Control of Cu(II) and Cu(III) states in N-confused porphyrin by protonation/deprotonation at the peripheral nitrogen

Article abstract of DOI:10.1021/ja0356075

The square-planar structure of a diamagnetic Cu(III) complex of tetrakis(pentafluorophenyl)-substituted N-confused porphyrin was elucidated by X-ray diffraction analysis. The facile interconversion between Cu(II) (1) and Cu(III) (2) complexes was achieved by chemical oxidation/reduction, and the Cu(III)/Cu(II) redox potential was controlled by adding anions through the hydrogen-bonding interaction to the peripheral NH. Copyright

Full text of DOI:10.1021/ja0356075

Published on Web 09/06/2003
Control of Cu(II) and Cu(III) States in N-Confused Porphyrin by Protonation/
Deprotonation at the Peripheral Nitrogen
Hiromitsu Maeda,† Yuichi Ishikawa,‡ Tomoyuki Matsuda,‡ Atsuhiro Osuka,† and Hiroyuki Furuta#,*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan,
Department of Applied Chemistry, Faculty of Engineering, Oita UniVersity, Oita 870-1192, Japan, and Department
of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity, Fukuoka 812-8581, Japan
Received April 14, 2003; E-mail: hfuruta@cstf.kyushu-u.ac.jp
Chart 1. Anionic Forms of Porphyrin and Its Analogues
Porphyrin and corrole are well-known aromatic macrocyclic
ligands that form neutral square-planar complexes with a variety
of divalent and trivalent metals, respectively.1,2 The difference in
the metal valency results from the number of protons inside the
macrocyclic cores, in principle (Chart 1). When the metal oxidation
state rises from 2+ to 3+ in the porphyrin complex, the coordination
of a counteranion to the axial position on the metal usually takes
place. To maintain square-planarity for the complexes having a
different oxidation state of metal, ion transfer from or to the
porphyrinoid ring needs to occur. For such purpose, N-confused
porphyrin (NCP)3 is a promising ligand because NCP is known to
complex with the tri- and divalent metals according to the metal
species in a neutral square-planar fashion by using two different
tautomeric forms.4 To date, however, attempts to isolate the
complexes with different oxidation states of the same metal have
not been successful due to the presence of a counteranion or the
high reactivity of the inner carbon.5 In this Communication, we
show the first X-ray structure of a Cu(III)-NCP complex that
proved to have square-planarity similar to that of a Cu(II) complex.
The interconversion between the Cu(II) and Cu(III) complexes by
chemical oxidation and reduction is also reported. To the best of
our knowledge, this is the first example of porphyrinoid complexes
in which the metal ion exists in two different oxidation states with
the same neutral square-planar coordination environment.
Scheme 1. Interconversion between Cu(II) Complex (1) and
Cu(III) Complex (2) by Detachment and Attachment of the
Peripheral NH
The Cu(II) complex of NCP was synthesized by using meso-
pentafluorophenyl-substituted NCP, whose square-planar structure
was elucidated by X-ray crystallography.6 When the Cu(II) complex
(1) was treated with 1.5 equiv of 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) in CHCl3, the solution changed from yel-
lowish-brown to red immediately. After silica gel column chro-
matography, a new complex (2) was isolated quantitatively (Scheme
1). The first clue for the diamagnetic Cu(III)-NCP complex came
from the 1H NMR spectrum of 2 in CDCl3, which showed a singlet
at 9.41 ppm, ascribable to the outer R-CH of the confused pyrrole
ring, and the remaining â-CH signals appeared at 8.88 (doublet)
and 8.72-8.67 ppm (multiplet). In accordance with this result, the
ESR signals of the paramagnetic Cu(II) complex 1 (giso ) 2.09, g|
) 2.12, [A|] ) 168 G in toluene (4 mM) at 77 K)7 disappeared
upon the addition of quantitative amounts of DDQ. The obtained
Cu(III) complex was easily reduced by p-toluenesulfonylhydrazide
to afford the Cu(II) complex (1) (see Supporting Information).
The UV/vis absorption spectra of the Cu(II) and Cu(III)
complexes (1, 2) and the colors in solutions are shown in Figure 1.
The absorption spectrum of 2 showed a split Soret band at 453.0
nm, 11 nm red-shifted as compared to that of 1 (441.5 nm); in
contrast, the Q-bands are blue-shifted and appeared in the region
Figure 1. UV/vis absorption spectra and the naked colors of the solutions
(inset) of 1 (dotted line, left) and 2 (solid line, right) in CH2Cl2.
from 521 to 632 nm. A similar blue-shift of the Q-bands is observed
in Ag(III)-NCP complexes.6,7
The structure of the Cu(III) complex 2 was explicitly determined
by single-crystal X-ray analysis (Figure 2).8 The crystal parameters
of 2 are quite similar to those of 1, but the average bond lengths of
Cu-C and Cu-N, 1.961(7) and 1.975(8) Å, are shorter than those
of 1 (1.980(9) and 2.018(9) Å)6 and comparable to the Cu(III)
complex of cis-doubly N-confused porphyrin, cis-N2CP (Cu-C,
1.939(3), 1.934(4) Å; Cu-N, 1.969(3), 1.954(4) Å).9 The absence
of counteranions in the crystal indicates that the complex is neutral.
The absorption spectrum of the crystal after the X-ray measurements
was intact, which excluded the possibility of contamination of Cu-
(II) complex 1 or reduction to 1 during the X-ray irradiation.
Interestingly, the electrochemical interconversion between the
two NCP complexes, 1 and 2, was controlled by the addition of
anions. The redox potentials for the Cu3+/Cu2+ couple of 1 in the
presence of 0.1 M tetrabutylammonium salts (TBAX, X ) Cl-,
† Kyoto University.
‡ Oita University.
# Kyushu University.
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10.1021/ja0356075 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
transferred to the central metal, consequently stabilizing the M(III)
oxidation state, and the latter may widen the potential window for
the metal oxidation. Compared with the Cu(III) complex of cis-
N2CP,9 the complex 2 was less stable and gradually transformed
to the Cu(II) complex in CHCl3 or CH2Cl2 solution, suggesting
that the Cu(III)/Cu(II) redox couple lies near or on the border of
M(II) and M(III) complexation in this C6F5-substituted N-confused
porphyrin ligand. As the control of the metal valencies by outer
stimuli may offer a variety of applications, including molecular
switches16 and anion sensors,17 we think the present NCP ligand
system is promising for such use.
Figure 2. X-ray single-crystal structure of 2: (a) top and (b) side views.
Meso substituents are omitted for clarity in the side view. The thermal
ellipsoids were scaled to the 30% probability levels. Due to the disorder of
the nitrogen at a confused pyrrole ring, one of the eight possible forms is
shown.
Acknowledgment. The authors thank Prof. Yoshio Hisaeda and
Mr. Isao Aritome at Kyushu University for X-ray measurement.
H.M. thanks JSPS for a Research Fellowship for Young Scientists.
Supporting Information Available: Synthetic procedures and
spectral data of the Cu(III) complex (2); 1H NMR, ESR, and absorption
spectra; anion binding details (DPV, titration curve, ESR) (PDF); and
X-ray crystallographic data for 2 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) Scheidt, W. R. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 3, Chapter
16.
(2) Erben, C.; Will, S.; Kadish, K. M. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000;
Vol. 2, Chapter 12.
Figure 3. (a) Shifts of the redox potential (vs Fc+/Fc) coupled with Cu3+
/
(3) (a) Furuta, H.; Asano, T.; Ogawa, T. J. Am. Chem. Soc. 1994, 116, 767-
768. (b) Chmielewski, P. J.; Latos-Graz˘yn´ski, L.; Rachlewicz, K.; Głowiak,
T. Angew. Chem., Int. Ed. Engl. 1994, 33, 779-781. (c) Latos-Graz˘yn´ski,
L. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, 2000; Vol. 2, Chapter 14. (d) Furuta,
H.; Maeda, H.; Osuka, A. Chem. Commun. 2002, 1795-1804.
(4) Furuta, H.; Ishizuka, T.; Osuka, A.; Dejima, H.; Nakagawa, H.; Ishikawa,
Y. J. Am. Chem. Soc. 2001, 123, 6207-6208.
Cu2+ in the differential pulse voltammetry (DPV) of 1 (1 mM) in CH2Cl2
-
upon addition of (a) Cl-, (b) ClO4-, and (c) PF6 as tetrabutylammonium
salts (0.1 M), and (b) a possible anion-binding mode of 1.
ClO4-, and PF6-) were 0.03, 0.14, and 0.15 V (vs Fc+/Fc, 1 mM
in CH2Cl2), respectively, which were well correlated to the anion
binding affinity of the complex 1 (Figure 3).10,11 The finding that
the ESR spectrum of the Cu(II) complex 1 in CH2Cl2 did not change
in the presence of Cl- ruled out the possibility of anion coordination
to the center Cu(II) metal in this system. The large negative potential
shift observed for Cl- might be attributable to a hydrogen-bonding
interaction with the outer NH of the confused pyrrole ring.12
Transmission of the partial negative charge from the peripheral
hydrogen-bonded anion, NsH‚‚‚X-, to the Cu cation through the
inner carbon of the confused pyrrole ring might be occurring.
Similar tuning of the electronic state of the center metal by proton
stimuli was demonstrated in the Sb(V)-NCP complex.13 It is
(5) (a) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Inorg. Chem. 1997, 36, 840-
845. (b) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Inorg. Chem. 2000, 40,
5475-5482. (c) Bohle, D. S.; Chen, W.-C.; Hung, C.-H. Inorg. Chem.
2002, 41, 3334-3336.
(6) Maeda, H.; Osuka, A.; Ishikawa, Y.; Aritome, I.; Hisaeda, Y.; Furuta, H.
Org. Lett. 2003, 5, 1293-1296.
(7) Interestingly, the profile of the spectrum of 1 in CHCl3 (ref 6) differs
largely from that in CH2Cl2. Currently, we are investigating this solvent
effect.
(8) Crystal data for 2: C44H7N4F20Cu, Mw ) 1035.09, trigonal R3h(h) (No.
148), a ) b ) 20.0878(10) Å, c ) 23.882(3) Å, V ) 8430.4(9) Å3, Dc )
1.835 g/cm3, Z ) 9, R ) 0.0681, wR ) 0.17578, GOF ) 1.038 (I >
2.0σ(I)).
(9) Furuta, H.; Maeda, H.; Osuka, A. J. Am. Chem. Soc. 2000, 122, 803-
807.
(10) The association constants of 1 with Cl-, Br-, ClO4-, and PF6- in CH2Cl2
were determined as 4.9 × 104, 6.9 × 103, 360, and 50 M-1, respectively,
from the absorption spectral changes (see Supporting Information).
(11) The oxidation potential using TBABr salt cannot be determined due to
the overlap with the oxidation wave of Br-.
noteworthy that the estimated Cl- binding affinity, 4.9 × 104 M-1
,
is unusually high for a single hydrogen-bonding system; thus, we
believe that the contribution of a zwitterionic resonance form14 and
additional interaction between Cl- and the electron-deficient C6F5
group nearest to the outer NH also participate in this anion-binding
system.15
(12) In the 1H NMR spectra of diamagnetic Ni(II) complex6 (2.0 × 103 M) in
CDCl3, the outer NH signal shifted to the low field from 10.05 to 14.65
ppm upon the addition of 1 equiv of TBACl.
(13) In the Sb(V) complexes, the bond lengths between the center Sb and axial
ligands are changed according to the protonation/deprotonation at the
peripheral nitrogen. Liu, J.-C.; Ishizuka, T.; Osuka, A.; Furuta, H. Chem.
Commun. 2003, 1908-1909.
(14) Xiao, Z.; Patrick, B. O.; Dolphin, D. Chem. Commun. 2002, 1816-1817.
(15) Alkorta, I.; Rozas, I.; Elguero, J. J. Am. Chem. Soc. 2002, 124, 8593-
8598.
(16) Fabbrizzi, L.; Licchelli, M.; Mascheroni, S.; Poggi, A.; Sacchi, D.; Zema,
M. Inorg. Chem. 2002, 41, 6129-6136.
(17) (a) Mizuno, T.; Wei, W.-H.; Eller, L. R.; Sessler, J. L. J. Am. Chem. Soc.
2002, 124, 1134-1135. (b) Nielsen, K. A.; Jeppesen, J. O.; Levillain, E.;
Becher, J. Angew. Chem., Int. Ed. 2003, 42, 187-191.
At present, the correlation between the redox potentials for the
M(III)/M(II) couple and the preferential M(II) or M(III) coordination
of NCP is not clearly understood. Similar DDQ treatment of Ni-
(II)- and Pd(II)-NCP complexes only resulted in the recovery of
the starting M(II) materials. The electron-withdrawing C6F5 group
at the meso position raises both the acidity of the outer NH and
the oxidation potential of the π-ring system. The former may
increase the electronic density of inner carbon that is directly
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