Monitoring Ligand Substitution in (Catalytically Active) Metal Complexes with Bodipy-Tagged Diimines and NHC Ligands

Article abstract of DOI:10.1021/acs.organomet.9b00130

The reaction of 2,6-dimethyl-4-(Bodipy-8-yl)aniline with 1,4-dioxane-2,3-diol provides the respective diimine 3, followed by ring closure with paraformaldehyde, resulting in the imidazolium salt 4·HCl containing two fluorophores. The NHC metal complexes (

Full text of DOI:10.1021/acs.organomet.9b00130

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pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Monitoring Ligand Substitution in (Catalytically Active) Metal
Complexes with Bodipy-Tagged Diimines and NHC Ligands
Oliver Halter, Jonas Spielmann, Yuki Kanai, and Herbert Plenio*
Organometallic Chemistry, TU Darmstadt, Alarich-Weiss-Str. 12, 64287 Darmstadt, Germany
S
* Supporting Information
ABSTRACT: The reaction of 2,6-dimethyl-4-(Bodipy-8-yl)aniline with 1,4-
dioxane-2,3-diol provides the respective diimine 3, followed by ring closure with
paraformaldehyde, resulting in the imidazolium salt 4·HCl containing two
fluorophores. The NHC metal complexes ([IrCl(cod)(4)], [Ir(cod)(4)(py)]-
OTf, [IrCl(CO)2(4)], [PdCl(allyl)(4)], [PdCl2(Clpy)(4)], [AuCl(4)], and
[NiCl(Cp)(4)]) were prepared. The complexes with (Au, Pd) are fluorescent.
Weakly fluorescent complexes with Ir undergo fluorogenic reactions with CO
and H2. The oxidative addition of H2 to [Ir(cod)(4)]OTf leads to a fluorescence gain of 8.2 and is suitable for the detection of
hydrogen. Following activation of the diimine complex [PdCl(CH3)(3)] with NaBArF, the formed [Pd(CH3)(3)]BArF is a
competent ethene polymerization catalyst. The addition of (pyridine, CH3CN, CO) or olefins (ethene, 1-hexene, styrene) to
the activated complex results in a pronounced increase in the fluorescence because of metal−ligand interactions and the
modulation of photoinduced electron transfer quenching. The fluorescence response was used for the determination of the
respective association constants with the donor molecules: 1-hexene (K = 9 L·mol−1) and CH3CN (K = 4 L·mol−1); styrene is a
poor ligand (K = 0.04 L·mol−1). Based on the ν(CO) of the [Mo(CO)4(3)] complex, the Bodipy-8-yl group is a moderately
electron-withdrawing group, comparable to the Br substituent.
̈
single-molecule studies by Herten, Kramer et al. on copper
INTRODUCTION
■
complexes motivated organometallic chemists.6 Blum et al.
extensively established the use of fluorescence microscopy in
transition-metal chemistry,7 reporting in detail on various
ruthenium-based olefin metathesis transformations.8 Gold-
smith et al. studied the initiation dynamics of surface-
immobilized palladium-based cross-coupling catalysts.9
ROMP reactions using fluorophore-tagged Grubbs−Hoveyda
The fluorescence light emitted from an excited fluorophore
closely connected to a receptor unit can provide valuable
information upon binding of a specific analyte by the host.1
The fluorescence of the probe is modulated by the binding
event, depending on the relative highest occupied molecular
orbital (HOMO)−lowest unoccupied molecular orbital
(LUMO) energies of the fluorophore and receptor.2 This is
compatible with a photoinduced electron transfer (PET)
quenching mechanism.3 Consequently, transition metal-based
fluorescent receptors appear to be especially useful because the
HOMO−LUMO energies of the metal can be manipulated by
various elementary organometallic reactions (e.g., oxidative
addition, ligand substitution, reductive elimination, insertion or
migration, and redox reactions).4 For a given fluorophore,
changes in the electron density at the near-by transition metal
lead to pronounced changes in the fluorescence, when the
respective HOMO−LUMO energies of the fluorophore and
metal are in the same range.5 Consequently, fluorophores are
useful reporter groups for (catalytically active) metal
complexes. A catalyst is present in very small amounts relative
to the substrates, whose conversion it is expected to catalyze,
and it can be difficult to obtain useful information on this
species by less sensitive spectroscopic techniques such as
NMR.
catalysts and the properties of the fluorescent polymer were
10
̈
investigated by Woll, Mecking et al.
However, this work is mainly limited to molecules deposited
on a surface. We have decided to focus on ensemble
measurements in solution because this better reflects the
specific conditions met in homogeneous catalysis.11 A highly
useful fluorescent reporter group appears to be Bodipy (boron
dipyrromethene, bdp).12 It possesses many favorable proper-
ties (such as a high extinction coefficient, excellent
fluorescence quantum yield, highly diverse functionalization
chemistry, lipophilicity, and chemical stability),13 which render
it useful for applications in organometallic chemistry.
Fluorophore-tagged (mostly Bodipy) metal complexes are
suitable for studying ligand substitution reactions in
solution14especially those involving catalytically active
complexes (Scheme 1).11,15 This provides information on the
initiation of olefin metathesis catalysts (Scheme 1, a),16
activating steps in gold-catalyzed transformation of alkynes
(Scheme 1, b),17 the fluorogenic detection of CO (Scheme 1,
The excellent detection sensitivity of fluorescence spectros-
copy and the fact that only fluorescent molecules are observed
can even provide information on individual fluorophore
molecules. Based on this, fluorescence microscopy has become
a useful tool for probing catalytic reactions.1c Ground-breaking
Received: February 28, 2019
© XXXX American Chemical Society
A
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Scheme 1. Fluorogenic Reactions Involving Fluorophore-
Tagged Transition-Metal Complexes
RESULTS AND DISCUSSION
■
a
Synthesis of Bodipy-Substituted Diimines and Azo-
lium Salts. 4-Amino-3,5-dimethylbenzaldehyde25 was synthe-
sized from 2,6-dimethylaniline on a decagram scale via a Duff
reaction26 in good yields (Scheme 2). Bodipy 2 is available
Scheme 2. Synthesis of a Bodipy-Substituted Azolium Salt 4·
a
HCl
a
Reagents and conditions: (a) 2,6 dimethylaniline, urotropine, AcOH,
reflux 4 h; (b) 3-ethyl-2,4-dimethylpyrrole, CH2Cl2, 8 h, para-
chloranil, 12 h, Et3N, BF3·OEt2; (c) 1,4-dioxane-2,3-diol and formic
acid cat. and ethanol, reflux, 24 h; (d) paraformaldehyde, ethyl
acetate, Me3SiCl, 70 °C, 48 h.
a
using the established one-pot procedure utilizing aldehyde 1.
Protection of the amino group is not required because this
group does not interfere with Bodipy synthesis. The procedure
reported here is a significant improvement over the previously
reported synthesis of 227 and provides multigram amounts of
Bodipy 2 in two steps from cheap commercially available
materials. The formation of diimines from anilines with glyoxal
is a very convenient reaction but tends to be less facile for the
condensation of sterically demanding anilines with electron-
withdrawing groups.28 Therefore, the reaction of 2 leading to
diimine 3 requires special precautions such as strictly
anhydrous reaction conditions, freshly dried molecular sieve
to remove the water formed in the reaction, and the use of 1,4-
dioxane-2,3-diol as an anhydrous glyoxal surrogate.29 Then,
diimine 3 can be obtained in good yields of 67%. Cyclization of
diimine 3 to the respective imidazolium salt according to the
very useful, general Hintermann procedure provides the
respective azolium salt 4·HCl,30 which serve as convenient
precursors for NHC ligands.31,32
Synthesis of Metal Complexes with Diimine 3 and
NHC 4 and Electronic Properties. Palladium complexes
with diimine 3 were prepared according to the general
procedure (Scheme 3)33 by treating the respective metal
source with diimine 3. The two complexes [PdCl2(3)] and
[PdCl(Me)(3)] are formed in good yields. Following the
activation with a Lewis acid, the latter complex forms an active
ethene polymerization catalyst.34 Diimine complex [Mo-
(CO)4(3)] can be prepared by heating a tetrahydrofuran
(THF) solution of 3 and Mo(CO)6.35
bdp and bdp′ denote Bodipy groups with different peripheral
substituents, bdp and bdp′ form a FRET pair, exc. = excitation, em. =
emission, dansyl = 5-(dimethylamino)naphthalene-1-sulfonylamide,
Tf = trifluoromethanesulfonate, L in AuL = the same Bdp-PCy2
ligand.
c),14a,18 the oxidative addition of H2 to a Crabtree-like catalyst
(Scheme 1, d),19 or the formation of dimeric metal complexes
in the course of gold-catalyzed cyclization reactions via the
observation of a FRET signal (Scheme 1, e).20 Bodipy-tagged
olefin metathesis catalysts have also been employed to probe
enyne-reactions or substrate−catalyst association.21 Bodipy-
substituted platinum complexes were employed in photo-
catalysis.22
Up until now, we have only synthesized ligands and the
derived organometallic complexes containing one Bodipy per
metal atom. However, certain metals like palladium and
ruthenium tend to quench the fluorescence of the appended
fluorophore very efficiently, resulting in “dark” complexes,14a
while only a few tagged palladium complexes show modest
fluorescence quantum yields.23 We were hoping that ligands
with two fluorophore molecules per palladium, would render
brightly fluorescent complexes, in which the fluorophores are
useful probes to study the rich catalytic chemistry of palladium
complexes. Based on this concept, we want to report here on
the facile synthesis of diimine and NHC ligands containing two
Bodipy units, the derived metal complexes, and the
fluorescence evolution during the interaction of olefins with
Bodipy-tagged Brookhart-type catalysts24 for ethene polymer-
ization.
Several new metal complexes utilizing the Bodipy-tagged
NHC ligand 4 were prepared (Scheme 4) according to general
literature procedures: [IrCl(cod)(4)],36 [Ir(cod)(4)(py)2]-
OTf,19 [IrCl(CO)2(4)],36 [PdCl(allyl)(4)],37 [PdCl2(Clpy)-
B
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a
potentials ΔE1/2(Bodipy/Bodipy+) = +1.04 V (Ea − Ec = 91
mV) and ΔE1/2(Bodipy/Bodipy−) = −1.33 V (Ea − Ec = 94
mV) are very close to those of 1,3,5,7-tetramethyl-2,6-diethyl-
8-pentyl Bodipy (ΔE1/2 = −1.33, +1.02 V).42 The orthogonal
8-aryl group in [IrCl(cod)(4)] has little influence on the redox
potential. The pronounced electron-withdrawing effect of the
Bodipy substituent on the redox potential of iridium is due to
the π-face interaction of the N-aryl ring with the transition
metal.43
Scheme 3. Synthesis of Bodipy-Diimine Metal Complexes
In metal complexes of diimine 3 (Scheme 3), the N-aryl
rings have a less favorable orientation relative to the metal
center concerning potential π-face interactions. The electronic
communication of the Bodipy substituents with the metal in
the respective diimine complexes should thus be different. In
order to establish this, Bodipy-substituted complex [Mo-
(CO)4(3)] and the closely related diimine-Mo(CO)4 com-
plexes with 4-R = H or Br were studied.35b The determination
of the donor properties of the diimine ligands via redox
potentials of the respective metal complexes is not convenient
because the diimines themselves are redox-active.35b Recently,
Brenna et al. successfully employed infrared spectroscopy and
ν(CO) for the determination of the donor properties of
substituted phenanthrolines.44 Concerning the related diimine,
we decided to compare the respective [(diimine)Mo(CO)4]
complexes with 4-R = Bodipy ([Mo(CO)4(3)]) and the
closely related complexes with 4-R = H and 4-R = Br. The
respective ν(CO) were determined to be 4-R = H 2008 cm−1,
4-R = Br 2015 cm−1, and 4-R = Bodipy (=[Mo(CO)4(3)]
(2015 cm−1)).35a In the absence of significant π-face
interactions of the substituted N-aryl groups, the electronic
influence of the Bodipy substituent on the metal is much
weaker (comparable to a halide) than in the respective NHC
complexes.
a
Reagents and conditions: (a) [PdCl(CH3)(cod)] or
[PdCl2(CH3CN)2], CH2Cl2, rt, N2, 24 h, yield (R = CH3) 61%, (R
= Cl) 71%. (b) Mo(CO)6, diimine 3, THF, reflux, 18 h, yield 44%.
(4)], [AuCl(4)],38 and [NiCl(Cp)(4)].39 All complexes are
air- and moisture-stable compounds.
The cyclic voltammogram of [IrCl(cod)(4)], namely, the
Ir(I/II) redox potential, was determined to measure the donor
properties of the NHC-ligand substituted with two electron-
deficient Bodipy units.28,40 The metal-centered quasi-reversible
redox potential Ir(I/II) was shown to be ΔE1/2 = +0.93 V (Ea
− Ec = 80 mV). This redox potential is close to that of the
related [IrCl(cod)(NHC)] complex with the highly electron-
withdrawing −SO2(tolyl) group (ΔE1/2 = +0.92 V).28 Based
on this, the Bodipy-8-yl substituent behaves like a highly
electron-withdrawing group with a Hammett constant close to
that of σp(SO2Et) = 0.77.41 Bodipy itself is characterized by
two (reversible) redox events corresponding to the oxidation
and the reduction of the Bodipy, which depend on the nature
of the substituents at the fluorophore.42 The two redox
Fluorescence Properties. All new Bodipy-tagged com-
pounds reported here show the same absorbance (λmax = 526
nm) and emission maxima (λem = 538 nm). Obviously, none of
the remote substituents in the different compounds has a
a
Scheme 4. Synthesis of NHC-Bodipy Metal Complexes
a
Reagents and conditions: (a) 4·HCl, [PdCl(allyl)]2 and K2CO3 acetone, 60 °C, 4 h; (b) PdCl2, 4·HCl, Cl-pyridine solvent, K2CO3, 80 °C; 24 h;
(c) [AuCl(Me2S)], 4·HCl, K2CO3, 60 °C, acetone; (d) [IrCl(cod)]2, CH2Cl2, 40 °C, 4·HCl, Ag2O; (e) [IrCl(cod)(4)], CO, 30 min, rt, CH2Cl2;
(f) [IrCl(cod)(4)], [Ag(OTf)], CH2Cl2, rt, 2 h; (g) 4·HCl, Cp2Ni, THF, reflux, 24 h.
C
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significant influence on the optical properties of the
fluorophore. The fluorescence quantum yields of the newly
synthesized compounds were determined (Table 1).45 Diimine
gold complexes to better understand the initial steps leading to
gold-catalyzed alkyne transformations.17
Fluorogenic Reactions of (NHC)Ir Complexes with CO
and H2. The reaction of [IrCl(cod)(4)] with CO resulting in
[IrCl(CO)2(4)] is a fluorogenic reaction and leads to a
pronounced increase in the fluorescence intensity (Figure 1)
Table 1. Fluorescence Quantum Yields for Diimine 3,
Azolium Salt 4·HCl, and the Derived Metal Complexes in
a
1,2-Dichloroethane (c = 1.0 × 10−6 mol·L−1)
compound
fluorescence quantum yield ϕ
3
0.49
0.003
0.06
0.02
0.65
0.06
0.58
0.05
0.67
0.60
0.38
0.03
[PdCl2(3)]
[PdCl(CH3)(3)]
[Mo(CO)4(3)]
4·HCl
[IrCl(cod)(4)]
[IrCl(CO)2(4)]
[Ir(cod)(py)(4)]OTf
[AuCl(4)]
[Pd(allyl)Cl(4)]
[PdCl2(Clpy)(4)]
[NiCl(Cp)(4)]
Figure 1. Fluorescence vs time plot for the reaction of [IrCl(cod)(4)]
(c = 1.0 × 10−6 mol·L−1 in 1,2-dichloroethane) with CO
(fluorescence gain = 11). The fluorescence of the initial complex is
arbitrarily set to 1.0.
a
The absorbance and excitation maxima for all compounds are λexc
526 nm and λem = 538 nm.
=
3 is characterized by a strong fluorescence, but for [PdCl2(3)]
and [Mo(CO)4(3)], a pronounced quenching of the
fluorescence is observed. Fortunately, [PdCl(CH3)(3)] dis-
plays a decent fluorescence brightness (ϕ = 0.06).
by a factor of 11. However, because the initial level of
fluorescence is already high (for the reasons mentioned for the
Pd complexes), the relative increase in the fluorescence is less
pronounced than in complexes with a single Bodipy.14a,18
Previously, we had reported the first fluorogenic reaction
involving H2.19 This is based on the oxidative addition of H2 to
a Crabtree catalyst-type complex. The complex studied then
(Scheme 1d) is characterized by a relatively large distance
between the fluorophore and transition-metal quencher,
leading to lower efficiency of the PET quenching and based
on this to a relatively high fluorescence level of the initial
cationic iridium complex prior to the addition of H2.
The obvious approach for a higher signal gain is to decrease
the distance between the transition-metal quencher and the
fluorophore. This should lead to a decreasing fluorescence
level of the initial cationic [Ir(cod)(NHC-bdp)(py)] complex.
Based on this criterion, the new complex [Ir(cod)(py)(4)]OTf
might offer a stronger fluorescence response upon addition of
H2. The reaction with H2 and the formation of the Ir(III)
complex leads to a pronounced increase in the fluorescence by
a factor of 8.2 (Figure 2). However, this gain is slightly smaller
than the fluorescence gain of 11.7 observed previously.19
Obviously, the effect of the closer vicinity of the Bodipy to the
transition metal is overcompensated by the presence of two
fluorophore molecules rather than just a single one. The
obvious solution would be to synthesize a complex closely
related to [Ir(cod)(py)(4)]OTf, but with only a single Bodipy
unit. Unfortunately, this synthetic target so far has eluded all
our synthetic efforts.
Most of the NHC-metal complexes and the azolium salt
itself display a strong fluorescence, namely, the complexes
[IrCl(CO)2(4)], [AuCl(4)], [Pd(allyl)Cl(4)], and
[PdCl2(Clpy)(4)] (Table 1). The strong fluorescence of the
two palladium complexes is remarkable because a previously
reported (NHC-Bodipy)Pd complex is characterized by a
modest fluorescence quantum yields (ϕ = 0.03).14a This bright
fluorescence offers chances for the monitoring cross-coupling
reactions46 or ligand exchange reactions47 via fluorescence
spectroscopy.11 The most significant difference between
[Pd(allyl)Cl(4)]/[PdCl2(Clpy)(4)] and the previous Pd
complexes is that the latter contains only a single Bodipy
unit per palladium, which leads to very different fluorescence
behavior. We expect that the quenching of two fluorophores by
a single palladium should be less efficient than the interaction
of one palladium with a single fluorophore. The proximity of
the two Bodipy units does not seem to have a significant effect
on the fluorescence properties because the conjugation path
between the two dyes is interrupted.48
We have shown previously for related (NHC-Bodipy)gold
complexes5 that changes in the electron density at the metal
center are very important for the fluorescence of the respective
(Bodipy-NHC)metal complexes. Based on a systematic study
of the fluorescence, it was concluded that PET quenching
appears to be the dominant quenching mechanism in the
various Bodipy-tagged transition-metal complexes.5 Conse-
quently, the systematic variation of the electron density at the
transition metal allows the facile manipulation of the
fluorescence.18 Especially intermediate fluorescence quantum
yields such as in [PdCl(CH3)(3)] offer interesting analytic
opportunities because a decrease or an increase in the electron
density leads to corresponding changes in the fluorescence
emission, both of which are easily observable. Such an
approach was recently utilized in Bodipy-tagged phosphine−
Reactions of [PdCl(CH3)(3)] and [PdCl(allyl)(4)] with
Thiolates. The interpretation of the fluorescence evolution in
the various reactions of Bodipy-tagged metal complexes
reported here is based on a predominately PET-based
fluorescence quenching. This has been demonstrated pre-
viously for [(NHC-bdp)AuCl] complexes employing the
chloride-to-thiolate substitution of RC6H4SH with electroni-
cally variable R groups5 and should also be established for the
complexes reported here.
D
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previously.5 Based on the observation that an increase in the
electron density leads to a decrease in the fluorescence, an
acceptor PET mechanism likely is the dominant quenching
pathway in the NHC metal complexes reported here.5
Interaction of Ligands with [Pd(Me)(3)]BArF. The
activation of [PdCl(Me)(3)]50 with NaBArF (sodium tetrakis-
(3,5-bis(trifluoromethyl)phenyl)borate)51 generates [Pd(Me)-
(3)]BArF, in which the strongly coordinating chloride is
replaced by the very weakly coordinating BArF− counterion.
The activation of the palladium complex leads to the in situ
formation of the cationic complex [Pd(Me)(3)]+BArF−
resulting in a minor dip in the fluorescence from 1.0 to 0.8
(Figure 4, inset). This emission change is comparable to the ca.
Figure 2. Fluorescence vs time plot for the reaction of [Ir(cod)(4)-
(py)]OTf (c = 1.0 × 10−6 mol·L−1 in 1,2-dichloroethane) with H2
(fluorescence gain = 8.2). The fluorescence of the initial complex is
arbitrarily set to 1.0.
We first tested the reaction of [PdCl(CH3)(3)] with
thiolates. However, in the presence of an excess of PhSH,
the diimine complex decomposes (as evidenced by NMR
spectroscopy) to produce Pd-thiolates and an unknown
diimine−thiol adduct. The reactions of [PdCl(allyl)(4)] with
six different thiols RC6H4SH (R = CF3, Cl, H, tBu, OMe,
NMe2) lead to the clean formation of the respective
[Pd(SR)(allyl)(4)] (evidenced by NMR spectroscopy).
Closely related palladium-thiolates with a phosphine instead
of the NHC ligand are known.49 Electron-deficient thiols react
very fast, while the electron-rich thiols are converted
progressively slower. It appears that the acidity of the thiol is
important for a fast reaction with the metal complex.
Figure 4. Fluorescence vs time plot for the reaction of diimine 3 (c =
1.0 × 10−6 mol·L−1 in 1,2-dichloroethane) with [PdCl(CH3)(cod)]
followed by the activation with NaBArF (see inset for magnified
view), followed by the addition of ethene and polymerization. The
fluorescence of [PdCl(CH3)(3)] is arbitrarily set to 1.0.
Replacing chloride by electron-rich thiolates leads to a
significant decrease in the fluorescence because of the
formation of the electron-rich Pd-thiolates (Figure 3). This
25% decrease in fluorescence upon substitution of a
coordinating chloride in [AuCl(Bodipy-PCy2)] with a weakly
coordinating triflate ion.17 Next, the coordination of donor
molecules by the cationic palladium complex can be followed
via the fluorescence evolution of the Bodipy tag. In general, the
addition of ligands results in a significant increase in the
fluorescence intensity. First, the reaction of two strong ligands
L = pyridine and CO with the cationic palladium complex was
tested (Scheme 5). In both reactions, the addition of a large
excess ligand to the cationic palladium led to the fast formation
of the respective complexes [Pd(Me)(3)(L)]BArF. For L = py,
the fluorescence increases 12-fold (gain = 44), for L = CO
(gain = 6). The closely related [Pd(Me)(diimine)(CO)]BArF
complexes are well established in the literature.52
It was tried to determine the equilibrium constant of the
pyridine complex by lowering the amount of added pyridine.
However, at c([Pd(Me)(3)]BArF) = 1.0 × 10−6 mol·L−1, even
0.1 μL of pyridine led to the nearly quantitative formation of
the respective pyridine complex. Further lowering the amount
of pyridine results in very long reaction times. Based on this,
the equilibrium constant K(py) was estimated to be larger than
105 L·mol−1. CH3CN is a much weaker ligand toward cationic
late transition metals than CO and pyridine53 and its binding
strength toward the cationic diimine Pd complexes was
previously found to be in the same range as that of simple
alkenes.54 The use of labile ligands in such complexes is
important because in order to form highly active polymer-
ization catalysts, the respective donor ligand should be
efficiently replaced by the olefin to be polymerized. The
addition of CH3CN to in situ generated [Pd(CH3)(3)]BArF
Figure 3. log fluorescence vs time plot for the reactions of
[PdCl(allyl)(4)] (c = 1.0 × 10−6 mol·L−1) with thiolates RC6H4SH
(R = CF3, Cl, H, tBu, OMe, NMe2) (1000 equiv) in the presence of
̈
Hunig base (100 equiv). The fluorescence of the initial [PdCl(allyl)-
(4)] is arbitrarily set to 1.0.
decrease depends on the nature of R and is more pronounced
for electron-rich thiols than for the electron-deficient thiols.
The final fluorescence intensities of the respective thiolates are
correlated with the Hammett constants41 of the respective R
groups (Hammett plot, Figure S52, Supporting Information).
In conclusion, our experiments provide convincing arguments
for the importance of PET quenching in the palladium
complexes studied heresimilar to the gold thiolates reported
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Scheme 5. Association of Ligand L with Cationic Complex [Pd(Me)(3)]+BArF− (L = py, CH3CN, Et2O or Olefins such as
Ethene, 1-Hexene, Styrene)
leads to an up to 50-fold increase in the fluorescence.
Saturation, because of the nearly quantitative formation of
[Pd(Me)(3)(CH3CN)]BArF, is reached after addition of ca.
105 equiv of CH3CN. The quantitative evaluation of the
titration data via the Scatchard method55 provides the
respective equilibrium constant K(CH3CN) = 4.2 L·mol−1.
The stability constants for THF (K = 1.7 L·mol−1) and Et2O
(K = 1.1 L·mol−1) were determined accordingly and are
weaker than those of CH3CN.56 This explains why the Et2O
adduct of the nontagged analogue of complex [Pd(Me)(3)-
(Et2O)]BArF was reported to be highly labile.54
with ethene has no effect on the fluorescence. Another
supportive observation is that the fluorescence−time trace
reaches a constant value after ca. 5 min exposure to ethene.
Because of the extremely small amounts of the palladium
complex contained in the fluorescence cuvette (ca. 2.0 × 10−9
mol), direct proof of the polymerization reaction in the cuvette
could not be obtained. In separate experiments on a larger
scale (under otherwise similar conditions), it was shown that
the Bodipy-tagged complex [PdCl(Me)(3)] activated with
NaBArF is a competent ethene polymerization catalyst, leading
to the formation of polyethene (Supporting Information).
Probably the most interesting step in the sequence of
reactions leading to the polymerization of ethene is the
association of an olefin with the activated palladium complex.
This step can be followed conveniently by fluorescence
spectroscopy because it is associated with a pronounced
change in the fluorescence.
Because ethene is a gas under ambient conditions, control
over the concentration of this gas below the saturation level in
the cuvette solution is experimentally difficult.59 However, the
concentration of liquid olefins in the reaction mixture can be
adjusted precisely and consequently reactions with 1-hexene,
styrene, and methacrylate were studied. The stepwise addition
of 1-hexene to a solution of [Pd(CH3)(3)]BArF leads to a
pronounced increase in the fluorescence (Figure 5) corre-
Fluorescence Evolution of [Pd(Me)(3)]BArF during
the Reaction with Olefins. The interaction of olefins with
the cationic palladium complex is more interesting than those
of the simple ligands (CO, py or CH3CN) because cationic
[Pd(CH3)(diimine)]+ species are a competent polymerization
catalyst.24,34a,57 The sequence of reactions leading to the
polymerization of ethene with [Pd(Me)(3)]BArF can be
monitored by recording the fluorescence of the reaction of
Bodipy-tagged diimine ligand 3 and [PdCl(Me)(cod)] (Figure
4) followed by the activation with NaBArF and followed by
ethene addition. To a solution of diimine 3 (c = 1.0 × 10−6
mol·L−1) in 1,2-dichloroethane was added [PdCl(CH3)(cod)],
leading to the rapid formation of [PdCl(Me)(3)]. This
reaction is accompanied by a pronounced decrease in the
fluorescence level from 22.5 for the diimine to 1.0 for
[PdCl(Me)(3)]. The reaction with NaBArF leads to the
modest decrease in fluorescence (see previous section) and the
formation of [Pd(Me)(3)]BArF. The addition of ethene to
this activated complex results in a pronounced increase in the
fluorescence to 4.9. Adding more ethene after 30 min reaction
time (re-saturation) has no effect on the fluorescence. The
change in the fluorescence should initially be due to the
formation of [Pd(Me)(3)(ethene)]BArF. At the same time,
the activated complex is a polymerization catalyst and the
insertion of ethene into the Pd−alkyl bond leads to the
formation of the respective complexes [Pd((C2H4)nMe)(3)-
(ethene)]+. Because such complexes are known to be the
resting states in palladium-catalyzed ethene polymerization
reactions, the fluorescence is dominated by those spe-
cies.24,54,58 All complexes with n = 0 and n > 0 are expected
to display the same fluorescence properties because any
changes in the remote alkyl substitution should be negligible
concerning the PET quenching of the fluorophore.
Figure 5. Fluorescence intensity vs time plot for the reactions of
[PdCl(Me)(3)] (c = 1.0 × 10−6 M) in 1,2-dichloroethane (2 mL)
with NaBArF (100 equiv) followed by addition of 1-hexene; 1 μL
corresponds to 4000 equiv of 1-hexene. The fluorescence of
[PdCl(Me)(3)]is arbitrarily set to 1.0.
The equilibrium constant determined via this approach
should correspond to the equilibrium constant for the simple
reaction, [Pd(Me)(3)]BArF + olefin = [Pd(Me)(3)(olefin)]+
+ BArF−, under the following assumptions: the different
insertion products [Pd((C2H4)nMe)(3)(ethene)]+ have the
same affinity to ethene independent of the value of n. This
should hold true in a good approximation for low degrees of
polymerization. Furthermore, c(ethene) should be virtually
constant throughout the reaction. This condition is met
because the olefin is present in ca. 105-fold excess relative to
the cationic palladium species. Furthermore, the re-saturation
sponding to the formation of [Pd(CH3)(3)(1-hexene)]BArF
and of the respective insertion products. The reaction of the
olefin with the activated palladium complex appears to be an
equilibrium reaction. The calculated equilibrium constant is
K(1-hexene) = 9 L·mol−1. Closely related complexes (without
the Bodipy tag) can polymerize 1-hexenealbeit at a much
slower rate than ethene.60 However, in the fluorescence
experiments, we do not observe any significant polymer
formation with 1-hexene. The respective association constant
for styrene were determined in the same way: K(styrene) =
0.04 L·mol−1.61 Methacrylate was also tested, but the metal
F
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Article
7.26 ppm, 13C NMR = 77.23 ppm), dimethyl sulfoxide (1H NMR =
2.50 ppm, 13C NMR = 39.51 ppm), CD2Cl2 (1H NMR = 5.32 ppm),
or hexafluorobenzene (19F NMR = −164.90 ppm). Mass spectra were
recorded on the Finnigan MAT95 spectrometer using electron
ionization. UV−vis spectra were recorded on the Analytik Jena
SPECORD 600 UV−vis spectrometer, and fluorescence spectra were
recorded on a J&M TIDAS S700/CCD UV/NIR 2098 spectrometer
combined with a J&M TIDAS LSM monochromator with 75 W
xenon light source and thermo-controlled cuvette holder. Samples for
emission and absorption measurements were contained in a 1 cm × 1
cm quartz cuvette (Hellma Analytics, lot. 111-10-40). Cyclic
voltammetry was performed using a standard electrochemical
instrumentation consisted of an EG&G 273A-2 potentiostat−
galvanostat. A three-electrode configuration was employed. The
working electrode was a Pt disk (diameter 1 mm) sealed in soft glass
with a Pt wire as a counter electrode. The pseudo reference electrode
was a Ag wire. Potentials were calibrated internally against the formal
potential of ferrocene (+0.46 V vs Ag/AgCl) or octamethylferrocene
(−0.01 V vs Ag/AgCl). All cyclic voltammograms were recorded in
dry CH2Cl2 under an atmosphere of argon, supporting electrolyte
NnBu4PF6 (c = 0.1 mol/L).
binding is even weaker than styrene and an association
constant could not be determined. Based on our fluorescence
experiments, the binding of 1-hexene is approx. twice as strong
as that of CH3CN, which is compatible with literature
data.54,62 The stability constants for propene and PhCN
determined by Kurosawa et al. for cationic cyclopentadienyl-
palladium complexes also are almost the same.63
Following the activation of [PdCl(CH3)(3)] with NaBArF,
the coordination of donor ligands leads to a pronounced
increase in the fluorescence as a consequence of the increased
electron density at the “cationic” palladium. This is indicative
of a donor-PET quenching mechanism for the diimine
complexes as opposed to an acceptor-PET reported for the
NHC metal complexes.64 With a view to the much weaker
donicity of diimines compared to NHC ligands, this is
plausible.40b
CONCLUSIONS
■
Based on the facile synthesis of Bodipy-substituted aniline 2,
numerous transition-metal NHC and diimine complexes
containing two Bodipy fluorophores were synthesized and
the variation of the fluorescence of the appended fluorophore
during various ligand substitution reactions monitored. In
NHC(4)-metal complexes, the increase of electron density on
the metal leads to a decrease in the fluorescence. In the related
diimine(3)-metal complexes, similar reactions led to an
increase in the fluorescence intensity. This is indicative of
acceptor-PET quenching for the former and donor-PET for the
latter metal complexes. The changes in the fluorescence are
analytically useful for monitoring chemical transformations at
the metal center in the course of catalytic transformations at
catalytically relevant concentrations of the respective metal
complex. The activated [Pd(Me)(3)]BArF coordinates donor
solvents and olefins. Changes in the fluorescence enable the
determination of the respective association constants for the
metal−ligand interaction. The binding of CO and pyridine to
the cationic palladium complex is much stronger than that of
ligands such as 1-hexene > CH3CN > THF > Et2O. Styrene
and methacrylate are very weak ligands. This work has
demonstrated the usefulness of fluorescence spectroscopy as an
analytic tool in fluorophore-tagged metal complexes, leading to
a better understanding of the chemistry at the metal center in a
catalytically relevant concentration range.
Synthesis of 4-Amino-3,5-dimethylbenzaldehyde 1. In a 1 L
round-bottom flask, 2,6-dimethylaniline (20.52 g, 0.1693 mol, 1
equiv) and urotropine (47.4 g, 0.338 mol, 2 equiv) were dissolved in a
mixture (600 mL) of AcOH/H2O (3:1) and heated to reflux for 4 h.
The mixture was allowed to cool down to room temperature. Next,
H2O (100 mL) was added, and the reaction mixture was extracted
four times with CH2Cl2. The combined organic layers were washed
with saturated aq. NaHCO3 solution and brine. The organic solution
was dried over MgSO4 and filtered off. The volatiles of the filtrate
were evaporated under reduced pressure. The remaining residue was
carefully purified by column chromatography (Cy/EA = 1:1). The
product is a yellowish oily solid 16.4 g (0.110 mol, 66%).
1H NMR (CDCl3, 500 MHz): δ 9.71 (s, 1H, CH), 7.47 (s, 2H,
CH), 4.21 (s, 2H, NH2), 2.21 (s, 6H, CH3). 13C NMR (CDCl3, 126
MHz): δ 191.05, 149.31, 130.93, 127.02, 120.96, 14.46.
Synthesis of Bodipy-Aniline 2. 3-Ethyl-2,4-dimethylpyrrole
(2.45 mL 18.09 mmol, 2.05 equiv) and 4-amino-3,5-dimethylbenzal-
dehyde (8.83 mmol, 1 equiv) were dissolved in 100 mL of CH2Cl2
containing a catalytic amount of trifluoroacetyl (12 drops). The
resulting mixture was stirred for 9 h at room temperature under a N2
atmosphere. para-Chloranil (3.91 g, 15.89 mmol, 1.8 equiv) was
added to the reaction mixture, while cooling the flask because of the
exothermic reaction. The dark suspension was stirred at room
temperature for 12 h, and then, Et3N (12.41 mL, 88.29 mmol, 10
equiv) added. Stirring was continued for 2 h at room temperature and
next BF3·OEt2 (16.34 mL, 132.42 mmol, 15 equiv) was added to the
stirred solution. After 3 h, the reaction mixture was transferred to a 2
L separation funnel, washed with saturated NaHCO3 solution (1.5 L),
and extracted with CH2Cl2 (1 L). The organic phase was dried over
magnesium sulfate. The volatiles were removed under reduced
pressure, and the remaining residue was purified by column
chromatography (Cy/EA 4:1) to obtain the desired dark reddish
product (1.69 g, 4.00 mmol, 41%).
EXPERIMENTAL SECTION
■
General Experiment. All reactions involving transition-metal
complexes were conducted in oven-dried glassware. Reactions were
performed in Schlenk flasks under a positive pressure of argon or
nitrogen. The flasks were fitted with rubber septa, and gas-tight
syringes with stainless steel needles or double-cannula were used to
transfer air- and moisture-sensitive liquids.
1H NMR (CDCl3, 500 MHz): δ 6.81 (s, 2H, CH), 3.73 (s, 2H,
NH2), 2.52 (s, 6H, CH3), 2.30 (q, 4H, CH2), 2.22 (s, 6H, CH3), 1.37
(s, 6H, CH3), 0.99 (t, 6H, CH3). 13C NMR (CDCl3, 126 MHz): δ
153.03, 143.14, 141.75, 138.67, 132.42, 131.53, 128.05, 125.08,
122.33, 17.67, 17.23, 14.77, 12.57, 12.16. 19F NMR (CDCl3, 471
MHz): δ −148.84 (q, 4F, BF2). ESI HR-MS m/z: calcd for
C25H33BF2N3 [M + H]+, 424.27301; found, 424.27370.
Synthesis of Diimine 3. An oven-dried Soxhlet extractor with a
Dimroth condenser connected to a 100 mL Schlenk flask was used for
the synthesis of compound 3. The Soxhlet extractor was charged with
30 g of carefully dried 3 Å-molecular sieve (100 °C for 2 d at 0.1
mbar, followed by 300 °C for 30 min at 0.1 mbar). Bodipy-aniline 2
(1.97 g, 4.65 mmol, 2.1 equiv), 1,4-dioxan-2,3-diol (262.3 mg, 2.21
mmol, 1 equiv), and 30 drops formic acid were suspended in dry
ethanol (60 mL). The reaction mixture was heated to 110 °C for 48 h.
Precipitation of the desired diimine 3 begins after ca. 1 h of reaction
Materials. All chemicals were purchased as reagent grade from
commercial suppliers and used without further purification, unless
otherwise noted. THF was dried under sodium and distilled under
argon atmosphere. 1,2-Dichloroethane was dried over CaH2. All
solvents for synthesis were stored over molecular sieves. Solvents
stored over molecular sieve are only suitable for fluorescence
measurements after careful filtrations because small amounts of
molecular sieve dust can lead to high levels of scattered excitation
light. Preparative chromatography was performed using Merck silica
60 (0.063−0.2 mm).
Instrumentations. The chemical shifts are given in parts per
million on the delta scale (δ) and are referenced to tetramethylsilane
(1H, 13C NMR = 0.0 ppm), the residual peaks of CDCl3 (1H NMR =
G
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time. After the 48 h, the reaction mixture was cooled to −10 °C, and
the precipitate was filtered off. The residue was washed with cold abs.
ethanol (6 mL) and n-pentane (6 mL). Compound 3 is an orange
solid (1.35 g, 1.56 mmol, 67%).
126.66, 83.22, 64.92, 32.65, 29.50, 18.50, 18.30, 17.28, 17.22, 14.79,
14.62, 13.06, 12.73, 12.06. 19F NMR (CDCl3, 471 MHz): δ −81.21
(s, 3F, CF3), −148.84 (m, 4F, BF2). ESI HR-MS m/z: calcd for
C61H74IrB2F4N6 [M − H]+, 1410.57415; found, 1410.57436.
Synthesis of [IrCl(CO)2(4)]. To a Schlenk flask (10 mL) equipped
with stirring bar and septum were added [IrCl(cod)(4)] (40 mg,
0.0329 mmol) and dry CH2Cl2 (5 mL). A balloon with carbon
monoxide was connected through a cannula, and CO was bubbled
through the stirred solution during 30 min. The reaction mixture was
concentrated under reduced pressure, then pentane was added, and
the resulting precipitate was filtered off and washed with pentane. The
resulting product is an orange solid (30 mg, 0.0258 mmol, yield 78%).
1H NMR (300 MHz, CDCl3): δ 7.35 (s, 2H, N−CH−CH−N),
7.19 (s, 4H, 4H m-Ar), 2.55 (s, 12H), 2.33−2.30 (m, 20H, 4× CH3,
4× CH2 Bodipy), 1.49 (s, 12H), 1.05−0.98 (m, 12H, 4× CH3
Bodipy). 13C NMR (126 MHz, CDCl3): δ 179.75, 176.87, 168.48,
155.07, 153.34, 139.54, 138.81, 137.67, 137.32, 136.88, 133.53,
132.70, 131.11, 128.71, 123.82, 18.82, 17.24, 14.83, 14.70, 12.95,
12.78, 12.64, 12.36. 19F NMR (471 MHz, CDCl3): δ −148.77,
−148.98 (m, 4F, BF2). ESI HR-MS m/z: calcd for
C55H63IrB2ClF4N6O2 [M − H]+, 1165.44470; found, 1165.44588.
Synthesis of [PdCl(allyl)(4)]. 4·HCl (50 mg, 0.0545 mmol, 1
equiv), [PdCl(allyl)]2 (10.0 mg, 0.0273 mol, 0.5 equiv), and K2CO3
(22.6 mg, 0.164 mmol, 3 equiv) were suspended in acetone (3 mL).
Next, the mixture was stirred at 60 °C for 4 h. The volatiles were
removed in vacuo, and CH2Cl2 (3 mL) was added to the residue. The
mixture was filtered through a silica plug and the filtrate dried in
vacuo. The solution was evaporated and purified by column
chromatography (Cy/EA 1:1). An orange solid was obtained (40
mg, 0.0376 mmol, 69%).
1H NMR (CDCl3, 500 MHz): δ 8.22 (s, 2H, CH), 7.04 (s, 4H,
CH), 2.54 (s, 12H, CH3), 2.33 (q, 8H, CH2), 2.26 (s, 12H, CH3),
1.42 (s, 12H, CH3), 1.01 (t, 12H, CH3). 13C NMR (CDCl3, 126
MHz): δ 163.84, 153.33, 150.33, 140.18, 138.40, 132.55, 131.05,
128.30, 127.59, 18.38, 17.24, 14.76, 12.64, 12.10. 19F NMR (CDCl3,
471 MHz): δ −148.84 (m, 4F, BF2). ESI HR-MS m/z: calcd for
C52H63B2F4N6 [M]+, 869.52310; found, 869.52447.
Synthesis of Imidazolium Salt 4·HCl. A 25 mL Schlenk flask
was charged with diimine 3 (300 mg, 0.345 mmol, 1 equiv),
paraformaldehyde (10.5 mg, 0.349 mmol, 1.01 equiv), Me3SiCl (37.6
mg, 44 μL, 0.347 mmol, 1 equiv), and abs. EtOAc (12 mL). The
reaction mixture was heated to 70 °C for 48 h. The suspension was
cooled to room temperature, and the solvent was removed under
reduced pressure. The residue was purified by column chromatog-
raphy (CHCl3/MeOH 10:1). The desired product is an orange solid
(163 mg, 0.178 mmol, 51%).
1H NMR (300 MHz, CDCl3): δ 11.97 (s, 1H, CH), 7.64 (s, 2H,
CH), 7.28 (s, 4H, CH), 2.53 (s, 12H, CH3), 2.37 (s, 12H, CH3), 2.31
(t, 8H, CH2), 1.47 (s, 6H, CH3), 1.41 (s, 6H, CH3), 1.01 (t, 6H,
CH3), 0.97 (t, 6H, CH3). 13C NMR (126 MHz, CDCl3): δ 155.66,
153.54, 141.66, 139.79, 139.48, 137.49, 136.88, 135.85, 133.89,
133.46, 132.85, 130.83, 129.96, 129.65, 124.13, 77.16, 18.08, 17.20,
17.13, 14.75, 14.60, 13.06, 12.73, 12.58, 12.28. 19F NMR (471 MHz,
CDCl3): δ −148.69, −148.89 (m, 4F, BF2). ESI HR-MS m/z: calcd
for C53H63B2F4N6 [M]+, 881.52310; found, 881.52488.
Synthesis of [IrCl(cod)(4)]. To a Schlenk tube (10 mL) equipped
with a stirring bar and 4·HCl (50 mg, 0.0545 mmol, 1 equiv) were
added Ag2O (6.3 mg, 0.0273 mmol, 0.5 equiv) and CH2Cl2 (5 mL).
The reaction mixture was heated to 40 °C for 3 h, and the solvent was
removed under reduced pressure. The remaining residue was
suspended in toluene. Next, [IrCl(cod)]2 (17.3 mg, 0.0273 mmol,
0.5 equiv) was added to the suspension and allowed to react for 4 h at
100 °C. The solution was filtered through Celite, evaporated, and
purified by chromatography (EA). The volatiles were removed to give
product [IrCl(cod)(4)] as an orange solid (60 mg, 0.0593 mmol,
90%).
1H NMR (500 MHz, CDCl3): δ 7.28 (s, 2H, N−CH−CH−N),
7.19−7.09 (m, 4H, 4H m-Ar), 4.85 (m, 1H, allyl), 3.94−3.87 (m, 1H,
allyl), 3.32 (d, J = 6.5 Hz, 1H, allyl), 2.79 (d, J = 13.5 Hz, 1H, allyl),
2.53 (m, 12H, 4× CH3), 2.34−2.27 (m, 20H, 4× CH3, 4× CH2
Bodipy), 1.97 (d, J = 12.0 Hz, 1H, allyl), 1.45 (s, 6H, 2× CH3), 1.37
(s, 6H, 2× CH3), 1.08−0.92 (m, 12H, 4× CH3 Bodipy). 13C NMR
(126 MHz, CDCl3): δ 184.82, 154.69, 153.60, 139.04, 138.73, 138.66,
137.60, 137.25, 137.08, 133.32, 132.80, 130.87, 130.37, 128.47,
123.38, 114.19, 72.85, 49.30, 29.81, 18.52, 18.49, 17.24, 17.16, 14.77,
14.71, 12.70, 12.61, 12.56, 12.18, 1.13. 19F NMR (282 MHz, CDCl3):
δ −148.79, −149.13 (m, 4F, BF2). ESI HR-MS m/z: calcd for
C56H67B2F4N6Pd [M]+, 1027.457882; found, 1027.460295.
Synthesis of [PdCl2(Clpy)(4)]. A Schlenk flask equipped with a
stirring bar was loaded with imidazolium salt 4·HCl (50 mg, 0.0545
mmol, 1 equiv), PdCl2 (9.67 mg, 0.0545 mmol, 1 equiv), and K2CO3
(37.7 mg, 0.273 mmol, 5 equiv). 3-Chloropyridine (1.0 mL) was
added, and the mixture was stirred at 80 °C in a sealed flask overnight.
The reaction mixture was filtered through Celite, and the filtrate was
evaporated under reduced pressure. The orange residue was dissolved
in a minimum amount of CH2Cl2, filtered, and precipitated with n-
pentane. The product was obtained as an orange solid (54 mg, 0.0461
mmol, 85%).
1H NMR (CDCl3, 500 MHz): δ 7.19 (s, 4H, CH), 6.98 (s, 2H,
CH), 4.29 (s, 2H, CH), 3.13 (s, 2H, CH), 2.56 (s, 6H, CH3), 2.55 (s,
6H, CH3), 2.46 (s, 6H, CH3), 2.34 (m, 14H, CH3/CH2), 1.90−1.80
(m, 4H, CH2), 1.67 (s, 6H, CH3), 1.61−1.46 (m, 4H, CH2), 1.36 (s,
6H, CH3), 1.025 (t, 6H, CH3), 1.015 (t, 6H, CH3). 13C NMR
(CDCl3, 126 MHz): δ 180.51, 154.82, 153.48, 139.32, 139.11, 137.50,
136.83, 133.28, 132.83, 130.76, 130.63, 129.12, 127.94, 124.10, 84.44,
77.42, 77.16, 76.91, 51.74, 33.74, 29.19, 19.99, 18.47, 17.28, 17.24,
14.78, 13.27, 12.75, 12.63, 11.85. 19F NMR (CDCl3, 471 MHz): δ
−148.90 (m, 4F, BF2). ESI HR-MS m/z: calcd for C61H74IrB2F4N6
[M]+, 1181.57210; found, 1181.57433.
Synthesis of [Ir(cod)(4)(py)]OTf. To a flame-dried Schlenk flask
containing [IrCl(cod)(4)] (41.2 mg, 33.9 mmol, 1 equiv) was added
CH2Cl2 (5 mL), followed by pyridine (4.1 μL, 4.0 mg, 50.8 μmol, 1.5
equiv) and [Ag(CF3SO3)] (8.7 mg, 33.9 μmol, 1 equiv). The reaction
mixture was stirred for 1 h and then filtered through a Celite plug.
The filtrate was evaporated under reduced pressure. The solid residue
was dissolved in a minimum amount of CH2Cl2 and precipitated from
diethyl ether. The precipitate was collected by filtration and dried
under vacuum. The product was obtained as an orange microcrystal-
line solid (46.8 mg, 33 μmol, yield 98%).
1H NMR (500 MHz, CD2Cl2): δ 8.62 (d, J = 2.4 Hz, 1H), 8.52
(dd, J = 5.6, 1.4 Hz, 1H), 7.75 (dt, J = 8.4, 1.7 Hz, 1H), 7.32 (s, 2H),
7.26 (s, 5H), 7.24−7.18 (m, 1H), 2.59−2.50 (m, 16H), 2.47 (s, 14H),
2.39 (q, J = 7.6 Hz, 4H), 2.31 (q, J = 7.6 Hz, 4H), 1.62 (s, 7H), 1.56
(s, 7H), 1.05 (t, J = 7.5 Hz, 6H), 0.99 (t, J = 7.5 Hz, 7H). 13C NMR
(126 MHz, CD2Cl2): δ 154.68, 154.11, 153.35, 150.69, 149.79,
140.16, 139.64, 138.73, 138.61, 138.56, 137.48, 133.74, 133.50,
132.56, 131.37, 130.86, 128.98, 125.14, 124.98, 30.29, 19.67, 17.64,
17.61, 15.05, 15.00, 13.29, 12.89, 12.62. 19F NMR (471 MHz,
CD2Cl2): δ −147.78, −147.99 (m, 4F, BF2). ESI HR-MS m/z: calcd
for C58H67B2Cl3F4N7Pd [M]+, 1170.36751; found, 1170.37199.
Synthesis of [AuCl(4)]. To a round-bottom flask (5 mL)
equipped with a stirring bar were added imidazolium salt 4·HCl
(50 mg, 0.0545 mmol, 1 equiv), [AuCl(Me2S)] (16.1 mg, 0.0545
mmol, 1 equiv), K2CO3 (7.5 mg, 0.0545 mmol, 1 equiv), and acetone
(3 mL). The flask was sealed, and the reaction mixture was stirred at
60 °C for 12 h. The solution was filtered, evaporated, and purified by
column chromatography (Cy/EA). The volatiles were removed to
1H NMR (500 MHz, CDCl3): δ 8.23 (d, 2H, CH), 7.85 (t, 1H,
CH), 7.47 (t, 2H, CH), 7.35 (s, 2H, CH), 7.27 (s, 2H, CH), 7.25 (s,
2H, CH), 4.01 (m, 2H, CH2), 3.28 (m, 2H, CH2), 2.57 (s, 6H, CH3),
2.55 (s, 6H, CH3), 2.38 (s, 6H, CH3), 2.32 (t, 4H, CH2), 2.31 (t, 4H,
CH2), 2.00 (m, 4H, CH2), 1.81 (s, 6H, CH3), 1.77 (s, 6H, CH3), 1.70
(m, 4H, CH2), 1.30 (s, 6H, CH3), 1.05 (t, 6H, CH3), 1.00 (t, 6H,
CH3). 13C NMR (126 MHz, CDCl3): δ 174.55, 155.40, 154.20,
150.74, 139.14, 138.61, 138.55, 138.27, 138.00, 137.59, 137.21,
136.40, 133.93, 133.13, 131.08, 130.05, 129.68, 129.39, 126.71,
H
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Article
provide complex [AuCl(3)] as a yellow solid (16 mg, 0.0144 mmol,
yield 26%).
ethylene during ca. 60 min. A stock solution of precatalyst
[PdCl(CH3)(3)] (c = 2.5 × 10−5 M, 1 mL) in 1,2-dichloroethane
was injected with stirring to initiate polymerization. After 24 h, the
polymerization was quenched via the addition of MeOH (5 mL),
1H NMR (500 MHz, CDCl3): δ 7.35 (s, 2H, N−CH−CH−N),
7.22 (s, 4H, 4H m-Ar), 2.54 (d, J = 3.8 Hz, 12H), 2.37−2.29 (m, 8H,
4× CH2 Bodipy), 2.24 (s, 12H), 1.49 (s, 6H), 1.42 (s, 6H), 1.05−
0.96 (m, 12H, 4× CH3 Bodipy). 13C NMR (126 MHz, CDCl3): δ
175.12, 155.50, 153.25, 139.99, 138.35, 138.25, 137.65, 137.03,
136.48, 133.97, 132.64, 131.17, 129.99, 129.06, 122.32, 18.06, 17.27,
17.19, 14.83, 14.66, 12.92, 12.77, 12.62, 12.39. 19F NMR (471 MHz,
CDCl3): δ −148.79, −149.00 (m, m, 4F, BF2). ESI HR-MS m/z:
calcd for C53H63AuB2ClF4N6 [M − H]+, 1113.45852; found,
1113.45894.
1
leading to the precipitation of the polymer. H NMR (500 MHz,
CDCl3): δ 1.36−1.17 (m, CH2), 0.89−0.82 (m, CH3). 13C NMR
(126 MHz, CDCl3): δ 32.10, 30.21, 29.87, 29.52, 27.29, 26.92, 22.85,
14.27.
Synthesis of [Mo(CO4)(3)]. A Schlenk flask equipped with a
stirring bar was loaded with diimine 3 (32.9 mg, 0.0379 mmol, 1
equiv) and Mo(CO)6 (10.0 mg, 0.0379 mmol, 1 equiv). THF (6 mL)
was added and heated to reflux for 18 h. The solvent was removed
under vacuum, and the remaining red residue was purified by flash
column chromatography (Cy/EA 8:1). Evaporation of the solvent
under reduced pressure provides the desired product as a dark violet
solid (18 mg, 0.0167 mmol, yield 44%).
Synthesis of [NiCl(Cp)(4)]. A Schlenk flask equipped with a
stirring bar was loaded with imidazolium salt 4·HCl (50 mg, 0.0545
mmol, 1 equiv) and nickelocene (10.2 mg, 0.0545 mmol, 1 equiv).
Next, THF (10 mL) was added and the reaction mixture was heated
to reflux for 24 h. The solvent was removed under vacuum, and the
resulting red residue was purified by column chromatography.
Evaporation of the solvent under reduced pressure provides the
desired product as a dark reddish solid (35 mg, 0.0337 mmol, yield
62%).
1H NMR (500 MHz, C6D6): δ 7.26 (s, 2H), 6.63 (s, 4H), 2.73 (s,
6H), 2.67 (s, 6H), 2.18 (q, J = 7.5 Hz, 4H), 2.11−2.06 (m, 16H),
1.65 (s, 6H), 1.51 (s, 6H), 0.92 (t, J = 7.6 Hz, 6H), 0.84 (t, J = 7.6 Hz,
6H). 13C NMR (126 MHz, C6D6): δ 158.91, 155.62, 153.53, 152.94,
139.87, 139.45, 136.82, 134.74, 133.79, 132.59, 131.91, 131.05,
129.45, 129.10, 17.97, 17.45, 17.31, 14.98, 14.75, 12.98, 12.89, 12.66,
12.27 (CO resonances could not be observed. This has been reported
before in the literature for similar complexes).65 ESI HR-MS m/z:
calcd for C56H63B2F4MoN6O4 [M + H]+, 1079.408163; found,
1051.409190. IR ν(CO): 2015, 1916, 1885 cm−1.
1H NMR (500 MHz, CD2Cl2): δ 7.34 (s, 4H, 4H m-Ar), 7.25 (s,
2H, N−CH−CH−N), 4.70 (s, 5H, CH, Cp), 2.54 (s, 12H), 2.46−
2.34 (m, 8H, 4× CH2 Bodipy), 2.26 (s, 12H), 1.85 (s, 6H), 1.49 (s,
6H), 1.05 (t, J = 7.6 Hz, 12H, 4× CH3 Bodipy). 13C NMR (126 MHz,
CD2Cl2): δ 167.99, 155.00, 154.04, 140.29, 140.06, 139.70, 138.62,
138.40, 137.51, 133.92, 133.53, 131.27, 131.06, 129.20, 125.61, 92.70,
18.88, 17.63, 15.03, 15.00, 13.38, 12.95, 12.86, 12.37. 19F NMR (282
MHz, CD2Cl2): δ −147.72, −148.07 (m, 4F, BF2). ESI HR-MS m/z:
calcd for C58H67B2F4Ni [M − Cl]+, 1003.48974; found, 1003.49013.
Synthesis of [PdCl2(3)]. In an oven-dried Schlenk flask, diimine 3
(50 mg, 0.0576 mmol, 1 equiv) and [PdCl2(CH3CN)2] (14.9 mg,
0.0576 mmol, 1 equiv) were dissolved in CH2Cl2 (10 mL). An orange
solid immediately precipitated from the solution, and the reaction
mixture was left to stir for 12 h. The resulting suspension was filtered
off and washed with diethyl ether. The resulting solid was dried under
reduced pressure to give an orange solid (43 mg, 0.0411 mmol, 71%).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00130.
■
S
Full experiment, NMR spectra, fluorescence measure-
ments, fluorescence spectra, cyclic voltammetry, and
mass spectrometry (PDF)
1
The [PdCl2(3)] is poorly soluble in CDCl3. H NMR (300 MHz,
AUTHOR INFORMATION
Corresponding Author
*E-mail: plenio@tu-darmstadt.de.
CDCl3): δ 8.35 (s, 2H, NCH−CHN), 7.12 (s, 4H, 4× H m-Ar),
2.53 (s, 12H, 2× CH3 Bodipy), 2.43 (s, 12H, 2× CH3 Bodipy), 2.29
(q, J = 6.9, 6.4 Hz, 8H, 4× CH2 Bodipy), 1.44 (s, 6H, CH3 Bodipy),
1.37 (s, 6H, CH3 Bodipy), 0.97 (t, J = 6.9 Hz, 12H, 2× CH3 Bodipy).
13C NMR insufficient solubility. ESI HR-MS m/z: calcd for
C54H65B2ClF4N7Pd [M−Cl + CH3CN]+, 1050.41416; found,
1050.41716.
Synthesis of [PdCl(CH3)(3)]. To a solution of diimine 3 (50 mg,
0.0576 mmol, 1 equiv) in dry CH2Cl2 (10 mL) was added
[Pd(cod)MeCl] (16.0 mg, 0.0604 mmol, 1.05 equiv). After stirring
the mixture for 12 h at room temperature, the volatiles were
evaporated to give an orange residue. The residue was dissolved in a
minimum amount of dichloromethane and added to diethyl ether.
Pentane was added to precipitate the product. The remaining solid
was dried in vacuo to provide the product as an orange solid (36 mg,
0.0351 mmol, 61%).
■
ORCID
Oliver Halter: 0000-0002-6680-0276
Yuki Kanai: 0000-0001-9561-861X
Herbert Plenio: 0000-0002-2257-983X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the DFG via grant Pl178/18-1
and Pl178/18-2. Generous support with gold salts by Heraeus
is acknowledged. We wish to thank B. Sc. Markus Frank for the
synthesis of two [(N,N-diaryl-diimine)Mo(CO)4] complexes.
1H NMR (500 MHz, CD2Cl2): δ 8.48 (s, 1H, NCH−CHN),
8.34 (s, 1H, NCH−CHN), 7.18 (s, 2H, 2× H m-Ar), 7.11 (s,
2H, 2× H m-Ar), 2.51 (s, 6H, CH3 Bodipy), 2.50 (s, 6H, CH3
Bodipy), 2.39 (s, 6H, 2× CH3 Bodipy), 2.37−2.31 (m, 14H, CH3
Bodipy, 2× CH2 Bodipy), 1.48 (d, J = 12.1 Hz, 6H, 2× CH3 Bodipy),
1.45 (s, 6H, 2× CH3 Bodipy), 1.03−0.99 (m, 12H, 4× CH3 Bodipy),
0.69 (s, 3H, CH3−Pd). 13C NMR (126 MHz, CD2Cl2): δ 165.52,
161.04, 155.01, 154.13, 153.44, 147.32, 146.79, 140.76, 140.35,
139.62, 139.55, 138.44, 138.34, 135.68, 134.88, 133.95, 133.47,
133.13, 131.67, 131.35, 130.75, 129.98, 129.07, 128.56, 18.97, 18.42,
17.60, 15.05, 15.01, 14.95, 12.91, 12.51, 12.47, 3.01. ESI HR-MS m/z:
calcd for C55H68B2F4N7Pd [M−Cl + CH3CN]+, 1030.46878; found,
1030.47204.
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