Double-helical dinuclear bis(dipyrromethene) complexes formed by self-assembly

Article abstract of DOI:10.1021/jo000886p

Bis(dipyrromethene) ligands linked by an alkyl spacer between β and β' positions are shown to give helical dimers or monomers, dependent upon the length of the alkyl linker, upon complexation. Ligands consisting of methylene, ethylene, and propylene linkers -(CH₂)(n) - (n = 1, 2, and 3) give helical dimers, while longer linking chains (n = 4, 5, or 6) give monomers or mixtures of dimers and monomers. X-ray crystal structures of the dimeric zinc complexes (n = 1, 2, and 3) reveal that the angles between dipyrromethene planes and the extent of helicity in the complexes differ as the length of the linker varies. The extent of helicity was assessed and found to be dependent upon the length and, specifically, the conformational preferences of the alkyl spacer unit. The presence of an ethylene linker gave complexes of greatest helicity. The use of a methylene spacer gave less helical structures upon complexation, while propylene spacers gave only slightly helical complexes. Our studies identify the crucial importance that the conformational preferences of the β-β' alkyl spacer group plays in the coordination algorithm of self-assembly to form dipyrromethene based complexes.

Full text of DOI:10.1021/jo000886p

7870
J . Org. Chem. 2000, 65, 7870-7877
Dou ble-Helica l Din u clea r Bis(d ip yr r om eth en e) Com p lexes F or m ed
by Self-Assem bly
Alison Thompson and David Dolphin*
Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia, Canada
ddolphin@qlt-pdt.com
Received J une 9, 2000
Bis(dipyrromethene) ligands linked by an alkyl spacer between â and â′ positions are shown to
give helical dimers or monomers, dependent upon the length of the alkyl linker, upon complexation.
Ligands consisting of methylene, ethylene, and propylene linkers -(CH2)n- (n ) 1, 2, and 3) give
helical dimers, while longer linking chains (n ) 4, 5, or 6) give monomers or mixtures of dimers
and monomers. X-ray crystal structures of the dimeric zinc complexes (n ) 1, 2, and 3) reveal that
the angles between dipyrromethene planes and the extent of helicity in the complexes differ as the
length of the linker varies. The extent of helicity was assessed and found to be dependent upon the
length and, specifically, the conformational preferences of the alkyl spacer unit. The presence of
an ethylene linker gave complexes of greatest helicity. The use of a methylene spacer gave less
helical structures upon complexation, while propylene spacers gave only slightly helical complexes.
Our studies identify the crucial importance that the conformational preferences of the â-â′ alkyl
spacer group plays in the coordination algorithm of self-assembly to form dipyrromethene based
complexes.
The preparation of supramolecular structures by self-
in the solid state, rendering X-ray crystallography useless
if crystals of adequate quality cannot be grown. Addition-
ally, the resulting salts are frequently difficult to purify
using chromatography.
Multi-porphyrin architectures have also been reported,
albeit sometimes created through the use of covalent
interactions, which show promise in electronic and pho-
tonic applications.9 A recent report has suggested the
possible application of self-assembled crystalline porphy-
rin lattices with nano-dimension channels running through
them as alternatives to inorganic zeolites.10 Porphyrins
assembly is a subject of much current interest as an
approach to materials with novel physicochemical prop-
erties.1 This strategy takes its lead from nature which
typically utilizes noncovalent or metal-ligand interac-
tions to organize molecules into large assemblies. This
facilitates the construction of molecular architectures, the
size and complexity of which would often be impossible
to imagine using traditional organic syntheses involving
covalent bonds. Bioinorganic chemists have long since
recognized the importance and usefulness of metal-
ligand interactions, since nature uses metals in a mul-
titude of roles, and organic chemists are slowly recog-
nizing the practicalities of self-assembly strategies
involving noncovalent bonding.
One approach to supramolecular architectures is metal-
ion assisted self-assembly.2 There are many reported
examples of coordination of metal ions by polybipyridine
ligands to generate interesting two and three-dimen-
sional arrays including rods,3 grids,4 cages,5 helices,6
ladders,7 and rings.8 Bipyridines are neutral ligands,
which produce charged complexes upon coordination to
metals at any oxidation state (> M0). Consequently,
counterions are needed to generate neutral species, but
unfortunately such counterions often give rise to disorder
(4) Some representative examples can be found in these articles and
the references therein: Weissbuch, I.; Baxter, P. N. W.; Cohen, S.;
Cohen, H.; Kjaer, K.; Howes, P. B.; Als-Nielsen, J .; Hanan, G. S.;
Schubert, U. S.; Lehn, J .-M.; Leiserowitz, L.; Lahav, M. J . Am. Chem.
Soc. 1998, 120, 4850-4860. Garcia, A. M.; Romero-Salguero, F. J .;
Bassani, D. M.; Lehn, J .-M.; Baum, G.; Fenske, D. Chem. Eur. J . 1999,
5, 1803-1808. Baxter, P. N. W.; Lehn, J .-M.; Fischer, J .; Youinou, M.-
T. Angew. Chem., Int. Ed. Engl. 1994, 33, 2284-2287.
(5) Some representative examples can be found in these articles and
the references therein: Power, K. N.; Hennigar, T. L.; Zaworotko, M.
J . J . Chem. Soc., Chem. Commun. 1998, 595-596. Fujita, M.; Yu, S.-
Y.; Kusukawa, T.; Funaki, H.; Ogura, K.; Yamaguchi, K. Angew. Chem.,
Int. Ed. Engl. 1998, 37, 2082-2085.
(6) Some representative examples can be found in these articles and
the references therein: El-ghayouy, A.; Harriman, A.; De Cian, A.;
Fischer, J .; Ziessel, R. J . Am. Chem. Soc. 1998, 120, 9973-9974.
Rapenne, G.; Patterson, B. T.; Sauvage, J .-P.; Keene, F. R. J . Chem.
Soc., Chem. Commun. 1999, 1853-1854. Baum, G.; Constable, E. C.;
Fenske, D.; Housecroft, C. E.; Kulke, T. Chem. Eur. J . 1999, 5, 1862-
1873. Lehn, J .-M.; Rigault, A. Angew. Chem., Int. Ed. Engl. 1988, 27,
1095-1097. Yamamoto, M.; Takeuchi, M.; Shinkai, S.; Tani, F.; Naruta,
Y. J . Chem. Soc., Perkin Trans. 2 2000, 9-16.
(7) Some representative examples can be found in these articles and
the references therein: Baxter, P. N. W.; Hanan, G. S.; Lehn, J .-M. J .
Chem. Soc., Chem. Commun. 1996, 2019-2020. Garcia, A. M.; Bassani,
D. M.; Lehn, J .-M.; Baum, G.; Fenske, D. Chem. Eur. J . 1999, 5, 1234-
1238.
* To whom correspondence should be addressed. Tel: (604) 822-
4571. Fax: (604) 822-9678.
(1) Philp, D.; Stoddart, J . F. Angew. Chem., Int. Ed. Engl. 1996, 35,
1155-1196. Lehn, J .-M. Supramolecular Chemistry Concepts and
Perspectives; VCH: Weinheim, Germany, 1995.
(2) Baxter, P. N. W. Comprehensive Supramolecular Chemistry;
Pergamon: Oxford, 1996; Chapter 6. Fujita, M. Comprehensive Su-
pramolecular Chemistry; Pergamon: Oxford, 1996; Chapter 7.
(3) Some representative examples can be found in these articles and
the references therein: Wa¨rnmark, K.; Baxter, P. N. W.; Lehn, J .-M.
J . Chem. Soc., Chem. Commun. 1998, 993-994. Barigelletti, F.;
Flamigni, L.; Calogero, G.; Hammarstro¨m, L.; Sauvage, J .-P.; Collin,
J .-P. J . Chem. Soc., Chem. Commun. 1998, 2333-2334.
(8) Some representative examples can be found in these articles and
the references therein. Funeriu, D. P.; Lehn, J .-M.; Baum, G.; Fenske,
D. Chem. Eur. J . 1997, 3, 99-104. Whiteford, J . A.; Stang, P. J .; Huang,
S. D. Inorg. Chem. 1998, 37, 5595-55601. Hasenknopf, B.; Lehn, J .-
M.; Baum, G.; Kneisel, B. O.; Fenske, D. Angew. Chem., Int. Ed. Engl.
1996, 35, 1838-1840.
10.1021/jo000886p CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/21/2000

Dinuclear Bis(dipyrromethene) Complexes
J . Org. Chem., Vol. 65, No. 23, 2000 7871
Sch em e 1a
are useful ligands for complexation reactions since they
generate resonance stabilized dianions which readily
coordinate to metal ions to give metalloporphyrins.
However, the difficulties often encountered in the prepa-
ration of functionalized porphyrins and the structural
limitations of the tetra-pyrrolic ring itself inhibits the
usefulness of porphyrins in self-assembly strategies.
Dipyrromethenes are essentially the building blocks
of porphyrins, but possess greater diversity and flexibility
as a result of their nonmacrocyclic nature. Our use of
these molecules as ligands for supramolecular complex-
ation reactions has allowed us to prepare new and
interesting architectures.11 We herein present a detailed
report of some of our work in this area and discuss some
of the features which control the self-assembly process.
a
Key: (a) HCHO (aq), MeOH, HCl (concd), 90%; (b) H2, Pd/C,
THF, Et3N; (c) HBr (aq), MeOH, 60% over two steps.
the factors dominant in the coordination algorithm
operating during self-assembly.
Ligands 2-716 each contain two dipyrromethene units
separated by an alkyl chain (n ) 1-6). We wished to
investigate the structure and geometry of the complexes
resulting from reaction of these ligands with metal ions.
We have previously shown that similar ligands (n ) 3,
4, 6, 8, 12), with octyl rather than ethyl â-substituents,
form dinuclear and mononuclear complexation products
depending upon the length of the linking alkyl chain.11
The dipyrromethene unit 1 is a fully conjugated flat
monoanionic ligand, useful in metal chelation reactions.12
Consequently, dipyrromethene moieties linked by a
bridging unit are useful as ligands for the formation of
well-defined architectures through self-assembly. Fuhrop
et al. reported that a mixture of 1:1 and 2:2 octaethyl
formylbiliverdinate/zinc complexes could be separated by
hand and X-ray crystallographic analysis showed that the
2:2 complex existed as a dinuclear helix.13 Similarly, the
X-ray structure of 1,2,3,7,8,12,13,17,18,19-decamethyl-
biladiene-a,c revealed a helical structure.14 As part of our
ongoing studies to investigate the use of bis(dipyrro-
methene) ligands in self-assembly strategies we have
recently shown that ligands consisting of two dipyr-
romethene units linked at the R- or â-positions generate
either trinuclear, dinuclear, or mononuclear helical struc-
tures upon complexation,11 depending upon the exact
structure of the dipyrromethene ligands used. This
followed original work by one of us in 1965 which
suggested that a 2:2 bis(dipyrromethene) ligand/CoII
complex existed in a helical conformation.15 The purpose
of our current study is to investigate the effects of varying
length of spacer between â-linked dipyrromethene units
upon the structure of resultant complexes and determine
Reaction of benzyl 2,4-dimethylpyrrole-5-carboxylate
with acidified aqueous formaldehyde in methanol gave
the key intermediate 8. Removal of the benzyl group gave
the corresponding acid which then underwent decarbox-
ylation and in-situ coupling with 3-ethyl-2-formyl-4-
methylpyrrole in the presence of hydrobromic acid to give
ligand 2 as shown in Scheme 1. All reactions proceeded
in excellent yield, with extremely facile isolation proce-
dures.
Ligands 3-4 were prepared as shown in Scheme 2.16
Preparation of the ethylene linked ligand proved to be
the most difficult with simple coupling reactions of benzyl
2,4-dimethylpyrrole-5-carboxylate with oxalyl chloride
proving extremely messy. It was eventually found that
reaction of the crude acid chloride generated from 9 with
ethyl-2,4-dimethylpyrrole-5-carboxylate in CH2Cl2 cata-
lyzed by SnCl4 under carefully controlled conditions gave
the required intermediate 11. Work up was achieved by
addition of 10% NaHSO3, since aqueous HCl, NaOH or
neutral workup gave homologated coupling products. It
is unclear why workup using NaHSO3 solution is so
successful. Reduction, debenzylation and coupling as
before gave ligand 3. Ligand 4 was prepared using the
same route but with higher yields due to a more facile
coupling procedure and greater solubility of the inter-
mediates (Scheme 2). Ligands 5-7 were prepared as
previously reported using similar strategies.16
The ligands were each reacted with excess ZnII and CoII
in the presence of NaOAc, as described in the Experi-
mental Section and shown in Scheme 3. Ligand 2 was
also reacted with CuII. Yields of the resulting complexes
were excellent in every case. The reactions were moni-
tored by UV-vis spectroscopy and complexation was seen
to occur immediately by an λmax red shift of 16-24 nm.
Complete complexation also occurs when stoichiometric
amounts of M(OAc)2 are used, but the reaction is much
slower. The complexes were isolated as stable green/
orange powders with a metallic luster. Solutions of the
complexes in CH2Cl2 or CHCl3 were stable for >6 months
in the dark, but very slow photobleaching was observed
(9) Some representative examples can be found in these articles and
the references therein. Sugiura, K.-I.; Tanaka, H.; Matsumoto, T.;
Kawai, T.; Sakata, Y. Chem. Lett. 1999, 1193-1194. Funatsu, K.;
Imamura, T.; Ichimura, A.; Sasaki, Y. Inorg. Chem. 1998, 37, 4986-
4995. Kuroda, Y.; Kato, Y.; Ogoshi, H. J . Chem. Soc., Chem. Commun.
1997, 469-470. Stang, P. J .; Fan, J .; Olenyuk, B. J . Chem. Soc., Chem.
Commun. 1997, 1453-1454. Kumar, R. K.; Balasubramanian, S.;
Goldberg, I. Inorg. Chem. 1998, 37, 541-552. Ikeda, C.; Nagahara,
N.; Motegi, E.; Yoshioka, N.; Inoue, H. J . Chem. Soc., Chem. Commun.
1999, 1759-1760.
(10) Diskin-Posner, Y.; Goldberg, I. J . Chem. Soc., Chem. Commun.
1999, 1961-1962.
(11) Thompson, A.; Rettig, S. J .; Dolphin, D. J . Chem. Soc., Chem.
Commun. 1999, 631-632. Zhang, Y.; Thompson, A.; Rettig, S. J .;
Dolphin, D. J . Am. Chem. Soc. 1998, 120, 13537-13538.
(12) Bruckner, C.; Zhang, Y.; Rettig, S. J .; Dolphin, D. Inorg. Chim.
Acta 1997, 263, 279-286. Taylor, E. C., J ones, R. A., Eds. Pyrroles;
J ohn Wiley & Sons: New York, 1990.
(13) Struckmeier, G.; Thewalt, U.; Fuhrhop, J .-H. J . Am. Chem. Soc.
1976, 98, 278-279.
(14) Sheldrick, W. S.; Engel, J . J . Chem. Soc., Chem. Commun. 1980,
5-6.
(15) Dolphin, D. H. Ph.D. Thesis, The University of Nottingham,
1965. Dolphin, D.; Harris, R. L. N.; Huppatz, J . L.; J ohnson, A. W.;
Kay, I. T.; Leng, J . J . Chem. Soc. C 1966, 98-106. Dolphin, D.; Harris,
R. L. N.; Huppatz, J . L.; J ohnson, A. W.; Kay, I. T. J . Chem. Soc. C
1966, 30-40.
(16) Paine, J . B., III; Dolphin, D. Can. J . Chem. 1978, 56, 1710-
1712.

7872 J . Org. Chem., Vol. 65, No. 23, 2000
Thompson and Dolphin
Sch em e 2a
a
Key: (a) ClC(O)Cl, reflux; (b) CH2Cl2, SnCl4, 23%, n ) 0, 85%, n ) 1 over two steps; (c) borane, THF, 23%, n ) 0, 94%, n ) 1; (d)
BnONa, BnOH, 209 °C, 94%, n ) 0, 95%, n ) 1.
Sch em e 3
Ta ble 1. Oligom er ic Ra tios of Zn II Com p lexes
higher oligomers are actually present in solution, al-
though it should be noted that they are not observed
using NMR spectroscopy. We strongly suspect that these
higher order oligomers are merely artifacts of the ana-
lytical method used. The mono- and dinuclear complexes
of ligands 5 and 6 were not separated, since TLC analysis
showed that this would be extremely difficult via chro-
matography, and recrystallization proved unfruitful.
Crystals of 17-19 and 24 suitable for X-ray analysis
were grown by slow diffusion of MeOH into chloroform
solutions of the complexes. We thus present a series of
X-ray crystal structures and compare the conformations
adopted by the complexes as the length of the alkyl
spacing group is increased.17 In 17 and 19 disordered
single molecules of CH2Cl2 and CHCl3 respectively reside
within the asymmetric unit, but this does not detract
from our studies involving bulk structural comparison
mononuclear/
dinucleara
ligand
n
product(s)
2
3
4
5
6
7
1
2
3
4
5
6
17
18
19
20
21
22
0:1
0:1b
0:1
1:2
2:1
1:0c
a
Oligomeric ratios were determined from EI m/s studies.
Approximate values are given, since the relative affinites of the
b
mono- and dinuclear species were not determined. 2% trinuclear
complex was also observed. c 2% dinuclear complex was also
observed.
to occur over a period of several weeks if the solutions
were left exposed to light.
Analysis of the complexes, primarily by EI mass
spectrometry, revealed that both dinuclear and mono-
nuclear complexes had been produced. The results for ZnII
complexation reactions are given in Table 1. Practically
identical results were obtained for compounds obtained
from CoII complexation. Table 1 shows that as the length
of the spacer unit increases, so the formation of the
mononuclear complex becomes favored. Presumably, this
arises since the longer spacers are sufficiently flexible
to allow the two dipyrromethene units within one ligand
to fold around a single metal ion to generate a mono-
nuclear product. Shorter chain lengths are incapable of
such folding and instead give dinuclear complexes. We
have previously reported that mononuclear complexes
were generated using ligands with octylene and docecyl-
ene alkyl spacers.11 In each case, the coordination algo-
rithm operates during self-assembly such as to satisfy
both the coordination geometry requirements of the metal
ion and the conformational preferences of the ligand. In
similar studies using MALDI spectrometry, higher order
oligomers were also observed.11 It is unclear whether the
(17) Crystal data for 17, C59H69N8Zn2Cl3, 1125.35: red platelet,
primitive triclinic, space group P1h (#2), a ) 11.898(3) Å, b ) 14.766(3)
Å, c ) 17.601(3) Å, R ) 79.838(4)°, â ) 74.264(2) °, γ ) 79.171(2)°,
V ) 2897.5(9) Å3, Z ) 2, T ) -100 °C, 2θmax ) 50.2°, Rigaku/ADSC
CCD diffractometer, Mo KR radiation (λ ) 0.71069 Å), R (on F, 7294
reflections with
I
> 3σ(I)) ) 0.080, Rw (on F2, all 9636 unique
reflections) ) 0.181, gof (F2) ) 1.87, 641 variables. Crystal data for
18, C60H72N8Zn2, 1036.04: green platelet, C-centred monoclinic, space
group C2/c (#15), a ) 11.6977(7) Å, b ) 22.059(7) Å, c ) 21.994(1) Å,
â ) 100.302(2) °, V ) 5584(1) Å3, Z ) 4, T ) -93 °C, 2θmax ) 57.0°,
Rigaku/ADSC CCD diffractometer, Mo KR radiation (λ ) 0.71069 Å),
R (on F, I > 3.00σ(I)) ) 0.048, Rw (on F, I > 3.00σ(I), 2457 reflections)
) 0.048, gof ) 2.39, 316 variables. Crystal data for 24, C60H72N8Co2,
1023.15: red-brown platelet, C-centred monoclinic, space group C2/c
(#15), a ) 11.630(1) Å, b ) 22.058(2) Å, c ) 21.949(2) Å, â ) 101.022-
(4) °, V ) 5526.7(9) Å3, Z ) 4, T ) -93 °C, 2θmax ) 50.5°, Rigaku/
ADSC CCD diffractometer, Mo KR radiation (λ ) 0.71069 Å), R (on F,
1920 reflections with I >3σ(I)) ) 0.063, Rw (on F2, all 7159 unique
reflections) ) 0.181, gof (F2) ) 0.97, 316 variables. Crystal data for
19, C64H78N8Zn2Cl6, 1302.85: red block crystal, primitive monoclinic,
space group P21/ a (#14), a ) 7.6470(9) Å, b ) 34.305(5) Å, c ) 12.577-
(3) Å, â ) 89.851(3) °, V ) 3299.3(8) Å3, Z ) 2, T ) -100 °C, 2θmax
)
50.5°, Rigaku/ADSC CCD diffractometer, Mo KR radiation (λ ) 0.71069
Å), R (on F, 2197 reflections with I >3σ(I)) ) 0.072, Rw (on F2, all 9671
unique reflections) ) 0.186, gof (F2) ) 1.27, 357 variables.

Dinuclear Bis(dipyrromethene) Complexes
J . Org. Chem., Vol. 65, No. 23, 2000 7873
Ta ble 2. Atom -Atom Dista n ces for Com p lexes 17-19
dipyrromethene defined planes of one ligand and an
apparent twist angle of 60°. Complex 19 has an angle of
117° between the planes defined by the dipyrromethene
units of one ligand and an apparent twist angle of 15°.
Hence, the extent of helicity decreases in the order 18 >
17 > 19 as noted above and the N-N distances map this
trend (Table 2).
a n d 24
complex
N(1)-N(4) (Å)
N(2)-N(3) (Å)
Zn(1)-Zn(2) (Å)
17
18
19
24
10.76
9.12
13.46
8.97
5.85
4.98
8.78
4.95
8.17
6.63a
11.29a
6.54b
a
b
Two major factors constitute the coordination algo-
rithm of formation for these dipyrromethene complexess
the coordination geometry requirements of the metal ion
and the preferred conformation of the ligand. The ligands
only differ by length of the alkyl spacer unit and so the
structural differences between the complexes formed
must be due to the preferred conformation of the ligands
involved. Each ligand contains largely equivalent dipyr-
romethene units, but consideration of the conformation
of the alkyl bridging units serves to rationalize our
observations. Complexes 17-19 all show slightly dis-
torted tetrahedral geometry around the ZnII centers.
The â,â′-methylene linkage between the two dipyr-
romethene binding units in 17 has limited conformational
possibilities since sp3 hybridization must be maintained.
In fact, the angle between the two dipyrromethene units
across the methylene bridge in 17 is 114°, indicative of
slight distortion from tetrahedral to accommodate the
large dipyrromethene substituents. The dipyrromethene
units in 18 experience a pseudo gauche interaction of
angle 68° across the â-â′ ethylene linkage. Clearly, the
dipyrromethene units suffer significant steric interaction,
but this is presumably a thermodynamically more stable
conformation (for the whole molecule) than the alterna-
tive anti conformation which would result in immense
strain in the N-Zn bonds and ligand conformation due
to the relatively short ethylene linker. In 17 and 18 the
dipyrromethene binding units are thus oriented such that
complexation occurs on widely differing planes for each
end of the molecule, resulting in twisted helical struc-
tures. The propylene linker in 19 has an anti-anti
conformation with anti angles of 178° and 176°. As a
consequence, the dipyrromethene binding units are ori-
ented on planes only 15° apart, thus leading to only slight
twisting and a small extent of helicity in the resulting
complex. The 2,4-dimethyl groups on the pyrrolato ring
in all the dipyrromethene units are placed in effectively
opposing positions either side of the â-linking position.
Consequently their combined effect upon the adopted
conformation is limited. We thus conclude that it is the
conformational preferences of the alkyl linking units,
rather than the length per se, which dominate the
coordination algorithm and are responsible for the struc-
tures of the complexes obtained.
Zn(1)-Zn(1)* (Å) value. Co(1)-Co(2)* (Å) value.
F igu r e 1. One ligand plus zinc taken from crystal structures
of 17, 18, and 19 and atoms Zn(1), N(1), and C(4) overlayed
using HyperChem.
of the complexes. Each complex crystallizes in a helical
dinuclear structure, with 18 showing greatest helicity,
followed by 17 and then 19, which is only slightly helical.
The Zn-N bond lengths for all complexes are in the range
1.96-2.00 (5) Å.
Analysis of the crystal structures reveals that the
dipyrromethene units in one ligand of 17 is planar, as
expected through conjugation, while the other ligand
contains a dipyrromethene unit slightly distorted from
planarity, the origin for which occurs at the methine
positions. Zinc dinuclear complex 18 and cobalt dinuclear
complex 24 have C2 symmetry and are essentially
identical when the structures are overlayed. One dipyr-
romethene unit within each ligand is planar, while the
other suffers slight distortion from planarity. Conversely,
both dipyrromethene units within the ligands of 19, are
essentially planar. This is presumably reflective of bond
strain within the complexes containing methylene and
ethylene spacers, as the coordination geometry require-
ments of the metal ion are met and the preferred ligand
conformations are compromised. Conversely, the longer
propylene spacer allows for satisfaction of the coordina-
tion geometry requirements of ZnII, while maintaining
the preferred planar ligand conformation.
Table 2 shows some interesting atom-atom lengths for
each of the complexes. Complex 19 has the longest N(1)-
N(4) distance, followed by 17 and 18. This is easily
appreciated by analysis of Figure 1, which shows a
3-atom overlayed view18 of one ligand plus ZnII from each
of 17, 18 and 19 and clearly indicates the relative degrees
of twist and bend within each of the three complexes.
Complex 18 has the shortest N1-N4 distance. This is
due to an angle of 36° between the two average planes
defined by the dipyrromethene units of a single ligand,
resulting in the near proximity of the dipyrromethene
units and hence a short N(1)-N(4) distance. The angle
of apparent twist along the axis of the molecule for 18 is
80°.19 Complex 17 has an angle of 90° between the two
The ligands and ZnII complexes were also analyzed by
1H and 13C NMR spectroscopy. Further characterization
and spectral assignment of ligand 2 and complex 17 was
achieved by use of NOE spectroscopy and the results are
shown in Table 3. These results were imperative to our
novel use of chiral lanthanide shift reagents for 1H
analysis of enantiomeric helical dipyrromethene com-
plexes20 and reveal interesting features which support
the presence of helical structures in solution. Experi-
(19) The angle of apparent twist along the axis of the molecule is
equivalent to the improper torsion angle of atoms N(4)-C(17)-N2-
C(6) for 18. Equivalent measurements for 17 and 19 were calculated
using the improper torsion angle for atoms N4-C(16)-N(2)-C(6) and
N(4)-C(18)-N(2)-C(6), respectively. This measurement serves as a
method for comparing the structures of the complexes obtained.
(18) Using HyperChem 5.11, the atoms N1, C4 and Zn1 were
overlayed in merged files containing single ligands plus metal from
the crystal structures of 17, 18 and 19.

7874 J . Org. Chem., Vol. 65, No. 23, 2000
Thompson and Dolphin
Cl/MeOH for 3 h. Similary, the Co:Co dinuclear complex
was refluxed with 10.0 equiv of Zn(OAc)2‚2H2O under
identical conditions. In both cases, approximately 2% of
the mixed Zn:Co complex was observed by EI mass
spectrometry analysis of the products.
These experiments, although only qualitative since the
relative affinities of the complexes under EI mass spec-
trometry conditions are unknown, reveal several impor-
tant features of the complexation process. A schematic
is shown in Figure 3. Although only the (productive) steps
toward formation of the obtained dinuclear complex are
shown, it is acknowledged that the dipyrromethene
ligands may (and almost certainly do) complex metal ions
in a variety of other, nonproductive, conformations. All
alternative steps must be fully and readily reversible
with complexation ultimately operating through k1 and
k2 as shown, since a single dinuclear product 17 is
obtained. From the experiments described above, it can
F igu r e 2. NOE correlations for 2 and 17.
Ta ble 3. 1H NMR Sign a l Assign m en ts for 2 a n d 17
signal
2 δ (ppm)
17 δ (ppm)
A
B
C
D
E
F
G
H
7.60
2.10
2.73
1.19
7.17
2.20
2.62
3.60
7.06
1.97
2.61
1.15
6.93
2.18
1.38
3.36
be concluded that k2 > k1 and that k2 . k-2
.
In summary, we have prepared a series of dipyrro-
methene based ZnII and CoII complexes of helical di-
nuclear and mononuclear architecture. The conformation
of four helical dinuclear complexes have been confirmed
by X-ray crystallography and compared by overlay meth-
ods. The presence of helical structures in solution is
supported by NMR spectroscopic results. The extent of
helicity was assessed and found to be dependent upon
the length and conformational preferences of the alkyl
spacer unit between the two dipyrromethene moieties.
Our studies have thus identified the crucial importance
that the conformational preferences of the â-â′ alkyl
spacer group plays in the coordination algorithm of self-
assembly to form our dipyrromethene based complexes.
Current work is being directed toward the design and
preparation of more exotic dipyrromethene based 3-di-
mensional complexes, and is guided by our understanding
of the importance and nature of alkyl spacer conforma-
tional preferences. Additionally we are working toward
the synthesis of dipyrromethene ligands connected by
rigid spacer units. The physicochemical properties of
dipyrromethene based ligands and complexes are cur-
rently under investigation.
ments showed that complexation of 2 to form 17 gave
1
rise to upfield shifts for all H signals. As expected, the
R-signals shift dramatically (0.54 ppm), while those
further removed from the site of complexation experience
less change upon complexation. Remarkably, the methyl
signal at 2.62 ppm in ligand 2 shifts to 1.38 ppm in
complex 17. Analysis of the X-ray crystal structure of 17
G
shows -CH3 to lie in a position such as to suffer
anisotropic effects due to the ring current of a dipyr-
romethene moiety within the other ligand, as noted by
others in analogous systems.6 Interestingly, CH3G-CH3F′
NOE enhancements (Figure 2) are observed for both the
ligand and the complex, indicative of steric interference
between the two dipyrromethene units within a single
ligand.21 This steric interaction presumably contributes
to the coordination algorithm of self-assembly.
To examine some kinetic and thermodynamic proper-
ties of the complexation reaction, the complexation of
ligand 2 with ZnII was conducted under a variety of
conditions. Temperature variations (-78 to 60 °C), reac-
tion time (5 min to 72 h) and order of addition did not
affect the nature of products formedsin each case,
dinuclear complex 17 was formed in quantitative yield,
suggesting it to be both the thermodynamic and kinetic
product.
Additionally, 1.0 equiv of ligand 2 was reacted with
0.5 equiv of Zn(OAc)2‚2H2O under standard conditions.
After 30 min, 0.5 equiv of of Co(OAc)2‚4H2O was added
and the reaction continued for a further 30 min and then
worked up as usual. EI mass spectrometry showed the
Zn:Zn and Co:Co dinuclear complexes to be present in a
1:1 ratio, with the Zn:Co dinuclear species making up
approximately 10% of the total yield. In another experi-
ment, 1.0 equiv of ligand 2, 0.5 equiv of Zn(OAc)2‚2H2O
and 0.5 equiv of Co(OAc)2‚4H2O were reacted under
standard conditions. EI mass spectrometry showed a
1:2:1 ratio for the Zn:Zn, Zn:Co, and Co:Co dinuclear
complexes, respectively. Furthermore, the Zn:Zn complex
was refluxed with 10.0 equiv of Co(OAc)2‚4H2O in CH3-
Exp er im en ta l Section
4,4′-Meth ylen ebis[3,5-dim eth yl-bis(ph en ylm eth yl)ester ]-
1H-p yr r ole-2-ca r boxyla te (8).16 To a solution of benzyl 2,4-
dimethylpyrrole-5-carboxylate22 (250 mg, 1.09 mmol) in MeOH
(20 mL) at room temperature was added 37% aqueous form-
aldehyde (2.6 equiv) and concentrated HCl (1 mL). After the
mixture was stirred for 14 h, a white precipitate had appeared
and TLC analysis showed the absence of starting material.
The product was isolated by filtration, washed with MeOH (10
mL), water (10 mL), and MeOH (10 mL), and then air-dried
(231 mg, 90%): mp > 230 °C; δH(200 MHz; CDCl3) 2.05 (3H,
s), 2.23 (3H, s), 3.47 (1H, s), 5.29 (2H, s), 7.29-7.43 (5H, s),
8.48 (1H, br s); m/z EI 470 (M+, 68); found M+, 470.2212,
C
29H30N2O4 requires 470.2206. Anal. Calcd for C29H30N2O4: C,
74.04; H, 6.38; N, 5.96. Found: C, 73.92; H, 6.42; N, 6.10.
3,3′-(Meth a n ed iyl)bis[2,4-d im eth yl-5-[(3-eth yl-4-m eth -
yl-2H-p yr r ol-2-ylid en e)m eth yl]-, Dih yd r obr om id e 1H-
P yr r ole (2). To a suspension of bipyrrole 8 (529 mg, 1.13
mmol) in THF (30 mL) were added NEt3 (1 drop) and 10% Pd/C
catalyst (20 mg). The reaction mixture was then exposed to
1.0 atm of hydrogen for 14 h, whereby all the starting material
was seen to have dissolved and TLC indicated the absence of
(20) Thompson, A.; Dolphin, D. Nuclear Magnetic Resonance Studies
of Helical Dipyrromethene-Zinc Complexes. Org. Lett. 2000, 1315-
1318.
(21) Although only shown in one direction for clarity, all NOE
enhancements indicated in Figure 3 were observed in both directions.
(22) Robinson, J . A.; McDonald, E.; Battersby, A. R. J . Chem. Soc.
1985, 1699-1709.

Dinuclear Bis(dipyrromethene) Complexes
J . Org. Chem., Vol. 65, No. 23, 2000 7875
F igu r e 3. Schematic formation of dinuclear complex 17 from ligand 2 and ZnII.
remaining starting material. The catalyst was removed by
filtration and washed with THF (5 mL) and MeOH (5 mL). To
the filtrate was added 3-ethyl-2-formyl-4-methylpyrrole (317
mg, 2.31 mmol), and the resultant mixture was stirred and
briefly heated gently with a heat gun to ensure full dissolution.
A solution of 50% HBr (1 mL) in MeOH (2 mL) was added to
the reaction mixture and stirring continued for 30 min. Careful
partial removal of the solvents in vacuo resulted in precipita-
tion of the orange/red product, which was isolated by filtration
and washed with cold MeOH (5 mL) (411 mg, 60%): mp dec
>230 °C; UV-vis λmax(CH2Cl2)/nm 498 (11, ꢀ 199 000), 456 (4);
δH(200 MHz; CDCl3) 1.19 (3H, t, J ) 7.6), 2.10 (3H, s), 2.20
(3H, s), 2.62 (3H, s), 2.73 (2H, q, J ) 7.6), 3.60 (1H, s), 7.17
(1H, s), 7.60 (1H, s), 13.07 (1H, br s), 13.32 (1H, br s); δC(75
MHz; CDCl3) 9.98 (CH3), 10.71 (CH3), 13.47 (CH3), 16.21 (CH3),
18.42 (CH2), 19.59 (CH2), 120.91 (CH), 124.39, 125.25, 126.27,
126.83, 141.06 (CH), 142.97, 148.53, 155.45; m/z EI 440 (M -
2HBr)+, 67); found M+, 440.2934, C29H36N4 requires 440.2940.
Anal. Calcd for C29H38N4Br2: C, 57.82; H, 6.36; N, 9.30.
Found: C, 57.54; H, 6.33; N, 9.04.
Zin c, Bis[m -[[3,3′-(m eth a n ed iyl)bis[5-[(3-eth yl-4-m eth -
yl-2H-p yr r ol-2-ylid en e-k N)m eth yl]-2,4-d im eth yl-1H-p yr -
r ola to-k N]](2-)]]d i- (17). To a stirred solution of ligand 2 (100
mg, 0.17 mmol) in CHCl3 (1 mL) at room temperature was
added a solution of Zn(OAc)2‚2H2O (182 mg, 0.83 mmol) and
NaOAc‚3H2O (113 mg, 0.83 mmol) in MeOH (1 mL). The
reaction mixture was stirred for 30 min, and the solvents were
then removed in vacuo. CH2Cl2 (10 mL) and water (10 mL)
were then added, and the organic layer was separated, washed
with water (10 mL), and dried over MgSO4. Filtration through
a short plug of silica gel eluting with CH2Cl2 and then 1:10
MeOH/CH2Cl2 and removal of the solvents in vacuo gave the
title compound as an orange powder with a green metallic
luster (80 mg, 96%): mp dec >230 °C; UV-vis λmax(CH2Cl2)/
nm 522 (11, ꢀ 220 000), 474 (4); δH(400 MHz; CDCl3) 1.15 (3H,
t, J ) 7.6), 1.38 (3H, s), 1.97 (3H, s), 2.18 (3H, s), 2.61 (2H, q,
J ) 7.6), 3.36 (1H, s), 6.93 (1H, s), 7.06 (1H, s); δC(75 MHz;
CDCl3) 9.88 (CH3), 10.00 (CH3), 15.23 (CH3), 16.50 (CH3), 18.07
(CH2), 20.44 (CH2), 122.09 (CH), 122.78, 126.81, 135.77, 136.60,
138.02, 142.03, 146.03, (CH), 160.17; m/z EI 1008 (M+, 100),
504 (M2+, 22); found M+, 1008.4102, C58H68N864Zn68Zn re-
quires 1008.4107. Anal. Calcd for C58H68N8Zn2‚1.5H2O: C,
67.63; H, 6.46; N, 10.88. Found: C, 67.32; H, 6.61; N, 10.73.
Cobalt, Bis[m-[[3,3′-(m eth an ediyl)bis[5-[(3-eth yl-4-m eth -
yl-2H-p yr r ol-2-ylid en e-k N)m eth yl]-2,4-d im eth yl-1H-p yr -
r ola to-k N]](2-)]]d i- (23). Following the general procedure as
for 17, ligand 2 (100 mg, 0.17 mmol) in CHCl3 (1 mL) was
reacted with Co(OAc)2‚4H2O (207 mg, 0.83 mmol) and NaOAc‚
3H2O (113 mg, 0.83 mmol) in MeOH (1 mL). Workup as for
17, followed by filtration through a short plug of basic alumina
eluting with CH2Cl2 and then 1:10 MeOH/CH2Cl2 followed by
removal of the solvents in vacuo, gave the title compound as
a metallic green powder (64 mg, 78%): mp dec >230 °C; UV-
vis λmax(CH2Cl2)/nm 522 (11, ꢀ 188 000), 488 (5); m/z EI 994
(M+, 100), 497 (M2+, 18); found M+, 994.4200 C58H68N8Co2
requires 994.4231.
as for 17, ligand 2 (60 mg, 0.10 mmol) in CHCl3 (1 mL) was
reacted with Cu(OAc)2‚H2O (100 mg, 0.50 mmol) and NaOAc‚
3H2O (68 mg, 0.50 mmol) in MeOH (1 mL). Workup as for 17
gave the title compound as a metallic green powder (43 mg,
86%): mp dec >230 °C; UV-vis λmax(CH2Cl2)/nm 520 (very
broad, 5, ꢀ 57 000), 494 (4); m/z EI 1004 (M+, 67), 502 (M2+
,
1); found M+, 1004.4149 C58H68N863Cu65Cu requires 1004.4141.
4,4′-[2-Oxo-(1,2-eth a n ed iyl)]bis[3,5-d im eth ylbis(eth yl)-
ester ]-1H-p yr r ole-2-ca r boxyla te (11). Oxalyl chloride (3
mL) was added to pyrrole 9 (300 mg, 1.33 mL) and the reaction
mixture refluxed for 2 h, after which time all of the starting
material pyrrole had dissolved and effervescence was no longer
apparent. The oxalyl chloride was removed in vacuo and any
remaining traces of oxalyl chloride removed by successive
addition and removal in vacuo of CHCl3 (3 × 10 mL). The
resulting crude acid chloride and ethyl-2,4-dimethylpyrrole
carboxylate36 (223 mg, 1.33 mmol) were dissolved in dry CH2-
Cl2 (4 mL) under N2 and cooled to 0 °C. SnCl4 (312 µL, 1.33
mmol) was then added dropwise. After 15 min, the dark red
reaction mixture was poured onto a solution of 10% NaHSO3
(30 mL), stirred for 15 min, and then extracted with CH2Cl2.
The resultant solution was dried over MgSO4, filtered and the
solvent removed in vacuo. Purification was achieved by chro-
matography on silica gel eluting with 3:7 ethyl acetate/hexane
to give the title product as a white powder (109 mg, 22%, 83%
based on recovered ethyl-2,4-dimethylpyrrole carboxylate): mp
212-214 °C; δH(200 MHz; CDCl3) 1.27-1.42 (6H, m), 2.16 (3H,
s), 2.18 (3H, s), 2.51 (3H, s), 2.64 (3H, s), 3.79 (2H, s), 4.21-
4.42 (4H, m), 8.69 (1H, br s), 9.05 (1H, br s); δc(75 MHz; CDCl3)
10.85, 11.73, 12.99, 14.45, 14.57, 15.16, 38.37, 59.63, 60.43,
115.23, 117.14, 118.03, 123.39, 127.66, 128.88, 130.83, 138.10,
161.75, 195.10; m/z EI 374 (M+, 13); found M+, 374.1848;
C
20H26N2O5 requires 374.1842. Anal. Calcd for C20H26N2O5: C,
64.15; H, 7.00; N, 7.48. Found: C, 64.12; H, 7.01; N, 7.38.
4,4′-(1,2-E t h a n ed iyl)b is[3,5-d im et h ylb is(et h yl)est er ]-
1H-p yr r ole-2-ca r boxyla te (13). To a suspension of bipyrrole
11 (400 mg, 1.07 mmol) under N2 in THF (50 mL) and ethyl
acetate (50 mL) was added NaBH4 (65 mg, 1.71 mmol). BF3‚
OEt2 (217 µL, 1.71 mmol) was then added dropwise. After 12
h at room temperature, much of the starting material was seen
to remain undissolved. Nevertheless, the excess reagent was
quenched by the cautious addition of acetic acid (1 mL)
followed by water (20 mL). Removal of the organic solvents in
vacuo, addition of CH2Cl2 (20 mL), separation of the organic
layer, drying over MgSO4, filtration, and removal of the solvent
in vacuo gave the crude product, which was purified by
chromatography on silica gel eluting with gradient 15:85 to
25:75 ethyl acetate:hexane (87 mg, 23%, 84% based on
recovered starting material): mp > 230 °C; δH(200 MHz;
CDCl3) 1.33 (3H, t, J ) 7.8), 1.92 (3H, s), 2.19 (3H, s), 2.44
(2H, s), 4.28 (2H, q, J ) 7.8), 8.45 (1H, br s); δc(75 MHz; CDCl3)
10.31, 11.01, 14.56, 24.82, 59.58, 116.54, 121.26, 127.33,
129.84; m/z EI 360 (M+, 14); found M+, 360.2049, C20H28N2O4
requires 360.2049)). Anal. Calcd for C20H28N2O4: C, 66.64;
H, 7.83; N, 7.77. Found: C, 66.70; H, 7.76; N, 7.74.
4,4′-(1,2-Eth an ediyl)bis[3,5-dim eth yl-bis(ph en ylm eth yl)-
ester ]-1H-p yr r ole-2-ca r boxyla te (14).16 To a solution of
bipyrrole 13 (200 mg, 0.56 mmol) in BnOH (3 mL) at 209 °C
was added a concentrated solution of BnONa in BnOH until
Cop p er , Bis[m -[[3,3′-(m et h a n ed iyl)b is[5-[(3-et h yl-4-
m eth yl-2H-p yr r ol-2-ylid en e-k N)m eth yl]-2,4-dim eth yl-1H-
p yr r ola to-k N]](2-)]]d i- (29). Following the general procedure

7876 J . Org. Chem., Vol. 65, No. 23, 2000
Thompson and Dolphin
a drop in reaction mixture temperature was no longer observed
upon addition. The solution was allowed to cool significantly
and then poured onto a 1:1 mixture of MeOH/water (20 mL)
and extracted with CH2Cl2; the organic fractions were then
back-extracted with water (3 × 20 mL). The resulting organic
fraction was dried over MgSO4 and filtered, and the solvent
was removed in vacuo to give the crude product. Purification
by recrystallization from CH2Cl2/hexane gave the desired
compound as white needles (253 mg, 95%): mp > 230 °C; δH-
(200 MHz; d6-DMSO) 1.91 (3H, s), 2.11 (3H, s), 2.36 (2H, s),
5.24 (2H, s), 7.27-7.48 (5H, m), 11.07 (1H, br s); δC(75 MHz;
d6-DMSO) 10.30, 10.45, 24.60, 64.22, 115.19, 120.55, 126.68,
127.62, 127.77, 128.45, 131.26, 137.14, 160.55; m/z EI 484 (85);
found M+, 484.2357, C30H32N2O4 requires 484.2362. Anal.
Calcd for C30H32N2O4: C, 74.36; H, 6.66; N, 5.78. Found: C,
74.27; H, 6.84; N, 6.00.
1H-P yr r ole, 3,3′-(1,2-Eth a n ed iyl)bis[2,4-d im eth yl-5-[(3-
eth yl-4-m eth yl-2H-p yr r ol-2-ylid en e)m eth yl]-, Dih yd r o-
br om id e (3). Following the procedure as for 2, a solution of
bipyrrole 14 (100 mg, 0.21 mmol) in THF (10 mL) and NEt3
(1 drop), with catalyst 10% Pd/C (10 mg), was exposed to 1.0
atm hydrogen for 3 h. Workup as before was followed by the
addition of 3-ethyl-2-formyl-4-methylpyrrole (59 mg, 0.43
mmol) and 50% HBr (200 µL) in MeOH (1 mL). Isolation as
for 2 gave the product as a dark red/brown powder (118 mg,
91%): mp dec >230 °C; UV-vis λmax(CH2Cl2)/nm 494 (5, ꢀ 161
000), 458 (3); δH(400 MHz; CDCl3) 1.17 (3H, t, J ) 7.7), 2.08
(6H, s), 2.58 (3H, s), 2.63 (2H, s), 2.67 (2H, q, J ) 7.7), 7.10
(1H, s), 7.58 (1H, d, J ) 3.3), 8.14 (1H, br s), 8.29 (1H, br s);
m/z EI 454 ((M - 2HBr)+, 57), 227 (M - 2HBr)2+, 100); found
M+, 454.3093, C30H38N4 requires 454.3097. Anal. Calcd for
over MgSO4 and filtered. Purification was achieved via chro-
matography over silica gel eluting with gradient 25:75 to 35:
65 ethyl acetate/hexane, to give the title product as a white
powder (783 mg, 85%): mp 196-198 °C; δH(200 MHz; CDCl3)
1.27-1.40 (6H, m), 2.21 (3H, s), 2.27 (3H, s), 2.48 (3H, s), 2.54
(3H, s), 2.69-2.90 (4H, m), 4.21-4.38 (4H, m), 8.52 (1H, br s),
8.85 (1H, br s); δC(75 MHz; CDCl3) 10.67, 11.49, 12.79, 14.42,
14.56, 15.19, 18.52, 43.39, 59.67, 60.41, 116.88, 117.94, 120.85,
123.33, 126.98, 129.05, 129.82, 138.05, 161.81, 197.54; m/z EI
388 (M+, 62); found M+, 388.2000, C21H28N2O5 requires
388.1998. Anal. Calcd for C21H28N2O5: C, 64.93; H, 7.27; N,
7.21. Found: C, 64.91; H, 7.28; N, 7511.
4,4′-(1,2-P r op a n ed iyl)bis[3,5-d im eth ylbis(eth yl)ester ]-
1H-p yr r ole-2-ca r boxyla te (15). Following the procedure as
for 13, bipyrrole 12 (200 mg, 0.52 mmol) was dissolved in a
mixture of THF (2 mL) and ethyl acetate (2 mL), and NaBH4
(40 mg, 1.04 mmol) and BF3‚OEt2 (132 µL, 1.04 mmol) were
added. After being stirred at room temperature for 30 min,
the excess reagent was quenched by the addition of acetic acid
(1 mL) and water (10 mL). Removal of the organic solvents in
vacuo, addition of CH2Cl2 (10 mL), separation of the organic
layer, drying over MgSO4, filtration, and removal of the
solvents in vacuo gave the crude product, which was purified
by recrystallization from CH2Cl2/hexane to give the title
product as fine white needles (183 mg, 94%): mp 211-213 °C;
δH(200 MHz; CDCl3) 1.33 (3H, t, J ) 7.9), 2.15 (3H, s), 2.22
(3H, s), 2.29-2.41 (3H, m), 4.28 (2H, q, J ) 7.9), 8.67 (1H, br
s); δc(75 MHz; CDCl3) 10.54, 11.45, 14.56, 23.80, 31.72, 59.56,
116.70, 121.86, 126.97, 129.39, 161.75; m/z EI 374 (M, 48);
found M+, 374.2001, C21H30N2O4 requires 374.2206. Anal.
Calcd for C21H30N2O4: C, 67.35; H, 8.07; N, 7.48. Found: C,
66.97; H, 8.07; N, 7.35.
C
30H40N4Br2‚1.5H2O: C, 57.61; H, 6.61; N, 8.86. Found: C,
57.93; H, 6.60; N, 8.87.
4,4′-(1,2-P r opan ediyl)bis[3,5-dim eth yl-bis(ph en ylm eth -
yl)ester ]-1H-p yr r ole-2-ca r boxyla te (16).16 Following the
procedure as for dipyrrole 14, dipyrrole 15 (200 mg, 0.52 mmol)
was benzylated. Workup and recrystallization as described for
X gave white crystals of 16 (250 mg, 94%): mp 204-205 °C;
δH(200 MHz; d6-DMSO) 1.32-1.54 (1H, m), 2.10 (3H, s), 2.12
(3H, s), 2.21-2.36 (2H, m), 5.24 (2H, s), 7.29-7.48 (5H, m),
11.10 (1H, br s); δC(75 MHz; d6-DMSO) 10.53, 10.87, 23.33,
31.58, 64.19, 115.36, 120.99, 126.24, 127.64, 127.74, 128.39,
130.68, 137.09, 160.50; m/z EI 498 (M+, 25); found M+,
498.2510, C31H34N2O4 requires 498.2519. Anal. Calcd for
Zin c, Bis[m -[[3,3′-1,2-eth a n ed iyl)bis[5-[(3-eth yl-4-m eth -
yl-2H-p yr r ol-2-ylid en e-k N)m eth yl]-2,4-d im eth yl-1H-p yr -
r ola to-k N]](2-)]]d i- (18). Following the general procedure as
for 17, ligand 3 (110 mg, 0.18 mmol) in CHCl3 (1 mL) was
reacted with Zn(OAc)2‚2H2O (196 mg, 0.89 mmol) and NaOAc‚
3H2O (121 mg, 0.89 mmol) in MeOH (1 mL). Workup as for
17, followed by rapid chromatography on basic alumina eluting
with 3:7 CH2Cl2/hexane, gave the title compound as an orange
powder with a metallic green luster (53 mg, 57%): mp >230
°C; UV-vis λmax(CH2Cl2)/nm 518 (11, ꢀ 242 000), 470 (6); δH-
(400 MHz; CDCl3) 1.12 (3H, t, J ) 7.6), 1.35 (3H, s), 1.88 (3H,
s), 2.24 (3H, s), 2.34 (1H, d J ) 10.0), 2.57 (2H, q, J ) 7.6),
2.73 (1H, d, J ) 10), 6.82 (1H, s), 6.92 (1H, s); δC(75 MHz;
CDCl3) 9.96 (CH3), 10.10 (CH3), 14.19 (CH3), 16.55 (CH3), 18.12
(CH2), 25.31 (CH2), 121.85 (CH), 122.78, 128.52, 135.53, 136.73,
138.96, 141.67, 145.87 (CH), 159.71; m/z EI 1552 (2), 1036 (M+,
94), 517 (M2+, 100); found M+, 1036.4434, C60H72N864Zn68Zn
requires 1036.4420; found M+, (trinuclear complex) 1552.6596,
C
31H34N2O4: C, 74.67; H, 6.87; N, 5.62. Found: C, 74.59; H,
6.99; N, 5.70.
1H -P yr r ole,3,3′-(1,3-p r op a n ed iyl)b is[2,4-d im et h yl-5-
[(3-et h yl-4-m et h yl-2H-p yr r ol-2-ylid en e)m et h yl]-, Dih y-
d r obr om id e (4). Following the procedure as for 2, a solution
of bipyrrole 16 (800 mg, 1.56 mmol) in THF (30 mL) and NEt3
(1 drop) with catalyst 10% Pd/C (20 mg) was exposed to 1.0
atm of hydrogen for 3 h. Workup as before was followed by
the addition of 3-ethyl-2-formyl-4-methylpyrrole (439 mg, 3.20
mmol) and 50% HBr (1 mL) in MeOH (5 mL). Isolation as for
2 gave the product as a dark red/brown powder (866 mg,
88%): mp dec >230 °C; UV-vis λmax(CH2Cl2)/nm 490 (11,
ꢀ 165 000), 460 (6); δH(400 MHz; CDCl3) 1.38 (3H, t, J ) 7.7),
1.53-1.64 (1H, m), 2.08 (3H, s), 2.25 (3H, s), 2.44 (2H, q, J )
7.7), 2.62-2.71 (5H, m), 7.13 (1H, s), 7.57 (1H, d, J ) 3.5),
13.09 (1H, br s), 13.28 (1H, br s); δC(75 MHz; CDCl3) 9.92
(CH3), 10.45 (CH3), 13.30 (CH3), 16.22 (CH3), 18.37 (CH2), 23.78
(CH2), 29.87 (CH2), 120.60 (CH), 123.85, 125.80, 127.28, 128.87,
139.67 (CH), 143.19, 147.69, 156.58; m/z EI 468 ((M - 2HBr)+,
12); found M+, 468.3245, C31H40N4 requires 468.3253. Anal.
C
C
90H108N1264Zn268Zn requires 1552.6652. Anal. Calcd for
60H72N8Zn2‚H2O: C, 68.37; H, 7.08; N, 10.63. Found: C,
68.50; H, 6.81; N, 10.35.
Cob a lt , Bis[m -[[3,3′-(1,2-et h a n ed iyl)b is[5-[(3-et h yl-4-
m eth yl-2H-p yr r ol-2-ylid en e-k N)m eth yl]-2,4-dim eth yl-1H-
p yr r ola to-k N]](2-)]]d i- (24). Following the general procedure
as for 23, ligand 3 (100 mg, 0.16 mmol) in CHCl3 (1 mL) was
reacted with Co(OAc)2‚4H2O (202 mg, 0.81 mmol) and NaOAc‚
3H2O (110 mg, 0.81 mmol) in MeOH (1 mL). Workup as for
23 gave the title compound as a green metallic powder (77
mg, 94%): mp dec >230 °C; UV-vis λmax(CH2Cl2)/nm 516 (11,
ꢀ 193 000), 484 (6); m/z EI 1022 (M+, 100), 511 (38, M2+); found
M+, 1022.4523, C60H72N8Co2 requires 1022.4544.
4,4′-[2-Oxo-(1,2-eth a n ed iyl)]bis[3,5-d im eth yl-bis(p h en -
ylm eth yl)ester ]-1H-p yr r ole-2-ca r boxyla te (12).16 Follow-
ing the general procedure as for the preparation of 11, pyrrole
10 (566 mg, 2.37 mmol) was refluxed with oxalyl chloride (5
mL) for 20 min to give the crude acid chloride. Isolation as
previously, followed by reaction in dry CH2Cl2 (6 mL) with
ethyl 2,4-dimethylpyrrolecarboxylate (396 mg, 2.37 mmol) and
SnCl4 (555 µL, 4.74 mmol) at 0 °C gave the coupled product
after 15 min, as monitored by TLC. The reaction mixture was
then poured onto 2 M HCl, stirred for 20 min, and then
extracted with CH2Cl2 (50 mL). The reaction mixture was dried
Calcd for
Found: C, 57.88; H, 6.54; N, 8.63.
C31H42N4Br2‚H2O: C, 57.41; H, 6.84; N, 8.64.
Zin c, Bis[m -[[3,3′-(1,3-p r op a n ed iyl)b is[5-[(3-et h yl-4-
m eth yl-2H-p yr r ol-2-yliden e-kN)m eth yl]-2,4-d im eth yl-1H-
p yr r ola to-k N]](2-)]]d i- (19). Following the general procedure
as for 17, ligand 4 (120 mg, 0.19 mmol) in CHCl3 (1 mL) was
reacted with Zn(OAc)2‚2H2O (209 mg, 0.95 mmol) and NaOAc‚
3H2O (130 mg, 0.95 mmol) in MeOH (1 mL). Workup as for
17 gave the title compound as an orange powder with a green
metallic luster (112 mg, 90%): mp decomp >230 °C; UV-vis
λ
max(CH2Cl2)/nm 506 (11, ꢀ 240 000), 480 (6); δH(400 MHz;
CDCl3) 1.16 (3H, t, J ) 7.6), 1.37-1.59 (1H, m), 1.61 (3H, s),

Dinuclear Bis(dipyrromethene) Complexes
J . Org. Chem., Vol. 65, No. 23, 2000 7877
1.97 (3H, s), 2.18 (3H, s), 2.19-2.25 (2H, m), 2.64 (2H, q, J )
7.6), 6.80 (1H, s), 7.04 (1H, s); δC(75 MHz; CDCl3) 10.12 (2CH3),
14.82 (CH3), 16.67 (CH3), 18.24 (CH2), 23.45 (CH2), 31.27 (CH2),
122.15 (CH), 122.76, 129.02, 135.78, 136.91, 138.53, 141.95,
146.10 (CH), 159.41; m/z EI 1064 (M+, 100), 532, (M2+, 65);
found M+, 1064.4686, C62H76N864Zn68Zn requires 1064.4733.
Anal. Calcd for C62H76N8Zn2‚2H2O‚2CHCl3: C, 57.41; H, 6.17;
N, 8.37. Found: C, 57.39; H, 5.80; N, 8.28.
Coba lt, Bis[m -[[3,3′-(1,3-p r op a n ed iyl)bis[5-[(3-eth yl-4-
m eth yl-2H-p yr r ol-2-ylid en e-k N)m eth yl]-2,4-d im eth yl-1H-
p yr r ola to-k N]](2-)]]d i- (25). Following the general procedure
as for 23, ligand 4 (100 mg, 0.16 mmol) in CHCl3 (1 mL) was
reacted with Co(OAc)2‚4H2O (209 mg, 0.81 mmol) and NaOAc‚
3H2O (130 mg, 0.81 mmol) in MeOH (1 mL). Workup as for
23 gave the title compound as a metallic green powder (65
mg, 77%): mp dec >230 °C; UV-vis λmax(CH2Cl2)/nm 512
(ꢀ 210 000); m/z EI 1050 (M+, 100), 525, (M2+, 34); found M+,
1050.4852, C62H76N8Co2 requires 1050.4860.
Ack n ow led gm en t. This work was supported by the
Natural Sciences and Engineering Council of Canada.
The authors would like to acknowledge Dr. Brian
Patrick (University of British Columbia) for X-ray
crystallographic work.
Su p p or tin g In for m a tion Ava ila ble: Full X-ray crystal-
lographic data and ORTEP diagrams for 17-19 and 24.
Experimental data for ligands 5-7 and complexes 20-22 and
26-28. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O000886P