Maximizing Property Tuning of Phosphorus Corrole Photocatalysts

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Maximizing Property Tuning of Phosphorus Corrole Photocatalysts through a Trifluoromethylation Approach Xuan Zhan,† Peter Teplitzky,† Yael Diskin-Posner,‡ Mahesh Sundararajan,§,∥ Zakir Ullah,§,∥ Qiu-Cheng Chen,† Linda J. W. Shimon,‡ Irena Saltsman,† Atif Mahammed,† Monica Kosa,†,⊥ Mu-Hyun Baik,*,§,∥ David G. Churchill,*,†,§,∥ and Zeev Gross*,† Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/19/19. For personal use only.

†

Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel Department of Chemical Research Support, Weizmann Institute of Science, Rehovot, 76100, Israel § Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS), Daejeon 34141, Republic of Korea ∥ Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡

S Supporting Information *

ABSTRACT: An eight-member series of CF3-substituted difluorophosphorus corroles was prepared for establishing a structure−activity profile of these high-potential photosensitizers. It consisted of preparing all four possible isomers of the monosubstituted corrole and complexes with 2-, 3-, 4-, and 5-CF3 groups on the macrocycle’s periphery. The synthetic pathway to these CF3-substituted derivatives, beginning with (tpfc)PF2, involves two different initial routes: (i) direct electrophilic CF 3 incorporation using FSO2CF2CO2Me and copper iodide, or (ii) bromination to achieve the 2,3,8,17,18pentabrominated compound using excess bromine in methanol. Crystallographic investigations revealed that distortion of the original planar macrocycle is evident even in the monosubstituted case and that it becomes truly severe for the penta-CF3-substituted derivative 5. There is a shift in redox potentials of about 193 mV per -CF3 group, which decreases to only 120 mV for the fifth one in 5. Differences in the electronic spectra suggest that the Gouterman four orbital model decreases in relevance upon gradual -CF3 substitution, a conclusion that was corroborated by DFT calculations. The very significant energy lowering of the frontier orbitals suggested that photoexcitation should lead to a highly oxidizing photocatalyst. This hypothesis was proven true by finding that the most synthetically accessible CF3-substituted derivative is an excellent catalyst for the photoinduced conversion of bromide to bromine (phenol, toluene, and benzene assay).

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INTRODUCTION Perfluorinated alkyls and aryls are intriguing in how they compare with standard hydrocarbon frameworks and fragments.1−23 These substituted versions bring distinct stability, polarizability, sterics, and electronics to bear on a given organic system, compared to their hydrocarbon counterparts (e.g., -CF3 versus -CH3 and -C6F5 vs -C6H5). The compact nature of the trifluoromethyl (-CF3) group (Hammett parameters of -CF3: σm = 0.43, σp = 0.54), when compared to other related groups such as, e.g., the pentaf luorophenyl group (-C6F5: σm = 0.26 σp = 0.27), has been utilized in molecular design.7−22 The -CF3-containing organic systems have led to intriguing reports about photolysis, radical behavior, skeleton rearrangement, and metal complexation for motifs as simple and straightforward as benzene.2−4,7 The -CF3 group vis-a-vis the -CH3 group (-CH3: σm = −0.07, σp = −0.17) is meta-directing and strongly deactivating. CF3 substitutions with various polypyrrolic macrocycles (e.g., porphyrinoids) have been reported.7−22 In 1999, a synthetic breakthrough in the corrole field involved the ability to obtain sufficient yields of corrole starting materials for further explorations. This was undertaken with perfluorinated hydrocarbon substituents in mind. By providing the © XXXX American Chemical Society

incipient meso positions of the macrocycle (5,10,15-positions) with -C6F5 groups (Figure S1), these robust and strongly electron-withdrawing groups helped alleviate the innate electron richness of the macrocycle. Sufficient product formation was enabled which helped unlock much chemistry since this report (1999); even more efficient methodologies have since been reported.23 While -CF3 substitution has recently emerged in the corrole literature, such exploration into macrocycle β-CF3 substitution and stepwise substitution is still in its infancy.24−41 The factors governing relative derivative stability and photoreactivity, which often lead to theoretical forays, have not been addressed yet. The rational design involving -CF3 substitution as an electron withdrawing group parameter continues unabated for deepening the research impact of porphyrinoids in photocatalysis and other applications. Since interests in photosynthesis and photocatalysis reach far into engineering and economic factors; the phosphorus(V) corrole offers clear options for inexpensive photosensitization/photocatalysis purposes.44−58 The PF2 Received: February 13, 2019

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DOI: 10.1021/acs.inorgchem.9b00436 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthetic Routes Applied for Obtaining the -CF3 Substituted Complexes 1a−1d, 2, 3, 4, and 5a

a

Umemoto’s reagent is 5-(trifluoromethyl)dibenzothiophenium tetrafluoroborate. The chemical structure of theoretically studied compound 8 is shown here also.

high pressure and under mercury lamp irradiation and with a constant oxygen purge.64 Lastly, solid graphite electrodes were employed for oxidizing bromide in wastewater.67 There is also a set of relevant and insightful reports from the Nocera laboratory which investigated the halogen photoreductive elimination from metallic centers (M−X activation). Examples of these involve late transition metal centers, namely, nickel,70 rhodium,69,71 palladium,69 and gold.68 Literature on the investigation of corroles as photocatalyst systems for halogen formation is very rare. Therefore, through this research and based on our inexpensive and metal-free phosphocorrole platform bearing varying electron-withdrawing substituents, we are seeking new insights, more efficient synthetic methods, and molecular tools for photocatalysis and photosensitization.

corrole (composed of only C, H, N, P, and F atoms), on top of the benefits of having a catalyst with no element that raises potential environmental or health concerns, is a thermodynamically and photostable platform29,34,36,37 that is conveniently addressed through theoretical studies. In particular, the use of DFT calculations and the consideration and comparison of the MO theoretical studies allow for a rationalization, not only of the geometry and changing of subtle electronics within a closely related set of compounds but also of fueling of a predictive power. Through a comparison of frontier orbitals, the reactivity behavior dynamics can be better understood; general trends and lessons can be formulated that are directly applicable to other macrocyclic systems. The ability to oxidize small molecules and feedstock simply and efficiently by way of channeling solar energy is a meaningful pursuit for industry and the world. The storage of species as ions on large scale and, for example, with regard to safety, utility, and ease of processing can not only help cut costs but also improve safety protocols; throughout the chemical sciences and industry, safety issues are critically important, and new ways to improve safety are important to consider. The halogens (X2), as with molecular oxygen, are excellent oxidants. Bromine oxidation reports from engineering, as well as reports from the chemical sciences, differ in their approach, conditions, and chemical composition.59−71 There are some relevant reports regarding bromide oxidation. First, in order to produce halogen, a literature report emphasizes the use of an n-type MoY2 (Y = S, Se)-based semiconducting photoanode which used LiBr as the halide source in aqueous media.59 Next, ruthenium complexes were employed to sensitize photochemical oxidation of bromide to bromine with the highest reported turnover number (TON) of 230.61,65,66 Some papers are more slanted toward engineering. In one report, chlorophyll was used for halide-to-halogen transfer in the air−salt interface within aqueous solutions.62 Next, an amorphous MnOx material was tested as a photocatalyst for bromine formation; in this study, methyl bromide served as the starting material and bromine source.58 Furthermore, the bromide-to-bromine photooxidation, without catalyst using natural water from the Dead Sea was studied at

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RESULTS AND DISCUSSION In order to confidently draw conclusions and identify general trends that connect the structure of corroles to properties and activity, a reasonably large library of compounds must first be assembled that is diverse in composition while maintaining a common general structural pattern. Systematic studies of corroles with 1 to 8 non-hydrogen elements on the β-pyrrole C atoms are almost unheard of, however, simply because these compounds are notoriously difficult to prepare. Selective substitution of 5,10,15-triarylcorroles is an enormous challenge that is best appreciated by the number of possible regioisomers: 4, 16, 28, 42, 28, 16, 4, and 1 for 1, 2, 3, 4, 5, 6, 7, and 8 identical substituents of the periphery, respectively.72 Thus, our goal was to prepare a significant number of CF3-substituted corroles, by not focusing on developing high yielding syntheses but on obtaining reasonable amounts of analytically pure derivatives. This approach outlined in Scheme 1 was highly rewarding: eight new complexes were isolated and fully characterized. Synthesis and Identification of Products. The last two decades have evidenced tremendous achievements regarding methodologies for introducing -CF3 groups into organic compounds; and the number of commercially available trifluoromethylating reagents has increased accordingly.73 The latter may be categorized as electrophilic, nucleophilic, B


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Inorganic Chemistry

Figure 1. 19F and 1H NMR spectra (in CDCl3, at 400 and 377 MHz, respectively) of (tpfc)PF2 and the four mono-CF3-substituted isomers, with (a) the three distinct spectral regions: the axial-F, -CF3, and para-F atom regions, and (b) of the β-pyrrole CH groups.

group.74 This was first tested on (tpfc)PF2, which has no leaving group on the β-pyrrole C atoms required for nucleophilic aromatic substitution reactions of the SNAr type. The results provided a clear-cut picture: the P−F and C−F bonds are inert, while (to our surprise) the C−H bonds did react under these conditions. All four possible mono-CF3 regioisomers were obtained,72 in a total yield of 33%, with a small preference for the formation of 1b (11%) relative to the other three products (7−8%). These results indicate that the reaction proceeded through a radical mechanism, consistent with the much larger reactivity of CH bonds relative to fluorine bound to carbon and heteroatoms. Utilization of a combination of 1H and 19F NMR spectroscopy led to a confident identification of the products as described below. 19 F NMR spectroscopy is an extremely powerful tool for product identification in this particular series of compounds, as the chemical shifts of the P-coordinated F atoms, the βpyrrole-CF3 substituents, and the meso-C6F5 groups are found

and radical reagents as they provide access to active nonfree + [CF3], −[CF3], and [CF3] species, with the difference in reactivity directed not only by the reagent but also by additives and reaction conditions. Regarding corroles, only the nucleophilic pathway was hitherto reported, by using Chen’s reagentmethyl-2,2-difluoro-2-(fluorosulfonyl)acetate (FSO2CF2CO2Me)for converting either C−Br or C−I bonds on the β-pyrrole positions to C−CF3.24,42,43 The emphasis of this work was on how CF3 substitution affects the (photo)physical and chemical properties of phosphorus corroles, which are most stable and safely characterized when the central P atom is coordinated by two axial fluorides on top of the four N atoms in the equatorial plane. Taking into account that (FSO2CF2CO2Me) is a precursor of CuCF3 under reaction conditions shown in Scheme 1, one concern that needed to be addressed was to determine whether its combination with (corrole)PF2 compounds will lead to CF3/F substitution reactions on either the P atom or the C6F5 C


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Inorganic Chemistry

element) the C2H (equivalent to C18H, as long as a C2 symmetry axis does exist) resonance is always positioned at the lowest field (e.g., 9.46 ppm in (tpfc)PF2) and (ii) the CF3 group induces further upfield shifts of vicinal protons, it is quite clear that the spectrum wherein the unique doublet is found at the lowest field (9.82 ppm, Δδ = 0.36 ppm relative to its position in (tpfc)PF2) corresponds to compound 1b. The doublet for C3H in the spectrum of the already identified 1a appears at 9.17 ppm, which is 0.13 ppm shifted (Δδ) relative to its position in (tpfc)PF2. The proton represented by the doublet is always strong under the influence of the neighboring -CF3 group: in the other cases, the doublets at 9.28 and 9.44 ppm (Δδ = 0.36 and 0.34 ppm, respectively) that appear in the spectra of compounds 1c and 1d correspond to the C8H and C7H atoms, respectively, and are influenced by the CF3 group positioned at C7H and C8H, respectively. The validity of these analyses was confirmed by chemical shift calculations (Table 1, right column) and eventually also by the crystal structures of 1b and 1c (vide infra). Realizing that the results obtained with FSO2CF2CO2Me do point toward a radical-like trifluoromethylation, (tpfc)PF2 was treated with a classical reagent for that purpose: the electron− donor−acceptor (EDA) complex formed between Umemoto’s reagent (5-(trifluoromethyl)dibenzothiophenium tetrafluoroborate) and N-methylmorpholine (NMM), which was reported to react with both arenes and pyrroles via radical trifluoromethylation.75 In contrast with expectation, there was a large selectivity for 1b which was obtained in 50% yield (Scheme 1). Even in the absence of base, the reaction of (tpfc)PF2 af forded 1b in only a slightly lower yield of 40%, clearly testifying to an electrophilic CF3 precursor, as C3 is well-known to be the most nucleophilic carbon atom in this type of corrole. With 1b easily accessible and with decent amounts in hand, it was treated with FSO2CF2CO2Me as to afford the bis-CF3-substituted compound 2. Its identity as the 3,17-substituted isomer was relatively straightforward, based on the high symmetry that came into effect by only one kind of CF3 group (at −37 ppm) and the 2:1 ratio of p-F atoms from the three C6F5 groups due the presence of a C2 axis. This conclusion was confirmed by Xray crystallography as well (vide infra). Corroles with more than two CF3 groups could not be obtained from (tpfc)PF2, so their synthesis was addressed by nucleophilic substitution of C−Br bonds by FSO2CF2CO2Me/ CuI. Access to the required precursor was provided by treating a methanolic solution of (tpfc)PF2 with excess bromine, leading to isolation of the novel penta-brominated corrole P− Br5 in 60% yield. Its identification as the 2,3,7,17,18pentabromo derivative was structural and rationalized based on the substantially larger reactivity of pyrroles A and D in electrophilic substitution and the J-coupling constants in excess of 4.5 Hz for protons on the B and C pyrroles. The treatment of P−Br5 with FSO2CF2CO2Me under prolonged heating of 20 h also led, on top of the desired product 5 (8% yield), to the tetra- and tris-CF3 substituted compounds 4 (3% yield) and 3 (2% yield). While the mechanism responsible for the production of the two latter complexes was not investigated, we note the precedence for dehalogenation under similar conditions.24 The number of CF3 substituents was easily deduced from the corresponding 19F NMR spectra: five, four, and three for compounds of the namesake 5, 4, and 3. Safe identification of compound 4 was based on a consideration of symmetry (the presence of only two magnetically different CF3 groups therein), and the main indication for the structure of

to be separated from each other by tens of ppm (Figure 1a). The first evidence for CF3 introduction into (tpfc)PF2 was the appearance of new resonances in the 19F NMR spectrum, at chemical shifts that are clearly distinct from those of the aromatic C6F5 groups: −53.2 to −54.2 ppm vs −136.4 to −162.9 ppm, respectively. That each compound (1a−1d) contains only one CF3 group was deduced by integration relative to the C6F5 groups, while the unaltered relative o/p/mC6F5 ratio clearly ruled out nucleophilic substitution on them. The P-coordinated F atoms in compounds 1a−1d appeared as doublets centered at about −37 ppm and with 1JF−P values of about 830 Hz. This was confirmed by obtaining the 31P NMR spectrum, which revealed chemical shifts of about −181 ppm and triplets with identical coupling constants. Another indication for a single CF3 group is the change of molecular symmetry from C2v to Cs, which can be spotted easily in the para-F regime. Unlike the 2:1 ratio in (tpfc)PF2, the 1:1:1 ratio of three chemically distinctive C6F5 groups clearly shows up in the four P-chelates of the mono-CF3-substituted corroles. Regarding the distinction between the four regioisomers, there is one strong clue: the CF3 singlet in 1a is much sharper than in the other three compounds. Based on what has been shown for the β-pyrrole H atoms of the free base H3(tpfc) and the corresponding gallium(III) chelate (tpfc)Ga,72 the broad signals are attributable to unresolved through-space coupling with the C6F5 groups. Only a CF3 group in position C2 (=C18) does not experience this effect, and this pointed toward compound 1a as the 2-substituted corrole (Scheme 1). Analysis of the 1H NMR spectra (Figure 1b, Table 1) is less straightforward but very rewarding because it allows for Table 1. Differences in Chemical Shifts of H Atoms Adjacent to the CF3 Group in the Mono-CF3-substituted Compounds 1a−1d Relative to the Same Atom in the Nonsubstituted Species (tpfc)PF2, as Deduced by 1H NMR Spectral Analysis (experimental) and by Computation (DFT) complex

H atom adjacent to -CF3 group

Δ (ppm) experimental

Δ (ppm) DFT

1a 1b 1c 1d

C3H C2H C8H C7H

0.13 0.36 0.36 0.34

0.15 0.32 0.50 0.33

identification of all the regioisomers. The first step was chemical shift assignment of (tpfc)PF2; each of the four nonchemically shift identical H atoms appears as a tripletlooking doublet of doublets (dd) due to coupling with the vicinal H and the quite remote P atom, with coupling constants that are coincidentally almost identical. This was proven by recording a P-decoupled spectrum (Figure S9), which together with DFT calculations allowed for a safe determination of the chemical shifts of C2H, C3H, C7H, and C8H (Spectra and full data are provided in the SI). A single H/CF3 substitution eliminates the C2 symmetry axis; the reduced molecular symmetry leads to seven nonequivalent resonances, of which only the one that is adjacent to the -CF3 group will now appear as a doublet rather than a doublet of doublets due to its 5JF−P coupling with the central phosphorus only. This pattern is clearly present in all the spectra shown in Figure 1b, in which only one doublet shows up for compounds 1a−1d. Considering that (i) in all (tpfc)M complexes (M = chelated D


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Inorganic Chemistry

expected increase in Lewis acidity of the central P atom upon CF3 substitution comes mostly into play for the axial ligand binding affinity. The calculations produced bond lengths that are generally in good agreement with the single crystal structures, although slightly longer. The trend of decreasing P−F is reproduced reliably, but the calculations also reveal that the changes in P−N bond lengths are irregular: decreasing upon monosubstitution and increasing in the bis- and pentaand mostly so in the octa-substituted case. A plausible reason for the latter effect is that the series of compounds is not truly harmonic, by means of significant differences in the corrole’s skeleton. The above hypothesis was confirmed by realizing that there is an increasingly severe distortion from planarity as the CF3 functional groups are introduced, as illustrated in Figure 2. This is noticeable even for the mono-CF3-substituted complexes 1b and 1c, less pronounced for the bis-CF3substituted derivative 2 due to its higher symmetry (C2v) than the other compounds (Cs), and most evident in derivative 5. The extent of saddling (χ1− χ4) and ruffling (ψ1− ψ2) can be determined from X-ray data and calculated gas phase geometries (Figure S12, Table S4). To help gauge these angles, the experimental values (Table S5) show a pronounced distortion for 5. Three large angles (deg) for saddling (χ1 = 17.7, χ2 = 15.1, χ4 = 13.8) were determined. The total of the absolute values of the four saddling angles (Σ|χn|) shows that the distortion of compound 5 is most prominent with a summed value of 52.8°. Next is 1c (45.9°) which has a large χ2 angle (χ2 = 21.6°), followed by 1b. Compound 2 (18.9°) and the nonsubstituted compound (tpfc)PF2 (14.4°) show the least saddling. There are arguably also crystal packing forces at play, but the effect made by a single CF3 group substituent in the C7 position (1c) is an important observation with which to follow up in future studies compared to the effect seen for 1b. The DFT studies show more modest effects but agree with the general trend. Based on the calculations, the changes are subtle when 3 is compared to (tpfc)PF2. In 5 and 8, a much more significant ruffling can be seen. A space-filling model shows clearly that these structural distortions are due to steric clashes between the proximal CF3 groups positioned at the C2 and C18 β-carbons. Among the four monofunctionalized species 1a− 1d, we note that species 1a is energetically the most stable species owing to its lowest degree of steric hindrance between

compound 3 was that its singlet appeared at 9.07 ppm, at much higher field than signals for the compounds in which the proton adjacent to CF3 is on C2 or C18 such as in 1b (9.82 ppm) and 2 (9.78 ppm). Geometrical and Electronic Structures. The isolation and investigations of the four monosubstituted isomeric species 1a−1d is an important achievement in porphyrinoid chemistry,76,77 and so is their assignment, together with the multiply substituted compounds. On top of the abovementioned NMR spectroscopy-based identification of all the compounds, four of them provided high quality crystals by slow evaporation from chloroform/n-hexane solutions. This not only allowed their structural identity to be confirmed by Xray crystallography but also provided deeper insights into how the CF3 groups affect the geometrical parameters of the corroles and their electronic structure, based on the data summarized in Table 2. Density functional theory (DFT) Table 2. P−N and P−F Bond Lengths in the New Phosphorus Corrole Complexes in the Previously Reported (tpfc)PF237 Based on X-ray Crystallography and DFT Calculations (at the B3LYP-D3/LACVP* level X-ray

complex

DFT

P−F average bond length (Å)

P−N average bond length (Å)

P−F bond length (Å)

P−N bond length (Å)

(tpfc) PF2 1b

1.618 (2)

1.812 (3)

1.641

1.823−1.843

1.608 (4)

1.816 (8)

1.820−1.842

1c

1.609 (5)

1.809 (8)

2 5 8

1.608 (2) 1.603 (8) -NA-

1.818 (3) 1.823 (7) -NA-

1.637, 1.639 1.638, 1.640 1.635 1.633 1.629

1.807−1.849 1.837−1.834 1.820−1.850 1.838−1.855

calculations were also applied, as to shine additional light on and also for obtaining data for the nonaccessible compound 8 bearing eight CF3 groups in the macrocycle periphery. A comparison of the experimental P−F bond lengths shows that, relative to (tpfc)PF2, they decrease mostly upon the introduction of the first CF3 group as in 1b and 1c and much less so as a response to the additional ones present in 2 and 5. The P−N bond lengths were less sensitive, suggesting that the

Figure 2. Increased distortion (a) found for compound 5 in comparison with other structurally characterized derivatives (Table S8) and the same compounds as on the left but displayed without the 5,10,15-aryl rings and the F atoms of the CF3 groups as to clearly show the macrocycle distortion (b), together with the corresponding drawings of the chemical structures. E


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Inorganic Chemistry the CF3 group and the meso-C6F5 groups. Species 1b and 1c are +3.8 kcal mol−1 and +6.5 kcal mol−1 higher in energy than 1a. This indicates that both the radical-like and, moreover, the electrophilic trifluoromethylation reactions of (tpfc)PF2 are controlled not by thermodynamic considerations but rather by kinetics, as 1b is seen to form with both low and very high selectivity, respectively. The isolation and study of compounds 1a−1d help in determining individual electronic contributions present for the different positions (Table S1), which may be investigated by electronic absorption and emission spectra accompanied by studies of the frontier molecular orbitals from the DFTcalculations. Not surprisingly, all investigated corroles displayed typical UV−vis spectra and two emission peaks; notable differences and distinctive features could still be identified. The Soret- and Q-bands, as well as the emission maximum red-shifted slightly from 1a to 1d (Table S1). The collapsing and splitting of the Q-band in the absorption spectrum centered at ∼575 nm could also be correlated with the change in the CF3 group position from 1a to 1d (Figure S2). The exact electronic basis for this was sought by, among other means, an inspection of the aspects of the outer edge of the corrole (below). To better understand these and other features, one must first examine the frontier orbitals that are responsible for the electronic spectra of the corrole core. Gouterman proposed in the 1960s that the absorption spectra of porphyrins can be understood by focusing on the two highest occupied and two lowest unoccupied molecular orbitals.78 Applying the same principles to the corroles, the four Gouterman orbitals can be identified, as shown in Figure 3. This model works best for

point for understanding the electronic spectra of these functionalized corroles. The HOMO and HOMO−1 of (tpfc)PF2 shown in Figure 3 are easily recognized as the analogues of the a1u and a2u orbitals of the porphyrin system (D4h symmetry), although the HOMO shows a significant departure from an ideal a1u orbital in the lower portion of the corrole ring where no pentafluorophenyl group is attached. Nonetheless, these two orbitals, which house the electrons that will be excited during absorption, are very close in energy: −5.85 and −5.74 eV, respectively. The acceptor orbitals, which are the unoccupied degenerate π* orbitals of eg symmetry in the porphyrin, are the LUMO and LUMO+1 (also shown in Figure 3). Lower symmetry and the presence of the pentafluorophenyl moieties induce significant mixing and polarization and lift the degeneracy by about ∼0.4 eV, but the π* character can be clearly seen at the bottom half of the molecule (C16−C19, C1−C4) in the LUMO diagram and top half of the molecule (C6−C14) in the LUMO+1 diagram (Figure 3). To address the question of how much these four orbitals contribute to the absorption behavior of the corrole derivatives, time-dependent DFT studies were carried out. These uncovered properly predicted excitation energies and red-shifts in most of the series, indicative that the primarily contributors are indeed the Gouterman orbitals. This however was not the case for species 5 and 8, in which non-Gouterman orbitals make a significant contribution to the transitions in the Soret region due to the negligible orbital energy gaps between HOMO−1 and HOMO−2 orbitals and mixing of these two orbitals, enabled by the lack of symmetry. The latter aspect is not obvious for 8, which in principle could have the same symmetry group as (tpfc)PF2 and 2, both of which are not so due to the large deviation of the macrocycle plane from planarity. The same phenomenon is apparently also the reason why the trends in P−F and more so in P−N bond lengths are much less obvious than predicted. Experimentally, the electronic spectra of the complexes change very much upon CF3-substitution, not only in terms of red shifts of the near-UV (Soret) and near-IR (Q) bands but also in their relative intensity. This is demonstrated in Figure 4, which displays the spectra of the whole series under identical concentrations and clearly shows the intensity decrease of the Soret band (400−420 nm) and the increase of the Q-band (520−620 nm). To deduce the reason(s) for these phenomena, a more in-depth analysis of the DFT calculations was carried out. The prototype frontier orbitals shown in Figure 4 and the MOs of other species provided in the Supporting Information emphasize the fact that these frontier orbitals are π and π* MOs. The CF3 groups are strongly σdirecting in their inductive effect, and adding them to in-plane positions limits their influence on the σ-orbital space, which extends across the corrole ring in a two-dimensional and fairly uniform fashion. Thus, there are no specific orbital−orbital interactions between the CF3-ligand and the frontier orbitals in the π electron space. The impact is electrostatic in nature. Thus, the general expectation is that both the doubly occupied and unoccupied frontier orbitals should be impacted in a similar fashion, namely, pushed to lower energy in a uniform fashion, as illustrated in Figure 4 (right). The extent of stabilization depends on two factors: (i) How close the main lobes of the orbitals are to the electron-withdrawing CF3 group position, which are placed at the molecular edges (C1−C19) in this case. Thus, the closer the main lobes of the individual MO are to the rim of the corrole, the more stabilization it will

Figure 3. Frontier molecular orbitals that form the basis of Gouterman’s four orbital model as shown for (tpfc)PF2.

symmetric species in which the frontier orbitals split cleanly and do not mix with other orbitals due to symmetry constraints. In the current system, the lack of symmetry complicates the situation and it was not clear if the Gouterman model is sufficient for describing the electronic absorptions and emissions, but at the very least, it offers a good starting F


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Inorganic Chemistry

Figure 4. Electronic spectra for compounds (tpfc)PF2, 1b, 2, 3, 4, and 5 under the same concentration in acetonitrile; the inset shows the Soret band region (a). Frontier molecular orbital analysis (FMO analysis) for compounds (tpfc)PF2 to 8 showing the relative decrease in orbital energies as a function of the number of β-CF3 groups added (b).

Figure 5. (a) Cyclic voltammetry measurements for 1b, 2, 3, 4, and 5 as well as (tpfc)PF2 in acetonitrile (HPLC grade) containing 0.5 mM phosphorus(V) corrole complexes and 0.1 M TBAP (Fluka, for electrochemical analysis) as the electrolyte under argon. A conventional threeelectrode system consisting of a glassy carbon working electrode, a platinum wire which served as the counter electrode, and silver wire separated from the bulk solution by a sample holder with a porous glass frit in 0.1 M TBAP/0.01 M AgNO3 solution. Electrochemical measurements were recorded with an EmStat3+ electrochemical system. All the potentials are referenced vs the Fc+/0 (ferrocenium, and neutral ferrocene) redox potential added as an internal standard (E1/2 = 0.0975 V vs Ag/Ag+ in 0.1 M TBAP/0.01 M AgNO3 solution). A scan rate of 100 mV/s was applied. (b) Plot of the 1st reduction potential vs the number of -CF3 substituents yielded a slope of 193 mV/n(CF3) where the data for complex 5 is not included.

nitrogen, whereas in the LUMO the lobes at the edges of the corrole are larger. As a consequence, the unoccupied frontier orbitals will experience a greater stabilization than the occupied orbitals. Similarly, since the unoccupied orbitals are higher in energy than the occupied ones, they are more polarizable and can therefore more actively distort their composition to react to the induced partial charge at the edge of the corrole. Both effects contribute to decreasing the HOMO−LUMO gap, which can be seen clearly in Figure 4(b). As the number of CF3 groups is increased, the HOMO− LUMO gap decreases by ∼0.3 eV from 2.73 eV in (tpfc)PF2 to 2.41 eV in the hypothetical species 8. In complex 5, the gap is 2.55 eV, thus affording a reduction of the HOMO−LUMO gap by 0.14 eV across the experimentally accessible corroles. Whereas the change of this energy gap is modest, the change of orbital energies in absolute terms is much more pronounced with the HOMO energy changing by nearly a full eV from −5.74 eV to −6.68 eV in (tpfc)PF2 and 5, respectively. Interestingly, the HOMO−2 orbital energies are not significantly altered and remain nearly at the same energy of

experience. (ii) The one-electron density associated with each MO can of course shift toward the rim, thus decreasing the distance between the one-electron density and the increasingly more positively charged outer rim of the corrole. To a first order approximation, the polarizability of the one-electron density associated with each MO is directly proportional with the orbital energy. That is, the higher the orbital energy of a MO, the easier it is to move that electron associated with that MO from its original location. The two factors mentioned above are intuitive and generally valid but have an important consequence for the HOMO− LUMO gap across the series of corroles studied here. First, because the nitrogen atoms close to the center of the corrole have higher electronegativity than carbon, they pull the low energy orbitals more toward the center of the corrole, whereas the higher lying antibonding orbitals are pushed away from the core. This effect can be clearly seen in the different lobe sizes of the π and π* MOs, HOMO−1, and LUMO of the representative orbitals shown in Figure 3. In the HOMO−1, the π orbitals of the pyrrole show a larger lobe closer to the G


Article

Inorganic Chemistry

Figure 6. (a) CV traces of 5, P-Br5, and Ga-Br8 in 0.5 M TBAP/acetonitrile using silver nitrite as the reference electrode, with the corresponding redox potential values. (b) Soret bands of corroles 5, Ga-Br8, and P-Br5 in acetonitrile, under identical concentrations. (c) Turnover number (TON) of bromine formation detected by UV−vis after irradiation (λmax = 400 nm and λmax = 450 nm) for 2 h with 5, Ga-Br8, and P-Br5. (d) Turnover number (TON) of brominated phenol (phenol assay) detected by GC after irradiation (λmax = 400 nm) for 2 h with 5, Ga-Br8, and PBr5. (e) Turnover number (TON) of benzyl bromide (toluene assay) detected by GC after irradiation (λmax = 400 nm and λmax = 450 nm) for 3 h with 5, Ga-Br8, and P-Br5. (f) Turnover number (TON) of bromobenzene (benzene assay) detected and determined by GC after irradiation for 24 h. The concentrations of 5, P-Br5, and Ga-Br8 were maintained at the same UV−vis absorbance of OD = 5.0 at 400 nm in acetonitrile solution in all photocatalysis assays. Reactions were performed under aerobic conditions, while no products were formed when the identical reactions were performed under an Ar atmosphere.

−7.43 to −7.67 eV, in (tpfc)PF2 and 5, respectively. This orbital in all calculated complexes is predominantly the πorbital located on the meso-C6F5 groups (Figure S11). Using the rules outlined above, it is easy to understand that this MO is only slightly affected by the induction-based change of the partial charge at the outer edge of the corrole framework. One interesting consequence of the orbital energy shifts is that the Gouterman four orbital model breaks down in the higher functionalized species 5 and in an exaggerated fashion in the hypothetical species 8, as detailed in Table S6. For instance, the HOMO−2 and HOMO−1 orbitals in species 8 and 5 are separated by just 0.32 and 0.77 eV, whereas for (tpfc)PF2, this gap is 1.58 eV. Moreover, a number of the occupied orbitals (HOMO−2 to HOMO−5) are within 1 eV in 5 (Table S6) and can participate in the electronic transitions, as our TDDFT calculations confirm. Thus, the Gouterman four-orbital model clearly does not hold in these cases. This seems to be the main reason for the changes in intensity of the absorption maxima, as the Soret and Q bands originate from the symmetry-allowed and symmetry-forbidden mixing, respectively, of the four electronic transitions. The redox properties of the substituted corroles were determined by cyclic voltammetry; they revealed an interesting pattern, as illustrated in Figure 5. The redox potentials of (tpfc)PF2 are already more positive than the analogous Al and Ga chelates of the same corrole, (tpfc)Al and (tpfc)Ga, respectively.79 As expected, the reduction potentials become increasingly positive with each CF3 functionality shifting the reduction potential by 193 mV, offering a quantitative assessment of the inductive effect that the CF3 group imposes on the corrole core. Most important and consistent with one of the main driving forces for carrying out of the current studies, the shift in redox potentials of the 193 mV/CF3 group is much larger than that observed for F, Cl, Br, and I, which is only about 50 mV/halide. Also notable is that whereas the fifth CF3

group causes an additional shift of the redox potential, it is significantly smaller: 120 mV, relative to 193 mV. This is attributable to the significant distortion of the macrocycle in 5, preventing the full electron-withdrawing effect of the additional CF3 group to come into effect, a phenomenon seen many years ago for halogeno-substituted porphyrins.80 Finally, this pattern can be confirmed by other spectroscopic features, such as the 1JP−F coupling constant that shows a linear correction (R2 = 0.98) with both (i) the number of CF3 groups and (ii) the first reduction potential (Figure S7). A linear relationship was also observed between the P−F bond lengths (average) and the 1JP−F coupling constant (R2 = 0.93), while the correlation with the chemical shift of the axial fluorine atoms was least perfect (R2 = 0.70) (Figure S8). While in simple chemical systems such as hydrocarbons (C−H) there are important and straightforward relationships between 1H and 13C NMR spectroscopic chemical shifts and atom orbital hybridizations, the issue of correlating crystallographic bond length distances and solution chemical shift (δ) values can often be problematic. The difficulty in locating the proton within an electron density map has been a long-standing issue with X-ray crystallographic structures (and often addressed through a neutron diffraction structural study); and intra- or intermolecular hydrogen bonding effects may also play a part in creating artificially long or short distances.81 There is actually an advantage when considering C−F bond length analysis compared to C−H bonds:82 fluorines contain many electrons and are easily located during the crystallographic solution. Changes in element−fluorine bond lengths (P−F) and 19F NMRS chemical shift (δ) are hence intriguing in the present set of phosphorus corroles. As a simple means of comparison, the D3h-symmetric PF5 molecule possesses an experimental axial bond length of 1.577 Å which is on average longer than the equatorial one (1.534 Å). While only one 19F NMRS chemical shift (δ −77 ppm) is observed due to H


Article

Inorganic Chemistry fluxional processes (Berry pseudurotation),84 the chemical shifts are calculated as Fax = −62.71 and Feq = −71.47.83 Thus, the longer bond (axial) is more downfield shifted, whereas, for the corrole systems under study, the longer the P−F bond, the more upfield the δ value (Figure S8). One must also take into account the very large and strongly distance-dependent diamagnetic current effect of the macrocycle, which might also be largely influenced by the extent of its deviation from planarity. In order to examine the utility of these electron-deficient corroles, 5, P-Br5, and the fully brominated Ga corrole Ga-Br8 (Figure S15) were examined as photocatalysts for bromination of organic molecules. This was inspired by our recent invention, in which we have tested several metallocorroles (including the fully brominated phosphorus corrole) and deduced that Ga-Br8 is most efficient because its photoexcited state has the most oxidizing power.85 Using HBr, we first tested the efficacy of 5 and P-Br5(Figure S15) relative to GaBr8, based on the conclusions reached from work with the latter: the direct correlation between high oxidation potentials and efficacy. The first evaluation assay was based on quantification of bromine formed in catalyst-containing CH3CN/HBr solutions, relying on the UV−vis absorbance of the Br3− species (ε259 nm = 55000 L mol−1 cm−1). Two LED lamps, emitting at 400 and 450 nm, were examined, with the latter being in clear favor of Ga-Br8, as the two other complexes hardly absorb the longer wavelength (Figure 6b). Nevertheless, the order of efficacy was 5 > P-Br5 > Ga-Br8 (Figure 6c, red bars), which is consistent with their redox potentials (Figure 6a: E1/2 = 1.59 V, 1.22 V, and 0.75 V, respectively) rather than absorption properties. This effect was amplified very much by using the LED lamp that emits at 400 nm, a wavelength that is more efficiently absorbed by 5 relative to P−Br8 and (even more so) Ga−Br8 (Figure 6c, black bars). Very similar results were obtained by trapping the in situ formed bromine by phenol, with catalytic turnover numbers (TON) of 1840, 528, and 92 for photocatalysts 5, P-Br5, and Ga-Br8, respectively (Figure 6d). Encouraged by the remarkably high photocatalytic activity of 5, our attention was driven to two much more challenging reactions: photocatalytic bromination of sp3 and sp2 CH bonds by bromide, using toluene and benzene as representative examples. Benzyl bromide was formed from toluene and bromobenzene from benzene, with a nondisputable superior efficiency found for 5: 4−10 times larger TON numbers relative to Ga−Br8 (Figures 6e, 6f: 320 vs 80 for toluene and 106 vs 11 for benzene). The formation of benzyl bromide from toluene is easily rationalized, since benzylic CH bonds are known to be very reactive toward bromine radicals that are formed from bromine or N-bromo succinimide (NBS) or via our methodology of photocatalytic activation of bromide. More surprising is the transformation of benzene to bromobenzene, as this reaction does not happen spontaneously via electrophilic attack of bromine (like the case for phenol). We note however that the combination of NBS and acids is a very efficient system for the ring bromination of aromatic compounds.86 Another alternative is that the species oxidized by the photoexcited catalyst in this case is not the bromide but the benzene, in a reaction scenario proposed by Fukuzumi and Ohkubo.87 The rationale for the superiority of 5 as photocatalyst is that its HOMO is notably lower in energy than the other two control systems, probed by both experiment (the redox

potentials, Figure 6a) and theory (the above DFT calculations, Figure 4). During photoirradiation, these sensitizers are excited and one of the two electrons from the HOMO is elevated to the LUMO. The oxidative work is accomplished by the hole in the orbital that is the HOMO in the ground state, while the cycle is completed by reduction of oxygen by the electron from the LUMO. Thus, to a first approximation, the HOMO energy that was probed in the ground state is the redox active orbital in the excited state; therefore, it is easy to understand that the oxidative power of 5 is superior to that of the other sensitizers. In the comparative MO study above, the internal mechanism that leads to the lowering of the HOMO-levels was explained, while the HOMO−LUMO gap was also found to decrease. This strategy is quite useful, as there is a red shift in the absorption wavelength in the examined series; that is, the most powerful oxidant 5 requires the most red-shifted and, thus, least powerful energy source. This is a direct consequence of the distinctive effect of the CF3 substitutionwhich may be more general to other macromolecular chemical systems giving rise to lower HOMO energies and smaller HOMO− LUMO gaps at the same time. It is unclear whether or not it is helpful for photocatalysis that the Gouterman’s four orbital model is no longer applicable in these electron-deficient systems and the excitation becomes more complicated and involves the π orbitals of the pentafluorophenyl groups. This feature remains to be investigated in future work. It is however clear that 5 bears outstanding electronic properties and performance for photocatalysis that arise from multiple CF3 substitutions; comparison to the analogous pentabrominated species P-Br5 verifies the importance of the involvement of the CF3 substituent.

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CONCLUSIONS Our study offers a rare systematic and thorough examination of the effects that an increasingly larger degree of CF 3 derivatization has on an organic/inorganic system. These findings are highly relevant in solar energy research. For example, the plausible but rarely tested hypothesis that the addition of CF3 groups to photoactive corrole type of sensitizers significantly lowers the frontier orbital energies has been quantitatively confirmed.42,88 We disclose the electronic foundation for the somewhat counterintuitive assertion that the most powerful oxidant shows the more red-shifted absorption. The relationship between the HOMO energy levels, HOMO−LUMO gaps, oxidative power of the excited state, and the degree of CF3 functionalization has been quantified and conceptualized for the first time by combining a difficult, laborious, but nonetheless successful synthesis and isolation of the substituted corroles with DFT calculations/ analysis, electrochemistry, and photophysical measurements. Importantly, the corrole derivative with the greatest number of β-CF3 groups demonstrates great photocatalytic efficiency in bromination of phenol, toluene, and even benzene, by bromide rather than bromine. This study helps to rationalize the electronic withdrawing contribution available in certain types of photocatalysis arising from any of the four distinct corrole βpositions. Lastly, compound 5 bears out our hypothesis and demonstrates outstanding electronic properties for photosensitization that arise from β-CF3 substitution. The economical consideration of using a catalyst composed of only sustainable elements (H, C, N, F, P) rather than heavier atoms that may be more expensive and/or more toxic is also noteworthy. Ongoing efforts are hence devoted to modifying I


Article

Inorganic Chemistry

OD = 5.0 at 400 nm) was charged with phenol (24 mg, 0.1 M) and aqueous 47% HBr (58 μL, 0.2 M) and placed in a 2.5 mL vial made of Pyrex. The stirred solution was irradiated by a LED lamp (200 W, λmax = 395−405 nm, source was 15 cm distance from vial) for 2 h, at 36 °C (internal temperature in reaction solution). Prior to injection to the GC for product determination, the mixture was treated with solid K2CO3 as to neutralize the acid and to dry the solution; nitrobenzene (5 μL, 49 μM) was added as an external reference. Identification of reaction products and determination of their response factors, relative to nitrobenzene, were performed by using pure products that were either purchased from a commercial supplier or prepared independently. The retention times checked by the GC for acetonitrile, phenol, nitrobenzene, 2-bromophenol, and 4-bromophenol were determined to be 1.3 min, 3.8 min, 6.5 min, 5.8 min, and 13.4 min. Photocatalysis (Toluene Assay). An acetonitrile solution (2.5 mL, with catalyst concentration (5 or Ga-Br8 or P−Br5) that leads to OD = 5.0 at 400 nm) was charged with toluene (26 μL, 0.1 M) and aqueous 47% HBr (58 μL, 0.2 M) and placed in a 2.5 mL Pyrex vial. The stirred solution was irradiated by an LED lamp (200 W, λmax = 395−405 nm, or 150 W with λmax = 450 nm, source 15 cm from the sample vial) for 3 h, at 36 °C (internal temperature of the reaction solution). Prior to injection to the GC for product determination, the mixture was treated with solid K2CO3 to neutralize the acid and to dry the solution, and nitrobenzene (5 μL, 49 μM) was added as an external reference. Identification of reaction products and determination of their response factors relative to nitrobenzene were performed by using pure products that were either purchased from a commercial source or prepared independently. The retention times checked by the GC for acetonitrile, toluene, nitrobenzene, and benzyl bromide were determined to be around 1.3 min, 1.8 min, 6.5 min, and 6.8 min. Photocatalysis (Benzene Assay). An acetonitrile solution (2.5 mL, with catalyst concentration (5 or Ga-Br8 or P−Br5) that leads to OD = 5.0 at 400 nm) was charged with benzene (90 μL, 0.4 M) and aqueous 47% HBr (116 μL, 0.4 M) and placed in a 2.5 mL vial made of Pyrex. The stirred solution was irradiated by an LED lamp (200 W, λmax = 395−405 nm, source placed 15 cm from the vial) for 24 h, at 36 °C (the internal temperature in the reaction solution). Prior to sample injection for product determination by GC, the mixture was treated with solid K2CO3 to neutralize the acid and to dry the solution; nitrobenzene (5.0 μL, 49 μM) was added as an external reference. Identification of reaction products and determination of their response factors relative to nitrobenzene were performed by using pure products that were either purchased from a commercial supplier or prepared independently. The retention times checked by the GC for acetonitrile, benzene, nitrobenzene, and bromobenzene were determined to be ca. 1.3 min, 1.5 min, 6.5 min, and 3.1 min, respectively. Synthetic Methods. The corrole complex (tpfc)PF2 and Ga-Br8 were synthesized based on previously reported literature.37 Synthesis of Mono-CF3 Substituted Complexes 1a−1d. A flask loaded with a DMF solution (5 mL) of (tpfc)PF2 (50 mg, 52 μmol) and copper(I) iodide (0.30 g, 1.56 mmol) (Aldrich) was heated and stirred at 100 °C under argon atmosphere for 15 min. Then, methyl 2,2-difluoro-2-(fluorosulfonyl) acetate (0.20 mL, 1.56 mmol) was added into this solution. After reaction for 24 h, the solvent was evaporated and the reaction residue was redissolved in dichloromethane (25 mL) and washed by distilled water three times. Then, after filtration and evaporation of the organic phase, the reaction mixture was dissolved by very small amounts of dichloromethane and subjected to PLC (preparative layer plates) using n-hexane:ethyl acetate (6.5:1 by volume) as an eluent. Four target compounds were collected from the PLC with silica format each constituting a different band with somewhat different Rf value. The scratched silica powder was suspended in a small amount of dichloromethane and filtrated. Pure reddish products were obtained after evaporation. The Rf values and isolated yields of 1a−1d were mentioned below: 1a (0.66, 7%), 1b (0.53, 11%), 1c (0.33, 7%), 1d (0.30, 8%). X-ray quality crystals of

reaction conditions as to increase the selectivity toward the most active catalyst and obtaining it in larger yields.

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EXPERIMENTAL SECTION

Materials. Experimental reagents (Aldrich) and solvents were used without further purification. Silica gel 60 (230−400 mesh) was used for the column chromatography. Physical Methods. 1H, 19F, and 31P NMR spectra were recorded on a Bruker Avance III 400 spectrometer equipped with a 5 mm, automated tuning, and matching broad band probe (BBFO) with zgradients. Chemical shifts are reported in ppm relative to the residual hydrogen atoms in the deuterated solvent CDCl3 (δ = 7.26 ppm). Absorption spectra of the samples were measured on an Agilent 8454 spectrophotometer. Mass spectra for the compounds were performed on a Bruker maXis Impact mass spectrometer, using APPI (atmospheric pressure photoionization) direct probe in either the positive or negative mode. GC (gas chromatography, type: Sion-4201, Israel) was used for detection of chlorinated phenol. Spectroscopy. Steady-state emission spectra were recorded on a Cary Eclipse fluorimeter, at room temperature. Quantum yields (Φf) of emission were calculated by using tetraphenylporphyrin89 (TPP) in toluene at room temperature (Φ = 0.11) as an external standard. The fluorescence lifetimes (τ) were measured by the combined spectrometer consisting of the EDINBURGH LIFESPEC II and the picosecond pulsed diode laser: EDINBURGH EPL-405 in toluene at room temperature. The radiation constants (Kr, Kr = Φf/τ) and the irradiation constants (Knr, Knr = (1 − Φf)/ τ)90 were measured and are shown in Table S1. Electrochemistry. Cyclic voltammetry measurements were carried out in acetonitrile (HPLC grade) containing 0.5 mM of those phosphorus(V) complexes and 0.1 M TBAP (Fluka, for electrochemical analysis) as the electrolyte under argon. A conventional three-electrode system was used consisting of a glassy carbon working electrode, a platinum wire which served as the counter electrode, and silver wire separated from the bulk solution by a sample holder with a porous glass frit in 0.1 M TBAP/0.01 M AgNO3 solution. Electrochemical measurements were recorded with an EmStat3+ electrochemical system. All the potentials are referenced vs the Fc+/0 (ferrocenium, and neutral ferrocene) redox potential added as an internal standard (E1/2 = 0.092 V vs Ag/Ag+ in 0.1 M TBAP/0.01 M AgNO3 solution). The potentials can be referenced vs NHE or SCE scale by adding +0.548 or +0.298, respectively. Crystal Structure Determinations. Data were collected at 100 K on a dual source Rigaku XtaLab PRO diffractometer equipped with PILATUS 200 detector, with [λ(Cu Kα) = 1.54184 Å] radiation. Data were processed with CrysAlisPRO.91 Structures were solved with SHELXT92 and further refined with full matrix least-squares based on F2 with SHELXL.93 Hydrogens were calculated in riding mode. In compound 4 the PLATON/SQUEEZE94 protocol was used to remove the contribution of disordered solvent molecules. All crystallographic data is presented in Table S8 (shown in the Supporting Information). DFT Calculations. Geometry optimizations of all species were carried out using DFT95 as implemented in the Jaguar 9.1 Suite96 of ab initio quantum chemistry programs. Geometry optimizations were performed with the B3LYP97 functional including Grimme’s D3 dispersion correction98 and in conjunction with the LACVP** basis set.99 Analytical vibrational frequencies within the harmonic approximation were computed with the above-mentioned basis set to confirm proper convergence to well-defined minima on the potential energy surface. Frontier orbital analysis were evaluated by additional single point calculations on each optimized geometry using the same method and Dunning’s correlation consistent triple-ζ basis set cc-pVTZ.100 We employed the standard set of optimized radii in ORCA 4.0.1101 for H (1.300 Å), C (2.000 Å), N (1.830 Å), F (1.720 Å), and P (2.106 Å). Chimera 1.12 version102 is used to plot the frontier orbitals. Photocatalysis (Phenol Assay). An acetonitrile solution (2.5 mL, with catalyst concentration (5 or Ga-Br8 or P-Br5) that leads to J


Article

Inorganic Chemistry

9.04 (t, J = 4.75 Hz, 1H), 8.89 (t, J = 4.64 Hz, 1H). 19F-NMR (377 MHz, CDCl3): δ = −33.86 (d, J = 846.40 Hz, 2F), −136.13 − −136.33 (m, 6 F, ortho-F), −149.69 (t, J = 21.02 Hz, 1F, para-F), −149.88 (t, J = 21.04 Hz, 1F, para-F), −150.37 (t, J = 20.98 Hz, 1F, para-F), −160.76 − −161.34 (m, 6F, meta-F). 31P NMR: −179.77 (t, J = 842.52 Hz, 1P). MS+(APCI, positive mode) for C37H3Br5F17N4P: m/z = 1255.5700 (calculated), 1257.5679 (calculated), 1255.5659 (observed), 1257.5640 (observed). UV−vis (toluene) λmax (ε) [nm (× 104 cm−1 M−1)]: 399 (5.4), 420 (24), 542 (1.7), 586 (3.2). Synthesis of Bis-CF3-Substituted Complex 2. A flask loaded with a DMF solution (5 mL) of 1b (30 mg, 31 μmol) and copper(I) iodide (0.17 g, 0.93 mmol) (Aldrich) was heated and stirred at 100 °C under an argon atmosphere for 15 min. Then, methyl 2,2-difluoro-2(fluorosulfonyl) acetate (0.12 mL, 0.93 mmol) was added into this solution. After reaction for 10 h, the solvent was evaporated and the reaction residue was redissolved in dichloromethane (25 mL) and was washed by distilled water three times. Then, after filtration, evaporation of the organic phase reduced the reaction mixture to a residue which was then dissolved into a very small amount of dichloromethane for use with PLC (preparative layer plate) in which n-hexane:ethyl acetate (7:1 by weight) was used as the eluent. The target compound was collected from the PLC. The scratched silica powder was suspended in a small amount of dichloromethane and then filtrated. A pure reddish product was then obtained after evaporation. The Rf values and isolated yield of 2 were mentioned below: 2 (0.58, 10%). X-ray quality crystals of 2 were obtained by slow evaporation from chloroform/n-hexane solutions. 2: 1H NMR (400 MHz, CDCl3): δ = 9.78 (d, J = 3.60 Hz, 2H), 8.94 (t, J = 4.90 Hz, 2H), 8.85 (t, J = 4.05 Hz, 2H). 19F-NMR (377 MHz, CDCl3): δ = −36.69 (d, J = 836.62 Hz, 2F), −53.62 (s, 6F), −134.56 − −137.53 (m, 6F, ortho-F), −149.09 (t, J = 23.07 Hz, 1F, para-F), −149.92 (t, J = 23.51 Hz, 2F, para-F), −159.96 (m, 2F, meta-F), −161.79 (m, 4F, meta-F). 31P NMR: −179.46 (t, J = 838.01 Hz, 1P).MS+ (APCI, positive mode) for C39H6F23N4P: m/z = 997.9963 (calculated), 997.9906 (observed). UV−vis (toluene) λmax (ε) [nm (× 104 cm−1 M−1)]: 386 (5.29), 407 (26.20), 569 (3.73), 578 (3.62). Synthesis of Multi-CF3 Substituted Complexes 3, 4, and 5. A flask loaded with a DMF solution (5 mL) of P-Br5 (50 mg, 36 μmol) and copper(I) iodide (0.200 g, 1.08 mmol) (Aldrich) was heated and stirred at 100 °C under argon atmosphere for 15 min. Then, methyl 2,2-difluoro-2-(fluorosulfonyl)acetate (0.14 mL, 1.08 mmol) was added into this solution. After reaction for 20 h, the solvent was evaporated and the reaction residue was redissolved in dichloromethane (25 mL) and was washed by distilled water three times. Then, after filtration and evaporation of the organic phase, the reaction mixture was dissolved by a very small amount of dichloromethane to enable a transfer onto PLC (preparative layer plates) in which n-hexane:ethyl acetate (7:1 by weight) was used as the eluent. Three bluish target compounds were collected from the PLC with silica format in different bands with different Rf values. The scratched silica powder was dissolved in a small amount of dichloromethane and filtrated. Pure reddish products were obtained after evaporation. The Rf values and isolated yields of 3, 4, and 5 were measured as 3 (0.36, 2%), 4 (0.29, 3%), and 5 (0.58, 8%). The X-ray quality crystal of 5 were grown by slow evaporation from chloroform/ n-hexane solutions. 3: 1H NMR (400 MHz, CDCl3): δ = 9.07 (d, J = 4.76 Hz, 1H), 8.93 (t, J = 4.09 Hz, 1H), 8.72−8.64 (m, 3H). 19F-NMR (377 MHz, CDCl3): δ = −36.84 (d, J = 839.74 Hz, 2F), −49.10 (q, J = 12.90 Hz, 3F), −50.08 (m, 3F), −53.78 (overlapping doublets, 3F), −136.09 (m, 6F, ortho-F), −148.37 (t, J = 20.09 Hz, 1F, para-F), −149.05 (t, J = 22.21 Hz, 1F, para-F), −149.48 (t, J = 24.20 Hz, 1F, para-F), −159.09 (m, 2F, meta-F), −159.49 (m, 2F, meta-F), −161.46 (m, 2F, meta-F). 31P NMR: −180.43 (t, J = 841.32 Hz, 1P). MS− (APCI, negative mode) for C40H5F26N4P: m/z = 1065.9837 (calculated), 1066, 903 (observed). UV−vis (toluene) λmax (ε) [nm (× 104 cm−1 M−1)]: 389 (3.8), 408 (11.33), 544 (0.85), 587 (2.92). 4: 1H NMR (400 MHz, CDCl3): δ = 8.66 (t, J = 4.80 Hz, 2H), 8.51 (t, J = 4.41 Hz, 2H). 19F-NMR (377 MHz, CDCl3): δ = −34.40 (d, J = 850.84 Hz, 2F), −49.47 (s, 6F), −50.15 (s, 6F), −136.16 (m,

1b and 1c were obtained by slow evaporation from chloroform/nhexane solutions. Alternative Method for the Synthesis of Mono-CF3-Substituted Complex 1b. A 5 mL vial was equipped with a magnetic stir bar and was charged with a solution of (tpfc)PF2 (50 mg, 52 μmol), Umemoto’s reagent (36 mg, 104 μmol), and NMM (12 μL, 104 μmol) in DMF (2 mL). After stirring at room temperature for 1 h, the mixture was added into a separatory funnel containing a mixture of H2O (10 mL) and Et2O (10 mL). The aqueous layer was washed by Et2O (3 × 15 mL). The organic phase portions were merged and dried with anhydrous sodium sulfate. The crude product/residue was then purified by flash chromatography on silica gel (eluent:dichloromethane: n-hexane = 1:4) to obtain a reddish product carefully characterized as 1b (see main text). 1a: 1H NMR (400 MHz, CDCl3): δ = 9.53 (t, J = 3.70 Hz, 1H), 9.17 (d, J = 4.50 Hz, 1H), 9.09−9.02 (m, 3H), 8.89−8.86 (overlapping doublets, 2H). 19F-NMR (377 MHz, CDCl3): δ = −37.37 (d, J = 826.57 Hz, 2F), −54.17 (s, 3F), −135.90 − −136.39 (m, 6 F, ortho-F), −150.05 (t, J = 20.90 Hz, 1F, para-F), −150. Twenty-five (t, J = 22.62 Hz, 1F, para-F), −150.55 (t, J = 22.62 Hz, 1F, para-F), −159.69 − −160.57 (m, 6F, meta-F). 31P NMR: −181.07 (t, J = 826.57 Hz, 1P). MS−(APCI, negative mode) for C38H7F20N4P: m/z = 930.0089 (calculated), 930.0705 (observed). UV−vis (toluene) λmax (ε) [nm (× 104 cm−1 M−1)]: 385 (2.80), 406 (15.42), 576 (1.93). 1b: 1H NMR (400 MHz, CDCl3): δ = 9.82 (d, J = 3.62 Hz, 1H), 9.48 (t, J = 3.27 Hz, 1H), 9.09 (t, J = 4.80 Hz, 1H), 9.06 (t, J = 5.31 Hz, 1H), 8.92 (t, J = 4.97 Hz, 1H), 8.88−8.85 (overlapping doublets, 2H). 19F-NMR (377 MHz, CDCl3): δ = −36.88 (d, J = 827.36 Hz, 2F), −53.36 (s, 3F), −135.57 − −136.37 (m, 6 F, ortho-F), −150.22 (t, J = 22.86 Hz, 1F, para-F), −150.49 (t, J = 22.06 Hz, 1F, para-F), −150.70 (t, J = 21.35 Hz, 1F, para-F), −159.68 − −162.54 (m, 6F, meta-F). 31P NMR: −180.02 (t, J = 827.36 Hz, 1P). MS− (APCI, negative mode) for C38H7F20N4P: m/z = 930.0089 (calculated), 929.9489 (observed). UV−vis (toluene) λmax (ε) [nm (× 104 cm−1 M−1)]: 385 (5.22), 406 (27.26), 564 (2.84), 576 (2.88). 1c: 1H NMR (400 MHz, CDCl3): δ = 9.52 (t, J = 3.20 Hz, 1H), 9.48 (t, J = 3.13 Hz, 1H), 9.28 (d, J = 4.48, 1H), 9.08 (t, J = 4.93 Hz, 1H). 9.05 (t, J = 4.42 Hz, 1H), 8.91 (t, J = 4.23, 1H), 8.87 (t, J = 4.54, 1H). 19F-NMR (377 MHz, CDCl3): δ = −36.90 (d, J = 828.90 Hz, 2F), −53.24 (s, 3F), −136.42 (ddd, J = 29.30, 29.20, 17.70 Hz, 6F, ortho-F), −150.02 − −151.40 (m, 3F, para-F), −160.61 (dd, J = 23.20, 8.30 Hz, 4F, meta-F), −162.85 (td, J = 24.80, 9.40 Hz, 2F, meta-F). 31P NMR: −182.47 (t, J = 828.90 Hz, 1P). MS−(APCI, negative mode) for C38H7F20N4P: m/z = 930.0089 (calculated), 929.9924 (observed). UV−vis (toluene) λmax (ε) [nm (× 104 cm−1 M−1)]: 388 (3.75), 409 (23.68), 558 (1.23), 580 (1.37). 1d: 1H NMR (400 MHz, CDCl3): δ = 9.54 (t, J = 5.60 Hz, 2H), 9.44 (d, J = 5.30 Hz, 1H), 9.09−9.04 (m, 3H), 8.75 (t, J = 4.60 Hz, 1H). 19F-NMR (377 MHz, CDCl3): δ = −36.98 (d, J = 827.45 Hz, 2F), −53.27 (s, 3F), −135.89 (d, J = 24.81 Hz, 2F, ortho-F), −136.19 (m, 4F, ortho-F), −149.96 (t, J = 23.19 Hz, 1F, para-F), −150.44 (t, J = 21.32 Hz, 1F, para-F), −150.94 (t, J = 21.57 Hz, 1F, para-F), −160.13 (m, 2F, meta-F), −160.42 (m, 2F, meta-F), −162.73 (m, 2F, meta-F). 31P NMR: −181.79 (t, J = 827.45 Hz, 1P). MS−(APCI, negative mode) for C38H7F20N4P: m/z = 930.0089 (calculated), 929.9807 (observed). UV−vis (toluene) λmax (ε) [nm (× 104 cm−1 M−1)]: 389 (2.41), 410 (17.13), 554 (0.64), 580 (0.78). Synthesis of Pentabrominated P-Br5. (tpfc)PF2 (50 mg, 52 μmol) and 1 mL of Br2 solution were introduced into a 25 mL round flask containing a magnetic stirrer. Then, ca. 15 mL of methanol was added before stirring was commenced. After 10 h, this reaction mixture was first diluted with 100 mL of dichloromethane and then evaporated down to a residue. The residue was chromatographed on a silica gel column (eluent:dichloromethane: hexane = 1:3) to give the corrole product as the first red eluate. Then, this fraction was evaporated and dried/precipitated to afford red crystals (yield: 60%) through a recrystallization step using the solvent mixture (dichloromethane:hexane = 1:1). The X-ray structural information is shown below. 1H NMR (400 MHz, CDCl3): δ = 9.17 (d, J = 4.84 Hz, 1H), K


Article

Inorganic Chemistry 6F, ortho-F), −148.27 (t, J = 23.38 Hz, 1F, para-F), −148.71 (t, J = 21.43 Hz, 2F, para-F), −158.98 (m, 2F, meta-F), −161.46 (m, 4F, meta-F). 31P NMR: −180.18 (t, J = 844.07 Hz, 1P). MS− (APCI, negative mode) for C41H4F29N4P: m/z = 1133.9711 (calculated), 1133.9501 (observed). UV−vis (toluene) λmax (ε) [nm (× 104 cm−1 M−1)]: 390 (6.64), 411 (14.04), 548 (1.76), 595 (6.11). 5: 1H NMR (400 MHz, CDCl3): δ = 9.09 (d, J = 4.90 Hz, 1H), 8.72 (t, J = 4.8 Hz, 1H), 8.47 (t, J = 4.49 Hz, 1H). 19F-NMR (377 MHz, CDCl3): δ = −35.20 (d, J = 854.97 Hz, 2F), −49.72 (s, 3F), −49.88 (s, 3F), −50.17 (s, 6F), −53.08 (s, 3F), −135.50 (d, J = 23.98 Hz, 2F, ortho-F), −135.92 (d, J = 20.36 Hz, 2F, ortho-F), −136.12 (d, J = 18.07 Hz, 2F, ortho-F), −147.36 (t, J = 22.15 Hz, 1F, para-F), −147.94 (overlapping doublets, 2F, para-F), −160.19 (m, 2F, metaF), −160.65 (m, 2F, meta-F), −160.77 (m, 2F, meta-F). 31P NMR: −177.54 (t, J = 853.49 Hz, 1P). MS− (APCI, negative mode) for C42H3F32N4P: m/z = 1201.9584 (calculated), 1201.9245 (observed). UV−vis (toluene) λmax (ε) [nm (× 104 cm−1 M−1)]: 393 (7.12), 414 (21.54), 551 (1.62), 591 (5.07).

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significant intellectual input, and some writing and preparation of figures; Z.U.: extensive theoretical calculations, discussions, intellectual input, and some contributed text; Q.-C. C.: Extensive technical assistance with electrochemistry, important discussions and graphics preparation; L.J.W.S.: structural characterization of all compounds and preparation of tables and files; I.S.: extensive technical assistance with synthesis, photocatalysis, and other technical issues; A.M.: assistance with synthesis, extensive involvement with photocatalysis and characterization, technical assistance, and editing of manuscript and experimental assistance; M.K., preliminary theoretical calculations and project design; M.B.: principal computational investigator, interpretation of computational results and discussions, extensive intellectual input, preparation of text, and editing of the manuscript; D.G.C.: extensive involvement with project design, assistance with synthesis, interpretation of spectra, performance of crystallizations and assistance with crystallography and extensive writing; Z.G.: principal investigator, synthetic laboratory space provider, relevant proposal writing etc., project design, manuscript writing and extensive editing, and extensive intellectual contributions.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00436. (i) All experimental data; (ii) reproductions of 1H and 19 F NMR spectra; (iii) collated UV−vis absorption and emission spectra; (iv) APCI mass spectra; (v) GC measurements from the chloride oxidation trials; (vi) details regarding theoretical calculations and additional figures and tables from DFT results; (vii) details of single-crystal X-ray crystallographic studies and additional molecular structure diagrams (PDF)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.G. acknowledges the support of this research by a grant from the Israel Science Foundation. D.G.C. acknowledges Z.G., the Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, and support from KAIST for facilitating his sabbatical year. Research conducted by M.B. was supported by the Institute for Basic Science (IBS-R010-D1) in Korea. M.S. gratefully acknowledges B.A.R.C. for sanctioning Extraordinary Leave (EOL).

Accession Codes

CCDC 1587845−1587846, 1818660, 1819699, and 1850162 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qiu-Cheng Chen: 0000-0001-5113-4832 Linda J. W. Shimon: 0000-0002-7861-9247 Mu-Hyun Baik: 0000-0002-8832-8187 David G. Churchill: 0000-0002-2520-4638 Present Address ⊥

PSI-MSC Fellow at the Paul Scherrer Institut PSI, 5232 Villigen, PSI, Switzerland.

Author Contributions

X.Z.: project design, undertaking of complete synthesis, extensive separation and purification of compounds, obtaining all experimental data and compiling of entire spectroscopic characterization of all compounds reported herein; P.T.: project design, discussions, and preliminary theoretical calculations; Y.D.-P.: structural characterization of all compounds and preparation of tables and files, etc. for publication; M.S.: extensive theoretical calculations and discussions, L


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