A Phosphino-Carboxylic Acid-Based Ru Dimeric Complex

Article abstract of DOI:10.1002/ejic.201701382

A new supramolecular Ru dimer is described, notably by X-ray crystallography, ESI-MS and solution IR spectroscopy. The properties and C–H bond activation catalytic activity of this new H-bonded binuclear complex are discussed.

Full text of DOI:10.1002/ejic.201701382

DOI: 10.1002/ejic.201701382
Full Paper
Supramolecular Ru Dimers
A Phosphino-Carboxylic Acid-Based Ru Dimeric Complex
Christian K. Rank,[a] Tatjana Wall,[a,b] Fabian Dietrich,[a,b] Marko Vidovic,[a,b]
Matthias P. Klein,[a,b] Yu Sun,[a] Gereon Niedner-Schatteburg,*[a,b] Markus Gerhards,*[a,b] and
Frederic W. Patureau*[a]
Abstract: A new supramolecular Ru dimer is described, notably The properties and C–H bond activation catalytic activity of this
by X-ray crystallography, ESI-MS and solution IR spectroscopy. new H-bonded binuclear complex are discussed.
Introduction
for C–H bond-activation catalytic activity through optimization
of ligand effects. We thus report here a new phosphino-carb-
oxylic acid-based Ru dimeric complex. Previously, only a AuI H-
bonded binuclear analogous structure had been described by
Schmidbaur and independently by Puddephatt (Complex [1]2,
Scheme 1).[5]
The field of C–H bond activation has grown exponentially over
the past decade.[1] This reactivity concept is developing rapidly
because it allows the direct-step and atom-economical con-
struction of molecular complexity compared to classical cross-
coupling techniques. The most popular catalysts are often
based on Pd, Rh, or Ru organometallic complexes, which is due
in part to their redox versatility.[1] Of the three, Ru is particularly
interesting for future developments because it is the least ex-
pensive metal. The vast majority of Ru-catalyzed C–H bond-
activation systems however rely solely on [(p-cymene)RuCl2]2,
which is the most commercially available RuII source.[2] Some
recent work by Larossa,[3] and also by us,[4] suggest that
[(p-cymene)RuCl2]2 may not always be the most optimal precat-
alyst. Some other works have also suggested that the addition
of a phosphine ligand may in some cases significantly increase
the catalytic efficiency of [(p-cymene)RuCl2]2 in some C–H aryl-
ation coupling reactions.[2h] Therefore, there is much potential
Results and Discussion
Reacting [(p-cymene)RuCl2]2 and commercially available 4-(di-
phenylphosphanyl) benzoic acid in CDCl3 instantaneously af-
fords complex [2] at room temperature. 31P NMR shows a single
characteristic peak at +25.2 ppm. The X-ray structure and the
H-bonding dimeric disposition are shown in Figure 1.
Although we had expected a direct interaction between the
carboxylic moieties and the Ru centers, the distance between
Ru and the closest oxygen atom forbids any such interpretation.
For comparison, in the binuclear gold complex [1]2, the inter-
molecular O–O distance is 2.637 and 2.611 Å (two dimers in the
unit cell),[5] compared to 2.619 Å in the X-ray structure of the
free ligand 4-(diphenylphosphanyl) benzoic acid.[5c] Thus, AuI
coordination does not seem to greatly affect the H-bonding
between the two carboxylic acid moieties. In binuclear Ru com-
plex [2]2, the O–O intermolecular distance is 2.666 Å, suggest-
ing a slightly weaker H-bonding between the carboxylic moie-
ties. The robustness of binuclear complex [2]2 was then probed
with ESI MS (Figure 2).
Clearly, complex [2] remains dimeric under ESI ionization
conditions. In contrast, complex [3] resulting from the coordina-
tion of 4-(diphenylphosphanyl) benzoic methyl ester with [(p-
cymene)RuCl2]2, displays a characteristic mononuclear ESI MS
pattern (Figure 3). The solid state ATR spectrum of the binuclear
complex [2]2 shows two bands at around 3055 cm–1 and
2963 cm–1, on top of a broader signal (Figure 4). DFT calcula-
tions show that the band of the monomer is expected at
3591 cm–1, whereas the double H-bonded structure such as
found in the X-ray of complex [2]2 should yield a very red-
shifted and intense band at 2938 cm–1. The comparison of
measured and calculated spectra is thus consistent with the
existence of the H-bonded structure in the solid state. In con-
Scheme 1. Comparison of the solid-state structures of carboxylic acid bridged
phosphine Au and Ru dimers.
[a] FB Chemie & Research Center Optimas, TU Kaiserslautern,
Erwin-Schrödinger Str. 52, 67663 Kaiserslautern, Germany
E-mail: gns@chemie.uni-kl.de
gerhards@chemie.uni-kl.de
patureau@chemie.uni-kl.de
[b] Forschungszentrum Optimas, TU Kaiserslautern
Kaiserslautern, Germany
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201701382.
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results show the relative fragility of the H-bonded dimer, even
in moderately polar dichloromethane solution (see Supporting
Information).
Figure 3. ESI-MS of monomeric complex [3], largest fragment (zoom) belong-
ing to [RuCl2(C10H14)P(C6H5)2(C6H4)COOCH3]+ according to simulation (be-
neath).
Figure 1. X-ray structure of binuclear complex [2]2, ORTEP view 30 %,
hydrogen atoms and two solvent molecules (dichloromethane) are omitted
for clarity.
Figure 4. Neat ATR-IR spectrum of binuclear complex [2]2.
Intrigued, we then looked at rationalizing the FTIR spectrum
of the acetonitrile solution of complex [2], displaying the two
O–H stretching bands at 3630 and 3540 cm–1, respectively (see
Supporting Information). This indicates that such originally di-
meric structures may be arranged very differently in solution,
particularly in polar solvents. H-bonding between the carb-
oxylic acid moieties and the chlorides was notably considered,
however the correct attribution for the observed bands could
not be convincingly calculated so far, despite our best efforts.
UV/Vis and fluorescence spectra were also acquired, notably in
KBr, in cyclohexane and acetonitrile, respectively (Figure 5). It
Figure 2. (Top) ESI-MS of binuclear complex [2]2 (top), and largest frag-
ment (zoom), which simulations suggest should correspond to
[(C29H29ClO2PRu)2Cl]+ (beneath).
trast, an acetonitrile solution of [2] yielded two O–H IR stretches should be noted that the spectra in acetonitrile are strongly
at respectively 3630 and 3540 cm–1, whereas a dichloro- dependent on the concentration, indicating that some dynamic
methane solution of [2] yielded a broad O–H IR stretch at association/dissociation processes may take place in that sol-
3494 cm–1. A similar IR profile was obtained in CD2Cl2. These vent (see Supporting Information for details).
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a cooperative fashion to the acceleration of the catalytic activ-
ity. Thus, the increased catalytic activity is solely due to a phos-
phine-accelerating effect, a phenomenon already observed in
other Ru catalyzed C–H activation reactions.[2h]
Scheme 2. Catalytic activity in an intermolecular cross dehydrogenative C–N
bond forming reaction with carbazoles.
In order to confirm this trend in a second test reaction, com-
plexes [2] and [4] were evaluated as catalysts in the notorious
Ru-catalyzed arylation of phenyl pyridines (Scheme 3).[7] Like-
wise, no significant differences were observed in terms of cata-
lytic behavior, both ligand systems affording high yields of the
C–H arylated products. Moreover, similar ratios of mono-/diaryl-
ation were observed.
Conclusions
In conclusion, we reported here a new phosphino-carboxylic
acid based Ru dimeric complex. So far, only the AuI analogue
had been known (complex [1]2). The dimeric H-bonded charac-
ter of complex [2]2 could be unambiguously confirmed in the
solid state with X-ray crystallography (see also ESI MS). The dou-
ble H-bonding system seems to change significantly in solution,
as evidenced by solution IR studies (in acetonitrile or alterna-
tively in dichloromethane). Absorption/emission spectra of the
complex in both apolar (cyclohexane) and polar media (aceto-
nitrile) do not show significant differences, suggesting a mostly
Figure 5. A) Fluorescence spectra of complex [2] in KBr matrix, with different
excitation wavelengths. B) UV/Vis spectra (black) and fluorescence (red) spec-
tra of complex [2] in cyclohexane. C) in acetonitrile.
We finally evaluated the catalytic activity of binuclear com- monomeric disposition in dilute conditions. The catalytic activ-
plex [2]2, in a Ru catalyzed intermolecular dehydrogenative C– ity in a cross-dehydrogenative C–N bond forming reaction is
N bond forming dimerization reaction, which we had previously superior to previously reported [(p-cymene)RuCl2]2. However,
developed (Scheme 2).[6] Complex [2] showed significantly su- this does not arise from a cooperative effect of the carboxylic
perior activity (47 % isolated yield of the bicarbazole) compared moieties, or any sort of proximal cooperativity between the two
to naked [(p-cymene)RuCl2]2 (in all reaction conditions that Ru centers that would be enforced by the μ-bridging ligand,
were tested, 40 % yield or less were generally obtained within since the control triphenylphosphine ligand performed just as
the first 24 h reaction time).[6] However, control monomeric well. The Ru-catalyzed C–H arylation of phenylpyridine also
complex [4] based on simple triphenylphosphine showed the showed similar activity for the two ligand systems. However,
same increased catalytic efficiency. This suggests that the carb- these results show that [(p-cymene)RuCl2]2 is not an optimal
oxylic acid functional group does not significantly contribute in pre-catalyst for such cross dehydrogenative coupling reactions,
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Scheme 3. Catalytic activity in the Ru catalysed C–H arylation of phenyl pyridines.
7.1.[10] This procedure yield two structures, which are used to gener-
ate dimers with different symmetries. The dimerization via the carb-
oxyl moieties is considered as well as chelation of one ruthenium
by the second molecule. For all structures vibrational spectra are
calculated, scaled with the factor 0.9608[11] and convolved with a
Gaussian (FWHM = 15 cm–1). Characterization of C1–N bicarbazole
product: see ref.[7] UV/Vis/Luminescence spectroscopy: For all
measurements, only spectroscopy grade solvents from Fluka were
used. All measured solutions had a concentration in the range of
2 × 10–5 mol L–1, 2 × 10–4 mol L–1 and 2 × 10–3 mol L–1 and were
prepared with the common Schlenk technique under Argon atmos-
phere. The absorption spectra were recorded with a Perkin–Elmer
Lambda 900 double beam UV/Vis/NIR spectrophotometer. Cylindri-
cal quartz cuvettes with a path length of 1 cm were used. For detec-
tion of the emission a Horiba Jobin Yvon Fluorolog 3–22 τ and
1 cm × 1 cm quartz cuvettes were applied.
and that better performance can still be achieved by ligand
optimization. Such ligand optimization efforts for improved cat-
alytic activity are currently under way in our laboratory.
Experimental Section
Catalysis experiments were carried out in 170 mL dried reaction
vessels with sealed Teflon screw caps under air, unless otherwise
specified. NMR spectra were obtained on Bruker AMX 400 or on
Bruker Avance 600 systems using CDCl3 as solvents, with proton
and carbon resonances at 400/600 MHz and 101/151 MHz, respec-
tively. Coupling constants (J) are quoted in Hz. Flash chromatogra-
phy was performed on silica gel (40–63 mesh) by standard tech-
nique. HRMS data were determined on a WATERS GCT-PremierTM
mass spectrometer. Electrospray ionization mass spectrometry (ESI-
MS) was performed with an ion trap instrument (Bruker amazon
ETD). The investigated cations were produced in the positive elec-
trospray ionization mode. The scan speed was 4650 m/z s–1 in maxi-
mum-resolution scan mode (0.18 fwhm/m/z). The scan range was
at least 70 to 3000 m/z. Sample solutions of complexes [2] and
Complex [1]2: see ref.[5]
Complex [2]2: [(p-cymene){(C6H5)2P(C6H4)CO2H}RuCl2]2: (p-cym-
ene)RuCl2 (commercial from TCI-Chemicals, 0.66 mmol, 202 mg) was
united with 4-(diphenylphosphanyl)benzoic acid (commercial from
Sigma Aldrich, 0.66 mmol, 203 mg) and submitted with CH2Cl2
(10 mL) under air. The solution was crushed with hexane (excess)
and filtered through a P4-frit. Afterwards the orange solid was dried
in vacuo, (0.60 mmol, 377 mg, 91 %). 1H NMR (400 MHz, CDCl3): δ =
8.01–7.91 (m, 4 H), 7.84–7.79 (m, 4 H), 7.46–7.37 (m, 6 H), 5.23 (d,
[3] in acetonitrile at concentrations of approximately 10–5
M were
continuously infused into the ESI chamber at a flow rate of
2 μL min–1 by using a syringe pump. N2 gas was used as the drying
gas at a flow rate of 3.0 to 4.0 L min–1 heated to 220 °C. The solu-
tions were sprayed at a nebulizer gas pressure of 0.2 to 0.28 bar
with the electrospray needle held at 4.5 kV. Helium was used as a
buffer gas with a partial pressure of ca. 3 × 10–3 mbar inside the
ion trap. Bruker trapControl 7.2 software controlled the instrument
and data analysis was performed with Data Analysis 4.2 software.
Theoretical methods: initial structures of the monomer are gener-
ated by application of the simulated annealing algorithm of the
XTB package.[8] The obtained minimum structures are optimized
with B3LYP-D3(BJ)/def2-TZVP using the Berny algorithm of Gaussian
09[9] and the energies and gradients calculated by Turbomole
3
3
3J = 6.1 Hz, 2 H), 4.99 (d, J = 5.5 Hz, 2 H), 2.86 (hept, J = 6.9 Hz),
1.86 (s, 3 H), 1.11 (d, J = 6.9 Hz, 6 H) ppm. 31P{1H} NMR (162 MHz,
3
CDCl3): δ = 25.26 (s, P) ppm. C29H29Cl2O2Ru (581.52): calcd. C 56.87,
H 4.77; found C 55.62, H 4.98, MS-ESI (CH3CN): see Figure 2. IR (neat):
ν = 3055, 2963, 1714, 1690, 1599, 1562, 1483, 1470, 1434, 1386,
˜
1275, 1214, 1181, 1088, 1018, 855, 766, 747, 695, 674 cm–1. Crystalli-
zation from CH2Cl2.
Complex [3]: [(p-cymene){(C6H5)2P(C6H4)CO2CH3}RuCl2]: (p-cym-
ene)RuCl2 (commercially available from TCI-Chemicals, 0.33 mmol,
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200 mg) was united with methyl 4-(diphenylphosphanyl)benzoic
methyl ester (0.25 mmol, 80.1 mg) and submitted with CH2Cl2
(3 mL) under air. The solution was crushed with hexane (excess)
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3
H), 7.45–7.37 (m, 6 H), 5.21 (d, 3J = 6.1 Hz, 2 H), 4.98 (d, J = 5.5 Hz,
2 H), 3.88 (s, 3 H), 2.85 (sept, 3J = 6.95 Hz, 1 H), 1.86 (s, 3 H), 1.10 (d,
3J = 6.9 Hz, 6 H) ppm. 31P{1H} NMR (162 MHz, CDCl3): δ = 24.84 ppm.
C30H31Cl2O2PRu (626.52): calcd. C 57.51, H 4.99; found C 56.97, H
5.43. MS-ESI (CH3CN): see Figure 3. IR (neat): ν = 3052, 2958, 1718,
˜
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Acknowledgments
The Deutsche Forschungsgemeinschaft (DFG)-funded transre-
gional collaborative research center SFB/TRR 88 “Cooperative
effects in homo and heterometallic complexes” (http://
3MET.de), DFG funded project PA 2395/2-1, and COST Action
CA15106 (CHAOS), and state research center OPTIMAS are ac-
knowledged for financial support.
[8] S. Grimme, C. Bannwarth, P. Shushkov, J. Chem. Theory Comput. 2017,
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[9] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
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katsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G.
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Keywords: Binuclear complexes · Cooperative catalysis ·
Hydrogen bonding · Cross-dehydrogenative coupling ·
Carbazole
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Received: November 26, 2017
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