Copper(ii) and nickel(ii) hexafluorophosphate complexes derived from a monoanionic porphyrin analogue: Solvato- and thermochromism of the Ni complexes by spin-interconversion

Article abstract of DOI:10.1039/b921609j

Phenanporphodimethene (1) is a porphyrin analogue which has a dipyrromethene unit replaced by a 1,10-phenanthroline moiety. This modification effectively coverts a dianionic porphyrin to a monoanionic porphyrin analogue with a porphyrin-like N4 coordination sphere. The ligand 1 forms copper and nickel hexafluorophosphate complexes, Cu-1 and Ni-1, respectively. X-ray crystallographic analysis of Cu-1 indicates that the Cu(ii) complex is tetracoordinated by two pyrrolic nitrogen atoms and two phenanthrolic nitrogen atoms and includes a non-bonding PF₆counter-anion. The Ni-1 complex has similar geometry with a tetracoordinate square-planar structure in non-coordinating solvents such as CHCl₃. In coordinating solvents such as MeOH, the coordination structure adopts an octahedral geometry. These results indicate that Ni-1 can be converted from a low-spin to high-spin configuration by the coordination of two solvent molecules to the nickel center. This solvatochromic conversion of Ni-1 is accompanied by thermochromic behavior resulting from the transformation between square planar and octahedral configurations in THF solution. The redox peak responsible for the nickel-centered redox reaction of Ni-1 is observed at -0.66 V (vs. Fc/Fc⁺) in CH₂Cl₂solution, which indicates generation of low valent Ni(i) species. Thus, Ni-1 may be useful for future investigations as a novel structural model of the active site of cofactor F₄₃₀in methyl-coenzyme M reductase. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b921609j

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Dalton
Transactions
An international journal of inorganic chemistry
www.rsc.org/dalton
Volume 39 | Number 10 | 14 March 2010 | Pages 2453–2736
ISSN 1477-9226
HOT ARTICLE
Naruta et al.
Copper(ii) and nickel(ii)
hexafluorophosphate complexes
PERSPECTIVE
Layfield and Solomon
derived from a monoanionic porphyrin The coordination chemistry of silyl-
1477-9226(2010)39:10;1-C
analogue
substituted allyl ligands

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PAPER
www.rsc.org/dalton | Dalton Transactions
Copper(II) and nickel(II) hexafluorophosphate complexes derived from a
monoanionic porphyrin analogue: Solvato- and thermochromism of the Ni
complexes by spin-interconversion†
Masatoshi Ishida, Yoshinori Naruta* and Fumito Tani
Received 15th October 2009, Accepted 4th December 2009
First published as an Advance Article on the web 13th January 2010
DOI: 10.1039/b921609j
Phenanporphodimethene (1) is a porphyrin analogue which has a dipyrromethene unit replaced by a
1,10-phenanthroline moiety. This modification effectively coverts a dianionic porphyrin to a
monoanionic porphyrin analogue with a porphyrin-like N4 coordination sphere. The ligand 1 forms
copper and nickel hexafluorophosphate complexes, Cu-1 and Ni-1, respectively. X-ray crystallographic
analysis of Cu-1 indicates that the Cu(II) complex is tetracoordinated by two pyrrolic nitrogen atoms
and two phenanthrolic nitrogen atoms and includes a non-bonding PF6 counter-anion. The Ni-1
complex has similar geometry with a tetracoordinate square-planar structure in non-coordinating
solvents such as CHCl3. In coordinating solvents such as MeOH, the coordination structure adopts an
octahedral geometry. These results indicate that Ni-1 can be converted from a low-spin to high-spin
configuration by the coordination of two solvent molecules to the nickel center. This solvatochromic
conversion of Ni-1 is accompanied by thermochromic behavior resulting from the transformation
between square planar and octahedral configurations in THF solution. The redox peak responsible for
the nickel-centered redox reaction of Ni-1 is observed at -0.66 V (vs. Fc/Fc+) in CH2Cl2 solution, which
indicates generation of low valent Ni(I) species. Thus, Ni-1 may be useful for future investigations as a
novel structural model of the active site of cofactor F430 in methyl-coenzyme M reductase.
a tendency to stabilize lower oxidation states of the coordinated
Introduction
metal ions.9
Chemical modifications of the porphyrin skeleton involving the
Examples of skeleton-modified N4-donor porphyrinoid ligands
replacement of a pyrrole unit with other aromatic rings have
with pyrrole moieties replaced by different nitrogen donor units
such as pyridine2 are relatively rare and the properties of their
metal ion complexes and redox potentials have not yet been well
characterized. Metal complexes with monoanionic tetradentate
ligands10 exhibit fascinating properties which have led us to
undertake the synthesis and investigation of new metal complexes
with monoanionic porphyrin-like ligands constructed by the
pyrrole replacement strategy.
revealed unusual and interesting physical properties relating to
aromatic characteristics, redox potentials, magnetic features and
metal-binding behavior.1 Several porphyrin-related macrocycles
have been synthesized which contain pyridine,2 benzene,3 phenol
(oxybenzi),4 and other moieties5 in place of a pyrrole subunit
of the porphyrin macrocycle in order to investigate the diverse
electronic features of the natural porphyrin ring. In this work, we
investigate the relationships between the ligand charge state of
porphyrin analogues and the physico-chemical characteristics of
the corresponding metal complexes. It is known that the charge
state of the coordination core of porphyrin-like macrocycles can
produce greater variances in complexation of transition metal
cations and redox properties thereof. Whereas natural porphyrins
are dianionic, corrole is a trianionic tetrapyrrole macrocycle which
has the ability to stabilize high-oxidation states of transition
metal ions, such as Mn and Cu for example, which may be
employed as oxidation catalysts.6 Other heteroporphyrins7 with
partially modified cores designated N3X (where X = O, S, Se,
Te) and N-alkylated (arylated) porphyrins8 represent well known
monoanionic porphyrinoids. These N-substituted porphyrins show
Recently, we have successfully synthesized phenanpor-
phodimethene (1) as a novel monoanionic porphyrin analogue
which has a dipyrromethene unit replaced by a 1,10-phenathroline
moiety.11 1,10-phenanthroline is a remarkable heteroaromatic
compound which has the ability to form stable complexes with
a variety of metal ions. It has been widely employed as a bidentate
ligand unit in applications such as catalysis,12 as a therapeutic agent
for Alzheimer’s disease,13 as a component of solar cells,14 and as
a component of enzyme model systems.15 Furthermore, studies
of the coordination chemistry of dipyrrins16 (dipyrromethenes)
have attracted interest with respect to development of lumines-
cent compounds such as the boron difluoride dipyrrin complex
(BODIPY).17 Our porphyrin analogue (1) provides a square planar
N4 donor environment by combining both heme (dipyrrin) and
non-heme (1,10-phenanthroline) units in the macrocycle. This
hybrid coordination platform is expected to provide uncommon
coordination properties as a result of its small coordination sphere,
and reduced charge (-1) relative to natural porphyrins. In the
present study, we report the synthesis and characterization of novel
Institute for Materials Chemistry and Engineering, Kyushu Univer-
sity, Fukuoka, 812-8581, Japan. E-mail: naruta@ms.ifoc.kyushu-u.ac.jp;
Fax: +81-92-642-2731
† CCDC reference number 751278 for Cu-1. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b921609j
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Cu(II) (Cu-1) and Ni(II) (Ni-1) hexafluorophosphate complexes of
1, which have porphyrin-like N4 coordination in both Cu-1 and
Ni-1. This modulation of the electronic characteristics of Ni-1
provides noteworthy low- and high-spin interconversion as solva-
tochromism in various solvents and thermochromic properties in
THF solution. The structural and electrochemical features of Ni-1
indicate that it may be useful as a model of the nickel-dependent
enzyme cofactor F430 of methyl coenzyme M-reductase.18
Results and discussion
Syntheses of Cu(II) and Ni(II) complexes
Ligand 1 was synthesized as described previously.11 In the com-
plexation reaction with metal ions, 1 produced Cu(II) and Ni(II)
complexes upon reaction with the corresponding metal acetate
salts [MII(OAc)2] (M = Cu, Ni) in a chloroform–methanol solution
conducted with short refluxing in the presence of 2,6-lutidine as a
base. The reddish orange reaction mixture converts to blue (Cu)
and green (Ni) as the reaction proceeds. An additional anion is
required to balance the charge of the metal dication, because
the ligand acts as a monoanionic ligand coordinating through
the porphyrin-like four nitrogen atom donors. The products
were washed with NaPF6 to exchange the counter anion of the
complexes, and then purified by SiO2 column chromatography to
afford Cu-1 and Ni-1 as M(II) hexafluorophosphate complexes
(M = Cu, Ni) in 81 and 63% yields, respectively (Scheme 1). The
metal insertion was confirmed by HR-FAB mass spectra of Cu-1
and Ni-1, revealing [M]+ m/z = 798.1780 for Cu-1 and 793.1837
for Ni-1. The complexes were further characterized by elemental
analysis.
Fig. 1 The molecular structure of Cu-1 with thermal ellipsoids at
50% probability level. (a) top view: Hydrogen atoms and solvents are
omitted for clarity and (b) side view: Substituents at the meso-positions
and the counter anion are omitted for clarity. Selected bond lengths
˚
(A) and angles (deg): Cu–N(1) 2.009(2), Cu–N(2) 2.031(2), Cu–N(3)
1.931(2), Cu–N(4) 1.942(2), Cu–O(ax) 2.289(2), N(1)–Cu–N(2) 82.21(10),
N(1)–Cu–N(3) 164.19(10), N(2)–Cu–N(3) 91.04(11), N(1)–Cu–N(4)
90.70(10), N(2)–Cu–N(4) 163.16(9), N(3)–M–N(4) 91.82(11), N(1)–Cu–O
94.04(11), N(2)–Cu–O 97.22(10), N(3)–Cu–O 101.01(10), N(4)–Cu–O
98.56(10).
19
˚
(tetraphenylporphyrin) copper(II) complex (av. Cu–N; 1.981 A)
and a (1,10-phenanthroline-methionine) copper(II) perchlorate
20
Scheme 1 Synthesis of Cu-1 and Ni-1.
˚
MeOH adduct (Cu–N; 1.978–2.034 A). The average Cu–N3,4
separation distances are significantly longer than the Cu–N1,2
separation ones. This indicates that the copper ion lies in an
asymmetric Cu–N coordination environment. Each of the N–Cu–
N bond angles also reflects these features. In particular, the N1–
Cu–N2 bond angle is smaller than the corresponding N3–Cu–N4
bond angle at the opposite position.
The absorption spectrum of Cu-1 in CH2Cl2 exhibits a
porphyrin-like Q band at 590 nm (log e = 4.43) which can
be assigned as a ligand p–p* transition (Fig. 2).21 The band is
significantly red-shifted and its intensity is enhanced relative to
that of ligand 1, which has a maximum absorbance at 489 nm (log
e = 4.28). The zinc complex of the quinoidal porphyrin bearing
meso-exocyclic double bonds has demonstrated the significant
red-shift of the longer absorption band compared to the normal
porphyrins due to the extension of p-conjugation through the
meso-substituents and the inductive effect of these electron-
withdrawing moieties.22 Since their structures are different from
those of our complexes, direct comparison between them is
X-Ray structural and spectroscopic characterization of Cu-1
The molecular structure of Cu-1 determined by X-ray crys-
tallographic analysis is shown in Fig. 1, and crystal data are
given in Table 1. The macrocycle framework of Cu-1 has a
gable-type non-planar structure with a dihedral angle of 124◦
between the 1,10-phenanthroline plane and the dipyrromethene
plane. One methanol molecule provided by the recrystallization
solvent occupies the axial position of the copper ion, which
adopts a distorted square pyramidal geometry. The copper ion
˚
lies only 0.268 A above the N4 mean plane. The relatively small
displacement of the copper ion from the N4 plane is caused
by tight binding of the copper ion which has a smaller ionic
˚
radius (0.73 A). The Cu–N (phenanthroline) bond separations
˚
are 2.009 (2) and 2.031(2) A and the Cu–N (pyrrole) bond
˚
separations are 1.931(2) and 1.942(2) A. The latter values are
in agreement with the corresponding Cu–N bond distances of a
2652 | Dalton Trans., 2010, 39, 2651–2659
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Table 1 Crystallographic data for Cu-1
Compound
Cu-1·PF6·2CH3OH
Crystal Dimensions/mm
Formula Weight
Formula
0.57 ¥ 0.15 ¥ 0.11
1009.37
C45H43O10N4F6PCu
Blue
Color
Crystal System
Space Group
Triclinic
P -1
Lattice Parameters:
˚
a/A
12.227(5)
12.954(5)
15.455(7)
109.09(3)
106.78(3)
92.90(3)
2186(1)
-140
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
Fig. 3 EPR spectrum of Cu-1 in CH2Cl2 at 25 ◦C. (microwave frequency
9.2124, microwave power 1 mW, modulation amplitude 4 G, modulation
frequency 100 kHz.)
3
˚
V/A
Temperature/◦C
Z
2
Dc/g cm-3
1.532
6.26
1038
from the typical Cu(II) porphyrin complexes (e.g., CuTPP: gꢀ =
2.190, g^ = 2.045 Aꢀ = 615 MHz),26 because Cu-1 has distorted
square-planar geometry with asymmetric coordination between
the copper center and the four nitrogen atoms.
m(Mo-Ka)/cm-1
F(000)
No. of observed reflections
No. of unique reflections
Reflections used
Reflection/Parameter Ratio
No. of Variables
R1 [I > 2s(I)]a
wR2 (all data)b
GOF
39888
9739
9739
30.82
647
0.088
0.245
0.886
1H-NMR spectroscopic characterization of Ni-1
Although suitable single crystals of Ni-1 for X-ray diffraction
measurements could not be obtained, the molecular structure
of Ni-1 is expected to have a similar conformational geometry
to Cu-1, based on the results of the 1H-NMR spectroscopic
experiment. The 1H-NMR spectrum of Ni-1 in CDCl3 indicates a
typical diamagnetic square planar nickel(II) complex as shown in
Fig. 4b. Compared with ligand 1, the spectrum of Ni-1 in CDCl3
has similarity in the chemical shift and splitting of each of the
resonances (Fig. 4a). The simple resonance patterns include sets
of peaks of the phenanthroline moiety (at 8.1, 8.2 and 8.8 ppm),
the meso-phenyl group (at 7.3–7.5 ppm), and two sets of the b-
pyrrole units (at 6.8 ppm) in an aromatic proton region. The sets of
the methylene and methyl proton peaks of the ester moiety appear
in the high field region of the spectrum with equal intensity as
expected for the Cs symmetric binding mode of the ligand. The
disappearance of the inner NH resonance peak is also expected
due to the symmetric coordination of the nickel ion into the core
of Ni-1. Furthermore, the DFT optimized structure of cationic
Ni-1 at the singlet ground state (S = 0) is quite similar to the X-ray
crystal one of Cu-1. (Fig. 5) Therefore, the coordination geometry
of Ni-1 in CDCl3 could be square planar.
a R1 = RꢀFo| - |Fcꢀ/R|Fo| b wR2 = {Rw(|Fo| - |Fꢀ)2/RwFo
}
1/2; w =
2
2
2
2 -1
1/s (Fo) = {s c(Fo) + p2/4·Fo } .
Fig. 2 UV-vis absorption spectra of 1 (dotted line) and Cu-1 (full line) in
CH2Cl2 at 25 ◦C. [1] = [Cu-1] = 2 ¥ 10-5 M.
difficult. The quinoid structures in our complexes, however, may
contribute to their absorption spectra to some extent.
Solvato- and thermo-chromisms of Ni-1
The effective magnetic moment (meff) determined by the Evans
method23 at room-temperature is 1.81. The magnitude of meff
is consistent with the spin-only value for an independent d9
copper(II) center with S = 1/2 (1.7–2.2 mB).24 Moreover, the ESR
spectrum of Cu-1 in CH2Cl2 solution at room temperature includes
four lines with a nuclear hyperfine spin 3/2, having gꢀ and g^
values of 2.122 and 2.019, respectively, which are less than 2.3
in agreement with the covalent character of the metal ligand
bond (Fig. 3). The relationship gꢀ > g^ is typical of d9 copper(II)
complexes in a ground state doublet with the unpaired electron
When Ni-1 is dissolved in MeOH, the solution color is changed
from dark green to bluish purple. The UV-vis absorption spectra
of the CH2Cl2 solution of Ni-1 containing different amounts of
MeOH indicate blue-shifting of the Q-like band from 620 to
569 nm in a manner dependent upon the amount of MeOH
(Fig. 6). This implies that a new species is generated in MeOH
solution. The 1H-NMR spectrum of this MeOH solution of
Ni-1 demonstrates formation of a paramagnetic nickel species
(Fig. 4c). Latos-Graz˙yn´ski et al. have reported that a core-modified
porphyrin with an incorporated phenolate donor affords a four
coordinated diamagnetic Ni(II) complex and that, upon treatment
with gaseous hydrogen chloride, the complex is converted into a
paramagnetic Ni(II) complex.27 The observed pattern of chemical
residing in a dx
orbital.25 The hyperfine Aꢀ splitting of 83.8
2
2
-y
indicates that the electron is interacting with only one copper
nucleus. The g and Aꢀ values of Cu-1 are significantly different
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Fig. 4 1H-NMR spectra of (a) 1 in CD2Cl2 (inlet; inner NH resonance) and Ni-1 (b) in CDCl3 and (c) in CD3OD, at 25 ◦C.
shifts is similar to the high spin (HS, where S = 1) Ni(II)
complexes of a porphyrin analogue,28 with b-pyrrole resonances in
the downfield region of 10–30 ppm and the 1,10-phenanthroline
resonances in the 45–55 ppm range. The effective magnetic
moment of the nickel species at room temperature was determined
by the Evans method to be 3.28 mB. This value was concluded to be
consistent with that of common HS Ni(II) complexes29 (2.8–4.0 mB),
indicating that the spin state of the nickel ion in methanol solution
is high-spin (HS). The conversion of spin states of Ni(II) from low-
spin (LS, where S = 0) to HS must be due to the conversion
from square planar to the octahedral geometry as a result of axial
ligation (in the HS state, the d8 ion presents two unpaired electrons
in the z2 and x2-y2 orbitals, which are at similar energy levels).30
The absorption spectra of Ni-1 in the other coordinating solvents
(e.g., DMSO and DMF) have similar spectral features such as blue-
shifting of the Ni-1·solv adducts, which implies formation of the
6-coordinated (6cHS) Ni(II) species (Table 2). Binding constants
(Kb) for the coordinated complexes with solvents are defined in eqn
(1), where AL and AH are the absorbance values corresponding
to the LS and HS Ni(II) species, respectively. Kb values were
calculated using the following relationship:
Fig. 5 Optimized structure of Ni-1 obtained by B3LPY DFT Calculation.
(a) top view and (b) side view: Hydrogen atoms and substituents at the
˚
meso-positions are omitted for clarity. Selected bond lengths (A) and angles
(deg); Ni–N1 1.947, Ni–N2 1.935, Ni–N3 1.884, Ni–N4 1.885, N1–Ni–N2
83.83, N2–Ni–N3 90.74, N3–Ni–N4 93.16, N4–Ni–N1 91.30.
Kb = [Ni-1·solv]/[Ni-1] = (AL - A)/(A - AH)
(1)
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Table 2 Summary of spectral and electrochemical data for 1, Cu-1 and Ni-1 in various solvents at 25 ◦
Absorption, almax/nm Potentials, bE/V
C
Comp.
CH2Cl2
MeOH
DMF
DMSO
Ox(1)
Ox(2)
Red(1)
Red(2)
Red(3)
1
489
590
620
0.81
0.84
0.92
1.09
—
—
-1.27
-0.77
-0.66
-1.58
-1.14
-1.03
Cu-1
Ni-1
-1.59
-1.29
569
570
576
a
lmax for visible bands. Measurement at room temperature. b Peak potentials determined by DPV in CH2Cl2 at a scan rate of 50 mV s-1.
-80 ◦C, the solution color gradually changes from blue to purple.
This color change is accompanied by an increase in intensity
of the higher energy peak at 573 nm (Fig. 7). This observed
spectrum of Ni-1 in THF at -80 ◦C is similar to that of the 6cHS
Ni-1 species observed in coordinating solvents such as MeOH,
DMF and DMSO (Table 2). The formation of 6cHS Ni-1·THF
adduct was further evidenced by determination of the effective
magnetic moment at -40 ◦C to be 2.47 mB.32 The thermochromism
would be associated with the reversible temperature-dependent
transformation between the 4cLS and 6cHS structures. Therefore,
this temperature-dependent spectral change can be rationalized
by the formation of the 6cHS Ni(II) species when the temperature
is decreased.
Fig. 6 UV-vis absorption spectral change of Ni-1 in CH2Cl2 upon
titration of MeOH, [Ni-1] = 2 ¥ 10-5 M, at 25 ◦C. Inlet shows the
photographs of the solution of Ni-1 in MeOH (left) and CH2Cl2 (right).
The Kb values (M-1) calculated from the slopes of the plots
are 0.038 for MeOH, 0.132 for DMF, and 0.230 for DMSO,
respectively. The similarity of these values indicates that the Ni-
1·solv adducts have essentially the same coordination geometry.
In non-coordinating solvents such as CH2Cl2, the formation of
a 4-coordinate square-planar (4cLS) Ni(II) complex is preferred,
whereas in coordinating solvents such as MeOH, the correspond-
ing 6-coordinate octahedral 6cHS Ni(II) complex with an axial
solvent molecule is probably formed. Although we attempted to
further characterize the structure of the 6cHS Ni(II) complex of
Ni-1 by cold-spray-ionization mass spectroscopy (CSI-MS), the
mass peak of the complex (which includes an axially-coordinated
solvent molecule) could not be observed due to the small binding
constants of the corresponding solvent molecules. However, the
structure of Cu-1 exhibits coordination of methanol to the
copper center as evidenced by X-ray crystallographic analysis.
A supramolecular complex containing a methanol-coordinated
HS Ni(II) species with a monoanionic tripyrrin ligand has been
reported.31 Therefore, Ni-1 might be a 6cHS Ni(II) complex in
coordinating solvents, as a result of the transformation from a
4cLS square planar complex to a 6cHS octahedral complex.
The difference between the Kb values of various solvents reflects
thermal behavior. Ni-1 exhibits thermochromism in THF, which
is a weaker coordinating solvent than MeOH. Two absorption
maxima emerge at 613 and 575 nm in the spectrum of Ni-1 in THF
at 20 ◦C. This result may indicate the existence of both 4cLS and
6cHS Ni(II) species. Upon cooling of the THF solution of Ni-1 to
Fig. 7 UV-vis absorption spectral change of Ni-1 observed upon lowering
the temperature form 20 to -80 ◦C in THF. [Ni-1] = 2 ¥ 10-5 M. Inlet shows
the photographs of the solution of Ni-1 in THF at low temperature (left)
and room temperature (right).
Redox properties of Ni-1
To gain insight into the electronic character of the porphyrin-like
macrocycle, the redox potentials of Ni-1 were measured by cyclic
voltammetry (CV) and differential pulse voltammetry (DPV) in
CH2Cl2 containing 0.1 M nBu4NClO4 (TBAP). All potentials
were referenced to the ferrocene/ferrocenium (Fc/Fc+) couple
as a standard. The data are summarized in Table 2. The cyclic
voltammogram of the Ni(II) complex (Ni-1) exhibits one oxidation
wave at Eox = 0.92 V and three reduction waves at Ered = -0.66,
-1.03 and -1.29 V (vs. Fc/Fc+), as shown in Fig. 8. The metal free
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reductions to occur without requiring large energy expenditure for
geometric rearrangements.
Relevance of Ni-1 to nickel dependent cofactor F430
The nickel dependent cofactor F430 of methyl-coenzyme M
reductase18 catalyzes the reduction of methyl-coenzyme M (CH3-
S-CoM) with coenzyme B (HS-CoB) to methane and the het-
erodisulfide CoM-S-S-CoB in the following equation:
CoB-S-H + CH3-S-CoM → CoB-S-S-CoM + CH4
(2)
This reaction represents the key final step of the process of
methanogenesis in Archaea. The highly reduced nickel hydrocor-
phin, a nonplanar monoanionic porphyrin-related macrocycle of
cofactor F430, exhibits intriguing spin interconversions37 and redox
properties.38 Nickel complexes of oxaporphyrin39 and nonpor-
phyrinoid tetraazamacrocyles40 have been developed as synthetic
models of Ni(I) F430. Ghosh et al. have recently investigated
symmetrized monoanionic porphyrin analogues such as oxapor-
phyrin (N3O coordination) and pyriporphyrin (N4 coordination
with a pyrrole unit replaced by pyridine). These compounds were
found to provide excellent models for theoretical investigation.41
Moreover, on the basis of spectroscopic evidence, the proposed
catalytic mechanism of cofactor F430 includes reduction of the
nickel center to the Ni(I) state as an active species.42 The redox
potential of the Ni(II)/Ni(I) couple in aqueous F430 is known to
be between -0.6 and -0.7 V (vs. NHE)43 The potential of the
Ni(II)/Ni(I) couple of Ni-1 was estimated to be approximately
-0.3 V (vs. NHE),44 which are close to the native potentials. Thus,
our nickel complex, Ni-1 is expected to provide a useful candidate
for development of a synthetic model of the Ni-dependent cofactor
Fig. 8 Cyclic voltammograms of Ni-1 in CH2Cl2 containing 0.1 M
nBu4NClO4 at room temperature. Conditions: [Ni-1] = 1 mM; working
electrode GC; counter electrode, Pt wire; reference electrode, Ag/Ag+;
scan rate 50 mV s-1.
ligand (1) gives rise to two oxidation waves at Eox = 0.81 and 1.09 V
and two reduction waves at Ered = -1.27 and -1.58 V. The redox
process in Ni-1 observed at -0.66 V represents the redox couple
of the metal center, NiII/NiI. This assignment is rationalized as
follows: if the reduction potential of Ni-1 was assigned to be a
ligand-centered process, the potential gap (1.58 V) between the
first reduction and the first oxidation of the ligand would be too
small relative to that of the free base ligand, 1 (2.08 V). Thus, the
monoanionic coordination core of the porphyrin analogue would
be consequently expected to stabilize the lower oxidation state of
the transition metals. The Ni(II) porphyrin complex (e.g., NiTPP,33
tetraphenylporphyrin) has a reduction peak at -1.75 V (vs. Fc/Fc+,
in CH2Cl2, 0.1 M TBAP). The one electron-reduced species of
the Ni(II) porphyrin complex is assigned as an equilibrated form
between the Ni(I) porphyrin anion and the Ni(II) porphyrin p-
anion radical.34 The positive shift of the nickel-centered redox
peak of Ni-1 relative to that of NiTPP indicates that reduction
of the nickel center in Ni-1 occurs more easily than reduction
of NiTPP. Moreover, the metal-centered reduction potential of
Cu-1 determined by DPV is -0.77 V. A similar positive shift
of the CuI/CuII redox couple is observed due to the nature of
the macrocycle (Table 2). On the other hand, metal-centered
oxidation potentials were not observed for both Ni-1 and Cu-1
when compared with metal-centered oxidation potentials of ligand
1. The replacement of half of the skeleton of the porphyrin core
with the 1,10-phenanthroline moiety reduces the ligand charges
of the macrocycle from -2 to -1. This clearly affects the redox
behavior of the macrocycle to stabilize the low-valent Ni(I) species.
The stability of this species may also be increased as a result of
the flexibility of the modified macrocycle. The ion-binding cavity
F430
.
Conclusion
We have described the preparation of copper(II) and nickel(II) com-
plexes of a heme/non-heme hybrid porphyrin analogue, phenan-
porphodimethene (1) in good yield. The combination of 1,10-
phenanthroline and a dipyrrin moiety as structural components of
a porphyrin-like macrocycle afforded a N4-monoanionic coordi-
nation sphere capable of accommodating copper(II) and nickel(II)
ions. The molecular structure of Cu-1 was characterized by X-ray
crystallographic analysis, which demonstrated a distorted square-
planar coordination structure with a hexafluorophosphate counter
anion. The nickel complex, Ni-1, has similar geometry to Cu-1,
which are characterized by DFT calculation and demonstrate an
intriguing conversion between 4cLS and 6cHS configurations of
the nickel center promoted by the solvent axial coordination. This
is accompanied by a distinct solution color change representing
solvatochromism. Moreover, the thermochromic behavior of Ni-1
was also observed in THF solution. The electrochemical analysis
of Ni-1 indicates a positive shift of the nickel(I/II) redox potential
relative to that of the NiTPP. This implies stabilization of the
low valent oxidation state of the nickel center due to the intrinsic
coordination environment of the macrocycle (e.g., flexibility and
electronic charge of macrocycle). These remarkable redox features
indicate that Ni-1 may give a structural model of the nickel
dependent cofactor F430 in methyl coenzyme M reductase. These
chemical modifications of the porphyrin framework will provide
˚
of natural porphyrins has a radius greater than 2.0 A. In contrast,
the square-planar 4cLS Ni(II)–N(pyrrole) bond distance of our
˚
complex is approximately 1.8–1.9 A, and the coordination of Ni(II)
by a porphyrin introduces strain (ruffling) into the macrocycles.35
We expect that this effect may be amplified in our modified ligand
1. The rigid coordination environment of the natural porphyrin
is unfavorable with regard to accommodation of a Ni(I) center,
which has a larger ionic radius, and, as a result, the ligand-
centered reduction event becomes energetically more favorable.
Thus the redox properties of nickel complexes can be controlled
by the rigidity of the macrocycle.36 Our ligand 1 is flexible at the
meso-exocyclic double bonds and more readily allows Ni-centered
2656 | Dalton Trans., 2010, 39, 2651–2659
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an attractive strategy for furnishing new metal coordination
environments and fine-tuning metal complexes as catalysts and
biomimetic enzyme models.
(d, J = 7.1 Hz, 2H, Ph-H), 6.80 (d, J = 4.6 Hz, 2H, Pyrr-H),
6.76 (d, J = 4.6 Hz, 2H, Pyrr-H), 4.18–4.36 (m, 8H, CH2), 1.31 (t,
J = 7.3 Hz, 6H, CH3), 1.23 (t, J = 7.3 Hz, 6H, CH3) ppm. 13C-
NMR (125 MHz, CDCl3) d 163.48, 162.40, 154.04, 151.05, 147.83,
145.41, 139.81, 138.21, 137.56, 135.55, 135.02, 133.30, 130.39,
130.26, 130.03, 129.58, 128.30, 128.04, 127.29, 122.24, 63.28,
62.95, 13.96, 13.79 ppm. HR-MS (FAB) m/z = 793.1837 (found),
793.1808 (calcd. For C43H35N4O8NiPF6). UV-vis (CH2Cl2): lmax
[nm] (log e [M-1 cm-1]) = 444 (3.92), 622 (4.25); (MeOH): lmax [nm]
(log e [M-1 cm-1]) = 310 (4.41), 573 (4.39). Elemental analysis
(Found: C 51.71; H 3.81; N 5.38. C43H35N4O8NiPF6+CH2Cl2
requires C 51.59; H 3.69; N 5.47%).
Experimental
Instruments
1H-NMR spectra were recorded on a JEOL JMX-GX400 (400
MHz) spectrometer. 13C-NMR spectra were recorded on a JEOL
ECA500 (500 MHz) spectrometer. Chemical shifts (d–scale) are
provided with reference to the peak of residual non-deuterated
chloroform (7.26 ppm, for 1H-NMR spectra, 65.00 ppm, for 13C-
NMR). High resolution mass (HR-MS) spectra were obtained
using a JEOL LMS-HX-110 spectrometer. FAB-MS spectra were
measured using 3-nitrobenzyl alcohol (NBA) as a matrix. UV-
vis absorption spectra were recorded on a Shimadzu UV-3100PC
spectrometer and an Agilent 8453 diode array spectrometer
equipped with a Unisoku thermostatic cell holder for variable
temperature measurement. Cold-spray ionization time-of-flight
(CSI-TOF) mass spectra were recorded using a JEOL JMS-
T100CS spectrometer. ESR spectra were recorded on a JEOL
JES-TE 300 spectrometer at room temperature. The magnetic field
was calibrated from the hyperfine coupling constants of Mn(II)
ion doped in MgO powder (86.9 Gauss). Cyclic voltammetric
measurements were carried out on a BAS 100B/W electrochemical
analyzer using a conventional three-electrode cell for samples
(1 mM) dissolved in dry CH2Cl2 containing 0.1 M nBu4NClO4
(TBAP) under an Ar atmosphere. A glassy-carbon working
electrode and a platinum wire as a counter electrode with an
Ag/Ag+ reference electrode were used in all experiments. The
potentials were calibrated with ferrocenium/ferrocene.
Cu-1. The copper complex, Cu-1 was prepared by a procedure
similar to the procedure for preparing Ni-1. Compound 1 (0.1 g,
0.14 mmol), a slight excess of copper(II) acetate dihydrate and a
few drops of 2,6-lutidine were treated in CHCl3–MeOH solution.
The reaction mixture was combined with water and extracted by
CHCl3. The organic layer was washed several times with saturated
NaPF6 aqueous solution. The resulting solution was dried over
Na2SO4 and evaporated to dryness. The residue purified by SiO2
column chromatography (CHCl3 + 5% MeOH) gave Cu-1 as a
blue powder. Yield: 102 mg (81%); UV-vis (CH2Cl2): lmax [nm]
(log e [M-1 cm-1]) = 590 (4.43). HR-MS (FAB) m/z = 798.1780
(found), 798.1751 (calcd. for C43H35N4O8CuPF6). Elemental anal-
ysis (Found: C 53.68; H 3.73; N 5.79. C43H35N4O8CuPF6 requires
C 54.69; H 3.74; N 5.93%).
Theoretical calucations
The calculations were performed with the Gaussian03 program.45
The geometry optimization of cationic Ni-1 at the singlet ground
state (S = 0) was performed by using the B3LYP DFT method
with the 6-31G(d) basis set for C, N, O and H atoms and the
LANL2DZ basis set for Ni and Cu atom without any symmetry
restriction.46 All calculations of the complexes were performed
Materials
Generalchemicals wereusedwithoutfurther purification. Solvents
were used after appropriate distillation or purification. Analytical
thin-layer chromatography (TLC) was performed on Merck silica
gel 60 pre-coated aluminium sheets. Column chromatography was
carried out using silica gel 60 N (60 mesh, Wako chemical) or
neutral alumina (Wako chemical). The ligand used in this study
was prepared according to the following procedure.11
-
without the counter anion of hexafluorophosphate (PF6 ).
X-Ray crystallographic determination
The X-ray experiments for Cu-1 were carried out using a
Rigaku RAXIS Rapid imaging plate area detector with graphite-
Synthetic procedure
˚
Ni-1. Compound 1 (0.1 g, 0.14 mmol) was dissolved in CHCl3
(30 ml) and a methanol solution (5 ml) of nickel(II) acetate (excess)
and a few drops of 2,6-lutidine were added to the solution. The
mixture was refluxed for 15 min under a nitrogen atmosphere.
After cooling, the reaction mixture was combined with water and
then extracted with CHCl3. The organic layer was washed with a
saturated NaPF6 aqueous solution several times to exchange the
counter ion of the nickel complex. This was followed by washing
with deionized water. The solution obtained from this process was
dried over Na2SO4 and evaporated to dryness. The residue was
purified by SiO2 column chromatography (CHCl3 + 4% MeOH)
monochromated MoKa radiation (l = 0.71075 A). The crystals
were mounted on a glass fiber. The data were collected for Cu-1
(2q = 54.8◦) at -140 ◦C. Refraction data were corrected for both
Lorentz and polarization effects. The structures were solved by the
direct method and refined anisotropically for non-hydrogen atoms
by full-matrix least-squares calculations. All hydrogen atoms were
located at the calculated positions, assigned a fixed displacement
˚
and constrained to ideal geometry with C–H = 0.95 A. The
thermal parameters of calculated hydrogen atoms were related to
those of their parent atoms by U(H) = 1.2Ueq(C,N). The crystal-
lographic calculations were performed using the crystal structure
software package of the Rigaku Corporation [CrystalStructure
Ver. 3.8.2: Crystal structure analysis package, Rigaku (3–9-12
Akishima, Tokyo, Japan) and Rigaku/MSC (9009 New Trails
Dr, The Woodlands, TX 77381 USA)].
1
and produced Ni-1 as a green powder. Yield: 80 mg (63%); H-
NMR (400 MHz, CDCl3) d 8.78 (d, J = 8.6 Hz, 2H, Phen-H),
8.19 (s, 2H, Phen-H), 8.13 (d, J = 8.6 Hz, 2H, Phen-H), 7.50 (dd,
J = 1.5, 7.1 Hz, 1H, Ph-H), 7.45 (d, J = 7.3 Hz, 2H, Ph-H), 7.37
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Acknowledgements
IM was supported by a Grant-in-Aid for Global COE Program,
“Science for Future Molecular Systems” from MEXT (Japan) and
this work is supported by Grants-in-Aid for Scientific Research (S)
(#17105003, YN) and for Priority Area (#20036041, YN) form
JSPS. We thank Dr Onitsuka (Kyushu University) for assistance
with the CSI-MS measurement.
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©