A binuclear Mn(ii) complex as an efficient catalyst for transamidation of carboxamides with amines

Article abstract of DOI:10.1039/c3ra45176c

A binuclear Mn(ii) complex has been synthesized and characterized by different structural methods. The complex contains two unique oxo-bridged metal centres and has been explored as an excellent catalyst for transamidation of carboxamides with amines under solvent-free conditions.

Full text of DOI:10.1039/c3ra45176c

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A binuclear Mn(II) complex as an efficient catalyst
for transamidation of carboxamides with amines†
Cite this: RSC Adv., 2014, 4, 1155
*
Divya Pratap Singh, Bharat Kumar Allam, Krishna Nand Singh and Vinod Prasad Singh
Received 17th September 2013
Accepted 12th November 2013
DOI: 10.1039/c3ra45176c
www.rsc.org/advances
A binuclear Mn(II) complex has been synthesized and characterized by transamidation under mild conditions, aluminium, copper,
different structural methods. The complex contains two unique oxo- cerium, zirconium, hydroxylamine hydrochloride, ammonium
bridged metal centres and has been explored as an excellent catalyst bromide, L-proline and boric acid catalyzed methods have been
for transamidation of carboxamides with amines under solvent-free recently developed by different research groups.9–12 Hypervalent
conditions.
iodine catalyzed13 and 1,4-dioxane mediated14 transamidation
protocols have also been explored by our group. In order to
further advance these studies, we chose to investigate the
catalytic use of a newly synthesized binuclear manganese(II)
complex for this process. Recently, manganese based catalysts
have emerged as effective promoters for various organic trans-
formations15 and bio-chemical reactions,16 but their use for a
demanding transamidation reaction has not been explored so
far. Establishing a practical synthetic protocol for trans-
amidation using manganese based catalysts would thus be
highly desirable.
We report herein the synthesis of a binuclear manganese(II)
complex, [Mn(Hbpoh)(OAc)(H2O)]2$6H2O, where, H2bpoh ¼
(N01E,N02E)-N01,N02-bis(phenyl (pyridin-2-yl)methylene)oxalo-
hydrazide, which has been characterized by spectroscopic and
single crystal X-ray methods. An investigation on the catalytic
role of the complex towards the transamidation of carbox-
amides with amines has also been undertaken under solvent-
free conditions.
Schiff bases are privileged ligands with important applications.1
The redox rich chemistry of manganese centres is usually
related to their function and applications in metalloenzymes,
catalysis, imaging, neurotoxicity, and magnetic materials.2 The
Mn(II) ions also play a key role in the development of medical
diagnostic tools of lower toxicity. High-spin Mn(II) materials can
efficiently enhance the relaxation rates of water protons and
thus act as contrast agents for magnetic resonance imaging
(MRI).2c
The synthesis and catalytic applications of manganese(II)
binuclear complexes have been developed as an important
research area.3 It is well-known that electronic communication
between manganese centres within a binuclear complex affects
their redox properties. The existence of a labile coordination
site is an important prerequisite for the involvement of an
inner-sphere electron transfer mechanism in the activation of
small molecules by the metal centres. It has been investigated
that the binuclear Schiff base complexes of transition metal
ions have the potential of catalyzing reactions more effectively
with different chemo-, regio- or stereo-selectivity than those of
corresponding mononuclear ones due to the synergic effect of
two metal ions.4–7
Single crystal X-ray structure of the complex (Fig. 1) shows
that out of the two pC]O groups present in ligand, one bonds
with Mn(II) in the form of enolate ion and the other pC]O
remains free. The Mn–O–Mn bridged complex has two octahe-
dral Mn centres, each coordinated with the ligand through
azomethine–nitrogen, pyridine–nitrogen and a carbonylate–
oxygen, and form two ve membered chelate rings. In addition,
one water molecule and one acetate ion occupy the remaining
octahedral coordination sites of each metal centre. The average
bond lengths of the complex are: O(1)–C(13) ¼ 1.260(4), N(3)–
N(2) ¼ 1.393(5), N(2)–C(6) ¼ 1.286(5), N(3)–C(13) ¼ 1.310(5),
N(1)–C(1) ¼ 1.340(6), C(5)–N(1) ¼ 1.351(4), C(5)–C(6) ¼ 1.480(6),
Transamidation is a valuable tool for the preparation of bio-
inspired materials and represents one of the most convenient
and straightforward method for exchanging the constituents of
two different amide groups.8 To accomplish an efficient
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi
221005, India. E-mail: singvp@yahoo.co.in; Tel: +91 9450145060
˚
C(6)–C(7) ¼ 1.496(4) A, which are longer or shorter than those of
† Electronic supplementary information (ESI) available: Experimental details and
spectral data. CCDC 885263 and 957204. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3ra45176c
the corresponding distances in ligand and suggest considerable
delocalization of charge.
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Fig. 2 Intra-molecular C–H/p interactions in the complex along ‘b’
axis.
Fig. 1 ORTEP diagram of [Mn(Hbpoh)(OAc)(H2O)]2$6H2O showing
atomic numbering scheme with ellipsoid of 30% probability (hydrogen
atoms and solvent cage are omitted for clarity).
while new n(C]N) and n(C–O) bands appear at 1547 and
1250 cmꢁ1 as a result of enolization of one carbonyl group
during complex formation. In the complex, one of the n(C]N)
shis towards lower wave number,5 indicating coordination
with azomethine–nitrogen.
The Mn–O(1), Mn–N(1) and Mn–N(2) bond distances are
˚
˚
˚
2.209(4) A, 2.307(4) A and 2.323(2) A, respectively, which fall in
the range reported for Mn(II) distorted octahedral complexes.17
The cis-angles in the basal plane involving the bridged car-
bonylate–oxygen and azomethine–nitrogen (O(1)–Mn(1)–N(2) ¼
66.92(7)ꢀ), (N(2)–Mn (1)–N(1) ¼ 69.19(7)ꢀ) show deviation from
the ideal value of 90ꢀ suggesting that the octahedral geometry is
slightly distorted due to chelation effect.18 The C(6)–N(2)
The complex shows meff value 5.95 B.M. corresponding to ve
unpaired electrons. Electronic spectra of the complex exhibits
two d–d bands of very weak intensity at 620 and 495 nm which
6
6
may be assigned to A1g
/ /
4T1g(G) and A1g 4T2g(G) sug-
gesting an octahedral environment around the metal ion.21
To investigate the catalytic efficiency of the complex, a model
transamidation reaction between acetamide and aniline to
afford product 3j was conducted by varying different conditions
(Table 1).
˚
distance (1.286(5) A) in the complex is longer than the ligand
˚
(1.294(5) A) due to coordination of azomethine–nitrogen to the
˚
metal ion. The N(3)–C(13) distance (1.310(5) A) in the complex is
˚
signicantly shorter than the free ligand (1.352(5) A) as a result
of deprotonation of immine group and formation of pC]N.
The addition of complex was assumed to facilitate the
transamidation reaction through the coordination and activa-
tion of amide oxygen. Initially 5 mol% of the complex was
screened at 100 ꢀC under solvent-free conditions affording 70%
˚
The O(1)–C(13) bond length (1.260(4) A) is signicantly longer
˚
than the free ligand (1.208(5) A) due to enolization of pC]O
group during complexation. These evidences suggest that the p
electrons originated from deprotonation of the oxamido group,
are delocalized to form a conjugated system including N(3),
C(13) and O(1). The torsion angles N(1)–C(5)–C(6)–C(7)
[172.6(4)ꢀ], O(1)–C(13)–N(3)–N(2) [–1.4(6)ꢀ], N(1)–C(5)–C(6)–N(2)
[–8.1(6)ꢀ] and N(3)–N(2)–C(6)–C(7) [–2.5(6)ꢀ] indicate that O(1)–
N(2), N(1)–N(2) and N(3)–C(7) are syn-periplanar to each other
but N(1)–C(7) is anti-periplanar to each other.
Table 1 Reaction optimizationa
Entry
Catalyst mol (%)
Solvent
T (ꢀC)
Yield 3jb (%)
1
2
3
4
5
6
7
8
Complex (5)
Complex (5)
Complex (5)
Complex (5)
Complex (5)
Complex (5)
Complex (3)
Complex (10)
MnCl2$4H2O (5)
Mn(OAc)2$4H2O (5)
MnO2 (5)
—
—
—
Toluene
tAmOH
DMSO
—
—
—
—
—
—
—
100
120
130
120
120
120
120
120
120
120
120
120
120
120
70
82
80
40
55
37
75
82
30
40
35
30
n.r
n.r
The molecular structure of the complex is stabilized by
intra-molecular C–H/p interactions (Fig. 2), and different
intra- and inter-molecular hydrogen bonding interactions
between water molecules and pC]O of ligand (Fig. S4†). The
pyridine ring proton approaches the centroid of the phenyl ring
resulting in C–H/p interaction with a contact distance of
9
3.034 A,19 while phenyl ring proton approaches the centroid of
˚
10
11
12
13
14
the pyridine ring resulting in C–H/p interaction with a
˚
contact distance of 3.202 A.
Mn(NO3)2$4H2O (5)
H2bpoh (10)
—
The IR spectral bands observed at 3387, 1690 and 1582 cmꢁ1
in the free ligand are assigned to n(N–H), n(C]O) and n(C]N),
respectively.20 The n(N–H), n(C]N), and n(C]O) bands in the
metal complex appear at the same position as in the ligand,
—
a
Using acetamide (10 mmol) and aniline (10 mmol) for 24 h. b Isolated
yield.
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product yield aer 24 h (entry 1). When the temperature was considerably (3h & 3l). Halogen substituted aromatic amines
increased to 120 ꢀC using the same conditions, a commendable also participated well in the reactions (3b & 3p). A typical
increase in the yield (82%) was noticed (entry 2). The reaction heterocyclic amine, 2-aminopyridine underwent the trans-
was then carried out using different solvents viz. toluene, amidation efficiently to deliver the N-(pyridin-2-yl)formamide in
tAmOH and DMSO, but all of them ever provided a much lower 79% yield (3d). As regards the steric effects, aromatic amine
yield (entries 4–6). A lower catalyst loading led to a lower yield, having ortho substituent gave rise to lower product yield (3m).
whereas a high catalyst loading did not improve the yield Dicyclohexylamine did not react at all. A little decrease in the
(entries 7 and 8). The reaction failed to proceed in the absence yield was noticed for hydroxyl substituted aromatic amines
of complex, thus necessitating the use of the catalyst (entry 14). (3c, 3g & 3o).
The ligand H2bpoh (10 mol%) alone was also found to be
ineffective (entry 13). Various Mn(II) salts such as MnCl2$4H2O,
Mn(OAc)2$4H2O, MnO2 and Mn(NO3)2$4H2O were also tested as
Conclusions
catalysts but the reactions invariably ended with low product The synthesis and characterization of a binuclear Mn(II)
yields (30–40%, entries 9–12). To extend the scope and versa- complex have been undertaken, and the catalytic efficacy of the
tility of the present ndings, different carboxamides were synthesized complex has been successfully explored for the
allowed to react with diverse aliphatic and aromatic amines transamidation reaction.
under the optimized set of reaction conditions to deliver the
corresponding products 3a–3r efficiently in good to excellent
yields (50–89%, Table 2).
Acknowledgements
The transamidation reactions of formamide and acetamide The authors D. P. S. and B. K. A. thank Council of Scientic and
underwent smoothly with different amines namely morpholine, Industrial Research (CSIR), New Delhi for nancial assistance in
4-chloroaniline, 3-hydroxyaniline, 2-aminopyridine, N-methyl- the form of Senior Research Fellowship.
aniline, 3-methylaniline, 4-hydroxyaniline, 4-methoxyaniline,
benzylamine, 2-metoxyaniline and phenylhydrazine to deliver
the corresponding formylated and acetylated amines (Table 2).
Notes and references
Aromatic primary amines substituted with electron-donating
groups at para-position enhanced the product yield
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