Impact of distortion of porphyrins on axial coordination in (porphyrinato)zinc(II) complexes with aminopyridines as axial ligands

Article abstract of DOI:10.1002/ejic.200801040

A series of (porphyrinato)zinc(II) compounds were synthesized with use of 5,10,15,20-tetraphenylporphyrin (H₂TPP), 2,3,5,10,12,13,15,20- octaphenylporphyrin (H₂OPP), and 2,3,5,7,8,10,12,13,15,17,18,20- dodecaphenylporphyrin (H2DPP). Those compounds form complexes with aniline, pyridine, and 3- and 4-aminopyridines as axial ligands. X-ray crystallography was performed on the complexes with 3-aminopyridine (3-AP) and 4-aminopyridine (4-AP) as axial ligands. 3-Aminopyridine was revealed to bind through the amino group to the Zn(OPP), exhibiting intermolecular π-π interaction between 3-AP and one of the pyrrole rings and intermolecular NH-π interactions of the coordinated amino group with two β-phenyl groups of an adjacent molecule. In solu-tion, the aminopyridines form a single species at ambient temperature and are assumed to have pyridine coordination through the aromatic pyridine nitrogen atom. Variable-temperature NMR spectroscopy in CD₂Cl ₂ indicates that two different species exist at lower temperatures, suggesting that amino-bound complexes of 3-AP can be formed as a metastable species in solution, which is stabilized in the crystal as a result of noncovalent interactions. The binding constants of aminopyridines to the three kinds of (porphyrinato)zinc complexes reveal enhancement of the axial ligation by virtue of the distortion of the porphyrin ring. Wiley-VCH Verlag GmbH & Co. KGaA, 2009.

Full text of DOI:10.1002/ejic.200801040

FULL PAPER
DOI: 10.1002/ejic.200801040
Impact of Distortion of Porphyrins on Axial Coordination in
(Porphyrinato)zinc(II) Complexes with Aminopyridines as Axial Ligands
Takahiko Kojima,*[a][‡] Tatsuaki Nakanishi,[a] Tatsuhiko Honda,[a] Ryosuke Harada,[a][‡‡]
Motoo Shiro,[b] and Shunichi Fukuzumi*[a]
Keywords: Porphyrinoids / N ligands / Coordination modes / Zinc / Molecular distortion
A series of (porphyrinato)zinc(II) compounds were synthe-
sized with use of 5,10,15,20-tetraphenylporphyrin (H2TPP),
tion, the aminopyridines form a single species at ambient
temperature and are assumed to have pyridine coordination
through the aromatic pyridine nitrogen atom. Variable-tem-
perature NMR spectroscopy in CD2Cl2 indicates that two dif-
ferent species exist at lower temperatures, suggesting that
amino-bound complexes of 3-AP can be formed as a meta-
stable species in solution, which is stabilized in the crystal as
a result of noncovalent interactions. The binding constants of
aminopyridines to the three kinds of (porphyrinato)zinc com-
plexes reveal enhancement of the axial ligation by virtue of
the distortion of the porphyrin ring.
2,3,5,10,12,13,15,20-octaphenylporphyrin
(H2OPP),
and
2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (H2DPP).
Those compounds form complexes with aniline, pyridine,
and 3- and 4-aminopyridines as axial ligands. X-ray crystal-
lography was performed on the complexes with 3-aminopyr-
idine (3-AP) and 4-aminopyridine (4-AP) as axial ligands. 3-
Aminopyridine was revealed to bind through the amino
group to the Zn(OPP), exhibiting intermolecular π–π interac-
tion between 3-AP and one of the pyrrole rings and intermo-
lecular NH–π interactions of the coordinated amino group
with two β-phenyl groups of an adjacent molecule. In solu-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
organic[4,5] and enzymatic reactions.[6] The Lewis acidity of
the ZnII center can be regulated by the coordination envi-
ronments and donor sets of the ligands applied. (Porphyr-
inato)zinc(II) complexes form dominantly five-coordinate
square-pyramidal structures.[7] They have served not only
as electron donors in photoinduced electron transfer, but
also as building blocks of assemblies. The axial ligation in
(porphyrinato)ZnII complexes is not as strong as that in
(porphyrinato)transition-metal complexes without dσ elec-
Introduction
Porphyrins, as extended π-conjugated molecules, are
known to exhibit planar structures. The introduction of pe-
ripheral substituents to the porphyrin skeleton, however,
can induce molecular distortion in various manners involv-
ing saddling, ruffling, waving, and doming.[1,2] This por-
phyrin ring distortion has been suggested to exert certain
effects to control the functions of iron centers in heme pro-
teins and heme enzymes in biological systems[1] and chloro-
phylls.[3] Thus, it is important to demonstrate systematic
and quantitative treatments of the influence of the por-
phyrin distortion on the characteristics of porphyrinato
complexes.
trons, such as CoIII and CrIII [8]
However, strong donors
.
such as pyridine derivatives can bind fairly strongly to the
ZnII center. On the basis of this behavior, the introduction
of pyridine compounds as axial ligands of (porphyrinato)-
ZnII complexes has been examined for various purposes,
including template synthesis of multiporphyrin com-
pounds,[9] self-organization of (porphyrinato)ZnII com-
plexes to form multiporphyrin arrays,[10] and photoinduced
charge separation systems.[11]
To consider the strength of axial coordination in (por-
phyrinato)ZnII complexes, we need to pay attention to the
Lewis acidity of the ZnII center to construct a stable archi-
tecture by taking advantage of the axial ligation. However,
no report has appeared to clarify the influence of the distor-
tion of porphyrin ligands in (porphyrinato)ZnII complexes
on the Lewis acidity, which can be reflected in the binding
constants of axial ligands such as pyridine derivatives.
Thus, we examined the impact of porphyrin distortion
on the strength of the axial coordination in (porphyrinato)-
As a metal center of a metalloporphyrin, the Zn2+ ion is
also one of the most prevailing metal ions. The Zn2+ ion has
been known to contribute as a Lewis acid in many kinds of
[a] Department of Material and Life Science, Graduate School of
Engineering
Osaka University and SORST (JST)
2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Fax: +81-6-6879-7370
E-mail: kojima@chem.eng.osaka-u.ac.jp
fukuzumi@chem.eng.osaka-u.ac.jp
[b] X-ray Research Laboratory, Rigaku Corporation,
3-9-12 Matsubara, Akishima-shi, Tokyo 196-8666, Japan
[‡] Present address: Department of Chemistry, Graduate School of
Pure and Applied Sciences, University of Tsukuba, Tsukuba,
Ibaraki 305-8571, Japan
E-mail: kojima@chem.tsukuba.ac.jp
[‡‡]Present address: Center for Future Chemistry, Kyushu Univer-
sity, Moto-oka, Fukuoka 819-0395, Japan
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T. Kojima, T. Nakanishi, T. Honda, R. Harada, M. Shiro, S. Fukuzumi
FULL PAPER
ZnII complexes by adopting tetraphenylporphyrin (H2TPP), of the adjacent molecule, showing an interatomic distance
2,3,5,10,12,13,15,20-octaphenylporphyrin (H2OPP),[12] of 3.48 Å from the opposite side of the 3-AP coordination
and 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin site relative to the porphyrin plane. No such intermolecular
(H2DPP)[13] as porphyrin ligands showing different extents interaction is found for T4.
of distortion and aminopyridines as axial ligands. (Por-
phyrin)ZnII complexes can be useful probes to evaluate
these effects, since ZnII is in a d10 configuration without
change of spin states, and the σ-donating ability of the pyr-
role nitrogen atoms in the porphyrins directly reflects on
the equilibrium of the axial ligation. Thus, we report herein
crystal structures of (aminopyridine)(porphyrinato)ZnII
complexes and quantitative evaluation of axial coordination
of aminopyridine derivatives in solution to exemplify the
influence of porphyrin ring distortion. In the course of our
research, we found unusual coordination of 3-aminopyr-
idine, which binds in the crystal to the ZnII center within
the OPP2– ligand through the amino nitrogen atom rather
than the pyridine nitrogen atom.
Figure 1. ORTEP drawing of T3 with selected numbering scheme
with thermal ellipsoids at 50% probability. All hydrogen atoms are
omitted for clarity.
Results and Discussion
Crystal Structures of (Aminopyridine)(porphyrinato)zinc
Complexes
(Porphyrinato)ZnII complexes with aminopyridine deriv-
atives were synthesized by the reaction of corresponding
(porphyrinato)ZnII precursor complexes with aminopyr-
idine in chloroform at room temperature in nearly quantita-
tive yields. Crystal structures of all the complexes were de-
termined by X-ray crystallography. Selected bond lengths
[Å] and the deviation of the ZnII center from the least-
square planes of the 24 atoms of the porphyrin cores are
summarized in Table 1.
Crystal structures of ZnII(TPP) complexes with 3-amino-
pyridine (3-AP), [Zn(3-AP)(TPP)] (T3), and with 4-amino-
pyridine (4-AP), [Zn(4-AP)(TPP)] (T4), are shown as
ORTEP drawings in Figures 1 and 2, respectively. T4 crys-
tallizes in the acentric space group P212121 to give a chiral
structure. In both complexes, the aminopyridine derivatives
coordinate to the ZnII centers through the aromatic nitro-
gen atoms. The bond lengths of four Zn–N bonds in the
Figure 2. ORTEP drawing of T4·CHCl3 with selected numbering
scheme with thermal ellipsoids at 50% probability. All hydrogen
equatorial plane are nearly the same for the two complexes,
but the axial bond length [2.083(5) Å] in T4 is shorter than
that [2.128(1) Å] in T3. The displacement of the Zn ion
atoms are omitted for clarity.
The crystal structures of ZnII(OPP) complexes with 3-
from the porphyrin mean plane is larger in T3 than that in AP (O3) and 4-AP (O4) are depicted in Figures 3 and 4,
T4. In the crystal of T3, the amino group of 3-AP exhibits respectively. To our surprise, they exhibit totally different
intermolecular NH–π interaction[14] with the meso position crystal structures. In sharp contrast to other crystal struc-
Table 1. Selected bond lengths [Å] and displacements [Å] for complexes T3, T4, O3, O4, D3, and D4.
T3
T4
O3
O4
D3
D4
Zn1–N1
Zn1–N2
Zn1–N3
Zn1–N4
Zn1–N(axial)
Zn displacement
2.080(1)
2.088(1)
2.080(1)
2.071(1)
2.128(1)
0.4453(3)
2.083(4)
2.079(4)
2.098(4)
2.065(4)
2.083(5)
0.364
2.122(2)
2.018(2)
2.133(2)
2.025(2)
2.246(2)
0.327
2.034(3)
2.085(3)
2.077(3)
2.087(3)
2.077(3)
2.127(3)
0.509
2.076(2)
2.063(3)
2.111(3)
2.070(3)
2.067(3)
0.471
2.166(4)
[a]
[a]
2.131(8)
0.314
[a] These values are unavailable owing to the symmetry of the molecule.
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(Porphyrinato)zinc(II) Complexes with Aminopyridines as Axial Ligands
tures, the crystal structure of O3 was revealed to involve
the axial ligation of 3-AP at the amino nitrogen atom rather
than the pyridine nitrogen atom, as shown in Figure 3(top).
The coordination of the amino nitrogen atom in aminopyr-
idine derivatives is unprecedented, allowing us to access a
very unique structural motif of the pyridine-bound (por-
phyrinato)ZnII complex. The 3-AP ligand in O3 exhibits
intramolecular π–π interaction with the pyrrole ring includ-
ing N1 and displays interatomic distances of 3.21, 3.37, and
3.63 Å. The dihedral angle between the pyridine plane of 3-
AP and the pyrrole ring including N1 is estimated to be
22.80° and the intercentroid distance of those two aromatic
rings is estimated to be 3.678 Å. In the case of O4, 4-AP
coordinates to the Zn center through the aromatic pyridine
nitrogen atom. In the crystal, the opposite side of the
Zn(OPP) moiety to the 4-AP binding site is occupied by
two chloroform molecules of crystallization as shown in
Figure 4. The molecular orientation of O4 in the crystal is
disordered to give rise to two apparent molecules of 4-AP
with a population of 0.5 each.
Figure 4. ORTEP drawing of O4·2CHCl3 with selected numbering
scheme with thermal ellipsoids at 50% probability. All hydrogen
atoms are omitted for clarity.
The synthesis of D3 was somewhat different from those
of the other complexes because of its low solubility in
CH2Cl2. Crystal structures of the Zn(DPP) complexes with
3-AP (D3) and 4-AP (D4) are presented in Figures 5 and 6,
respectively. In each case, the DPP ligand exhibits severe
saddle distortion, which is typical for this ligand. In both
cases, the aminopyridine ligands coordinate to the ZnII cen-
ters through the aromatic nitrogen atoms. In the crystal of
D4, the amino group of the 4-AP ligand forms NH–π inter-
actions with pyrrole nitrogen atoms of an adjacent mole-
cule, showing interatomic distances of 3.05 and 3.45 Å from
the opposite side of the 4-AP coordination site relative to
the porphyrin plane. In contrast, no such intermolecular
interactions are recognized for the amino groups of D3 in
the crystal.
Figure 3. (top) ORTEP drawing of O3 with selected numbering
scheme with thermal ellipsoids at 50% probability. All hydrogen
atoms are omitted for clarity. (bottom) Dimeric structure in the
crystal with NH–π interactions (dotted white lines). All hydrogen
atoms, except amino hydrogen atoms, are omitted for clarity.
In the crystals of both O3 and O4, the amino group of
the 3- or 4-aminopyridine ligands is wedged into the cleft
made by two β-phenyl groups attached to one of the pyrrole
rings of an adjacent molecule to form intermolecular NH–
π interactions.[14] Such interaction found in the crystal of
O3 is demonstrated in Figure 3 (bottom) as dotted lines. As
can be seen in Figure 3 (bottom), O3 forms a dimeric unit
by noncovalent interactions. The interatomic distances
(N···C) are 3.52 and 3.35 Å for O3 and 3.34 Å and 3.39 Å
for O4.
Figure 5. ORTEP drawing of D3 with selected numbering scheme
with thermal ellipsoids at 50% probability. All hydrogen atoms are
omitted for clarity.
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FULL PAPER
Figure 6. ORTEP drawing of D4 with selected numbering scheme
with thermal ellipsoids at 50% probability. All hydrogen atoms are
omitted for clarity.
As a general feature of the structures presented above,
the bonds of the axial ligand for 3-AP are longer than those
for 4-AP, as listed in Table 1. This difference results from
the stronger basicity of the pyridine nitrogen atom in 4-AP
than that in 3-AP [pKa(3-AP) = 6.04 and pKa(4-AP) = 9.11
at 25 °C].[15] As for the porphyrinato ligands, the order of
the bond lengths for the axial ligands is DPP Ͻ TPP Ͻ
OPP. This result is consistent with the order of the displace-
Figure 7. Displacements of atoms from the mean porphyrinato
ment of the Zn2+ ion from the porphyrinato mean plane. plane (in units of 0.01 Å).
The displacement of the Zn2+ ion is the largest in D3
(0.509 Å) and the smallest in O4 (0.314 Å). Those values
Coordination of Aminopyridine Ligands in CH2Cl2
To examine the structures of the complexes in solution,
listed in Table 1 are larger than that observed in
[Zn(CH3CN)(DPP)] [0.233(1) Å].[16] As the tendency of the
ZnII displacements, coordination of 3-AP causes larger dis- we measured variable-temperature 1H NMR spectra in
placement than that of 4-AP.
CD2Cl2. However, the solubility of D3 was poor and did
The crystal structure determination described above al- not allow us to obtain its 1H NMR spectrum. The (porphyr-
lowed us to estimate the distortion of the porphyrinato inato)Zn complexes of 3-AP and 4-AP exhibit the same
rings in the complexes. The displacements of the 24 atoms spectral pattern for signals due to the porphyrinato rings in
of the porphyrinato core from the mean porphyrinato plane all cases, indicating that the axial ligands undergo a fast
of each complex are given in Figure 7. In the cases of T3 binding/dissociation equilibrium on the NMR timescale at
and T4, the TPP ligand exhibit slight saddle distortion as room temperature. By lowering the temperature to –90 °C,
represented by the signs of displacements at the β-positions we could observe another set of signals, which could be
of the pyrrole rings: Two of those trans to each other show assigned to the ligated species. The variable-temperature 1H
the same directions of displacements. The DPP complexes NMR spectra of T4 are shown in Figure 8 as a typical ex-
D3 and D4 exhibit larger saddle distortion than that found ample. As the temperature is lowered, signals due to coordi-
in the TPP complexes, and the saddle distortion is much nated 4-AP sharpen and are observed clearly. A doublet
larger than that observed in [Zn(CH3CN)(DPP)].[16] In the assigned to the 2-H and the 6-H atoms emerge at δ =
case of the OPP ligand, O3 and O4 show smaller distortion 1.84 ppm (–90 °C) with a large upfield shift caused by
than that found in the TPP complexes and different distor- strong shielding derived from the porphyrinato π-electron
tions from each other: The OPP ligand is almost planar but ring current, indicating the coordination of the pyridine ni-
slightly saddle-distorted in O3; the OPP ligand in O4 exhib- trogen atom in 4-AP. Another doublet ascribed to the 3-H
its a slight wave distortion. We also found a similar extent and the 5-H atoms is observed at δ = 4.58 ppm (–90 °C)
of such wave distortion in the crystal structure of with less shielding by the porphyrin ring current. In ad-
[Cr(OPP)(piperidine)2]ClO4.[17]
dition, a broad singlet at δ = 3.42 ppm (–90 °C) is assigned
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(Porphyrinato)zinc(II) Complexes with Aminopyridines as Axial Ligands
to the NH protons of the amino group of 4-AP. In the case
As an overview, the binding constants are in the order
of O3, its NMR spectrum at –90 °C exhibits a singlet as- aniline Ͻ pyridine Ͻ 3-AP Ͻ 4-AP for the axial ligands
signed to the 2-H atom at δ = 1.70 ppm and a doublet due examined and TPP2– Ͻ OPP2– Ͻ DPP2– for the porphyr-
to the 6-H atom at δ = 1.91 ppm with large upfield shift. inato ligands. The order of the binding constants de-
This result indicates that 3-AP coordinates in solution pending on the axial ligands is consistent with that of their
through the pyridine nitrogen atom rather than the amino pKa values: aniline (4.87) Ͻ pyridine (5.23) Ͻ 3-AP (6.04)
nitrogen atom, which is in sharp contrast to its crystal Ͻ 4-AP (9.11), at 25 °C.[13] Linear correlations of the
structure depicted in Figure 3.
logKbind values relative to the pKa values were observed as
shown in Figure 10. A similar tendency has been reported
for Zn(TPP) complexes.[15] The slope of the plot (+0.236)
for the Zn(TPP) complexes is consistent with that reported
previously,[18] but the gradients for the Zn(OPP) (+0.314)
and Zn(DPP) (+0.268) complexes are different from that of
Zn(TPP) complexes. Although the theoretical background
of the gradients of those relationships has not yet been es-
tablished, our results indicate that the slope is not necessar-
ily the same but that it depends on the porphyrin involved.
Figure 8. Variable-temperature 1H NMR spectra for T4 in CD2Cl2.
To determine binding constants (Kbind) of aniline, pyr-
idine, 3-AP, and 4-AP to (porphyrinato)ZnII complexes, we
conducted spectroscopic titrations in CH2Cl2 at a concen-
tration of the (porphyrinato)Zn complexes of 3.0ϫ10–5

at room temperature. The spectral change in the course of
the spectroscopic titration is depicted in Figure 9 for D4 as
a representative example. In all cases, the axial coordination
of the ligands induced redshifts in the Soret and Q bands
in the absorption spectra. The binding constants obtained
are summarized in Table 2.
Figure 10. Relationship between the logarithm of binding constants
of pyridine derivatives and their pKa values (pyridine, 5.23; 3-AP,
6.04; 4-AP, 9.11): Zn(TPP) (᭿), Zn(OPP) (᭡), and Zn(DPP) (᭹).
We performed DFT calculations to clarify the influence
of the distortion of the porphyrinato ligands on the binding
constants. The structure optimizations were made on four-
coordinate Zn(TPP), Zn(OPP), and Zn(DPP) structures
based on the crystal structures of [Zn(4-AP)(TPP)], [Zn(4-
AP)(OPP)], and [Zn(CH3CN)(DPP)]. The degrees of distor-
tion of the porphyrinato ligands were estimated by root-
mean-square out-of-plane displacement (∆RMS), which is
defined by Equation (1):[19]
(1)
Figure 9. UV/Vis spectral change of Zn(DPP) (3.0ϫ10–5 ) upon
addition of various concentrations of 4-AP (0, 6ϫ10–6, 1.2ϫ10–5,
1.8ϫ10–5, 2.4ϫ10–5, 3.0ϫ10–5, 3.6ϫ10–5, 4.2ϫ10–5, 4.8ϫ10–5,
5.4ϫ10–5, and 6.0ϫ10–5 ) in CH2Cl2 at room temperature. Inset:
absorption changes at 680 nm.
where δi is the orthogonal displacement of atom i in the
macrocycle from the mean plane including all 24 atoms of
the porphyrinato macrocycle. As can be seen in Figure 11,
Table 2. Binding constants of various axial ligands to (porphyrinato)ZnII complexes.[a]
∆RMS [Å]
Aniline
Pyridine
3-AP
4-AP
Zn(TPP)
Zn(OPP)
Zn(DPP)
0.06
0.44
0.57
(1.6Ϯ0.1)ϫ102
(1.9Ϯ0.5)ϫ102
(3.2Ϯ0.4)ϫ102
(1.1Ϯ0.1)ϫ104
(1.4Ϯ0.2)ϫ104
(4.6Ϯ0.2)ϫ104
(1.8Ϯ0.1)ϫ104
(3.2Ϯ0.5)ϫ104
(1.1Ϯ0.0)ϫ105
(9.2Ϯ0.4)ϫ104
(2.5Ϯ0.2)ϫ105
(6.2Ϯ0.0)ϫ105
[a] Measured in CH2Cl2 at room temperature. [(porphyrinato)Zn] = 3.0ϫ10–5 .
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T. Kojima, T. Nakanishi, T. Honda, R. Harada, M. Shiro, S. Fukuzumi
FULL PAPER
zinc acetate. Other chemicals were purchased from commercial
sources and used without further purification.
the binding constants are proportional to the ∆RMS values
for pyridine, 3-AP, and 4-AP in every porphyrinato system
employed here. These results indicate that the degree of por-
phyrin distortion can regulate the strength of the axial coor-
dination. The ring distortion should enforce lone pairs of
nitrogen atoms in the pyrrole moieties to direct out of
plane, and this ill-direction of the lone pairs can weaken the
interaction between the ZnII center and the nitrogen lone
pairs to enhance the Lewis acidity of the ZnII center. Thus,
the more the porphyrinato ring is distorted as reflected in
a larger ∆RMS value, the stronger the Lewis acidity of the
ZnII center becomes.
Zn(OPP): H2OPP (100 mg, 0.109 mmol) and Zn(AcO)2·2H2O
(239 mg, 1.09 mmol) were heated at reflux in CHCl3/MeOH (5:3,
v/v; 80 mL) for 2 h. The solvents were evaporated under reduced
pressure. The residue was dissolved in a minimum amount of
CHCl3 and chromatographed on a silica gel column by using
CHCl3 as the eluent. The first red fraction was collected, and the
solvent was evaporated. The product was recrystallized from
CHCl3/hexane to afford a purple crystalline solid. Yield: 0.104 g
(97%). 1H NMR (270 MHz, CDCl3, 298 K): δ = 8.50 (s, 4 H, β-
H), 7.76 (d, 8 H, meso-phenyl o-H), 7.20 (m, 12 H, meso-phenyl
m,p-H), 7.03 (d, 8 H, β-phenyl o-H), 6.89 (m, 12 H, β-phenyl m-
and p-H) ppm. UV/Vis (CH2Cl2): λmax = 430, 558 nm.
[Zn(3-AP)(TPP)] (T3): Zn(TPP) (20 mg, 0.030 mmol) and 3-ami-
nopyridine (3.4 mg, 0.036 mmol) were dissolved in CHCl3/hexane
(2:3, v/v; 15 mL). A purple crystalline solid was obtained by slow
concentration of the solution. Yield: 21 mg (91%).
C49H34N6Zn·0.5H2O (781.25): calcd. C 75.33, H 4.51, N 10.75;
found C 75.20, H 4.48, N 10.79. 1H NMR (300 MHz, CD2Cl2,
233 K): δ = 8.86 (s, 8 H, β-H), 8.17 (s, 8 H, phenyl o-H), 7.73 (m,
12 H, phenyl m-, p-H), 5.69 (d, J = 8 Hz 1 H, pyridyl 4-H), 2.54
(s, 2 H, NH2), 1.72, 1.68 (d, J = 11 Hz 2 H, 6-H and 2-H) ppm.
The signal of the pyridyl 5-H proton overlapped with the residual
solvent peak at δ = 5.32 ppm. UV/Vis (CH2Cl2): λmax = 428, 563,
603 nm.
Figure 11. Correlations between the logarithm of binding constants
of pyridine derivatives and root-mean-square out-of-plane dis-
placements of porphyrin ligands [Zn(TPP), 0.06; Zn(OPP), 0.44;
Zn(DPP), 0.57]: pyridine (᭿), 3-AP (᭡), and 4-AP (᭹).
[Zn(4-AP)(TPP)] (T4): Zn(TPP) (20 mg,0.030 mmol) and 4-amino-
pyridine (3.4 mg, 0.036 mmol) were dissolved in CHCl3/hexane
(2:3, v/v; 15 mL). A purple crystalline solid was obtained by slow
concentration of the solution. Yield: 23 mg (98%).
C49H34N6Zn·0.5CHCl3·1.5H2O (858.95): calcd. C 69.21, H 4.40, N
9.78; found C 68.99, H 4.09, N 9.69. 1H NMR (300 MHz, CD2Cl2,
233 K): δ = 8.85 (s, 8 H, β-H), 8.17 (d, J = 7 Hz 8 H, phenyl o-H),
7.74 (m, 12 H, phenyl m-, p-H), 4.63 (d, J = 7 Hz, 2 H, pyridyl m-
H), 3.48 (s, 2 H, NH), 1.91 (d, J = 6 Hz, 2 H, pyridyl o-H) ppm.
UV/Vis (CH2Cl2): λmax = 428, 564, 604 nm.
Conclusions
We have determined a series of crystal structures of (por-
phyrinato)ZnII complexes with aminopyridine isomers and
demonstrated a novel coordination mode of 3-aminopyr-
idine involving coordination of the amino nitrogen atom
rather than the more σ-donating pyridine nitrogen atom.
The ZnII complexes of TPP2–, OPP2–, and DPP2– showed
different degrees of distortion of the porphyrinato rings.
Among them, the DPP2– ligand exhibited the largest non-
planarity as a saddle distortion and the largest displace-
ment of the Zn2+ ion from the porphyrinato mean plane.
In solution, NMR measurements revealed that all the ami-
nopyridine derivatives underwent coordination of the aro-
matic nitrogen atom in all porphyrinato systems at room
temperature. The strength of binding of these ligands is reg-
ulated not only by the basicity of the pyridine nitrogen
atom to control the ability of σ-donation but also the dis-
[Zn(3-AP)(OPP)] (O3): Zn(OPP) (30 mg, 0.030 mmol) and 3-ami-
nopyridine (3.4 mg, 0.036 mmol) were dissolved in CHCl3/hexane
(1:1, v/v; 20 mL). A purple crystalline solid was obtained by slow
concentration of the solution. Yield: 27 mg (84%).
C73H50N6Zn·2H2O (1112.66): calcd. C 78.80, H 4.89, N 7.55; found
1
C 78.65, H 4.92, N 7.81. H NMR (300 MHz, CD2Cl2, 233 K): δ
= 8.42 (s, 4 H, β-H), 7.78 (d, J = 12 Hz, 8 H, meso-phenyl o-H),
7.19 (m, 12 H, meso-phenyl m-, p-H), 6.96 (d, J = 5 Hz, 8 H, β-
phenyl o-H), 6.85 (m, 12 H, β-phenyl m-, p-H), 5.76 (d, J = 8 Hz,
1 H, pyridyl 4-H), 5.48 (t, J = 7 Hz, 1 H, pyridyl 5-H), 2.26 (s, 2
H, NH), 1.99 (d, 1 H, pyridyl 6-H), 1.84 (s, 1 H, pyridyl 2-H) ppm.
UV/Vis (CH2Cl2): λmax = 443, 575, 626 nm.
[Zn(4-AP)(OPP)] (O4): Zn(OPP) (30 mg, 0.030 mmol) and 4-ami-
nopyridine (3.4 mg, 0.036 mmol) were dissolved in CHCl3/hexane
tortion of the porphyrinato ligands to regulate the Lewis (1:1, v/v; 20 mL). A purple crystalline solid was obtained by slow
concentration of the solution. Yield: 31 mg (97%).
C73H50N6Zn·1.5CHCl3·2.5H2O (1300.74): calcd. C 68.79, H 4.38,
N 6.46; found C 68.78, H 4.23, N 6.43. 1H NMR (300 MHz,
CD2Cl2, 233 K): δ = 8.42 (s, 4 H, β-H), 7.80 (ABq, JAB = 15 Hz,
8 H, meso-phenyl o-H), 7.19 (m, 12 H, meso-phenyl m-, p-H), 7.05
(d, J = 5 Hz, 8 H, β-phenyl o-H), 6.84 (m, 12 H, β-phenyl m,p-H),
4.82 (d, J = 6 Hz, 2 H, pyridyl m-H), 3.63 (s, 2 H, NH), 2.15 (d, J
= 6 Hz, 2 H, pyridyl o-H) ppm. UV/Vis (CH2Cl2): λmax = 443, 576,
626 nm.
acidity of the metal center.
Experimental Section
Materials: H2OPP was prepared by a cross-coupling reaction be-
tween phenylboronic acid and 2,3,12,13-tetrabromo-5,10,15,20-tet-
raphenylporphyrin according to the procedures described by Chan
et al.[12] Zn(TPP) was synthesized by the reaction of H2TPP with
732
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 727–734

(Porphyrinato)zinc(II) Complexes with Aminopyridines as Axial Ligands
Table 3. Crystallographic data for complexes T3, T4, O3, O4, D3, and D4.
T3
T4
O3
O4
D3
D4
Empirical formula
Formula mass
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C49H34N6Zn
772.23
monoclinic
P21/n
14.5281(17)
17.209(2)
14.7967(18)
90
93.770(6)
90
C50H35Cl3N6Zn C73H50N6Zn
C75H52Cl6N6Zn C97H66N6Zn
C92H64N6Zn
1318.86
monoclinic
P21/a
18.198(3),
28.373(5),
18.424(3)
90
110.855(3)
90
8889(3)
4
891.61
orthorhombic
P212121
9.690(3)
17.667(5)
25.200(3)
90
1076.62
monoclinic
P21/c
17.573(1)
12.9073(6)
24.716(2)
90
1315.37
monoclinic
C2/m
1380.93
triclinic
P1
¯
13.6586(16)
20.8378(14)
11.0276(9)
90
8.4808(19)
17.858(4)
17.858(4)
83.901(5)
81.562(5)
76.306(5)
4172.8(17)
2
α [°]
β [°]
90
90
106.200(3)
90
100.287(4)
90
γ [°]
V [Å3]
3691.5(8)
4
4314.0(19)
4
5383.5(6)
4
3088.2(5)
2
Z
T [K]
133
1.389
0.711
133
1.373
0.798
183.1
1.328
0.509
123.1
1.414
0.708
123.1
1.099
0.342
123.1
0.985
0.318
Dcalcd. [gcm–1]
µ [mm–1]
Total reflections
Unique reflections
Observed reflections [IϾ2σ(I)]
Parameters
Final R indices[a]
53232
8441
7318
506
R1 = 0.0310
wR2 = 0.0872
43114
9874
8057
554
R1 = 0.0875
wR2 = 0.2193
1.55/–1.13
1.032
12236
12198
9424
721
R1 = 0.0579
wR2 = 0.1799
1.52/–0.57
1.000
10272
2859
2766
250
R1 = 0.0721
wR2 = 0.1817
0.87/–0.58
1.087
32971
17072
13466
1104
R1 = 0.0624
wR2 = 0.1949
0.93/–0.53
1.171
84930
20120
15222
939
R1 = 0.0836[b]
wR2 = 0.2298[b]
1.27/–0.59
1.083
Largest difference peak/hole [e Å–3] 0.38/–0.36
Goodness of fit on F2
1.114
[a] R1 for IϾ2σ(I) and wR2 for all data. [b] The SQUEEZE program was applied.
[Zn(3-AP)(DPP)] (D3): To
a
solution of [Zn(DPP)] (30 mg,
fractometer with monochromated Mo-Kα (λ = 0.7107 Å) radiation
at –140 °C up to 2θmax = 55.0°. Diffraction data for O3–D4 were
obtained with a Rigaku Mercury CCD area detector with mono-
chromated Mo-Kα (λ = 0.7107 Å) radiation at –90 °C up to 2θmax
= 55.0°. In the case of O4, the data used for the structure refine-
ment was limited to 2θmax = 51.0° because of the quality of the
crystal. The structures were solved by direct methods and expanded
by using Fourier techniques. All non-hydrogen atoms were refined
anisotropically. Refinement was carried out with full-matrix least
squares on F including anomalous dispersion effects. All calcula-
tions were performed by using the Crystal Structure crystallo-
graphic software package,[21] and structure refinements were made
by using SHELX-97.[22] Crystallographic data are summarized in
Table 3. For the structure refinement for D4, the SQUEEZE pro-
gram[23] was applied to improve the refinement by subtracting se-
verely disordered solvent molecules of crystallization (CH2Cl2 and
n-hexane) from the diffraction pattern. CCDC-706362 (T3),
-706363 (T4), -706364 (O3), -706365 (O4), -706366 (D3), and
-706367 (D4) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
0.023 mmol) in CH2Cl2 (2 mL), was added a solution of 3-amino-
pyridine (7.5 mg, 0.080 mmol) in CH3CN (1 mL) to obtain a two-
layered solution. In the course of the diffusion of the CH3CN solu-
tion, single crystals of D3 were obtained. Yield: 22 mg (82%).
C97H66N6Zn·3H2O (1435.07): calcd. C 81.19, H 5.06, N 5.86; found
C 81.40, H 5.01, N 5.55. UV/Vis (CH2Cl2): λmax = 478, 619,
675 nm.
[Zn(DPP)(4-AP)] (D4): To
a solution of [Zn(DPP)] (30 mg,
0.023 mmol) in toluene (2 mL), was added a solution of 4-amino-
pyridine (7.5 mg, 0.080 mmol) in CH3OH (0.3 mL). Vapor dif-
fusion of hexane into the mixed solution gave single crystals of D4.
Yield: 20 mg (73%). C97H66N6Zn·3H2O (1435.07): C 81.19, H 5.06,
N 5.86; found C 81.20, H 5.12, N 6.02. UV/Vis (CH2Cl2): λmax
478, 621, 676 nm.
=
Physical Measurements: Absorption spectra were recorded with a
Jasco V-570 spectrophotometer in CH2Cl2 in the range of 200–
800 nm at room temperature. 1H NMR spectra in CD2Cl2 were
measured with JEOL GX-400 (400 MHz), EX-270 (270 MHz), and
A-300 (300 MHz) spectrometers. Variable-temperature NMR spec-
tra were obtained with a JEOL A-300 spectrometer.
Electrochemical Measurements: Cyclic voltammograms were mea-
sured in CH2Cl2 (0.1  [nBu4N]PF6 as an electrolyte) at room tem-
perature with a BAS model ALS-720 potentiostat by using a plati-
num wire as a counter electrode, a glassy carbon electrode as a
working electrode, and Ag/AgNO3 as a reference electrode.
Acknowledgments
This work was supported by Grants-in-Aid (Nos. 18033033,
19205019, and 19750034), and a Global COE program, “the
Global Education and Research Center for Bio-Environmental
Chemistry” from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan, is gratefully acknowledged. T. N. was
supported by a JSPS predoctoral fellowship (No. A208410), for
which thanks are due. We also appreciate support from the Japan
Science and Technology Agency (JST) through the “Potentiality
verification stage” program.
DFT Calculations: All DFT calculations were performed at the
B3LYP/6-31G(d) level of theory with use of Gaussian 03[20] on a
32-processor QuantumCubeTM. Initial structures of four-coordi-
nate [Zn(TPP)], [Zn(OPP)], and [Zn(DPP)] were constructed on the
basis of the crystal structures of T4, O4, and [Zn(CH3CN)(DPP)].
X-ray Crystallography: Single crystals were mounted on glass capil-
laries with silicon grease. Diffraction data for T3 and T4 were col-
lected with
a Rigaku R-AXIS RAPID Imaging Plate dif-
Eur. J. Inorg. Chem. 2009, 727–734
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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733

T. Kojima, T. Nakanishi, T. Honda, R. Harada, M. Shiro, S. Fukuzumi
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Received: October 22, 2008
Published Online: January 15, 2009
734
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Eur. J. Inorg. Chem. 2009, 727–734