Iridium complexes with a new type of N^N′-donor anionic ligand catalyze the N-benzylation of amines via borrowing hydrogen

Article abstract of DOI:10.1002/aoc.6003

The development of efficient and eco-friendly methods for the synthesis of elaborate amines is highly desired as they are valuable chemicals. The catalytic alkylation of amines using alcohols as alkylating agents, through the so-called borrowing hydrogen process, satisfies several of the principles of green chemistry. In this paper, four neutral half-sandwich complexes of Ru(II), Rh(III), and Ir(III) have been synthesized and tested as catalysts in the N-benzylation of amines with benzyl alcohol. The new derivatives contain a N^N′ anionic ligand derived from 5-(pyridin-2-ylmethylene)hydantoin (Hpyhy) that has never been tested in metal complexes with catalytic applications. In particular, the Ir derivatives, [(Cp*)IrX(pyhy)] (X = Cl or H), exhibit high activity along with good selectivity in the process. Indeed, the scope of the optimized protocol has been proved in the benzylation of several primary and secondary amines. The selectivity towards monoalkylated or dialkylated amines has been tuned by adjusting the amine:alcohol ratio and the reaction time. Experimental results support a mechanism consisting of three consecutive steps, two of which are Ir catalyzed, and a favorable condensation step without the assistance of the catalyst. Moreover, an unproductive competitive pathway can operate when the reaction is performed in open-air vessels, due to the irreversible release of H₂. This route is hampered when the reaction is carried out in close vessels, likely because the release of H₂ is reversed through metal-based heterolytic cleavage. From our viewpoint, these results show the potential of the new catalysts in a very attractive and promising methodology for the synthesis of amines.

Full text of DOI:10.1002/aoc.6003

Received: 24 May 2020
DOI: 10.1002/aoc.6003
Revised: 3 August 2020
Accepted: 25 August 2020
F U L L P A P E R
Iridium complexes with a new type of N^N0-donor anionic
ligand catalyze the N-benzylation of amines via borrowing
hydrogen
Margarita Ruiz-Castañeda1 | Ana M. Rodríguez1
| Ahmed H. Aboo2
|
Blanca R. Manzano1
| Gustavo Espino3
| Jianliang Xiao2 | Félix A. Jalón1
1Departamento de Química Inorgánica,
Orgánica y Bioquímica, Facultad de
Químicas-IRICA, Universidad de Castilla-
La Mancha, Avda. Camilo J. Cela 10,
Ciudad Real, 13071, Spain
2Department of Chemistry, University of
Liverpool, Liverpool, L69 7ZD, UK
3Departamento de Química, Facultad de
Ciencias, Universidad de Burgos, Plaza
Misael Bañuelos s/n, Burgos, 09001, Spain
The development of efficient and eco-friendly methods for the synthesis of
elaborate amines is highly desired as they are valuable chemicals. The catalytic
alkylation of amines using alcohols as alkylating agents, through the so-called
borrowing hydrogen process, satisfies several of the principles of green chemis-
try. In this paper, four neutral half-sandwich complexes of Ru(II), Rh(III), and
Ir(III) have been synthesized and tested as catalysts in the N-benzylation of
amines with benzyl alcohol. The new derivatives contain a N^N0 anionic ligand
derived from 5-(pyridin-2-ylmethylene)hydantoin (Hpyhy) that has never been
tested in metal complexes with catalytic applications. In particular, the Ir
derivatives, [(Cp*)IrX(pyhy)] (X = Cl or H), exhibit high activity along with
good selectivity in the process. Indeed, the scope of the optimized protocol has
been proved in the benzylation of several primary and secondary amines. The
selectivity towards monoalkylated or dialkylated amines has been tuned by
adjusting the amine:alcohol ratio and the reaction time. Experimental results
support a mechanism consisting of three consecutive steps, two of which are Ir
catalyzed, and a favorable condensation step without the assistance of the
catalyst. Moreover, an unproductive competitive pathway can operate when
the reaction is performed in open-air vessels, due to the irreversible release of
H2. This route is hampered when the reaction is carried out in close vessels,
likely because the release of H2 is reversed through metal-based heterolytic
cleavage. From our viewpoint, these results show the potential of the new
catalysts in a very attractive and promising methodology for the synthesis
of amines.
Correspondence
Félix A. Jalón, Departamento de Química
Inorgánica, Orgánica y Bioquímica,
Facultad de Químicas-IRICA, Universidad
de Castilla-La Mancha, Avda. Camilo
J. Cela 10, 13071 Ciudad Real, Spain.
Email: felix.jalon@uclm.es
Funding information
FEDER funds and Plan Propio de I+D+i-
UCLM,Grant/AwardNumber:2014/10340;
Junta de Castilla y León, Grant/Award
Number:BU087G19; Ministerio de Ciencia,
Innovación y Universidades, Grant/Award
Numbers: RED2018-102471-T,
RTI2018-100709-B-C21
K E Y W O R D S
amines, benzylation, borrowing hydrogen, hydantoin, iridium, transfer hydrogenation
1 | INTRODUCTION
development of advantageous methods to produce more
elaborate amines is of great interest.[1–3] Classical
Amine groups are ubiquitous in natural products,
pharmaceuticals, dyes, food additives, and other fine
chemicals of commercial interest, so that the
methods for the preparation of amines include the
following: (a) reduction of nitro,[4–7] nitrile or amide
groups[8]; (b) functionalization of alkenes or alkynes[9,10]
;
Appl Organomet Chem. 2020;e6003.
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© 2020 John Wiley & Sons, Ltd.
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RUIZ-CASTAÑEDA ET AL.
(c) alkylation of simple amines with alkyl halides[11,12]
;
iridacyles,[13] Rh and Ru,[21,24–28] as well as other
transition metals[29,30] and heterogeneous systems,[31–33]
Catalysts active even in water and ionic liquids[34] or in
solvent-free conditions[35] have been reported, but the
majority of catalysts are used in organic solvent media.
However, many of these processes suffer from incomplete
amine conversion of the intermediate imine. As an
alternative to the BH methodology, in recent years,
TM-free N-alkylation reactions in basic media have
been reported.[36] This procedure may work through an
initial alcohol oxidation by air oxygen,[37,38] and it is
autocatalyzed by the resulting carbonyl derivative
(aldehyde or ketone). Other alternative is the previous
addition of and aldehyde[39] or a ketone.[40] The transfer
hydrogen step takes place by a Meerwein–Ponndorf–
Verley process or by a hemiaminal intermediate.[41]
Although this alternative method offers the advantage of
not needing a metallic catalyst, suffers from drawbacks as
long reaction times (e.g., 24 h), more exigent temperature
conditions (130 ꢀC to 180 ꢀC) and relatively high loadings
of base (20–40 mol%), whose nature and accompanying
cation is critical in most of the reported examples.[39]
The catalytic N-alkylation of amines employing
alcohols as alkylating reagents through BH exhibits
several advantages compared with classical methods:
(a) it generates water as the only by-product, with the
consequent favorable contribution to the atom economy
and eco-friendly principles; (b) avoids the use of often
toxic alkyl halides as alkylating agents; (c) prevents the
formation of overalkylation products when preparing sec-
ondary amines or in other words allows to control the
degree of alkylation as a function of the reaction stoichi-
ometry[17]; and (d) avoids the use of reductants compared
with the reductive amination of carbonyl compounds.[13]
The ligands used in the metal complexes for BH pro-
cesses are diverse although classical ligands such as phos-
phines are very common and half-sandwich complexes
with Cp or arene ligands are also used frequently.[17]
NHC ligands,[21,42] or diamido ligands,[43] such as those
developed by Noyori and Ikariya, have been reported as
well. The derivative 5-(pyridin-2-ylmethylene)hydantoin
(Hpyhy) (see Scheme 2) has the appropriate conforma-
tion to form stable chelate complexes with transition
metals, and, in fact, it has been used as proligand to syn-
thesize nickel and copper complexes.[44,45] The ligand
pyhy is a N^N0 anionic ligand that could be considered
reminiscent of the widely used Noyori–Ikariya ligands or
even the N^C cyclometallating ligands.[13] However,
metal derivatives with the ligand pyhy have never been
explored in BH processes nor in other catalytic processes.
Thus, in this work, we have addressed the synthesis
of four novel half-sandwich complexes of Ru(II), Rh(III),
and Ir(III) with pyhy, including a hydrido derivative. We
and
(d)
reductive
amination
of
carbonyl
compounds.[13–15]
Alternatively, it is possible to obtain secondary or ter-
tiary amines through alkylation of pre-existing primary
or secondary amines, respectively, using alcohols as the
alkylating agents. This methodology, called “borrowing
hydrogen (BH)” or “hydrogen autotransfer,”[16] implies
the use of less toxic and inexpensive starting materials
and produces water as the only by-product.[17] These
advantages have enormously boosted the interest of the
scientific community in this process. BH is mediated usu-
ally by homogeneous catalysts, although heterogeneous
systems have also been used,[6] and was first described by
Grigg et al.,[18] Watanabe et al.,[19] and Murahashi
et al.[20] in an independent way using [Ru], [Ir], or
[Rh] complexes. This one-pot process occurs through
three in situ consecutive reactions as shown in Scheme 1:
Reaction I involves the dehydrogenation of the alcohol to
give an aldehyde or ketone; Reaction II is the condensa-
tion between the primary or secondary amine and the
aldehyde or ketone to give an imine or iminium ion; and
Reaction III consists in the hydrogenation of the imine or
iminium to give the secondary or tertiary amine.[17]
Hence, the alcohol plays a dual role, as it acts as both the
alkylating agent and the H2 donor, which makes this BH
chemical sequence a very efficient process in terms of
atom economy. The condensation reaction is promoted
by acidic species and except in a few cases does not
involve the participation of the metallic catalyst.[21]
Nevertheless, the BH methodology requires the participa-
tion of metal complexes able to promote processes I and
III.[22,23] During the last years, several groups have
contributed to the development of this strategy with
metal complexes, mainly of Ir, including highly active
SCHEME 1 Sequence of reactions in the one-pot N-alkylation
of amines with alcohols through borrowing hydrogen. The process
indicated with the dotted arrow represent the global process, but
the reaction takes place through three consecutive steps (I, II, and
III) (The square in M+-☐ means a vacant coordination site)

RUIZ-CASTAÑEDA ET AL.
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SCHEME 2 Synthesis and structure of the
Ir(III), Ru(II), and Rh (III) complexes bearing
ligand pyhy−
have demonstrated that some of the complexes are active
in the catalytic N-benzylation of a variety of amines, and
we have studied the influence of different factors on the
efficiency and chemoselectivity of the process. A com-
plete conversion of the starting amine is reported in
many of the experiments, and optimization studies have
allowed to get good to excellent chemoselectivity without
the presence of the imine intermediate in the reaction
products. Moreover, different NMR and catalytic experi-
ments have shed light on the mechanism of this
transformation.
room
temperature
in
a
mixture
of
dichloromethane/ethanol (Scheme 2). The hydrido Ir(III)
derivative [(Cp*)IrH(pyhy)] [4] was synthesized by
reacting [1] with an excess of HCOONa in a mixture of
DMSO:H2O (1:5) at room temperature (Scheme 2).
The four derivatives are chiral (C1 symmetry), and
they were obtained as racemic mixtures of R and S enan-
tiomers. The compounds were isolated in good yields as
yellow ([1] and [3]) or red ([2] and [4]) solids and are air
and moisture stable and soluble in common organic sol-
vents such as DMSO, CHCl3, and 2,2,2-trifluoroethanol
(TFE).
1
The new complexes have been characterized by H
2 | RESULTS AND DISCUSSION
and 13C{1H} NMR. In addition, FT-IR, and FAB+ MS
spectra were also recorded, and elemental analysis was
performed. The crystal structures of [1], [2], and the
hydride derivative [4] were determined by single-crystal
X-ray diffraction.
2.1 | Synthesis and characterization of
proligand and complexes
1
The
proligand
5-(pyridin-2-ylmethylene)hydantoin
The main features in the H NMR spectra of these
(Hpyhy), formed from the heterocycles pyridine and
hydantoin, was prepared as previously reported in the lit-
erature by reaction of pyridine-2-carbaldehyde and
hydantoin.[46] The Rh(III) and Ir(III) neutral complexes
of general formula [(Cp*)MCl(pyhy)] (M = Ir, [1];
M = Rh, [3]) were obtained by reacting the respective
dimer [(Cp*)M(μ-Cl)Cl]2 (M = Rh or Ir) with the
proligand Hpyhy at room temperature in a mixture of
acetone/ethanol or dichloromethane/ethanol (Scheme 2).
The Ru(II) neutral p-cymene complex [(η6-p-cym)RuCl
(pyhy)], [2], was prepared by reacting the dimer [(η6-p-
cym)Ru(μ-Cl)Cl]2 with the same proligand Hpyhy at
derivatives are (i) a broad singlet at 7.4–7.5 ppm for [1]
and [4] and at 8.17 ppm for [2] in CDCl3 attributed to
the N H group of the hydantoin moiety. (ii) Singularly,
[4] exhibits a singlet at −9.5 ppm (CDCl3) for the hydride
group. (iii) In the case of [2], the C1 symmetry is reflected
in the existence of four resonances for the aromatic
p-cymene protons and two doublets for the isopropylic
methyl groups.
The IR spectra, apart from the two different bands for
the vibrations of the two C O groups,[46] exhibit the
expected band for the stretching vibration of the NH
group. The position of this band is sensitive to the

4 of 18
RUIZ-CASTAÑEDA ET AL.
formation of hydrogen bonds, and indeed, it appears at
different positions depending on the complex (3159 cm−1
for [1], 3366 cm−1 for [2], 3149 cm−1 for [3], and
3318 cm−1 for [4]).
From FAB+ spectrometry, it has been found that the
base peak corresponds to fragments of the type [M − Cl]+
for all the complexes with the exception of the hydrido
derivative [4] where the pseudo-molecular cation, [M]+,
is detected. Complementary NMR techniques and ele-
mental analysis are also in agreement with the expected
structures for these complexes.
The crystal structures of [1]ꢁCHCl3, [2]ꢁ
(CH3CH2OH)0.5, and [4] were determined by X-ray dif-
fraction. Single crystals of [2]ꢁ(CH3CH2OH)0.5 were
obtained from a saturated chloroform solution of [2].
Surprisingly, single crystals of [1]ꢁCHCl3 were formed in
a saturated chloroform solution of [4]. In the case of [4],
the single crystals were obtained by slow diffusion of a
dimethylformamide solution of [1] in an aqueous solu-
tion of HCOONa. The ORTEP diagrams are shown in
Figures 1–3, whereas selected bond lengths and angles
with estimated standard deviations are gathered in
Table 1, and relevant crystallographic parameters are
given in Table S1. A full list of bond lengths (Å) and
angles (ꢀ) for these structures is compiled in Tables S5–
S7. In the case of [2]ꢁ(CH3CH2OH)0.5, two independent
molecules (Molecules 1 and 2 with Ru(1) and Ru(2),
respectively) are present in the unit cell. The molecular
structures of the three derivatives exhibit the expected
three-legged piano-stool design, with a π-bonded ligand
(Cp* or p-cymene), plus the pyhy N^N0-donor ligand for-
ming a six-membered metallacycle and a monodentate
FIGURE 2 ORTEP diagram for the molecular structure of [2]
(Molecule 1). Thermal ellipsoids are shown at the 30% probability.
Only the R enantiomer is shown. Hydrogen atoms have been
omited for clarity. Solvent molecules have been omited as well
FIGURE 3 ORTEP diagram for the molecular structure of [4].
Thermal ellipsoids are shown at the 30% probability. Only the R
enantiomer is shown. Hydrogen atoms, except the hydride have
been omited for clarity
group (chloride or hydride). The M N(1) (pyridine)
bonding distances are longer than the respective M N
(2) (hydantoin) distances in all the cases, likely as a con-
sequence of the strong σ-donor character of the anionic
N of hydantoin. The M Cl bond distances for [1] and [2]
and the metal-centroid lengths for the three derivatives
are conventional when compared with those reported for
FIGURE 1 ORTEP diagram for the molecular structure of [1].
Thermal ellipsoids are shown at the 30% probability. Only the R
enantiomer is shown. Hydrogen atoms have been omitted for
clarity. Solvent molecules have been omited as well

RUIZ-CASTAÑEDA ET AL.
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RUIZ-CASTAÑEDA ET AL.
interaction with the stated H and O atoms. The value of
the bite angle also depends on the metal (around 84ꢀ for
Ir derivatives and 87ꢀ to 88ꢀ for the two independent Ru
molecules). The reported values for the Ni and Cu com-
plexes with the pyhy ligand are higher (92ꢀ to 93ꢀ).[44,45]
The 3D architecture in the crystal structures of these
complexes is sustained by hydrogen bonding interactions
and by other weak interactions such as π–π stacking con-
tacts. In particular, the structure of [1] features dimeric
entities consisting of two molecules of [1] that interact
with each other through double, strong, and identical
hydrogen bonds, N(3) H(3A)ꢁꢁꢁO(2) (Figure S2). The 3D
structure of [2] exhibits dimeric entities as well, but in
this case, they are the result of double hydrogen bonding
interactions, N(3) H(3A)ꢁꢁꢁCl(1), and π–π stacking con-
tacts between the hydantoin rings (Molecule 1, Figure S3).
In the crystal structure of [4] one of the main motifs is a
double chain based on the hydrogen bonding interactions
N(3) H(3A)ꢁꢁꢁO(1) and C(4) H(4)ꢁꢁꢁO(2) (Figure S4).
CHART 1 β angle
similar complexes.[47–52] It is noteworthy that the hydride
atom has been located in the Fourier difference map and
refined freely.
The hydantoin part of the ligand is essentially planar,
reflecting an electron delocalization around this ring. The
same situation has been found in the Ni and Cu com-
plexes previously described.[44,45] The respective distance
ranges determined for this ring are reflected in
Figure S1a where it is observed that distances B and D
are the longest. In accordance with the electron delocali-
zation, there is no pyramidalization on the NH group
(sum of the angles around the N atom = 358.5ꢀ to 360ꢀ).
The dihedral angle (α) between the two rings of the pyhy
ligand in general is low (14.8ꢀ for [1] and 6.3ꢀ and 11.6ꢀ
for Molecules 1 and 2 of [2]), but it is higher for the
hydrido complex [4] (27.7ꢀ). This fact could reflect differ-
ent degrees of delocalization in the metallacycle. Interest-
ingly, there is a clear correlation between the α angle and
the distance C(5) C(6) (distance F in Figure S1a). When
the angle is higher (complexes [1] and [4]), this distance
is longer (1.45–1.46 Å) whereas for the more planar
ligands, the distance is shorter (two molecules of [2],
1.44 Å) (the value expected for a Csp2–Csp2 distance is
1.47 Å) reflecting in these last cases a higher degree of
delocalization (Figure S1b). On the other hand, the β
angle (see Table 1 and Chart 1) that reflects the orienta-
tion of the N^N0 ligand with respect the N–M–N plane
depends essentially on the metal with the highest values
for the Ir compounds (33.8ꢀ for [1] and 34.1ꢀ for [4]) and
lower values for the Ru complex [2] (6.3ꢀ and 11.6ꢀ), a
fact that reflects a possible steric hindrance between both
the C(1) H(1) and mainly the O(2) groups and the
methyl groups of the Cp* ring. In the case of the two mol-
ecules of [2], the p-cymene ring is rotated in such a way
that the C(16) C(17) axis is oriented to reduce the steric
2.2 | Catalytic activity
2.2.1 | Optimization of the reaction
conditions
As previously stated, in this work, we decided to test the
catalytic potential of our complexes in BH reactions.
Thus, we chose the N-alkylation of aniline, 5a, with
benzyl alcohol, 6, as the model reaction (Scheme 3) and
performed the initial trials with [1] as the precatalyst,
using conditions similar to those recently reported by
some of us.[13]
To our delight, complex [1] was active in the process
and provided a moderate conversion (33%) along with a
high selectivity for benzylamine 7a (30%) versus imine 8a
(3%) when the reaction was carried out in 2,2,2-TFE at
100 ꢀC in the presence of K2CO3 (5% mol) and using a
20% excess of aniline in a sealed reaction tube under N2
during 7 h (Entry 1, Table 2). The beneficial role of TFE
was demonstrated when this reaction was carried out in
the absence of solvent (Entry 2). Both conversion and
benzylamine production decreased notably (Entry 2 vs.
Entry 1). After that, we analyzed the influence of several
factors over the performance of precatalyst [1] in this
transformation, aiming to optimize the yield of 7a.
First, we probed different carbonates, hydroxides, and
alkoxides as alternative bases in TFE (Entries 2–14;
SCHEME 3 Catalytic borrowing hydrogen reaction
studied for the N-alkylation of aniline using benzyl alcohol

RUIZ-CASTAÑEDA ET AL.
7 of 18
TABLE 2 Base optimization for the N-alkylation of aniline using benzyl alcohol in TFE
Entry
1
2a
Base
%Conversion
%Amine (product)
%Imine
K2CO3
33
16
37
37
6
30
13
34
34
4
3
3
3
3
2
-
K2CO3
3
Cs2CO3
Cs2CO3
Na2CO3
CaCO3
4b
5
6
0
-
7
(NH4)2CO3
(Fe3)2CO3ꢁFeCO3
CdCO3
0
-
-
8
0
-
-
9
0
-
-
10
11
12
13
14
KOH
11
21
0
8
2
2
-
NaOH
18
-
NH4HCO3
tBuOK
tBuONa
8
5
3
3
13
10
Note: Aniline (0.6 mmol), benzyl alcohol (0.5 mmol), [1] (1 mol%, 5 μmol), base (5 mol%, 0,025 mmol), TFE (2.6 mL), 100 ꢀC, 7 h.
aIn the absence of any solvent (neat conditions); with identical ratio of the different components but starting with 18 μmol of [1].
b120 ꢀC; conversions were determined by 1H NMR spectroscopy.
Table 2). Unfortunately, we only made a small improve-
ment in this study by replacing K2CO3 with Cs2CO3 (37%
conversion, Entry 3, Table 2). In addition, we established
that an excess of base is beneficial for both the conversion
and the selectivity of the prototype reaction (Table S2).
Specifically, the use of two equivalents of either K2CO3 or
Cs2CO3 increased the conversion up to 68% and 67%,
respectively (Entries 1–2, Table 3). In conclusion, we
decided to use K2CO3 in a 2:1 molar ratio (base/limiting
substrate), as the ideal additive due to its lower price
compared to Cs2CO3.
Then, the model reaction was tested in the presence
of [1] in several protic (ethanol) and aprotic (toluene and
DMF) solvents, apart from TFE, using K2CO3 as the base
(Table 3, Entries 3–5). However, no reaction was
observed in these alternative media. The use of other
bases in water was neither satisfactory (Entries 6 and 7,
Table 3), although a small conversion was obtained when
using the combination water/Cs2CO3 (Entry 8, Table 3).
Hence, we concluded that TFE is essential to promote
the catalytic activity of [1] in this reaction and
anticipated that it could play a multiple role in the
TABLE 3 Optimization of conditions (solvent and base).
Entry
Base
Solvent
TFE
%Conversion
%Amine (product)
%Imine
1
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOH
KOH
67
68
0
62
65
-
4
1
-
2
TFE
3
Toluene
EtOH
DMF
H2O
4
5a
1
-
-
0
-
-
6
0
-
-
7
H2O
0
-
-
8
9b
Cs2CO3
K2CO3
H2O
9
3
6
1
TFE
50
48
Note: Aniline (0.6 mmol), benzyl alcohol (0.5 mmol), [1] (1 mol%, 5 μmol), base (2 equiv., referred to the amount of benzyl alcohol, 1 mmol),
solvent (2.6 mL), 100 ꢀC, 7 h; conversion were determined by 1H NMR spectroscopy.
aIn DMF, the result was analyzed using thin-layer chromatography (TLC) and compared with a product of another reaction.
b80 ꢀC.

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RUIZ-CASTAÑEDA ET AL.
corresponding catalytic cycle, as discussed below in the
mechanistic proposal. The crucial role of TFE has been
previously observed by some of us in the alkylation of
amines with alcohols or in the synthesis of nitrogen
heterocycles when using complexes of the type [Cp*IrCl
(C^N)] as precatalysts.[13]
The effect of the temperature on the production of 7a
was examined as well. Indeed, we determined that a
decrease in the reaction temperature from 100 ꢀC to
80 ꢀC is detrimental for the conversion (Entries 2 and
9, Table 3). However, a rise in this parameter up to
120 ꢀC was neither satisfying (Entries 3 and 4, Table 2).
Therefore, the ideal temperature for this transformation
was established as 100 ꢀC.
conditions was performed, and the unreacted substrates
were recovered from the reaction crude after 7 h
(Table 4, Entry 5).
As some authors[38,39] have reported that an aldehyde
can substitute a metal complex as catalyst in TM-free N-
alkylation reactions in basic media, two additional cata-
lytic experiments were performed replacing the metal
precatalyst with benzaldehyde (10 mol%) both in the
presence (Entry 10, Table 4) and in the absence of base
(Entry 11). The conversion in both cases was 0%, and
minor amounts or imine were observed in both tests.
2.2.2 | Catalyst screening
Further improvements were achieved using an excess
of benzyl alcohol. Thus, we obtained a conversion of 97%
with a high selectivity (94% of 7a) when using a benzyl
alcohol/aniline molar ratio of (2:1) (Entry 3, Table 4).
This conversion dropped down when 10 mol% of NaCl
(NaCl/[1] molar ratio = 10/1) was added to the reaction
mixture (Entry 4), reflecting that substitution of the chlo-
ride ligand is important in the triggering of the catalytic
reaction.
The conditions established in the last experiment
(Entry 3, Table 4) seem to be optimal for the
N-benzylation of aniline in the presence of [1]. In conse-
quence, we used those conditions for additional
experiments aiming to compare the catalytic activity of
complexes [1]–[4] and to explore the substrate scope. A
blank experiment without catalyst (24 h) using these
All the new complexes were tested in the benzylation of
aniline using the optimized conditions (Table 4). The cat-
alytic activities of complexes [2] and [3] in the model
reaction were disappointing compared with that of [1].
In fact, [2] and [3] gave conversion values of 13% and
15%, respectively (Entries 7 and 8 in Table 4). However,
the Ir(III) hydride species [4] exhibited quantitative con-
version with ideal selectivity towards the monoalkylated
amine (Entry 9), indicating that [4] could be the active
catalytic intermediate when using [1]. The same result in
terms of activity and selectivity was also found in the
presence of [1] after 12 h (Entry 6, Table 4). Conse-
quently, the Ir(III) precatalyst [1] was chosen for the sub-
sequent studies because it is more stable and more easily
prepared than [4].
TABLE 4 Optimization of alcohol/aniline ratio and precatalyts screening.
Entry
Precatalyts
Benzyl alcohol (mmol)
Aniline (mmol)
%Conversion
%Amine (product)
%Imine
1
[1]
[1]
[1]
[1]
-
0.5
0.5
1
0.6
1
68
64
97
35
0
65
61
94
35
0
1
2
3
0
0
-
2
3
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
4a
5b
6c
7
1
1
[1]
[2]
[3]
[4]
-
1
>99
13
15
>99
0
>99
9
1
4
4
-
8
1
11
>99
-
9
1
10d
11e
1
4
8
-
1
0
-
Note: Precatalyst (1 mol%, with respect to the starting material in default, 5 μmol), K2CO3 (2 equiv. with respect the limiting substrate,
1 mmol), TFE (2.6 mL), 7 h, 100 ꢀC; conversion were determined by 1H NMR spectroscopy.
aIdentical conditions of Entry 3 but adding 10 mol % of NaCl.
b24 h. Identical conditions as in “Note” but without precatalyst.
c12 h.
dConditions as in “Note” but without a metallic precatalyst and using benzaldehyde (10 mol%).
eConditions as in “Note” but without a metallic precatalyst and without base and using benzaldehyde (10 mol%).

RUIZ-CASTAÑEDA ET AL.
9 of 18
for 5i, probably due to the steric bulk of two Me groups
in the benzyl position of the starting amine that pre-
cludes the easy formation of the iminium intermediate.
Finally, we tried some aliphatic secondary amines,
5j–5q, with contrasting results. For instance, amines
5j–5l afforded the expected monobenzyl derivatives with
excellent yields (Entries 10–12 in Table 5), and
pyrrolidine, 5m, gave the monoalkylated product in a
57% yield (Entry 13, Table 5). It is remarkable the case of
the reaction with 5k that shows, in addition to high selec-
tivity, a performance that exceeds those obtained with
the highly active iridacycles in comparable experimental
conditions.[13] On the contrary, amine 5n and other steri-
cally hampered secondary amines, such as 5o–5q, did not
undergo any degree of transformation (Entries 14–17 in
Table 5).
SCHEME 4 Amine scope: general conditions and products for
the N-benzylation of amines using benzyl alcohol
2.2.3 | Substrate scope
The potential of this protocol for the N-benzylation of
amines was demonstrated by reacting several types of
amines with benzyl alcohol in the presence of [1] under
the previously established optimized conditions, unless
otherwise stated (see Scheme 4 and Table 5).
Moreover,
the
reaction
between
1,2,3,4-tetrahydroisoquinoline (5k) and benzyl alcohol
was carried out in the optimal conditions for the N-
benzylation of the amine, but in the presence of air
instead of N2, resulting in a conversion > 99%. Therefore,
we concluded that the use of inert atmosphere is dispens-
able in this protocol.
Furthermore, the synthetic viability of the established
protocol was demonstrated by isolating benzyl amines,
7a, 7g, 8g, 7j, 7k, 7l, and 7i from the respective reactions
with amines 5a, 5g, 5j, 5k, 5l, and 5i (Table 5) as the
ammonium chloride salts.* In all the cases, the isolated
yields were higher than 80% except for the products
obtained from 5g (Entry 7).
First, we tested diverse anilines (5b–5f in Table 5)
and obtained excellent conversion values for those with
electron-donating groups in the para position of the phe-
nyl group, 5b ( OMe) and 5c ( Me), or those with mod-
erate electron effects, 5d ( Cl) (Entries 2–4 in Table 5).
Nevertheless, the chemoselectivity of the transformations
with 5b and 5c was not as good as that observed for ani-
line, 5a, because dibenzylated products were formed in
significant yields for both substrates (compare Entry
1 with Entries 2 and 3 in Table 5). Aniline 5e with an
electron-withdrawing group ( NO2) in the para position
of the phenyl ring gave a low conversion (30%) of the
monoalkylated product (Entry 5 in Table 5) as previously
reported for other catalytic systems.[13] The bulky aniline
5f with a methyl group in the ortho position of the phenyl
ring provided also a low conversion value (compare
Entries 3 and 6 in Table 5).
On the other hand, it is worth emphasizing that for-
mation of dialkylated amines is favored for anilines with
electron-donating groups (Entries 2 and 3), whereas it is
not observed for anilines with electron-withdrawing
groups or for aniline (Entries 1, 4 and 5). This tendency
can be rationalized as a result of the high nucleophilic
nature of the amines resulting from the first benzylation,
7b and 7c, which facilitates the second benzylation
reaction.
2.2.4 | Optimization of selectivity
First at all, we decided to extend the reaction time to
24 h, using the standard conditions established in
Table 5, in order to increase the selectivity towards the
dialkylation of some of our amines. Amines 5b, 5c, 5g,
and 5h were selected for this study since the selectivity
after 7 h was moderate or low and hence improvable. As
expected, the relative ratio of the dialkylated amine
increased up to certain extend, although the selectivity
towards the dialkylated products remained far from opti-
mal (Table 6, Entries 1–4).
With the goal of refining the selectivity towards the
monoalkylated amines of this methodology, we decided
to monitor the benzylation of p-anisidine (5b) and
p-toluidine (5c) using short reaction times. The results
for times of 1, 2, 4, 7, and 24 h can be found in Tables S3
Then, we probed several aliphatic primary amines
such as 5g–5i (Entries 7–9 in Table 5). These experiments
provided full conversion values with good or excellent
selectivity for the monobenzyl product. The less ham-
pered amines 5g and 5h allowed the formation of small
amounts of dibenzyl products while this is not the case
*The substrates chosen have been those with almost a total conversion
and among them those that present the best chemoselectivity towards
the formation of the monoalkylated amine.

10 of 18
RUIZ-CASTAÑEDA ET AL.
TABLE 5 N-Alkylation of amines using benzyl alcohol (substrate scope).
Entry
Amine
% Conversion
97b
% Amine product monoalkylated/dialkylated
Isolated yield (%)a
1
94/0
84
2
3
4
>99
>99
70
58/42
70/30
70/0
-
-
-
5
30
30/0
-
6
7
58
50/8
-
>99
84/16
75/15
8
>99
>99
>99
89/11
>99/0
>99/0
-
92
80
9
10
11
12
13
>99
>99
57
>99/0
>99/0
57/0
84
90
-

RUIZ-CASTAÑEDA ET AL.
11 of 18
TABLE 5 (Continued)
Entry
Amine
% Conversion
% Amine product monoalkylated/dialkylated
Isolated yield (%)a
14
0
-
-
15
16
17
0
0
0
-
-
-
-
-
-
Note: Amine (0.5 mmol), benzyl alcohol (1 mmol), K2CO3 (2 equiv.), [1] (1 mol%), TFE (2.6 ml), T = 100 ꢀC, 7 h; conversion values were
determined by 1H NMR spectroscopy. Conversions are referred to the starting amine. All the experiments that gave dialkylated amines were
checked by mass spectrometry.
aIsolated yields were calculated for the hydrochloride salts.
b3% of imine was detected in solution for this substrate.
and S4 for 5b and 5c, respectively. The corresponding
data are depicted in Figure S6 for 5b and Figure 4 for 5c.
In the case of 5b (Entry 1 of Tables 6 and S3), the
highest amount of monoalkylated amine is achieved after
2 h (86%). At this time, only 6% of dialkylated amine and
an 8% of the starting p-anisidine are present in
solution. Better results were obtained for p-toluidine 5c
(Entry 2 of Tables 6 and S4 and Figure 4) with a 94/6
ratio (monoalkylated/dialkylated amine) upon 2 h of
reaction and a 100% of monoalkyaled amine after 1 h, in
both cases with a quantitative conversion of the amine.
For the bulky amines 5g and 5h, the products ratio at
2 h was also determined (Entries 3 and 4, Table 6). The
selectivity towards the monalkylated amine was
improved relative to that obtained upon 7 h for 5g but
not for 5h. In any case, quantitative conversions and
good selectivity were already obtained for these
substrates at 7 h.
1H NMR spectrum of complex [1] in TFE-d3 exhibited
resonances that did not change their position when five
equivalents of NBu4Cl were added, and therefore, they
were assigned to the chlorido complex [1]. The addition
TABLE 6 Time dependence of the ratio of
monoalkylated/dialkylated amine products obtained from 5b, 5c,
5g, and 5h at different reaction times.
Entry
Amine
2 h
7 h
24 h
1
86/6a
58/42
32/68
2
3
94/6
70/30
84/16
47/63
68/32
90/10
Besides, in the case of 5b, an increase in the initial
alcohol:amine ratio to a 10:1 value was also assayed to
achieve a better conversion to the dibenzylamine. A ratio
of 4.6:1 of the dialkylated/monoalkylated amines was
obtained, although benzaldehyde (17%) was also found in
the reaction mixture (see Entry 7 of Table S3) due to the
use of an excess of alcohol.
4
89/11
89/11
80/20
Note: Amine (0.5 mmol), benzyl alcohol (1 mmol), K2CO3 (2 equiv.),
[1] (1 mol%), TFE (2.6 ml), T = 100ꢀC. Reaction time 2, 7, or 24 h;
conversion values were determined by 1H NMR spectroscopy.
Conversions were determined with respect to the starting amine.
a8% of p-anisidine remains unreactive.
2.3 | Mechanistic proposal
In order to delve into the mechanism of the benzylation
process, we performed some additional experiments. The

12 of 18
RUIZ-CASTAÑEDA ET AL.
FIGURE 4 N-Alkylation of p-toluidine (5c)
using benzyl alcohol (1:2 ratio)
of the salt only produced a broadening of two of these
resonances, a fact that could be due to equilibria
involving hydrogen bonds, either intermolecular or with
the chloride anions of NBu4Cl (Figure S7). In contrast,
the addition of five equivalents of AgBF4 to the initial
solution of [1] led to the disappearance of the stated
resonances and the appearance of new signals for a
species labeled as 1A (see Figure S8) that could be a
TFE-solvato derivative or an unsaturated complex
stabilized by the presence of the Ir N (hydantoin)
amido bond.
Complex [1] in TFE-d3 reacts with K2CO3 (the base
used in our catalytic processes). When 10 equiv. of the
base were added to a solution of the precatalyst in an
NMR tube, an equilibrium was established between
[1] and a major species that putatively was assigned
to the carbonato compound [Cp*Ir(OCO2)(pyhy)]− (1B)
(Figure S5). A 2/8 ratio between the chlorido and the
carbonato species was found after 2 days of reaction.
Attempts to isolate this new species in its pure form
failed due to its insolubility in conventional solvents such
as CHCl3, acetone, or methanol and its lability in others,
such as DMSO, in which the formation of two major
species was observed.
In another NMR experiment (see Figures S9 and S10
for the different regions of the spectra), benzyl alcohol
(5 equiv.) was added to a TFE-d3 solution of [1], and no
changes were observed in the 1H NMR spectrum of the Ir
complex. When K2CO3 (5 equiv.) was added, the stated
equilibrium between the chlorido and the carbonato
species was observed. Additionally, three other minor Ir
species appeared in the spectrum (x, y, and z) that can be
due to the reaction of the carbonato-complex with benzyl
alcohol. They were clearly observed, at room tempera-
ture, in the spectrum regions corresponding to the
methine-bridge protons of the pyhy ligand and the Cp*
(Figure S10), although in this last region resonances of
x and y exhibit the same chemical shift. Besides, the
incipient formation of benzaldehyde was also observed
(1.4% relative to the concentration of all the Ir species).
Heating at 60 ꢀC for 5 min produced an increase in the
signals of the aldehyde (3%) and mainly in those of
the carbonato-complex 1B that represented 55% of the Ir
mixture. The resonances of [1] were small at this
moment. At 60 ꢀC, the resonances of the three minor
products (x–z) broadened, together with the residual
resonances of [1]. We tentatively assigned these reso-
nances as due to species resulting from the substitution
of the chloride or the carbonate ligand by CF3CH2O− or
PhCH2O− (1D) and the adduct with benzyl alcohol (1C),
that could be, all together, in dynamic interchange with
[1] at 60 ꢀC.
Then, aniline (2.5 equiv.) was subsequently added. At
room temperature, although 1B was the major compo-
nent, other four Ir compounds were visible in the spec-
trum (labeled as h–k in Figure S10). After an increase in
temperature to 60 ꢀC for 5 min, the resonances of 1B
almost disappeared while the intensity of resonances of
i–k species increased, achieving the respective h–k
species a ratio of 3/29/38/30.
The resonances for the hydrido complex [4] were not
observed along these experiments, reflecting a fast evolu-
tion of this species in the presence of the amine, which
indicates that when the imine intermediate is present, [4]
acts as a good nucleophile.
In an independent experiment, we proved that the
benzyl alcohol undergoes dehydrogenation to produce
benzaldehyde (6%) under the optimal catalytic conditions
in the presence of complex [1] ([1] 1 mol %, TFE, 100 ꢀC,
K2CO3, 7 h) but in the absence of aniline. Consequently,
we proved that this reaction may well be the first step of
the alkylation process, even though from a thermody-
namic point of view it is likely that subsequent steps push
the dehydrogenation equilibrium forward.

RUIZ-CASTAÑEDA ET AL.
13 of 18
A mechanistic interesting point is to know if the con-
densation step of the starting amine and the aldehyde
intermediate occur with or without the assistance of the
catalyst. On one side, this condensation has been consid-
ered rapid and easily accomplished without the interven-
tion of the catalyst by some authors.[53–55] Other authors
have reported that this process is more favored by
water[56] or alcohols[57] than by some metallic species. In
contrast, other theoretical[21,58,59] and experimental[21]
studies indicate that the condensation step takes place in
the coordination sphere of the catalyst. In order to delve
fact that although has been previously pointed out, is not
usually studied.[13]
With all this experimental information in mind, we
tentatively formulate a mechanistic proposal for this one-
pot catalytic process with the following steps (Scheme 5):
(a) activation of the Ir–Cl precatalyst [1] by dissociation
of the chloride anion to form intermediate 1A that could
be a TFE-solvato species or to have a vacant site. The
formation of the carbonato-complex 1B is also reasonable
from our experimental data and can act as a resting state
of the precatalyst [1]; (b) coordination of the benzyl
alcohol to the metal ion to produce the Ir-OHR species
1C; (c) deprotonation of the coordinated benzyl alcohol
which affords the Ir-alkoxide intermediate 1D;
(d) formation of the Ir-hydride intermediate, [4], together
1
this question, we monitored by H NMR the reaction of
benzaldehyde and p-toluidine in a molar ratio 2:1 in TFE
media in the absence of catalyst, and we found that this
reaction was completed at room temperature in 5 min
(see Figure S11). Therefore, the condensation of benzal-
dehyde and an arylamine seems to take place quickly in
TFE and without the need of either the catalyst or the
base. Interestingly, it has been observed in our catalytic
amine alkylation experiments that as long as the starting
arylamine is not completely consumed, benzaldehyde is
not present.
Finally, considering that the catalytic experiments
were performed in sealed reaction vessels and in order to
get information about the possible participation of
released H2 in the process, a catalytic experiment of the
N-alkylation of p-anisidine using benzyl alcohol in an
open reaction vessel (coupled to a condenser) was per-
formed (2 h, 100 ꢀC, see Table S3, Entry 3). If the results
are compared with the similar experiment in a sealed
reaction vessel (Entry 2), a clear reduction in the amine
conversion (from 92% to 58%) was observed. Thus, it is
concluded that hydrogen gas may be released and reused
when the catalytic process is performed in close vessels, a
with
benzaldehyde,
through
β-elimination;
(e) condensation between the benzaldehyde and the ani-
line, assisted by TFE, to form an imine; and (f) activation
of the imine through the formation of a hydrogen
bonding imine-TFE adduct and hydride transfer from the
Ir–H intermediate, [4], to produce the final amine and
regenerate the active Ir species 1A. The result of the
aforementioned catalytic test performed in an open vessel
indicates that the hydrido intermediate, [4], can alterna-
tively react with protons from the media (CF3COOH or
HCO3−) to generate species 1A and H2. From our view,
this unproductive pathway is disfavored in close vessels
due to the reversibility of reaction (step g in Scheme 5)
but can really compete with the productive pathway in
open vessels.
As aforementioned, the crucial role of TFE in this
process could stem from its participation in several steps
of the mechanism. First, it could intercede in the activa-
tion of the Ir Cl bond of [1] and the carbonato-complex
1B through hydrogen bonding interactions.[13,60] In other
words, we postulate that the polarity and hydrogen bond-
ing ability of TFE could promote the dissociation of the
chloride or carbonate anions to afford the coordination of
benzyl alcohol due to the assumed high solvation energy
of Cl− and CO3 in this solvent (step a in Scheme 5).
2−
Second, we speculate that the acidic nature of TFE could
also catalyze the condensation reaction (step e). In third
place, TFE could activate the imine bond (C N) through
hydrogen bonding (step f), facilitating the hydride
transfer from the Ir H intermediate. The activation of
the imine intermediate has been previously considered as
a necessary step for an effective hydride transfer to
imines.[61–63] In Scheme 5, we also assume that TFE is
presumably the proton donor in the amine formation
(step f), based on its pKa value (11.4) lower than that of
−
benzyl alcohol (15.4). The participation of the HCO3
SCHEME 5 Proposed mechanism for the N-benzylation of
aniline with benzyl alcohol through borrowing hydrogen
(BH) methodology
anion as proton donor (pKa = 10.33) cannot be ruled out,
although its concentration in the medium is lower than

14 of 18
RUIZ-CASTAÑEDA ET AL.
that of TFE. Finally, TFE can also participate as the
proton transfer agent in step g.
4 | EXPERIMENTAL SECTION
It is interesting to note that in the catalytic tests,
the presence of imine is only observed in one case
(Entry 1, Table 5) suggesting that the hydrogen transfer
to the imine is also a fast process.
4.1 | General methods
All manipulations were carried out under an atmosphere
of dry oxygen-free nitrogen using standard Schlenk tech-
niques. Solvents were distilled from the appropriate dry-
ing agents and degassed before use. Elemental analyses
were performed with a Thermo Quest FlashEA 1112
microanalyzer and IR spectra on a Shimadzu IRPrestige-
21 IR spectrometer equipped with a Pike Technologies
ATR. The FAB+ mass spectrometry measurements were
made with a Thermo MAT95XP mass spectrophotometer
3 | CONCLUSIONS
To conclude, we have synthesized and characterized four
new half-sandwich complexes of Ru(II), Rh(III), and
Ir(III) with the deprotonated form of the proligand
5-(pyridin-2-ylmethylene)hydantoin. In addition, we have
demonstrated that this kind of N^N0 anionic ligand, never
used previously in homogeneous catalysis, leads to
iridium(III) derivatives (chloride and hydride) which are
active catalysts in the one-pot monobenzylation or
dibenzylation of amines using benzyl alcohol as the
alkylating agent. It has been verified that the chlorido
precatalyst [Cp*IrCl(pyhy)] is stable in the presence of air
under catalytic conditions. The optimized conditions for
this protocol included the use of K2CO3 and TFE as base
and solvent, respectively. In general, with primary or
secondary amines, good to excellent yields were obtained
for the monoalkylated amines although the presence of
electron-withdrawing or ortho-substituted groups in
anilines or steric hindrance in secondary amines reduced
the conversions. The selectivity towards the mono-
alkylated amines was good in general and it was
improved using short reaction times reaching to selectiv-
ity values between 90% and 100% even with quantitative
conversion in one case. It was verified that the presence
of donor groups in anilines, longer reaction times or
higher alcohol:amine ratios increased the ratio of the dia-
lkylated amines. Different NMR and catalytic experi-
ments gave information about the mechanism of the
process that includes the detection of some iridium inter-
mediates and of the aldehyde formed by dehydrogenation
of the alcohol. It is demonstrated that the condensation
of benzaldehyde and p-toluidine in TFE even at room
temperature is a fast process that takes place in the
absence of catalyst. The potential release of H2 from the
reaction between the Ir H species and protons under
open vessel conditions seems to be deleterious for the
alkylation reaction. However, under pressure, this
unproductive process is in some way reversed or ham-
pered. The demonstration of the activity in BH of the
iridium compounds with the anionic pyhy ligand opens
the way to the formation of new complexes with modified
ligands including substituents either in the pyridine ring
or in the NH group that could lead to complexes with
improved catalytic performance.
1
with magnetic sector. H and 13C{1H} NMR spectra were
recorded on Varian Innova 500, Varian Unity 300, and
Varian Gemini 400. Chemical shifts (ppm) are relative to
TMS (1H, 13C NMR). The atom numbering is reflected in
Scheme 2. Coupling constants (J) are in Hertz. H–1H
1
COSY spectra: standard pulse sequence with an acquisi-
tion time of 0.214 s, pulse width of 10 ms, relaxation
delay of 1 s, 16 scans, 512 increments. For H–13C g–
1
HMBC and g–HMQC spectra, the standard Varian pulse
sequences were used (VNMR 6.1 C software). The spectra
were acquired using 16,096 (1H) and 25,133.5 Hz (13C)
widths; 16 transits of 2048 data points were collected for
each of the 256 increments. NOESY spectra were
acquired using 8000 Hz width, and 16 transits of 2048
data points were collected for each of the 256 increments,
with a pulse time of 1 s and mixing time of 1 s. In the
NMR analysis, s, d, and bs denote singlet, doublet and
broad signal, respectively. Unless otherwise stated, the
13C{1H} NMR signals are singlets. For mass spectrometry,
a GC-EI Varian 3800 GC coupled with a Varian Triple
Quad 1200L detector was used; 20 and 70 eV were tested
as power source for the electronic ionization of the sam-
ple, and representative changes in the analyzed signals
and in the ionization level of the sample were not
observed. The experimental conditions for GC were as
follows: column factor IV (30 m × 0.25 mm × 0.25 μm).
Helium flux: 1 mL min−1; injector. All the ammonium
chloride salts resulting as reaction products were vacuum
dried for several hours prior to being weighed and ana-
lyzed. The metal complexes [(p-cymene)RuCl2]2,[64]
[Cp*RhCl2]2,[65]
[Cp*IrCl2]2,[65]
and
5-(pyridin-
2-ylmethylene)hydantoin[46] were prepared following
published procedures.
4.2 | X-ray crystallography
A summary of crystal data collection and refinement
parameters for all compounds are given in Table S1. The
single crystals for [1]ꢁCHCl3, [2]ꢁ0.5 EtOH, and [4] were

RUIZ-CASTAÑEDA ET AL.
15 of 18
0
mounted on a glass fiber and used for data collection on
a Bruker X8 APEX II CCD-based diffractometer equipped
with a graphite monochromated MoKα radiation source
(λ = 0.71073 Å). The data reduction was performed with
the APEX3 software,[66] and an absorption correction was
performed with the program SADABS.[67] Crystal
structures were solved by direct methods using the SIR97
program[68] and refined by full-matrix least squares on F2
including all reflections using anisotropic displacement
parameters by means of the WINGX crystallographic
package.[69,70] All hydrogen atoms were included into the
model at geometrically calculated positions and refined
using a riding model except H(1a) of [4], which was
taken from the Fourier difference map and refined freely.
CCDC 1,868,126–1,868,128, respectively, for complexes
[1], [2], and [4] contain the supplementary crystallo-
graphic data for this paper.
4J(H,H)
=
1.5 Hz, 1H, H3 Pyr), 7.03 (ddd,
3J(H,H) = 7.7 Hz, J(H,H) = 5.8 Hz, J(H,H) = 1.4 Hz,
3
4
0
1H, H5 Pyr), 5.97 (s, 1H, CH6), 1.54 (s, 15H, 5 × MeCp*
)
1
ppm. H NMR (500 MHz, TFE-d3, 25ꢀC): δ = 8.73 (d,
0
3J(H,H) = 5.5 Hz, 1H, H6 Pyr), 7.80 (t, J(H,H) = 6.8 Hz,
3
0
0
1H, H4 Pyr), 7.37 (d, 3J(H,H) = 8.4 Hz, 1H, H3 Pyr), 7.21 (t,
0
3J(H,H) = 6.8 Hz, 1H, H5 Pyr), 6.20 (s, 1H, CH6), 1.50 (s,
15H, 5 × MeCp*) ppm. 13C{1H} NMR of [1] (101 MHz,
DMSO-d6, 25ꢀC): δ = 167.75 (C5Hyd), 159.76 (C4Hyd),
0
0
157.53 (C6 Pyr), 151.90 (C2 Pyr), 140.41 (C2Hyd), 138.15
0
0
0
(C4 Pyr), 125.06 (C3 Pyr), 123.30 (C5 Pyr), 100.08 ( CH ),
86.70 (Cq-Cp*), 9.18 (MeCp*) ppm. 13C{1H} NMR, DMSO
adduct of [1] (101 MHz, DMSO-d6, 25ꢀC): δ = 166.89
0
0
(C5Hyd), 161.25 (C4Hyd), 157.70 (C6 Pyr), 151.79 (C2 Pyr),
0
0
140.69 (C2
or C4 Pyr), 139.46 (C2
or C4 Pyr), 127.86
Hyd
Hyd
0
0
0
0
(C3
or C5 Pyr), 125.34 (C3
or C5 Pyr), 102.36
Pyr
Pyr
( C6H ), 95.29 (Cq-Cp*), 9.13 (MeCp*
)
ppm. IR
(νmax/cm−1): 3159, (br), ν(N H); 3061, (br), ν(CHCp*);
1695, (s), ν(C(4) O(4)); 1624, (s), ν(C(2) O(2)); 1595, (s),
ν(C C); 1340, (vs), δ(N H); 1122, (s), ν(C N C);
763, (s), δ(C H)ip; 640, (s), δ(C H)oop. MS (FAB+): m/z
4.3 | Synthesis of the new complexes
4.3.1 | Synthesis of [Cp*IrCl(pyhy)] ([1])
(assign., rel. int. %): 516 [(M
−
Cl)+, 100.0],
363 [(M − L)+,7.2]. Crystals suitable for X-ray diffraction
were obtained by slow evaporation from a saturated
chloroform solution of [4] at 20ꢀC. [1] is soluble in TFE,
DMSO, CDCl3, and MeOH. It is insoluble in toluene,
water, and acetone.
To a solution of Hpyhy (491.9 mg, 2.6 mmol) in 40 mL of
acetone:ethanol (40:60) mixture, [Cp*IrCl2]2 (868.4 mg,
1.09 mmol) was added, and the reaction mixture was
stirred for 24 h at room temperature. The yellow precipi-
tate obtained was filtered and washed with an acetone:
ethanol mixture (40:60) (2 × 10 mL) and dried in
vacuum. [1] was obtained as a yellow solid (1100 mg,
92%). Anal. Calcd. for C19H21ClIrN3O2: C, 41.41; H,
4.3.2 | Synthesis of [(p-cym)RuCl(pyhy)]
([2])
1
3.84; N, 7.63. Found: C, 41.56; H, 3.79; N, 7.61. The H
and 13C{1H} NMR spectra in DMSO-d6 reflect the pres-
ence of two compounds: [1] and a DMSO adduct. When
tetrabutylammonium chloride is added, only the reso-
To a solution of Hpyhy (491.9 mg, 2.6 mmol) in a
CH2Cl2:ethanol (40:60) mixture (40 mL), [(p-cym)RuCl2]2
(667.5 mg, 1.09 mmol) was added, and the reaction mix-
ture was stirred for 24 h at room temperature. A yellow
precipitate was formed in a red solution. This solution
was filtered and evaporated in vacuum to 10 ml to precip-
itate a solid that was filtered, washed with CH2Cl2
(5 mL), and vacuum dried. A red solid was obtained
(636.7 mg, 64%). Anal. Calcd. for C19H20ClN3O2Ru: C,
49.53; H, 4.39; N, 9.16. Found: C, 49.55; H, 4.32; N, 9.12.
1H NMR (400 MHz, CDCl3, 25ꢀC): δ = 9.16 (dd,
1
nances of [1] are observed. H NMR of [1] (400 MHz,
DMSO-d6, 25ꢀC): δ = 10.83 (s, 1H, NHHyd), 8.61 (dd,
0
4
3J(H,H) = 5.9 Hz, J(H,H) = 1.4 Hz, 1H, H6 Pyr), 7.89 (td,
0
3J(H,H) = 7.6 Hz, 4J(H,H) = 1.5 Hz, 1H, H4 Pyr), 7.48 (dd,
0
3J(H,H) = 7.8 Hz, J(H,H) = 1.4 Hz, 1H, H3 Pyr), 7.20
4
(ddd, 3J(H,H)
=
7.4 Hz, 3J(H,H)
=
5.9 Hz,
0
4J(H,H) = 1.5 Hz, 1H, H5 Pyr), 5.91 (s, 1H, CH6), 1.42 (s,
1
15H, 5 × MeCp*) ppm. H NMR of the DMSO adduct of
0
4
[1] (400 MHz, DMSO-d6, 25ꢀC): δ = 11.31 (s, 1H, NH),
3J(H,H) = 5.9 Hz, J(H,H) = 1.4 Hz, 1H, H6 Pyr), 8.17 (bs,
0
8.71 (dd, 3J(H,H) = 6.0 Hz, 4J(H,H) = 1.5 Hz, 1H, H6 Pyr),
1H, NHHyd), 7.61 (ddd, 3J(H,H)
=
7.7 Hz,
0
0
8.14 (td, J(H,H) = 7.8 Hz, J(H,H) = 1.6 Hz, 1H, H4 Pyr),
4J(H,H) = 1.5 Hz, 1H, H4 Pyr), 7.10 (dd, 3J(H,H) = 7.9 Hz,
3
4
0
0
7.82 (dd, 3J(H,H) = 7.9 Hz, 4J(H,H) = 1.2 Hz, 1H, H3 Pyr),
4J(H,H)
=
1.3 Hz, 1H, H3 Pyr), 7.00 (ddd,
7.41 (ddd, 3J(H,H) = 7.6 Hz, 3J(H,H) = 5.7 Hz,
3J(H,H) = 7.4 Hz, J(H,H) = 5.9 Hz, J(H,H) = 1.5 Hz,
3
4
0
0
4J(H,H) = 1.6 Hz, 1H, H5 Pyr), 6.22 (s, 1H, CH6), 1.55 (s,
1H, H5 Pyr), 5.97 (s, 1H, CH6), 5.69 (d, J = 6.0 Hz, 1H,
15H, 5 × MeCp*) ppm. 1H NMR (400 MHz, CDCl3, 25ꢀC):
H(p-cym)), 5.60 (d, J = 6.0 Hz, 1H, H(p-cym)), 5.37 (d,
J = 6.0 Hz, 1H, H(p-cym)), 5.29 (d, J = 6.0 Hz, 1H,
H(p-cym)), 2.56 (sept, 1H, CHiPr(p-cym)), 2.33 (s, 3H,
MeTol(p-cym)), 1.18 (d, J = 6.9 Hz, 3H, MeiPr(p-cym), 1.13
3
4
δ = 8.70 (dd, J(H,H) = 5.8 Hz, J(H,H) = 1.3 Hz, 1H,
0
H6 Pyr), 7.68 (td, J(H,H) = 7.6 Hz, J(H,H) = 1.8 Hz, 1H,
3
4
0
H4 Pyr), 7.42 (s, 1H, NHHyd), 7.16 (dd, J(H,H) = 8.0 Hz,
3

16 of 18
RUIZ-CASTAÑEDA ET AL.
(d, J = 6.9 Hz, 3H, MeiPr(p-cym) ppm; 13C {1H} NMR
(101 MHz, CDCl3, 25ꢀC): δ = 167.1 (C5Hyd), 161.5
0.50 mmol) was added, and the reaction mixture was
stirred for 24 h at room temperature. It was evaporated in
vacuum up to 3 ml to precipitate the product and washed
with H2O (3 × 1 mL) affording a pure product as a red
solid (11.7 mg, 42%). Anal. Calcd. for C19H22IrN3O2: C,
44.18; H, 4.29; N, 8.13. Found: C, 43.87; H, 4.34; N, 8.14.
1H NMR (500 MHz, acetone-d6, 25ꢀC): δ = 10.42 (br, 1H,
0
0
(C4Hyd), 158.4 (C6 Pyr), 154.0 (C2 Pyr), 141.8 (C2Hyd), 137.5
0
0
0
00
(C4 Pyr), 125.6 (C3 Pyr), 121.2 (C5 Pyr), 103.0 (C1 or
00
00
00
C4 (p-cym)), 101.8 (C1 or C4 (p-cym)), 100.7 ( C6H ),
85.4 (CH(p-cym)), 85.0 (CH(p-cym)), 84.7(CH(p-cym)),
81.7
(CH(p-cym)),
31.0
(CHiPr(p-cym)),
22.5
(MeiPr(p-cym)), 19.0 (MeTol(p-cym)) ppm. IR (νmax/cm−1):
3366 (br), ν(N H); 3128, ν(C C H); 3074, ν(Carom H);
2962, ν(C H); 1709 (s), ν(C(4) O(4)); 1630 (s), ν(C(2)
O(2)); 1599, ν(C C) (s); 1333(s), δ(N H); 1130 (s),
ν(C N C); 786, δ(C H)ip; 650, δ(C H)oop. MS
(FAB+): m/z (assign., rel. int. %): 459 [(M)+, 18.4],
424 [(M − Cl)+, 100.0]. Crystals suitable for X-ray
diffraction were obtained by a saturated chloroform
solution of [2] at 20ꢀC. [2] is soluble in CHCl3 and
insoluble in CH2Cl2.
NHHyd), 8.71 (dd, J(H,H) = 6.0 Hz, J(H,H) = 1.1 Hz,
3
4
0
3
4
1H, H6 Pyr), 7.78 (td, J(H,H) = 7.6 Hz, J(H,H) = 1.6 Hz,
0
0
1H, H4 Pyr), 7.30 (d, 3J(H,H) = 7.8 Hz, 1H, H3 Pyr),
6.89 (ddd, 3J(H,H) = 7.4 Hz, 3J(H,H) = 6.0 Hz,
0
4J(H,H) = 1.4 Hz, 1H, H5 Pyr), 5.83 (s, 1H, CH6), 1.65
1
(s, 15H, 5 × MeCp*), −9.60 (s, 1H, Ir H) ppm. H NMR
3
(500 MHz, CDCl3, 25ꢀC): δ = 8.70 (dd, J(H,H) = 5.7 Hz,
0
4J(H,H) = 1.4 Hz, 1H, H6 Pyr), 7.60 (td, J(H,H) = 8.0 Hz,
3
0
4J(H,H) = 1.7 Hz, 1H, H4 Pyr), 7.52 (br, 1H, NHHyd), 7.06
0
(dd, 3J(H,H) = 8.2 Hz, 4J(H,H) = 1.1 Hz, 1H, H3 Pyr), 6.74
(ddd, 3J(H,H)
=
7.2 Hz, 3J(H,H)
=
5.7 Hz,
0
4J(H,H) = 1.4 Hz, 1H, H5 Pyr), 5.96 (s, 1H, CH6), 1.68
1
4.3.3 | Synthesis of [Cp*RhCl(pyhy)] ([3])
(s, 15H, 5 × MeCp*), −9.48 (s, 1H, Ir H) ppm. H NMR
60
(500 MHz, TFE-d3, 25ꢀC): δ = 8.82 (br, 1H, H Pyr), 7.74
0
To a solution of Hpyhy (141.6 mg, 0.75 mmol) in 8 mL of
CH2Cl2:ethanol (40:60) mixture, [Cp*RhCl2]2 (193.2,
0.31 mmol) was added, and the reaction mixture was
stirred for 24 h at room temperature. The yellow
precipitate obtained was filtered and washed with
aCH2Cl2:ethanol mixture (40:60) (1 mL) and dried in
vacuum. A yellow solid was obtained (192 mg, 99%).
Anal. Calcd. for C19H21ClN3O2Rh: C, 49.42; H, 4.58; N,
9.10. Found: C, 49.24; H, 4.55; N, 9.03. 1H NMR
(t, 3J(H,H)
=
8.6 Hz, 1H, H4 Pyr), 7.27 (dd,
0
0
3J(H,H) = 7.0 Hz, 1H, H3 Pyr), 6.94 (br, 1H, H5 Pyr), 6.24
(s, 1H, CH6), 1.62 (s, 15H, 5 × MeCp*), −9.48 ppm (s,
1H, Ir H) ppm. 13C{1H} NMR (126 MHz, DMSO-d6:ace-
tone-d6 1:2, 25ꢀC): δ = 167.82 (C5Hyd), 160.33 (C4Hyd),
0
0
159.55 (C6 Pyr), 153.86 (C2 Pyr), 142.69 (C2Hyd), 136.81
0
0
0
(C4 Pyr), 124.75 (C3 Pyr), 121.81 (C5 Pyr), 100.35 ( C6H),
87.01(Cq-Cp*), 9.76 (MeCp*
ppm. IR (νmax/cm−1):
)
3318 (br) ν(N H); 2095, (m), ν(Ir H(3)); 1726, (s),
ν(C(4) O(4)); 1697, (s), ν(C(2) O(2)). MS (FAB+): m/z
(assign., rel. int. %): 516 [(M)+, 100.0]. Crystals
suitable for X-ray diffraction were obtained by slow
diffusion of [1], dissolved in dimethylformamide, in a
water solution of HCOONa at 20ꢀC. [4] is soluble in
acetone, TFE, and CDCl3. It is insoluble in MeOH
and water.
3
(500 MHz, CDCl3, 25ꢀC): δ = 8.77 (d, J(H,H) = 5.5 Hz,
0
1H, H6 Pyr), 7.68 (dd, 3J(H,H) = 7.0 Hz, 4J(H,H) = 1.5 Hz,
0
1H, H4 Pyr), 7.40 (s, 1H, NHHyd), 7.20 (d,
0
3J(H,H) = 7.8 Hz, 1H, H3 Pyr), 7.14 (dd, 3J(H,H) = 5.5 Hz,
0
3J(H,H) = 1.5 Hz, 1H, H5 Pyr), 6.07 (s, 1H, CH6), 1.55 (s,
15H, 5 × MeCp*) ppm. This product was not soluble
enough in conventional NMR solvents to obtain a
13C{1H} NMR spectrum and in DMSO-d6 indications of a
transformation were observed. IR (νmax/cm−1): 3149, (br),
ν(N H); 3096, (br), ν(CHCp*); 1744, (s), ν(C(4) O(4));
1690, (s), ν(C(2) O(2)); 1620, (s), ν(C C); 1340, (s),
δ(N H); 1124, (s), ν(C N C); 775, (s), δ(C H)ip;
648, (s), δ(C H)oop. MS (FAB+): m/z (assign., rel. int.
%): 461 [(M)+, 11.6], 426 [(M − Cl)+, 100.0]. [3] is
slightly soluble in CHCl3 and CH3CN. It is insoluble in
toluene and acetone. It is soluble but instable in TFE
and DMSO.
4.4 | General procedure for the catalytic
N-alkylation of aniline with benzyl alcohol
All reactions were carried out under nitrogen, using
Schlenk−vacuum line techniques in a multireactor
carousel 12 plus reaction Station™ Radleys that allow set
12 assays simultaneously. Every pressure tube was loaded
with the properly salt, [1] (2.8 mg, 5 μmol), the necessary
benzyl alcohol, the aniline derivative, and TFE (2.6 mL).
The resulting mixtures were degassed and flushed with
nitrogen and stirred at 100ꢀC for 7 h. The reactions were
stopped on ice and aliquots placed in an NMR tube
containing CDCl3 (0.3 mL) after solvent was removed
under vacuum. Conversions were determined by 1H
4.3.4 | Synthesis of [Cp*IrH(pyhy)] ([4])
To a solution of complex [1] (30 mg, 0.0544 mmol) in a
DMSO:H2O (1:5) mixture (6 ml), HCOONa (34 mg,

RUIZ-CASTAÑEDA ET AL.
17 of 18
NMR. Calculations for the conversion values are referred
to the reactant in default, that is, benzyl alcohol in
Entries 1 and 2 and aniline in Entry 3. Moreover, the
aldehyde formed was not taken into account in the
calculation of the conversion percentage.
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The data that supports the findings of this study are
available in the supplementary material of this article.
ORCID
Ana M. Rodríguez https://orcid.org/0000-0001-7122-
1431
Blanca R. Manzano https://orcid.org/0000-0002-4908-
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Gustavo Espino https://orcid.org/0000-0001-5617-5705
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How to cite this article: Ruiz-Castañeda M,
Rodríguez AM, Aboo AH, et al. Iridium complexes
with a new type of N^N0-donor anionic ligand
catalyze the N-benzylation of amines via borrowing
hydrogen. Appl Organomet Chem. 2020;e6003.
https://doi.org/10.1002/aoc.6003