Ruthenium(0) complexes with triazolylidene spectator ligands: Oxidative activation for (de)hydrogenation catalysis

Article abstract of DOI:10.1016/j.jorganchem.2015.02.022

Transmetallation of silver triazolylidene intermediates with the ruthenium(0) precursor [Ru(Cp=O) (CO)₂]₂ afforded low-valent ruthenium(0) complexes containing a triazole-derived NHC ligand (Cp=O = 3,4-di(4-methoxyphenyl)-2,5-dipheny

Full text of DOI:10.1016/j.jorganchem.2015.02.022

Accepted Manuscript
Ruthenium(0) complexes with triazolylidene spectator ligands: Oxidative activation for
(de)hydrogenation catalysis
Cristiana Cesari, Rita Mazzoni, Helge Müller-Bunz, Martin Albrecht
PII:
S0022-328X(15)00090-X
10.1016/j.jorganchem.2015.02.022
JOM 18922
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 15 January 2015
Revised Date: 9 February 2015
Accepted Date: 13 February 2015
Please cite this article as: C. Cesari, R. Mazzoni, H. Müller-Bunz, M. Albrecht, Ruthenium(0) complexes
with triazolylidene spectator ligands: Oxidative activation for (de)hydrogenation catalysis, Journal of
Organometallic Chemistry (2015), doi: 10.1016/j.jorganchem.2015.02.022.
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ACCEPTED MANUSCRIPT
Ruthenium(0) complexes with triazolylidene spectator ligands: oxidative activation for
(de)hydrogenation catalysis
Cristiana Cesari,a,b Rita Mazzoni,b Helge Müller-Bunz,a Martin Albrecht*,a
a) School of Chemistry & Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland
b) Dipartimento di Chimica Industriale “Toso Montanari”, viale Risorgimento 4, 40136 Bologna,
Italy
Author contact: E-mail: martin.albrecht@ucd.ie; Fax: +353 17162501
In memoriam A. E Shilov in recognition and admiration of his pioneering work on C–H bond
activation.
Abstract
Transmetallation of silver triazolylidene intermediates with the ruthenium(0) precursor
[Ru(Cp=O)(CO)2]2 afforded low-valent ruthenium(0) complexes containing a triazole-derived NHC
ligand (Cp=O = 3,4-di(4-methoxyphenyl)-2,5-diphenyl-cyclopentadienone). Protonation of the
carbonyl group of the Cp=O ligand significantly reduces the
π character of the Ru–CO bond as
deduced from νCO analysis. The new triazolylidene ruthenium(0) complexes were evaluated as
catalyst precursors in transfer hydrogenation of 4-fluoro-acetophenone and in the acceptorless
dehydrogenation of benzyl alcohol. Low activities were noted, though in both reactions, catalytic
performance is markedly increased when cerium(IV) was added. Electrochemical analysis indicate
that activation of the catalyst precursor proceeds via cerium-mediated oxidation of the ruthenium
center, which facilitates dissociation of a CO ligand to enter the catalytic cycle. Such oxidative
activation of catalyst precursors may be of more general scope.
Keywords
Ruthenium – N-heterocyclic carbenes – mesoionic triazolylidenes – oxidative activation – alcohol
dehydrogenation – transfer hydrogenation
1

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Introduction
N-Heterocyclic carbenes (NHCs) have become some of the most popular ligands in transition metal
chemistry due to their efficiency as ancillary ligands to improve catalytic activity.1 Their success is
due to a combination of unique properties, such as their easily tunable electronic and steric
properties that influence the metal center and which allow catalytic activity to be rationally
optimized. The exploitation of such concepts is particularly appealing because the synthesis of NHC
ligand precursors as well as NHC metal complexes is fairly simple and highly versatile.2
Accordingly, a variety of catalytic reactions have strongly benefited from introduction of NHC
ligands, such as ruthenium-catalyzed olefin metathesis,3 palladium-catalyzed cross-coupling
reactions,4 iridium-catalyzed reductions and oxidations,5 and gold-catalyzed activation of π-bonds,
to name but a few.6 Apart from olefin metathesis, NHC ruthenium complexes have shown catalytic
activity in various redox transformations,7 including: transfer hydrogenation,8 hydrogenation of
olefins9 and esters,10 asymmetric hydrogenation,11 amide synthesis from alcohols and nitriles,12
dehydrogenation of esters and imines from alcohols,13 racemization of chiral alcohols,14 oxidation
of alcohols15 and water oxidation.16 Most of the literature on NHC ruthenium chemistry features
ruthenium(II) complexes, while low-valent NHC ruthenium(0) systems are restricted to a few
examples based on either [Ru3(CO)12] or [Ru(CO)2(PPh3)3] as precursors.17 Recently, a
straightforward approach towards NHC ruthenium(0) complexes has been developed,18 which is
based on a dimeric ruthenium(0) cyclopentadienone dicarbonyl dimer.19 Cleavage of the dimer in
the presence of a free carbene or a silver carbene precursor provided access to a new class of
ruthenium complexes.
Here we have expanded this synthetic methodology to 1,2,3-triazolylidenes as strong donor NHC-
type ligands.20 Triazolylidenes offer a vast synthetic flexibility due to their convenient accessibility
through [2+3] dipolar cycloaddition of alkynes and azides.21 In addition, their enhanced donor
properties as compared to more commonly utilized imidazolylidenes may further destabilize the
ruthenium(0) oxidation state and hence facilitate substrate activation by metal/ligand cooperation in
analogy to the reduced portion of Shvo’s catalyst.22 Thus, it will be of particular interest to evaluate
the propensity of the new triazolylidene ruthenium(0) complexes to dehydrogenate substrates via
formal H2 transfer, as this process may lead to efficient catalysts for acceptorless oxidation (alcohol
dehydrogenation) or transfer hydrogenation.
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Results and Discussion
Synthesis of triazolylidene Ru complexes
The triazolium salts 1a-d, readily accessible by “click” cycloaddition of the corresponding alkynes
and azides and subsequent alkylation,21 were successfully transformed into the silver triazolylidene
intermediates as reported previously.23 Transmetalation with the low-valent ruthenium precursor
afforded complexes 3a–d in very high yields (Scheme 1). Complexes 3 constitute a class of
ruthenium(0) complexes which contain exclusively carbon-donor ligands.
1. Ag2O
Ph
2.
Ar
CO
CO
O
Ar
Ph
O
Ru
Ru
Ph
Ph
OC
Ph
Ar
Ar
(Ar = C6H4-4-OMe)
R' R''
Ar
Ar
OC
O
R
N
N
I–
R'
2
Ph
R
R
Ru
+
CO
CO
N
a
b
c
d
Me Ph Me
Bu Bu Me
Bu Bu Bu
Ph Me Me
N
N
R''
R'
N
R''
1
3
Scheme 1. Synthesis of cyclopentadienone triazolylidene ruthenium(0) complexes 3a-d.
Formation of complexes 3a–d was established by 1H NMR, 13C{1H} NMR, and IR spectroscopy as
well as ESI-MS and for representative examples, by single crystal X-ray diffraction analysis. IR
spectroscopy provided a particularly convenient methodology for monitoring the progress of the
transmetalation reaction, as the Ru–CO vibrations of the precursor complex shifted distinctly by 20
cm–1 to lower energy upon triazolylidene coordination (νCO = 2018, 1967 cm–1 in 2, and 1999, 1938
cm–1 in 3b, Table 1). The bathochromic shift is slightly but consistently more pronounced when the
triazolylidene ligand contained only alkyl substituents, and less strong when phenyl substituents are
present, irrespective of the connectivity pattern. The carbene ligand in 3a with the phenyl group
attached to the carbon induced the same shift as the analogous carbene with the phenyl group
attached to nitrogen (3d). These data suggest that the CO stretch vibration may be used as a probe
for the qualitative assessment of the carbene donor properties.23,24 In line with this notion,
coordination of 2-imidazolylidenes induces a slightly smaller bathochromic shift (∆ν = 4 cm–1),
which reflects their weaker donor properties when compared to triazolylidenes.20a,b Potentially,
backbonding from the electron-rich ruthenium(0) center to the carbene ligand may affect the CO
stretch vibration and may thus complicate a simple linear correlation between IR frequencies and
donor properties.
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Table 1. Relevant carbonyl stretch vibrations (cm–1) of the complexes
complex
Ru–CO
Cp=O
2018, 1967
2004, 1945
1999, 1938
1998, 1937
2003, 1944
2036, 1979
---
2
3a
3b
3c
1577
1583
1584
1578
---
3d
3a + HBF4
Further evidence of the formation of complexes 3a–d was obtained from NMR data, in particular
the disappearance of the low field signal of the triazolium precursor in the 1H NMR spectrum and
the downfield shift of the carbenic resonance in the 13C NMR spectra. This nucleus resonates in the
154–158 ppm range and hence some 20 ppm higher field than in ruthenium(II) complexes,
indicative for a low valent and electron-rich metal center.
Unambiguous structural evidence was obtained by X-ray diffraction analysis of single crystals of
complexes 3a and 3d. The molecular structure reveals the expected piano-stool geometry with the
cyclopentadienone ligand occupying one face and the two CO ligands and the triazolylidene
forming the three legs (Fig. 1). Bond lengths and angles (Table 2) are very similar in both
complexes, suggesting only marginal steric consequences upon swapping the methyl and phenyl
wingtip groups in the triazolylidene ligand. As expected, the Ru–Ctrz bond is longer in complexes
3a and 3d than in analogues featuring a higher-valent ruthenium(II) metal center (∆d = 0.06 Å).15
Table 2. Selected bond lengths (Å) and angles (deg) for complexes 3a and 3d.
3a
3d
Ru–C1
Ru–C42
2.1196(12)
1.8965(13)
1.8825(13)
1.930(<1)
95.18(5)
91.66(6)
116.44(3)
125.76(4)
127.83(4)
90.99(6)
1.2464(14)
2.120(2)
1.878(2)
1.898(2)
1.932(<1)
91.08(8)
95.90(8)
115.02(5)
125.60(7)
127.44(6)
92.96(9)
1.239(2)
Ru–C43
Ru–Cg a)
C1–Ru–C42
C1–Ru–C43
C1–Ru–Cg a)
C42–Ru–Cg a)
C43–Ru–Cg a)
C42–Ru–C43
C11–O1
a) Cg = centroid of cyclopentadienone ligand
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Figure 1. ORTEP representations (50% probability level, hydrogen atoms omitted for clarity) of complexes 3a (a) and
3d (b).
Catalytic alcohol oxidation
The triazolylidene Ru(0) complex 3a was evaluated as catalyst precursor for the oxidation of
alcohols using benzyl alcohol (BnOH) as model substrate. While ruthenium(II) systems with a
cymene spectator ligand gave attractive conversions even in the absence of oxidants or base,15 runs
performed with 3a in the absence of such additives were essentially non-productive with <5%
conversion (Table 3, entry 1). Better results were obtained upon adding [Ce(NH3)6](NO3)2 (CAN)
as oxidizing agent to activate the catalyst precursor 3a.25
Table 3. Ruthenium/cerium-catalyzed BnOH oxidation a)
OH
O
cat [Ru]
CAN
H
1,2-C6H4Cl2
150 °C
Entry
Ru0 complex
mol% CeIV
---
10
5
2.5
10
5
2.5
10
yield (2 h)
<5%
25%
12%
12%
57%
24%
20%
19%
yield 24 h
<5%
80%
1
2
3
4
5
6
7
8
3a
---
---
---
3a
3a
3a
2
42%
31%
97% b)
85%
84%
81%
a) General conditions: 0.19 mmol BnOH, 5 mol% Ru0 complex, 3 mL 1,2-C6H4Cl2, mol% CeIV and Ru relative to
BnOH; 150 °C, yield of PhCHO determined by 1H NMR spectroscopy by using anisole as internal standard; b) reaction
complete after 8 h.
5

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Initial background measurements indicated that in the absence of a ruthenium complex, CAN
catalyzes the oxidation of BnOH to benzaldehyde on its own under the applied reaction conditions.
Background conversions reached 80% when 10 mol% CAN was used, and dropped to 40% and
30% approximately, if the CAN loading was reduced to 5 and 2.5 mol%, respectively (entries 2–4,
Fig. S1–S3). Addition of complex 2, i.e. the carbene-free ruthenium(0) precursor, had no notable
effect and yields and conversions mirrored those of CAN only (entry 8). Possibly, the lack of a
stabilizing ligand for ruthenium(II) induces rapid decomposition, supported also by the brown color
that rapidly developed upon heating the mixture. In contrast, complex 3a induced a considerable
accelaration (entries 5–7). For example, with 5 mol% 3a and 2.5 mol% CAN, 84% conversion was
accomplished within 24 h (cf. 31% conversion in the background reaction). At higher CAN loading
(10 mol%), the catalytic competence of 3a is evident in particular at early stages of the oxidation.
After 2 h, 57% conversion was observed with the Ru0/CeIV couple (25% with CeIV only, 0% with
Ru0 only), and oxidation was essentially complete after less than 8 h. While the selectivity is
typically high and conversion occurs selectively to the desired aldehyde, further improvement of the
catalytic performance of 3a may need a focus on an oxidant for ruthenium that is less catalytically
active than CAN.
Catalytic transfer hydrogenation
In addition to alcohol dehydrogenation, we were also interested to see whether complexes 3 are
active in transfer hydrogenation. A model reaction employed complex 3a as catalyst precursor and
4-fluoroacetophenone as substrate under standard transfer hydrogenation conditions,26 i.e. refluxing
iPrOH as hydrogen source (Table 4).
Table 4. Catalytic transfer hydrogenation of 4-fluoroacetophenone a)
O
OH
cat [Ru]
additive
iPrOH
reflux
F
F
entry
1
2
3
4
5
6
7
Ru0 complex
3a
2
3a
2
3a
2
additive b)
---
---
KOH
KOH
HBF4
HBF4
CAN
CAN
yield (%) 8h
0
96
0
95
<5
55
59
0
yield (%) 24h
<5
n.d. c)
6
n.d. c)
31
59
89
7
3a
2
8
9
10
CAN + HBF4
CAN + KOH
42
6
44
17
3a
3a
6

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a) General conditions: Ruthenium complex (5 mol% Ru), iPrOH (5 mL), reflux; yield of alcohol determined by 1H
NMR spectroscopy; b) quantities of additives as follows: HBF4 1 molequiv., KOH 2 molequiv., CAN 1 molequiv. per
ruthenium center; c) n.d. = not determined due to complete conversion after 8 h.
Catalytic runs were performed in particular for comparing the catalytic activity of the triazolylidene
ruthenium(0) complex 3a and the triazolylidene-free dimeric precursor 2 under various conditions.25
In the absence of any additives (entries 1,2), complex 3a is inactive and essentially no conversion
was observed, while precursor 2 reaches almost complete conversion within 8 h in pure iPrOH.
Addition of KOH did not change the relative performance and conversions were identical to those
in iPrOH only (entries 3,4). These results suggest that KOH is not a suitable additive for the
activation of the ruthenium center in complex 3a, probably because the electron-rich ruthenium(0)
center has no affinity for binding a hard alkoxide ligand, nor for dissociation of a CO ligand due to
the high donor properties of the triazolylidene ligand, which are assumed to enhance Ru-CO
backbonding. Conversely, the activity observed for complex 2 probably arises from splitting of the
dimeric precursor into two monomeric forms which, in the presence of a hydrogen source lead to a
derivative of Shvo’s catalyst (A, Figure 2).22 Such complexes are well-known to catalyze the
transfer hydrogenation of a broad range of ketones.22
Ph
Ph
O
Ph
O
Ph
Ar
Ar
O+
H
H
Ar
Ar
OH
Ph
H
Ph
Ar
Ar
CO
CO
Ph
Ar
Ar
OC
OC
Ph
Ru+
Ru
Ru
Ru
CO
CO
CO
CO
N
N
N
N
Ph
N
Ph
N
A
B
C
Figure 2. Shvo-type catalyst A resulting from cleavage of dimer 2 in iPrOH, and cationic complexes B and C from
protonation of complex 3a, featuring a ruthenium(0) and a ruthenium(II) center, respectively.
In an attempt to labilize the CO ligands, a proton source was introduced. Protonation at the dienone
system has been reported and may provide a cationic structure with a positive charge either at the
oxygen (complex B, Fig. 2) or at the ruthenium center (C), thus inducing formal oxidation of the
metal center to ruthenium(II) and depletion of electron density, which in turn is expected to
substantially reduce the π character of the Ru–CO bond. Stoichiometric experiments using HCl (10
molequiv.) or HBF4 (5 molequiv.) in CH2Cl2 with complex 3a indeed gave cationic complexes in
which the carbonyl stretch vibration was significantly shifted to higher frequency (νCO = 2036 and
1979 cm–1, cf. 2004 cm–1, 1945 cm–1 in 3a), and the ketone C=O vibration observed in 3a for the
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Cp=O ligand disappeared in the cationic complexes. These data strongly suggesting formation of a
cyclopentadienyl-type ligand as in complex C. Transfer hydrogenation in the presence of HBF4 (1
equiv) did indeed increase the catalytic activity of complex 3 (entry 5), however the 30%
conversion after 24 h is unremarkable when compared with other ruthenium carbene systems.27 The
dimeric complex 2 performs significantly less in the presence of acid (entry 6), in agreement with
the formation of a Shvo-type active system from formal dihydrogen transfer rather than protonation.
Best result for complex 3a were obtained when using CAN as an oxidizing agent. Under these
conditions, oxidation of the metal center from ruthenium(0) to ruthenium(II) takes place (see
below), which weakens the bonding of CO and hence produces a methodology to activate the
complex. In the presence of CAN, conversion of 4-fluoroacetophenone reaches 58% after 8 h and
89% after 24 h (entry 7). In contrast, CAN efficiently quenches the activity of complex 2 (entry 8).
Hence, conditions have been developed that favor catalytic transfer hydrogenation using the dimer 2
but not 3a (entries 1,2), and inversely, conditions that favor activity of complex 3a but not 2 (entries
7,8). These results demonstrate the orthogonality induced upon bonding of a triazolylidene ligand.
Addition of either base or acid to the CAN-activated complex 3a are not advantageous and
conversions were consistently lower (entries 9,10). Variation of the amount of CAN did not result
in further enhancement of activity. Both lowering the amount to 0.25 or 0.5 equiv relative to 3a as
well as increasing the amount to 2, 4 or 6 equivalents resulted in slower and generally incomplete
conversion (Fig. S4). In particular using excess CAN lead to substantial catalyst deactivation after
around 5 h.
Electrochemical measurement
Electrochemical analysis of the triazolylidene ruthenium(0) complex 3a by cyclic voltammetry
measurements reveal a quasi-reversible oxidation of 3a in CH2Cl2 solution with E1/2 = +0.952(±3)
vs SCE (Fig. 3, Fig. S5). The redox process likely involves a Ru0/RuI oxidation which is according
to the peak-current ratio reversible at higher scan rates (ipc/ipa = 0.96 at 200 mV s–1) and less
reversible at slower scan rates (ipc/ipa = 0.56 at 20 mV s–1), suggesting a slow chemical
transformation of the oxidized species (EC mechanism).28 The ipc/ipa ratio at different scan rates fits
excellently with an irreversible first-order reaction after the oxidation process, with a rate constant k
= 0.58 s–1 for the homogeneous follow-up reaction (correlation > 0.99, Table S1, Fig. S6). These
data are in agreement with a metal-centered oxidation process and subsequent slow dissociation of
CO from the coordination sphere due to the lower electron-density upon ruthenium oxidation (cf.
CAN-mediated activity of 3a in transfer hydrogenation). Consistent with this model,
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electrochemical analysis of 3a measured in MeCN shows a largely irreversible oxidation process
(Epa = +0.96 V, ipc/ipa = 0.64) even at high scan rates (500 mV s–1; Fig. S7). The coordinating
properties of MeCN may thus accelerate the substitution of CO and thus the EC mechanism.
Figure 3. Cyclic voltammograms for 3a in CH2Cl2 at different scan rates (potential vs SCE, Fc as internal standard).
Since the catalytic activity of the ruthenium complex 3a was considerably enhanced when pre-
activated with CAN, both for transfer hydrogenation and alcohol dehydrogenation, a series of
electrochemical measurements was performed on 3a in the presence of CAN. When 0.5 molequiv
CAN were added to a solution of 3a in MeCN, differential pulse voltammetry revealed three
oxidation processes (Fig. 4). The first process at E = +0.90 V pertains to the Ru0/RuI oxidation of
complex 3a and was also observed in measurements without CAN,29 and the second process with E
= +1.06 V corresponds to a cerium-centered oxidation.30 A third oxidation process at E = +1.22 V
with equal ratio to the first process is assumed to belong to a new species resulting from oxidation
of complex 3a and has thus tentatively been attributed to a RuI/RuII oxidation. The equal relative
intensities of the first and last oxidation processes remain constant over several hours, suggesting an
equilibrium. The 1:1 ratio indicates equal quantities of two ruthenium species, which agrees well
with the provision of 0.5 molequiv CAN at the onset of the experiment and suggests a complete
one-electron transfer from ruthenium(0) to cerium(IV), thus resulting in 50% [3a]+ and 50% of the
parent complex 3a. In support of such a model, no oxidation process at +0.9 V was detected after
addition of 1 molequiv CAN, thus indicating the absence of significant amounts of the original
complex 3a. Instead, only an oxidation process at E = +1.22 V was detected, together with the
cerium-centered oxidation. These measurements are thus consistent with a CAN-mediated oxidation
of the ruthenium-center in 3a to ruthenium(II) by consumption of two equivalents of cerium(IV),
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and concomitant release of CO (EC mechanism) as a process for entering the catalytic cycle. Such
processes require the presence of a stabilizing donor ligand such as the triazolylidene used in this
study.
Figure 4. Relevant section of differential pulse voltammetry analysis of complex 3a in presence of different equivalent
of CAN: 0 equiv (orange dashed line), 0.5 equiv (blue bold line) and 1.0 equiv (red thin line; all measurements in
MeCN at 50 mV s–1, potential vs SCE, Fc as internal standard).
Conclusions
A set of ruthenium(0) complexes 3 has been prepared which contain differently substituted
triazolylidene ligands, thus demonstrating the suitability of such strongly donating NHC ligands to
stabilize low-valent metal centers. Catalytic activity in hydrogen transfer reactions has been
evidenced, though oxidative activation with CAN as a potent auxiliary is required. For alcohol
dehydrogenation, both CAN and the ruthenium complex are pre-catalysts and the relative quantities
of the two components can be used to increase the relevance of either the Ce- or the Ru-catalyzed
transformation. Detailed electrochemical analyses lend support to a fast two-step oxidation of the
ruthenium center of 3 in order to enter the catalytic cycle, an activation process that is
complementary to that of the dimeric ruthenium carbonyl complex 2. Upon addition of a
triazolylidene ligand, loss of CO appears to be essential for the generation of an open coordination
site for substrate coordination and catalytic turnover. Triazolylidene ligands with their strongly
mesoionic character may be particularly useful for facilitating the metal oxidation and for entailing
such catalyst activation. This concept may become particularly useful when employing other low-
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valent complexes as catalyst precursors and may provide an interesting opportunity for exploiting
base metals in catalysis.
Experimental Section
General comments. The syntheses of the ruthenium complexes were carried out under an inert
atmosphere of N2 using Schlenk techniques and dry solvents. Purifications were performed in air
1
13
using commercial solvents. H and C{1H} NMR spectra were recorded at room temperature on
Varian spectrometers operating at 400 or 500 MHz unless stated otherwise. Chemical shifts (δ in
ppm, J in Hz) were referenced to SiMe4. Signal assignments are based on homo- and heteronuclear
(multiple-bond) correlation spectroscopy. Elemental analyses were performed by the
Microanalytical Laboratory at the University College Dublin, Ireland. Infra-red spectra were
recorded on a Perkin Elmer FTIR at 1 cm–1 resolution. Electrochemical studies were carried out on
a Metrohm Autolab PGSTAT101 potentiostat using a gas-tight three electrode cell under an argon
atmosphere. A platinum disk with 3.8 mm2 surface area was used as the working electrode and was
polished before each measurement. The reference electrode was Ag/AgCl, the counter electrode
was a Pt wire. In all experiments, [Bu4N]PF6 (100 mM in dry CH2Cl2 or MeCN) was used as
supporting electrolyte with analyte concentrations of approximately 1 mM. The redox potentials
were referenced to ferrocenium/ferrocene (Fc+/Fc; E1/2 = 0.46 V vs. SCE in CH2Cl2; E1/2 = 0.40 V
vs. SCE in MeCN).31 The ruthenium precursors 219 and the triazolium salts 1a–d were prepared
according to literature procedures.15 All other reagents were purchased from commercial sources
and were used as received, unless otherwise stated.
General procedure for the synthesis of the ruthenium complexes 3. The triazolium salts 1a-d
were reacted at room temperature with Ag2O (0.5 eq.) in CH2Cl2 solution under inert atmosphere
and protected from light. After 2 h, the dimeric ruthenium precursor 2 (0.5 eq.) was added to the
mixture and stirring continued for 2 h. The mixture was then filtered through a short pad of Celite
and filtrate was evaporated to yield the triazolylidene Ru(0) complexes 3a-d in typically
quantitative yield. Suitable crystals for X-Ray diffraction were obtained by slow diffusion of
pentane into a CH2Cl2 solution of the complex.
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Analytical data for dicarbonyl-(η4-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-
dienone)(1,3-dimethyl-4-phenyl-1,2,3-triazol-ylidene)ruthenium (3a).
1H NMR (300 MHz, CD2Cl2): δ = 7.59–7.51 (m, 4H, CHaryl), 7.32–7.01 (m, 15H, CHaryl + CHPh),
6.58–6.50 (m, 4H, CHaryl), 3.63 (s, 6H, OCH3), 3.61, 3.56 (2 × s, 3H, NCH3); 13C{1H} NMR (400
MHz, CD2Cl2): δ = 203.11 (Ru–CO), 169.08 (CCp=O), 159.31 (COMe), 157.57 (Ru–Ctrz), 150.38
(Ctrz–Ph), 136.51 (Caryl–Cp), 134.45 (CHaryl), 131.83, 130.48, 129.33 (3 × CPh), 128.46 (CHaryl),
128.19 (CHaryl), 125.93 (Caryl), 124.90 (CHaryl), 113.34 (CHaryl), 103.91 (C2,5 Cp), 80.59 (C3,4 Cp),
55.77 (OCH3), 42.11, 37.48 (2 × NCH3). IR (CH2Cl2) νCO: 2004 cm–1, 1945 cm–1, 1577 cm–1. ESI-
MS (m/z) = 776 [M+H]+; 798 [M+Na]+. Anal. Calcd (%) for C43H35N3O5Ru (774.82): C, 66.66; H,
4.55; N 5.42. Found: C, 65.95; H, 4.64; N 5.34.
Analytical data for dicarbonyl-(η4-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-
dienone)(1,4-dibutyl-3-methyl-1,2,3-triazolylidene)ruthenium (3b).
1H NMR (300 MHz, CDCl3): δ = 7.8–7.2 (m, 4H, CHaryl), 7.37–6.63 (m, 10H, CHaryl), 6.62–6.50
(m, 4H, CHaryl), 3.81 (s, 3H, NCH3), 3.63 (s, 6H, OCH3), 3.61 (m, 2H, NCH2), 1.87–1.03 (m, 10H, -
–CH2–), 0.73 (m, 6H, CH2CH3). 13C{1H} NMR (400 MHz, CDCl3): δ = 202.90 (CO), 158.44
(COMe), 154.01 (Ru–Ctrz), 148.55 (Ctrz–Bu), 135.59 (Caryl), 133.71 (CHaryl), 129.11 (CHaryl), 127.37
(CH), 125.25 (Caryl), 125.06 (CHaryl), 112.87 (CHaryl), 103.85 (C2,5 Cp), 78.43 (C3,4 Cp), 55.02
(OCH3), 53.31 (NCH2), 36.05 (NCH3), 32.08 (NCH2CH2), 31.24 (Ctrz-CH2), 23.55 (Ctrz-CH2CH2),
22.38, 19.39 (2 × CH2CH3), 14.05, 13.60 (2 × CH2CH3). IR (CH2Cl2) νCO: 1999 cm–1, 1938 cm–1,
1583 cm–1.
Analytical data for dicarbonyl-(η4-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-
dienone)(1,3,4-tributyl-1,2,3-triazolylidene)ruthenium (3c).
1H NMR (300 MHz, CD2Cl2): δ = 7.78–7.64 (m, 4H, CHaryl), 7.21–7.03 (m, 10H, CHaryl), 6.69–6.61
(m, 4H, CHaryl), 4.10–3.98 (m, 2H, NCH2), 3.70 (s, 6H, OCH3), 3.70–3.58 (m, 2H, NCH2), 1.79–
1.04 (m, 14H, CH2 Bu), 0.95–0.67 (m, 9H, CH2CH3); 13C{1H} NMR (400 MHz, CD2Cl2): δ = 204.09
(CO), 169.47 (CCp=O), 159.45 (COMe), 157.58 (Ru–Ctrz), 148.96 (Ctrz–Bu), 136.81 (Caryl), 134.60
(CHaryl), 129.90 (CHaryl), 128.10 (CH), 126.36 (Caryl), 125.80 (CHaryl), 113.56 (CHaryl), 104.48 (C2,5
Cp), 79.07 (C3,4 Cp), 55.82 (OCH3), 50.13 (NCH2), 34.96 (NCH3), 32.77, 32.67 (2 x NCH2CH2),
32.19 (Ctrz-CH2), 24.28 (Ctrz-CH2CH2), 23.18, 20.36, 20.17 (3 × CH2CH3), 14.63, 14.14, 14.09 (3 ×
CH2CH3). IR (CH2Cl2) νCO: 1998 cm–1, 1937 cm–1, 1584 cm–1.
12

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Analytical data for dicarbonyl-(η4-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-
dienone)(1-phenyl-3,4-dimethyl-1,2,3-triazol-ylidene)ruthenium (3d).
1H NMR (300 MHz, CDCl3) δ (ppm): 7.59–7.49 (m, 4H, CHaryl), 7.28–6.92 (m, 15H, CHaryl
+
CHPh), 6.64–6.55 (m, 4H, CHaryl), 3.87 (s, 3H, NCH3), 3.66 (s, 6H, OCH3), 1.79 (s, 3H, Ctrz–CH3);
13C{1H} NMR (400 MHz, CDCl3): δ = 202.41 (CO), 167.75 (CCp=O), 159.84 (Ru–Ctrz), 158.27
(COMe), 145.38 (Ctrz–Me), 139.18 (Caryl), 135.45–112.64 (Caryl), 133.57, 129.54, 127.31 (3 × CPh),
103.34 (C2,5 Cp), 79.72 (C3,4 Cp), 54.95 (OCH3), 35.99 (NCH3), 10.53 (trz–CH3). ESI-MS (m/z): 798
[M+Na]+. IR (CH2Cl2) νCO: 2003 cm–1, 1944 cm–1, 1578 cm–1.
General condition for alcohol oxidation: Complex 3a (7.4 mg, 9.5 ꢀmol, 5 mol%), 1,2-
dichlorobenzene (3 mL), and the appropriate amount of [Ce(NH3)6](NO3)2 dissolved in MeCN (0.5
mL) were stirred at reflux for 15 min. Benzyl alcohol (20 ꢀL, 190 ꢀmol) was then added. Aliquotes
(0.05 mL) were taken from the mixture at selected intervals, diluted with CDCl3 (0.5 mL) and
conversions were determined by 1H NMR spectroscopy.
General conditions for transfer hydrogenation: Complex 3a (12 mg, 15 ꢀmol, 5 mol%), CAN (8
mg, 15 ꢀmol, 5 mol%) and iPrOH (5 mL) were stirred at reflux for 15 min. Then 4-
fluoroacetophenone (36 ꢀL, 300 ꢀmol) was added and samples were taken at regular intervals.
Aliquots (ca. 0.05mL) were diluted with CDCl3 (0.5 mL) and conversions were determined by 1H
NMR spectroscopy.
Crystallographic details. Crystal data for complexes 3a and 3d were collected by using an Agilent
Technologies SuperNova A diffractometer fitted with an Atlas detector using Mo-Kα radiation
(0.71073 Å; 3a) or Cu-Kα radiation (1.54184 Å; 3d). A complete dataset was collected, assuming
that the Friedel pairs are not equivalent. An analytical numeric absorption correction was
performed.32 The structure was solved by direct methods using SHELXS-9733 and refined by full-
matrix least-squares fitting on F2 for all data using SHELXL-97.33 Hydrogen atoms were added at
calculated positions and refined by using a riding model. Their isotropic temperature factors were
fixed to 1.2 times (1.5 times for methyl groups) the equivalent isotropic displacement parameters of
the carbon atom the H-atom is attached to. Anisotropic thermal displacement parameters were used
for all nonhydrogen atoms. Crystallographic details are compiled in the supporting information
(Table S2).
13

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Acknowledgements
Financial support for this work is acknowledged from Science Foundation Ireland through the
Synthesis and Solid State Pharmaceutical Center (SSPC), by the European Research Council (ERC
CoG 615653) and the Marco Polo scheme of the University of Bologna for supporting an exchange
visit to CC.
Appendix A. Supplementary material
Time-conversion profiles for catalytic reactions, electrochemical data, and crystallographic details.
CCDC number 1043258 and 1043259 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
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Only for use as graphical abstract entry
Ruthenium(0) complexes containing a strongly donating 1,2,3-triazolylidene ligand were prepared
and evaluated as precursors for hydrogen transfer catalysis. Both dehydrogenation and transfer
hydrogenation are catalyzed by these complexes when activated with cerium(IV) as oxidant;
electrochemical analysis lend support to a ruthenium oxidation process in the catalyst activation
step.
18

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Highlights:
-
-
-
-
low-valent ruthenium(0) triazolylidene complexes prepared
catalytic activity in hydrogen transfer reaction upon activation with cerium(IV)
dehydrogenation and transfer hydrogenation is catalyzed
catalyst activation process is detailed electrochemically
1

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Supplementary Material
Ruthenium(0) complexes with triazolylidene spectator ligands: oxidative activation for
(de)hydrogenation catalysis
Cristiana Cesari,a,b Rita Mazzoni,b Helge Müller-Bunz,a Martin Albrecht*,a
a) School of Chemistry & Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland
b) Dipartimento di Chimica Industriale “Toso Montanari”, viale Risorgimento 4, 40136 Bologna,
Italy
Author contact: E-mail: martin.albrecht@ucd.ie; Fax: +353 17162501
S1

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Catalytic time-conversion profiles
Figure S1. Comparison of conversion for the alcohol oxidation of BnOH with 2 equivalents (per
Ru center) of CAN using as pre-catalyst the ruthenium dimer (), the carbene complex 3a (), and
the blank without any precursor ()
Figure S2. Comparison of conversions for the alcohol oxidation of BnOH with complex 3a as pre-
catalyst (), and the blank without any precursor () with 0.5 molequiv CAN (left) and 1
molequiv CAN (right) relative to 3a.
S2

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Figure S3. Comparison of conversions for the alcohol oxidation of BnOH with 5 mol% complex 3a
as pre-catalyst with 2 equivalents (),1 equivalent () and 0.5 equivalent () of CAN as activator.
Figure S4. Comparison of the catalytic results for the transfer hydrogention of 4-
fluoroacetophenone with 3a in the presence of different amounts of CAN. Best results were
obtained with stoichiometric quantities of the oxidazing agent; lower amounts of CAN give a lower
conversion, probably due to a higher induction time. Higher CAN concentrations induced catalyst
deactivation after 4–8 h and gave incomplete conversions. A blank experiment using 10 mol% CAN
showed no conversion.
S3

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Electrochemical analysis
Figure S5. Cyclic Voltammetry of 3a in CH2Cl2 at 200 mV s–1 (left) and in the presence of
ferrocene (right): Epc = 1.013 V and Epa = 0.894 V, hence E1/2 = 0.953 V (∆E = 120 mV); ipa = 17.0
µA and ipc = 16.3 µA, thus ipc/ipa = 0.96.
Table S1. Scan rate dependence of the +0.95 V oxidation of complex 3a
scan rate
E1/2 (V)
0.946
0.953
0.954
0.956
0.953
∆E (mV)
90.3
ipc/ipa
0.56
0.80
0.84
0.91
0.96
20 mV s–1
50 mV s–1
75 mV s–1
100 mV s–1
200 mV s–1
95.2
111
110
120
Figure S6. Analysis of ipc/ipa ratios at different scan rates provides a linear fit for a coupled first-
order irreversible follow-up reaction to the oxidation (EC mechanism) with a first-order rate
constant k = 0.58 s–1 (t = time between passing the E1/2 potential and reaching the switching
potential Emax, here 0.95 V and 1.15 V, respectively).
S4

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Figure S7. Cyclic Voltammetry diagrams of 3a in MeCN with different scan rates
Figure S8. Cyclic voltammogram of CAN measured in MeCN at 100 mV s–1.
S5

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Crystallographic details
Table S2. Crystallographic details for complexes 3a and 3d
CCDC No.
1043258
1043259
Empirical formula
Formula weight
Temperature
C43H35N3O5Ru
774.81
C43H35N3O5Ru
774.81
100(2) K
100(2) K
Wavelength
0.71073 Å
Monoclinic
P21/n (#14)
a = 14.2501(1) Å
1.54184 Å
Triclinic
Crystal system
Space group
P–1 (#2)
Unit cell dimensions
a = 11.59996(7) Å
α = 96.3043(6)°
β = 107.6103(6)°
γ = 105.4593(6)°
b = 12.8432(1) Å β = 91.0445(6)° b = 12.4497(1) Å
c = 19.6923(2) Å
3603.42(5) Å3
c = 13.3559(1) Å
1733.40(2) Å3
2
1.484 Mg m–3
4.088 mm–1
796
Volume
Z
4
Density (calculated)
Absorption coefficient
F(000)
1.428 Mg m–3
0.486 mm–1
1592
Crystal size
0.3520 x 0.2699 x 0.1758 mm3
2.86 to 32.88°
0.2053 x 0.1485 x 0.0444 mm3
3.55 to 76.91°
θ range for data collection
Reflections collected
Independent reflections
Completeness to θ = 32.00°
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness–of–fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
Largest diff. peak and hole
112232
69936
12821 [R(int) = 0.0412]
98.9 %
7272 [R(int) = 0.0366]
99.4 %
Analytical
Analytical
0.932 and 0.882
Full–matrix least–squares on F2
12821 / 0 / 609
1.062
0.867 and 0.588
Full–matrix least–squares on F2
7272 / 0 / 473
1.063
R1 = 0.0276, wR2 = 0.0633
R1 = 0.0361, wR2 = 0.0680
0.651 and –0.562 e Å–3
R1 = 0.0262, wR2 = 0.0657
R1 = 0.0280, wR2 = 0.0667
0.842 and –0.659 eÅ–3
S6