Lewis acid-catalysed formation of two-dimensional phthalocyanine covalent organic frameworks

Article abstract of DOI:10.1038/nchem.695

Covalent organic frameworks (COFs) offer a new strategy for assembling organic semiconductors into robust networks with atomic precision and long-range order. General methods for COF synthesis will allow complex building blocks to be incorporated into these emerging materials. Here we report a new Lewis acid-catalysed protocol to form boronate esters directly from protected catechols and arylboronic acids. This transformation also provides crystalline boronate ester-linked COFs from protected polyfunctional catechols and bis(boronic acids). Using this method, we prepared a new COF that features a square lattice composed of phthalocyanine macrocycles joined by phenylene bis(boronic acid) linkers. The phthalocyanines stack in an eclipsed fashion within the COF to form 2.3 n pores that run parallel to the stacked chromophores. The material's broad absorbance over the solar spectrum, potential for efficient charge transport through the stacked phthalocyanines, good thermal stability and the modular nature of COF synthesis, show strong promise for applications in organic photovoltaic devices.

Full text of DOI:10.1038/nchem.695

ARTICLES
PUBLISHED ONLINE: 20 JUNE 2010 | DOI: 10.1038/NCHEM.695
Lewis acid-catalysed formation of two-dimensional
phthalocyanine covalent organic frameworks
Eric L. Spitler and William R. Dichtel
*
Covalent organic frameworks (COFs) offer a new strategy for assembling organic semiconductors into robust networks
with atomic precision and long-range order. General methods for COF synthesis will allow complex building blocks to be
incorporated into these emerging materials. Here we report a new Lewis acid-catalysed protocol to form boronate esters
directly from protected catechols and arylboronic acids. This transformation also provides crystalline boronate ester-linked
COFs from protected polyfunctional catechols and bis(boronic acids). Using this method, we prepared a new COF that
features a square lattice composed of phthalocyanine macrocycles joined by phenylene bis(boronic acid) linkers. The
phthalocyanines stack in an eclipsed fashion within the COF to form 2.3 nm pores that run parallel to the stacked
chromophores. The material’s broad absorbance over the solar spectrum, potential for efficient charge transport through
the stacked phthalocyanines, good thermal stability and the modular nature of COF synthesis, show strong promise for
applications in organic photovoltaic devices.
he continuing development of organic semiconductors1 will their potential fully. Since they were first reported in 2005, all
bring about flexible displays2, radiofrequency identification boronate ester-linked COFs have been synthesized through the
T
tags3, improved lighting technologies4, efficient sensors5,6 solvothermal condensation of polyfunctional boronic acids and
and economically competitive solar cells7,8. In addition to their catechols. However, only 2,3,6,7,10,11-hexahydroxytriphenylene
low cost, one of the most attractive aspects of organic electronic (HHTP) and four 1,2,4,5-tetrahydroxybenzene derivatives have
materials is the promise of tuning the properties of a device produced crystalline materials, and HHTP is the only building
through rational chemical design and synthesis. Known structure– block used more than once. Reports of new boronate ester-
property relationships and computational tools enable predictable linked COFs ceased after an initial flurry of activity. This lack of
tuning of the bandgaps and frontier molecular orbital energies of progress is attributable to undesirable features of compounds
organic semiconductors. However, control of the packing and that contain multiple catechol moieties. Polyfunctional catechols
long-range order is also critical for efficient charge transport are prone to oxidation and are often sparingly soluble in organic
through the material9. Efforts in crystal engineering produced solvents, factors that hinder both the preparation of useful
examples of cofacially packed pentacene10 and tetrathiafulvalene11 quantities of functionalized monomers and their incorporation
derivatives, but to predict the crystal structures of small organic into COFs.
molecules reliably remains an unsolved challenge12. Varying the
Here we report a new general method for the synthesis of boro-
identity or positions of substituents to tune electronic properties nate ester-linked COFs that avoids the direct use of insoluble and
can induce unpredictable changes in long-range order, which unstable polyfunctional catechol reactants. Using this method we
limits the generality of molecular design strategies.
prepared a 2D network of cofacially stacked phthalocyanines, chro-
COFs comprise an emerging class of materials that can overcome mophores that absorb strongly and have been employed in both
these challenges13–15. COFs incorporate organic subunits into peri- bulk heterojunction22,23 and dye-sensitized solar cells24, as well as
odic two-dimensional (2D) and three-dimensional porous crystalline in many other applications25–27. The phthalocyanine COF forms
structures held together by covalent bonds rather than by non- an eclipsed 2D square lattice as determined by powder X-ray diffrac-
covalent interactions. These linkages provide robust materials with tion (PXRD), surface area analysis and ultraviolet–visible, near
precise and predictable control over composition, topology and por- infrared and fluorescence spectroscopies. This material represents
osity, features first exploited for gas-storage applications16. The rela- a key first step towards forming COF-based bulk heterojunctions
tive geometries of the reactive groups in the starting materials that feature structurally precise and large surface area interfaces
determine the COF’s topology, which does not change significantly between complementary organic semiconductors.
as other functional groups are varied. 2D COFs predictably assemble
functional aromatic systems into cofacially stacked morphologies Results and discussion
ideal for transporting excitons or charge carriers through the The direct formation of boronate esters from protected catechols
material17,18. Although COFs linked by boroxines13,18, imines19 and presents an attractive alternative for COF synthesis because the pro-
borosilicates20 are reported, the most widely studied COFs are tecting groups can decrease the compound’s polarity, prevent auto-
linked by boronate ester moieties14,21. The boronate ester-linked xidation and confer enhanced solubility. Although we are aware of
materials are particularly promising for organic electronics, in part only a single example of a related transformation28, we found
because they incorporate two distinct molecular components that (Fig. 1) that phenylboronic acid 1 reacts cleanly with catechol acet-
allow their composition and porosity to be varied independently.
Despite their great promise, the limited generality of synthetic presence of substoichiometric amounts of the Lewis acid BF3 OEt2.
methods for COFs represents a significant barrier to realizing The reaction takes place over the course of a few hours at
onide 2 to form the corresponding catechol boronate ester 3 in the
.
Department of Chemistry and Chemical Biology, Cornell University, Baker Laboratory, Ithaca, New York 14853-1301, USA. e-mail: wdichtel@cornell.edu
*
672
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.695
ARTICLES
HO
OH
millimolar concentrations of starting materials in several ether and
non-coordinating solvents at mild temperatures (20–85 8C), and
does not require the rigorous exclusion of oxygen or water. We
next confirmed that this transformation can produce crystalline
COFs by synthesizing COF-5 and COF-10, previously reported by
Yaghi and co-workers13,29, through the condensation of HHTP
tris(acetonide) and the appropriate bis(boronic acid). Spectral data
of the materials obtained under Lewis acid-catalysed conditions
matched those reported previously (see Supplementary Fig. S22).
The phthalocyanine tetra(acetonide) 4 (Fig. 2) is a suitable equiv-
alent tetrafunctional catechol for the formation of COFs under the
H*
H*
B
*H
BF3•OEt2
*H
*H
(0.6 equiv.)
O
*H
O
O
B
+
CDCl3,
room temperature
O
1
2
3
Ph
O
B
O
O
H*
B
Ph
*H
O
O
*H
B
Before addition
of BF3•OEt2
H*
*H
.
above BF3 OEt2-catalysed boronate esterification conditions.
*H
O
O
Multigram quantities of 4 were obtained by modifying a synthetic
procedure reported by Breslow and co-workers30. Phthalocyanine
4 is moderately soluble in many organic solvents and stable
under ambient conditions. In contrast, the corresponding
octahydroxyphthalocyanine was reported as a highly insoluble
solid that must be stored under an inert atmosphere to prevent its
oxidation31,32. Pc-PBBA COF was synthesized by combining 4
(Pc) (32 mg), 1,4-phenylenebis(boronic acid) (PBBA) (5, 18.5 mg)
After addition
of BF3•OEt2
(0.6 equiv. 18 h)
B
9.0
8.5
8.0
7.5
7.0
6.5
6.0
δ (ppm)
.
Figure 1 | BF3 OEt2 catalyses the formation of 2-phenyl-1,3,2-benzodi-
.
oxaborole (3) from phenylboronic acid (1) and catechol acetonide (2). The
partial 1H NMR spectra (300 MHz, 298 K, CDCl3) of the reaction mixture
and BF3 OEt2 (0.015 ml) in a 1:1 mixture of mesitylene and
1,2-dichloroethane (DCE) in a flame-sealed glass reaction vessel.
The sealed tube was placed in a 120 8C oven for six days. The result-
ing green precipitate was collected by filtration and washed with
anhydrous MeCN to isolate Pc-PBBA COF in 48% yield. The
isolated COF displayed excellent thermal stability to 500 8C, as
determined by thermogravimetric analysis.
.
before and 18 hours after the addition of BF3 OEt2 showed a clean conversion
of 1 (which rapidly dehydrates to the corresponding boroxine when dissolved
in CDCl3) and 2 to the corresponding boronate ester 3. No resonances that
correspond to free catechol were observed in spectra taken at
intermediate conversion.
a
O
O
N
N
N
O
O
O
O
HN
+
(HO)2B
B(OH)2
NH
N
N
N
5
BF3•OEt2
Mesitylene, DCE
120 ºC, 6 days
4
b
O
O
B
B
O
O
O
O
N
N
N
N
N
N
O
O
O
O
O
O
HN
NH
B
HN
NH
B
B
B
O
O
N
N
N
N
N
N
O
O
O
O
O
O
O
O
B
B
B
B
N
N
N
N
N
N
O
O
O
B
O
O
B
O
O
O
B
NH
HN
NH
HN
B
N
N
N
N
N
N
O
O
O
O
B
B
Pc-PBBA COF
Figure 2 | Lewis acid-catalysed boronate ester formation conditions provide an eclipsed 2D COF that consists of phthalocyanines linked by PBBA. a, The
Lewis acid-catalysed deprotection–condensation protocol was used to form the Pc-PBBA COF from phthalocyanine tetra(acetonide) 4 and PBBA 5.
b, Pc-PBBA COF exhibits a square lattice with 2D sheets that form eclipsed stacks.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.695
ARTICLES
22.85 Å
e
a
100
001
220
210
200
f
300
b
c
g
d
0
5
10
15
20
25
30
35
2θ (degrees)
Figure 3 | X-ray diffraction confirms that the Pc-PBBA COF is a crystalline material with an eclipsed 2D structure. a, The experimental PXRD pattern
(black; the major observed reflections are labelled) and Pawley refined (red) pattern of Pc-PBBA COF. b, A difference plot between the experimental and
refined diffraction patterns shows excellent agreement. c, A simulated PXRD pattern for a Pc-PBBA COF square lattice shows good agreement with the
experimental and refined patterns. d, A simulated PXRD for a theoretical 2D staggered structure (g) does not agree with the experimental and refined
patterns. e,f, The crystal parameters extracted from the Pawley-refined PXRD are displayed on a model of the Pc-PBBA lattice. g, Theoretical 2D
staggered structure.
The PXRD pattern of Pc-PBBA COF (Fig. 3a, black) indicated calculated lattice parameters (a ¼ b ¼ 22.96 Å, c ¼ 3.34 Å) from
that it is a crystalline material consistent with the long-range this model were also quite close to the measured and refined par-
structure depicted in Fig. 2. The most intense peak at 2u ¼ 3.848 ameters (a ¼ b ¼ 22.85 Å, c ¼ 3.34 Å).
corresponds to the (100) and (010) diffractions of the square
We also considered an alternative staggered 2D arrangement
lattice. The minor diffraction peaks at 7.688, 8.528, 11.568 and (Fig. 3g) in which the phthalocyanine units of adjacent sheets are
26.648 correspond to the (200), (210), (300) and (001) diffractions, horizontally offset by a distance of a/2 and b/2. The simulated
respectively. None of the observed peaks correspond to the phthalo- PXRD pattern for this arrangement (Fig. 3d) does not match the
cyanine or the acid starting materials (see Supplementary Fig. S9). experimental data. We attribute the formation of the eclipsed struc-
Pawley refinement of the observed PXRD pattern profile using ture to the strong tendency for phthalocyanine units to form cofacial
the Reflex Plus module of the Materials Studio version 4.4 aggregates that are reinforced by stabilizing B–O interactions
suite of programs33 (Fig. 3a, red) produced the unit-cell parameters between adjacent layers.
a ¼ b ¼ 22.85 Å and c ¼ 3.34 Å (Fig. 3e,f) and confirmed the
Two distinct crystal morphologies were observed by scanning
assignment of the observed reflections. The refined profile electron microscopy imaging of Pc-PBBA COF, one with a striated
matches the observed powder diffraction pattern very well, with rectangular prism shape that averaged approximately 1 mm in
profile-fitting factors that converge to a weighted R-factor (wRp) length and 200–300 nm in thickness and one of flattened irregular
of 9.72% and Rp of 6.46%. The difference plot (Fig. 3b) also indicates plates 2–4 mm long (Fig. 4). The presence of these two phases
a good fit in which the only deviations appear in the very low angle may account for line broadening in the PXRD pattern, and could
region, where background interference is greatest.
be the result of different nucleation or growth environments in
Simulation of the PXRD pattern of the eclipsed 2D Pc-PBBA the reaction vessel.
lattice (Fig. 3c) showed good agreement with the experimental
The Fourier transform infrared (FTIR) spectrum of Pc-PBBA
data. We constructed a unit-cell precursor that consisted of a phtha- COF obtained in attenuated total reflectance (ATR) mode indicated
locyanine macrocycle functionalized with phenylboronate esters at the formation of boronate ester rings through the strong B–O stretch
each of the four termini (Fig. 3e) and optimized its geometry at 1,328 cm21 (see Supplementary Fig. S3). An analogous band at
(hydrogen atoms were omitted). A tetragonal crystal of D4h 1,344 cm21 appeared in the spectrum of 5, although the two
(P4/mmm) symmetry was then generated with initial lattice par- spectra were otherwise quite different. The hydroxyl bands of the
ameters a and b that corresponded to the distance between the cen- COF were attenuated greatly relative to the starting materials, con-
troids of phenylene units on opposite sides of the cell (about 23 Å). sistent with free hydroxyl groups at the edges of the crystallites or
Initially, the interlayer spacing c was set at 3.33 Å, the p–p stacking at defects. The phthalocyanine acetonide 4 shared many infrared
distance in boron nitride34. The crystal geometry was reoptimized, absorbances with the Pc-PBBA COF material, although the
and the resulting simulated powder diffraction pattern matched methyl C–H stretches from the acetonide-protecting groups were
the experimental peak positions and intensities quite well. The notably absent from the COF spectrum. Also, we obtained an
674
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.695
ARTICLES
a
b
1µm
300nm
Figure 4 | Scanning electron micrographs of the Pc-PBBA COF display two crystal morphologies. a,b, These include (a) rectangular prisms about 1
length and (b) flat sheets of 2–4 m long.
mm in
m
FTIR spectrum of a crude sample of octahydroxyphthalocyanine phases of cofacially aligned phthalocyanine discotic mesogens43. The
.
(produced by treating 4 with BF3 OEt2 in the absence of 5). Its COF spectrum is red-shifted from that of 4 by a small amount, which
FTIR spectrum is distinct from that of the COF (see probably arises from differences in aggregation geometry as well as
Supplementary Fig. S4). These observations provide strong evidence the electron-withdrawing nature of the boronate esters relative to
for the formation of the boronate ester-linked material.
the acetonide functionalities. The changes in absorbance properties
The porosity and surface area of the material was measured by exhibited by Pc-PBBA COF, particularly the broadening of the
nitrogen-gas adsorption. An adsorption isotherm between 0 and 1 absorbance to enhance absorption in the 450–600 nm region and
atmosphere was generated after evacuation under continuous the tailing into the near infrared, make these materials promising
vacuum for 12 hours at 180 8C (Fig. 5). We observed a sharp initial candidates for the collection of solar energy.
uptake at low pressures as the small pores filled, followed by a more
Photoluminescence measurements of Pc-PBBA COF also
gradual uptake over the remainder of the pressure range. suggested an H-aggregated structure. CH2Cl2 solutions of 4 fluoresce
Desorption occurred with a small hysteresis at about P/P0 ¼ 0.5 strongly with a small Stokes shift (17 nm, lem ¼ 708 nm), typical of
(P0 ¼ 1 atm). The Langmuir surface-area model was applied to the monomeric phthalocyanines of similar structure. The solid samples
P/P0 ¼ 0.02–0.06 (16–43 mm Hg) region of the isotherm, where of Pc-PBBA COF were non-emissive, as observed for other phthalo-
the initial adsorption was most linear. The calculated Langmuir cyanine H-aggregates (see Supplementary Fig. S21). Also, these
surface areafrom these datawas 506 m2 g21, which is slightly low rela- results can be understood by exciton theory44. We expected 4 to
tive to other reported COFs, but still well within the values of other form non-emissive aggregates in the solid state, but found that
micro- and mesoporous materials, such as zeolites and several both powders and drop-cast films of 4 fluoresced very weakly at
metal–organic frameworks35–40. The increased density of a square 820 nm. We hypothesize that the four acetonide groups distort the
lattice combined with the relatively small phenylene linker unit cofacial aggregates of 4, which results in weakly allowed emission
results in a material of lower surface area. This trend has been from a dimer or aggregate of higher order. This process was not
observed in series of COFs with gradually increasing pore size26,27
.
observed in the more structurally precise COF. The changes in
The Barrett–Joyner–Halenda (BJH) adsorption–desorption both absorption and emission spectra of the phthalocyanine moieties
model was used to study the pore size and volume distributions observed in the COF materials strongly suggest cofacial stacking.
(see Supplementary Fig. S15)41,42. Pore-distribution plots revealed
a maximum pore area of 469 m2 g21 at a width of 2.12 nm and a
12
maximum pore volume of 0.258 cm3 g21 at a width of 2.17 nm.
Peaks at larger pore sizes likely result from uptake in structural
defects or slipped sheets along the micropore walls. The BJH
model is most appropriate for mesoporous materials with pore
sizes between 2 and 300 nm. Pc-PBBA COF has a predicted pore
width of approximately 2 nm, which is at the lower limit. Even
with this limitation, the pore data match predictions from the
Materials Studio program reasonably well.
400
Langmuir surface area =
506 13 m2 g–1
11
10
9
300
200
100
0
8
y = 0.193x +2.82
R2 =0.999
7
6
5
20
25
35
45
15
30
40
P (mmHg)
Adsorption
Phthalocyanines strongly absorb visible light and are thus deep
blue or green compounds, depending on the identity (or absence)
of a metal ion coordinated to the four central nitrogen atoms. The
electronic absorption spectra of dilute CH2Cl2 solutions of 3
(ꢀ1026 M) are typical of non-aggregated, free-base phthalocyanines.
The sharp peaks at 653 and 691 nm located within the broad absorp-
Desorption
0
0.2
0.4
0.6
0.8 1
P/P0
tion band from 500 to 725 nm (Q band) are hallmarks of monomeric Figure 5 | Nitrogen-adsorption isotherms suggest that the Pc-PBBA COF is
phthalocyanine macrocycles. Diffuse reflectance spectra (Fig. 6) microporous. The isotherm indicates reversible nitrogen uptake with a small
obtained from powders of both Pc-PBBA COF and 4 showed a hysteresis at about P/P0 ¼ 0.5 (P0 ¼ 1 atm). The linear portion of the plot
blue shift of these maxima (11 nm for the COF and 66 nm for 4, between 0.02 and 0.06 was used to calculate a Langmuir surface area of
respectively) consistent with the formation of cofacially stacked 506 m2 g21 (inset; Q ¼ gas adsorbed per unit mass of analyte). The small
H-aggregates, as well as broadening of the Q band into the near infra- phenylene spacer unit and large phthalocyanine core result in a unit cell that
red. Similar blue shifts and spectral band broadening were observed is only about 75% free space; even so, reasonably high surface areas were
for solutions of aggregated phthalocyanines and in liquid crystalline observed for this COF.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.695
ARTICLES
centrifugation. The trituration and centrifugation steps were repeated to provide the
phthalocyanine tetra(acetonide) 4 (620 mg, 52%) as a dark indigo-blue solid.
Matrix-assisted laser desorption–ionization mass spectrometry (MALDI-MS):
802.20 (Mþ). Infrared (powder, ATR): 2,990, 2,921, 2,850, 1,764, 1,716, 1,682, 1,603,
1,474, 1,447, 1,409, 1,386, 1,376, 1,250, 1,214, 1,073, 1,026, 1,002, 979, 852, 812, 785,
737, 714 cm21. Ultraviolet–visible, 2.08 mM in CH2Cl2, l (nm) (log 1 (M21 cm21)):
691 (5.09), 653 (5.02), 638 (4.68, shoulder), 592 (4.34), 425 (4.53), 347 (4.89), 294
(4.76). Ultraviolet–visible (powder, praying mantis diffuse reflection accessory
(DRA)): 609, 396, 331, 289 nm. Analytically calculated for C44H34N8O8: C, 65.83; H,
4.27; N, 13.96. Found: C, 64.66; H, 3.91; N, 13.10. The MALDI-MS and ultraviolet–
a
0.8
0.6
0.4
0.2
0
4
Pc-PBBA COF
visible absorption spectra of 4 match those reported previously47
.
Pc-PBBA COF. Phthalocyanine acetonide 4 (32 mg, 0.040 mmol) and PBBA
(5, 18.5 mg, 0.112 mmol) were loaded into a 1 dram screw-cap vial and suspended in
a mixture of mesitylene and DCE (1:1, 1.5 ml). The dark-blue mixture was sonicated
for 15 minutes. Boron trifluoride etherate (15 ml, 0.12 mmol) was added, and the
mixture was sonicated for another 15 minutes. The dark, heterogeneous mixture was
transferred to a pre-scored KIMAX-51 borosilicate glass ampoule (5 ml, body length
37 mm, outer diameter 17 mm, neck length 51 mm) and flash frozen in a liquid
nitrogen bath. The ampoule neck was flame-sealed in air using a propane torch,
which reduced the total length by 20–30 mm. After warming to room temperature
the suspension was placed in a gravity convection oven at 120 8C for six days. The
reaction was cooled to room temperature, the ampoule was broken at the scored neck
and the mixture was poured onto qualitative filter paper (medium porosity) on a
Hirsch filter funnel and filtered under vacuum. The resulting dark solid was washed
with anhydrous CH3CN (4 ml) and dried in air. The solid was loaded into a 1 dram
screw-cap vial and suspended in anhydrous CH3CN (3 ml) overnight, and again
recovered by filtration to yield Pc-PBBA COF as a dark-green solid (16 mg, 48%).
Infrared (powder, ATR): 3,272, 1,605, 1,523, 1,469, 1,439, 1,373, 1,328, 1,272, 1,187,
1,079, 1,018, 866, 849, 810, 739, 709, 653 cm21. PXRD (2u (relative intensity)): 3.848
(100), 7.768 (17), 8.568 (8), 11.688 (8), 26.528 (19), 26.648 (19). Ultraviolet–visible
(powder, praying mantis DRA): 671, 325, 293 nm. Analytically calculated for
(C11H5BN2O2)n: C, 63.52; H, 2.42; N, 13.47. Found: C, 53.54; H, 2.41; N, 11.02.
Elemental analysis of boronate COFs typically give lower carbon values to those from
the formation of non-combustible boron carbide by-products13. We also anticipate a
similar formation of boron nitride by-products, which lower the nitrogen value. The
presence of boron was confirmed by a characteristic B 1s peak in the X-ray
photoelectron spectrum with a binding energy of 192.839 eV and an abundance of
4.66% (calculated: 5.33% excluding hydrogens).
250
450
650
850
1,050
1,250
λ (nm)
b
1.4×105
1.2×105
1.0×105
8.0×104
6.0×104
4.0×104
2.0×104
0
250
350
450
550
λ (nm)
650
750
Figure 6 | Pc-PBBA COF absorbs light over a broad range of the visible and
near-infrared spectral regions. a,b Blue shifts of the absorption maxima of
phthalocyanine acetonide powder 4 (a, blue) and the Pc-PBBA COF (a, red)
relative to CH2Cl2 solutions of 4 (b) are consistent with stacked
phthalocyanines. Spectral broadening of the phthalocyanine Q bands results
in a material that absorbs light over most of the solar spectrum.
Received 14 January 2010; accepted 6 May 2010;
published online 20 June 2010
Phthalocyanine J-aggregates (offset stacks) show red-shifted absorp-
tion spectra and are emissive45. Furthermore, disordered phthalocya-
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Phthalocyanine tetra(acetonide) 4. Phthalonitrile30 7 (1.20 g, 5.99 mmol; see
Supplementary Information) was dissolved in 1-pentanol (20 ml) and lithium metal
granules (420 mg, 60 mmol) were added at room temperature with vigorous stirring.
The mixture was heated to reflux (140 8C) for five hours under a nitrogen
atmosphere. During this time, the reaction mixture turned very dark green. Then the
mixture was cooled to room temperature, and glacial acetic acid (20 ml) added as the
mixture was stirred. After 30 minutes the solution was concentrated under vacuum.
The resulting green residue was dissolved in a mixture of CHCl3 and MeOH (15:1,
100 ml) and washed with brine (3 × 100 ml) and H2O (1 × 100 ml). The dark-
green organic layer was dried with MgSO4 and concentrated to a volume of 50 ml.
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The authors declare no competing financial interests. Supplementary information and
chemical compound information accompany this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://npg.nature.
com/reprintsandpermissions/. Correspondence and requests for materials should be
addressed to W.R.D.
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