Platinum(II) and platinum(IV) porphyrin pincer complexes: Synthesis, structures, and reactivity

Article abstract of DOI:10.1021/om100644n

Pt(II) and Pt(IV) porphyrin pincer complexes were synthesized and characterized. Their structures and electronic properties were influenced by the valence state of platinum at the outer coordination site. The considerable distortion in the porphyrin core of the Pt(II) complex was relieved in the Pt(IV) complex, due to the change of the coordination geometry of platinum. Oxidation of the phenylplatinum(II) pincer complex with iodine induced a facile carbon-carbon bond formation reaction at the meso position via reductive elimination.

Full text of DOI:10.1021/om100644n

Organometallics 2010, 29, 3997–4000 3997
DOI: 10.1021/om100644n
Platinum(II) and Platinum(IV) Porphyrin Pincer Complexes: Synthesis,
Structures, and Reactivity
Keita Yoshida,† Shigeru Yamaguchi,‡ Atsuhiro Osuka,*,‡ and Hiroshi Shinokubo*,†
†Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku,
Nagoya 464-8603, Japan and, and ‡Department of Chemistry, Graduate School of Science, Kyoto University,
Sakyo-ku, Kyoto 606-8502, Japan.
Received July 3, 2010
Summary: Pt(II) and Pt(IV) porphyrin pincer complexes were
synthesized and characterized. Their structures and electronic
properties were influenced by the valence state of platinum at
the outer coordination site. The considerable distortion in the
porphyrin core of the Pt(II) complex was relieved in the Pt(IV)
complex, due to the change of the coordination geometry of
platinum. Oxidation of the phenylplatinum(II) pincer complex
with iodine induced a facile carbon-carbon bond formation
reaction at the meso position via reductive elimination.
pincer complexes bearing a Pt center, which takes a stable
octahedral tetravalent state in contrast to a Pd center. Here
we disclose the synthesis, structure, and reactivity of Pt(II)
and Pt(IV) porphyrin pincer complexes.
Results and Discussion
Scheme 1 depicts the synthesis of Pt(II) and Pt(IV) por-
phyrin pincer complexes. The pincer ligand 1 was prepared
according to our reported procedure.2 The reaction of 1 with
K2PtCl4 in a toluene/DMF solution proceeded smoothly to
provide the Pt(II) porphyrin pincer complex 2 in 96% yield
as a green solid. Upon treatment of 2 with CuCl2 in a
CH2Cl2/MeOH solution, 2 was readily oxidized to afford
Pt(IV) porphyrin pincer complex 3 in 48% yield. None of the
proton signals assignable to the meso protons of 2 and 3 were
observed in the 1H NMR spectra, clearly indicating metala-
tion at the meso positions. X-ray diffraction analyses un-
ambiguously elucidated the structures of 2 (Figure 1a,b) and
3 (Figure 1c,d). The macrocycles of 2 and 3 assume distorted
structures (the saddle-type conformation) induced by the
outer metalation, similarly to Pd porphyrin pincer com-
plexes. However, the degree of distortion is substantially
larger in 2 than in 3. The mean plane deviations of 2 and 3
Since the first report of pincer type complexes in the late
1970s, these complexes have attracted considerable atten-
tion, owing to the intriguing properties imparted by the pin-
cer ligands.1 Usually such ligands comprise a central anionic
aryl ring bearing two supporting coordinating groups at the
ortho positions. Various arenes have been employed for the
anionic ring in pincer type complexes. Although porphyrins
have been extensively investigated aromatic macrocycles,
due to their promising properties as functional materials,
there had been no example of pincer complexes bearing por-
phyrins as anionic aromatic backbones until our recent
reports.2-4 We have reported the synthesis and structural
elucidation of these types of pincer complexes with a square-
planar Pd(II) as an outer metal.2,3 Porphyrins are certainly
known to be conformationally flexible and quite susceptible
to peripheral conjugation. Hence, it is intriguing how the
structure and properties of porphyrin pincer complexes are
altered by the coordination geometry and the valence state of
the metal at the outer coordination site. In this context, we
have explored the synthesis and reactivity of porphyrin
˚
from the 4N plane are 0.41 and 0.25 A, respectively. This
change is a result of the different coordination geometries of
the outer metal, which are square planar for 2 and octahedral
for 3, respectively. The notable difference in the coordination
˚
geometries is bond lengths of the Cmeso-Pt bonds, 1.957 A
˚
for 2 and 2.206 A for 3, presumably due to the absence of π
back-donation of the d orbital of platinum to the macrocycle
in 3 (Table S1, Supporting Information).
*To whom correspondence should be addressed. E-mail: hshino@
apchem.nagoya-u.ac.jp (H.S.); osuka@kuchem.kyoto-u.ac.jp (A.O.).
(1) (a) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750. (b) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. Rev. 2005, 105,
2527. (c) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759.
(d) Singleton, J. T. Tetrahedron 2003, 59, 1837.
Then, replacement of the chlorine atom in 2 with another
ligand was examined to reveal the influence of an ancillary
ligand on the property of the complex. The exchange of the
chloride ligand of 2 with 10 equiv of phenylmagnesium
bromide provided the novel porphyrin pincer complex 4 in
76% yield (Scheme 2). The X-ray crystal structure is shown
in Figure 1e,f. The macrocycle takes a saddle conformation,
and the mean plane deviation of 4 from the 4N plane is 0.39
(2) Yamaguchi, S.; Katoh, T.; Shinokubo, H.; Osuka, A. J. Am.
Chem. Soc. 2007, 129, 6392.
(3) (a) Yamaguchi, S.; Katoh, T.; Shinokubo, H.; Osuka, A. J. Am.
Chem. Soc. 2008, 130, 14440. (b) Yamaguchi, S.; Shinokubo, H; Osuka, A.
Inorg. Chem. 2009, 48, 795. (c) Matano, Y.; Matsumoto, K.; Nakano, Y.;
Uno, H.; Sakaki, S.; Imahori, H. J. Am. Chem. Soc. 2008, 130, 4588.
(4) Several examples were reported for pincer complexes bearing
porphyrins without a direct metal-porphyrin connection: (a) Huck,
W. T. S.; Rohrer, A.; Anikumar, A. T.; Fokkens, R. H.; Nibbering,
N. M. M.; van Veggel, F. C. J. M.; Reinhoudt, D. N. New J. Chem. 1998,
165. (b) Dixon, I. M.; Collin, J. P. J. Porphyrins Phthalocyanines 2001, 5,
600. (c) Suijkerbuijk, B. M. J. M.; Lutz, M.; Spek, A. L.; van Koten, G.; Klein
Gebbink, R. J. M. Org. Lett. 2004, 6, 3023. (d) Suijkerbuijk, B. M. J. M.;
Tooke, D. M.; Lutz, M.; Spek, A. L.; Jenneskens, L. W.; van Koten, G.; Klein
Gebbink, R. J. M. J. Org. Chem. 2010, 75, 1534.
˚
A. In general, the trans influence of a phenyl group is
stronger than that of chloride. Thus, the Cmeso-Pt bond in
4 is elongated (2.028 A) in comparison to that of 2. With 4 in
˚
hand, we then attempted to synthesize Pt(IV) porphyrin
pincer complexes via oxidation of 4. Upon treatment of 4
with iodine, the vivid color change of green for 4 to red was
observed on the TLC analysis (Scheme 2). On the basis of
ESI-TOF mass and 1H NMR analyses, the product was
r
2010 American Chemical Society
Published on Web 08/09/2010
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3998 Organometallics, Vol. 29, No. 17, 2010
Yoshida et al.
Scheme 1. Synthesis of Pt(II) and Pt(IV) Porphyrin Pincer
Complexesa
Scheme 2. Ligand Exchange Reaction and Reductive Elimina-
tion Reaction of the Pt Porphyrin Pincer Complexesa
a Reagents and conditions: (a) PhMgBr, THF, room temperature;
(b) I2, THF, room temperature.
a Reagents and conditions: (a) K2PtCl4, toluene/DMF, 100 °C;
(b) CuCl2, CH2Cl2/MeOH, room temperature.
Figure 2. UV-vis absorption spectra of 1 (purple), 2 (red), 3
(green), 4 (blue), and 5 (black) in CH2Cl2.
In the case of the diaminobenzene-based NCN Pt pincer
complexes, analogous cis-dihalophenylplatinum(IV) com-
plexes are known to be stable.5 Thus, the unique structure
and electronic state of porphyrin pincer complexes might be
a reason for this facile reductive elimination reaction.
The valence state of the outer platinum center influences
the absorption and electrochemical properties of the com-
plexes. The difference in electronic states of the Pt porphyrin
pincer complexes 2-4 is apparent in the UV-vis absorption
spectra (Figure 2). The Soret and Q bands of these complexes
exhibit red shifts relative to those of 1 and 5. The degrees of
the red shifts are different, depending on the valence states of
the Pt centers. The Q bands of complexes 2 and 4 appear at
wavelengths longer than that of 3. DFT calculations indicate
that the HOMO-LUMO gaps of 2 and 4 are smaller than
that of 3, mainly due to the HOMO energies of 2 and 4 being
substantially higher than that of 3 (Figure 3). This situation
can be explained by destabilization of the HOMOs of 2 and 4
because of the electron-rich Pt(II) center. The cyclic voltam-
mograms of 1-4 were measured, and the potentials are given
in Table 1. Irreversible oxidation waves for 1 and 4 and
Figure 1. X-ray crystal structures of Pt porphyrin pincer com-
plexes and meso-phenyl β-dipyridyl porphyrin: (a) top view of 2;
(b) side view of 2; (c) top view of 3; (d) side view of 3; (e) top view
of 4; (f) side view of 4; (g) top view of 5; (h) side view of 5. The
thermal ellipsoids are scaled to the 50% probability level. meso-
Aryl substituents and hydrogen atoms are omitted for clarity.
assigned to be the meso-phenyl-substituted porphyrin 5, the
structure of which was confirmed by X-ray diffraction
analysis (Figure 1g,h). At this stage, we propose that reduc-
tive elimination from the possible diiodophenylplatinum(IV)
intermediate A would afford the coupling product 5 and PtI2.
(5) van Koten, G.; Terheiden, J.; van Beek, J. A. M.; Wehman-
Ooyevaar, I. C. M.; Muller, F.; Stam, C. H. Organometallics 1990, 9, 903.

Note
Organometallics, Vol. 29, No. 17, 2010 3999
Figure 3. Energy diagrams and pictorial representations of HOMOs and LUMOs for 2-4, calculated at the B3LYP/LANL2DZ level.
Table 1. Oxidation and Reduction Potentials of 1-4 in CH2Cl2 (in
V vs Ferrocene/Ferrocenium Ion Pair)a
carbon-carbon bond formation via reductive elimination.
Further investigation of porphyrin pincer complexes bearing
a variety of outer transition metals is currently underway in
our laboratory.
complex
Eox1
Eox2
Ered1
Ered2
1
2
3
4
0.59b
0.38
0.61
0.32b
-1.64
-1.63
-1.27b
-1.69
Experimental Section
0.87
-1.66
General Considerations. 1H NMR spectra were recorded on
a Varian INOVA-500 (500 MHz) spectrometer, and chemical
shifts were reported as the δ scale in ppm relative to CHCl3 as
internal reference for 1H NMR (δ 7.260 ppm). UV/vis absorp-
tion spectra were recorded on a Shimadzu UV-2550 spectro-
meter. Mass spectra were recorded on a Bruker microTOF
instrument using the positive mode ESI-TOF method for
acetonitrile solutions. Unless otherwise noted, materials ob-
tained from commercial suppliers were used without further
purification.
a These values were measured by cyclic voltammetry using a platinum
working electrode, a platinum-wire counter electrode, and an Ag/
AgClO4 reference electrode. The measurements were carried out in
CH2Cl2 solutions containing 0.10 M Bu4NPF6 as a supporting electro-
lyte. b Irreversible waves.
reversible oxidation waves for 2 and 3 were observed. The
first oxidation potentials of 2 (0.38 V) and 4 (0.32 V) are
substantially lower than that of 1 (0.59 V) because of back-
donation of Pt(II) to the macrocycle. In sharp contrast, com-
plex 3 exhibits the first oxidation potential at 0.61 V. The
higher oxidation potential of 3 is probably accounted for by
the electronically positive metal center of the high oxidation
state. This trend again matches well with the theoretical
calculations. Reversible reduction waves were observed for
1, 2, and 4, which are at almost the same potentials (-1.64 V
for 1, -1.63 V for 2, and -1.69 V for 4). In the case of 3, how-
ever, an irreversible reduction wave was observed at -1.27 V.
Such an irreversible reduction wave is frequently observed in
Pt(IV) complexes, which can be assigned as a reduction
process of the Pt(IV) center to a Pt(II) center.6
Compound 2. A toluene/DMF solution (5 mL/5 mL) of 1 (109
mg, 0.1 mmol) and K2PtCl4 (209 mg, 0.5 mmol) was stirred for
15 h at 105 °C under an N2 atmosphere. The resulting mixture
was diluted with CH2Cl2, washed with water, extracted with
CH2Cl2, and dried through Na2SO4. Removal of solvents in
vacuo, followed by recrystallization from CH2Cl2/CH3CN,
afforded 2 in 96% yield (126 mg, 95.5 μmol) as a green solid.
1H NMR (CDCl3): δ 9.98 (d, J = 6.3 Hz, 2H, Py), 8.88 (s, 2H, β),
8.54 (d, J = 4.8 Hz, 2H, β), 8.49 (d, J = 4.8 Hz, 2H, β), 7.97 (d,
J = 4.8 Hz, 2H, β), 7.93 (d, J = 1.1 Hz, 4H, Ar-o), 7.82 (m, 4H,
Py and Ar-o), 7.77 (d, J = 4.3 Hz, 1H, Py), 7.69 (t, J = 1.1 Hz,
1H, Ar), 7.11 (t, J = 4.4 Hz, 2H, Py), 1,53 (s, 36H, t-Bu), and
1.48 (s, 18H, t-Bu). UV/vis (CH2Cl2; λmax, nm (ε, M-1 cm-1)):
406 (38 000), 478 (120 000), 576 (14 000), 628 (8000). HR-MS
(ESI-MS): m/z 1279.5177, calcd for (C72H77N6NiPt)þ
1279.5212 (M - Cl)þ.
Conclusion
In conclusion, the synthesis of the Pt(II) and Pt(IV)
porphyrin pincer complexes has been achieved. The struc-
tures of the complexes have been revealed by X-ray diffrac-
tion analyses. Importantly, the valence state of the outer
Pt center has a significant influence on the structure of por-
phyrinic macrocycles and electrochemistry of the complexes.
Oxidation of the phenylplatinum(II) complex induced
Compound 3. Compound 2 (90.8 mg, 69 μmol) and
CuCl2 2H2O (118 mg, 0.69 mmol) were dissolved in CH2Cl2/
3
MeOH and stirred at room temperature for 19 h. The mixture
was washed with water, extracted with CH2Cl2, and dried
through Na2SO4. Solvents were removed in vacuo. The mixture
was passed through a short silica column (AcOEt and hexane as
an eluent). Recrystallization from CH2Cl2/CH3CN provided 3
1
(46.6 mg, 33.6 μmol) as a green solid in 49% yield. H NMR
(CDCl3): δ 10.16 (d, J = 5.4 Hz, 2H, Py), 9.06 (s, 2H, β), 8.60 (d,
J = 5.1 Hz, 2H, β), 8.55 (d, J = 4.8 Hz, 1H, β), 8.06 (d, J = 6.5
Hz, 2H, Py), 7.93 (m, 2H, Py and Ar-o), 7.83 (d, J = 1.9 Hz, 4H,
Ar-o), 7.79 (t, J = 1.7 Hz, 2H, Ar-p), 7.71 (t, J = 1.7 Hz, Ar-p),
(6) (a) Mink, L. M.; Neitzel, M. L.; Bellomy, L. M.; Falvo, R. E.;
Boggess, R. K.; Trainum, B. T.; Yeaman, P. Polyhedron 1997, 16, 2809.
(b) Crespo, M.; Grande, C.; Klein, A.; Font-Bardia, M.; Solans, X.
J. Organomet. Chem. 1998, 563, 179.

4000 Organometallics, Vol. 29, No. 17, 2010
Yoshida et al.
7.23 (t, J = 6.8 Hz, 2H, Py), 1.53 (s, 36H, t-Bu), and 1.48 (s, 18H,
t-Bu). UV/vis (CH2Cl2; λmax, nm (ε, M-1 cm-1)): 363 (34 000),
387 (37 000), 467 (160 000), 571 (16 000), 615 (12 000). HR-MS
(ESI-MS): m/z 1349.4589, calcd for (C72H77N6NiPtCl2)þ
1349.4577 (M - Cl)þ.
direct methods and refined by full-matrix least squares on F2
using SHELXTL. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Crystallographic data:
for 2, C74H77Cl3N6NiPt, Mw = 1410.57, orthorhombic, space
˚
˚
group Pbca (No. 61), a = 19.056(3) A, b = 13.8732(19) A, c =
3
˚
˚
Compound 4. Under an N2 atmosphere, to a THF solution (4 mL)
of 2 (105 mg, 80 μmol) was added a THF solution of phenylmagne-
sium bromide (0.58 M, 0.69 mL) at room temperature and the
mixture was stirred at room temperature for 1.5 h. The reaction was
quenched by aqueous NH4Cl, and the organic layer was dried
through Na2SO4. After removal of solvents in vacuo, the
mixture was separated by column chromatography on silica
(AcOEt/hexane as an eluent). Recrystallization from CH2Cl2/
CH3CN afforded 4 (82.2 mg, 60.6 μmol) in 76% yield as a
green solid. 1H NMR (CDCl3): δ 8.93 (s, 2H, β), 8.65 (d, J =
6.1 Hz, 2H, Py), 8.53 (d, J = 5.0 Hz, 2H, β), 8.48 (d, J =
5.0 Hz, 2H, β), 8.00 (d, J = 7.2 Hz, 2H, Py), 7.95 (d, J = 1.6 Hz,
1H, Ar-o), 7.83 (d, J = 1.9 Hz, 4H, Ar-o), 7.76 (t, J = 1.9 Hz,
2H, Ar-o), 7.72 (t, J = 6.6 Hz, 2H, Py), 7.68 (t, J = 1.9 Hz, 2H,
Ar-p), 7.52 (d, J = 6.4 Hz, 2H, Ph-o), 7.21 (t, J = 7.4 Hz, 2H,
Ph-m), 7.05 (t, J = 7.1 Hz, 1H, Ph-p), 6.64 (t, J = 5.6 Hz, 2H,
Py), 1.54 (s, 36H, tert-butyl), and 1.47 (s, 18H, tert-butyl)
ppm; UV/Vis (CH2Cl2): λmax (ε [M -1 cm -1]) = 371 (42000),
413 (50000), 482 (140000), 577 (18000), and 630 (9000) nm;
HR-MS (ESI-MS): m/z = 1356.5648, calcd for (C78H82N6-
NiPt)þ = 1356.5605 (M - Cl)þ.
49.033(7) A, V = 12963(3) A , Z = 8, Dcalcd = 1.446 Mg/cm3,
T = 90 K, R = 0.0929 (I > 2.0σ(I)), Rw = 0.2128 (all data),
GOF = 1.125 (I > 2.0σ(I)); for 3, C73H78Cl6N6NiPt, Mw
1505.91, monoclinic, space group C2/c (No. 15), a = 42.745(5)
=
˚
˚
˚
A, b = 14.683(5) A, c = 27.725(5) A, β = 125.799(5)°, V =
14113(6) A , Z = 8, Dcalcd = 1.418 Mg/cm3, T = 153 K, R =
3
˚
0.0519 (I > 2.0σ(I)), Rw = 0.1491 (all data), GOF = 1.032 (I >
2.0σ(I)); for 4, C73H78Cl6N6NiPt, Mw = 1955.14, triclinic, space
˚
˚
group P1 (No. 2), a = 15.130(5) A, b = 17.343(5) A, c =
3
˚
˚
17.879(5) A, β = 103.735(5)°, V = 4502(2) A , Z = 2, Dcalcd
=
=
1.418 Mg/cm3, T = 153 K, R = 0.0786 (I > 2.0σ(I)), Rw
0.2320 (all data), GOF = 1.081 (I > 2.0σ(I)); for 5,
C90H92Cl2N6Ni, Mw = 1387.31, monoclinic, space group P21/
˚
n (No. 24), a = 12.014(5) A, b = 33.877(5) A, c = 18.949(5) A,
˚
˚
3
˚
β = 101.791(5)°, V = 7549(4) A , Z = 4, Dcalcd = 1.221 Mg/
cm3, T = 153 K, R = 0.0595 (I > 2.0σ(I)), Rw = 0.1792 (all
data), GOF = 1.042 (I > 2.0σ(I)).
Theoretical Calculations. All calculations were carried out
using the Gaussian 03 program.7 Initial geometries of the
model compounds for 2-4 were obtained from their X-ray
structures, but all 3,5-di-tert-butylphenyl substituents were
replaced with hydrogen. Optimizations were performed with
Becke’s three-parameter hybrid exchange functional and the
Lee-Yang-Parr correlation functional (B3LYP),8 employing
a basis set consisting of LANL2DZ. Optimized geometries
appear to be in agreement with the crystal structures. The
molecular orbital diagrams were calculated at the B3LYP/
LANL2DZ level.
Compound 5. To a CH2Cl2 solution (1 mL) of 4 (27.5 mg, 20
μmol) was added iodine (22.2 mg, 87 μmol) in THF (1 mL) via
syringe. The mixture was stirred at room temperature for 12 h
under an N2 atmosphere. The mixture was washed with water,
extracted with CH2Cl2, and dried through Na2SO4. The solvents
were removed in vacuo. Short column chromatography on silica
(AcOEt/hexane as an eluent), followed by recrystallization from
CH2Cl2/CH3CN, provided 5 (19.4 mg, 17 μmol) as a red solid in
84% yield. 1H NMR (CDCl3): δ 8.84 (s, 2H, β), 8.71 (d, J = 5.0
Hz, 2H, β), 8.69 (d, J = 4.9 Hz, 2H, β), 8.38 (d, J = 4.5 Hz, 2H,
Py), 7.89 (m, 6H, Ar-o), 7.70 (t, J = 1.5 Hz, 1H, Ar-p), 7.67 (t,
J = 1.5 Hz, 2H, Ar-p), 7.53 (d, J = 6.0 Hz, 2H, Py), 7.10 (t, J =
7.5 Hz, 2H, Py), 6.82 (t, J = 5.5 Hz, 2H, Py), 6.75-6.60 (m, Ph),
1.49 (s, 18H, tert-butyl), and 1.47 (s, 36H, tert-butyl). UV/vis
(CH2Cl2; λmax, nm (ε, M-1 cm-1)) 306 (18 000), 431 (160 000),
542 (13 000). HR-MS (ESI-MS): m/z 1161.6028, calcd for
(C78H83N6Ni)þ 1161.6027 (M þ H)þ.
Acknowledgment. This work was supported by
Grants-in-Aid for Scientific Research (Nos. 20037034
and 21685011) from the MEXT of Japan and the Global
COE Program in Chemistry of Nagoya University. H.S.
gratefully acknowledges financial support from the Tor-
ay Science Foundation. S.Y. acknowledges the JSPS
Research Fellowships for Young Scientists.
Supporting Information Available: Figures giving spectral
data for 2-5 and cyclic voltammograms for 1-4, a table giving
bond lengths and angles for 2-4, and CIF files giving crystal-
lographic data for 2-5. This material is available free of charge
via the Internet at http://pubs.acs.org.
X-ray Crystallographic Analyses. Single crystals of 2-5 sui-
table for X-ray diffraction analyses were grown by vapor
diffusion of CH3CN into a 1,2-dichloroethane solution of 2,
of CH3CN into a CHCl3 solution of 3, of CH3CN into a CHCl3
solution of 4, and of CH3CN into a chlorobenzene solution of 5.
The diffraction data were collected on a Bruker SMART APEX
CCD diffractometer with graphite-monochromated Mo KR
(7) Frisch, M. J. et al. Gaussian 03, Revision B.05; Gaussian, Inc.,
Pittsburgh, PA, 2003.
(8) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
˚
radiation (λ = 0.710 73 A). An empirical absorption correction
was applied by using SADABS. The structure was solved by