Synthesis, spectroscopic characterization and hydroxylation of Mn(II) complexes with bis(2-pyridylmethyl)benzylamine

Article abstract of DOI:10.1016/j.saa.2009.01.014

Two new Mn(II) complexes of bis(2-pyridylmethyl)benzylamine (bpa) were synthesized and characterized by elemental analyses, IR and UV-visible spectroscopies, thermal analyses and ES-MS. These complexes are stable in air with the formula of [(pba)₂

Full text of DOI:10.1016/j.saa.2009.01.014

Spectrochimica Acta Part A 73 (2009) 25–28
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopic characterization and hydroxylation of
Mn(II) complexes with bis(2-pyridylmethyl)benzylamine
Jun-Feng Li, Qiu-Yun Chen∗
School of Chemistry and Chemical Engineer, Jiangsu University, 301 Xufu Road, Zhenjiang, 212013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new Mn(II) complexes of bis(2-pyridylmethyl)benzylamine (bpa) were synthesized and charac-
terized by elemental analyses, IR and UV–visible spectroscopies, thermal analyses and ES-MS. These
complexes are stable in air with the formula of [(pba)2Mn2Cl2(␮-Cl)2] (1) and [(pba)2Mn2(H2O)2(␮-Ac)2]
(Ac)2 (2). The spectroscopic titration results show that the complexes could react with H2O2 resulting
active oxidants, which could cause the intramolecular aromatic hydroxylation. The hydroxylated ligand
(pba-OH) was confirmed by ES-MS and HPLC.
Received 19 September 2008
Received in revised form 16 December 2008
Accepted 15 January 2009
Keywords:
Manganese(II) complex
Spectroscopy
Hydroxylation
Synthesis
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
O2CCH3)3L2]+ (L = bis(pyridylmethyl)amine) were studied for their
ability to disproportionate hydrogen peroxide [11]. The com-
Biomimetic oxygenation reactions using the model com-
pounds of non-heme dioxygenases have attracted much attention
in the communities of bioinorganic and oxidation chemistry
[1,2]. Hydroxylation belongs to the oxygen transfer reactions
introducing the hydroxyl group (–OH) into organic molecules,
primarily via the substitution of functional groups or hydro-
gen atoms. Selective hydroxylation of aromatic compounds
is among the most challenging chemical reactions in syn-
thetic chemistry and has gained steadily increasing attention
during recent years, particularly because of the use of hydroxy-
lated aromatics as precursors for pharmaceuticals [3]. Recently,
intramolecular aromatic and aliphatic ligand hydroxylation as
well as intermolecular phenolate oxygenation, H-atom abstrac-
tion by the models of tyrosinase has been studied extensively
in connection with the catalytic mechanism of tyrosinase
[4–6].
Manganese complexes bearing non-heme ligands, such as
salen- and tacn-derived ligands have shown promise as ver-
satile catalysts in olefin epoxidation and alkane hydroxylation
[7–9]. The dinuclear manganese complexes with pyridine and
(pyridylmethyl)amine ligands were able to catalyze the oxida-
tion of several alkenes to the corresponding epoxides using H2O2
as oxidants [10]. Binuclear manganese(II) complexes [Mn2(␮-
plex [Mn2(␮-Cl)2L2Cl2] was also synthesized although the ability
of this complex to disproportionate hydrogen peroxide was
not reported [12]. Up to now the intramolecular hydroxylation
catalyzed by the manganese(II) complexes were not reported.
Here we report the synthesis, spectroscopic characterization and
intramolecular aromatic hydroxylation of Mn(II) complexes with
bis(2-pyridylmethyl)benzylamine (bpa).
2. Experimental
2.1. Materials and instrumentation
All reagents are of commercial grade and used as received. IR
spectra were recorded on a Nicolet 210 spectrometer in KBr pel-
lets. Elemental analyses were performed by the PerkinElmer 240.
The molar electrical conductivities in DMF solution containing
10−4 mol L−1 complex were measured at 25 0.1 ◦C using a BSA-A
conductmeter. Thermogravimetric analysis was carried out in nitro-
gen atmosphere with a heating rate of 10 ◦C min−1 using a Shimadzu
DT-40 thermal analyzer. 1H NMR and 13C NMR spectra were mea-
sured on a Bruker 400 MHz spectrometer. The electronic absorption
spectra were recorded in the 900–190 nm region using the UV-2450
spectrophotometer. The electrospray-mass spectral (ES-MS) were
determined on a Finnigan LCQ mass spectrograph and the concen-
tration of the samples were about 1.0 ␮mol L−1. The diluted solution
was electrosprayed at a flow rate of 5 × 10−6 L−1 min−1 with a nee-
dled voltage of +4.5 kV. The mobile phase was methanol.
∗
Corresponding author. Tel.: +86 511 88791800; fax: +86 511 88791602.
E-mail address: chenqy@ujs.edu.cn (Q.-Y. Chen).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.01.014

26
J.-F. Li, Q.-Y. Chen / Spectrochimica Acta Part A 73 (2009) 25–28
Table 1
IR bands (cm−1) of the ligand pba and the complexes (1) and (2).
Compound
pba
ꢀ(OH)
ꢀ( C–H)
ꢀ(C–H)
ꢀ(C C, or C N)
ꢀ(COO−
)
ꢀ (py or ph)
3060 (w)
2925 (w)
1589 (m)
1472 (m)
1569 (m)
1432 (m)
760 (m)
699 (w)
(1)
(2)
3058 (w)
3058 (w)
2929 (w)
2929 (w)
1604 (m)
1478 (m)
1570 (m)
1440 (m)
770 (m)
704 (m)
3385 (br)
1604 (m)
1478 (m)
1418 (s)
1570 (s, br)
770 (m)
704 (m)
2.2. Synthesis of the ligand and the Mn(II) complexes
method. The Mn(II)–pba complexes (1) (0.05 mmol) was stirred
with an 100 equiv molar amount of hydrogen peroxide in acetoni-
trile (5 ml) at 0 ◦C for 2 h, then the reaction mixture was acidified to
pH 1 by adding conc. HCl to desalt. The solution was concentrated to
5 ml and the residual material was dissolved into NH3 aqueous solu-
tion, the aqueous solution was extracted with CH2Cl2 (3 × 20 ml),
dried over Na2SO4. After concentrated, oil products were obtained
and analyzed by ES-MS and HPLC.
2.2.1. Synthesis of the bis(2-pyridylmethyl)benzylamine (pba)
A mixture of di(2-picolyl)amine (6 mmol, 1.2 g), benzyl bromide
(6 mmol, 1.0 g) and Et3N (6 mmol, 625 mg) in 25 ml of CH2Cl2 was
stirred at room temperature for 12 h. Then the solution was washed
with 20 ml of 1 M NaOH and water, dried with sodium sulfate and
concentrated to give oil liquid after column chromatography on sil-
ica gel in 80% yield. % Found: C, 78.93; H, 6.65; N, 14.47. % Calc: C,
78.86; H, 6.62; N, 14.52 for C19 H19 N3. 1H NMR (400 MHz, CDCl3):
ı = 3.7 (s, 2H, CH2); 3.9 (s, 4H, CH2N), 7.1–8.5 (m, 13H, H–Ar). 13C
NMR (400 MHz, CDCl3): ı = 58 (CH2); 60 (CH2N), 118–120 (C–Ar),
118, 136, 148 158, 200 (C-py). IR (KBr) (cm−1): ␯( CH) 3060 m,
␯(–CH2–) 2925 m, ␯(C C, C N) 1589 s, 1569 m, ␦(CH, pyridine)
760 s. UV–vis (H2O/nm) (ε × 10−4/dm3 mol−1 cm−1): 203 (4.12),
260 (1.75).
3. Results and discussion
3.1. Characterization of the Mn(II) complexes
The elemental analysis, IR and UV data suggest that the ratio
of metal/L in the two complexes are 1:1, and the composition of
these complexes is [(pba)2Mn2Cl2(␮-Cl)2] (1), [(pba)2Mn(H2O)2(␮-
Ac)2](Ac)2 (2). The molar conductivity value of the complexes in
DMF indicates the complex (1) is a non-electrolyte and the com-
plex (2) is 1:2 electrolyte. The two manganese(II) complexes are
soluble in water and common polar organic solvents, insoluble in
dichloromethane and ethyl ether.
2.2.2. Synthesis of [(pba)2Mn2Cl2(ꢁ-Cl)2] (1)
To a stirred ethanol solution (5 ml) of bis(2-pyridylmethyl)
benzylamine (290 mg, 1 mmol), a solution of MnCl2·4H2O (213 mg,
1 mmol) in ethanol (5 ml) was added dropwise. The mixture was
heated at 80 ◦C for 2 h and then cooled to room temperature. The
solution was filtered and the filtrate was kept in the pyramidal bot-
tle with ether diffusion to precipitate [(pba)2Mn2Cl2(␮-Cl)2]. Yield:
75%. % Found: C, 55.04; H, 4.62; N, 10.18; Mn, 13.32. % Calc: C,
54.96; H, 4.61; N, 10.12; Mn, 13.23 for C38H38Cl4Mn2N6. UV–vis
(H2O/nm) (ε × 10−4/dm3 mol−1 cm−1): 218 (1.46), 261 (1.26). ꢂm
(DMF/ohm−1 cm2 mol−1): 9.6.
3.1.1. IR spectra and mode of bonding
The IR spectral data of the ligand (pba) and complexes (1) and
(2) are listed in Table 1. The IR spectrum of the complex (2) shows
a broad band at 3414 cm−1 assigned to ꢀ(OH) of the coordinated
water, which are confirmed by elemental and thermal analyses.
The bands between 2819 and 3060 cm−1 for complexes (1) and
(2) can be assigned to the stretching vibration of saturated hydro-
carbon and arene C–H in the IR spectra. The vibrational spectra of
complexes can be used to detect the coordination of the pyridyl
nitrogens of the ligands by comparison of the pyridyl ring and in
plane vibrations for the free ligands. The pyridyl ring vibration
bands at 1589, 1569, 1472 cm−1 for complexes (1) and (2) were
shifted to 1604, 1570 and 1478 cm−1, respectively. The ␦(CH) vibra-
tion band (760 cm−1) of pyridyl ring for the complexes was shifted
to 770 cm−1. These shifts can be explained by the fact that the nitro-
gen atoms of pyridyl ring of the ligands donate a pair of electrons
each to the central metal forming coordinate covalent bond [12–14].
The strong and wider band at 1570 cm−1 and the band at 1418 cm−1
for the complex (2) belong to the stretching asymmetric ꢃas(COO−)
2.2.3. Synthesis of the [(pba)2Mn2(H2O)2(ꢁ-Ac)2] (Ac)2 (2)
The compound was synthesized by following the same proce-
dure as described in Section 2.2.2 except MnAc2·4H2O was used.
Yield: 70%. % Found: C, 57.58; H, 5.62; N, 8.68; Mn, 11.50. % Calc:
C, 57.50; H, 5.66; N, 8.75; Mn, 11.44 for C46H54Mn2N6O10. UV–vis
(H2O/nm) (ε × 104/dm3 mol−1 cm−1): 219 (1.52), 261 (1.00). ꢂm
(DMF/ohm−1 cm2 mol−1): 256.
2.3. Reaction of complexes (1) and (2) with hydrogen peroxide
Reaction process of complexes (1) and (2) with H2O2 at 273 K
(0 ◦C) in CH3CN solution was measured by spectroscopic titration
Table 2
Mass spectra data for the Mn complexes (1) and (2) in methanol solution.
Complexes
m/z (Relative abundances)
Fragments
[(pba)2Mn2Cl2(␮-Cl)2] (1)
380.00 (100)
[Mn(pba)(Cl)]+
+
[(pba)2Mn2(H2O)2(␮-Ac)2](Ac)2 (2)
840.75 (70)
822.83 (13)
424.00 (100)
394.00 (47)
290.00 (88)
[Mn2(pba)2(H2O)(Ac−
)
2 + OH−
]
+
[Mn2(pba)2(Ac− 2 + OH−
) ]
[Mn(pba) + OH− + 2CH3OH]+
[Mn(pba)(Ac−)(H2O) + HAc]+
[(pba) + H+]+
[pba-OH]
306.00 (100)
307.00 (20) (isotopic abundance)
290. 0 (62)
[(pba-OH) + H+]+
[(pba-OH) + H+]+
[pba + H+]+

J.-F. Li, Q.-Y. Chen / Spectrochimica Acta Part A 73 (2009) 25–28
27
Fig. 1. TG curves in the temperature range 0–1000 ◦C for the complex (1).
frequencies and the symmetric vibration ꢃsym(COO−), respectively.
The ꢄꢀ(COO−) between the observed asymmetric ꢀas(COO−) and
the symmetric ꢀs(COO−) bands could provide important informa-
tion about the different binding modes of the CH3COO− group
[15,16]. The difference between ꢃas(COO−) and ꢃsym(COO−) is about
152 cm−1, suggesting that the acetate groups coordinate to the
metal atoms only as bidentate ligands [16,17].
3.1.2. Mass spectra
The positive-ion electrospray mass spectra in methanol for the
complexes (1) and (2) were shown in Table 2. The main peak at
m/z = 380.00 (100) for the neutral complex [(pba)2Mn2Cl2(␮-Cl)2]
(1) corresponds to the species [Mn(pdpa)(Cl)]+, which was formed
by losing the coordinated Cl− anion. No other species was found
indicating the complex (1) is stable in the ES-MS condition. The
dominant peak of complex (2) at m/z = 840.75 (70) corresponds
to the species [Mn2(pba)2(H2O)(Ac−)2 + OH−]+ which indicates the
existence of the complex (2). The main peak at m/z = 424.00 cor-
responds to the mononuclear species [Mn(pba) + OH− + 2CH3OH]+,
which was formed by losing the coordinated H2O and Ac− and bind-
ing one OH− and two methanol molecules. The peak at m/z = 290.00
(88) corresponds to the species [(pba) + H+]+ formed by losing the
coordinated Mn(II) ion, which indicates the complex (2) is unstable
in the ES-MS condition.
3.1.3. Thermal studies
Thermal analysis (TG) curves of the complexes (1) and (2) in
the range 0–1000 ◦C were showed in Figs. 1 and 2. The complex
[(pba)2Mn2Cl2(␮-Cl)2] (1) shows four decomposition steps in the
temperature range 0–1000 ◦C (Fig. 1). The 8.55% weight loss in the
temperature range 0–250 ◦C corresponds to the loss of two Cl−
anions (calcd. 8.57%). The loss (29.00%) of two pyridylmethyl group
(2pyCH2–) results from the ligands (pba) and one Cl− ion in the
Fig. 3. (a) UV–vis spectral of complex (1) 1 × 10−4 mmol/ml in acetonitrile at
273 K (0 ◦C) with addition of H2O2 over 3 min (line a: CH
0 mmol/ml; line k:
,
O
2
CH
2.94 × 10−4 mmol/ml). (b) UV–vis spectral of complex2(2) 1 × 10−4 mmol/ml
O
,
2
in 2acetonitrile at 273 K (0 ◦C) with addition of H2O2 over 3 min (line a: CH
O
,
2
2
0 mmol/ml; line l: CH 2 , 3.23 × 10−5 mmol/ml). (c) UV–vis spectral of the complex
O
2
(1) 2.5 × 10−3 mmol/ml in acetonitrile at 0 ◦C with addition of H2O2 over 3 min (line
a: CH
0 mmol/ml; line h: CH 2 , 8.82 × 10−5 mmol/ml).
O
,
O
2
2
2
range 250–350 ◦C (calcd. 28.97%). Other decomposition steps in
the range 375–925 ◦C corresponds to the further decomposition of
the two pba ligands in the complex (1). The 15.78% weight loss in
the range 350–575 ◦C indicate the further loss of one benzyl group
(calcd. 15.80%).
The forth step of decomposition occurs at a temperature range
from 580 to 940 ◦C corresponds to the loss of one benzyl amine
(phCH2NH2) with a weight loss 21.98% (calcd. 22.03%). So the data
Fig. 2. TG curves in the temperature range 0–1000 ◦C for the complex (2).

28
J.-F. Li, Q.-Y. Chen / Spectrochimica Acta Part A 73 (2009) 25–28
of thermal analysis shows the existence of the dinuclear man-
ganese(II) complex (1).
which lead to the intramolecular aromatic hydroxylation. This
experimental result is helpful to understand the mechanisms of
oxygen transfer reactions catalyzed by non-heme dioxygenases.
The thermal decomposition of the [(pba)2Mn2(H2O)2(␮-Ac)2]
(Ac)2 (2) proceeds approximately with three decomposition steps.
There is no obvious weight loss in the 0–115 ◦C. The first step falls in
the range of 115–225 ◦C, which is assigned to loss of two coordinated
H2O molecules and two CH3COO− anions with a weight loss 16.11%
(calcd. 16.03%). The weight loss 44.72% in the range of 225–475 ◦C
is assigned to loss of four pyridylmethyl group (4pyCH2–) and one
CH3COO− anions (calcd. 44.65%). The final decomposition step in
the range of 475–900 ◦C corresponds to the loss of one benzyl amine
(phCH2NH2) with a weight loss 10.99% (calcd. 11.13%). The thermal
decomposition data obtained supports the proposed structure of
the complex (2).
Acknowledgements
Financial support from National Science Foundation of China
(20777029B0702), distinguished scholar science foundation of
Jiangsu University (06JDG050) and the foundation of State Key Lab-
oratory of Coordination Chemistry, Nanjing University.
References
[1] T. Punniyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 105 (2005) 2329.
[2] W. Nam, Acc. Chem. Res. 40 (2007) 522.
[3] K.D. Koehntop, S. Marimanikkuppam, M.J. Ryle, R.P. Hausinger, LQ Jr., J. Biol.
Inorg. Chem. 11 (2006) 63.
[4] E.I. Solon, R. Sarangi, J.S. Woertink, A.J. Augustine, J. Yoon, S. Ghosh, Acc. Chem.
Res. 40 (2007) 581.
3.2. Reaction of complexes (1) and (2) with H2O2 by
spectroscopic titration
[5] T. Matsumoto, H. Furutachi, M. Kobino, M. Tomii, S. Nagatomo, T. Tosha, T.
Osako, S. Fujinami, S. Itoh, T. Kitagawa, M. Suzuki, J. Am. Chem. Soc. 128 (2006)
3874.
[6] A. Kunishita, J. Teraoka, J.D. Scanlon, T. Matsumoto, M. Suzuki, C.J. Cramer, S.
Itoh, J. Am. Chem. Soc. 129 (2007) 7248.
[7] E.M. McGarrigle, D.G. Gilheany, Chem. Rev. 105 (2005) 1563.
[8] J.R. Lindsay Smith, B.C. Gilbert, A.M. Payeras, J. Murray, T.R. Lowdon, J. Oakes,
R.P. Iprats, P.H. Walton, J. Mol. Catal. A: Chem. 251 (2006) 114.
[9] K. Nehru, S.J. Kim, I.Y. Kim, M.S. Seo, Y. Kim, S.-J. Kim, J. Kim, W. Nam, Chem.
Commun. (2007) 4623.
[10] M.S. Niassary, F. Farzanch, M. Ghandi, L. Turkian, J. Mol. Catal. A: Chem. 157
(2000) 18.
[11] I. Romero, L. Dubois, M.N. Collomb, A. Deronzier, J.M. Latour, J. Pecaut, Inorg.
Chem. 41 (2002) 1795.
[12] I. Romero, M.N. Collomb, A. Deronzier, A. Llobet, E. Perret, J. Pécaut, L.L. Pape,
J.M. Latour, J. Eur. Inorg. Chem. 1 (2001) 69.
[13] T.I.A. Gerber, J. Coord. Chem. 56 (2003) 1397.
Reaction of complexes (1) and (2) with H2O2 at 273 K (0 ◦C)
in CH3CN solution by spectroscopic titration was showed in
Fig. 3(a)–(c). In the UV–vis spectrum, the two absorption bands at
around 218 and 261 nm were assigned to ␲–␲* and n–␲* intrali-
gand transition [18]. The addition of H2O2 to the complexes (1) and
(2) solution promoted an absorbance increase in the 200–800 nm
(Fig. 3(a)–(c)), typical of metal center oxidation [19]. The bands at
261 nm slowly increase with broadening while the bands at 218 nm
experience a slight increase in intensity with slight red shift. These
changes suggest the presence of a slight amount of a protonated
form, or the formation of a mixture of oxo bridged species on the
interaction of Mn complexes with H2O2 [20,21].
The intramolecular hydrogen atom abstraction and H2O2
rebound or direct H2O2 insertion mechanism through the inter-
mediate for the benzylic hydroxylation reaction was proposed by
Itoh and co-workers, although such an intermediate could not be
detected at room temperature [21–25]. Intramolecular aromatic lig-
and hydroxylation was unambiguously confirmed by the modified
ligands pba-OH (m/z = 306) isolated by demetalation of the final
products (Table 2). The yield of hydroxylated ligand pba-OH was
evaluated as about 65% based on the manganese complex (1) by
comparing the dimension of the peak at 17 min (bpa) with that at
19 min (pba-OH) in HPLC.
[14] E.G. Bakalbassis, C.A. Tsipis, A.P. Bozopoulos, D.W. Dreissig, H. Hartl, J. Mrozinski,
Inorg. Chem. 30 (1991) 2801.
[15] P.S. Mukherjee, D. Ghoshal, E. Zangrando, T. Mallah, N.R. Chaudhuri, Eur. J. Inorg.
Chem. (2004) 4675.
[16] C.E. Xanthopoulos, M.P. Sigalas, G.A. Katsoulos, C.A. Tsipis, A. Terzis, A. Hountas,
Inorg. Chim. Acta 214 (1993) 153.
[17] R. Cao, Q. Shi, D. Sun, M. Hong, W. Bi, Y. Zhao, Inorg. Chem. 41 (2002) 6161.
[18] S. Chandra, S. Sharma, Trans. Met. Chem. 27 (2002) 732.
[19] M.R. Maurya, A.K. Chandrakar, S. Chand, J. Mol. Catal. A: Chem. 270 (2007)
225.
[20] S. Groni, G. Blain, R. Guillot, C. Policar, E.A. Mallart, Inorg. Chem. 46 (2007) 1951.
[21] M.R. Maurya, A.K. Chandrakar, S. Chand, J. Mol. Catal. A: Chem. 274 (2007) 192.
[22] S. Itoh, H. Nakao, L.M. Berreau, T. Kondo, M. Komatsu, S. Fukuzumi, J. Am. Chem.
Soc. 120 (1998) 2890.
[23] J.A. Halfen, V.G. Young Jr., W.B. Tolman, J. Am. Chem. Soc. 118 (1996) 10920.
[24] S. Mahapatra, S. Kaderli, A. Llobet, Y.M. Neuhold, T. Palanche, J.A. Halfen, V.G.
Young Jr., T.A. Kaden, L. Que Jr., A.D. Zuberbuhler, W.B. Tolman, Inorg. Chem. 36
(1997) 6343.
[25] S. Itoh, T. Kondo, M. Komatsu, Y. Ohshiro, C. Li, N. Kanehisa, Y. Kai, S. Fukuzumi,
J. Am. Chem. Soc. 117 (1995) 4714.
4. Conclusion
The dinuclear Mn(II) complexes of bis(2-pyridylmethyl)
benzylamine (bpa) could react with H2O2 resulting active oxidants,