observation is in perfect agreement with previous reports13 and
has been attributed to the slower electron transfer kinetics that
characterizes molecules featuring bulkier dendritic structures
around their electroactive sites.
4
In summary, we have constructed [ < Ru > ] dendrimers
possessing an overall neutral charge. The loss of external
counterions in these metallomacromolecules had a marginal
effect on their spectra, as well as stability and physical
properties. However, the solubilities of these neutral species
decreased in polar solvents, such as MeOH and H
plexes 19–22 are all slightly soluble in H O, whereas, 1 and 2
are insoluble in H O. The neutral metallodendrimers have
internally balanced carboxylates, which are weaker counter ions
2
O. Com-
2
Fig. 1 CV responses for 1.0 m
0, and (f) 1 (the smaller current observed was due to the limited solubility
of 1 that presented in DMF) in Et NBF (0.1 ) in DMF at 25 °C; scan rate
M
solutions of (a) 21, (b) 22, (c) 2, (d) 19, (e)
2
2
4
4
M
than most others like Cl , NO
2
2
and PF . Thus, addition of
2
3
6
00 mV s2
.
1
2
inorganic salts to these neutral species allows convenient
interchange between external counter ions. Notably, 1 and 2 are
not soluble in DMF, but quickly go into solution upon addition
dialysis, the desired constructs were supported by the sig-
13
nificant downfield shift ( C NMR) of all terpyridine carbons.
2
2
6
or PF . Another important aspect is that
of salts like BF
these neutral metallomacromolecules were more easily ionized
i.e. lower laser power) and gave stronger signals in MALDI-
TOF mass spectrometry in comparison to the tert-butyl and acid
4
The tert-butyl groups were removed from 19 or 21 by treatment
9
with formic acid, affording complexes 20 (98%) or 22 (97%),
(
respectively. The characteristic downfield shift of 13C NMR
(
Dd = 4 ppm) of the carbonyl carbon supports the transforma-
2
2
metallodendrimers possessing external Cl or PF
6
counter
tion; the retention of the external alkyl ester moieties was
evidenced by the presence of the ethyl signals. Final neutral
complexes 1 (92%) or 2 (97%) were formed by addition of a
ions. The observed intramolecular proton transfer during the
redox process stands to give insight into the potential chemistry
within such macromolecular constructs.
We gratefully acknowledge support of this work from the
National Science Foundation (DMR-96-22609; BIR-95-12208)
and the Army Research Office (DAAH04-95-1-0373;
DAAH04-96-1-0306).
2
slight excess of KOH into a H O–MeOH solution of 20 or 22.
After dialysis, the desired neutral complexes 1 and 2 were
analyzed (0.00% of Cl), and only one downfield shift (Dd = 3.4
ppm for 1 and 4.2 ppm for 2) for the internal acid carbonyl
carbon supports the formation of the carboxylate carbon
centers. The UV–VIS spectra of the macromolecules possessing
the four [ < Ru > ] linkages are analogous to related systems.5
All the intermediate molecules and final complexes exhibit
correct molecular weights. MALDI-TOF mass spectra of 19 and
,10
Notes and references
†
All new compounds exhibited satisfactory spectral, elemental and mass
data.
240 36 4
Selected data for 1: C233H N O40Ru ; M = 4589.0; mp > 230 °C
2
1 exhibited low sensitivity and broad signals with all attempted
matrices.
Electrochemical experiments with these metallodendrimers
‡
(decomp.); Found: C, 60.61; H, 5.45; N, 10.23; Cl, 0.00; requires: C, 60.98;
gave further insight to their electrocatalytic potential. Fig. 1(a)
H, 5.27; N, 10.99; Cl, 0.00%; m/z (MALDI-TOF, IAA matrix) 4590 [M +
H ].
+
shows the two reversible waves that characterize the cathodic
11
For 2: C313H372N40O80Ru ; M = 6378.9; mp > 210 °C (decomp.);
CV response of the two terpyridine ligands of 21 . After
internal deprotection of the tert-butyl groups, the presence of
the carboxylic acid moieties in 22 results in a merging of the two
redox waves and the virtual disappearance of the corresponding
anodic signal [Fig. 1(b)]. Based on previous studies of the
electrochemical reduction of pyridine and its derivatives,12 the
observed irreversibility is due to an electrochemical–chemical
reaction in which a proton from the vicinal carboxylic acid
group quenches the aromatic anion radical resulting in a
probable 1,4-reduction of one of the pyridine rings of each
terpyridine ligand. This explanation is further supported by CV
experiments with neutral dendrimer 2; as seen in Fig. 1(c), the
lack of neighboring acidic protons, readily available in 22,
results in the recovery of the typical ‘two wave’ reversible
response of the terpyridine ligands.
4
Found: C, 58.49; H, 5.86; N, 8.71; Cl, 0.00; requires: C, 58.94; H, 5.88; N,
+
8
.78; Cl, 0.00%; m/z (MALDI-TOF, IAA matrix) 6381 [M + H ].
1
For recent reviews see: G. R. Newkome, C. N. Moorefield and F.
Vögtle, Dendritic Molecules: Concepts, Syntheses, Perspectives, VCH,
Weinheim, 1996; V. Balzani, S. Campagna, G. Denti, A. Juris, S.
Serroni and M. Venturi, Acc. Chem. Res., 1998, 31, 26; H.-F. Chow,
T. K. K. Mong, M. F. Nongrum and C.-W. Wan, Tetrahedron, 1998, 54,
8543; V. V. Narayanan and G. R. Newkome, Top. Curr. Chem., 1998,
19; P. L. Boulas, M. Gómez-Kaifer and L. Echegoyen, Angew. Chem.,
Int. Ed., 1998, 37, 216; C. Gorman, Adv. Mater., 1998, 10, 295; A.
Matthews, A. N. Shipway and J. F. Stoddart, Prog. Polym. Sci., 1998,
2
3, 1; E. C. Constable, Chem. Commun., 1997, 1973.
G. R. Newkome and C. D. Weis, Org. Prep. Proced. Int., 1996, 28,
85.
2
4
3
4
K. Landsteiner and J. Van Der Scheer, J. Exp. Med., 1938, 67, 709.
J. Klausner and B. Bodansky, Synthesis, 1972, 453.
CV experiments on the first tier dendrimer series 19, 20 and
1
showed similar voltammetric responses to those discussed
above. By careful inspection of Table 1, however, the second
values than
5 G. R. Newkome and E. He, J. Mater. Chem., 1997, 7, 1237.
6 S. Ram and R. E. Ehrenkaufer, Tetrahedron Lett., 1984, 25, 3415.
7 E. C. Constable and M. D. Ward, J. Chem. Soc., Dalton Trans., 1996,
tier assemblies 21 and 2 show slightly larger DE
p
2
8, 485.
those corresponding to the first tier series 19 and 1. This
8
9
G. R. Newkome and X. Lin, Macromolecules, 1991, 24, 1443.
S. Chandrasekaran, A. F. Kluge and J. A. Edwards, J. Org. Chem., 1977,
Table 1 CV data for 1, 2, 19–22
4
2, 3972.
1
0 G. R. Newkome, E. He and L. A. Godínez, Macromolecules, 1998, 31,
4382.
Terpyridines
RuIII/RuII
E
/V)a
/V)a
/V)a
1/2/V (DE
p
11 G. R. Newkome, R. Güther, C. N. Moorefield, F. Cardullo, L.
Echegoyen, E. Pérez-Cordero and H. Luftmann, Angew. Chem., Int. Ed.
Engl., 1995, 34, 2023.
12 A. P. Tomilov, S. G. Mairanovskii, M. Y. Fioshin and V. A. Smirnov,
Electrochemistry of Organic Compounds, Wiley, NY, 1972.
13 P. J. Dandlilker, F. Diederich, J.-P. Gisselbrecht, A. Louati and M.
Gross, Angew. Chem., Int. Ed. Engl., 1995, 34, 2725; C. B. Gorman,
B. L. Parkhurst, W. Y. Su and K.-Y. Chen, J. Am. Chem. Soc., 1997,
119, 1141.
Compound
E
1/2/V (DE
p
E
1/2/V (DE
p
1
2
9
0
1
1
2
2
21.945 (0.082)
21.759 (0.068)
0.627 (0.073)
0.629 (0.057)
0.624 (0.059)
0.640 (0.100)
0.627 (0.097)
0.634 (0.089)
b
b
I
I
21.950 (0.072)
21.788 (0.077)
21.751 (0.088)
2
2
21.950 (0.086)
c
c
I
I
21.935 (0.102)
21.767 (0.106)
a
E/V vs. Fe/Fe . Same conditions as those described in Fig. 1. b Irreversible
+
cathodic peak at 21.945 V. c Irreversible cathodic peak at 21.968 V.
Communication 8/06509H
28
Chem. Commun., 1999, 27–28