Cross-conjugated bis(porphryin)s: Synthesis, electrochemical behavior, mixed valency, and biradical dication formation

Article abstract of DOI:10.1021/jo991046h

The synthesis and characterization of seven (Zn(II))₂bis(porphyrin) molecules are described. The molecular structures of two bis(porphyrin)s'(6 and 7) were determined by X-ray diffraction methods. Four of the compounds have their porphyrin moie

Full text of DOI:10.1021/jo991046h

9124
J . Org. Chem. 1999, 64, 9124-9136
Cr oss-Con ju ga ted Bis(p or p h r yin )s: Syn th esis, Electr och em ica l
Beh a vior , Mixed Va len cy, a n d Bir a d ica l Dica tion F or m a tion
David A. Shultz,* Hyoyoung Lee,† R. Krishna Kumar, and Kevin P. Gwaltney
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
Received J une 29, 1999
The synthesis and characterization of seven (ZnII)2bis(porphyrin) molecules are described. The
molecular structures of two bis(porphyrin)s (6 and 7) were determined by X-ray diffraction methods.
Four of the compounds have their porphyrin moieties attached in a meta-fashion to a substituted
benzene ring (1-4), two have porphyrin rings attached in a gem-fashion to a carbon-carbon double
bond (6 and 7), and one bis(porphyrin) (5) has a p-phenylene coupler. Bis(porphyrin)s 1, 4, and 5
contain tetraaryl-type porphyrins, while 2, 3, 6, and 7 contain triarylethynylporphyrins. Except
for exciton coupling in the unoxidized species, interaction between porphyrins is greater in the
triarylethynyl-type bis(porphyrin)s than in the tetraaryl-type bis(porphyrin)s regardless of the
number of bonds through which the interaction propagates. The cyclic voltammetry of the bis-
(porphyrin)s was examined, and a varying degree of interaction between electrophores within the
series was found. Redox splitting was observed for porphyrin ring oxidations of 3, 6, and 7,
suggesting interaction between the oxidized porphyrin rings beyond simple electrostatic repulsions.
No such redox splitting was observed for any of the tetraaryl-type bis(porphyrin)s (1, 4, and 5). We
conclude that two porphyrin radical cations interact best when (1) the interaction pathway is short
(∆E1/2(6/7) > ∆E1/2(2/3)), (2) π-overlap between the electrophore and coupler is maximized (minimal
bond torsions: ∆E1/2(6/7) > ∆E1/2(4/5)), and (3) the electron demand of the coupler matches that of
the spin carrier (∆E1/2(3) > ∆E1/2(2)). One-electron oxidized triarylethynyl-type bis(porphyrin)s
exhibit spectral features characteristic of mixed-valent compounds. In two cases (6•+ and 7•+), near-
IR bands near 8300 cm-1 were observed and are tentatively assigned to intervalence transitions.
Singly oxidized tetraaryl-type bis(porphyrin)s exhibit no such near-IR transitions, and electronic
absorption spectra recorded during electrochemical oxidations are marked by isosbestic points,
suggesting negligible interaction between the two halves of the molecule, consistent with cyclic
voltammetric results. Two-electron oxidation of 2-7 yields biradical dications whose frozen solution
EPR spectra lack fine structure, but exhibit ∆ms ) 2 transitions characteristic of exchange-coupled
S ) 1 states. Oxidation of 1 yields a biradical in which both exchange coupling and dipolar
interaction between unpaired electrons are presumably very weak; consequently no ∆ms ) 2 is
observed. The results of variable-temperature EPR spectroscopy of 22•2+-72•2+ suggest either a triplet
ground state or a singlet-triplet degeneracy. As a consequence of our results, we hypothesize that
the exchange interaction, and therefore the singlet-triplet gap, in a biradical di-ion can be adjusted
to favor the triplet state by simple substituent effects.
In tr od u ction
bene spins, resulting in a quartet ground state. Kamachi
has reported verdazyl-substituted porphyrins attached
to polymer backbones that exhibit only weak antiferro-
magnetic interactions.1,3 Iwamura has also studied the
magnetic behavior of pyridine nitroxides coordinated to
CrIII tetraphenylporphyrin.4 Miller and Epstein have been
studying MnIIporphyrin-TCNE complexes, many of which
order magnetically.5-17
Multispin molecular assemblies featuring metallopor-
phyrins are promising components of molecule-based
magnetic materials. Iwamura prepared free-base por-
phyrins and metalloporphyrins with one to four phenyl-
carbene groups.1,2 The phenylcarbenes can be ferromag-
netically or antiferromagnetically coupled through the
tetraphenylporphyrin core. For example, the syn-dicar-
bene (free-base and zinc-metalated) obeyed the Curie law
for a quintet ground state, while the anti-isomer is a
singlet ground state with the quintet 54 cal higher in
energy. In addition, the CuII monocarbene compound
showed ferromagnetic coupling between metal and car-
(3) Kamachi, M. Nihon Butsuri Gakkaishi 1987, 42, 351.
(4) Kitano, M.; Koga, N.; Iwamura, H. J . Chem. Soc., Chem.
Commun. 1994, 447.
(5) Miller, J . S.; Calabrese, J . C.; McLean, R. S.; Epstein, A. J . Adv.
Mater. 1992, 4, 498.
(6) Bo¨hm, A.; Vazquez, C.; McLean, R.; Calabrese, J .; Kalm, S.;
Manson, J .; Epstein, A.; Miller, J . Inorg. Chem. 1996, 35, 3083.
(7) Brandon, E. J .; Kollmar, C.; Miller, J . S. J . Am. Chem. Soc. 1998,
120, 1822.
(8) Brandon, E. J .; Arif, A. M.; Burkhart, B. M.; Miller, J . S. Inorg.
Chem. 1998, 37, 2792.
* To whom correspondence should be addressed. E-mail:
shultz@chemdept.chem.ncsu.edu. Fax: (919) 515-8920. Phone: (919)
515-6972. World Wide Web: http://www2.ncsu.edu/ncsu/chemistry/
das.html.
(9) Brinckerhoff, W. B.; Morin, B. G.; Brandon, E. J .; Miller, J . S.;
Epstein, A. J . J . Appl. Phys. 1996, 83, 6974.
(10) Buschmann, W.; Vazquez, C.; Ward, M.; J ones, N.; Miller, J .
Chem. Commun. 1997, 409.
† Current address: Center for Smart Supramolecules (CSS), Pohang
University of Science and Technology (POSTECH), San 31 Hyojadong,
Pohang 790-784, South Korea.
(1) Iwamura, H. Adv. Phys. Org. Chem. 1990, 26, 179.
(2) Koga, N.; Iwamura, H. Nihon Kagaku Kaishi 1989, 1456.
(11) Girtu, N. A.; Wynn, C. M.; Sugiura, K.-I.; Miller, J . S.; Epstein,
A. J . Synth. Met. 1997, 85, 1703.
10.1021/jo991046h CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/20/1999

Cross-Conjugated Bis(porphyrin)s
J . Org. Chem., Vol. 64, No. 25, 1999 9125
Ta ble 1. Su m m a r y of Cr ysta l Da ta , Da ta Collection , a n d
Refin em en t Deta ils
To extend interactions in materials utilizing porphy-
rins, we have prepared bis(porphyrin)s 1-7. In addition
to being useful as building blocks for two- or three-
dimensional coordination polymers, the cross-conjugated
molecules can be oxidized to yield biradical dications that
are exchange coupled by ferromagnetic coupling units.18,19
Herein, we describe the preparation of several new bis-
(porphyrin)s, and present EPR spectra that are consistent
with exchange coupling in six out of seven of the corre-
sponding biradical dications.
bis(porphyrin)
bis(porphyrin)
6‚1.5(C7H8)
7‚4(C7H8)
empirical formula
fw
C
115.50H105N8Zn2
C137H128N8Zn2
2017.21
1735.81
cryst size, mm
temp, K
cryst syst
space group
a, Å
0.06 × 0.06 × 0.34 0.08 × 0.12 × 0.18
158(2)
158(2)
triclinic
P1h
triclinic
P1h
12.9399(2)
18.3283(3)
25.4184(3)
103.937(1)
101.406(1)
107.230(1)
5347.7(1)
2
13.0929(2)
19.0849(1)
25.1560(1)
69.751(1)
78.647(1)
72.949(1)
5606.23(9)
2
b, Å
c, Å
R, deg
Resu lts
â, deg
γ, deg
V, Å3
Syn th esis. Bis(porphyrin)s 6 and 7 were prepared as
described previously,19,20 while the syntheses of bis-
(porphyrin)s 1-5 are shown below. Phenylacetylene-
substituted porphyrin 16 was prepared as described by
Lindsey,21 and coupled to 5-tert-butyl-1,3-diiodobenzene,
8,22 to give 1 in good yield. 5-Iodo-10,15,20-trimesitylpor-
phyrin, 17,23 was coupled to 5-tert-butyl-1,3-bis-(2′-tri-
methylsilanylethynyl)benzene, 9, and 1,5-dimethoxy-2,4-
bis(trimethylsilanylethynyl)benzene, 11, using our in situ
alkyne deprotection protocol20 to provide bis(porphyrin)s
2 in 68% and 3 in 65% yield, respectively.
Z
µ, mm-1
0.496
0.483
D
calcd, g/cm3
1.078
1.195
F(000)
radiation
λ, Å
1828
Mo KR
0.710 73
2132
Mo KR
0.710 73
70 891
22 680
1182
0.0851
0.1723
0.871
no. of reflns collected 70 300
no. of unique reflns
no. of params
R
13 974
1144
0.1042
0.2919
0.721
wR2
(∆F)max, e/Å3
Gable bis(porphyrin) 4 and p-phenylene-linked bis-
(porphyrin) 524 were prepared by Suzuki couplings23 of
iodide 17 to diboronic esters 13 and 15,25 respectively,
prepared using the method of Miyaura.25 The prepara-
tions of dibromides 1026 and 1227 have been reported.
For EPR studies, biradical dications 12•2+-72•2+ were
prepared by oxidation of 1-7, respectively, with 2 equiv
of tris(4-bromophenyl)amminium hexachloroantimonate.
(12) Epstein, A. J .; Miller, J . S. Synth. Met. 1996, 80, 231.
(13) Molecular Magnetism: From Molecular Assemblies to the
Devices; Coronado, E.; Delhaes, P., Gatteschi, D., Miller, J . S., Eds.;
Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996; Vol.
321.
(14) Yates, M. L.; Arif, A. M.; Manson, J . L.; Kalm, B. A.; Burkhart,
B. M.; Miller, J . S. Inorg. Chem. 1998, 37, 840.
(15) Wynn, C. M.; Girtu, M. A.; Brinckerhoff, W. B.; Sugiura, K.-I.;
Miller, J . S.; Epstein, A. J . Chem. Mater. 1997, 9, 2156.
(16) Miller, J . S.; Vazquez, C.; Epstein, A. J . J . Mater. Chem. 1995,
5, 707.
(17) Miller, J . S.; Epstein, A. J . Angew. Chem., Int. Ed. Engl. 1994,
33, 385.
(18) Shultz, D. A.; Lee, H.; Gwaltney, K. P.; Sandberg, K. A. Mol.
Cryst. Liq. Cryst. 1999, 334, 459.
2 equiv of N(4-BrC6H4)3SbCl6
bis(porphyrin)
8
CH2Cl2
(19) Shultz, D. A.; Lee, H.; Gwaltney, K. P. J . Org. Chem. 1998, 63,
7584.
(20) Shultz, D. A.; Gwaltney, K. P.; Lee, H. J . Org. Chem. 1998, 63,
4034.
[bis(porphyrin)]2•2+[SbCl6
]
2
-
(21) Lindsey, J . S.; Prathapan, S.; J ohnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941.
(22) Sasaki, S.; Iyoda, M. Chem. Lett. 1995, 11, 1011.
(23) Shultz, D. A.; Gwaltney, K. P.; Lee, H. J . Org. Chem. 1998, 63,
769.
(24) Yang, S. I.; Lammi, R. K.; Seth, J .; Riggs, J . A.; Arai, T.; Kim,
D.; Bocian, D. F.; Holten, D.; Lindsey, J . S. J . Phys. Chem. B 1998,
102, 9426.
(25) Ishiyama, T.; Murata, M.; Miyaura, N. J . Org. Chem. 1995, 60,
7508.

9126 J . Org. Chem., Vol. 64, No. 25, 1999
Ta ble 2. Selected Geom etr ic P a r a m eter sa for 6 a n d 7
Shultz et al.
geometric param
bis(porphyrin) 6
bis(porphyrin) 7
geometric param
bis(porphyrin) 6
bis(porphyrin) 7
Zn1‚‚‚Zn2
11.230
6.250
175.1(10)
170.2(10)
125.4(9)
174.2(9)
11.314
6.366
177.8(5)
172.5(4)
123.1(4)
172.5(4)
C96-C97-C100
122.6(9)
177.7(10)
111.8(8)
4.3
3.3
100.92
123.9(4)
174.4(5)
112.9(3)
3.3
2.4
103.21
C2‚‚‚C49
C96-C95-C49
C2-C99-C98
C99-C98-C97
C98-C97-C100
C97-C96-C95
C96-C97-C98
C98-C97-C100-C105
C96-C97-C100-C101
C2‚‚‚C97‚‚‚C49
a
Distances in angstroms and bond/dihedral angles in degrees.
X-r a y Cr ysta llogr a p h y. Crystals of both 6 and 7 were
grown by diffusion of pentane into a toluene solution of
the bis(porphyrin).28 ORTEPs of 6 and 7 are shown in
Figures 1 and 2, respectively. A summary of the crystal-
lographic data for both 6 and 7 is given in Table 1, and
selected geometric parameters are given in Table 2.
Electr och em istr y. Bis(porphyrin)s 1-7 were exam-
ined by cyclic voltammetry (CV), and the results are
displayed in Figures 3 and 4, and in Table 3. Only the
redox waves associated with the porphyrin/porphyrin•+
couples are shown. The two-electron nature of these
waves was confirmed by controlled-potential coulometry.
Electr on ic Absor p tion Sp ectr oscop y. Electronic
absorption spectra of 1-7 are shown in Figure 5, and
spectroscopic data for these compounds as well as model
compound Zn(tetramesitylporphyrin), Zn TMP , are col-
lected in Table 4. Bis(porphyrin)s 2-7 show split Soret
(B) bands, while bis(porphyrin) 1 does not. Numerical
values for Soret splitting were determined by fitting the
Soret bands to a four-Gaussian model.28
Electronic absorption spectra recorded between 20000
cm-1 (500 nm) and 7000 cm-1 (1429 nm) of Zn TMP •+
1
,
2•2+, 42•2+, and 52 •2+ are shown in Figure 6, while spectra
of 22•2+, 32•2+, 62•2+, and 72•2+ are shown in Figure 7.
Pertinent spectroscopic data are collected in Table 4. The
spectra in Figures 6 and 7 were collected at various
intervals during controlled-potential coulometry. The
singly oxidized forms 22•+, 32•+, 62•+, and 72•+ exhibit
unique absorption bands (marked with an asterisk) that
are absent in both the un-oxidized forms and the doubly
oxidized forms, and are independent of concentration.
EP R Sp ectr oscop y. X-band EPR spectra of 12•2+
-
72•2+ generated by oxidation with 2 equiv of tris(4-
bromophenyl)amminium hexachloroantimonate recorded
at 298 K are single, broad lines with unresolved hyperfine
structure and a peak-to-peak separation of ca. 7 G.
Spectra recorded at 77 K also consist of single lines and
thus lack fine structure, but exhibit ∆ms ) 2 transitions
characteristic of triplet states, as shown in Figure 8. As
noted previously,19 chemical oxidation of 6 is sometimes
accompanied by some decomposition; however, a ∆ms )
2 transition is observed. The temperature dependence of
the doubly integrated ∆ms ) 2 transitions (Curie plots)
are shown in Figure 9. Pertinent data are collected in
Table 5.
Discu ssion
Molecu la r Design Elem en ts. The bis(porphyrin)s
presented were prepared for several purposes. For ex-
ample, we wish to use these molecules for the construc-
tion of coordination polymers with interesting magnetic
(26) Kohn, M.; Lo¨ff, G. Monatsh. Chem. 1924, 45, 589.
(27) Ishida, T.; Iwamura, H. J . Am. Chem. Soc. 1991, 113, 4238.
(28) See the Supporting Information for spectroscopic and crystal-
lographic data.

Cross-Conjugated Bis(porphyrin)s
J . Org. Chem., Vol. 64, No. 25, 1999 9127
F igu r e 1. ORTEP of bis(porphyrin) 6. Hydrogens have been omitted for clarity.
F igu r e 2. ORTEP of bis(porphyrin) 7. Hydrogens have been omitted for clarity.
properties, and we are also interested in using porphyrin
π radical cations as spin-containing components of high-
spin molecules.18-20,29 Important structural features of
1-7 are the mesityl groups. The nearly orthogonal
arrangement of mesityl rings with the porphyrin ring30
reduces the possibility of aggregation, and after oxidation
prevents delocalization of spin density into the mesityl
groups, thus maximizing spin density in the coupler.
(29) Shultz, D. A.; Sandberg, K. A. J . Phys. Org. Chem. 1999, 12,
10.
(30) Scheidt, W. R.; Lee, Y. J . In Structure and Bonding, Vol. 64;
Buchler, J . W., Ed.; Springer-Verlag: Berlin, 1987; pp 1-70.

9128 J . Org. Chem., Vol. 64, No. 25, 1999
Shultz et al.
Oxidation of a variety of meso-tetraarylmetalloporphy-
rins gives radical cations having significant spin densities
on the meso-carbons (SOMOs are a2u-like).18,29,31-38 To
favor intramolecular high-spin coupling of the unpaired
electrons of bis(porphyrin) biradical dications, we at-
tached the porphyrins to π-systems (couplers) to give
cross-conjugated molecules whose topologies are identical
to those of known high-spin organic molecules.39 That is,
12•2+-42•2+ are m-phenylene-type biradicals, and 62•2+ and
72•2+ are trimethylenemethane-type (TMM-type) biradi-
cals whose general structures are illustrated below.
The biradical 52•2+ serves as a measure of the topology
dependence of spin-spin coupling since the unpaired
spins should antiferromagnetically couple (below left). Of
course, bond torsions between the porphyrin rings and
the phenylene coupler will modulate the strength of this
interaction. However, spin polarization still predicts
antiferromagnetic coupling (below right).40-42
F igu r e 3. Cyclic voltammograms for 1-7 as 1 mM solutions
in CH2Cl2 (except [5] ) 0.5 mM). The supporting electrolyte
is tetra-n-butylammonium hexafluorophosphate (100 mM).
Scan rate 100 mV/s.
Finally, 3 differs from 2 in that two electron-donating
methoxy groups have replaced the tert-butyl group of the
(31) Fajer, J .; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J .
Am. Chem. Soc. 1970, 92, 3451.
(32) Atamian, M.; Wagner, R. W.; Lindsey, J . L.; Bocian, D. F. Inorg.
Chem. 1988, 27, 1510.
(33) Prendergast, K.; Spiro, T. G. J . Phys. Chem. 1991, 95, 9728.
(34) Skillman, A. G.; Collins, J . R.; Loew, G. H. J . Am. Chem. Soc.
1992, 114, 9538.
(35) Song, H.; Reed, C. A.; Scheidt, W. R. J . Am. Chem. Soc. 1989,
111, 6865.
(36) Erler, B. S.; Scholz, W. F.; Lee, Y. L.; Scheidt, W. R.; Reed, C.
A. J . Am. Chem. Soc. 1987, 109, 2644.
(37) Fujii, H. Inorg. Chem. 1993, 32, 875.
(38) Fujii, H. J . Am. Chem. Soc. 1993, 115, 4641.
(39) (a) Shultz, D. A. In Magnetic Properties of Organic Materials;
Lahti, P., Ed.; Marcel Dekker: New York, 1999. (b) West, A. P., J r.;
Silverman, S. K.; Dougherty, D. A. J . Am. Chem. Soc. 1996, 118, 1452.
(c) Borden, W. T.; Davidson, E. R. Acc. Chem. Res. 1996, 29, 67. (d) Li,
S.; Ma, J .; J iang, Y. J . Phys. Chem. 1996, 100, 4775. (e) Rajca, A. Chem.
Rev. 1994, 94, 871. (f) Ichimura, A.; Koga, N.; Iwamura, H. J . Phys.
Org. Chem. 1994, 7, 207. (g) Borden, W. T.; Iwamura, H.; Berson, J .
A. Acc. Chem. Res. 1994, 24, 109. (h) Iwamura, H. Pure Appl. Chem.
1993, 65, 57. (i) Iwamura, H.; Koga, N. Acc. Chem. Res. 1993, 26, 346.
(j) Dougherty, D. A.; J acobs, S. J .; Silverman, S. K.; Murray, M.; Shultz,
D. A.; West, A. P., J r.; Clites, J . A. Mol. Cryst. Liq. Cryst. 1993, 232,
289. (k) J acobs, S. J .; Shultz, D. A.; J ain, R.; Novak, J .; Dougherty, D.
A. J . Am. Chem. Soc. 1993, 115, 1744. (l) Dougherty, D. A. Acc. Chem.
Res. 1991, 24, 88. (m) Iwamura, H. Adv. Phys. Org. Chem. 1990, 26,
179. (n) Kinetics and Spectroscopy of Carbenes and Biradicals; Platz,
M. S., Ed.; Plenum Press: 1990. (o) Iwamura, H. Pure Appl. Chem.
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4587.
F igu r e 4. Cyclic voltammograms for 2 and 3 as 1 mM
solutions in CH2Cl2. The supporting electrolyte is tetra-n-
butylammonium hexafluorophosphate (100 mM). Scan rate 60
mV/s.
Also, the meso-ethynyl groups of 2, 3, 6, and 7 are
important for spatially separating the porphyrin rings
from the coupler to allow coupler/porphyrin coplanarity
necessary for maximal spin-spin coupling in the biradi-
cal.
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Cross-Conjugated Bis(porphyrin)s
J . Org. Chem., Vol. 64, No. 25, 1999 9129
Ta ble 3. Cyclic Volta m m etr ic Da ta for Bis(p or p h yr in )s 1-7a
c
e
bis(porphyrin)
E1/2(1)b
E1/2(2)
∆E1/2
Ep,c(1)d
Ep,a(1)
Ep,c(2)
Ep,a(2)
∆Ep
0.11
0.13
1
2
3
4
5
6
7
0.59
0.59
0.50
0.58
0.59
0.53
0.53
0.59
0.59
0.59
0.58
0.59
0.65
0.62
0
0
0.53
0.52
0.44
0.52
0.52
0.49
0.48
0.64
0.65
0.54f
0.64
0.65
0.58f
0.58f
0.09e
0
0.53f
0.64
0.10, 0.11
0.12
0
0.13
0.12e
0.09e
0.60f
0.57f
0.70
0.68
0.09, 0.10
0.10, 0.11
a
0.1 mM CH2Cl2 solution of 1-7 (except [5] ) 0.5 mM) with 100 mM tetra-n-butylammonium hexafluorophosphate as supporting
electrolyte. Scan rate 120 mV/s. E1/2 ) (Ep,a − Ep,c)/2; potentials in volts vs Ag/AgNO3 ( 20 mV. c Difference between first and second
b
oxidations as determined by the appearance of CV; see the text. Peak potential (c ) cathodic, a ) anodic). e (20 mV. f (10 mV.
d
twisted ca. 40°,30 so that disruption of porphyrin-coupler
conjugation is expected for 12•2+, 42•2+, and 52•2+ 45
Con-
.
sequently, spin-spin coupling should be attenuated by
both bond torsions and distance in 12•2+ and by bond
torsions alone in both 42•2+ and 52•2+ 46
.
F igu r e 5. Electronic absorption spectra for bis(porphyrin)s
1-7.
coupler. Thieren has shown that phenyl ring substitution
of phenylethynylporphyrins dramatically affects the por-
phyrin ring oxidation potential.43 We are particularly
interested in modulating spin-spin coupling in biradical
di-ions by controlling electron density with standard
electron-donating or electron-withdrawing groups. This
modulation is reasonable since simple MO theory predicts
charge and spin are delocalized over the same atoms of
a π-radical ion, and therefore spin density might be
manipulated by controlling charge density.44
The interaction pathway for spin-spin coupling in the
cross-conjugated bis(porphyrin) biradical dications de-
creases from 12•2+ (shortest pathway between porphyrin
meso-carbons ) 16 bonds) to 22•2+/32•2+ (shortest pathway
between porphyrin meso-carbons ) 8 bonds) to 62•2+/72•2+
(shortest pathway between porphyrin meso-carbons ) 6
bonds) to 42•2+ (shortest pathway between porphyrin
meso-carbons ) 4 bonds), so spin-spin coupling should
increase in this order. However, it is well-established that
meso-phenyl substituents in porphyrin radical cations are
The crystal structures of both 6 and 7 are made up of
discrete molecules of the bis(porphyrin)s. A toluene
molecule caps one face of each porphyrin. Thus, the
toluene molecules fill the cavities formed by the discrete
bis(porphyrin)s. Within the molecules of 6 and 7, the bond
distances and angles are considered normal. The molec-
ular structures of 6 and 7 are quite similar; therefore,
only the description of 6 is presented below.
(43) LeCours, S. M.; DiMagno, S. G.; Therien, M. J . J . Am. Chem.
Soc. 1996, 118, 11854.
(44) Computational results corroborate our hypothesis: Shultz, D.
A.; Sandberg, K. A.; Dilday, J . S. J . Phys. Chem., to be submitted.

9130 J . Org. Chem., Vol. 64, No. 25, 1999
Shultz et al.
Ta ble 4. UV-Vis Sp ectr oscop ic Da ta for 1-7, Zn II-Tetr a m esitylp or p h yr in (Zn TMP ), a n d Bir a d ica ls 12•2+-52•2+ a n d
72•2+ a
bis(porphyrin)
ω
max/cm-1 (log ꢀ)b
ω
max/cm-1 (log ꢀ)c
bis(porphyrin)
ω
max/cm-1 (log ꢀ)b
ω
max/cm-1 (log ꢀ)c
1
23 697 (6.03)
20 576 (3.68)
19 455 (3.86)
18 182 (4.66)
16 892 (3.91)
5
23 810 (5.44)
19 531 (3.63)
18 182 (4.24)
17 007 (3.59)
23 365 (5.27)
e
[609 cm-1
]
12•2+
18 797d
52•2+
18 182 (4.17)
16 000d
14 881 (4.17)
15 576 (4.63)
14 948 (4.63)
12 788 (4.60)
11 455 (4.61)
13 089d
11 534 (4.13)
2
23 148 (5.72)d
20 040 (3.69)
6
23 095 (5.56)d
18 868 (4.03)
22 523 (5.90)
18 975 (4.00)
17 668 (4.65)
16 260 (4.79)
22 422 (5.77)
17 637 (4.55)
16 207 (4.65)
e
e
[442 cm-1
]
[502 cm-1
]
22•2+
16 181 (4.71)
62•2+
14 472 (4.56)
14 728 (4.73)
13 514 (4.74)
11 587 (4.74)
10 730d
12 019 (4.56)
9 671d,f
8 562f
11 990f
3
23 148 (5.54)d
18 797 (4.04)
7
23 148 (5.49)d
18 868 (3.99)
22 422 (5.65)
17 544 (4.51)
16 026 (4.92)
22 422 (5.67)
17 606 (4.50)
16 207 (4.61)
11 848 (4.21)
e
e
[742 cm-1
]
[591 cm-1
]
32•2+
18 868 (4.88)
72•2+
16 207d
14 085 (4.84)
12 870 (4.84)
11 723d
10 142d
9 497d,f
8 271f
10 070 (4.80)
12 300f
4
24 038 (5.99)
20 747 (3.76)
Zn TMP
23 753 (5.70)
19 531 (3.39)
18 182 (4.27)
17 065 (3.31)
23 202 (5.94)
19 531 (3.97)
18 182 (4.80)
17 007 (4.01)
e
[824 cm-1
]
42•2+
19 305 (4.34)
Zn TMP •+
19 342 (3.56)
17 953 (3.70)
15 773 (3.99)
14 815 (3.95)
11 682 (3.17)
18 018 (4.41)
15 798 (4.57)
14 815 (4.58)
11 628 (4.01)
a
d
CH2Cl2 solutions. b Soret region (unoxidized bis(porphyrin)s). c Q-band region (unoxidized bis(porphyrin)s). Shoulder. e Soret splitting.
f Tentatively assigned as an intervalence transition: only observed for the singly oxidized, radical cation oxidation state; see the text.
The orientation of the two porphyrin moieties with
respect to each other are governed to a major extent by
the geometry of the bridging group, especially the ethene
coupler. The C96-C97-C98 bond angle of 111.8° is less
than the ideal bond angle of 120°. This, along with the
distortion of the acetylenic bonds, renders the two por-
phyrin centers closer than might otherwise be expected.
The C2-C97-C49 angle of 100.9° is about 19° less than
the ideal value. The porphyrin cores are almost planar:
the mean deviation from the least-squares plane includ-
ing all 25 atoms is 0.029 Å for the porphyrin containing
atom Zn1, while it is 0.094 Å for the porphyrin containing
atom Zn2. The dihedral angle between the two porphyrin
planes is 50.4°. The two porphyrin planes cannot be
coplanar due to the overlap of the mesityl groups of both
porphyrins.
(45) For papers discussing the effects of torsion on exchange
coupling, see: (a) Dvolaitzky, M.; Chiarelli, R.; Rassat, A. Angew.
Chem., Int. Ed. Engl. 1992, 31, 180. (b) Kanno, F.; Inoue, K.; Koga,
N.; Iwamura, H. J . Am. Chem. Soc. 1993, 115, 847. (c) Silverman, S.
K.; Dougherty, D. A. J . Phys. Chem. 1993, 97, 13273. (d) Okada, K.;
Matsumoto, K.; Oda, M.; Murai, H.; Akiyama, K.; Ikegami, Y.
Tetrahedron Lett. 1995, 36, 6693. (e) Adam, W.; van Barneveld, C.;
Bottle, S. E.; Engert, H.; Hanson, G. R.; Harrer, H. M.; Heim, C.; Nau,
W. M.; Wang, D. J . Am. Chem. Soc. 1996, 118, 3974. (f) Fujita, J .;
Tanaka, M.; Suemune, H.; Koga, N.; Matsuda, K.; Iwamura, H. J . Am.
Chem. Soc. 1996, 118, 9347. (g) Okada, K.; Imakura, T.; Oda, M.;
Murai, H.; Baumgarten, M. J . Am. Chem. Soc. 1996, 118, 3047. (h)
Barone, V.; Bencini, A.; di Matteo, A. J . Am. Chem. Soc. 1997, 119,
10831. (i) Nakazono, S.; Karasawa, S.; Iwamura, H. Angew. Chem.,
Int. Ed. Engl. 1998, 37, 1550. (j) Shultz, D. A.; Boal, A. K.; Lee, H.;
Farmer, G. T. J . Org. Chem. 1999, 64, 4386. (k) Shultz, D. A. In
Magnetic Properties of Organic Materials; Lahti, P., Ed.; Marcel
Dekker: New York, 1999. (l) Ab initio calculations explain antiferro-
magnetic coupling in m-phenylene-linked structures; see: Fang, S.;
Lee, M.-S.; Hrovat, D. A.; Borden, W. T. J . Am. Chem. Soc. 1995, 117,
6727. (m) For several beautiful examples of conformational J modula-
tion in metal complexes exchange coupled through organic ligands,
see: McCleverty, J . A.; Ward, M. D. Acc. Chem. Res. 1998, 31, 842.
(46) Sessler reported a gable bis(porphyrin) in which the porphyrin
rings were twisted 55° relative to the m-phenylene coupler: Sessler,
J . L.; J ohnson, M. R.; Creager, S. E.; Fettinger, J . C.; Ibers, J . A. J .
Am. Chem. Soc. 1990, 112, 9310.
However, using Chem-3D models of the crystal struc-
tures of 6 and 7, the smallest porphyrin ring torsion
angles possible are ca. 25° relative to the Coupler. At
smaller dihedral angles, meso-mesityl groups attached
to different porphyrins are in van der Waals contact.
However, both widening of the C96-C97-C98 bond
angle to 120° and increasing the alkyne bond angles to
180° allow porphyrin ring torsions of ca. 5° relative to
the coupler. Thus, in solution, we believe that the
porphyrin rings of both 6 and 7 are able to achieve near
coplanarity, and sterics should play a relatively minor
role in spin-spin coupling in 22•2+/32•2+ and 62•2+/72•2+
.
Electr och em istr y. For 1, 2, 4, and 5 the quasirevers-
ible, first oxidation of each porphyrin occurs at nearly
the same potential. Thus, E1/2(1) and E1/2(2) constitute a
single, “two-electron” wave, indicating that interaction
between the radical cations is negligible. By the term,
“two-electron” wave, we do not mean that E1/2(1) ) E1/2(2),
rather that ∆E1/2 is too small to be observed by cyclic

Cross-Conjugated Bis(porphyrin)s
J . Org. Chem., Vol. 64, No. 25, 1999 9131
F igu r e 8. X-band EPR spectra (∆ms ) 2 transitions) of
biradical dications 22•2+-72•2+ recorded at 77 K as solutions
in methylene chloride.
F igu r e 6. Electronic absorption spectra for model compound
Zn TMP 2•2+, and tetraaryl-type bis(porphyrin) biradical dica-
tions 12•2+, 42•2+, and 52•2+
.
F igu r e 9. Curie plots (normalized intensities) for biradical
dications 22•2+-72•2+
.
Ta ble 5. X-Ba n d EP R Sp ectr oscop y (77 K) Resu lts for
Bir a d ica ls 12•2+-72•2+ a
biradical
∆ms ) 2
no
yes: J g 0
ues: J g 0
yes: J g 0
yes: J g 0
yes: J g 0
yes: J g 0
spectral type
|D/hc| (cm-1
)
12•2+
22•2+
32•2+
42•2+
52•2+
62•2+
72•2+
double doublet
biradical
biradical
biradical
biradical
biradical
biradical
e0.0007
e0.0007
e0.0007
e0.0007
e0.0007
e0.0007
a
Biradical dications prepared by oxidation with 2 equiv of tris-
4-bromophenylamminium hexachloroantimonate; [biradical dica-
tion] ) 10 mM in CH2Cl2.
F igu r e 7. Electronic absorption spectra for triarylethynyl-
type bis(porphyrin) biradical dications 22•2+, 32•2+, and 72•2+
.
voltammetry. Isolation of the electroactive porphyrin
rings is affected by bond torsions of the meso-phenyl rings
in 1, 4, and 5, as predicted. In bis(porphyrin) 2, however,
no such torsion-induced isolation of the porphyrins exists,
and the lack of redox splitting (E1/2(1) ≈ E1/2(2)) must be
due to weak delocalization through the meso-ethynyl
group into the tert-butyl-m-phenylene coupler. In fact,
Swager et al. noted minimal redox splitting in ethyne-
bridged p-phenylenediamines, and concluded that ethyne
groups affect localization of positive charge, and the
observed ∆E1/2 values were attributed solely to electro-
static repulsions.47
However, oxidation of the porphyrin rings in 3, 6, and
7 occurs at different potentials (split redox waves: E1/2(1)
(47) Zhou, Q.; Swager, T. M. J . Org. Chem. 1995, 60, 7096.

9132 J . Org. Chem., Vol. 64, No. 25, 1999
Shultz et al.
* E1/2(2)), and we contend that the ethynyl groups are
efficient at promoting communication beyond simple
electrostatic repulsion. Figure 4 shows the cyclic voltam-
mograms of 2 and 3 and better illustrates the difference
in redox behavior. Bis(porphyrin)s 2 and 3 are isostruc-
tural and differ only in the electron demand of the coupler
group. Indeed it is this difference that causes the change
from a “two-electron oxidation wave” (2) to two, sequen-
tial, one-electron waves (3). The electron-donating meth-
oxy groups stabilize positive charge by resonance dona-
tion, while the m-tert-butyl is less effective at stabilization.
The negative shift in oxidation potentials of 3 compared
to 2 (ca. 80 mV) indicates the cation-stabilizing effect of
the methoxy groups. Since the methoxy groups of 3
stabilize the radical cation, charge density should in-
crease in and/or near the coupler group and therefore
closer to the second porphyrin ring. Thus, interaction of
the electrophores (electroactive moieties) in 3 is enhanced
relative to 2 and the redox events occur at different
potentials. In essence, 3•+ represents a class II48 mixed-
valent organic compound,49-61 and our results parallel
quite nicely those of Sutton and Taube for bis(pyridyl)-
bridged RuII/RuIII complexes.62 Two of their complexes are
shown below. Complex 19, despite the greater number
of atoms in the ligand compared to 20, is a mixed-valence
complex where the metal ions interact more strongly than
in 20.
The fact that electron-donating groups in 3 increase
charge delocalization into/toward the coupler implies that
spin density is increased in the coupler, and enhanced
spin-spin coupling should be observed in biradical
dications with electron-rich couplers.64 We therefore
predict that exchange coupling in 3 will be greater than
that of 2. Unfortunately, in the present study we cannot
prove this hypothesis. Future work will focus on dem-
onstration of enhancing exchange coupling in biradical
di-ions.44
Neither gable65 m-phenylenebis(porphyrin) 4 nor p-
phenylenebis(porphyrin) 524 exhibit redox splitting. This
is noteworthy for two reasons. First, despite the fact that
the porphyrin units are closer together in cross-conju-
gated 4 (and electrostatic interaction is greater) than in
cross-conjugated 6/7, no redox splitting is observed for
4. We believe that redox splitting in 6/7 is the result of
interaction propagated through the π-system, and these
compounds in their monooxidized forms, like 3•+, are class
II mixed-valence molecules.66 Second, the redox waves
for 4 and 524 are indistinguishable. Others24 and we
attribute this to large phenylene bond torsions in both 4
and 5 that insulate each half of the molecule, hence the
single redox wave. Again, there are parallels with
inorganic species such as those discussed above: torsion
between coupler and electrophore results in attenuated
π-π overlap and therefore weak interaction between
electrophores.62,67 Since bond torsions modulate conjuga-
tion, the absence of redox splitting in 4/5 lends credence
to our conclusion that interaction in the biradical dica-
tions (and radical cations) is effectively propagated
through the ethyne-containing π-system.
Similar to the results for 2 and 3, Sutton and Taube
observed redox splitting in 19 that was absent in 20. Both
the subtlety and the importance of matching Coupler
electron demand with electrophore electron demand is
illustrated dramatically in 2/3 since both the π-topology
and conformation of the couplers are identical and only
substituents are different.
∆E1/2 values similar to those observed here have been
attributed to simple electrostatic repulsions;47,63 however,
since no such splitting is observed for redox couples of 4
or 5, as discussed below, we believe that the origin of
the redox splitting in the present case is through-bond.
To summarize, the two radical cations interact best
when (1) the interaction pathway is short (∆E1/2(6/7) >
E1/2(2/3)), (2) π-overlap between the electrophore and
coupler is maximized (minimal bond torsions: ∆E1/2(6/
7) > E1/2(4/5)), and (3) the electron demand of the coupler
matches that of the spin carrier (∆E1/2(3) > E1/2(2)). Point
3 suggests that electron-rich couplers should promote
effective interaction of radical cations and electron-poor
couplers should promote effective interaction of radical
anions.64 We hypothesize that a necessary consequence
of our results is that the exchange interaction (and
therefore the singlet-triplet gap) in a biradical di-ion can
be adjusted by straightforward substituent effects. We
feel also that our hypothesis concerning enhanced ex-
change coupling when electron demand of the coupler
matches those of the charged spin carriers is related to
enhanced electron-transfer rates across electron-donor-
substituted π-conjugated bridges observed by Wasielew-
ski et al.68
(48) Robin, M. B.; Day, P. Adv. Inorg. Radiochem. 1967, 10, 247.
(49) Bonvoisin, J .; Launay, J . P.; Rovira, C.; Veciana, J . Angew.
Chem., Int. Ed. Engl. 1994, 33, 2106.
(50) Bonvoisin, J .; Launay, J . P.; Vanderauweraer, M.; DeSchryver,
F. C. J . Phys. Chem. 1994, 98, 5052.
(51) Bonvoisin, J .; Launay, J . P.; Verbouwe, W.; Vanderauweraer,
M.; DeSchryver, F. C. J . Phys. Chem. 1996, 100, 17079.
(52) Coronado, E.; Galan-Mascaros, J . R.; Gimenez-Saiz, C.; Gomez-
Garcia, C. J .; Triki, S. J . Am. Chem. Soc. 1998, 120, 4671.
(53) Domingo, V. M.; Castaner, J . J . Chem. Soc., Chem. Commun.
1995, 895.
(54) Domingo, V. M.; Castan˜er, J .; Riera, J .; Brillas, E.; Molins, E.;
Martinez, B.; Knight, B. Chem. Mater. 1997, 9, 1620.
(55) Karafiloglou, P.; Launay, J . P. J . Phys. Chem. A 1998, 102,
8004.
Electr on ic Sp ectr oscop y. Bis(p or p h yr in )s. The
split Soret bands of bis(porphyrin)s 2-7 indicate excitonic
coupling of the two chromophores, while the lack of
(64) An alternative possibility would be charge-spin separation (for
examples of charge-spin separation, see: Niemz, A.; Rotello, V. M. J .
Am. Chem. Soc. 1997, 119, 6833. Wudl, F. J . Am. Chem. Soc. 1981,
103, 7064). In this case, charge density would increase in the coupler,
but spin density would not increase in the coupler. Thus, electron-
poor couplers might more strongly couple radical cations. Either way,
substituent effects could modulate exchange coupling.
(65) Tabushi, I.; Sasaki, T. Tetrahedron Lett. 1982, 23, 1913.
(66) We do not mean to imply that 1•+, 2•+, 4•+, and 5•+ are not mixed
valence (the IT band could be in the IR region), but rather that
delocalization in 3•+, 6•+, and 7•+ is greater.
(56) Le Mest, Y.; L’Her, M.; Hendricks, N. H.; Kim, K.; Collman, J .
P. Inorg. Chem. 1992, 31, 835.
(57) Rajca, S.; Rajca, A. J . Am. Chem. Soc. 1995, 117, 9172.
(58) Sedo´, J .; Ruiz, D.; VidalGancedo, J .; Rovira, C. J .; Bonvoisin,
J .; Launay, J . P.; Veciana, J . Adv. Mater. 1996, 8, 748.
(59) Sedo´, J .; Ruiz, D.; VidalGancedo, J .; Rovira, C. J .; Bonvoisin,
J .; Launay, J . P.; Veciana, J . Mol. Cryst. Liq. Cryst. 1997, A306, 125.
(60) Sedo´, J .; Ruiz, D.; VidalGancedo, J .; Rovira, C. J .; Bonvoisin,
J .; Launay, J . P.; Veciana, J . Synth. Met. 1997, 85, 1651.
(61) Utamapanya, S.; Rajca, A. J . Am. Chem. Soc. 1991, 113, 9242.
(62) Sutton, J . E.; Taube, H. Inorg. Chem. 1981, 20, 3125.
(63) Ferrere, S.; Elliot, C. M. Inorg. Chem. 1995, 34, 5818 and
references therein.
(67) Ward, M. D. Chem. Soc. Rev. 1995, 121.
(68) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R.
Nature 1998, 396, 60.

Cross-Conjugated Bis(porphyrin)s
J . Org. Chem., Vol. 64, No. 25, 1999 9133
splitting in 1 suggests that the porphyrin chromophores
are sufficiently separated so as to preclude exciton
coupling. Interestingly, bis(porphyrin) 1swhich lacks
Soret splittingsalso does not exhibit a ∆ms ) 2 transition
when doubly oxidized (see below).
The exciton splitting of the Soret band of gable bis-
(porphyrin) 4 (824 cm-1) is larger than exciton splitting
in both the phenylene- (2 and 5, but not 3) and TMM-
coupled molecules (6, but not 7), which in turn is greater
than that in 1, which lacks measurable splitting. This
variation in exciton splitting is the expected order for an
interaction between chromophores that is modulated
primarily by interchromophore distance and contrasts
nicely the variation in redox behavior that is modulated
primarily through π-bonds. The exceptions (3 and 7)
indicate an additional interaction that is not a function
of distance alone.
mixed-valent forms appearing as shoulders near 9600
cm-1. These spectral features could be caused by the fact
that normal modes involved in intramolecular electron
transfer in 6•+/7•+ are of greater frequency than the
corresponding metal-centered modes of inorganic metal
complexes.73 Thus, in the present case, Hush’s equations
might not apply. However, a number of organic mixed-
valent compounds are known to exhibit Hush-regime IT
bands.49-55,57-61 In addition, the special pair radical
cations of the photosynthetic reaction center exhibit
unique near-IR bands, one of which is near 8000 cm-1
(1200 nm).74-84 Perhaps 6•+/7•+ are uniquely suited as
model compounds for studying the special-pair radical
cations of the photosynthetic reaction center. In any case,
additional spectroscopic studies are warranted to clearly
understand the nature of the near-IR transitions of 6•+
/
7•+, and efforts along these lines are underway.
Electr on ic Sp ectr oscop y. Oxid ized Bis(p or p h y-
r in )s. Electronic absorption spectra of 12•2+-72•2+ re-
corded after controlled-potential coulometry at ca. 0.7 V
vs Ag/AgNO3 are shown in Figures 6 and 7 and are
characteristic of porphyrin cation radicals: Soret bands
that are hypochromically and bathochromically shifted
(not shown), and multiple transitions in the visible and
near-infrared (near-IR) regions.69 The triarylethynylpor-
phyrin biradical dications 22•2+, 32•2+, 62•2+, and 72•2+
exhibit near-IR bands considerably more intense and red-
shifted than their oxidized tetraarylporphyrin counter-
parts.
EP R Sp ectr oscop y. None of the biradicals exhibited
fine-structure, precluding estimation of the zero-field-
splitting parameters, |D/hc|, from positions of ∆ms ) 1
transitions and suggesting that the dipole-dipole inter-
actions between unpaired electrons in the biradicals are
weak. Collman and co-workers described several cofacial
bis(porphyrin) biradicals, only one of which showed zero-
field splitting.56 This 1,8-anthracene-linked bis(porphy-
rin) biradical had |D/hc| ≈ 0.0047 cm-1 (∼50 G) despite
a ca. 5 Å interporphyrin separation and cofacial align-
ment.56,85 A point-dipole estimate86 of |D/hc| with r ) 5
Å gives |D/hc| ) 0.021 cm-1 (225 G) much greater than
Collman’s observed D value. Thus, we might expect small
zero-field-splitting parameters for our biradical dications.
However, clearly visible near half-field are ∆ms ) 2
transitions, the signature transition of a triplet state,
The bulk electrolyses of the tetraaryl-type bis(porphy-
rin)s 1, 4, and 5 result in absorption spectra that are
nearly identical to those of Zn TMP , and are marked by
isosbestic points (Figure 6). Both of these observations
are consistent with negligible interaction between the two
halves of the oxidized molecules.
confirming spin-spin coupling in biradicals 22•2+-72•2+
,
as shown in Figure 8. Notably, 12•2+ does not exhibit a
∆ms ) 2 transition, indicating that dipolar coupling is
negligible, and mirroring the lack of excited-state inter-
action in unoxidized 1 suggested by the lack of splitting
of the Soret band.
As seen in Figure 7, the singly oxidized forms 2•+, 3•+
,
6•+, and 7•+ (recorded midway through the bulk electroly-
sis) exhibit unique electronic absorption bands (marked
by an asterisk) characteristic of class II mixed-valent
compounds,48 and are shifted compared to those of
Scheidt’s mixed-valent [ZnIIoctaethylporphyrin/ZnIIocta-
ethylporphyrin2•+] dimers.70 Since the unique transitions
for 2•+ and 3•+ are obscured by other transitions, we will
focus on the near-IR transitions for 6•+ and 7•+ that
The appearance of ∆ms ) 2 transitions for 22•2+-72•2+
permits estimation of D values from the relative intensi-
ties of the ∆ms ) 1 and ∆ms ) 2 transitions.87 These
calculations give upper limits of 0.0013 cm-1 (14 G) for
|D/hc for 22•2+, 32•2+, and 72•2+. Clearly, the calculated D
appear near 8300 cm-1. The near-IR transitions for 6•+
/
7•+ can also be observed when exhaustive oxidation at
0.7 V vs Ag/AgNO3 is followed by dilution with an
equimolar amount of unoxidized 6/7. Thus, compropor-
tionation slightly favors the mixed-valent state, and Kcom
is estimated to be ca. 72 for the equilibrium71
(73) Haberkorn, R.; Michelbeyerle, M. E.; Marcus, R. A. Proc. Natl.
Acad. Sci. U.S.A. 1979, 76, 4185.
(74) Binstead, R. A.; Hush, N. S. J . Phys. Chem. 1993, 97, 13172.
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7503.
(76) Gasyna, Z.; Schatz, P. N. J . Phys. Chem. 1996, 100, 1445.
(77) Lendzian, F.; Huber, M.; Isaacson, R. A.; Endeward, B.; Plato,
M.; Bonigk, B.; Mobius, K.; Lubitz, W.; Feher, G. Biochim. Biophys.
Acta 1993, 1183, 139.
(78) Osuka, A.; Maruyama, K. Chem. Lett. 1987, 825.
(79) Reimers, J . R.; Hush, N. S. Inorg. Chim. Acta 1994, 226, 33.
(80) Reimers, J . R.; Hush, N. S. Chem. Phys. 1995, 197, 323.
(81) Reimers, J . R.; Hutter, M. C.; Hush, N. S. Photosynth. Res. 1998,
55, 163.
(82) Scheidt, W. R.; Cheng, B. S.; Haller, K. J .; Mislankar, A.; Rae,
A. D.; Reddy, K. V.; Song, H. S.; Orosz, R. D.; Reed, C. A.; Cukiernik,
F.; Marchon, J . C. J . Am. Chem. Soc. 1993, 115, 1181.
(83) Stocker, J . W.; Hug, S.; Boxer, S. G. Biochim. Biophys. Acta
1993, 1144, 325.
(84) Wynne, K.; Haran, G.; Reid, G. D.; Moser, C. C.; Dutton, P. L.;
Hochstrasser, R. M. J . Phys. Chem. 1996, 100, 5140.
(85) Collman’s cofacial bis(porphyrin) biradicals (ref 56) have â-octa-
ethyl substitution, and the SOMOs are most likely a1u-like. However,
this should alter neither the fact that the two unpaired electrons are
delocalized in porphyrin planes that are ca. 5 Å apart, nor the validity
of the D-value estimate.
Kcom
7 + 72•2+ y z 2 72•+
From the transition energy, apparent extinction coef-
ficient, and spectral width of the intervalence transitions
(IT), barrier heights and mixing coefficients can be
calculated using well-known relations.72 However, the IT
bands are far narrower than predicted by Hush’s treat-
ment: ∆ωj1/2(calcd) ≈ 4400 cm-1; ∆ωj1/2(exptl) ≈ 700 cm-1
.
In addition, an additional absorption band exists in the
(69) The Porphyrins; Dolphin, D., Ed.; Academic Press: New York,
1978; Vols. 1-7.
(70) Brancato-Buentello, K. E.; Kang, S.-J .; Scheidt, W. R. J . Am.
Chem. Soc. 1997, 119, 2839.
(71) Calculated from Kcom ) exp(n1n2F∆E1/2/RT)}.
(72) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 301.
(86) Keana, J . F.; Dinerstein, R. J . J . Am. Chem. Soc. 1971, 93, 2808.
(87) Weissman, S. I.; Kothe, G. J . Am. Chem. Soc. 1975, 97, 2537.

9134 J . Org. Chem., Vol. 64, No. 25, 1999
Shultz et al.
values are clearly an overestimation as fine structure
should be evident for |D/hc| ) 0.0013 cm-1. Simulations
show that D values of 0.0007 cm-1 (7 G), comparable to
the spectral line width in frozen solution, lacks fine
structure in the ∆ms ) 1 region. The interelectronic
distances, ca. 12 Å, estimated from the D value86 are close
to the interporphyrin ring separations (meso-carbon to
meso-carbon distances, ca. 7-10 Å), estimated from
molecular models.
The lack of visible fine structure for 42•2+ and 52•2+ is
somewhat surprising since the point dipole model sug-
gests that the D values for these biradicals should be ca.
8 times the D value for 22•2+. Perhaps the biradicals in
this study serve as examples where the point dipole
approximation is inaccurate or perhaps invalid as alluded
to above.88
4, and 5 contain tetraaryl-type porphyrins, while 2, 3, 6,
and 7 contain triarylethynylporphyrins. Except for exci-
ton coupling in the unoxidized species, interaction be-
tween porphyrins is greater in the triarylethynyl-type
bis(porphyrin)s than in the tetraaryl-type bis(porphyrin)s
regardless of the number of bonds through which the
interaction propagates. The electronic absorption spectra
of 2-7 exhibit Soret splitting characteristic of interacting
chromophores, while the spectrum of 1 exhibits no Soret
splitting.
The cyclic voltammetry of the bis(porphyrin)s was
examined, and a varying degree of interaction between
electrophores within the series was found. Redox splitting
was observed for 3, 6, and 7, suggesting interaction
between the electroactive porphyrin rings beyond simple
electrostatic repulsions. No redox splitting was observed
for any of the tetraaryl-type bis(porphyrin)s 1, 4, and 5.
It is concluded that two porphyrin radical cations interact
best when (1) the interaction pathway is short (∆E1/2(6/
7) > E1/2(2/3)), (2) π-overlap between the electrophore and
coupler is maximized (minimal bond torsions: ∆E1/2(6/
7) > E1/2(4/5)), and (3) the electron demand of the coupler
matches that of the spin carrier (∆E1/2(3) > E1/2(2)).
One-electron-oxidized triarylethynyl-type bis(porphy-
rin)s exhibit spectral features characteristic of mixed-
valent compounds. In two cases (6•+ and 7•+), near-IR
bands near 8300 cm-1 were observed and are tentatively
assigned to intervalence transitions. Singly oxidized
tetraaryl-type bis(porphyrin)s 1•+, 4•+, and 5•+ exhibit no
such near-IR transitions, and electronic absorption spec-
tra recorded during electrochemical oxidations are marked
by isosbestic points, suggesting negligible interaction
between the two halves of the molecule, consistent with
cyclic voltammetric results.
Two-electron oxidation of 2-7 yields biradical dications
whose frozen-solution EPR spectra lack fine structure,
but exhibit ∆ms ) 2 transitions characteristic of exchange-
coupled S ) 1 states. Oxidation of 1 yields a biradical in
which exchange coupling and dipolar interactions are
presumably very weak; consequently no ∆ms ) 2 is
observed. The results of variable-temperature EPR spec-
troscopy of 22•2+-72•2+ suggest triplet ground states and/
or singlet-triplet degeneracies. No topological depen-
dence of the Curie plot on coupler topology was
observed: both 42•2+ and 52•2+ gave linear Curie plots,
despite the prediction of a singlet ground state for the
latter. We believe this is consistent with bond torsions
localizing the spins, precluding effective (measurable)
coupling.
The variation of EPR signal intensity as a function of
temperature (a Curie plot)89 can sometimes be used to
evaluate the interaction of unpaired electrons. The Curie
plots for 22•2+ and 42•2+-72•2+ shown in Figure 9 are all
linear within experimental error. In such cases one can
only state that the exchange coupling constant, J (2J )
singlet-triplet gap), is consistent with both ferromag-
netic coupling (J > 0) and a singlet-triplet degeneracy
(J ) 0). Ferromagnetic coupling is expected for cross-
conjugated, planar biradical dications (22•2+, 32•2+, 62•2+
,
and 72•2+). However, negligible or even antiferromagnetic
J might be expected for biradical dications in which
considerable torsion between the spin-containing por-
phyrin radical cations and the phenylene couplers (42•2+
,
5
2•2+) exists.45 That linear Curie plots are observed for
both 42•2+ and 52•2+ indicates that either the meso-carbon
spin densities (F ≈ 0.16)31,32 are insufficient for substan-
tive exchange coupling (J ≈ Fi‚Fj)90,91 or bond torsions
attenuate the coupling, and the spin polarization mech-
anism is too weak to affect measurable coupling by the
EPR method. We believe the latter to be true since spin
densities of ca. 0.16 are large enough to result in
measurable exchange coupling.92-95 However, there are
examples in the literature where spin-containing units
appear to prefer no coupling (J ) 0) over antiferromag-
netic coupling.96,97 Additional experimentation is neces-
sary to fully evaluate the exchange coupling in the
biradical dications presented here, and efforts along these
lines are underway.
Con clu sion s
The synthesis and characterization of seven (ZnII)2bis-
(porphyrin) molecules were described. Bis(porphyrin)s 1,
As a consequence of our results, we hypothesize that
the exchange interaction, and therefore the singlet-
triplet gap, in a biradical di-ion can be adjusted to favor
the triplet state by simple substituent effects.64
(88) An alteration of Keana’s relationship (ref 86) is necessary for
accurate estimation of interelectronic separation in dinitroxide biradi-
cals. Gleason, W. B.; Barnett, R. E. J . Am. Chem. Soc. 1976, 98, 2701.
Perhaps in the present cases, neither relationship accurately correlates
r and |D/hc|.
Exp er im en ta l Section
(89) Berson, J . A. Diradicals; Wiley: New York, 1982.
(90) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
(91) Rajca, A. Chem. Rev. 1994, 94, 871.
Gen er a l P r oced u r es. Chemicals were purchased from
Aldrich Chemical Co. unless noted otherwise. Diborane pinacol
ester was purchased from Frontier Scientific Inc., Logan, CO.
Solvent distillations, synthetic procedures, and electrochem-
istry were carried out under an argon atmosphere. THF was
distilled from sodium benzophenone ketyl prior to use. Meth-
ylene chloride was distilled from calcium hydride. Electro-
chemical experiments and EPR spectroscopy were performed
as described previously.98
(92) Mitsumori, T.; Inoue, K.; Koga, N.; Iwamura, H. J . Am. Chem.
Soc. 1995, 117, 2467.
(93) Kanno, F.; Inoue, K.; Koga, N.; Iwamura, H. J . Phys. Chem.
1993, 97, 13267.
(94) Akita, T.; Mazaki, Y.; Kobayashi, D.; Koga, N.; Iwamura, H. J .
Org. Chem. 1995, 60, 2092.
(95) Yoshioka, N.; Lahti, P. M.; Kaneko, T.; Kuzumaki, Y.; Tsuchida,
E.; Nishide, H. J . Org. Chem. 1994, 59, 4272.
(96) Carilla, J .; J ulia´, L.; Riera, J .; Brillas, E.; Garrido, J . A.;
Labarta, A.; Alcala´, R. J . Am. Chem. Soc. 1991, 113, 8281.
(97) Silverman, S. K.; Dougherty, D. A. J . Phys. Chem. 1993, 97,
13273.
(98) Shultz, D. A.; Farmer, G. T. J . Org. Chem. 1998, 63, 6254.

Cross-Conjugated Bis(porphyrin)s
J . Org. Chem., Vol. 64, No. 25, 1999 9135
X-r a y Cr ysta llogr a p h y. Da ta Collection for Bis(p or -
p h yr in )s 6 a n d 7. A crystal of suitable dimensions was
mounted on a standard Siemens SMART CCD-based X-ray
diffractometer equipped with a normal focus Mo-target X-ray
tube (λ ) 0.710 73 Å) operated at 2000 W power (50 kV, 40
mA). The X-ray intensities were measured at 158(2) K.
Analysis of the data showed negligible decay during data
collection; the data were corrected for absorption using an
empirical method (SADABS: Sheldrick, G. M., SHELXTL,
Siemens, Madison, WI, 1995). The structure was solved and
refined with the Siemens SHELXTL (version 5.10) software
package.
Str u ctu r e Deter m in a tion for Bis(p or p h yr in ) 6. The
structure was solved and refined using the space group P1h with
Z ) 2 for the formula C115.5H105N8Zn2, which includes a total
of 1.5 toluene solvates disordered over 4 positions. All non-
hydrogen atoms, with the exception of the lattice solvates, were
refined anisotropically. Hydrogen atoms were placed in ideal-
ized positions. The final full-matrix refinement based on F2
converged at R1 ) 0.1042 and wR2 ) 0.2919 (based on
observed data) and R1 ) 0.1458 and wR2 ) 0.3249 (based on
all data); the largest peak/hole in the final difference map was
+0.72/-0.54 Å.
through a short flash column (SiO2, CH2Cl2), followed by
purification by radial chromatography (SiO2, 5% CH2Cl2/
pentane), giving the product as a white solid (782 mg, 70%):
1H NMR (CDCl3) δ 7.52 (s, 1H), 6.32 (s, 1H), 3.88 (s, 6H), 0.23
(s, 18H); 13C NMR (CDCl3) δ 162.0, 139.4, 104.8, 100.3, 97.2,
94.7, 56.0, 0.1; IR (film) νmax 2152 cm-1. Anal. Calcd for
C18H26O2Si2: C, 65.40; H, 7.29. Found: C, 65.22; H, 8.06.
5-ter t-Bu tylben zen e-1,3-d ibor on ic Acid Dip in a col Es-
ter , 13. A 25 mL Schlenk flask containing PdCl2(dppf) (50 mg,
0.06 mmol), KOAc (607 mg, 6.16 mmol), diborane pinacol ester
(574 mg, 2.26 mmol), and 1227 (300 mg, 1.03 mmol) in DMF
(11 mL) was prepared in a glovebox. The reaction mixture was
heated to 80 °C for 11 h. After cooling, DMF was removed by
bulb-to-bulb distillation, and the reaction residue was dissolved
in CH2Cl2. The residue was passed through a short Celite
column and then purified by triethylamine-deactivated silica
gel column chromatography (1% triethylamine, 90% CH2Cl2/
petroleum ether). The resulting product was recrystallized
with n-pentane and CH2Cl2, giving 13 as a white solid (215
mg, 55%): 1H NMR (CD2Cl2) δ 7.96 (s, 1H), 7.87 (s, 2H), 1.34
(s, 33H); 13C NMR (CD2Cl2) δ 150.1, 139.3, 134.7, 84.3, 35.1,
31.8, 25.3. Anal. Calcd for C22H36O4Br2: C, 68.43; H, 9.39.
Found: C, 68.25; H, 9.38.
Str u ctu r e Deter m in a tion for Bis(p or p h yr in ) 7. The
structure was solved and refined using the space group P1h with
Z ) 2 for the formula C137H128N8Zn132, which includes a total
of three toluene solvates. Two of these were located on
difference Fourier maps. PLATON identified three additional
voids, which may presumably contain additional disordered
solvent. These voids cumulatively reflect a potential solvent
volume of 681.4 Å3, which represents a total of 12.2% of the
lattice volume. The total electron count per cell is 81.9, and
introducing this correction resulted in a dramatic improvement
in the reported residuals (10.6% for wR2 and 3.8% for R1 based
on all data). These voids likely contain an additional toluene
solvate, but this is not definitive. The empirical formula,
formula weight, density, absorption coefficient, and F(000)
should all therefore be treated with caution. All non-hydrogen
atoms, with the exception of the lattice solvates were refined
anisotropically. Hydrogen atoms were placed in idealized
positions. The final full matrix refinement based on F2
converged at R1 ) 0.0851 and wR2 ) 0.1723 (based on
observed data); R1 ) 0.1261 and wR2 ) 0.1934 (based on all
data) and the largest peak/hole in the final difference map was
+0.87/-0.58 Å.
1-t er t -B u t y l-3,5-b is (t r im e t h y ls ila n y le t h y n y l)b e n -
zen e, 9. A 100 mL Schlenk flask containing 1227 (1.02 g, 3.48
mmol), Pd(PPh3)4 (201 mg, 0.174 mmol), CuI (20 mg, 0.104
mmol), piperidine (1.7 mL, 17.4 mmol), and trimethylsilyl-
acetylene (1.5 mL, 10.4 mmol) in THF (30 mL) was subjected
to three freeze-pump-thaw cycles. The tightly sealed reaction
flask was heated to reflux for 60 h. The mixture was allowed
to cool, and saturated aqueous NH4Cl was added. The organic
layer was dried over Na2SO4, the solvent removed in vacuo,
and the residue run through a short flash column (SiO2, CH2-
Cl2), followed by purification by radial chromatography (SiO2,
pentane), giving 9 as a light yellow oil (988 mg, 87%): 1H NMR
(CDCl3) δ 7.43 (s, 3H), 1.30 (s, 9H), 0.25 (s, 18H); 13C NMR
(CDCl3) δ 151.3, 132.8, 129.2, 122.9, 104.7, 94.1, 34.7, 31.1,
0.01; IR (film) νmax 2144 cm-1. Anal. Calcd for C20H30Si2: C,
73.54; H, 9.25. Found: C, 73.58; H, 9.34.
Ben zen e-1,4-d ibor on ic Acid Dip in a col Ester , 15.25 A 25
mL Schlenk flask containing PdCl2(dppf) (44.6 mg, 0.06 mmol),
KOAc (540 mg, 5.46 mmol), diborane pinacol ester (508 mg,
2.0 mmol), and p-diiodobenzene (300 mg, 0.91 mmol) in DMF
(10 mL) was prepared in a glovebox. The reaction mixture was
heated to 80 °C for 19 h. After cooling, DMF was removed by
bulb-to-bulb distillation, and the reaction residue was dissolved
in CH2Cl2 and passed through a short Celite column. Purifica-
tion was achieved by triethylamine-deactivated silica gel
column chromatography (1% triethylamine/petroleum ether).
The resulting product was recrystallized with n-pentane and
CH2Cl2, giving 15 as a white solid (232 mg, 99%). This
compound was identical to that reported by Ishiyama.25
5-ter t-Bu t yl-1,3-d ip h en ylet h yn yl-4′,4′′-b is[Zn (II)-t r i-
m esitylp or p h yr in yl]ben zen e, 1. A 100 mL Schlenk flask
containing ethyneporphyrin 1621 (141 mg, 0.17 mmol), diiodo-
benzene 822 (31 mg, 0.8 mmol), toluene (40 mL) and triethyl-
amine (8 mL) was sparged with argon at 35 °C for 30 min.
Pd2(dba)3 (22 mg, 0.02 mmol) and AsPh3 (52 mg, 0.17 mmol)
were then added. After the mixture was stirred for 1 h, the
solvent was removed in vacuo, and ether and saturated NH4-
Cl were added. The layers were separated, and the organic
portion was dried over Na2SO4, filtered, and evaporated. The
residue was subjected to flash chromatography (SiO2, 1%
diethyl ether/petroleum ether) and MPLC (1% diethyl ether/
petroleum ether), giving 1 (100 mg, 70%): 1H NMR (CD2Cl2)
δ 8.92 (d, 4H, J ) 4.4 Hz), 8.77 (d, 4H, J ) 4.7 Hz), 8.71 (br s,
8H), 8.27 (d, 4H, J ) 4.7 Hz), 7.99 (d, 4H, J ) 4.7 Hz), 7.79
(br s, 1H), 7.79 (br s, 2H), 7.30 (br s, 12H), 2.62 (br s, 18H),
1.84 (br s, 36H), 1.49 (s, 9H); 13C NMR (CD2Cl2) δ 152.7, 150.6,
150.5, 150.4, 150.3, 143.9, 139.7, 139.6, 138.1, 135.2, 132.5,
131.7, 131.6, 131.2, 130.4, 129.7, 128.2, 124.0, 122.8, 119.8,
119.5, 119.5, 90.5, 90.1, 35.4, 31.6, 21.9, 21.8; UV-vis (CHCl3)
λmax/nm (log ꢀ) 294 (4.81), 402 (4.93), 422 (6.03), 486 (3.68),
514 (3.86), 550 (4.66), 592 (3.91); MS-FAB C120H102N8Zn2 calcd
exact mass 1782.6788, obsd 1782.6810.
3-ter t-Bu tyl-1,5-[2′(Zn (II)-tr im esitylp or p h yr in yl)eth y-
n yl]ben zen e, 2. A 50 mL flask containing iodoporphyrin 1723
(233 mg, 0.28 mmol), K2CO3 (86 mg, 0.62 mmol), piperidine
(0.28 mL, 2.83 mmol), Pd(PPh3)4 (13.1 mg, 0.01 mmol), CuI
(1.1 mg, 0.006 mmol), 9 (86 mg, 0.62 mmol), THF (30 mL),
and distilled methanol (3 mL) was pump/purged 10 times, and
then refluxed for 16 h. Once cool, ether and saturated NH4Cl
were added. The organic layer was washed two times with
saturated NaCl solution, separated, and dried over Na2SO4,
and the solvent was removed in vacuo. The product was
purified by flash chromatography (alumina, 5% CH2Cl2/0.1%
diethyl ether/petroleum ether to 20% CH2Cl2/2% diethyl ether/
petroleum ether), giving 2 (124 mg, 68%): 1H NMR (CD2Cl2)
δ 9.94 (d, 4H, J ) 4.4 Hz), 9.40 (d, 4H, J ) 4.7 Hz), 8.67 (br s,
8H), 8.59 (br s, 1H), 8.24 (d, 2H, J ) 1.5 Hz), 7.35 (s, 8H), 7.30
(s, 4H), 2.66 (s, 12H), 2.63 (s, 6H), 1.90 (s, 24H), 1.88 (s, 12H),
1,5-Dim et h oxy-2,4-b is(t r im et h ylsila n ylet h yn yl)b en -
zen e, 11. A 100 mL Schlenk flask containing dibromide 1026
(1.0 g, 3.38 mmol), Pd(PPh3)4 (195 mg, 0.17 mmol), CuI (19.3
mg, 0.10 mmol), and piperidine (1.67 mL, 16.9 mmol) in THF
(50 mL) was sparged with argon for 5 min and pump/purged
five times. The reaction mixture was heated to reflux. After
15 min, 1 equiv of trimethylsilylacetylene (1.43 mL, 10.14
mmol) was added quickly with argon purging followed by an
additional 3 equiv of trimethylsilylacetylene every 5 h there-
after until a total of 12 equiv had been added. After an
additional 24 h at reflux, the mixture was allowed to cool, and
ether and saturated aqueous NH4Cl were added. The layers
were separated, the organic layer was dried over Na2SO4, and
the solvent was removed in vacuo. The residue was run

9136 J . Org. Chem., Vol. 64, No. 25, 1999
Shultz et al.
1.66 (s, 9H); 13C NMR (CD2Cl2) δ 153.2, 152.8, 151.0, 150.4,
150.2, 139.8, 139.4, 138.3, 132.1, 131.9, 131.7, 131.5, 129.4,
128.3, 125.3, 121.3, 120.6, 99.3, 96.3, 93.5, 35.7, 31.8, 22.0, 21.8;
IR (film) νmax 2193 cm-1; UV-vis (CHCl3) λmax/nm (log ꢀ) 353
(4.41), 432 (5.72), 444 (5.90), 499 (3.69), 527 (4.00), 566 (4.65),
615 (4.79); MS-FAB C108H94N8Zn2 calcd exact mass 1630.6183,
obsd 1630.6163.
1H), 8.88 (d, 4H, J ) 4.4 Hz), 8.71 (s, 8H), 8.63 (s, 2H), 7.32
(s, 4H), 7.27 (s, 8H), 2.65 (s, 12H), 2.63 (s, 6H), 1.93-1.72 (m,
45H); 13C NMR (CD2Cl2) δ 150.5, 150.1, 140.8, 139.5, 139.3,
139.2, 137.6, 132.5, 131.3, 131.1, 127.8, 119.1, 118.7, 35.4, 32.1,
22.0, 21.7; UV-vis (CH2Cl2) λmax/nm (log ꢀ) 307 (4.61), 353
(4.45), 416 (5.99), 431 (5.94), 482 (3.76), 512 (3.97), 550 (4.80),
588 (4.01); MS-FAB C104H94N8Zn2 calcd exact mass 1582.6184,
obsd 1582.6281.
2,4-Dim eth oxy-1,5-bis[2′(Zn (II)-tr im esitylpor ph yr in yl)-
eth yn yl]ben zen e, 3. A 100 mL Schlenk flask containing 11
(40 mg, 0.12 mmol), iodoporphyrin 1723 (216 mg, 0.25 mmol),
and K2CO3 (74 mg, 0.27 mmol) in THF (20 mL) was sparged
with argon for 5 min. Piperidine (0.24 mL, 1.2 mmol), Pd-
(PPh3)4 (14 mg, 0.01 mmol), CuI (1.2 mg, 0.003 mmol), and
distilled methanol (2.6 mL) were added, and the reaction
mixture was pump/purged five times with argon and then
refluxed for 36 h. Once cool, the reaction mixture was run
through a short flash chromatography column (SiO2, CH2Cl2/
THF), and then purified by flash chromatography (SiO2, 30-
50% CH2Cl2/petroleum ether), giving 3 (128 mg, 65%): 1H
NMR (CD2Cl2) δ 9.99 (d, 4H, J ) 4.7 Hz), 8.86 (d, 4H, J ) 4.7
Hz), 8.64 (s, 8H), 8.58 (s, 1H), 7.33 (d, 12H, J ) 14.7 Hz), 6.95
(s, 1H), 4.39 (s, 6H), 2.63 (d, 18H, J ) 13.1 Hz), 1.87 (d, 36H,
J ) 7.3 Hz); 13C NMR (CD2Cl2) δ 163.0, 152.4, 150.6, 150.3,
149.9, 139.6, 139.2, 138.1, 136.7, 131.8, 131.6, 131.3, 128.1,
120.6, 120.3, 106.8, 100.5, 96.4, 96.1, 92.6, 57.0, 21.7, 21.6;
UV-vis (CHCl3) λmax/nm (log ꢀ) 432 (5.54), 446 (5.65), 532
(4.04), 570 (4.51), 624 (4.92); MS-FAB C106H90N8O2Zn2 calcd
exact mass 1634.58, obsd 1634.58.
3-ter t-Bu tyl-1,5-bis[Zn (II)-tr im esitylp or p h yr in yl]ben -
zen e, 4. A 25 mL Schlenk flask containing 13 (51 mg, 0.13
mmol), iodoporphyrin 1723 (250 mg, 0.29 mmol), Pd(PPh3)4 (80
mg, 0.007 mmol), and K3PO4 (93 mg, 0.44 mmol) in DMF (5
mL) was prepared in a glovebox. The reaction mixture was
heated to 100 °C for 12 h. DMF was removed by bulb-to-bulb
distillation, and the residue was dissolved in CH2Cl2 and
poured into a separatory funnel. The ether solution was
washed with saturated aqueous NH4Cl, and the layers were
separated. The organic layer was dried over Na2SO4 and
filtered and the solvent removed in vacuo. The residue was
run through a short flash chromatography column (SiO2, 1%
THF/CH2Cl2). The solvents were removed, and the residue was
dissolved in CH2Cl2. After cooling and filtration, the purple
solid was purified by radial chromatography (SiO2, 5-10%
CH2Cl2/petroleum ether), giving 4 as a purple solid (126 mg,
60%): 1H NMR (CDCl3) δ 9.35 (d, 4H, J ) 4.4 Hz), 8.97 (s,
1,4-Bis[Zn (II)-tr im esitylp or p h yr in yl]ben zen e, 5.24 A 25
mL Schlenk flask containing 1525 (27 mg, 0.11 mmol), 1723 (179
mg, 0.21 mmol), Pd(PPh3)4 (6 mg, 0.007 mmol), and K3PO4 (74
mg, 0.35 mmol) in DMF (4 mL) was prepared in a glovebox.
The reaction mixture was heated to 100 °C for 24 h. DMF was
removed by bulb-to-bulb distillation, and the residue was
dissolved in CHCl3. After cooling in the freezer, 5 was filtered
off as a purple solid (193 mg, 60%): UV-vis (CH2Cl2) λmax/nm
(log ꢀ) 294 (4.81), 402 (4.93), 422 (6.03), 486 (3.68), 514 (3.86),
550 (4.66), 592 (3.91); MS-FAB C100H86N8Zn2 calcd exact mass
1526.5558, obsd 1526.5505.
Ack n ow led gm en t. This work was funded by the
National Science Foundation (Grant CHE-9501085)
under the CAREER Program, and by Research Corp.
(Grant CS0127) under the Cottrell Scholars Program.
D.A.S. also thanks the Camille and Henry Dreyfus
Foundation for a Camille Dreyfus Teacher-Scholar
Award. Mass spectra were obtained at the Mass Spec-
trometry Laboratory for Biotechnology. Partial funding
for the Facility was obtained from the North Carolina
Biotechnology Center and the National Science Founda-
tion. The MALDI-TOF mass spectrometer was funded
in part by the North Carolina Biotechnology Center. We
thank Dr. J eff Kampf (Department of Chemistry, Uni-
versity of Michigan) for determining the crystal struc-
tures of 6 and 7, and Professor Stefan Franzen (De-
partment of Chemistry, North Carolina State University)
for stimulating discussions and fitting Soret bands.
Su p p or tin g In for m a tion Ava ila ble: Spectral and crys-
tallographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
J O991046H