1,1′-Bis(pyrazol-4-yl)ferrocenes: Potential Clip Ligands and Their Supramolecular Structures

Article abstract of DOI:10.1002/ejic.201600644

Two ferrocene derivatives with pyrazoles appended at their C-4 position to both cyclopentadienyl (Cp) rings have been synthesized, namely, 1,1′-bis(1H-pyrazol-4-yl)ferrocene (H₂^HL) and 1,1′-bis(3,5-dimethyl-1H-pyrazol-4-yl)ferrocene (H₂^MeL). In the solid state, these are shown crystallographically to form supramolecular aggregates through intermolecular NH···N hydrogen bonds in either a dimeric (H₂^HL) or trimeric (H₂^MeL) arrangement. Variable-temperature NMR spectroscopy and diffusion-ordered spectroscopy (DOSY) evidenced the presence of [H₂^MeL]₃trimers in [D₈]toluene solution, whereas both H₂^HL and H₂^MeL exist as monomers in deuterated N,N-dimethylformamide ([D₇]DMF) even at low temperatures. The kinetic parameters for the NH tautomerism have been determined. In their dianionic forms, both of these hybrid ferrocene/pyrazole molecules serve as ligands towards Cu^I, Ag^I, and Au^Iions. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) revealed the formation of [M₂^RL]₃hexametallic complexes, the structures of which are suggested to have D_3hsymmetry with two metallomacrocyclic [M(μ-pz)]₃(pz = pyrazolate) decks connected by three ferrocene clips. M?ssbauer spectra and preliminary luminescence data were collected for the proligands and the resulting complexes. Owing to the insolubility of the coinage-metal complexes, the electrochemical properties could be measured only for the two H₂^RL proligands.

Full text of DOI:10.1002/ejic.201600644

DOI: 10.1002/ejic.201600644
Full Paper
Ferrocenyl Ligands
1,1′-Bis(pyrazol-4-yl)ferrocenes: Potential Clip Ligands and Their
Supramolecular Structures
Mattia Veronelli,[a] Sebastian Dechert,[a] Anne Schober,[a] Serhiy Demeshko,[a] and
Franc Meyer*[a]
Abstract: Two ferrocene derivatives with pyrazoles appended rameters for the NH tautomerism have been determined. In
at their C-4 position to both cyclopentadienyl (Cp) rings have their dianionic forms, both of these hybrid ferrocene/pyrazole
been synthesized, namely, 1,1′-bis(1H-pyrazol-4-yl)ferrocene molecules serve as ligands towards CuI, AgI, and AuI ions. Ma-
(H2HL)
and
1,1′-bis(3,5-dimethyl-1H-pyrazol-4-yl)ferrocene trix-assisted laser desorption/ionization mass spectrometry
(H2MeL). In the solid state, these are shown crystallographically (MALDI MS) revealed the formation of [M2 L]3 hexametallic
to form supramolecular aggregates through intermolecular complexes, the structures of which are suggested to have D3h
NH···N hydrogen bonds in either a dimeric (H2HL) or trimeric symmetry with two metallomacrocyclic [M(μ-pz)]3 (pz = pyr-
(H2MeL) arrangement. Variable-temperature NMR spectroscopy azolate) decks connected by three ferrocene clips. Mössbauer
and diffusion-ordered spectroscopy (DOSY) evidenced the pres- spectra and preliminary luminescence data were collected for
ence of [H2MeL]3 trimers in [D8]toluene solution, whereas both the proligands and the resulting complexes. Owing to the insol-
H2HL and H2MeL exist as monomers in deuterated N,N-dimethyl- ubility of the coinage-metal complexes, the electrochemical
R
R
formamide ([D7]DMF) even at low temperatures. The kinetic pa- properties could be measured only for the two H2 L proligands.
Introduction
metallophilic contacts are the main factors responsible for the
fascinating luminescence properties associated with this class
of compounds.[3,7a–7c,9a,10] In the past few years, different stud-
ies have focused on the possibility to obtain coinage-metal
complexes with enhanced metallophilic contacts through the
targeted design of ligands.[11] Recently, we introduced the use
Pyrazoles and pyrazolates hold a prominent position as ligands
in coordination chemistry, and their complexes with mono-
valent coinage metals (CuI, AgI, and AuI) have attracted wide-
spread interest.[1] In combination with these monovalent metal
ions, the pyrazolate anion [pz]– forms compounds with struc-
tures that range from polymeric chains[2] to oligonuclear metal-
lomacrocycles of different nuclearities. In the latter group, the
planar trimeric ring [M(μ-pz)]3 is by far the most recurrent mo-
tif,[1,3] though different examples of tetrameric and hexameric
species have also been reported.[1b,3,4] The structure adopted
by each complex is dependent on the metal ion,[3] the substitu-
ents at the heterocycle,[3] and, in some cases, also on the spe-
cific synthetic and crystallization procedures.[4a,4b,5] A second
aspect of interest is the ability of pyrazolate coinage-metal
complexes [M(μ-pz)]n to act as supramolecular synthons. Inter-
molecular d10–d10 interactions usually lead to the formation of
R
of 1,1′-bis(pyrazol-3-yl)ferrocene molecules (H2 L′; Figure 1) as
ligands towards CuI and AgI ions.[12] The resulting hexametallic
species are characterized by two decks of M3N6 metallomacro-
cycles, which are forced to adopt an eclipsed conformation, be-
cause the ferrocene (Fc) units act as clips. Furthermore, the pro-
R
ligands H2 L′ themselves form hydrogen-bonded supramolec-
ular structures in the solid state and in solution.
[2,6]
ordered supramolecular aggregates of polymeric [M(μ-pz)]∞
or trimeric [M(μ-pz)]3 units in the solid state.[7] If the electronic
characteristics of the ligands are changed, these complexes can
act either as π-acids or π-bases,[7c] and several supramolecular
adducts with aromatic molecules[7c,7d,8] or metal-based Lewis
acids[9] have been reported. Moreover, intra- and intermolecular
Figure 1. Representations of the pyrazole-substituted ferrocene frameworks
with the heterocycles attached at the 3-position (H2RL′, left)[12] and at the 4-
position (H2RL, right); R = general substituent.
Ferrocene-based molecules have been investigated widely,
and numerous ferrocene derivatives with N-heterocyclic substit-
uents have been reported.[13] These compounds have been
used as ligands towards various metal ions[13] or have been
designed to form supramolecular structures through hydrogen
bonds.[14] Quite surprisingly, despite the well-established chem-
istry of both ferrocenes and pyrazoles, examples of hybrid mol-
[a] Institut für Anorganische Chemie, Georg-August-Universität Göttingen,
Tammannstrasse 4, 37077 Göttingen, Germany
E-mail: franc.meyer@chemie.uni-goettingen.de
http://www.meyer.chemie.uni-goettingen.de
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600644.
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ecules composed of one ferrocene unit and two appended pyr- ular NH···N hydrogen bonds. H2HL crystallizes as a dimer (Fig-
azole rings are still very rare.[12,15] Moreover, in all of these com- ure 2, top) in which the four pyrazole units adopt a saddlelike
pounds, the metallocene is connected to the 3- or 5-position disposition similar to those observed for common tetrameric
of the heterocycle, and examples of connectivity through the pyrazoles.[17] The same kind of arrangement was encountered
pyrazole C-4 atom are limited to monosubstitution.[14c,16] In the recently in the solid-state structure of 1,1′-bis(5-methyl-1H-pyr-
present work, we fill this gap with the report of two distinct azol-3-yl)ferrocene (H2MeL′; Figure 1).[12] Consequently, each
R
1,1′-bis(pyrazol-4-yl)ferrocenes (H2 L; Figure 1), namely, 1,1′- pyrazole ring of H2HL interacts with both heterocycles of a sec-
bis(1H-pyrazol-4-yl)ferrocene (H2HL) and 1,1′-bis(3,5-dimethyl- ond molecule of H2HL, and the two ferrocenyl units point in
1H-pyrazol-4-yl)ferrocene (H2MeL). These proligands were char- two perpendicular directions to give a structure with crystallo-
acterized both in the solid state (X-ray diffraction and Möss- graphic C2 symmetry. Both possible enantiomers are found in
bauer spectroscopy) and in solution (NMR spectroscopy and the crystal structure, as the NH protons show a 1:1 disorder
cyclic voltammetry). Furthermore, their potential use as ligands between the nitrogen atoms of the central belt of the dimer.
towards monovalent CuI, AgI, and AuI ions was tested, and the
resulting complexes were identified by mass spectrometry.
Results and Discussion
Proligands
The syntheses of the two proligands H2HL and H2MeL are based
on a Negishi coupling protocol and were inspired by the proce-
dure developed by Mochida et al. for 4-ferrocenyl-1-tritylpyraz-
ole.[14c] Ferrocene was deprotonated twice at the 1- and 1′-
positions by the nBuLi/tetramethylethylenediamine (nBuLi/
TMEDA) system (Scheme 1), and the subsequent addition of
ZnCl2 led to the formation of dizincated ferrocene A. Cross-
coupling between this species and the appropriate tetrahydro-
pyran-protected (THP-protected) 4-iodopyrazole [4-iodo-1-
(tetrahydropyran-2-yl)-1H-pyrazole (HI) or 4-iodo-3,5-dimethyl-
1-(tetrahydropyran-2-yl)-1H-pyrazole (MeI)] was conducted in
the presence of Pd(PPh3)4 (5 mol-% with respect to the starting
ferrocene). After the workup of the reaction mixture and acidic
deprotection of the pyrazole NH moieties, the target products
were isolated in overall yields of 41 % (H2HL) and 32 % (H2MeL).
Figure 2. Supramolecular arrangements of H2HL (top) and H2MeL (bottom) in
the solid state. The dashed lines emphasize intermolecular N–H···N hydrogen
bonds.
In contrast, H2MeL crystallizes as a trimeric aggregate (Fig-
ure 2, bottom). The cyclic H-bonded arrangement of three pyr-
azole units found in each of the two decks of [H2MeL]3 has been
reported previously for several other pyrazoles.[17a,17b] In the
present case, two such (Hpz)3 decks are stacked at a separation
of ca. 3.73 Å because of the three ferrocene linkers that serve
as clips. Only one tautomer of (H2MeL)3 was observed in the
crystalline material, that is, the one in which the two pyrazole
rings of the same H2MeL molecule have the protons on opposite
nitrogen atoms. This results in a structure with an approximate
(noncrystallographic) D3 symmetry.
Scheme 1. Synthetic pathway for the proligands H2HL and H2MeL.
Both proligands were successfully crystallized, and their mo-
lecular structures were determined by X-ray diffraction. In both
cases, a synperiplanar eclipsed conformation of the pyrazole
moieties in the individual molecules was found (Figures 2 and
S20–S21). The heterocycles are almost coplanar with the adja-
cent cyclopentadienyl (Cp) rings for H2HL (dihedral angles of
7.0° and 11.2°), whereas the interplanar angles are larger in
H2MeL (they range from 17.6° to 23.0°; three independent mol-
ecules per unit cell), most likely because of the presence of
The stabilities of these supramolecular aggregates of H2HL
steric hindrance between the Cp rings and the methyl substitu- and H2MeL in solution were investigated by NMR spectroscopy
ents at the pyrazole ring. Remarkably, both molecules form su- with the same (but deuterated) solvents employed for the crys-
pramolecular aggregates in the solid state through intermolec- tallization process, that is, deuterated N,N-dimethylformamide
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([D7]DMF) and [D8]toluene for H2HL and H2MeL, respectively. The pyrazole in hexamethylphosphoramide (HMPA: ΔG‡
=
345
1H NMR spectrum of H2HL at room temperature shows only four 63 kJ mol–1)[19] and DMSO (ΔG‡
= 62 kJ mol–1;[20] from the
362
signals (two originating from the Cp protons, one from the above parameters, the ΔG‡ values for H2HL are calculated to be
3-/5-Hpz atoms, and one assigned to NH groups; see Figure S1, 58.1 and 59.6 kJ mol–1 at 345 and 362 K, respectively). However,
top), whereas a total of seven peaks would be expected if the direct comparisons between different systems are difficult, be-
structure found in the solid state was retained in solution. Thus, cause the pyrazole tautomerism is believed to proceed by mul-
at room temperature, NH tautomerism and, most likely, rotation timolecular pathways rather than a unimolecular one[17b] and
of the pyrazole rings around the bond linking them with the is, thus, sensitive to solvent and concentration.[17b] The negative
ferrocenyl moieties are operative. As the sample is cooled to value of ΔS‡ for H2HL is also consistent with a multimolecular
temperatures below 240 K, the peak at δ ≈ 7.6 ppm (3-/5-Hpz) transition state.
splits into two resonances of equal integration (Figure S1);
The presence of trimeric [H2MeL]3 species was detected in
1
therefore, the NH tautomerism becomes slow on the NMR time [D8]toluene solution. By measuring the H DOSY NMR spectra
scale. At the same temperature, the Cp protons remain pairwise at 298 and 193 K and by using the residual protons of the
equivalent, either because they are quite far from the pyrazole solvent as an internal standard, it was demonstrated that H2MeL
NH group and, thus, accidently isochronous or because of the increases its volume by a factor of ca. 2.9 upon cooling (see
free rotation of the pyrazole rings along the 1-CFc/4-Cpz bond Supporting Information for details). This value is in agreement,
that links the pyrazole and ferrocene subunits. Even at the low- within experimental error, with the trimerization of H2MeL at
1
est temperature limit of [D7]DMF (223 K), H2HL exists as a mono- low temperatures. As expected, the appearance of the H NMR
mer in solution, as ascertained by 1H diffusion-ordered NMR spectrum of H2MeL is temperature-dependent. At 298 K, the
spectroscopy (DOSY), which does not reveal any change of the proligand exists in a monomeric form, and the NH tautomerism
volume of the proligand between 298 and 223 K (see below is fast on the NMR time scale; therefore, a highly symmetric
and the Supporting Information for details). The formation of molecule is detected (Figure 3, top). Below 270 K, the tautomer-
a supramolecular aggregate of H2HL in solution is most likely ism becomes sufficiently slow that the three CH resonances of
prevented by the high polarity and the H-bond acceptor prop- the spectrum each split into two resonances of equal integra-
erties of the solvent [D7]DMF. Unfortunately, NMR spectroscopy tion; therefore, a total of seven signals can be seen at 213 K (at
investigations in different media were not possible because of intermediate temperatures, some peaks originating from the Cp
the insolubility of the proligand in solvents other than DMF protons overlap and are difficult to distinguish; see Figure 3).
and dimethyl sulfoxide (DMSO). By examining the temperature These experiments indicate that the same kind of supramolec-
dependence of the signal of the 3-/5-Hpz protons, the kinetic ular aggregate is present in both the solid state and, at low
parameters associated with the NH···N tautomeric equilibrium temperatures, in solution. However, the NMR spectroscopy data
were determined.[18] From the coalescence temperature Tc = did not permit us to determine the tautomeric form of the tri-
240 K, the Gibbs activation energy was estimated to be mer in solution, which could have either D3 or D3h symmetry.
47.8 kJ mol–1 (see the Supporting Information for details). The NMR spectroscopy experiments also allowed us to estimate the
entropy and enthalpy of activation were derived by linewidth Gibbs activation energy for the tautomeric equilibrium of the
analysis, which gave ΔS‡ = (–90.9 4.1) J K–1 mol–1 and ΔH‡ = pyrazole moieties (ΔG‡ ≈ 54.2 kJ mol–1, TC ≈ 270 K for the CH3
(26.7 1.06) kJ mol–1 (Eyring plot shown in the Supporting In- resonances; see the Supporting Information for details). The en-
formation). The derived activation energy is in line with the tropy and enthalpy of activation for the same process could not
values reported for the tautomeric equilibrium of unsubstituted be determined, as the plot of ln(k/T) versus 1/T (Eyring plot; k
Figure 3. 1H NMR spectra of H2MeL recorded in [D8]toluene at different temperatures. The peak at δ = 2.08 ppm is due to the residual protons of the solvent.
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determined by linewidth analysis between 243 and 193 K) was a single quadrupole doublet was observed with isomeric shifts
not linear. This is, because the H2MeL system undergoes two and quadrupole splitting values typical for low-spin FeII and
distinct dynamic processes in the temperature range consid- similar to those reported for ferrocene[22] and 4-ferrocenyl-3,5-
ered in the order of the NMR time scale, namely, NH tautomer- dimethylpyrazole.[16a] As an example, the spectrum recorded for
ism and aggregation to form the trimeric supramolecular aggre- H2HL is shown in Figure 5, and Mössbauer parameters for pro-
gates. As expected, the formation of supramolecular structures ligands H2MeL and all coinage-metal complexes (see below) are
of H2MeL in solution is sensitive to the solvent employed and listed in Table 1.
can be prevented in more polar media capable of H-bonding.
1H DOSY NMR spectroscopy of H2MeL in [D7]DMF did not show
any increase of the volume between 298 and 223 K, and the
NH tautomerism also remained fast on the NMR time scale in
this temperature range (Figure S2).
The electrochemical properties of both H2HL and H2MeL were
investigated by cyclic voltammetry (CV) in DMF/0.1
M
[nBu4N]PF6 solutions. Both compounds show an electrochemi-
cally reversible anodic process associated with the oxidation of
the ferrocenyl moieties (linear dependence of the current on
the square root of the scan rate; see Figures S5 and S6). The
oxidation potential of H2HL is E1/2 = –156 mV versus Fc/Fc+ (Fig-
ure 4, left), whereas E1/2 is –179 mV for H2MeL (Figure 4, right;
the dependence of the peak separation ΔEp on the scan rate ν
is due to uncompensated solution resistance under the experi-
mental conditions). The presence of two pyrazole units clearly
renders the iron center of each proligand more electron-rich
than that in the parent ferrocene and, thus, easier to oxidize.
The slightly lower oxidation potential of H2MeL can be rational-
ized by the better electron-donating ability of the methyl sub-
stituents compared with that of the H-atoms present in H2HL. It
is interesting to compare the oxidation potentials found for the
two new proligands discussed here with that found for 1,1′-
bis(5-methyl-1H-pyrazol-3-yl)ferrocene (H2MeL′; Figure 1), which
we reported recently (E1/2 = –50 mV in DMF).[12] Despite the
structural similarity of the systems, the oxidation of H2MeL′ is
shifted anodically by more than 100 mV. Thus, the different
connectivity of the pyrazole rings with the ferrocene moiety
influences the electronic situation at the iron atom; the trends
of the CV data are in line with the highest electron density of
the pyrazole heterocycle at the 4-position.[21]
Figure 5. Mössbauer spectrum of solid H2HL at 80 K; the blue line represents
simulation with
0.54 mm s–1 and quadrupole splitting ΔEQ
2.23 mm s–1
a
δ
=
=
.
Table 1. Mössbauer parameters [mm s–1] of the two proligands H2RL and their
complexes.
δ
ΔEQ
H2HL
H2MeL
0.54
0.55
0.54
0.56
0.55
0.56
0.54
0.55
0.49
2.23
2.37
2.25
2.35
2.24
2.36
2.28
2.36
2.41
[Cu2HL]3
[Cu2MeL]3
[Ag2HL]3
[Ag2MeL]3
[Au2HL]3
[Au2MeL]3
Ferrocene[22]
To further characterize H2HL and H2MeL, their Mössbauer
spectra were collected for solid material at 80 K. In both cases,
Figure 4. Cyclic voltammograms of H2HL (left) and H2MeL (right) in DMF/0.1
1/2(H2HL) = –156 mV, E1/2(H2MeL) = –179 mV.
M
[nBu4N]PF6 versus Fc/Fc+ at different scan rates (ν = 20, 50, 100, 200, 500 mV s–1);
E
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Complexes
species in solution (by NMR spectroscopy and cyclic voltamme-
try). We tentatively attribute the insolubility to multiple inter-
molecular d10–d10 interactions that effectively lead to polymeric
structures in the solid state, as was also observed for
[Cu2MeL′]3.[12]
The two new hybrid ferrocene/pyrazole proligands H2HL and
H2MeL were used, in their dianionic forms [RL]2–, as ligands to-
wards monovalent CuI, AgI, and AuI ions. The direct reactions
R
of H2 L with
2 equiv. of the appropriate metal salt
{[Cu(MeCN)4][BF4], AgBF4, or AuCl(SMe)2, respectively} in the
presence of Et3N led to the formation of compounds of general
formula [M2 L]3 (M = CuI, AgI, AuI). With proligand H2MeL, the
R
reactions were performed in MeOH for the copper complex and
in MeCN for the AgI and AuI salts to avoid any facile oxidation
of the ferrocene unit (FeII to FeIII) in the presence of the latter
ions. The redox potential of the Ag/Ag+ couple is highly solvent-
dependent and reaches its minimum in MeCN (E1/2 = 4 mV vs.
Fc/Fc+).[23] Owing to its lower solubility, H2HL required mixtures
of the above solvents with DMF. The isolated complexes were
characterized by matrix-assisted laser desorption/ionization
time-of-flight mass spectrometry (MALDI-TOF MS). In all cases,
the mass spectra showed an intense peak assigned to the hexa-
Figure 7. Proposed structure of [M2RL]3 (M = CuI, AgI, AuI; R = H, Me).
R
The Mössbauer spectra of all [M2 L]3 complexes are essen-
R
R
metallic [M6 L3]+ ions and a series of signals originating from
tially identical to the spectra of the free proligands H2 L; there-
R
[Mx L2]+ (x = 3, 4) fragments (Figures 6 and S7–S12). As anti-
fore, the electronic situation at the iron centers is not affected
significantly when the appended pyrazole rings coordinate to
monovalent coinage-metal ions. The Mössbauer parameters are
compiled in Table 1.
cipated, these results are consistent with the formation of com-
R
plexes of general formula [M2 L]3. It is reasonable to assume
that these complexes adopt molecular structures featuring two
decks of M3N6 metallomacrocycles held together by the three
As discussed in the Introduction, the luminescence proper-
clipping ferrocenyl spacers (Figure 7), similarly to the supra- ties of multinuclear coinage-metal pyrazolates are of particular
molecular structure of the protic proligand [H2MeL]3 shown in interest and have been studied intensively.[3,7a–7c,9a,10a] There-
Figure 2. This assumption is supported by the similarity of the fore, preliminary investigations of the luminescence properties
mass spectra of the present series of complexes with those of of the six new complexes were performed for solid (powder)
R
the recently reported class of hexametallic complexes [M2 L′]3 samples. In all cases, upon excitation with UV light, a weak
with the closely related ferrocenyl/pyrazole hybrid ligands emission centered in the violet region of the visible spectrum
[RL′]2– (Figure 1), for which the solid-state structures could be was recorded (see the spectrum of [Ag2HL]3 in Figure 8 as an
authenticated by X-ray diffraction for two examples (M = CuI, example; the other spectra are shown in Figures S15–S19). Un-
R = Me and M = AgI, R = CF3).[12] Unfortunately, all of the coin- der the same conditions, the proligands themselves are weakly
age-metal complexes of the new ligands [RL]2– are practically emitting at similar wavelengths (Figures S13 and S14). Thus, it
insoluble in common organic solvents [such as acetone, CH2Cl2, is assumed that the transitions involved in the luminescence
CHCl3, MeOH, MeCN, MeNO2, DMF, N,N-dimethylacetamide processes are mainly ligand-centered with only minor contribu-
(DMA), and DMSO]; therefore, we have not yet obtained single tions from the coinage-metal ions and metallophilic d10–d10 in-
crystals suitable for X-ray diffraction or studied the hexametallic teractions.
Figure 6. Positive-ion MALDI-TOF mass spectrum of Cu6HL3 in DMF; the inset shows the experimental and simulated isotopic pattern for the molecular ion
peak at m/z = 1329.7, which corresponds to [Cu6HL3]+. The peaks at m/z =822.9 and 885.8 are assigned to ions [Cu3HL2]+ and [Cu4HL2]+, respectively.
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lead to different electrochemical properties and to different su-
pramolecular arrangements that could also prevent the inter-
molecular d10–d10 interactions that are likely responsible for the
low solubilities of the compounds. On the other hand, metallo-
cene linkers other than ferrocene could modulate the spacing
of the two M3N6 decks. Studies in these directions are ongoing.
Experimental Section
General: The syntheses of proligands and CuI complexes were per-
formed under an anaerobic and anhydrous atmosphere of dry ar-
gon by standard Schlenk techniques. Tetrahydrofuran (THF) was
dried with sodium and potassium in the presence of benzo-
phenone; MeOH, MeCN, and DMF were dried with CaH2; hexane
was dried with molecular sieves with an MBRAUN solvent purifica-
tion system. NMR spectra were recorded with Bruker Avance
300 MHz and Bruker Avance 400 MHz spectrometers. Chemical
shifts (δ) are reported in ppm and are referenced to the residual
proton and carbon signals of the solvents (CDCl3: δH = 7.26 ppm,
δC = 77.16 ppm; CD2Cl2: δH = 5.32 ppm, δC = 53.84 ppm; [D6]DMSO:
Figure 8. Photoluminescence spectra of [Ag2HL]3. The emission spectrum (red)
was recorded after excitation at λ = 365 nm, and the emission was monitored
at λ = 415 nm for the excitation spectrum (blue).
δH = 2.50 ppm, δC = 39.52 ppm; [D8]toluene: δH = 2.08 ppm, δC
20.43 ppm; [D7]DMF: δH = 2.75 ppm, δC = 29.76 ppm). EI mass
=
Conclusions
In this work, we have successfully synthesized hybrid ferrocene/ spectra were recorded with a Finnigan MAT 8200 instrument.
MALDI-TOF mass spectra were recorded with a Bruker Autoflex
Speed instrument. Cyclic voltammograms were measured with a
PerkinElmer 263A potentiostat controlled by the Electrochemistry
Powersuit software; a three-electrode arrangement was used with
pyrazole molecules in which each of the ferrocene Cp rings is
equipped with a pyrazole connected through its 4-position. In
the solid state, H2HL and H2MeL form supramolecular aggre-
gates, either dimeric [H2HL]2 (C2 symmetry) or trimeric [H2MeL]3
(noncrystallographic D3 symmetry), held together by intermo-
lecular NH···N hydrogen bonds. For H2MeL, the trimers are also
present in solution ([D8]toluene) at low temperature, whereas
the aggregation of H2HL is likely prevented by the high polarity
a glassy carbon working electrode, an Ag/0.01
M AgNO3 reference
electrode and a Pt wire counter electrode. All cyclic voltammograms
were referenced externally to Fc/Fc+, for which the cyclic voltammo-
gram of Fc was recorded under identical conditions immediately
before or after the electrochemical experiments. The Mössbauer
spectra were recorded with a 57Co source in an Rh matrix with an
of the solvent ([D7]DMF) that is required to dissolve the com-
R
pound. In their dianionic forms, H2 L serve as ligands towards alternating constant acceleration Wissel Mössbauer spectrometer
monovalent CuI, AgI, and AuI ions. On the basis of MALDI MS
operated in the transmission mode and equipped with a Janis
closed-cycle helium cryostat. The isomer shifts are given relative to
iron metal at ambient temperature. The simulation of the experi-
mental data was performed with the Mfit program with Lorentzian
line doublets.[24] Elemental analyses were performed by the analyti-
cal laboratory of the Institute of Inorganic Chemistry at Georg Au-
gust University with an Elementar Vario EL III instrument. 4-Iodo-
1H-pyrazole was obtained from a commercial source (abcr GmbH),
4-iodo-3,5-dimethyl-1H-pyrazole was obtained either from a com-
mercial source (abcr GmbH) or synthesized by an adapted literature
results and literature precedence for related systems, the com-
plexes are proposed to have the general formula [M2 L]3 and
R
to be composed of two eclipsed decks of M3N6 metallomacro-
cycles linked by the three ferrocenyl moieties; such molecular
structures would likely feature a prismatic arrangement of six
coinage-metal ions and overall D3h symmetry. Solution studies
R
were hampered by the low solubilities of these [M2 L]3 com-
plexes. The substituents at the pyrazole rings (H or Me) exert
the expected effects on the electrochemical redox potentials of procedure (see below).[25] Details of the X-ray crystallographic struc-
the ferrocene moiety in the free proligands H2HL and H2MeL,
ture determinations can be found in the Supporting Information.
CCDC 1482525 (for H2HL), and 1482526 (for H2MeL) contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre.
and the 57Fe Mössbauer parameters of all of the new com-
pounds are insensitive to the identity of the pyrazole substitu-
ents R or the coinage-metal ions M and are similar to the Möss-
bauer parameters of the parent ferrocene itself. Unfortunately,
on the basis of preliminary data, the luminescence properties
4-Iodo-3,5-dimethyl-1H-pyrazole:
3,5-Dimethyl-1H-pyrazole
R
of the solid compounds [M2 L]3 (M = CuI, AgI, AuI) are unspec-
(5.0 g, 52.0 mmol, 1.0 equiv.) was dissolved in H2O (300 mL). I2
(6.60 g, 26.0 mmol, 0.5 equiv.) and H2O2 (50 % aqueous solution,
1.8 mL, 31.7 mmol, 0.6 equiv.) were added, and the mixture was
stirred at room temperature for 24 h. A precipitate formed and was
separated by filtration, washed with H2O (50 mL), and dissolved in
CH2Cl2. The organic solution was washed with a saturated aqueous
solution of sodium thiosulfate to remove unreacted iodine and
dried with MgSO4, and the solvent was removed under reduced
tacular, as was also observed for the related series of complexes
R
[M2 L′]3.[12]
The results presented in this article enlarge the still small
family of 1,1′-bis(pyrazolyl)ferrocene molecules and delineate a
promising strategy for the controlled stacking of pyrazole-
based M3N6 metallomacrocycles. The introduction of pyrazole
substituents other than H and Me (either more electron-with-
pressure to give the title compound as a white solid (8.8 g,
drawing or bulkier) or the oxidization of the FeII centers could 39.6 mmol, 76 %). H NMR (CDCl3, 300 MHz): δ = 2.26 (s, 6 H, CH3)
1
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Full Paper
ppm. 13C NMR (CDCl3, 75 MHz): δ = 13.1 (CH3), 62.7 (4-Cpz), 146.7 vacuo; for H2MeL, the solid was first collected by filtration, washed
(3-/5-Cpz) ppm.
with NaHCO3(aq), and finally dried in vacuo.
H2HL: The product was obtained as an ocher powder (2.10 g,
6.6 mmol, 41 %). Crystals suitable for X-ray diffraction were ob-
tained by vapor diffusion of EtOH into a solution of the compound
4-Iodo-1-(tetrahydropyran-2-yl)-1H-pyrazole (HI): 4-Iodo-1H-pyr-
azole (8.5 g, 43.8 mmol, 1.0 equiv.), 3,4-dihydro-2H-pyran (8 mL,
87.7 mmol, 2.0 equiv.), and p-toluenesulfonic acid (75 mg,
0.436 mmol, 0.01 equiv.) were dissolved in MeCN (20 mL), and the
mixture was heated to reflux for 3 h. After cooling to room tempera-
ture, the solution was diluted with AcOEt (50 mL), washed with a
saturated aqueous solution of NaHCO3, and dried with MgSO4. The
solvent was then removed under reduced pressure to afford a dark
brown oil. The crude material was purified by Kugelrohr distillation
(110 °C, 5 × 10–3 mbar), and the product was obtained as a colorless
1
in DMF. H NMR ([D6]DMSO, 300 MHz): δ = 4.02 (t, J = 1.8 Hz, 4 H,
3-/4-HFc), 4.28 (t, J = 1.8 Hz, 4 H, 2-/5-HFc), 7.55 (s, 4 H, CHpz), 12.63
(br., 2 H, NH) ppm. 13C NMR ([D6]DMSO, 75 MHz): δ = 67.6 (2-/5-
CFc), 68.8 (3-/4-CFc), 79.3 (1-CFc), 117.3 (4-Cpz), 130.7 (3-/5-Cpz) ppm.
1H NMR ([D7]DMF, 300 MHz): δ = 4.06 (s, 4 H, 3-/4-HFc), 4.37 (s, 4 H,
2-/5-HFc), 7.67 (s, 4 H, CHpz), 12.80 (s, 2 H, NH) ppm. MS (EI): m/z
(%) = 318.0 (100) [M]+. C16H14FeN4·1/3DMF (342.5): calcd. C 59.61,
H 4.81, N 17.72; found C 59.31, H 4.52, N 17.49.
1
oil (11.9 g, 42.8 mmol, 97 %). H NMR (CDCl3, 300 MHz): δ = 1.59–
H2MeL: The product was obtained as an orange powder (1.94 g,
5.16 mmol, 32 %). Crystals suitable for X-ray diffraction were ob-
tained by slow cooling of a hot solution of the compound in tolu-
1.73 (m, 3 H), 1.99–2.10 (m, 3 H), 3.64–3.73 (m, 1 H), 4.00–4.07 (m,
1 H), 5.35–5.39 (m, 1 H), 7.54 (s, 1 H, 3-Hpz), 7.66 (s, 1 H, 5-Hpz) ppm.
13C NMR (CDCl3, 75 MHz): δ = 22.3 (5-CTHP), 25.0 (4-CTHP), 30.5 (6-
CTHP), 57.3 (4-Cpz), 67.8 (3-CTHP), 87.9 (1-CTHP), 132.4 (5-Cpz), 144.6
(3-Cpz) ppm.
1
ene. H NMR ([D6]DMSO, 300 MHz): δ = 2.19 (s, 12 H, CH3), 4.11 (s,
4 H, 3-/4-HFc), 4.26 (s, 4 H, 2-/5-HFc), 12.05 (br., 2 H, NH) ppm. 13C
NMR ([D6]DMSO, 75 MHz): δ = 12.2 (CH3), 67.2 (2-/5-CFc), 68.3 (3-/4-
CFc), 80.3 (1-CFc), 110.9 (4-Cpz) ppm; the signal for 3-/5-Cpz could not
be detected. 1H NMR (CDCl3, 300 MHz): δ = 2.29 (s, 12 H, CH3), 4.26
4-Iodo-3,5-dimethyl-1-(tetrahydropyran-2-yl)-1H-pyrazole (MeI):
4-Iodo-3,5-dimethyl-1H-pyrazole (7.5 g, 33.8 mmol, 1.0 equiv.), 3,4-
dihydro-2H-pyran (6.2 mL, 68.0 mmol, 2.0 equiv.), and p-toluene-
sulfonic acid (58 mg, 0.34 mmol, 0.01 equiv.) were dissolved in
MeCN (20 mL), and the mixture was heated to reflux for 3 h. After
cooling to room temperature, the solution was diluted with AcOEt
(50 mL), washed with a saturated aqueous solution of NaHCO3, and
dried with MgSO4. The solvent was then removed under reduced
pressure to afford a brownish solid. The crude material was washed
with several small portions of cold MeOH (–40 °C) until a white
powder resulted (6.51 g, 21.23 mmol, 63 %). More product was ob-
tained by leaving the filtrate at –30 °C overnight (2.07 g, 6.76 mmol,
20 %). Overall yield: 8.58 g, 28.03 mmol, 83 %. 1H NMR (CDCl3,
300 MHz): δ = 1.54–1.92 (m, 3 H), 1.87–1.92 (m, 1 H), 2.23 (s, 3 H,
CH3), 2.33 (s, 3 H, CH3), 2.07–2.10 (m, 1 H), 2.36–2.44 (m, 1 H), 3.59–
1
(s, 4 H, 3-/4-HFc), 4.48 (s, 4 H, 2-/5-HFc), 12.46 (br., 2 H, NH) ppm. H
NMR (CD2Cl2, 400 MHz): δ = 2.28 (s, 12 H, CH3), 4.28 (s, 4 H, 3-/4-
HFc), 4.52 (s, 4 H, 2-/5-HFc) ppm. 1H NMR ([D8]toluene, 400 MHz): δ =
2.16 (s, 12 H, CH3), 4.09 (s, 4 H, 3-/4-HFc), 4.26 (s, 4 H, 2-/5-HFc) ppm.
1H NMR ([D7]DMF, 400 MHz): δ = 2.30 (s, 12 H, CH3), 4.16 (t, J =
2 Hz, 4 H, 3-/4-HFc), 4.36 (t, J = 2 Hz, 4 H, 2-/5-HFc), 12.14 (br., 2 H,
NH) ppm. MS (EI): m/z (%) = 474.1 (100) [M]+. C20H22FeN4·1/7toluene
(387.4): calcd. C 65.10, H 6.02, N 14.46; found C 64.79, H 6.28, N
14.21.
General Procedure for the Preparation of the Copper Com-
plexes: The appropriate proligand H2RL (1.0 equiv.) and
[Cu(MeCN)4][BF4] (2.0 equiv.) were dissolved in dry MeOH (5 mL for
H2MeL) or MeOH/DMF (3:1; 12 mL for H2HL). After 3 min, degassed
Et3N (0.1 mL) was added dropwise, and a suspension formed imme-
diately. The reaction mixture was stirred for 2 h. The resulting solid
was separated by filtration, washed with MeOH, and dried in vacuo.
Owing to the insolubility of the products in any solvent, single-
crystalline material of analytical purity could not be obtained.
3
3.66 (m, 1 H), 4.02–4.07 (m, 1 H), 5.22 (dd, JH,H = 10.2/2.4 Hz, 1 H)
ppm. 13C NMR (CDCl3, 75 MHz): δ = 12.1 (CH3), 14.3 (CH3), 23.0
(5-CTHP), 25.1 (4-CTHP), 29.5 (6-CTHP), 65.0 (4-Cpz), 68.1 (3-CTHP), 85.4
(1-CTHP), 141.5 (5-Cpz), 150.0 (3-Cpz) ppm.
General Procedure for the Preparation of 1,1′-Bis(1H-pyrazol-4-
yl)ferrocene (H2HL) and 1,1′-Bis(3,5-dimethyl-1H-pyrazol-4-
[Cu2HL]3: 1.0 equiv., 0.314 mmol. The product was obtained as a
dark green solid (117 mg, 0.088 mmol, 84 %). MS (MALDI-TOF): m/z
(%) = 822.9 (30) [Cu3L2]+, 885.8 (65) [Cu4L2]+, 1329.7 (100) [Cu6L3]+.
yl)ferrocene (H2MeL): nBuLi (1.6
M in hexane, 21 mL, 33.6 mmol,
2.1 equiv.) and TMEDA (5.1 mL, 34.0 mmol, 2.1 equiv.) were mixed
in hexane (20 mL), and solid ferrocene (3.0 g, 16.1 mmol, 1.0 equiv.)
was added after 15 min. The resulting solution was stirred over-
night, and an orange suspension formed. The solvent was then re-
moved under reduced pressure, and the residue was dissolved in
THF (30 mL). The solution was cooled to 0 °C, and anhydrous ZnCl2
(4.83 g, 35.4 mmol, 2.2 equiv.) was added in one portion. After 1 h
of stirring, a solution containing the appropriate protected iodo-
pyrazole (8.97 g, 32.3 mmol, 2.0 equiv. of HI for H2HL; 9.87 g,
32.2 mmol, 2.0 equiv. of MeI for H2MeL) and Pd(PPh3)4 (900 mg,
5 mol-%) in THF (20 mL) was added to the reaction mixture. The
resulting mixture was stirred at room temperature for 2.5 d, and
then a saturated NaOH aqueous solution (100 mL) was added, and
the layers were separated. The aqueous phase was extracted with
CH2Cl2 (3 × 80 mL), the combined organic phases were dried with
MgSO4, and the solvent was removed under reduced pressure. The
crude material was purified by silica column chromatography with
AcOEt as the eluent (Rf = 0.49 for H2HL-THP; 0.53 for H2MeL-THP).
The compound was deprotected by dissolving the THP derivate in
MeOH (10 mL) and ethanolic HCl (10 mL) and stirring the solution
overnight. For H2HL, the resulting suspension was neutralized with
NaHCO3 in water and filtered, and the resulting solid was dried in
[Cu2MeL]3: 1.0 equiv., 0.198 mmol. The product was obtained as an
orange-brown solid (82 mg, 0.0547 mmol, 83 %). 1H NMR (CDCl3,
400 MHz): δ = 2.19 (s, 12 H, CH3), 4.27 (s, 4 H, 3-/4-HFc), 4.41 (s, 4 H,
1
2-/5-HFc) ppm. H NMR ([D7]DMF, 300 MHz): δ = 2.07 (s, 12 H, CH3),
4.67 (s, 4 H, 3-/4-HFc), 4.81 (s, 4 H, 2-/5-HFc) ppm. MS (MALDI-TOF):
m/z (%) = 935.0 (24) [Cu3L2 + H]+, 998.0 (100) [Cu4L2]+, 1497.9 (35)
[Cu6L3]+.
General Procedure for the Preparation of the Silver Complexes:
The appropriate proligand H2RL (1.0 equiv.) was dissolved in MeCN
(10 mL for H2MeL) or MeCN/DMF (3:1; 20 mL for H2HL), and a solu-
tion of AgBF4 (2.0 equiv.) in MeCN (2 mL) was added. After 3 min,
Et3N (0.1 mL) was added dropwise, and a precipitate formed imme-
diately. The mixture was stirred for 1 h, and then the solid was
separated by filtration, washed with MeCN and MeOH, and dried in
vacuo.
[Ag2HL]3: 1.0 equiv., 0.261 mmol. The product was obtained as an
orange/red solid (107 mg, 0.067 mmol, 72 %). MS (MALDI-TOF): m/z
(%) = 741.6 (86), 769.6 (100), 818.4 (83), 955.9 (60) [Ag3L2 + H]+,
1063.8 (53) [Ag4L2]+, 1100.9 (70), 1595.7 (73) [Ag6L3]+.
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H. V. R. Dias, Inorg. Chem. 2005, 44, 8200–8210; d) H. V. R. Dias, C. S. P.
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[11] a) T. Jozak, Y. Sun, Y. Schmitt, S. Lebedkin, M. Kappes, M. Gerhards, W. R.
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Z. Lv, G.-h. Chen, D. Li, Chem. Eur. J. 2011, 17, 4113–4117; c) A. C. Jahnke,
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C48H36Ag6Fe3N12·DMF (1668.7): calcd. C 36.71, H 2.60, N 10.91;
found C 37.35, H 2.71, N 10.45.
[Ag2MeL]3: 1.0 equiv., 0.222 mmol. The product was obtained as a
dark red solid (102 mg, 0.058 mmol, 78 %). MS (MALDI-TOF): m/z
(%) = 672.6 (100), 1068.0 (11) [Ag3L2 + H]+, 1175.9 (22) [Ag4L2]+,
1763.8 (66) [Ag6L3]+. C60H60Ag6Fe3N12 (1763.96): calcd. C 40.85, H
3.43, N 9.53; found C 40.71, H 3.59, N 8.84.
General Procedure for the Preparation of the Gold Complexes:
The appropriate proligand H2RL (1.0 equiv.) and AuCl(SMe2)
(2.0 equiv.) were mixed in MeCN (10 mL for H2MeL) or MeCN/DMF
(3:1; 10 mL for H2HL). After the addition of Et3N (0.1 mL), the reac-
tion mixture was stirred overnight. The resulting solid was then
separated by filtration, washed with MeCN and THF, and dried in
vacuo. Owing to the insolubility of the products in any solvent,
single-crystalline material of analytical purity could not be ob-
tained.
[Au2HL]3: 1.0 equiv., 0.088 mmol. The product was obtained as a
green solid (52 mg, 0.024 mmol, 82 %). MS (MALDI-TOF): m/z (%) =
2130.0 (100) [Au6L3]+.
[Au2MeL]3: 1.0 equiv., 0.142 mmol. The product was obtained as a
brown solid (100 mg, 0.0435 mmol, 92 %). MS (MALDI-TOF): m/z
(%) = 1336.1 (12) [Au3L2]+, 1532.1 (100) [Au4L2]+, 1907.2 (13) [Au4L3
+ 3H]+, 2102.2 (30) [Au5L3 + H]+, 2298.1 (31) [Au6L3]+.
[12] M. Veronelli, S. Dechert, S. Demeshko, F. Meyer, Inorg. Chem. 2015, 54,
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Acknowledgments
Financial support by the Georg-August-Universität Göttingen is
gratefully acknowledged.
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Keywords: Coinage metals · Nitrogen heterocycles ·
Metallocenes · Oligonuclear complexes · Supramolecular
chemistry
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Received: May 31, 2016
Published Online: ■
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Full Paper
Ferrocenyl Ligands
Supramolecular double deckers of the
common pyrazolate-based [M(μ-pz)]3
metallomacrocycles (M = CuI, AgI, AuI;
pz = pyrazolate) are assembled from
ferrocene derivatives with two C-4-ap-
pended pyrazole rings. The ferrocene/
M. Veronelli, S. Dechert,
A. Schober, S. Demeshko,
F. Meyer* ............................................... 1–9
1,1′-Bis(pyrazol-4-yl)ferrocenes: Po-
tential Clip Ligands and Their Supra-
molecular Structures
R
pyrazole proligands H2 L form dimeric
or trimeric supramolecular aggregates
through
intermolecular
NH···N
hydrogen bonds, both in the solid
state and in solution.
DOI: 10.1002/ejic.201600644
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