Dendritic encapsulation-roles of cores and branches

Article abstract of DOI:10.1016/S0040-4020(03)00435-6

Examples of new dendrimers are presented. In the first, the effect on electron transfer rate attenuation is investigated for two dendrimer isomers that differ only in the linkage (ortho- versus meta-linked) of the phenyl ether units within one generation

Full text of DOI:10.1016/S0040-4020(03)00435-6

Tetrahedron 59 (2003) 3853–3861
Dendritic encapsulation-roles of cores and branches
Tyson L. Chasse, Joshua C. Yohannan, Namjin Kim, Qun Li, Zemin Li
*
and Christopher B. Gorman
Department of Chemistry, North Carolina State University, Box 8204 Raleigh, NC 27695-8204, USA
Received 8 October 2002; revised 20 December 2002; accepted 22 January 2003
Abstract—Examples of new dendrimers are presented. In the first, the effect on electron transfer rate attenuation is investigated for two
dendrimer isomers that differ only in the linkage (ortho- versus meta-linked) of the phenyl ether units within one generation of the structure.
Second, the effect of encapsulation on electrochemical and luminescence behavior of a new type of rhenium selenide cluster core dendrimer
is illustrated. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
conformation of the dendrimer, its concomitant ability to
shield sterically the core moiety and how this shielding is
manifest in the physical properties of the molecule. To this
end, we have embarked on several investigations of
dendrimer structure–property relationships relevant to this
question. Results of several approaches are reported here.
The phenomenon of encapsulation manifests itself across
the realms of materials science and life science. The need to
separate, to contain and to release under controlled
conditions is found in structures both natural and artificial.
These include cells, micelles/liposomes, host–guest com-
plexes, and core-shell structures. The effect of the
encapsulation can be shielding (e.g. steric protection from
the outer environment) and/or engulfing (e.g. partitioning
into an inner environment). At the molecular level, the mode
of encapsulation can be either covalent or non-covalent.
2. Results and discussion
Our general strategy is to prepare cluster core dendrimers in
which (1) the effect of dendrimer encapsulation on the
electrochemical, magnetic and/or photophysical properties
can be observed and (2) the structure of the dendron arms is
modified so as to maximize the encapsulating effect. One
might imagine that the ideal scenario for this is to find the
cluster at the geometric center of the dendrimer with the
arms somehow uniformly disposed around the cluster. This
general architecture is shown in Figure 1. Thus, it is of
interest to vary both the structure of the arms and of the
Dendrimers have been explored extensively as molecular
scale encapsulants. This area has been discussed critically in
the literature by both us1 and by Frechet et al. Small
2
´
molecule encapsulation may have utility in catalysis,3
efficient binding in macromolecular host structures,4–10
and molecular scale imprinting.11 In covalent encapsulation,
the dendrimer serves to sterically shield and/or protect a
moiety typically found at its core. This type of dendritic
shielding has been used to mimic metalloproteins12–17 and
in electron transfer rate attenuation which is potentially
important in molecular electronics.18,19
Recently, we have reported several types of metal cluster
core dendrimers.1,12,18,20–23 The properties of these clusters
have been exploited to probe dendrimer conformation and to
understand the effects of encapsulation on electronic
properties of the molecules. An open question is how
differing structures of dendrimers and differing environ-
ments into which they have been placed influence the
Keywords: dendrimer; encapsulation; isomers; conformation; rhenium;
selenide; iron sulfer.
*
Corresponding author. Tel.: þ1-919-515-4252; fax: þ1-919-515-8920;
Figure 1. Generic encapsulation scheme highlighting the roles of the core
(to be encapsulated) and the dendrons (to affect the encapsulation).
e-mail: chris_gorman@ncsu.edu
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00435-6

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T. L. Chasse et al. / Tetrahedron 59 (2003) 3853–3861
Scheme 1.
encapsulated cluster. Approaches for both types of variation
are described here.
We recently showed this in a simple way by comparing
redox active, iron–sulfur cluster core dendrimers that had
alternatively stiff and flexible repeat units.1,23 An aestheti-
cally satisfying way to probe the influence of structure on
encapsulation via conformation is through the study of
constitutional isomers. The synthesis24 and study of
dendrimer isomers has had some precedent including the
2.1. Dendrimer isomers
The primary structure of the dendrimer influences confor-
mation, which, in turn, influences encapsulation behaviors.
Scheme 2.

T. L. Chasse et al. / Tetrahedron 59 (2003) 3853–3861
3855
Scheme 3.
study of linear versus hyperbranched structures,25,26 supra-
molecular organization of different dendrimer isomers,27
stereoisomers in dendrimers (e.g. cis versus trans azo-
benzene linkages in dendrimers)28 and isomeric metallo-
dendrimers.29 Ideal dendrimer isomers differ only in their
primary structure. Changes in the primary structure of the
dendrimer can result in a change in its conformation (e.g.
the disposition of the arms around the core and the relative
degree to which the core is buried in the dendrimer). This
change in conformation could be reflected in a change in the
measured degree of encapsulation of the core. For example,
in the case of an electroactive core (such as is discussed
here), a change in the kinetics and/or thermodynamics of
electron transfer to/from that core is expected.
The question emerges as to what primary structural
elements in dendrimers might be most efficacious at
influencing the conformation of these molecules. We and
others30– 33 have hypothesized that primary structural
elements that enforce backfolding should increase the
Table 1. Electrochemical data obtained for dendrimer isomers
Structure
CAa
CVb
OSWVc
e
f
Do (£106 cm2/s)d
E1/2
ko (£103 cm/s)
a
˚
RH (A)
13 (extended)
14 (backfolded)
2.78(0.05)
4.26(0.17)
11.86(0.34)
7.57(0.32)
21494(4)
21618(2)
4.79(0.92)
0.64(0.15)
0.48(0.03)
0.50(0.005)
Values in parentheses represent the magnitude of the 90% confidence intervals of these values.
a
Data determined from chronoamperometry.
Data determined from cyclic voltammetry.
Data determined from Osteryoung square wave voltammetry.
Diffusion coefficient.
Stokes–Einstein radius calculated from the diffusion coefficient.
Thermodynamic redox potential.
b
c
d
e
f

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T. L. Chasse et al. / Tetrahedron 59 (2003) 3853–3861
Scheme 4.
degree of steric congestion around the core moiety of the
dendrimer. This behavior should lead to more effective
encapsulation. To probe this hypothesis experimentally, two
redox-active cluster core dendrimer isomers were prepared
as illustrated in Schemes 1–3. Using a combination of
cyclic voltammetry, chronoamperometry and Osteryoung
square wave voltammetry as described previously,23
diffusion coefficients and heterogeneous electron transfer
rate constants for these molecules were determined and are
reported in Table 1.
equation, the backfolded isomer is also found to be smaller
than the more extended-isomer. This observation is
consistent with the hypothesis that backfolding results in
more steric congestion. Interestingly, it is the smaller
molecule that has the lower rate of electron transfer. This
perhaps counterintuitive observation can be rationalized by
suggesting that it is the relative degree to which the redox
active iron–sulfur cluster is buried within the molecule,
rather than the overall size of the molecule that is most
relevant to the attenuation of the rate of electron transfer.
Several observations can be made from these data that are
consistent with our hypothesis that units in the outer
generation that were backfolded (in this case ortho-linked)
resulted in better encapsulation than units that were more
extended (in this case meta-linked). A lower rate of electron
transfer is observed for the backfolded isomer compared to
the more extended isomer. Moreover, the backfolded isomer
is more difficult to reduce (e.g. it has a more negative redox
potential) than the more extended isomer, consistent with
the core being found in a more compact, hydrophobic
dendrimer environment. As the reduction of interest here is
the conversion of a di-anion to a tri-anion, this increase in
effective charge is expected and found to be impeded by the
hydrophobic dendrimer environment compared to the more
polar solvent (DMF). This process has been discussed
previously by us relative to several redox cluster core
dendrimers.1
2.2. Octahedral cluster core dendrimers
Discrete clusters offer a variety of interesting and poten-
tially useful electrochemical, luminescent and magnetic
properties. To harness these properties in new materials,
efficient synthetic protocols are required to functionalize
them appropriately. We have, for example, described ligand
exchange chemistry that can be used to prepare Mo6Cl8
cluster core dendrimers.22
Zheng et al. have reported the efficient synthesis of
dendrimers composed of Re6Se8 units.34–36 Given our
interest in electroactive encapsulation, we performed
cyclic voltammetry on these molecules. However, only
irreversible electrochemistry was observed. Postulating that
the focal group moiety bonding to the cluster could have a
significant effect on its electrochemical kinetics, and that the
pyridyl linkage employed by Zheng et al. might be non-
ideal in this regard, we explored the synthesis and
electrochemistry of Re6Se8 cluster core dendrimers that
Estimating the hydrodynamic radius of these two molecules
from their diffusion coefficients using the Stokes–Einstein

T. L. Chasse et al. / Tetrahedron 59 (2003) 3853–3861
3857
Figure 2. Cyclic voltammograms of rhenium selenide cluster core
molecules. Conditions: 100 mV/s scan rate, 2 mM solution in propylene
carbonate containing 300 mM tetrabutyl ammonium tetrafluoroborate
supporting electrolyte. Potentials are referenced to Ag/100 mM AgNO3/
MeCN.
Figure 3. Fluorescence spectra of rhenium selenide cluster core molecules
using an excitation wavelength of 436 nm. The zero and first generation
dendrimer spectra were recorded in dichloromethane solution and the hexa-
acetonitrile complex was recorded in acetonitrile solution.
employed different linkages between the dendron and the
cluster core.
3. Conclusions
Overall, these results indicate several intriguing properties
of metal cluster core dendrimers. First, the structure of the
dendron ligands around the cluster has a measurable effect
on encapsulation behaviors such as electron transfer rate
attenuation. This effect is illustrated in two dendrimer
isomers in which the structure of the linkage is alternatively
backfolded and extended. Second, a variety of metal cluster
cores can be incorporated into dendrimer architectures
provided efficient ligand exchange chemistry can be
effected. This chemistry has allowed us to probe electronic
encapsulation and luminescence behaviors in rhenium
selenide cluster core dendrimers.
We determined synthetically that a benzonitrile linking
group provided efficient ligand exchange chemistry and
retained quasi-reversible electrochemical kinetics for the
Re6Se8L6 moiety (L¼para-methoxy benzonitrile). Dendri-
mer versions of these clusters were prepared to determine
the encapsulating effect of six dendritic arms octahedrally
disposed around an inorganic cluster. The synthesis of the
dendrons and the ligand exchange to form the Re6Se8-
cluster core dendrimers are shown in Scheme 4. Conditions
for the ligand exchange involved longer reaction times and
higher temperatures than were necessary for the analogous
ligand exchange reactions around iron–sulfur clusters.
Yields of the resulting dendrimers were modest, mostly as
the result of repeated precipitations to ensure the purity of
these molecules
4. Experimental
Instrumentation for routine characterization. NMR and
FAB-MS were used as previously reported.38 Fluorescence
experiments were carried out on an ISS PC1 photon
counting spectrofluorimeter with and ILC PS300-1 25
D.C. ampere illuminator power supply. UV–Vis spectra
were recorded using a Hewlett Packard 8452A diode array
spectrometer.
A preliminary investigation of the electrochemical behavior
of these dendrimers was performed using cyclic volt-
ammetry. The results of these experiments are shown in
Figure 2 One can observe the quasi-reversible electro-
chemical behavior (DE¼125 mV at 100 mV/sec scan rate
for Re6Se8(p-methoxy benzonitrile)26þ/3þ) for the “zero
generation” compound. In contrast, essentially irreversible
electrochemical behavior is observed for the analogous
“first generation” molecule.
Electrochemical apparatus. Electrochemical experiments
were carried out on a Bioanalytical Systems CV-50W
Voltammetric Analyzer. The three-electrode cell consisted
of a Pt disk working electrode (geometric area of
0.0201 cm2), a Pt auxiliary electrode, and a homemade,
non-aqueous Ag/AgNO3 reference electrode (Ag wire
contacting a DMF solution of 0.01 M AgNO3 and 0.1 M
supporting electrolyte, tetraethylammonium tetrafluorobo-
rate, TEAF4B). All electrochemical experiments were
carried out in a nitrogen-filled Vacuum Atmospheres
controlled atmosphere box at room temperature. Millimolar
concentrations of the analytes were dissolved in DMF,
which contained 0.1 M TEAF4B supporting electrolyte.
Rhenium selenide clusters of this type are known to
photoluminesce efficiently.37 Using the compound Re6Se8-
(CH3CN)26þ as an actinometer, the relative quantum yields
of the G0 and G1 ligated molecules were obtained by
exciting at 436 nm and observing the relative fluor-
escence.37 The data are presented in Figure 3. The similarity
of the three fluorescence spectra indicates that the para-
alkoxy benzonitrile linkage in the molecules under study did
not affect the relative luminescence efficiency or color of the
rhenium selenide moiety.

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T. L. Chasse et al. / Tetrahedron 59 (2003) 3853–3861
Cyclic voltammetry. Potential sweep rates ranged from 25 to
400 mV/s for analytes displaying quasireversible electro-
chemical behavior at these scan rates
filtrate under reduced pressure gave the product, which was
purified as described below.
Compound 3 Purified by column filtration using silica gel
and hexanes/ethyl acetate/DCM 80:15:5 as eluent. The
product was a yellow-white, sticky solid. Yield: 86%
Osteryoung square wave voltammetry. Osteryoung square
wave voltammetry was performed using a step height of
4 mV, sweep width amplitude of 25 mV, and a frequency of
5, 10, 15, and 20 Hz. Difference current was iteratively fit
using FSQPLT software (provided by J. J. O’Dea of the
J. Osteryoung Group)
1
(16.9 g). H NMR (CDCl3) d (ppm): 3.93 (s, 3H), 4.77 (s,
4H), 4.93 (s, 8H), 5.12 (s, 6H), 6.45 (t, 1H, J¼2.2 Hz), 6.54
(t, 2H, J¼2.2 Hz), 6.74 (d, 4H, J¼2.2 Hz), 6.77 (d, 2H,
J¼2.2 Hz), 7.25 – 7.45 (m, 32H). 13C NMR (CDCl3) d
(ppm): 166.59, 160.20, 159.94, 152.58, 139.94, 139.13,
136.99, 136.85, 128.58, 128.49, 128.31, 127.99, 127.88,
127.59, 127.42, 125.49, 109.49, 106.74, 106.23, 101.94,
76.67, 74.95, 71.32, 70.08, 69.84, 52.30. HRFAB (m/z):
1090.4266 Calcd for C71H62O11: 1090.4292. Anal. Calcd
for C71H62O11: C, 78.16; H, 5.69; Found: C, 78.17; H, 5.77.
Chronoamperometry. Chronoamperometry was carried out
using a pulse width of 900, 1000, and 1500 ms and a
potential step height of 800 mV centered about E1/2 for each
molecule studied.
4.1. Synthesis of dendrimer isomers
Compound 4 Purified by column filtration through silica gel
with an eluent of 70% hexanes/30% ethyl acetate. The
product was a yellow-white, sticky solid. Yield: 92.5%
(3.95 g). 1H NMR (CD3CN) d (ppm): 3.70 (s, 3H), 4.57 (s,
4H), 4.97 (s, 8H), 5.12 (s, 4H), 5.37 (s, 2H), 6.31 (d, 2H,
J¼8.8 Hz), 6.58 (d, 4H, J¼8.1 Hz), 7.00 (t, 1H, J¼8.8 Hz),
7.02–7.21 (m, 24H), 7.28–7.35 (m, 6H), 7.36 (s, 2H). 13C
NMR (CD3COCD3) d (ppm): 52.09, 61.16, 64.31, 70.67,
70.90, 106.26, 106.52, 109.67, 115.08, 116.89, 124.97,
127.71, 128.01, 128.23, 128.49, 128.93, 129.14, 130.46,
130.92, 138.38, 138.75, 144.78, 154.36, 159.48, 159.76,
167.32. Anal. Calcd for C71H62O11: C, 78.16; H, 5.69;
Found: C, 78.25; H, 5.75.
Reagents were purchased from Aldrich and used without
further purification unless otherwise noted. Compounds 139
and 240 were prepared as previously described.
S-Tetrahydropyranyl-4-hydroxy thiophenol into an oven
dried flask were weighed p-hydroxythiophenol (1 equiv.,
25.67 g) and pyridinum-p-toluenesulfonate (0.05 equiv.,
2.5 g), dissolved in dry dichloromethane. The solution was
cooled in an ice bath and to the cold solution was added
dihydropyran (2.74 equiv., 50 mL) dropwise with stirring
over a period of 30 min. After the addition, the mixture was
allowed to warm to room temperature and stirring continued
overnight. The mixture was then washed with 10% aq.
NaOH, water, saturated NaCl, and dried over anhydrous
sodium sulfate. The salts were filtered off and the filtrate
rotary evaporated to dryness. Yield: 62.2% (36.9 g). The
crude bis-THP substituted p-hydroxythiophenol (1 equiv.,
36.9 g) was dissolved in methanol and the solution cooled in
an ice bath. To the cold solution, was added 6 M HCl
(1.3 equiv., 29 mL). The reaction mixture was stirred for 4 h
while monitoring using TLC. Methanol was removed in
vacuo and the residue taken with water and the organics
extracted into dichloromethane. Removal of solvent gave a
sticky product, which was subsequently heated with hexane,
and the supernatant solution was decanted hot. A white
precipitate was obtained on cooling the hexane solution.
The process was repeated several times and the white,
crystalline, solid collected. Yield: 40% (10.5 g). HRFAB
(m/z): 210.1888. Calcd for C11H14SO2: 210.2964, mp 87–
898C, 1H NMR (CDCl3) d (ppm): 1.50–2.10 (m, 6H), 3.50–
3.60 (m, 1H), 4.15–4.25 (m, 1H), 5.04 (t, 1H, J¼4.5 Hz),
6.60–6.75 (m, 2H), 7.30–7.40 (m, 2H).
Compound 9 Purified by column filtration through silica gel
with an eluent of 70% hexanes/25% ethyl acetate/5% DCM.
The product was a white, sticky solid. Yield: 75% (1.33 g).
1H NMR (CD3COCD3) d (ppm): 1.50–2.05 (m, 6H), 3.45
(m, 1H), 4.05 (m, 1H), 4.82 (s, 4H), 4.95 (s, 8H), 4.99 (s,
2H), 5.00 (m, 1H), 5.05 (s, 2H), 5.11 (s, 4H), 6.45 (t, 1H,
J¼2.2 Hz), 6.53 (t, 2H, J¼2.2 Hz), 6.78 (d, 4H, J¼2.2 Hz),
6.81 (d, 2H, J¼2.2 Hz), 6.91 (t, 4H, J¼8.8 Hz), 7.25–7.45
(m, 32H). 13C NMR (CDCl3) d (ppm): 160.19, 159.95,
158.45, 153.04, 140.36, 139.57, 137.05, 136.88, 134.27,
132.65, 128.57, 128.49, 127.98, 127.87, 127.60, 125.94,
115.44, 107.55, 106.75, 106.11, 101.80, 86.28, 75.06, 71.36,
70.25, 70.06, 69.86, 64.73, 31.61, 25.62, 21.78. Anal. Calcd
for C81H74O11S: C, 77.49; H, 5.95; Found: C, 77.45; H,
6.04.
Compound 10 Purified by column filtration through silica
gel using hexane 80%/ethyl acetate 15%/DCM 5% as
eluent. Product was a white, crystalline solid. Yield: 84%
1
(1.68 g). H NMR (CD3CN) d (ppm): 1.5–1.87 (m, 6H),
4.1.1. General method for nucleophilic coupling reac-
tion. To a solution of the phenol derivative (1 equiv.), dry
acetone (50–60 equiv.), potassium carbonate (1.1 equiv.
per OH in phenol), and 18-crown-6 (0.01–0.02 equiv.), was
added a benzyl halide derivative (1.1–1.2 equiv. per OH in
phenol). The mixture was then refluxed for 2–3 days under
nitrogen with vigorous stirring. The mixture was then
cooled and concentrated to half the volume. Water was
added to dissolve the salts and the organics extracted into
ethyl acetate, DCM, or ether. The organic extract was
washed with brine or NaHCO3 and dried over anhydrous
Na2SO4 or MgSO4. Filtration and concentration of the
3.49 (m, 1H), 4.08 (m, 1H), 4.63 (s, 4H), 4.75 (s, 2H), 4.98
(s, 8H), 5.02 (m, 1H), 5.08 (s, 4H), 5.19 (s, 2H), 6.38 (d, 2H,
J¼8.1 Hz), 6.63 (d, 4H, J¼8.1 Hz), 6.76 (s, 2H), 6.85 (d,
2H, J¼8.8 Hz), 7.02 (t, 1H, J¼8.1 Hz), 7.11–7.25 (m,
26H), 7.28–7.39 (m, 8H). 13C NMR (CDCl3) d (ppm):
22.49, 26.17, 32.22, 61.17, 63.72, 65.56, 70.89, 70.99,
71.28, 86.99, 106.28, 106.52, 106.54, 108.31, 115.58,
115.94, 117.33, 127.57, 127.67, 127.80, 128.02, 128.15,
128.73, 129.02, 129.70, 130.32, 131.29, 134.97, 137.88,
138.62, 139.88, 154.65, 159.33, 159.48, 159.53, 159.62.
Anal. Calcd for C81H74O11S; C, 77.49; H, 5.95; S, 2.55;
Found: C, 77.74; H, 6.14; S, 2.24.

T. L. Chasse et al. / Tetrahedron 59 (2003) 3853–3861
3859
4.1.2. General method for reduction. The ester (1 equiv.)
was cannulated into a solution of LiAlH4 (1.3 equiv.) in dry
THF (20.8 equiv.) cooled in an ice-water bath stirring
continuously for 2 hours. The reaction was quenched with
H2O, 15% NaOH, or aqueous NH4Cl. This was followed by
addition of 6 M HCl to dissolve the salts. After the reaction
was complete (a few hours for 5 and overnight for 6) the
reaction was carefully worked up with water and the solids
filtered by passing through a bed of Celite. The organics
were extracted into ethyl acetate or dichloromethane. The
organic layer was dried over anhydrous Na2SO4, filtered,
rotary evaporated, and placed under vacuum overnight. The
crude alcohol was purified as described below.
Compound 8 To a solution of the alcohol 6 (1.1 equiv.,
2.80 g) in THF or DCM cooled in an ice bath, was added, by
cannulating, a solution of carbon tetrabromide (2.5 equiv.,
2.19 g) and tri-phenyl phosphine (2.5 equiv., 1.73 g) in THF
or DCM, dropwise while stirring. The temperature was
allowed to rise to ambient over 2 h. The reaction was
quenched with water and stirred for an additional 2 h. The
organics were extracted with dichloromethane and washed
with saturated NaCl and dried using Na2SO4 or MgSO4. The
mixture was filtered and concentrated. The compound was a
yellow-white solid and was used without further purifi-
cation. Yield: 63% (1.85 g). 1H NMR (CD3COCD3) d
(ppm): 4.35 (s, 2H), 4.57 (s, 4H), 4.99 (s, 8H), 5.13 (s, 4H),
5.34 (s, 2H), 6.31 (d, 2H, J¼8.8 Hz), 6.66 (d, 4H,
J¼8.8 Hz), 6.83 (s, 2H), 6.94 (t, 1H, J¼8.1 Hz), 7.08–
7.28 (m, 26H), 7.38 (d, 6H, J¼7.3 Hz). 13C NMR
(CD3COCD3) d (ppm): 60.78, 63.69, 70.67, 70.73, 106.31,
106.57, 109.12, 115.10, 117.04, 127.70, 127.83, 127.06,
128.18, 128.67, 128.80, 128.93, 129.99, 130.58, 138.17,
138.72, 154.39, 159.27, 159.59. Anal. Calcd for
C70H61O9Br: C, 74.66; H, 5.46; Found: C, 74.70; H, 5.66.
Compound 5 Purified by column filtration through silica gel
using hexane 75%/ethyl acetate 20%/DCM 5% as eluent
and resulted in a white solid. Yield: 90% (11.38 g). 1H NMR
(CDCl3) d (ppm): 3.75 (t, OH, J¼5.9 Hz), 4.58 (d, 2H,
J¼5.9 Hz), 4.77 (s, 4H), 4.91 (s, 8H), 5.06 (s, 2H), 5.09 (s,
4H), 6.44 (t, 1H, J¼2.2 Hz), 6.53 (t, 2H, J¼2.2 Hz), 6.70 (s,
2H), 6.72 (d, 4H, J¼2.2 Hz), 6.80 (d, 2H, J¼2.2 Hz), 7.25–
7.41 (m, 30H). 13C NMR (CDCl3) d (ppm): 65.39, 69.82,
70.04, 71.27, 75.03, 101.77, 103.39, 106.07, 106.73, 127.58,
127.84, 127.95, 128.46, 128.55, 136.86, 139.60, 152.93,
159.92, 160.15. Anal. Calcd for C70H62O10: C, 79.08; H,
5.89; Found: C, 79.31; H, 6.12.
4.1.3. Deprotection of tetrahydropyranyl protected
thiols. Tetrahydropyranyl (THP) protected thiol 9 or 10
(1 equiv.) was dissolved in tetrahydrofuran. Excess of silver
nitrate was added. Water was then added to the mixture until
a bright yellow precipitate formed. The mixture was then
stirred for 20 min and diluted ten-fold with methylene
chloride. NaSH (solid, 10 equiv.) was added and stirred
vigorously for thirty minutes. The result was a black
precipitate, which was removed by filtration. The precipitate
was washed twice with methylene chloride. The aqueous
and the organic phases were separated and the organic phase
washed three times with aq. ammonium chloride solution.
The product was dried over magnesium sulfate. Remaining
methylene chloride was evacuated under vacuum. A singlet
Compound 6 Purified by silica gel column chromatography
using 25% ethyl acetate/5% DCM/hexanes as eluent and
resulted in a white, crystalline solid. Yield: 87% (1.28 g). 1H
NMR (CD3COCD3) d (ppm): 3.87 (t, OH, J¼5.9 Hz), 4.44
(d, 2H, J¼5.9 Hz), 4.58 (s, 4H), 4.98 (s, 8H), 5.15 (s, 4H),
5.35 (s, 2H), 6.31 (d, 2H, J¼8.8 Hz), 6.61 (d, 4H,
J¼8.1 Hz), 6.78 (s, 2H), 6.92 (t, 1H, J¼8.8 Hz), 7.11–
7.24 (m, 26H), 7.37 (d, 6H, J¼6.6 Hz). 13C NMR
(CD3COCD3) d (ppm): 60.46, 63.50, 64.76, 70.51, 105.99,
106.21, 106.47, 115.29, 117.26, 127.47, 127.67, 127.86,
128.51, 128.77, 129.67, 130.28, 137.10, 138.07, 138.65,
154.29, 159.14, 159.43. Anal. Calcd for C70H62O10: C,
79.08; H, 5.89; Found: C, 78.96; H, 5.81.
1
around 3.4 ppm in the H NMR spectrum identified the
resulting thiol.
Compound 11 The product was a white solid. Yield: 88%
1
(0.17 g). H NMR (CDCl3) d (ppm): 3.31 (s, SH), 4.73 (s,
Compound 7 To a solution of the alcohol 5 (1 equiv., 2.94 g)
and proton sponge (1.37 equiv., 0.81 g) dissolved in
methylene chloride and cooled in an ice bath, was added
thionyl chloride (1.48 equiv., 0.3 mL) dropwise while
stirring. The mixture was then allowed to warm to room
temperature and stirring continued. After 2 h, a color change
was observed and the mixture was quenched with water.
The organics were extracted with methylene chloride three
times. The combined organic extracts were washed with
brine, dried over anhydrous sodium sulfate, filtered and
concentrated. The chloride product resulted in a yellow-
white solid and was carried on without further purification.
4H), 4.86 (2£s, 10H), 5.03 (2£s, 6H), 6.41 (t, 1H,
J¼2.2 Hz), 6.49 (t, 2H, J¼2.2 Hz), 6.68 (m, 6H), 6.76 (d,
2H, J¼2.2 Hz), 6.79 (d, 2H, J¼8.8 Hz), 7.18 (t, 2H,
J¼8.8 Hz), 7.25–7.48 (m, 28H).
Compound 12 The product was a white solid. Yield: 91%
1
(0.09 g). H NMR (CDCl3) d (ppm): 3.84 (s, SH), 4.49 (s,
4H), 4.57 (s, 2H), 4.90 (s, 8H), 5.16 (s, 4H), 5.38 (s, 2H),
6.20 (d, 2H, J¼8.8 Hz), 6.58 (d, 4H, J¼8.8 Hz), 6.71–6.92
(m, 9H), 7.03–7.33 (m, 30H).
4.1.4. General method for ligand exchange. To a Schlenk
flask, 1 equiv. of (nBu4N)2[Fe4S4(S-tBu)4]41 was added
under inert atmosphere in a dry box. The flask was sealed
with a rubber septum and parafilm and transferred to a
double manifold vacuum line and purged with argon. To a
second Schlenk flask sealed with a rubber septum and
parafilm, the thiol 11 or 12 (4.8 equiv.) was added and
dissolved in anhydrous dimethylformamide under inert
atmosphere to a concentration of 0.1 M. The thiol solution
was then transferred to the flask containing the iron–sulfur
cluster via a canula needle. The flask containing the two
1
Yield: 70% (2.09 g). H NMR (CDCl3) d (ppm): 4.50 (s,
2H), 4.77 (s, 4H), 4.92 (s, 8H), 5.04 (s, 2H), 5.07 (s, 4H),
6.44 (t, 1H, J¼2.2 Hz), 6.53 (t, 2H, J¼2.2 Hz), 6.68 – 7.73
(m, 6H), 6.77 (d, 2H, J¼2.2 Hz), 7.25–7.4 (m, 30H). 13C
NMR (CDCl3) d (ppm): 70.4, 70.6, 71.9, 75.6, 102.3, 102.4,
106.7, 107.3, 109.1, 128.2, 128.5, 128.6, 129.0, 129.1,
133.7, 137.4, 137.6, 139.1, 140.0, 140.1, 153.5, 160.5,
160.7. HRFAB (m/z): 1080.3959; Calcd for C70H61O9Cl:
1080.4004. Anal. Calcd for C70H61O9Cl: C, 77.73; H, 5.68;
Found: C, 77.92; H, 5.84.

3860
T. L. Chasse et al. / Tetrahedron 59 (2003) 3853–3861
components was sealed with a glass stopper and stirred for
30 min. The flask was placed under vacuum slowly to avoid
bumping, while stirring continuously at 408 C overnight.
The flask was then transferred into a dry box for further use.
119.96, 128.34, 128.66, 128.90, 129.02, 129.26, 134.70,
138.16, 142.32, 142.92, 157.52, 157.80, 163.16. Anal. Calcd
for C85H81NO7: C, 83.13; H, 6.73; N, 1.13; Found: C, 82.87;
H, 6.73; N, 1.24.
Compound 13 The product was a glossy, black solid. Yield
1
90% (0.16 g). H NMR (DMSO-d6) d (ppm): 0.93 (t, 24H,
4.1.6. General procedure for synthesis of rhenium
selenide cluster core molecules. [Re6Se8(CH3CN)6]-
(SbF6)2 (0.1353 g, 0.0562 mmol) was combined with
J¼7.3 Hz), 1.32 (m, 16H), 1.56 (m, 16H), 3.13 (m, 16H),
4.73 (m, 16H), 4.81–5.05 (m, 56H), 5.06 (s, 8H), 5.75 (br,
8H), 6.41 (s, 4H), 6.49 (s, 8H), 6.69 (m, 24H), 6.86 (s, 8H),
7.15 – 7.35 (m, 120H), 7.82 (s, 8H). Anal. Calcd for
C336H332O40Fe4N2S8: C, 73.13; H, 6.28; Found: C, 74.04;
H, 6.06.
4-methoxybenzonitrile
(0.0582 g,
0.437 mmol,
7.78 equiv.) and dissolved in dry 1,2-dichlorobenzene
(6 mL). The mixture was heated under N2 at 1108C
overnight. The solvent was removed by vacuum at 608C.
The residue was then dissolved in dichloromethane. This
residue was precipitated with diethyl ether and the solid was
collected. This precipitation was repeated until only one
spot on TLC (ether) was observed on the baseline. The
solution was then filtered through a celite plug. The solvent
was allowed to evaporate off yielding a red/orange solid.
Compound 14 The product was a glossy, black solid. Yield
86%. 1H NMR (DMF-d6) d (ppm): 0.93 (t, 24H, J¼7.3 Hz),
1.39 (m, 16H), 1.76 (m, 16H), 3.38 (m, 16H), 4.71 (m, 16H),
4.88 (m, 16H), 5.04–5.19 (m, 32H), 5.36 (m, 16H), 5.82 (br,
8H), 6.44 (m, 8H), 6.69 (m, 16H), 6.93 (m, 8H), 7.04 (m,
12H), 7.19–7.40 (m, 120H), 7.76 (br, 8H). MALDI-TOF
(m/z): 5025.90; Calcd for C336H332O40Fe4N2S8: 5027.36.
Anal. Calcd for C336H332O40Fe4N2S8: C, 73.13; H, 6.28; S,
4.65; Found: C, 74.51; H, 6.27; S, 4.37.
[Re6Se8(p-CH3OC6H4CN)6](SbF6)2, Compound 19. Yield:
1
89.5 mg, 71%. H NMR (CD2Cl2) d (ppm): 3.92 (s, 18H),
7.12 (d, 12H, J¼8.8 Hz), 7.76 (m, 12H). ESI-MS (m/z):
Calculated for [Re6Se8(p-CH3OC6H4CN)6]2þ1273.46;
Found: 1273.83.
4.1.5. Rhenium selenide cluster core dendrimers.
General considerations. All reagents were used as received
except hydriodic acid, which was purchased as either
55–57% or 47% and then diluted according to the certificate
of analysis concentration. The compounds [Re6Se8(CH3-
CN)6](SbF6)2,42 G1-OMs (15)43 and G2-OMs (16)43 were
prepared according to literature procedures.
[Re6Se8(p-G1OC6H4CN)6](SbF6)2, Compound 20. Yield:
59.2 mg, 40.5%. 1H NMR (CD2Cl2) d (ppm): 1.54 (m, 12H),
1.62 (s, 18H), 2.12 (br, 12H), 2.21 (m, 12H), 4.01 (t, 12H,
J¼6.0 Hz), 5.03 (s, 24H), 6.89 (d, 24H, J¼8.8 Hz), 7.04 (d,
12H, J¼9.2 Hz), 7.14 (d, 24H, J¼8.8 Hz), 7.30–7.43 (m,
60H), 7.70 (d, 12H, J¼8.8 Hz). ESI-MS (m/z): Calculated
for [Re6Se8(p-G1OC6H4CN)6]2þ 2535.53; Found: 2538.83.
Anal. Calcd for C228H210N6O18F12Re6Se8Sb2: C, 49.36; H,
3.79; N, 1.52; Found: C, 49.49; H, 3.95; N, 1.64.
Compound 17 G1-OMs (15) (0.5345 g, 1.007 mmol),
4-cyanophenol (0.1268 g, 1.064 mmol), K2CO3 (1.7019 g,
4.028 mmol), and 0.05 equiv. (0.53 g, 2 mmol) of 18-
crown-6 was added to a round bottom flask. Acetone
(40 mL) was added and the solution was refluxed overnight
under N2. The solution was cooled to room temperature and
the solvent was removed under reduced pressure. The
remaining solid was dissolved in dichloromethane (20 mL).
Deionized H2O (20 mL) was added and the organic layer
was collected. Extraction with dichloromethane was
repeated three times on the aqueous layer. The organic
layer was dried with MgSO4 and the crude product was
purified by flash chromatography (CH2Cl2). Yield: 5.0 g,
[Re6Se8(p-G2OC6H4CN)6](SbF6)2, Compound 21. Yield:
1
310 mg, 46%. H NMR (CD2Cl2) d (ppm): 1.59 (br, 90H),
2.17 (br, 36H), 3.85 (br, 24H), 3.96 (br, 12H), 5.00 (s, 48H),
6.75 (d, 24H, J¼7.6 Hz), 6.86 (m, 48H), 6.98–7.12 (m,
24H), 7.29–7.41 (m, 120H), 7.66 (d, 12H, J¼8.8 Hz). ESI-
MS (m/z): Calculated for [Re6Se8(p-G2OC6H4CN)6]2þ
4602.20; Found: 4603.40.
1
Acknowledgements
90%. H NMR (CDCl3) d (ppm): 1.62 (m, 5H), 2.20 (m,
2H), 3.94 (t, 2H, J¼6.6 Hz), 5.03 (s, 4H), 6.89 (d, 6H,
J¼8.7 Hz), 7.13 (d, 4H, J¼8.7 Hz), 7.30–7.44 (m, 10H),
7.56 (d, 2H, J¼8.7 Hz). 13C NMR (CD2Cl2) d (ppm): 25.38,
25.44, 28.28, 38.69, 45.46, 56.55, 70.23, 70.71, 70.88,
99.27, 114.94, 116.69, 128.33, 128.69, 128.83, 128.98,
129.28, 136.61, 137.39, 138.12, 142.65, 157.59, 165.89.
Anal. Calcd for C38H35NO3: C, 82.47; H, 6.37; N, 2.53;
Found: C, 82.26; H, 6.46; N, 2.58.
This work was supported by the National Science
Foundation (CHE-9900072). We thank Rakesh Sachdeva
for assistance in preparing synthetic intermediates.
References
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