Heteroleptic 1,4-Diazabutadiene Complexes of Ruthenium: Synthesis, Characterization and Utilization in Catalytic Transfer Hydrogenation

Article abstract of DOI:10.1002/ejic.202000859

Reaction of [Ru(trpy)Cl₃] with 1,4-diazabutadienes (p-RC₆H₄N=C(H)-(H)C=NC₆H₄R-p; R = OCH₃, CH₃, H and Cl; abbreviated as L-R) in refluxing ethanol in the presence of triethylamine has afforded a family of complexes, isolated as perchlorate salts, of type [Ru(trpy)(L-R)Cl]ClO₄ [depicted as complexes 1 (R = OCH₃), 2 (R = CH₃), 3 (R = H) and 4 (R = Cl)]. Crystal structures of complexes 1, 2 and 4 have been determined, and structure of complex 3 has been optimized by DFT method. The 1,4-diazabutadiene ligand in each complex is bound to ruthenium as a N,N-donor forming five-membered chelate. Complexes 1–4 catalyze transfer hydrogenation of aryl aldehydes to the corresponding alcohols with high (ca. 10⁶) TON. They are also found to catalyze transfer hydrogenation of aryl ketones to corresponding secondary alcohols, but with much less efficiency. Catalytic transfer hydrogenation of nitroarenes to the corresponding amines has also been achieved.

Full text of DOI:10.1002/ejic.202000859

European Journal of Inorganic Chemistry
Accepted Article
Title: Heteroleptic 1,4-Diazabutadiene Complexes of Ruthenium:
Synthesis, Characterization and Utilization in Catalytic Transfer
Hydrogenation
Authors: Rumpa Saha, Aparajita Mukherjee, and Samaresh
Bhattacharya
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000859
Link to VoR: https://doi.org/10.1002/ejic.202000859
01/2020

10.1002/ejic.202000859
European Journal of Inorganic Chemistry
Manuscript No.: ejic.202000859
Heteroleptic 1,4-Diazabutadiene Complexes of Ruthenium: Synthesis,
Characterization and Utilization in Catalytic Transfer Hydrogenation
Rumpa Saha, Aparajita Mukherjee, and Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University,
Kolkata - 700 032, India. E-mail: samareshb.chemistry@jadavpuruniversity.in
Abstract: Reaction of [Ru(trpy)Cl3] with 1,4-diazabutadienes (p-RC6H4N=C(H)-
(H)C=NC6H4R-p; R = OCH3, CH3, H and Cl; abbreviated as L-R) in refluxing ethanol in
the presence of triethylamine has afforded a family of complexes, isolated as perchlorate
salts, of type [Ru(trpy)(L-R)Cl]ClO4 [depicted as complexes 1 (R = OCH3), 2 (R = CH3),
3 (R = H) and 4 (R = Cl)]. Crystal structures of complexes 1, 2 and 4 have been
determined, and structure of complex 3 has been optimized by DFT method. The 1,4-
diazabutadiene ligand in each complex is bound to ruthenium as a N,N-donor forming
five-membered chelate. Complexes 1 – 4 catalyze transfer hydrogenation of aryl
aldehydes to the corresponding alcohols with high (~106) TON. They are also found to
catalyze transfer hydrogenation of aryl ketones to corresponding secondary alcohols, but
with much less efficiency. Catalytic transfer hydrogenation of nitroarenes to the
corresponding amines has also been achieved.
________________________________________________________________________
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
2
Introduction
Hydrogenation of substrates is a key step in many organic transformations. Though
addition of molecular hydrogen has always been there as a familiar protocol, transfer
hydrogenation has gradually emerged as a favorite alternative for many such
reductions,[1-6] primarily owing to the convenience associated with this method. Several
transition metal complexes are reported to catalyze transfer hydrogenation reactions.[7]
However, efficiency of bivalent ruthenium complexes in catalyzing transfer
hydrogenation reactions is particularly noteworthy.[8] The Ru-catalyzed transfer
hydrogenation reactions are known to proceed through the intermediacy of a ruthenium-
hydrido species. Hence complexes having a Ru-Cl bond, which is known to provide a
Ru-H fragment in situ upon reaction with primary or secondary alcohols,[9] are especially
useful as catalyst-precursors. In the present study, which has originated from our
continued interest in developing new ruthenium-based catalysts,[10] our objectives have
been to synthesize a new group of ruthenium complexes with a Ru-Cl moiety, and
explore their catalytic potential in transfer hydrogenation reactions.
As ruthenium(II) prefers to remain hexa-coordinated in majority of its
complexes, choice of ligands that occupy the other five coordination sites of the metal
center in the Ru-Cl moiety is crucial. A combination of tridentate and bidentate
ligands is found to be very convenient for this purpose. Standard chelating tridentate
ligands, as well as arene ligands with ability to display 6-mode of binding, are often
utilized to occupy three coordination sites; while chelating bidentate ligands of
different types are used to take up the remaining two sites. Ligands with recognized
-acid character are particularly useful in this regard, as they can stabilize the bivalent
state of ruthenium. Herein we have planned to use 2,2′:6′,2′′-terpyridine (abbreviated as
trpy) as the tridentate ligand, and as bidentate ligand we have chosen a group of 1,4-
diazabutadiene ligands, abbreviated in general as L-R, where R depicts the para-
substituent in the aryl ring. The trpy ligand is well known to form stable chelate (I) with
ruthenium(II).[11] The 1,4-diazabutadiene ligands are reported to bind to metal centers as
bidentate N,N-donors forming five-membered chelate ring (II).[12] The -diimine
fragment in these ligands has considerable -acidity,[13] and hence chelation by them is
expected to stabilize the bivalent state of ruthenium.[14] As the source of ruthenium
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
3
[Ru(trpy)Cl3] has been selected, firstly as it can provide the Ru(trpy)Cl moiety, and
secondly as it is well known to undergo facile displacement of two coordinated chlorides
by chelating bidentate ligands (associated with one-electron reduction at the metal
N
N
N
N
N
N
M
I
trpy
R
N
N
R
R
N
N
R
M
II
R = OCH3, CH3,
H and Cl
L-R
center).[11] Reaction of [Ru(trpy)Cl3] with the 1,4-diazabutadiene ligands indeed afforded
a family of complexes of type [Ru(trpy)(L-R)Cl]+. In this paper we describe the
formation and characterization of these complexes; their electronic spectral and
electrochemical properties; and their ability to serve as precursors in catalytic transfer
hydrogenation of aryl aldehydes, aryl ketones and nitroarenes.
Results and Discussion
Syntheses and Structures
As outlined in the introduction, the primary goal of this study was to synthesize a group
of mixed ligand ruthenium(II) complexes of type [Ru(trpy)(L-R)Cl]+. Accordingly
reactions of [Ru(trpy)Cl3] with the chosen 1,4-diazabutadiene ligands were carried out in
refluxing ethanol in the presence of triethylamine. The reactions proceeded smoothly to
afford the targeted [Ru(trpy)(L-R)Cl]+ complexes, which were isolated as perchlorate
salts in good yields. Preliminary characterizations (microanalysis, mass, NMR and IR) of
the new complexes are found to be consistent with their general [Ru(trpy)(L-R)Cl]ClO4
formula. 2,2′:6′,2′′-Terpyridine being a symmetric tridentate ligand and the 1,4-
diazabutadienes being symmetric bidentate ligands, these new complexes can have only
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
4
R
N
N
Cl
N
Ru
Ru(trpy)
N
N
Ru
ClO4
N
N
N
R
Complex
OCH3
CH3
H
1
2
3
4
R
Cl
Scheme 1. Geometry and numbering of the complexes.
one possible geometry, which, along with numbering of the complexes, is shown in
Scheme 1. Solid state structures of complexes 1, 2 and 4 were determined by X-ray
crystallography. Single crystals of complex 3 were not obtained even after repeated
attempts. Crystal structure of complex 1 is shown in Figure 1, and selected bond
Figure 1
parameters are listed in Table 1. Crystal structures of complexes 2 and 4 selected bond
parameters associated with them are presented in Figure S1 and Table S1 (Supporting
Information). The structures show that the 1,4-diazabutadiene ligands are coordinated to
ruthenium in the expected chelating mode (II, M = Ru) with a bite angle of ~78°. The C-
C and C-N bond lengths within the five-membered chelate of the coordinated 1,4-
diazabutadiene ligands deviate significantly from localized C-C single bond length and
C=N double bond length, which is attributable to the combined influence of
delocalization of the -cloud and back-donation from the electron-rich low-spin d6
ruthenium center. In all the crystal structures one imine-nitrogen (N2), that is trans to the
coordinated chloride, is found to be closer to ruthenium than the other imine-nitrogen
(N1), which is trans to the central trpy-nitrogen (N3); presumably due to trans effect. The
bond parameters within the Ru(trpy) fragment are all found to be quite usual.11 The
perchlorate ion was located outside the coordination sphere in all three crystal structures.
In complexes 1 and 2 it was found be highly disordered, while in complex 4 it did not
show any disorder. As complexes 1 – 4 were synthesized similarly and they show similar
properties (vide infra), the complex 3 is assumed to have a similar structure as the other
three complexes. The structure of the complex cation in 3 was optimized by DFT method,
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
5
which is shown in Figure S2 (Supporting Information) and some computed bond
parameters are listed in Table S2 (Supporting Information). The computed bond
parameters for complex 3 compare well with those found in the crystal structures of its
congeners. In all these complexes 1 – 4 ruthenium is nested in a N5Cl coordination
environment, which is distorted significantly from ideal octahedral geometry as reflected
in the bond parameters around the metal center.
Spectral Properties
Magnetic susceptibility measurements show that complexes 1 – 4 are diamagnetic, which
1
is consistent with the +2 oxidation state of ruthenium (low-spin d6, S = 0) in them. H
NMR spectra of the complexes were recorded in CD3CN solution, and the NMR data are
presented in the experimental section. Majority of the signals arising from the
coordinated 1,4-diazabutadiene and trpy ligands could be identified, while few could not
be because of overlap problem. For example, in complex 1 two distinct signals for the
methoxy groups are observed at 3.58 and 3.86 ppm; and two sharp singlets at 8.56 and
9.03 ppm are observed for the two azomethine protons. Similarly in complex 2, two
methyl signals at 2.07 and 2.41 ppm, and two azomethine proton signals at 8.59 and 9.06
ppm, are observed. Appearance of these signals is consistent with the absence of any C2
symmetry in these complexes. Signals from the Ru-coordinated trpy ligand are observed
within 7.4 – 8.3 ppm in all four complexes. With reference to the plane containing the
Ru(L-R)Cl moiety, that is orthogonal to and bisects the Ru(trpy) plane, six signals are
expected from the trpy ligand, out of which three to four could be distinctly seen while
the remaining ones appeared as overlapping signals.
Infrared spectra of complexes 1 – 4 show many bands of varying intensities
within 4000 - 450 cm-1. Comparison with the spectrum of the starting [Ru(trpy)Cl3]
complex shows four common bands near 1602, 1448, 1386 and 770 cm-1, which are
obviously due to the Ru-bound trpy ligand. This comparison with the spectrum of
[Ru(trpy)Cl3] also shows presence of few additional bands near 1495, 1120, 1015, 735
and 623 cm-1 in the spectra of complexes 1 – 4, of which the two bands near 1120 and
623 cm-1 are attributable to the perchlorate anion. The remaining bands near 1495, 1015
and 735 cm-1 are assignable to the Ru-coordinated 1,4-diazabutadiene ligand. The NMR
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
6
and infrared spectral data of the new mixed-ligand complexes are therefore consistent
with their compositions.
Complexes 1 – 4 are found to be readily soluble in polar solvents such as acetone,
acetonitrile, dimethylformamide and dimethylsulfoxide, producing intense wine-red or
purple solutions. Electronic spectra of these complexes were recorded in acetonitrile
solution, and the spectral data are presented in Table 2. Each complex shows four distinct
absorptions spanning over the visible and ultraviolet regions. A representative spectrum
is shown in Figure S3 (Supporting Information). Ruthenium(II) complexes of -acid
ligands are known to display intense metal-to-ligand charge-transfer (MLCT) transitions
in the visible region. In the present group of complexes ruthenium is chelated by -acid
ligands of two types, viz. trpy and 1,4-diazabutadiene, and hence MLCT transitions
involving both these ligands are probable. To gain insight into the nature of the observed
absorptions, TDDFT calculations have been performed on all four complexes using the
Gaussian 09 program package.[15],[16] The results of the TDDFT calculations are found to
be similar for all four complexes. The main calculated transitions for complexes 1 – 4 and
compositions of the molecular orbitals associated with these transitions are presented in
Table S3 – Table S10 and Figure S4 – Figure S7 (Supporting Information). The results
for complex 1 are discussed here as a representative case. The lowest energy absorption
at 701 nm is attributable largely (~85%) to a HOMO → LUMO transition, and based on
the nature of these two participating orbitals this electronic excitation is assignable to a
MLCT transition, where the 1,4-diazabutadiene ligand is serving as the accepting ligand.
This absorption is found to have much less HOMO-2 → LUMO and HOMO-3 → LUMO
character, and hence this dominant MLCT transition has some ILCT (intra-ligand charge-
transfer), LLCT (ligand-to-ligand charge-transfer) and IMCT (intra-metal charge-
transfer) characters. The next two absorptions at 519 nm and 403 nm are found to be
primarily due respectively to HOMO-2 → LUMO (~67%) and HOMO-4 → LUMO
(~87%) transition; and both have dominant an ILCT character involving the 1,4-
diazabutadiene (L-OCH3) ligand. The fourth absorption at 322 nm is mostly (~69%) due
to a HOMO-5 → LUMO+1 transition, and is best described as a LLCT transition from
1,4-diazabutadiene (L-OCH3) to trpy.
Electrochemical Properties
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10.1002/ejic.202000859
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Electrochemical properties of complexes 1 – 4 have been studied by cyclic voltammetry
in acetonitrile solution (0.1 M TBHP). Each complex shows an oxidative response on the
positive side of SCE, and three reductive responses on the negative side. Cyclic
voltammetric data are given in Table 2, and a representative voltammogram is shown in
Figure S8 (Supporting Information). The first oxidative response, observed within 1.03 –
1.12 V, is assigned to Ru(II)-Ru(III) oxidation. This oxidation is found to be reversible
with a peak-to peak separation (Ep) close to 70 mV.[17] The Ru(II)-Ru(III) oxidation
potential is found to be fairly sensitive to the nature of the substituent R in the 1,4-
diazabutadiene ligands, the potential increases with increasing electron-withdrawing
nature of R. The three reductive responses observed on the negative side of SCE are
believed to be ligand centered, but their assignment to specific sites has been rather
difficult owing to the redox non-innocent nature of both the organic ligands. The diimine
moiety in the 1,4-diazabutadiene ligands is known to undergo two one-electron
reductions with a gap of about 500 mV.[18] Hence the first (-0.56 to -0.70 V) and the third
(-1.21 to -1.39 V) reductive responses are tentatively assigned to reductions of the
diimine fragment. The second reduction (-0.74 to -0.89 V) is assigned, by comparing
with cyclic voltammetric properties of reported ruthenium(II)-trpy complexes, to
reduction of trpy.[11] The reductive responses are not reversible, and hence the observed
shift in their potential could not be interpreted as manifestation of effect of the substituent
R in the 1,4-diazabutadiene ligands.
Catalytic Transfer Hydrogenation
As outlined earlier, our second objective was to evaluate catalytic potential of the
synthesized [Ru(trpy)(L-R)Cl]+ complexes towards transfer hydrogenation of selected
substrates. We first decided to try aryl aldehydes as substrate, as they are known to
undergo smooth Ru-catalyzed transfer hydrogenation.[1b,e;8] We began our study by
examining the transfer hydrogenation of 4-chlorobenzaldehyde to 4-chlorobenzyl alcohol
using complex 1 as the catalyst precursor. All the experimental parameters were
systematically varied to reach an optimum yield of the product, and after extensive
optimization (Table S11; Supporting Information) it was found that 0.00002 mol%
catalyst, 1.0 mol% NaOtBu as base, 1-propanol as solvent, a reaction temperature of 85
°C, and a reaction time of 1 h furnished an excellent yield (98%) of the expected product
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
8
(entry 22).[19] The other three complexes (2 – 4) were found to show catalytic efficiency
similar to that of complex 1 (entries 25-27), and hence only the results obtained with
complex 1 as the catalyst precursor are presented here.
Scope of the transfer hydrogenation of aldehydes is summarized in Table 3. Using
the optimized the reaction conditions, transfer hydrogenation has been performed on a
thirteen different aldehydes. Using benzaldehyde and substituted benzaldehydes as
substrates (S1 – S7), the corresponding alcohols (P1 – P7) were obtained in very good (85
– 99%) yields; and the average turn-over number was remarkably high (~ 4.6 × 106).
Electronic nature of the substituent(s) on the phenyl ring was varied, but the yields
remained rather indifferent to it. When relatively bulkier substrates (S8 and S9) were used,
the product alcohols were obtained in significantly lower yields. It is interesting to note
that a substrate having both aldehyde and olefin fragments (S10) was found to undergo
selective hydrogenation of the aldehyde function to furnish the corresponding alcohol
(P10) in a decent (86%) yield. However, for substrates with a recognized donor site next
to the -CHO group (S11 – S13), the expected product alcohols (P11 – P13) were not
obtained at all, presumably due to catalyst deactivation via coordination of these
substrates that are well known for their ability to form chelates.
The observed catalytic transfer hydrogenation of aryl aldehydes is believed to
follow the sequences illustrated in Scheme 2, which are essentially similar to those
proposed earlier.[1b,e] The [Ru(trpy)(L-R)Cl]+ catalyst-precursor, which is depicted as A
with only the Ru-Cl fragment shown, undergoes reaction with 1-propanol in the initial
step to generate the intermediate B, in which the n-propoxide ion is coordinated to
Scheme 2
ruthenium through oxygen, and thus entry into the catalytic cycle takes place. This
species B then undergoes a -hydride elimination from the coordinated n-propoxide
ligand to afford the hydrido species C. Attachment of the aldehyde to the metal center is
believed to take place next to generate species D, which is followed by insertion of the
aldehyde substrate into the Ru–H bond to generate the corresponding aryloxo species E.
In the final step, elimination of the product alcohol takes place with simultaneous
regeneration of species B, and the catalytic cycle thus continues.
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
9
The remarkable catalytic efficiency of complexes 1 – 4 in transfer hydrogenation
of aryl aldehydes encouraged us to try similar hydrogenation of aryl ketones to the
corresponding secondary alcohols. At first we tried transfer hydrogenation of 4-
fluoroacetophenone under the same experimental condition used for the reduction of the
aryl aldehydes, but the expected alcohol was not obtained at all (Table S12, entry 1;
Supporting Information). The catalyst loading had to be increased to 0.5 mol% in order to
obtain the product alcohol in good (98%) yield (Table S12, entry 3; Supporting
Information). With this catalyst loading (0.5 mol%) reduction of twelve ketones was
attempted, and the results are presented in Table 4. Using acetophenone and 4-substituted
acetophenones as substrates (S1 – S7), the corresponding secondary alcohols (P1 – P7)
were obtained in good (65 – 99%) yields. When cyclohexanone (S8) and benzophenone
(S9) were used as substrates the corresponding alcohols (P8 and P9) were also obtained in
good yields. Cyclohex-2-en-1-one (S10), a substrate with two reducible fragments, was
found to undergo selective reduction of the carbonyl function to afford the corresponding
alcohol (P10) in a relatively lower yield. When two aryl ketones with a recognized donor
site next to the carbonyl function, viz. 2-hydroxyacetophenone (S11) and 2-acetylpyridine
(S12), were tried as substrates, the expected reduction did not take place at all, probably
due to catalyst deactivation via coordination of these substrates. In view of the relative
difficulty in reduction of ketone compared to that of aldehyde,[20] the average turn-over
number (1.6 × 102) observed in the present study is decent. The catalytic efficiency of our
complexes in transfer hydrogenation of aldehydes and ketones is better than many of the
reported ruthenium-catalysts.[1b,e;2a,c,h,i;20b,c] For comparison of the catalytic efficiency of
our complexes, we prepared the well-known [Ru(trpy)(bpy)Cl]ClO4 complex and tested
it as catalyst precursor (0.00002 mol%) under the same experimental condition for the
transfer hydrogenation of 4-chlorobenzaldehyde. The expected reduced product, viz. 4-
chlorobenzylalcohol, was obtained in 31% yield, which indicates that compared to 2,2ʹ-
bipyridine, the 1,4-diazabutadiene ligand favors the catalytic transfer hydrogenation.
Next we planned to try reduction of nitroarenes with our Ru-catalyst. While
catalytic reduction of nitroarenes by conventional methods is still in practice,[21] transfer
hydrogenation is becoming popular as a convenient alternative.[6] At first we tried
transfer hydrogenation of 4-nitrotoluene using complex 1 as the catalyst precursor. We
did a thorough screening of the reaction parameters (Table S13, Supporting Information),
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
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and after the extensive screening it was found that 5 mol% catalyst loading, 1-propanol as
solvent, 20 mol% KOtBu as base, 100° C reaction temperature and 12 h reaction time
afforded aniline as the only reduced product in 97% yield (entry 10). Though aniline
could also be obtained in 98% yield with 4 mol% catalyst loading, the reaction required
much longer (21 h) time (entry 9). Hence we accepted conditions in entry 10 as optimized
ones. Scope of this catalytic reduction of nitroarenes is shown in Table 5. We attempted
reduction of fourteen different nitroarenes. When nitrobenzenes having substituent at
ortho or para position (S1 – S5) were taken as substrates, the corresponding anilines (P1 –
P4) were obtained in excellent yields except for P5, where the yield was slightly lower.
Changing size of the aryl fragment from phenyl (S1) to naphthyl (S6) did not have any
observable effect as the products (P1 and P6) were obtained in same yield. When 1,4-
dinitrobenzene (S7) or 1,3-dinitrobenzene (S8) were tried as substrate, both the nitro
groups were found to undergo reduction to amine furnishing products (P7 and P8) in
>70% yield. We then took 1-chloro-2,4-dinitrobenzene (S9), where the two nitro groups
are distinguishable, and obtained the product (P9) with complete reduction of the nitro
group next to chloro substituent, while the second nitro group remained unaffected.
Though the exact reason behind this observation remains to be understood, preferential
approach of the nitro group at 2-position to the metal center, assisted by the chloro
substituent through hydrogen bonding with the coordinated ligand(s), seems plausible.
With 4-nitrobenzaldehyde as substrate (S10), that has another reducible function (viz.
aldehyde) besides nitro, reduction of both the functions was observed affording product
(P10) in 94% yield. However, when 4-nitrostyrene (S11) was taken as substrate, products
with reduction of only nitro group (P11a) and with reduction of both nitro and olefin
fragments (P11b) were obtained in comparable yields. This result indicates that under the
chosen experimental condition, reduction of olefin in presence of nitro group is relatively
difficult. Styrene could be reduced completely with much less catalyst loading. When 2-
nitrophenol (S12), 2-nitroaniline (S13) and 4-nitroaniline (S14) were tried as substrates, the
corresponding expected reduced products were not at all obtained, indication catalyst
poisoning via coordination through phenolic-oxygen or amine-nitrogen.
The observed catalytic transfer hydrogenation of nitroarenes to the corresponding
amines is assumed to proceed through steps shown in Scheme 3, which are basically
drawn from those reported earlier.[6a-d] The steps up to formation of the hydrido species C
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
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Scheme 3
are same as in Scheme 2. Attachment of the substrate and hydride transfer take place in
the following two steps, generating species D and E respectively. Interaction with 1-
propanol happens next leading to elimination of nitrosobenzene along with water and
generation of species B. This catalytic cycle thus continues. Transformation of
nitrosobenzene to N-phenylhydroxylamine to aniline takes place via similar catalytic
transfer hydrogenation.[6] The observed catalytic efficiency of our complexes in the
transfer hydrogenation of nitroarenes is better than,[6a,c] or comparable to,[6d] the recently
reported methods. It is also relevant to mention here that our catalytic protocol is superior
to most of the reported methods as it furnishes aryl amines as the only reduction product
instead of a mixture of products. For instance, when [Ru(trpy)(bpy)Cl]+ was utilized as
catalyst precursor for transfer hydrogenation of 4-nitrotoluene, the expected reduced
product, p-toluidine, was not obtained at all. Instead the azoxy derivative (p-CH3-C6H4-
N=N(O)-C6H4-CH3-p) was obtained in 99% yield. Use of 1,4-diazabutadiene as ancillary
ligand is thus found to help selective reduction of nitroarenes to aminoarenes.
Conclusions
A group of heteroleptic 1,4-diazabutadiene (L-R) complexes of ruthenium of type
[Ru(trpy)(L-R)Cl]ClO4 has been synthesized and the scope of these complexes as catalyst
precursor for transfer hydrogenation of aryl aldehydes, aryl ketones and nitroarenes has
been examined. The Ru-Cl fragment in these complexes is found to be highly effective
for their utilization as pre-catalyst in transfer hydrogenation reactions using 1-propanol as
the provider of hydrogen. The ruthenium-hydrido species C (see Scheme 2), generated in
situ, serves as an excellent catalyst for the transfer hydrogenation of aryl aldehydes to the
corresponding alcohols with high turn-over number (~ 106). Aryl ketones, which are
known as relatively difficult to reduce, could also be hydrogenated, but with less
efficiency (turn-over number ~ 102). Catalytic transfer hydrogenation of nitroarenes was
also done smoothly, and while ruthenium-catalyzed transfer hydrogenation of nitroarenes
usually furnishes a mixture of amino-, azo- and azoxy-products, our catalytic protocol is
found to afford the corresponding aryl amines as the only reduction product. The
demonstrated ability of this group of ruthenium complexes as efficient catalyst precursor
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
12
for transfer hydrogenation indicates further scope of their utilization for reduction of
other functional groups under relatively mild homogeneous condition.
Experimental Section
Materials: Ruthenium trichloride was obtained from Arora Matthey, Kolkata, India, and
2,2′:6′,2′′-terpyridine (trpy) was purchased from Sigma-Aldrich. [Ru(trpy)Cl3] was
prepared by following a reported procedure.[22] Glyoxal was obtained from SD Fine
Chem, Mumbai, India, and the 4-R-anilines (R = OCH3, CH3, H and Cl) were procured
from Merck, India. The 1,4-diazabutadiene ligands (L-R) were prepared by condensation
of glyoxal with the 4-R-anilines in hot methanol. Tetrabutylammonium
hexafluorophosphate (TBHP) procured from Aldrich and AR-grade acetonitrile procured
from Merck (India) were used in electrochemical work. All other chemicals and solvents
were reagent grade commercial materials and were used as received.
Physical Measurements: Microanalyses (C, H, and N) were performed using a Heraeus
Carlo Erba 1108 elemental analyzer. Mass spectra were recorded with a Micromass LCT
electrospray (Qtof Micro YA263) mass spectrometer. Magnetic susceptibilities were
1
measured using a Sherwood MK-1 balance. H NMR spectra were recorded in CD3CN
solution on Bruker Avance DPX 400 and 500 NMR spectrometers using TMS as the
internal standard. IR spectra were recorded on a Perkin Elmer Spectrum Two IR
spectrometer, with samples prepared as KBr pellets. Electronic spectra were recorded on
a JASCO V-630 spectrophotometer. Electrochemical measurements were made using a
CH Instruments model 600A electrochemical analyzer. A platinum disc working
electrode, a platinum wire auxiliary electrode and an aqueous saturated calomel reference
electrode (SCE) were used in the cyclic voltammetry experiments. All electrochemical
experiments were performed at 298 K under a dinitrogen atmosphere. Geometry
optimization by density functional theory (DFT) method and electronic spectral analysis
by time-dependent density functional theory (TDDFT) calculation were performed using
the Gaussian 09 (B3LYP/GEN) package.[16] GC-MS analyses were performed using a
Perkin Elmer CLARUS 680 instrument.
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10.1002/ejic.202000859
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Preparation of [Ru(trpy)(L-OCH3)Cl]ClO4: To a solution of L-OCH3 (92 mg, 0.34
mmol) in 40 mL of hot ethanol [Ru(trpy)Cl3] (100 mg, 0.23 mmol), LiCl (100 mg 2.38
mmol) and NEt3 (0.5 mL, 0.41 mmol) were added. The mixture was heated at reflux for 8
h under a dinitrogen atmosphere. The resulting red solution was filtered to remove any
insoluble material. To the filtrate few drops of aqueous saturated NaClO4 solution was
added, and it was kept in the refrigerator for 24 h. [Ru(trpy)(L-OCH3)Cl]ClO4 separated
as a red crystalline precipitate, which was collected by filtration, washed with a small
volume of ice-cold water, and dried in a desiccator under vacuum. Yield: 73% (122 mg).
Anal. Calcd. for C31H27N5O6Cl2Ru: C: 50.47; H: 3.66; N: 9.49; found C: 50.71; H: 3.63;
N: 9.52%. MS (ESI), positive mode: [M-ClO4]+, 638.1. 1H NMR (500 MHz, CD3CN):[23]
 (ppm) = 3.58 (s, 3H), 3.86 (s, 3H), 5.74 (d, J = 6.5 Hz, 2H ), 6.38 (d, J = 7.0 Hz, 2H),
7.08 (d, J = 7.0 Hz, 2H), 7.47 (t, J = 7.0 Hz, 2H), 7.85 (d, J = 5.5 Hz, 2H), 7.90 (t, J = 8.0
Hz, 1H), 7.93-8.11 (4H)*, 8.18 (d, J = 8.0 Hz, 2H), 8.23 (d, J = 8.0 Hz, 2H), 8.56 (s, 1H),
9.03 (s, 1H). IR (wave number, cm-1): 624, 636, 736, 770, 1015, 1085, 1120, 1145, 1246,
1284, 1396, 1448, 1499 and 1602.
Preparation of [Ru(trpy)(L-CH3)Cl]ClO4: This complex was synthesized by following
the same above procedure, using L-CH3 instead of L-OCH3. Yield: 67% (108 mg).
Anal. Calcd. for C31H27N5O4Cl2Ru: C: 52.76; H: 3.82; N: 9.92; found C: 52.64; H: 3.90;
1
N: 9.97%. MS (ESI), positive mode: [M-ClO4]+, 606.1. H NMR (500 MHz, CD3CN): 
(ppm) = 2.07 (s, 3H), 2.41 (s, 3H), 5.67 (d, J = 6.5 Hz, 2H ), 6.66 (d, J = 8.0 Hz, 2H),
7.36 (d, J = 8.0 Hz, 2H), 7.47 (t, J = 6.5 Hz, 2H), 7.80-7.92 (5H)*, 7.97 (t, J = 8.0 Hz,
2H), 8.15 (d, J = 8.0 Hz, 2H), 8.22 (d, J = 8.0 Hz, 2H), 8.59 (s, 1H), 9.06 (s, 1H). IR
(wave number, cm-1): 624, 636, 735, 770, 1015, 1085, 1120, 1145, 1245, 1284, 1396,
1448, 1497 and 1602.
Preparation of [Ru(trpy)(L-H)Cl]ClO4: To a solution of L-H (71 mg, 0.34 mmol) in
40 mL of hot ethanol [Ru(trpy)Cl3] (100 mg, 0.23 mmol), LiCl (100 mg 2.38 mmol) and
NEt3 (0.5 mL, 0.41 mmol) were added. The mixture was heated at reflux for 8 h under N2
atmosphere. The resulting violet solution was filtered to remove any insoluble material
and few drops of aqueous saturated NaClO4 solution was added to it and the solvent was
evaporated to dryness under reduced pressure. The dark solid, thus obtained, was washed
with cold water to remove excess NaClO4, and dried in a desiccator. The dry solid was
This article is protected by copyright. All rights reserved.

10.1002/ejic.202000859
European Journal of Inorganic Chemistry
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dissolved in a minimum volume of acetonitrile, and it was subjected to purification by
column chromatography using neutral alumina. With 1:2 acetonitrile-benzene as eluant a
purple band separated, which was collected. Evaporation of the solvents under reduced
pressure afforded [Ru(trpy)(L-H)Cl]ClO4 as a crystalline solid. Yield: 67%. Anal. Calcd.
for C29H23N5O4Cl2Ru: C 51.40; H 3.40; N 10.34; found C: 51.47; H: 3.36; N: 10.27%.
1
MS (ESI), positive mode: [4-ClO4]+, 578.0. H NMR (400 MHz, CD3CN):  (ppm) =
5.84 (d, J = 7.6 Hz, 2H ), 6.94 (t, J = 8.0 Hz, 2H), 7.11 (t, J = 7.6 Hz, 1H), 7.23 (t, J = 7.6
Hz, 1H), 7.5-7.7 (6H)*, 7.94 (d, J = 6.8 Hz, 2H), 8.0-8.1 (3H)*, 8.22 (d, J = 8.4 Hz, 2H),
8.30 (d, J = 8 Hz, 2H), 8.73 (s,1H), 9.18 (s, 1H). IR (wave number, cm-1): 625, 636, 734,
769, 1015, 1085, 1120, 1144, 1246, 1282, 1395, 1448, 1495 and 1602.
Preparation of [Ru(trpy)(L-Cl)Cl]ClO4: To a solution of L-Cl (95 mg, 0.34 mmol) in
40 mL of hot ethanol [Ru(trpy)Cl3] (100 mg, 0.23 mmol), LiCl (100 mg 2.38 mmol) and
NEt3 (0.5 mL, 0.41 mmol) were added. The mixture was heated at reflux for 8 h under N2
atmosphere. The resulting purple solution was filtered to remove any insoluble material,
and the solvent was evaporated under reduced pressure. The dark solid, thus obtained,
was dissolved in a minimum volume of acetonitrile and few drops of aqueous saturated
NaClO4 solution were added and it was kept in the refrigerator for 5 days. [Ru(trpy)(L-
Cl)Cl]ClO4 separated as a pinkish-red crystalline precipitate, which was collected by
filtration, washed with a small volume of ice-cold water, and dried in a desiccator under
vacuum. Yield: 70% (118 mg). Anal. Calcd for C29H21N5O4Cl4Ru: C: 46.64; H: 2.81; N:
9.38; found C: 46.59; H: 2.86; N: 9.32%. MS (ESI), positive mode: [M-ClO4]+, 648.0.
1H NMR (400 MHz, CD3CN):  (ppm) = 5.82 (d, J = 8.4 Hz, 2H ), 6.93 (d, J = 8.4 Hz,
2H), 7.51 (t, J = 6.4 Hz, 2H), 7.60 (d, J = 8.8 Hz, 2H), 7.83 (d, J = 5.6 Hz, 2H), 7.95-7.99
(3 H)*, 8.04 (t, J = 8.2 Hz, 2H), 8.23 (d, J = 8.0 Hz, 2H), 8.28 (d, J =8.0 Hz, 2H), 8.70
(s, 1H), 9.15 (s, 1H). IR (wave number, cm-1): 624, 638, 771, 750, 833, 1009, 1088, 1120,
1145, 1212, 1284, 1390, 1449, 1479 and 1602.
X-ray Crystallography: Single crystals of [Ru(trpy)(L-OCH3)Cl]ClO4 and [Ru(trpy)(L-
CH3)Cl]ClO4 were harvested from ethanol solutions of the respective complexes,
obtained directly from the synthetic reactions, after keeping them in the refrigerator for
24 h. Single crystals of [Ru(trpy)(L-Cl)Cl]ClO4 were obtained by slow evaporation of
solvent from an acetonitrile solution of the complex. Selected crystal data and data
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
15
collection parameters for complexes 1, 2 and 4 are deposited as Table S14 (Supporting
Information). Data on all the crystals were collected on a Bruker SMART CCD
diffractometer. X-ray data reduction, structure solution and refinement were done using
the SHELXS-97 and SHELXL-97 packages.[24] The structures were solved by the direct
methods. CCDC 2024110-2024112 contain the supplementary crystallographic data for
this paper.
General Procedure for the Catalytic Transfer Hydrogenation of Aldehydes and
Ketones. In a typical run, an oven-dried 10 mL round-bottomed flask was charged with
the aldehyde or ketone, a known mol percent of the catalyst, and a known mol percent of
NaOtBu dissolved in 1-propanol (5 mL). The flask was placed in a preheated oil bath at
the required temperature. After the specified time, the flask was removed from the oil
bath and water (20 mL) was added, and extracted with diethyl ether (3  10 mL). The
combined organic layers were washed with water (3  10 mL), dried with anhydrous
Na2SO4, and filtered. Diethyl ether was removed under vacuum and the residue obtained
was dissolved in hexane and analyzed by GC-MS.
General Procedure for the Catalytic Transfer Hydrogenation of Nitroarenes: In a
typical run, an oven-dried 10 mL round-bottomed flask was charged with the nitroarene,
a known mol percent of the catalyst, and KOtBu (20.0 mol%) dissolved in 1-propanol (6
mL). The flask was placed in a preheated oil bath at the required temperature. After the
specified time, the flask was removed from the oil bath and water (20 mL) was added,
and extracted with diethyl ether (3  10 mL). The combined organic layers were washed
with water (3  10 mL), dried with anhydrous Na2SO4, and filtered. Diethyl ether was
removed under vacuum and the residue obtained was dissolved in toluene and analyzed
by GC-MS.
Supporting Information: Crystal structures of complexes 2 and 4 (Figure S1) and
selected bond parameters (Table S1); DFT optimized structure of complex cation in 3
(Figure S2) and some computed bond parameters (Table S2); electronic absorption
spectrum of complex 1 (Figure S3); the main calculated transitions and compositions of
the molecular orbitals associated with the transitions for complexes 1 – 4 (Table S3 –
Table S10), and contour plots of the molecular orbitals (Figures S4 – S7); cyclic
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
16
voltammogram of complex 2 (Figure S8); optimization for transfer hydrogenation of
aldehyde (Table S11); variation of experimental parameters for transfer hydrogenation of
ketone (Table S12); optimization for transfer hydrogenation of nitroarene (Table S13);
crystallographic data for complexes 1, 2 and 4 (Table S14), and X-ray crystallographic
data in CIF format.
Acknowledgments
The authors thank the reviewers for their constructive comments, which have been very
helpful in preparing the revised manuscript. Financial assistance received from the
Council of Scientific and Industrial Research (CSIR), New Delhi [Sanction No.
01(2994)/19/EMR-II] and JU-RUSA 2.0 program (Ref. No.: R-11/96/19) is gratefully
acknowledged. The DST-FIST and DST-PURSE programs of the Department of
Chemistry, Jadavpur University, are also gratefully acknowledged for providing financial
and infrastructural supports. R.S. thanks the University Grants Commission (UGC), New
Delhi, for her fellowship (Ref. No.: 19/06/2016(i)EU-V). A.M. thanks the Council of
Scientific and Industrial Research (CSIR), New Delhi, for her fellowship [Grant No.:
09/096(0962)/2019-EMR-I].
Keywords: Ruthenium Complexes of 1,4-Diazabutadienes; Crystal Structures; Catalytic
Transfer Hydrogenation; Aryl Aldehydes and Ketones to Alcohols; Nitroarenes to Aryl
Amines
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10.1002/ejic.202000859
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17
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M. J. Williams, Chem. Asian J. 2010, 5, 538-542; d) M. Zhao, Z. Yu, S. Yan, Y.
Li, Tetrahedron Lett. 2009, 50, 4624-4628.
[21] a) Z-J. Yao, J-W. Zhu, N. Lin, X-C. Qiao, W. Deng, Dalton Trans. 2019, 48,
7158-7166; b) R. Nandhini, B. S. Krishnamoorthy, G. Venkatachalam, J.
Organomet. Chem. 2019, 903, 120984-120992; c) W-G Jia, H. Zhang, T. Zhang,
S. Ling, Inorg. Chem. Commun. 2016, 66, 15-18; d) W-G. Jia, H. Zhang, T.
Zhang, D. Xie, S. Ling, E-H. Sheng, Organometallics 2016, 35, 503-512; e) J.
Mondal, S. K. Kundu, W. K. H. Ng, R. Singuru, P. Borah, H. Hirao, Y. Zhao, A.
Bhaumik, Chem. Eur. J. 2015, 21, 19016-19027.
[22] B. P. Sullivan, J. M. Calvert, T. J. Meyer, Inorg. Chem. 1980, 19, 1404-1407.
[23] Chemical shifts for the NMR data are given in ppm; multiplicity of the signals
along with the associated coupling constant and number of hydrogen(s) are given
in parentheses; overlapping signals are marked with an asterisk (*).
[24] (a) G. M. Sheldrick, SHELXS-97 and SHELXL-97, Fortran programs for crystal
structure solution and refinement, University of Göttingen: Göttingen, Germany;
1997; (b) G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112-122.
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10.1002/ejic.202000859
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Table 1. Selected bond distances and bond angles for complex 1.
Bond distances (Å)
Ru1-N1
Ru1-N2
Ru1-N3
Ru1-N4
Ru1-N5
Ru1-Cl1
2.085(3)
1.999(3)
2.075(3)
1.971(3)
2.067(3)
2.3874(10)
C1-N1
C3-N1
C1-C2
C2-N2
C10-N2
1.311(5)
1.427(4)
1.411(5)
1.311(4)
1.429(4)
Bond angles (°)
N3-Ru1-N4
N1-Ru1-N2
N2-Ru1-Cl1
N1-Ru1-N4
77.94(10)
172.32(7)
174.32(10)
79.19(10)
79.14(11)
158.21(11)
N4-Ru1-N5
N3-Ru1-N5
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10.1002/ejic.202000859
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Table 2. Electronic spectral and cyclic voltammetric data of the complexes.
Complex
Electronic spectral dataa
max, nm (, M-1cm-1)
Cyclic voltammetric datac

E1/2, V (Ep, mV)
701 (500), 519 (10700),
403 (10700), 322 (20900)
706 (400), 515 (9800),
364b (8900), 321 (22600)
714 (300), 513 (8000),
361b (5600), 314(22100)
722 (300), 513 (10700),
362b (9300), 311 (23800)
1.03 (73), -0.70 (67),
-0.89 (67), -1.39 (90)
1.06 (79), -0.69 (68),
-0.87 (65), -1.32 (85)
1.08 (89), -0.64 (66),
-0.82 (68), -1.29 (126)
1.12 (77), -0.56 (66),
-0.74 (66), -1.21 (168)
1
2
3
4
a In acetonitrile solution.
b Shoulder.
c Solvent, acetonitrile; supporting electrolyte, TBHP; scan rate, 50 mVs-1;
E1/2 = 0.5 (Epa + Epc), where Epa anodic peak potential and Epc cathodic peak
potential; Ep = Epa - Epc.
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10.1002/ejic.202000859
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Table 3. Transfer hydrogenation of aldehydes.a,b
CHO
OH
Catalyst
R
R
1-Propanol, NaOtBu
85 °C, 1 h
Sn
Pn
OH
OH
OH
OH
OH
OH
H3C
H3CO
P1, 99%
P2, 95%
P3, 92%
F
Cl
Br
P4, 99%
P5, 98%
P6, 85%
OH
OCH3
OH
OH
H3CO
P7, 96%
P8, 75%
P9, 50%
OH
CHO
S10
P10, 86%
CHO
OH
OH
OH
S11
P11, 0%
OH
CHO
NH
NH
S12
P12, 0%
CHO
OH
N
N
S13
P13, 0%
a Reaction conditions: Catalyst: complex 1, 0.00002 mol%; substrate, (0.7 mmol);
base (1.0 mol%); solvent (5.0 mL).
b Product yield determined by GCMS.
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10.1002/ejic.202000859
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Table 4. Transfer hydrogenation of ketones.a,b
O
OH
R
H
R
Catalyst
1-Propanol, NaOtBu
85 °C, 1 h
R'
R'
Sn
Pn
OH
CH3
OH
CH3
OH
H
H
H
CH3
H3CO
H3C
P1, 99%
H
P2, 95%
H
P3, 91%
H
OH
CH3
OH
CH3
OH
CH3
F
Cl
Br
P4, 98%
H
P5, 86%
P6, 78%
OH
H
OH
CH3
OH
H
I
P7, 65%
P8, 65%
P9, 85%
O
OH
H
S10
O
P10, 32%
H
OH
CH3
OH
CH3
OH
S11
P11, 0%
H
O
OH
CH3
CH3
N
N
S12
P12, 0%
a Reaction conditions: Catalyst: complex 1, 0.5 mol%; substrate, (0.7 mmol);
base (1.0 mol%); solvent (5.0 mL).
b Product yield determined by GCMS.
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10.1002/ejic.202000859
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Table 5. Transfer hydrogenation of nitroarenes to corresponding amines.a,b
NO2
NH2
Catalyst
R
R
1-Propanol, KOtBu
100 °C, 12 h
Sn
Pn
NH2
NH2
NH2
Cl
CH3
P1, 99%
P2, 92%
P3, 95%
NH2
NH2
NH2
HOOC
ClOC
P4, 99%
NO2
P5, 82%
P6, 99%
NH2
O2N
O2N
H2N
S7
P7, 73%
NO2
H2N
NH2
S8
P8, 72%
O2N
NO2
Cl
O2N
NH2
Cl
S9
P9, 95%
NO2
NH2
OHC
HOH2C
S10
P10, 94%
NO2
NH2
NH2
+
S11
P11a, 53%
P11b, 47%
NO2
OH
NH2
OH
P12, 0%
P13, 0%
P14, 0%
S12
NO2
NH2
NH2
NH2
S13
NO2
NH2
H2N
H2N
S14
a Reaction conditions: Catalyst: complex 1, 5.0 mol%; substrate (0.7 mmol);
base (20.0 mol%); solvent (6.0 mL).
b Determined by GCMS.
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
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Figure 1. Crystal structure of complex 1. The perchlorate ion and hydrogen atoms are
omitted for clarity.
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
27
Ru Cl
(A)
H
HO
H
C2H5
HCl
H
R
H
O
H
HO
H
C2H5
Ru
(B)
O
H
C2H5
H
HO
H
C2H5
H
R
H
Ru
(E)
Ru
(C)
H
O
H
R
O
H
H
R
Ru
(D)
O
Scheme 2. Probable mechanism for the observed transfer hydrogenation reaction. In the
pre-catalyst (A), besides the Ru-Cl fragment no other coordinated ligand is
shown.
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
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Ru Cl
(A)
H
HO
H
C2H5
NO
HCl
H
H2O
+
O
H
C2H5
Ru
(B)
O
H
C2H5
H
HO
H
C2H5
OH
Ru
(E)
Ru
(C)
H
O
N
NO2
H
O
Ru
(D)
O
N
NO
NHOH
NH2
Ru
H
Ru
H
1-Propanol
1-Propanol
C2H5CHO
C H CHO
H2O
+
2
5
Scheme 3. Probable mechanism for the observed transfer hydrogenation reaction. In the
pre-catalyst (A), besides the Ru-Cl fragment no other coordinated ligand is
shown.
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10.1002/ejic.202000859
European Journal of Inorganic Chemistry
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for
Table of Contents
Heteroleptic 1,4-Diazabutadiene Complexes of Ruthenium: Synthesis,
Characterization and Utilization in Catalytic Transfer Hydrogenation
Rumpa Saha, Aparajita Mukherjee, and Samaresh Bhattacharya*
Graphical Abstract
Reaction of 1,4-diazabutadienes (L-R) with [Ru(trpy)Cl3] in refluxing ethanol in the
presence of triethylamine affords complexes of type [Ru(trpy)(L-R)Cl]+ (isolated as
perchlorate salts), which are found to efficiently catalyze transfer hydrogenation of aryl
aldehydes and aryl ketones to the corresponding alcohols, and nitroarenes to the
corresponding aryl amines.
Ar NH2
H
HO
R
N
N
R
H/R'
R
R = OCH3, CH3,
H and Cl
[Ru(trpy)Cl3]
H/R'
R
O
Ar NO2
Key Topic:
1,4-Diazabutadiene Complexes of Ruthenium as Transfer Hydrogenation Catalyst
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