A convergent synthetic approach using sterically demanding aryldipyrrylmethanes for tuning the pocket sizes of cofacial bisporphyrins.

Article abstract of DOI:10.1021/ic025507k

Meso substitution opposite to the spacer provides a convenient approach for tuning the pocket sizes of pillared cofacial bisporphyrins. The synthesis and coordination chemistry of xanthene and dibenzofuran anchored platforms structurally modified with 2,6-dimethoxyaryl groups are described. Comparative structural analysis of xanthene derivatives confirms the ability of the trans-aryl groups to adjust the vertical dimension of the cofacial cleft: 7 (C(97)H(106)Cl(4)N(8)O(5)), monoclinic, space group P2(1)/c, a = 28.8353(12) A, b = 17.1139(7) A, c = 17.5978(7) A, beta = 98.826(1) degrees, Z = 4; 8 (C(101)H(123)Cl(2)N(8)O(11.5)Zn(2)), monoclinic, space group P2(1)/n, a = 14.5517(6) A, b = 22.9226(10) A, c = 28.5155(13) A, beta = 90.312(14) degrees, Z = 4; 12 (C(99)H(102)Cl(14)N(8)O(5)Mn(2)), monoclinic, space group P2/c, a = 19.5891(3) A, b = 15.0741(2) A, c = 33.2019(6) A, beta = 91.947(10) degrees, Z = 4. The convenience and versatility of this synthetic method offers intriguing opportunities to specifically tailor the binding pockets of cofacial bisporphyrins for the study of small-molecule activation within a proton-coupled electron transfer framework.

Full text of DOI:10.1021/ic025507k

Inorg. Chem. 2002, 41, 3008−3016
A Convergent Synthetic Approach Using Sterically Demanding
Aryldipyrrylmethanes for Tuning the Pocket Sizes of Cofacial
Bisporphyrins
Christopher J. Chang,† Yongqi Deng,† Shie-Ming Peng,‡ Gene-Hsiang Lee,‡ Chen-Yu Yeh,‡ and
,†
Daniel G. Nocera*
Departments of Chemistry, 6-335, Massachusetts Institute of Technology,
77 Massachusetts AVenue, Cambridge, Massachusetts 02139, and
National Taiwan UniVersity, Taipei 01764, Taiwan, Republic of China
Received January 15, 2002
Meso substitution opposite to the spacer provides a convenient approach for tuning the pocket sizes of pillared
cofacial bisporphyrins. The synthesis and coordination chemistry of xanthene and dibenzofuran anchored platforms
structurally modified with 2,6-dimethoxyaryl groups are described. Comparative structural analysis of xanthene
derivatives confirms the ability of the trans-aryl groups to adjust the vertical dimension of the cofacial cleft: 7
(C H Cl4N O ), monoclinic, space group P21/c, a ) 28.8353(12) Å, b ) 17.1139(7) Å, c ) 17.5978(7) Å, â )
97 106
8 5
98.826(1)°, Z ) 4; 8 (C H Cl2N O11.5Zn2), monoclinic, space group P21/n, a ) 14.5517(6) Å, b ) 22.9226(10)
101 123
8
Å, c ) 28.5155(13) Å, â ) 90.312(14)°, Z ) 4; 12 (C H Cl14N O Mn2), monoclinic, space group P2/c, a )
99 102
8 5
19.5891(3) Å, b ) 15.0741(2) Å, c ) 33.2019(6) Å, â ) 91.947(10)°, Z ) 4. The convenience and versatility of
this synthetic method offers intriguing opportunities to specifically tailor the binding pockets of cofacial bisporphyrins
for the study of small-molecule activation within a proton-coupled electron transfer framework.
Introduction
Such systems readily provide a face-to-face geometry with
little lateral displacement between porphyrin subunits, while
the single bridge allows for a range of binding geometries
through distortion of the pocket “bite”. For the past 25
years, the cofacial Pacman systems have been limited to two
spacers, anthracene (DPA)4 and biphenylene (DPB).5 These
constructs have been quite prominent as electrocatalysts
for the reduction of oxygen and other small molecules,3-5,10
though the arduous synthesis of their prerequisite dialde-
hyde bridges has hampered extensive investigation. In
addition, the vertical pocket sizes of DPA and DPB bispor-
phyrins differ by only ca. 1 Å, offering a restricted window
of conformational flexibility for exploring structure/function
relationships.
The continuing search for synthetic platforms to catalyze
the multielectron transformation of small-molecule substrates
has prompted significant attention toward the study of cofa-
cial bisporphyrins and their transition-metal complexes.1,2 At
the forefront of this research is the development of cofacial
“Pacman” bisporphyrins anchored by a single rigid pillar.1-13
* Author to whom correspondence should be addressed. E-mail: nocera@
mit.edu.
† Massachusetts Institute of Technology.
‡ National Taiwan University.
(1) Collman, J. P.; Wagenknecht, P. S.; Hutchinson, J. E. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1537-1554.
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R.; Seok, W. K.; Ibers, J. A.; L’Her, M. J. Am. Chem. Soc. 1992,
114, 9869-9877.
Our interest in the activation of small-molecule substrates
within a proton-coupled electron transfer framework14,15 has
(4) Chang, C. K.; Abdalalmuhdi, I. J. Org. Chem. 1983, 48, 5388-5390.
(5) Chang, C. K.; Abdalmuhdi, I. Angew. Chem., Int. Ed. Engl. 1984, 23,
164-165.
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Brandes, S.; Hutchinson, J. E.; Collman, J. P. J. Am. Chem. Soc. 1992,
114, 9877-9889.
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C.; Gouerac, P.; Knop, Y.; Collman, J. P. J. Am. Chem. Soc. 1995,
117, 11721-11729.
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Khalifa, M.; Saillard, J. Y. J. Am. Chem. Soc. 1997, 119, 6905-6106.
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3008 Inorganic Chemistry, Vol. 41, No. 11, 2002
10.1021/ic025507k CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/07/2002

Tuning the Pocket Sizes of Cofacial Bisporphyrins
led us to develop new cofacial bisporphyrins supported by
xanthene (DPX)16 and dibenzofuran (DPD) scaffolds.17 These
systems display vertical pocket sizes spanning over 5 Å in
metal-metal distances with little to no lateral displacements.
In particular, the DPD platform provided the first direct
structural evidence for the Pacman effect within a pillared
cofacial bisporphyrin motif through a comparative structural
analysis of its bizinc(II) and bisiron(III) µ-oxo complexes,17
which showed that this platform can open and close its
binding pocket by a vertical distance of over 4 Å in the
presence of appropriate substrates. Moreover, dicobalt(II)
complexes of both DPD and DPX are efficient electrocata-
lysts for the selective four-electron reduction of oxygen to
water despite their large difference (ca. 4 Å) in metal-metal
distances, demonstrating that the above-mentioned vertical
flexibility of these platforms has important ramifications for
the dynamic binding, activation, and release of substrates
and products.18
The success of pillared cofacial bisporphyrins for multi-
electron small-molecule activation is based on their ability
to provide a reaction pocket of suitable size and flexibility.
Typically, such structural attributes have been largely
controlled by the choice of spacer. In this report, we present
a complementary approach for tuning the pocket sizes of
pillared bisporphyrins by the selective introduction of steri-
cally demanding aryl groups opposite the cofacial bridge.
The synthesis and coordination chemistry of xanthene and
dibenzofuran anchored platforms structurally modified with
2,6-dimethoxyaryl groups (DPXM ) diporphyrin xanthene
methoxyaryl and DPDM ) diporphyrin dibenzofuran meth-
oxyaryl, respectively) are described. Crystallographic studies
for the development of novel cofacial bisporphyrin systems
for proton-coupled small-molecule activation chemistry.
Experimental Section
Materials. Silica gel 60 (70-230 and 230-400 mesh, Fisher)
was used for column chromatography. Analytical thin-layer chro-
matography was performed using silica gel (precoated sheets, 0.2
mm thick). Solvents for synthesis were of reagent grade or better,
and were dried according to standard methods.19 Spectroscopic
experiments employed dichloromethane (spectroscopic grade, Bur-
dick & Jackson), which was stored over 4 Å molecular sieves under
high vacuum or in a glovebox. Starting materials for syntheses were
obtained as follows: 2-(ethyloxycarbonyl)-3-ethyl-4-methylpyrrole
(1) was obtained from ethyl isocyanoacetate20 and 3-acetoxy-2-
nitropentane21,22 by the method of Barton and Zard,23 and 4,5-bis-
[4,4′-diethyl-3,3′-dimethyl-2,2′-dipyrrylmethyl]-9,9-dimethyl-
xanthene (2)18 and 4,6-bis[4,4′-diethyl-3,3′-dimethyl-2,2′-dipyrryl-
methyl]dibenzofuran (3)17 were prepared according to previous
reports. All other reagents were used as received. Mass spectral
analyses were carried out by the University of Illinois Mass Spec-
trometry Laboratory and the MIT Department of Chemistry Instru-
ment Facility. Elemental analyses were performed at Michigan State
University and Quantitiative Technologies, Inc. (Whitehouse, New
Jersey).
(2,6-Dimethoxyphenyl)bis(5-ethoxycarbonyl-4-ethyl-3-methyl-
2-pyrryl)methane (4). A mixture of 2,6-dimethoxybenzaldehyde
(5.0 g, 30.1 mmol) and 1 (10.9 g, 60.2 mmol, 2 equiv) in absolute
ethanol (150 mL) containing concentrated HCl (2.5 mL) was
refluxed under nitrogen for 18 h and cooled in an ice bath. The
precipitate was filtered and washed with cold methanol to give 4
1
as a pale-pink powder (13.8 g, 89% yield). H NMR (300 MHz,
CDCl3, 25 °C): δ ) 9.07 (br s, 2H, NH), 7.18 (t, J ) 8.4 Hz, 1H,
ArH), 6.60 (d, J ) 8.4 Hz, 2H, ArH), 6.19 (s, 1H, CH), 4.23 (q, J1
) 7.2 Hz, J2 ) 7.2 Hz, 4H, CH2), 3.78 (s, 6H, CH3), 2.69 (q, J1 )
7.2 Hz, J2 ) 7.5 Hz, 4H, CH2), 1.81 (s, 6H, CH3), 1.31 (t, J ) 7.2
Hz, 6H, CH3), 1.06 (t, J ) 7.5 Hz, 6H, CH3).
(2,6-Dimethoxyphenyl)bis(4-ethyl-3-methyl-2-pyrryl)-
methane (5). Powdered NaOH (12 g) in water (15 mL) was
combined with 4 (12.0 g, 23.5 mmol) suspended in ethylene glycol
(120 mL). The mixture was refluxed under nitrogen for 16 h and
cooled to room temperature. The precipitate was collected by
suction filtration, washed with water until the washings became
neutral, and dried under vacuum over P2O5. Product 5, which was
obtained as a goldenrod solid (8.5 g, 99% yield), was stored under
1
nitrogen at -20 °C. H NMR (300 MHz, CDCl3, 25 °C): δ )
8.15 (br s, 2H, NH), 7.00 (t, J ) 8.4 Hz, 1H, ArH), 6.47 (d, J )
8.4 Hz, 2H, ArH), 6.24 (d, J ) 2.4 Hz, 2H, CH), 6.07 (s, 1H, CH),
3.63 (s, 6H, CH3), 2.29 (q, J1 ) 7.5 Hz, J2 ) 7.8 Hz, 4H, CH2),
1.70 (s, 6H, CH3), 1.31 (t, J ) 7.2 Hz, 6H, CH3).
(2,6-Dimethoxyphenyl)bis(5-formyl-4-ethyl-3-methyl-2-pyr-
ryl)methane (6). Under a nitrogen atmosphere, phosphorus oxy-
chloride (14 mL) was added dropwise to a solution of 5 in
anhydrous DMF (50 mL) at -10 °C. After stirring at -10 °C for
30 min, the resulting mixture was allowed to warm to room
temperature and stirred for an additional 2 h. The reaction was
reveal that the trans-aryl groups serve as molecular “teeth”
within the cleft of the cofacial bisporphyrin “bite”, affording
a powerful approach for systematically tailoring the steric
and electronic properties of the binding pockets through
minimal changes along the porphyrin superstructure. The
versatility of this synthetic methodology opens new avenues
(14) Chang, C. J.; Brown, J. D. K.; Chang, M. C. Y.; Baker, E. A.; Nocera,
D. G. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, Germany, 2001; Vol. 3.2.4, pp 409-461.
(15) Cukier, R. I.; Nocera, D. G. Annu. ReV. Phys. Chem. 1998, 49, 337-
369.
(19) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinmann: Oxford, 1996.
(20) Harman, G. D.; Wenstock, L. M. Org. Synth. 1988, VI, 620-624.
(21) Tindall, J. B. Ind. Eng. Chem. 1941, 33, 65-66.
(22) Sprang, C. A.; Egering, E. F. J. Am. Chem. Soc. 1942, 64, 1063-
1064.
(16) Chang, C. J.; Deng, Y.; Heyduk, A. F.; Chang, C. K.; Nocera, D. G.
Inorg. Chem. 2000, 39, 959-966.
(17) Deng, Y.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc. 2000, 122,
410-411.
(18) Chang, C. J.; Deng, Y.; Shi, C.; Chang, C. K.; Anson, F. C.; Nocera,
D. G. Chem. Commun. 2000, 1355-1356.
(23) Barton, D. H. R.; Zard, S. Z. J. Chem. Soc., Chem. Commun. 1985,
1098-1100.
Inorganic Chemistry, Vol. 41, No. 11, 2002 3009

Chang et al.
concentrated to half its original volume, and methanol (200 mL)
and Na2CO3 (20 g) were added. The mixture was heated at 60 °C
for 30 min and cooled to room temperature. The precipitate was
filtered and washed with water until the washings became neutral.
Recrystallization from methanol afforded 6 as an ochre solid (7.2
solution was refluxed for 5 h, and the solvent was removed by
rotary evaporation. The remaining solid was taken up in a 1:1
mixture of dichloromethane/water (150 mL). The organic layer was
decanted and taken to dryness. The crude material was purified by
flash column chromatography (silica gel, 1:1 dichloromethane/
hexanes) followed by recrystallization from dichloromethane/
methanol to afford analytically pure 9 as a brick red solid (53 mg,
87% yield). Anal. Calcd for C95H98N8O5Cu2: C, 73.19; H, 6.34;
N, 7.19. Found: C, 73.05; H, 6.24; N, 7.14. HRFABMS (M+):
calcd for C95H98N8O5Cu2 m/z 1556.645, found 1556.644.
1
g, 98% yield). H NMR (300 MHz, CDCl3, 25 °C): δ ) 9.48 (s,
2H, CHO), 9.14 (br s, 2H, NH), 7.21 (t, J ) 8.4 Hz, 1H, ArH),
6.60 (d, J ) 8.4 Hz, 2H, ArH), 6.17 (br s, 1H, CH), 3.76 (s, 6H,
CH3), 2.65 (q, J1 ) 7.5 Hz, J2 ) 7.8 Hz, 4H, CH2), 1.78 (s, 6H,
CH3), 1.15 (t, J ) 7.2 Hz, 6H, CH3).
4,5-Bis[(2,3,13,17-tetraethyl-3,7,12,18-tetramethyl-15-(2,6-
dimethoxyphenyl)-5-porphyrinyl)]-9,9-dimethylxanthene, H4-
(DPXM) (7). A suspension of 2 (2.19 g, 3.28 mmol) and 6 (2.80
g, 6.63 mmol) in freshly distilled methanol (400 mL) was purged
with nitrogen for 20 min. A solution of p-toluenesulfonic acid (1.2
g) in methanol (20 mL) was added dropwise over a period of 1 h.
The resulting red mixture was stirred in the dark under nitrogen
for 2 days. Solid o-chloranil (0.9 g) was added in one portion, and
stirring was continued in air for 24 h. The mixture was concentrated
to half its original volume and the precipitate was suction-filtered
and washed with cold methanol and water. The solid was redis-
solved in chloroform (100 mL), a saturated methanolic solution of
Zn(OAc)2‚2H2O (10 mL) was added, and the solution was refluxed
for 30 min. The solvent was removed and the remaining residue
was purified by flash column chromatography (silica gel, 1:1
dichloromethane/hexanes). The bright red band eluted was collected,
taken to dryness, and redissolved in dichloromethane (150 mL).
The solution was vigorously stirred with 6 N HCl (16 mL) for 15
min and neutralized with a 10% aqueous sodium carbonate solution,
and the mixture was stirred for an additional 15 min. The organic
phase was separated, washed with water (3 × 50 mL), dried over
Na2SO4, and taken to dryness. Recrystallization from dichlo-
romethane/methanol afforded pure 7 as a royal purple microcrys-
Ni2(DPXM) (10). A mixture of 7 (53 mg, 0.037 mmol) and NiCl2
(102 mg, mmol) in 6 mL of DMF was refluxed under nitrogen for
4 h. The solvent was removed under vacuum, and the remaining
solid was taken up in a 1:1 mixture of dichloromethane/water (150
mL). The organic layer was decanted and taken to dryness.
Purification by flash column chromatography (silica gel, 1:1
dichloromethane/hexanes) followed by recrystallization from dichlo-
romethane/methanol gave analytically pure 10 as a ruby red
microcrystalline powder (46 mg, 81% yield). 1H NMR (500 MHz,
CDCl3, 25 °C): δ ) 7.83 (d, 2H, ArH), 7.76 (s, 4H, meso), 7.58
(t, J ) 7.5 Hz, 2H, ArH), 7.16 (t, J ) 7.8 Hz, 2H, ArH), 7.04 (d,
J ) 8.7 Hz, 2H, ArH), 6.82 (d, J ) 7.2 Hz, 2H, ArH), 6.68 (d,
J ) 8.4 Hz, 2H, ArH), 4.11 (s, 6H, OCH3), 3.03 (s, 6H, OCH3),
2.72 (m, 4H, CH2), 2.50 (m, 4H, CH2), 2.24 (m, 8H, CH2), 2.21 (s,
6H, CH3), 2.17 (s, 12H, CH3), 2.12 (s, 12H, CH3), 1.15 (t, J ) 7.2
Hz, 6H, CH3), 1.03 (t, J ) 7.2 Hz, 6H, CH3). Anal. Calcd for
C95H98N8O5Ni2: C, 73.65; H, 6.38; N, 7.23. Found: C, 73.53; H,
6.22; N, 7.42. HRFABMS (M+): calcd for C95H98N8O5Ni2 m/z
1546.637, found 1546.636.
Fe2Cl2(DPXM) (11). In a drybox, a 100-mL flask equipped with
a condenser was charged with 7 (202 mg, 0.141 mmol), 2,6-lutidine
(0.4 mL), FeBr2 (495 mg), THF (25 mL), and benzene (12 mL).
The mixture was refluxed under nitrogen for 12 h, opened to air,
and brought to dryness under vacuum. The residue was taken up
in a 1:1 mixture of dichloromethane/water (50 mL). The organic
layer was separated, washed with water (3 × 75 mL), and stirred
with a 5:1 mixture of saturated aqueous NaCl and HCl (72 mL)
for 90 min. The organic layer was decanted, and solvent was
removed under vacuum. The remaining solid was purified by
column chromatography (silica gel, dichloromethane to 5% methanol/
dichloromethane) and retreated with aqueous NaCl and HCl as
described above. The organic phase was separated, washed with
water (3 × 75 mL), dried over Na2SO4, and taken to dryness.
Recrystallization from dichloromethane and ether afforded pure 11
as a brown powder (189 mg, 83% yield). Anal. Calcd for
C95H98N8O5Cl2Fe2: C, 70.68; H, 6.12; N, 6.94. Found: C, 70.72;
H, 6.15; N, 7.00. HRFABMS ([M - Cl]+): calcd for C95H98N8O5-
Cl2Fe2 m/z 1577.605, found 1577.599.
1
talline powder (1.57 g, 34% yield). H NMR (500 MHz, CDCl3,
25 °C): δ ) 8.01 (s, 4H, meso), 7.95 (d, J ) 9.9 Hz, 2H, ArH),
7.76 (t, J ) 8.4 Hz, 2H, ArH), 7.28 (t, J ) 7.5 Hz, 2H, ArH), 7.19
(d, J ) 8.7 Hz, 2H, ArH), 6.98 (d, J ) 7.2 Hz, 2H, ArH), 6.80 (d,
J ) 8.7 Hz, 2H, ArH), 4.17 (s, 6H, OCH3), 3.33 (m, 4H, CH2),
3.02 (s, 6H, OCH3), 2.70 (m, 4H, CH2), 2.53 (s, 12H, CH3), 2.33
(s, 6H, CH3), 2.25 (s, 12H, CH3), 1.70 (m, 4H, CH2), 1.28 (t, J )
7.2 Hz, 6H, CH3), 0.729 (t, J ) 7.2 Hz, 6H, CH3), -2.27 (br s,
2H, NH), -3.68 (br s, 2H, NH). HRFABMS (MH+) calcd for
C95H102N8O5 m/z 1435.798, found 1435.804.
Zn2(DPXM) (8). A saturated methanolic solution of Zn(OAc)2‚
2H2O (1 mL) was added to a solution of 7 (20 mg, 0.014 mmol) in
chloroform (5 mL). The reaction was refluxed for 30 min and taken
to dryness. The solid residue was purified by flash column
chromatography (silica gel, 1:1 to 3:1 dichloromethane/hexanes)
followed by recrystallization from dichloromethane/methanol to
deliver analytically pure 8 as a scarlet microcrystalline powder in
essentially quantitative yield. 1H NMR (500 MHz, CDCl3, 25 °C):
δ ) 8.40 (s, 4H, meso), 7.83 (d, J ) 7.8 Hz, 2H, ArH), 7.50 (t, J
) 8.4 Hz, 2H, ArH), 7.12 (t, J ) 6.9 Hz, 2H, ArH), 6.88 (d, J )
8.4 Hz, 2H, ArH), 6.73 (d, J ) 6.9 Hz, 2H, ArH), 6.63 (d, J ) 8.4
Hz, 2H, ArH), 3.63 (s, 6H, OCH3), 3.35 (m, 4H, CH2), 3.07 (m,
8H, CH2), 2.88 (s, 6H, OCH3), 2.71 (m, 4H, CH2), 2.33 (s, 6H,
CH3), 2.23 (s, 24H, CH3), 1.24 (t, J ) 7.2 Hz, 6H, CH3), 1.06 (t,
J ) 7.2 Hz, 6H, CH3). Anal. Calcd for C95H98N8O5Zn2: C, 73.02;
H, 6.32; N, 7.17. Found: C, 73.06; H, 6.56; N, 7.17. HRFABMS
(M+): calcd for C95H98N8O5Zn2 m/z 1558.624, found 1558.630.
Cu2(DPXM) (9). To a 12-mL chloroform solution of 7 (56 mg,
0.039 mmol) was added a solution of Cu(OAc)2‚2H2O (130 mg)
and potassium acetate (145 mg) in 12 mL of methanol. The resulting
Mn2Cl2(DPXM) (12). A solution of 7 (202 mg, 0.141 mmol)
and Mn(OAc)2‚4H2O (495 mg) in DMF (15 mL) was refluxed in
air for 3 h. The reaction was cooled to room temperature and taken
to dryness. The residue was taken up in a 1:1 mixture of
dichloromethane/water (50 mL). The organic layer was separated,
washed with water (3 × 75 mL), and stirred with a 5:1 mixture of
saturated aqueous NaCl and HCl (72 mL) for 90 min. The organic
layer was decanted and solvent was removed under vacuum. The
remaining solid was purified by column chromatography (silica gel,
dichloromethane to 5% methanol/dichloromethane) and retreated
with aqueous NaCl and HCl as described above. The organic phase
was separated, washed with water (3 × 75 mL), dried over Na2-
SO4, and taken to dryness. Recrystallization from dichloromethane
and ether afforded pure 12 as a brown solid (202 mg, 89% yield).
Anal. Calcd for C95H98N8O5Cl2Mn2: C, 70.76; H, 6.13; N, 6.95;
3010 Inorganic Chemistry, Vol. 41, No. 11, 2002

Tuning the Pocket Sizes of Cofacial Bisporphyrins
Scheme 1a
a
(a) Ethanol, HCl, reflux; (b) NaOH, ethylene glycol, reflux; (c) (i) POCl3/DMF; (ii) Na2CO3; (d) (i) 6, p-TsOH, methanol; (ii) o-chloranil; (e) MX2,
reflux. (f) (i) 6, p-TsOH, methanol; (ii) o-chloranil; (g) Zn(OAc)2‚2H2O, chloroform/methanol, reflux.
Cl, 4.40. Found: C, 70.35; H, 6.56; N, 6.84; Cl, 4.40. HRFABMS
([M - Cl]+): calcd for C95H98N8O5Cl2Mn2 m/z 1575.611, found
1575.611.
separated, washed with water (3 × 25 mL), dried over Na2SO4,
and taken to dryness. Recrystallization from dichloromethane/
methanol afforded pure 13 as a plum solid (250 mg, 11% yield).
1H NMR (500 MHz, CDCl3, 25 °C): δ ) 9.78 (s, 4H, meso), 8.66
(d, J ) 7.5 Hz, 2H, ArH), 7.76 (t, J ) 6.9 Hz, 2H, ArH), 7.70 (d,
J ) 7.2 Hz, 2H, ArH), 7.58 (t, J ) 7.8 Hz, 2H, ArH), 6.83 (d, J )
8.2 Hz, 2H, ArH), 6.80 (d, J ) 8.2 Hz, 2H, ArH), 3.80 (m, 16H,
CH2), 3.37 (s, 6H, OCH3), 3.17 (s, 6H, OCH3), 2.43 (s, 12H, CH3),
2.36 (s, 12H, CH3), 1.58 (m, 24H, CH3), -2.83 (br s, 4H, NH).
HRFABMS (M+): calcd for C92H96N8O5 m/z 1393.798, found
1393.758.
Zn2(DPDM) (14). A saturated methanolic solution of Zn(OAc)2‚
2H2O (1 mL) was added to a solution of 13 (50 mg, 0.0359 mmol)
in chloroform (5 mL). The reaction mixture was refluxed for 30
min and taken to dryness. The solid residue was purified by flash
column chromatography (silica gel, 3:1 dichloromethane/hexanes)
followed by recrystallization from dichloromethane/methanol to
deliver analytically pure 14 as a bright red powder in essentially
quantitative yield. 1H NMR (500 MHz, CDCl3, 25 °C): δ ) 9.72
(s, 4H, meso), 8.65 (d, J ) 7.5 Hz, 2H, ArH), 7.74 (m, 4H, ArH),
7.58 (t, J ) 8.1 Hz, 2H, ArH), 6.85 (d, J ) 7.8 Hz, 2H, ArH), 6.79
(d, J ) 8.4 Hz, 2H, ArH), 3.77 (m, 16H, CH2), 3.41 (s, 6H, OCH3),
4,6-Bis[(2,3,13,17-tetraethyl-3,7,12,18-tetramethyl-15-(2,6-
dimethoxyphenyl)-5-porphyrinyl)]-dibenzofuran, H4(DPDM) (13).
A suspension of 3 (1 g, 1.60 mmol) and 6 (2.8 g, 3.20 mmol) in
freshly distilled methanol (300 mL) was purged with nitrogen for
15 min. A solution of p-toluenesulfonic acid (0.67 g) in methanol
(15 mL) was added dropwise over a period of 90 min. The resulting
red mixture was stirred in the dark under nitrogen for 2 days. Solid
o-chloranil (533 mg) was added in one portion, and stirring was
continued in air for 24 h. The mixture was taken to dryness, and
the solid was redissolved in chloroform (50 mL). A saturated
methanolic solution of Zn(OAc)2‚2H2O (5 mL) was added, and the
solution was refluxed for 1 h. The solvent was removed and the
remaining residue was purified by repeated flash column chroma-
tography (silica gel, 1:1 dichloromethane/hexanes). The bright red
band that eluted was collected, taken to dryness, and redissolved
in dichloromethane (60 mL). The solution was vigorously stirred
with 6 N HCl (6 mL) for 25 min. The solution was neutralized
with a 10% aqueous sodium carbonate solution, and the mixture
was stirred for an additional 15 min. The organic phase was
Inorganic Chemistry, Vol. 41, No. 11, 2002 3011

Chang et al.
3.09 (s, 6H, OCH3), 2.43 (s, 12H, CH3), 2.34 (s, 12H, CH3), 1.59
(m, 24H, CH3). Anal. Calcd for C92H92N8O5Zn2: C, 72.67; H, 6.10;
N, 7.37. Found: C, 72.44; H, 5.84; N, 7.49. HRFABMS (M+):
calcd for C95H98N8O5Zn2 m/z 1516.577, found 1516.567.
General Details of X-ray Data Collection and Reduction.
X-ray diffraction data were collected using a Siemens three-circle
diffractometer equipped with a CCD detector. Measurements were
carried out at -90 °C using Mo KR (λ ) 0.71073 Å) radiation,
which was wavelength selected with a single-crystal graphite
monochromator. Four sets of data were collected using ω scans
and a -0.3° scan width. All calculations were performed using a
PC workstation. The data frames were integrated to hkl/intensity,
and final unit cells were calculated by using the SAINT v.4.050
program from Siemens. The structures were solved and refined with
the SHELXTL v.5.03 suite of programs developed by G. M.
Sheldrick and Siemens Industrial Automation, Inc., 1995.
X-ray Structure of H4(DPXM)‚2CH2Cl2 (7). A 0.50 mm ×
0.31 mm × 0.30 mm royal purple crystal of plate morphology was
obtained from slow diffusion of methanol into a dichloromethane
solution of the compound. The crystal was coated in STP and
mounted onto a glass fiber. A total of 43593 reflections were
collected in the θ range 2.14-26.37°, of which 17101 were unique
(Rint ) 0.0342). Hydrogen atoms were placed in calculated positions
using a standard riding model and were refined isotropically. The
largest peak and hole in the difference map were 0.934 and -0.502
e Å-3, respectively. The least squares refinement converged
normally, giving residuals of R1 ) 0.0683 and wR2 ) 0.1850,
with GOF ) 1.051.
Figure 1. Crystal structure of H4(DPXM) (7). Thermal ellipsoids are drawn
at the 25% probability level. Hydrogen atoms and solvent molecules within
the lattice have been omitted for clarity.
X-ray Structure of Zn2(DPXM)‚5CH3OH‚1.5H2O‚CH2Cl2 (8).
A 0.10 mm × 0.26 mm × 0.40 mm brick red crystal of needle
morphology was obtained from slow diffusion of methanol into a
dichloromethane solution of the compound. The crystal was coated
in STP and mounted onto a glass fiber. A total of 48278 reflections
were collected in the θ range 2.32-26.38°, of which 19020 were
unique (Rint ) 0.0394). The Patterson method was used to locate
the zinc atoms; all remaining atoms were placed using the difference
Fourier map. Hydrogen atoms were placed in calculated positions
using a standard riding model and were refined isotropically. A
disordered ethyl group attached to C(28) was assigned half-
occupancy in two different conformations. Oxygen atoms O(8) and
O(11) from two disordered methanol solvent molecules were
assigned half-occupancy in two different conformations. The largest
Figure 2. Crystal structure of Zn2(DPXM) (8). Thermal ellipsoids are
drawn at the 25% probability level. Hydrogen atoms and solvent molecules
within the lattice have been omitted for clarity.
peak and hole in the difference map were 1.208 and -0.931 e Å-3
,
respectively. The least squares refinement converged normally,
giving residuals of R1 ) 0.0854 and wR2 ) 0.2323, with GOF )
1.022.
X-ray Structure of Mn2Cl2(DPXM)‚4CHCl3 (12). A 0.50 mm
× 0.25 mm × 0.20 mm dark brown crystal of needle morphology
was obtained from slow diffusion of hexane into a chloroform
solution of the compound. The crystal was coated in STP and
mounted onto a glass fiber. A total of 38526 reflections were
collected in the θ range 1.23-23.24°, of which 14012 were unique
(Rint ) 0.0446). The Patterson method was used to locate the
manganese atoms; all remaining atoms were placed using the
difference Fourier map. Hydrogen atoms were placed in calculated
positions using a standard riding model and were refined isotro-
pically. The largest peak and hole in the difference map were 1.486
and -1.021 e Å-3, respectively. The least squares refinement
converged normally, giving residuals of R1 ) 0.0812 and wR2 )
0.2045, with GOF ) 1.101.
Figure 3. Crystal structure of Mn2Cl2(DPXM) (12). Thermal ellipsoids
are drawn at the 25% probability level. Hydrogen atoms and solvent
molecules within the lattice have been omitted for clarity.
300, Mercury 300, or Inova 500 spectrometer at 25 °C. All chemical
shifts are reported using the standard δ notation in parts per million;
positive chemical shifts are to higher frequency from the given
reference. Absorption spectra were obtained using a Cary-17
1
Physical Measurements. H NMR spectra were collected in
CDCl3 (Cambridge Isotope Laboratories) at the MIT Department
of Chemistry Instrumentation Facility (DCIF) using either a Unity
3012 Inorganic Chemistry, Vol. 41, No. 11, 2002

Tuning the Pocket Sizes of Cofacial Bisporphyrins
Table 1. Crystallographic Data for 7, 8, and 12
7
8
12
empirical formula
fw
T (K)
C97H106Cl4N8O5
1605.72
183(2)
C101H123Cl2N8O11.5Zn2
1834.71
183(2)
C99H102Cl14N8O5Mn2
2090.10
183(2)
λ (Å)
0.71073
monoclinic
P21/c
28.8353(12)
17.1139(7)
17.5978(7)
90
98.826(1)
90
8581.4(6)
4
0.71073
monoclinic
P21/n
14.5517(6)
22.9226(10)
28.5155(13)
90
90.3121(14)
90
9511.6(7)
4
1.281
0.624
3884
0.71073
monoclinic
P2/c
19.5891(3)
15.0741(2)
33.2019(6)
90
91.947(10)
90
9798.5(3)
4
1.420
0.696
4336
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F
calcd (g cm-3
)
1.243
0.196
3408
abs coeff (mm-1
F(000)
)
reflns collcd
ind reflns
max/min transm
43593
48278
38526
17101 (Rint ) 0.0342)
0.9828/0.8316
17101/0/1043
1.051
19020 (Rint ) 0.0394)
0.8944/0.6748
19020/4/1078
1.022
14012 (Rint ) 0.0446)
0.2752/0.2252
13991/0/1154
1.101
data/restraints/params
GOFb on F2
2
R1a (Fo ) (>2σ(I))
0.0683
0.0854
0.0812
wR2b ) 0.19
0.0833
wR2b ) 0.1994
0.1283
wR2b ) 0.1460
0.1026
2
R1a (Fo ) (all data)
2
2
a R1 ) ∑||Fo - |Fc||/∑|Fo|. b GOF ) (∑w(Fo - Fc )2/(n - p))1/2 where n is the number of data and p is the number of parameters refined.
Table 2. Selected Geometric Lengths (Å) for 7, 8, and 12
spectrophotometer modified by On-Line Instrument Systems (OLIS)
7
8
12
to include computer control or a Spectral Instruments 440 spec-
trophotometer.
Ct(1)-N(1) 2.129
Ct(1)-N(2) 2.014
Ct(1)-N(3) 2.118
Ct(1)-N(4) 2.021
Ct(2)-N(5) 2.013
Ct(2)-N(6) 2.119
Ct(2)-N(7) 2.007
Ct(2)-N(8) 2.114
Zn(1)-N(1) 2.063(4) Mn(1)-N(1) 2.015(5)
Zn(1)-N(2) 2.062(4) Mn(1)-N(2) 1.999(5)
Zn(1)-N(3) 2.066(4) Mn(1)-N(3) 2.015(5)
Zn(1)-N(4) 2.057(4) Mn(1)-N(4) 1.996(5)
Zn(2)-N(5) 2.047(4) Mn(2)-N(5) 2.004(7)
Zn(2)-N(6) 2.049(5) Mn(2)-N(6) 2.014(5)
Zn(2)-N(7) 2.065(4) Mn(2)-N(7) 2.010(4)
Zn(2)-N(8) 2.068(4) Mn(2)-N(8) 2.017(5)
Results and Discussion
Synthesis. The synthetic strategy for the preparation of
pillared cofacial bisporphyrins containing a single aryl group
trans to the spacer is outlined in Scheme 1. We employ the
convergent three-branch approach developed by Chang4 and
modified by Collman;3 the stepwise approach described here
allows for the synthesis of meso and/or â-substituted
derivatives with more precise control over statistical methods
(e.g., Adler-Longo, Lindsey) that have been previously used
to prepare â-free cofacial bisporphyrins bearing meso-aryl
substituents.24,25
Aryl-substituted dipyrrylmethane dialdehydes are conve-
niently supplied in a three-step protocol from a pyrrole and
an aldehyde; the 2,6-dimethoxyphenyl derivative is an
exemplary system. Ester-protected dipyrrylmethane 4 is
obtained in 89% yield by acid-catalyzed reaction of 2,6-
dimethoxybenzaldehyde with pyrrole 1 in boiling ethanol.
Deprotection of the R-ethyl esters of 4 with sodium
hydroxide in ethylene glycol gives R-free dipyrrylmethane
5 in almost quantitative yield. Vilsmeier formylation of 5
using POCl3/DMF followed by base hydrolysis produces 6
in 98% yield.
O(1)-C(52) 1.375(3) Zn(1)-O(6) 2.175(5) Mn(1)-Cl(8) 2.400(2)
O(1)-C(53) 1.376(3) Zn(2)-O(7) 2.252(6) Mn(2)-Cl(2) 2.409(2)
Zn(2)-O(8) 2.459(17)
the highest reported for these types of pillared compounds.
1
The H NMR spectrum of 7 is consistent with a splayed
structure in solution, as the internal NH-pyrrolic resonances
(δ -2.27, -3.68 ppm) are in the same range as an analogous
monomeric xanthene porphyrin (δ -3.10, -3.29 ppm).16
Homobimetallic coordination complexes of 7 are prepared
smoothly by direct reaction with metal salts. The binuclear
zinc(II) complex Zn2(DPXM) (8) is obtained in quantitative
yield by reaction of 7 with Zn(OAc)2‚2H2O in methanol/
1
chloroform mixtures. Complex 8 was characterized by H
NMR, high-resolution mass spectrometry, and elemental
analyses. The biscopper(II) complex Cu2(DPXM) (9) is
prepared in excellent yield (87%) using Cu(OAc)2‚2H2O and
potassium acetate in methanol/chloroform mixtures. Complex
9 gave satisfactory mass spectral and elemental analyses.
Dinickel complex Ni2(DPXM) (10) is synthesized in good
yield (81%) using Adler’s method (NiCl2/DMF). The dia-
Cyclization of xanthene tetrapyrrole 2 with 6 in the
presence of p-toluenesulfonic acid (PTSA) followed by
oxidation with o-chloranil affords bisporphyrin 7 (DPXM
) diporphyrin xanthene methoxyaryl) after workup and
purification. The yield for the final coupling step (34%) is
1
magnetic compound was characterized by H NMR, high-
resolution mass spectrometry, and elemental analyses. Bis-
chloroiron(III) compound 11 is delivered in good yield (83%)
from FeBr2 and 2,6-lutidine in a THF/benzene solvent system
followed by anion exchange with aqueous NaCl and HCl.
Satisfactory high-resolution mass spectral data and elemental
(24) Collman, J. P.; Tyvoll, D. A.; Chng, L. L.; Fish, H. T. J. Org. Chem.
1995, 60, 1926-1931.
(25) Naruta, Y.; Sasayama, M. J. Chem. Soc., Chem. Commun. 1994, 2667-
2668.
Inorganic Chemistry, Vol. 41, No. 11, 2002 3013

Chang et al.
Table 3. Selected Geometric Angles (deg) for 7, 8 and 12
7
8
12
N(1)-Mn(1)-N(2)
N(1)-Ct(1)-N(2)
N(1)-Ct(1)-N(3)
N(2)-Ct(1)-N(3)
N(1)-Ct(1)-N(4)
N(2)-Ct(1)-N(4)
N(3)-Ct(1)-N(4)
N(7)-Ct(2)-N(8)
N(7)-Ct(2)-N(6)
N(8)-Ct(2)-N(6)
N(7)-Ct(2)-N(5)
N(8)-Ct(2)-N(5)
N(6)-Ct(2)-N(5)
C(53)-O(1)-C(52)
C(52)-C(51)-C(25)
C(53)-C(41)-C(5)
C(50)-C(51)-C(25)
C(42)-C(41)-C(5)
C(51)-C(52)-O(1)
C(41)-C(53)-O(1)
83.3
179.4
96.5
97.0
179.3
83.2
83.4
96.5
177.8
177.7
96.5
83.7
119.6(2)
121.3(2)
122.3(2)
120.0(2)
119.3(2)
115.0(2)
115.0(2)
N(1)-Zn(1)-N(2)
N(1)-Zn(1)-N(3)
N(2)-Zn(1)-N(3)
N(1)-Zn(1)-N(4)
N(2)-Zn(1)-N(4)
N(3)-Zn(1)-N(4)
N(7)-Zn(2)-N(8)
N(7)-Zn(2)-N(6)
N(8)-Zn(2)-N(6)
N(7)-Zn(2)-N(5)
N(8)-Zn(2)-N(5)
N(6)-Zn(2)-N(5)
N(1)-Zn(1)-O(6)
N(2)-Zn(1)-O(6)
N(3)-Zn(1)-O(6)
N(4)-Zn(1)-O(6)
N(5)-Zn(2)-O(7)
N(6)-Zn(2)-O(7)
N(7)-Zn(2)-O(7)
N(8)-Zn(2)-O(7)
86.6(2)
166.3(2)
92.0(2)
92.1(2)
169.9(2)
86.9(2)
86.5(2)
92.5(2)
172.0(2)
176.1(2)
92.8(2)
87.7(2)
99.2(2)
94.6(2)
94.5(2)
95.5(2)
94.2(2)
92.5(2)
89.7(2)
83.6(5)
91.9(2)
163.1(2)
86.4(2)
86.6(2)
168.3(2)
91.7(2)
92.4(2)
85.3(2)
162.2(2)
166.6(2)
86.3(2)
91.9(2)
98.2(1)
94.3(2)
98.7(1)
97.4(1)
97.1(1)
102.6(2)
96.3(1)
95.2(1)
N(1)-Mn(1)-N(3)
N(2)-Mn(1)-N(3)
N(1)-Mn(1)-N(4)
N(2)-Mn(1)-N(4)
N(3)-Mn(1)-N(4)
N(7)-Mn(2)-N(8)
N(7)-Mn(2)-N(6)
N(8)-Mn(2)-N(6)
N(7)-Mn(2)-N(5)
N(8)-Mn(2)-N(5)
N(6)-Mn(2)-N(5)
N(1)-Mn(1)-Cl(1)
N(2)-Mn(1)-Cl(1)
N(3)-Mn(1)-Cl(1)
N(4)-Mn(1)-Cl(1)
N(5)-Mn(2)-Cl(2)
N(6)-Mn(2)-Cl(2)
N(7)-Mn(2)-Cl(2)
N(8)-Mn(2)-Cl(2)
Table 4. Crystallographically Derived Intradimer Geometrical Features
analyses accompanied 11. Lastly, bischloromanganese(III)
complex 12 is afforded in high yield (89%) by an Adler
reaction of 7 with Mn(OAc)2‚4H2O in DMF with subsequent
anion exchange in the same manner as the bischloroiron-
(III) derivative (aqueous NaCl/HCl).
for DPX, DPD, and DPXM Compoundsa
inter-
planar torsional a-b
c-d
dist
(Å)
M-M Ct-Ct angle
twist
(deg)
dist
(Å)
(Å)
(Å)
(deg)
H4(DPX)30
na
na
na
3.708
7.775
5.913
4.002
8.220
6.104
3.863
7.587
5.987
4.982
4.7
23.0
16.9
4.4
24.6
32.9
20.8
14.3
1.9
26.0
7.9
1.2
13.1
19.2
3.552 4.324
4.853 5.654
4.675 4.602
4.619 4.272
4.800 5.577
4.676 4.677
4.656 4.485
The use of dipyrrylmethane dialdehyde 6 is not limited to
bisporphyrin templates containing a xanthene pillar. For
example, the dibenzofuran tetrapyrrole undergoes cyclization
with 6 in the presence of PTSA to give the corresponding
cofacial bisporphyrin H4(DPDM) (13) (DPDM ) diporphyrin
dibenzofuran methoxyaryl) in 11% yield. As observed for 7
and the parent bisporphyrin H4(DPD), the chemical shift for
the internal NH-pyrrolic resonances (δ -2.83 ppm) indicates
an open structure in solution.17 Zinc insertion proceeds in
quantitative yield using the standard method (Zn(OAc)2‚
2H2O, methanol/dichloromethane) to generate Zn2(DPDM)
(14). Complex 14 was fully characterized by 1H NMR, high-
resolution spectrometry, and elemental analyses.
H4(DPD)30
H4(DPXM) (7)
Zn2(DPX)16
Zn2(DPD)17
Zn2(DPXM) (8)
Mn2(DPXM) (12) 5.378
a Metrics were derived as follows. Macrocyclic centers (Ct) were
calculated as the centers of the four-nitrogen (4-N) planes for each
macrocycle. Interplanar angles were measured as the angle between the
4-N least-squares planes. Torsional twists were measured as the angle
between the two meso-carbon to spacer bonds. The a-b and c-d separations
represent the distances between the bridge carbons attached to the porphyrin
meso positions and those porphyrin meso-carbons, respectively.
7. Both ring systems display a saddle conformation in the
solid state. The porphyrin subunit containing N(1) to N(4)
has a mean deviation from planarity of 0.23 Å with meso
carbons alternately displaced from the porphyrin mean plane,
ranging from 0.39 Å above to 0.38 Å below the 24-atom
macrocyclic unit. The macrocycle containing N(5) to N(8)
exhibits a slightly greater mean deviation from planarity (0.29
Å). Similar to the ring containing N(1) to N(4), the porphyrin
with N(5) to N(8) contains meso carbons alternately dis-
placed from the macrocyclic mean plane, ranging from 0.30
Å above to 0.36 Å below the 24-atom ring subunit. The
internal pyrrolic hydrogen atoms are in their calculated
positions, and are disordered over all the nitrogen atoms.
An interesting feature of 7 is the small butterfly fold of the
xanthene backbone along its center O(1)-C(47) axis. For
7, the butterfly angle is 5.5°; in contrast, the parent porphyrin
H4(DPX) has a large butterfly distortion of 36.1°.30
Structural Chemistry. The structural consequences of a
single aryl group trans to the spacer have been investigated
using single-crystal X-ray analysis. In order to provide a
systematic study, three structures of the DPXM framework
have been determined for comparison with known DPX and
DPD congeners: the free-base bisporphyrin 7, compound 8
containing the non-redox-active zinc(II) metal, and complex
12 bearing redox-active manganese centers.
The molecular structures of 7, 8, and 12 are shown in
Figures 1-3, respectively. Crystallographic data and selected
geometrical measurements are given in Tables 1-4. Trends
in bond lengths and angles of macrocyclic core structures
and side chains agree well with those observed in related
systems such as H4(DPX), H4(DPD), H4(DPA), and
H4(DPB).1,3,9-11,16-18,26-29 Structural highlights of the core
structures for 7, 8, and 12 are as follows:
(26) Fillers, J. P.; Ravichandran, K. G.; Abdalmuhdi, I.; Tulinsky, A.;
Chang, C. K. J. Am. Chem. Soc. 1986, 108, 417-424.
(27) 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,
4134-4142.
(28) Clement, T. E.; Nurco, D. J.; Smith, K. M. Inorg. Chem. 1998, 37,
1150-1160.
(29) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin) 1987, 64, 1-70.
(30) Chang, C. J.; Baker, E. A.; Pistorio, B. J.; Deng, Y.; Loh, Z.-H.; Miller,
S. E.; Carpenter, S. D.; Nocera, D. G. Inorg. Chem. 2002, in press.
3014 Inorganic Chemistry, Vol. 41, No. 11, 2002

Tuning the Pocket Sizes of Cofacial Bisporphyrins
Figure 4. Comparative views of the crystal structures of Zn2(DPX), 8 and Zn2(DPD): (a) side view, perpendicular to the bridge plane; (b) side view, along
the bridge plane. Alkyl side groups and hydrogen atoms have been omitted for clarity. Table 4 footnote defines the methods by which the crystallographically
derived geometric features were measured.
8. A conformational analysis reveals inequivalent ring
systems; however, both macrocycles of 8 display a saddle
conformation. The core containing Zn(1) has a more
pronounced saddle distortion with a mean deviation from
planarity of 0.16 Å. The ring with Zn(2) has a mean deviation
from planarity of 0.11 Å. The zinc(II) ions are situated in
different coordination environments. Zn(1) is five-coordinate,
binding an axial methanol solvate within the bisporphyrin
pocket at a distance of 2.175(5) Å. The Zn(II) core adopts
an approximate square-pyramidal geometry, as the N-Zn-N
bond angles are 90° ( 3.5° and the N-Zn-O bond angles
are 90° ( 9.2°. The zinc center is displaced slightly from
the porphyrin mean plane toward the axial ligand (0.21 Å).
The average equatorial Zn-N bond length of 2.062(4) Å is
in good agreement with literature values for five-coordinate
zinc(II) porphyrins.31 In contrast, Zn(2) is six-coordinate,
binding an axial methanol solvate outside the bisporphyrin
pocket and an axial water molecule inside the cofacial cleft.
The Zn-O bond lengths for the axial methanol and water
are 2.252(6) and 2.459(17) Å, respectively. The Zn(II) center
assumes a distorted octahedral geometry, and is displaced
0.11 Å toward the outer face of the bisporphyrin toward the
axial methanol; the N-Zn-N bond angles are 90° ( 3.2°
and the N-Zn-O bond angles are 90° ( 5.5°. The average
equatorial Zn-N bond length is 2.057(5) Å.
respectively. The average Mn-N bond length is 2.009(5)
Å. Both Mn(III) macrocycles exhibit a pronounced nonplanar
distortion. The macrocycle containing Mn(1) has a mean
deviation from planarity of 0.70 Å, while the ring with
Mn(2) has a mean deviation from planarity of 0.64 Å.
Taken together, the crystallographic data reveal that the
introduction of a single aryl group trans to the spacer can
considerably alter the pocket sizes of pillared cofacial
bisporphyrins. The molecular structures of homologous
biszinc(II) complexes of the DPX, DPXM, and DPD series
shown in Figure 4 confirm that this type of substitution for
the DPXM framework affords a family of bisporphyrin
platforms that exhibit vertical pocket sizes that are intermedi-
ate to those of the parent DPX and DPD systems. This is
clearly illustrated by the observed center-to-center distances,
which vary from 5.0 to 6.1 Å for the DPXM platforms. For
comparison, the DPX and DPD systems have center-to-center
distances of 3.8-4.7 Å and 7.6-8.2 Å, respectively. The
pocket size enlargements of the DPXM series compared to
their DPX counterparts are predominantly due to vertical
changes, revealed by the interplanar angles between the two
porphyrinic subunits. The DPXM series has interplanar
angles in the range of 16.9-32.9°. In contrast, the ring-
parallel DPX complexes display interplanar angles of less
than 5°. Horizontal changes are modest, as the torsional
angles for the structures of 7, 8, and 12 are all less than 26°.
12. The pentacoordinate Mn centers assume a highly
distorted square-pyramidal geometry, as the N-Mn-N bond
angles are 90° ( 4.7° and the N-Mn-Cl bond angles are
90° ( 10.3°. The axial chlorides are coordinated to the
Mn(III) ions outside the cofacial pocket with an average
Mn-Cl bond length of 2.405(2) Å. The metal centers
Mn(1) and Mn(2) are displaced out of the porphyrin planes
toward the axial chloride ligands by 0.25 and 0.27 Å,
Concluding Remarks
The success of cofacial bisporphyrins for efficient mul-
tielectron catalysis has traditionally relied on the choice of
spacer to provide a suitable cavity for substrate activation.
In contrast, our results reveal that strategic modification of
the porphyrin superstructure by substitution trans to the
spacer can also dramatically tune the vertical pocket sizes
of cofacial bisporphyrins. To this end, the synthesis and
(31) Kadish, K. M.; Smith, K. M.; Guilard, R. The Porphyrin Handbook;
Academic Press: San Diego, 2000.
Inorganic Chemistry, Vol. 41, No. 11, 2002 3015

Chang et al.
coordination chemistry of methoxyaryl-modified xanthene
(DPXM) and dibenzofuran (DPDM) systems has been
achieved. In particular, crystallographic analysis confirms
the ability of the DPXM platform to provide cofacial pockets
with vertical pocket sizes that span the parent DPX and DPD
platforms. The modular nature of this synthetic methodology
should provide ready access to more intricate structural
modifications, as the prerequisite aryldipyrrylmethanes are
readily obtained in large gram-scale quantities. The applica-
tion of these and related systems for the study of proton-
coupled small-molecule activation is underway.
Acknowledgment. C.J.C. thanks the MIT/Merck Foun-
dation and the National Science Foundation for predoctoral
fellowships. We thank P. Landing for inspiring discussions.
This work was supported by the National Institutes of Health
(Grant GM 47274).
Supporting Information Available: X-ray crystallographic file
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
IC025507K
3016 Inorganic Chemistry, Vol. 41, No. 11, 2002