Synthesis and Characterization of a Binuclear Copper(II

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Cite This: Inorg. Chem. 2017, 56, 12665-12669

Synthesis and Characterization of a Binuclear Copper(II) Naphthoisoamethyrin Complex Displaying Weak Antiferromagnetic Coupling James T. Brewster II,†,§ Gonzalo Anguera,†,§ Matthew D. Moore,† Brian S. Dolinar,‡ Hadiqa Zafar,† Grégory D. Thiabaud,† Vincent M. Lynch,† Simon M. Humphrey,† and Jonathan L. Sessler*,† †

Department of Chemistry, The University of Texas at Austin, 105 East 24th Street, Stop A5300, Austin, Texas 78712-1224, United States ‡ Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United States S Supporting Information *

binuclear metal-ion species. Here, we report the synthesis and characterization of the dicopper(II) naphthoisoamethyrin complex. This complex, but not the starting ligand, displays aromatic character as a result of an apparent two-electron oxidation of the original 24π-electron antiaromatic core, as inferred by UV−vis spectroscopy, X-ray crystallography, and cyclic voltammetry (CV). As detailed below, weak antiferromagnetic coupling is seen between the copper(II) ions, as supported by variable-temperature (VT) electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry. Treatment of the free-base form of 1 with copper acetate in methanol/CH2Cl2 (1:1, v/v) open to air at room temperature yielded the binuclear copper complex 2 as a dark-purple-red solid with metallic luster in 43% [5 equiv of Cu(OAc)2] or 65% [50 equiv of Cu(OAc)2] yield, after removal of the solvent and purification by flash chromatography over basic aluminum oxide (15% acetone/CH2Cl2; Scheme 1).31 The recovered starting

ABSTRACT: The reaction between a naphthylbipyrrolecontaining hexaphyrin-type expanded porphyrin and copper acetate affords a bench-stable dicopper(II) complex. UV−vis spectroscopy, cyclic voltammetry, and X-ray crystallographic analysis measurements provide support for the conclusion that this complex displays aromatic features. A weak antiferromagnetic exchange interaction between the binuclear copper(II) ions is evidenced by variable-temperature electron paramagnetic resonance and by fitting of the bulk magnetic susceptibility to a dimer model, yielding J = −5.1 cm−1.

P

orphyrins represent one of the most privileged scaffolds found within nature. These and related tetrapyrrolic macrocycles play key roles in many of the biochemical pathways essential for life. Not surprisingly, tetrapyrroles are the focal point of innumerable research programs across the chemical and medical communities.1 Significant efforts have been made to elaborate the porphyrin periphery,2 as well as to modify the porphyrin core.3 In the context of the latter paradigm, an emphasis has been placed on the so-called expanded porphyrins. These analogues are typically characterized by a more extensive π system than is present in naturally occurring tetrapyrroles and often offer larger internal lacunae for metal complexation. To date, a number of mono- as well as homo- and heterobinuclear ion complexes of expanded porphyrins have been described.4−12 Binuclear metal-ion systems are of particular interest because of their potential utility as biomimetic enzyme models and transition-metal catalysts,13−18 as well as single-molecule magnets.19−21 Recently, the incorporation of naphthyl building blocks has proven to be useful in rigidifying the basic scaffolds of several classic expanded porphyrins. This has allowed new electronic pathways to be stabilized within conjugated systems and has typically led to a bathochromic shift in the corresponding absorption profiles.22−28 The attendant rigidification might also allow the metalation chemistry to be better controlled.29 With such general considerations in mind, we were keen to explore whether the naphthylbipyrrole expanded porphyrin, known as naphthoisoamethyrin (1),30 would support the formation of © 2017 American Chemical Society

Scheme 1. Synthesis of Dicopper Complex 2

material (ca. 30%) was also collected by flushing the column with acetone and then resubjected to the reaction conditions to furnish additional product in similar yields (ca. 40%). Initial attempts at studying the dicopper complex by VT 1H NMR failed to yield any perceptible signals, even upon expansion of the chemical shift range [±100 ppm; cf. Supporting Information (SI) S2]. As prepared, ligand 1 showed a UV−vis spectrum that differs from previously characterized amethyrin systems. For instance, a strong Soret band at 418 nm (ε = 45400 M−1 cm−1), Received: July 3, 2017 Published: October 9, 2017 12665

DOI: 10.1021/acs.inorgchem.7b01669 Inorg. Chem. 2017, 56, 12665−12669

Communication

Inorganic Chemistry

τ5 was also calculated as an index of trigonality using the method of Addison and co-workers.38 In an ideal trigonal bipyramid, τ5 = 1, whereas the dicopper complex shows a deviation to 0.72 for Cu1 and 0.50 for Cu2 (cf. SI S3b). The pyrrole nitrogen N−Cu distances reveal an asymmetric coordination environment and are 1.930 Å (N5−Cu2), 1.936 Å (N2−Cu1), 1.996 Å (N1− Cu1), 2.017 Å (N6−Cu2), 2.068 Å (N3−Cu1), and 2.080 Å (N4−Cu2), respectively. The CuII−CuII distance of 2.752 Å is slightly shorter than the sum of the van der Waals radii for copper(II), leading us to consider the possibility of Cu−Cu interactions.39,40 Moreover, the four Cu−Cl bond distances are nonequivalent at 2.309 Å (Cu2−Cl2), 2.362 Å (Cu1−Cl1), 2.467 Å (Cu1−Cl2), and 2.556 Å (Cu2−Cl1). The bond angle (θ) of the bridging chlorides is 67.93° for Cu1−Cl1−Cu and 70.29° for Cu1−Cl2−Cu2. Other than the bridging halides, no counterions are seen; this is consistent with what would be expected for a dicopper(II) complex of a two-electron-oxidized form of 1.41 CV studies of the free-base ligand 1 (1 mM) were performed in dry acetonitrile in the presence of 0.1 M NBu4ClO4 purged with nitrogen at room temperature. All potentials are referenced to the silver (Ag/AgNO3) redox couple. Two quasi-reversible single-electron oxidation waves are seen that are attributed to oxidation of the 24π-electron antiaromatic ligand to the corresponding 22π-electron aromatic species; these features are seen at 0.17 and 0.37 V at a scan rate of v = 0.2 V s−1 (Figure 3a).

accompanied by three slightly smaller absorbance peaks at 457, 490, and 530 nm (ε = 38200, 38500, and 38000 M−1 cm−1, respectively), is seen, along with a broad absorbance band spanning the 750−950 nm spectral range (Figure 1). Upon

Figure 1. UV−vis spectrum of ligand 1 (---) and complex 2 () recorded in chloroform.

metalation, an easy-to-visualize color change from a dark-redorange to dark-red-purple was observed. This is accompanied by a slight increase in the molar absorptivity in chloroform and a bathochromic shift in the Soret band, which now appears at 517 nm (ε = 50400 M−1 cm−1). A smaller absorbance feature at 583 nm (ε = 22100 M−1 cm−1) is also seen, along with a prominent Q-type absorption band at 811 nm (ε = 15000 M−1 cm−1). Considered in concert, these spectral features are taken as initial evidence that oxidation has occurred to produce a 22π-electron aromatic system. Crystals suitable for X-ray diffraction (XRD) were grown via the slow evaporation of a concentrated solution of complex 2 in hexanes/chloroform (1:1, v/v).32 As illustrated in Figure 2,

Figure 3. Cyclic voltammograms of (a) ligand 1 and (b) complex 2.

CV studies of the binuclear copper complex 2 (1 mM), carried out in the presence of 0.1 M NBu4ClO4 in acetonitrile purged with nitrogen at room temperature with a scan rate of v = 0.2 V s−1, revealed four quasi-reversible reduction peaks at −0.15, −0.33, −0.65, and −0.84 V, respectively (Figure 3b). These features are ascribed to the CuII/I redox cycle and the reduction of the ligand from the 22π-electron aromatic form to give a metalated 24π-electron antiaromatic species that is electronically analogous to the free-base ligand 1. EPR spectroscopic studies of complex 2 carried out at 90 K in frozen toluene/chloroform (1:1, v/v) revealed features readily assignable to two coupled copper(II) ions in a rhombic geometry with anisotropic g values of gx = 2.59, gy = 2.07, and gz = 1.78. The ascribed g values and rhombic geometry were retained from 90 to 175 K, thus suggesting minimal temperature-dependent switching of the electronic ground state within these temperatures (cf. SI S4).42 As can be seen from an inspection of Figure 4, the spectrum of 2 yields a classic EPR pattern for a triplet state (S = 1), with both ΔMs ± 1 and ΔMs ± 2 (half-field transition; g = 4.45) features being observed. This is consistent with a coupled pair of copper(II) ions. To probe the putative magnetic exchange between the copper ions, analogous studies were carried out from 90 to 175 K and at room temperature (ca. 290 K). In this case, the signal intensity

Figure 2. ORTEP plot (thermal ellipsoids set at 50% probability) of the single-crystal XRD structure of the copper complex 2 viewed from the (a) top and (b) side. Hydrogen atoms are removed for clarity.

naphthoisoamethyrin distorts slightly from planarity so as to complex better the two copper(II) ions.29,33 Analogous to previously reported metal complexes within the amethyrin class of expanded porphyrins, all six pyrroles of naphthoisoamethyrin act as donors for the coordinated copper ions.9,11,33−36 For Cu1, equatorial N2, Cl1, and Cl2 donor atoms, along with axial N1 and N3 atoms, define the coordination sphere. For Cu2, the equatorial positions are defined by N5, Cl1, and Cl2, with N4 and N6 serving as axial ligands. Following the approach of Mutterties and Guggenberger,37 wherein the shape-determining dihedral angle (e3) is used to describe the geometry of a fivecoordinate sphere about a metal center, the resulting complex adopts a distorted trigonal-bipyramidal geometry. In an ideal trigonal bipyramid, the e3 angle is 53.1°, whereas the dicopper complex of naphthoisoamethyrin displays an e3 angle of 41.92° for Cu1 and 40.37° for Cu2 (cf. SI S3a). The structural parameter 12666

DOI: 10.1021/acs.inorgchem.7b01669 Inorg. Chem. 2017, 56, 12665−12669

Communication

Inorganic Chemistry

In conclusion, a weakly antiferromagnetic binuclear copper(II) naphthoisoamethyrin complex has been prepared and characterized by UV−vis spectroscopy, CV, VT EPR, SQUID magnetometry, and XRD analysis. Copper(II) complexation under aerobic conditions leads to a two-electron oxidation and generation of a formally aromatic species, as inferred from the spectroscopic features of the resulting complex. The present study helps to illustrate further the utility of expanded porphyrins in coordinating biologically relevant metal cations in well-defined coordination modes. The information obtained from studies of these new complexes can inform analyses of natural metalcontaining systems while providing synthetic species whose features are unique in their own right.

Figure 4. VT X-band EPR spectra at 90 K () and 290 K (---) of complex 2 recorded in toluene/chloroform (1:1, v/v). Experimental settings were amplitude modulation = 10 G and microwave power = 2 mW.

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ASSOCIATED CONTENT

S Supporting Information *

decreased until the observed EPR spectrum was devoid of discernible signals, leading us to suggest a weak antiferromagnetic interaction between the copper ions (Figure 4). The field-cooled temperature-dependent magnetic susceptibility of 2 was measured under an external applied field = 1.0 kOe upon warming from 1.8 to 300 K. The resulting plots of χm versus T and 1/χm versus T are shown in Figure 5. Application of the

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01669. Detailed experimental procedures, analytical data, spectra, and X-ray crystallographic data (PDF) Accession Codes

CCDC 1538501 and 1556648 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 [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

James T. Brewster II: 0000-0002-4579-8074 Gonzalo Anguera: 0000-0002-9364-5769 Matthew D. Moore: 0000-0001-6401-6667 Brian S. Dolinar: 0000-0002-8228-4590 Hadiqa Zafar: 0000-0003-0683-8907 Simon M. Humphrey: 0000-0001-5379-4623 Jonathan L. Sessler: 0000-0002-9576-1325

Figure 5. Molar magnetic susceptibility (χm) of 2 under an applied field of 10 kOe (×) and the fit to the Van Vleck equation (dashed line). The fit to Curie−Weiss law from 300 to 100 K is shown for 1/χm on the right axis with θ = −37 K.

Author Contributions

Curie−Weiss law to the high-temperature data gave θ = −37 K, indicative of a weak through-space antiferromagnetic exchange interaction. Copper(II) dimers are well studied and capable of being modeled with the modified Van Vleck equation (see the SI for details). The excellent fit obtained across the entire T range in this work (Figure 5, dashed line) rules out the presence of any minor monomeric impurities often found in dimeric systems (see the SI for details).43,44 The experimental χm data was fitted to the modified van Vleck equation by fixing the value of ⟨g⟩ = 2.17 obtained directly from the EPR experiments described above. The crystal structure of 2 indicates that there should only be one J value, and magnetic superexchange is mediated by single-atom chlorine bridges. Least-squares refinement of the model with S = 0 and J as the only variable yielded a coupling constant J = −5.1 cm −1 (−7.4 K). This result is consistent with weak antiferromagnetic exchange and parallels what is found in other μ2-chloride-bridged copper complexes.44,45 For confirmation, a sample was subject to a second round of preparative purification and tested; it displayed essentially the same magnetic behavior. Further details of the magnetometry studies and curve fittings are provide in the SI.

§

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (Grant CHE1402004 to J.L.S.) and the Robert A. Welch Foundation (F-0018 to J.L.S.). The MPMS-3 Quantum Design SQUID magnetometer was purchased by funds provided by Texas A&M University Vice President of Research. G.A. thanks Fundación Ramón Areces (Madrid, Spain) for a postdoctoral fellowship. The authors thank Prof. Kim R. Dunbar (Texas A&M University) for her help in facilitating magnetic susceptibility measurements. J.T.B. thanks Samuel Xie for assistance with the EPR and Angela Spangenberg for assistance with the VT 1H NMR spectroscopic studies.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01669 Inorg. Chem. 2017, 56, 12665−12669

Communication

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DOI: 10.1021/acs.inorgchem.7b01669 Inorg. Chem. 2017, 56, 12665−12669