Synthesis, properties, and crystal structure of a novel anthracene-bridged molybdenum-zinc porphyrin dimer

Article abstract of DOI:10.1021/ic010323b

The synthesis and properties of novel anthracene-bridged porphyrin dimers having an oxomolybdenum(V) porphyrin unit, H₂(DPA)[Mo^VO(OMe)] (1) and (DPA)[Mo^VO(OMe)][Zn¹¹(MeOH)] (2), and the relevant monomer porphyrin complexes Mo^VO(MPP)OMe (3) and Zn¹¹(MpP) (4) are presented. An oxomolybdenum(V) unit was introduced into one of the two porphyrins in DPA to give 1, which has a free-base porphyrin unit. By introducing a zinc(II) ion to the free-base part, a mixed-metal complex of 2 was prepared and isolated. The structure of 2 was analyzed by X-ray crystallography (2.7/6CH₂Cl₂, triclinic, P1 (no. 2), a = 15.2854(12) A, b = 19.9640(15) A, c = 13.6915(12) A, α = 90.968(3)°, β = 113.108(4)°, ≥ = 96.501(4)°, Z = 2, R1 = 9.9, wR2 = 19.2). The structure of 2 demonstrated that a methanol is stably coordinated to the Zn(II) ion with the aid of a hydrogen bond to the methoxo ligand on the Mo(V) ion in the binding pocket of DPA. The electrochemical measurements of 2 suggested that the methanol was kept in the pocket of DPA in solution even at the reduced state of the molybdenum ion.

Full text of DOI:10.1021/ic010323b

Inorg. Chem. 2002, 41, 1170−1176
Synthesis, Properties, and Crystal Structure of a Novel
Anthracene-Bridged Molybdenum−Zinc Porphyrin Dimer
Tetsuaki Fujihara, Kiyoshi Tsuge, Yoichi Sasaki, Yasuhiro Kaminaga, and Taira Imamura*
DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity,
Sapporo 060-0810, Japan
Received March 23, 2001
The synthesis and properties of novel anthracene-bridged porphyrin dimers having an oxomolybdenum(V) porphyrin
II
unit, H (DPA)[MoVO(OMe)] (1) and (DPA)[MoVO(OMe)][Zn (MeOH)] (2), and the relevant monomer porphyrin
2
complexes MoVO(MPP)OMe (3) and Zn (MPP) (4) are presented. An oxomolybdenum(V) unit was introduced into
II
one of the two porphyrins in DPA to give 1, which has a free-base porphyrin unit. By introducing a zinc(II) ion to
the free-base part, a mixed-metal complex of 2 was prepared and isolated. The structure of 2 was analyzed by
X-ray crystallography (2‚7/6CH Cl2, triclinic, P1h (no. 2), a ) 15.2854(12) Å, b ) 19.9640(15) Å, c ) 13.6915(12)
2
Å, R ) 90.968(3)°, â ) 113.108(4)°, γ ) 96.501(4)°, Z ) 2, R1 ) 9.9, wR2 ) 19.2). The structure of 2
demonstrated that a methanol is stably coordinated to the Zn(II) ion with the aid of a hydrogen bond to the methoxo
ligand on the Mo(V) ion in the binding pocket of DPA. The electrochemical measurements of 2 suggested that the
methanol was kept in the pocket of DPA in solution even at the reduced state of the molybdenum ion.
Introduction
electron reactivity from dioxygen to water.3-7 Diporphyrins
containing various metal ions such as Mn,8 Ru,9 or Os10 also
show extremely metal-specific reactivities toward small
molecules in the binding pocket between two porphyrin units.
Especially, the introduction of different metal ions to
diporphyrins is the most effective to give rise to singular
properties that must be caused by intramolecular interactions
between the two metal ions.11-14 Very recently, in the
Much attention has been paid to cofacial diporphyrins due
to their reactivities with small molecules.1 In particular, face-
to-face diporphyrins attached by a rigid spacer such as
anthracene (DPA), biphenylene (DPB), or o-phenylene
(Gable type) have given unique properties.1-10 For instance,
biscobalt porphyrin dimers having rigid spacers display four-
(1) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1534-1554 and references therein.
(2) Abbreviations: H4DPA ) 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-
tetramethylporphyrinyl)]anthracene, H4DPB ) 1,8-bis[5-(2,8,13,17-
tetraethyl-3,7,12, 18-tetramethylporphyrinyl)]biphenylene, MPP )
5-phenyl-2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinato di-
anion, DPTBTMP ) 5,15-diphenyl-2,8,12,18-tetra-n-butyl-3,7,13,17-
tetramethylporphyrinato dianion, TPP ) 5,10,15,20-tetraphenylpor-
phyrinato dianion, 5-NO2-OEP ) 5-nitro-2,3,7,8,12,13,17,18-octaeth-
ylporphyrinato dianion, (DPA′)4- ) anion of 1,8-bis[5-(2,8,12,18-
tetrahexyl-3,7,13,17-tetramethyl-15-phenyl)porphyrin]anthracene, im
) imidazolate, TBAP ) tetra-n-butylammonium perchlorate.
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106, 2725-2726. (b) Liu, H.-Y.; Abdalmuhdi, I.; Chang, C. K.; Anson,
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1170 Inorganic Chemistry, Vol. 41, No. 5, 2002
10.1021/ic010323b CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/09/2002

Synthesis of an Anthracene-Bridged Porphyrin Dimer
Table 1. Crystallographic Data for 2
empirical formula
temp/K
cryst syst
space group
a/Å
C80H84N8O3MoZn‚7/6CH2Cl2
193
triclinic
P1h (No. 2)
15.2854(12)
19.9640(15)
13.6915(12)
90.968(3)
113.108(4)
96.501(4)
3809.5(5)
2
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
Z
d
calcd/g cm-3
1.28
no. of unique reflns
no. of obsd reflns
R1,wR2, %a
16489
9602, I > 2σ(I)
9.9, 19.2
Figure 1. Anthracene-bridged porphyrin dimers having an oxomolybde-
num(V) porphyrin unit and relevant monomer porphyrin complexes.
a R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo2 - Fc )2]/ ∑[w(Fo )2]}1/2
,
2
2
preparation of this paper, new diporphyrins of gallium-
gallium, gallium-ruthenium, and gallium-cobalt DPA
complexes were reported.15 The study demonstrated that in
the Ga-Ga complex an unprecedented double π-π* fluo-
rescence was aroused from the two lowest energy absorption
Q-bands and that in the Ga-Ru and Ga-Co complexes
luminescences were efficiently quenched by Ru(II) and
Co(II) centers.
Molybdenum in porphyrin rings can take various oxidation
states from +2 to +6.16,17 In their high oxidation states, +4,
+5, and +6, the molybdenum porphyrins show a strong
affinity for oxygen species and display unique photochemical
and electrochemical reactivities.18-20
We now pay attention to heterometallic diporphyrins
having a high-valent molybdenum(V) ion. Since the bi-
phenylene spacer is short enough to form a metal-metal
bond,21 the anthracene spacer has been selected in the present
study to use the binding pocket effectively. In this paper,
the synthesis, characterization, and properties of novel
anthracene-bridged porphyrin dimers having an oxomolyb-
denum(V) porphyrin unit, H2(DPA)[MoVO(OMe)] (1) and
(DPA)[MoVO(OMe)][ZnII(MeOH)] (2), are reported (Figure
1).
w ) {σ2(Fo ) + [0.03(max(Fo , 0) + 2Fc )/3]2}-1
.
2
2
2
Analysis, Hokkaido University. ESR spectra were recorded on a
JEOL JES-FE3X spectrometer. Cyclic voltammograms were re-
corded at a scan rate of 100 mV/s with a HOKUTO DENKO HZ-
3000 voltammetric analyzer. Working and counter electrodes were
a Pt disk and a Pt wire, respectively. As the reference a Ag/AgCl
electrode was used. The complexes (ca. 1 mM) were dissolved in
CH2Cl2 containing 0.1 M TBAP, and the solutions were deoxy-
genated with a stream of argon gas.
X-ray Crystallography. The crystals of (DPA)[MoVO(OMe)]-
[ZnII(MeOH)] (2) were obtained as CH2Cl2 solvates (2‚7/6CH2Cl2).
Data were collected on a Rigaku RAXIS-RAPID diffractometer
with an imaging plate area detector using graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å) at 193 K. To determine the cell
constants and the orientation matrix, two oscillation photographs
were taken with an oscillation angle of 2°. Intensity data were
collected by taking oscillation photographs (total oscillation range
220°, 44 frames, and exposure time 180 s). A numerical absorption
correction was applied.
The crystal structure was solved by a direct method and expanded
by the Fourier method. The structure was refined by full-matrix
least-squares refinement on F2. All non-hydrogen atoms of 2 were
refined anisotropically. Dichloromethane molecules were inclueded
as crystal solvents, which were found to be disordered and were
refined isotropically. All hydrogen atoms, with the exception of
those of crystal solvents, were located on the calculated positions
and not refined. All calculations were performed using the teXsan
crystallographic software package.22 The crystallographic data of
2‚7/6CH2Cl2 are summarized in Table 1.
Materials. Mo(CO)6 was purchased from Aldrich. Silica gel
(Wakogel C-300HG) and alumina (Woelm, neutral, activity III)
were used for column chromatography. Other agents and solvents
were purchased from Wako and used as received. The porphyrin
ligands of H4DPA11,23 and H2MPP23 were prepared according to
previous reports. The elemental analysis of each compound was
satisfactory, and all the spectral data agreed well with the previous
results.
Experimental Section
Methods. UV-vis spectra were recorded on a Hitachi UV-3000
spectrophotometer. IR spectra were recorded on a Hitachi 270-50
infrared spectrophotometer. 1H NMR spectra were measured with
a JEOL EX270 FT-NMR spectrophotometer. FAB-MS spectra and
elemental analysis were carried out at the Center for Instrumental
(15) Harvey, P. D.; Proulx, N.; Martin, G.; Drouin, M.; Nurco, D. J.; Smith,
K. M.; Bolze, F.; Gros, C. P.; Guilard, R. Inorg. Chem. 2001, 40,
4142-4148.
(16) Matsuda, Y.; Murakami, Y. Coord. Chem. ReV. 1988, 92, 157-192.
(17) Brand, H.; Arnold, J. Coord. Chem. ReV. 1995, 140, 137-168.
(18) (a) Chevrier, B.; Diebold, T.; Weiss, R. Inorg. Chim. Acta 1976, 19,
L57-L59. (b) Ledon, H.; Bonnet, M. J. Chem. Soc., Chem. Commun.
1979, 702-704. (c) Mentzen, B. F.; Bonnet, M. C.; Ledon, H. J. Inorg.
Chem. 1980, 19, 2061-2066.
H2(DPA)[MoVO(OMe)] (1). H4DPA (100 mg, 8.8 × 10-5 mol)
and an excess of Mo(CO)6 (200 mg, 7.6 × 10-4 mol) were added
to the mixed solvent of decalin (36 mL) and n-octane (9 mL). The
mixture was refluxed for 2 h under N2. After the reaction mixture
was filtered, the filtrate was passed through an alumina column
(neutral, activity III, 2 × 10 cm). The first band eluted with neat
(19) Hasegawa, K.; Imamura, T.; Fujimoto, M. Inorg. Chem. 1986, 25,
2154-2160.
(20) (a) Tachibana, J.; Imamura, T.; Sasaki, Y. J. Chem. Soc., Chem.
Commun. 1993, 1436-1439. (b) Tachibana, J.; Imamura, T.; Sasaki,
Y. Bull. Chem. Soc. Jpn. 1998, 71, 363-369. (c) Fujihara, T.; Hoshiba,
K.; Sasaki, Y.; Imamura, T. Bull. Chem. Soc. Jpn. 2000, 73, 383-
390. (d) Fujihara, T.; Myougan, K.; Ichimura, A.; Sasaki, Y.; Imamura,
T. Chem. Lett. 2001, 186-187.
(22) teXsan: Single-Crystal Structure Analysis Software, Version 1.6;
Molecular Structure Corp.: The Woodlands, 1993.
(23) Chang, C. K.; Abdalmuhdi, I. J. Org. Chem. 1983, 48, 5388-5390.
(21) Collman, J. P.; Kim, K.; Garner, J. M. J. Chem. Soc., Chem. Commun.
1986, 1711-1713.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1171

Fujihara et al.
Scheme 1
CH2Cl2 was discarded. The second red band eluted with 1%
MeOH-CH2Cl2 (v/v) was collected, and the eluate was evaporated
to dryness. The residue was dissolved again in a small amount of
CH2Cl2 and crystallized by diffusion of MeOH. The resulting purple
crystals were filtered, washed with MeOH, and dried under reduced
pressure at 100 °C for 2 h. Yield: 30 mg (27%). Anal. Calcd for
C79H83N8O2Mo: C, 74.57; H, 6.57; N, 8.81. Found: C, 73.70; H,
6.75; N, 8.78. UV-vis (CH2Cl2): λmax/nm (ꢀ/103 M-1 cm-1) 345
(sh), 406 (136), 450 (55.8), 502 (14.1), 538 (8.52), 568 (16.0), 598
(5.81), 625 (2.78). ESR (CH2Cl2): gav ) 1.97, AMo ) 46 × 10-4
cm-1, AN ) 2.24 × 10-4 cm-1. IR (KBr): ν/cm-1 902 (νModO),
478 (νMosO), 3296 (νNsH). FAB-MS: m/z+ ) 1242 ([H2(DPA)-
(MoO)]+).
(DPA)[MoVO(OMe)][ZnII(MeOH)] (2). Method 1. A 30 mg
sample of Zn(OAc)2‚2H2O in MeOH (5 mL) was added to a
solution of 1 (30 mg, 2.3 × 10-5 mol) in CH2Cl2 (50 mL). After
the reaction mixture was refluxed for 30 min, the resulting solution
was evaporated to dryness. The residue was again dissolved in
CH2Cl2 followed by filtration to remove excess Zn(OAc)2. The
filtrate was passed through an alumina column (neutral, activity
III, 2 × 10 cm). The first fraction eluted with CH2Cl2 was collected,
and the eluate was evaporated to dryness. The residue was dissolved
again in a small amount of CH2Cl2 containing MeOH (ca. 2%)
and crystallized by diffusion of n-pentane. The obtained purple
crystals were washed with n-pentane and dried under reduced
pressure at 100 °C for 4 h. Yield: 25 mg (79%). Anal. Calcd for
C80H85N8O3MoZn: C, 70.24; H, 6.26; N, 8.19. Found: C, 69.25;
H, 6.46; N, 8.07. UV-vis (0.5% MeOH-CH2Cl2): λmax/nm (ꢀ/
103 M-1 cm-1) 341 (81.1), 414 (249), 458 (41.4), 545 (16.8), 575
(16.9), 598 (4.7). ESR (CH2Cl2): gav ) 1.97, AMo ) 43 × 10-4
cm-1, AN ) 2.43 × 10-4 cm-1. IR (KBr): ν/cm-1 916 (νModO),
428 (νMosO). FAB-MS: m/z+ ) 1304 ([(DPA)(MoO)Zn]+).
Method 2. H2(DPA)[Zn]14 (50 mg, 4.2 × 10-5 mol) and an
excess of Mo(CO)6 (100 mg, 3.8 × 10-4 mol) were added to the
mixed solvent of decalin (36 mL) and n-octane (9 mL). The mixture
was refluxed for 2 h under N2, cooled, and filtered. The filtrate
was passed through an alumina column (neutral, activity III, 2 ×
10 cm). The first red band, which was eluted with 1% MeOH-
CH2Cl2 (v/v), was collected, and the eluate was evaporated to
dryness. The residue was dissolved again in a small amount of
CH2Cl2 containing MeOH (ca. 2%) and crystallized by diffusion
of n-pentane. The purple crystals obtained were washed with
n-pentane and dried under reduced pressure at 100 °C for 4 h.
Yield: 26 mg (47%).
ZnII(MPP) (4). H2MPP in CH2Cl2 was treated with Zn(OAc)2‚
2H2O in MeOH to give Zn(MPP) quantitatively. Anal. Calcd for
C38H40N4Zn: C, 73.84; H, 6.52; N, 9.06. Found: C, 73.92; H, 6.71;
N, 9.16. 1H NMR (CDCl3, 270 MHz): δ/ppm 10.16 (s, 2H, meso
H), 10.06 (s, 1H, meso H), 8.08 (d, 2H, phenyl H), 7.75 (m, 3H,
phenyl H), 4.05 (dq, 8H, CH2CH3), 3.65 (s, 6H, CH3), 2.46 (s, 6H,
CH3), 1.90 (t, 6H, CH2CH3), 1.77 (t, 6H, CH2CH3). UV-vis
(CH2Cl2): λ/nm (ꢀ/103 M-1 cm-1) 404 (424), 534 (18.1), 569 (17.3).
FAB-MS: m/z+ ) 616 ([Zn(MPP)]+).
Results and Discussion
Synthesis of DPA Complexes with an Oxomolybde-
num(V) Unit. The outline of the syntheses of the DPA
complexes of 1 and 2 is summarized in Scheme 1.24 The
insertion of molybdenum ion into H4DPA was carried out
using Mo(CO)6 as a metal source. By changing both the
reflux time and the amount of metal source, the mono-
molybdenum porphyrin complex of 1 was successfully
isolated.25 The introduction of zinc(II) ion into the free-base
porphyrin unit of 1 with Zn(OAc)2‚2H2O gave 2. 2 was also
prepared from H2(DPA)[Zn] as described in the Experimental
Section.11-15 In the preparation method, the monozinc
complex of H2(DPA)[Zn] was directly treated with Mo(CO)6
to yield 2. Successive treatments of 2 in CH2Cl2 with dilute
HClaq led to the monomolybdenum complex of 1.
MoVO(MPP)(OMe) (3). H2MPP (200 mg, 3.6 × 10-4 mol) and
an excess of Mo(CO)6 (200 mg, 7.6 × 10-4 mol) were added to
the mixed solvent of decalin (36 mL) and n-octane (9 mL). The
mixture was refluxed for 2 h under N2, cooled, and filtered. The
filtrate was passed through a silica gel column (Wakogel C-300HG,
2 × 20 cm). The first band eluted with CH2Cl2 contained the free-
base porphyrin. The second green band eluted with 2% MeOH-
CH2Cl2 (v/v) was collected, and the eluate was evaporated to
dryness. The residue was dissolved again in a small amount of
CH2Cl2 and crystallized by diffusion of n-pentane. The brown-green
powder was filtered, washed with n-pentane, and dried under
reduced pressure at 100 °C for 2 h. Yield: 87 mg (34%). Anal.
Calcd for C44H28N4O3Mo: C, 67.33; H, 6.23; N, 8.05. Found: C,
67.10; H, 6.42; N, 8.08. UV-vis (CH2Cl2): λmax/nm (ꢀ/103 M-1
cm-1) 345 (49.3), 446 (87.3), 564 (15.6), 598 (5.77). ESR
(CH2Cl2): gav ) 1.96, AMo ) 43 × 10-4 cm-1, AN ) 2.23 × 10-4
cm-1. IR (KBr): ν/cm-1 902 (νModO), 452 (νMosO). FAB-MS: m/z+
) 666 ([MoO(MPP)]+).
The elemental analyses of 1-4 were consistent with their
respective compositions. The FAB-MS spectrum of 1
indicated that an oxomolybdenum unit was introduced into
one of two porphyrin units in DPA. The insertion of a zinc
ion was also confirmed by FAB-MS; that is, 2 showed the
parent peak at 1304 (m/z+), whereas 1 had the parent peak
at 1242 (m/z+).
1 and 2 showed characteristic ESR signals of oxomolyb-
denum(V) porphyrins, that is, six weak signals derived from
95,97Mo (I ) 5/2) and a strong signal due to the molybdenum
(24) Preliminary X-ray structure analysis of 1 showed that the oxo ion
coordinated to the central molybdenum ion is directed to the outside
of the dimer core; the same is true for 2. The data will be reported
somewhere after refinements.
(25) Another green band, which was eluted with 5% MeOH/CH2Cl2, was
obtained. The FAB-MS spectrum for the solid material shows a peak
at m/z+ ) 1351, which corresponds to bis(oxomolybdenum) DPA
complexes. The characterization of the complex is now in progress.
1172 Inorganic Chemistry, Vol. 41, No. 5, 2002

Synthesis of an Anthracene-Bridged Porphyrin Dimer
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) of 2
molybdenum unit zinc unit
bond
length/Å
bond
length/Å
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-N(1)
Mo(1)-N(2)
Mo(1)-N(3)
Mo(1)-N(4)
1.722(6)
1.993(6)
2.065(6)
2.090(7)
2.090(7)
2.069(8)
Zn(1)-O(3)
Zn(1)-N(5)
Zn(1)-N(6)
Zn(1)-N(7)
Zn(1)-N(8)
2.180(6)
2.086(7)
2.064(7)
2.061(7)
2.045(7)
molybdenum unit
zinc unit
atoms
angle/deg
atoms
angle/deg
O(1)-Mo(1)-O(2)
O(1)-Mo(1)-N(1)
O(1)-Mo(1)-N(2)
O(1)-Mo(1)-N(3)
O(1)-Mo(1)-N(4)
O(2)-Mo(1)-N(1)
O(2)-Mo(1)-N(2)
O(2)-Mo(1)-N(3)
O(2)-Mo(1)-N(4)
N(1)-Mo(1)-N(2)
N(1)-Mo(1)-N(3)
N(1)-Mo(1)-N(4)
N(2)-Mo(1)-N(3)
N(2)-Mo(1)-N(4)
N(3)-Mo(1)-N(4)
172.3(3)
98.2(3)
91.6(3)
89.9(3)
96.2(3)
89.0(2)
85.7(3)
82.9(3)
86.5(3)
92.3(3)
171.9(3)
87.1(3)
87.4(3)
172.2(3)
92.1(3)
O(3)-Zn(1)-N(5)
O(3)-Zn(1)-N(6)
O(3)-Zn(1)-N(7)
O(3)-Zn(1)-N(8)
N(5)-Zn(1)-N(6)
N(5)-Zn(1)-N(7)
N(5)-Zn(1)-N(8)
N(6)-Zn(1)-N(7)
N(6)-Zn(1)-N(8)
N(7)-Zn(1)-N(8)
89.2(2)
97.7(3)
103.1(3)
96.6(2)
90.1(3)
167.7(3)
88.6(3)
87.7(3)
165.6(3)
90.5(3)
The total feature of the structure of 2 is very similar to that
of (DPA)[Ga(OMe)][Ru(MeOH)(CO)].15
The central molybdenum atom is displaced by 0.13 Å
toward the oxo ligand from the mean plane of the 24-atom
porphyrin core, and by 0.143 Å from the mean plane de-
fined by the four nitrogen atoms. The bond lengths of
ModO (1.722(6) Å) and MosN (average 2.08 Å) are in
the range of those of oxomolybdenum(V) porphyrins with a
monodentate axial ligand.26,27 The MosO(OMe) distance
(1.993(6) Å) is slightly longer than that observed in MoVO-
(DPTBTMP)(OMe) (1.89(9) Å).27 The longer distance must
be due to the presence of the hydrogen bond between the
methoxo ion and the methanol coordinated to the zinc ion
(vide infra).
The zinc center of 2 adopts an approximate square-
pyramidal geometry. The zinc atom is displaced by 0.23 Å
toward the methanol ligand from the mean plane of the 24-
atom porphyrin core, and by 0.24 Å from the mean plane of
the four nitrogen atoms. Although the average Zn-N
distance (2.06 Å) is comparable to those of pentacoordinated
zinc(II) porphyrins with a methanol as an axial ligand,28-30
the relatively short distance of Zn-O(MeOH) (2.180(6) Å)
supports the presence of the hydrogen bond.29
Figure 2. Molecular structure of 2: (a) side view, parallel to the bridge
plane; (b) side view, perpendicular to the bridge plane. Methy and ethyl
groups of the DPA unit were omitted for clarity.
nucleus with I ) 0. The strong signal splits into nine
hyperfine lines because of the interaction with the four
nitrogen nuclei (I ) 1).16 A corresponding monomeric
complex of 3 also showed the characteristic ESR signals.
These results clearly indicate that the valence states of
molybdenum ion in 1-3 are +5.
Crystal Structure of (DPA)[MoVO(OMe)][ZnII(MeOH)]
(2). A crystal of 2 suitable for X-ray diffraction study was
obtained by the diffusion of n-pentane into a CH2Cl2 solution
of 2 containing a small amount of MeOH. Figure 2 shows
the molecular structure of 2. The selected bond lengths and
bond angles around the central metal ions are listed in Table
2.
One of the porphyrin rings is metalated by a molybdenum
ion and the other by a zinc ion. The molybdenum ion is
hexacoordinated with an oxo and a methoxo ligand. Although
there are two possible conformations of an oxo ligand bound
to a molybdenum ion, that is, outside or inside the DPA unit,
the crystal structure clarified that the terminal oxo ligand is
positioned outside the DPA unit. The zinc ion is penta-
coordinated by a methanol at the fifth position. Both the
methoxo and methanol ligands are situated inside the DPA
unit. The molybdenum porphyrin unit is oriented toward the
outside, and the zinc porphyrin unit is largely bent with
respect to the anthracene unit. Dihedral angles between the
anthracene mean plane and each porphyrin mean plane of
24 atoms are 84.1° (Mo side) and 68.3° (Zn side). The
dihedral angle between two porphyrin units is up to 31.8°.
(26) (a) Ledon, H. J.; Mentzen, B. Inorg. Chim. Acta 1978, 31, L393-
L394. (b) Imamura., T.; Furusaki, A. Bull. Chem. Soc. Jpn. 1990, 63,
2726-2727. (c) Hamstra, B. J.; Cheng, B.; Ellison, M. K.; Scheidt,
W. R. Inorg. Chem. 1999, 38, 3554-3561.
(27) van der Dijk, M.; Morita, Y.; Pertovic, S.; Sanders, G. M.; van der
Plas, H. C.; Stam, C. H.; Wang, Y. J. Heterocycl. Chem. 1992, 29,
81-86.
(28) (a) Senge, M. O.; Kalisch, W. W. Inorg. Chem. 1997, 36, 6103-
6116. (b) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.;
Renner, M. W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851-
8857.
(29) (a) Liang, Y.; Chang, C. K. Tetrahedron Lett. 1995, 36, 3817-3820.
(b) Bag, N.; Chern, S-.S.; Peng, S-.M.; Chang, C. K. Inorg. Chem.
1995, 34, 753-756.
(30) Senge, M. O.; Eigenbrot, C. W.; Brennan, T. D.; Shusta, J.; Scheidt,
W. R.; Smith, K. M. Inorg. Chem. 1993, 32, 3134-3142.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1173

Fujihara et al.
Table 3. Conformational Parameters for Cofacial Bisporphyrin Complexes
complex
Ct-Ct/Åa
M-M/Å
tilt/degb
R/degc
Sr/Åd
Sp/Åe
ref
(DPA)[MoO(OMe)][Zn(MeOH)] (2)
(DPA)[Ga(OMe)][Ru(MeOH)(CO)]
(DPA)[Ga(OMe)]2
(DPA)[Ga(µ-OH)-Ru(CO)]
(DPA)[Ni]2
(DPA)[Lu(OH)]2‚MeOH
(DPA′)[Fe2(µ-im)(Him)2]Cl
(DPB)[Co][Al(OEt)]
6.00
6.48
4.55
4.26
4.56
5.67
5.96
4.08
3.94
5.89
6.17
5.25
3.95
4.57
3.52
5.96
4.37
4.13
31.8
22.6
7.2
13.0
2.4
19.7
28.4
7.4
21.8
19.1
29.4
7.6
31.7
14.1
14.2
29.8
25.8
5.57
6.12
3.96
4.22
3.87
5.49
5.78
3.54
3.54
2.23
2.12
2.23
0.56
2.39
1.39
1.46
2.03
1.71
f
15
15
15
31
32
33
11
12
(DPB)[Cu][MnCl]
5.2
a The macrocycle center (Ct) was determined as the average position of the four nitrogens for each macrocycle. Ct-Ct is the distance between the two
centers. b The angle (tilt) was calculated as the dihedral angle between the two 24-atom mean planes of the porphyrins. c The slip angle (R) was calculated
as the average angle between the vector connecting the two “Ct” centers and the unit vectors normal to the two macrocyclic 24-atom mean plane; that is,
R ) (R1 + R2)/2. d The inter-ring separation was defined as Sr ) (Ct-Ct) cos R. e The slip parameter was determined as Sp ) (Ct-Ct) sin R. f This work.
Figure 3. Diagram illustrating the coordination environment of 2.
It is interesting that a methanol exists inside the DPA
cavity, though the inside of the DPA pocket is sterically
crowded. The distance between the oxygen atom (OMe)
Figure 4. UV-vis spectra of 1 (a) and the relevant monomer compounds
(b) in CH2Cl2 at room temperature: dashed line, 3; dotted line, H2MPP;
coordinated to the molybdenum(V) ion and that (MeOH) to
the zinc(II) ion is 2.61 Å as illustrated in Figure 3. The short
contact between the two oxygen atoms also supports the
presence of the hydrogen bond. The angles Mo-O(2)-O(3)
and Zn-O(3)-O(2) are 120.9(2)° and 126.4(3)°, respec-
tively. The hydrogen atom must locate on the line between
two oxygen atoms, though the X-ray analysis was not able
to determine the position accurately. The reason the methanol
is captured inside the DPA cavity is that the methanol ligand
is stabilized by both the coordination bond and the hydrogen
bond. This result does not contradict with the data of Zn(5-
NO2-OEP)(MeOH), in which the intermolecular O-O dis-
tance of two methanols coordinated to two adjacent zinc(II)
ions is 2.83 Å.30
solid line, calculated sum of two spectra.
iron(III)33 complexes, the distortions are symmetrical with
respect to the two porphyrin units, or take the simple open
conformation of the DPA unit. In contrast, asymmetrical
structures appear to be specific for the heterometallic
diporphyrins of 2 and (DPA)[Ga(OMe)][Ru(MeOH)(CO)]15
having a methoxo ion and a methanol in the pocket; that is,
the two porphyrin rings are open and distorted asymmetri-
cally. Both the diporphyrins also have a large center-to-center
distance (6.00 Å for 2 and 6.48 Å for (DPA)[Ga(OMe)]-
[Ru(MeOH)(CO)]).
Infrared and UV-Vis Spectroscopy. 1 gives the IR
bands of ModO and MosO at 902 and 478 cm-1,
respectively. These values are comparable to the stretching
of oxomolybdenum(V) porphyrins having an alkoxo ligand,16
and those of 3. 1 also shows a weak absorption band due to
inner NH stretching of the free-base porphyrin unit at 3296
cm-1. In the case of 2, the band of MosO stretching (428
cm-1) is shifted by 50 cm-1 to lower wavenumbers compared
to that of 1. This shift must result from the formation of the
Table 3 summarizes several parameters of the structure
and the conformation of the diporphyrin complexes.11,12,15,31-33
In the homometallic diporphyrins of the lutetium(III)32 and
(31) Fillers, J. P.; Ravichandran, K. G.; Abdalmuhdi, I.; Tulinsky, A.;
Chang, C. K. J. Am. Chem. Soc. 1986, 108, 417-424.
(32) Lachkar, M.; Tabard, A.; Brandes, S.; Guilard, R.; Atmani, A.; De
Cian, A.; Fischer, J.; Weiss, R. Inorg. Chem. 1997, 36, 4141-4146.
(33) Naruta, Y.; Sawada, N.; Tadokoro, M. Chem. Lett. 1994, 1713-1716.
1174 Inorganic Chemistry, Vol. 41, No. 5, 2002

Synthesis of an Anthracene-Bridged Porphyrin Dimer
Figure 6. Cyclic voltammograms of 2 in 0.1 M TBAP-CH2Cl2. The
oxidation process was displayed in the potential ranges -1.0 to +1.3 V (a)
and -1.0 to +1.6 V (b). The reduction process was shown in the potential
range +0.2 to -1.5 V (c).
Scheme 2
Figure 5. UV-vis spectra of 2 (a) and the relevant monomer compounds
(b) in 0.5% MeOH-CH2Cl2 at room temperature: dashed line, 3; dotted
line, 4; solid line, calculated sum of two spectra.
hydrogen bond between the methoxo ligand and the methanol
ligand, as found in the crystal structure.
Figure 4a shows a UV-vis spectrum of 1 in CH2Cl2.
Although 1 has many absorption bands at 345, 406, 450,
502, 538, 568, 598, and 625 nm, the spectrum was clearly
analyzed as the overlap of those of 3 and H2MPP. 3 has
absorption bands at 345, 446, 564, and 598 nm, and H2MPP
at 402, 501, 535, 570, and 623 nm, as shown in Figure 4b.
The result suggests that there are few interactions between
the two porphyrin units.
The UV-vis spectrum for the CH2Cl2 solution of 2 at
concentrations lower than 5 × 10-6 M shows the Soret band
of the zinc porphyrin unit at 407 nm. However, by an
increase in the concentration to ca. 10-4 M or by the addition
of 0.5% MeOH to the solution at low concentrations, the
Soret band red-shifted to 414 nm (Figure 5a). This spectrum
is different from the superposition of the components of 3
and 4 (Figure 5b). Since the red shift of the Soret band of
zinc(II) porphyrins is generally observed by the coordination
of bases such as pyridine, the UV-vis spectrum for high
concentrations of 2 in solution is explained by the coordina-
tion of a methanol to the zinc(II) porphyrin. In 4 itself, the
addition of the same amount of MeOH to the CH2Cl2 solution
caused no red shifts. Thus, the relatively stable coordination
of methanol to zinc(II) ion must be specific to 2.34
are displayed in Figure 6. In the potential range between 0
and +1.2 V in Figure 6a, two reversible oxidation peaks
were observed at +0.49 and +0.96 V (E1/2 ) half-wave
potential). These peaks (I and II) are due to the formation
of the monocation and dication radicals of the zinc(II)
porphyrin unit, respectively, as compared to 4. Remarkably,
the first oxidation process of 2 to form monocation radical
shows a large negative shift (∆E ) 180 mV), in comparison
to the oxidation of 4 at +0.67 V (E1/2), due to the
coordination of methanol to the zinc(II) ion.35 When the
potential was scanned positively up to +1.6 V as shown in
Figure 6b, another reversible oxidation peak was observed
at +1.47 V (E1/2 , peak III), which was assigned to the ring-
centered oxidation of the molybdenum porphyrin unit.
The negative scan from the potentials over +0.96 V gave
two small reduction peaks at around +0.6 and -0.2 V aside
Electrochemistry. The redox behavior for ca. 1 × 10-3
M mixed-metal complex of 2 is summarized in Scheme 2.
The voltammograms of 2 in three different potential ranges
(35) For the first oxidation of Zn(TPP)(L) (L ) nitrogenous base) in CH2Cl2
containing 0.1 M TBAP and 1.0 M ligand, the half-wave potentials
are in the narrow range of +0.74 to +0.83 V vs SCE. In this system,
the values are not so different from that (+0.78 V) of Zn(TPP) without
axial ligands. Kadish, K. M.; Shiue, L. R.; Rhodes, R. K.; Bottomley,
L. A. Inorg. Chem. 1981, 20, 1274-1277.
(34) Although the UV-vis spectrum of 2 showed the Soret band at 414
nm at concentrations higher than 10-4 M in neat CH2Cl2, the
determination of the molar absorptivity of 2 was performed for a
solution containing 0.5% (v/v) MeOH to get accurate values.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1175

Fujihara et al.
from the peak at -0.78 V (Figure 6b). Since [ZnII(MPP)]+
in 4 is reduced at +0.67 V and the MoVO(MPP)ClO4
complex prepared independently has a reversible reduction
peak of Mo(IV)/Mo(V) at -0.2 V, the two peaks at around
+0.6 and -0.2 V in 2 should be ascribed to the ring-centered
reduction of the zinc porphyrin unit without methanol ligands
and the reduction of the Mo(IV)/Mo(V) of the molybdenum
porphyrin unit without methoxo groups, respectively. When
potentiostatic electrolysis of 2 was carried out at around +1
V followed by negative scans, the two peaks at +0.6 and
-0.2 V increased in intensity with the increase of the
electrolysis time. The results indicate that by the oxidation
of 2 at the potentials over +0.96 V, both the methoxo ligand
and the methanol ligand are in some amounts released from
each metal ion.
positive scan gave almost the same oxidation peaks as the
first one. The appearance of the reversible peak at -0.78 V
in the system of 2 indicates that both the methoxo ligand
and the methanol ligand are still coordinated to each metal
ion even after the reduction. The two ligands must be
sustained with the help of the DPA pocket and the hydrogen
bond.
Conclusion
Novel anthracene-bridged porphyrin dimers having an
oxomolybdenum(V) unit, H2(DPA)[MoVO(OMe)] (1) and
(DPA)[MoVO(OMe)][ZnII(MeOH)] (2), and the relevant
monomer complexes of MoVO(MPP)OMe (3) and ZnII(MPP)
(4) were synthesized and characterized. The structure of the
mixed-metal complex of 2 was determined by X-ray crystal-
lographic analysis. The results clearly indicate that a
methanol molecule stably exists inside the DPA pocket due
to both the coordination to the zinc(II) ion and the hydrogen
bond with the methoxo ion. The UV-vis and cyclic
voltammetric measurements of 2 suggest that the structure
is retained even in solution. The study of other mixed-metal
complexes having redox-active metal ions is now in progress.
In the direct negative scan in the potential range of +0.2
to -1.5 V, a reversible reduction peak (peak IV, E1/2
)
-0.78 V) was observed as shown in Figure 6c. ESR
measurements revealed that by the potentiostatic electrolysis
of 2 at -1.0 V the characteristic ESR signal of the Mo(V)
unit decreased in intensity, indicating the formation of a
diamagnetic Mo(IV) porphyrin unit. The CV wave at -0.78
V is significantly different from that of the monomer system
of MoVO(TPP)OMe, which gave an irreversible reduction
peak.36 The irreversibility of MoVO(TPP)OMe is owed to
the release of the methoxo ligand after the reduction of the
molybdenum(V) ion.36 In the system of 2, the repeated
Supporting Information Available: X-ray crystallographic data
of 2 in CIF format and figures showing the full ORTEP drawing
and schematic diagrams of perpendicular displacement of porphyrin
cores of 2. This material is available free of charge via the Internet
at http://pubs.acs.org.
(36) (a) Kadish, K. M.; Malinski, T.; Ledon, H. J. Inorg. Chem. 1982, 21,
2982-2987. (b) Malinski, T.; Hanley, P. M.; Kadish, K. M. Inorg.
Chem. 1986, 25, 3229-3235.
IC010323B
1176 Inorganic Chemistry, Vol. 41, No. 5, 2002