Resorcinol-templated synthesis of a cofacial terpyridine in crystalline π-stacked columns

Article abstract of DOI:10.1021/ol200532t

Chemical equations presented. A resorcinol achieves a templated stereospecific and near-quantitative synthesis of a cofacial terpyridine in the solid state. The solid-state synthesis occurs in one-dimensional π-stacked columns with reactivities highly sen

Full text of DOI:10.1021/ol200532t

ORGANIC
LETTERS
2011
Vol. 13, No. 9
2260–2262
Resorcinol-Templated Synthesis of a
Cofacial Terpyridine in Crystalline
π-Stacked Columns
ꢀ
Saikat Dutta, Dejan-Kresimir Bucar, and Leonard R. MacGillivray*
ꢀ
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242-1294,
United States
len-macgillivray@uiowa.edu
Received February 27, 2011
ABSTRACT
A resorcinol achieves a templated stereospecific and near-quantitative synthesis of a cofacial terpyridine in the solid state. The solid-state
synthesis occurs in one-dimensional π-stacked columns with reactivities highly sensitive to peripheral groups attached to the template.
Small-molecule templates based on resorcinol (res)1 can
assemble olefins into discrete hydrogen-bonded assemblies
Scheme 1. Supramolecular Construction of hh-TPC
in the solid state that become covalently fixed via [2 þ 2]
photodimerizations2 to give architecturally rich molecules
difficult to achieve in solution (e.g., [n]ladderanes) (Scheme 1).
Here, we report the use of a res to construct the cofacial
terpyridine (TPY) hh-TPC stereospecifically and in near
quantitative yield in a solid. Cofacial molecules such as hh-
TPC have been studied for over three decades as platforms
to bind metal ions and recognize molecules with applica-
tions in catalysis, biomimicry, and self-assembly (e.g.,
porphyrins).3 Despite significant advances, the solution-
phase synthesis of cofacial molecules is often tedious,
requiring multiple steps that proceed in low yields. Nguyen
and Mirkin have shown how the synthetic problem can be
addressed supramolecularly using a weak-link approach to
coordination chemistry.4 Since a cyclobutane ring from a
[2 þ 2] cycloaddition stacks two aromatics in close proxi-
mity, we endeavored to use a res to stack TPE noncova-
lently in a cocrystal1 and then covalently fix the olefins to
generate hh-TPC. Although TPY has widespread appeal
ꢀꢀ ꢁ
(1) MacGillivray, L. R.; Papaefstathiou, G. S.; Friscic, T.; Hamilton,
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(2) (a) Bhogala, B. R.; Captain, B.; Parthasarathy, A.; Ramamurthy,
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1025–1074.
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Rose, E.; Schmittou, E. R.; Ibers, J. A. J. Am. Chem. Soc. 1981, 103, 516–
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r
10.1021/ol200532t
Published on Web 04/13/2011
2011 American Chemical Society

as a building block in supramolecular chemistry,5 only two
cocrystals of a TPY had, to our surprise,6 been reported
while a photodimerization of a TPY had not been de-
scribed in the solid state or solution. We show the assembly
process of TPE and a series of res affords unexpected
reactive hydrogen-bonded columns7 that afford hh-TPC
stereospecifically and in near-quantitative yield when a
specific combination of template shape, hydrogen-bond-
ing, and π-stacking of TPE is satisfied within the columnar
structures. The ability of hh-TPC to form cofacial com-
plexes with transition metals is also reported.
An X-ray structure analysis of (5-I-res) (TPE) revealed
the components to assemble, as expected, via OꢀH
3
N
(O N: 2.76 A, 2.77 A, 2.94 A, 2.96 A) hydrogen bonds
3 3 3
˚
˚
˚
˚
3 3 3
(Figure 2). The assembly process, however, generated 1D
columns6 with adjacent olefins linked by offset 5-I-res
molecules. Each TPE stacked head-to-head and adopted
the s-trans,s-trans conformation, with the 2-pyridyl groups
forming hydrogen bonds to the res. The CdC bonds were
˚
separated at 3.79 and 3.82 A, which positioned TPE to
react to give hh-TPC in the solid.
To determine the feasibility to construct hh-TPC in a
solid, we first studied the photoreactivity of pure TPE.8
Single crystalswereobtainedfrom hot hexaneafter cooling
toroomtemperaturein1 day. TheX-raystructurerevealed
the olefin to pack in a geometry expected to render TPE
photostable (Figure 1).9 The olefin adopted an s-trans,s-
trans conformation5 with nearest neighbor CdC bonds
˚
separated by 6.07 A (centroid-to-centroid), well beyond
the limit for photodimerization.8 Upon exposure to med-
ium-pressure Hg broadband UV radiation, TPE was
photostable.
Figure 2. X-ray structure of (5-I-res) (TPE): (a) schematic, (b)
3
hydrogen bonding, and (c) overhead and side-on.
That TPE in (5-I-res) (TPE) reacted togivehh-TPC was
3
confirmed by X-ray diffraction. Single crystals of 2(5-I-
res) (hh-TPC) formed by allowing a CH3NO2 solution of
3
the reacted solid to evaporate over 1 day. An X-ray
analysis confirmed hh-TPC with TP groups stacked at
˚
5.21 A and twisted by 1.8° (Figure 3a), a geometry
Figure 1. Structure of TPE: (a) s-trans,s-trans conformation and
(b) packing.
comparable to cofacial TP frameworks.10 Remarkably, the
components form a discrete six-component assembly sus-
tained by eight OꢀH N hydrogen bonds (O N 2.73,
3 3 3
3 3 3
˚
2.77, 2.77, 2.85 A), with two 5-I-res that link two molecules of
hh-TPC (Figure 3b). The generation of hh-TPC represents
the first synthesis of a cofacial molecule in a solid.
Whereas pure TPE is photostable, TPE reacted in near-
quantitative yield to give a cyclobutane product when 5-I-
res was a template. Co-crystallization of TPE with 5-I-res
from CH3NO2 afforded crystals of (5-I-res) (TPE) upon
evaporation. When powdered (5-I-res) (TPE) was UV-
3
3
irradiated for 3 days, TPE formed a cyclobutane in 98%
yield, as evidenced by the disappearance of the olefinic
protons (δ = 7.71, 7.55 ppm) and appearance of two
cyclobutane protons (δ = 4.75, 5.00 ppm) (Supporting
Information). The emergence of two peaks suggested that
TPE reacted to give a head-to-head photoproduct.1
(5) (a) Constable, E. C. Chem. Soc. Rev. 2007, 36, 246–253. (b)
Flamigni, L.; Collin, J.-P.; Sauvage, J.-P. Acc. Chem. Res. 2008, 41,
857–871.
(6) (a) Messina, M. T.; Metrangolo, P.; Resnati, G.; Quici, S.; Pilati,
T. Supramol. Chem. 2001, 12, 405–410. (b) Liantonio, R.; Logothetis,
T. A.; Messina, M. T.; Metrangolo, P.; De Santis, A.; Pilati, T.; Resnati,
G. Collect. Czech. Chem. Commun. 2002, 67, 1373–1382.
Figure 3. X-ray structure of 2(5-I-res) (hh-TPC): (a) hh-TPC
3
and (b) wireframe of six-component assembly (space-fillings
insets).
ꢀꢀ ꢁ
(7) Friscic, T.; MacGillivray, L. R. Mol. Cryst. Liq. Cryst. 2006, 456,
155–162.
(8) Novel synthesis based on: Kadjane, P.; Charbonniere, L.; Camer-
ꢂ
(10) (a) Crowley, J. D.; Bosnich, B. Eur. J. Inorg. Chem. 2005, 2015–
2025. (b) Betley, T. A.; Wu, Q.; Van Voorhis, T.; Nocera, D. G. Inorg.
Chem. 2008, 47, 1849–1861.
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Org. Lett., Vol. 13, No. 9, 2011
2261

To gain insight into the assembly and reactivity of TPE,
the olefin was cocrystallized with the parent res and 4,6-di-I-
res. X-ray analyses revealed columns, demonstrating ro-
bustness of the π-stacked structure. For res, a H2O molecule
assembled in 2(res) 2(TPE) (H2O), participating in hydro-
res can covalently fix olefins in infinite, as well as discrete,
self-assembled structures.1
Preliminary studies demonstrate that the TPY units that
span the cyclobutane ring of hh-TPC support cofacial
complexes with transition-metal ions (Figure 5). When
hh-TPC was reacted with either ZnCl2 or Cu(NO3)2 from
CH3OH solutions of DMF and H2O, colorless and green
crystals of [Zn2(hh-TPC)2Cl4] and [Cu4(hh-TPC)2(NO3)8]
formed after slow evaporation in 5 days, respectively. In
[Zn2(hh-TPC)2Cl4], each TPY group chelates, in the s-cis,
s-cis conformation,2,3 a ZnCl2 moiety with the Zn(II) ions
in distorted trigonal bipyramidal geometries and separated
3
3
˚
˚
gen bonds that bridge adjacent res (O O: 2.67 A, 2.80 A)
3 3 3
˚
and TPE molecules (O N: 2.83 A; O O: 2.67) (Figure
4a). The CdC bonds were parallel and separated by 3.84
3 3 3
3 3 3
˚
and 4.46 A. UV-irradiation produced hh-TPC in 75% yield.
For 4,6-diI-res, the res interacted, similar to (5-I-res) (TPE),
directly with each TPE via the OꢀH groups in (4,6-di-I-
3
˚
res) (TPE) (O N: 2.83 A; 2.67) (Figure 4b). In contrast
3
3 3 3
to (5-I-res) (TPE), however, the CdC bonds adopted a
3
˚
by 7.65 A (Figure 5a). In [Cu4(hh-TPC)2(NO3)8], each
˚
slipped and antiparallel geometry, being separated by 4.63 A.
TPE was photostable in (4,6-di-I-res) (TPE).
cofacial complex forms a U-shaped tetranuclear assembly
with metal ions, which sit in an octahedral geometry,
3
˚
˚
Cu2) and 6.69 A
separated by 6.76
A
(Cu1
(Cu1 Cu1) (Figure 5b). Studies are underway to
3 3 3 3
3 3 3 3
further elucidate the coordination behavior of hh-TPC.
Figure 5. X-ray structures: (a) [Zn2(hh-TPC)2Cl4] and (b)
[Cu4(hh-TPC)2(NO3)8] (color code: Cl, green; Zn, brown; Cu,
orange; O, red).
Figure 4. X-ray structures of stacked columns and olefin geo-
metries: (a) 2(res) 2(TPE) (H2O) and (b) (4,6-diI-res) (TPE).
3
3
3
The photostability of TPE in (4,6-di-I-res) (TPE) can be
3
In this paper, the first synthesis of a cofacial molecule
has been achieved in the solid state. hh-TPC has been
generated 1D columns stereospecifically and in near-quan-
titative yield. The reactivity depends on secondary inter-
actions of the template while the cofacial TP supports
metal complexes. Our results add to a limited, yet growing,
list of unusual molecules (i.e., ladderanes) constructed by
fixing olefins in solids using templates.1 We continue to
identify targets of increasing complexity.
attributed to van der Waals interactions of the ortho
I-atoms along the exterior of each column. The interac-
tions impart a larger separation distance, as revealed by
˚
separations of adjacent res molecules [O O (A) 5-I-res
3 3 3
3.2 (98% yield), res 4.6 (75% yield), 4,6-diI-res 4.9 (0%
yield)] and, thus, disrupt stacking of TPE in each column
(Figure 4b). For 2(res) 2(TPE) (H2O), such “secondary”
interactions are absent (Figure 4a), with the “smaller” res
allowing the H2O molecule to fill space in the column.
Indeed, the ability of hh-TPC to form in near-quantitative
3
3
Acknowledgment. We thank the National Science
Foundation (L.R.M., DMR-0133138) for support. We
thank Elizabeth Elacqua for collection of spectral data.
yield in (5-I-res) (TPE) appears to be akin to hitting a
3
“supramolecular sweet spot”11 where template shape,
hydrogen bonds, and π-stacking of TPE are satisfied to
allow the C-atoms to move12 and fully react in the solid.
These observations are important since they suggest that a
Supporting Information Available. Details of synth-
1
eses, H NMR spectra, and X-ray structure determina-
tions. This material is available free of charge via the
Internet at http://pubs.acs.org
(11) Gibb, B. C. Angew. Chem., Int. Ed. 2003, 42, 1686–1687.
(12) Garcia-Garibay, M. A. Angew. Chem., Int. Ed. 2007, 46, 8945–8947.
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