From an eight-component self-sorting algorithm to a trisheterometallic scalene triangle

Article abstract of DOI:10.1021/ja108419k

Using motifs from 3-fold completive self-sorting in an eight-component library, we report on the design and fabrication of a fully dynamic trisheterometallic scalene triangle, a demanding supramolecular structure that complements the so far known triangul

Full text of DOI:10.1021/ja108419k

Published on Web 10/25/2010
From an Eight-Component Self-Sorting Algorithm to a Trisheterometallic
Scalene Triangle
Kingsuk Mahata, Manik Lal Saha, and Michael Schmittel*
Center of Micro and Nanochemistry and Engineering, Organische Chemie I, UniVersita¨t Siegen,
Adolf-Reichwein-Str. 2, D-57068 Siegen, Germany
Received September 17, 2010; E-mail: schmittel@chemie.uni-siegen.de
Scheme 2. Synthesis of the Supramolecular Scalene Triangles Tn
Abstract: Using motifs from 3-fold completive self-sorting in an
eight-component library, we report on the design and fabrication
of a fully dynamic trisheterometallic scalene triangle, a demanding
supramolecular structure that complements the so far known
triangular structures.
To fit with the trend of evolution (“EVolutionary processes are
anamorphic, or complexity-generating”),1 artificial self-assembly will
need to master vastly enhanced complexity and diversity issues, for
example by enlarging the number of different components and
interactions.2 While biological self-assembly follows intricate orthogo-
nal self-sorting algorithms,3-5 artificial self-sorting is still limited in
orthogonality. Indeed, most of the known self-sorting processes entail
either multiple species or a single assembly along with excess
ligand(s).3-5 In contrast, we seek to merge all members of a library
in a single multicomponent aggregate employing completiVe5 and
integratiVe4 self-sorting. As an example we elaborated the self-
assembly of a dynamic bimetallic trapezoid from a six-component
A1A2D1D2D3D4 (A ) acceptor, D ) donor) library.5 Extending the
above conceptual insights to self-sorting in an eight-component
A1A2A3D1D2D3DD4D5 library (Scheme 1), we report herein on the
fabrication of a trisheterometallic scalene triangle, a demanding and
until now unrealized supramolecular structure (Scheme 2) that comple-
ments the so far known triangles (monometallic and equilateral,6
bisheterometallic,7,8 and isosceles9,10).
Self-sorting in metallosupramolecular structures is managed by
various factors, such as steric and electronic effects, π-π interac-
tions, and metal-ion coordination specifics. For the self-sorting
eight-component library depicted in Scheme 1, we chose to blend
our previous A1A2D1D2D3D4 library5 with the pyridine-zinc
porphyrin binding motif.11 To our delight, full orthogonality of the
pyridine-zinc porphyrin binding motif with the two other individual
heteroleptic metal-ligand combinations was established. Only 3
out of 35 plausible combinations, i.e. [Zn(1)(2)]2+, [Cu(4)(5)]+, and
[(3)(6)], were afforded while making full use of all library members
in a 3-fold completiVe self-sorting (readily derived from H NMR
analysis; see Supporting Information). Obviously, none of the
available bi- or tridentate ligands can amalgamate readily with the
zinc porphyrin 6 due to steric bulk, all the more as this would lead
to uncoordinated nitrogen ligands in the overall mixture, which
would violate the maximum site occupancy rule.3a
1
With the complexes [Zn(1)(2)]2+, [Cu(4)(5)]+, and [(3)(6)] repre-
senting the three corners of a scalene triangle, all tools are available
for designing the three different sides along with their binding units
(Scheme 2). Thus, we instated 1 and 6 as the termini of the
phenanthroline-porphyrin hybrid 7 being readily accessible Via
Sonogashira cross-coupling (Supporting Information). Along a known
procedure,5 2 and 4 were merged in the terpyridine-phenanthroline
ligand 8, while the complexation properties of 3 and 5 were
amalgamated within the phenanthroline-pyridine hybrid 9. Details of
the synthesis are contained in the Supporting Information. For all
ligands, spacers were chosen to render the building blocks 7-9 unequal
in length.
Scheme 1. Self-Sorting in a A1A2A3D1D2D3D4D5 Library
In a first set of experiments all components (7, 8, 9, Zn2+, and
Cu+) were mixed in 1:1:1:1:1 ratio and refluxed for 2 h in
acetonitrile/DCM (2:1). After obtaining a clear dark-violet solution
the reaction mixture was characterized by electrospray ionization
mass spectroscopy (ESI-MS), 1H NMR, COSY, diffusion-ordered
spectroscopy (DOSY), elemental analysis, and differential pulse
voltammetry (DPV). ESI mass spectra (Figure 1) showed only peaks
that were in full agreement with T1 ) [CuZn(7)(8)(9)](OTf)2(PF6).
Most importantly, the full integrity of the scalene triangle T1 was
unambiguously proven by intense signals at m/z ) 894.9, associated
with [CuZn(7)(8)(9)]3+ and at m/z ) 1415.5, associated with
[CuZn(7)(8)(9)](PF6)2+
.
Data from 1H NMR and DOSY NMR (Supporting Information)
further supported the structural assignment of T1. A single diffusion
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J. AM. CHEM. SOC. 2010, 132, 15933–15935 15933

C O M M U N I C A T I O N S
Figure 1. ESI-MS of the scalene triangle T1. Inset: Experimental (black
line) and calculated (red line) isotopic distribution of [CuZn(7)(8)(9)]3+
.
coefficient as well as a single set of signals provided evidence for
the clean formation of the scalene triangle T1 in solution. In order
to evaluate the connectivity of the ligands in T1 we paid special
attention to several characteristic proton resonances. For example,
the pyridine protons (R, ꢀ) of 9 in T1 experienced a diagnostic
upfield shift from 8.61 to 2.62 ppm and from 7.38 to 5.66 ppm in
the 1H NMR, respectively, a typical shift of pyridine protons upon
axial coordination to a zinc porphyrin.11 Thus, following our design,
ligand 7 is indeed connected to 9 by a zinc porphyrin-pyridine
interaction (Scheme 2). Diagnostically shifted b-H, b′-H protons
of 7 yielded further information regarding the connectivity in T1.
The ca. 0.60 ppm upfield shifts of b-H and b′-H protons (from 7.40
and 7.50 ppm in 7 to 6.78 and 6.95 ppm in T1) are indicative of a
[Zn(7phenAr2)(8terpy)]2+ complex.5,12
Figure 2. Energy minimized structure of the scalene triangle T1. Coun-
teranions are not included.
As all attempts to obtain a crystal structure of Tn were met with
failure, MM+ force field computations and molecular dynamics on
Tn provided some insight about their structure as scalene triangles.
Taking the metal-metal distance as a measure, the three metal
corners of T1 are separated by 1.44, 1.60, and 1.84 nm in the energy
minimized structure (Figure 2) and by 1.44, 1.66, and 1.84 nm in
T2 (Supporting Information), nicely illustrating the geometrical
scalene arrangement of Tn.
In conclusion, we report on the fabrication of two scalene
triangles Tn that were designed along the eight components of a
3-fold completiVe self-sorting library. The triangles are scalene from
both a geometrical and a compositional point of view. Precise tuning
of steric and electronic effects, π-π interactions, and metal-ion
specifics led to the formation of a single species in solution
excluding other aggregates. To the best of our knowledge, T1 and
T2 are the first supramolecular scalene triangles with three different
self-assembled corners. Furthermore, T2 is the first trisheterometallic
scalene triangle. Such structural diversity in a rather simple
supramolecular architecture points the way to promising devices
with electronically different subunits.14
The suggested structure requires that T1 is chiral due to the
stereogenic [Cu(8phen)(9phenAr2)]+ unit. As a result, several groups
being homo- or enantiotopic in the individual ligands become
diastereotopic in T1. For example, the four methoxy groups of 7
show up as four singlets at 2.85-2.93 ppm. Their shift is indicative
of a [Zn(7phenAr2)(8terpy)]2+ complex.5 Likewise, the two mesityl
protons (x′) become diastereotopic in T1 (δ ) 5.92, 6.10 ppm). As
these protons show up at δ ) 6.92 ppm in 9, their characteristic
upfield shift in T1 confirms the [Cu(8phen)(9phenAr2)]+ complexation.7
The assortment of the metal ions in the two metal exchanging
corners of the scalene triangle was interrogated by DPV probing
the Cu+ oxidation wave. Due to the diagnostically different redox
potentials of [Cu(4)(5)]+ (E1/2 ) 0.44 VSCE), [Cu(1)(4)]+ (0.29 VSCE
)
and [Cu(1)(2)]+ (-0.21 VSCE),5 a mixture of copper(I) complexes
would show several copper(I) oxidation waves. A single oxidation
wave at 0.76 VSCE in T1 (Supporting Information) confirmed the
presence of only one type of copper(I) complex, pointing persua-
sively to the formation of [Cu(8phen)(9phenAr2)]+. A combination of
ESI-MS, 1H NMR, DPV, DOSY, and elemental analysis thus
unambiguously provided evidence for the clean formation of scalene
triangle T1.
Acknowledgment. We thank the Deutsche Forschungsgemein-
schaft and the University of Siegen for financial support.
Supporting Information Available: Experimental procedures and
spectroscopic data are provided for 7, 9, and all triangular assemblies
Tn. This material is available free of charge via the Internet at http://
pubs.acs.org.
Despite the many different entities potentially arising from five
donor and three acceptor units, the exclusive formation of T1 based
on thermodynamic equilibration is no surprise in light of the 3-fold
completiVe self-sorting described in Scheme 1 and the design criteria
applied to ligands 7-9. Thus, it seems to be a promising strategy
for future multicomponent structure design to first probe completiVe
self-sorting in a library of mononuclear cornerstones and then to
merge the motifs in multiligand building blocks for integratiVe self-
sorting.
References
(1) Wicken, J. S. J. Theor. Biol. 1979, 77, 349.
(2) (a) Schmittel, M.; Mahata, K. Angew. Chem., Int. Ed. 2008, 47, 5284. (b)
Granzhan, A.; Riss-Johannessen, T.; Scopelliti, R.; Severin, K. Angew.
Chem., Int. Ed. 2010, 49, 5515.
(3) (a) Kra¨mer, R.; Lehn, J.-M.; Marquis-Rigault, A. Proc. Natl. Acad. Sci.
U.S.A. 1993, 90, 5394. (b) Ulrich, S.; Lehn, J.-M. J. Am. Chem. Soc. 2009,
131, 5546. (c) Northrop, B. H.; Zheng, Y.-R.; Chi, K.-W.; Stang, P. J.
Acc. Chem. Res. 2009, 42, 1554. (d) Jiang, W.; Scha¨fer, A.; Mohr, P. C.;
Schalley, C. A. J. Am. Chem. Soc. 2010, 132, 2309. (d) Parimal, K.;
Witlicki, E. H.; Flood, A. H. Angew. Chem., Int. Ed. 2010, 49, 4628. (e)
Molla, M. R.; Das, A.; Ghosh, S. Chem.sEur. J. 2010, 16, 10084. (f) Ulrich,
S.; Petitjean, A.; Lehn, J.-M. Eur. J. Inorg. Chem. 2010, 1913.
(4) Jiang, W.; Schalley, C. A. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10425.
(5) Mahata, K.; Schmittel, M. J. Am. Chem. Soc. 2009, 131, 16544.
(6) (a) Zangrando, E.; Casanova, M.; Alessio, E. Chem. ReV. 2008, 108, 4979.
(b) Maran, U.; Britt, D.; Fox, C. B.; Harris, J. M.; Orendt, A. M.; Conley,
H.; Davis, R.; Hlady, V.; Stang, P. J. Chem.sEur. J. 2009, 15, 8566. (c)
Song, X.; Liu, X.; Oh, M.; Lah, M. S. Dalton Trans. 2010, 39, 6178. (d)
Miras, H. N.; Chakraborty, I.; Raptis, R. G. Chem. Commun. 2010, 46,
2569.
In another experiment, we replaced Zn2+ by Hg2+ following a
similar protocol to fabricate the trimetallic scalene triangle T2. 7,
8, 9, Hg2+, and Cu+ were mixed in a 1:1:1:1:1 ratio and refluxed
1
for 2 h in acetonitrile/DCM ) 2:1. As for T1, H NMR and ESI-
MS data supported the clean formation of the trisheterometallic
scalene triangle T2. As Hg2+ binds more strongly than Zn2+ in
[M(terpy)(phenAr2)]n+ complexes,13 we tested metal exchange in
T1 upon addition of Hg2+ (1 equiv). Complete transformation of
T1 to T2 was noticed within one day as evidenced from ESI-MS.
(7) Schmittel, M.; Mahata, K. Chem. Commun. 2008, 2550.
9
15934 J. AM. CHEM. SOC. VOL. 132, NO. 45, 2010

C O M M U N I C A T I O N S
(8) Ghosh, S.; Turner, D. R.; Batten, S. R.; Mukherjee, P. S. Dalton Trans.
2007, 1869.
(9) (a) Schmittel, M.; Mahata, K. Inorg. Chem. 2009, 48, 822. (b) Schmittel,
M.; Mahata, K. Chem. Commun. 2010, 46, 4163.
(12) Terpy ) [2,2′;6′,2′′]terpyridine; phen ) [1,10]-phenanthroline; phenAr2
) 2,9-diaryl[1,10]-phenanthroline.
(13) Schmittel, M.; Kalsani, V.; Kishore, R. S. K.; Co¨lfen, H.; Bats, J. J. Am.
Chem. Soc. 2005, 127, 11544.
(10) Jiang, J.; MacLachlan, M. J. Org. Lett. 2010, 12, 1020.
(11) (a) Kishore, R. S. K.; Paululat, T.; Schmittel, M. Chem.sEur. J. 2006, 12,
8136. (b) Kishore, R. S. K.; Kalsani, V.; Schmittel, M. Chem. Commun.
2006, 3690, and references cited therein.
(14) (a) Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4763. (b) Kurth,
D. G. Sci. Technol. AdV. Mater. 2008, 9, 014103.
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