The Planar Cyclooctatetraene Bridge in Bis-Metallic Macrocycles

Article pubs.acs.org/IC

The Planar Cyclooctatetraene Bridge in Bis-Metallic Macrocycles: Isolating or Conjugating? Susovan Bhowmik,† Monica Kosa,† Amir Mizrahi,‡,† Natalia Fridman,† Magal Saphier,‡ Amnon Stanger,† and Zeev Gross*,† †

Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel Chemistry Department, Nuclear Research Centre Negev, Beer-Sheva, Israel

‡

S Supporting Information *

ABSTRACT: A minor modification of the reported procedure for the synthesis of a corrole dimer that is fused by the cyclooctatetraene (COT) unit, (H3tpfc)2COT, allowed for its isolation in 18% yield. Of the two redox isomers that this interesting macrocycle does form, the current focus is on the reduced form, in which each subunit resembles that of monomeric corroles with a trianionic N4 coordination core. The corresponding bis-gallium(III) complex was prepared as an entry into the potentially rich coordination chemistry of (H3tpfc)2COT. Both X-ray crystallography and DFT calculations disclosed that the COT moiety is essentially planar with very unusual nonalternating C−C bonds. The same holds true for the bis-gallium(III) complexes [(Ga-tpfc)2]COT(py)2 and [(Gatpfc)2]COT(py)4, obtained with one and two pyridine molecules coordinated to each metal ion, respectively. The electronic spectra of both the free base and the gallium(III) complexes display an extremely low energy band (λmax at 720−724 nm), which points toward extensive π delocalization through the COT bridge. This aspect was fully addressed by examining the interactions between the two corrole subunits in terms of electrochemistry and DFT calculations of the oxidized and reduced macrocycle. The new near-IR bands that appear upon both oxidation (λmax 1250 nm) and reduction (λmax 1780 nm) serve as additional supporting evidence for this conclusion.

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INTRODUCTION Cyclic π-conjugated systems have attracted immense interest in terms of their applications as molecular materials having conductive or nonlinear optical properties, as building blocks for supramolecular structures, and for complexation with metals or ions.1 Bimetallic catalysis allows sequential or cooperative participation of two metal components leading to enhanced reaction rates, better selectivity, and sometimes new types of reactions.2 Synergistic and/or cooperative activations through multiple metal centers are quite common in biocatalysis by enzymes, including for example tyrosinase, superoxide dismutase, methane monooxygenase, ribonucleotidereductase, urease, and phosphohydrolase.3 Metallocorroles as non-precious-metal catalysts for O2 reduction, water oxidation, and CO2 reduction are considered key to meet the future energy crisis.4 The initial reports describing reliable synthetic protocols for corroles have initiated extensive research into their use as chromophores, sensors, and catalysts.5 Aiming to synthesize an acyclic π-conjugated system with bimetallic coordination sites, Osuka and co-workers synthesized a cyclooctatetraene (COT)bridged corrole dimer via a multistep process that relied on regiospecific palladium-catalyzed oxidative coupling reactions.6 The same dimer was also prepared by Barata et al., by heating the monomer in 1,2,4-trichlorobenzene under a strict inert atmosphere.7 The bridging of two corroles by the antiaromatic © XXXX American Chemical Society

COT moiety is particularly intriguing, as it might strongly affect possible electronic communication between the highly redox active macrocycles. Evidence for extensive π delocalization in (H3tpfc)2COT is an extremely low energy band, absent in monomeric corroles. Shortcomings regarding previous publications are that the crystal structure of the free base was not obtained, there was no focused discussion on bond lengths and bond angles of the COT unit, and the investigation of the coordination chemistry was limited to the bis-cobalt(III) complex. We now report the X-ray structures of the free base corrole (H3tpfc)2COT and its bis-gallium(III) complexes, obtained with either one or two axial pyridine molecules coordinated to each gallium(III) ion. The latter were selected as representative examples of coordination complexes with non-redox-active metal ions. Our focus was on how the inter-ring interactions in the corrole dimer influence the structure and properties of the individual corrole centers and how the absorption spectra bear the imprint of its electronic structure. These aspects were addressed both by experiment and by DFT computations. The structures and properties of the new complexes were compared with those of their monomeric analogues. This study provides Received: December 14, 2016

A

DOI: 10.1021/acs.inorgchem.6b02944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of (H3tpfc)2COT and the Corresponding Pyridine-Coordinated Gallium(III) Complexes [(Gatpfc)2]COT(py)2 and [(Ga-tpfc)2]COT(py)4

the basis for a discussion of the axial binding affinity to the bisGaIII corrole dimer, the conjugation within and through the COT ring at the center of the dimer, and most importantly, the correlation between the structure and redox processes of the complexes.

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RESULTS Synthesis and Structure. The free base COT-bridged corrole dimer (H3tpfc)2COT was synthesized by simple modification of the procedure reported earlier:7 heating the monomeric corrole H3(tpfc) in 1,2,4-trichlorobenzene at 200 °C for 24 h in air rather than under anaerobic conditions. The rationale was that the dimerization of the corrole is an oxidative transformation. Indeed, the yield improved from 11% under N2 to 18% in air. The insertion of gallium into the macrocycle was achieved by treating (H3tpfc)2COT with GaCl3 in pyridine under inert conditions, allowing for the isolation of [(Ga-tpfc)2]COT(py)2 (one pyridine molecule coordinated to each metal center) in 76% yield (Scheme 1) via crystallization from noncoordinating solvents.8a,b Recrystallization in pyridine led to quantitative conversion to [(Ga-tpfc)2]COT(py)4, with two pyridine molecules coordinated to each gallium(III) ion. The electronic spectrum of the free base corrole dimer (H3tpfc)2COT is unique in terms of a very intense band at about 720 nm. Metalation by gallium(III) has hardly any effect on that band, while the near-UV maximum (Soret band) is red-shifted from 396 to 403 nm and more clearly split (Figure S1 in the Supporting Information). Treatment of a CH2Cl2 solution of [(Ga-tpfc)2]COT(py)2 with an excessive amount of pyridine affected the spectrum very little, and even the spectrum in pure pyridine was very similar (Figure S2 in the Supporting Information). Accordingly, the affinity of each gallium ion in [(Ga-tpfc)2]COT(py)2 for an additional axial pyridine as to form [(Ga-tpfc)2]COT(py)4 could not be evaluated by UV−vis spectroscopy. Dark brown crystals suitable for an X-ray diffraction study were grown at room temperature by slow diffusion of hexane into a dichloromethane solution of (H3tpfc)2COT, which crystallized in a triclinic crystal system with space group P1̅. Figure 1 depicts the X-ray structure of (H3tpfc)2COT. Its corrole subunits resemble that of monomeric H3(tpfc) very much: the same tautomer, wherein both directly connected pyrroles are protonated, is present in both subunits, the individual pyrrole rings turn up and down, and the NH proton that is opposite to the nonprotonated pyrrole deviates by as much as 0.843 Å from

Figure 1. (A) Top and (B) side views of (H3tpfc)2COT (without the C6F5 groups), with 50% thermal contours.

its own pyrrole ring. The corrole subunits are still nearly coplanar, in contrast to a previously computed structure that suggested a large dihedral angle between them.6b However, the bridging COT ring is perfectly planar and all C−C bonds that are shared by COT and the corrole subunits are significantly shorter than those that bridge between them, by 0.030, 0.025, and 0.016 Å. The COT unit may hence be analyzed as two sets of three adjacent bonds with double-bond character that are connected to each other by two single bonds. The full data for the COT moiety and a corresponding pictorial representation are provided in Table S2 in the Supporting Information and Figure 3, for both the free base (H3tpfc)2COT and its five- and six-coordinated bisgallium(III) complexes. Dark green X-ray-quality crystals were obtained by slow diffusion of hexane into a dichloromethane solution of the bisgallium(III) complex. Figure 2 depicts the X-ray structure of [(Ga-tpfc)2]COT(py)2, which crystallized in the triclinic crystal system with space group P21/c. Each subunit displays a fivecoordinate domed complex with the Ga ion 0.38 Å above the N4 coordination core and toward the axial pyridine (Figure 2B). The relative orientation of the axial pyridines is anti, similar to what has been observed for the bis-Co(PPh3) complex of the same corrole dimer.6a Selected crystallographic parameters are reported in Table 1, and information on the COT moiety is provided in Table S2 in the Supporting Information and Figure 3. B


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Inorganic Chemistry

Figure 3. C−C bond lengths determined by X-ray crystallography within the COT moiety of (A) (H3tpfc)2COT, (B) [(Ga-tpfc)2]COT(py)2, and (C) [(Ga-tpfc)2]COT(py)4.

Figure 2. (A) Top view of [(Ga-tpfc)2]COT(py)2 and (B) side view (without the meso-C6F5 groups and coordinated pyridine ligands), with 50% thermal contours.

coordinated complexes.8b The more than 0.1 Å difference between the two gallium−pyridine distances in [(Ga-tpfc)2]COT(py)4 is, however, unique to it. The above phenomenon is further reflected in a slight out of plane deviation of 0.05 Å of the metal ion, toward the more strongly bound pyridine. The out of plane displacement of gallium in five-coordinate [(Ga-tpfc)2]COT(py)2 is 0.38 Å, similar to typical values of 0.301−0.411 Å obtained for monomeric (pyridine)gallium(III) corroles. Altogether, the affinity of the bis-gallium corrole dimer for a sixth ligand appears to be relatively low, which is the reason that a reliable binding constant could not be obtained. The low sixth-ligand affinity is further corroborated by the computed ligand binding energies (vide infra). The peculiarity mentioned earlier of the bridging COT moiety in the free base comes into effect even more in both the five- and six-coordinate bis-gallium(III) complexes. Inspection of Figure 3 discloses two sets of three adjacent C−C bonds that are of practically identical lengths (1.432 Å in [(Ga-tpfc)2]COT(py)2 and 1.422 Å in [(Ga-tpfc)2]COT(py)4) and are significantly shorter than the two bonds that connect the corrole subunits (1.453 Å in [(Ga-tpfc)2]COT(py)2 and 1.445 Å in [(Gatpfc)2]COT(py)4). An identical trend was also obtained by the computational results, as may be appreciated from Table S2 in the Supporting Information. The phenomena of the three adjacent bonds of equal lengths is quite different from the situation in the monomeric corrole, where a clear alternation exists, with lengths of 1.415, 1.428, and 1.418 Å for the relevant bonds. The apparent cumulene-like organization of bonds in the dimers does not, however, affect the bond angles, which differ by

X-ray quality-crystals were also obtained when [(Ga-tpfc)2]COT(py)2 was dissolved in pyridine and allowed to slowly evaporate. The diffraction analysis of the dark green crystals revealed the formation of the six-coordinate complex [(Gatpfc)2]COT(py)4 (Figure S3 in the Supporting Information). Each metal ion is coordinated by two pyridine molecules, which form somewhat different bond lengths with it (2.252 and 2.364 Å) and are in an almost perfect coplanar arrangement. The metal ion is now much more within the N4 coordination core, only 0.05 Å out of plane toward the axial ligand that is bound more strongly to it. The relative orientation of the shorter bound pyridine ligands in each of the metal centers is anti. Selected crystallographic parameters are reported in Table 1, and the information on the COT moiety is provided in Table S2 in the Supporting Information and Figure 3. The Ga−Npy distances in the five-coordinate [(Ga-tpfc)2]COT(py)2 and in its monomeric analogue (Ga-tpfc)(py) are practically identical (2.033(4) and 2.037(17) Å, respectively) and quite short relative to gallium−pyridine bonds in completely inorganic complexes (e.g., 2.121−2.151 Å in {Ga(N3)3(py)3},9 while the Ga−Nc bond length of 1.9345(3) Å in the former is somewhat shorter than the 1.9404(17) Å in the latter.8a A much more significant shortening of the average Ga−Nc bonds to 1.9082(5) is seen for the six-coordinate [(Ga-tpfc)2]COT(py)4, which is accompanied by larger Ga−Npy distances of 2.252(7) and 2.364(7) Å for the two pyridines that are trans to each other. The comparison with previously reported monomeric gallium(III) corroles reveals very similar trends of shortening of Ga−Nc and elongation of Ga−Npy bonds upon moving from five- to six-

Table 1. Selected Structural Parameters for (H3tpfc)2COT, [(Ga-tpfc)2]COT(py)2, and [(Ga-tpfc)2]COT(py)4, Presented Together with Those of the Monomeric Analogues H3tpfc, (Ga-tpfc)(py), and [Ga-tpfc(NO2)2](Py)2 compound H3tpfc (Ga-tpfc)(py) [Ga-tpfc(NO2)2](py)2g (H3tpfc)2COT [(Ga-tpfc)2]COT(py)2 [(Ga-tpfc)2]COT(py)4

Ga−Nc (Å)a

Ga−Npy (Å)a

ΔGa23 (Å)b

ΔGa4N (Å)c

1.940(17) 1.922

2.0370(17) 2.233(6), 2.284(6)

0.50 0.13

0.41 0.05

1.935(3) 1.908(5),

2.033(4) 2.252(7), 2.364(7)

0.45 0.06

0.38 0.05

Δ23 (Å)d

Cβ e

Cmf

ref

0.012 0.07 0.04 0.14 0.05 0.04

0.16 0.10 0.05 0.22 0.07 0.03

0.05 0.02 0.01 0.07 0.05 0.04

5j 8a 8b this work this work this work

a Bond distances. bDisplacement of Ga from the least-squares plane defined by the C19N4 skeleton. cDisplacement of Ga from the least-squares plane of the N4 corrole core. dAverage displacement of individual atoms from the least-squares C19N4 plane. eAverage displacement of β-carbon atoms from the least-squares plane of the C19N4 skeleton. fAverage displacement of meso carbon atoms from the least-squares plane of the C19N4 skeleton. g This complex was chosen as reference since the non-nitrated Ga(tpfc) crystallizes with only one pyridine axial ligand.

C


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full symmetry groups are C2v and D2h, respectively, which are expected to show up as four doublet of doublet signals in a 2:2:1:1 ratio in the former case and as two doublet of doublet signals in an 8:4 ratio in the latter case. This was however not the case, as although the spectra obtained in CDCl3 and pyridine-d5 displayed different chemical shifts, their o-F patterns (at −137 to −138 ppm) were identical and consistent with pentacoordination. The apparent reason, which is further supported by the electronic spectra and crystallographic data analyzed earlier, is the very low affinity of gallium(III) for the sixth ligand in [(Gatpfc)2]COT(py)4.

only 1−2° relative to the monomer (Figure S4 and Table S1 in the Supporting Information).

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NMR CHARACTERIZATION NMR spectra of the five-coordinate bis-gallium corrole dimer [(Ga-tpfc)2]COT(py)2 were recorded in noncoordinating CDCl3 and also in pyridine-d5, in which it spontaneously forms the six-coordinate [(Ga-tpfc)2]COT(py)4 complex (Figure S5 in the Supporting Information). The 1H NMR spectra revealed one singlet (4H) and two doublets (4H each), completely consistent with the structures. The coordinated pyridines could not be observed in the spectra of both complexes, because of fast exchange with pyridine-d5 in the latter case and excessive broadening in the former case due to fast on−off rates. The latter statement was confirmed by adding small amounts of nonlabeled pyridine to the CDCl3 solution, which only appeared as broad bands therein. The singlet of the six-coordinate complex is more downfield by a full 1 ppm than that for the five-coordinate complex, and the same trend holds for the two doublets. 19 F NMR is usually an ideal tool for deducing the symmetry of metallocorroles and for distinguishing between five- and sixcoordinate complexes, because the two o-F (and to a lesser extent also the m-F) atoms in each C6F5 ring experience different and identical magnetic environments, respectively. The first aspect was fulfilled by virtue of the 4:2 ratio between the p-F atoms at around −150 ppm due to the C2 axis present in both [(Gatpfc)2]COT(py)2 and [(Ga-tpfc)2]COT(py)4 (Figure 4). The

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ELECTROCHEMICAL PROPERTIES The cyclic voltammogram (CV) of [(Ga-tpfc)2]COT(py)2 (Figure S6 in the Supporting Information) discloses two oxidation processes with half-wave potentials (E1/2) of +0.82 and +1.04 V in dichloroethane (DCE) solution, due to ligandcentered oxidations. Under the same conditions, the monomeric (Ga-tpfc)(py) displays only one oxidation at E1/2 = 0.98 V: i.e., between two oxidation potentials of the dimer. The oxidation potentials suggest that [(Ga-tpfc)2]COT(py)2 could be oxidized to the corresponding cation radical by tris(4-bromophenyl)aminium hexachloroantimonate, whose E1/2 value is 1.11 V vs Ag/AgCl in CH2Cl2. This hypothesis was checked by in situ treatment of [(Ga-tpfc)2]COT(py)2 with increasing amounts (up to about 3 equiv) of the oxidizing agent (Figure 5B). The most important observations were the practically complete disappearance of the characteristic corrole dimer 720 nm band and the appearance of a significantly weaker new band with λmax 840 nm. In fact, the same changes occur when [(Gatpfc)2]COT(py)2 is kept in aerobic solution for a prolonged time, which is indicative of its easy oxidation. Supporting evidence for that conclusion and additional insight were obtained by controlled spectroelectrochemistry (Figure 5A). Fixing the potential at 1.2 V, i.e. higher than the second redox potential of [(Ga-tpfc)2]COT(py)2, induced spectral changes similar to those obtained by chemical oxidation, but with a significantly more intense band at 840 nm. This indicates that the electrochemical oxidation led to the doubly oxidized {[(Gatpfc)2]COT(py)2}2+ complex more efficiently than the chemical oxidation. Changing the potential to 0.6 V, i.e., below the first redox potential of the complex, fully restored the spectrum of [(Ga-tpfc)2]COT(py)2 (Figure S7 in the Supporting Information), which testifies to the stability of the doubly oxidized complex. Partially oxidized solutions of [(Ga-tpfc)2]COT(py)2

Figure 4. 19F NMR spectra (400 MHz, 298 K) obtained for [(Gatpfc)2]COT(py)2: (upper) in CDCl3; (lower) in pyridine-d5.

Figure 5. (A) Spectroelectrochemistry of [(Ga-tpfc)2]COT(py)2 (0.125 mM) in 0.2 M TBAP/dichloroethane solution at an applied potential of +1.2 V (1440 s, scanned every 60 s). (B) UV−vis changes upon titration of [(Ga-tpfc)2]COT(py)2 with tris(4-bromophenyl)aminium hexachloroantimonate, in CH2Cl2 solution at 298 K. D


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Inorganic Chemistry

Figure 6. Display of the scans obtained during the spectroelectrochemistry of [(Ga-tpfc)2]COT(py)2 (0.25 mM) at (A) 10−80 s and (B) 80−300 s. Conditions: 0.2 M TBAP/pyridine, applied potential of −1.2 V.

Figure 7. (A) Changes in the far-visible and NIR spectra of [(Ga-tpfc)2]COT(py)2 upon gradual addition of tris(4-bromophenyl)aminium hexachloroantimonate, in CH2Cl2. (B) Gradual addition of NaBH4, in pyridine at 298 K.

The aforementioned chemical oxidation and reduction of [(Ga-tpfc)2]COT(py)2 also led to new absorption bands in the near-IR region: very broad bands between 1000 and 1400 nm with a maximum at 1250 nm and one much sharper band with λmax 1780 nm, respectively (Figure 7).

that are formed when solutions are left in open to air were also reduced by applying the same procedure. CV examinations of [(Ga-tpfc)2]COT(py)2 at negative potentials in noncoordinating solvents revealed redox processes that were not fully reversible and its electrochemistry was hence examined in pyridine, in which [(Ga-tpfc)2]COT(py)4 is formed. This uncovered two processes with half-wave potentials of −0.71 and −1.1 V (Figure S8 in the Supporting Information). The large separation between the ligand-centered reductions is indicative of efficient conjugation through the COT core. Chemical reduction was achieved by slow addition of a pyridine solution of NaBH4 to a pyridine solution of [(Ga-tpfc)2]COT(py)2, and the process was followed by UV−vis spectroscopy (Figure S9 in the Supporting Information). The dominant 720 nm band [(Gatpfc)2]COT(py)4 decreased somewhat in intensity and shifted slightly to the red (λmax 735 nm), in contrast to the chemical oxidation (Figure 5B), where a new peak appeared at 840 nm. The red-shifted peak at 735 nm appears to have broadened considerably. The spectroelectrochemical reduction of the bis-gallium(III) complex in pyridine bears the imprint of its chemical reduction: a red shift of the peak position from 720 to 735 nm and diminished absorption intensity, accompanied by the formation of a new low-intensity band at 790 nm (Figure 6A). A clear isosbestic point is observed for that transformation during the first 80 s only. Continuation of the reduction for prolonged times (80− 300 s) induced a further intensity lowering of the 735 nm band along with a red shift of the 790 nm band to 803 nm and an increase in intensity (Figure 6B), with new isosbestic points. These results may hence be analyzed as stepwise formation of the singly and doubly reduced complexes: i.e., {[(Ga-tpfc)2]COT(py)4}− and {[(Ga-tpfc)2]COT(py)4}2−.

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COMPUTATIONAL CHARACTERIZATION Structure. Selected geometry parameters for the bisgallium(III) complexes of the directly bridged corrole dimer (without the meso-C6F5 groups) are shown in Scheme 2, together with that of the monomeric complexes. Complexes 1 and 4 were calculated without any additional ligands, complexes 2 and 5 with one axial pyridine per gallium, and complexes 3 and 6 with two axial ligands on each gallium. Two possible geometries that differ in the relative positioning of the coordinated pyridine in 2, on the same and opposite sides of the macrocycle, were calculated. Since they were determined to have practically identical energies, only the latter is presented, as it is the one obtained experimentally. The inspection of bond lengths within the bridging COT moiety reveals two sets, composed of six 1.439 ± 0.005 Å bonds that are shared with the corrole subunits and the two that bridge between them and display bond lengths of 1.453−1.454 Å. The almost equal bond lengths of the former set points toward very efficient electronic delocalization within each one of the monocorrole units. Electronic communication between the two corrole subunits is still possible through the somewhat longer bonds that connect them to each other, an aspect that was further corroborated by the shapes and the energies of the frontier molecular orbitals (FMO) of the whole system (vide infra). A comparison of the characteristic corrole bonds connecting the directly bound pyrrole rings in the analogous monomeric and dimeric gallium complexes reveals that they have about the same E


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Inorganic Chemistry

form 3, ΔH4 of eq 4, are −58.11 and −13.84 kcal/mol. The twice as large values of ΔH3 vs ΔH1 and of ΔH4 vs ΔH2 indicate that the complexation of pyridine molecule to each one of the gallium atoms could proceed independently and that there is no electronic coupling between the two gallium(III) centers in 1. Frontier Molecular Orbitals (FMOs) of the Model Neutral Complexes. The FMOs of 2 and 5 are presented in Figure 8 (occupied and unoccupied molecular orbitals). At the

Scheme 2. Geometry of Model Compounds 1−6 Optimized at the UB3LYP/6-31g(d,p) Level of Theorya

a

Pentafluorophenyl substituents at the meso positions were replaced by hydrogen atoms, in order to reduce the computational cost.

length in all possible coordination states ranging from 1.432 to 1.444 Å. However, the two bonds adjacent to it are significantly shorter, 1.423−1.427 Å in the monomeric 4−6, than in the dimers 1−3, 1.439−1.444 Å. Complexation Enthalpies of Monomeric and Dimeric Gallium(III) Complexes with Pyridine. The complexation enthalpies were computed according to eqs 1−4 and summarized in Table 2, with negative complexation enthalpies indicative of

Figure 8. Computed (PBE1PBE/6-31g(d,p)//B3LYP/6-31g(d,p)) molecular orbital shapes (0.02 e/au3) and energies (eV) of 2 and 5.

PBE1PBE/6-31g(d,p) level of theory, the shapes of the HOMO and HOMO-1 orbitals of 5 are very similar to those of the previously computed full complex (i.e., with the C6F5 groups, see Figure S10 in the Supporting Information, and without the coordinating pyridine molecule)8a and other corroles and porphyrins,10 corresponding to the well-known Gouterman four-orbital model.11 For the unoccupied molecular orbitals, while LUMO+1 is consistent with the same model, there is substantial contribution of the pyridine ligand to the LUMO, LUMO+2, and LUMO+3 in 5 (Figure 8). The FMO of the dimer 2 may be viewed as the combination on the FMO of the monomer 5. The HOMO and HOMO-3 of 2 are the two possible combinations of two HOMO-1 orbitals of 5 and the HOMO-1 and HOMO-2 of 2 of two HOMO orbitals of 5. The LUMO and LUMO+3 of 2 are distributed on the entire biscorrole macrocycle; LUMO+1 and LUMO+2 are located exclusively on the pyridine ligands. Very similar shapes of FMOs persist for the full model of 2: i.e., with 6 pentafluorophenyl substituents at the meso positions (Figure S10). Additional calculations at the B3LYP/6-31g(d,p) and CAMB3LYP/6-31g(d,p) levels of theory reveal different contributions of the pyridine ligand orbitals to the unoccupied molecular orbitals of model compounds 2 and 5 (Figure 8 and Figure S12 and S13 in the Supporting Information). Such dependence of FMOs on the DFT exchange correlation potential, particularly of heteroaromatic small molecules, was previously addressed by Egger et al. and Marom et al.12,13 At all standard exchange correlation functionals considered in this study, i.e. PBE1PBE (i.e. PBE0),14 B3LYP,15 and CAM-B3LYP,16 the conjugation

Table 2. Computed (UB3LYP/6-31g(d,p), kcal/mol) Complexation Enthalpies of Pyridine Molecules with Monomeric and Dimeric Gallium(III) Corroles equation

4 + Py → 5

complexation enthalpy ΔH, kcal/mol

(1)

ΔH1 = H5 − [H4 + HPy] 5 + Py → 6 (2)

−28.94

ΔH2 = H6 − [H5 + HPy] 1 + 2Py → 2 (3)

−6.73

ΔH3 = H2 − [H1 + 2HPy] 2 + 2Py → 3 (4)

−58.11

ΔH4 = H3 − [H2 + 2HPy]

−13.84

energetically favorable complex formation. For the coordination of one pyridine molecule to 4 as to form 5, the enthalpy according to eq 1 (ΔH1) is −28.94 kcal/mol. The affinity for completion of the coordination sphere by a sixth ligand to from 6 according to eq 2 is significantly lower (ΔH2 = −6.73), consistent with earlier reported experimentally determined binding constants.8a The computed complexation enthalpy of two pyridine molecules to each one of the Ga atoms in 1 to form 2, ΔH3 of eq 3, and that of additional two pyridine molecules to F


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Inorganic Chemistry between the two corrole subunits in model system 2 is observed with the occupied FMOs HOMO, HOMO-1, HOMO-2, and HOMO-3 spanning the two corrole subunits. The conjugation between the two corrole subunits in 2 leads to a reduced HOMO−LUMO (Kohn−Sham orbitals eigenvalue) gap of 2.42 eV in 2 vs 2.78 eV in 5 at the PBE1PBE/6-31g(d,p) level of theory (the HOMO−LUMO gap of 5 is calculated to be 2.78 eV from the data in Figure 8). Computed Electronic Structures of Reduced and Oxidized 2. Upon single-electron reduction of 2, the extra one-electron density appears to be distributed over the entire biscorrole moiety, according to the total spin density representation shown in Figure 9a, in agreement with the shape of the LUMO

Figure 10. Computed (PBE1PBE/6-31g(d,p)//B3LYP/6-31g(d,p)) UV−vis spectra of 2 and 5. See the Supporting Information for individual transitions.

model 2, the electron-withdrawing pentafluorophenyl substituents were replaced by hydrogen atoms and also to the variations in absolute computed values as a function of the choice of DFT potential (see Figures S14−S17 in the Supporting Information for computed UV spectra of 2 and 5 using several DFT potentials). The computed electronic spectrum of the oxidized 2, i.e. [2]+, shows that the 600 nm band is replaced by a red-shifted signal whose λmax value is 697 nm (Figure 11). In addition to the 697

Figure 9. Spin density at the PBE1PBE/6-31g(d,p)//B3LYP/631g(d,p) level of (a) [2]−, (b) [2]+, (c) singlet-[2]2−, and (d) triplet[2]2−.

orbital of the neutral 2 (Figure 8), the active orbital during the reduction process. The computed NBO charges of the neutral 2 and its reduced form, i.e. the anion radical [2]−, indicate that the extra one-electron density in [2]− is distributed over the biscorrole moiety (see Table S4 in the Supporting Information for atomic charges). Similarly to one-electron reduction, the oneelectron oxidation results in the distribution of the spin density over the entire bis-corrole moiety in the resulting cation radical, i.e. [2]+ (Figure 9b; see Table S4 for computed atomic charges). The spin density distribution of [2]+ is in agreement with the shape of the HOMO orbital of 2 (Figure 8): i.e., the active orbital in the oxidation process. The two-electron reduction of 2 could result in either ferromagnetic (triplet) or antiferromagnetic (singlet) coupling between the two extra electrons in [2]2−. The free energy difference, ΔG, between the two molecules in different electronic states was determined to be only 1.45 kcal/mol, with the triplet state being slightly lower in energy at the UB3LYP/6-31g(d,p) level of theory. The spin distribution for both viable electronic states (Figure 9c,d) shows homogeneous distribution of the spin density over the bis-corrole with contribution from the pyridine ligands. Computed Electronic Spectra. The computed UV−vis spectra of 2 clearly disclose the new low-energy transition at ∼600 nm (Figure 10), in agreement with the experimentally observed new low-energy transition at 720 nm. The difference in the absolute values of computed and measured low-energy transitions is 120 nm, which can be attributed to the fact that, in

Figure 11. Computed (PBE1PBE/6-31g(d,p)//B3LYP/6-31g(d,p)) UV−vis spectra of (dotted blue line) 2 and (solid orange line) its oxidized form, [2]+.

nm band, an even lower energy 1492 nm band appears. The simulated electronic spectrum of [2]+ corroborates well the experimental trend obtained from the chemical as well as the voltage-controlled spectroelectrochemical oxidation of [(Gatpfc)2]COT(py)2. The computed UV spectra of singly and doubly reduced 2, i.e. [2]− and [2]2−, are shown in Figure 12. They reveal red shifts of the lowest energy band from ∼600 nm in the neutral complex to 605 nm (main) and 668 nm (minor band) upon one-electron reduction and 615 nm for the two-electron-reduced species. For [2]− and [2]2− the computed lowest energy transitions occur at 1375 and 1340 nm, correspondingly.

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DISCUSSION Intrigued by previous publications about the cyclooctatetraene (COT) fused corrole (H3tpfc)2COT,6,7 we set out to examine several aspects of this interesting and potentially useful kind of bis-metallic binding macrocycle. In terms of improved accessibility, we have focused on the one-pot route from the G


Article

Inorganic Chemistry

delocalization through the COT bridge and a consequentially reduced HOMO−LUMO gap. Cyclic voltammetry examinations further confirmed that hypothesis, as each of the complexes displayed large separations (of at least 200 mV) between the first and second redox processes obtained at both positive and negative potentials. A combination of chemical reductions and oxidations with voltage-controlled spectroelectrochemistry allowed for the identification of spectroscopic changes that accompany these processes. Both the directions and the magnitude of the spectral changes that the characteristic longwavelength dimer bands undergo were reproduced by the DFT calculations. The same holds for the new NIR bands obtained upon both oxidation and reduction of the complexes. The spin density in both the one-electron-oxidized and one-electronreduced complexes was determined to be distributed over the entire bis-corrole moiety, which explains why the first and second redox processes are so well separated. Doubly oxidized/reduced complexes were also formed by applying reasonable potentials; the DFT calculations suggest that there is no significant energy difference between the conceivable triplet and singlet states of the doubly reduced bis-corrole. Finally, the delocalization in [(Ga-tpfc)2]COT(py)2 was evaluated by ACID calculations,23 which confirmed the conjugation of the two corrole subunits through the two bonds connecting them (Figure S22 in the Supporting Information). To address the potentially interesting issue of aromaticity/ antiaromaticity within the COT unit, we have applied an NICS scan22 on top of ACID. While NICS(1)zz yields paratropic values, NICS(1)π,zz (calculated from the σ-only model) suggests very diatropic values (Figures S23 and S24 in the Supporting Information). While such a large influence of metals on NICS behavior has been observed before,22e it remains clear (see also ACID results) that the discussed eight-membered ring is paratropic. However, whether or not this paratropicity originates from the π electrons (i.e., antiaromaticity) has yet to be verified. This study provides new and valuable insight into the structural and electronic features of COT-bridged corroles and their corresponding complexes with non-redox-active elements. It hence paves the way for investigating the much more complicated systems in which this highly noninnocent ligand is bound to transition-metal complexes. Studies in this direction are being actively pursued in our laboratories, not least because of the potential potency of such bimetallic complexes as catalysts for important electrocatalytic processes.

Figure 12. Computed (PBE1PBE/6-31g(d,p)//B3LYP/6-31g(d,p)) UV−vis spectra of (dotted blue line) 2 and its singly and doubly reduced forms (solid green) 2•− and (solid violet) triplet-22−.

monomeric corrole H3(tpfc), since the very elegant rational synthesis that leads to an overall yield of 50% from H3(tpfc) still requires three steps and expensive chemicals. With the realization that the heat-driven transformation of H 3 (tpfc) into (H3tpfc)2COT is actually an oxidative process (dehydrogenation), simply running the reaction under aerobic rather than anaerobic conditions increased the chemical yield from 11% to 18%. Of the two redox isomers that exist for the dimer, only that in which the two subunits resemble monomeric corroles in terms of a triprotonic N4 coordination core was obtained. This similarity was confirmed by X-ray crystallography, which revealed that each subunit adopts the same (one of two possible) tautomeric form of H3(tpfc). The phenomenon of one particular NH bond that is extremely deviating from the plane defined by its own pyrrole moiety, known to be responsible for the unusually large NH acidity of H3(tpfc), is also present in (H3tpfc)2COT. The crystal structure further allowed for a novel insight into an aspect that was not previously discussed: the structural parameters of the formally antiaromatic COT moiety that bridges the corrole subunits. This revealed that this eight-carbon ring is perfectly planar with two sets of three adjacent short C−C bonds that are connected to each other by two longer bonds, dramatically different from the case for free COT with its alternating single and double C−C bonds and tube-like D2d conformation. On the basis of previous calculations,6b the 720 nm band of (H3tpfc)2COT was attributed to the HOMO− LUMO transition. The computed UV spectrum and FMO shape and energies, in this study, confirm no further absorptions beyond the new band at 720 nm (Figures S20 and S21 in the Supporting Information). Considering the relatively little prior work on this subject,17−19 we chose to first focus on a non-transition-metal ion, of which gallium was selected because it serves as a prototype for monomeric metallocorroles.20,21 Two kinds of bis-gallium(III) complexes were isolated and fully characterized, five-coordinate [(Ga-tpfc)2]COT(py)2 and six-coordinate [(Ga-tpfc)2]COT(py)4, with one and two axial pyridine molecules on each metal ion, respectively. Both X-ray crystallography and DFT calculations disclosed that the COT moiety in both complexes is essentially planar and displays very unusual nonalternating C− C bonds. The unique bond that connects two pyrroles in Ga(tpfc)py is significantly longer than the two adjacent to it, while their equivalent lengths in the bis-gallium complexes points toward more efficient conjugation within each one of the monocorrole units. The intense far-visible band, present only in the dimers, was confirmed by DFT to be due to extensive π

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EXPERIMENTAL SECTION

Materials. All routine chemical reagents and solvents were purchased from commercial sources and were purified by standard procedures before use. The directly doubly linked corrole dimer was synthesized by modifying the literature method,6 as outlined below. The details regarding the DFT calculations are provided in the Supporting Information. Synthesis. Synthetic Methods. The synthetic details for the preparation of H3(tpfc) and Ga(tpfc)(py) have been provided in previous publications.5a,8a Synthesis of (H 3 tpfc) 2 COT. A solution of 5,10,15-tris(pentafluorophenyl)corrole (200 mg, 250 μmol) in 1,2,4-trichlorobenzene (2 mL) was refluxed at 200 °C for 24 h in open air. TLC of the reaction mixture revealed some starting corrole and three new compounds. The desired product was separated by column chromatography using a 2/1 hexane/dichloromethane mixture as eluent (36 mg, 18% yield). UV−vis (CH2Cl2): λmax (ε × 10−4) 396 (5.17), 433 (3.90), 475 (2.88), 721 (5.18) nm. All of its spectroscopic features were identical with those previously reported.7 H


Article

Inorganic Chemistry Synthesis of [(Ga-tpfc)2]COT(py)2. A solution of (H3tpfc)2COT (100 mg) in pyridine (30 mL) was added to a large excess of flame-dried GaCl3, and the reaction mixture was heated to reflux for 1 h under argon, followed by evaporation of the solvent. Excess inorganic salts were removed by flash chromatography (silica, CH2Cl2/methanol 100/1), followed by recrystallization from CH2Cl2 and n-heptane in the presence of 1 drop of pyridine to give 90 mg (76% yield) of pure crystals. UV−vis (CH2Cl2): λmax (ε × 10−4) 403 (4.96), 436 (4.06), 618 (1.9), 656 (1.8), 724 (4.99) nm. 1H NMR (CDCl3, 400 MHz): δ 9.46 (s, 4H, β pyrrole H); 8.76 (d, J = 4.4 Hz, 4H, β pyrrole H); 8.56 (d, J = 4.4 Hz, 4H, β pyrrole H). 19F NMR (CDCl3, 377 MHz): δ −137.6 (dd, J1 = 7.8, J2 = 24 Hz, 8 F, ortho-F), −137.82 (dd, J1 = 8.67, J2 = 24.88 Hz, 4 F, ortho-F); −153.23 (t, J = 24 Hz, 4F, para-F), −153.75 (t, J = 12.4 Hz, 2F, para-F), −162.17 (m, 12F,meta-F). Synthesis of [(Ga-tpfc)2]COT(py)4. [Ga(tpfc)(py)]2COT was dissolved in pyridine, and when this solution was left open for slow evaporation, it provided a quantitative yield of pure crystals. UV−vis (CH2Cl2): λmax (ε × 10−4) 407 (4.9), 725 nm (4.5). 1H NMR (pyridined5, 400 MHz): δ 10.68 (s, 4H, β pyrrole H); 9.28 (d, J = 4.4 Hz, 4H, β pyrrole H); 9.06 (d, J = 4.4 Hz, 4H, β pyrrole H), 19F NMR (CDCl3, 377 MHz): δ −137.32 (dd, J1 = 7.2, J2 = 25 Hz, 4 F, ortho-F), −137.82 (dd, J1 = 7.54, J2 = 25.63 Hz, 8 F, ortho-F); −152.61 (t, J = 21.49 Hz, 4F, para-F), −153.26 (t, J = 21.48 Hz, 2F, para-F), −161.56 (m, 12F, meta-F)

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Shinokubo, H.; Takagi, A.; Kawai, T.; Matsumoto, T.; Yoon, Z. S.; Kim, D. Y.; Ahn, T. K.; Kim, D.; Muranaka, A.; Kobayashi, N.; Osuka, A. A Directly Fused Tetrameric Porphyrin Sheet and Its Anomalous Electronic Properties That Arise from the Planar Cyclooctatetraene Core. J. Am. Chem. Soc. 2006, 128, 4119−4127. (f) Ooi, S.; Tanaka, T.; Park, K. H.; Kim, D.; Osuka, A. Triply Linked Corrole Dimers. Angew. Chem., Int. Ed. 2016, 55, 6535−6539. (2) (a) van den Beuken, E. K.; Feringa, B. L. Bimetallic catalysis by late transition metal complexes. Tetrahedron 1998, 54, 12985−13011. (b) Urbach, F. L. The Properties of Binuclear Copper Centres in Model and Natural Compounds. In Metal Ions in Biological Systems; Sigel, H., Ed.; Dekker: New York, 1981; Vol. 13, p 73. (3) (a) Reedijk, J.; Bioinorganic Catalysis; Dekker: New York, 1993. (b) Bertini, I.; Gray, H. B.; Lippard, S. J.; Selverstone Valentine, J. Bioinorganic Chemistry; University Science Books: Mill Valley, CA, 1994. (c) Karlin, K. D.; Tyeklfir, Z.; Bioinorganic Chemistry of Copper; Chapman & Hall: New York, 1993. (d) Vigato, P. A.; Tamburini, S.; Fenton, D. E. The Activation of Small Molecules by Dinuclear Complexes of Copper and Other Metals. Coord. Chem. Rev. 1990, 106, 25−170. (e) Feringa, B. L.; Gelling, O.-J.; Rispens, M. T.; Lubben, M. In Transition Metals in Supramolecular Chemistry; Fabrizzi, L., Poggi, A., Eds.; Kluwer Academic: Dordrecht, The Netherlands, 1994; Vol. C 448, pp 171−190. (4) (a) Levy, N.; Mahammed, A.; Kosa, M.; Gross, Z.; Elbaz, L. Metallocorroles as Nonprecious-Metal Catalysts for Oxygen Reduction. Angew. Chem., Int. Ed. 2015, 54, 14080−14084. (b) Kadish, K. M.; Fremond, L.; Ou, Z.; Shao, J.; Shi, C.; Anson, F. C.; Burdet, F.; Gros, C. P.; Barbe, J.-M.; Guilard, R. Cobalt(III) Corroles as Electrocatalysts for the Reduction of Dioxygen: Reactivity of a Monocorrole, Biscorroles, and Porphyrin−Corrole Dyads. J. Am. Chem. Soc. 2005, 127, 5625− 5631. (c) Dogutan, D. K.; Stoian, S. A.; McGuire, R.; Schwalbe, M.; Teets, T. S.; Nocera, D. G. Hangman Corroles: Efficient Synthesis and Oxygen Reaction Chemistry. J. Am. Chem. Soc. 2011, 133, 131−140. (d) Mahammed, A.; Mondal, B.; Rana, A.; Dey, A.; Gross, Z. The cobalt corrole catalyzed hydrogen evolution reaction: surprising electronic effects and characterization of key reaction intermediates. Chem. Commun. 2014, 50, 2725−2727. (e) Chatterjee, S.; Sengupta, K.; Hematian, S.; Karlin, K. D.; Dey, A. Electrocatalytic O2-Reduction by Synthetic Cytochrome c Oxidase Mimics: Identification of a “Bridging Peroxo” Intermediate Involved in Facile 4e−/4H+ O2-Reduction. J. Am. Chem. Soc. 2015, 137, 12897−12905. (f) Schöfberger, W.; Faschinger, F.; Chattopadhyay, S.; Bhakta, S.; Mondal, B.; Elemans, J. A. A. W.; Müllegger, S.; Tebi, S.; Koch, R.; Klappenberger, F.; Paszkiewicz, M.; Barth, J. V.; Rauls, E.; Aldahhak, H.; Schmidt, W. G.; Dey, A. A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water. Angew. Chem., Int. Ed. 2016, 55, 2350−2355. (5) (a) Gross, Z.; Galili, N.; Saltsman, I. The first direct synthesis of corroles from pyrrole. Angew. Chem., Int. Ed. 1999, 38, 1427−1429. (b) Paolesse, R.; Jaquinod, L.; Nurco, D. J.; Mini, S.; Sagone, F.; Boschia, T.; Smith, K. M. 5,10,15-Triphenylcorrole: a product from a modified Rothemund reaction. Chem. Commun. 1999, 1307−1308. (c) Gryko, D. T. A simple, rational synthesis of meso-substituted A2B-corroles. Chem. Commun. 2000, 2243−2244. (d) Gryko, D. T. Recent Advances in the Synthesis of Corroles and Core-Modified Corroles. Eur. J. Org. Chem. 2002, 2002, 1735−1743. (e) Paolesse, R.; Marini, A.; Nardis, S.; Froiio, A.; Mandoj, F.; Nurco, D. J.; Prodi, L.; Montalti, M.; Smith, K. M. Novel routes to substituted 5,10,15-triarylcorroles. J. Porphyrins Phthalocyanines 2003, 07, 25−36. (f) Gryko, D. T.; Fox, J. P.; Goldberg, D. P. Recent advances in the chemistry of corroles and core-modified corroles. J. Porphyrins Phthalocyanines 2004, 08, 1091−1105. (g) Gross, Z.; Gray, H. B. Oxidations Catalyzed by Metallocorroles. Adv. Synth. Catal. 2004, 346, 165−170. (h) Paolesse, R. Corrole: the little big Porphyrinoid. Synlett 2008, 2008, 2215−2230. (i) Gryko, D. T. Adventures in the synthesis of meso-substituted corroles. J. Porphyrins Phthalocyanines 2008, 12, 906−917. (j) Gross, Z.; Galili, N.; Simkhovich, L.; Saltsman, I.; Botoshansky, M.; Bläser, D.; Boese, R.; Goldberg, I. Solvent-Free Condensation of Pyrrole and Pentafluor-

ASSOCIATED CONTENT

S Supporting Information *

CCDC 1475449 ((H3tpfc)2COT), 1475450 ((Ga-tpfc)2COT(Py)2), and 1475451 ((Ga-tpfc)2COT(Py)4) contain supplementary crystallographic data for this paper; these data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02944. Details of computational methods, xyz coordinates of the computed models, additional UV spectra, and NBO charges (PDF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Z.G.: [email protected]. ORCID

Zeev Gross: 0000-0003-1170-2115 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Pazy foundation. S.B. acknowledges a Schulich Postdoctoral fellowship for his research. REFERENCES

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The Planar Cyclooctatetraene Bridge in Bis-Metallic Macrocycles

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