Multifunctional "clickates" as versatile extended heteroaromatic building blocks: Efficient synthesis via click chemistry, conformational preferences, and metal coordination

Article abstract of DOI:10.1002/chem.200701240

Click chemistry has been utilized to access 2,6-bis(1-aryl-1,2,3-triazol-4- yl)pyridines (BTPs) as versatile extended heteroaromatic building blocks for their exploitation in supramolecular chemistry, in particular foldamer and ligand design. In addition to their high-yielding synthesis using Cu ^I-catalyzed Huisgen-type 1,3-dipolar cycloaddition reactions the formed triazole moieties constitute an integral part of the BTP framework and encode both its pronounced conformational preferences as well as its chelating ability. A diverse set of symmetrical and non-symmetrical BTPs carrying electron-donating and -withdrawing substituents at both terminal aryl and the central pyridine moieties has efficiently been synthesized and could furthermore readily be postfunctionalized with amphiphilic side chains and porphyrin chromophores. In both solution and solid state, the BTP scaffold adopts a highly conserved horseshoelike anti-anti conformation. Upon protonation or metal coordination, the BTP scaffold switches to the chelating syn-syn conformation. Iron and europium complexes have been prepared, successfully characterized by single-crystal X-ray diffraction analysis, and investigated with regard to their spin state and luminescent properties. The extended heteroaromatic BTP scaffold should prove useful for the design of responsive foldamer backbones and the preparation of new magnetic and emissive materials.

Full text of DOI:10.1002/chem.200701240

DOI: 10.1002/chem.200701240
Multifunctional “Clickates” as Versatile Extended Heteroaromatic Building
Blocks: Efficient Synthesis via Click Chemistry, Conformational Preferences,
and Metal Coordination
Robert M. Meudtner,[a, b] Marc Ostermeier,[a] Richard Goddard,[b]
Christian Limberg,*[a] and Stefan Hecht*[a, b]
Abstract: Click chemistry has been uti-
lized to access 2,6-bis(1-aryl-1,2,3-tri-
ACHTREUNG
non-symmetrical BTPs carrying elec-
tron-donating and -withdrawing sub-
stituents at both terminal aryl and the
central pyridine moieties has efficiently
been synthesized and could further-
more readily be postfunctionalized
with amphiphilic side chains and por-
phyrin chromophores. In both solution
and solid state, the BTP scaffold
adopts a highly conserved horseshoe-
like anti–anti conformation. Upon pro-
tonation or metal coordination, the
BTP scaffold switches to the chelating
syn–syn conformation. Iron and europi-
um complexes have been prepared,
successfully characterized by single-
crystal X-ray diffraction analysis, and
investigated with regard to their spin
state and luminescent properties. The
extended heteroaromatic BTP scaffold
should prove useful for the design of
responsive foldamer backbones and the
preparation of new magnetic and emis-
sive materials.
blocks for their exploitation in supra-
molecular chemistry, in particular fol-
damer and ligand design. In addition to
their high-yielding synthesis using CuI-
catalyzed Huisgen-type 1,3-dipolar cy-
cloaddition reactions the formed tri-
ACHTREUNGazole moieties constitute an integral
Keywords: click chemistry · coordi-
nation chemistry · cycloaddition ·
part of the BTP framework and encode
both its pronounced conformational
preferences as well as its chelating abil-
ity. A diverse set of symmetrical and
foldamers
chemistry
·
supramolecular
Introduction
ordination framework, which has been utilized extensively
for the design of various supramolecular architectures,[1b] op-
toelectronic[1b,4] and magnetic[5] materials as well as enantio-
selective catalysts.[6] However, variation of the ligand scaf-
fold is often cumbersome, and therefore facile, modular, and
versatile syntheses of these and related new ligand families
are still needed.
The ability of the coordination chemist to prepare new
metal complexes for various applications is inevitably tied to
the accessibility of a versatile ligand set. Tridentate pyri-
dine-centered heteroaromatic ligands, most notably terpyri-
dines (tpy),[1] bis(oxazolinyl)pyridines (py-box),[2] and bis-
(pyrazolyl)pyridines,[3] represent a particularly privileged co-
The CuI-catalyzed Huisgen-type 1,3-dipolar cycloaddition
of terminal alkynes and organic azides,[7,8] frequently refer-
red to as the “click reaction”,[9] provides an extremely effi-
cient method to connect various functional entities.[10] Re-
cently, this versatile reaction has been applied in various
fields ranging from the biological[11] to the materials scien-
ces.[12] While in most cases the formed 1,4-disubstituted
1,2,3-triazole serves as mere connecting unit, only occasion-
ally the heterocycle itself is used as an integral structural
[a] Dipl.-Chem. R. M. Meudtner, Dipl.-Chem. M. Ostermeier,
Prof. Dr. C. Limberg, Prof. Dr. S. Hecht
Department of Chemistry, Humboldt-Universität zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
Fax : (+49)30-2093-6940
E-mail: christian.limberg@chemie.hu-berlin.de
sh@chemie.hu-berlin.de
ment.[13]
eleACHTREUNG
[b] Dipl.-Chem. R. M. Meudtner, R. Goddard, Prof. Dr. S. Hecht
Max-Planck-Institut für Kohlenforschung
The integration of several triazole building blocks within
a larger structure constitutes a promising approach for the
design of advanced chelating ligands, that is, “clickates”.[14–16]
Here, we report the facile and highly modular synthesis of
Kaiser-Wilhelm-Platz1, 45470 Mülheim an der Ruhr (Germany)
Supporting information for this article (68 pages of extensive synthet-
ic details and characterization data) is available on the WWW under
http://www.chemeurj.org/ or from the author.
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FULL PAPER
Table 1. Synthesis of 2,6-bis(1-aryl-1,2,3-triazol-4-yl)pyridines (BTPs) as
outlined in Scheme 1.
2,6-bis(1-aryl-1,2,3-triazol-4-yl)pyridines (BTPs), featuring
two integral triazole moieties bridged by a central pyridine
core, thus creating a new ligand family and key building
block for elongated heteroaromatic foldamer strands[17]
(Figure 1). Our approach allows diverse substituents to be
readily introduced at various positions in the ligand scaffold
either during or after the “click reaction”, thereby enabling
the preparation of a multitude of symmetrical and non-sym-
metrical functional BTP derivatives with defined conforma-
tional preferences and rich coordination chemistry.[18]
Compound
R1
R2
Yield[%][a]
4a
4b
4c
4d
4e
4 f
4g
5a
5b
5c
5d
CO2Tg[b]
CO2Tg
CO2Tg
CO2Tg
CO2Tg
CO2Tg
CO2Tg
OTg
CH3
I
NO2
quant.[c]
98
95
N
A
95
CO2Et
OCH3
n-C10H21
CH3
quant.
88[c]
96
quant.[c]
85
OTg
OTg
OTg
I
NO2
86
94
N
A
Results and Discussion
[a] Yield of isolated product. [b] Tg=-(CH2CH2O)3CH3. [c] 20 mol% Na
asc. were used.
Framework synthesis: 2,6-Diethynylpyridines carrying either
electron-donating or electron-withdrawing groups were al-
lowed to react with diverse electron-rich and electron-poor
aryl azides (Scheme 1, Table 1).[19] Although frequently ali-
phatic azides and alkynes serve as substrates in the “click”
reaction, we were pleased to note that after screening of the
reaction conditions practically quantitative yields were ob-
tained for aromatic substrates, independent of the electronic
nature of the substituents. To maintain solubility of the
products throughout the reaction (and for purposes of solvo-
phobic foldamer design[20]) triethylene glycol side chains
were introduced onto the pyridine components.
Non-symmetrical BTPs carrying two different triazole
fragments were also generated in excellent yields by a se-
quential coupling approach (Scheme 2). The mono-protect-
ed 2,6-diethynyl-pyridine 6 was coupled to the first azide
fragment to furnish monocycloadduct 7, which was depro-
tected and subsequently coupled to another azide fragment
to yield non-symmetrical, donor–acceptor substituted BTP 9
in 98% overall yield. The triisopropylsilyl (TIPS) protecting
group proved stable under the applied click reaction condi-
tions, yet it could easily be removed in the consequent acti-
vation step allowing for successive cycloaddition. This repet-
itive synthetic route provides the basis for the preparation
of more complex structures, in particular larger oligomers
(foldamers).[21]
Scheme 1. Synthesis of 2,6-bis(-1aryl-1,2,3-triazol-4-yl)pyridines (BTPs)
4a–g and 5a–d (Tg=-(CH2CH2O)3CH3, TBTA=tris(benzyltriazolyl-
ACHTREUNGmethyl)amine, Na asc.=sodium ascorbate).
Figure 1. “Clickates” based on 2,6-bis(1,2,3-triazol-4-yl)pyridines (BTPs) provide a highly functional kinked foldamer building block and extended ligand
scaffold, readily accessible via “click chemistry”.
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S. Hecht, C. Limberg et al.
Conformational preferences: In analogy to the known con-
formational preference of 2,2’-bipyridines,[22] the BTP skele-
ton is expected to predominantly adopt the anti–anti confor-
mation due to favorable electrostatic interactions, whereas
repulsive interactions due to lone pair repulsion disfavor the
syn–syn conformation (Figure 1). DFT calculations on a 4-
(2-pyridyl)-1,2,3-triazole model system were carried out and
predict a significant stabilization of the anti conformer by
6.4 kcalmolÀ1 over the syn conformer in the gas phase.[19]
Experimental verification of the predicted strong conforma-
tional preference for the anti–anti conformation was ob-
tained both in the solid state and in solution (Figure 2). Sev-
eral independent single-crystal X-ray structural analyses
reveal the highly conserved horseshoe-type flat arrangement
of the BTP core. In NMR measurements on the non-sym-
metrical BTP derivative 9 in solution practically no NOE
between the distant pyridyl and triazolyl protons could be
detected.
The repulsive interaction between the lone pairs of adja-
cent N atoms can be overcome by protonation and metala-
tion, respectively, thereby taking advantage of the chelating
effect. Hence, as anticipated and conceptually envisaged in
Figure 1, the anti–anti conformation of the BTP skeleton
can be switched to the syn–syn conformation by protonation
as shown by NOE experiments on BTP derivative 5b
(Figure 3).[19]
Scheme 2. Synthesis of non-symmetrical BTP 9 (Tg=-(CH2CH2O)3CH3,
TBTA=tris(benzyltriazolylmethyl)amine, Na asc.=sodium ascorbate,
TBAF=tetraACHTREUNG(n-butyl)ammonium fluoride).
The BTP framework proved chemically robust and could
be further functionalized, both at its central pyridine as well
as at the terminal aryl moieties (Scheme 3). For example,
the bisalkylated BTP ester 4g could be easily saponified to
yield the free carboxylic acid 10a, while Pd0-catalyzed cross-
coupling of 4b with the appropriate porphyrin-based acety-
lene readily provided 10b, in which two zinc porphyrins are
linked by the BTP scaffold. In both structures conformation-
al switching of the BTP scaffold by changing pH or com-
plexation of metal ions (vide infra) can be utilized either to
alter self-assembly behavior or to tune distances and relative
orientation of large chromophores.[21]
Coordination chemistry: First investigations into the coordi-
nation chemistry of the BTP scaffold were performed to
compare it with well-known tridentate polyazaromatic li-
gands, such as bis(pyrazolyl)pyridines, from which the
À
parent BTP can be derived via formal replacement of a C
H group within the pyrazole residue by a nitrogen atom.
To clarify the influence of this modification on the ligand
field strength, subsequent to initial UV/Vis titrations,[19] the
iron complex [FeCAHRTUNEG(5c)2]ACHRTE(UGN OTf)2 (OTf=trifluoromethanesulfo-
nate) was synthesized (Scheme 4). While the triethylene
Scheme 3. Postfunctionalization of the BTP scaffold (Tg=-(CH2CH2O)3CH3).
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Multifunctional “Clickates”
FULL PAPER
rarely allow single-crystal X-ray diffraction analysis. None-
theless, the molecular structure of [FeACTHERNGU(5c)2]ACHTRE(UGN OTf)2 (Figure 4)
À
could be determined. It shows unexpectedly short Fe N
bonds averaging 1.93 Š, which point to a low-spin state at
116 K (high-spin complexes of the type [Fe(L)2]2+ typically
exhibit bond lengths between 2.1 and 2.2 Š),[23] consistent
with the red-brown color of the compound. No thermochro-
mic effects are observed on warming to room temperature
1
and accordingly, H NMR spectroscopic investigations per-
formed in CD3CN solution at room temperature again indi-
cate a low-spin state,[19] a finding that is surprising, consider-
ing that related systems with bis(pyrazolyl)pyridine ligands
usually switch from their low-spin ground states into the
high-spin states at temperatures significantly below room
temperature[3] or even at 30 K after irradiation, referred to
as light-induced excited spin state trapping (LIESST).[24]
Thus, the ligand field strength of 5c is higher than that of
bis(pyrazolyl)pyridine and more comparable to that of ter-
pyridine.[25] Future research will aim at tuning the electronic
properties of the BTP ligand framework (R and R’ substitu-
ents in Scheme 4) to obtain iron complexes that undergo
spin crossovers in an accessible temperature range or in-
duced by light, which can subsequently be attached to surfa-
ces via the readily adjustable tether at the pyridyl moiety.[21]
Another area where terpyridine, bis(pyrazolyl)pyridine,
and their related bis(benzimidazolyl)pyridine derivatives
have been extensively employed is the sensitization of lan-
thanide (Ln) luminescence, whose direct excitation is not
possible due to Laporte-forbidden 4f–4f transitions.[26] Suita-
ble organic chromophores (ligands) with a significant ab-
sorption cross-section in the UVCAHTRE(UNG Vis) are thus required as
antennae that efficiently transfer their excitation energy to
the excited state(s) of the lanthanide ion and thus lead to
“sensitized” light emission. Ideally, ligands should: 1) form
stable complexes (for practical applications under physiolog-
ical conditions), 2) enable efficient ligand-to-lanthanide
energy transfer (through compatible energy levels), and 3)
prevent nonradiative deactivation of the lanthanide excited
Figure 2. Conformational preference of BTPs for the anti–anti conforma-
tion in the solid: a) top view overlay of four independent crystal struc-
tures of 4a, 4c, 5b, and 10a (oval denotes highly conserved horseshoe-
like conformation) as well as in solution: b) intensity of NOE contacts in
compound 9 (CH2Cl2, 278C).
À
glycol side chains at the BTP scaffold are certainly advanta-
geous for solubility, foldamer design, and subsequent surface
attachment, they naturally hamper crystallization due their
inherent flexibility and poor packing ability and therefore
states, for instance by O H oscillators of coordinated or
closely diffusing water molecules.[27] A strategy that has
been successfully pursued in the past relies on the organiza-
tion of three tridentate binding sites around nine-coordinate
Figure 3. Conformational switching from the anti–anti to the syn–syn conformation induced by protonation, as revealed from the relative intensities of
NOE contacts in compound 5b before and after addition of trifluoroacetic acid (TFA) in CD2Cl2 at 278C (Tg=-(CH2CH2O)3CH3).
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9837

S. Hecht, C. Limberg et al.
Scheme 4. Preparation of iron and europium complexes (Tg=-(CH2CH2O)3CH3, OTf=trifluoromethanesulfonate).
Figure 4. Molecular structure of [Fe
co-crystallized [Fe(OTf)(H2O)(MeCN)4]OTf and solvent molecules have
been omitted for clarity. OTf=trifluoromethanesulfonate). Selected
ACHRTU(NEG 5c)2]ACHTRE(UGN OTf)2 (hydrogen atoms, anions,
A
N
ACHTREUNG
À
À
À
bond lengths [Š] and angles [8]: N1 Fe 1.913(4), N2 Fe 1.921(3), N3 Fe
À
À
À
1.945(3), N4 Fe 1.908(4), N5 Fe 1.937(4), N6 Fe 1.936(4); N1-Fe-N2
80.0(2), N1-Fe-N3 81.1(2), N1-Fe-N4 178.2(2), N1-Fe-N5 101.2(2), N1-Fe-
N6 98.4(2), N2-Fe-N3 160.9(2), N2-Fe-N4 99.1(2), N2-Fe-N5 91.5(2), N2-
Fe-N6 91.8(2), N3-Fe-N4 99.9(2), N3-Fe-N5 89.5(2), N3-Fe-N6 93.7(2),
N4-Fe-N5 80.4(2), N4-Fe-N6 80.0(2), N5-Fe-N6 160.4(2).
Figure 5. a) Molecular structure of [EuACHTRENG(U 5a)3]ACHTRE(UGN OTf)3 (Hydrogen atoms,
anions, and co-crystallized solvent molecules have been omitted for clari-
ty. OTf=trifluoromethanesulfonate). b) Tricapped, trigonal prismatic co-
ordination of the metal center. Selected bond lengths [Š] and angles [8]:
LnIII ions.[28,29] To test the potential of BTPs in this context a
threefold excess of 5b was allowed to react with Eu
ACHTRE(UNG OTf)3
À
À
À
À
N1 Eu1 2.574(8), N2 Eu1 2.531(8), N3 Eu1 2.513(8), N4 Eu1 2.598(8),
À
À
À
À
for the synthesis of compound [Eu(5b)3](OTf)3 (Scheme 4),
A
N
N5 Eu1 2.520(8), N6 Eu1 2.514(8), N7 Eu1 2.572(8), N8 Eu1 2.507(8),
À
N9 Eu1 2.499(7); N1-Eu-N2 62.9(3), N1-Eu-N3 64.2(2), N1-Eu-N4
which was fully characterized; however, severe disordering
of the cations within its crystals prohibited the exact analysis
of the molecular structure.[19] However, when 5a was em-
ployed as the ligand, the crystal structure of the resulting
complex [EuACHTREUNG(5a)3]ACHTREUNG(OTf)3 (Scheme 4) could be determined
and the result is displayed in Figure 5a.
The metal center is coordinated by all nine N atoms of
the three terdentate ligands with a tricapped trigonal pris-
matic geometry (Figure 5b). The two triangular faces of the
122.4(3), N1-Eu-N5 136.7(3), N1-Eu-N6 74.2(3), N1-Eu-N7 119.8(3), N1-
Eu-N8 135.7(2), N1-Eu-N9 74.3(3), N2-Eu-N3 127.1(3), N2-Eu-N4
135.4(3), N2-Eu-N5 147.2(3), N2-Eu-N6 78.8(3), N2-Eu-N7 74.6(3), N2-
Eu-N8 78.3(3), N2-Eu-N9 89.0(3), N3-Eu-N4 74.7(3), N3-Eu-N5 79.4(3),
N3-Eu-N6 85.2(3), N3-Eu-N7 137.5(2), N3-Eu-N8 146.3(3), N3-Eu-N9
79.1(2), N4-Eu-N5 63.9(3), N4-Eu-N6 63.5(3), N4-Eu-N7 117.8(3), N4-
Eu-N8 71.7(3), N4-Eu-N9 135.6(3), N5-Eu-N6 127.3(3), N5-Eu-N7
72.6(2), N5-Eu-N8 87.6(3), N5-Eu-N9 76.6(3), N6-Eu-N7 137.2(3), N6-
Eu-N8 78.2(3), N6-Eu-N9 148.4(3), N7-Eu-N8 64.1(3), N7-Eu-N9 63.8(3),
N8-Eu-N9 128.0(3).
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Multifunctional “Clickates”
FULL PAPER
trigonal prism are not exactly eclipsed but slightly staggered
and are formed by the six coordinating N atoms belonging
to the triazole units, while the pyridyl donors form the three
caps. Comparing this structure with the related structure of
[Eu(L’)3]ACHTREUNG
(PF6)3 (L’=2,6-bis(1H-pyrazol-3-yl)pyridine)[28b] it
À
can be noted that the Eu Npyridyl distances are very similar
À
in both complexes, whereas the Eu Ntriazole distances in [Eu-
(5a)3]3+ are somewhat shorter than the Eu Npyrazole distan-
ces in [Eu(L’)3]3+. This is a reflection of the fact that in
[Eu(L’)3]3+ the ligands are not exactly planar but exhibit
slight twists between the adjacent aromatic rings, and inter-
estingly the same observation has been made for [Eu-
À
ACHTREUNG ACHTREUGN
(terpy)3]3+ as well.[29] The ligand scaffolds in [Eu(5a)3]3+ are
merely planar, however, which indicates that the binding
pockets provided by the BTPs are more favorable for the
(BTP)3]3+, thus leading to stron-
formation of complexes [LnACHTREUNG
ger metal–ligand interactions. In summary, the BTP ligand
provides superior donor capabilities and enhanced steric
freedom in comparison to established tridentate azaaromatic
ligands such as terpy (in the case of [EuACHTREUNG
(pyrazolyl)pyridines (in the case of [Fe
tively.
ACHTREUNG
[EuACHTREUNG(5b)3]ACHTREUNG(OTf)3 displays a characteristic red-orange emis-
sion both in the solid state (crystal and powder) and in solu-
tion (Figure 6). The BTP scaffold efficiently absorbs in the
UV and sensitizes the typical narrow lanthanide ion emis-
3+
sion, in agreement with the few reported spectra of EuL3
ions.[30] Again, substitution of the BTP ligand framework
should allow convenient tuning of the luminescence proper-
ties of the respective lanthanide complexes and their cova-
lent attachment to either biomacromolecules (labeling for
diagnostics) or solid substrate surfaces.[21]
Conclusion
Figure 6. Emission properties of europium complex [Eu
croscope views showing [Eu(5b)3](OTf)3 single crystals a) in daylight,
and b) exposed to excitation by a 254 nm UV lamp as well as cuvettes
containing CH2Cl2 solutions of [Eu(5b)3](OTf)3 c) in daylight, and d) ex-
posed to excitation by a 254 nm UV lamp. e) Absorption and emission
(lexc =265 nm) spectra of [Eu(5b)3](OTf)3 in CH2Cl2 at 258C.
ACHRTUNEG(5b)3]CAHTRE(UNG OTf)3: Mi-
We have developed “clickates” based on the BTP motif as a
new privileged framework for supramolecular chemistry.
While our approach benefits from its synthetic ease and ver-
satility, the resulting BTP derivatives display strong yet
switchable conformational preferences and reveal rich coor-
dination chemistry. Ongoing efforts in our laboratories are
primarily focused on constructing larger oligomers and poly-
mers as new responsive foldamer backbones and exploring
the coordination chemistry to utilize the BTP scaffold for
the generation of magnetic and emissive materials, as well
as switchable amphiphiles, chromophore arrays, and supra-
molecular polymers.[21]
G
ACHTREUNG
U
ACHTREUNG
G
ACHTREUNG
and the Max Planck Society (MPG), while preparation and investigation
of the metal complexes has been performed at HU Berlin (since 11/2006)
supported by the Fonds der Chemischen Industrie, the BMBF, and the
Dr. Otto Rçhm Gedächtnisstiftung.
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2006.
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ties, and Applications (Eds.: S. Hecht, I. Huc), Wiley-VCH, Wein-
heim, 2007, pp. 1–33.
[18] During the preparation of this manuscript, Flood and co-workers
published a communication on the synthesis of two BTP ligands, de-
rived from alkyl azides and unsubstituted pyridines, and some as-
pects of their coordination chemistry, see: Y. Li, J. C. Huffman,
A. H. Flood, Chem. Commun. 2007, 2692–2694.
[9] The concept of “click chemistry” is described in: H. C. Kolb, M. G.
Finn, K. B. Sharpless, Angew. Chem. 2001, 113, 2056–2075; Angew.
Chem. Int. Ed. 2001, 40, 2004–2021.
[19] For details, see Supporting Information.
[10] For reviews, see: a) H. C. Kolb, K. B. Sharpless, Drug Discovery
Today 2003, 8, 1128–1137; b) V. D. Bock, H. Hiemstra, J. H. van
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Properties, and Applications (Eds.: S. Hecht, I. Huc), Wiley-VCH,
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[11] For prominent examples using in situ “click chemistry”, see: a) in-
hibition of HIV-1 protease: M. Whiting, J. Muldoon, Y.-C. Lin, S. M.
Silverman, W. Lindstrom, A. J. Olson, H. C. Kolb, M. G. Finn, K. B.
Sharpless, J. H. Elder, V. V. Fokin, Angew. Chem. 2006, 118, 1463–
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beling of Escherichia coli: A. L. James, D. A. Tirrell, J. Am. Chem.
Soc. 2003, 125, 11164–11165; c) bioconjugation to cowpea mosaic
virus: Q. Wang, T. R. Chan, R. Hilgraf, V. V. Fokin, K. B. Sharpless,
M. G. Finn, J. Am. Chem. Soc. 2003, 125, 3192–3193.
[12] For reviews about the application of “click chemistry” in polymer
and materials science, see: a) J.-F. Lutz, Angew. Chem. 2007, 119,
1036–1043; Angew. Chem. Int. Ed. 2007, 46, 1018–1025; b) W. H.
Binder, R. Sachsenhofer Macromol. Rapid Commun. 2007, 28, 15–
54; c) D. Fournier, R. Hoogenboom, U. S. Schubert, Chem. Soc. Rev.
2007, 36, 1369–1380. Prominent examples include: d) dendrimer
synthesis: P. Wu, A. K. Feldman, A. K. Nugent, C. J. Hawker, A.
Scheel, B. Voit, J. Pyun, J. M. J. Frꢁchet, K. B. Sharpless, V. V. Fokin,
Angew. Chem. 2004, 116, 4018–4022; Angew. Chem. Int. Ed. 2004,
43, 3928–3932; e) dendronization of linear polymers: B. Helms, J. L.
Mynar, C. J. Hawker, J. M. J. Frꢁchet, J. Am. Chem. Soc. 2004, 126,
15020–15021; f) postfunctionalization of linear polymers: P. L.
Golas, N. V. Tsarevsky, B. S. Sumerlin, K. Matyjaszewski Macromo-
lecules 2006, 39, 6451–6457; g) crosslinked polymer glue: D. D.
Dꢂaz, S. Punna, P. Holzer, A. K. McPherson, K. B. Sharpless, V. V.
Kokin, M. G. Finn, J. Polym. Sci.: Part A 2004, 42, 4392–4403;
h) stabilization of organogels: D. D. Dꢂaz, K. Rajgopal, E. Strable, J.
Schneider, M. G. Finn, J. Am. Chem. Soc. 2006, 128, 6056–6507;
i) functionalization of single-walled carbon nanotubes: H. Li, F.
Cheng, A. M. Duft, A. Adronov, J. Am. Chem. Soc. 2005, 127,
14518–14524.
[21] Results of these investigations will be detailed separately.
[22] High level ab initio calculations indicate that anti (trans) bipyridine
(N-C-C-N dihedral angle=1808) is approximately 5.9 kcalmolÀ1
more stable than its twisted syn (cis) conformer (N-C-C-N dihedral
angle=44.98) in the gas phase, see: S. T. Howard, J. Am. Chem. Soc.
1996, 118, 10269–10274.
2+
[23] FeL2 complexes with L=bis(pyrazolyl)pyridine: a) J. M. Holland,
J. A. McAllister, Z. Lu, C. A. Kilner, M. Thornton-Pett, M. A. Hal-
crow, Chem. Commun. 2001, 577–578; b) J. M. Holland, J. A.
McAllister, C. A. Kilner, M. Thornton-Pett, A. J. Bridgeman, M. A.
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Chem. 1994, 106, 2109–2141; Angew. Chem. Int. Ed. Engl. 1994, 33,
2+
2024–2054. LIESST in related FeL2 complexes with L=bis(pyra-
zol-1-yl)pyridine: b) V. A. Money, I. Radosavljevic Evans, M. A.
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[25] E. C. Constable, G. Baum, E. Bill, R. Dyson, R. van Eldik, D.
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[13] The most notable exception includes peptidomimetics; for a review,
see: a) Y. L. Angell, K. Burgess, Chem. Soc. Rev. 2007, 36, 1674–
1689. Prominent examples include: b) W. S. Horne, M. K. Yadav,
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[14] To our knowledge, the only example incorporating multiple triazole
fragments into the design of chelating ligands via “click chemistry”
is the tris(benzyltriazolylmethyl)amine (TBTA) ligand: T. R. Chan,
R. Hilgraf, K. B. Sharpless, V. V. Fokin, Org. Lett. 2004, 6, 2853–
2855. This publication also mentions the synthesis of 2,6-bis(1-
benzyl-1,2,3-triazol-4-yl)pyridine.
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Received: August 9, 2007
Published online: October 19, 2007
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