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

Article abstract of DOI:10.1021/acs.inorgchem.9b00348

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 NiTPPF₁₀·(py)₂] and low-spin four-coordinate (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.

Full text of DOI:10.1021/acs.inorgchem.9b00348

Article
pubs.acs.org/IC
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 Nather, 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 Mu
̈
lheim an der Ruhr, Germany
⊥Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstrasse 8, 07743 Jena, Germany
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 NiTPPF10·(py)2] and low-spin four-
coordinate (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.
controversies.12,14 Characterizations have been restricted to
INTRODUCTION
■
transient absorption or resonance Raman spectroscopy.10,12,14
Axial coordination to metal porphyrins has been studied from a
It is widely accepted that the properties of the CN5 species are
similar to those of the CN6 species. Most importantly, the
number of different perspectives for several decades.1,2 Those
porphyrins are crucial to a variety of biological processes or
UV−vis and NMR spectra are expected to be almost
technological applications such as photoswitchable magnetic
identical.18 There is also a general agreement that the CN5
resonance imaging contrast agents,3,4 solar energy conversion,
complexes are high-spin because no evidence for a CN5 low-
especially dye-sensitized photovoltaics,5,6 hydrogen-evolution
spin complex has been observed.12 However, UV−vis titrations
catalysis,7 and redox catalysis.8,9 Numerous studies have been
conducted on meso- and β-substituted nickel(II) porphyrins.
of electron-deficient porphyrins with very strong ligands
exhibit a lack of true isosbestic points, indicating the presence
Similar to their iron counterparts, the spin state and thus their
of an additional species with an absorption profile different
magnetic properties are strongly dependent on the coordina-
from those of the CN4 and CN6 complexes.14−16 This
tion of axial ligands.1,2,10 Without axial ligands, nickel(II)
observation strongly suggests the presence of a significant
porphyrins exhibit a diamagnetic low-spin state because of the
amount of CN5 species, with a UV−vis spectrum different
2
doubly occupied dz orbital of the central nickel ion. Upon
from that of the CN6 complex, contradicting previous
coordination of at least one axial ligand, one electron is
assumptions.14
2
2
2
transferred from the dz orbital to the dx −y orbital to give rise
to a paramagnetic high-spin species (S = 1).11
EXPERIMENTAL SECTION
Upon the addition of ligands to nickel(II) porphyrins, five-
coordinate (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
■
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
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00348
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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(2,3,4,5,6-
pentafluorophenyl)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 py-
containing 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
Ni−Npy
Ni−NPorph. Ni−N4 plane
Porphyrin·Ligand
NiTPPF10 (2)*
CN
(Å)
(Å)
(Å)
4
4
5
6
6
6
6
6
6
1.92
1.93
2.05
2.04
2.05
2.05
2.06
2.05
2.05
0
21
NiTPPF20
0
1*
2.11
2.18
2.21
2.22
2.19
2.22
2.23
0.26
0.08
0
0
0
(Z)-5·MeOH4
2·(py)2*
Scheme 1. Synthesis of the CN5 Porphyrin 1
21
NiTPPF20·(py)2
NiTPPF20·(4-(NMe2)py)2
NiTPPF20·(4-CNpy)2
NiTPPF20·(4-(NO2)py)2
15
15
0
0
15
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
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.
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 bath-
ochromic 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).
Figure 1. ORTEP plot of the CN5 porphyrin 1 with labeling of
selected atoms and displacement ellipsoids drawn at the 50%
probability level.
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

Inorganic Chemistry
Article
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).
1
Figure 3. H 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) porphyr-
ins 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).
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).
1H 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 meso-
substituents 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
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

Inorganic Chemistry
Article
= 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.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00348.
■
S
Additional spectroscopic data, experimental and compu-
tational details, and characterization data (PDF)
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) chang-
ing 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).
Accession Codes
CCDC 1886208−1886211 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: rherges@oc.uni-kiel.de.
■
ORCID
Natalia Levin: 0000-0001-9567-8604
Winfried Plass: 0000-0003-3473-9682
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.
̈
Christian Nather: 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.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge funding from the Deutsche
Forschungsgemeinschaft for the Collaborative Research Center
SFB 677 Function by Switching.
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DOI: 10.1021/acs.inorgchem.9b00348
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