Synthesis of Imines via Reactions of Benzyl Alcohol with Amines Using Half-Sandwich (η6-p-cymene) Ruthenium(II) Complexes Stabilised by 2-aminofluorene Derivatives

Article abstract of DOI:10.1002/aoc.5200

A new class of half-sandwich (η⁶-p-cymene) ruthenium(II) complexes supported by 2-aminofluorene derivatives [Ru(η⁶-p-cymene)(Cl)(L)] (L?=?2-(((9H-fluoren-2-yl)imino)methyl)phenol (L¹), 2-(((9H-fluoren-2-yl)imino)methyl)-3-methoxyphenol (L²), 1-(((9H-fluoren-2-yl)imino)methyl)naphthalene-2-ol (L³) and N-((1H-pyrrol-2-yl)methylene)-9H-fluorene-2-amine (L⁴)) were synthesized. All compounds were fully characterized by analytical and spectroscopic techniques (IR, UV–Vis, NMR) and also by mass spectrometry. The solid state molecular structures of the complexes [Ru(η⁶-p-cymene)(Cl)(L²)], [Ru(η⁶-p-cymene)(Cl)(L³)] and [Ru(η⁶-p-cymene)(Cl)(L⁴)] revealed that the 2-aminofluorene and p-cymene moieties coordinate to ruthenium(II) in a three-legged piano-stool geometry. The synthesized complexes were used as catalysts for the dehydrogenative coupling of benzyl alcohol with a range of amines (aliphatic, aromatic and heterocyclic). The reactions were carried out under thermal heating, ultrasound and microwave assistance, using solvent or solvent free conditions, and the catalytic performance was optimized regarding the solvent, the type of base, the catalyst loading and the temperature. Moderately high to very high isolated yields were obtained using [Ru(η⁶-p-cymene)(Cl)(L⁴)] at 1?mol%. In general, microwave irradiation produced better yields than the other two techniques irrespective of the nature of the substituents.

Full text of DOI:10.1002/aoc.5200

Received: 24 April 2019
DOI: 10.1002/aoc.5200
Revised: 30 July 2019
Accepted: 1 August 2019
F U L L P A P E R
Synthesis of Imines via Reactions of Benzyl Alcohol with
Amines Using Half‐Sandwich (η6‐p‐cymene) Ruthenium(II)
Complexes Stabilised by 2‐aminofluorene Derivatives
Govindasamy Vinoth1
| Sekar Indira1
| Madheswaran Bharathi1
|
Anandhan Durgadevi1 | Ravikumar Abinaya1 | Luis G. Alves2
|
Ana Margarida Martins3
| Kuppannan Shanmuga Bharathi1
1 Department of Chemistry, School of
Physical Sciences, Periyar University,
Periyar Palkalai Nagar, Salem
636011Tamil Nadu, India
2 Centro de Química Estrutural,
Associação do Instituto Superior Técnico
para a Investigação e Desenvolvimento,
Av. Rovisco Pais, 1, 1049‐003 Lisbon,
Portugal
A new class of half‐sandwich (η6‐p‐cymene) ruthenium(II) complexes
supported by 2‐aminofluorene derivatives [Ru(η6‐p‐cymene)(Cl)(L)] (L = 2‐
(((9H‐fluoren‐2‐yl)imino)methyl)phenol (L1), 2‐(((9H‐fluoren‐2‐yl)imino)
methyl)‐3‐methoxyphenol (L2), 1‐(((9H‐fluoren‐2‐yl)imino)methyl)naphtha-
lene‐2‐ol (L3) and N‐((1H‐pyrrol‐2‐yl)methylene)‐9H‐fluorene‐2‐amine (L4))
were synthesized. All compounds were fully characterized by analytical and
spectroscopic techniques (IR, UV–Vis, NMR) and also by mass spectrometry.
The solid state molecular structures of the complexes [Ru(η6‐p‐cymene)(Cl)
(L2)], [Ru(η6‐p‐cymene)(Cl)(L3)] and [Ru(η6‐p‐cymene)(Cl)(L4)] revealed that
the 2‐aminofluorene and p‐cymene moieties coordinate to ruthenium(II) in a
three‐legged piano‐stool geometry. The synthesized complexes were used as
catalysts for the dehydrogenative coupling of benzyl alcohol with a range of
amines (aliphatic, aromatic and heterocyclic). The reactions were carried out
under thermal heating, ultrasound and microwave assistance, using solvent
or solvent free conditions, and the catalytic performance was optimized regard-
ing the solvent, the type of base, the catalyst loading and the temperature.
Moderately high to very high isolated yields were obtained using [Ru(η6‐p‐
cymene)(Cl)(L4)] at 1 mol%. In general, microwave irradiation produced better
yields than the other two techniques irrespective of the nature of the
substituents.
3 Centro de Química Estrutural, Instituto
Superior Técnico, Av. Rovisco Pais, 1,
1049‐001 Lisbon, Portugal
Correspondence
Kuppannan Shanmuga Bharathi,
Department of Chemistry, School of
Physical Sciences, Periyar University,
Periyar Palkalai Nagar, Salem‐636011,
Tamil Nadu, India.
Email: nksbharathi@periyaruniversity.ac.
in
Funding information
Fundação para a Ciência e a Tecnologia,
Grant/Award Number: UID/QUI/00100/
2019; University Grants Commission,
Grant/Award Number: UGC‐BSR, NO: F.
30‐319/2016
KEYWORDS
imine synthesis, microwave energy, ruthenium catalysts, sonochemical energy, thermal energy
1 | INTRODUCTION
required for the production of a massive diversity of valu-
able biological active products.[3,4]
The development of efficient synthetic methods of imines
is an important target owing to its widespread applica-
tions in pharmaceuticals, fungicides, agricultural and
industrial products[1,2] (Figure 1). Furthermore, imines
and their nitrogen containing derivatives are increasingly
Imines may be obtained through several reactions
such as addition, condensation, cycloaddition, asymmet-
ric organo‐catalysis, cross‐dehydrogenative coupling and
multicomponent reactions.[1] But still the imination reac-
tion is a challenge, due to the formation of side‐products
Appl Organometal Chem. 2019;e5200.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5200

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VINOTH ET AL.
FIGURE 1 Biological active
compounds containing imine functional
groups
or wastes in several synthetic pathways. To overcome
these drawbacks, acceptorless dehydrogenative methodol-
ogies involving the open‐air oxidative coupling of an alco-
hol and an amine have been recently developed.[5]
Through this procedure benzyl alcohol is converted into
benzaldehyde that, in turn, reacts with amines leading
to the formation of imines via an unstable hemiaminal
intermediate (Scheme 1).
sieves leading to imines as the only products (C)6c;
Ramesh and co‐workers reported the synergic catalytic
activity of a bimetallic system bridged by two hydrazide
nitrogens (D)6d and a mononuclear ruthenium (II) com-
plex containing the benzhydrazone ligand (E).6e These
studies showed that the catalytic activity is strongly influ-
enced by the nature of the arene, the chelating ligands
and the vacant coordination sites provided by leaving
groups.[18,19]
This work describes new Schiff‐base and pyrrolate‐
imine ruthenium complexes as catalysts for the synthesis
of imines using benzyl alcohol and amines as starting
materials. The effect of thermal heating, sonochemical
and microwave irradiation on the catalytic activity is also
discussed.
Several homogeneous and heterogeneous transition
metal catalysts for the synthesis of imines, such as Ru,[6]
Ir,[7] Fe,[8] Mn,[9] Os,[10] Co,[11] Cu,[12] Au/TiO2,[13]
Pd/DNA,[14] Pd/ZrO2,
MOF[16] were reported. Ruthe-
[15]
nium complexes, well known for their effective catalytic
activity in huge assortment of organic transformation
reactions,[17] are among the most useful catalysts for
acceptorless dehydrogenative coupling of imines (see
Figure 2). Milstein and co‐workers reported an efficient
Ru‐PNP pincer catalyst that shows high turnover number
and only generates hydrogen gas and water as co‐
products (A)6a Gelman et. al reported a bifunctional Ru‐
PCP pincer complex bearing a dibenzobarrelene‐based
ligand (B)6b; Bera et. al described a new type of direct
metal–metal bonded diruthenium complex bridged by a
naphthyridine functionalized N‐heterocyclic carbene
(NHC) ligand that was used in the presence of molecular
2 | EXPERIMENTAL SECTION
2.1 | General considerations
Elemental analyses were performed with a Vario EL
CHN elemental analyser. The melting points were mea-
sured with the aid of Boetius micro‐heating label. FT‐IR
spectra (4000–600 cm−1) were accomplished on a Bruker
SCHEME 1 Direct synthesis of imines
from benzyl alcohol and amines

VINOTH ET AL.
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FIGURE 2 Ruthenium(II) catalysts for the synthesis of imines using benzyl alcohol and amines
783 spectrometer by direct utilization. Electronic spectra
were obtained on a Cary 300 Bio UV–Vis Varian spectro-
photometer from the solutions of chloroform, 10−3 M, in
quartz cuvettes (1 cm optical path) in the range of 800–
200 nm. NMR spectra were recorded using a Bruker
Advances III HD Nanobay 400 MHz FT‐ NMR spectrom-
eter at 295 K, referenced internally to the residual proton‐
solvent (1H) or solvent (13C) resonances, and reported in
parts per million (ppm) relatively to tetramethylsilane
(0 ppm). Sonication was performed using an Ultrasonic
cleaner, sonica 2200 MHS3 (model no: 090.003.003). Con-
vection microwave oven (MC2846SL, LG28L) was used to
carry out the reactions.
and the solvent partially evaporated. A yellow precipitate
was obtained on standing. It was washed with cold etha-
nol and dried in a vacuum desiccator. Yield: 84%; m.p.:
240 °C; FT‐IR: N‐H, 3008 cm−1, C=N, 1659 cm−1. UV–
Vis (CHCl3, λmax [nm] (10−3 ε [M−1 cm−1]): 408 (147),
1
274 (391). H NMR (400 MHz, CDCl3) (δ ppm): 10.62 (s,
1H, NH), 8.65 (s, 1H, HC=N), 7.90–6.82 (m, 10H, ArH),
3.98 (s, 2H, CH2). ESI‐MS: m/z = 259.29 [M + H]+ (Cald.
258.32).
2.3 | General procedure for the syntheses
of half‐Sandwich (6‐p‐cymene)
ruthenium(II) complexes (1–4)
Commercially available RuCl3.3H2O was used as sup-
plied from SRL Pvt. Ltd. All solvents and reagents were
acquired from Merck or Aldrich. The starting precursor
[Ru(η6‐p‐cymene)Cl2]2 was prepared according to
literature.[20] (2‐(((9H‐fluoren‐2‐yl)imino)methyl)phenol
To a solution of [Ru(η6‐p‐cymene)Cl2]2 (0.1 g, 1 mmol) in
dichloromethane (10 ml) was added a solution of the cor-
responding 2‐aminofluorene derivative (HL1–4) (2 mmol)
in dichloromethane (5 ml) followed by a few drops of
triethylamine. The reaction mixture was stirred at room
temperature for 5 hr. A gradual color change from red-
dish orange to orange was observed. The solvent was
evaporated to dryness under reduced pressure affording
an orange solid. The complexes were crystallized from
CH2Cl2/hexane solutions.
(HL1),
2‐(((9H‐fluoren‐2‐yl)imino)methyl)‐3‐
methoxyphenol (HL2) and 1‐(((9H‐fluoren‐2‐yl)imino)
methyl)naphthalene‐2‐ol (HL3) were synthesized by the
reported procedure.[21]
2.2 | Synthesis of the 2‐aminofluorene
derivative HL4
2.3.1 | Synthesis of [Ru(η6‐p‐cymene)(cl)
(L1)] (1)
Pyrrole‐2‐carboxaldehyde (0.1 g, 1.0 mmol) was dissolved
in ethanol (10 ml) and 2‐aminofluorene (0.2 g, 1.0 mmol)
was added to the solution. This mixture was stirred at
room temperature for 3 hr. The solution turned yellow
Yield: 78%; m.p.: 190 °C; Anal. calcd. For C30H28NOClRu:
C, 64.86; H, 5.01; N, 2.29. Found: C, 64.91; H, 5.08; N,

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VINOTH ET AL.
2.52. FT‐IR: C=N, 1602 cm−1; C‐O, 1386 cm−1. UV–Vis
(CHCl3, λmax [nm] (10−3 ε [M−1 cm−1]): 439 (713), 334
(6034), 262 (2154). H NMR (400 MHz, CDCl3) (δ ppm):
2.3.3 | Synthesis of [Ru(η6‐p‐cymene)(cl)
(L3)] (3)
1
8.79 (s, 1H, HC=N), 7.92 (s, 1H, ArH), 7.83 (t,
J = 4.0 Hz, 1H, ArH), 7.61–7.60 (overlapping, 2H, ArH),
7.43 (t, J = 7.6 Hz, 1H, ArH), 7.35 (t, J = 7.2 Hz, 1H,
ArH), 7.22 (d, J = 8.0 Hz, 1H, ArH), 7.03–7.00 (overlap-
ping, 2H, ArH), 6.99 (d, J = 8.0 Hz, 1H, ArH), 6.43 (t,
J = 7.2 Hz, 1H, ArH), 5.37 (d, J = 5.6 Hz, 1H, p‐cym
ArH), 5.26 (d, J = 5.6 Hz, 1H, p‐cym ArH), 5.02 (d,
J = 5.6 Hz, 1H, p‐cym ArH), 4.23 (d, J = 5.6 Hz, 1H, p‐
cym ArH), 3.99 (s, 2H, CH2), 2.67 (sept, J = 6.7 Hz, 1H,
p‐cym ArCH(CH3)2), 2.13 (s, 3H, p‐cym Ar(CH3)), 1.19
(d, J = 6.8 Hz, 3H, p‐cym ArCH(CH3)2), 1.13 (d,
J = 6.8 Hz, 3H, p‐cym ArCH(CH3)2). 13C NMR
(100 MHz, CDCl3) (δ ppm): 165.2 164.0, 162.7 (L1 CqPhO,
CH=N and CqNFluorene), 157.7, 150.3, 144.2, 143.9, 140.8,
137.2, 135.4, 129.2, 127.1, 125.2, 123.8, 122.7, 120.9,
119.8, 118.2, 114.3 (L1 Cq,Fluorene, CqArCCH=N and CHAr),
101.7, 97.5 (p‐cym CqAr), 86.3, 84.1, 83.9, 80.1 (p‐cym
CHAr), 37.1 (CH2), 22.8, 21.6 (p‐cym ArCH(CH3)2 and
Ar(CH3)), 8.8, 8.7 (p‐cym ArCH(CH3)2). ESI‐MS:
m/z = 554.5210 [M + H]+ (Cald. 555.08).
Yield: 85%; m.p.: 210 °C; Anal. calcd. For C34H30NOClRu:
C, 67.37; H, 4.95; N, 2.30. Found: C, 67.47; H, 4.99; N,
2.31. FT‐IR: C=N, 1616 cm−1; C‐O, 1383 cm−1. UV–Vis
(CHCl3, λmax [nm] (10−3 ε [M−1 cm−1]): 458 (827), 334
1
(6006), 264 (2126). H NMR (400 MHz, CDCl3) (δ ppm):
8.66 (s, 1H, HC=N), 7.94 (s, 1H, ArH), 7.86 (d,
J = 8.0 Hz, 1H, ArH), 7.72 (d, J = 8.0 Hz, 1H, ArH),
7.63 (d, J = 8.8 Hz, 1H, ArH), 7.57 (d, J = 8.0 Hz, 1H,
ArH), 7.45–7.43 (overlapping 2H, ArH), 7.32–7.30 (over-
lapping, 2H, ArH), 7.18–7.12 (overlapping, 4H, ArH),
5.42 (d, J = 5.2 Hz, 1H, p‐cym ArH), 5.31 (d, J = 3.2 Hz,
1H, p‐cym ArH), 5.02 (d, J = 5.6 Hz, 1H, p‐cym ArH),
4.25 (d, J = 5.6 Hz, 1H, p‐cym ArH), 4.01 (s, 2H, CH2),
2.67 (sept, J = 6.8 Hz, 1H, p‐cym ArCH(CH3)2), 2.15 (s,
3H, p‐cym Ar(CH3)), 1.20 (d, J = 6.8 Hz, 3H, p‐cym
ArCH(CH3)2), 1.12 (d,
J = 6.8 Hz, 3H, p‐cym
ArCH(CH3)2). 13C NMR (100 MHz, CDCl3) (δ ppm):
168.8, 167.9, 166.0 (L3 CqPhO, CH=N and CqNFluorene),
158.8, 157.6, 147.0, 145.1, 143.5, 141.6, 140.2, 135.3,
134.8, 128.9, 127.0, 126.6, 125.4, 124.0, 122.8, 121.9,
120.0, 119.8, 118.6, 108.2 (L3 Cq,Fluorene, CqArCCH=N, CqAr
and CHAr), 101.6, 97.5 (p‐cym CqAr), 86.4, 84.7, 84.3, 80.2
(p‐cym CHAr), 37.1 (CH2), 22.8, 21.6 (p‐cym ArCH(CH3)2
and Ar(CH3)), 8.8, 8.7 (p‐cym ArCH(CH3)2). HR‐MS:
m/z = 606.1110 [M + H]+ (Cald. 606.1132).
2.3.2 | Synthesis of [Ru(η6‐p‐cymene)(cl)
(L2)] (2)
Yield: 84%; m.p.: 174 °C; Anal. calcd. For
C31H30NO2ClRu: C, 63.38; H, 5.11; N, 2.23. Found: C,
63.63; H, 5.16; N, 2.39. FT‐IR: C=N, 1596 cm−1; C‐O,
2.3.4 | Synthesis of [Ru(η6‐p‐cymene)(cl)
1383 cm−1
.
UV–Vis (CHCl3, λmax [nm] (10−3
ε
(L4)] (4)
1
[M−1 cm−1]): 446 (483), 331 (6008), 264 (2079). H NMR
(400 MHz, CDCl3) (δ ppm): 8.14 (s, 1H, HC=N), 7.92 (s,
1H, ArH), 7.85–7.81 (overlapping, 4H, ArH), 7.43 (t,
J = 7.6 Hz, 1H, ArH), 7.36 (t, J = 7.6 Hz, 1H, ArH), 6.76
(d, J = 7.6 Hz, 1H, ArH), 6.62 (d, J = 8.0 Hz, 1H, ArH),
6.36 (t, J = 7.2 Hz, 1H, ArH), 5.38 (d, J = 6.0 Hz, 1H, p‐
cym ArH), 5.31 (d, J = 5.6 Hz, 1H, p‐cym ArH), 5.03 (d,
J = 5.6 Hz, 1H, p‐cym ArH), 4.30 (d, J = 5.2 Hz, 1H, p‐
cym ArH), 3.97 (s, 2H, CH2), 3.84 (s, 3H, OCH3), 2.68
(sept, J = 6.9 Hz, 1H, p‐cym ArCH(CH3)2), 2.12 (s, 3H,
p‐cym Ar(CH3)), 1.42 (d, J = 7.2 Hz, 3H, p‐cym
Yield: 88%; m.p.: 182 °C; Anal. calcd for C28H27N2ClRu:
C, 63.55; H, 5.11; N, 5.25. Found: C, 63.67; H, 5.15; N,
5.30. FT‐IR: C=N, 1563 cm−1. UV–Vis (CHCl3, λmax
[nm] (10−3 ε [M−1 cm−1]): 427 (739), 365 (1293), 267
1
(1043). H NMR (400 MHz, CDCl3) (δ ppm): 8.64 (s, 1H,
HC=N), 7.82–7.77 (overlapping, 2H, ArH), 7.67 (s, 1H,
ArH), 7.62–7.57 (overlapping, 2H, ArH), 7.40 (t,
J = 7.6 Hz, 1H, ArH), 7.33–7.31 (overlapping, 2H, ArH),
6.82 (d, J = 3.6 Hz, 1H, ArH), 6.37 (d, J = 2.4 Hz, 1H,
ArH), 5.45 (d, J = 6.0 Hz, 1H, p‐cym ArH), 5.19 (d,
J = 5.6 Hz, 1H, p‐cym ArH), 5.12 (d, J = 5.6 Hz, 1H, p‐
cym ArH), 4.97 (d, J = 5.6 Hz, 1H, p‐cym ArH), 3.96 (s,
2H, CH2), 2.39 (sept, J = 6.8 Hz, 1H, p‐cym ArCH(CH3)2),
2.20 (s, 3H, p‐cym Ar(CH3)), 1.07 (d, J = 6.8 Hz, 3H, p‐
cym ArCH(CH3)2), 0.99 (d, J = 6.8 Hz, 3H, p‐cym
ArCH(CH3)2). 13C NMR (100 MHz, CDCl3) (δ ppm):
ArCH(CH3)2), 1.41 (d,
J = 7.2 Hz, 3H, p‐cym
ArCH(CH3)2). 13C NMR (100 MHz, CDCl3) (δ ppm):
168.3, 165.1, 163.8 (L2 CqPhO, CH=N and CqNFluorene),
157.7, 156.6, 152.5, 144.9, 143.5, 141.8, 140.4, 127.1,
126.9, 125.2, 123.9, 120.0, 119.8, 118.0, 115.0, 113.2 (L2
Cq,Fluorene, CqArCCH=N and CHAr), 101.9, 97.5 (p‐cym
CqAr), 86.2, 84.1, 83.9, 80.2 (p‐cym CHAr), 56.1 (OCH3),
37.1 (CH2), 22.8, 21.6 (p‐cym ArCH(CH3)2 and Ar(CH3)),
8.7, 8.5 (p‐cym ArCH(CH3)2. HR‐MS: m/z = 586.1019
[M + H]+ (Cald. 586.108).
161.2, 159.6, 156.3, 153.6 (L4 CqNPyrrole, CHNPyrrole
CH=N and CqNFluorene), 144.2, 143.3, 141.1, 140.9, 139.6,
127.0, 126.7, 125.1, 121.8, 120.0, 119.8, 118.1, 114.0 (L4
Cq,Fluorene and CHAr), 100.8, 99.9 (p‐cym CqAr), 84.4,
,

VINOTH ET AL.
5 of 12
82.4, 81.7, 79.8 (p‐cym CHAr), 37.0 (CH2), 22.6, 21.6 (p‐
cym ArCH(CH3)2 and Ar(CH3)), 8.6, 8.2 (p‐cym
ArCH(CH3)2). HR‐MS: m/z = 551.0807 [M + Na]+ (Cald.
551.0798).
collected using the graphite monochromated Mo‐Kα radi-
ation (λ = 0.71073 Å) on a Bruker AXS‐KAPPA APEX II
diffractometer equipped with an Oxford Cryosystem
open‐flow nitrogen cryostat. Cell parameters were
retrieved using the Bruker SMART software and refined
using Bruker SAINT on all observed reflections.[22]
Absorption corrections were applied using SADABS.[23]
The structures were solved by direct methods using
SIR97.[24] Structure refinement was done using
SHELXL.[25] These programs are part of the WinGX soft-
ware package version 1.80.05.[26] The hydrogen atoms
were inserted in fixed positions and allowed to refine rid-
ing on the parent carbon atom except on the NH groups
that were located in the electron density map. Torsion
angles, mean square planes and other geometrical param-
eters were calculated using SHELX.[25] Crystallographic
and experimental details of data collection and crystal
structure determinations are available in Table 1. Illustra-
tions of the molecular structures were made with ORTEP‐
3 for Windows.[27] The poor diffracting power and crystal
quality of 2 (as attested by the Rint value obtained) pre-
cluded the final refinement to lower the R values.
2.4 | Synthesis of imines via benzyl
alcohol with amines exploitation of various
methods
i. Conventional thermal heating method
A
mixture of benzyl alcohol (1 mmol), mono
amine/diamine (1 mmol/0.5 mmol), complex
4
(0.01 mmol), Cs2CO3 (0.2 mmol) in toluene (5 ml) was
refluxed at 110 °C with constant stirring for 12 hr. The
reaction mixture was allowed to cool to room tempera-
ture and diluted with ethyl acetate (10 ml). Two layers
were formed upon the addition of water (5 ml) and the
organic layer was separated, dried over anhydrous
Na2SO4 and concentrated in vacuum. The product was
purified by column chromatography on a silica gel of
100–200 mesh using a hexane/ethyl acetate (3:1) mixture
as eluent. The product was analysed by a proton NMR
spectrum and the isolated yield was calculated.
Data for structures 2, 3 and 4 were deposited in CCDC
under the deposit numbers 1858584, 1858585 and
1858586, respectively, and can be obtained by free of
charge from The Cambridge Crystallographic Data Centre.
ii. Ultrasonic irradiation
A mixture of benzyl alcohol (1 mmol), mono‐ or diamine
(1 mmol or 0.5 mmol, respectively), complex 4 (0.01 mmol)
and Cs2CO3 (0.2 mmol) was sonicated at a frequency of
50 kHz and an input power of 170 W using an US bath at
90 °C. The progress of the reaction was monitored by
TLC. The solution was filtered and concentrated under
reduced pressure and the product was purified by the col-
umn chromatography on a silica gel of 100–200 mesh using
a mixture of hexane/ethyl acetate (3:1) as eluent.
3 | RESULTS AND DISCUSSION
A collection of 2‐aminofluorene imine derivatives were
synthesised by the reactions of 2‐aminofluorene with
salicylaldehyde (HL1), o‐vanillin (HL2), 2‐hydroxy‐1‐
naphthaldehyde (HL3) and pyrrole‐2‐carboxaldehyde
(HL4). The compounds HL1–3 were previously
described.[21] Using a similar methodology, N‐((1H‐
pyrrol‐2‐yl)methylene)‐9H‐fluorene‐2‐amine, HL4, was
isolated in 84% yield. The treatment of [Ru(η6‐p‐cymene)
Cl2]2 with HLi (i = 1–4) in a 1:2 molar ratio led to the for-
mation of complexes of general formula [Ru(η6‐p‐cymene)
(Cl)(Li)] (1–4) in high yields (Scheme 2).
All ligand precursors and ruthenium complexes are air
and moisture stable and readily soluble in organic sol-
vents like acetonitrile, benzene, chloroform, dichloro-
methane, dimethylformaide, dimethylsulfoxide, ethanol
and methanol. The new compounds were characterised
by elemental analysis and spectroscopic methods (IR,
UV–Vis and NMR). Ruthenium(II) complexes 2–4 were
further characterised by the single crystal X‐ray
diffraction.
iii. Microwave irradiation
Catalyst 4 (1 mol%), benzyl alcohol (1 mmol), mono‐ or
diamine (1 mmol/0.5 mmol) and Cs2CO3 (0.2 mmol) were
placed in a microwave vial containing a stirrer bar. The
mixture was irradiated at 115 °C for 120 min. The crude
product was purified by column chromatography on sil-
ica gel 100–200 mesh using a mixture of hexane/EtOAc
(3:1) as eluent.
2.5 | General procedures for X‐ray
crystallography
The IR spectra of the ligand precursors and of the
ruthenium(II) complexes display sharp bands at
1659 cm−1 and 1314 cm−1 that are assigned to the
Suitable crystals of compounds 2, 3 and 4 were coated and
selected in Fomblin® oil, mounted on a loop and data

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VINOTH ET AL.
TABLE 1 Crystal data and structure refinement for compounds 2–4
Compound
2
3
4
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit Cell Dimensions:
a (Å)
C31 H30 Cl N O2 Ru
585.08
C34 H30 Cl N O Ru
605.11
C28 H27 Cl N2 Ru
528.04
150(2)
150(2)
150(2)
0.71073
0.71073
0.71073
Orthorhombic
Pbca
Monoclinic
P21/c
Monoclinic
P21/c
15.113(1)
19.461(2)
7.6151(6)
19.054(2)
90
13.949(1)
8.0893(6)
21.084(1)
90
b (Å)
14.323(1)
c (Å)
23.844(2)
α(°)
90
β(°)
90
106.447(5)
90
105.454(3)
90
γ(°)
90
Volume (Å3)
5161.4(7)
2708.2(5)
4
2293.1(3)
4
Z
8
Calculated density (g m−3
)
1.506
1.484
1.530
Absorption coefficient (mm−1
)
0.741
0.706
0.819
F (000)
2400
1240
1080
Crystal size (mm)
0.06 x 0.08 x 0.14
0.20 x 0.22 x 0.30
3.206–29.643
0.04 x 0.10 x 0.22
2.939–25.735
Theta range for data collection (°) 2.599–25.752
Limiting indices
−16 ≤ h ≤ 18, −17 ≤ k ≤ 17,
−29 ≤ l ≤ 29
−27 ≤ h ≤ 21, −10 ≤ k ≤ 9,
−23 ≤ l ≤ 26
−16 ≤ h ≤ 17, −9 ≤ k ≤ 9,
−25 ≤ l ≤ 23
Reflections collected/unique [Rint
Completeness to θ (%)
Refinement method
]
39481/4913 [0.1776]
28171/7620 [0.0588]
21036/4360 [0.0670]
99.9 (θ = 25.242)
99.8 (θ = 25.242)
99.9 (θ = 25.242)
Full‐matrix least squares on F 2 Full‐matrix least squares on F 2 Full‐matrix least squares on F 2
Data/restraints/parameters
Goodness‐of‐fit on F 2
Final R indices [I > 2σ(I)]a
R indices (all data)a
4913/0/329
7620/0/346
4360/0/292
0.928
1.021
0991
R1 = 0.0519, wR2 = 0.0995
R1 = 0.1233, wR2 = 0.1160
Multi‐scan
R1 = 0.0352, wR2 = 0.0769
R1 = 0.0500, wR2 = 0.0815
Multi‐scan
R1 = 0.0346, wR2 = 0.0776
R1 = 0.0494, wR2 = 0.0823
Multi‐scan
Absorption correction
Largest diff. Peak/hole (e Å−3
)
0.584 and − 0.832
0.472 and − 0.439
0.991 and − 0.684
aR1 = Σ||F0|‐|Fc||/Σ|F0|; wR2 = {Σ[w(F2 – F2c)2]/Σ[w(F20)2]}1/2
.
0
stretching of azomethine (νC=N) and phenolic (νC‐O
)
(νC=N) is shifted to lower wavenumber, in the region of
1616–1563 cm−1, attesting the coordination of the imine
nitrogen to ruthenium(II).[28]
bonds, respectively. A broad band at 3453 cm−1 and a
sharp band at 3008 cm−1 are due to the vibrations of
the νO‐H and νN‐H bonds of the ligand precursors, respec-
tively. The disappearance of the latter bands and the
simultaneous increase in the C‐O stretching band in the
region of 1386–1383 cm−1 indicate the coordination to
the metal through the oxygen of the phenolate or
naphtholate moieties in the case of complexes 1–3 and
through the nitrogen of the pyrrolate fragment in com-
plex 4. Moreover, the azomethine stretching band
The electronic spectra of the ligand precursors (HLi)
display two absorption bands observed at 274–262 nm
and 408–348 nm that are due to π‐π* and n‐π* charge
transfer transitions, respectively. Analogously to the pre-
viously reported ruthenium(II) p‐cymene complexes,[29]
the spectra of complexes 1–4 show three characteristic
peaks, namely high‐intensity π‐π* bands at 267–262 nm,
medium‐intensity n‐π* bands at 365–331 nm and also

VINOTH ET AL.
7 of 12
SCHEME 2 Synthesis of ruthenium(II) p‐cymene complexes 1–4
bands at 458–427 nm that are assigned to the Ru(dπ)‐to‐
(Lπ*) charge transfer transitions (MLCT).[23]
and δ 157.6 ppm, respectively. The two isopropyl methyl
carbons (C (CH3)2) of p‐cymene are observed at δ 8.8–
8.1 ppm and the methine carbons (CH (CH3)2) appear
at δ 22.8–22.6 ppm. In the complexes, the methoxy
(OCH3), methylene (CH2), and p‐cymene methyl (CH3)
carbons are observed close to δ 56 ppm, δ 37 ppm, and
δ 21 ppm, respectively.The 13C NMR and mass spectra
of Figure S9‐S17 see the supporting information.
The solid‐state molecular structures of complexes 2–4
were determined by single crystal X‐ray diffraction.
Compound 2 crystalizes in the orthorhombic space group
Pbca while both 3 and 4 crystalize in the monoclinic
space group P21/c. ORTEP depictions of the molecular
structures of 2–4 are displayed in Figures 3–5, respec-
tively, and relevant distances and angles are shown in
Table 2.
All complexes display a typical three‐legged piano‐
stool geometry with distances between ruthenium and
the ring centroids (Ru‐PhCT) ranging from 1.666(2) to
1.6688(8) Å. The distances between the metal centre and
the chloride and the bidentate ligands are with the usual
ranges observed for this type of bonds.[30] These values
reveal that no steric or electronic effects were perceived
in the bidentate N,O‐ and N,N‐ligand moieties. In gen-
eral, the structural parameters obtained for 2–4 agree
with those reported for other [Ru(η6‐p‐cymene)(Cl)(L)]
complexes.[31]
The proton NMR spectra of the ligand precursors dis-
play several resonances in the range δ 9.38–6.37 ppm
assigned to the aromatic protons and singlets in the region
of δ 9.40–8.59 ppm that are attributed to the azomethine
protons (HC=N). The methylene protons of fluorene moi-
ety and the methoxy group of HL2 give rise to singlets at δ
3.98–3.94 ppm and δ 3.85 ppm, respectively. The OH and
NH resonances of the phenol, naphthol or pyrrole groups
appear at low field (δ 15.70–13.43 ppm and δ 10.62 ppm),
in accordance with its acidic properties. As expected, the
latter protons are not observed in the proton NMR spectra
of the ruthenium complexes. The down field shift to δ 8.79–
8.14 ppm registered for the azomethine protons also con-
firms the coordination to ruthenium. The resonances
assigned to the other protons of the ligands L1–4 are similar
to those of the ligand precursors. In agreement with the
structures proposed for complexes 1–4, the p‐cymene pro-
tons gave rise to 4 doublets in the range δ 5.42–4.22 ppm
and the 1H NMR spectra of Figure S1‐S8 see the supporting
information.
The 13C NMR spectra of complexes 1–4 further support
the proposed structures. The peaks assigned to the car-
bons of the azomethine (HC=N) and the
phenolate/naphtholate groups of the ligands[21] are
shifted downfield and appeared at δ 168.8–165.1 ppm

8 of 12
VINOTH ET AL.
FIGURE 3 ORTEP diagram of 2 with
thermal ellipsoids at 40% probability level.
Hydrogen atoms are omitted for clarity
FIGURE 4 ORTEP diagram of 3 with
thermal ellipsoids at 40% probability level.
Hydrogen atoms are omitted for clarity
FIGURE 5 ORTEP diagram of 4 with
thermal ellipsoids at 40% probability level.
Hydrogen atoms are omitted for clarity

VINOTH ET AL.
9 of 12
TABLE 2 Selected bond lengths (Å) and angles (°) for com-
pounds 2–4
and benzene also led to high conversions of 80% and
78%, respectively (entries 3, 1).
The comparison of catalysts 1–4 was carried out in
toluene at 110 °C for 12 h, using benzyl alcohol, aniline
and Cs2CO3 as reference reagents. In these conditions,
all catalysts are active within the range 73–95% yield
of imine (see Table 4). The higher activity is shown by
Ru‐N
Ru‐Cl
Ru‐X
Ru‐[C6Plane]
2
3
4
2.081(4)
2.083(2)
2.104(2)
Cl‐Ru‐N
84.5(1)
2.431(1)
2.4255(5)
2.4095(8)
N‐Ru‐X
87.6(2)
2.043(3)
2.055(1)
2.052(2)
1.666(2)
1.6688(8)
1.676(1)
X‐Ru‐Cl
85.8(1)
TABLE 4 Screening of half‐sandwich ruthenium complexes 1–4
2
3
4
for imines formation via benzyl alcohol and anilinea
85.15(5)
85.35(7)
87.70(6)
76.7(1)
84.24(4)
86.67(8)
Entry
Catalyst
Yieldb (%)
1
2
3
4
[Ru(p‐cymene)(Cl)(L1)] (1)
[Ru(p‐cymene)(Cl)(L2)] (2)
[Ru(p‐cymene)(Cl)(L3)] (3)
[Ru(p‐cymene)(Cl)(L4)] (4)
73
85
88
95
3.1 | Catalytic studies for the synthesis of
imines via reactions of benzyl alcohol and
amines
aConditions: benzyl alcohol (1 mmol), aniline (1 mmol), catalyst 1–4
(1.0 mol%), Cs2CO3 (0.2 mmol) in the presence of toluene (5 ml) at 110 °C
for 12 hr.
Complexes 1–4 were used as catalysts for the syntheses of
imines from reactions of benzyl alcohol with amines.
These reactions were carried out using different energy
supply methods, namely, thermal heating, sonochemical
and microwave irradiation. The suitability and effective-
ness of the three methods are compared. The catalytic
reactions were optimised having in consideration the cat-
alyst, the catalysts loading, the temperature, the solvent
and the base.
In order to evaluate the role of the solvent on the
imination reaction, benzyl alcohol and aniline were used
as model substrates and the reactions were carried out in
the presence of Cs2CO3 and the ruthenium catalyst 4. The
results are listed in Table 3. Among the various solvents
tested, the best solvent to perform the reaction is toluene
(entry 4), which led to an excellent conversion of 95%.
Highly polar solvents as acetonitrile, dichloromethane,
DMSO and chloroform (entries 6, 2, 5 and 7, respectively)
led to low conversions, in the range 43–57%. Methanol
bIsolated yield.
TABLE 5 Effect of the catalyst loadinga
Entry
Ruthenium catalyst (mol %)
Yield2(%)a
1c
2
‐
05
89
95
91
72
57
0.5
1.0
1.5
2.0
2.5
3
4
5
6
aConditions: benzyl alcohol (1 mmol), aniline (1 mmol), catalyst 4 (0.5–
2.5 mol%), Cs2CO3 (0.2 mmol) in the presence of toluene (5 ml) at 110 °C
for 12 hr.
bIsolated yield.
cAbsence of catalyst.
TABLE 3 Screening of different solvents for the synthesis of iminesa
Entry
Solvent
Yieldb (%)
1
2
3
4
5
6
7
Benzene
78
48
80
95
57
42
56
Dichloromethane
Methanol
Toluene
Dimethyl sulfoxide
Acetonitrile
Chloroform
aConditions: benzyl alcohol (1 mmol), aniline (1 mmol), catalyst 4 (1.0 mol%), Cs2CO3 (0.2 mmol) in the presence of solvent (5 ml) at 110 °C for 12 hr.
bIsolated yield.

10 of 12
VINOTH ET AL.
complex 4 that displays the best ligand donor set. This
result suggests that the oxidative addition of the benzyl
alcohol, which is the first step for the formation of the
aldehyde is also the limiting step of the imination reac-
tion. Complexes 2 and 3 display essentially the same
activity and are better catalysts than 1. The better per-
formance of complex 2 in comparison with 1 is in line
with the requirement of good electron donor sets
already justified, but the possibility of a stereochemical
protection of the ortho‐methoxy group may not be
excluded, as the bulkiness of the o‐OMe group may
avoid dimerization thought phenolate bridges. This pro-
cess may also be restricted by the naphthoic moiety of
complex 3 and is likely the reason for the similar activ-
ity of complexes 2 and 3.
The effect of the concentration of the catalyst (see
Table 5) was assessed using complex 4 between 0.5 and
2.5 mol%. A maximum activity was obtained at 1 mol%
catalyst. The observation of a decrease in the reaction
yield for the higher ruthenium concentration is consistent
with the assumption that the most active catalysts are
monomeric species.
TABLE 6 Screening of different bases for the synthesis of iminesa
Entry
Base
Yieldb (%)
1
2
3
4
5c
6
7
Na2CO3
NaH
83
78
67
77
13
95
86
NaOH
KOH
‐
Cs2CO3
K2CO3
aConditions: benzyl alcohol (1 mmol), aniline (1 mmol), catalyst 4 (1.0 mol%),
base (0.2 mmol) in the presence of toluene (5 ml) at 110 °C for 12 hr.
bIsolated yield.
cAbsence of base.
TABLE 7 Assessment of substrate scope for the synthesis of imines using catalyst 4a
4c1, (e95, f92, g99)%
4c2, (e83, f82, g84)%
4c3, (e89, f82, g85)%
4c6, (e90, f90, g91)%
4c4, (e88, f86, g85)%
4c5, (e89, f91, g89)%
4c9, (e67, f66, g69)%
4c12, (e83, f83, g95)%
4c8, (e95, f74, g76)%
4c7, (e71, f77, g71)%
4c10, (e60, f57, g69)%
4c11, (e, f, g NR)h
%
4c13, (e65, f65, g88)%
4c14, (e75, f64, g90)%
4c17, (e74, f52, g80)%
4c15, (e83, f62, g64)%
4c18, (e, f, g NR)h
%
4c16, (e72, f87, g90)%
[a] Conventional method 1: toluene (5 ml), 110 °C, 12 hr.; Ultrasonic method 2: solvent‐free, 90 °C, 6.5 hr.; Microwave method 3: solvent‐free, 115 °C,
2 h.; the total amount of substrates (benzyl alcohol and amine), catalyst 4 and Cs2CO3 for all the three methods is the same. Isolated yield [%] for conventional
method (e), ultrasonic method (f) and microwave method (g). [h] no reaction.

VINOTH ET AL.
11 of 12
The requirement of a base to initiate an efficient
yields. Microwave irradiation revealed an excellent
method to perform the generality of the catalytic reac-
tions having in account these reactions are faster than
the ones thermally activated, lead to better or the same
yields and are carried out in solvent‐free conditions.
There are several positive points that should be men-
tioned about the study carried out in this work, namely,
the use of an easily prepared and easily to handle catalyst,
the use of solvent‐free conditions, a simple workup proce-
dure, a catalytic process that only produces water as by‐
product and does not require the use of molecular sieves
and good to high conversion for a wide range of amines.
imination reaction was confirmed by the low yield of
13% obtained in the absence of base when benzyl alcohol,
aniline and catalyst 4 were reacted for 12 h, in toluene at
110 °C (see Table 6, entry 5). The efficiency of several
bases, as metal carbonates (K2CO3, Na2CO3 and Cs2CO3),
hydroxides (NaOH and KOH) and sodium hydride was
made from the comparison of reactions carried out in
the same experimental conditions. Strong bases, such as
potassium hydroxide, sodium hydroxide and sodium
hydride gave moderate conversions (entries 2, 3 and 4
respectively), but lower than the ones obtained for reac-
tion using metal carbonates. From the latter, caesium car-
bonate proved being the most useful base leading to an
excellent yield, 95% (entry 6).
ACKNOWLEDGEMENTS
Having established an optimised protocol for the cata-
lytic synthesis of imines we tested the reaction using sev-
eral amines to evaluate the scope of application of our
procedure using 3 different energy supply methodologies,
namely traditional heating, sonochemical and microwave
The authors are grateful for the financial support from
the University Grants Commission, (UGC‐BSR, NO: F.
30‐319/2016) Government of India, New Delhi, India.
The first author acknowledges Periyar University, Salem,
Tamil Nadu for providing University Research Fellow-
ship (URF). A. M. Martins and L. G. Alves thank
Fundação para a Ciência e a Tecnologia for funding
(UID/QUI/00100/2019).
1
irradiation. The results are listed in Table 7 and the H
NMR spectra of Figure S18‐S24 see the supporting infor-
mation. The first aspect that is worth noting is that in
general the reactions display moderately high to very
high yields, the only exceptions being the secondary ali-
phatic amines diethanolamine (4c11) and piperazine
(4c18). Electron attractor p‐substituted anilines (4c2‐4c4)
react with lower yields than electron donor para‐ or
ortho‐substituted ones (4c5‐4c8), which in turn are
slightly less reactive than aniline. For these reactions,
there are no significant differences between thermal
heating, sonication or microwave irradiation, except in
the case of 2,6‐diisopropylaniline that displays ortho
bulky groups and shows a clear beneficial effect of ther-
mal heating. The same preference is observed for the
reaction of ethylenediamine (4c15). All other reactions
(4c12–14, 4c16, 4c17) show a beneficial effect when car-
ried out under microwave irradiation. This result is
important not only because the substrates are either
functionalised or more extended aromatic amines but
also because the reactions are carried out without solvent
addition and require short reaction times.6b, d, 13, 32
ORCID
Govindasamy Vinoth
https://orcid.org/0000-0002-3109-
9993
Sekar Indira https://orcid.org/0000-0001-8167-8953
Madheswaran Bharathi https://orcid.org/0000-0002-7569-
8381
Luis G. Alves https://orcid.org/0000-0002-7938-9850
Ana Margarida Martins
3922-5501
Kuppannan Shanmuga Bharathi
0001-8154-0196
https://orcid.org/0000-0002-
https://orcid.org/0000-
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How to cite this article: Vinoth G, Indira S,
Bharathi M, et al. Synthesis of Imines via Reactions
of Benzyl Alcohol with Amines Using Half‐
Sandwich (η6‐p‐cymene) Ruthenium(II) Complexes
Stabilised by 2‐aminofluorene Derivatives. Appl
Organometal Chem. 2019;e5200. https://doi.org/
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