Complexes of click-derived bistriazolylpyridines: Remarkable electronic influence of remote substituents on thermodynamic stability as well as electronic and magnetic properties

Article abstract of DOI:10.1002/chem.201000721

2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) ligands with substitution patterns ranging from strongly electrondonating to strongly electron-accepting groups, readily prepared by means of Cu-catalyzed 1,3-dipolar cycloaddition (the "click" reaction), were investigated with regard to their complexation behavior, and the properties of the resulting transition-metal compounds were compared. Metal-btp complexes of 1:1 stoichiometry, that is, [Ru-(btp)Cl₂(dmso)] and [Zn(btp)Br₂], could be isolated and were crystallographically characterized: they display octahedral and trigonal-bipyramidal coordination geometries, respectively, and exhibit high aggregation tendencies due to efficient π-π stacking leading to low solubilities. Metal-btp complexes of 1:2 stoichiometry, that is, [Fe-(btp) ₂]²⁺ and [Ru(btp)₂]²⁺, could also be synthesized and their metal centers show the expected octahedral coordination spheres. The iron compounds exhibit quite a complex magnetic behavior in the solid state including spin crossover near room temperature, and hysteresis and locking into high-spin states on tempering at 400 K, depending on the substituents on the btp ligands. Cyclic voltammetry studies of [Ru(btp) ₂]²⁺ reveal strong modulation of the oxidation potentials by more than 0.6 V and a clear linear correlation to the Hammett constant (σ_para) of the substituent at the pyridine core. Isothermal titration calorimetry was used to measure the thermodynamics of the Fe ^II-btp complexation process and enabled accurate determination of the complexation enthalpies, which display a linear relationship with the σ_para values for the terminal phenyl substituents. Detailed NMR spectroscopic studies finally revealed that in the case of Fe^II complexation, dynamics are rapid for all investigated btp derivatives in acetonitrile, while replacing Fe^II by Ru^II or changing the solvent to dichloromethane effectively slows down ligand exchange. The results nicely demonstrate the utility of substituent parameters, originally developed for linear free-energy relationships to explain reactivity in organic reactions, in coordination chemistry, and to illustrate the potential to customdesign btp ligands and complexes thereof with predictable properties. The fast equilibration of the [Fe-(btp)₂]²⁺ complexes together with their tunable stability and interesting magnetic properties should enable the design of dynamic metallosupramolecular materials with advantageous properties.

Full text of DOI:10.1002/chem.201000721

DOI: 10.1002/chem.201000721
Complexes of Click-Derived Bistriazolylpyridines: Remarkable Electronic
Influence of Remote Substituents on Thermodynamic Stability as well as
Electronic and Magnetic Properties
Marc Ostermeier,[a] Marie-Anne Berlin,[a] Robert M. Meudtner,[a] Serhiy Demeshko,[b]
Franc Meyer,[b] Christian Limberg,*[a] and Stefan Hecht*[a]
Abstract: 2,6-Bis(1,2,3-triazol-4-yl)pyri-
dine (btp) ligands with substitution pat-
terns ranging from strongly electron-
donating to strongly electron-accepting
groups, readily prepared by means of
Cu-catalyzed 1,3-dipolar cycloaddition
(the “click” reaction), were investigat-
ed with regard to their complexation
behavior, and the properties of the re-
sulting transition-metal compounds
were compared. Metal–btp complexes
of 1:1 stoichiometry, that is, [Ru-
exhibit quite a complex magnetic be-
havior in the solid state including spin
crossover near room temperature, and
hysteresis and locking into high-spin
states on tempering at 400 K, depend-
ing on the substituents on the btp li-
gands. Cyclic voltammetry studies of
ents. Detailed NMR spectroscopic
studies finally revealed that in the case
of FeII complexation, dynamics are
rapid for all investigated btp deriva-
tives in acetonitrile, while replacing
FeII by RuII or changing the solvent to
dichloromethane
effectively
slows
[Ru
G
down ligand exchange. The results
nicely demonstrate the utility of sub-
stituent parameters, originally devel-
oped for linear free-energy relation-
ships to explain reactivity in organic re-
actions, in coordination chemistry, and
to illustrate the potential to custom-
design btp ligands and complexes
thereof with predictable properties.
The fast equilibration of the [Fe-
of the oxidation potentials by more
than 0.6 V and a clear linear correla-
tion to the Hammett constant (spara) of
the substituent at the pyridine core.
Isothermal titration calorimetry was
used to measure the thermodynamics
of the FeII–btp complexation process
and enabled accurate determination of
the complexation enthalpies, which dis-
play a linear relationship with the spara
values for the terminal phenyl substitu-
ACHTUNGTRENNUNG(btp)Cl2ACHTUNGTRENNUNG(dmso)] and [ZnCATHUNGTREN(UNGN btp)Br2],
could be isolated and were crystallo-
graphically characterized: they display
octahedral and trigonal-bipyramidal
coordination geometries, respectively,
and exhibit high aggregation tenden-
cies due to efficient p–p stacking lead-
ing to low solubilities. Metal–btp com-
plexes of 1:2 stoichiometry, that is, [Fe-
AHCTUNGTRENNUNG
(btp)2]2+ complexes together with their
tunable stability and interesting mag-
netic properties should enable the
design of dynamic metallosupramolec-
ular materials with advantageous prop-
erties.
Keywords: click chemistry · coordi-
nation chemistry · isothermal titra-
tion calorimetry · substituent ef-
fects · tridentate ligands
ACHTUNGTRENNUNG ACHTUNGTRENNUNG
(btp)2]2+ and [Ru(btp)2]2+, could also
be synthesized and their metal centers
show the expected octahedral coordi-
nation spheres. The iron compounds
[a] Dipl.-Chem. M. Ostermeier, Dipl.-Chem. M.-A. Berlin,
Dr. R. M. Meudtner, Prof. Dr. C. Limberg, Prof. Dr. S. Hecht
Department of Chemistry
Introduction
Humboldt-Universitꢀt 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
For a given metal ion the chemical and physical properties
of its complexes are determined by the surrounding ligand
environment. Consequently, the development of tailor-made
ligands continues to be a very active field of research, since
the design of novel ligands is the key to new and improved
properties and applications. One ligand motif, which has
been utilized extensively in the past, contains a central pyri-
dine unit flanked by heteroaromatic residues, such as pyridyl
(i.e., 2,2’:6’,2’’-terpyridine (tpy)),[1] oxazolinyl (i.e., bis(oxa-
zolinyl)pyridine (pybox)),[2] and pyrazolyl.[3] These tridentate
[b] Dr. S. Demeshko, Prof. Dr. F. Meyer
Institut fꢁr Anorganische Chemie
Georg-August Universitꢀt
Tammannstrasse 4, 37073 Gçttingen (Germany)
Supporting information (containing experimental details) for this arti-
cle is available on the WWW under http://dx.doi.org/10.1002/
chem.201000721.
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Chem. Eur. J. 2010, 16, 10202 – 10213

FULL PAPER
ligands have enabled the design of various macromolecular
and supramolecular architectures,[1b] optoelectronic[1b,4] and
magnetic[5] materials as well as enantioselective catalysts.[6]
However, variation of the ligand scaffolds proves cumber-
some in many cases, and therefore the facile and modular
synthesis of a broad range of representatives of the above-
mentioned ligand families remains an important yet difficult
task. To address this issue, we have recently applied click
chemistry[7,8] to synthesize 2,6-bis(1-aryl-1,2,3-triazol-4-yl)-
pyridine (btp)[9] compounds as versatile extended heteroaro-
matic building blocks to exploit their conformational prefer-
ences in macromolecular and supramolecular chemistry[10]
and to utilize the btp ligand scaffold in coordination chemis-
try.[9,11] A diverse set of symmetrical and nonsymmetrical
btp molecules carrying electron-donating and -withdrawing
substituents at both terminal aryl moieties and the central
pyridine core has efficiently been synthesized, and in pre-
liminary studies the complexation behavior of two represen-
tatives in the presence of Fe2+ and Eu3+ ions has been in-
vestigated. During the course of these initial studies one
lution and present the structures as well as the electronic
and magnetic properties of the resulting complexes. In par-
ticular, we show that the complexation thermodynamics as
well as the redox properties are strongly influenced by the
ligand electronics and can be quantitatively correlated to
classical substituent effects at the terminal aryl and central
pyridine moieties, that is, R1, R2, and R3 shown in Figure 1.
Results and Discussion
The btp ligands: The employed btp ligands were prepared
by using our recently described procedure[9] through a two-
fold click reaction of various terminal aryl azides possessing
electronically different substituents ranging from strongly
ꢀ
ꢀ
donating ( NMe2) to strongly accepting ( NO2), with sever-
al 2,6-diethynylpyridine cores, again possessing either donat-
ing or accepting substituents in their 4-position, that is, btp
ligand series 1a–d and 2a–c, respectively.[9] Complementary
to our previous work, three additional btp ligands possessing
no central triethylene glycol chains (Tg), that is, btp ligands
3a–c, were prepared[13] to facilitate crystallization and struc-
tural refinement of the derived metal complexes. Due to the
strong conformational preference of the btp scaffold to
adopt the anti,anti conformation, all free ligands crystallize
in a horseshoe-like kinked structure (see Figures 5–7 in the
Supporting Information).
[FeACHTUNGTRENNUNG ACHTUNGTRENNUGN
(btp)2]2+ complex as well as one [Eu(btp)3]3+ complex
were prepared and could be characterized by single-crystal
X-ray structure analysis. Whereas the latter Eu complex ex-
hibited a characteristic luminescence in solution and in the
solid state, the Fe complex surprisingly exhibited a low-spin
configuration at room temperature, which illustrates that the
field strength of the particular btp derivative is significantly
higher than the one of bis(pyrazolyl)pyridine[3] yet compara-
ble to the field strength of tpy.[12]
In view of these initial results, we were interested in a
thorough analysis of the coordination chemistry of a diverse
set of btp ligands (Figure 1). Here, we report on the com-
plexation behavior of a wide range of btp derivatives in so-
Metal–btp complexes with a metal-to-ligand ratio of 1:1:
The metal centers of the complexes reported so far with btp
ligands of the type shown in Figure 1 are coordinatively sa-
turated by btp ligands exclusively[9] or by a combination of
btp and tpy.[11c] Hence, no free coordination sites are avail-
able to, for instance, coordinate other ligands for structural
modifications or to bind substrates for activation during the
course of catalysis; note that, for instance, complexes of bis-
AHCTUNGTREG(NNUN imino)pyridines, which may be considered as a related
ligand class, with a metal-to-ligand ratio of 1:1 proved effi-
cient catalysts for polymerization/oligomerization reac-
tions.[14] We were therefore also interested in metal–btp
complexes with more asymmetric coordination spheres. As
RuII in combination with polydentate N-heteroaromatic li-
gands, in particular tpy derivatives, can lead to advantageous
optoelectronic phenomena exploited in light-harvesting ap-
plications,[4,15] we set out to investigate the synthesis of RuII–
btp complexes. Starting from the common precursor [RuCl2-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
A
R
CHTUNGTRENNUNG
precipitate. The conversion can be accelerated by heating
the reaction mixture at reflux, from which the formed com-
plexes then precipitate within a few hours. Importantly,
solely 1:1 complexes were obtained by this procedure, inde-
pendently of the molar ratio between the metal precursor
and the btp ligand employed. In the case of btp ligands car-
rying no triethylene glycol units, that is, 3a–c, problems with
Figure 1. Tuning the electronics of tridentate 2,6-bis(1-aryl-1,2,3-triazol-4-
yl)pyridine (btp) ligands (coordinating N atoms shown in bold) by vary-
ing R1, R2, R3 (gray circles) from electron-donating groups (EDG) to
electron-withdrawing groups (EWG) to control metal coordination and
complex properties (the free ligand is shown in the energetically less fa-
vorable syn,syn conformation); Tg=(CH2CH2O)3CH3.
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C. Limberg, S. Hecht et al.
their markedly lower solubilities in acetonitrile can be cir-
cumvented by the use of dichloromethane as the solvent,
which allows for quantitative complex formation within 24 h
of reflux. After filtration, washing with diethyl ether or di-
chloromethane, respectively, gave the orange-red complexes
in very good yields and in analytically pure form, as evi-
denced by elemental analysis, ESI-MS, and NMR spectros-
copy. However, single crystals could only be obtained in the
sor (ZnBr2) in acetonitrile to a solution of the ligand (1a) in
acetonitrile. This procedure led to cocrystallization of [Zn-
AHCTUNGTERG(NNUN 1a)Br2] (Figure 3) and ZnBr2.
case of [Ru
N
2ACHTUNGTRENNUNG(dmso)]: Careful layering of solutions of
3c and [RuCl CHTUNGTRENNUNG
of diffusion occurring during this process allowed for in situ
complex formation and crystal growth at the interface. This
procedure led to cocrystallization of [Ru
ACHUTGTNRNE(NUG 3c)Cl2CAHTUNGTRENN(UGN dmso)]
(Figure 2) and 3c as revealed by X-ray diffraction.
Figure 3. Molecular structure of [Zn
ACHTUNGERTN(NUNG 1a)Br2] (hydrogen atoms and coc-
rystallized ZnBr2 and CH3CN solvent molecules have been omitted for
clarity).[17] Selected bond lengths [ꢃ] and angles [8]: Zn N1 2.27(3), Zn
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
N2 2.10(2), Zn N3 2.31(2), Zn Br1 2.377(6), Zn Br2 2.388(6); N1-Zn-
N2 74.8(9), N1-Zn-N3 149.8(8), N1-Zn-Br1 96.2(7), N1-Zn-Br2 94.5(6),
N2-Zn-N3 75.1(8), N2-Zn-Br1 123.4(6), N2-Zn-Br2 110.5(6), N3-Zn-Br1
99.0(5), N3-Zn-Br2 97.3(6), Br1-Zn-Br2 126.0(2).
Figure 2. Molecular structure of [Ru
ACHUTNGTRNENUG(3c)Cl2ACHTUNGTERN(NUGN dmso)] (hydrogen atoms and
a cocrystallized molecule of 3c have been omitted for clarity).[17] Selected
ꢀ
ꢀ
ꢀ
bond lengths [ꢃ] and angles [8]: Ru N1 2.077(7), Ru N2 2.015(9), Ru
ꢀ
ꢀ
ꢀ
N3 2.044(8), Ru Cl1 2.428(2), Ru Cl2 2.399(2), Ru S 2.275(3); N1-Ru-
N2 78.6(3), N1-Ru-N3 156.8(3), N1-Ru-Cl1 86.6(2), N1-Ru-Cl2 93.7(2)
N1-Ru-S 103.1(3), N2-Ru-N3 78.2(4), N2-Ru-Cl1 90.4(3), N2-Ru-Cl2
89.2(3), N2-Ru-S 178.0(3), N3-Ru-Cl1 93.8(3), N3-Ru-Cl2 85.7(3), N3-
Ru-S 100.2(3), Cl1-Ru-Cl2 179.44(8), Cl1-Ru-S 88.59(9), Cl2-Ru-S
91.79(9).
As envisaged, again only one btp ligand coordinates to
the zinc center, which additionally binds two bromido li-
gands, such that the coordination sphere of the Zn ion can
be described as distorted square pyramidal (t[19] =0.46).
Whereas the overall structure resembles the analogous
[ZnCl2ACHTUNGTRNEUNG
(tpy)] complex[20] and the central Zn–pyridine bond
lengths in both complexes are comparable, the bonds of the
terminal triazole N atoms to the Zn center are more than
0.1 ꢃ longer than the 2-pyridyl–Zn bonds in the tpy com-
plex (2.185(3) ꢃ).
The observed very low solubilities in common organic sol-
vents, enabling the facile isolation of the 1:1 metal–btp com-
plexes, appear to originate from extensive p–p stacking in-
teractions[21] between the btp entities leading to a layered ar-
rangement in the crystalline solid (Figure 4). The distances
between the individual yet not exactly parallel planes of the
As expected, the btp ligand binds in a tridentate meri-
dional mode at the ruthenium center, the coordination
sphere of which is completed by two chlorido ligands and
one dmso molecule interacting through its S atom. Hence, a
distorted octahedral geometry results and the main devia-
tion from an ideal structure is caused by the positioning of
the N atoms of the triazole rings: The N1-Ru-N3 angle
amounts to 156.88 and is therefore much smaller than the
ideal 1808 of an undistorted octahedron. The remaining X-
Ru-X’ angles with X and X’ oriented in trans positions (N2-
Ru-S 178.08, Cl1-Ru-Cl2 179.48) are very close to 1808. The
overall structural parameters of the complex are astonish-
ingly similar to the respective tpy complex, that is, [RuCl2-
metal–btp frameworks amount to 3.3–3.7 ꢃ in [Zn
ACHTUNGTRENNUNG(1a)Br2]
and to only 3.1–3.3 ꢃ in [Ru(3a)Cl2A(dmso)], respectively.
A
CHTUNGTRENNUNG
The layered arrangement is additionally supported by hy-
ꢀ
drogen bonds between the MX2 moieties and C H units of
A
U
triazole and pyridine rings belonging to adjacent mole-
cules.[22]
stead of triflates (that had led to 1:2 complexes)[9] represents
a more general approach to 1:1 metal–btp complexes, a solu-
tion of ZnBr2 in acetonitrile was treated with a solution of
btp ligand 1a in acetonitrile. This immediately led to the
Metal–btp complexes with a metal-to-ligand ratio of 1:2
Synthesis and structure: The published syntheses for
[RuL2]2+ (with L=tpy[18b,23] or a more simple representative
of the btp ligand family with alkyl groups at the triazole
precipitation of [ZnACHTUNGTRENNUNG(1a)Br2] in the form of a white powder.
Due to the poor solubility of the compound in common or-
ganic solvents, attempts to grow crystals were again per-
formed by careful addition of a solution of the metal precur-
2A
source in combination with NH4PF6 dissolved in high-boiling
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Complexes of Click-Derived Bistriazolylpyridines
FULL PAPER
treatment
(dmso)] with 1a in the presence
of NH4PF6. [Ru(1a)(1b)](PF6)2
of
[RuACHTUNGTRENNUNG(1b)Cl2-
AHCTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
could be isolated as a lemon-
colored solid and 1H NMR
spectroscopy as well as ESI-MS
measurements[13] clearly showed
that two different ligands are
bound to each of the Ru cen-
ters, thus excluding a 1:1 mix-
ture of the corresponding ho-
moleptic complexes.
Figure 4. Packing diagram of [Zn
CH3CN are omitted for clarity) and [Ru
groups, dmso ligands, and hydrogen atoms are omitted for clarity).
ACHTUNGTRENNUNG
G
CHTUNGTRENNUNG
solvents. We found that our route developed for the prepa-
ration of 1:1 Ru–btp complexes (vide supra) can also be uti-
lized for the synthesis of 1:2 complexes when it is pursued in
the presence of AgOTf (OTf=trifluoromethanesulfonate),
which removes the more strongly bound chlorido ligands
from the coordination sphere of the Ru center.[24] Thus,
treatment of [RuCl
(2 equiv) yielded the complex [Ru
(dmso)4] with 1b (2 equiv) and AgOTf
(1b)2](OTf)2, which is
N
ACHTUNGTRENNUNG
highly soluble in DMF and therefore (in contrast to the 1:1
complexes) remains in solution, thereby facilitating separa-
tion from the AgCl precipitate. Isolation gave [Ru
ACHTUNGERTN(NUNG 1b)2]-
ACHTUNGTRENNUNG
tonitrile and was fully characterized, yet no single crystals
could be obtained.
By utilizing ligand 3b, which lacks the solubilizing triethy-
lene glycol chain, [Ru
heating [RuCl2A(dmso)4] and 3b (2 equiv) in ethylene glycol
at reflux followed by the addition of an aqueous solution of
NH4PF6. In a similar fashion complexes [Ru(btp)2](PF6)2
ACHTUTGNRENUNG(3b)2]ACHTUNTGERN(NUGN PF6)2 could be synthesized by
CHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
with btp=3a, 3c, and 1b were prepared, and for all of these
compounds single crystals suitable for X-ray diffraction
could be grown. Exemplarily, the molecular structure of
Figure 5. Molecular structure of [Ru
ACHTUGTNREN(UNG 3b)2]ACHUTNTGREN(NUGN PF6)2 (hydrogen atoms, PF6
anions, and cocrystallized solvent molecules have been omitted for clari-
ty).[17] Selected bond lengths [ꢃ] and angles [8]: Ru N1 2.066(9), Ru N2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2.023(7), Ru N3 2.056(9), Ru N4 2.045(8), Ru N5 2.031(7), Ru N6
2.046(8); N1-Ru-N2 78.6(3), N1-Ru-N3 157.0(3), N1-Ru-N4 92.7(3), N1-
Ru-N5 101.6(3), N1-Ru-N6 91.7(3), N2-Ru-N3 78.5(3), N2-Ru-N4
104.0(3), N2-Ru-N5 178.2(3), N2-Ru-N6 98.9(3), N3-Ru-N4 92.1(3), N3-
Ru-N5 101.3(3), N3-Ru-N6 92.6(3), N4-Ru-N5 77.8(3), N4-Ru-N6
157.1(3), N5-Ru-N6 79.3(3).
[RuACHTUNGTRENNUNG(3b)2]ACHTUNGTRENNUNG(PF6)2 is shown in Figure 5; the other structures
are depicted in Figures 10–13 in the Supporting Information.
They are all very similar and resemble the structures previ-
ously found for [MACTHNUTRGNEUNG
(btp)2]2+ (M=Ru, Fe):[9,11a] The Ru
center is coordinated by two 3b ligands in a distorted-octa-
hedral fashion, and the distances between the Ru center and
the N atoms of the triazole rings are slightly longer than
those involving the N atoms of the pyridine rings. In gener-
al, the 1:2 complexes—unlike the 1:1 complexes (vide
supra)—are readily soluble in polar organic solvents, such as
acetonitrile and benzonitrile, as neither a layered structure
with p–p interactions nor a network of hydrogen bonds can
be formed.
The selective formation of 1:1 complexes (even under
forcing conditions or in the presence of an excess amount of
btp) on the one hand, and the possibility to introduce a
second btp ligand at the Ru center (through the addition of
AgOTf or heating followed by the addition of NH4PF6) on
the other hand, can be utilized for the preparation of com-
plexes containing two different btp ligands. This is exempli-
An important aspect of the btp coordination chemistry is
related to the question of how the substituents on the aryl
rings of the terminal triazole units and those on the central
pyridyl entity influence the coordination behavior of the li-
gands and thus the properties of the resulting complexes. If
significant substituent effects are unraveled, facile tuning of
the ligand framework could be achieved. In the following
sections, we discuss this possibility to electronically modu-
late the btp framework.
Magnetic properties: A first indication for the relevance of
this aspect came from a closer inspection of the magnetic
behavior of [Fe
spin transition was expected to occur at elevated tempera-
G
G
fied by the synthesis of complex [RuACHTGNURTEN(NGUN 1a)ACHUTNRTEGGN(NUN 1b)]ACHTNUGTERN(NUNG PF6)2 through
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C. Limberg, S. Hecht et al.
tures.[9] Indeed, when [Fe
ACHTUNTRGENNUG(1c)2]ACHTUGNTREN(NUGN OTf)2 was heated to 2008C
the red-brown powder changed its color to yellow. Surpris-
ingly, the original red-brown color could not be re-estab-
lished by cooling and not even by storage at ꢀ308C for sev-
eral weeks or at ꢀ1968C for several days. It could be regen-
erated, though, by dissolving the complex in acetonitrile at
room temperature. Whereas in solution thermally induced
spin transitions proceed gradually, in the solid state their
devolution can vary quite significantly, depending on wheth-
er long-range cooperative effects become operative.[25] For a
closer inspection of the spin transition at higher tempera-
tures we measured cMT (cM = molar magnetic susceptibility)
at a magnetic field of 5000 Oe in the temperature range 5–
400 K (Figure 6).
Figure 6. Temperature dependence of cMT for [FeACHTUNRGTENNUG(1c)2]AHCUTGNTREN(NUNG OTf)2 (tempera-
Figure 7. Mçssbauer spectra of [Fe
a) “red” low-spin [Fe(1c)2]
(OTf)2: d=0.36 mmsꢀ1 and DE0 =0.96 mmsꢀ1
b) “yellow” high-spin [Fe(1c)2](OTf)2 resulting from heating of the red
sample (a) to 2008C for 30 min.
ACHUTNGTRENGUN(1c)2]ACHTUTGNREN(NUGN OTf)2 measured at 80 K.
ture-independent paramagnetism, TIP=1795ꢄ10ꢀ6 cm3 molꢀ1 is subtract-
G
R
.
*
ed). Progress of T program: =1st hysteresis loop: 5–400–5 K, gray
N
ACHTUNGTRENNUNG
~
area; = 2nd hysteresis loop: 5–400–5 K, dashed area; ·=1 h at 400 K;
*
=3rd hysteresis loop: 400–5–400 K, white area.
(Figures 21–24 in the Supporting Information). Its d and
DE0 values are similar to those obtained for octahedral
Starting from low-spin [FeACHTNUGTRENN(UG 1c)2]ACHTUGNTERN(NUNG OTf)2 held at 5 K, raising
the temperature has nearly no effect on cMT until reaching
325 K when cMT increases sharply, reaching a value of
1.90 cm3 Kmolꢀ1 at 400 K. Recooling of the sample leads to
a sharp decrease of the magnetic moment that parallels the
increase on heating, but with a hysteresis at a 25–50 K lower
temperature, and this process proved to be partially reversi-
ble in a further heating–cooling cycle (Figure 6). The low-
temperature plateau of approximately 0.1 cm3 Kmolꢀ1 might
indicate the trapping of part of the sample (about 3%) in
the high-spin state, possibly already in the course of the
work-up procedure,[13] and the fact that tempering (vide
infra) increases the value of this plateau seems to support
this. Dislocations and defects in the lattice are usually held
responsible for such behavior in the case of a crystalline ma-
terial.[25]
iron(II) low-spin complexes, such as [FeACTHNGUETRNNU(G 3-bpp)2]-
AHCTUNGTRENNUNG
(OTf)2·H2O[26] (3-bpp=2,6-bis(pyrazolyl-3-yl)pyridine; d=
0.29 mmsꢀ1, DE0 =0.69 mmsꢀ1 at 140 K) or [Fe
ACHTUNGTRENNUNG
[27c]
(BF4)2
(1-bpp=2,6-bis(pyrazolyl-1-yl)pyridine;
0.39 mmsꢀ1, DE0 =0.75 mmsꢀ1 at 80 K), and the results thus
support the predominance of the low-spin state within the
temperature range of 7–295 K.[28]
The preliminary qualitative observations outlined above
seemed to suggest that warming of [FeACTHNUGTRENNUG(1c)2]ACHTUNGTREN(NUGN OTf)2 to higher
temperatures (2008C) causes irreversibility of the spin tran-
sition. Indeed, a magnetic measurement for a corresponding
“yellow sample” showed a typical high-spin behavior with
cMT varying only between 3.5 and 3.7 cm3 Kmolꢀ1 in the
temperature range 50–400 K (Figure 33 in the Supporting
Information). Consistently, a high-spin doublet dominates
the Mçssbauer spectrum of such a sample (Figure 7b, gray
area: d=1.10 mmsꢀ1 and DE0 =1.21 mmsꢀ1), while the in-
tensity of the original low-spin signal decreased to only
1.7% (Figure 7b, black area: d=0.40 mmsꢀ1 and DE0 =
1.04 mmsꢀ1).
To complement these findings, Mçssbauer spectra of pow-
dered [FeACHTUNGTRENNUNG(1c)2]ACHTUNGTRENNUNG(OTf)2 were recorded at 7, 80, 200, and
295 K. The spectrum at 80 K shows one doublet with d=
0.36 mmsꢀ1 and DE0 =0.96 mmsꢀ1 (Figure 7a), and increas-
ing the temperature to 295 K did not lead to basic changes
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Complexes of Click-Derived Bistriazolylpyridines
FULL PAPER
To test whether tempering at temperatures somewhat
far higher in the latter case of the more electron-donating
btp ligand and a quantitative analysis indicates a low-spin
contribution of only 11.3% (Figure 37 in the Supporting In-
formation). This is in agreement with the results of Mçssba-
uer measurements performed for powdered samples of [Fe-
ACHUTNGRENU(NG 1b)2]ACHTUNTGREN(NGUN OTf)2 at 7 and 80 K, which reveal a spin distribution
of 90% high spin and 10% low spin (Figures 27 and 28 in
the Supporting Information). Furthermore, whereas in the
lower than 2008C (473 K) has a similar effect, [Fe
ACHTUNGERTN(NUNG 1c)2]-
ACHTUNGTRENNUNG
already) was kept at 400 K for one hour and its magnetic be-
havior was investigated during this time (Figure 6): cMT in-
creased from 1.92 to 2.05 cm3 Kmolꢀ1, and on cooling it de-
creased only to 0.62 cm3 Kmolꢀ1 at room temperature
(0.33 cm3 Kmolꢀ1 before heating) and to 0.24 cm3 Kmolꢀ1 at
5 K (0.07 cm3 Kmolꢀ1 before heating). Consistently, a Mçss-
bauer spectrum recorded subsequently at 80 K now showed
beside the low-spin doublet (Figure 7a) a high-spin doublet
case of [FeACTHNUGTRENNU(G 1c)2]ACHTUNGTRENN(UGN OTf)2 initial heating cycles are reversible,
display a hysteresis, and become irreversible only after pro-
longed heating, the corresponding complex incorporating
methyl-substituted ligand 1b remains quantitatively in the
high-spin state once it has reached 400 K (Figures 36 and 37
as depicted in Figure 7b with
a partitioning of 9%
(Figure 25 in the Supporting Information). Hence, temper-
ing at 400 K or—more rigorously—short periods of heating
to 473 K (2008C) antagonize the reversibility of the low-
spin$high-spin transition. A possible reason for this is the
occurrence of structural rearrangements occurring at higher
in the Supporting Information). Finally, complex [Fe
ACHTUNGTRENNUNG(3a)2]-
AHCTUNGTRENNUNG
gands, was investigated with the expectation of an even
more dominating high-spin state.
temperatures that favor elongated Fe N bonds, characteris-
Notably, a cMT value of only 1.8 cm3 Kmolꢀ1 was deter-
mined at room temperature, and besides this, the study of
ꢀ
tic for high-spin complexes. Variable-temperature X-ray
analysis and powder X-ray diffraction were not possible. To
further investigate this phenomenon differential scanning
calorimetry (DSC) measurements were performed for the
temperature range 250–475 K (Figure 20 in the Supporting
Information), the results of which nicely complemented the
magnetic measurements, and they can be summarized as fol-
lows: 1) on heating from 217 to 400 K a broad endothermic
peak at 330 K with a shoulder at 360 K appears, which can
be assigned to a two-stage low-spin!high-spin transition, as
well as a small additional peak that corresponds to a phase
transition; 2) the phase transition at 390 K activates cooper-
ativity during the spin crossover, as confirmed by a further
magnetic measurement: If heating of the sample is stopped
at 375 K, in corresponding cooling–heating cycles cMT de-
creases and increases only slowly; once a sample has been
heated to 400 K, however, the spin crossover occurs far
more abruptly, cooperatively, and reversibly (compare Fig-
ures 20 and 30 in the Supporting Information); 3) on heating
of the sample to 473 K a third peak is observed at 430 K,
whereby the residual low-spin molecules are transferred into
the high-spin state, and hence the spin crossover proceeds in
three steps. Having passed the third step, the crossover to
the high-spin state becomes irreversible: on recooling to
224 K no features are observed anymore. Please note that
heating does not lead to appreciable decomposition of [Fe-
the temperature-dependent behavior of [Fe
vealed several astonishing effects (Figure 8): While initial
ACHUTNGTRNENUG(3a)2]CAHTUNGTERNN(UGN OTf)2 re-
Figure 8. Temperature dependence of cMT for [Fe
ACHTUGNETRN(NUGN 3a)2]ACHTUNGTRENN(UNG OTf)2 (TIP =
1609ꢄ10ꢀ6 cm3 molꢀ1 is subtracted). Progress of T program: =1st hyste-
*
*
*
resis loop: 300–140–300 K; =2nd hysteresis loop: 300–5–400 K; =3rd
*
hysteresis loop: 400–5–400 K; =4th hysteresis loop: 400–5–400 K.
cooling from room temperature to 5 K leads to a “normal”
steady decrease of cMT to a low-temperature plateau at
around 1.5 cm3 Kmolꢀ1, on annealing of this sample cMT de-
creases between 220 and 270 K, whereas only on further
warming is a spin-crossover behavior like that observed for
ACHTUNGTRENNUNG(1c)2]ACHTUNGTRENNUNG(OTf)2 as shown by thermogravimetric analysis
(Figure 19 in the Supporting Information).
Importantly, the partitioning of the high-spin state in a
solid sample and also the conditions required to completely
lock [Fe
N
[Fe
2.4 cm3 Kmolꢀ1 at 400 K, and on recooling—as in the case of
[Fe(1c)2](OTf)2—a hysteresis is observed, which on rean-
ACHTUTGNRENGU(N 1c)2]ACHTUNGTREN(NUNG OTf)2 (Figure 6) found. cMT reaches a value of
pendent on the substituents on the aryl rings. Whereas the
complex obtained from ligand 1c, carrying electron-accept-
ing NO2 groups, has a cMT value of 0.1 cm3 Kmolꢀ1 (com-
pare Figure 6) at room temperature, it amounts to
A
ACHTUNGTRENNUNG
nealing features a minimum again (Figure 8). With every
heating–cooling cycle the minimum at 220–270 K loses
depth but is always clearly visible. Since thermodynamically
it is not plausible that the partitioning of the low-spin state
becomes higher at any stage of the temperature range on
warming, the minimum should be associated with a structur-
3.5 cm3 Kmolꢀ1 in the case of [Fe
the Supporting Information), which differs from [Fe
(OTf)2 only by the replacement of the NO2 groups by
methyl substituents. Clearly the high-spin/low-spin ratio is
ACHUTGTNERNNUG(1b)2]ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
Chem. Eur. J. 2010, 16, 10202 – 10213
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10207

C. Limberg, S. Hecht et al.
al phase transition within this temperature range that favors
the low-spin state.[29] Hence, a DSC measurement was car-
ried out, which, however, only showed a maximum at 350 K,
which corresponds to the low-spin!high-spin transition. No
feature around 250 K was observed. These results concern-
The measured oxidation potentials clearly correlate with
the electron-donating ability of the substituent in the 4-posi-
tion attached to the central pyridine moiety of the btp
ligand, which leads to lower oxidation potentials when more
strongly donating substituents are incorporated.[30] For ex-
ing the solid-state magnetic behavior of [Fe
reveal several important aspects: The hysteresis and high-
spin trapping observed for [Fe(1c)2](OTf)2 are not due to
A
N
ample, the Ru2+ center in [Ru
ACHTNGUTREUNN(G 3a)2]ACTHNUGTREN(NUGN PF6)2 is oxidized to
Ru3+ at a potential of 0.58 V. This is significantly lower than
the potentials required to oxidize the three other complexes
and is consistent with the strongly electron-donating proper-
ties of the pyrrolidyl substituent in ligand 3a. On the other
hand, by attaching an electron-withdrawing ester substituent
A
ACHTUNGTRENNUNG
the OTg residue but apparently a property inherent to the
btp framework. Secondly, it becomes clear that for the solid-
state samples the substitution pattern of the btp ligands sen-
sitively influences the magnetic properties of the resulting
complexes, but the behavior is not predictable, since solid-
state packing interferes and mainly contributes to the over-
all effects observed. To eradicate solid-state effects and un-
ravel their intrinsic magnetic properties, the complexes [Fe-
as in [RuACHTUNGTRENNGU(3c)2]ACHTUNGTRNE(NUGN PF6)2 the metal center is oxidized at a signifi-
cantly higher potential of 1.19 V. Remarkably, simple varia-
tion of one substituent attached to the central pyridine
moiety of the btp scaffold enables shifting of the Ru2+ oxi-
dation potential over a range exceeding 0.6 V (Table 1).
ACHTUNGTRENNUNG(btp)2]ACHTUNGTRENNUNG(OTf)2 (btp=1c, 1b, 3a) were also investigated in so-
lution by using the Evans method, which indeed indicated a
clear trend: dissolved in acetonitrile at room temperature
Table 1. Half-wave potentials and peak potential differences of investi-
gated [RuACTHNUTRGNEUNG
(btp)2]2+ complexes versus the ferrocene/ferrocenium (fc/fc+)
[Fe
the solid state, and—also consistently—a high-spin contribu-
tion was found for [Fe(1b)2](OTf)2, albeit to a much smaller
extent than that in the solid state (1.6 mB; such differences
between the solid-state and solution behavior are rather
common[25]). The high-spin partioning was even more pro-
ACHTUNGTRENNUNG(1c)2]ACHTUNGTRENNUNG(OTf)2 again adopts a low-spin configuration as in
couple determined from cyclovoltammetry (Figure 9).
1
Complex
E = [V]
DEp [mV]
2
A
ACHTUNGTRENNUNG
[Ru
[Ru
[Ru
[Ru
N
R
0.58
0.89
1.06
1.19
102
86
98
ACHTUNGTRENNUNG
A
U
152
nounced in the case of [FeACHTNUGTRENNG(U 3a)2]ACHTUNGTERN(NNGU OTf)2 (3.4 mB), and there-
fore the ligand-field splitting decreases with increasing
donor character of the btp ligands. This should also be re-
flected in the redox properties of btp complexes (through
the positioning of the t2g orbital set), which was investigated
using Ru2+ instead of Fe2+ as the central atom, since the Ru
complexes persist as 1:2 complexes in solution, in contrast
to their highly dynamic Fe analogues (vide infra).
A more quantitative analysis of the data is possible by in-
volving classical linear free-energy relationships, which cor-
relate chemical-reaction parameters, such as equilibrium
constants or rates, with substituent effects (in p-conjugated
systems).[31] When plotting the measured half-wave poten-
tials against Hammettꢅs prominent spara values of the respec-
tive substituents,[32] a clear linear correlation was found (Fig-
ure 10).[31f,33] This finding illustrates that the electron-donat-
ing ability of the central pyridine moiety strongly influences
the oxidation potential of the bound metal center and fur-
thermore implies that a predictable fine-tuning of redox
Redox properties: To gain a better understanding of the in-
fluence that residues bound to the btp scaffold have on the
electronic properties of coordinated metal centers, cyclic
voltammetry measurements with various [Ru
complexes were performed. Cyclic voltammograms of [Ru-
(3a)2](PF6)2, [Ru(1b)2](PF6)2, [Ru(3b)2](PF6)2, and [Ru-
(3c)2](PF6)2 display qualitatively similar redox behavior yet
ACHTUNGTREN(NUG btp)2]ACHTUNGTREN(NUNG PF6)2
A
E
E
G
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
the characteristic one-electron oxidation waves are strongly
shifted depending on the ligand attached (Figure 9).
Figure 10. Half-wave potential of the investigated [Ru
determined from cyclovoltammetry (Figure 9, Table 1) in relation to the
(btp)2]2+ complexes
ACHTUNGTRENNUNG
ꢀ
ꢀ
Figure 9. Cyclic voltammetry of various Ru–(btp)2 complexes (1 mm,
200 mVsꢀ1, 0.1m (nBu)4NPF6, PhCN) versus the ferrocene/ferrocenium
(fc/fc+) couple.
spara values of their btp substituents R1 (spara
N
(
[32]
ꢀ
ꢀ
OMe)=ꢀ0.27; spara( H)=0; spara
G
The line shows
the linear fit (R=0.98836).
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Complexes of Click-Derived Bistriazolylpyridines
FULL PAPER
properties by substitution of the btp ligand framework is
possible.
By considering that pyridine-based ligands with two flank-
stability constants are derived from fitting the data, the cor-
responding complexation enthalpies can be obtained directly
from the measurements and are hence associated with a
very small error. Therefore, the complexation enthalpies
have been used to compare the substituent effects on the
thermodynamics of complex formation. The measured over-
all complexation enthalpies range from ꢀ13 to
ꢀ18 kcalmolꢀ1 depending on the ligand and its substitution
pattern. Donor substitution—not only at the central pyridine
but also at the terminal phenyl moieties—clearly increases
the exothermicity of complexation. Interestingly, series 1a–c
spans a much wider range (DDH=5.0 kcalmolꢀ1) than series
2a–c (DDH=1.5 kcalmolꢀ1), which illustrates the stronger
influence of the terminal substituents R2/R3 in the ether-sub-
stituted series 1a–c. Given a stronger variation of the R1
substituents, an even more pronounced alteration of the
complexation thermodynamics can be anticipated. Interest-
ingly, 1c shows a lower absolute enthalpy of complexation
than 2c.
ing imido units, namely, bisACHTNUTRGNE(UNG imino)pyridines, have proved
non-innocent in reductions of their corresponding [ML2]2+
complexes (M=Ni, Zn, Fe), as indicated by two reversible
reduction waves between ꢀ1 and ꢀ2 V,[34] we have also in-
vestigated the electrochemical reduction of [RuACTHNUGRTENN(UG btp)2]ACHTUNTGREN(NUNG PF6)2
complexes. In the case of [Ru
(3b)2]
E
tive current starts to flow and increases until ꢀ2.0 V (with a
shoulder at ꢀ1.5 V) but the corresponding process is not re-
versible. However, [RuACTHNUTRGENN(UG 3c)2]ACHTUGNTERN(NUGN PF6)2 exhibited two reversible
reduction waves at 1.06 and 1.27 V, even though 3c itself
cannot be reduced reproducibly. The seemingly contradicto-
ry findings stimulate efforts to carry out the two reduction
steps chemically to isolate and characterize the resulting
product(s) in the future.
Complex stability: Although the magnetic and electronic
properties of the various btp complexes are clearly strongly
influenced by substitution, we investigated the influence of
the differently substituted btp ligands on the stability of
their complexes. For this purpose, isothermal titration calo-
rimetry (ITC) measurements were performed to monitor
In both ligand series, the measured enthalpies of complex-
ation can be correlated to Hammettꢅs spara values
(Figure 11). The observed linear correlation is rather re-
the formation of Fe2+ complexes from [Fe
ACHTNUGTRENN(UG OTf)2ACHTUGNTRNE(NUGN CH3CN)2]
on treatment with two btp ligand series 1a–d and 2a–c. Fe2+
instead of Ru2+ was employed to ensure fast ligand-ex-
change kinetics, which is a prerequisite for reliable ITC
measurements. Although ITC has received rather little at-
tention in the field of coordination chemistry,[35] it can pro-
vide detailed insight into the thermodynamics of complexa-
tion processes and hence the strength of metal–ligand inter-
actions.[36] ITC measurements were carried out by titrating
small aliquots of [FeACHTNUGTRENNU(G OTf)2ACHTUNTGREN(NGUN CH3CN)2] into a solution of the
respective btp ligand in acetonitrile, and after averaging
three independent titration experiments on each ligand and
correction for the heat of dilution (Figure 17 in the Support-
ing Information), thermodynamic parameters describing the
complexation equilibria could be extracted (Table 2).
Figure 11. Complexation enthalpies for the formation of [FeACTHNUGRTENNUG(btp)2]ACHTUNGTRENNUNG(OTf)2
complexes of various btp ligands in CH3CN at 258C (Table 2) as a func-
tion of the spara values of their substituents R2 and R3.[32] Squares: 1a–c
(R1 =OTg), circles: 2a–c (R1 =COOTg). The lines show the fit of the
data (circles: R=0.98656; squares: R=0.99913).
Quantitative analysis of the data shows cumulative stabili-
ty constants (b) in the order of 106–108 mꢀ2 for solutions of
the [FeACHTUNGTRENNUNG(btp)2]ACHTUNGTRENNUNG(OTf)2 complexes in acetonitrile. Although the
markable since the terminal substituents are not in true res-
onance with the coordinating N atoms of the triazole moiet-
ies. This finding points to the possibility of charge delocali-
zation across the triazole ring system.[37] In addition, the ob-
served linearity indicates that within the investigated series
the entropic contribution to complexation does not vary sig-
nificantly and hence differences in complexation ability
most likely are enthalpic in origin.
Table 2. Cumulative stability constants and corresponding enthalpies for
formation of [Fe(btp)2](OTf)2 complexes in CH3CN at 258C measured by
ITC (average of three independent measurements) starting from an
excess amount of btp ligand (ꢁ0.125 mm) and subsequent [Fe(OTf)2-
(CH3CN)2] addition (55 steps of 1 mL each of a ꢁ3.125 mm solution).
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
Fe2+ +2
(btp)![Fe
ACHTUNGTRENNUNG
btp
b [mꢀ2
]
]
Clearly, the ITC results are in line with the CV measure-
ments showing a correlation between binding enthalpies and
redox potentials, both of which primarily depend on the do-
nating/accepting properties of the ligands by considering the
respective metal center (Fe or Ru). However, the ITC re-
sults, which indicate a strengthening of the Fe–btp interac-
tion with increasing electron density within the btp frame-
1a
1b
1c
1d
2a
2b
2c
ꢁ108
ꢁ108
ꢁ107
ꢁ107
ꢁ107
ꢁ107
ꢁ106
ꢀ18.13ꢂ0.14
ꢀ15.34ꢂ0.12
ꢀ13.08ꢂ0.13
ꢀ14.05ꢂ0.14
ꢀ15.30ꢂ0.07
ꢀ14.74ꢂ0.09
ꢀ13.82ꢂ0.13
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10209

C. Limberg, S. Hecht et al.
work, seem to contradict those of the magnetic measure-
ments. A strong interaction is usually associated with a
strong ligand field and thus with a large t2g–eg separation,
which should favor low-spin states. However, while the bind-
ing enthalpy of 1b (absolute value) is higher than the one of
1c, still the partioning of the low-spin state decreases on
going from 1c to 1b. This can be explained in a simplistic
way assuming that the binding enthalpies measured are
mainly due to Fe–N s interactions and will increase with in-
creasing “N base strength”, that is, with increasing electron
density. However, the substituents also sensitively influence
the energetic positions of the ligand p and p* orbitals and
thus their interaction with the metal t2g and eg orbitals,
which also determines the t2g–eg separation. For phenanthro-
line derivatives this has already led to counterintuitive
ligand-field splittings in the past.[38] Apparently, also in our
case, the substituents influence the contribution of the p in-
teractions to the t2g–eg separation more strongly and in an
opposite direction than the contribution of the s interac-
tions, which, however, mainly determine the binding enthal-
pies. Note that NO2 groups will reduce the s-donor capacity,
while the p-accepting ability is enhanced.
To gain further insight into the relative importance of ter-
minal and central substituents on complexation thermody-
namics, ITC experiments were also performed with deriva-
tive 1d, which carries a donor on one phenyl terminus while
carrying an acceptor on the other terminus. Within the
ether-substituted series (1a–d), 1d does not display an exact
intermediate complexation enthalpy but is rather similar to
1c, which suggests that acceptor substitution is dominating
the ligand behavior.
Figure 12. a) UV/Vis titration of 1b (3.51ꢄ10ꢀ5 m) with [Fe
ACHTUNGTRENNUNG(OTf)2-
(MeCN)2] (0!1.4 equiv) in acetonitrile at 258C. b) Molar extinction co-
efficient in relation to the Fe2+/btp ratio for 293 (squares), 345 (circles),
and 430 nm (triangles).
isosbestic points (l=269, 301, 312 nm) indicates a unimolec-
ular process and thus the presence of only two absorbing
species, further supported by a more rigorous analysis using
extinction difference diagrams[40] (Figure 14 in the Support-
ing Information). The absorption arising from the formed
complex (lmax =293, 345, 430 nm) continues to increase
Complexation equilibria: From the ITC curves showing pro-
nounced exotherms of equal intensity until half an equiva-
lent of [Fe
the Supporting Information), it is apparent that 1:2 com-
plexes, that is, [Fe
(btp)2]2+, are formed. Clearly, considera-
ACHTUNGTRENNUNG(OTf)2ACHTUNGTRENNUNG(CH3CN)2] has been added (Figure 18 in
ACHTUNGTRENNUNG
ble energy is released upon exchange of the weakly coordi-
nating acetonitrile/triflate ligands by the tridentate btp
upon addition of [FeACTHNUGTRENNU(G OTf)2ACHTUGNTRENUN(GN CH3CN)2] until a 1:2 ratio of
Fe/btp has been reached (Figure 12b), as expected on the
basis of the ITC measurements. Furthermore, the 1:2 stoi-
chiometry of the formed complex was verified by a Job
plot[41] (Figure 16 in the Supporting Information), and the
shape of the actual Job curve, which is known to depend on
the complex stability in a characteristic manner,[41c] was in
excellent agreement with the stability constant of 108 mꢀ2 de-
rived from the ITC measurements.
ligand framework. This observation is further supported by
[9]
the isolation of [FeACHTNUTGREN(NUNG 1c)]2CAHTUNGTNER(NUGN OTf)2 from one such reaction,
and subsequent UV/Vis titrations of 1b with [Fe
(CH3CN)2] (Figure 12) also corroborated these findings.
Addition of the Fe2+ ions is indicated by the evolution of
ACHTUNGERTN(NUNG OTf)2-
ACHTUNGTRENNUNG
three different bands: 1) the hypsochromically shifted ligand
p!p* absorption (lmax =293 nm), 2) the p!p* absorption
associated with the syn,syn conformation of the ligands
(lmax =345 nm), and 3) the MLCT transition (lmax
430 nm).[39] Note that the UV/Vis spectra of various [Ru-
ACHTUNGTRENNUNG
(btp)2]2+ complexes[13] show pronounced changes in the
energy and intensity of the MLCT transition, again pointing
to significant electronic modulation of the complexes by
ligand substitution, although in this case no simple linear
correlation was found (Figure 15 in the Supporting Informa-
tion). Relative to the analogous tpy-based Fe complexes, the
MLCT transition in the btp complexes is hypsochromically
shifted by approximately 100 nm. The presence of several
=
Complexation dynamics: As pointed out, [FeACTHUNGRTEN(NUG 1c)2]ACHTUNGTRENNUNG(OTf)2
adopts a low-spin configuration both in the solid state and
in solution (vide supra). Consistently, on dissolution of [Fe-
(OTf)2 in CD3CN the 1H NMR spectrum shows one
ACHUTGTNERNNUG(1c)2]ACHTUNGTRENNUGN
sharp signal set for the protons associated with the bound
btp ligand, with all resonances being shifted (to different ex-
tents) downfield relative to free 1c. Nevertheless, more thor-
ough NMR spectroscopic titration experiments to comple-
ment the UV/Vis experiments were carried out with ligand
1b (Figure 13a–e): [FeACHTUNGTRENNG(U 1b)2]ACHTUNGTREN(NUNG OTf)2 dissolved in CD3CN also
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Complexes of Click-Derived Bistriazolylpyridines
FULL PAPER
4 1b sample, and instead only a broadening of all signals be-
longing to the btp framework was observed (Figure 13d).
This finding could in principle be rationalized by a rapid
ligand exchange. To investigate the ligand-exchange dynam-
ics the sample was cooled to 243 K,[42] which led to severe
changes in the spectrum (Figure 13e). Several new resonan-
ces appear in the aromatic as well as aliphatic regions,
which suggests that now the signals of both the free and the
coordinated btp molecules can be detected due to the decel-
erated ligand exchange. Although most signals could not be
assigned unambiguously,[43] the observed splitting of the
signal associated with the terminal methyl group (signal g in
Figure 13) indicates that indeed complex and ligand coexist
in an approximate 1:2 ratio, as expected by the given stoichi-
ometry (note that the spectrum of the free btp ligand does
not undergo any changes upon cooling).
The results of the NMR spectroscopic investigations clear-
ly show that ligand-exchange dynamics for the btp/FeII
system in acetonitrile at room temperature are fast on the
NMR spectroscopic timescale, which represents a marked
difference to the corresponding tpy chemistry. Consequently,
addition of two equivalents of tpy to a solution of [Fe
(OTf)2 in CD3CN leads to two signal sets: one for [Fe-
(tpy)2](OTf)2 and one for uncomplexed 1b. In addition to a
ACHTUNGTRENNUNG(1b)2]-
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
fast ligand exchange, further equilibria in solution are possi-
bly involved, considering that ESI/MS measurements per-
formed with [Fe
(amongst others) peaks for [Fe
(1b)2A
(OTf)]+ (Figure 42 in the Supporting Information).[44]
ACHUTGTNERNNUG(1b)2]ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
CHTUNGTRENNUNG
This naturally raises the question of the role of acetonitrile
in these dynamics, especially considering that it is needed
for the occupation of the free coordination sites generated.
Indeed, performing the same NMR spectroscopic experi-
ments in CD2Cl2 revealed that the ability of the solvent to
coordinate to the metal center is crucial for the establish-
ment of the equilibrium described above: On dissolution of
Figure 13. 1H NMR spectra of a) 1b, b) [Fe
(OTf)2A(CH3CN)2], d) [Fe(1b)2](OTf)2 +21b at RT, and e) [FeACHTUNGTRENNNUG
ACHUTGTNERNNUG(1b)2]ACHTUNGTRENNUNG
G
N
G
R
[FeACHTUNGTRENNU(G 1b)2]ACHTUNGTERN(NUGN OTf)2 together with 1b in CD2Cl2 two very sharp
A
sets of signals are observed, one of which is identical to the
spectrum displayed by free 1b. Hence, it is possible to influ-
ence the complex stability and the possibility of ligand ex-
change by the solvent, which may be utilized for the design
of more complex structures in the future. Furthermore it
should be pointed out that the corresponding Ru complexes
are also stable in acetonitrile in which no ligand exchange
could be detected, a prerequisite for reasonable CV data
(vide supra).
resentation, signals f and g are not shown in their full height. Signals
marked with * can be attributed to impurities, those marked with D
belong to CH3CN.
primarily exhibits low-spin character (vide supra) and
beyond that offers enhanced solubility relative to [Fe
(OTf)2.
Similarly as for [Fe
tween the spectra of the isolated complex [Fe
(Figure 13b) and the 1:2 mixture of [Fe(OTf)
ACHTUNGERTN(NUNG 1c)2]-
ACHTUNGTRENNUNG
ACHUTNRGEN(NUG 1c)2]ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
N
CHTUNGTRENNUNG
and ligand 1b (Figure 13c). Upon addition of two further
equivalents of ligand 1b to a 1:2 sample, giving rise to a Fe/
btp ratio of 1:4, mainly the 1:2 complex and free ligand
would be expected to be present in solution, and it is inter-
esting to note that performing such an NMR spectroscopic
experiment with the tpy ligand indeed leads to two sets of
signals at room temperature: one for free tpy and one for
Conclusion
By exploiting the synthetic versatility of click chemistry, btp
ligands with a wide range of substitution patterns were pre-
pared in a modular fashion. The redox properties as well as
the thermodynamic stability of their resulting transition-
metal complexes, in particular FeII and RuII, display an ex-
cellent correlation to the electronic properties of the ligands.
The observed linear relationship with Hammettꢅs substituent
[Fe
ACHTUNGTRENNUNG(tpy)2]ACHTUNGTRENNUNG
set appeared within the spectrum of a [Fe
N
CHTUNGTRENNUNG
Chem. Eur. J. 2010, 16, 10202 – 10213
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10211

C. Limberg, S. Hecht et al.
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constants enables the precise fine-tuning of btp ligands to
yield btp complexes with predictable electronic properties.
For example, oxidation potentials of [RuACTHNUTRGNEUNG
(btp)2]2+ complexes
can predictably be tuned within a range of 610 mV by
simple ligand substitution. The magnetic behavior of the
iron complexes in solution is sensitively and consistently in-
fluenced by the substitution pattern, too, while the magnetic
investigation of solid samples revealed very interesting fea-
tures such as hystereses, high-spin locking through temper-
ing, and “low-spin minima”, the origin of which remains to
be understood.
When comparing the btp ligand scaffold with established
tridentate nitrogen-based ligands, in particular tpy, btp li-
gands maintain considerable complex stabilities while offer-
ing the advantage of much faster ligand-exchange dynamics.
This should lead to improved properties in supramolecular
polymeric materials,[1b,45] as the self-healing is accelerated.
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Acknowledgements
[15] J.-P. Sauvage, J.-P. Collin, J.-C. Chambron, S. Guillerez, C. Coudret,
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The authors thank C. Knispel, P. Neubauer, and Dr. B. Ziemer (HU
Berlin) for carrying out the single-crystal X-ray structure analyses and
Dr. H. Hennig and Prof. N. P. Ernsting for their help with using the iso-
thermal titration calorimeter. Generous support by ERA Chemistry
(Project “SurConFold”), the German Research Foundation (DFG via
SFB 765), the Fonds der Chemischen Industrie, the Bundesministerium
fꢁr Bildung, Wissenschaft, Forschung und Technologie, and the Dr. Otto
Rçhm Gedꢀchtnisstiftung is gratefully acknowledged. Wacker Chemie
AG, BASF AG, Bayer Industry Services, and Sasol Germany are thanked
for generous donations of chemicals.
[17] CCDC-759265 (3a), -759266 (3b), -759267 ([Ru
-759268 ([Ru(1b)2](PF6)2), -759269 ([Zn(1a)(Br)2]), -759270 (3c),
-759271 ([Ru(3c)2](PF6)2), -759272 ([Ru(3a)2](PF6)2), and 759273
([Ru(3c)(dmso)Cl2]) contain the supplementary crystallographic
ACHTUNGTREN(NUG 3b)2]ACHTUNGTRENNUNG(PF6)2),
A
R
ACHTUNGTRENNUNG
A
E
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif.
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A
U
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 10202 – 10213

Complexes of Click-Derived Bistriazolylpyridines
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T
(OTf)2 with d=0.36 mmsꢀ1 is compara-
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
A
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ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
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AHCTUNGTRENNUNG
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ACHTUNGTRENNUNG
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Received: March 23, 2010
Revised: May 20, 2010
Published online: July 23, 2010
Chem. Eur. J. 2010, 16, 10202 – 10213
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10213