Comparing Isomeric Tridentate Carbazole-Based Click Ligands: Metal Complexes and Redox Chemistry

Article abstract of DOI:10.1002/chem.201704858

Two novel bis(triazolyl)carbazole ligands Hbtc1 (3,6-di(tert-butyl)-1,8-bis[(1-(3,5-di(tert-butyl)phenyl)-1,2,3-triazol-4-yl)]-9H-carbazole) and Hbtc2 (3,6-di(tert-butyl)-1,8-bis[(4-(3,5-di(tert-butyl)phenyl)-1,2,3-triazol-1-yl)]-9H-carbazole), differing

Full text of DOI:10.1002/chem.201704858

DOI: 10.1002/chem.201704858
Full Paper
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Click chemistry
Comparing Isomeric Tridentate Carbazole-Based Click Ligands:
Metal Complexes and Redox Chemistry
Iweta Pryjomska-Ray, Denise Zornik, Michael Pꢀtzel, Konstantin B. Krause, Lutz Grubert,
Beatrice Braun-Cula, Stefan Hecht,* and Christian Limberg*[a]
Abstract: Two novel bis(triazolyl)carbazole ligands Hbtc1
(3,6-di(tert-butyl)-1,8-bis[(1-(3,5-di(tert-butyl)phenyl)-1,2,3-tria-
zol-4-yl)]-9H-carbazole) and Hbtc2 (3,6-di(tert-butyl)-1,8-
bis[(4-(3,5-di(tert-butyl)phenyl)-1,2,3-triazol-1-yl)]-9H-carba-
zole), differing in the regiochemistry of triazole attachment,
have been synthesized by Cu-catalyzed azide-alkyne cyclo-
addition, the so-called “click-reactions”. Metalation with Ru,
Zn, and Ni precursors led to the formation of M(btc)2 com-
plexes (M=Ru, Zn, Ni), with two deprotonated ligands coor-
dinating to the metal center in tridentate fashion, forming
almost perfectly octahedral coordination spheres. The redox
properties of M(btc)2 complexes have been investigated by
cyclic voltammetry, UV/Vis spectroscopy, spectroelectro-
chemistry, and chemically. The CV of the ruthenium com-
plexes revealed three quasi-reversible one-electron oxidation
processes, one assigned as the RuII/III couple and two origi-
nating from ligand-based oxidations. The CVs of both Zn
and Ni complexes contained only two oxidation waves cor-
responding to the oxidation of the two ligands. The oxida-
tion potentials of complexes derived from Hbtc1 ligands
were found to be 300–400 mV lower than those of the cor-
responding complexes derived from Hbtc2, reflecting the
significant difference in donation through the N(2) or N(3)
atom of the triazole moiety.
Introduction
Since the discovery that the Huisgen 1,3-dipolar cycloaddition
between terminal azides and alkynes can be catalyzed by
copper salts (CuAAC), and then proceeds with exquisite con-
version and regioselectivity as a “click reaction”,[1] it has
emerged as one of the most powerful synthetic tools to create
covalent linkages in (bio)materials. However, the resulting 1,4-
disubstituted 1,2,3-triazoles are of interest beyond serving as a
link between two entities, and in fact many applications of
CuAAC nowadays aim at exploiting specific features of the
generated triazole moieties.[2] The fact that 1,2,3-triazoles are
remarkably stable and readily tunable had soon suggested
their utilization as a coordination site to engage in the forma-
tion of transition metal complexes and thus triggered ligand
design based on CuAAC.[3] Within the last ten years a plethora
of such “click ligands” with different denticities and numbers
of triazole donors has been reported.[2c] We have contributed
to this field with the first “clickates” based on 2,6-bis(1-aryl-
1,2,3-triazol-4-yl)pyridines (btps, Scheme 1) as a versatile motif
Scheme 1. Previously reported bis(1,2,3-triazolyl)pyridine (btp),[4,5] where
R1 =CO2Tg, OTg (Tg=-(CH2CH2O)3CH3), R2 =CH3, I, NO2, N(CH3)2, CO2Et, OCH3,
nC10H21, and bis(1,2,3-triazolyl)carbazole (Hbtc) ligands studied in this work.
for coordination and supramolecular chemistry.[4] Btps with a
wide range of substitution patterns were prepared in a modu-
lar fashion, and the redox properties as well as thermodynamic
stabilities of their transition-metal complexes, in particular with
FeII and RuII, display an excellent correlation with the electronic
properties of the ligands.[5] Moreover it turned out that the btp
motif has very similar binding properties as compared to ter-
pyridine (terpy), with similar bond angles and lengths deter-
mined for RuII complexes. Due to such structural observations,
1,2,3-triazoles are often regarded as easy-to-modify pyridine
surrogates, but one has to bear in mind, that compared to pyr-
idine they are somewhat weaker s- and p-donors, as well as p-
acceptors.[2c] Hence, replacement of the peripheral pyridyl units
in terpy by 1,2,3-triazole residues in btp leads to a destabiliza-
tion of the LUMO (p*) and thus increases the energy of the
3MLCT for low-spin d6-metal complexes.[2c,5,6]
[a] Dr. I. Pryjomska-Ray, Dr. D. Zornik, Dr. M. Pꢀtzel, K. B. Krause, Dr. L. Grubert,
Dr. B. Braun-Cula, Prof. S. Hecht, Prof. C. Limberg
Department of Chemistry and IRIS Adlershof
Humboldt Universitꢀt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: sh@chemie.hu-berlin.de
christian.limberg@chemie.hu-berlin.de
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under https://doi.org/10.1002/
chem.201704858.
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Based on our experience with btps, we considered exploit-
ing the respective modular approach for the construction of
new, electronically different ligands to extend the scope of our
tridentate systems. In particular, we envisioned to replace the
central pyridyl unit by a carbazole entity giving rise to a
bis(1,2,3-triazolyl)carbazole (Hbtc, Scheme 1) ligand, for which
we anticipated significantly altered properties and coordina-
tion chemistry as exemplarily detailed below for ruthenium:
1) In contrast to the charge-neutral btp, the deprotonated
btcꢀ is an anionic ligand, which would allow to access rare ex-
amples of overall neutral bis(tridentate) RuII complexes,[7]
which could be purified and vacuum-processed by evapora-
tion/sublimation.
Scheme 2. Synthetic routes to the Hbtc ligand precursors.
2) It is known for bis(tridentate) RuII complexes that the bite
angle of the ligand (158.88 for terpy, 156.78 for btp)[3a] has a
strong influence on the electronic structure, as it determines
the distortion from the ideal octahedron (z compression/xy
elongation)–this in turn determines the relative positioning of
(tert-butyl)phenyl azide with the complementary 1,8-diethynyl-
carbazole.[9] Hbtc2, on the other hand, was obtained by start-
ing from the 1,8-diamino-carbazole derivative,[13] which was
converted to the corresponding 1,8-bisazide, followed by
CuAAC with the bis(tert-butyl)phenyl acetylene.[14]
3
3
the MC and MLCT states.[8] Changing from a pyridyl to a car-
bazole unit can be expected to enlarge the bite angle signifi-
cantly, which will increase the donor strength of the triazole
units.
Complex synthesis and structures: All initial attempts to
deprotonate the carbazole NH unit, applying various different
bases, prior to a subsequent reaction with a ruthenium precur-
sor provided the desired products only in very low yields, due
to incomplete deprotonation, reprotonation by the environ-
ment, ligand decomposition, and nonuniform complexation.
Finally, the Ru(btc)2 products were obtained in satisfactory
yields through a route in analogy to one established for a tri-
dentate pyrrole ligand,[7c] that is, in one step, by reaction of
the Hbtc with RuCl2(DMSO)4 in the presence of NEt3 as a base
and hot EtOH/THF as the solvent. After reflux for 24 h under
anaerobic conditions, the desired complexes Ru(btc1)2 and
Ru(btc2)2 precipitated from the reaction mixture in good yields
as dark red powders. Interestingly, formation of the homoleptic
complexes with a metal to ligand ratio of 1:2 was always ob-
served, irrespective of the precursor ratio used in the reaction.
Successful metalation was indicated by the absence of a
sharp band for the n(NH) stretching vibration at 3379 cmꢀ1 in
the IR spectra of the crude products, and the absence of a sin-
3) Likewise, the negatively charged amido N atom of the car-
bazole is a much stronger donor than the pyridyl N atom in
btp, which will increase the ligand field strength in z direction.
Typically, the two neighboring N atoms of a triazole differ in
their donor strengths, with the N(2) being weaker than the
N(3), and hence we sought to compare the two possible var-
iants of Hbtc (Scheme 1): Hbtc1, where the terminal triazoles
are connected to the carbazole through their C(4) atoms and
thus the coordination involves the N(3) atoms, and Hbtc2,
where the connection is inverted and involves the N(3) leading
to coordination through the N(2) atoms.
Whilst Hbtc2 represents a completely new type of ligand, it
should be noted that one derivative of Hbtc1 (with benzyl in-
stead of aryl residues at the triazole N atoms) has been report-
ed as a ligand of an in situ generated Cu complex used for cy-
anide sensing.[9] Very recently corresponding Co complexes
have been investigated as well.[12] Another recently published
compound formally belongs to the family of Hbtc ligands and
was used as a precursor for bis(carbene) CNC pincer ligands;
however, note that NNN tridentate coordination is not possible
as the N(3) atom is alkylated.[10]
1
glet peak at 12.2 ppm in the H NMR spectra, which are caused
by the NꢀH unit of the free carbazole. The compositions of the
Ru(btc)2 compounds were further verified by ESI-MS and in the
1
case of Ru(btc2)2, also by NMR spectroscopy. H NMR spectra
Here, we present a comparative study of the two Hbtc li-
gands (Scheme 1) based on first investigations on the coordi-
nation behavior of the btc2 anion and an extension of the
scarce knowledge on btc1 complexes. Both ligands proved
redox active, and through detailed examination of correspond-
ing Ru, Zn, and Ni complexes we were able to reveal common
properties and differences of the two isomeric ligand systems.
recorded for Ru(btc1)2 were never interpretable (independently
of the treatment or the way of workup), presumably as the
complex is rather sensitive towards oxidation. Consistently,
even crystals of Ru(btc1)2 showed an EPR signal, as they obvi-
ously also contained the oxidized version.
Both Ru(btc1)2 and Ru(btc2)2 are almost insoluble in most or-
ganic solvents, but readily dissolve in chlorinated or fluorinated
solvents (CHCl3, CH2Cl2, C6H5F, etc.). Therefore, methylene chlo-
ride has been selected as the solvent for crystallization. Single
crystals suitable for X-ray diffraction could be grown through
slow evaporation of the volatiles from saturated solutions of
the complexes. From the fact that those of Ru(btc1)2 showed
Results and Discussion
Ligand synthesis: Synthesis of both ligands involved facile
two-fold click reactions of the corresponding terminal building
blocks to the carbazole core (Scheme 2). On the one hand,
Hbtc1 was synthesized by CuAAC of the in situ generated bis-
+
an EPR signal, indicating the presence of Ru(btc1)2 cations,
which, however, could not be further elucidated, it had to be
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concluded that there may be some ambiguity about the deter-
mined bond lengths and angles. These data are thus not dis-
cussed, whereas the principal structure is shown in the Sup-
porting Information (Figure S5). The molecular structure of
Ru(btc2)2 is however reliable, and shown in Figure 1. In both
cases the btc ligands are bound to the ruthenium center in tri-
dentate modes, forming almost perfectly octahedral coordina-
tion geometries around the RuII ions (average angle N-Ru-N in
Ru(btc2)2 equals 89.94(12)8). Comparison of the bond lengths
in the carbazole backbone of Hbtc2 and coordinated btc2ꢀ li-
gands suggests that the ligands are bound as monoanions.
The average RuꢀN(carbazole) bond length equals 2.007(3) ꢂ,
which is, as expected, slightly shorter than typical N(pyridine)
bonds found in ruthenium complexes of btps (2.02 ꢂ). Due to
the favorable positioning of the donors, the RuꢀNtrz bonds
found in Ru(btc2)2 are also slightly shorter than corresponding
bond lengths observed for Ruꢀbtp complexes (2.06 ꢂ), with
minimal differences observed within one and the same ligand:
The distance RuꢀN(1) (2.014(3) ꢂ) is slightly shorter than Ruꢀ
N(5) (2.027(3) ꢂ). The bite angles found in Ru(btc2)2 are signifi-
cantly larger (177.43(13) and 178.89(11)8), than corresponding
angles in Ru-btp complexes (156.78) and enable the almost
perfect octahedral arrangement of the donors.
For comparing the redox properties of Ru(btc1)2 and
Ru(btc2)2 (see below) we have prepared the analogous zinc(II)
complexes Zn(btc1)2 and Zn(btc2)2 by treating Hbtc with ZnEt2,
as well as the high-spin complexes Ni(btc1)2 and Ni(btc2)2 (an
Evans measurement revealed two unpaired electrons per Ni
center) by reacting NiBr2·DME (DME=1,2-dimethoxyethane)
with Hbtc in the presence of NEt3. Both Hbtc1 derived com-
plexes, Zn(btc1)2 and Ni(btc1)2, have been crystallized success-
fully. While the data obtained for Zn(btc1)2 were not sufficient
to allow for a structure discussion, the molecular structure of
Ni(btc1)2 is depicted, with selected bond lengths and angles, in
Figure 2, as an example of a complex that contains the btc1
ligand.
Figure 2. a) View of the crystal structure of Ni(btc1)2·4(acetone) all hydrogen
atoms and solvent molecules omitted for clarity. Selected bond lengths [ꢂ]
and angles [8]: NiꢀN1 2.032(5), NiꢀN2 2.071(4), N2ꢀN3 1.318(7), N3ꢀN4
1.338(7), N1-Ni-N1’ 180.0, N2-Ni-N2’ 175.1(3); b) Fragment of the crystal
structure showing nitrogen atom numbering scheme in complex Ni(btc1)2.
As expected, the Ni(btc1)2 complex exhibits the same struc-
ture as its ruthenium analogs. The nickel center is located in
an almost ideal octahedral coordination sphere with an aver-
age N-Ni-N bond angle of 90.03(15)8. A large bite angle, similar
to the one found in Ru(btc2)2, is observed (175.18 in Ni(btc1)2
vs. 178.898 in Ru(btc2)2), imposed by the steric constraint pro-
vided by the ligand. Notably, the average NiꢀN(carbazole)
bond in Ni(btc1)2 is found to be slightly longer (2.032(5) ꢂ)
than the corresponding RuꢀN(carbazole) bond in Ru(btc2)2
(2.007 ꢂ). Similarly, the NiꢀN(triazole) bond was found to be
much longer than the analogous CoꢀN(triazole) bonds in a
Co(btc)2(BF4) complex (2.07 ꢂ and 1.94 ꢂ, respectively).[12]
Figure 1. a) View of the crystal structure of Ru(btc2)2; all hydrogen atoms
omitted for clarity. Selected bond lengths [ꢂ] and angles [8]: RuꢀN3 2.008(3),
RuꢀN10 2.006(3), RuꢀN1 2.014(3), RuꢀN5 2.027(3), RuꢀN8 2.024(3), RuꢀN12
2.016(3), N1ꢀN2 1.378(4), N1ꢀN7 1.317(4), N4ꢀN5 1.369(4), N5ꢀN6 1.305(4),
N8ꢀN14 1.314(4), N8ꢀN9 1.371(4), N11ꢀN12 1.370(4), N12ꢀN13 1.313(5), N8-
Ru-N12 177.43(13), N5-Ru-N1 178.89(11), N3-Ru-N10 179.75(15); b) Packing
diagram of the unit cell for complex Ru(btc2)2; c) Fragment of the crystal
structure showing nitrogen atom numbering scheme in Ru(btc2)2.
Cyclic voltammetry (CV): The redox properties of the Ru
complexes have been examined by cyclic voltammetry, at a
platinum working electrode for CH2Cl2 solutions, containing
[NBu4][PF6] as supporting electrolyte. The peak potentials de-
termined for the observed electron transfers are summarized
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to donor.[7c] The same is the case here and therefore we assign
the first transition to the RuII/RuIII couple. This means that the
next two oxidation events involve either both ligands, or only
one of them, whereas the second one corresponds to a RuIII/
RuIV couple.
Table 1. Summary of electrochemical data.
1
3
Epa [V]
Epa2 [V]
btcox1
Epa [V]
MII!MIII
btcox2
Hbtc1[a]
0.735
0.741
ꢀ0.311
ꢀ0.062
ꢀ0.104
ꢀ0.230
Me-btc1[a]
To support these assignments and further decipher the
redox behavior, the electrochemical behavior of Zn(btc2)2 was
studied, since the CV measurements should exclusively reflect
the behavior of the ligand, due to the presence of the redox-
inert metal center. The cyclic voltammogram of Zn(btc2)2
(Figure 4) exhibits two quasi-reversible waves at positive po-
[btc1]-Bu4N+[a]
Ru(btc1)2
Zn(btc1)2
0.722
0.739
0.106
0.009
[b]
ꢀ0.879
ꢀ0.454
[b]
[b]
Ni(btc1)2
Hbtc2[a]
0.975 (irr)
1.056
ꢀ0.069
0.297
0.338
0.212
Me-btc2[a]
[btc2]-Bu4N+[a]
0.984 (qr)
1.150
0.574
[b]
Ru(btc2)2
Zn(btc2)2
Ni(btc2)2
[b]
[b]
0.447
Me-carbazole[a]
3,6-(tBu)2-9-Me-carbazole[a]
0.768 (qr)
0.671
[a] Experiment performed in MeCN; [b] Experiment performed in CH2Cl2.
in Table 1 and referenced versus the ferrocene/ferrocenium po-
tential.
The cyclic voltammogram of Ru(btc2)2 (ꢀ1.15 V!1.25 V, see
Figure 3) exhibits three quasi-reversible redox waves centered
at ꢀ0.45, 0.29, and 1.15 V versus Fc/Fc+, which all belong to
oxidations. One of these events certainly belongs to the RuII!
RuIII transition and related compounds are compared for the
assignment. [Ru(btp)2]2+ compounds were found to show
these transitions between 0.58 and 1.19 V, dependent on the
substituents at the triazole unit,[5] which already demonstrates
the significant influence of even subtle changes within one
and the same ligand class. For [Ru(terpy)2]2+ this redox event
was found at +0.86 V,[11] which is in the range observed for
the btp complexes. However, these are dicationic complexes,
which are not as appropriate for comparison as neutral [L2Ru]
compounds, where—as in Ru(btc1)2 and Ru(btc2)2—the nega-
tive charge is localized at the ruthenium centers. A neutral
bis(2,5-di(2-pyridyl)pyrrolato)-ruthenium(II) complex was found
to be oxidized at a potential as negative as ꢀ0.36 V, which was
attributed to both the effect of overall charge and to the stabi-
lization of the harder RuIII state by the harder, charged pyrrola-
Figure 4. Cyclic voltammograms of Zn(btc)2 complexes in CH2Cl2.
tentials, namely at 0.34 and 0.57 V versus Fc/Fc+, which sup-
ports assignment of the two redox events at positive poten-
tials in the CV of Ru(btc2)2 as ligand-centered oxidations, and
at the same time that the first one at negative potential
(ꢀ0.45 V) corresponds to oxidation of RuII to RuIII. Location of
the second redox wave representing ligand oxidation in
Zn(btc2)2 at much lower potential than in the analogous Ru-
complex (Ru(btc2)2) may be attributed to the fact that the Zn
center with completely filled d-orbitals, does not mediate elec-
tronic communication between the ligands to the same extent
as the RuII ion, so that the first oxidation does not affect the
second one significantly. Location of the first one also at more
negative potential on moving from Zn to Ru is attributed to
the fact that Ru is at that stage already oxidized to the oxida-
tion state +III.
A solution of Ru(btc1)2 in CH2Cl2 displays three quasi-reversi-
ble redox waves at ꢀ0.87, ꢀ0.06, and 0.74 V. By analogy with
Ru(btc2)2 two waves at higher potential can be assigned to
ligand oxidation and the one at ꢀ0.87 V originates from RuII to
RuIII oxidation (Scheme 3). This assignment is consistent with
the presence of two quasi-reversible waves, originating from li-
gands oxidation processes, in a CV of the corresponding zinc
complex (Zn(btc1)2) at ꢀ0.104 and 0.106 V. Also, for the homo-
leptic octahedral cobalt complexes of a btc1 derivative, three
redox events (ꢀ0.79, 0.38, and 0.59 V vs. Ag/AgNO3) have been
recently reported.[12]
The CVs of the corresponding Ni complexes look very similar
as compared to those of the Zn complexes, but appear at yet
more negative potentials (see Table 1). A conceivable explana-
Figure 3. Cyclic voltammograms of Ru(btc)2 complexes in CH2Cl2.
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Scheme 3. Electron transfer processes occurring in Ru(btc1)2 complexes.
tion would be a more ionic character of the NiꢀN(carbazole)
bonds as compared to the corresponding ZnꢀN(carbazole)
bonds.
transitions, typical for unsaturated hydrocarbons. For the
ruthenium complexes–apart from intense bands in the UV
region representing ligand-based transitions–additional bands
in the visible and infrared region of the spectrum are also ob-
served. Figure 5 shows the spectra of free ligand Hbtc2, as well
as Ru(btc2)2. The bands at 446, 546, and 582 nm are presuma-
bly transitions with MLCT character, as evident from their high
absorption coefficients (Table 2). For comparison, correspond-
ing btp complexes show 2–3 bands in the region between
320–480 nm, in dependence of the residues attached.
The CVs of the ligands Hbtc1 and Hbtc2 were also measured
for comparison. Whereas, Hbtc1 revealed a reversible one-elec-
tron oxidation wave at 0.73 V corresponding to the formation
of the radical cation Hbtc1/Hbtc1·+, Hbtc2 only exhibited an ir-
reversible oxidation at 0.97 V.
To provide a better stability to the one-electron oxidized
form of the Hbtc1 and Hbtc2 ligands, their methylated forms
(Me-btc1 and Me-btc2) were also synthesized and studied by
CV. Indeed, both Me-btc1 and Me-btc2 reveal a single one-
electron reversible redox event at 0.741 and 1.056 V, respec-
tively. Notably, these values compare well with the second
one-electron oxidation events of Ru(btc1)2 and Ru(btc2)2
(Table 1) at 0.739 and 1.150 V, respectively, which supports
their assignment as a predominantly ligand-based process. The
CVs of the in situ generated anionic forms (btc1ꢀ and btc2ꢀ)
(using addition of Bu4NOH to a solution of Hbtc in an electro-
chemical cell) were also determined for comparison purposes
(Table 1, Figure S25 and S26 in Supporting Information).
Comparison of the redox properties of metal complexes
with btc1 and btc2 reveals the significant difference in redox
stability of the complexes, depending on the connection mode
of the triazoles to the carbazole moiety, that is, either through
C(4) in btc1 or through N(3) in btc2, and the resulting distinctly
different coordination to the metal center. The oxidation
events in the ruthenium, zinc, and nickel complexes of btc1
are located at significantly lower potentials (between 300–
400 mV) than in the corresponding btc2 complexes. Hence,
the electron density at the carbazole N atom of btc1 is signifi-
cantly higher than in case of btc2. The rather negative poten-
tial of the RuII!RuIII transition in Ru(btc1)2 explains the difficul-
ties that we encountered when attempting to isolate this com-
pound in its fully reduced form in bulk, since it is rather easily
oxidized.
Figure 5. UV/Vis absorption spectra of Hbtc2 and Ru(btc2)2 in CH2Cl2.
Redox chemistry: While studying the UV/Vis properties of
Ru(btc2)2 we noted that, in contrast to Hbtc2, Ru(btc2)2 is sen-
sitive to UV-light and is oxidized in chlorinated solvents. Irradi-
ation of CH2Cl2 (or CHCl3) solutions of both Ru-complexes with
UV-light (190–370 nm) leads to a color change from red to
brown, and the NMR spectra recorded afterwards clearly indi-
cated formation of a paramagnetic species. This was accompa-
nied also by significant changes in spectral features within the
UV/Vis spectrum. As shown in Figure 6, two new bands are de-
veloping in the visible region of the spectrum at 406 and
490 nm and two in the near infrared region at 755 and
845 nm, originating from LMCT or LLCT. At the same time the
three bands in the visible region of the spectrum of Ru(btc2)2,
UV/Vis Spectroscopy: Optical spectra of the Hbtcs and their
metal complexes have been measured in CH2Cl2 over the
range 200–1000 nm. The spectra of both Hbtcs show intense
bands in the UV-region, originating from intra-ligand p!p*
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observed for Ru(btc2)2, and the formation of four new intense
bands at 400, 485, 752, and 836 nm, and hence to formation
of a product displaying basically the same UV/Vis spectrum as
the one obtained after irradiation, which we assign as
[Ru(btc2)2]+.
Table 2. UV/Vis spectral data of the Hbtc ligands and their Ru complexes
in CH2Cl2 solution.
lmax [nm] (e [dm3 molꢀ1 cmꢀ1])
Hbtc1
318
361
377
(23500)
413
(32300)
432
(59500)
318
(71400)
(17200)
530
(7100)
587
(40400)
522
(8000)
(18100)
570
(5800)
750
(5200)
638
(8000)
Accordingly, chemical oxidation under all conditions tested
stopped after the first oxidation event seen in the CV, although
some reagents should have also been capable of realizing the
second oxidation. As in the CV, the first redox event is also
chemically reversible: upon addition of one equivalent of
[CoCp2] (E8’=ꢀ1.33 V vs. Fc) to the oxidized complexes, rapid
regeneration of the bands at 446, 546, and 582 nm could be
observed.
Ru(btc1)2
[Ru(btc1)2]+
Ru(btc1)2]2+
890
(9100)
696
900
(15000)
(11500)
Hbtc2
307
(58500)
446
(29000)
406
(36500)
369
353
(22000)
546
(11800)
490
(26800)
429
367
(25500)
582
(11000)
755
(9500)
682
Ru(btc2)2
[Ru(btc2)2]+
Ru(btc2)2]2+
Consistently, spectroelectrochemical studies showed that the
UV/Vis spectroscopic changes noted above are associated with
the first redox event (RuII!RuIII, Figure 7) and passing the
second one leads to further increasing absorption in the area
where the most redshifted bands appear, as well as evolution
of a band at even longer wavelengths (Figure 8). Importantly,
the spectra belonging to Ru(btc2)2, [Ru(btc2)2]+, and
[Ru(btc2)2]2+ can reversibly be generated while moving along
the CV.
844
(16000)
813
(14800)
(13400)
(8200)
(13000)
Figure 6. Evolution of UV/Vis spectra upon irradiating a solution of Ru(btc2)2
in CH2Cl2 (0.1 mm) with UV-light (190 to 400 nm) over 300 s. Spectra collect-
ed at the intervals of 2 s.
Figure 7. Changes in the UV/Vis spectra of Ru(btc2)2 during cyclic voltamme-
try. Difference spectrum of Ru(btc2)2 while moving through the first oxida-
tion wave in 0.1m Bu4NPF6/CH2Cl2, Pt-mesh, dE/dt=10 mVsꢀ1
c=5ꢃ10ꢀ4 mollꢀ1, cuvette d=0.5 mm.
,
at 446, 546, and 582 nm (see above), are decreasing. No such
changes are observed upon irradiation with visible light. Iso-
sbestic points indicate a unimolecular conversion.
An ESI/MS of the solution after UV-irradiation revealed the
presence of only one species with a value of m/z 1678.97, and
isotopic distribution consistent with the formulation
[Ru(btc2)2]+, which, however, is also found in the ESI/MS spec-
trum of Ru(btc2)2. It is known that certain dyes reductively
cleave dichloromethane upon photolysis, thereby generating a
radical cation of the dye. The same is conceivable in our case,
that is, UV-light populates an excited state with a ligand-cen-
tered reducing electron, which after being transferred to the
chlorinated solvent leaves behind an oxidized ruthenium com-
plex. Indeed, the complex proved photostable in fluoroben-
zene, but treating a fluorobenzene solution of Ru(btc2)2 under
anaerobic conditions with chemical oxidants such as ferroceni-
um tetrafluoroborate (E8’=0.0 V), [N(C6H4Br-4)3]SbCl6 (E8’=
0.70 V vs. Fc), cerium ammonium nitrate (CAN), or bulk elec-
trolysis (1.2 V) led to the disappearance of the bands typically
Figure 8. Changes in the UV/Vis spectra during cyclic voltammetry. Mathe-
matically subtracted difference spectrum of Ru(btc2)2 while moving through
the second oxidation wave in 0.1m Bu4NPF6/CH2Cl2, Pt-mesh, dE/
dt=10 mVsꢀ1, c=5ꢃ10ꢀ4 mollꢀ1, cuvette d=0.5 mm.
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EtOH/H2O (7:1, 120 mL). The mixture was refluxed for 2 h. After
cooling to room temperature, the mixture was extracted with pe-
troleum ether (5ꢃ50 mL), and the combined organic layers were
washed with aqueous ethylenediaminetetraacetic acid (EDTA) solu-
tion (2ꢃ100 mL) and brine (1ꢃ100 mL). After evaporation of the
solvent, 3,5-di-tert-butylphenyl azide (2.07 g, 8.97 mmol, 100%
crude) was obtained as an oil, which was used without further pu-
rification. The crude azide (2.070 g, 8.97 mmol), 3,6-di-tert-butyl-
1,8-diethynyl-9H-carbazole[9] (1.277 g, 3.90 mmol), ascorbic acid
(77 mg, 0.390 mmol), CuSO4 (97 mg, 0.390 mmol), and tris(benzyl-
triazolylmethyl)amine (TBTA, 207 mg, 0.390 mmol) were dissolved
in CH2Cl2/tertBuOH/H2O (4:4:1, 50 mL) and stirred at room temper-
ature overnight. The mixture was diluted with CH2Cl2 (200 mL), the
organic layer was separated, washed with EDTA solution (3ꢃ
200 mL), and dried over MgSO4. After evaporation of the solvent
under reduced pressure, the residue obtained was subjected to
column chromatography (petroleum ether/ethyl acetate=20:1) to
Investigation of a solution of Ru(btc2)2 in CH2Cl2 before and
after exposure to UV-light using electron paramagnetic reso-
nance (EPR) furthermore confirmed that the oxidation to
[Ru(btc2)2]+ is metal-centered. The X-band EPR spectrum of
Ru(btc2)2 at 77 K (Figure S38 in Supporting Information) was
found to be predominantly silent, typical for RuII, with minor
species having the characteristic rhombic S=1= EPR signal. Ex-
2
posure of the sample to UV-light gives rise to a strong signal
of rhombic lineshape with g1 =2.28, g2 =2.12, g3 =1.98, charac-
teristic of a RuIII low-spin species with octahedral metal config-
uration. Significant g anisotropy Dg=g1–g3 =0.3 clearly dem-
onstrates a large contribution from the metal center to the
paramagnetism of this species.
Analogous studies for Ru(btc1)2 reflected great similarity
with the properties of Ru(btc2)2. Exposition of DCM solutions
to UV-light triggered disappearance of the three bands in the
visible region (413, 530, and 570 nm) characteristic for
Ru(btc1)2 and appearance of new bands at 432, 587, 750, and
890 nm. Several isosbestic points indicated the temporary pres-
ence of only two species, with ultimate formation of the singly
oxidized complex, as also confirmed by spectroelectrochemical
studies (see Figure S29). The same features were observed in
the spectra recorded after reaction of Ru(btc1)2 with various
chemical oxidizing agents (Figure S27), and similarly to com-
plex Ru(btc2)2, the redox process occurring in the presence of
chemical oxidants is fully reversible: Reduction of one-electron
oxidized species by one equivalent of [CoCp2] recreates the
original spectrum of complex Ru(btc1)2.
obtain Hbtc1 as
a
colorless powder (1.08 g, 35%); 1H NMR
(500.130 MHz, CD2Cl2): d=1.45 (s, 36H), 1.56 (s, 18H), 7.60 (s, 2H),
7.79 (s, 2H), 7.89 (s, 4H), 8.22 (s, 2H), 8.56 (s, 2H), 12.17 (s,
1H) ppm (for assignment see Figure S7); 13C NMR (125.758 MHz,
CD2Cl2): d=31.5, 32.3, 35.1, 35.6, 112.9, 115.7, 117.1, 118.0, 120.6,
123.4, 124.3, 136.1, 137.3, 142.6, 148.4, 153.4 ppm (Figure S8); ele-
mental analysis calcd (%) for C52H67N7 C 79.04, H 8.55, N 12.41;
found: C 78.69, H 8.56, N 12.41.
Synthesis of Hbtc2: NaNO2 (1.338 g, 19.389 mmol) in H2O (10 mL)
was added to a solution of 1,8-diamino-3,6-di-tert-butyl-9H-carba-
zole[13] (2.00 g, 6.463 mmol), in EtOH/H2O (4:1, 50 mL) over 30 min
at 08C and subsequently stirred at this temperature for 1 h. NaN3
(1.260 g, 19.389 mmol) in H2O (10 mL) was added to the reaction
mixture at 08C within 30 min and subsequently stirred at ambient
temperature for 1 h. The mixture was extracted with ethyl acetate
(3ꢃ50 mL) and the combined organic phases were dried with
MgSO4. After column chromatography (petroleum ether/ethyl ace-
tate=20:1) 1,8-diazido-3,6-di-tert-butyl-9H-carbazole (0.896 g, 38%
crude) was obtained, which was used without further purification.
The crude azide (1.230 g, 3.403 mmol), 3,5-di-tert-butylphenyl acet-
ylene[14] (2.188 g, 10.209 mmol), ascorbic acid (67 mg, 0.340 mmol),
1m aqueous CuSO4 (0.25 mL, 0.340 mmol), and TBTA (180 mg,
0.340 mmol) in CH2Cl2/tertBuOH/H2O (4:4:1, 35 mL) were stirred at
ambient temperature overnight. The mixture was diluted with
CH2Cl2 (200 mL), the organic phase was separated, washed with
aqueous EDTA solution (3ꢃ200 mL), and dried over MgSO4. After
evaporation of the solvent under reduced pressure the residue ob-
tained was subjected to column chromatography (petroleum
ether/ethyl acetate=20:1) to obtain Hbtc2 as a colorless powder
Conclusion
We report here two new ligand systems, namely bis(triazole)-
carbazoles btc1 and btc2, that were accessed by “click reac-
tions”. Although they are isomeric and differ only in the posi-
tion of one N atom in the triazole units, the properties of their
metal complexes M(btc)2 (M=Ru, Zn, Ni) are remarkably differ-
ent. Each of the complexes undergoes two reversible ligand
centered oxidation events (at the carbazole N atoms). The cor-
responding oxidation waves appear at 300–400 mV lower po-
tentials for the btc1 complexes indicating a significantly higher
electron density at the carbazole N atoms. For a given btc
ligand, the potentials decrease on going from Ru!Zn!Ni.
The ruthenium complexes are oxidized at the metal centers
first, though, before ligand oxidation occurs. With their redox
activities that can readily be tuned by the regiochemistry of tri-
azole attachment, these pincers are anticipated to be valuable
supporting ligands in heteroleptic complexes, to be employed
in catalysis.
1
(1.30 g, 48%). H NMR (500.130 MHz, CD2Cl2): d=1.45 (s, 36H), 1.57
(s, 18H), 7.52 (s, 2H), 7.80 (s, 2H), 7.92 (s, 4H), 8.27 (s, 2H), 8.57 (s,
2H), 11.37 (s, 1H) ppm (for assignment see Figure S9); 13C NMR
(125.758 MHz, CD2Cl2): d=31.2, 31.7, 34.9, 113.7, 116.7, 117.0, 120.3,
121.2, 122.8, 125.7, 129.4, 129.9, 143.5, 148.3, 151.6 ppm (Fig-
ure S10); elemental analysis calcd (%) for C52H67N7 C 79.04, H 8.55,
N 12.41; found: C 78.44, H 8.67, N 11.89.
Synthesis of Me-btc1 and Me-btc2: NaH (15 mg, 0.375 mmol) was
added at 08C to a solution of Hbtc1 and Hbtc2, respectively,
(198 mg, 0.25 mmol) in DMF (2 mL). The mixture was allowed to
stir at ambient temperature for 2 h, followed by dropwise addition
of iodomethane (23 mL, 0.375 mmol). After stirring overnight, H2O
(5 mL) was added. The precipitated solid was isolated by filtration,
washed with H2O and subjected to column chromatography (pe-
troleum ether/ethyl acetate=10:1) to obtain Me-btc1 and Me-
btc2, respectively, as colorless powder.
Experimental Section
Ligand syntheses
Synthesis of Hbtc1: 1-Bromo-3,5-di-tert-butyl-benzene (2.340 g,
8.690 mmol) was added to a suspension of ascorbic acid (172 mg,
0.869 mmol), CuI (331 mg, 1.738 mmol), N,N’-dimethylethylenedia-
mine (153 mg, 1.738 mmol), and NaN3 (1.13 g, 17.38 mmol) in
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Me-btc1:
160 mg
(88%);
mp=316.1–316.88C;
1H NMR
Synthesis of Zn(btc2)2: The product was synthesized following the
procedure described for [Zn(btc1)2]. After drying of the yellow
solid in vacuum [Zn(btc1)2] could be obtained in pure form in 79%
(500.130 MHz, CDCl3): d=8.13 (s, 2H), 8.11 (d, J=1.95 Hz, 2H), 7.58
(d, J=1.80 Hz, 4H), 7.35 (d, J=1.80 Hz, 2H), 7.28 (t, J=1.80 Hz,
2H), 2.48 (s, 3H), 1.33 (s, 18H), 1.21 (s, 36H) ppm (for assignment
see Figure S11); 13C NMR (125.758 MHz, CDCl3): d=152.9, 147.2,
142.4, 139.9, 136.8, 127.0, 125.0, 123.1, 121.0, 116.9, 115.4, 113.8,
37.0, 35.2, 34.7, 32.0, 31.4 ppm (Figure S12). HRMS-ESI+: calcd. for
[C53H70N7+]: m/z 804.5639; found: m/z 804.5703.
1
yield (26 mg); H NMR (CD2Cl2): d=8.45 (d, J=1.72 Hz, 4H), 8.25 (s,
4H), 7.62 (d, J=1.72 Hz, 4H), 7.25 (t, J=1.81 Hz, 4H), 7.10 (d, J=
1.81 Hz, 8H), 1.56 (s, 36H), 1.21 ppm (s, 72H) (for assignment see
Figure S19); 13C{1H} (CD2Cl2): d=151.3, 147.8, 140.4, 138.8, 129.2,
129.0, 122.6, 122.0, 120.0, 117.7, 117.3, 113.3, 35.0, 35.0, 32.3,
31.6 ppm (Figure S20); ESI-MS calcd. for [Zn(btc2)2H+]: m/z
1644.01; found: m/z 1644.06.
Me-btc2:
145 mg
(79%);
mp=323.6–324.18C;
1H NMR
(500.130 MHz, CDCl3): d=8.13 (d, J=1.85 Hz, 2H), 7.99 (s, 2H), 7.54
(d, J=1.75 Hz, 4H), 7.53 (d, J=1.95 Hz, 2H), 7.45 (t, J=1.70 Hz,
2H), 3.18 (s, 3H), 1.42 (s, 18H), 1.32 (s, 36H) ppm (for assignment
see Figure S13); 13C NMR (125.758 MHz, CDCl3): d=151.5, 148.9,
143.3, 134.9, 129.1, 125.5, 123.9, 123.3, 122.8, 120.8, 120.3, 118.6,
60.4, 35.0, 34.8, 31.9, 31.5 ppm (Figure S14); HRMS-ESI+ calcd. for
[C53H70N7+]: m/z 804.5639; found: m/z 804.5726.
Synthesis of Ni(btc1)2: Triethylamine (0.2 mL) was added to a solu-
tion of NiBr2·DME (11.7 mg, 38 mmol) and Hbtc1 (60 mg, 76 mmol)
in degassed THF (10 mL). An immediate color change of the solu-
tion from orange to red was observed. The reaction mixture was
further stirred at room temperature overnight, and the precipitated
yellow solid was collected by filtration. This crude product was
washed with small portions of THF, followed by extraction with
Et2O. Evaporation of the solvent and drying under vacuum provid-
ed Ni(btc1)2 as an orange solid in 82% yield (51 mg). Single crystals
of Ni(btc1)2 can be grown by slow evaporation of the solvent from
a DCM/acetone solution under an inert atmosphere. ESI-MS calcd.
for [Ni(btc1)2+H]+: m/z 1636.02; found: m/z 1636.02; calcd. for
[Ni(btc1)2 + Na]+: m/z 1658.00; found: m/z 1658.00; elemental
analysis calcd (%) for NiC104H132N14: C 76.31, H 8.13, N 11.98; found
C 75.47, H 8.32, N 11.60; for the C deviation see comment for
Ru(btc1)2; magnetic moment (in C6D6, 300 MHz) meff =2.63 mB (2 un-
paired electrons per Ni).
Syntheses of metal complexes
Synthesis of Ru(btc1)2: RuCl2(DMSO)4 (30 mg, 0.06 mmol), Hbtc1
(100 mg, 0.12 mmol), and triethylamine (ca. 0.3 mL) were refluxed
overnight in a mixture of degassed ethanol (10 mL) and THF (5 mL)
with stirring under an inert atmosphere. The reaction mixture was
cooled to ꢀ158C and the precipitated solid collected by filtration
to afford the title compound as dark red precipitate. The resulting
product was dissolved in a minimum amount of CH2Cl2 and filtered
through Celite. Slow removal of CH2Cl2 under a stream of dinitro-
gen caused the crystallization of the product as red plates. The red
microcrystals were collected by filtration and washed with pentane
(70 mg, 68%); ESI-MS: calcd. for [Ru(btc1)2]+: m/z 1679.99; found:
m/z 1678.97; elemental analysis calcd (%) for RuC104H132N14 C 74.38,
H 7.92, N 11.68; found: C 72.34, H 7.91, N 11.28. For all btc com-
plexes correct H and N values were found, whereas the C values
were (for unknown reasons) too low, despite many attempts for
different samples. For the following compounds: Ru(btc2)2,
Zn(btc1)2 and Zn(btc2)2 purity has been demonstrated by NMR
spectroscopy.
Synthesis of Ni(btc2)2: The product was synthesized following the
procedure described for [Ni(btc1)2] but employing Hbtc2. Yield:
41% (34.2 mg); ESI-MS calcd. for [Ni(btc2)2+H]+: m/z 1636.02;
found: m/z 1636.02; calcd. for [Ni(btc2)2 + Na]+: m/z 1658.00;
found: m/z 1658.00; elemental analysis calcd (%) for NiC104H132N14:
C 76.31, H 8.13, N 11.98; found C 75.61, H 8.35, N 11.74; for the C
deviation see comment for Ru(btc1)2; magnetic moment (in C6D6,
300 MHz) meff =2.84 mB (2 unpaired electrons per Ni).
Acknowledgements
Synthesis of Ru(btc2): The product was synthesized following the
procedure described for [Ru(btc1)2] but employing Hbtc2. It was
obtained as a dark red solid. Single crystals of Ru(btc2)2 were
grown by slow evaporation of DCM under an inert atmosphere
The authors thank the Humboldt-Universitꢀt zu Berlin for finan-
cial support. We are also grateful to the COST action CM1305:
ECOSTBio.
1
(65 mg, 63%); H NMR (CD2Cl2): d=8.61 (s, 4H), 8.49 (s, 4H), 7.74
(br, 4H), 7.19 (t, J=1.85 Hz, 4H), 7.04 (d, J=1.88 Hz, 8H), 1.59 (s,
36H), 1.16 ppm (s, 72H) (for assignment see Figure S15); 13C{1H}
(CD2Cl2): d=151.0, 146.9, 128.5, 122.2, 119.4, 117.0, 34.6, 31.0 ppm
(Figure S16); ESI-MS calcd. for [Ru(btc1)2]+: m/z 1679.99; found: m/
z 1678.98; elemental analysis calcd (%) for RuC104H132N14 C 74.38, H
7.92, N 11.68; found: C 72.08, H 7.85, N 11.26; for the C deviation
see comment for Ru(btc1)2.
Conflict of interest
The authors declare no conflict of interest.
Keywords: carbazole · click chemistry · redox behavior ·
ruthenium · triazole
Synthesis of Zn(btc1)2: Zn(Et)2 as a 1m solution in hexane (0.15 mL,
0.15 mmol) was added to
a solution of Hbtc1 (100 mg,
0.126 mmol) in THF (2 mL). Stirring the reaction mixture at ambient
temperature overnight led to the formation of a yellow precipitate.
After filtration the precipitate was washed with THF (3 mL) and
hexane (3 mL) and dried under vacuum to obtain Zn(btc1)2 in 76%
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4H) 7.68 (d, J=1.91 Hz, 4H), 7.29 (t, J=1.74 Hz, 4H), 7.00 (d, J=
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Version of record online: && &&, 0000
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FULL PAPER
&
Click chemistry
I. Pryjomska-Ray, D. Zornik, M. Pꢀtzel,
K. B. Krause, L. Grubert, B. Braun-Cula,
S. Hecht,* C. Limberg*
&& – &&
Comparing Isomeric Tridentate
Carbazole-Based Click Ligands: Metal
Complexes and Redox Chemistry
Two isomers with a click: After com-
plexation of isomeric bis(triazolyl)carba-
zoles, differing only in the triazole link-
age, they represent redox active ligands,
and the triazole positioning sensitively
influences the redox properties of the
complexes. These novel pincers are thus
anticipated to be valuable supporting li-
gands in heteroleptic complexes for cat-
alytic applications.
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