Rhenium-Imido Corroles

Metallocorroles involving 5d transition metals are currently of interest as near-IR phosphors and as photosensitizers for oxygen sensing and photodynamic therapy. Their syntheses, however, are often bedeviled by capricious and low-yielding protocols. Against this backdrop, we describe rhenium-imido corroles, a new class of 5d metallocorroles, synthesized simply and in respectable (∼30%) yields via the interaction of a free-base corrole, Re2(CO)10, K2CO3, and aniline in 1,2,4-trichlorobenzene at ∼190 °C in a sealed vial under strict anaerobic conditions. The generality of the method was shown by the synthesis of six derivatives, including those derived from meso-tris(pentafluorophenyl)corrole, H3[TPFPC], and five different meso-tris(p-X-phenyl)corroles, H3[TpXPC], where X = CF3, F, H, CH3, OCH3. Single-crystal X-ray structures obtained for two of the complexes, Re[TpFPC](NPh) and Re[TpCF3PC](NPh), revealed relatively unstrained equatorial Re–N distances of ∼2.00 Å, a ∼ 0.7-Å displacement of the Re from the mean plane of the corrole nitrogens, and an Re–Nimido distance of ∼1.72 Å. Details of the corrole skeletal bond distances, diamagnetic 1H NMR spectra, relatively substituent-independent Soret maxima, and electrochemical HOMO–LUMO gaps of ∼2.2 V all indicated an innocent corrole macrocycle. Surprisingly, unlike several other classes of 5d metallocorroles, the Re-imido complexes proved nonemissive in solution at room temperature and also failed to sensitize singlet oxygen formation, indicating rapid radiationless deactivation of the triplet state, presumably via the rapidly rotating axial phenyl group. By analogy with other metal-oxo and -imido corroles, we remain hopeful that the Re-imido group will prove amenable to further elaboration and thereby contribute to the development of a somewhat challenging area of coordination chemistry.


■ INTRODUCTION
The 5d metallocorroles are an unusual class of complexes that encapsulate a large 5d transition metal ion within a sterically constrained macrocyclic ligand. 1,2 Unsurprisingly in view of the structural mismatch inherent in their structures, their syntheses require highly specific conditions of reagent, solvent, and temperature and typically afford products in poor yields. Even the best conditions for gold insertion are capricious, unusually sensitive to impurities, and afford yields of only about 20%, 3−6 while the yields for platinum insertion are considerably worse, 7,8 well under 5%. These shortcomings notwithstanding, the 5d metallocorroles are of considerable interest. Once synthesized, they are surprisingly rugged. In addition, a number of them exhibit room-temperature near-IR phosphorescence, 9−14 which has led to applications as oxygen sensors 10−12 and as photosensitizers in photodynamic therapy 13,15 and dye-sensitized solar cells. 13 Particularly promising in this regard are the rhenium(V)-oxo corroles, which are accessible both simply and in fairly high yields. 16 Presented herein are a new class of 5d metallocorroles rhenium-imido corroleswhich have now been synthesized in respectable yields. Like Mo V O, 17 Re V O, 16 and Ta(V)-imido 18 corroles, the present complexes may also serve as starting materials for new axial ligation chemistry, an aspect of 5d metallocorroles that is still in its infancy. 19,20 ■ RESULTS AND DISCUSSION Synthesis and Proof of Composition. Given the extreme oxophilicity of rhenium, the synthesis of rhenium-imido corroles is a potentially tricky proposition. After a fair amount of trial and error, the reaction conditions that we ultimately came up with proved simple. A free-base meso-triarylcorrole, Re 2 (CO) 10 , K 2 CO 3 , and aniline, upon heating at ∼190°C in 1,2,4-trichlorobenzene under strict exclusion of oxygen, afforded the desired rhenium(V)-phenylimido corroles in 28−35% yields along with smaller quantities (<10%) of the corresponding Re V O corroles (Scheme 1). The generality of the method was shown by the synthesis of six derivatives, including those derived from meso-tris(pentafluorophenyl)corrole, H 3 [TPFPC], and five different meso-tris(p-X-phenyl)-corroles, H 3 [TpXPC], where X = CF 3 , F, H, CH 3 , OCH 3 . The formation of the desired phenylimido complexes seemed immediately plausible upon mass spectrometric analysis of the crude products, an inference that was soon bolstered by 1 H and 19 F NMR spectroscopy. Although 1 H NMR spectra showed a number of broad signals at room temperature, they sharpened at −20°C, allowing essentially full assignment of the spectra. This sharpening reflects slowing of meso-phenyl rotation at low temperature, resulting in nonequivalent o, o′ and m, m′ signals, a phenomenon that has also been observed in Re V O, 16 Os VI N, 21 and other metallocorroles with a squarepyramidal metal center ( Figure 1).
X-ray Crystal Structures. Unambiguous proof of composition and structure of the new complexes came from two single-crystal X-ray analyses (Table 1) 30 UV−Vis and Electrochemical Studies. Electronic absorption spectroscopy ( Figure 4) and electrochemical studies ( Figure 5), summarized in Table 2, provided additional electronic-structural insight. Qualitatively, the electronic absorption spectra of the new complexes resemble those of their Re V O, 16 Os VI N, 21 Au III , 5 and Pt IV congeners, 8 consisting of sharp Soret bands, distinctive double-humped Q bands, and a couple of weaker features in between. Furthermore, the 434 nm Soret maxima are essentially independent of the para substituent on the meso-phenyl groups, an observation that has been empirically correlated with an innocent corrole 3−   indicative of an innocent macrocycle and of purely ligandcentered oxidation and reduction processes. 30,43 Interestingly, both the oxidation and reduction potentials of the present imido complexes are some 100−150 mV downshifted relative to those of their oxo congeners, 16 an apparent reflection of the greater π-donating ability of the axial imido ligand. Photophysical Studies. The photophysical properties of three of the Re-imido corroles, i.e., Re[TpCF 3 PC](NPh), Re[TPC](NPh), and Re[TpOCH 3 PC](NPh), were investigated in anoxic toluene at room temperature and in a 2:3 v/v toluene/tetrahydrofuran frozen glass at 77 K. In contrast to rather emissive ReO corroles, 14 the new Re-NPh complexes were found to be nonemissive at room temperature ( Figure  S19). However, experiments at 77 K revealed moderately strong phosphorescence (with quantum yields of 2.8−3.4%) of all the complexes (Figures 6, S20, and S21; Table 1). The emission of the Re-NPh corroles is located in the NIR part of the spectrum (λ max = 749−761 nm), as for previously reported rhenium(V)-oxo corroles. The excitation spectra (Figures 6, S20, and S21) match very well with the absorption spectra of the dyes (Figure 4). At 77 K, the intensity decay profile is biexponential, with two components of around 45 and 100 μs (Table 3). Interestingly, the relative contributions of the components (∼33 and 67%, respectively) are very similar for all the dyes.
In addition to the photophysical studies, singlet oxygen sensitization was investigated for one of the new complexes, Re[TPC](NPh), using a literature method. 11,44 The method utilizes 9,10-dimethylanthracene as a singlet oxygen acceptor and methylene blue as a standard (whose quantum yield for singlet oxygen formation is 0.48). Formation of singlet oxygen by Re[TPC](NPh) was not detected ( Figure S22). By comparison, we previously showed that even moderately phosphorescent Os VI N corroles are potent singlet oxygen sensitizers (quantum yields of singlet oxygen formation 0.76−    Moreover, we also observed weak room-temperature phosphorescence after immobilization of the complex within a rigid matrix (polystyrene, Figure S23). Notably, polystyrene is rather similar to toluene in terms of polarity and its compatibility with the rhenium complex, and also to the frozen glass in terms of its presumed ability to inhibit rotation of the axial phenyl substituent. Interestingly, the intensity decay profile in polystyrene is monoexponential, with τ 0 = 75 μs (anoxic conditions).

■ CONCLUSION
In summary, we have developed a simple and general synthesis of Re-imido corroles, a welcome addition, in our opinion, in an area that is still bedeviled by capricious and low-yielding synthetic protocols. In addition to corrole peripheral substituents, the axial phenylimido group may provide an additional handle for structural diversification. Two singlecrystal X-ray structures, electronic absorption spectra, and electrochemical studies are all strongly indicative of an innocent Re V (NPh)-corrole 3− formulation for the new complexes. Interestingly, unlike several other classes of 5d metallocorroles, including ReO, OsN, Au, and Pt(IV) corroles, the Re-imido corroles proved nonemissive in anoxic toluene at room temperature. In a frozen glass at 77 K, however, they proved moderately phosphorescent, with quantum yields of 2.8−3.4%. The complexes also do not sensitize singlet oxygen formation in solution at room temperature, suggesting rapid radiationless deactivation of the triplet state, presumably as a result of rapid rotation of the axial phenyl group. By analogy with other metal-oxo and metal-imido corroles, we are optimiztic that the present complexes will help develop new axial ligation chemistry, an aspect of 5d metallocorroles that is still rather underdeveloped.
Standard Analytical Methods. UV−visible−NIR spectra were recorded on an HP 8453 spectrophotometer. 1 H NMR spectra were recorded on 400 MHz Bruker Avance III HD spectrometer equipped with a 5 mm BB/ 1 H SmartProbe at 253 K in CDCl 3 or CD 2 Cl 2 and referenced to residual CHCl 3 at 7.26 ppm and CH 2 Cl 2 at 5.31 ppm. Mass spectra were recorded on a Thermo Scientific LTQ Orbitrap XL spectrometer with an ION-MAX electrospray ion source in positive mode.
Cyclic voltammetry was carried out at 298 K with an EG&G Model 263A potentiostat having a three-electrode system−a glassy carbon working electrode, a platinum wire counterelectrode, and a saturated calomel reference electrode (SCE). Anhydrous CH 2 Cl 2 (Aldrich) was used as solvent and tetrakis(n-butyl)ammonium perchlorate, recrystallized twice from absolute ethanol, and dried in a desiccator for at least 2 weeks, was used as the supporting electrolyte. The reference electrode was separated from the bulk solution using a fritted-glass bridge filled with the solvent/supporting-electrolyte mixture. The electrolyte solution was purged with argon for at least 2 min, and all measurements were carried out under an argon blanket. All potentials were referenced to the SCE. Elemental analyses were obtained from Atlantic Microlab, Inc.
General Procedure for the Synthesis of Re[TpXPC](NPh). To a 20 mL microwave vial containing 1,2,4-trichlorobenzene (10 mL) and a magnetic stirring bar was added a free-base corrole, H 3 [TpXPC] or H 3 TPFPC (0.125 mmol), Re 2 (CO) 10 (0.25 mmol), aniline (0.3 mL), and potassium carbonate (100 mg). The contents were sealed and deoxygenated with a flow of argon (via needles) for 10 min. The argon line was removed, and the vial was heated in an oil bath at 190°C for 16 h with constant stirring. Completion of the reaction was indicated by the disappearance of the Soret absorption of the freebase corrole and appearance of a new Soret maximum at ∼436 nm. Upon cooling, the reaction mixture was loaded directly on to silica gel column with n-hexane as the mobile phase. The 1,2,4-trichlorobenzene was first removed by eluting with pure hexane. Different solvent mixtures were then used to elute the various reddish Reimido/ReO corrole mixtures, namely, 3:1 v/v n-hexane/dichloromethane for X = CF 3 , H, CH 3 , F, and for TPFPC; 2:1 v/v n-hexane/ dichloromethane for X = OCH 3 . All fractions with λ max ∼ 436 nm were collected and dried. The resulting products, which still consisted of mixtures of Re-imido and ReO corroles, were then subjected to preparative thin-layer chromatography on silica plates with 3:1 nhexane/dichloromethane (for X = CF 3 , H, CH 3 , F, and TPFPC) or 2:1 n-hexane/dichloromethane (for X = OCH 3 ). The first light-red band corresponded to the desired Re-imido corroles and a second dark-red band to the ReO corroles.
X-ray quality crystals of the two Re-imido corroles (X = F and CF 3 ) were obtained by slow diffusion of methanol vapor into concentrated benzene solutions of the complexes. The yields and analytical details for the different Re-imido corroles are as follows.   49 and refined by full-matrix least-squares on F 2 (SHELXL-2018). 50 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were geometrically calculated and refined as riding atoms.
For Re[TpCF 3 PC](NPh), CELL_NOW was used to determined the two orientation matices. The relationship between the components was found to be 180°about the reciprocal axis 1 0 0. The data were integrated using the two matrices in SAINT and TWINABS was used to produce a merged HKLF4 file for structure solution and initial refinement and an HKLF5 file for final structure refinement. TWINABS indicated the twin ratio to be 83:17. Displacement parameter restraints were used to model the CF 3 group containing C40, F7−F9, and F7′−F9′.
Photophysical Studies. The photophysical properties of the Reimido corroles were measured on a Fluorolog 3 fluorescence spectrometer (Horiba, Japan) equipped with an NIR-sensitive R2658 photomultiplier (Hamamatsu, Japan). The spectra were corrected for the sensitivity of the photomultiplier and smoothing was (adjusting averaging function) applied to eliminate noise due to low signals. For measurements in toluene, the dye solutions in a sealable quartz cell (Hellma Analytics, Mulheim, Germany) were deoxygenated by bubbling argon (5.0, Linde gas, Austria) for 15 min. Measurements at 77K were conducted in a 2:3 v/v toluene/ tetrahydrofuran frozen glass using low-temperature accessories from Horiba. Luminescence quantum yields under these conditions were evaluated relative to N,N′-bis(2,6-diisopropylphenyl)-1,6,7,12-tetraphenoxyperylene-3,4,9,10-tetracarboxylic acid diimide ("fluorescent red", Kremer Pigmente, Germany), assuming Φ = 100% in toluene/ tetrahydrofuran glass at 77 K (Φ = 96% in chloroform at room temperature). 51 The excitation wavelength of 543 nm was used for all the complexes and the reference dye. Luminescence decay times were measured on the Fluorolog 3 spectrometer equipped with a DeltaHub module (Horiba Scientific) controlling a SpectraLED-460 lamp and with DAS-6 software for data analysis.
Singlet oxygen generation by Re[TPC](NPh) was studied using a previously described protocol. 11 The assay makes use of 9,10dimethylanthracene, which as a singlet oxygen acceptor decomposes at a rate proportional to the singlet oxygen quantum yield of the sensitizer.