggesting that coordination of the metal ion does not significantly alter the electronic structure of the ligands. Photoisomerization Studies of Ligands 3−5. The reversible photoswitching properties of the new ligands 3−5 and the Ru(II) complexes 6a,b and 7 have next been examined by UV−vis spectroscopy. As a representative example, the D


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Organometallics

Figure 4. UV−vis absorption spectra of the ligands 3−5 before and after 2 min irradiation at 365 nm.

ligands 3−5 are comparable to those of the recently reported arylazopyrazole-based molecular photoswitches.20,51 The back cis to trans photoisomerizations of ligands 3, 4b, and 5 have also been studied upon green light irradiation in a manner similar to that for ligand 4a and are given in Figures S1−S4, S6, and S7 in the Supporting Information. Photoisomerization Studies of Complexes 6a,b and 7. Although arylazopyrazoles have recently been shown to undergo efficient trans to cis isomerization, their capacity to isomerize when coordinated to a metal ion has not been explored yet. We therefore performed preliminary photoirradiation experiments on the arylazopyrazole-conjugated Ru(II) arene complexes 6a,b and 7. The photoisomerization properties of the complexes have been examined by UV−vis spectroscopy, monitoring the changes occurring in their absorption spectra during irradiation (Figure 5). As shown in Figure 5, the Ru(II) complexes show photoisomerization behavior similar to that of the free ligands. Irradiating dichloromethane solutions of the Ru(II) complexes with UV light at 365 nm generated the cis form, characterized by absorptions in the visible region at λmax ∼450 nm. Upon photoisomerization, a decrease in the intensity of the π−π* transition and an increase in the n−π* transition was observed. It was observed that the rate of the trans to cis isomerization of the complexes is slightly slower than those of the free ligands. The PSS was reached within 4 min in comparison to the 2 min for the free ligands. The cis to trans ratio of the Ru(II) complexes at the PSS is estimated to be 80:20 (6a), 79:21 (6b), and 70:30 (7), as determined by using a method similar to that for ligand 4a.52−54 Notably, upon coordination to the metal ion each of the ligands retained their characteristic absorption properties. This similar behavior of the complexes and the free ligands may suggest that coordination of the metal ions did not significantly alter the electronic structure of the free ligands. To ensure that the observed photoisomerization of the complexes was fully reversible, a relaxation experiment was carried out in a manner similar to that for the free ligands. The reverse cis to trans photoisomerization of the Ru(II) complexes was studied by irradiation with 530 nm light. After irradiation of

Similar absorption spectral changes were observed for the other ligands, as shown in Figure 4 and Figures S1−S6 in the Supporting Information. The estimated percent conversions, the band maxima, and the absorption spectral data for the ligands and Ru(II) complexes are also given in Table S2 in the Supporting Information. The light-induced reversible photoswitching process is described in Scheme 3. Scheme 3. Photoisomerization Process of Ligand 4a and Complex 6a

The photoinduced reverse cis to trans photoisomerization properties of the ligands 3−5 have next been examined by irradiation with green (530 nm) light. To test for photoreversibility, UV-exposed dilute solutions of the samples were further irradiated with 530 nm light and the process was followed by acquiring UV−vis spectra at regular time intervals. As depicted in Figure 3b, irradiation of a solution of ligand 4a with 530 nm light at different time intervals led to the full recovery of the absorption spectrum of the trans isomer. The observed cis to trans photoisomerization properties of the E


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Organometallics

Figure 5. UV−vis absorption spectra of the Ru(II) complexes 6a,b and 7 before and after 4 min irradiation at 365 nm.

The data obtained showed that, at room temperature, the thermal cis to trans isomerization rates of the metal complexes were much faster (less stable cis isomer) than those of the free ligands. This kind of behavior has been observed with related azobenzene-containing metal complexes.12,13 The half-lives (t1/2) for the complexes were determined to be 224 min (6a), 165 min (6b), and 131 min (7). Furthermore, a somewhat lower thermal stability was observed for the cis isomer of complex 7 in comparison to the free ligand 5. Presumably, this is due to the slight decomposition observed in the thermal relaxation process of complex 7. Generally, although a direct comparison cannot be made due to the different natures of the ligands described here, the thermal stabilities of the cis isomer of complexes 6a,b are comparable to or even better than those of recently reported azobenzenecontaining Ru(II) arene complexes.12,13 The data obtained also showed that the thermal stabilities of ligands 4a,b are lower than that of the recently reported benzylsubstituted arylazopyrazole photoswitch (albeit measured with a different solvent and temperature) but have thermal stabilities comparable to that of a naphthyl-substituted photoswitch.51 A higher thermal stability of the cis isomer was observed for ligand 3 (2310 min) and ligand 5 (1386 min). As previously observed for other pyrazole ring substituted azopyrazoles, the nature of the substituents on the pyrazole nitrogen has a significant influence on the thermal stabilities of the new photoswitches described here.20,51 The photostability of the reversible trans to cis photoisomerization of the complexes was investigated using 6a as a representative example. To test for photostability, the UV−vis absorption spectra of a dilute solution of 6a was monitored by alternate irradiation of the sample with 365 nm (2 min) followed by 530 nm light (10 min). Complex 6a was stable under these conditions (Figure S5 in the Supporting

solutions of the complexes with 530 nm light, spectra that match the original spectra assigned for the trans isomer were regenerated (Figures S4, S6, and S7 in the Supporting Information), indicating that the process was fully reversible. In all, the UV−vis studies clearly showed trans to cis isomerization of the ligands 3−5 and the Ru(II) arene complexes upon irradiation with UV light and the reverse reaction under green light. Thermal Cis to Trans Isomerization of 3−5 and Complexes 6a,b and 7. The thermal cis to trans isomerization rates of the arylazopyrazole-based ligands 3−5 and the corresponding Ru(II) complexes 6a,b and 7 were measured in CH3CN at 25 °C by UV−vis spectroscopy. First, acetonitrile solutions of ligands 3−5 and the complexes 6a,b and 7 were irradiated for 2 min at 365 nm and kept in the dark. Then, the reverse cis to trans isomerization process was actively monitored by UV−vis spectroscopy, registering the spectra at regular time intervals until the initial spectrum was restored. The value of absorbance at the high-intensity band (π−π*) was used to calculate the first-order rate constants (k) and half-lives (t1/2) using Monkowius’ procedure,8 and the data for this process are given in Table 1. Table 1. Thermal Kinetic Data for the Cis to Trans Isomerization at 25 °C

a

compound

λmax (nm)

3 4a 4b 5 6a 6b 7

330 337 342 344 331 342 340

k (min−1) 3.00 2.20 1.77 5.00 3.10 4.20 5.30

× × × × × × ×

10−4 10−3 10−3 10−4 10−3 10−3 10−3

half-life (min) 2310 315 392 1386 224 165 ∼131a

Approximate value due to slight decomposition in solution. F


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Organometallics

Figure 6. 1H NMR spectral changes in the aromatic region of complex 6a (2.0 × 10−3 M) in CD3CN solution: (a) spectrum of trans-6a before irradiation (trans-6a only); (b) spectrum of 6a after irradiation with UV light at 365 nm for 30 min.

Figure 7. Optimized calculated structures of the trans and cis forms of (a) ligand 4a and (b) Ru(II) complex 6a.

photoirradiation. Likewise, the doublet signal in the 1H NMR spectra of the CH proton adjacent to the pyridine nitrogen moiety was shifted upfield from 9.04 to 8.97 ppm. These shifts in NMR signals could be ascribed to the alteration of the chemical environment in the structure after the photoisomerization reactions. The signal for the CH proton adjacent to the pyridine nitrogen was then integrated to calculate the percent composition of the cis isomer produced. The percent conversion of the cis isomer was determined to be 82% for complex 6a by 1H NMR, which is similar to data obtained using UV−vis spectrscopy. Computational Studies. To gain further insight into the isomerization behavior and optical properties of the ligands and the Ru(II) complexes, computational modeling of both the cis and trans forms of the compounds was performed by the DFT method at the B3LYP/6-311++G(d,p) level using the Gaussian 09 package.55 As representative examples, the lowest-energy conformers of the gas-phase optimized geometries for the trans and cis forms of ligand 4a and its Ru(II) p-cymene complex 6a are shown in Figure 7. The results from the calculations reveal

Information), and no signs of photodegradation were detected during the 10 cycles of irradiation. Investigation of the Photoisomerization Process of Complex 6a by 1H NMR Spectroscopy. The photoisomerization process of the complexes is further evidenced by 1H NMR spectroscopy, following the changes occurring in the spectra of the Ru(II) cymene complexes subjected to UV light irradiation. As a representative example, Figure 7 shows changes in the portions of the 1H NMR spectra of complex 6a in CD3CN before and after irradiation. In the trans form of complex 6a, the aromatic protons of the p-cymene moiety display four distinct signals: two doublets at 5.85 and 5.25 ppm corresponding to H2 and H3 and two doublet signals at 5.60− 5.55 ppm corresponding to H5 and H6, respectively (Figure 6). Upon irradiation with green (530 nm) light for 30 min the aromatic protons resonated as four doublets in the upfield region at 4.80 (H2), 5.45 (H3), 5.65 (H5), and 5.70 (H6) ppm, providing evidence in favor of the formation of the cis isomer. Analogously, the methylene protons located on the pyridine moiety (H7) shifted upfield from 5.30 to 5.00 ppm after G


Organometallics

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CONCLUSION In summary, we report herein the synthesis, characterization, and photoswitching behavior of new benzothiazole- and pyridine-functionalized arylazopyrazole-based ligands and their half-sandwich ruthenium(II) p-cymene complexes. One of the complexes (6a) formed crystals suitable for X-ray structural determination, exhibiting a typical piano-stool coordination geometry around the Ru(II) center. The detailed photoisomerization studies reveal that the ligands and the Ru(II) complexes undergo fast trans to cis isomerization upon irradiation with 365 nm UV light, reaching favorable photostationary states. The reverse cis to trans isomerization of the compounds can be nearly quantitatively restored by irradiation with 530 nm light. The results also show that coordination of the Ru(II) does not significantly alter the reversible trans to cis isomerization properties of the ligands, with ligands and metal complexes displaying essentially similar photoswitching behavior. Furthermore, variation of the substituents on the N-1 position of the pyrazole ring has a minor influence on the photoisomerization properties of the ligands and metal complexes. However, the presence of different substituents on the pyrazole ring has a significant influence on the thermal stabilities of the compounds investigated. Since Ru(II) arene complexes have known diverse catalytic applications and have recently been increasingly considered for the development of metal-based pharmaceutical agents, we believe that adding a light-switchable moiety in their ligand structure may enable modulating their functionality and enhance their potential in the domains of catalysis and biological applications.

that the trans form of the azopyrazole unit in both the ligands and the Ru(II) complexes adopts a nearly planar geometry with C−NN−C dihedral angles of ∼180°(Figure 7 and Figure S8 in the Supporting Information). On the other hand, the computed geometry of the cis isomers of both the ligands and the Ru(II) complexes indicates significant twisting of the phenyl ring of the azo unit in order to minimize the steric interaction between the methyl group of the pyrazole ring (Figure 7 and Figure S8). The optimized geometries of the ligands are similar to those for previously reported arylazopyrazole-based molecular switches reported by Fuchter and coworkers.20 The predicted structure is quite similar to that of the experimentally determined X-ray crystal structure of complex 6a. The optimized bond lengths of complex 6a are in the range of those observed for Ru−N(1) (2.205 Å vs 2.128 Å), Ru− N(2) (2.172 Å vs 2.115 Å), Ru−Cl (2.428 Å vs 2.390 Å), and N(4)−N(5) (1.258 Å vs 1.259 Å). The calculated energy values for both the ligands and Ru(II) p-cymene complexes also predict that the cis form of the arylazopyrazole is higher in energy than the trans configuration. For instance, the results from the computations predict that the trans isomer of ligand 4a is 16.4 kJ/mol more stable than the cis isomer. Likewise, the trans form of the Ru(II) p-cymene complex 6a is calculated to be 18 kJ/mol more stable than the cis form. The computed energy difference between the free ligand 4a and its Ru(II) complex 6a is minor, indicating that the incorporation of the Ru(II) p-cymene group has very little effect on the thermodynamics of the trans to cis isomerization. This minor calculated energy difference is in good agreement with the observed experimental findings that both the ligands and Ru(II) p-cymene complexes show similar isomerization behavior in solution. A similar trend is observed for the other ligands and Ru(II) complexes. The lowest-energy conformers for the trans and cis forms of the other ligands (3, 4b, znd 5) and the Ru(II) p-cymene complexes (6b and 7) are depicted in Figure S8 in the Supporting Information. Using the optimized structures, a comparative analysis of the absorption spectra of the ligands was also performed by timedependent (TD) DFT calculations. These calculations were conducted in dichloromethane in a manner similar to the conditions under which the experimental absorption profiles were measured. The computed values of the electronic transitions giving rise to the main absorption features for the arylazopyrazole-based ligands are compiled in Table S3 in the Supporting Information. In agreement with the experimental results, the UV−vis absorption data predicted from the TDDFT calculations reveal the presence of two bands for the π−π* transitions and one band for the n−π* transitions in all of the compounds studied (Table S3). However, the calculated absorption wavelength values of the ligands were significantly red shifted in comparison to the experimentally observed values. For instance, the main absorption spectra (π−π* transition) of ligand 4a show the presence of an intense band centered at λcalcd 355.73 nm which is ascribed to a highest occupied molecular orbital (HOMO) → lowest occupied (LUMO) transition. A minor HOMO-1 → LUMO transition centered at λcalcd 306.65 nm was also observed. The computed value of the high-intensity band (λcalcd 355.73 nm) is red-shifted by ∼20 nm in comparison to the experimentally determined λmax value at 337 nm.

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EXPERIMENTAL SECTION

All reactions were carried out under nitrogen. All chemicals and solvents were purchased from commercial sources and used as received. [(η6-cymene)RuCl2]256 and 3-(2-phenylhydrazono)pentane2,4-dione (1) were prepared according to literature methods.20 1H NMR and 13C{1H} NMR spectra were recorded on a JEOL Eclipse3 400 MHz spectrometer using solvent resonances as internal references. Spectroscopic Measurements. Dry acetonitrile and dichloromethane solvents were used for all UV−vis measurements. UV−vis absorption spectra were recorded on a Varian Cary 50 BIO spectrometer. Photoisomerization was induced by irradiating dichloromethane solutions of the chelating ligands or ruthenium(II) cymene complexes in a quartz cuvette at room temperature with a hand-held 6 W 365 nm UV lamp, followed immediately by recording the absorption spectrum. For the reverse isomerization process, irradiation at 530 nm was carried out using a Shimadzu RF-5301 PC spectrofluorometer equipped with a 150 W Xe lamp. Determination of the Percentage of Cis by UV−Vis Spectroscopy. The percent conversion of the cis isomer of the ligands and Ru(II) complexes at the photostationary state formed by UV irradiation was estimated on the basis of the equation52−54

% cis =

A trans − Acis × 100 A trans

(1)

where Atrans is the absorbance for the trans isomer at λmax before light irradiation and Acis is the absorbance for the cis isomer at the same wavelength measured at the photostationary state.52−54 Determination of the Percentage of Cis by NMR for 6a. A concentrated solution of 6a (2.0 × 10−3 M) in CD3CN was prepared in an NMR tube, and the initial proton spectrum was recorded before irradiation. Then, the solution was irradiated with 365 nm light for 30 min in an NMR tube. After the irradiation was completed, the solution was quickly placed in the NMR spectrometer and the proton spectrum was acquired at 25 °C. The CH proton adjacent to the pyridine nitrogen was then integrated to calculate the percent composition of the cis isomer produced. H


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Organometallics X-ray Structure Determination. Single crystals of complex 6a were grown by layering a dichloromethane solution with petroleum ether. Crystals were mounted on glass fibers. All measurements were made using graphite-monochromated Cu Kα radiation on a BrukerAXS three-circle diffractometer, equipped with a SMART Apex II CCD detector. Initial space group determination was based on a matrix consisting of 120 frames. The data were reduced using SAINT +,57 and empirical absorption correction was applied using SADABS.58 The X-ray structure of 6a was solved using direct methods. Leastsquares refinement was carried out on F2. Two crystallographically independent molecules were present. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in riding positions and refined isotropically. Structure solution, refinement, and the calculation of derived results were performed using the SHELXTL package of computer programs59 and Shelxle.60 Despite attempts to crystallize 6a from a variety of solvents, in each case the resulting crystals were found to be solvates in which the included solvent was highly disordered and could not be modeled. Therefore, Platon-Squeeze was used to mask the solvent-based reflection data during final refinement.61 A total potential solvent-accessible void volume of 880.4 Å3 was found, corresponding to 339 e−/unit cell. It is likely that this volume corresponds to two molecules of hexane, which was used as the cocrystallization solvent. Details of the X-ray experiment and crystal data are summarized in Table S1 in the Supporting Information. Selected bond lengths and bond angles are given in Figure 1. Computational Details. All density functional theory (DFT) calculations were performed with the Gaussian09 package at the B3LYP level.55 LanL2DZ and 6-311G**(d,p) basis sets were used for ruthenium(II) and all other atoms, respectively. All structures are gasphase optimized geometries. The absence of imaginary frequencies was checked on all calculated structures to confirm that they were true minima. Computed structures were illustrated using Avogadro.55 Calculations were performed on the Comet cluster at SDSC via Extreme Science and Engineering Discovery Environment (XSEDE) Comet Rapid Access Project (to S.M.C.), with the GridChem 2.0.0 (http://www.gridchem.org/) computational gateway, which is supported by the NSF. Photochemical Studies. Stock solutions of the chelating ligands 3, 4a,b, and 5 and ruthenium(II) p-cymene complexes 6a,b and 7 (2.0 × 10−5 M) were prepared in dichloromethane. Electronic absorption spectral experiments were performed in a quartz cuvette of 1 cm path length. Solutions of both the ligands and metal complexes were irradiated by UV light (365 nm). Half-lives for the thermal cis to trans back-isomerization reactions were determined by following procedures similar to those described in the literature.8,20 The rate constant k for the reaction is determined by plotting ln{(Atrans − Apss)/(Atrans − At)} vs time (Atrans = absorbance of the pure trans isomer; Apss = absorbance at the photostationary state; At = absorbance at the interval time t).8 Assuming the photoisomerization process obeys a first-order reaction, the slope of the best-fit line gives the rate k for the reverse reaction. The half-life of the complexes and ligands is then calculated using the following equation: t1/2 = ln(2)/k. Synthesis of 1-(2-Aminoethyl)-4-(phenyldiazenyl)-3,5-dimethyl-1H-pyrazole (3). 3,5-Dimethyl-4-(phenyldiazenyl)-1H-pyrazole (2; 1.0 g, 10.0 mmol) was dissolved in 25 mL of anhydrous acetonitrile, 1.80 g (45.0 mmol) of sodium hydroxide was added to this solution, and the mixture was stirred for 30 min at ambient temperature. The reaction mixture was refluxed, and to this mixture was slowly added a suspension of 2-chloroethylamine. After the solution was refluxed for 12 h, it was cooled to room temperature and the precipitate that formed was removed by filtration. The solvent of the filtrate was removed by rotary evaporation to give a yellow solid in 75% yield. 1H NMR (CDCl3, 400 MHz): δ 7.79 (d, J = 7.8 Hz, 2H), 7.47 (t, J = 8.12 Hz, 2H), 7.38 (t, J = 7.32 Hz, 2H), 4.35 (t, J = 5.96 Hz, 2H, CH2), 3.92 (t, J = 5.96 Hz, 2H, CH2), 2.64 (s, 3H, CH3, Pz), 2.51 (s, 3H, CH3, Pz). 13C NMR (CDCl3, 100 MHz): δ 156.0 (Pz), 152.2 (Py), 149.4 (Pz), 143.0 (Pz), 140.0 (Py), 137.1 (Py), 135.4 (Py), 132.0 (2C, Py), 123.3 (2C, py), 122.7 (Py), 121.1 (Py), 54.7

(CH2Py), 14.0 (CH3Pz), 13.6 (CH3Pz). Anal. Calcd for C13H17N5, :C, 64.17; H, 7.04; N, 28.78. Found: C, 63.92; H, 7.01; N, 28.35. General Procedure for the Synthesis of Ligands 4a,b. 3,5Dimethyl-4-(phenyldiazenyl)-1H-pyrazole (2; 1.0 g, 10.0 mmol) was dissolved in 25 mL of anhydrous acetonitrile. To this solution were added 0.50 g of 2-chloromethylpyridine hydrochloride, 0.20 g of KOH, and 0.10 mg of TBAB. The mixture was refluxed for 24 h. The resulting mixture was filtered, and the solvent was removed by rotary evaporation, giving a sticky brown solid product in 80% yield. The solid was washed several times with hexane and diethyl ether solvents to obtain a dry solid. 1-(2-Methylenepyridyl)-4-(phenyldiazenyl)-3,5-dimethyl-1H-pyrazole (4a). 1H NMR (CDCl3, 400 MHz): δ 8.57 (d, J = 6.00 Hz, 1H Py), 7.79 (d, J = 8.7 Hz, 2H, Py), 7.64 (t, J = 7.8 Hz, 1H, Py), 7.46 (m, 2H, Py), 7.39 (m, 1H, Py), 7.20 (d, J = 8.2 Hz, 1H, Py), 6.95 (d, J = 7.8 Hz, 1H, Py), 5.40 (s, 2H, CH2Py), 2.57 (s, 3H, CH3, Pz), 2.54 (s, 3H, CH3, Pz). 13C NMR (CDCl3, 100 MHz): δ 156.0 (Pz), 152.2 (Py), 149.4 (Pz), 143.0 (Pz), 140.0 (Py), 137.1 (Py), 135.4 (Py), 132.0 (2C, Py), 123.3 (2C, py), 122.7 (Py), 121.1 (Py), 54.7 (CH2Py), 14.0 (CH3Pz), 13.6 (CH3Pz). Anal. Calcd for C17H17N5: C, 70.07; H, 5.90; N, 24.03. Found: C, 69.84; H, 5.84; N, 23.67. 1-(2-Methylenepyridyl)-4-((4-bromophenyl)diazenyl)-3,5-dimethyl-1H-pyrazole (4b). Ligand 4b was synthesized following similar procedures as in 4a. 1H NMR (CDCl3, 400 MHz): δ 8.58 (d, J = 6.8 Hz, 1H Py), 7.65 (m, 3H, Py), 7.57 (m, 2H, Py), 7.21 (d, J = 6.8 Hz, 1H, Py), 6.97 (d, J = 7.8 Hz, 1H, Py), 5.41 (s, 2H, CH2Py), 2.56 (s, 3H, CH3, Pz), 2.52 (s, 3H, CH3, Pz). 13C NMR (CDCl3, 100 MHz): δ 156.0 (Pz), 152.2 (Py), 149.4 (Pz), 143.0 (Pz), 140.0 (Py), 137.1 (Py), 135.4 (Py), 132.0 (2C, Py), 123.3 (2C, py), 122.7 (Py), 121.1 (Py), 54.7 (CH2Py), 14.0 (CH3Pz), 13.6 (CH3Pz). Anal. Calcd for C17H16N5Br: C, 55.15; H, 4.36; N, 18.92. Found: C, 54.91; H, 4.31; N, 18.55. Synthesis of 1-(2-Benzothiazolyl)-4-(phenyldiazenyl)-3,5-dimethyl-1H-pyrazole (5). 2-Hydrazinobenzothiazole (1.0 g, 1 equiv) was added to a solution of 3-(2-phenylhydrazino)-pentane-2,4-dione (1, 1 equiv) dissolved in ethanol, and the mixture was refluxed for 5 h. The resulting solution was rotary evaporated to give a bright yellow solid product in 90% yield. 1H NMR (CDCl3, 400 MHz): δ 7.93 (d, J = 7.8 Hz, 1H), 7.85 (d, J = 7.8 Hz, 2H), 7.49 (d, J = 7.8 Hz, 2H), 7.35−7.46 (m, 4H), 3.17 (s, 3H, CH3, Pz), 2.60 (s, 3H, CH3, Pz). 13C NMR (CDCl3, 100 MHz): δ 162.6 (Pz), 154.8 (Py), 153.0 (Pz), 147.3 (Pz), 144.3 (Pz), 139.1 (Py), 134.3 (Py), 131.7 (Py), 130.5 (2C, Py), 127.9 (Py), 126.3 (Py), 124.1 (Py), 123.6 (2C, Py), 122.8 (Py), 16.4 (CH3Pz), 13.7 (CH3Pz). Anal. Calcd for C18H15N5S: C, 64.84; H, 4.53; N, 21.01. Found: C, 65.08; H, 4.68; N, 21.06. General Procedure for the Synthesis of Ru(II) p-Cymene Complexes 6a,b and 7. The appropriate ligands dissolved in dichloromethane (5.0 mL) were added to a solution of the [(η6-pcymene)RuCl2]2 dimer in 10.0 mL of dichloromethane. The mixture was stirred for 24 h at room temperature. The solution was concentrated to ∼3.0 mL, and the corresponding product was precipitated with cold diethyl ether and filtered off. The solid product was dissolved in dichloromethane and recrystallized by layering with hexane or petroleum ether. [(η6-p-Cymene)](1-(2-methylenepyridyl)-4-(phenyldiazenyl)-3,5dimethyl-1H-pyrazole)chlororuthenium(II) Chloride (6a). 1H NMR (CDCl3, 400 MHz): δ 8.93 (d, J = 5.5 Hz, 1H, Py), 7.98 (d, J = 7.32 Hz, 1H, Py), 7.92 (t, J = 6.9 Hz, 1H, Py), 7.75 (d, J = 7.32 Hz, 1H, Py), 7.39−7.48 (m, 4H, Pz), 6.41 (d, J = 6.0 Hz, 1H, Ar), 6.20 (d, J = 5.9 Hz, 1H, Ar), 6.0 (s, 2H, CH2Py), 5.97 (d, J = 6.0 Hz, 1H, Ar), 5.79 (d, J = 6.0 Hz, 1H, Ar), 2.94 (sept, J = 6.8 Hz, 1H, H(CH3)2, Ar), 2.86 (s, 3H, CH3, Ar), 2.84 (s, 3H, CH3, Pz), 2.19 (s, 3H, CH3, Pz), 1.32 (d, J = 6.9 Hz, 3H, CH3, Ar), 1.21 (d, J = 6.9 Hz, 3H, CH3, Ar). 13C NMR (CDCl3, 100 MHz): δ 156.3 (Pz), 155.8 (Py), 153.0 (Py), 149.6 (Pz), 142.0 (Pz), 140.1 (Py), 135.8 (Py), 130.7 (2C, Py), 129.1 (2C, Pz), 126.7 (Pz), 125.2 (Pz), 122.2 (2C, Pz), 106.1 (Ar), 101.5 (Ar), 86.3 (Ar), 84.8 (Ar), 84.1 (Ar), 83.8 (Ar), 30.7 (CH, Ar), 22.7 (CH3, Pz), 22.5 (CH3, Pz), 18.7 (CH3, Ar), 16.3 (CH3, Ar), 11.0 (CH3, Ar). Anal. Calcd for C27H31N5RuCl2: C, 54.27; H, 5.23; N, 11.72. Found: C, 54.61; H, 5.29; N, 11.66. I


Article

Organometallics [(η 6 -p-Cymene)](1-(2-methylenepyridyl)-4-((4-bromophenyl)diazenyl)-3,5-dimethyl-1H-pyrazole)chlororuthenium(II) Chloride (6b). 1H NMR (CDCl3, 400 MHz): δ 8.89 (d, J = 5.0 Hz, 1H, Py), 8.02 (d, J = 7.32 Hz, 1H, Py), 7.88 (t, J = 7.32 Hz, 1H, Py), 7.55−7.62 (m, 4H, Pz), 7.39 (t, J = 6.4 Hz, 1H, Pz), 6.33 (d, J = 5.5 Hz, 1H, Ar), 6.16 (d, J = 5.5 Hz, 1H, Ar), 6.08 (s, 2H, CH2Py), 5.96 (d, J = 5.5 Hz, 1H, Ar), 5.78 (d, J = 5.3 Hz, 1H, Ar), 2.90 (sept, J = 6.9 Hz, 1H, H(CH3)2, Ar), 2.83 (s, 3H, CH3Pz), 2.82 (s, 3H, CH3Ar), 2.15 (s, 3H, CH3Ar), 1.30 (d, J = 6.9 Hz, 3H, CH3, Ar), 1.19 (d, J = 6.9 Hz, 3H, CH3, Ar). 13C NMR (CDCl3, 100 MHz): δ 156.1 (Pz), 155.5 (Py), 151.6 (Py), 149.5 (Pz), 142.4 (Pz), 140.0 (Py), 135.6 (Py), 132.1 (2C, Py), 126.8 (Pz), 125.1 (Pz), 124.6 (Pz), 123.5 (2C, py), 105.9 (Ar), 101.4 (Ar), 86.1 (Ar), 84.6 (Ar), 84.0 (Ar), 83.6 (Ar), 54.5 (CH2Py), 30.6 (CH, Ar), 22.5 (CH3, Pz), 22.3 (CH3, Pz), 18.5 (CH3, Ar), 16.2 (CH3, Ar), 10.9 (CH3, Ar). Anal. Calcd for C27H30N5RuBrCl2: C, 47.94; H, 4.47; N, 10.35. Found: C, 48.36; H, 4.52; N, 10.80. [(η6-p-Cymene)](1-(2-benzothiazolyl)-4-(phenyldiazenyl)-3,5-dimethyl-1H-pyrazole)chlororuthenium(II) Chloride (7). 1H NMR (CDCl3, 400 MHz): δ 7.83 (t, J = 7.8 Hz, 2H), 7.76 (t, J = 7.8 Hz, 2H), 7.04−7.39 (m, 5H), 5.89 (d, J = 6.4 Hz, 1H, Ar), 5.83 (d, J = 6.4 Hz, 1H, Ar), 5.70 (d, J = 5.96 Hz, 1H, Ar), 5.55 (d, J = 6.4 Hz, 1H, Ar), 2.93 (sept, J = 6.7 Hz, 1H, H(CH3)2, Ar), 2.60 (s, 6H, CH3, Pz), 2.06 (s, 3H, CH3, Ar), 1.87 (d, J = 6.8 Hz, 3H, CH3, Ar), 1.32 (d, J = 6.8 Hz, 3H, CH3, Ar). 13C NMR (CDCl3, 100 MHz): δ 153.1 (Pz), 151.5 (Pz), 145.8 (Pz), 142.8 (Pz), 137.5 (Py), 132.8 (Py), 130.2 (Py), 129.0 (Py), 128.7 (2C, Py), 126.4 Py), 124.8 (Py), 122.6 (Py), 122.1 (2C, Py), 121.3 (Py), 101.2 (Ar), 96.7 (Ar), 81.3 (2C, Ar), 80.5 (2C, Ar), 30.6 (Cym), 22.1 (2C, Cym), 18.9 (Cym), 14.8 (Pz), 12.2 (Pz). Anal. Calcd for C28H25N5SRuCl2: C, 52.91; H, 3.96; N, 11.02. Found: C, 53.27; H, 4.04; N, 10.88.

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tometer facility and helping with X-ray data analysis and solving of the crystal structure.

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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.organomet.7b00493. UV−vis absorption spectra from photoisomerization studies, orbital assignments of the most intense transitions obtained in the TD-DFT calculations, calculated structures, NMR spectra, and X-ray crystallographic data for 6a (PDF) Accession Codes

CCDC 1556869 contains 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 data_ [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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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*K.G.: e-mail, [email protected], tel, 757727-5475. ORCID

Kesete Ghebreyessus: 0000-0003-4273-7556 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. National Science Foundation (NSF) for financial support through the PREM (Award Number 1523620) and CREST (HRD-1137747) grants. The authors also thank Dr. Robert D. Pike of the College of William and Mary for providing access to a single-crystal X-ray diffracJ


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