Diastereoselective generation of triple-stranded helicates induced by gem-dimethyl groups on a linker

Article abstract of DOI:10.1039/c2dt12316a

The diastereoselectivity of the self-assembly of bis(dipyrromethene) ligands with trivalent metals was improved by introducing gem-dimethyl groups to the linker. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt12316a

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Diastereoselective generation of triple-stranded helicates induced by
gem-dimethyl groups on a linker†
Zhan Zhang, Yakun Chen and David Dolphin*
Received 1st December 2011, Accepted 12th January 2012
DOI: 10.1039/c2dt12316a
helicates.9 Interestingly but not surprisingly, when bis(dipyrro-
The diastereoselectivity of the self-assembly of bis(dipyrro-
methene) ligands with trivalent metals was improved by
introducing gem-dimethyl groups to the linker.
methene) ligand 3 with a gem-dimethyl-substituted linker was
employed, helicates were generated predominantly with heating.
In addition, running the reaction at room temperature did not
noticeably change the helicate/mesocate ratio, although the
yields reduced.
Geminal dialkyl groups have long been used to enhance the
rate of cyclization reactions. Such an effect, commonly referred
to as the Thorpe–Ingold effect10 or the gem-dialkyl effect, has
wide applications in synthetic organic chemistry.11,12 More
recently, gem-dialkyl groups were also employed to manipulate
the conformation of a copolymer so that the photophysical prop-
erties could be improved.13,14 However, using gem-dialkyl
groups to control the outcome of a supramolecular self-assembly
process is rarely seen in the literature. Herein we report that
replacing a methylene linker with a gem-dimethyl-substituted
one can dramatically improve the diastereoselectivity of such
metal-directed self-assembly.
The proligand, bis(dipyrromethene) 3-H2 was synthesized in an
overall 72% yield starting from a quantitative reduction of the
β,β′-linked diphenyldipyrromethane 4 with excess NaBH4. The
crucial precursor, tetrapyrrole 5, was subsequently prepared
through acid-catalyzed condensation under conditions similar to a
Lindsey condensation15 (Scheme 1). Upon oxidation with DDQ,
5 was converted to 3-H2 at room temperature, but a minor amount
of the scrambled product 5-phenyldipyrromethene was also iso-
lated. However, use of the lower potential chloranil considerably
reduced the scrambling. After chromatography, the free base bis
(dipyrromethene) 3-H2 was obtained as a dark brown solid.
In the presence of triethylamine as base, proligand 3-H2
reacted with trivalent metal ions such as Fe3+, Co3+, Mn3+, Ga3+
and In3+ to yield red-purple products. Due to their nonpolar
nature, such compounds precipitated out of the reaction media
Metal-directed self-assembly of three strands of ligands can in
principle lead to two types of well-defined helical complexes—
helicates and mesocates.1,2 Helicates have the same configuration
at each coordinate center while mesocates have alternate
configurations. Since the first report of a triple-stranded mesocate
by Albrecht and Kotila,3 considerable efforts have been made to
understand and, as a goal, to control the formation of helicate
versus mesocate.1,2,4–7 It was reported that either the length of a
linker or a guest molecule can dramatically affect the helicate/
mesocate ratio.4–6,8 However, the distinct inherent natures of
different ligands tend to exclude a general rule governing the
processes. In our study toward triple-stranded poly(dipyrro-
methene) supramolecular complexes, we found that the same
ligand such as 1 and 2 (Fig. 1) led to both noninterconvertible
diastereomers.7,9 In addition, reaction temperature had a signifi-
cant influence on the product ratio of M223 complexes. Increas-
ing the reaction temperature greatly favored the generation of
Fig. 1 Structure of proligands 1-H2·2HBr, 2-H2 and 3-H2.
Department of Chemistry, University of British Columbia, 2036 Main
Mall, Vancouver, British Columbia, V6T 1Z1, Canada. E-mail: david.
dolphin@ubc.ca; Fax: +(1) 604-822-9678; Tel: +(1) 604-822-4571
†Electronic supplementary information (ESI) available. CCDC
815374–815377. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt12316a
Scheme 1 Synthesis of proligand 3-H2.
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Table 1 Reactions of proligand 3-H2 with trivalent metals
Table 2 Metal–nitrogen (M–N) bond lengths, distances between
metals (M–M), twist angles and helical pitches of the helicates
Helicate
Fe233
Co233
Ga233
Mn233
Mean M–N (Å)
M–M (Å)
Twist angle (degree)
Helical pitch (Å)
1.969
7.630
104
1.935
7.567
105
2.051
7.715
101
2.084
7.700
98
26.4
25.9
27.5
28.3
Entry
M
MS
h : m
Total yield (%)
1
2
3
4
5
Fe
1547
1553
1545
1575
1665
—
16 : 1
—
8 : 1
8 : 1
52
28
58
70
29
Co
Mn
Ga
In
when methanol was used as solvent. Mass analysis using
MALDI-TOF shows that all the compounds have a mass corre-
sponding to M233, suggesting the generation of triple-stranded
complexes. The complexation of ligand 3 with metals is also evi-
1
denced by H NMR spectroscopy. Upon coordination, the broad
N–H signal disappeared and the two pyrrole protons, which gave
signals at 7.69 and 7.38 ppm in the proligand, had a pronounced
upfield shift in the complexes. Interestingly, a significant
downfield shift from less than 133 ppm to 150 ppm was also
observed for one 13C signal. Meanwhile, the presence of two
series of signals indicated that both helicate and mesocate were
generated from the same ligand. However, the ratios of the dia-
stereomers differ significantly from those of M223 complexes
obtained under the same conditions,9 since only one of the com-
plexes is now predominantly generated (Table 1). This allowed
us to isolate the predominant isomer from the mixture through
recrystallization. Although the exact product ratios of the para-
magnetic complexes (Fe233 and Mn233) could not be determined
using NMR spectroscopy, recrystallization also enabled separ-
ation of the predominant diastereomer. As discussed in our
earlier report,9 such a reaction is not simply under thermodyn-
amic control as usually found in self-assembly processes.
However, changes in the reaction media (methanol, chloroform
or DMF) or reaction temperatures (at 25 °C, 65 °C or 150 °C)
did not lead to any change in the product ratios as they did in the
reactions of proligand 2-H2.
X-ray analysis of the predominant diastereomers shows they
all have a helical structure where each metal is octahedrally coor-
dinated to three dipyrromethene segments of the three ligand
strands (ESI†). The crystal data show that the metal centers of
the M233 helicates share very similar features with those of the
M213 and M223 helicates in the solid state. The M233 (M = Fe,
Co and Ga) helicates have comparable N–M–N bond angles and
similar M–N bond lengths to the M213 and M223 helicates
(Table 2).7,9 In all cases, these data compare well with the
reported mononuclear complexes.16–18 Meanwhile the twist
angles of the gem-dimethyl-substituted bis(dipyrromethene)
M233 helicates are greater than those of the corresponding
methylene-bridged M213 (M = Fe, Co) or M223 (M = Co, Ga)
helicates. Accordingly, the M–M distances of the M233 com-
plexes are slightly shorter than that of the corresponding methyl-
ene-linked helicates.7,9
Fig. 2 ORTEP structures of helicates Mn233. The thermal ellipsoids
are scaled to the 50% probability level. Note: Phenyl groups and hydro-
gen atoms are omitted for clarity. The complex is a racemic mixture.
Jahn–Teller effect was observed in the Mn233 helicate (Fig. 2).
The mean length of the two elongated Mn–N bonds is 2.231 Å
while the mean length of the other four Mn–N bonds is 2.010 Å.
The axial elongation indicates the Mn3+ is in a high-spin state,
which is also in agreement with the reported ground state of a
mononuclear complex.19 Similar elongations were reported for a
porphyrin Mn(III) complex,20 a tetraazacyclotetradecane Mn(III)
complex21 and
a
bis(N-(2-picolyl)picolinamido) Mn(III)
complex.22 To our knowledge, this is the first X-ray structure of
a Mn(III) tris(dipyrromethene) complex.
In order to gain more insight into the effect of the gem-
dimethyl groups, theoretical calculations were carried out at the
B3LYP/6-31+G(2df,2pd) level of theory using the Gaussian03
package23 (ESI†). After geometry optimization, the calculated
linker C–C–C bond angles of the “S” conformers (asymmetric
conformers) of proligands 2-H2 and 3-H2 are 113.6 and 108.7
degrees, and the average lengths of the linker C–C bonds are
1.51 and 1.52 Å, respectively. These calculated data are in good
agreement with those determined from X-ray diffraction exper-
iments.24 The “C” conformers (conformers with a mirror plane)
of proligands 2-H2 and 3-H2 were also optimized at the same
level of theory. Thermodynamically, the “C” conformer of proli-
gand 2-H2 is less stable than the “S” one by 1.1 kcal mol−1
,
while the energy difference is 1.8 kcal mol−1 for 3-H2, again
with “S” conformer being more stable. This indicates that, at the
same temperature, “S” conformer is more populated for 3-H2
compared to 2-H2. We suggest the different population of the
“S” conformers might be responsible for the distinct helicate/
mesocate ratios. However, whether a kinetic or thermodynamic
effect is responsible for the rate-promoting effect of geminal sub-
stitution is still under debate in the case of cyclization.11,12
Compared to the ideal octahedral coordination of the Fe233,
Co233 and Ga233 helicates, a tetragonal elongation caused by a
4752 | Dalton Trans., 2012, 41, 4751–4753
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2 M. Albrecht, Chem. Rev., 2001, 101, 3457–3498.
Similarly, a full understanding of the gem-dimethyl effect on this
complicated multi-step self-assembly cannot be reached at this
point.
3 M. Albrecht and S. Kotila, Angew. Chem., 1995, 107, 2285–2287;
M. Albrecht and S. Kotila, Angew. Chem., Int. Ed. Engl., 1995, 34,
2134–2137.
Similar to M213 and M223 complexes, the M233 helicates also
have a strong absorption around 500 nm and their molar extinc-
tion coefficients are usually greater than 105 M−1 cm−1 (ESI†).
Compared to proligand 3-H2, both the major band around
500 nm and relatively weak band around 320 nm of the helicates
are significantly more intense.
In summary, unlike the methylene-linked ligand 2, the gem-
dimethyl-substituted ligand 3 led to predominant formation of
helicates with trivalent metals at room temperature. This suggests
that supramolecular self-assembly of such systems is a process
that is very sensitive not only to the length, location and rigidity
of a linker but to the degree of substitution on the linker carbon
as well. Thus all of these criteria need to be considered in the
future design of similar ligands in order to affect the stereoselec-
tivity of the self-assembly.
4 M. Albrecht, Chem.–Eur. J., 2000, 6, 3485–3489.
5 M. Albrecht, I. Janser, H. Houjou and R. Frohlich, Chem.–Eur. J., 2004,
10, 2839–2850.
6 J. Xu, T. N. Parac and K. N. Raymond, Angew. Chem., 1999, 111, 3055–
3058; J. Xu, T. N. Parac and K. N. Raymond, Angew. Chem., Int. Ed.,
1999, 38, 2878–2882.
7 Z. Zhang and D. Dolphin, Chem. Commun., 2009, 6931–6933.
8 S. Goetz and P. E. Kruger, Dalton Trans., 2006, 1277–1284.
9 Z. Zhang and D. Dolphin, Inorg. Chem., 2010, 49, 11550–11555.
10 R. M. Beesley, C. K. Ingold and J. F. Thorpe, J. Chem. Soc., 1915, 107,
1080–1106.
11 M. E. Jung and J. Piizzi, Chem. Rev., 2005, 105, 1735–1766.
12 P. G. Sammes and D. J. Weller, Synthesis, 1995, 1205–1222.
13 T.-Y. Luh and Z. Hu, Dalton Trans., 2010, 39, 9185–9192.
14 M.-Y. Yeh and T.-Y. Luh, Chem.–Asian J., 2008, 3, 1620–1624.
15 B. J. Littler, M. A. Miller, C. H. Hung, R. W. Wagner, D. F. O’Shea, P.
D. Boyle and J. S. Lindsey, J. Org. Chem., 1999, 64, 1391–1396.
16 S. M. Cohen and S. R. Halper, Inorg. Chim. Acta, 2002, 341, 12–16.
17 V. S. Thoi, J. R. Stork, D. Magde and S. M. Cohen, Inorg. Chem., 2006,
45, 10688–10697.
18 C. Bruckner, Y. J. Zhang, S. J. Rettig and D. Dolphin, Inorg. Chim. Acta,
1997, 263, 279–286.
Acknowledgements
19 P. A. Daugherty, J. Glerup, P. A. Goodson, D. J. Hodgson and
K. Michelson, Acta Chim., Scand., 1991, 45, 244–253.
20 J. F. Kirner and W. R. Scheidt, Inorg. Chem., 1975, 14, 2081–2086.
21 S. Mossin, H. O. Sorensen and H. Weihe, Acta Crystallogr., 2002, C58,
204–206.
22 S. Hazra, S. Naskar, D. Mishra, S. I. Gorelsky, H. M. Figgie,
W. S. Sheldrich and S. K. Chattopadhyay, Dalton Trans., 2007, 4143–
4148.
This work was supported by the Natural Sciences and Engineer-
ing Research Council (NSERC) of Canada. The authors thank
Dr. Brian Patrick in the X-ray crystallography lab of the Depart-
ment of Chemistry at UBC. The authors also thank the NMR
and mass spectroscopy labs of the Department of Chemistry at
UBC.
23 M. J. Frisch et al., Gaussian 03, Revision E.01, Gaussian, Inc., Walling-
ford CT, 2004.
24 The average linker C–C–C bond angles in Co223, Ga223, Fe233, Co233,
Mn233 and Ga233 helicates are 113.8°, 114.3°, 108.9°, 109.0°, 108.8°,
109.1°; the average linker bond lengths are 1.50 Å, 1.51 Å, 1.52 Å,
1.52 Å, 1.52 Å, 1.52 Å, respectively.
References
1 C. Piguet, G. Bernardinelli and G. Hopfgartner, Chem. Rev., 1997, 97,
2005–2062.
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