Synthesis and Structural Determination of a Porphyrinatoplatinum(II): Meso-Tetrakis(4-t-butylphenyl)porphyrinatoplatinum(II)

Article abstract of DOI:10.1246/bcsj.76.2123

A substituted porphyrinatoplatinum(II) meso-tetrakis(4-t-butylphenyl)porphyrinatoplatinum(II), has been spectroscopically and structurally characterized. The ¹HNMR signal from the β-protons of the pyrrole ring is observed as a singlet with a full-width at half maximum of 1.6 Hz, contrary to the reported quasi-triplet resonance attributable to the ¹H-¹⁹⁵Pt long-range coupling. The porphyrin nucleus is nearly planar in the solid state. All macrocycle atoms lie within approximately 0.06 A of the least squares mean plane with a pseudo-wave core conformation. The intramolecular Pt-N distances are 2.019(3) A.

Full text of DOI:10.1246/bcsj.76.2123

Ó 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 2123–2127 (2003) 2123
Synthesis and Structural Determination of a Porphyrinatoplatinum(b):
meso-Tetrakis(4-t-butylphenyl)porphyrinatoplatinum(b)
#
Masamichi Umemiya, Ken-ichi Sugiura, Hitoshi Miyasaka,##
ꢀ;
Tomohiko Ishii, and Masahiro Yamashita
#
ꢀ;
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University,
1-1 Minami-Ohsawa, Hachi-Oji, Tokyo 192-0397
Received May 19, 2003; E-mail: sugiura@porphyrin.jp
A substituted porphyrinatoplatinum(b), meso-tetrakis(4-t-butylphenyl)porphyrinatoplatinum(b), has been spectro-
1
scopically and structurally characterized. The H NMR signal from the ꢀ-protons of the pyrrole ring is observed as a
singlet with a full-width at half maximum of 1.6 Hz, contrary to the reported quasi-triplet resonance attributable to the
1H–195Pt long-range coupling. The porphyrin nucleus is nearly planar in the solid state. All macrocycle atoms lie within
ꢀ
approximately 0.06 A of the least squares mean plane with a pseudo-wave core conformation. The intramolecular Pt–N
ꢀ
distances are 2.019(3) A.
Platinum(b) porphyrin (PtbPor) has been attracting a lot of
attention from the viewpoint of coordination chemistry.1 The
large metal-to-porphyrin ꢁ-bond confers specific properties
to this metal complex,2,3 e.g., the electrophilic behavior of
the porphyrinato ligand1,4 and the metal-centered redox
reaction.1,5 Additionally, a closed-shell low-spin d8-configura-
tion and a large spin–orbit interaction associated with the large
atomic weight of platinum produce red phosphorescence at
room temperature at approximately 630 nm.6 This specific
character has led to the discovery of the electroluminescent
(EL) properties of 2,3,7,8,12,13,17,18-octaethylporphyrinato-
platinum(b) (PtbOEP) by two independent groups,7 Forrest
and co-workers in 19988 and Friend and co-workers in 1999.9
Combined with green (G) and blue (B) light emitting dyes,
the red (R) light of PtbPor is expected to be used in full-color
RGB EL displays.7d Furthermore, the long triplet life time
and the luminescence from this spin state make PtbPor com-
pounds suitable for use as triplet sensitizers10 and as oxygen
sensors.11 These findings confirm that PtbPor is a key compo-
nent molecule for contemporary interdisciplinary advanced
material chemistry.
by methanogenic organisms.12 Presently, it is postulated for
this specific redox reaction that a small ionic radius of divalent
four-square-coordinated nickel(b) induces the Ni(a)/Ni(b) re-
dox couple accompanying the large conformational distortion
of the ligand.13
To understand the unique functions of PtbPor described
above, a discussion based on the structure of PtbPor seems
essential. However, to our surprise, PtbPors with well-charac-
terized structures are limited only to meso-tetraphenylpor-
phyrinatoplatinum(b) (PtbTPP)14 and PtbOEP15 (Chart 1). Be-
cause the planarity of the porphyrin nucleus of these two com-
plexes was reported to be ruffled for the former and planar for
the latter, the nature of the PtbPor complex remains unclear and
confusing. With this information in mind, we performed
a structural determination of a platinum(b) porphyrin deriva-
tive, meso-tetrakis(4-t-butylphenyl)porphyrinatoplatinum(b)
(PtT4tBuPP), the corresponding free base (H2T4tBuPP), and
the metal complexes of which are widely used16–22 because
of their enhanced solubilities in usual organic solvents, attribut-
able to the four t-butyl groups. In this study, our goal is to in-
vestigate this class of materials further.
In addition, it should be interesting to discuss the relation-
ship between the distortion of the porphyrin nucleus and the na-
ture of the incorporated metal, particularly group-10 metals
such as nickel, palladium, and platinum. Studies on the corre-
sponding nickel(b) complexes have been conducted in order to
mimic and/or to elucidate the function of Factor-430, a key
component of the active site of methylcoenzyme M-reductase,
which catalyzes the final chemical step of methane formation
Experimental
General. Pyrrole (TCI), 4-t-butylbenzaldehyde (TCI), pro-
pionic acid (Kishida Chemical Co., Ltd.), and platinum(b) chloride
(Kojima Chemical Co., Ltd.; purity >99%) were used as received.
Infrared spectra were recorded on a Shimadzu FTIR-8400 instru-
ment using the KBr pellet technique in the range of 650 to 4000
cmꢁ1. Absorption spectra were recorded on a Hitachi U-3500. La-
ser desorption/ionization time-of-flight mass spectra were record-
ed on a Kratos Analytical KOMPACT PROBE. Thermal proper-
ties were studied on a Rigaku TG-8120 thermogravimetric
analyzer. TGA was performed using freshly prepared PhCl solvat-
ed crystal filtered from PhCl and dried under ambient conditions
for 30 min. Samples were placed in an aluminum pan and heated
to 450 ꢂC at 15 ꢂC minꢁ1 under a continuous flow of nitrogen (10
mL minꢁ1).
# CREST, Japan Science and Technology Corporation (JST), Ka-
waguchi Center Building, 4-1-8, Honcho, Kawaguchi, Saitama
332-0012
## PREST, Japan Science and Technology Corporation (JST), Ka-
waguchi Center Building, 4-1-8, Honcho, Kawaguchi, Saitama
332-0012
Published on the web November 15, 2003; DOI 10.1246/bcsj.76.2123

2124 Bull. Chem. Soc. Jpn., 76, No. 11 (2003)
Porphyrinatoplatinum(b)
Table 1. Crystal Data and Structure Refinement Parameters
for [PtbT4tBuPP] PhCl
ꢃ
Identification code
[PtbT4tBuPP] PhCl
ꢃ
Formula
C66H65ClN4Pt
1144.81
182
monoclinic
P21=n (#14)
22.603(5)
9.679(2)
24.084(4)
91.80(2)
5266(1)
Formula weight/g molꢁ1
Temperature/K
Crystal system
Space group
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
ꢀ/ꢂ
Cell volume/A
ꢀ 3
Z
4
Crystal size/mm
Dcalc/Mg mꢁ3
0:50 ꢄ 0:33 ꢄ 0:08
1.444
0.71069
ꢀ
Wavelength/A
Absorption coefficient/mmꢁ1 27.51
R1
R1 for all data
Crystallization solvent
0.036
0.060
Chlorobenzene/MeOH
ꢀ
Mo-Kꢄ radiation (ꢃ ¼ 0:71070 A, 50 kV, 200 mA) equipped with
a Rigaku low-temperature device. The data were collected using
the !–2ꢅ scan technique to a maximum 2ꢅ value of 55ꢂ. Neutral
atom scattering factors were taken from Cromer and Waber.25
26
Anomalous dispersion effects were included in Fcalc
;
the values
for ꢁf0 and ꢁf00 were those of Creagh and McAuley.27 The val-
ues for the mass attenuation coefficient are those of Creagh and
Hubbel.28 All calculations were performed using the teXsan29
crystallographic software package of Molecular Structure Corpora-
tion.
Chart 1.
Results and Discussion
meso-Tetrakis(4-t-butylphenyl)porphyrin (H2T4tBuPP).
This free base23 was prepared according to the reported method24
using pyrrole, 4-t-butylbenzaldehyde, and propionic acid.
1H NMR (CDCl3) ꢂ 8.87 (s, 8H), 8.15 (d, J ¼ 8:2 Hz, 8H), 7.76
(d, J ¼ 8:2 Hz, 8H), 1.61 (s, 36H), and ꢁ2:75 (bs, 2H). IR
(KBr) 3315, 1475, 993, 982, 968, and 802 cmꢁ1. UV–vis (CHCl3)
ꢃ (log ") 421 (5.54), 516 (4.16), 551 (3.92), 593 (3.66), and 651
(3.63) nm.
meso-Tetrakis(4-t-butylphenyl)porphyrinatoplatinum(b)
(PtbT4tBuPP). 515 mg of H2T4tBuPP (0.61 mmol) and 816 mg of
PtCl2 (3.1 mmol, 5 eq.) were dissolved in 30 mL of freshly distilled
benzonitrile. The mixture was purged with nitrogen and refluxed
for 2 h. The solvent was removed under reduced pressure. The res-
idue was extracted with CHCl3 and filtered through a Celite1 pad.
The filtrate was concentrated to approximately 20 mL and subject-
ed to silica-gel chromatography eluted by CHCl3. The most rapid-
ly eluted orange band was collected. The solvent was concentrated
to about 50 mL under reduced pressure and 50 mL of MeOH was
added to obtaine 627 mg of the product (quant.). 1H NMR (CDCl3)
ꢂ 8.78 (s, 8H), 8.07 (d, J ¼ 8:2 Hz, 8H), 7.73 (d, J ¼ 8:2 Hz, 8H),
and 1.59 (s, 36H). IR (KBr) 1369, 1015, 814, and 797 cmꢁ1. UV–
vis (CHCl3) ꢃ (log ") 404 (5.41), 510 (4.39), and 538 (3.66) nm.
X-ray Experiment. A single crystal of PtbT4tBuPP suitable
for diffraction study was isolated from chlorobenzene–MeOH
mixed solvent. The crystal data of the complexes are summarized
in Table 1. The temperature was calibrated with an Anritsu HFT-
50 thermometer. Data for the complex was collected on a Rigaku
AFC7R four-circled diffractometer with graphite monochromated
Synthesis. The free base porphyrin, H2T4tBuPP,23 was pre-
pared under the usual Adler–Long reaction conditions using
commercially available 4-t-butylbenzaldehyde.24 The metalla-
tion reaction was performed by using platinum(b) chloride
(PtbCl2) in boiling benzonitrile30 to give orange PtbT4tBuPP
in quantitative yield. Laser desorption time-of-flight mass
spectroscopic data clearly support the obtained compound to
be PtbT4tBuPP.
It was pointed out by Milgrom that the 1H NMR peak of the
ꢀ-protons of PtbPors is comprised of three peaks with 1:4:1 rel-
ative intensity, i.e., the central main peak attributable to the
complexes containing 194Pt or 196Pt (both I ¼ 0) is flanked by
two satellite peaks about 6–8 Hz away on either side of it.31
The satellite peaks are attributable to the long-range H–195Pt
1
(I ¼ 1=2) coupling. However, in our study, the peak attributa-
ble to the ꢀ-protons was observed as a singlet with a full-width
at half maximum of 1.6 Hz (vide infra). Other signals attribut-
able to the attached phenyl groups, 8.07 (ortho-H) and 7.73
(meta-H) ppm, are in the range observed for the TPPs.31
In the IR spectra, the absorption at 3315 cmꢁ1, attributable to
N–H stretching for H2T4tBuPP, was not observed for
PtbT4tBuPP. The well-resolved C–H rocking vibrations of
the pyrrole ring of H2T4tBuPP, 993, 982, and 968 cmꢁ1
,
changed to a strong singlet at 1015 cmꢁ1 after the metallation,
consistent with the previously reported IR spectra of PtTPP.2
The vibration at 1475 cmꢁ1, indicating the –C=N– stretching

M. Umemiya et al.
Bull. Chem. Soc. Jpn., 76, No. 11 (2003) 2125
vibration of the free base, also shifted to 1360 cmꢁ1 after the
metallation.
This PtbPor, in which four phenyl groups are attached per-
pendicularly to the porphyrin nucleus, also acts as a ‘‘porphyrin
sponge’’ by incorporating one chlorobenzene (PhCl) molecule,
as characterized by the single-crystal diffraction study (vide
infra).32 This finding is in marked contrast to that of a structur-
ally similar PtbTPP forming a non-solvated crystal.14 Ther-
mogravimetric analysis (TGA) showed that the weight loss
started at room temperature, indicating that the PhCl is loosely
incorporated in the crystal lattice (vide infra). A steady weight
loss was observed below 153 ꢂC.
Structure. The structure contains one crystallographically
independent porphyrin molecule and one PhCl molecule. The
crystal structure of PtbT4tBuPP is shown in Fig. 1 and some im-
portant structural parameters of the complex are summarized in
Table 2 together with the data of reference complexes including
PtbTPP14 and PtbOEP,15 as well as other typical metallopor-
phyrins incorporating group-10 metals.
Fig. 2. Formal diagram of the macrocycle of PtbT4tBuPP.
ꢀ
The out-of-plane displacements (in units of 0.01 A) of each
atom from the 24-atom mean plane are shown.
One of the most important structural parameters that best
characterizes the complex is the average metal–nitrogen bond
cleus,34 and slightly smaller than that for porphyrinatopalladi-
b
distance (dM{N) of 2.019(3) A for Pt T4 BuPP, which is compa-
t
ꢀ
b
35
ums (2.014 A). The order of the average dM{Ns for d8-metal-
ꢀ
14
ꢀ
rable to that reported for Pt OEP (2.012(3) A), and slightly
ꢀ
ꢀ
ꢀ
loporphyrins, 1.96 A (for Ni) < 2.013 A (for Pt) ꢅ 2.014 A (for
15
larger than that for Pt TPP (2.008(2) A) and the value of
b
ꢀ
Pd), may be related to the order of the mean ionic radius for
8
ꢀ
2.010 A for the ideal reference metalloporphyrin with the least
ꢀ
four square-planar d -metals, i.e., 0.63 A (for Ni) < 0.74 A
ꢀ
strain.33 Furthermore, this distance is much larger than the typ-
36
ꢀ
(for Pt) < 0.78 A (for Pd).
ꢀ
ical value of 1.96 A for nickel derivatives with a planar nu-
From Figs. 1 and 2, it is clear that PtbT4tBuPP is nearly pla-
nar in the solid state. All macrocycle atoms lie within approx-
ꢀ
imately 0.06 A of the least squares mean plane (Fig. 2). Smith
and co-workers classified the distortions of the porphyrin nu-
cleus into four types: saddle, wave, ruffle, and dome.37
PtbT4tBuPP is considered to have the pseudo-wave core con-
formation, in which the line through C(9)–Pt(1)–C(19) acts as
an edge and/or trough of the wave.
A mean plane was fitted to the 24 atoms of the porphyrin nu-
cleus defined according to Schomaker and his co-workers.38
The root-mean-square out-of-plane displacement of the 24-
atom macrocycle (ꢁ)34,39 is 0.027 A. Although this value is
ꢀ
b
ꢀ
larger than that for Pt OEP (0.009 A), a molecule that is closely
packed due to strong ꢁ–ꢁ interactions, PtbT4tBuPP is more
planar than the structurally similar PtbTPP. The deformation
of the latter complex is considered to be a result of packing
forces. It is well known that the deformation of the porphyrin
nucleus decreases the dM{N bond distances of NibPor systems.33
Although only three crystal structures exist, including that of
the present study, a similar tendency is also observed for PtbPor
systems.
The dihedral angles between the porphyrin plane and the aryl
groups are 75.5(1)ꢂ, 87.9(1)ꢂ, 72.6(1)ꢂ, and 86.3(1)ꢂ for Ph-1,
Ph-2, Ph-3, and Ph-4, respectively (Fig. 1). These values are
within the range of those observed for the other TPPs.40
The packing structure of PtbT4tBuPP is shown in Fig. 3.
Each t-butyl group contacts the t-butyl group of the adjacent
PtbT4tBuPP. This attractive van der Waals attractive force pro-
duces the two-dimensional perfect sheet structure in the ac-
plane. In the sheet, molecules pack like a window mill. The
solvent molecule, PhCl, is incorporated in the space between
two molecules. No short atomic contacts exist between PhCl
and PtbT4tBuPP. This is the main reason why the solvent
Fig. 1. ORTEP drawings of PtbT4tBuPP. Thermal ellip-
soids are 50% probability levels.
Table 2. Comparison of Important Structural Parameters of
PtbT4tBuPP with Those of Related Compounds
ꢀ
dM{N/A
ꢀ aÞ
ꢁ/A
Reference
PtbT4tBuPP
PtbTPP
2.019(3)
2.008(2)
2.012(3)
2.009(9)
2.014(3)
1.931(2)
1.958(2)
1.952(8)
1.929(3)
0.027
0.226
0.009
0.226
0.014
0.264
0.018
0.029
0.298
(this work)
14
15
30a
30b
41
42
43
44
PtbOEP
PdbTPP
PdbOEP
NibTPP
NibOEP (triclinic-A)
NibOEP (triclinic-B)
NibOEP (tetragonal)
a) The root-mean-square out-of-plane displacement of the 24
atoms (see Ref. 34 and 39).

2126 Bull. Chem. Soc. Jpn., 76, No. 11 (2003)
Porphyrinatoplatinum(b)
Fig. 3. Top view of the crystal packing of PtbT4tBuPP. Solvent molecules, chlorobenzene, are shown by dark circles.
molecule is weakly incorporated in the crystal, evaporating
even at room temperature. This sheet is stacked in the b-direc-
Raman spectroscopy34 and single-crystal diffraction methods.37
Second, the crystal structure provided herein is, to our
knowledge, the third one of PtbPors. The core deformation
of PtbT4tBuPP was estimated from the ꢁ-value,34,39 and is be-
tween those of PtbOEP and PtbTPP (Table 2). The ‘‘true’’ con-
formation of PtbPor is still unclear, i.e., whether planar or bent.
Our results suggest that platinum can also induce flexible core
conformations, as observed for porphyrinatonickels.41–44 This
suggests the potential utility of PtbPors in estimating the bio-
logical function of Factor-430 using its strong luminescent
character, the possibility of detecting unknown reduced
[PtaPor]ꢁ and [Pt0Por]2ꢁ species, and the creation of an artifi-
cial enzyme based on PtbPors.
Because of the enhanced solubility in standard organic sol-
vents and the well-characterized structural information provid-
ed by this work, PtbT4tBuPP is expected to be widely used as a
standard compound in the field of PtbPor chemistry. We are
currently working on further modifications of the porphyrinato
ligand and its excited state chemistry.
ꢀ
tion with an interplanar distance of 4.84 A (¼ b=2).
Conclusions
A substituted platinum(b) porphyrin was structurally and
spectroscopically characterized. Two paramount conclusions
can be drawn from our investigation.
First, the 1H NMR signal of the ꢀ-hydrogen atoms of the por-
phyrin nucleus was a sharp singlet at 8.78 ppm. This is in
marked contrast to the previously observed quasi-triplet reso-
nance.31 It is well known that the spin coupling constant main-
ly depends on the dihedral angle between the atoms, i.e., the
spin coupling constant provides information about the confor-
mation of the molecule. Hence, the different observations de-
scribed above might result from the conformational differences
among the PtbPors. Although the dihedral angle dependent
coupling constant of the metalloporphyrin system is not known,
our findings may suggest a new methodology to estimate the
planarity of the porphyrin nucleus using long-range spin cou-
pling between 195Pt and ꢀ-protons, along with frequently used
This work was supported in part by Grants-in-Aid for Scien-
tific Research (Category: B-12440178 to K.-i.S.) from the Min-

M. Umemiya et al.
Bull. Chem. Soc. Jpn., 76, No. 11 (2003) 2127
B. D. Berezin, Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol.,
29, 58 (1986); Chem. Abstr., 107, 134102w (1990).
24 G. H. Barnett, M. F. Hudson, and K. M. Smith, J. Chem.
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25 D. T. Cromer and J. T. Waber, ‘‘International Tables for X-
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istry of Education, Culture, Sports, Science and Technology.
This research was also financially supported by grant from
the Toray Science Foundation (to K.-i.S.). We appreciate the
technical assistance provided by Mr. Kazuo Wakasugi, Tokyo
Metropolitan University.
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39 The root-mean-square out-of-plane displacement ꢁ is de-
fined as follows:29
vffiffiffiffiffiffiffiffiffiffiffiffiffiffi
17 K.-i. Sugiura, A. M. Arif, D. K. Rittenberg, J. Schweizer, L.
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(1997).
18 H. Brand, J. A. Capriotti, and J. Arnold, Inorg. Chem., 33,
4334 (1994).
u
24
2
u
t
X
ꢂi
24
ꢁ ¼
;
i¼1
where ꢂi is the orthogonal displacement of the macrocycle atom i
from the mean plane. The sum includes all 24 atoms of the porphy-
rin macrocycle.
19 J. Jiang, W. Liu, W.-F. Law, and D. K. P. Ng, Inorg. Chim.
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22 K. W. Mak and K. S. Chan, J. Am. Chem. Soc., 120, 9686
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23 E. M. Kuvshinova, O. A. Golubchikov, A. S. Semeikin, and
40 H. L. Anderson, Chem. Commun., 1999, 2323.
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42 E. F. Meyer, Jr., J. Am. Chem. Soc., 96, 2095 (1974).
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44 E. F. Meyer, Jr., Acta Crystallogr., Sect. B, 28, 2162 (1972).