Spirometallodendrimers: Terpyridine-based intramacromolecular cyclization upon complexation

Article abstract of DOI:10.1039/b204512e

The first examples of metallodendritic spiranes have been obtained via incorporation of single terpyridine units within each dendritic quadrant.

Full text of DOI:10.1039/b204512e

Spirometallodendrimers: terpyridine-based intramacromolecular
cyclization upon complexation
George R. Newkome,* Kyung Soo Yoo and Charles N. Moorefield
Departments of Chemistry and Polymer Science, The University of Akron, Akron, Ohio 44325, USA.
E-mail: newkome@uakron.edu; Tel: +330-972-6458
Received (in Purdue, IN, USA) 10th May 2002, Accepted 2nd August 2002
First published as an Advance Article on the web 27th August 2002
The first examples of metallodendritic spiranes have been
obtained via incorporation of single terpyridine units within
each dendritic quadrant.
identified by the presence of a new methyl peak at 51.5 ppm, as
well as the complete disappearance of tert-butyl markers.
Reduction of the nitro moiety (Raney-Ni, EtOH, 120 psi, 40 °C,
48 h) gave the desired amine 8c, which is supported (13C NMR)
by the chemical shift for C4° from 92.9 to 53.6 ppm, confirming
the desired C4°NO2 to C4°NH2 transformation and the molecular
peak (MALDI-TOF9) at m/z 1386.27 [M + Na+] (calcd m/z
1386.54 [M + Na+]).
Dendrimers are of special interest in macromolecular chemistry
due to interesting utilitarian properties resulting from their well-
defined parameters.1 In particular, the highly branched metal-
lodendrimers2 have been investigated for their magnetic,
electronic, photooptical, and catalytic properties.3 Herein, we
report the construction of new dendrons containing 2,2A:6A,2B-
terpyridine units, their assembly into dendrimers, and sub-
sequent intramolecular metallocyclization resulting in the
corresponding bis-terpyridine Ru(II) complexes {–(Ru)–}. Sim-
ilar architectures have been crafted using transition metal
coordination at the spirane junction for investigation as
molecular motors4 and muscles.5
First and 2nd generation dendrimers were accessed via
treatment of dendrons 3 and 8c, respectively, with 6,6-bis(car-
boxy-2-oxabutyl)-4,8-dioxaundecane-1,11-dicarboxylic acid10
(DCC, DMF, 25 °C, 72 h). Their spectra (13C NMR) exhibited
the expected downfield shift [52.3 (3) and 53.5 ppm (8c) to 57.0
(G1) and 56.9 ppm (G2)] for the signals assigned to the new
C4°NHCO corroborating amidation and the peaks (MALDI-
TOF) at m/z 3077.14 [M + Na+] (calcd 3077.72 [M + Na+]) for
G1 and m/z 5829.17 [M + Na+] (calcd 5829.51 [M + Na+]) for
G2.
Treatment of each generation with two equivalents of
RuCl3·3H2O (EtOH, N-ethylmorpholine) followed by addition
of excess NH4PF6 afforded, after chromatography, the bis-
Ru(II) spirometallodendrimers 1a (41%) and 2 (47%), re-
spectively (Fig. 1). The absence of any free terpyridine moiety,
the observed downfield shift of all terpyridine carbons as well as
Synthesis of the 1st generation dendron 3 was acheived by
treatment of 1-amino-4A-O-terpyridinylpentane6 with 1 equiva-
lent of acryloyl chloride (Et3N, THF) to give the corresponding
acrylamido analogue, which after Michael addition with
CH3NO2 (Triton B) afforded the nitro-terpyridine as confirmed
(13C NMR) by the appearance of a new peak for primary CNO2
at 74.5 ppm. Treatment of this precursor with a slight excess of
tert-butyl acrylate (Triton B, CHCl3, 25 °C, 24 h) gave the nitro
functionalized dendron, which was supported by the appearance
of the chemical shift for CNO2 at 92.3 ppm, followed by
reduction with Raney-Ni (EtOH, 40 °C) to afford the corre-
sponding desired amine 3. Characterization included an upfield
chemical shift (13C NMR) for the C4° from 92.3 to 52.3 ppm; the
molecular peak m/z 677.82 [M + H+] (calcd m/z 677.87 [M +
H+]) in ESI-MS further supported the assignment.
The larger dendron 8 (Scheme 1) was obtained starting with
the addition of benzyl acrylate to CH3NO2 [di(isopropyl)ethyla-
mine] to afford the initial benzyl g-nitrobutanoate, which was
then treated with tert-butyl acrylate (Triton B, THF, 25 °C, 16
h) to afford the 2+1 functionally differentiated triester 4. The
structure was confirmed by the appearance of a chemical shift
(13C NMR) for the C4° at 91.9 ppm and the molecular ion peak
at m/z 480.43 [M + H+] (calcd m/z 480.58 [M + H+], ESI-MS).
Hydrolysis of tert-butyl groups (HCO2H, 25°C, 24 h) afforded
the corresponding diacid 5 supported by disappearance (13C
NMR) of tert-butyl peaks and appearance of a signal for the acid
carbonyl group (CNO) at 173.3 ppm. Treatment of 5 with
Behera’s amine7,8 (DCC, DMF, 25 °C, 24 h) gave the bisamide
6 supported by the downfield shift (13C NMR) of the signal at
52.9 to 57.4 ppm attributed to the C4°NHCO moiety. Monoacid
7 was then obtained by hydrogenolysis (Pd/C, MeOH, 25 °C, 24
h) and was confirmed (13C NMR) by the disappearance of
benzyl absorption and the formation of a new peak at 174.0 ppm
corresponding to the new CO2H group and the molecular peak
at m/z 1073.35 [M + H+] (calcd m/z 1073.33 [M + H+]) in ESI-
MS.
Reaction of hexaester 7 with amine 3 (DCC, DMF, 25 °C, 24
h) gave the nitro tert-butyl ester 8a as confirmed by the
formation of a new peak at 170.5 ppm for the new amide carbon.
Failure to reduce (Raney-Ni, EtOH) the nitro moiety in 8a
prompted its transesterification (cat. H2SO4, MeOH, 60 °C, 24
h). The corresponding methyl ester 8b was subsequently
Scheme 1
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Fig. 1 Bis-Ru(II) spirometallodendrimers 1a and 2.
Additional supporting evidence of the facile intramolecular
cyclization was obtained by reacting the 1st tier tetraterpyridyl
dendrimer with two equivalents of FeCl2·4H2O to give (92%)
the pure, corresponding Fe-spirodendrimer 1b (MALDI-TOF
m/z 3301.2 for [M + 4Cl]; calcd 3301.4). The higher yield
presumably results from not having to reduce the Fe(II) rather
than the complexation, reduction, and cyclization for Ru(III).
In summary, these examples of macromolecular ring closures
are anticipated to provide entrance to dendrimer-based molec-
ular devices via the incorporation of differing metals, ligands,
and metal oxidation states.
Fig. 2 CV response of 0.1 mM solutions of the spirometallodendrimers. (a)
1a and (b) 2 (0.1 M Et4NTFB, DMSO, 25 °C; scan rate 200 mV s21).
We greatfully acknowledge financial support from the Office
of Naval Research (N000-14-99-1-0082) and the National
Science Foundation (DMR-9901393).
Notes and references
1 G. R. Newkome, C. N. Moorefield and F. Vögtle, Dendrimers and
Dendrons: Concepts, Syntheses, Applications, Wiley-VCH, Weinheim,
Germany, 2001.
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1689; F. J. Stoddart and T. Welton, Polyhedron, 1999, 18, 3575; M.
Venturi, A. Credi and V. Balzani, Coord. Chem. Rev., 1999, 185–186,
233; G. E. Oosterom, J. N. H. Reek and P. C. J. Kamer, Angew. Chem.,
Int. Ed., 2001, 40, 1828; V. Balzani, P. Ceroni, A. Juris, M. Venturi, S.
Campagna, F. Puntoriero and S. Serroni, Coord. Chem. Rev., 2001,
219–221, 545; D. Astruc, J.-C. Blais, E. Cloutet, L. Djakovitch, S.
Rigaut, J. Ruiz, V. Sartor and C. Valério, Top. Curr. Chem., 2000, 210,
229; E. C. Constable, Chem. Commun., 1997, 1073.
3 D. Astruc and F. Chardac, Chem. Rev., 2001, 101, 2991.
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5 J.-P. Collin, C. Dietrich-Buchecker, P. Gaviña, M. C. Jimenez-Molero
and J.-P. Sauvage, Acc. Chem. Res., 2001, 34, 477.
6 G. R. Newkome, E. He, L. A. Godínez and G. R. Baker, J. Am. Chem.
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9 The presence of Na+ was facilitated by the addition of F3CCO2Na in the
MALDI-TOF matrix.
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Fig. 3 Space-filling representation of the minimized structure of the
spirometallodendrimer 1.
the assignable, symmetric 13C NMR spectra and MALDI-TOF
mass spectra (molecular peaks at m/z 3691 [M 2 PF6]+ for 1a
and m/z 3186 [M 2 2PF6]2+ for 2) supported the assigned intra-
molecular-based structures. Four major absorption bands (lmax
;
242, 267, 306, 486 nm) were observed for these [- < Ru > -]
connected metallodendrimers; the molar absorptivities (e) of the
complexes 1a and 2 have similar values and are indicative of
two molecular assembles possessing the same number of
[- < Ru > -] moieties. Notably, the molar absorptivities e = 3.55
3 104 and 3.45 3 104 dm3 mol21 cm21 for 1a and 2 (lmax
=
486 nm) are ca. two times as strong as Constable’s mono Ru(II
)
complex.11
The cyclic voltammograms of both 1a (a) and 2 (b) (Fig. 2)
were similar and exhibited two quasi-reversible waves at
negative potentials corresponding to redox processes on two
electroactive terpyridine groups.12
Molecular modelling (Fig. 3) of the spirodendrimer 1
revealed an open structure. The metal to metal distance is
estimated to be 25.76 Å and center carbon to metal distance to
be 13.38 Å. The overall longest distance is about 34.5 Å.
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