Synthetic Approaches to an Isostructural Series of Redox-Active, Metal Tris(bipyridine) Core Dendrimers

Article abstract of DOI:10.1021/jo0351116

Several types of six-armed, metal tris(bipyridine) core dendrimers were synthesized. Bis-4,4′-alkoxy bipyridine dendrons were prepared and employed to make tris(bipyridine) dendrimers. Although the ruthenium-centered and iron-centered dendrimers displayed

Full text of DOI:10.1021/jo0351116

Syn th etic Ap p r oa ch es to a n Isostr u ctu r a l Ser ies of Red ox-Active,
Meta l Tr is(bip yr id in e) Cor e Den d r im er s
Young-Rae Hong and Christopher B. Gorman*
Department of Chemistry, North Carolina State University, Box 8204,
Raleigh, North Carolina 27695-8204
chris_gorman@ncsu.edu
Received J uly 29, 2003
Several types of six-armed, metal tris(bipyridine) core dendrimers were synthesized. Bis-4,4′-alkoxy
bipyridine dendrons were prepared and employed to make tris(bipyridine) dendrimers. Although
the ruthenium-centered and iron-centered dendrimers displayed quasi-reversible cyclic voltammetry,
the analogous cobalt-centered complex did not. The synthesis of 4,4′-disubstituted bipyridines
2
containing -CH OR groups proceeded in low yield. The reactions of the dicarbanion of 4,4′-dimethyl
bipyridine prepared with LDA and mesylate, triflate, and bromide groups were found to result in
no or poor yields of carbon-carbon bond formation. Use of KDA in place of LDA resulted in much
higher yields of dendritic bipyridines.
In tr od u ction
between two different core moieties. The idea of an
isostructural series allows for the comparison of den-
drimers with a similar conformational manifold; thus, the
Redox active core dendrimers are useful in several
structural and behavioral studies of this class of mol-
ecules. By probing the rate and driving force for electron
transfer to/from these molecules, one can learn about how
the dendritic arms encapsulate the core. This information
can facilitate conclusions about how the primary struc-
ture of the dendrimer leads to its conformations. Encap-
sulation of a core by a dendrimer also has potential
utility, including mimicking metalloprotein behaviors,
attenuating molecular exciton quenching in luminescent
thin films, and understanding charge-trapping behaviors
in molecular electronics.1
(
8) Gorman, C. B.; Su, W. Y.; J iang, H.; Watson, C. M.; Boyle, P.
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We and others have sought to prepare several types of
redox-active core dendrimers to study these various
phenomena. These include illustrating the effect of
1
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(
16) Cardona, C. M.; Kaifer, A. E. J . Am. Chem. Soc. 1998, 120,
dendrimer architecture on rate and driving force for
4023-4024.
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,3,5,6
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comparison of structural features
7
in dendrimer isomers, and illustration of the behaviors
of several types of redox active cores in dendrimers
8
9-11
12-17
including Fe
4
S
4
, Mo
6
Cl
8
, Re
6
Se
8
,
ferrocene,
and
porphyrins.1
8-37
It is of interest to systematically vary the redox
potential in an isostructural series of dendrimers. This
set of molecules would permit various studies such as
measurement of homogeneous electron-transfer rates
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(
9
(
1
0.1021/jo0351116 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/17/2003
J . Org. Chem. 2003, 68, 9019-9025
9019

Hong and Gorman
SCHEME 1
effects of differences in redox potential can be separated
from those due to conformational differences between
molecules.
lustrated in this paper, no synthetic methodology could
be adopted from this work that was amenable to the
synthesis of an isostructural series of bipyridine-based,
redox active molecules. Synthetic and electrochemical
investigations are presented here that illustrate an
efficient route to an isostructural series of redox-active
metal tris(bipyridine) core dendrimers.
To this end, we sought to prepare a series of metal tris-
(bipyridine) core dendrimers that contain different cen-
tral metal atoms. Bipyridine-based metal complexes as
well as the related terpyridine- and phenanthroline-based
complexes have been used in the core, branches, and
periphery of dendrimers.3
8-53
Resu lts a n d Discu ssion
However, as will be il-
A. Den dr im er s Con tain in g a Bipyr idin e-OR Lin k-
a ge. Bis(phenoxy)isovalerate-based dendrons can be eas-
ily converted to molecules with an electrophilic mesylate
focal group (e.g., molecule 3). Thus, a bipyridine deriva-
tive presenting a nucleophilic group seemed a good way
to synthesize tris(bipyridine) core dendrimers. The route
employed to do so is shown in Scheme 1. We determined
that treatment with HBr/acetic acid at reflux converted
(
30) Bhyrappa, P.; Vaijayanthimala, G.; Suslick, K. J . Am. Chem.
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4
,4′-dimethoxy bipyridine to 4,4′-dihydroxy bipyridine in
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acceptable yield. This molecule reacted with the focally
substituted mesylate 3 in excellent yield to give 4. Thus,
from a synthetic standpoint, this route was extremely
attractive for the synthesis of the dendritic bipyridine.
Further, both 4,4′-dimethoxy bipyridine (generation zero
or G0) and 4 (generation 1 or G1) could be converted to
the ruthenium, iron, and cobalt tris(bipyridine) complexes
(Scheme 2).
The goal in preparing and studying these molecules
was to access at least three metal tris(bipyridine) com-
plexes with different redox potentials and quasi-revers-
ible electron-transfer kinetics. These molecules were thus
examined by cyclic voltammetry (Figure 1). The ruthe-
nium and iron complexes (5 and 6) showed the type of
electrochemistry desired. Although the cobalt complex (7)
showed the expected, ligand-centered reduction below
(
(
(
Chem.-Eur. J . 1996, 2, 1085-1091.
(
(
39) Tzalis, D.; Tor, Y. Tetrahedron Lett. 1996, 37, 8293-8296.
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41) Lamba, J . J . S.; Fraser, C. L. J . Am. Chem. Soc. 1997, 119,
801-1802.
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Denti, G. Gazz. Chim. Ital. 1994, 124, 423-427.
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1
(
(
3
(
1
(
(
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(
(
47) Nierengarten, J . F. Chem.-Eur. J . 2000, 6, 3667-3670.
48) Zhou, X. L.; Tyson, D. S.; Castellano, F. N. Angew. Chem., Int.
-
1500 mV, the metal-centered reduction was irreversible.
Thus, although synthetically attractive, this complex did
not display the desired electrochemical behavior.
Ed. 2000, 39, 4301.
49) Ghaddar, T. H.; Wishart, J . F.; Kirby, J . P.; Whitesell, J . K.;
Fox, M. A. J . Am. Chem. Soc. 2001, 123, 12832-12836.
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1, 3578-3586.
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Nierengarten, J . F. Chem. Commun. 2002, 2830-2831.
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P. L.; Abruna, H. D. Macromolecules 2003, 36, 1272-1278.
(
2
B. Den d r on s Con t a in in g a Bip yr id in e-CH OR
Lin k a ge. It was hypothesized that the alkoxy-bipyri-
(
4
(
(53) McClenaghan, N. D.; Passalacqua, R.; Loiseau, F.; Campagna,
S.; Verheyde, B.; Hameurlaine, A.; Dehaen, W. J . Am. Chem. Soc. 2003,
125, 5356-5365.
(
9
020 J . Org. Chem., Vol. 68, No. 23, 2003

Synthetic Approaches to Dendrimers
SCHEME 2
of dialkyl bipyridines (Scheme 4) from the alkyl bromide,
reaction of the analogous mesylate or triflate was unsuc-
cessful at producing the desired product. The G1 den-
dritic mesylate could, however, be converted into the
bromide (18) very efficiently using Finkelstein chemistry
(Scheme 5). However, the use of lithium diisopropyl
amide as base to deprotonate 4,4′-dimethyl bipyridine
followed by reaction with 18 led to a disappointingly low
yield of the desired product 19. From the model studies
above, we reckoned that the structure and reactivity of
the biscarbanion rather than the leaving group was to
blame for the poor yield. Indeed, when potassium diiso-
propyl amide was substituted, an excellent yield of 19
was obtained. This reaction was run and quenched at
-
78 °C given the known temperature sensitivity of the
5
6
KDA and likely that of the resulting potassium dicar-
banion. The disubstituted, G1 bipyridine (19) could be
converted into metal, tris(bipyridine) complexes of ru-
thenium, iron, and cobalt (Scheme 6).
Cyclic voltammetry was performed on both the G0 and
G1 ruthenium, iron, and cobalt tris(bipyridine) complexes
F IGURE 1. Cyclic voltammograms of compounds 5, 6, and 7
(Figure 2 and Table 1). All displayed quasi-reversible
(scan rate 200 mV/s, argon-purged acetonitrile solution, 100
electrochemical behavior. Thus, the alkyl linkage to the
bipyridine avoids the slow electrochemical kinetics found
in the alkoxy-substituted trisbipyridine cobalt complex.
We hypothesize that, in going from alkyl to alkoxy groups
on the bipyridines, the ligand field weakens as has been
mM tetrabutylammonium tetrafluoroborate supporting elec-
trolyte, platinum as working electrode).
dine linkage gave rise to the poor electrochemical behav-
ior for the cobalt complex, 7 (vide infra). To remedy this,
a route to an alkyl-substituted bipyridine derivative was
sought. Fraser et al. reported the conversion of 4,4′-
dimethyl 2,2′-bipyridine (10) to the electrophilic bisbro-
5
7
observed for alkoxy terpyridine cobalt complexes. This
effect causes the alkoxy cobalt complex to undergo a spin
crossover upon oxidation, which is rate limiting. Such
slowing of electrochemical kinetics has been observed in
cobalt bis[poly(pyrazolyl)borate] complexes.5 Further-
more, the G1 dendrimers displayed the expected lower
current than did their G0 analogues. This result is
consistent with a slowing of electron-transfer kinetics due
to dendritic encapsulation.1 A more complete set of
electrochemical results that compares rate attenuation
in these molecules as a function of generation will be
reported in due course.
momethyl derivative (12) via the bistrimethylsilylmethyl
8
bipyridine (11).54 Molecule 12 could then be reacted with
nucleophiles to form bipyridine derivatives. When this
molecule was reacted with the focally substituted den-
dritic alkoxide (Scheme 3), the yield was unacceptably
low (32%). Yields were not improved by varying the
hydride used (NaH versus KH), addition of 18-crown-6,
or variation of the solvent (dimethyl formamide versus
tetrahydrofuran). Because of these observations, this
approach was not pursued further.
-4
C. Den d r im er s Con t a in in g a Bip yr id in e-CH
2
R
Exp er im en ta l Section
Lin k a ge. Another method for the synthesis of substi-
2
,2′-Bip yr id in yl-4,4′-d iol (2). To a solution of 4,4′-dimeth-
tuted bipyridines is the direct deprotonation of 10 fol-
oxy-2,2′-bipyridine (1) (2.95 g, 14 mmol) in 170 mL of glacial
acetic acid was added 48 wt % HBr solution in water (24 mL,
lowed by reaction with electrophiles.5
4,55
We discovered
that, although this reaction is acceptable for the synthesis
(
56) Raucher, S.; Koolpe, G. A. J . Org. Chem. 1978, 43, 3794-3796.
(
54) Fraser, C. L.; Anastasi, N. R.; Lamba, J . J . S. J . Org. Chem.
997, 62, 9314-9317.
55) Griggs, C. G.; Smith, D. J . H. J . Chem. Soc., Perkin Trans. 1
982, 3041-3043.
(57) Hathcock, D. J .; Stone, K.; Madden, J .; Slattery, S. J . Inorg.
Chim. Acta 1998, 282, 131-135.
(58) Chanaka, D.; De Alwis, L.; Schultz, F. A. Inorg. Chem. 2003,
42, 3616-3622.
1
1
(
J . Org. Chem, Vol. 68, No. 23, 2003 9021

Hong and Gorman
SCHEME 3
SCHEME 4
[Ru (1)
3
](P F
6
)
2
(5). To a solution of 4,4′-dimethoxy-2,2′-
bipyridine (1) (0.50 g, 2.3 mmol) in 30 mL of pure ethanol was
added ruthenium-trichloride hydrate (40-43% Ru) (0.15 g,
0
.72 mmol) in 5 mL of pure ethanol. The reaction mixture was
refluxed for 5 days. The solvent was removed in vacuo. The
residue was dissolved in CH Cl , and a solution of NH PF
1.18 g, 7.23 mmol) in a small amount of methanol was added
2
2
4
6
(
dropwise with stirring. The mixture was stirred for 2 h and
extracted with water two times. The combined organic layers
were evaporated to dryness under reduced pressure. The
residue was dissolved in a small amount of CH Cl and added
2 2
dropwise with stirring to 150 mL of petroleum ether. The
orange precipitate was filtered, washed with ethanol, and
dried: yield 40% (0.30 g); 1H NMR (CD
CN) δ (ppm) 3.99
3
1
3
(
s, 6H), 6.94 (d, 2H), 7.52 (d, 2H), 7.98 (s, 2H); C NMR
CD CN) δ (ppm) 57.7, 111.9, 114.6, 153.3, 159.5, 167.9.
);
found, 892.68. FAB-MS (matrix, NBA); m/z calcd, 895.14 (M
PF ); found, 895.10. Anal. Calcd for C36 Ru: C,
1.59; H, 3.49; N, 8.08. Found: C, 41.48; H, 3.34; N, 8.11.
[F e(1) ](P F (6). To a solution of 4,4′-dimethoxy-2,2′-
bipyridine (1) (0.50 g, 2.3 mmol) in 30 mL of pure ethanol was
added (NH FeSO SO ‚6H O (0.28 g, 0.72 mmol) in 5 mL of
water. The reaction mixture was refluxed for 24 h. The solvent
was removed in vacuo. The residue was dissolved in CH Cl
and a solution of NH PF (1.18 g, 7.23 mmol) in a minimum
(
3
MALDI-TOF MS (matrix, DHB); m/z calcd, 895.14 (M - PF
6
-
4
6
36 12 6 6 2
H F N O P
1
40 mmol). The mixture was refluxed overnight. After the
3
6 2
)
mixture had cooled to room temperature, the solvent was
removed in vacuo. The residue was dissolved in water and
neutralized by adding aqueous ammonium hydroxide. This
produced a white solid that was filtered and dried. This was
used for the next step without further purification. Yield: 67%
4
)
2
4
4
2
2
2
,
4
6
(1.76 g).
amount of methanol was added dropwise while stirring. The
mixture was stirred for 2 h and extracted with water two
times. The combined organic layers were evaporated to dryness
under reduced pressure. The residue was dissolved in a
minimum amount of CH Cl and added dropwise while stirring
2 2
to 150 mL of petroleum ether. The purple precipitate was
G1-OBp y (4). To a suspension of 2 (0.50 g, 2.66 mmol) in
6
0 mL of dry acetone were added G1-OMs (3)(3.13 g, 5.9
mmol), anhydrous potassium carbonate (1.1 g, 7.98 mmol), and
a catalytic amount of 18-crown-6. The mixture refluxed and
stirred vigorously for 36 h. After the mixture had cooled to
room temperature, the solid residue was filtered and the
filtrate was evaporated to dryness under reduced pressure. The
crude product was purified by column chromatography with
filtered, washed with ethanol, and dried: yield 82% (0.59 g);
1
H NMR (CD
3
CN) δ (ppm) 4.01 (s, 6H), 6.98 (d, 2H), 7.23 (d,
CN) δ (ppm) 57.7, 111.9, 115.0,
155.6, 161.4, 169.1. MALDI-TOF MS (matrix, DHB); m/z calcd,
849.17 (M - PF ); found, 847.20. FAB-MS (matrix, NBA); m/z
calcd, 849.17 (M - PF ); found, 849.20. Anal. Calcd for
: C, 43.48; H, 3.65; N, 8.45. Found: C,
43.40; H, 3.62; N, 8.42.
[Co(1) ](P F (7). To a solution of 4,4′-dimethoxy-2,2′-
bipyridine (1) (0.50 g, 2.3 mmol) in 30 mL of pure ethanol was
2H), 8.06 (s, 2H); 13C NMR (CD
3
CH
(
(
2
Cl
2.62 g); H NMR (CDCl
m, 4H), 4.06 (t, 4H), 5.03 (s, 8H), 6.80 (br, 2H), 6.89 (d, 8H, J
6.9 Hz), 7.20 (d, 8H, J ) 6.9 Hz), 7.30-7.50 (m, 20H), 7.94
2
followed by 2:1 ethyl acetate-hexane eluent: yield 93%
1
3
) δ (ppm) 1.50-1.80 (m, 10H), 2.22
6
6
)
36 36 6 6 2
C H F12FeN O P
1
3
(
s, 2H), 8.47 (br, 2H, J ) 4.7 Hz); C NMR (CDCl
3
) δ (ppm)
2
1
5.0, 28.3, 38.6, 45.1, 68.8, 70.3, 107.2, 111.5, 114.4, 127.7,
28.1, 128.5, 128.8, 137.3, 142.0, 150.2, 156.9, 157.7, 166.3.
3
6 2
)
9
022 J . Org. Chem., Vol. 68, No. 23, 2003

Synthetic Approaches to Dendrimers
SCHEME 5
TABLE 1. Electr och em ica l Da ta
compound
Epa (mV) Epc (mV) E1/2 (mV) ∆E (mV)
[
[
[
[
[
[
[
[
[
[
[
Ru(1)3](PF6)2, 5
Fe(1)3](PF6)2, 6
+680
+500
irrev.a
+680
+491
+860
+649
-60
+579
+394
-211
+577
+400
+754
+547
-181
+805
+590
-143
+630
+447
NAb
101
106
NA
103
91
106
102
121
90
b
Co(1)3](PF6)2, 7
Ru(4)3](PF6)2, 8
Fe(4)3](PF6)2, 9
Ru(10)3](PF6)2, 20
Fe(10)3](PF6)2, 21
Co(10)3](PF6)2, 22
Ru(19)3](PF6)2, 23
Fe(19)3](PF6)2, 24
Co(19)3](PF6)2, 25
+629
+446
+807
+598
-121
+850
+638
-82
+895
+686
-20
96
123
a
Not determined in a fit of the irreversible wave. b Not ap-
plicable.
to dryness under reduced pressure. The residue was dissolved
in a minimum amount of CH Cl and added dropwise while
2 2
stirring to 150 mL of petroleum ether. The yellow precipitate
was filtered, washed with ethanol, and dried: yield 81% (0.58
1
g); H NMR (CD
(
3
CN) δ (ppm) 6.66 (s, 6H), 40.79 (s 2H), 77.08
s, 2H), 91.96 (br, 2H). MALDI-TOF MS (matrix, DHB); m/z
calcd, 852.17 (M - PF ); found, 852.21. FAB-MS (matrix,
NBA); m/z calcd, 852.17 (M - PF ); found, 852.21. Anal. Calcd
for C36 : C, 43.34; H, 3.64; N, 8.42. Found: C,
3.47; H, 3.64; N, 8.48.
Ru (4) ](P F (8). This complex was prepared analogous
to 5, alternatively using chloroform/ethanol solvent mixture
2:1 v/v), and was purified by neutral alumina column chro-
matography with 10:1 CH Cl -methanol eluent: yield 24%
0.11 g); H NMR (acetone-d
6
6
H
6 6 2
36CoF12N O P
F IGURE 2. Cyclic voltammograms of compounds 20, 21, and
4
2
2 (bottom) and 23, 24, and 25 (top) and CV of the three G0
[
3
6 2
)
complexes and the three G1 complexes (identical conditions
to those reported in Figure 1).
(
2
2
SCHEME 6
1
(
6
) δ (ppm) 1.50-1.80 (m, 10H),
2
2
2
7
1
.24 (m, 4H), 4.16 (t, 4H), 5.03 (s, 8H), 6.90 (d, 8H), 7.02 (d,
H), 7.13 (d, 8H), 7.10-7.43 (m, 20H), 7.74 (d, 2H), 8.24 (br,
H); 13C NMR (acetone-d
) δ (ppm) 24.7, 27.5, 37.9, 44.7, 69.7,
6
0.0, 111.5, 114.4, 127.7, 127.7, 128.0, 128.4, 128.6, 137.8,
42.0, 152.4, 157.1, 158.7, 166.4. MALDI-TOF MS (matrix,
DHB); m/z calcd, 3416.39 (M - PF
matrix, NBA); m/z calcd, 3416.39 (M - PF
Anal. Calcd for C216 Ru: C, 72.81; H, 5.77; N,
.36. Found: C, 72.60; H, 5.93; N, 2.34.
F e(4) ](P F (9). This complex was prepared analogous
6
); found, 3413.55. FAB-MS
(
6
); found, 3418.4.
H
204 12 6 18 2
F N O P
2
[
3
6 2
)
added Co(NO
3
)
2
‚6H
2
O (0.21 g, 0.72 mmol) in 5 mL of ethanol.
to 6, alternatively using chloroform/ethanol solvent mixture
(2:1 v/v). The purple solid was dissolved in a minimum amount
The reaction mixture was heated to reflux under Ar for 24 h.
The solvent was removed in vacuo. The residue was dissolved
of CH
2 2
Cl and added dropwise while stirring to 150 mL of
2 2 4 6
in CH Cl , and a solution of NH PF (1.18 g, 7.23 mmol) in a
toluene. The purple precipitate was filtered, washed with
ethanol, and dried: yield 39% (0.22 g); H NMR (acetone-d )
6
δ (ppm) 1.59-1.70 (br, 10H), 2.20-2.30 (m, 4H), 4.15-4.20
(br, 4H), 5.03 (s, 8H), 6.90 (d, 8H, J ) 7.2 Hz), 7.05-7.14 (m,
1
minimum amount of methanol was added dropwise while
stirring. The mixture was stirred for 2 h and extracted with
water two times. The combined organic layers were evaporated
J . Org. Chem, Vol. 68, No. 23, 2003 9023

Hong and Gorman
1
0H), 7.25-7.45 (m, 22H), 8.30 (s, 2H); 13C NMR (acetone-d
δ (ppm) 24.6, 27.5, 37.9, 44.7, 69.7, 70.0, 111.4, 114.4, 127.7,
6
)
-78 °C for 1 h, and a solution of G1-Br (18) (3.0 g, 5.82 mmol)
in 10 mL of THF was added while stirring at -78 °C was
continued for additional 2 h. The reaction was quenched with
methanol (2 mL) and added to 20 mL of saturated aqueous
1
1
27.8, 128.0, 128.4, 128.6, 137.8, 142.0, 154.8, 157.1, 160.6,
67.5. FAB-MS (matrix, NBA); m/z calcd, 3370.42 (M - PF );
6
found, 3372.20.
NH
extracted with CH
evaporated under reduced pressure, and the crude product was
purified by column chromatography with CH Cl followed by
10:0.2 CH Cl -ethyl acetate eluents: yield 96% (2.10 g); H
NMR (CDCl ) δ (ppm) 1.20-1.31 (m, 4H), 1.59 (s, 6H), 1.64-
4
Cl. The THF was removed in vacuo, and the residue was
G1-O-CH Bp y (14). To a suspension of KH (0.14 g, 3.5
2
2
2
Cl . The combined organic layers were
mmol) and a catalytic amount of 18-crown-6 in 20 mL of dry
THF at 0 °C was added G1-OH (13 ) (0.8 g, 1.77 mmol) in 10
mL of dry THF. After 1 h, 4,4′-bis-bromomethyl-2,2′-bipyridine
2
2
1
2
2
(12) (0.2 g, 0.58 mmol) in 5 mL of dry THF was added and
3
allowed to warm to room temperature. After 12 h, ethanol was
added slowly to quench the reaction followed by water. The
solution was extracted with ethyl acetate three times, and the
combined organic layers were evaporated to dryness under
reduced pressure. The crude product was purified by column
1.71 (m, 4H), 2.08 (m, 4H), 2.63 (t, 4H J ) 6 Hz), 5.03 (s, 8H),
6.89 (d, 8H, J ) 6.6 Hz), 7.07-7.12 (m, 10H), 7.31-7.46 (m,
20H), 8.24 (s, 2H), 8.53 (d, 2H, J ) 3.9 Hz); 13C NMR (CDCl
)
3
δ (ppm) 24.8, 28.1, 31.3, 35.6, 42.0, 45.2, 70.2, 114.3, 121.5,
124.1, 127.8, 128.1, 128.4, 128.8, 137.4, 142.5, 149.1, 153.1,
chromatography with CH
2
Cl
2
followed by ethyl acetate elu-
ents: yield 32% (0.2 g); H NMR (CDCl ) δ (ppm) 1.40-1.50
m, 4H), 1.50-1.70 (m, 6H), 2.10-2.20 (m, 4H), 3.50 (t, 4H),
72 2 4
156.1, 156.8. Anal. Calcd for C74H N O : C, 84.38; H, 6.89;
N, 2.66. Found: C, 84.49; H, 6.92; N, 2.76.
1
3
(
4
[Ru (10) ](P F ) (20). This complex was prepared analogous
3
6 2
1
.53 (s, 4H), 5.01(s, 8H), 6.87 (d, 8H), 7.13 (d, 8H), 7.28-7.46
to 5: yield 76% (0.63 g); H NMR (acetone-d ) δ (ppm) 2.56 (s,
6
(m, 22H), 8.30 (s, 2H), 8.65 (d, 2H).
6H), 7.38 (d, 2H, J ) 4.2 Hz), 7.83 (d, 2H, J ) 4.2 Hz), 8.67 (s,
2H); 13C NMR (acetone-d
) δ (ppm) 20.5, 125.2, 128.7, 150.2,
n -Octyl Mesyla te (16). To a solution of n-octyl alcohol (3.0
6
g, 23 mmol), triethylamine (9.7 mL, 69 mmol), and a catalytic
amount of 4-(dimethylamino)pyridine in CH Cl at 0 °C was
added methanesulfonyl chloride (5.3 g, 46 mmol). The mixture
was stirred for 1 h and then allowed to warm to room
temperature. After an additional 2 h, water was added to
quench the reaction. The solution was extracted with CH Cl
2 2
two times, and the combined organic layers were evaporated
to dryness under reduced pressure. The crude product was
151.0, 157.1. MALDI-TOF MS (matrix, DHB) m/z calcd, 799.17
(M - PF ); found, 799.58. FAB-MS (matrix, NBA); m/z calcd,
799.17 (M - PF ); found, 799.10.
F e(10) ](P F (21). This complex was prepared analogous
to 6: yield 84% (0.66 g); H NMR (acetone-d
2
2
6
6
[
3
6 2
)
1
6
) δ (ppm) 2.58 (s,
6
2
1
H), 7.39 (d, 2H, J ) 3.9 Hz), 7.51 (d, 2H, J ) 4.2 Hz), 8.69 (s,
13
H); C NMR (CD
2
Cl
2
) δ (ppm) 21.3, 124.7, 128.8, 151.5, 152.9,
);
58.7. FAB-MS (matrix, NBA) m/z calcd, 753.20 (M - PF
6
purified by plug column chromatography with CH
yield 83% (4.25 g); H NMR (CDCl
1
2
Cl
) δ (ppm) 0.90 (t, 3H), 1.20-
.85 (m, 12H), 3.00 (s, 3H), 4.20 (t, 2H). This spectrum matched
2
eluent:
found, 753.30.
Co(10) ](P F
g, 0.81 mmol) in 30 mL of ethanol was added 4,4′-dimethyl-
,2′-bipyridine (10) (0.50 g, 2.3 mmol). The reaction mixture
1
3
[
3
6
)
2
(22). To a solution of Co(NO
3
)
2
2
‚6H O (0.24
5
9
that published previously.
2
6
0,61
n -Octyl Tr ifla te (17).
To a solution of n-octyl alcohol
was refluxed under Ar for 24 h. The solution was then allowed
to cool to room temperature and filtered to remove any insol-
uble impurities. The solvent was removed in vacuo. The resi-
(
3.0 g, 23 mmol), triethylamine (9.7 mL, 69 mmol), and a
catalytic amount of 4-(dimethylamino)pyridine in CH Cl at
°C was added trifluoromethanesulfonic anhydride (13 g, 46
mmol). After 2 h, water was added to quench the reaction. The
solution was extracted with CH Cl two times, and the
2
2
0
2 2
due was dissolved in CH Cl and added dropwise with stirring
to 150 mL of petroleum ether. The yellow precipitate was
filtered and washed with ether and petroleum ether several
times. The yellow precipitate was dissolved in water and ex-
tracted with diethyl ether several times. To the combined aque-
2
2
combined organic layers were evaporated to dryness under
reduced pressure. The crude product was purified by plug
column chromatography with pentane eluent: yield 63% (4
g); H NMR (CDCl
4
ous layers was added a solution of KPF
in a small amount of water. The produced yellow precipitate
was filtered, dissolved in CH Cl , and extracted with water
6
(1.49 g, 8.14 mmol)
1
3
) δ (ppm) 0.90 (t, 3H), 1.30-1.88 (m, 12H),
.54 (t, 2H, J ) 6.6 Hz). This spectrum matched that published
2
2
61
previously.
two times. The combined organic layers were evaporated to
G1-Br (18). To a solution of G1-OMs (3) (1.0 g, 1.88 mmol)
in 50 mL of dry acetone were added NaBr (0.58 g, 5.64 mmol)
and a catalytic amount of tetrabutylammonium bromide. The
mixture was refluxed for 36 h. After the mixture cooled to room
temperature, the solid residue was filtered and the filtrate was
evaporated to dryness under reduced pressure. The residue
was dissolved in CH
The combined organic layers were dried with Na
evaporated to dryness under reduced pressure: yield 93% (0.90
1
dryness under reduced pressure: yield 75% (0.55 g); H NMR
(
9
acetone-d
6
) δ (ppm) 0.36 (s, 6H), 44.77 (s, 2H), 81.55 (s, 2H),
13
1.48 (br, 2H); C NMR failed to show all of the expected
signals, presumably due to fast nuclear relaxation by the
paramagnetic cobalt. FAB-MS (matrix, NBA) m/z calcd, 756.20
(
M - PF
C, 47.96; H, 4.02; N, 9.32. Found: C, 47.94; H, 4.09; N, 9.30.
Ru (19) ](P F (23). To a solution of 19 (0.54 g, 0.51 mmol)
6 6 2
); found, 756.20. Anal. Calcd for C36H36CoF12N P :
2
Cl
2
and extracted with water two times.
2
SO and
4
[
3
6 2
)
in 40 mL of ethylene glycol/DMF (1/2 v/v) was added ruthenium-
trichloride hydrate (40-43% Ru) (0.15 g, 0.72 mmol) in 5 mL
of DMF. The reaction mixture was refluxed for 24 h. The
solution was allowed to cool to room temperature and filtered
to remove any insoluble impurities. The solvent was removed
1
g); H NMR (CDCl
.36 (t, 2H), 5.04 (s, 4H), 6.89 (d, 4H, J ) 8.7 Hz), 7.11 (d, 4H,
J ) 8.7 Hz), 7.30-7.50 (m, 10H). This spectrum matched that
3
) δ (ppm) 1.60-1.80 (m, 5H), 2.10 (br, 2H),
3
6
2
published previously.
G1-Bp y (19). To a solution of potassium tert-butoxide (1 M
2 2
in vacuo. The residue was dissolved in CH Cl and added
in THF, 6.24 mL, 6.24 mmol) and diisopropylamine (0.93 mL,
6
added n-butyllithium (1.6 M in hexane, 3.64 mL, 5.82 mmol).
The mixture was stirred for 1 h at -78 °C, and then, a solution
of 4,4′-dimethyl-2,2′-bipyridine (10) (0.38 g, 2.08 mmol) in 10
mL of THF was added over 1 min. The mixture was stirred at
dropwise with stirring to 150 mL of petroleum ether. The
orange precipitate was filtered and washed with ether and
petroleum ether several times. The orange precipitate was
dissolved in a minimum amount of acetone and added while
.65 mmol) in 20 mL of THF cooled to -78 °C under Ar was
6
stirring to 200 mL of water. A solution of KPF (0.3 g, 1.65
mmol) in a minimum amount of water was then added to
the resulting orange red solution. This produced an orange
precipitate, which was subsequently filtered, dissolved in
CH Cl , and extracted with water two times. The combined
2 2
organic layers were evaporated under reduced pressure, and
the crude product was purified by column chromatography
(59) Chatti, S.; Bortolussi, M.; Loupy, A. Tetrahedron 2001, 57,
4
3
365-4370.
(
(
60) Aubert, C.; Begue, J . P. Synthesis 1985, 759-760.
61) Beard, C. D.; Baum, K.; Grakausk, V. J . Org. Chem. 1973, 38,
673-3677.
62) Wooley, K. L.; Hawker, C. J .; Fr e´ chet, J . M. J . J . Am. Chem.
Soc. 1991, 113, 4252-4261.
with CH
H NMR (CD
2
Cl
2
-ethyl acetate 10:0.2 eluent: yield 34% (0.20 g);
(
1
2
Cl ) δ (ppm) 1.22-1.27(br, 4H), 1.54-1.65 (br,
2
9
024 J . Org. Chem., Vol. 68, No. 23, 2003

Synthetic Approaches to Dendrimers
1
6
2
3
1
0H), 2.08-2.12 (br, 4H), 2.72 (t, 4H, J ) 8.4 Hz), 4.99 (s, 8H),
.85 (d, 8H, J ) 9 Hz), 7.08-7.15 (m, 10H), 7.31-7.46 (m,
may be perhaps due to paramagnetic shifting of certain nuclei
due to the presence of the cobalt as acceptable elemental and
mass spectral analyses were obtained. C NMR failed to show
all of the expected signals, presumably due to fast nuclear
relaxation by the paramagnetic cobalt. FAB-MS (matrix, NBA)
0H), 8.08 (s, 2H); 13C NMR (CD
Cl
) δ (ppm) 25.0, 27.7, 31.0,
13
2
2
5.5, 41.8, 70.1, 114.2, 123.9, 127.8, 128.0, 128.1, 128.4, 128.7,
37.5, 142.5, 150.7, 154.7, 156.6, 156.9. MALDI-TOF MS
(
matrix, DHB) m/z calcd, 3404.52 (M - PF
6
); found, 3410.46.
); found,
Ru: C, 75.09; H,
.13; N, 2.37. Found: C, 74.87; H, 6.23; N, 2.34.
F e(19) ](P F (24). This complex was prepared analogous
to 6, alternatively using chloroform/ethanol solvent mixture
m/z calcd, 3361.55 (M - PF ); found, 3362.60. Anal. Calcd for
6
FAB-MS (matrix, NBA) m/z calcd, 3404.52 (M - PF
3
6
6
C222H216F12CoN O P : C, 75.99; H, 6.20; N, 2.40. Found: C,
6
12
2
216 12 6 12 2
405.70. Anal. Calcd for C222H F N O P
7
5.86; H, 6.31; N, 2.35.
[
3
6 2
)
Ack n ow led gm en t. This work was supported in part
1
by the National Science Foundation (NSF CHE-9900072).
Mass spectra were obtained at the Mass Spectrometry
Laboratory for Biotechnology. Partial funding for the
Facility was obtained from the North Carolina Biotech-
nology Center and the National Science Foundation. We
thank Tyson Chasse for assistance with electrochemical
measurements and Professor David Shultz for insight
on the nature of spin crossover on electron-transfer
kinetics.
(2:1 v/v): yield 77% (0.40 g); H NMR (CD
2 2
Cl ) δ (ppm) 1.22
(
6
1
br, 4H), 1.54-1.65 (m, 10H), 2.10 (m, 4H), 2.73 (t, 4H, J )
.6 Hz), 4.99 (s, 8H), 6.86 (d, 8H, J ) 6.6 Hz), 7.09-7.13 (m,
0H), 7.31-7.41 (m, 22H), 8.11 (s, 2H); 13C NMR (CD
Cl ) δ
2
2
(
ppm) 25.1, 27.8, 31.0, 35.5, 41.9, 45.1, 70.2, 114.2, 127.7, 127.9,
28.1, 128.3, 128.6, 137.4, 142.4, 153.2, 155.6, 156.8, 158.7.
); found,
: C, 76.05; H,
.21; N, 2.40. Found: C, 75.50; H, 6.17; N, 2.34.
Co(19) ](P F (25). This complex was prepared analogous
to 22 alternatively using chloroform/ethanol solvent mixture
2:1 v/v): yield 97% (0.77 g); 1H NMR (CD
Cl ) δ (ppm) is
1
FAB-MS (matrix, NBA) m/z calcd, 3358.55 (M - PF
6
3
6
216 6 12 2
360.30. Anal. Calcd for C222H F12FeN O P
[
3
6 2
)
(
2
2
Su p p or tin g In for m a tion Ava ila ble: Experimental gen-
shown in the Supporting Information. Additional peaks were
always observed in these spectra despite repeated attempts
to purify this compound by silica gel and alumina chromatog-
raphy, reverse phase chromatography, selective precipitation,
and gel permeation chromatography. It is suspected that these
eral considerations and relevant 1H and 13C NMR data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O0351116
J . Org. Chem, Vol. 68, No. 23, 2003 9025