Amidation of aldehydes using mono-cationic half-sandwich rhodium(III) complexes with functionalized phenylhydrazone ligands

Article abstract of DOI:10.1016/j.jorganchem.2019.02.018

A series of mono-cationic half-sandwich rhodium(III) complexes have been synthesized in methanol using phenylhydrazone-derived ligands (L₁–L₆) and the starting precursor [(η⁵-C₅Me₅)₂Rh₂(μ-Cl)₂Cl₂] in a 2:1 molar ratio. The N,N′-phenylhydrazone complexes have been isolated as tetraphenylborate salts. All complexes were characterized by elemental analysis, FT-IR, UV–visible, NMR spectroscopy and mass spectrometry. The molecular structure of complex [(η⁵-C₅Me₅)Rh(L₁)Cl](BPh₄) (1) was confirmed by single-crystal X-ray structure analysis. Complex [(η⁵-C₅Me₅)Rh(L₃)Cl](BPh₄) (3) was used as an efficient catalyst for the amide formation reaction, with up to 99% conversion after 2 h in toluene at 110 °C in the presence of hydroxyl amine hydrochloride and sodium bicarbonate.

Full text of DOI:10.1016/j.jorganchem.2019.02.018

Accepted Manuscript
Amidation of aldehydes using mono-cationic half-sandwich rhodium(III) complexes
with functionalized phenylhydrazone ligands
Neelakandan Devika, Subbiah Ananthalakshmi, Nandhagopal Raja, Gajendra Gupta,
Bruno Therrien
PII:
S0022-328X(19)30071-3
DOI:
https://doi.org/10.1016/j.jorganchem.2019.02.018
JOM 20720
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 22 November 2018
Revised Date: 21 February 2019
Accepted Date: 21 February 2019
Please cite this article as: N. Devika, S. Ananthalakshmi, N. Raja, G. Gupta, B. Therrien, Amidation
of aldehydes using mono-cationic half-sandwich rhodium(III) complexes with functionalized
phenylhydrazone ligands, Journal of Organometallic Chemistry (2019), doi: https://doi.org/10.1016/
j.jorganchem.2019.02.018.
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ACCEPTED MANUSCRIPT
Amidation of aldehydes using mono-cationic half-sandwich rhodium(III) complexes with
functionalized phenylhydrazone ligands
Neelakandan Devikaa,b, Subbiah Ananthalakshmic, Nandhagopal Rajaa,d, Gajendra Guptaa, Bruno Therriena*
aInstitute of Chemistry, University of Neuchatel, Avenue de Bellevaux 51, CH-2000 Neuchatel, Switzerland.
bDepartment of Chemistry, BIT campus, Anna University, Tiruchirappalli 620 024, India.
cDepartment of Chemistry, Urumu Dhanalakshmi College, Tiruchirappalli 620 019, India.
dDepartment of Chemistry, University College of Engineering, Anna University, Ariyalur 621 704, India.
Abstract
A series of mono-cationic half-sandwich rhodium(III) complexes have been synthesized
in methanol using phenylhydrazone-derived ligands (L1–L6) and the starting precursor [(η5-
C5Me5)2Rh2(µ-Cl)2Cl2] in a 2:1 molar ratio. The N,N'-phenylhydrazone complexes have been
isolated as tetraphenylborate salts. All complexes were characterized by elemental analysis, FT-
IR, UV-visible, NMR spectroscopy and mass spectrometry. The molecular structure of complex
[(η5-C5Me5)Rh(L1)Cl](BPh4) (1) was confirmed by single-crystal X-ray structure analysis.
Complex [(η5-C5Me5)Rh(L3)Cl](BPh4) (3) was used as an efficient catalyst for the amide
formation reaction, with up to 99% conversion after 2 hours in toluene at 110°C in the presence
of hydroxyl amine hydrochloride and sodium bicarbonate.
Keywords: Phenylhydrazone-based ligands; Half-sandwich complexes; Homogenous catalysis;
Pentamethylcyclopentadienyl rhodium complexes; Amidation reaction.
*Corresponding author. Fax: +41 32 7182511.
E-mail address: bruno.therrien@unine.ch (B. Therrien).
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Introduction
Organometallic compounds are powerful catalytic agents in synthetic organic chemistry
[1]. Rhodium based complexes are very important for different catalytic reactions like the
transfer hydrogenation [2], hydroformylation [3], hydrosilylation [4], coupling reactions [5], ring
opening [6], ring closing [7], polymerisation [8], oxidation [9], isomerization [10], and others.
Both oxidative addition and reductive elimination reactions are readily accessible, and
accordingly, rhodium complexes have been widely used in the design of catalytic systems [11].
In parallel, ligand design is vital for the development of catalysts [12]. The structure of
the ligand dictates the environment of the metal centers and subsequently the catalytic activity.
Hydrazone-derived ligands possess distinct features that make them interesting for the design of
metal-based catalysts. The tri-atomic C=N–N structure of hydrazone introduces electrophilic and
nucleophilic characters to the ligand, which also offers great functional diversity. The structural
properties of the hydrazone group enable its use in various applications [13]. Furthermore,
chemical functionalization of the hydrazone group can increase its stability, modify its
coordination ability, and generate higher modularity, thus overall fine-tuning the steric and
electronic properties of the complex for optimization of the catalytic process [14].
The amide bond is present in many natural and synthetic polymers, as well as in peptides
and proteins [15]. Amide bonds are typically synthesized by acylation of amines with carboxylic
acid derivatives such as acid chloride, anhydride, active esters, etc... However, these methods
have the inconvenience of producing a stoichiometric amount of side products and of using
highly hazardous reagents [16]. Therefore, alternative methods for amide synthesis have been
developed, such as the Beckmann rearrangement [17], the Staudinger reaction [18], the Schmidt
reaction [19], the amino carbonylation of halo arenes [20], the iodonium-promoted alkyl halo
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nitro alkane amine coupling [21], the direct amide synthesis from alcohols with amines or
nitroarenes [22,23], the hydro amination of alkynes [24], the amidation of thioacids with azides
[25], as well as the trans-amidation of primary amides [26]. An elegant alternative pathway,
which is more atom economic and makes use of cheap and abundant starting materials, is the
oxidative amidation of aldehyde with amines [27], using metal-based catalysts [28-37]. These
catalytic reactions require large amount of catalysts and are generally performed at high
temperature [38-40].
In contrast to the considerable growth of literature on the chemistry of hydrazone
complexes, to the best of our knowledge, there are no reports available concerning the catalytic
transformation of aldehyde to amides by half-sandwich rhodium(III) phenylhydrazone
complexes containing N,N'-hydrazine-based donors. In view of the interest to understand the
behavior of these simple and inexpensive ligands towards rhodium, six half-sandwich rhodium
metal complexes containing hydrazine-derived ligands were synthesized. All complexes were
fully characterized by spectroscopic and spectrometric methods, and one complex was tested as
catalyst for the conversion of aldehydes to amides in the presence of NH2OH·HCl and NaHCO3.
Results and discussion
Six half-sandwich rhodium(III) complexes have been synthesized from the rhodium
starting precursor [(η5-C5Me5)2Rh2(µ-Cl)2Cl2], the phenyl hydrazone ligands (L1–L6) and sodium
tetraphenylborate in a 1:2:2 molar ratio in dry methanol under reflux for 6 hours (Scheme 1). The
complexes were isolated as tetraphenylborate salts. All complexes are air-stable, non-
hygroscopic in nature, partially soluble in water and highly soluble in common solvents such as
chloroform, dichloromethane, acetonitrile and dimethyl sulfoxide. The complexes were found to
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be diamagnetic, low spin in nature and characterized by elemental analysis, infrared, UV-visible,
1H NMR, 13C NMR spectroscopy and ESI mass spectrometry (see experimental part).
Scheme 1. Synthesis of half-sandwich rhodium complexes 1-6.
The coordination mode of the phenylhydrazone ligand to the rhodium metal center was
first investigated by infrared spectroscopy. The spectra of the ligands were compared to those of
the corresponding half-sandwich rhodium complexes. The phenylhydrazone ligands showed a
very strong absorption around 1600–1620 cm-1, corresponding to the azomethine (C=N) group.
After coordination, the azomethine frequency is shifted to lower frequency at around 1565–1600
cm-1. This is due to the reduction of the electron density upon coordination of the azomethine
group. Thus the infrared data confirmed the coordination of the nitrogen atom to the metal. In
addition, a medium intensity band in the 1015–1030 cm-1 region, which corresponds to
coordinated pyridyl groups, is observed in all complexes, together with the Rh–Cl stretching
frequency around 320 cm-1. The electronic spectra of the complexes were recorded between 800
and 200 nm in dichloromethane solution (10-4 M). All complexes show two to three absorption
bands: An intense absorption band centered at 390 nm, which is assigned to MLCT transitions,
and a band around 290–230 nm, which is attributed to n-π* or π-π* transitions.
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1H NMR spectra were recorded in CD3CN, and they confirm the proposed structure of
the complexes. The aromatic protons of the complexes appear as one doublet of triplet and one
triplet of doublet in the range of 8.82–8.79 ppm and 8.20–8.03 ppm, while the remaining
aromatic protons appeared as a multiplet in the range of 8.03–7.08 ppm. The protons of the
tetraphenyl borate counter ion appear as a multiplet (~7.30 ppm) and two triplets at ~7.03 and
~6.88 ppm, respectively. The complexes show a broad singlet in the range of 9.21–8.94 ppm
corresponding to the uncoordinated NH proton. The acetyl methyl proton of complexes 1, 2 and
3 appears as a sharp singlet at 2.60 ppm. Finally, the methyl protons of the
pentamethylcyclopentadienyl ring (Cp*) are observed as a sharp singlet at ~1.47 ppm. The
electrospray ionization mass spectra of 1-6 display a molecular ion peak, which corresponds,
after the loss of the BPh4 counter anion, to the cationic complexes [(η5-C5Me5)Rh(L)Cl]+.
The molecular structure of 1 was unambiguously confirmed by single-crystal X-ray
structure analysis. The ortep diagram of the mono-cationic complex [(η5-C5Me5)Rh(L1)Cl]+ is
presented in Figure S1, together with selected bond lengths and angles. The complex is chiral,
but crystallizes in the centrosymmetric space group P -1, thus displaying in the crystal both
enantiomers. The complex shows a typical pseudo-tetrahedral geometry (piano-stool) around the
metal center with an η5-coordinated Cp*, a chloride, and the N,N'-hydrazone ligand (L1). The
bond length between the rhodium and the chloride is 2.4160(6) Å, while the two Rh-N bond
lengths are almost identical at 2.099(2) Å (Rh-Npyridyl) and 2.101(2) Å (Rh-Nimine). Upon
coordination, the phenyl group of the hydrazine ligand is out of the pyridyl-azomethine plane by
-69.3(3)°. Overall, these geometrical parameters are consistent with those found in analogous
N,N’-coordinated rhodium pentamethylcyclopentadienyl compounds [41].
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In the crystal packing of 1, two mono-cationic complexes are facing each other, thus
forming dimers in the solid state. In these dimers (Figure S2), strong hydrogen bonds are
observed, involving both the N-H and the chloride. The N-H···Cl contact shows a N···Cl
distance of 3.328(2) Å, with a N-H···Cl angle of 157(2)°, while the C-H···Cl contact possesses a
C···Cl distance of 3.642(3) Å and a C-H···Cl angle of 141°. The separation between the two
cationic complexes is only 6.2120(8) Å (Rh···Rh distance).
Rhodium mediated amide formation is a very useful reaction in which an amide
derivative is formed by hydroxyl amine hydrochloride and an aldehyde [42]. Therefore, the
rhodium-based complexes 1-6 were tested as catalysts for this reaction. The initial conditions
used for the transformation of benzaldehyde to benzamide (Scheme 2) were based on our
previous studies dealing with analogous piano-stool catalysts [43].
Scheme 2: Organic transformation of aldehyde to amide with rhodium-based catalysts 1-6.
Even so, the nature of the solvent, the choice of the base, and the catalysts/substrate ratio
(see supporting information), were evaluated to confirm the initial catalytic conditions. The best
solvent was toluene, with a 90% conversion of benzaldehyde to benzamide (Table S1). Among
the following bases, NaHCO3, KHCO3, Na2CO3, K2CO3, NaOH, KOH, LiOH, K2CO3, Et3N and
tBuOK, sodium bicarbonate gave the best conversion (Table S2). Is it worth mentioning here that
in the absence of a base, the conversion of benzaldehyde to benzamide was not observed.
Finally, by decreasing the catalyst/substrate ratio below 1:500, the formation of side products
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(nitrile, carboxylic acid) appears, while above 1:500, the overall yield was reduced. Therefore,
1:500 seems to be the best compromise in term of catalyst/substrate ratio. Then, under these
conditions, the effect of complexes 1-6 on the amidation of benzaldehyde as a function of time
has been determined (Table 1).
Table 1. Effect of complexes 1-6 on the amidation of benzaldehyde (yields of benzamide as a
function of time).a
Complex
1h
45
63
80
55
61
68
2h
52
64
90
60
72
75
4h
65
70
92
69
79
85
6h
70
79
92
73
81
86
8h
72
81
93
76
82
88
1
2
3
4
5
6
aConditions: Reactions were carried out at 110°C using 1 mmol of benzaldehyde, 1 mmol of
b
NH2OH·HCl, 1 mmol of NaHCO3, 0.002 mmol of catalyst in 2 ml of toluene. Conversions were
determined using 1H NMR (integrations of Haldehyde vs Hamide).
The time-conversion profiles of complexes 1-6 under standard conditions show that
complex 3 is the best catalyst, reaching 93% formation of benzamide after 8 hours (Table 1).
However, we can also observed that extending the reaction time does not increase significantly
the yield, 90% conversion being already reached after 2 hours. The second best catalyst is
complex 6, which gives an 88% conversion after 8 hours. Then complexes 2 and 5 show similar
profiles, which are better than complexes 1 and 4. Interestingly, complexes 3 and 6 are the chloro
derivatives (see Scheme 1), while complexes 2 and 5 are the bromo derivatives. These results
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suggest that ligand optimizations is important for such N,N'-hydrazine-based complexes, and
functionalization of the ligand can play an important role on the catalytic activity.
Having now confirmed the reaction conditions for the amide formation (toluene at reflux,
2 hours, NaHCO3 as the base, and a catalyst/substrate ratio of 1:500) and identified the best
catalyst among the six complexes, we tested the catalytic activity of complex 3 on various
aldehydes: benzaldehyde, 2-naphthaldehyde, 4-chloro-benzaldehyde, 4-bromo-benzaldehyde, 4-
methyl-benzaldehyde, 4-methoxy-benzaldehyde, trans-cinnamaldehyde, and 2-thiophene
carboxaldehyde. The rates of conversion are listed in Table 2. As previously mentioned,
benzaldehyde gives benzamide with a 90% yield (entry 1). Introduction of additional aromaticity
to the aldehyde benzene ring (entry 2) decreases the yield to 85%, probably due to steric
constraints or electronic effects. Similarly when electron donating groups (methyl, methoxy) are
introduced at the para-position of benzaldehyde (entries 3 & 4) the yield decreases, while the
presence of electron withdrawing groups (Br, Cl, NO2) at the same position (entries 5–7)
increases the yields. High yields are also observed for cinnamaldehyde (entry 8), 2-pyridimide
(entry 9) and with 2-thiophene carboxamide (entry 10).
Table 2: One pot conversion of aldehydes to amides using catalyst 3.a
Entry
1
Aldehyde
Amide
Yieldb
90
2
85
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aAld
ehy
de
3
4
5
6
7
80
76
94
99
90
(1
mm
ol),
NH2
OH·
HCl
(1
mm
ol),
NaH
CO3
(1
mm
ol),
catal
yst
(0.0
02
mm
ol)
and
tolu
ene
(2
ml),
2
hour
s at
110
°C.
bIsol
ated
yiel
8
9
92
O
O
N
N
95
98
H
NH2
10
ds after column chromatography, and yields obtained by 1H NMR (integrations of Haldehyde vs Hamide).
Complex 3 shows good to excellent yields for the transformation of aldehydes to amides
under relatively mild conditions, low catalysts loading, and fast reaction time. In general, such
catalytic reactions require large amount of catalysts and high temperatures. The Wilkinson’s
complex has been found to catalyze the one-pot transformation of aldoximes to the
corresponding amides with a 5 mol% catalyst loading and a reaction temperature of 150°C [38].
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Similarly, Mizuno reported that aluminum supported rhodium hydroxide (Rh(OH)x/Al2O3) can
act as an effective catalyst for the synthesis of primary amides with a 4 mol% catalyst loading
and 160°C [39]. Recently, Hull reported a rhodium complex for amide formation reaction with
good yield and similar catalyst loading in the presence of styrene as the base [40]. Overall,
complex 3 compares well with previously reported rhodium-based catalysts for the formation
amides.
Conclusion
Six half-sandwich rhodium catalysts incorporating N,N'-hydrazine-based ligands have
been synthesized and characterized. Complex 3, [(η5-C5Me5)Rh(L3)Cl]BPh4, is an efficient
catalyst for the formation of amides from aldehydes in the presence of NH2OH·HCl and
NaHCO3 in toluene at reflux. This study confirms the versatility of rhodium-based catalysts in
organic reactions.
Experimental section
Materials and methods
RhCl3·nH2O, 1,2,3,4,5-pentamethylcyclopentadiene, hydroxylamine hydrochloride,
phenylhydrazone, acetyl pyridine, benzoyl pyridine, benzaldehyde, 1-naphthaldehyde, 4-
anisaldehyde, trans-cinnamaldehyde, 2-thiophenecarboxaldehyde and sodium bicarbonate were
obtained from Sigma Aldrich, while 4-nitrobenzaldehyde and 4-tolylaldehyde were purchased
from Fluka and 4-bromobenzaldehyde from Acros. All reagents were of analytical reagent grade
and used as received. The starting materials [(η5-C5Me5)2Rh2(µ-Cl)2Cl2] and the ligands were
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1
13
prepared according to published methods [44,45]. The H and C NMR spectra were recorded
with a Bruker Avance II 400 MHz spectrometer. Infrared spectra were recorded in the range of
4000-400 cm-1 with a Perkin-Elmer FTIR 1720 X-spectrometer. Electrospray mass spectra were
obtained in positive mode with a LCQ Finnigan mass spectrometer. Microanalyses were carried
out at the sophisticated analytical instrument facility Cochin (India), on a VarioEL III CHNS
analyzer (Table S4). UV−visible absorption spectra were recorded on an Uvikon 930
spectrophotometer.
General procedure for the synthesis of complexes 1-6.
One equivalent of the dimeric starting precursor [(η5-C5Me5)2Rh2(µ-Cl)2Cl2] was treated
with two equivalents of phenylhydrazone (L1-L6) in methanol (25ml). The mixture obtained was
refluxed for 6 hours. In all cases, the solution turned from yellow to dark red. Then, NaBPh4 was
added the hot solution to obtain the corresponding complex (1-6). On cooling, reddish brown
colored precipitates were obtained, and the products were isolated by filtration, and washed with
diethyl ether. The purity of the complexes was checked by TLC.
Complex 1, brown solid; Yield: 65%. IR (KBr, υ, cm-1): 1597 (C=N), 1021 (py). 1H NMR (400
MHz, CD3CN, 25°C): δ (ppm) = 8.92 (br s, 1H, NH), 8.79 (dt, 3JH-H = 5.6 Hz, 1H, Har), 8.19 (td,
3JH-H = 8 Hz, 1H, Har), 8.01 (d, 3JH-H = 7.6 Hz, 1H, Har), 7.81-7.77 (m, 1H, Har), 7.47 (dd, 3JH-H
=
7.6 Hz, 2H, Har), 7.41-7.36 (m, 2H, Har), 7.32-7.28 (m, 8H, HBar), 7.12-7.08 (m, 1H, Har), 7.03 (t,
3
3JH-H = 7.6 Hz, 8H, HBar), 6.87 (t, JH-H = 7.2 Hz, 4H, HBar), 2.59 (s, 3H, HMe), 1.45 (s, 15H,
HCp*). 13C{1H} NMR (100 MHz, CD3CN, 25°C): δ (ppm) = 153.7 (C=N), 152.0 (Car), 142.8
(Car), 139.7 (Car), 135.4 (Car), 128.7 (Car), 127.8 (Car), 125.3 (CBar), 122.1 (Car), 121.4 (Car),
116.4 (Car), 97.4 (d, 2JRh-C = 8.2 Hz, Ccp*), 15.0 (CH3), 7.7 (Ccp*). MS (ESI positive mode): m/z
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484 [M-BPh4]+. UV-vis (10-4 M, CH2Cl2) λmax (nm): 385 (ε = 2050 cm-1 mol-1 L), 266 (ε =
19230 cm-1 mol-1 L), 231 (ε = 29450 cm-1 mol-1 L). Anal. Calcd for C47H48BClN3Rh: C, 70.21;
H, 6.02; N, 5.23. Found: C, 70.19; H, 6.01; N, 5.15.
Complex 2, brown solid; Yield: 60%. IR (KBr, υ, cm-1): 1592 (C=N), 1021 (py). 1H NMR (400
MHz, CD3CN, 25°C): δ (ppm) = 9.03 (br s, 1H, NH), 8.79 (dt, 3JH-H = 5.6 Hz, 1H, Har), 8.20 (td,
3JH-H = 8 Hz, 1H, Har), 8.03(d, 3JH-H = 7.6 Hz, 1H, Har), 7.83-7.79 (m, 1H, Har), 7.53-7.50 (m, 2H,
3
Har), 7.44-7.40 (m, 2H, Har), 7.32-7.28 (m, 8H, HBar), 7.02 (t, JH-H = 7.6 Hz, 8H, HBar), 6.87 (t,
3JH-H = 7.2 Hz, 4H, HBar), 2.60 (s, 3H, HMe), 1.46 (s, 15H, HCp*). 13C{1H} NMR (100 MHz,
CD3CN, 25°C): δ (ppm) = 153.5 (C=N), 152.1 (Car), 142.1 (Car), 139.8 (Car), 135.4 (Car), 131.5
2
(Car), 128.1 (Car), 125.5 (Car), 125.3 (CBar), 121.4 (Car), 118.1 (Car), 97.5 (d, JRh-C = 8.1 Hz,
Ccp*), 15.1 (CH3), 7.8 (Ccp*). MS (ESI positive mode): m/z 564 [M-BPh4]+. UV-vis (10-4 M,
CH2Cl2) λmax (nm): 390 (ε = 860 cm-1 mol-1 L), 278 (ε = 8090 cm-1 mol-1 L), 232 (ε = 17650 cm-1
mol-1 L). Anal. Calcd for C47H47BBrClN3Rh: C, 63.93; H, 5.37; N, 4.76. Found: C, 63.95; H,
5.35; N, 4.75.
Complex 3, brown solid; Yield: 80%. IR (KBr, υ, cm-1): 1596 (C=N), 1019 (py). 1H NMR (400
MHz, CD3CN, 25°C): δ (ppm) = 9.01 (br s, 1H, NH), 8.79 (dt, 3JH-H = 5.6 Hz, 1H, Har), 8.19 (td,
3JH-H = 8.0 Hz, 1H, Har), 8.02-8.00 (m, 1H, Har), 7.81-7.78 (m, 1H, Har), 7.50-7.47 (m, 2H, Har),
7.40-7.37 (m, 2H, Har), 7.33-7.28 (m, 8H, HBar), 7.02 (t, 3JH-H = 7.6 Hz, 8H, HBar), 6.87 (t, 3JH-H
=
7.2 Hz, 4H, HBar), 2.59 (s, 3H, HMe), 1.45 (s, 15H, HCp*). 13C{1H} NMR (100 MHz, CD3CN,
25°C): δ (ppm) = 153.5 (C=N), 152.1 (Car), 142.8 (Car), 139.8 (Car), 135.5 (Car), 128.6 (Car),
2
128.0 (Car), 126.6 (Car), 125.5 (CBar), 121.4 (Car), 117.8 (Car), 116.9 (Car), 97.5 (d, JRh-C = 8.2
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Hz, Ccp*), 15.1 (CH3), 7.8 (Ccp*). MS (ESI positive mode): m/z 518 [M-BPh4]+. UV-vis (10-4 M,
CH2Cl2) λmax (nm): 383 (ε = 660 cm-1 mol-1 L), 292 (ε = 6800 cm-1 mol-1 L), 245 (ε = 15870 cm-1
mol-1 L). Anal. Calcd for C47H47BCl2N3Rh: C, 67.32; H, 5.65; N, 5.01. Found: C, 67.30; H, 5.60;
N, 5.02.
Complex 4, brown solid; Yield: 68%. IR (KBr, υ, cm-1): 1596 (C=N), 1030 (py). 1H NMR (400
MHz, CD3CN, 25°C): δ (ppm) = 8.91 (br s, 1H, NH), 8.81 (dt, 3JH-H = 5.6 Hz, 1H, Har), 8.02 (td,
3JH-H = 8.0 Hz, 1H, Har), 7.79-7.35 (m, 10H, Har), 7.33-7.28 (m, 8H, HBar), 7.24 (m, 2H, Har),
3
3
3
7.15 (t, JH-H = 7.2 Hz, 1H, Har), 7.02 (t, JH-H = 7.2 Hz, 8H, HBar), 6.87 (t, JH-H = 7.2 Hz, 4H,
HBar), 1.47 (s, 15H, HCp*). 13C{1H} NMR (100 MHz, CD3CN, 25°C): δ (ppm) = 154.0 (C=N),
152.2 (Car), 142.8 (Car), 139.6 (Car), 135.4 (Car), 131.4 (Car), 130.8 (Car), 130.1 (Car), 130.0 (Car),
129.9 (Car), 128.7 (Car), 128.6 (Car), 128.5 (Car), 127.6 (Car), 126.6 (Car), 125.3 (CBar), 123.0
(Car), 121.4 (Car), 117.7 (Car), 97.6 (d, 2JRh-C = 8.2 Hz, Ccp*), 7.6 (Ccp*). MS (ESI positive mode):
m/z 546 [M-BPh4]+. UV-vis (10-4 M, CH2Cl2) λmax (nm): 389 (ε = 820 cm-1 mol-1 L), 287 (ε =
9870 cm-1 mol-1 L), 232 (ε = 19350 cm-1 mol-1 L). Anal. Calcd for C52H50BClN3Rh: C, 72.11; H,
5.82; N, 4.85. Found: C, 72.15; H, 5.80; N, 4.86.
Complex 5, brown solid; Yield: 62%. IR (KBr, υ, cm-1): 1564 (C=N), 1030 (py). 1H NMR (400
MHz, CD3CN, 25°C): δ (ppm) = 8.93 (br s, 1H, NH), 8.82 (dt, 3JH-H = 5.6 Hz, 1H, Har), 8.02 (td,
3JH-H = 8.0 Hz, 1H, Har), 7.80-7.49 (m, 9H, Har), 7.32-7.28 (m, 8H, HBar), 7.27-7.24 (m, 2H, Har),
7.02 (t, 3JH-H = 7.6 Hz, 8H, HBar), 6.87 (t, 3JH-H = 7.2 Hz, 4H, HBar), 1.49 (s, 15H, HCp*). 13C{1H}
NMR (100 MHz, CD3CN, 25°C): δ (ppm) = 153.8 (C=N), 152.2 (Car), 142.1 (Car), 139.7 (Car),
135.4 (Car), 131.5 (Car), 128.0 (Car), 127.0 (Car), 125.3 (CBar), 121.4 (Car), 119.3 (Car), 115.1
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2
(Car), 97.7 (d, JRh-C = 8.2 Hz, Ccp*), 7.7 (Ccp*). MS (ESI positive mode): m/z 626 [M-BPh4]+.
UV-vis (10-4 M, CH2Cl2) λmax (nm): 398 (ε = 1620 cm-1 mol-1 L), 289 (ε = 16580 cm-1 mol-1 L),
235 (ε = 26240 cm-1 mol-1 L). Anal. Calcd for C52H49BBrClN3Rh: C, 66.09; H, 5.23; N, 4.45.
Found: C, 66.01; H, 5.20; N, 4.40.
Complex 6, brown solid; Yield: 69%. IR (KBr, υ, cm-1): 1590 (C=N), 1018 (py). 1H NMR (400
MHz, CD3CN, 25°C): δ (ppm) = 8.94 (br s, 1H, NH), 8.82 (dt, 3JH-H = 5.6 Hz, 1H, Har), 8.03 (td,
3JH-H = 8.0 Hz, 1H, Har), 7.80-7.54 (m, 7H, Har), 7.38-7.36 (m, 2H, Har), 7.32-7.28 (m, 8H, HBar),
3
3
7.25 (m, 2H, Har), 7.02 (t, JH-H = 7.6 Hz, 8H, HBar), 6.87 (t, JH-H = 7.2 Hz, 4H, HBar), 1.49 (s,
15H, HCp*). 13C{1H} NMR (100 MHz, CD3CN, 25°C): δ (ppm) = 153.8 (C=N), 152.2 (Car),
141.7 (Car), 139.7 (Car), 135.4 (Car), 131.5 (Car), 129.9 (Car), 128.4 (Car), 127.9 (Car), 125.3
2
(CBar), 121.4 (Car), 119.1 (Car), 97.7 (d, JRh-C = 8.2 Hz, Ccp*), 7.7 (Ccp*). MS (ESI positive
mode): m/z 580 [M-BPh4]+. UV-vis (10-4 M, CH2Cl2) λmax (nm): 390 (ε = 990 cm-1 mol-1 L), 292
(ε = 10090 cm-1 mol-1 L), 242 (ε = 19340 cm-1 mol-1 L). Anal. Calcd for C52H49BCl2N3Rh: C,
69.35; H, 5.48; N, 4.67. Found: C, 69.45; H, 5.41; N, 4.60.
General procedure for the catalytic transformation of aldehyde to amide
Compound 3 (0.002 mmol), the aldehyde (1 mmol), NH2OH·HCl (1 mmol) and NaHCO3
(1 mmol) were introduced in a dried schlenk tube and purged with N2. Then, to the mixture,
dried and degassed toluene (2 ml) was added and the mixture was stirred for about 10 minutes at
room temperature, before the solution was refluxed under stirring for 2 hours. The mixture was
cooled and the products were extracted with methanol and dichloromethane before being filtered
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through celite to remove the remaining catalyst and NaHCO3. The crude amide was purified
using column chromatography, and dried under vacuum.
Single-crystal X-ray structure analysis
Crystals of compound 1 were obtained at room temperature by the diffusion of pentane in
a dichloromethane solution (1:3). A crystal was mounted on a Stoe Image Plate Diffraction
system equipped with a
φ circle goniometer, using Mo-Kα graphite monochromated radiation (λ
= 0.71073 Å) with range 0-200º. The structure was solved by direct methods using the program
φ
SHELXS-97, while the refinement and all further calculations were carried out using SHELXL-
97 [46]. The H-atoms were found on Fourier difference map or included in calculated positions
and treated as riding atoms using the SHELXL default parameters. Examination of the structure
with PLATON [47] reveals voids corresponding to highly disordered solvent molecules, and
accordingly, a new data set was generated with the SQUEEZE algorithm [48]. Then, the
structure was refined to convergence. The non-H atoms were refined anisotropically, using
weighted full-matrix least-square on F2. Crystallographic details are summarized in Table S5.
Figure S1 was drawn with ORTEP [49] and Figure S2 with Mercury [50].
Supplementary material
Figures S1 and S2, Tables S1-S5.
CCDC-1876101 complex 1 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from
15

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the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax:
(internat.) +44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
Acknowledgement
ND and BT thank the University of Neuchatel for financial support. ND also thanks the Vice-
Chancellor, Registrar of Anna University Chennai and the Dean of the BIT campus for the
permission granted to carry out her research in Switzerland.
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HIGHLIGHTS
• The synthesis of half-sandwich phenylhydrazone-based catalysts is presented.
• A rhodium complex was characterized by single-crystal X-ray analysis.
• The complexes are active catalysts for the formation of amides from aldehydes.