Structure and Properties of a Five-Coordinate Nickel(II) Porphyrin

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

Structure and Properties of a Five-Coordinate Nickel(II) Porphyrin Florian Gutzeit,† Marcel Dommaschk,†,‡ Natalia Levin,§ Axel Buchholz,⊥ Eike Schaub,† Winfried Plass,⊥ Christian Näther,∥ and Rainer Herges*,† Otto Diels Institute for Organic Chemistry and ∥Department of Inorganic Chemistry, Christian-Albrechts-University Kiel, Otto-Hahn-Platz 4/6, 24118 Kiel, Germany § Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34−36, 45470 Mülheim an der Ruhr, Germany ⊥ Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstrasse 8, 07743 Jena, Germany Inorg. Chem. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 03/30/19. For personal use only.

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S Supporting Information *

ABSTRACT: Axial coordination in nickel(II) porphyrins has been thoroughly investigated and is well understood. However, isolated five-coordinate nickel(II) porphyrins are still elusive after 50 years of intense research, even though they play a crucial role as intermediates in enzymes and catalysts. Herein we present the first fully stable, thoroughly characterized five-coordinate nickel(II) porphyrin in solution and in the solid state (crystal structure). The spectroscopic properties indicate pure high-spin behavior (S = 1). There are distinct differences in the NMR, UV−vis, and redox behavior compared to those of high-spin six-coordinate [with two axial ligands, such as NiTPPF 10·(py)2] and low-spin fourcoordinate (NiTPPF10) nickel(II) porphyrins. The title compound, a strapped nickel(II) porphyrin, allows a direct comparison of four-, five-, and six-coordinate nickel(II) porphyrins, depending on the environment. With this reference in hand, previous results were reevaluated, for example, the switching efficiencies and thermodynamic data of nickel(II) porphyrin-based spin switches in solution.

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controversies.12,14 Characterizations have been restricted to transient absorption or resonance Raman spectroscopy.10,12,14 It is widely accepted that the properties of the CN5 species are similar to those of the CN6 species. Most importantly, the UV−vis and NMR spectra are expected to be almost identical.18 There is also a general agreement that the CN5 complexes are high-spin because no evidence for a CN5 lowspin complex has been observed.12 However, UV−vis titrations of electron-deficient porphyrins with very strong ligands exhibit a lack of true isosbestic points, indicating the presence of an additional species with an absorption profile different from those of the CN4 and CN6 complexes.14−16 This observation strongly suggests the presence of a significant amount of CN5 species, with a UV−vis spectrum different from that of the CN6 complex, contradicting previous assumptions.14

INTRODUCTION Axial coordination to metal porphyrins has been studied from a number of different perspectives for several decades.1,2 Those porphyrins are crucial to a variety of biological processes or technological applications such as photoswitchable magnetic resonance imaging contrast agents,3,4 solar energy conversion, especially dye-sensitized photovoltaics,5,6 hydrogen-evolution catalysis,7 and redox catalysis.8,9 Numerous studies have been conducted on meso- and β-substituted nickel(II) porphyrins. Similar to their iron counterparts, the spin state and thus their magnetic properties are strongly dependent on the coordination of axial ligands.1,2,10 Without axial ligands, nickel(II) porphyrins exhibit a diamagnetic low-spin state because of the doubly occupied dz2 orbital of the central nickel ion. Upon coordination of at least one axial ligand, one electron is transferred from the dz2 orbital to the dx2−y2 orbital to give rise to a paramagnetic high-spin species (S = 1).11 Upon the addition of ligands to nickel(II) porphyrins, fivecoordinate (CN5) and six-coordinate (CN6) complexes are formed.12 However, ligand exchange is very fast,13 and usually the first association constant (K1S) is smaller than the second (K2 > K1S). Hence, the CN5 complex is formed in low concentration and rapidly exchanges with the four-coordinate (CN4) and CN6 complexes.12,14−16 Thus, a CN5 complex could never be studied in isolation.8,11,12,17 The properties of the CN5 complexes have been the subject of discussions and © XXXX American Chemical Society

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EXPERIMENTAL SECTION

We designed and synthesized a “strapped porphyrin” with a bridge between two opposing meso positions. The bridge holds a pyridine (py) ligand in a perfect preorientation to bind to the central nickel ion giving rise to a well-defined CN5 porphyrin 1. Model calculations revealed that a py unit connected to two opposing meso positions via Received: February 6, 2019

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DOI: 10.1021/acs.inorgchem.9b00348 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry thiomethylbiphenyl units exhibits an optimal geometry for efficient coordination [Supporting Information (SI), section IV]. We synthesized the CN5 complex 5,15-[2′-[4″-(3‴,5‴pyridinylene)sulfonylmethylphenyl]phenyl]-10,20-bis(2,3,4,5,6pentafluorophenyl)nickel(II) porphyrin (1) in five steps with an overall yield of 1.7% via a bridged aldehyde approach (Scheme 1). Synthesis of the porphyrin core and subsequent strapping with the pycontaining bridge 9 was not successful (SI, section VI, compounds 12−16).19,20

Table 1. Selected Ni−Npy, Ni−NPorph., and Ni−N4 plane (Å) Distances from the Crystal Structures Presented in This Work (*) and Previously Published for CN4, CN5, and CN6 Porphyrins

Scheme 1. Synthesis of the CN5 Porphyrin 1

Porphyrin·Ligand

CN

NiTPPF10 (2)* NiTPPF2021 1* (Z)-5·MeOH4 2·(py)2* NiTPPF20·(py)221 NiTPPF20·(4-(NMe2)py)215 NiTPPF20·(4-CNpy)215 NiTPPF20·(4-(NO2)py)215

4 4 5 6 6 6 6 6 6

Ni−Npy (Å)

Ni−NPorph. (Å)

Ni−N4 plane (Å)

2.11 2.18 2.21 2.22 2.19 2.22 2.23

1.92 1.93 2.05 2.04 2.05 2.05 2.06 2.05 2.05

0 0 0.26 0.08 0 0 0 0 0

Compared to the corresponding CN6 complex NiTPPF10· (py)2 (2·(py)2), the distance between the nickel ion and py nitrogen atom in the CN5 porphyrin 1 is shortened by 0.11 Å and the distance between the ligand and porphyrin is increased by 0.15 Å resulting in the displacement of 0.26 Å above the four porphyrin nitrogen atoms (SI, section VIII). Density functional theory calculations prove that the displacement of the nickel ion is not due to strain induced by the bridge but an intrinsic property of CN5 nickel porphyrins (SI, section IV). All axial ligands in the structures presented in Table 1 are py derivatives with varying para substituents (NO2, CN, H, and NMe2). The electronic nature of the porphyrins (electron-deficient or electron-rich) and ligand-field strength of the axial ligands only have a minor impact on the geometry (crystal structure) and spectroscopic properties (UV−vis, NMR, and cyclic voltammetry). However, those properties strongly influence the association constants of axial coordination (K1S and K2), which determine the coordination geometry in solution (CN4 = low-spin; CN5 and CN6 = high-spin) and thus dictate the observed spectra.15 In a toluene solution at room temperature, porphyrin 1 is completely CN5. No decoordination is observed within the detection limit of our spectroscopic methods. Porphyrin 1 is the first CN5 nickel(II) porphyrin that can be fully characterized in solution. Upon the addition of 20000 equiv of trifluoroacetic acid (TFA) in toluene, the py unit was completely protonated and the CN4 square-planar complex 1·H+ was obtained. For the diamagnetic CN4 complex 1·H+, fully resolved diamagnetic NMR and UV−vis spectra are observed (Figures 2 and 3). For the CN4 compound 2, no changes were observed upon the addition of TFA (SI, section I). The absorption spectrum of the CN5 species 1 in toluene (Figure 2, red) shows distinct differences from that of the CN6 complex 1·py formed in the presence of py (Figure 2, green).14−16 Their extinction coefficients and band shapes are similar because of the identical high-spin state. Still bathochromic shifts of 5.5−27 nm are observed for all bands (Soret, Q) upon the second axial coordination step (K2, CN5 → CN6). The shifts arising from the first coordination (K1S, CN4 → CN5) are more pronounced (19−23 nm) because of the change of the spin state (low-spin → high-spin; strongly → slightly ruffled porphyrin). The spectral differences compared to those of other nickel(II) porphyrins are small for the CN4 and CN6 complexes (SI, section I).

Bridged porphyrin 1 was spectroscopically characterized, and a single-crystal structure was obtained (Figure 1). Structure 1 proves monoaxial coordination (CN5) in the solid state. No additional solvent molecules were observed in the crystal.

Figure 1. ORTEP plot of the CN5 porphyrin 1 with labeling of selected atoms and displacement ellipsoids drawn at the 50% probability level.

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RESULTS AND DISCUSSION The crystal structure of the CN5 porphyrin 1 shows that the nickel ion is displaced out of the porphyrin plane by 0.26 Å (calcd 0.25 Å), whereas for CN4 and CN6 porphyrins, the nickel ion is situated in the porphyrin plane. The positions and distances are almost independent of the porphyrin and ligand properties (Table 1). The nickel ion displacement in CN5 porphyrins was predicted by Raman measurements and is comparable to that of CN5 iron or zinc porphyrins.14,22,23 B

DOI: 10.1021/acs.inorgchem.9b00348 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

piperidine (pip) complexes 1·pip (δ = 52.5 ppm) and 2·(pip)2 (δ = 52.5 ppm) (Figure 3). Because coordination and decoordination of the axial ligands is fast on the NMR time scale, an average signal of between 9 and 53 ppm is observed for any mixture of nickel(II) porphyrins and axial ligands. The observed shift directly reflects the ratio between the diamagnetic and paramagnetic species. Thus, the equilibrium constants and thermodynamic parameters can be obtained. To apply this method for the stepwise coordination process (CN4 → CN5 → CN6), it was assumed that the pyrrole proton shift of the CN5 intermediate is identical with that of the CN6 complex [δ(CN5) ∼ δ(CN6)]. This approximation was applied because the pyrrole proton shift of the CN5 complex was experimentally inaccessible. The porphyrin 1 offers the opportunity to investigate the CN5 complex independently. The pyrrole protons of the porphyrin 1 resonate at δ = 49.5 ppm (toluene-d8, 298 K), 3.0 ppm lower than that in the CN6 complexes 1·pip and 2· (pip)2 (both 52.5 ppm; Figure 4). This overestimation of

Figure 2. UV−vis spectra of the CN5 porphyrin 1 in toluene (red, square pyramidal), in the presence of TFA (blue, square planar, CN4), and in py (green, octahedral, CN6). The spectra of the two high-spin complexes 1 and 1·py (CN5 and CN6) are similar in shape. Bathochromic shifts (5.5−27 nm) are observed for both coordination steps (CN4 → CN5 → CN6) but are more pronounced for the first step because of the change of the spin state (low-spin → high-spin).

Figure 3. 1H NMR (500 MHz, toluene-d8, 298 K, tetramethylsilane) signals of the β pyrrole protons of the three coordination geometries of nickel(II) porphyrins 1 and 2. From top to bottom: CN6, 2·(pip)2 and 1·pip; CN5, 1; CN4, 1·H+ and 2. The coordination geometry (CN5 vs CN6; Δδ = 3 ppm) outweighs the substituent effects (CN6, 1·pip vs 2·(pip)2; Δδ = 0 ppm).

The porphyrin 1 allows the investigation of the redox chemistry of axially coordinated high-spin nickel(II) porphyrins in standard solvents. Preliminary experiments show a less positive oxidation potential of the CN5 porphyrin 1 (1.17 eV) as compared to 2, offering promising applications as an efficient oxidation catalyst (SI, section II). 1 H NMR is a powerful tool to observe and quantify axial coordination to nickel(II) porphyrins. The β-pyrrole proton signals gradually shift from about 9 ppm (CN4, completely diamagnetic) to 53 ppm (CN6, completely paramagnetic) (at 298 K). These minimum (9 ppm) and maximum (53 ppm) shifts respectively are largely independent of the mesosubstituents and axial ligands.15,24−26 This was also found to be true for the CN4 porphyrins 1·H+ (δ = 8.6 ppm) and NiTPPF10 2 (δ = 8.7 ppm) and the corresponding CN6

Figure 4. (top) Association constants and thermodynamic parameters of the CN5 complexes RP 3−6, (bottom) compared to the thermodynamic parameters of the intermolecular association constants of NiTPPF20 with external py (CN4 → CN5; toluene-d8).

δ(CN5) by 3.0 ppm results in a slight underestimation of K1S (CN4 → CN5) and thus an overestimation of K2 (CN5 → CN6) for all previously investigated systems. The pronounced stability of a five-coordinate nickel(II) species (K1 ≫ K2) was also reported for a nonporphyrin system containg a planar tridentate benzohydrazide ligand at the nickel center.27 The magnetic moment of the CN5 porphyrin 1 was determined [μ C

DOI: 10.1021/acs.inorgchem.9b00348 Inorg. Chem. XXXX, XXX, XXX−XXX

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= 2.9 μB (Bohr magneton), toluene-d8, 298 K; SI, section III] by the Evans method and is in agreement with paramagnetic complexes like NiTPPF20·(py)2 (μ = 2.9 μB).15,28 The spin multiplicity (S = 1) of the CN5 porphyrin was proven in the solid state by magnetic measurements (SI, section IV). The high-temperature value of μeff = 3.05 μB is in full agreement with the data in solution. Recently, a new group of nickel(II) porphyrins was presented carrying a photochromic azopyridine ligand attached to one of the meso positions.4,26,29 These molecules, also referred to as record players (RPs; 3−6), undergo light-driven coordination-induced spin-state switching (LD-CISSS) changing between a CN4 complex [(E)-azopyridine] and a CN5 complex [(Z)-azopyridine]. The process is fully reversible at room temperature and compatible with a broad variety of solvents. So far, it was not possible to determine the association constant of the tethered azopyridine to the central nickel ion and thus to determine the thermodynamic parameters of intramolecular coordination of the CN5 complex. With the exact CN5 pyrrole shift in hand, the missing parameters were determined (Figure 4 and SI, section VII). Obviously, the association strongly depends on the para py substituent. Electron-donating substituents as methyl (5) or methoxy (6) groups increase the binding affinity. In contrast to previous assumptions, the intramolecular coordination of these RPs is basically quantitative at room temperature (99% and >99% for 5 and 6, respectively). Hence, RP 6 can be considered to be an almost perfect on−off spin switch.4 The thermodynamic parameters reveal the origin of the RP’s efficiency. The binding enthalpies (ΔH) are small compared to the association of external ligands to nickel(II) porphyrins, i.e., NiTPPF20. ΔH for the coordination of one 4-methoxypyridine to NiTPPF20 is −6.2 kcal·mol−1 and thus more exothermic than the intramolecular coordination of RP 6 (Figure 4).15 Because bond formation involves two molecules, it suffers from a massive entropic penalty of ΔS = −14 cal·mol−1·K−1.15 The RPs rely on a covalent connection of the porphyrin and ligand. The entropic penalty is therefore less severe, on average only half as large as that for the coordination of an external ligand.15 The real gain of the RP design is an entropic advantage.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00348. Additional spectroscopic data, experimental and computational details, and characterization data (PDF) Accession Codes

CCDC 1886208−1886211 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

Natalia Levin: 0000-0001-9567-8604 Winfried Plass: 0000-0003-3473-9682 Christian Näther: 0000-0001-8741-6508 Rainer Herges: 0000-0002-6396-6991 Present Address ‡

M.D.: University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft for the Collaborative Research Center SFB 677 Function by Switching.

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REFERENCES

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CONCLUSION

We presented the first stable, single-molecular CN5 nickel(II) porphyrin in the solid state and solution. The stable, squarepyramidal geometry was obtained by the introduction of a pyridine-containing bridge. The rigidity of the bridge constrains the porphyrin in a way that makes decoordination highly unfavorable. The crystal structure and solution UV−vis and NMR spectroscopy confirm the structure and properties of the paramagnetic complex. The spectroscopic data of this first stable CN5 nickel(II) porphyrin were used to determine the thermodynamic parameters of intramolecular coordination in porphyrins 3−6. UV−vis and NMR spectra of the CN5 porphyrin 1 can now be used to determine the association constants of similar compounds from titration studies where CN4, CN5, and CN6 species are in dynamic equilibrium. The free-base porphyrin 10 can be metalated with a variety of transition metals. Thus, CN5 porphyrins of iron(II)/iron(III), cobalt(II), and manganese(III) are obtainable in well-defined complexes for applications as catalysts, biological sensors, and photovoltaics.30 D

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DOI: 10.1021/acs.inorgchem.9b00348 Inorg. Chem. XXXX, XXX, XXX−XXX