Direct Synthesis of an Unprecedented Stable Radical of Nickel(II) 3,5

May 19, 2017 - Observation of the Strong Electronic Coupling in Near-Infrared-Absorbing Tetraferrocene aza-Dipyrromethene and aza-BODIPY with Direct F...
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Direct Synthesis of an Unprecedented Stable Radical of Nickel(II) 3,5Bis(dimedonyl)azadiisoindomethene with Strong and Narrow NearInfrared Absorption at λ ∼ 1000 nm Elena A. Makarova,† Yuriy V. Zatsikha,‡ Kelly M. E. Newman,§ Vinod K. Paidi,§ Valeria A. Beletsky,⊥ Johan van Lierop,*,§ Evgeny A. Lukyanets,*,† and Victor N. Nemykin*,‡,⊥ †

Organic Intermediates and Dyes Institute, B. Sadovaya str. 1/4, 123995 Moscow, Russia Department of Chemistry and §Department of Physics and Astronomy, University of Manitoba, Winnipeg Manitoba R3T 2N2, Canada ⊥ Department of Chemistry and Biochemistry, University of MinnesotaDuluth, Duluth, Minnesota 55812, United States ‡

S Supporting Information *

this protocol is that only aza-DIPYs with either aryl or heteroaryl substituents at the α positions can be synthesized.6 Herein, we report a new approach to synthesizing symmetrical azadiisoindomethenes with alicyclic substituents at the αisoindole positions (Scheme 1). In particular, we probed the

ABSTRACT: An unprecedented stable neutral radical nickel(II) complex of 3,5-bis(dimedonyl)azadiisoindomethene (1) was prepared by the direct reaction between 1,3-diiminoisoindoline and dimedone. A new radical complex 1 has an intense and narrow absorption at 1008 nm and can be reduced to a less stable anionic [1]− with a typical aza(dibenzo)boron dipyrromethene (aza-BODIPY) UV−vis spectrum. Complex 1, along with two other colored condensation reaction products 2 and 3, was characterized by spectroscopy and X-ray crystallography, while the paramagnetic nature of 1 was probed by EPR and SQUID methods. Complex 1 forms dimers in the solid state with short (∼3.16 Å) Ni---Ni contacts. Redox data on 1 are indicative of a reversible reduction process in this complex; its magnetism suggests a S = 1/2 state with the spin density delocalized over the aza-BODIPY core. The experimental data 1 and [1]− were correlated with the density functional theory (DFT) and time-dependent DFT calculations.

Scheme 1. Synthetic Pathway for the Preparation of 1−3

T

he search for a simple and inexpensive synthetic methodology for the preparation of stable functional dyes with an intense and narrow absorption band at ∼1000 nm and beyond is an extremely challenging task. It is even more difficult to create such a system with reversible electrochromic or chemochromic effects. Boron dipyrromethenes (BODIPYs) and boron azadipyrromethenes (aza-BODIPYs) are relatively new classes of organic chromophores that are popular as near-IR dyes, fluorescent markers and biomarkers, photodynamic therapy (PDT) agents, fluorescent sensors, photosensitizers for dyesensitized solar cells, light-harvesting modules for organic photovoltaics, and potential components for molecular electronics.1 Transition-metal complexes of azadipyrromethenes (aza-DIPYs) have good potential for applications in catalysis, PDT, and molecular electronics.2 The most common synthetic procedures for their synthesis include the use of 1,3-diaryl-4nitrobutan-1-one derivatives3 and the use of α-nitrosopyrroles.4 Alternative method is based on the reaction of phthalonitrile derivatives with arylmagnesium halides, which leads to the formation of α-diarylazadiisoindolylmethenes.5 The drawback of © 2017 American Chemical Society

reaction between dimedone and 1,3-diiminoisoindoline (DII) in the presence of nickel salt. DII was used as a readily available precursor for the preparation of azadibenzodipyrromethene, which is known to shift the characteristic absorption band to the near-IR (NIR) region.5 Dimedone was used as CH acid capable of a nucleophilic attack on the imino group of DII and a compound that can provide an additional coordination site toward a square-planar cavity for nickel coordination.7 Although we varied the reaction conditions, reactions always produced two major products of orange and violet colors in the absence of the template and a brown product in the presence of Received: May 4, 2017 Published: May 19, 2017 6052

DOI: 10.1021/acs.inorgchem.7b01140 Inorg. Chem. 2017, 56, 6052−6055

Communication

Inorganic Chemistry

(Scheme 1). The formation of a stable radical in transition-metal aza-DIPYs under direct synthesis conditions has not been reported. Indeed, the formation of stable radicals of polypyrrolebased functional dyes is extremely rare. The formation of stable radical BODIPYs and porphyrins under direct synthesis conditions has never been reported, while only a few reports are available for the direct synthesis of radical-containing phthalocyanines.9 Further confirmation of our assignment came from density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on 1−3 (Figures 2 and S13−S18). DFT predicts that the spin density in complex 1 is delocalized over the azadiisoindomethene, while TDDFT-predicted spectra correlate very well with the experimental UV−vis−NIR data. In particular, TDDFT calculations predict that the NIR band observed at ∼1000 nm should be dominated by α-HOMO → α-LUMO single-electron excitations, while the experimental band observed at 555 nm correlates well with TDDFT-predicted excited state 12, which mostly originates from β-HOMO−3 → β-LUMO single-electron excitations. Because some of the nickel(II) salen-type radical complexes are known to undergo valence tautomerism10 and because of the possible magnetic coupling expected for the dimers of 1 in a solid state, we have investigated 1 using SQUID magnetometry (SI and Figure S19). The room temperature data correlate well with the EPR spectrum of 1 and are consistent with the presence of delocalized π-cation radicals centered at the aza-BODIPY core. The temperature-dependent magnetism is indicative of more complex magnetochemical behavior, which will be discussed in detail along with the corresponding palladium and platinum complexes in a full report to follow. The redox properties of 1 were investigated using electrochemical, spectroelectrochemical, and chemical oxidation or reduction methods. Electrochemistry on 1 is indicative of a reversible reduction process observed at small negative potential (−0.58 V versus FcH/FcH+; Figure S23). This process explains an easiness of the formation of radical 1 under synthetic conditions. In general, aza-DIPYs and aza-BODIPYs have lower reduction potentials compared to the DIPYs and BODIPYs,11 while the introduction of electron-withdrawing groups into the BODIPY core results in a significant shift of the first reduction process toward positive values.12 In the case of 1, a combination of the electron-withdrawing azadiisoindomethene core and electron-withdrawing dimedone fragments leads to oxidation of 1 by the oxygen under ambient conditions. In addition, one reversible oxidation at −0.05 V and a second irreversible reduction (∼−1.2 V) process were observed in electrochemical experiments on 1 (Figure S23 and Table S1). Redox properties of complex 1 were probed by spectroelectrochemical and chemical reduction methods (Figures 3 and S24). Reduction under both spectroelectrochemical and chemical conditions results in identical transformation of the UV−vis−NIR spectrum of 1. During such reduction, bands at 1008 and 555 nm disappear, while the formation of a typical for aza-BODIPYs and transitionmetal aza-DIPYs band at 748 nm was clearly observed. This new band has a high intensity (ε = 90000 M−1 cm−1) and small bandwidth (890 cm−1). Thus, the reduction of 1 into [1]− restores the π system of the azadiisoindomethene chromophore. The reduction is fully reversible because exposure of the solution from the spectroelectrochemical experiment to air or simple filtration of a green solution of [1]− through a short layer of silica gel results in the formation of brown 1. Further confirmation of the diamagnetic nature of the [1]− was obtained from the 1H

the nickel salt template (Scheme 1). Spectroscopy and X-ray crystallography on 2 are indicative of [see the Supporting Information (SI) and Figures S1−S5] a condensation product between two molecules of dimedone and a single molecule of DII. X-ray crystallography suggests four regular carbonyl groups with CO bond distances of ∼1.22−1.24 Å, in complex 2. On the basis of experimental data (SI and Figures S6−S9), the violet compound was assigned to the β-isoindigo-containing bisdimedone product (Scheme 1).8 ESI MS (Figure S10) and X-ray crystallography (Figure 1) are indicative of the nickel ion in 1. A nickel ion has pseudo-square-

Figure 1. X-ray crystal structure of 1. All hydrogen atoms are omitted for clarity.

planar geometry with N2O2 coordination sphere. The Ni−O and Ni−N bond distances in 1 are almost equivalent (∼1.84−1.86 Å), and the two complexes form a dimer structure in the solid state with a Ni---Ni distance of ∼3.16 Å (Figure S11). Coordinated to the nickel ion, the C−O bond distances (∼1.27−1.28 Å) are longer, while C(O)−C bonds in 1 (∼1.40−1.41 Å) are significantly shorter than those in 2. No counterion was observed in structure of 1, which is consistent with the high mobility of this compound observed during chromatographic purification. The lack of a diamagnetic NMR spectrum and a clear isotropic signal with g ∼ 2.01 observed in the electron paramagnetic resonance (EPR) spectrum of 1 in solution (Figure S12) is clearly indicative of the π-systemcentered organic radical associated with this compound. The UV−vis−NIR spectrum of 1 (Figure 2) is also not typical of the expected azadiisoindomethene π system and is dominated by strong NIR absorption. Thus, we assigned complex 1 to a stable radical form of nickel 3,5-bis(dimedonyl)azadiisoindomethene

Figure 2. Experimental (top) and TDDFT-calculated (bottom) absorption spectra of radical 1. The DFT-predicted spin density for 1 is shown in inset. 6053

DOI: 10.1021/acs.inorgchem.7b01140 Inorg. Chem. 2017, 56, 6052−6055

Communication

Inorganic Chemistry

We tried to expand our new methodology for the preparation of transition-metal azadiisoindomethene on the other 3d ions. A similar condensation reaction between DII and dimedone in the presence of V3+, Cu2+, Co2+, and Zn2+ ions failed to form the expected transition-metal azadiisoindomethene, while the results for the Pd2+ and Pt2+ templates as well as the use of different CH acids for condensation with DII will be published in a full paper. In conclusion, we have observed the formation of an unusual and stable radical complex 1. A characteristic spectroscopic signature of 1 is the presence of an intense NIR band observed at 1008 nm. Radical 1 can be reversibly reduced to the diamagnetic complex [1]−, which has properties typical for aza-BODIPYs and transition-metal aza-DIPYs. Magnetochemical data on 1 are indicative of chromophore-centered spin delocalization at room temperature in this complex. The electronic structures and excited-state energies of 1 and [1]− were explored by DFT and TDDFT methods and are in good agreement with the experimental data.

Figure 3. Reduction of 1 into [1]− during titrations with NBu4+OH−/ NBu3 in acetonitrile.

NMR experiments (Figure S25). Finally, the UV−vis spectrum of [1]− correlates well with TDDFT calculations on this system (Figure 4). The most prominent band observed in [1]− at 748 nm is dominated by the classic HOMO → LUMO singleelectron transition.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01140. Preparation and characterization data for 1−3 (PDF) Accession Codes

CCDC 1521674 and 1521676 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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Figure 4. Experimental (top) and TDDFT-calculated (bottom) absorption spectra of the reduced anion [1]−.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.v.L.). *E-mail: [email protected] (E.A.L.). *E-mail: [email protected] (V.N.N.).

We also probed the spectroscopic signature for the 1 → [1]+ transformation using spectroelectochemical and chemical oxidation approaches (Figures 5 and S26). Removal of the

ORCID

Victor N. Nemykin: 0000-0003-4345-0848 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support by the NSF (Grants CHE-1464711, MRI1420373, and MRI-0922366), Minnesota Supercomputing Institute, University of Manitoba, and WestGrid to V.N.N. is greatly appreciated. V.N.N. and J.v.L. acknowledge support from the NSERC and CFI.

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REFERENCES

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Figure 5. Oxidation of radical 1 to [1]+ under spectroelecrtrochemcal conditions in a DCM/0.3 M TBAP system.

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DOI: 10.1021/acs.inorgchem.7b01140 Inorg. Chem. 2017, 56, 6052−6055