Construction of triangular metallomacrocycles: [M3(1,2-bis(2, 2′:6′,2″-terpyridin-4-yl-ethynyl)benzene)3] [M = Ru(II), Fe(II), 2Ru(II)Fe(II)]

Article abstract of DOI:10.1039/b409348h

Complexation of a predesigned (1,2-bis(2,2′:6′,2″- terpyridin-4-yl-ethynyl)benzene) ligand possessing a 60° angle between two terpyridines with transition metals [Fe(II) and Ru(II)] afforded the self-assembled, triangular metallomacrocycles. The Royal Soc

Full text of DOI:10.1039/b409348h

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COMMUNICATION
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Construction of triangular metallomacrocycles: [M3(1,2-
bis(2,29:69,20-terpyridin-4-yl-ethynyl)benzene)3] [M = Ru(II), Fe(II),
2Ru(II)Fe(II)]{
Seok-Ho Hwang,a Charles N. Moorefield,b Frank R. Fronczek,c Olena Lukoyanova,d Luis Echegoyend and
George R. Newkome*a
Received (in Columbia, MO, USA) 18th June 2004, Accepted 12th October 2004
First published as an Advance Article on the web 16th December 2004
DOI: 10.1039/b409348h
Complexation of a predesigned (1,2-bis(2,29:69,20-terpyridin-4-
yl-ethynyl)benzene) ligand possessing a 60u angle between two
terpyridines with transition metals [Fe(II) and Ru(II)] afforded
the self-assembled, triangular metallomacrocycles.
It has been well documented that the self-assembly of supramo-
lecular species1 is based on a combination of the inherent
Scheme 1 Reagents and conditions: (a) 2-methyl-3-butyn-2-ol, Pd(dba)2,
PPh3, CuI, Et3N; (b) KOH, MeOH/toluene; (c) Pd(PPh3)4, iPr2NH/
structural information, such as the ligand juxtapositioning,
incorporated within the polytopic building blocks. Stang,2
Fujita,3 Atwood,4 and others5–7 have elegantly demonstrated the
structural beauty arising from the application of such positioning.
The often high yields of thermodynamically favored, self-
assembled constructs confirm the methodology but the ligand–
metal bonds must possess some degree of lability in order to
incorporate a self-repairing mechanism so necessary to eliminate
mistakes generated from the initially formed kinetic products;
some degree of reversibility is critical in the early assembly stages.
We were thus surprised to isolate, in many cases, . 90% of
metallohexamers8–10 based on the formation of six tpy–Ru(II)–tpy
(where tpy 5 terpyridine) bonds, especially since this mode of
connectivity is not reversible under normal reaction conditions; in
general, intramolecular reactions were found to be favored over
intermolecular. Rigid cyclic trimetallic complexes with metal–metal
bonds or bridging ligands are known11 and their formation has
been shown to be influenced by several factors, including
concentration,12,13 stoichiometry14 or the presence of a template.13
Metallotriangles utilizing tpy–metal(II)–tpy connectivity are largely
unknown.
toluene.
cross coupling using [(C6H5)3]4Pd(0) in base solvent, to yield (41%)
4, as an air-stable, off-white solid. Vapor diffusion of hexane into a
CHCl3 solution of 4 afforded a single crystal for X-ray analysis.{
The ORTEP representation of 4 (Fig. 1) confirms the desired angle
of directionality (ca. 62u) between the ligands. One terpyridine lies
approximately in the same plane as the benzene moiety; the other
terpyridine is tilted ca. 30u out of this plane and the alkyne
connections are linear.
Reaction of a 1 : 1 mixture of ligand 4 and FeCl2?4H2O in
MeOH for 24 h at 25 uC (Scheme 2) gave the self-assembled
triangular Fe(II) metallomacrocycle 5, which revealed (1H NMR)
two doublets for the benzene moieties identical to that of the
starting ligand at d 5 8.04 and 7.78 supporting the symmetric
structure, in contrast to linear oligomers. Also observed was an
expected upfield shift for the doublet (d 5 7.10, Dd 5 21.48)
arising from the 6,60-tpyHs and downfield shift for the 39,59-tpyH
signals (d 5 9.19, Dd 5 0.54). Other diagnostic spectral attributes
(13C NMR) included the two distinct peaks at d 5 91.5 and 96.3
for the acetylene carbons. COSY and HETCOR spectra of the
bis(terpyridine) 4 and the self-assembled macrocycle 5 further
verified the peak assignments and the coupling patterns. This
Herein, we report the logical extension of this metallomacro-
cycle formation involving the preparation of bis(terpyridine)
monomer 4 whereby the two ligating moieties are rigidly held at
a 60u angle with respect to each other and the use of this monomer
for the construction of triangular metallomacrocycles employing
Fe(II) and Ru(II) as connecting centers.
Construction (Scheme 1) of the desired angular building block,
1,2-bis(2,29:69,20-terpyridin-4-yl-ethynyl)benzene
4 started with
ethynylation of 1,2-dibromobenzene to give diol 1 followed by
deprotection to afford 1,2-diethynylbenzene 2, which was subse-
quently reacted with 2.5 equivalents of 49-trifluoromethanesulfo-
nyl-2,29:69,20-terpyridine15 (4-tpyOTf; 3) via a palladium-catalyzed
{ Electronic supplementary information (ESI) available: NMR, Mass, UV
and elemental analysis data for 4, 5, 6 and 9. See http://www.rsc.org/
suppdata/cc/b4/b409348h/
Fig. 1 ORTEP drawing of bis(terpyridine) ligand 4.
Chem. Commun., 2005, 713–715 | 713
*newkome@uakron.edu;
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bis-Ru(III) adduct 7 with two equiv. of unmetalated bis(terpyr-
idine) 4 and 4-ethylmorpholine (0.2 mL) at 50 uC. Finally, reaction
of resultant oligomer with one equiv. of RuCl2?(DMSO)4 with
refluxing afforded the corresponding metallomacrocycle 6.
Structural supports (1H NMR, 13C NMR, mass spectrometry)
for the metallomacrocycle were obtained giving the same results as
with previous 6 made by the self-assembly method. This procedure
might also permit the specific introduction of different metal
centers (i.e., Os, Fe, Zn) in the periphery. In order to show another
example, reaction of one equiv. of FeCl2?4H2O with the
diamagnetic bis-complex 8 has been conducted to obtain the
desired heteronuclear metallomacrocycle 9. The heteronuclear
architecture of 9 exhibited (1H NMR) signals for the 39,59-tpyH
protons that were split into three kinds of peaks at d 5 9.18 (Fe),
9.05 (Fe/Ru), 9.01 (Ru) ppm in a 1 : 1 : 1 ratio. Also, there were
two doublets at d 5 6.93 (Ru), 6.87 (Fe) ppm in a 2 : 1 ratio for the
5,50-tpyHs position as additional supporting resonance. The mass
spectrum displayed the signals of multiple-charged entities ranging
Scheme 2 Reagents and conditions: (a) i) FeCl2?4H2O, MeOH, ii)
NH4PF6/MeOH; (b) i) RuCl2?(DMSO)4, MeOH, reflux, ii) NH4PF6/
MeOH.
triangular structure was further established by ESI-MS by peaks at
m/z 5 2659.7 [M + 2H 2 PF62]+ (Calcd. m/z 5 2659.27) resulting
from the loss of PF6, which is a known phenomenon.16 While
metallomacrocycle 5, initially isolated as the 6 Cl2 salt, exhibited
solubility in MeOH and hot H2O, its conversion to the 6 PF62 salt
facilitated solubility in MeCN, acetone, DMF, and DMSO.
The self-assembled Ru(II) counterpart 6 was prepared by
treatment of MeOH solution of monomer 4 with one equiv. of
RuCl2?(DMSO)4 over 36 h at 50 uC. The initial desired trimer was
obtained in approximately 70% yield, but after column chromato-
graphy followed by counterion exchange (Cl2 to PF62), pure
macrocycle 6 was isolated (ca. 30%). The complete absence (1H
NMR) of extraneous peaks excluded the presence of starting
materials, intermediates, and oligomeric materials; the diagnostic
shifts for the doublets (d 5 7.35, Dd 5 21.23) of 6,60-tpyHs, and
the 39,59-tpyHs (d 5 9.01, Dd 5 0.36), along with definitive ESI-
MS data (m/z 5 2796.4 [M + H 2 PF62]+), all support the
structural assignment.
from m/z 5 1302.1 [M 2 2PF6 ]
2 2+ to 337.3 [M 2 6PF62]6+charge
states.
Absorption spectra recorded for ethynyl-substituted complexes
in dilute CH3CN solution exhibit the expected absorption
transitions; in the case of 5, an intense ligand-centered p–p*
transition of terpyridine moieties is clearly apparent at lmax
5
283 nm (e 5 1.27 6 105) and 326 nm (e 5 1.26 6 105) while the
metal–ligand charge-transfer (MLCT) transition, which is
described as the promotion of an electron from the metal-centered
d-orbitals to an unfilled ligand-centered p* orbital17 has a
maximum (lmax) around 578 nm with molar absorption
coefficients at the peak of ca. 6.43 6 104 dm3 mol21 cm21 per
Fe(II)-triangle. Similarly, triangle 6 exhibited two ligand-centered
p–p* transitions of terpyridine moieties and an MLCT at
lmax 5 275 nm (e 5 7.28 6 104), 316 nm (e 5 8.01 6 104),
and 496 nm (e 5 4.23 6 104), respectively. The compound 9
exhibited two metal–ligand charge transfer (MLCT) bands
attributed to the Ru(II)- and Fe(II)-complexes at lmax 5 499 nm
(e 5 5.86 6 104), 582 nm (e 5 2.38 6 104), respectively. This
observation of its UV absorption spectrum may serve the
proposed structure of heteronuclear metallomacrocycle 9 as
further supporting information.
A stepwise approach (Scheme 3) was also undertaken, in which
the diamagnetic [Ru2(4)2][4Cl2], 8, was prepared by treatment of
The cyclic voltammogram (CV) of a 0.11 mM solution of the
Ru(II) triangle 6 in MeCN is shown in Fig. 2(b). Two terpyridine
ligand-centered reductions and one oxidation of the ruthenium
Scheme 3 Reagents and conditions: (a) RuCl3?3H2O, MeOH, reflux; (b) 2
equiv. 4, N-ethylmorpholine, MeOH, reflux; (c) i) RuCl2?(DMSO)4 or
FeCl2?4H2O, MeOH, reflux, ii) NH4PF6/MeOH.
Fig. 2 CV responses of 1 mM solutions of (a) Ru(II)–terpyridine
monocomplex 11; (b) triangular Ru(II) metallomacrocycle 6.
714 | Chem. Commun., 2005, 713–715
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Table 1 Redox potentials (V vs. Fc/Fc+) of triangular Ru(II)
metallomacrocycle 6 and model compound 11 in 0.1 M TBAPF6–
MeCN at glassy carbon electrode; scan rate 5 100 mV s21
coordination-directed triangle gave entry into shape-persistent,
densely packed architecture. Also, the stepwise contruction permits
specific introduction of different metal centers. Therefore, the
reversible redox characteristics and the heterogeneous metal core
structure suggest that they are ideal candidates for energy storage
devices and nanomachinery.
Complex
Ered (I)
Ered (II)
Eox (I)
6
Epc
Epa
E1/2
Epc
Epa
E1/2
21.46
21.39
21.43
21.55
21.49
21.52
21.72
21.61
—
21.80
21.65
—
0.92
0.98
0.95
0.89
0.95
0.92
The authors are grateful to the National Science
Foundation (DMR-0196231, DMR-0401780, CHE-0116041 and
CHE-0420987), the Korean Research Foundation [KRF-2003-
042-C00069 (GRN)], the Louisiana Board of Regents [LEQSF-
ENH-TR-13 (FRF)], the Air Force Office of Scientific Research
[F49620-02-1-0428,02 (GRN)], and the Ohio Board of Regents for
financial support.
11
center were observed and the corresponding potentials are listed in
Table 1. The first redox couple at 21.43 V with a peak-to-peak
separation (DEp) of 70 mV is reversible; whereas, the second redox
couple, exhibiting a very sharp oxidative peak at 21.61 V, is not.
This oxidative peak indicates adsorption of the reduction product
on the electrode surface. The oxidative couple at 0.95 V is
reversible with DEp of 40 mV and its current intensity is
Seok-Ho Hwang,a Charles N. Moorefield,b Frank R. Fronczek,c
Olena Lukoyanova,d Luis Echegoyend and George R. Newkome*a
aDepartment of Polymer Science, The University of Akron, Akron,
OH 44325, USA. E-mail: newkome@uakron.edu; www.dendrimers.com
bMaurice Morton Institute of Polymer Science, The University of Akron,
Akron, OH 44325, USA
comparable to the first reduction.
2
]
For comparison, the CV of model compound [Ru(tpy)2][2PF6
cDepartment of Chemistry, Louisiana State University, Baton Rouge,
LA 70803-1804, USA
(Scheme 4) exhibits similar behaviour to the CV of 6. From an
analysis of the CV of the first reduction peak intensities of 11
(Ipmono) and triangle 6 (Iptri), the peak current ratio (Ipmono : Iptri 5
1 : 1.4) was observed, taking into consideration the expected
difference in diffusion coefficients. This result closely agrees with
the expected theoretical ratio of 1 : 1.5, which suggests that in each
compound, all metal centers undergo reduction simultaneously
and the macromolecule contains the desired three Ru centers. The
presence of a single oxidation potential for the Ru(II/III) couple
within the solvent window at all scan rates suggests that three
ruthenium centers in the macrocycle are oxidized at the same
potential, as expected for non-interacting centers in identical
environments.
dDepartment of Chemistry, Clemson University, Clemson,
SC 29634-0973, USA
Notes and references
{ Crystal data for 4: C40H24N6, M 5 588.65, triclinic, space group P1,
a 5 10.8137(10), b 5 12.4510(10), c 5 13.061(2) s, a 5 68.247(5),
b 5 75.799(5), c 5 66.262(7), V 5 1485.6(3) s3, T 5 102 K, Z 5 2, m(Mo–
Ka) 5 0.080 mm21, 10773 independent reflections, Rint 5 0.038, R1 5 0.054,
wR2 5 0.141 (all data). CCDC 252799. See http://www.rsc.org/suppdata/cc/
b4/b409348h/ for crystallographic data in .cif or other electronic format.
1 J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
VCH, Weinheim, 1995.
2 S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100, 853.
3 M. Fujita and K. Ogura, Coord. Chem. Rev., 1996, 148, 249.
4 L. R. MacGillivray and J. L. Atwood, Angew. Chem., Int. Ed., 1999, 38,
1018.
5 B. J. Holliday and C. A. Mirkin, Angew. Chem., Int. Ed., 2001, 40, 2022.
6 S. J. Lee, A. Hu and W. Lin, J. Am. Chem. Soc., 2002, 124, 12948.
7 H. Jiang and W. Lin, J. Am. Chem. Soc., 2003, 125, 8084.
8 G. R. Newkome, T. J. Cho, C. N. Moorefield, G. R. Baker,
M. J. Saunders, R. Cush and P. S. Russo, Angew. Chem., Int. Ed.,
1999, 38, 3717.
Scheme 4 Model compound of triangular metallomacrocycle 6.
Reagents and conditions: (a) i) RuCl2?(DMSO)4, MeOH, reflux, ii)
NH4PF6/MeOH.
9 G. R. Newkome, T. J. Cho, C. N. Moorefield, R. Cush, P. S. Russo,
L. A. God´ınez and M. J. Saunders, Chem. Eur. J., 2002, 8, 2946.
10 G. R. Newkome, T. J. Cho, C. N. Moorefield, P. P. Mohapatra and
L. A. God´ınez, Chem. Eur. J., 2004, 10, 1493.
11 R.-D. Schnebeck, L. Randaccio, E. Zangrando and B. Lippert, Angew.
Chem., Int. Ed., 1998, 37, 119.
12 M. Schweiger, S. R. Seidel, A. M. Arif and P. J. Stang, Inorg. Chem.,
2002, 41, 2556.
13 S. B. Lee, S. Hwang, D. S. Chung, H. Yun and J.-I. Hong, Tetrahedron
Lett., 1998, 39, 873.
14 R.-D. Schnebeck, E. Freisinger, F. Glahe and B. Lippert, J. Am. Chem.
Soc., 2000, 122, 1381.
These metallomacrocycles possess potential internal steric
interactions that can be observed in the CPK representations.
Variable temperature 1H NMR spectra of [Fe3(4)3][6PF62] support
this observation in that peaks begin to broaden and shift as the
temperature is decreased and bis(terpyridine) complex rotation is
slowed.
15 K. T. Potts and D. Konwar, J. Org. Chem., 1991, 56, 4815.
16 E. C. Constable, C. E. Housecroft, M. Neuburger, A. G. Schneider and
M. Zehnder, J. Chem. Soc., Dalton Trans., 1997, 2427.
17 M. Beley, J.-P. Collin, J.-P. Sauvage, F. Heisel and A. Miech, J. Chem.
Soc., Dalton Trans., 1991, 3157.
In conclusion, we have demonstrated the formation and
characterization of a unique, self-assembled, triangle metalloma-
crocycle by using tpy–metal(II)–tpy connectivity, which is
stable and irreversible under the reaction conditions. This
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 713–715 | 715