Synthesis, characterization, and catalytic behavior of mono- and bimetallic ruthenium(II) and iridium(III) complexes supported by pyridine-functionalized N-heterocyclic carbene ligands

Article abstract of DOI:10.1016/j.inoche.2017.04.022

We have prepared and characterized five unreported ruthenium(II) and iridium(III) complexes supported by pyridine-functionalized N-heterocyclic carbene ligands including a bimetallic iridium(III) complex. When activated, all complexes are active catalysts for the transfer hydrogenation of acetophenone.

Full text of DOI:10.1016/j.inoche.2017.04.022

Inorganic Chemistry Communications 81 (2017) 27–32
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Synthesis, characterization, and catalytic behavior of mono- and
bimetallic ruthenium(II) and iridium(III) complexes supported by
pyridine-functionalized N-heterocyclic carbene ligands
a
a
a,1
b
a,
Isaac G. Smith , Joshua C. Zgrabik , Alexa C. Gutauskas , Danielle L. Gray , Gregory J. Domski ⁎
a
Augustana College, Rock Island, IL 61201, USA
University of Illinois, George L. Clark X-Ray Facility and 3M Materials Laboratory, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
We have prepared and characterized five unreported ruthenium(II) and iridium(III) complexes supported by
pyridine-functionalized N-heterocyclic carbene ligands including a bimetallic iridium(III) complex. When acti-
vated, all complexes are active catalysts for the transfer hydrogenation of acetophenone.
Received 31 March 2017
Received in revised form 21 April 2017
Accepted 23 April 2017
Available online xxxx
©
2017 Elsevier B.V. All rights reserved.
Keywords:
N-heterocyclic carbene
Ruthenium(II)
Iridium(III)
Transfer hydrogenation
Bimetallic
Since Arduengo's 1991 report of an isolable, free N-heterocyclic
carbene (NHC) [1], the coordination chemistry of NHCs has become
an exceptionally fertile area of research and has been extensively
reviewed [2–9]. In particular, chelating, functionalized NHC ligands
of metalloenzymes [23] to rate enhancements of industrially rel-
evant catalytic processes due to cooperation between metal cen-
ters [24–26] and chemiluminescence [27]. Compared to more
traditional organometallic ligands, chelating bimetallic NHC li-
gand architectures [28–35] and, in particular, bimetallic donor-
functionalized NHC ligands [36–43] are less developed. In light
of the apparent utility of bimetallic catalysts and the strong cata-
lytic track record of metal complexes supported by donor-func-
tionalized NHCs, further development of bimetallic complexes
supported by donor-functionalized NHCs is warranted.
Catalytic transfer hydrogenation of carbonyl compounds and im-
ines is an industrially important reaction [44,45] and a good test case
for catalytic activity since the reaction proceeds under benign condi-
tions (Trxn ≤ 82 °C, iPrOH as hydrogen source). Furthermore, the cat-
alytic cycle of transfer hydrogenation shares elementary steps with
other industrially relevant catalytic processes (e.g. dehydrogenation
of alcohols). Thus, a metal complex that exhibits catalytic activity for
transfer hydrogenation may be a promising catalyst for other impor-
tant reactions.
[
10,11] have been shown to effectively support a large number of
catalytically active metal centers and circumvent catalyst decom-
position by discouraging, for example, reductive elimination reac-
tions between NHC ligands and hydrocarbyl, hydride, and acyl
ligands [12]. Due, in part, to their facile preparation, pyridine-
appended NHC ligands have become an active area of research. To
date there have been numerous reports on the synthesis and cata-
lytic behavior of various late transition metal centers supported by
pyridine-functionalized NHCs [13–19]. Given the promising cata-
lytic potential of these complexes, further exploration of pyridine-
functionalized NHC ligand architectures is an important area of
continued research.
Another burgeoning area of research in the field of organome-
tallic synthesis and catalysis has been the development of bime-
tallic catalyst systems [20–22]. Bimetallic complexes are of
interest for diverse purposes: from elucidating the mechanisms
Noyori and co-workers showed that a ligand-appended electro-
phile can enhance the rate of transfer hydrogenation by facilitating
the migratory insertion of hydride to a metal-bound carbonyl com-
pound [46]. We hypothesized that the second metal center of a
bimetallic transition metal complex may play the role of secondary
electrophile in a manner similar to Noyori's “N-H effect” and yield
⁎
Corresponding author.
E-mail address: gregdomski@augustana.edu (G.J. Domski).
Present address: University of Illinois, College of Veterinary Medicine, USA.
1
http://dx.doi.org/10.1016/j.inoche.2017.04.022
387-7003/© 2017 Elsevier B.V. All rights reserved.
1

28
I.G. Smith et al. / Inorganic Chemistry Communications 81 (2017) 27–32
Scheme 1. Synthesis of mono- and bisimidazolium ligand precursors (1–3) and complexes 4–8.
similar rate enhancements for catalytic transfer hydrogenation.
Thus, we set out to prepare bimetallic complexes supported by
pyridine-functionalized NHC ligands. Herein we describe the
synthesis and characterization of several previously unreported
monometallic iridium(III) and ruthenium(II) complexes support-
ed by pyridine-functionalized NHC ligands, an analogous unre-
ported bimetallic iridium(III) complex, and the results of
catalytic transfer hydrogenation trials using these complexes as
precatalysts.
al. in which the arylimidazole (1,2) or arylbisimidazole (3) was
allowed to react with 2-bromopyridine at 160 °C under neat condi-
tions [47].
Compounds 1 [48] and 2 [49] have been prepared and character-
ized previously while, to our knowledge, 3 has not been reported.
The corresponding, air-stable metal complexes (4–8) were prepared
via transmetalation from the in situ-formed Ag(I)\\Br complexes
and either [Cp*IrCl
2 2 2 2
] or [p-cymeneRuCl ] followed by anion ex-
change with KPF . The monometallic complexes (4–7) were readily
6
The mono- and bisimidazolium ligand precursors (1–3; Scheme 1)
were prepared according to the method reported by Crabtree et
prepared at room temperature using dichloromethane as solvent,
while, due to low solubility in dichloromethane, the bisimidazolium
Fig. 1. Alkyl region of 1H NMR (400 MHz, d
-DMSO) for 4.
6

I.G. Smith et al. / Inorganic Chemistry Communications 81 (2017) 27–32
29
fact that the integration of this region matches well with the ex-
pected value of 10H. In the solid-state structure of 4 (Fig. 2), the
distance between hydrogen atoms of methyl groups a, a′, and e,
methine hydrogen d and their nearest non-bonded neighbor are
all within or near the sum of the van der Waals radii which indi-
cates significant steric congestion consistent with the complicated
1
H NMR spectrum.
The H NMR spectrum of compound 5, which bears an N-phenyl
1
group, suggests decreased steric congestion in the coordination
sphere as the signals corresponding to the isopropyl group attached
to p-cymene display first order splitting indicating free rotation of
the isopropyl group and methyl group. Likewise, the 1H NMR
spectrum for compound 7 was consistent with free rotation about
the N\\Cphenyl bond and of the Cp* as indicated by a singlet at
1
.39 ppm corresponding to the methyl groups of the Cp* moiety.
1
The H NMR spectrum of compound 8 was consistent with the highly
symmetrical solid-state structure of this compound with a relatively
uncongested coordination sphere about each iridium center; the
aromatic protons of the 2,5-dimethylbenzene linker appeared as a
2
H singlet at 7.92 ppm and the methyl protons of the linker appeared
as a 6H singlet at 2.44 ppm, while the Cp* methyl protons appeared
as a 30H singlet at 1.45 ppm.
The solid-state structures of compounds 4–8 were determined
via X-ray crystallography (Figs. 2–4). Compounds 4 and 5 exhibited
structural parameters similar to a N-3-butenyl pyridine-NHC
ruthenium(II) complex reported by Saha and co-workers [50]: the
Ru\\CNHC bond distance was within 0.02 Å, the Ru\\Npyr distance
was within 0.01 Å. However, the Ru\\Cl distances in compounds 5
and 6 were 0.2 Å greater than those for the compound reported by
Saha. This difference is likely due to the greater steric demand of
the N-mesityl and N-phenyl substituents compared to the N-3-
butenyl analog. Compounds 6–8 revealed similar solid-state
structures to the related pyrimidine compounds reported by
Crabtree and co-workers [51]. Specifically, the Ir-cent, Ir\\Npyr, and
Ir\\CNHC distances for 6 were all within 0.02 Å of those reported for
the analogous mesityl-appended pyrimidine complex, and the
CNHC\\Ir\\Cl (88.43(7)° for 6 vs. 86.98(19)°) and CNHC\\Ir\\Npyr
(76.91(9)° for 6 vs. 76.7(3)°) bond angles were similar. Compound
8 revealed a highly symmetric anti arrangement of iridium metal
centers in the solid state consistent with the 1H NMR spectrum
(vide supra).
Fig. 2. ORTEP diagram of 4. Thermal ellipsoids are shown at the 35% probability level. PF−
counterions and non-interacting hydrogen atoms have been omitted for clarity.
6
salt (3) was metalated at 60 °C in methanol and the anion exchange
was conducted in acetonitrile to yield 8. We have prepared several
other pyridine-functionalized, arene-linked bisimidazolium salts
including the benzene-linked analog of 3, however, the 2,5-
dimethylbenzene-linked salt was chosen for this study because it
was more soluble in methanol than analogous compounds. Possibly
due to greater air/moisture sensitivity under the comparatively
harsh reaction conditions (i.e. protic solvent, elevated temperature)
attempts to isolate the analogous bimetallic ruthenium(II) com-
pound have thus far been unsuccessful.
Compounds 4–8 were evaluated for activity as transfer hydroge-
nation catalysts. Under air-free conditions, upon activation with
1
The H NMR spectra of compounds 4 and 6 revealed hindered
rotation about the N\\Cmes bond as evidenced by separate signals
for the ortho-methyl groups and meta-hydrogens of the mesityl
ring. This is consistent with the solid-state structure of these
two compounds in which one side of the mesityl group is proxi-
mal to a metal bound chloride while the other is distal. For com-
pound 4 in particular, the coordination environment around the
metal center appears somewhat distorted and the rotation around
several C\\C bonds appears to be hindered due to steric conges-
tion. For example, the difference in Cl\\Ru-centroid angle be-
tween 4 and 5 is 2.21° while the difference in the Cl\\Ir-
centroid angles for 6 and 7 is only 0.27° thus indicating greater
distortion in the case of the ruthenium(II) complexes possibly as
a result of the bulky p-cymene ligand. Indeed, the entire alkyl re-
6
KOH and KPF , each complex showed activity for catalytic transfer
hydrogenation of acetophenone to form 1-phenylethanol (Table
1). Compounds 4–6 exhibited similar activities to the pyrimidine
analogs reported by Crabtree and co-workers [51] reaching
≥92.0% conversion and ≥88 turnovers (TON) in 4 h. In the case of
the ruthenium complexes 4 and 5, the phenyl-appended compound
5 exhibited a higher activity at t = 1 h (entries 2 vs. 4, Table 1); this
may be due to the lower steric demand of the phenyl moiety com-
pared to the mesityl group. Curiously, the opposite trend was ob-
served for the analogous iridium compounds (entries 6 vs. 8,
Table 1) in which the mesityl-appended complex (6) exhibited
−
1
−1
twice the turnover frequency of 7 (TOF = 86 h
vs. 38 h ).
One possible explanation for this observation is that the presumed
16 electron phenyl-appended catalytic intermediate may undergo
ortho-metalation of the N-phenyl moiety leading to catalyst deacti-
vation. This would not be surprising in light of the propensity of
electron deficient Ir(III)-centers to participate in C\\H activation
[52]. As noted earlier, we had hoped that bimetallic complex 8
would exhibit superior activity in transfer hydrogenation to its
monometallic analogs. We were disappointed to find that this
was not the case with 8 exhibiting only ca. 20% of the activity of
6 (entries 10 vs. 6, Table 1) and ca. 45% of the activity of 7 (entries
10 vs. 8, Table 1). Two possible factors may contribute to the low
1
gion of the H NMR spectrum for 4 is quite complicated (Fig. 1)
presumably due to hindered rotation of the p-cymene ring, and
also the C\\C bonds connected to methyl groups a, a′, c, c′, and
e. For example, whereas the signal for the isopropyl methyl
groups appear as a 6H doublet for compound 5, in compound 4
the signals for c and c′ appear as broad multiplets at 0.94 ppm
and 0.57 ppm, indicating that the protons on methyl groups c
and c′ are not equivalent on the NMR timescale. The signals for
protons of methyl groups labeled a, a′, and e and the methine
proton d are likewise complex and poorly resolved despite the

30
I.G. Smith et al. / Inorganic Chemistry Communications 81 (2017) 27–32
−
6
Fig. 3. ORTEP diagrams of 5–7. Thermal ellipsoids are shown at the 35% probability level. Hydrogen atoms and PF counterions have been omitted for clarity.
activity of the catalyst derived from 8: (1) the high steric demand
of the 2,5-dimethylbenzene linker makes it likely that the Ir(III)
centers remain isolated from one another and are thus incapable
of cooperation in the catalytic cycle, and (2) the presumed 16
electron intermediate generated from 8 may undergo ortho-
metalation via C\\H activation thereby leading to catalyst deacti-
vation. Future work will focus on investigating the nature of the
active species and interrogating the possibility of deleterious
C\\H activation and further developing bimetallic complexes in-
corporating aliphatic linkers which may lead to greater confor-
mational flexibility, circumvent ortho-metalation, and increase
catalytic activity.
complexes for transfer hydrogenation and other organic
transformations.
Acknowledgements
GJD thanks Augustana College (Faculty Research Fellowship
and Presidential Research Fellowship) and the American Chemical
Society's Petroleum Research Fund New Investigator Award
program (PRF # 51217-UNI3) for financial support. The authors
thank Stephen Dempsey, Elizabeth Gehrmann, Travis Goodwin,
and Matthew Klyman for help with synthesis and characterization
of 5 and 7.
We have synthesized and characterized five previously
unreported ruthenium(II) and iridium(III) complexes supported
Appendix A. Supplementary material
by pyridine-functionalized NHC ligands including
bisimidazolium salt (3) and the corresponding bimetallic complex
8). Each of the compounds has shown activity for catalytic trans-
fer hydrogenation upon activation with KOH and KPF . We plan to
explore different linker architectures for pyridine-functionalized
bisimidazolium compounds and less sterically demanding ancil-
lary ligands, expand the scope of bimetallic coordination com-
plexes supported by pyridine-functionalized NHCs to other late
metal centers, and evaluate the catalytic potential of these new
a novel
Procedures and characterization data for the compounds 3–8, proce-
dural details for catalytic trials, and X-ray crystallography procedures
and data are available as electronic supplementary material. Full cifs
for each structure are also available free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/structures. CCDC 1540972, 1540970, 1540973, 1540971, and
1540969 contains the supplementary crystallographic data for 4,
5, 6, 7, and 8, respectively, for this paper. Supplementary data
(
6

I.G. Smith et al. / Inorganic Chemistry Communications 81 (2017) 27–32
31
8
Ir(1)-cent
Ir(1)-C(1)
1.8245(14) Å
2.012(3) Å
2.4148(7) Å
2.120(3) Å
88.26(9)°
Ir(1)-Cl(1)
Ir(1)-N(3)
C(1)-Ir(1)-Cl(1)
C(1)-Ir(1)-N(3)
76.48(11)°
Fig. 4. ORTEP diagram of 8. Thermal ellipsoids are shown at the 35% probability level. Hydrogen atoms and PF−
counterion have been omitted for clarity.
6
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Table 1
6
Results of catalytic transfer hydrogenation trials using 4–8/KOH/KPF .
Entrya
Catalyst
Time (minutes)
% conversionb
TONc
TOF (h−1)d
[
1
2
3
4
5
6
7
8
9
4
4
4
5
5
6
6
7
7
30
60
240
60
240
30
240
30
240
30
240
1440
33.3
54.5
92.0
79.1
95.5
81.3
98.9
35.3
76.5
16.1
48.9
95.9
35
58
88
84
101
86
105
38
81
17
71
58
44
84
25
86
26
38
20
17
13
4
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General conditions: Catalyst (16 μmol), KOH (0.2 mmol), and KPF
6
(30 μmol) were
dissolved in 10 mL of iPrOH at 82 °C and acetophenone (1.7 mmol) was added.
b
Determined via gas chromatography.
mol product/mol metal (metal = Ru or Ir).
TON/time.
c
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[
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μmol of compound 8.
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