Ruthenium complexes of phosphine-amide based ligands as efficient catalysts for transfer hydrogenation reactions

Article abstract of DOI:10.1039/d0dt04401f

This work presents three mononuclear Ru(ii) complexes of tridentate phosphine-carboxamide based ligands providing a NNP coordination environment. The octahedral Ru(ii) ion shows additional coordination with co-ligands; CO, Cl and CH₃OH. All three Ru(ii) complexes were thoroughly characterized including their crystal structures. These Ru(ii) complexes were utilized as catalysts for the transfer hydrogenation of assorted carbonyl compounds, including some challenging biologically relevant substrates, using isopropanol as the hydrogen source. The binding studies illustrated the coordination of the isopropoxide ion by replacing a Ru-ligated chloride ion followed by the generation of the Ru-H intermediate that was isolated and characterized and was found to be involved in the catalysis.

Full text of DOI:10.1039/d0dt04401f

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Ruthenium complexes of phosphine–amide
based ligands as efficient catalysts for transfer
hydrogenation reactions†
Cite this: Dalton Trans., 2021, 50,
3269
Samanta Yadav, Paranthaman Vijayan, Sunil Yadav and Rajeev Gupta
*
This work presents three mononuclear Ru(II) complexes of tridentate phosphine–carboxamide based
ligands providing a NNP coordination environment. The octahedral Ru(II) ion shows additional coordi-
nation with co-ligands; CO, Cl and CH3OH. All three Ru(II) complexes were thoroughly characterized
including their crystal structures. These Ru(II) complexes were utilized as catalysts for the transfer hydro-
genation of assorted carbonyl compounds, including some challenging biologically relevant substrates,
using isopropanol as the hydrogen source. The binding studies illustrated the coordination of the isoprop-
oxide ion by replacing a Ru-ligated chloride ion followed by the generation of the Ru–H intermediate that
was isolated and characterized and was found to be involved in the catalysis.
Received 28th December 2020,
Accepted 29th January 2021
DOI: 10.1039/d0dt04401f
rsc.li/dalton
co-workers14 have successfully developed an effective cyclopen-
tadienyl ruthenium catalyst for the reduction of ketones via
Introduction
The hydrogenation of organic compounds is one of the most TH in a concerted pathway. The notable work of Henbest and
fundamental transformations in organic chemistry.1–3 The Mitchell and co-workers15 has illustrated the use of an [Ir–H]
reduction of a multiple bond can be achieved conventionally complex for the hydrogenation of cyclohexanones and
by using hydrogen gas; however, an attractive alternative is the α,β-unsaturated ketones to their corresponding alcohols using
use of an organic compound as a hydrogen donor, the so isopropanol as the hydrogen source. Sasson and Blum16 have
called transfer hydrogenation (TH).4,5 TH is a convenient shown that [RuCl2(PPh3)3] acts as an active catalyst for the
method as it requires neither hazardous high-pressurized biphasic TH of acetophenone with isopropanol. Chowdhury
hydrogen gas nor
a
sophisticated experimental setup.6 and Bäckvall17 have concluded that the [RuCl2(PPh3)3] cata-
Furthermore, many inexpensive hydrogen donors are readily lyzed TH reaction can be accelerated 103–104 times upon
available and easy to handle.7–9 In the TH of carbonyl com- adding a strong base.
pounds, many successful catalysts are based on transition
Although many reagents such as alcohols,18 formic acid–tri-
metals as they exhibit excellent activity and selectivity both ethylamine19 and sodium formate20 have been used as H2-
under the moderate temperature and reaction conditions.10 alternative hydrogenation agents of carbonyl compounds for
The emergence of late transition-metal based catalysts, par- the synthesis of secondary alcohols, isopropanol (2-propanol)
ticularly involving metals of groups 8, 9, 10 and 11, has has been found to be the best.21 2-Propanol is not only a safe,
resulted in many noteworthy catalysts.11 In particular, cheap, non-toxic and environment friendly hydrogen donor, but
N-heterocyclic carbene (NHC) compounds of ruthenium and also a convenient solvent of choice thus eliminating the require-
rhodium have been used as successful catalysts in the TH of ment of a separate solvent.21 The presence of a strong base such
carbonyl compounds.12 Recently, Ding and co-workers13 have as NaOH, KOH or KOtBu is usually necessary for most TH reac-
shown that the Ir(III) complex of a benzothiazole-based ligand tions while using 2-propanol.22 Primary alcohols, such as
can be used for the hydrogenation of acetophenone. Shvo and methanol and ethanol, are generally not employed as hydrogen
donors because of their unfavourable redox potential.23
In this work, we report Ru(II) complexes supported with
phosphine–carboxamide based tridentate ligands also contain-
ing other co-ligands (CO, Cl and CH3OH). The chelating ligands
coordinate the Ru(II) ion as NNP donors. These Ru(II) complexes
have been utilized for the transfer hydrogenation of assorted
carbonyl compounds, including some biologically relevant sub-
strates, using isopropanol as the hydrogen source. The mechan-
Department of Chemistry, University of Delhi, Delhi – 110 007, India.
E-mail: rgupta@chemistry.du.ac.in; http://people.du.ac.in/∼rgupta/
†Electronic supplementary information (ESI) available: Figures for FTIR, UV/Vis,
NMR and mass spectra; cyclic voltammetry; spectral titrations including fittings;
gas chromatograms and tables for X-ray data collection and bonding parameters.
CCDC 1993891–1993893. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/d0dt04401f
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istic investigations including binding studies illustrated the rings are more π-electron deficient heterocycles when com-
coordination of the isopropoxide ion to the Ru(II) center before pared to a pyridine ring.27 As a result, the Ru(II) ion is more
the generation of the Ru–H intermediate that was isolated and electron-deficient in complexes 2 and 3 and such a situation
characterized and was found to participate in the TH catalysis.
resulted in the coordination of two CO groups as better co-
ligands. Such a fact is clearly demonstrated in the FTIR spectra
wherein νCO bands for 2 and 3 were observed at higher energy
when compared to those for 1. This was due to an increase in
the electron donation ability of a pyridine ring in 1 as com-
pared to that of the quinoline and isoquinoline rings in 2 and 3.
The UV-visible spectra of complexes 1–3 in DMF displayed
λmax between 360 and 430 nm which are tentatively assigned to
ligand-to-metal charge transfer (LMCT) transitions (Fig. S19,
ESI†).26 All Ru(II) complexes have been thoroughly character-
Results and discussion
Synthesis and characterization of Ru(II) complexes
This work presents three novel closely related phosphine–car-
boxamide ligands; N-(2-(diphenylphosphanyl)phenyl)pyridine-
1-carboxamide (HL1), N-(2-(diphenylphosphanyl)phenyl)iso-
quinoline-1-carboxamide (HL2) and N-(2-(diphenylphosphanyl)
phenyl) quinoline-2-carboxamide (HL3). These ligands were
synthesized by the coupling of 2-aminodiphenylphosphine
with the respective carboxylic acid in pyridine using P(OPh)3.
All three ligands were obtained in high yield and thus the
present synthetic method is not only convenient but also high-
yielding when compared to the methods reported in the litera-
1
ized using H, 13C and 31P NMR spectra (Fig. S20–S28, ESI†).
The proton NMR spectrum of complex 1 showed a signal
corresponding to the CH3 group of the coordinated methanol
at 3.5 ppm (Fig. S20, ESI†). The 13C NMR spectra of complexes
1–3 showed CuO resonances between 196.6 and 204.2 ppm
which are comparable to those of other Ru(II) carbonyl com-
plexes (Fig. S23–S25, ESI†).26 The 31P NMR spectra of com-
plexes 1–3 displayed one sharp singlet between 49.8–50.5 ppm
for the phosphine group (Fig. S26–S28, ESI†). Such 31P signals
are considerably downfield shifted when compared to those of
the ligands (−19.21, −17.96 and −18.58 ppm for HL1, HL2 and
HL3, respectively) asserting the involvement of the phosphine
group in the coordination to the Ru(II) center.26 The ESI+ mass
spectra of complexes 1–3 showed the most abundant peak
assigned to the [M + H+]+ species (Fig. S29–S34, ESI†). In all
cases, the observed isotopic distribution patterns matched per-
fectly the simulation patterns.
1
ture.24 These ligands were characterized by FTIR, H, 13C and
31P NMR spectra as well as ESI+ mass spectra (Fig. S1–S15,
ESI†). The Ru(II) complexes 1–3 were respectively synthesized
by refluxing ligands HL1–HL3 with [Ru(CO)2(Cl)2]n in MeOH
(Scheme 1). In the resultant Ru(II) complexes, all three ligands
acted as tridentate ones and coordinated via anionic Namide
neutral Pdiphenylphosphine and neutral Npyridine/Nquinoline
isoquinoline sites via two five-membered chelate rings.
All three Ru(II) complexes were non-electrolytic in nature as
,
/
N
confirmed by their molar conductivity.25a The FTIR spectra of
the Ru(II) complexes (Fig. S16–S18, ESI†) showed the dis-
appearance of the N–H stretches (at 3271, 3226 and 3235 cm−1
for HL1, HL2 and HL3, respectively) and bathochromically
shifted CvOamide bands (1615–1627 cm−1) when compared to
those of the corresponding ligands (1678–1688 cm−1).25b,26
Both these features asserted the involvement of the deproto-
nated form of the amide group (Namidate) in the resulting Ru(II)
complexes.25b,26 The FTIR spectrum of complex 1 showed a
single peak for the terminally coordinated carbonyl group at
1939 cm−1 whereas complexes 2 and 3 exhibited two stretches
at 1984/2049 and 1974/2043 cm−1, respectively, due to the pres-
ence of two CO groups.25b,26 Both quinoline and isoquinoline
Crystal structures
The solid-state molecular structures of all three Ru(II) com-
plexes were determined by single crystal X-ray diffraction ana-
lyses (Fig. 1 and Tables S1 and S2, ESI†). The crystal structures
displayed the presence of a tridentate chelating ligand coordi-
nating in a NNP fashion in addition to assorted co-ligands (Cl,
CO and CH3OH). Notably, both complexes 2 and 3 showed a
very similar coordination environment around the Ru(II) center
with a NNP ligand, one chloride anion and two CO ligands. In
contrast, complex 1 showed a NNP ligand in addition to a
chloride anion and
a methanol molecule (Ru–OMeOH =
2.183 Å). For all three complexes, the Ru–Cl bond distance
Fig. 1 Thermal ellipsoidal representations (with 50% probability) of
ruthenium complexes 1–3. Only methanol hydrogen atoms (in pink
colour) are shown in complex 1 for clarity.
Scheme 1 Ligands HL1, HL2, and HL3 and their Ru(II) complexes 1–3
discussed in this work.
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varied between 2.408 and 2.422 Å.26 The crystal structures
revealed that the Ru(II) ion is present in a nearly octahedral
environment in all three complexes.26 Despite the anionic σ-
donation from the deprotonated Namidate group to the Ru(II)
ion, the Ru–Namidate bond distances (2.068, 2.073 and 2.088 Å
for complexes 1–3, respectively) were comparable to the Ru–
Table 1 Control and optimization experiments for transfer hydrogen-
ation reactions using acetophenone as a model substratea
Entry
Catalyst
Base
Solvent
Yieldb (%)
N
pyridine/Nquinoline/Nisoquinoline
bond
distances
(2.068–2.088 Å).28 These complexes showed Ru–CO bond dis-
tances between 1.811 and 1.895 Å.26,29 As expected, two phenyl
rings of the diphenylphosphine moiety were located above and
below the basal plane maintaining a tetrahedral geometry
around the phosphorus atom.30
All three Ru(II) complexes exhibited several notable features
that suggested their potential applications in catalysis. All
three complexes displayed the presence of labile co-ligands,
chloride anions, in all three cases in addition to a methanol in
complex 1. Such a fact suggests that a suitable substrate and/
or reagent may replace such labile ligand(s) creating interest-
ing catalysis opportunities.31 In all three complexes, NNP-
based coordination constituted one anionic and two neutral
1
2
3
4
5
6
7
8
—
KOH
KOH
KOH
—
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
MeOH
EtOH
DMF
DMSO
THF
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
0
5
4
[Ru(CO)2Cl2]n
RuH(CO)Cl(PPh3)3
2
2
2
2
2
2
2
2
2
2
1
1
3
3
2
8
Et3N
tBuOK
NaOH
KOH
KOH
KOH
KOH
KOH
KOH
NaOH
KOH
NaOH
KOH
KOH
20
35
54
94
12
20
0
0
0
42
91
48
90
85
9
10
11
12
13
14
15
16
17
18c
donors. Such
a situation provides a balanced electron
donation at the ruthenium center and is likely to stabilize vari-
able oxidation states of the ruthenium ion (cf.
electrochemistry).26,32 The presence of the diphenylphosphine
group creates steric hindrance that would allow a substrate to
only approach from the opposite side. The presence of pyri-
dine, quinoline and isoquinoline rings is likely to subtly alter
a Conditions: Catalyst, 0.02 mmol (1 mol%); acetophenone,
2.00 mmol; KOH, 1.00 mmol; i-PrOH, 5 mL; temperature, 80 °C; time,
6 h. b Determined by gas chromatography. c 0.5 mol% catalyst loading
with a reaction time of 12 h.
the electron density at the ruthenium center and may also resulted in a miniscule TH product (entry 4). Entries 5–7 show
t
have some effects on the catalysis.
that other bases (Et3N, BuOK and NaOH) were only moder-
ately successful. Satisfyingly, complex 2 in the presence of
KOH as a base and isopropanol as both the solvent and hydro-
Electrochemistry
All three complexes were subjected to cyclic voltammetric (CV) gen donor provided 94% of the TH product, 1-phenylethanol
studies in CH3OH (Fig. S35, ESI†). Complexes 1 and 2 exhibi- (entry 8). In comparison, both MeOH and EtOH had limited
ted an irreversible Ru3+/Ru2+ redox couple with Epa of 1.12 and success (entries 9 and 10), whereas, as expected, other solvents
1.08 V versus the Fc+/Fc couple, respectively.26,32 Complex 3, on were completely ineffective (entries 11–13).23 Entries 14–17
the other hand, showed a nearly reversible response with the display that complexes 1 and 3 were also effective TH catalysts
E12 value of 1.32 V (ΔEp = 90 mV). All three complexes also dis- with KOH as a base, although their catalytic efficiency was
played highly negative but reversible to quasi-reversible lower when compared to that of complex 2. When the catalyst
responses for the Ru2+/Ru+ couple with E12 values of −1.72 V loading of complex 2 was reduced to half, 85% product for-
(ΔEp = 180 mV), −1.48 V (ΔEp = 80 mV) and −1.61 V (ΔEp
=
mation was observed in 12 h (entry 18), compared to that
100 mV), respectively.26,32 The CV results suggest that the obtained in 6 h with a higher catalyst loading of 1 mol% (entry
phosphine–carboxamide ligands have considerably stabilized 8). In order to rule out the involvement of [Ru] nanoparticles
the Ru(II) state.26,32 The irreversible and/or quasi-reversible in catalysis, the mercury drop-test was carried out.33 The pres-
nature of the Ru3+/Ru2+ redox couple suggests considerable ence of one drop of Hg did not alter the outcome of the TH of
structural changes during the process of electron transfer.
acetophenone, producing 1-phenylethanol in 93% yield (data
not shown in Table 1).33 As complex 2 was a better catalyst, it
was used to explore the scope of TH catalysis with a variety of
Transfer hydrogenation
Subsequently, all three Ru(II) complexes were utilized for the carbonyl substrates (Table 2).
TH of assorted carbonyl compounds.4–6 Initially, reaction con-
As can be seen from Table 2, both the electron-withdrawing
ditions were optimized by varying the solvent, base, catalyst and electron-donating groups on the phenyl ring of a carbonyl
and catalyst loading (Table 1). As can be seen from entry 1, the compound were tolerated in the aforementioned TH catalysis,
presence of a catalyst is critical in driving the TH reaction as producing the corresponding alcohols as the products in excel-
its absence did not produce any product. Entries 2 and 3 show lent yields (entries 1–13).4,34c,d However, it was noted that the
that Ru(II) precursors, such as [Ru(CO)2Cl2]n and [RuH(CO)Cl substrates containing electron-withdrawing groups underwent
(PPh3)3], were largely ineffective as catalysts in promoting TH. faster reduction as compared to the ones having electron-
The presence of a suitable base is essential as its absence donating groups.34 This can be explained by correlating the
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Table 2 Substrate scope in the catalytic transfer hydrogenation reac-
tion of assorted carbonyl compounds using complex 2 as a catalysta
Entry
1
Substrate
Product
Yieldb (%)
94
2
3
4
5
92
88
85
85
Scheme 2 Transfer hydrogenation of some biologically relevant sub-
strates. The yield was determined by gas chromatography (Fig. S66–S69,
ESI†), whereas the products were characterized by 1H NMR spec-
troscopy (Fig. S70–S73, ESI†).
6
7
89
95
ditions, however, with 3 mol% catalyst loading. Interestingly,
the TH of coumarin resulted in complete conversion but pro-
duced both 2H-chromen-2-ol (ca. 60%) and chroman-2-ol (ca.
40%) (Fig. S70, ESI†).36 Notably, menthone was quantitatively
reduced (ca. 98%) to menthol with a 53 : 47 diastereomeric
ratio as determined using the 1H NMR spectrum (Fig. S71,
ESI†).35 Both camphor (in 77 : 23 diastereomeric ratio;
Fig. S72, ESI†) and 1-camphor sulphonic acid (in 67 : 33 dia-
stereomeric ratio; Fig. S73, ESI†) were reduced to their corres-
ponding alcohol products in decent yields (ca. 60–65%).34c The
success of these substrates substantiated the practical utility of
the present catalysts in promoting challenging TH reactions.
8
88
89
87
83
9
10
11
12
13
85
80
Substrate binding and mechanistic studies
Based on the literature and the present catalysis results, a ten-
tative mechanism for TH is proposed and presented in
Scheme 3.37 The first step involves base-assisted displacement
of the ligated chloride ion by the isopropoxide ion (−OiPr).37
The [Ru–OiPr] species then generates the ruthenium co-
ordinated hydride intermediate, [Ru–H], which is responsible
for the hydrogenation of the carbonyl substrate.38–40 In order
to prove the proposed mechanism, it was essential to sub-
stantiate the generation of [Ru–OiPr] and [Ru–H] species.41
Additionally, the possible labile nature of the co-ligand, the
chloride ion, in the Ru(II) complex is equally significant.
Therefore, the potential replacement of the chloride ion by the
isopropoxide ion was investigated by taking complexes 2 and 3
14
15
87
90
a Conditions: Substrate, 2.00 mmol; catalyst 2, 0.02 mmol; KOH,
1.00 mmol; PrOH, 5 mL; temperature, 80 °C, time, 6 h. b Determined
i
by gas chromatography (Fig. S36–S50, ESI†) and characterized by 1H
NMR spectroscopy (Fig. S51–S65, ESI†).
ease in hydride transfer from the in situ generated [Ru–H] as the representative examples. The case of complex 2 is
intermediate to the carbonyl substrate as the electrophilicity of explained here. Thus, when complex 2 was titrated with potass-
the carbonyl group is increased by the presence of an electron- ium isopropoxide in DMF,42 a clear transformation resulted as
withdrawing substituent.4–6,34 Entries 14 and 15 show that shown in Fig. 2. Prominent spectral changes were noted at
cyclic ketones can be conveniently converted to their respective 386, 454 and 536 nm for 2 as a function of the concentration
alcohols in high yield.
We then focussed on the TH of a few biologically relevant at ca. 440 nm, suggesting a neat transformation. Importantly,
substrates (Scheme 2) under the optimized reaction con- Job’s plot unambiguously justified
1 : 1 stoichiometry
of potassium isopropoxide. An isosbestic point was observed
a
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complex 3 and similar observations were noted including the
binding constant of 3.38 × 103 M−1 and a 1 : 1 stoichiometry
(Fig. S74, ESI†). In addition, binding studies confirmed that
the ligated –Cl ion has been replaced by the isopropoxide ion
on the ruthenium center.
The next challenge was to ascertain the generation of the
[Ru–H] species.38–40 For such a purpose, complexes 2 and 3
were selected. Thus, when 2 and 3 were refluxed with potass-
ium isopropoxide in CD3OD, brown-red to red products were
precipitated that were isolated. These products were respect-
ively found to be the corresponding Ru–H complexes 4 and 5
(Scheme 4). The proton NMR spectra of 4 and 5 showed the
presence of metal-bound hydride as the doublet at −9.48 (2JHP
:
25 Hz) and −10.00 ppm (2JHP: 21 Hz), respectively (Fig. S75 and
S76, ESI†).37 Similarly, the 13C NMR spectra of 4 and 5 were
somewhat similar to those of their precursors 2 and 3
although prominent shifts were noted for the ligated CO mole-
cules (Fig. S77 and S78, ESI†). The FTIR spectra of 4 and 5
illustrated the hydride stretches at 2039 and 2020 cm−1
,
respectively, whereas most of the other stretches were quite
similar to those of complexes 2 and 3 (Fig. S79 and S80, ESI†).
Both complexes 4 and 5 displayed absorption maxima at
430–440 nm (Fig. S81, ESI†). Collectively, these results indi-
cated the formation of Ru–H complexes from the in situ gener-
ated [Ru–OiPr] species.
Scheme 3 Proposed catalytic cycle for the transfer hydrogenation
reaction taking complex 2 as a representative example.
With isolated Ru–H complexes 4 and 5 in hand, the TH of
acetophenone was attempted. Importantly, both 4 and 5 were
greatly successful in the TH of acetophenone producing 1-phe-
nylethanol in quantitative yield (>99%) in 12 h. More impor-
tantly, such TH reactions were accomplished without the
requirement of any base, as anticipated for the Ru–H
species.37d–f These experiments unambiguously asserted that
the Ru–H species is involved in the TH of the carbonyl sub-
strate as illustrated in Scheme 3.
The isolated Ru–H complexes 4 and 5 further provided the
opportunity to explore the relationship between different elec-
tronic substituents present on a substrate and the rate con-
stant as well as the reaction yield (Fig. 3). In these experiments,
the following para-substituents on the phenyl ring of acetophe-
none were utilized: H, CH3, Cl and NO2. Importantly, the
Fig. 2 UV-Vis spectral titration of complex 2 with potassium isopropox-
ide. Inset a: Job’s plot showing a 1 : 1 binding stoichiometry between 2
and the isopropoxide ion. Inset b: Linear regression fitting curve for a
1 : 1 binding between 2 and the isopropoxide ion at λ = 386 nm. Inset c:
Temperature dependent binding of the isopropoxide ion with 2. All
studies were performed in DMF.
pseudo first order rate constant ([K × 10−3 min−1 46
of 7.11
]
(–CH3), 7.62 (–H), 8.73 (–Cl) and 9.97 (–NO2)) varied linearly
with respect to the Hammett constants (σ) yielding a positive
slope (blue triangles). Such a point supported the fact that the
substrates with electron-withdrawing groups favored TH more
between complex 2 and the isopropoxide ion (inset a).43 Such than the substrates with electron-donating groups.34,37d In
a fact was further supported by the linear regression fitting for
a 1 : 1 stoichiometry between the Ru(II) complex and the −OiPr
ion (inset b).43 The binding constant (Kb) was found to be 3.65
× 103 M−1 44
.
Further evidence was obtained from the tempera-
ture-dependent binding of the isopropoxide ion with complex
2 (inset c). As can be seen, the binding constant (Kb × 103 M−1
)
was found to increase with an increase in temperature in the
following order: 1.69 (20 °C), 3.38 (40 °C), 4.46 (60 °C) and
8.35 (80 °C), showing a nearly linear relationship.45 The
Scheme 4 Synthesis of Ru–H complexes 4 and 5 from complexes 2
binding of the isopropoxide ion was also investigated with and 3.
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Fig. 4 Selected part of the 31P NMR spectrum of complex 4 in DMSO-
d6 showing 31P peaks (a) at 69 ppm for pristine complex 4 and in the
presence of an equimolar amount of acetophenone after heating at
80 °C for (b) 1 h and (c) 6 h.
Fig. 3 Hammett plot illustrating the electronic effect of the substitu-
ents (H, CH3, Cl and NO2) at the para position of acetophenone on the
rate constant for a pseudo first order reaction (blue triangles) and on the
product yield for a 12 h TH reaction (black squares).
Synthesis of ligands
N-(2-(Diphenylphosphino)phenyl)pyridine-1-carboxamide (HL1).
2-Aminodiphenylphosphine (1.00 g, 3.60 mmol) and 2-picoli-
nic acid (0.443 g, 3.61 mmol) were taken in 5 ml of pyridine
and stirred at 100 °C followed by the addition of triphenylpho-
sphite (1.34 g, 4.30 mmol). After stirring the reaction mixture
for 12 h at 100 °C, the solvent was removed under reduced
pressure. The crude oily product thus obtained was washed
several times with cold water. The addition of diethyl ether
afforded an off-white product. Yield: 1.28 g (92%). Anal. calcd
for C24H19N2OP (382.39): C 75.38, H 5.01, N 7.33; found C
75.10, H 5.12, N 7.13. FTIR spectrum (Zn–Se ATR, cm−1): 3386
(O–H), 3271 (N–H), 3054 (C–H), 1688 (CvOasym), 1527
(CvOsym), 1431 (CvC), 694 (C–Hbending). 1H NMR spectrum
(400 MHz, CDCl3, 25 °C): δ (ppm) 10.85 (s), 8.54 (d, J = 4.8 Hz),
8.46 (dd, J = 8.2, 4.6 Hz), 8.18 (d, J = 6.9 Hz), 7.81 (t, J = 7.7 Hz),
7.37 (ddd, J = 12.8, 8.1, 4.1 Hz), 7.07 (t, J = 7.4 Hz), 6.98–6.90
(m). 13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ (ppm)
162.20, 150.04, 148.14, 141.03, 140.92, 137.46, 134.91, 134.16,
133.97, 133.87, 132.28, 130.22, 129.21, 128.77, 128.70 127.47,
126.34, 124.74, 122.31 and 121.52. 31P NMR spectrum
(160 MHz, CDCl3, 25 °C): δ (ppm) −19.23. MS spectrum (ESI+):
m/z calcd for [C24H20N2OP]+ 383.40, found 383.13.
N-(2-(Diphenylphosphino)phenyl)isoquinoline-3-carboxamide
(HL2). A similar method (as for HL1) was adopted with the fol-
lowing reagents: 2-aminodiphenylphosphine (1.00 g,
3.60 mmol), isoquinoline-1-carboxylic acid (0.62 g, 3.60 mmol)
and triphenylphosphite (1.10 g, 3.60 mmol). The isolated
crude product was dissolved in acetone and stored at 0 °C to
afford a pale pink crystalline product. Yield: 1.35 g (86%).
Anal. calcd for C28H21N2OP (432.45): C 77.77, H 4.89, N 6.48;
found C 77.83, H 5.00, N 6.61. FTIR spectrum (Zn–Se ATR,
cm−1): 3226 (N–H), 3046 (C–H), 1678 (CvOasym), 1578
(CvOsym), 1427 (CvC), 695 (C–Hbending). 1H NMR spectrum
(400 MHz, CDCl3, 25 °C): δ (ppm) 10.99 (s), 9.58 (d, J = 8.4 Hz),
8.42 (d, J = 5.5 Hz), 7.85 (dd, J = 21.1, 6.4 Hz), 7.72–7.61 (m),
7.46 (t, J = 7.1 Hz), 7.40–7.26 (m), 7.09 (t, J = 7.4 Hz), 7.00–6.94
fact, the ρ value from the Hammett plot was not only positive
but also quite large (+3.05) and supported the same.47 The
large positive ρ value further suggested the formation of a
negatively charged transition state during the reaction which
is better stabilized by the electron-withdrawing substituents.
Similar large ρ values of +4.7 and +3.1 have been reported in
the literature for the NaBH4 mediated reduction of substituted
benzaldehydes48a and acetophenones,48b respectively, implying
the more anionic nature of the transition state.
Further evidence was obtained from a similar plot but invol-
ving yields (92% (–CH3), 95% (–H), 96% (–Cl) and 99%
(–NO2))49 of the TH reaction and the Hammett constants (σ)
for the four substrates (Fig. S82–S85, ESI†). Such a plot further
showed a linear relationship, therefore supporting the rate
constant experiment.
To support the detachment of the phosphine group as
shown in Scheme 3, the time-dependent 31P NMR spectra of
complex 4 in the presence of a substrate, acetophenone, were
studied (Fig. 4). The original 31P signal for pristine 4, at
69 ppm (trace a), slowly disappeared with time, whereas two
new resonances were observed at ca. 49 and ca. 28 ppm corres-
ponding to the partially and fully detached phosphine groups
of the ligand (traces b and c). This experiment therefore
asserted the involvement of the inner-sphere reaction
mechanism37d–f as was also inferred from the Hammett
studies.
Experimental section
Chemicals and reagents
All chemicals and reagents were obtained from commercial
sources and were used without further purification.
2-Aminodiphenylphosphine was synthesized using the
reported method.50 [Ru(CO)2(Cl)2]n was synthesized as per the
literature method.51
3274 | Dalton Trans., 2021, 50, 3269–3279
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Paper
(m). 13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ (ppm) 0.1101 mmol). Yield: 0.052
g
(75%) Anal. calcd for
162.29, 149.66, 146.32, 141.00, 140.81, 137.31, 135.02, 134.94, C30H20N2O3PClRu (623.99): C 57.75, H 3.23, N 4.49; found: C
134.19, 133.99, 133.84, 130.25, 130.19, 130.06, 129.42, 129.25, 57.56, H 3.12, N 4.21. FTIR spectrum (Zn–Se ATR, cm−1): 3048
128.86, 128.78, 128.17, 127.72, 124.78, 121.56 and 118.72. 31P (νC–H), 2049 (νCO), 1984 (νCO), 1627 (νCvO). UV/Vis spectrum
NMR spectrum (160 MHz, CDCl3, 25 °C) δ (ppm) −17.96. MS (DMF, λmax (ε, mol−1 cm−1)): 430. 1H NMR spectrum (400 MHz,
spectrum (ESI+): m/z calcd for [C28H22N2OP]+ 433.14, found CDCl3, 25 °C): δ (ppm) 10.36–10.26 (d), 9.66–9.56 (dd), 8.55
433.02.
(dd, J = 6.1, 2.8 Hz), 7.97–7.77 (m), 7.64–7.54 (m), 7.51–7.42
N-(2-(Diphenylphosphino)phenyl)quinoline-3-carboxamide (m), 7.35 (ddd, J = 11.3, 7.7, 1.5 Hz), 7.20–7.15 (m). 13C NMR
(HL3). A similar method (as for HL1) was adopted with the fol- spectrum (100 MHz, CDCl3, 25 °C): δ 195.52, 190.75, 172.26,
lowing reagents: 2-aminodiphenylphosphine (1.00 g, 156.33, 155.47, 144.54, 137.82, 133.16, 132.44, 131.86, 131.42,
3.60 mmol), quinoline-2-carboxylic acid (0.62 g, 3.60 mmol) 130.99, 130.11, 129.70, 129.08, 128.66, 128.19, 127.19, 124.69,
and triphenylphosphite (1.10 g, 3.60 mmol). The isolated 123.41, 122.95 and 122.22. 31P NMR spectrum (160 MHz,
crude product was dissolved in acetone and stored at 0 °C to CDCl3, 25 °C): δ (ppm) 47.89. MS spectrum (ESI+, CH3OH): m/z
afford an orange crystalline product. Yield: 1.35 g (87%). Anal. calcd for [C30H20N2O3PClRu + H+] 625.
calcd for C28H21N2OP (432.45): C 77.77, H 4.89, N 6.48; found
[Ru(L3)(CO)2(Cl)] (3). A similar method as discussed for
C 77.80, H 5.09, N 6.69. FTIR spectrum (Zn–Se ATR, cm−1): complex 1 was followed using the following reagents: ligand
3355 (O–H), 3235 (N–H), 3047 (C–H), 1679 (CvOasym), 1569 HL3 (0.047 g, 0.1101 mmol) and [Ru(CO)2(Cl)2]n (0.025 g,
(CvOsym), 1433 (CvC), 694 (C–Hbending). 1H NMR spectrum 0.1101 mmol). Yield: 0.052
g (75%) Anal. calcd for
(400 MHz, CDCl3, 25 °C): δ (ppm) 11.06 (s), 8.50 (dd, J = 8.2, C30H20N2O3PClRu (623.99): C 57.75, H 3.23, N 4.49; found: C
4.6 Hz), 8.05 (d, J = 8.4 Hz), 7.85 (d, J = 7.4 Hz), 7.76 (t, J = 7.7 57.66, H 3.41, N 4.23. FTIR spectrum (Zn–Se ATR, cm−1): 3046
Hz), 7.61 (t, J = 7.5 Hz), 7.42–7.24 (m), 7.09 (t, J = 7.5 Hz), (νCH), 2043 (νCO), 1974 (νCO), 1650 (νCvO). UV/Vis spectrum
7.00–6.97 (m). 13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ (DMF, λmax (ε, mol−1 cm−1)): 401. 1H NMR spectrum (400 MHz,
(ppm) 163.75, 140.11, 137.61, 134.96, 134.87, 134.33, 134.21, CDCl3, 25 °C) δ (ppm) 9.56 (dd, J = 8.5, 5.5 Hz), 8.53 (ddd, J =
134.02, 133.74, 130.56, 130.10, 129.22, 128.84, 128.79, 128.71, 18.3, 15.0, 8.6 Hz), 8.05–7.90 (m), 7.72 (ddd, J = 8.0, 7.1, 0.9
128.57, 127.77, 127.77, 127.27, 126.97, 124.78, 124.73, and Hz), 7.61 (dt, J = 14.1, 4.3 Hz), 7.56–7.42 (m), 7.39–7.29 (m),
121.91. 31P NMR spectrum (160 MHz, CDCl3, 25 °C) δ (ppm) 7.22–7.14 (m). 13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ
−18.58. MS spectrum (ESI+): m/z calcd for [C28H22N2OP]+ 197.11, 189.95, 169.02, 160.41, 156.04, 145.97, 140.44, 134.81,
433.14, found 433.02.
134.62, 133.58, 133.42, 132.16, 131.67, 131.26, 129.20, 129.15,
128.76, 128.66, 128.16, 125.00, 123.64, 122.00. 31P NMR spec-
trum (160 MHz, CDCl3, 25 °C): δ (ppm) 49.63. MS spectrum
Synthesis of Ru(II) complexes
[Ru(L1)(CO)(Cl)(CH3OH)] (1). Ligand HL1 (0.084 g, (ESI+, CH3OH): m/z calcd for [C30H20N2O3PClRu + H+] 625.
0.22 mmol) was dissolved in methanol (15 mL) followed by the
[Ru(L2)(CO)2(H)] (4). Complex 2 (0.050 g, 0.08 mmol) was
addition of [Ru(CO)2(Cl)2]n (0.050 g, 0.22 mmol) and the reac- treated with KOiPr (0.063 g, 0.91 mmol) in CD3OD (1 mL) and
tion mixture was refluxed for 12 h. After that the solvent was refluxed at 70 °C for about an hour. A reddish-brown precipi-
removed under reduced pressure. The yellow crude product tate was obtained which was isolated by filtration and dried
was isolated after washing thrice with diethyl ether. The crude under vacuum. Yield 0.020
g (65%). Anal. calcd for
product was recrystallized from methanol after layering with C30H21N2O3PRu (589.55): C 57.75, H 3.23, N 4.49; found: C
diethyl ether that produced the crystalline product within 2–3 57.66, H 3.41, N 4.23. FTIR spectrum (Zn–Se ATR, cm−1): 2039
days. Yield: 0.105 g (83%). Anal. calcd for C26H22N2O3PClRu (νRu–H), 1927 (νCO), 1910 (νCO), 1610 (νCvO). UV/Vis spectrum
(577.97): C 54.03, H 3.84, N 4.85; found: 54.20, H 3.65, N 4.71; (DMF, λmax (ε, mol−1 cm−1)): 442. 1H NMR spectrum (400 MHz,
FTIR spectrum (Zn–Se ATR, cm−1): 3390 (O–HMeOH), 3058 DMSO, 25 °C): δ (ppm) 10.17 (d, J = 8.7 Hz, 1H), 9.42 (dd, J =
(νC–H), 1939 (νCO), 1620 (νCvO ), 1460 (νCvO ), 1428 (νC–H, 8.4, 4.8 Hz, 1H), 8.77 (d, J = 6.0 Hz, 1H), 8.13 (t, J = 7.1 Hz, 2H),
asym
sym
1
bending). UV/Vis spectrum (DMF, λmax (ε, mol−1 cm−1)): 365. H 8.00–7.88 (m, 3H), 7.85 (d, J = 8.4 Hz, 1H), 7.61–7.39 (m, 10H),
NMR spectrum (400 MHz, CDCl3, 25 °C): δ (ppm) 9.57 (dd, J = 7.16 (t, J = 7.3 Hz, 1H), −9.48 (d, J = 25.0 Hz, 1H). 13C NMR
8.6, 5.4 Hz), 8.65 (s), 8.34 (d, J = 7.9 Hz), 8.03 (t, J = 7.8 Hz), spectrum (100 MHz, DMSO, 25 °C): δ 211.32, 206.17, 169.39,
7.92 (dd, J = 12.2, 6.9 Hz), 7.55 (ddd, J = 21.5, 11.5, 4.9 Hz), 159.46, 156.63, 145.88, 139.85, 137.82, 133.64, 133.55, 133.33,
7.47–7.42 (m), 7.32 (dd, J = 10.5, 8.5 Hz), 7.16 (t, J = 7.4 Hz), 131.54, 130.52, 130.12, 129.70, 129.37, 128.99, 128.82, 128.68,
3.46 (s). 13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ 207.11, 125.79, 123.31, 122.49.
167.48, 152.39, 137.56, 136.02, 130.67, 130.57, 129.10, 128.91,
[Ru(L3)(CO)2(H)] (5). A similar method on an identical scale,
127.95, 127.77, 127.62, 126.06, 121.73, 120.91, 119.05, 87.21 as discussed for complex 4, was followed using complex 3. A
and 49.44. 31P NMR spectrum (160 MHz, CDCl3, 25 °C): δ red coloured product was isolated. Yield 0.026 g (74%). Anal.
(ppm) 46.17. MS spectrum (ESI+, CH3OH): m/z calcd for calcd for C30H21N2O3PRu (589.55): C 57.75, H 3.23, N 4.49;
[C25H19N2O3PClRu]+ 546.99, found 546.9924.
found: C 58.06, H 3.15, N 4.55. FTIR spectrum (Zn–Se ATR,
[Ru(L2)(CO)2(Cl)] (2). A similar method as mentioned for cm−1) 2020 (νRu–H), 1910 (νCO), 1906 (νCO), 1615 (νCvO). UV/Vis
complex 1 was adopted using the following reagents: ligand spectrum (DMF, λmax (ε, mol−1 cm−1)): 430. H NMR spectrum
1
HL2 (0.047 g, 0.110 mmol) and [Ru(CO)2(Cl)2]n (0.025 g, (400 MHz, DMSO, 25 °C): δ (ppm) 9.41 (dd, J = 8.3, 4.7 Hz,
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Dalton Transactions
1H), 8.78 (t, J = 9.3 Hz, 2H), 8.36 (d, J = 8.4 Hz, 1H), 8.21 (d, J =
(2-Amino-5-chlorophenyl)(phenyl)methanol. 1H NMR spec-
8.1 Hz, 1H), 8.06 (t, J = 7.7 Hz, 1H), 7.98–7.92 (m, 2H), 7.84 (t, trum (400 MHz, CDCl3, 25 °C): δ (ppm) 7.34 (m, J = 1.7, 5.2 Hz,
J = 7.4 Hz, 1H), 7.57–7.46 (m, 10H), 7.17 (t, J = 7.3 Hz, 1H), 8H), 5.80 (s, 1H), 3.48 (s, 1H).
−10.00 (d, J = 21 Hz, 1H). 13C NMR spectrum (100 MHz,
(2-Aminophenyl)(phenyl)methanol. 1H NMR spectrum
DMSO, 25 °C): δ 210.76, 204.52, 168.72, 160.04, 155.63, 147.02, (400 MHz, CDCl3, 25 °C): δ (ppm) 8.18 (d, J = 8.3 Hz, 1H), 7.87
140.11, 135.92, 135.57, 134.19, 133.16, 132.25, 131.90, 130.25, (d, J = 8.4 Hz, 1H), 7.72 (d, J = 15.4 Hz, 1H), 7.56–7.42 (m, 6H),
129.48, 129.09, 126.04, 123.85, 123.26, 122.61.
7.27 (s, 1H).
Bis(4-methoxyphenyl)methanol.
(400 MHz, CDCl3, 25 °C): δ (ppm) 7.27 (d, J = 9.0 Hz), 6.85 (d,
J = 9.0 Hz), 5.76 (s), 3.78 (s).
Cyclohexanol. H NMR spectrum (400 MHz, CDCl3, 25 °C):
1H
NMR
spectrum
General procedure for the transfer hydrogenation of carbonyl
substrates
1
A reaction mixture of 1 mol% catalyst, carbonyl substrate
(2 mmol) and KOH (1 mmol) in isopropyl alcohol (5 mL) was δ (ppm) 3.61 (dd, J = 8.7, 4.5 Hz, 1H), 1.89 (d, J = 5.5 Hz, 2H),
stirred at 80 °C for 6 h. After cooling, the mixture was diluted 1.79–1.72 (m, 2H), 1.55 (d, J = 11.7 Hz, 1H), 1.27 (d, J = 8.5 Hz,
with water and then extracted with ethyl acetate. The organic 5H).
1
layer was separated, washed with aqueous brine and dried over
Cycloheptanol. H NMR spectrum (400 MHz, CDCl3, 25 °C):
anhyd. Na2SO4. The removal of the solvent under reduced δ (ppm) 3.94–3.72 (m, 1H), 1.81 (d, J = 11.0 Hz, 2H), 1.67 (dd, J
pressure afforded an organic product that was purified by = 14.7, 6.3 Hz, 3H), 1.55–1.44 (m, 7H).
column chromatography on neutral alumina using 5% ethyl
2H-Chromen-2-ol and chroman-2-ol. 1H NMR (400 MHz,
acetate/hexanes solution. The organic products were identified CDCl3): δ (ppm) 12.41 (s), 9.35 (s), 8.72 (d, J = 8.4 Hz), 8.63 (s),
by gas chromatography (GC). A calibration plot was studied for 7.82 (s), 7.78 (d, J = 5.5 Hz), 7.67 (d, J = 5.5 Hz), 7.47 (d, J = 7.5
a mixture of 4-nitro acetophenone (substrate) and 1-(4-nitro- Hz), 7.41 (d, J = 7.6 Hz), 7.08 (s), 6.98 (s), 5.82 (d, J = 10.2 Hz),
phenyl)ethan-1-ol (product) and is presented in Fig. S86 5.78 (d, J = 10.2 Hz), 5.06 (s), 5.00 (s), 4.96 (s), 4.91 (d, J = 8.2
(ESI†).
Hz), 4.30 (d, J = 6.7 Hz), 4.06 (d, J = 6.8 Hz), 2.88–2.81 (m), 2.58
(dd, J = 14.1, 5.5 Hz).
NMR spectral characterization data for TH organic products
Menthol. 1H NMR (400 MHz, CDCl3): NMR (400 MHz,) δ
1-Phenylethan-1-ol. 1H NMR spectrum (400 MHz, CDCl3, (ppm) 3.44–3.40 (m), 3.40–3.35 (m), 2.20–2.15 (m), 2.14 (dd, J =
25 °C): δ (ppm) 7.34 (d, 3H), 7.27 (m, 1H), 4.89 (q, J = 6.5 Hz, 7.1, 3.0 Hz), 1.99–1.95 (m), 1.96–1.91 (m), 1.67 (dd, J = 5.6, 3.0
1H), 1.49 (d, J = 6.5 Hz, 3H).
Hz), 1.62 (dd, J = 6.7, 3.6 Hz), 1.59 (dd, J = 8.1, 4.8 Hz), 1.58
Diphenylmethanol. 1H NMR spectrum (400 MHz, CDCl3, (dd, J = 6.6, 3.2 Hz), 1.47–1.40 (m), 1.41–1.34 (m), 1.14–1.10
25 °C): δ (ppm) 7.41–7.29 (m, 8H), 7.28–7.22 (m, 2H), 5.82 (m), 1.10–1.06 (m), 0.99 (dd, J = 11.7, 3.8 Hz), 0.95 (d, J = 1.2
(s, 1H).
1-(p-Tolyl)ethan-1-ol. 1H NMR spectrum (400 MHz, CDCl3,
25 °C): δ (ppm) 7.26 (d, J = 8.1 Hz, 2H), 7.15 (d, J = 7.9 Hz, 2H), CDCl3) δ (ppm) 3.99–3.90 (m), 3.57 (dd, J = 7.3, 4.1 Hz), 2.32 (t,
4.85 (q, J = 6.4 Hz, 1H), 2.34 (s, 3H), 1.47 (d, J = 6.5 Hz, 3H). J = 3.4 Hz), 2.04 (t, J = 4.5 Hz), 1.94–1.91 (m), 1.89 (dd, J = 8.3,
Hz), 0.92 (s), 0.89 (s), 0.80 (s), 0.78 (s).
1,7,7-Trimethylbicyclo[2.2.1]heptan-2-ol. H NMR (400 MHz,
1
1
1-(4-Chlorophenyl)ethan-1-ol. H NMR spectrum (400 MHz, 4.0 Hz), 1.81 (s), 1.77 (s), 1.68 (d, J = 4.1 Hz), 1.66 (s), 1.63 (d,
CDCl3, 25 °C): δ (ppm) 7.41–7.26 (m, 4H), 4.87 (q, J = 6.5 Hz, J = 3.9 Hz), 1.60 (d, J = 3.5 Hz), 1.43 (d, J = 3.1 Hz), 1.41–1.39
1H), 1.46 (d, J = 6.4 Hz, 3H).
(m), 1.39–1.36 (m), 1.32 (dd, J = 5.1, 3.0 Hz), 1.28 (d, J = 3.6
1
1-(4-Bromophenyl)ethan-1-ol. H NMR spectrum (400 MHz, Hz), 1.26 (d, J = 3.8 Hz), 1.19 (s), 0.96 (s), 0.93 (d, J = 1.9 Hz),
CDCl3, 25 °C): δ (ppm) 7.31 (d, J = 8.3 Hz, 2H), 7.22 (d, J = 7.7 0.90 (s), 0.86 (s), 0.85 (s), 0.81 (s), 0.80 (s), 0.78 (s), 0.76 (s).
Hz, 2H), 4.91 (q, J = 6.5 Hz, 1H), 1.50 (d, J = 6.6 Hz, 3H).
1-(3-Bromophenyl)ethan-1-ol. H NMR spectrum (400 MHz, sulfonic acid. H NMR (400 MHz, CDCl3) δ (ppm) 3.36 (d, J =
CDCl3, 25 °C): δ (ppm) 7.34–7.28 (m, 3H), 7.25 (t, 1H), 4.9 (q, 20.1 Hz, 2H), 3.27 (d, J = 17.8 Hz, 1H), 2.90 (d, J = 8.4 Hz, 2H),
(2-Hydroxy-7,7-dimethylbicyclo[2.2.1]heptan-1-yl)methane-
1
1
1H), 1.49 (d, J = 6.5 Hz 3H).
2.78 (d, J = 11.6 Hz, 1H), 2.46 (s, 2H), 2.30 (d, J = 16.4 Hz, 1H),
1-(4-Nitrophenyl)ethan-1-ol. 1H NMR spectrum (400 MHz, 2.02 (s, 2H), 1.88 (s, 1H), 1.63 (s, 4H), 1.41 (s, 2H), 1.27 (s, 1H),
CDCl3, 25 °C): δ (ppm) 8.20 (d, J = 8.7 Hz, 2H), 7.55 (d, J = 1.11 (d, J = 5.3 Hz, 2H), 1.06 (dd, J = 46.5, 8.8 Hz, 12H), 0.78 (d,
8.7 Hz, 2H), 5.03 (q, J = 6.5 Hz, 1H), 1.52 (d, J = 6.5 Hz, 3H).
1-(2-Aminophenyl)ethan-1-ol. H NMR spectrum (400 MHz,
CDCl3, 25 °C): δ (ppm) 7.06 (d, J = 7.5 Hz, 2H), 6.71 (t, J = 7.4
J = 14.4 Hz, 6H).
1
Physical methods
Hz, 1H), 6.64 (d, J = 7.9 Hz, 1H), 4.88 (q, J = 6.6 Hz, 1H), The conductivity measurements were done in DMF using the
1.55 (d, J = 6.6 Hz, 3H).
1-(2,4-Dichlorophenyl)ethan-1-ol.
digital conductivity bridge from Popular Traders, India (model
spectrum number: PT-825). Elemental analysis data were obtained using
1H
NMR
(400 MHz, CDCl3, 25 °C): δ (ppm) 7.39 (m, 3H), 4.8 (q, J = 6.6 an Elementar Analysen System GmbH Vario EL-III instrument.
Hz, 1H), 1.48 (d, J = 6.5 Hz, 3H).
NMR measurements were done using a 400 MHz JEOL instru-
(4-Chlorophenyl)(phenyl)methanol. 1H NMR spectrum ment. FTIR spectra (Zn–Se ATR) were recorded using a
(400 MHz, CDCl3, 25 °C): δ (ppm) 7.33 (d, J = 4.3 Hz, 4H), 7.29 PerkinElmer Spectrum-Two FTIR spectrometer. The absorp-
(d, J = 1.7 Hz, 4H), 7.26 (dd, J = 5.2, 2.6 Hz, 1H), 5.81 (s, 1H).
tion spectra were recorded using a PerkinElmer Lambda 950
3276 | Dalton Trans., 2021, 50, 3269–3279
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spectrophotometer. Cyclic voltammetric experiments were per- of the Ru–H intermediate that was isolated and characterized
formed using a CHI electrochemical analyzer (Model 1120 A). and was found to participate in the catalysis. The straight-
The cell contained a glassy-carbon or Pt-disc working elec- forward ruthenium complexes exhibiting significant TH cata-
trode, a Pt wire auxiliary electrode and a Ag/Ag+ reference lytic performance suggest their potential in other challenging
electrode.52,53 The solution concentrations were ca. 1 mM in organic transformations that are presently being explored in
complex and ca. 0.1 M in tetrabutyl ammonium perchlorate the laboratory.
(TBAP).
Crystallography
Conflicts of interest
X-ray crystallographic data for all three complexes were col-
lected at 298 K with an Oxford CCD diffractometer having an
There are no conflicts to declare.
X-calibur sapphire measurement device equipped with a
graphite monochromatic MoKα radiation source (λ
=
Acknowledgements
0.71073 Å).54 The structures were initially solved by direct
methods using SIR-92 55 and then further refined by full-
matrix least-squares refinement techniques on F2 using
SHELXL-97.56 All the calculations were done in the WinGX
crystallographic module.57 The non-hydrogen atoms were
refined anisotropically, whereas all hydrogen atoms were
placed at the calculated positions in the last cycle of the refine-
ment process. In complex 1, attempts to assign the peaks of
electron density as ghost peaks near the metal center were
unsuccessful. The crystallographic data collection details and
structural solution parameters are presented in Table S1
(ESI†).
RG gratefully acknowledges the Council of Scientific
&
Industrial Research, New Delhi (01(2841)/16/EMR-II) and the
IoE Project from the University of Delhi for financial support.
The authors thank CIF-USIC of the University of Delhi for the
instrumental facilities. SY and PV respectively thank CSIR,
New Delhi for the JRF and SERB, New Delhi for N-PDF fellow-
ships. Reviewers’ comments were very helpful during the revi-
sion stage.
References
Kinetics
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Kinetics experiments were performed using the UV-Vis spectral
titration of complex 4 with different para-substituted acetophe-
nones.58 In the experiment, 40 μM solution of complex 4 was
taken followed by the gradual addition of aliquots (4 μM) of
acetophenone at an interval of one minute. The change in the
absorption spectra was measured at λmax
= 440 nm.
Subsequently, a graph was plotted between the logarithm of
the change in the absorbance with time and the slope pro-
vided the value of the rate constant. The respective rate con-
stants were plotted against the σ values for the electronic sub-
stituents: −0.17 (CH3), 0 (H), 0.23 (Cl), 0.77 (NO2).59,60 All
studies were performed in DMF solvent at 20 °C.
7 R. Noyori and S. Hashiguchi, Acc. Chem. Res., 1997, 30, 97–
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Conclusions
This work presented ruthenium complexes of phosphine–car- 10 (a) F. E. Fernández, M. C. Puerta and P. Valerga,
boxamide based tridentate ligands also containing other
monodentate co-ligands. These Ru(II) complexes have been
thoroughly characterized spectroscopically and crystallographi-
cally. These Ru(II) complexes were successfully utilized as
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