Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Halogen Photoelimination from SbV Dihalide Corroles Christopher M. Lemon,† Seung Jun Hwang, Andrew G. Maher, David C. Powers,‡ and Daniel G. Nocera* Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States S Supporting Information *
ABSTRACT: Main-group p-block metals are ideally suited for mediating two-electron reactions because they cycle between Mn and Mn+2 redox states, as the one-electron state is thermodynamically unstable. Here, we report the synthesis and structure of an SbIII corrole and its SbVX2 (X = Cl, Br) congeners. SbIII sits above the corrole ring, whereas SbV resides in the corrole centroid. Electrochemistry suggests interconversion between the SbIII and SbVX2 species. TD-DFT calculations indicate a HOMO → LUMO+2 parentage for excited states in the Soret spectral region that have significant antibonding character with respect to the Sb−X fragment. The photochemistry of 2 and 3 in THF is consistent with the computational results, as steady-state photolysis at wavelengths coincident with the Soret absorption of SbVX2 corrole lead to its clean conversion to the SbIII corrole. This ability to photoactivate the Sb−X bond reflects the proclivity of the pnictogens to rely on the PnIII/V couple to drive the two-electron photochemistry of M−X bond activation, an essential transformation needed to develop HX-splitting cycles.
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INTRODUCTION Solar fuel cycles rely on the production of hydrogen along an endergonic reaction profile that may be traversed with an input of solar light. Whereas water splitting is well-documented as a source for renewable H2,1,2 HX splitting (X = Cl, Br) to generate H2 for energy storage in the form of a redox flow battery3 or chemical fuel4 has been less explored. Both HX and H2O splitting store approximately the same amount of energy per electron (when X = Cl), but the HX-splitting reaction is kinetically easier to manage, as it is a two-electron, two-proton process, rather than the four-electron, four-proton process of water splitting.4,5 The photochemistry of metal−halogen (M−X) bond activation is the crucial kinetic and thermodynamic determinant to establish an HX-splitting photocycle. Most successful M−X photochemistry involves complexes of second and third row transition metals,5−12 though it has recently been shown that complexes of Ni can promote the phototransformation.13,14 M−X photoactivation has been extended to complexes containing main-group metals as well. Gabbai ̈ and co-workers have utilized bimetallic complexes comprising Sb paired with a transition metal (Pt or Pd).9,15,16 In these complexes, Sb serves as an ancillary site for halide coordination, and it works in concert with the transition metal to effect X2 elimination. Seferos and co-workers have observed elimination from a maingroup metal via the Te IV /Te II photocycle offered by telluorphene dihalides.17−19 As highlighted by this work, main-group p-block elements are ideal for managing the twoelectron photochemistry of halogen elimination, as the Mn/ Mn+2 couple prevails owing to the high energy of the intermediary Mn+1 redox level.20−23 © XXXX American Chemical Society
With the intent of further exploring the inherent twoelectron redox photochemistry of p-block elements as it pertains to M−X activation, we turned our attention to Sb, which derives an oxidation−reduction chemistry from the SbIII/ SbV redox couple. Corrole is an excellent ligating platform for Sb because it is trianionic and thus ideal for housing the +3 charge of the SbIII and SbVX2 centers that are central for establishing a two-electron photoredox cycle. Corroles, like porphyrins, are aromatic 18-π-electron tetrapyrrole macrocycles, but with a contracted 23-atom core that results from a direct pyrrole−pyrrole linkage. Facile one-pot synthetic methods have led to a surge in corrole chemistry24−26 and an attendant application of these molecules as catalysts, photosensitizers, and imaging agents with an emphasis on transitionmetal metallocorroles.27−29 Studies of main-group corroles targeting light main-group elements such as boron, aluminum, phosphorus, gallium, and germanium have revealed these complexes to be highly fluorescent, rendering them suitable probes for biological imaging in the cases of phosphorus and gallium.30 Heavier analogues of periods 5 and 6 have been less well studied (Sn,31,32 Sb,33,34 Bi,35,36 Pb37) or lacking (Tl), though such compounds are of interest because the π-aromatic ring of the corrole engenders large cross sections for the absorption of light, which may be channeled nonradiatively to the metal center to drive photochemical transformations of small molecules coordinated to the metal center. The photochemistry of Sb corrole complexes reveals them to be competent photosensitizers, akin to their Sb porphyrin counterparts.38 The excited state of SbV-oxo corroles may Received: February 7, 2018
A
DOI: 10.1021/acs.inorgchem.8b00314 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry generate singlet oxygen,33 and as a result, they have been used for the photoinactivation of mold spores.39 Additionally, Sb porphyrins40 and corroles41 have been utilized as photosensitizers for bromide oxidation. In both cases, bromide oxidation occurs via an outer-sphere electron transfer process that is coupled to the two-electron reduction of O2 to H2O2. As an alternative to manipulating the Sb−X bond by outer-sphere electron transfer, we show here that the Sb−X bond may be activated directly by photoexciting SbX2 corroles (X = Cl, Br; Scheme 1).
solution turned from green to blue-violet. After solvent removal, the compound was recrystallized from a 1/1 mixture of hexanes and CH2Cl2 (20 mL) overnight. Fine purple needles were collected on a glass frit and rinsed with hexane, affording 56 mg of 2. The collected filtrate was crystallized a second time to afford an additional 8 mg of the compound (91% total yield after two crystallizations). 1H NMR (500 MHz, CDCl3, 25 °C): δ 4.13 (s, 3H), 8.42 (d, J = 8.0 Hz, 2H), 8.54 (d, J = 8.1 Hz, 2H), 9.12 (d, J = 4.8 Hz, 2H), 9.14 (d, J = 4.2 Hz, 2H), 9.17 (d, J = 4.6 Hz, 2H), 9.68 (d, J = 4.2 Hz, 2H). 19F NMR (470 MHz, CDCl3, 25 °C): δ −162.03 (m, 4F), −152.04 (t, J = 20.9 Hz, 2F), −137.52 (dd, J1 = 23.5 Hz, J2 = 6.8 Hz, 4F). Anal. Calcd for M+, M = C39H15Cl2F10N4O2Sb 951.9451, found LD-MS 951.9410. UV−vis (toluene), λ in nm (ε in 103 M−1 cm−1): 424 (212), 540 (10), 577 (16), 609 (30). Emission (toluene, λexc 425 nm), λ in nm: 619, 672. [10-(4-Methoxycarbonylphenyl)-5,15-bis(pentafluorophenyl)corrolato]dibromoantimony(V) (3). In a 20 mL scintillation vial, 11 mg of 1 (12 μmol) was dissolved in 3 mL of CH2Cl2. A bromine solution was prepared by dissolving 50 mg of Br2 (0.31 mmol) in 5 mL of CH2Cl2. A 0.3 mL aliquot of this Br2 solution (19 μmol) was added to the corrole solution, and the color immediately changed from green to blue-violet. The resultant mixture was stirred for 3 min. After this time, TLC confirmed that starting complex 1 had been consumed. Solvent was removed to afford 11 mg (85% yield) of 3 as a purple solid. NMR demonstrated that the complex was pure, obviating the need for further purification. 1H NMR (500 MHz, CDCl3, 25 °C): δ 4.13 (s, 3H), 8.42 (d, J = 8.0 Hz, 2H), 8.54 (d, J = 8.1 Hz, 2H), 9.09 (d, J = 4.7 Hz, 2H), 9.11 (d, J = 4.2 Hz, 2H), 9.13 (d, J = 4.8 Hz, 2H), 9.68 (d, J = 4.3 Hz, 2H). 19F NMR (470 MHz, CDCl3, 25 °C): δ −162.02 (m, 4F), −151.98 (t, J = 21.0 Hz, 2F), −137.42 (dd, J1 = 23.6 Hz, J2 = 6.9 Hz, 4F). Satisfactory mass data could not be obtained because the compound readily dehalogenated during LD-MS; only compound 1 was observed in the mass spectrum. UV−vis (toluene), λ in nm (ε in 103 M−1 cm−1): 427 (138), 542 (7.2), 578 (11), 613 (19). Emission (toluene, λexc 425 nm), λ in nm: 623, 675. NMR and MS Methods. NMR spectra were recorded on a Varian Inova-500 NMR spectrometer at the Harvard University Department of Chemistry and Chemical Biology Laukien-Purcell Instrumentation Center. 1H NMR spectra were internally referenced to the residual solvent signal (δ 7.26 for CHCl3 in CDCl3),44 while 19F NMR spectra were externally referenced to α,α,α-trifluorotoluene (δ −63.72). Mass spectra were recorded on a Bruker UltrafleXtreme MALDI-TOF/TOF mass spectrometer in positive ion mode at the Small Molecule Mass Spectrometry Facility, part of the Harvard FAS Center for Systems Biology. All samples were internally calibrated using Bruker peptide calibration standard II (Bruker Biosciences) mixed with α-CHCA as a matrix. Electronic Spectroscopic Methods. UV−vis absorption spectra were acquired using a Cary 5000 spectrometer (Agilent). Steady-state emission spectra were recorded on a Photon Technology International (PTI) QM4 fluorometer equipped with a 150 W Xe arc lamp and a Hamamatsu R2658 photomultiplier tube. Relative quantum yields of Sb corroles in toluene (η = 1.4961) were determined using Nile blue in EtOH (η = 1.3611) as the reference (ϕref = 0.27)45 according to the equation
Scheme 1. Synthetic Scheme for the Oxidation of 1 Using PhICl2 or Br2 to Give the SbV Derivatives 2 and 3, Respectively
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EXPERIMENTAL SECTION
Materials. The following materials were used as received: hexanes, dichloromethane (CH2Cl2), toluene, ethanol (EtOH), pyridine, antimony(III) chloride, bromine, inhibitor-free tetrahydrofuran (THF), and Nile blue from Sigma-Aldrich and chloroform-d (CDCl3) from Cambridge Isotope Laboratories. Tetrabutylammonium hexafluorophosphate ([TBA][PF6]) from Sigma-Aldrich was recrystallized from EtOH and subsequently dried under vacuum prior to use. Acetonitrile (MeCN) for electrochemical experiments was obtained from a solvent drying system (Pure Process Technologies) and stored over 3 Å molecular sieves. Iodobenzene dichloride (PhICl2)42 and the free base corrole 10-(4-methoxycarbonylphenyl)-5,15-bis(pentafluorophenyl)corrole (1-H3)43 were prepared according to literature methods. [10-(4-Methoxycarbonylphenyl)-5,15-bis(pentafluorophenyl)corrolato]antimony(III) (1). In a 100 mL round-bottom flask, 99 mg of 1-H3 (0.13 mmol) and 200 mg of SbCl3 (0.877 mmol) were dissolved in 10 mL of pyridine. The dark green solution was heated at 100 °C for 30 min. More SbCl3 (442 mg, 1.94 mmol) was added to the reaction mixture, and it was heated at 100 °C for 2 h. Solvent was removed by rotary evaporation, and the residue was purified on a silica gel column using a 1/1 mixture of hexanes and CH2Cl2; the desired product eluted as a dark green solution. After solvent removal, 99 mg (87% yield) of 1 was isolated as a dark green solid. 1H NMR (500 MHz, CDCl3, 25 °C): δ 4.12 (s, 3H), 8.07 (bs, 1H), 8.46 (bs, 3H), 8.79 (d, J = 4.2 Hz, 2H), 8.84 (d, J = 4.7 Hz, 2H), 8.93 (d, J = 4.5 Hz, 2H), 9.33 (d, J = 4.2 Hz, 2H). 1H NMR (500 MHz, CDCl3, −50 °C): δ 4.10 (s, 3H), 8.07 (d, J = 8.0, 1H), 8.40 (d, J = 7.9, 1H), 8.44 (d, J = 8.0, 1H), 8.50 (d, J = 8.0, 1H), 8.81 (d, J = 4.1 Hz, 2H), 8.86 (d, J = 4.6 Hz, 2H), 8.96 (d, J = 4.4 Hz, 2H), 9.36 (d, J = 4.2 Hz, 2H). 19F NMR (470 MHz, CDCl3, 25 °C): δ −163.21 (m, 2F), −163.05 (m, 2F), −154.10 (t, J = 20.9 Hz, 2F), −138.82 (dd, J1 = 24.3 Hz, J2 = 7.8 Hz, 2F), −138.22 (dd, J1 = 24.6 Hz, J2 = 7.6 Hz, 2F). Anal. Calcd for M+, M = C39H15F10N4O2Sb 882.0074, found LD-MS 882.0079. UV−vis (toluene), λ in nm (ε in 103 M−1 cm−1): 445 (111), 460 (103), 542 (7.8), 557 (7.6), 603 (8.4), 650 (22). Emission (toluene, λexc 460 nm), λ in nm: 669, 730. [10-(4-Methoxycarbonylphenyl)-5,15-bis(pentafluorophenyl)corrolato]dichloroantimony(V) (2). In a 100 mL round-bottom flask, 65 mg of 1 (74 μmol) was dissolved in 20 mL of CH2Cl2 and 38 mg of freshly prepared PhICl2 (0.14 mmol) was added. The reaction mixture was stirred at room temperature for 1 h, protected from ambient light. Over the course of the reaction, the
2 ⎛ ∇ ⎞⎛ η ⎞ ϕsam = ϕref ⎜ sam ⎟⎜⎜ sam ⎟⎟ ⎝ ∇ref ⎠⎝ ηref ⎠
(1)
where ∇ is the slope of the plot of integrated fluorescence intensity versus absorbance (constructed from five points) and η is the refractive index of the solvent. Air-free corrole samples for transient absorbance spectra, triplet lifetimes, and photolysis experiments were prepared using three cycles of freeze−pump−thaw (FPT) to pressures below 10−5 Torr. Steady-state photochemical reactions were performed using a 1000 W high-pressure Hg/Xe arc lamp (Oriel). The beam was passed through a water-jacketed filter holder containing a 305 or 380 nm long-pass filter and then through an iris and collimating lens before entering the sample, which was in a quartz cuvette placed in a waterjacketed sample holder to maintain a constant temperature. B
DOI: 10.1021/acs.inorgchem.8b00314 Inorg. Chem. XXXX, XXX, XXX−XXX
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parameter exchange functional50−52 and the Lee−Yang−Parr nonlocal correlation functional (B3LYP),53 as implemented in the Gaussian 09, Revision D.01, software package.54 For nonmetal atoms (H, C, N, O, F, Cl, and Br), a polarized split-valence triple-ζ basis set that includes p functions on hydrogen atoms and d functions on other atoms (i.e., the 6-311G(d,p) or 6-311G** basis set) was used. A Wood−Boring55 quasi-relativistic effective core potential (i.e. MWB46) was used for Sb. Calculations were performed with a polarizable continuum (PCM) solvation model in toluene using a polarizable conductor calculation model (CPCM).56,57 Geometries were confirmed as local minima structures by calculating the Hessian matrix and ensuring that no imaginary eigenvalues were present. Excited state calculations were preformed using time-dependent DFT (TD-DFT)58−62 with the same functionals, basis sets, and solvation details as the ground state, but with the inclusion of diffuse functions on all nonmetal atoms (i.e., the 6-311++G** basis set). Excited state energies were computed for the 15 lowest singlet and triplet excited states. Optimized geometries and molecular orbitals were rendered in the program Avogadro.63 Simulated UV−vis spectra were generated in the program Gauss View 5 by broadening transition lines with Gaussian functions with a half-width of 0.06 eV. X-ray Crystallographic Details. Diffraction-quality crystals of 1 (from toluene) and 2 (from dichloromethane/pentane) were obtained at −30 °C in a N2-filled glovebox, affording crystals as green blocks and red plates, respectively. X-ray diffraction data for 1 were collected on a Bruker three-circle platform goniometer equipped with an Apex II CCD and an Oxford cryostream cooling device at 100 K. Radiation was generated from a graphite fine focus sealed tube Mo Kα source (0.71073 Å). X-ray diffraction data for 2 were collected on a vertically mounted Bruker D8 three-circle platform goniometer equipped with an Apex II CCD and an Oxford Diffraction Helijet cooling device (15 K) with synchrotron radiation (0.41328 Å) supplied by ChemMatCARS, located at the Advanced Photon Source, Argonne National Laboratory. Crystals were mounted on a glass fiber using Paratone-N oil. Data were collected as a series of φ and ω scans. Data were integrated using the Bruker SAINT software package and scaled with a multiscan absorption correction using SADABS.64 The structure was solved by intrinsic phasing methods using SHELXS-97 and refined against F2 on all data by full-matrix least squares with SHELXL-97.65 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed at idealized positions and refined using a riding model.
Electrochemical Methods. Electrochemical measurements were made in a glovebox under a nitrogen atmosphere using a CH Instruments 760D Electrochemical Workstation using CHI Version 10.03 software. Samples were prepared at concentrations of ∼1 mM with 0.1 M [TBA][PF6] as the supporting electrolyte in acetonitrile. Cyclic voltammograms (CVs) were recorded at a scan rate of 100 mV/ s using a glassy-carbon-button working electrode, a Ag-wire reference electrode, and a Pt-wire counter electrode. The CVs were internally referenced to ferrocene. Thin-layer UV−vis spectroelectrochemistry experiments were performed using a 0.5 mm path length quartz cell with an Ocean Optics USB4000 spectrophotometer and DT-Mini2GS UV−vis−NIR light source in conjunction with the CH electrochemical workstation. Samples were prepared at concentrations of ∼0.2 mM with 0.1 M [TBA][PF6] in acetonitrile. Bulk electrolysis was performed using a Pt-flag working electrode, a Ag-wire reference electrode, and a Pt-wire counter electrode. Femtosecond and Nanosecond Laser Methods. Femtosecond emission lifetime measurements were acquired using a Libra-F-HE (Coherent) chirped-pulse amplified Ti:sapphire laser system that has been previously described.46 The 800 nm laser output was used to pump an OperA Solo (Coherent) optical parametric amplifier (OPA); excitation pulses of 400 nm were produced via fourth-harmonic generation of the signal using a BBO crystal, and the pulse power was attenuated to 2−3 mW at the sample using neutral density filters. Emission lifetimes were measured on a Hamamatsu C4334 Streak Scope streak camera, which has been described elsewhere.47 The emission signal was collected over a 140 nm window centered at 630 nm with a 1 or 2 ns time window using a Stanford Research Systems DG535 delay generator. Nanosecond transient absorption (TA) spectra of corroles were acquired using a previously reported system.48,49 Pump light was provided by the third harmonic (355 nm) of a Quanta-Ray Nd:YAG laser (Spectra Physics) operating at 10 Hz. The pump light was passed through a BBO crystal in an optical parametric oscillator (OPO), yielding a visible frequency that was tuned to 650 nm for 1 or 610 nm for 2 and 3. Excitation light was attenuated to 0.5−2.0 mJ per pulse for all experiments using neutral density filters. Probe white light was generated using a 75 W Xe arc lamp (PTI). The probe beam was aligned with the sample while the laser pump beam was positioned at 15° with respect to the white light probe, and both beams were focused on the sample. After it exited the sample, the light entered an iHR320 monochromator (Horiba Scientific) and was dispersed by a blazed grating (500 nm, 300 grooves/mm) centered at 450 nm. The entrance and exit slits of the monochromator were set to provide a spectral resolution of 2 nm. TA spectra were collected using a gated intensified CCD camera (DH520-25F-01, Andor Technology). Acquisition delays and gate times for the CCD were set using a Stanford Research Systems DG535 delay generator, which was synchronized to the Q-switch output of the laser. The final data were calculated from a combination of four spectra: I (pump on/probe on), IF (pump on/probe off), I0 (pump off/probe on), and IB (pump off/probe off). The change in optical density from the resultant TA spectra was determined as
⎛ I − IF ⎞ ΔOD = − log⎜ ⎟ ⎝ I0 − IB ⎠
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RESULTS AND DISCUSSION Synthesis and Structure. The SbIII corrole (1) was prepared by heating the free-base corrole with SbCl3 in pyridine to give the product in 87% yield. The room-temperature 1H NMR spectrum of 1 exhibits two broad features that integrate to four protons, and they are attributed to the 10-aryl substituent. When the sample is cooled, the broad signals resolve into four doublets (Figure S1), indicating that each proton is chemically unique. This result implies out-of-plane metal coordination, rendering the two faces of the macrocycle inequivalent (Cs symmetry). At room temperature, the rotation of the meso-aryl ring results in broad signals. At low temperatures, the rotation is sufficiently slow on the NMR time scale to resolve the individual protons, but upon heating, rotation is fast and the signals coalesce. A similar observation has been ascribed to the out-of-plane coordination of BiIII to porphyrins.66−68 The asymmetry of 1 is also observed in the 19F NMR spectrum (see the Supporting Information), which displays two distinct signals for both the ortho and meta fluorine atoms, as a result of those atoms that reside above and below the corrole plane. The coordination geometry of 1 was confirmed by X-ray crystallography (Figure 1 and Table S1) and represents the first structurally characterized SbIII tetrapyrrole complex. The Sb center in 1 resides 0.974 Å above the mean N4 plane of the
(2)
which accounts for both sample emission and extraneous background light. In order to acquire these four spectra, pump and probe beams were selectively exposed to the sample using electronically controlled shutters (Uniblitz T132, Vincent Associates), which were triggered using a Stanford Research Systems DG535 delay generator synchronized to the Q-switch output of the laser. For singlewavelength kinetics measurements, the output signal from the sample was amplified by a photomultiplier tube (R928, Hamamatsu) and collected on a 1 GHz digital oscilloscope (9384CM, LeCroy); acquisition was triggered using a photodiode to collect scattered laser excitation light. Computational Details. Density functional theory (DFT) calculations were performed with the hybrid functional Becke threeC
DOI: 10.1021/acs.inorgchem.8b00314 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 2. Solid-state crystal structure of 2, showing one of the two corrole molecules in the asymmetric unit. Thermal ellipsoids are drawn at the 50% probability level; hydrogen atoms and solvent molecules have been removed for clarity.
Figure 1. Solid-state crystal structure of 1. Thermal ellipsoids are drawn at the 50% probability level; hydrogen atoms and solvent have been removed for clarity. The lower structure illustrates the out-ofplane coordination of the SbIII center.
corrole; this domed coordination motif is also observed for a BiIII corrole complex, in which the metal center sits 1.15 Å above the N4 plane.35,47 The out-of-plane coordination results from the large sizes of bismuth and antimony (90 pm ionic radius for SbIII and 110 pm for BiIII 69) in comparison to the N4 corrole cavity of the macrocycle. The SbIII corrole was oxidized with PhICl2 or Br2 to produce SbVX2 corrole compounds 2 and 3, respectively, in high yield (Scheme 1), thus adding to the halide series established with the previous preparation of the SbVF2 corrole.33 Unlike the asymmetric geometry of SbIII corrole 1, the 1H NMR spectra of both SbV derivatives exhibit symmetric resonances for the 10aryl substituent. This suggests that the metal center resides in the corrole plane, furnishing a compound with C2v symmetry and pseudo-octahedral geometry at the Sb center. The NMR result is confirmed by the solid-state structure of 2 (Figure 2 and Table S1). The corrole macrocycle exhibits a planar conformation with an average deviation from the mean 23atom corrole plane of 0.074 ± 0.072 Å. The SbV center resides only 0.022 Å out of the N4 plane, in contrast to the 0.974 Å displacement observed for 1. As expected for an oxidized metal center, Sb−N bond lengths (1.984−1.994 Å) are contracted relative to 1 (2.110−2.129 Å). The structural metrics of 2 are comparable to those reported for the SbVF2 corrole complex, which exhibits Sb−N bond lengths in the 1.971−1.979 Å range.33 As expected, the Sb−Cl bond distances (2.388 and 2.430 Å) are significantly longer than the analogous Sb−F bonds (1.932 and 1.940 Å). Optical Spectroscopy. The electronic absorption spectra of 1−3 (Figure 3) display intense Soret (B) bands in the near-
Figure 3. Steady-state absorption (solid lines) and emission (dashed lines) spectra of (a) 1 (blue line) (λexc 460 nm) and (b) 2 (red line) and 3 (green line) (λexc 425 nm), in aerated toluene. For compounds 2 and 3, emission spectra were recorded for absorbance-matched samples at the excitation wavelength (A425 = 0.2000 ± 0.0002).
UV region and weaker Q bands in the visible region. Compounds 2 and 3 exhibit an additional weak shoulder on the high-energy side of the Soret band. The low symmetry of 1 is reflected in the split Soret band in the absorption spectrum, resulting from lifting the degeneracy of the x,y polarization.47 Conversely, with their C2v symmetry, 2 and 3 (Figure 3b) display a single Soret band. D
DOI: 10.1021/acs.inorgchem.8b00314 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Compound 1 displays a weak emission band at 669 nm with a weak shoulder at ∼730 nm (Figure 3a). These features are assigned as fluorescence bands due to the small Stokes shift of 403 cm−1 with respect to the lowest energy Q band. The emission quantum yield is low (0.013%), and the lifetime is short ( 305 nm) of a freeze−pump− thawed sample of 3 in THF. The initial spectrum (green line) converts to that of 1 (brown line) over the course of 2 min. Spectra were recorded every 30 s.
S18). Prompt and quantitative photolysis of 2 and 3 to 1 is in evidence by the maintenance of isosbestic points during the course of the reaction. Shorter wavelengths were initially selected to excite the UV bands to the blue of the Soret. Clean conversion to photoproduct is also observed when irradiation wavelengths are limited to the Soret shoulder (λ >380 nm), albeit on longer time scales (Figure S19). The photochemical action spectrum was constructed by measuring quantum yields (ϕ) for a variety of excitation wavelengths (315, 410, 425, 435, 540, and 575 nm) using a ferrioxalate actinometer.80,81 These data are summarized in Figure 8 and Table S12. Across the action spectrum, 3 exhibits higher photoreactivity than 2, especially in the Soret spectral region. TD-DFT calculations predict four singlet states (S3−S6) in the Soret region of 3 (Table S9). Interestingly, S3 at 432 nm and S6 at 393 nm contain the largest contributions of LUMO+2 (70% and 84%, respectively), whereas S4 at 411 nm and S5 at 415 nm, which coincide with the Soret maximum, have
Figure 6. Illustration of the primary (97%) orbital transition associated with S5 of compound 2 that corresponds to the shoulder to the blue of the Soret band in the experimental spectrum. F
DOI: 10.1021/acs.inorgchem.8b00314 Inorg. Chem. XXXX, XXX, XXX−XXX
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mono- and dibrominated derivatives of 1.43 These results suggest that photogenerated bromine radicals react with the corrole, which effectively functions as an internal chemical trap. In summary, main-group corrole complexes provide the potential for two-electron photochemistry by converting between Mn and Mn+2 redox states. Pnictogen (Pn) metalloids such as Sb are well suited for a corrole ligand architecture because the 3− charge of the deprotonated corrole balances the 3+ charge of both the PnVX2 photoreactant and PnIII halogen photoelimination product. Indeed, SbVX2 corrole smoothly converts to SbIII corrole upon irradiation into the Soret spectral region in the presence of halogen atom traps. In the absence of a trap, the halogen atoms substitute on the periphery of the corrole ring. Trap-free halogen elimination may possibly be achieved with appropriate modification of the corrole ring, such as fluorination of the β-pyrrole positions.83
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Figure 8. Photochemical quantum yields (ϕ) for the reduction of 2 (red ●) and 3 (blue ■) in THF as a function of excitation wavelength. The absorption spectrum of 3 is included for reference. The numerical data are summarized in Table S12. Error bars are reflective of 1 standard deviation; it should be noted that some error bars are smaller than the size of the data point.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00314. Summaries of crystallographic data, 1H and 19F NMR spectra, TA spectra, cyclic voltammograms, and results of DFT and TD-DFT calculations (PDF)
significantly smaller LUMO+2 contributions (15% and 28%, respectively). The photochemical action spectrum of 3 seems to be consistent with these calculations, where the red and blue edges of the Soret band have higher photochemical quantum yields in comparison to the Soret maximum. Moreover, both compounds exhibit similar photoreaction quantum yields as excitation is moved deeper into the UV, where mixing of Sb, Sb−X, and corrole π-aromatic orbitals prevails in numerous orbitals. Compound 2 exhibits maximum activity in the UV region (ϕ315 = 0.17%), whereas 3 is most photoactive in the visible region (ϕ435 = 0.88%). The observed quantum yields of halogen elimination for 2 and 3 are similar to those reported for other main-group compounds (α-isoindigo tellurophene dihalides, ϕ505 = 0.17%;17 α-phenyl tellurophene dichloride, ϕ448 = 1.6%;18 an Sb−Pd bimetallic complex, ϕ354 = 0.58%16) and more photoactive than a related Sb porphyrin complex (ϕ545 = 0.1% for Br3− formation)40 but smaller than those for α-phenyl tellurophene dibromide (ϕ448 = 16.9%)18 and an Sb− Pt bimetallic complex (ϕ320 = 13.8%).9 The thermochemistry for halogen elimination from SbVX2 corrole is computationally predicted to be uphill (Tables S13− S15). The computational results may be experimentally validated by measuring the heat of reaction for the microscopic reverse transformation, the oxidation of 1 to 2 by chlorine. Figure S20 shows the thermogram for the oxidation of 1 with PhICl2, which simplifies thermochemistry measurements, as the oxidant can be weighed as a solid. A ΔHrxn value of −22.8 kcal/ mol was measured from the thermogram of Figure S20; using the heat of reaction for PhICl2 → PhI + Cl2 (ΔHrxn = +9.8 kcal/mol),82 ΔHrxn = +32.6 kcal mol−1 is measured for Cl2 elimination from 2. This stored energy, however, is not realized when halogen traps are employed such as THF, as is used here to drive the halogen elimination. We sought to avoid the need for a trap by performing the photochemistry in the solid state. Compound 3 was irradiated (λ >305 nm) in the solid state for 10 h under static vacuum, and evolved Br2 was measured colorimetrically using N,N-diethyl-1,4-phenylenediamine sulfate, as we have previously described.13 However, negligible quantities (
