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Ground-State Copper(III) Stabilized by N-Confused/N-Linked Corroles: Synthesis, Characterization, and Redox Reactivity Yogesh Kumar Maurya, Katsuya Noda, Kazuhisa Yamasumi, Shigeki Mori, Tomoki Uchiyama, Kazutaka Kamitani, Tomoyasu Hirai, Kakeru Ninomiya, Maiko Nishibori, Yuta Hori, Yoshihito Shiota, Kazunari Yoshizawa, Masatoshi Ishida, and Hiroyuki Furuta J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01876 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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Journal of the American Chemical Society

Ground-State Copper(III) Stabilized by N-Confused/N-Linked Corroles: Synthesis, Characterization, and Redox Reactivity Yogesh Kumar Maurya,† Katsuya Noda,† Kazuhisa Yamasumi,† Shigeki Mori,‡ Tomoki Uchiyama,§ Kazutaka Kamitani,⊥ Tomoyasu Hirai,⊥ Kakeru Ninomiya,║ Maiko Nishibori,║ Yuta Hori,⊥ Yoshihito Shiota,⊥ Kazunari Yoshizawa,⊥ Masatoshi Ishida,*,† and Hiroyuki Furuta*,† †

Department of Chemistry and Biochemistry, Graduate School of Engineering and Center for Molecular Systems, Kyushu University, Fukuoka 819-0395, Japan ‡ Advanced Research Support Center, Ehime University, Matsuyama 790-8577, Japan § Japan Synchrotron Radiation Research Institute, SPring-8, Hyogo 679-5198, Japan ⊥Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395, Japan ║Faculty of Engineering Sciences, Kyushu University, Fukuoka, 816-8580, Japan KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal that requires keywords, provide significant keywords to aid the reader in literature retrieval. ABSTRACT: Stable square planar organocopper(III) complexes (CuNCC2, CuNCC4, and CuBN) supported by carbacorrole-based tetradentate macrocyclic ligands with NNNC coordination cores were synthesized, and their structures were elucidated by spectroscopic means including X-ray crystallographic analysis. On the basis of their distinct planar structures, X-ray absorption/photoelectron spectroscopic features, and temperature-independent diamagnetic nature, these organocopper complexes can be preferably considered as novel organocopper(III) species. The remarkable stability of the high-valent Cu(III) states of the complexes stems from the closed-shell electronic structure derived from the peculiar NNNC coordination of the corrole modified frameworks, which contrasts with the redox-noninnocent radical nature of regular corrole copper(II) complexes with an NNNN core. The proposed structure was supported by DFT (B3LYP) calculations. Furthermore, a p-laminated dimer architecture linked through the inner carbons was obtained from the one-electron oxidation of CuNCC4. We envisage that the precise manipulation of the molecular orbital energies and redox profiles of these organometallic corrole complexes could eventually lead to the isolation of yet unexplored high-valent metal species and the development of their organometallic reactions.

INTRODUCTION Copper in the unusual trivalent oxidation state (i.e., Cu(III)) has been a subject of interest for the research community because of its pivotal role as reactive intermediate in the regulation of various biological processes, including electron transfer, O2 transport, and substrate transformation within copper-containing enzymes, that is, particulate methane monooxygenase (pMMO).1 Copper(III) species are also involved as synthetic key intermediates in bond forming reaction steps of organic catalytic processes, such as C–C/X cross coupling and conjugate addition reactions.2 However, in contrast to well-known noble metal systems like organopalladium(IV) or organoplatinum(IV),3 the relationship between the structure and reactivity of organocopper(III) species is not well understood because of the scarcity of examples of stable organocopper(III) complexes (Figure 1).2,4 In addition, the precise assignment of the physical oxidation states of copper atoms is not always straightforward, even with the availability of multiple spectroscopic techniques such as X-ray-based spectroscopy (e.g., X-ray absorption near edge structure, XANES).5a For this reason, the electronic configuration of some reported Cu(III) complexes has been revisited by comprehensive spectroscopic and high-level computational studies.5b,c

Figure 1. Examples of previously reported organocopper(III) complexes.

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(e.g., bonding character) of the organocopper(III) corrole species. Furthermore, the one-electron oxidation of the CuNCC4 derivative yielded a ligand-oxidized copper(III) corrole p-radical cation species, which exhibits a peculiar face-to-face dimeric organometallic corrole architecture.

The anionic nitrogen-based strong donor and steric properties of tetradentate ligands render them suitable for the stabilization of highvalent Cu(III) species.6 In this regard, macrocycles with constrained valence and multi-anionic (carbon) donor sites have been recently used, including the N-confused porphyrins reported by our group, O-confused derivatives by Latos-Grażyński et al., and biphenyl-embedded porphyrinoids by the groups of Srinivasan and Sessler (Figure 1).7 Nevertheless, details on the electronic effect and valence states of these complexes were not provided in these works. Among the representative macrocyclic ligands, corroles, which are ring-contracted porphyrin analogues with a formal 18p aromatic circuit, are known to be capable of stabilizing high metal oxidation states (e.g., Fe(IV)/(V), Mn(V)/(VI), or Cr(V)/(VI)), taking advantage of the distinct small trianionic N4 donor sphere.8 The unique electronic structure of metallocorroles has received great attention for their application as catalysts.9 In the case of copper derivatives, 8b,d,10 a substantial redoxnoninnocence character has been identified by theoretical studies, and accordingly, these copper corrole complexes (CuCor) are preferably described as an antiferromagnetically coupled unpaired d9 Cu(II) center and a singly occupied p-orbital of the corrole radical (i.e., CuIICor•2-) in their ground states (Figure 2a).11 This redox-noninnocent nature provides the CuCor complexes with characteristic features such as temperature-dependent NMR spectral shifts, intrinsic saddle-distorted macrocyclic geometry, narrower HOMO-LUMO gap, and unique reactivity that affords oxidative dimers via thermal reaction (Scheme S1, in the Supporting Information).11a In fact, this behavior appears to be general in many metallocorroles with open-shell electronic configurations (cf. eq. 1).10a,12

Figure 2. a) Chemical structures of novel organocopper corrole, CuNCC2, CuNCC4, and CuBN, and the regular corrole, CuCor (Ar = pentafluorophenyl); b) Illustration of the frontier molecular orbital diagrams of regular copper corrole and the organocopper modified congeners. RESULTS AND DISCUSSION Synthesis of Organocopper Corrole Complexes. The organocopper complexes CuNCC2, CuNCC4, and CuBN were prepared using the corresponding N-confused/N-linked corrole ligands containing NNNC donor cores, namely, NCC2, NCC4, and BN, respectively (Scheme 1). For the preparation of the ligands NCC2 and NCC4, optimization of the reported oxidative ring-closing reactions was required for the scalable production, since the reported yields were 5% and 18%, respectively. The predicted side-products may contain C–N fused tetrapyrrin derivatives and its oligomers according to the report.17 In the case of NCC4, the yield was remarkably improved to 61% when a milder oxidant, p-chloranil, was used in acetonitrile instead of 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ).14a,18 Similarly, the treatment of 1 with p-chloranil in acetonitrile under microwave irradiation afforded the desired NCC2 in 20% yield (see experimental details in Supporting Information).

Nevertheless, the ligand field effect on the noninnocence nature of copper corrole complexes still remains unclear and has thereby become a lively topic of debate in the chemistry of high-valent metallocorroles. In this context, we herein describe the use of novel unsymmetrical carbacorrole-based macrocycles that derived from distinctive core modifications of the parent corrole, which allowed for the variation of the strength of the Cu dx2-y2-p corrole orbital interactions through the manipulation of the orbital symmetry (Figure 2b).13 These structural modifications, which we have named N-confusion and neo-confusion approaches, afford isomeric mutant of corroles (e.g., NCC2, NCC4, and BN as ligands in Figure 2a) by displacing the inner nitrogen atoms to the periphery or to the p-circuit skeleton (Figure 2a).14 These ligands serve as trianionic organometallic ligands with a NNNC donor core that retains the formal 18p conjugation.15 Through the core mutation, we envisioned that the symmetry perturbation of the molecular orbitals would enable to raise the ligand-based b1 energy, and the concurrent strong sdonating effect of the carbanionic ligand would stabilize the Cu(III) species, as seen in some metal complexes of N-confused porphyrinoids (Figure 2b).7b,c,16 It should be noted that the organocopper(III) species of N-confused/N-linked corroles could be considered as suitable platforms for the investigation of their redox reactivity toward unexplored organocopper(IV) metal species.10g

Subsequently, the organocopper N-confused corroles were synthesized with a slight modification of the reported procedures.11a Complex CuBN was prepared following our previously reported procedure.15b For CuNCC2, the macrocycle NCC2 was dissolved in 1,2-dichlorethane/methanol solution and stirred with 10 equivalents of copper(II) acetate and sodium acetate to afford the corresponding complex in 34% yield (Scheme 1a). In the case of CuNCC4, a dimeric species (CuNCC4-dimer) was predominantly formed under identical conditions (vide infra; Scheme 2). In contrast, the treatment of one equivalent of copper(II) salt under inert atmosphere and low temperature successfully afforded the desired CuNCC4 in 41% (Scheme 1b). These organocopper complexes are surprisingly stable under ambient condition compared with the regular counterpart CuCor. The new complexes were characterized by using various spectroscopic methods such as 1H/19F NMR and UV/vis spectroscopy, high-resolution mass spectrometry, and X-ray crystallographic analysis. Thin needle-like crystals of CuNCC2, CuNCC4, and the CuNCC4-dimer complex suitable for Xray diffraction were grown from dichloromethane/acetonitrile, dichloromethane/hexane, and dichloromethane/hexane/methanol, respectively.

In this paper, we report a detailed spectroscopic characterization of novel ground-state organocopper(III) complexes (CuNCC2, CuNCC4, and CuBN) bearing N-confused/N-linked carbacorrole. The enhanced interaction between the copper center and the carbondonor site in the modified organocopper complexes was found to play an important role in the stabilization of the Cu(III) oxidation state. A concise approach that combines X-ray crystallography, variable-temperature NMR, XANES, XPS, absorption, and EPR spectroscopy with theoretical studies has been employed to elucidate the structural properties

Scheme 1. New Synthetic Protocols for a) NCC2 and b) NCC4 and their Copper Complexes


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Journal of the American Chemical Society cally above 40°) of the regular copper corroles is considered as a consequence of the remarkable overlap between the Cu(dx2-y2) orbital and the corrole p HOMO(b1).10c,f The abovementioned crystal structural features of CuNCC2, CuNNC4, and CuBN are indicative of the intrinsic ligand innocence nature with minimal d–p orbital interaction and the presence of a relatively smaller Cu(III) ion.21 The planar geometries of the organocopper complexes CuNCC2, CuNCC4, and CuBN stem most likely from the alteration in the genuine electronic structures of the regular derivatives. Table 1. Selected Bond Lengths (Å), Dihedral Angles (°), and Mean-Plane Deviations (m.p.d., Å) from the Crystal Structures of the Organocopper Corrole and Regular Corrole Complexes

Molecular Structures. The explicit geometric structures of the novel organocopper corrole complexes (CuNCC2 and CuNCC4) were elucidated by X-ray crystallographic analysis (Table S1). Two perspective views of CuNCC2, CuNCC4, and CuBN15b are depicted in Figure 3. The analysis of these structures revealed essentially planar geometries with the least out of plane displacements from the mean–plane having deviation values (m.p.d.; defined by 23 core atoms) of 0.020–0.051 Å (Figures 3 and S13, and Table 1). The copper atoms were located within the NNNC coordination spheres in each case. These coordination geometries are reflected in the structural parameters, revealing shorter Cu–N(pyrrole)/C(confused pyrrole) bond lengths (1.855, 1.873, and 1.862 Å in average for CuNCC2, CuNCC4, and CuBN, respectively) than those of regular CuCor (1.890 Å)19 and other copper corrole derivatives,8b,11d and smaller torsion angles (χ1–χ4; defined in Table 1), with the average saddling dihedral angles being 9.92°, 12.47°, and 3.85° for CuNCC2, CuNCC4, and CuBN, respectively. These latter values contrast sharply with those of regular copper corroles, which are severely saddle distorted (the average saddling dihedral angle of 39.18° and the m.p.d. value of 0.118 Å) due to the intrinsic structural feature of the noninnocent copper(II) corrole radicals.19,20 This dominant saddling nature (with χ typi-

Complexes Cu-N1 Cu-N2

CuNCC2 1.850(6) 1.865(6)

CuNCC4 1.869(9) 1.873(7)

CuBN 1.840(2) 1.878(3)

CuCor 1.883(4) 1.898(4)

Cu-N3/C Cu-N4/C

1.864(6) 1.841(6)

1.894(6) 1.854(7)

1.862(2) 1.866(3)

1.894(4) 1.883(4)

c1 c2 c3 c4

0.27 20.93 0.73 17.76

2.01 29.21 1.54 17.13

0.50 1.37 5.00 8.54

29.79 45.18 35.77 45.96

m.p.d.

0.037

0.051

0.020

0.118

Figure 3. Two perspective views (top and side) of the X-ray crystal structures of the organocopper corrole complexes: a) CuNCC2, b) CuNCC4, and c) CuBN.15b The thermal ellipsoids were scaled to the 50% probability levels. The confused pyrrolic rings are highlighted in green color for CuNCC2 and CuNCC4, and blue color for CuBN.



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the copper complexes.25 In particular, the pre-edge spectral feature for Cu(III) species has been considered as a golden rule according to the interpretation by Solomon et al., which states that the band appears at approximately 8980 eV and typically shifted to higher energy by ~2 eV relative to that of Cu(II) species, thus falling in the Cu(III) region.24a,c However, a recent re-evaluation of the origin of the pre-edge transition energy performed by Tomson et al. disclosed the complicated bonding situation for redox noninnocent copper species.5c Therefore, we used the discrete divalent CuNCP complex possessing an identical coordination NNNC core for comparison (Figure 5).26 As a result, the pre-edge energy peak of CuNCP was observed at 8977.4 eV, whereas the energy shifts were observed at 8979.6 eV for both CuNCC4 and CuBN, which is indicative of the preference for the trivalent copper oxidation state in the latter complexes. In the case of CuNCC2, the corresponding transition peak was virtually invisible, most likely because of the limited sensitivity of the instruments (see experimental section). However, the allowed rising transitions appeared in the curved shape as in the case of CuNCC4, which possesses an identical NNNC ligand field. The slightly different features in the rising region among the derivatives could be originated from the differences in the charge effect of the metal centers and the covalent character of the bonds between the metal center and the ligands, which might, in turn, have an effect in the proportion of hidden Cu(II) in the electronic structures. Unfortunately, the precise quantification of the percent of Cu(III) relative to Cu(II) character in the complexes was not achieved. However, time-dependent (TD)-DFT calculation supports the higher energy shifts of the pre-edge transitions of the organometallic species compared with that of CuNCP (vide infra).

X-ray Photoelectron and X-ray Absorption Spectroscopy. In an effort to elucidate the local electronic valence of the copper ions in the complexes, they were subjected to XPS analysis (Figure 4a–c). The corresponding spectra revealed that the resembled Cu(2p) binding energies were in the range of 935.4−936.7 eV for 2p3/2 and 955.5−956.6 eV for 2p1/2. Among the derivatives, the degree of the shifts of binding energies were found to vary depending on the supporting ligands, which may indicate nonidentical covalency in the metal–ligand bonds in the complexes. However, apparent positive shifts in energy were observed relative to the peak values of a copper(II) N-confused porphyrin complex (CuNCP; 934.0 eV for 2p3/2 and 954.0 eV for 2p1/2) (Figure 4d).22 Furthermore, the absence of shake-up satellite peaks of Cu(II) in the XPS of the new complexes is particularly diagnostic of the trivalent oxidation state of the copper ion.23 These features suggest the preference of the higher oxidation state of copper centers.

Variable-Temperature NMR Spectroscopy. The 1H NMR spectra recorded in either CDCl3 or toluene-d8 revealed the apparent diamagnetic nature of the organocopper complexes at 298 K (Figure 6). Consistently, no broadening of the 19F resonances was observed in any case (Figures S1b–S3b), and EPR silent spectra were obtained under identical conditions. The six b-protons and a confused a-pyrrolic CH proton gave rise to sharp signals in the range of 8.66–7.88 ppm for CuNCC2 or 7.94−7.54 ppm for CuNCC4. Likewise, the resonances attributable to the six b-pyrrolic CH protons of CuBN appeared in the region of 8.63– 8.21 ppm. The additional downfield-shifted broad signals around 10 ppm were attributed to the outer NH proton for both CuNCC2 and CuNCC4 (Figures S4 and S5). These chemical shift values are almost identical to those of the freebase congeners.14a Taken together, these spectroscopic features are indicative of the distinct aromaticity of the 18p macrocyclic ligands in the complexes.

Figure 4. Cu(2p) core-level XPS data of the organocopper complexes: a) CuNCC2 (red), b) CuNCC4 (green), c) CuBN (blue), and d) CuNCP (black). The asterisk indicates the satellite structure peaks of Cu(II).

Figure 5. Normalized Cu K-edge XANES spectra for the organocopper corrole complexes and a standard CuIINCP complex. Inset shows the magnified region of the Cu K pre-edge.

Figure 6. Partial 1H NMR spectra of the organocopper complexes a) CuNCC2, b) CuNCC4, and c) CuBN in CDCl3 at 298 K. The signals marked with circles in the spectrum of CuBN are assigned to the peripheral benzene CHs.

8

Further evidence of the preferential d Cu(III) ion present in CuNCC2, CuNCC4, and CuBN was supported by Cu K-edge XANES spectroscopy using synchrotron X-ray radiations (Figure 5).24 Two specific regions of the XANES spectrum, that is, pre-edge (1s-to-3d quadrupole transition) and the rising shoulder bands (1s to 4p electronic transition with shakedown contribution arising from ligand-to-metal chargetransfer transition), are relevant to ascertain the local oxidation state for

It is known that the regular copper corroles and their derivatives consist of an antiferromagnetically coupled Cu(II) center and a corrole radical cation.12a The fitting analysis of the temperature-dependent shift of a


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specific CH signal in the 1H NMR spectra allows the quantification of the corresponding energy gaps (DES-T) between the singlet and the thermally accessible paramagnetic triplet states.11c,12a In the case of the organocopper corrole complexes used in this work, very sharp resonances with negligible shifts ( CuNCC2 > CuBN, whereas more positive charges with large deviations on the surrounded carbon were found to decrease in the order CuBN > CuNCC4 > CuNCC2. The Wiberg bond indices also indicate the higher covalent character of the Cu−C bonds compared with those of the Cu−N bonds in the organometallic complexes. The overall valences of the copper centers, which were estimated to be in the range of 2.418−2.498, were higher than that of CuCor (UB3LYP method) (Table S2). Therefore, it can be concluded that the incorporation of the confused pyrrole rings effectively alters the electronic bonding orders in these organocopper complexes, thereby affecting the Lewis basicity of the ligands. The unique coordination environment of the N-confused/N-linked corrole analogues seems to play a substantial role in the stabilization of the singlet Cu(III) state in their ground states. We also analyzed the Cu K-edge XAS data by TD-DFT calculations using the ORCA program, which reproduced well the experimental spectra for the pre-edge regions of the organocopper complexes with correlation coefficients of R2 ≈ 1, though their overall energies are slightly overestimated (Figure S24). Considering that the energies and the cross-sections of pre-edge absorptions are known to be related to the physical oxidation states of the metals, the geometries of bonding, and the degree of covalency in the metal-ligand bonds, the calculated transitions reproduced the trends in the pre-edge (and rising-edge) energy features for the organocopper corrole complexes approximately at 8985.9 ± 0.1 eV with similar oscillator strengths (Figure S24a–c). This value is significantly larger than that of the related divalent complex CuNCP (8983.6 eV; Figure S24d). These transition features were also well described as dipole-forbidden (almost zero) and quadrupole-allowed (1.80–1.97 × 10−6) Cu 1s -> 3d transitions, which are fully consistent with the experimental spectra depicted in Figure 5.27 In this scenario, the red-shifted pre-edge energies for complexes CuNCC2, CuNCC4, and CuBN compared with that of the porphyrin congener CuNCP could be attributed to the predominance of Cu(III) under similar NNNC coordination environments. Finally, the distinct strong aromaticity of the organocopper complexes was found to be correlated to the ground singlet-state structures of the corrole ligand. The nucleus-independent chemical shift (NICS) values showed the overall aromatic character of the organocopper complexes (Figure S23). Consistent with the 1H NMR spectroscopic results, the extent of aromaticity was found to vary among the molecular structures in the order CuNCC4 < CuNCC2 » CuBN. Especially, the NICS values at the inner core were largely negative relative to those of CuCor (UB3LYP method). In addition, the bond length alteration of the p-conjugated macrocycles, which is a good index for aromaticity, was also taken into consideration.28 The harmonic oscillator model of aromaticity (HOMA) values calculated for CuNCC2, CuNCC4, and CuBN were 0.77, 0.66, and 0.75, respectively.29 These results provide further evidence for the presence of the closed-shell 18p aromatic electronic structures in the organocopper complexes as depicted in Figure 2a.



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Figure 7. Molecular orbital energy (eV) levels and respective frontier MOs for the organocopper complexes a) CuNCC2, b) CuNCC4, and c) CuBN obtained at the B3LYP/6-31G**/SDD level of theory. The values of the d-orbital contribution of each MO are given. Table 2. Comparison of the Selected Bond Lengths (Å) and Relative Energies (kcal/mol) obtained from the Optimized Geometries of the Closed-Shell Singlet States and Triplet States of the Organocopper Complexes at the (U)B3LYP/6-31G**/SDD Level of Theory Complex

CuNCC2 (singlet)

CuNCC2 (triplet)

CuNCC4 (singlet)

CuNCC4 (triplet)

CuBN (singlet)

CuBN (triplet)

Cu-N1

1.870

1.915

1.877

1.958

1.855

1.931

Cu-N2

1.877

1.946

1.903

1.919

1.897

1.924

Cu-N3/C

1.860

1.884

1.882

1.945

1.875

1.929

Cu-N4/C

1.862

1.951

1.856

1.887

1.868

1.906

DE(S-T)

0.000

+8.28

0.000

+10.67

0.00

+6.69

HOMO-LUMO gaps were determined to be DE = 2.15, 2.12, and 2.06 eV for CuNCC2, CuNCC4, and CuBN, respectively (Figure 7). In addition, the estimated electrochemical gaps are larger than those for the reported copper corroles (e.g., CuCor, DE = 0.81 V).10d,f,11a,19 This could be attributed to the distinct closed-shell electronic structures of the organocopper complexes that preserves their stability, whereas the redoxnoninnocent CuCor undergoes facile ligand redox processes.

Electrochemical Properties and Oxidative Reactivity. On the basis of the above results, the organocopper complexes can be best considered as ground-state Cu(III) species. In turn, the redox reactivity of the Cu(III) complexes, which could lead to higher oxidized species such as Cu(IV), should be worth exploring. Cyclic voltammograms of the organocopper corrole complexes CuNCC2, CuNCC4, and CuBN were recorded in CH2Cl2 containing 0.1 M TBAPF6 (Figures 8 and S25, and Table 3). The electrochemical features of the organocopper corroles were found to be prominently different to those of the regular corrole congener CuCor.11a,19 The corresponding oxidation potentials were observed at Eox(1) = +0.30, +0.19, and +0.34 V (vs. Fc+/Fc couple) for CuNCC2, CuNCC4, and CuBN, respectively. The organocopper complexes displayed quasi-reversible/irreversible reductions at Ered(1) = −1.12, −1.22, and −0.93 V, for CuNCC2, CuNCC4, and CuBN, respectively. The electrochemical potential values suggest a relatively more facile reduction for CuBN (by 190−290 mV) than for CuNCC2 and CuNCC4, due presumably to the resonance effect of the peripheral annulated benzene ring in the scaffold of the former derivative. The electrochemical HOMO-LUMO gaps for the organocopper complexes ranged from 1.42 to 1.26 V (Table 3). The trend observed in Table 3 is comparable with that obtained from the DFT calculations, in which the

In an attempt to obtain more detailed information of the electronic structures in their various redox states, spectroelectrochemistry and EPR spectroscopic analysis, combined with DFT calculations, were performed. On the basis of the spectroelectrochemical measurements, the oxidation processes were assigned as follows: (1) the macrocycle-based oxidation to give the Cu(III) corrole π-cation radicals for CuNCC2 and CuBN, as inferred from the optical spectral feature of the typical corrole p-radicals showing a significant decrease in the intensity of the Soret band and vibronic Q-bands (Figure S26),30 and (2) the corresponding one-electron oxidation of neutral organocopper complexes that was generated by titration of tris(4-bromophenyl)ammoniumyl hexachloroantimonate (so-called Magic Blue) as a chemical oxidant (0.70 V vs. Fc+/Fc in CH2Cl2) (Figure 9). The EPR spectra of the one-electron oxidized


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species in CH2Cl2 at room temperature displayed simple broad bands with g-tensors centered at g = 2.008 for both CuNCC2 and CuBN, attributable to the corrole-based radical as a CuIII–corrole•2- species (Figure 10a).31 The UB3LYP-based calculations supported the ligand-centered oxidation, in which the ligand exhibits a certain amount of corrole b1 radical character; the spin density profiles of the oxidized complexes show a majority of spin population over the ligand (Figure 10b). The vertical electronic excitation that contributed to the broad band in the absorption spectra was assigned to p-SOMO-based transitions on the basis of TDDFT calculations (Figures S27–S32 and Tables S5, S6, S8). Therefore, the relatively high energy of the Cu 3dx2-y2 orbital would allow to remove an electron from the frontier p-orbital, resulting in the formation of the Cu(III) radical cation species in the organocopper complexes as seen in silver(III) and gold(III) corroles (Figure 10c).32 Figure 9. UV-vis absorption spectral change of a) CuNCC2, b) CuNCC4, and c) CuBN upon gradual addition of the oxidant Magic Blue in CH2Cl2 at 298 K. The final spectra are shown in red. Formation of a Laminated Dimer of CuNCC4. In contrast to CuNCC2 and CuBN, the one-electron oxidation of CuNCC4 resulted in a distinct spectral change, consisting on the appearance of a new Soret-like band at l = 414 nm and broad Q-bands in the red region upon addition of the oxidant Magic Blue (Figure 9b).33 The absence of EPR active signals at room temperature for the oxidized CuNCC4 suggested the presence of diminished open-shell p-radical species. In fact, these absorption spectral features were observed in the dimeric complex of CuNCC4 (i.e., CuNCC4-dimer) obtained by the treatment of CuNCC4 with an excess of copper salts (Scheme 2 and Figure S12). In this reaction, the radical coupling reaction accompanied by the dehydrogenation of CuNCC4 could occur. In a similar manner, the chemical oxidation of CuNCC4 in CH2Cl2 with one equivalent of Magic Blue gave the corresponding CuNCC4-dimertff in 48% yield (Scheme 2).34 The mass spectrometric data of the product strongly supported the dehydrogenated dimer structure, as can be inferred from the identical value found at m/z = 1709.9451 to the theoretical one ([M]+; calcd for C74H14N8F30Cu2; 1709.9454) (cf. Supporting Information). This was further confirmed by an X-ray crystallographic analysis, which revealed the dimeric structure of CuNCC4-dimertff exhibiting a face-to-face laminated geometry directly linked through both inner carbons of the CuNCC4 fragments (Figure 11 and Table S3). The corresponding carbon–carbon bond length was determined to be 1.552(7) Å, and the angles around the carbon atoms were found to be approximately 112°, which suggests the presence of sp3-hybridized carbons in the corrole units. Such a skewed tweezer-like geometry with a copper–copper distance of 3.195 Å could induce shorter p-p distances, which affords unique conformational interactions.15b,35 It is important to note that each corrole macrocycle is reversely aligned in a transoid co-facial geometry for minimizing the steric repulsion of the pentafluorophenyl rings. Although the mechanism of the formation of CuNCC4-dimertff is unclear at this moment, the origin of the regioselective coupling of CuNCC4 can be attributed to the spin density distribution of the one-electron oxidized species of CuNCC4 (Figure 10b). A superior spin density coefficient localized on the inner carbon of the confused pyrrole ring could lead to the thermal C–C coupling of the CuNCC4 radical cation itself or to that of the neutral CuNCC4 under aerobic condition.36

Figure 8. Cyclic voltammograms of the organocopper complexes a) CuNCC2, b) CuNCC4, and c) CuBN in CH2Cl2 containing 0.1 M TBAPF6. Table 3. Potentials (V vs. Fc+/Fc) and the HOMO-LUMO gap (V) for the Organocopper Corrole Complexes in CH2Cl2 containing 0.1 M TBAPF6 (scan rate = 50 mV/s) Oxidation

Reduction

H-L gapb DE

ref

CuNCC2

0.30

−1.12

a

1.42

this work

CuNCC4

0.19a

−1.22a

1.41

this work

a

a

1.27

this work

−0.15c

0.81

19

Entry

a

(1) ox

E

CuBN

0.34

CuCor

0.66c

(1) red

E

−0.93

b

Determined by DPV measurement. Potential difference between the first oxidation and first reduction. cPotential values in acetonitrile solution.



Page 8 of 11

Scheme 2. Oxidative Dimerization of CuNCC4

Figure 10. a) EPR spectra of the one-electron oxidized organocopper complexes CuNCC2 (left) and CuBN (right) (5.5 mM) in CH2Cl2 at 298 K; b) DFT-derived (UB3LYP/6-31G**/SDD) spin density distribution plots for the one-electron oxidized organocopper complexes; c) Illustrative MO diagrams of the neutral organocopper complexes and the one-electron oxidized species, where L is modified corrole ligand.

Figure 11. Stick views of the X-ray crystal structure of CuNCC4dimertff: a) top view and b) side view. The top corrole core is colored in red and the bottom is in green. Solvent molecules are omitted for clarity.

In terms of the conformation of the dimer, we can consider other possible conformers, that is, CuNCC4-dimertpd, CuNCC4-dimercff, and CuNCC4-dimercpd, which would possess a parallel displaced rotational structure and cis-aligned structures of both types, respectively (Figures S33 and S34). Even though these conformers were not obtained in the current synthetic procedure, their formation cannot be ruled out. However, it should be noted that the DFT-based thermodynamic energy survey of the conformers indicates that the most stable structure is that of CuNCC4-dimertff (Table S4). The appearance of seven sharp and symmetric resonances in the 1H NMR spectrum of CuNCC4-dimertff suggests the inherent diamagnetic nature of the low spin Cu(III) fragment as well as an effective C2 molecular symmetry (Figures 12 and S3a). The resonance signal at 9.38 ppm was assigned to the confused a-pyrrolic CH, and the remaining resonances in the range of 8.38–7.48 ppm were assigned to pyrrolic b-CH protons on the basis of the 1H NMR integrals and cross peaks in the 1H-1H COSY spectrum (Figure S6). The absence of significant resonance shifts upon increasing the temperature evidences the lack of interconversion to the other rotamers, which is indicative of the highly stable interlock structure of the dimeric complex (Figure S10). The fact that the formation of CuNCC4-dimer is only favored in the case of CuNCC4 is most likely because of the lack of steric hindrance, since the confused inner carbon atom is located near the direct pyrrole-pyrrole linkage, which is unobstructed by bulky aryl groups.

Figure 12. Partial 1H NMR spectrum of CuNCC4-dimertff in CDCl3 at 298 K. CONCLUSION In this study, a series of stable organocopper complexes containing corrole ligands, CuNCC2, CuNCC4, and CuBN, were successfully synthesized and characterized by various spectroscopic techniques. The simple modification of the symmetry of the parent corrole molecular orbitals by a N-confusion synthetic approach offers a strong s-donor NNNC coordination environment of constrained valence that stabilizes the high-valent copper(III) ions. The distinct planar geometry of the macrocycles, temperature-independent diamagnetic nature, and large HOMO-LUMO energy profiles in the organometallic CuNCC2, CuNCC4, and CuBN complexes compared with regular copper corrole derivatives were ascertained by NMR and X-ray-based spectroscopy and electrochemical studies. These results are in striking contrast with the open-shell antiferromagnetically coupled copper(II) p-radical species commonly identified in the regular copper corrole complexes. Further studies on other organometallic complexes based on our carbacorrole ligands are desirable in order to gain a more accurate description of the ground-state electronic structures. The present study, however, provides a rationale design for the stabilization of high-valent metal complexes


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and mechanistic insights of their redox reactivities. Interestingly, the one-electron oxidation of CuNCC4 gave rise to a copper(III) corrole radical cation species that eventually afforded the unique dimer CuNCC4-dimertff through the oxidative coupling of the inner carbon atoms of the N-confused frameworks. Such stacked p-dimer could give access to three-dimensional p-space architectures for chiral materials and cooperative bis-metal catalytic applications.37

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxxx. The full experimental methods including detailed synthetic procedures and characterization data; NMR, FAB MS, UV-visible spectra; cyclic voltammetry traces, X-ray crystallographic details, XANES data, and DFT calculations (PDF) Crystallographic data for CuNCC2 (CIF) Crystallographic data for CuNCC4 (CIF) Crystallographic data for CuNCC4-dimertff (CIF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

ORCID Masatoshi Ishida: 0000-0002-1117-2188 Hiroyuki Furuta: 0000-0002-3881-8807 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The part of work was supported by Grant-in-Aids (JP15K13646 to H.F., JP16K05700 and JP17H05377 to M.I.) from Japan Society for the Promotion of Science (JSPS). The authors acknowledge Dr. J. Yang and Prof. T. Fujigaya (Kyushu Univ.) for their help in the XPS measurements, and Prof. A. Takahara (Kyushu Univ.) for his helpful comments. The synchrotron radiation experiments were performed at the BL14B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2016B1591) and the Kyushu University Beamline (BL06/SAGA-LS). The financial support from a bilateral program between JSPS and the National Research Foundation (NRF) of South Africa is also acknowledged.

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Ground-State Copper(III) Stabilized by N-Confused ... - ACS Publications

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