Structure, photochemistry and magnetic properties of tetrahydrogenated Schiff base chromium(III) complexes

Article abstract of DOI:10.1016/j.saa.2015.01.012

Four mononuclear chromium(III) complexes [Cr(L⁽¹⁾)(en)]Br_0.3Cl_0.7 (1), [Cr(L⁽¹⁾)(pr)]Cl (2), [Cr(L⁽²⁾)(en)]ClO₄ (3), [Cr(L⁽²⁾)(pr)]Cl (4) along with one dinuclear μ-methoxo [Cr(μ-OMe)(L₁)]₂ (5) were synthesized (en = 1,2-ethanediamine, pr = 1,3-diaminopropane H₂L⁽¹⁾ = Tetrahydrosalen = H₂[H₄]salen = N,N′-bis(2-hydroxybenzyl)-1,2-ethanediamine, H₂L⁽²⁾ = Tetrahydrosalpr = H₂[H₄]salpr = N,N′-bis(2-hydroxybenzyl)-1,3-diaminopropane). The competitive reactions in the presence of EDTA were carried out and the first-order rate constants k(1) = (5.2 ± 0.2) × 10^-3 h^-1 < k(2) = (6.7 ± 0.3) × 10^-3 h^-1 < k(3) = (8.0 ± 0.1) × 10^-3 h^-1 < k(4) = (9.5 ± 0.2) × 10^-3 h^-1 were obtained by spectroscopic measurements. In addition, photo-induced decomposition was monitored under irradiation of xenon lamp. The sequence of first-order rate constants is k′(1) = (4 ± 0.1) × 10^-4 s^-1 < k′(2) = (6 ± 0.3) × 10^-4 s^-1 < k′(3) = (1.1 ± 0.2) × 10^-3 s^-1 < k′(4) = (1.4 ± 0.2) × 10^-3 s^-1, which is in accordance with that of kinetics studies with EDTA. Dinuclear complex 5 exhibits a strong antiferromagnetic coupling with the J = -10.8 cm^-1.

Full text of DOI:10.1016/j.saa.2015.01.012

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 437–443
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Structure, photochemistry and magnetic properties of
tetrahydrogenated Schiff base chromium(III) complexes
⇑
⇑
Bin Liu , Jie Chai, SiSi Feng, BinSheng Yang
Institute of Molecular Science, Key Laboratory of Chemical Biology of Molecular Engineering of Education Ministry, Shanxi University, Taiyuan 030006, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
Five tetrahydrogenated Schiff base
Cr(III) complexes were composed.
The stability relates to the chelate
effect of rings.
The photochemistry reaction was
studied.
NH
NH
NH
NH
O
Cr
O
Cr
1
O
O
H2N
NH2
H2N
NH2
2
Complex 5 exhibits a strong
antiferromagnetic coupling.
NH
Cr
NH
NH
NH
O
O
NH
NH
O
O
Cr
O
O
NH2
O
Cr
3
O
NH2
O
O
Cr
5
H2N
H2N
NH
NH
4
a r t i c l e i n f o
a b s t r a c t
Four mononuclear chromium(III) complexes [Cr(L(1))(en)]Br0.3Cl0.7 (1), [Cr(L(1))(pr)]Cl (2), [Cr(L(2)
(en)]ClO4 (3), [Cr(L(2))(pr)]Cl (4) along with one dinuclear
-methoxo [Cr( -OMe)(L1)]2 (5) were synthe-
sized (en = 1,2-ethanediamine, pr = 1,3-diaminopropane H2L(1) = Tetrahydrosalen = H2[H4]salen = N,N0-bis
(2-hydroxybenzyl)-1,2-ethanediamine,
H2L(2) = Tetrahydrosalpr = H2[H4]salpr = N,N0-bis(2-hydroxy-
)
Article history:
Received 17 November 2014
Received in revised form 30 December 2014
Accepted 11 January 2015
l
l
Available online 17 January 2015
benzyl)-1,3-diaminopropane). The competitive reactions in the presence of EDTA were carried out
and
the
first-order
rate
constants
k(1) = (5.2 0.2) ꢁ 10ꢂ3 hꢂ1 < k(2) = (6.7 0.3) ꢁ 10ꢂ3 hꢂ1
<
Keywords:
k(3) = (8.0 0.1) ꢁ 10ꢂ3hꢂ1 < k(4) = (9.5 0.2) ꢁ 10ꢂ3 hꢂ1 were obtained by spectroscopic measurements. In
addition, photo-induced decomposition was monitored under irradiation of xenon lamp. The sequence of
first-order rate constants is k0(1) = (4 0.1) ꢁ 10ꢂ4 sꢂ1 < k0(2) = (6 0.3) ꢁ 10ꢂ4 sꢂ1 < k0(3) = (1.1 0.2) ꢁ
10ꢂ3 sꢂ1 < k0(4) = (1.4 0.2) ꢁ 10ꢂ3 sꢂ1, which is in accordance with that of kinetics studies with EDTA. Dinu-
Chromium(III)
Tetrahydrogenated Schiff base
Kinetics
Photochemistry
Chelate effect
Magnetism
clear complex 5 exhibits a strong antiferromagnetic coupling with the J = ꢂ10.8 cmꢂ1
.
Ó 2015 Elsevier B.V. All rights reserved.
Introduction
geometries afforded to metal centers by these multidentate species
has sustained their widespread interest over the past 150 years. It
should be noted that hydrogenation of tetradentate Schiff bases
will not only increase the donor strength of the internal nitrogen
donors, but also increase the ligand flexibility [2]. Taylor and Reg-
linski have studied the effect of donor groups and geometry on the
redox potential of tetrahydrogenated Schiff base complexes by sys-
tematically increasing the size of the polymethylene chain
between the internal nitrogen donors [1,3]. Although the transition
metal complexes of tetrahydrogenated ligand H2[H4]salen [4] and
H2[H4]Salpr [3] (H2[H4]salen = N,N0-bis(2-hydroxybenzyl)-1,2-eth-
Transition metal complexes with oxygen and nitrogen donor
Schiff bases are of particular interest because of their unusual
con-figurations, structural lability and sensitivity to molecular
environments as functional materials [1]. Although the tetraden-
tate Schiff-base ligands containing N2O2 donors are easy to coordi-
nate to transition- and non-transition-metal ions, the coordination
⇑
Corresponding authors. Tel.: +86 351 7016358.
E-mail addresses: liubin@sxu.edu.cn (B. Liu), yangbs@sxu.edu.cn (B. Yang).
http://dx.doi.org/10.1016/j.saa.2015.01.012
1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

438
B. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 437–443
anediamine, H2[H4]salpr = N,N0-bis(2-hydroxybenzyl)-1,3-diami-
nopropane) have been fully studied, few Cr(III) complexes with
salen2ꢂ, [H4]salen2ꢂ, salpr2ꢂ or [H4]Salpr2ꢂ have been reported
[5–7]. It is well known that Cr(III) complexes with Schiff bases
and theirs derivatives play an important role in catalytic reactions,
photochemistry, artificial nucleases, carbohydrate metabolism.
Interested by the thermodynamic, kinetic, and electronic prop-
erties of Cr(III) complexes with salicylate and en ligands [8], we ini-
tiated an investigation of oligomeric Cr(III) compounds containing
the tetrahydrogenated ligand [H4]salen2ꢂ or [H4]Salpr2ꢂ and eth-
anediamine (en) or 1,3-diaminopropane (pr). The use of Zn as a
reducing agent can turn the inert Cr(III) to reactive Cr(II). The com-
bination of these polydentate ligands can lead to the formation of
stable chelates. In this paper five tetrahydrogenated Schiff base
Cr(III) complexes were synthesized for the first time. The possible
steric effects including the number of chelate rings and the size of
chelate ring upon the kinetics stability and photochemistry reac-
tion were investigated by spectroscopic measurements. The mag-
netic properties of a dinuclear Cr(III) complex were also explored.
and methanol (20 mL) was added. After an hour’s refluxing, 1,3-
diaminopropane (pr, 2 mL, 24.0 mmol) was added to the solution,
the solution was kept stirring and refluxing for 0.5 h, purple pre-
cipitate was obtained. Filtered through the funnel, purple mini-
crystal (complex 2) was obtained. Anal. Cacld. for
C
20H32ClCrN4O3: C, 51.78; H, 6.95; N, 12.08. Find: C, 51.74; H,
6.99; N, 12.11. ESI-MS spectra: m/z = 396.16 (Fig. S4), [M]+; [M]+
calculated: 396.16. IR: 3271, 3113, 2930, 1595, 1480, 1276, 1080,
760 cmꢂ1
.
[Cr(L(2))(en)]ClO4 (3)
H2L(2) (143 mg, 0.5 mmol), CrCl3ꢃ6H2O (133 mg, 0.5 mmol) and
granular (Mesh size 20) zinc (0.10 g, 1.5 mmol) were refluxing in
methanol (20 mL) for an hour, then en (1 mL, 15.0 mmol) was
added to the solution, the solution was kept stirring and refluxing
for 0.5 h, purple precipitate was obtained. The solution was filtered
through the funnel; purple minicrystal and aubergine filtrate were
obtained. Sodium perchlorate (0.12 g, 1.0 mmol, note: perchlorate
salts are explosive) was added to the aubergine filtrate carefully
in room temperature, purple crystal were obtained (complex 3)
over night. Anal. Cacld. for C19H28ClCrN4O2: C, 52.96; H, 6.32; N,
13.00. Find: C, 52.98; H, 6.218; N, 13.03. IR: 3303, 3244, 1595,
Experimental
1479, 1286, 1107, 766 cmꢂ1
.
General procedure
[Cr(L(2))(pr)]ClꢃCH3OH (4)
Unless otherwise stated all chemicals were commercially
obtained and used without further purification. All manipulations
were performed under aerobic conditions. The ligands H2L(1) and
H2L(2) (H2L(1) = Tetrahydrosalen, H2L(2) = Tetrahydrosalpr) were
obtained from methods given in the literature (see the ESI) [1c].
All other chemicals were of analytical grade.
H2L(2) (143 mg, 0.5 mmol), CrCl3ꢃ6H2O (133 mg, 0.5 mmol) and
granular (Mesh size 20) zinc (0.10 g, 1.5 mmol) were refluxing in
methanol (20 mL) for an hour, then pr (2 mL, 24 mmol) was added
to the solution, keep refluxing for 0.5 h and purple precipitate was
obtained. The result solution was filtered and the aubergine filtrate
was placed, purple crystal (complex 4) was obtained over night.
Anal. Cacld. for C21H34ClCrN4O3: C, 52.77; H, 7.17; N, 11.72. Find:
1H NMR spectra were recorded on Bruker-300 MHz spectrome-
ters, the chemical shifts (d) were reported as ppm in DMSO. ESI-MS
spectra were recorded on an Agilent 6520 Accurate-Mass Q-TOF
LC/MS mass spectrometer. UV–visible (UV–Vis) spectra were mea-
sured with a Varian 50 BIO spectrophotometer. Elemental analyses
were measured on a Vario EL III analyzer. IR spectra were recorded
with a Bruker TENSOR 21 FT-IR spectrophotometer.
C, 52.79; H, 7.14; N, 11.69. IR: 3107, 2934, 1596, 1482, 1278 cmꢂ1
.
X-ray structure determination
For each sample, a single crystal with suitable dimensions for X-
ray diffraction analyses was mounted on a glass rod, the crystal
data were collected with a Siemens (Bruker) SMART CCD diffrac-
tometer using monochromated Mo-Ka radiation (0.71073 Å) at
Synthesis
293 K. In all cases intensity data were measured by thin-slice
x-
[Cr(L(1))(en)]Br0.3Cl0.7ꢃCH3OH (1), [Cr(L(1))(
-OMe)]2ꢃ2CH3OH (5)
l
or - and -scans. The method used to solve the structure was
x
u
Complexes 1 and 5 were formed from the same batch. H2L(1)
(136 mg, 0.5 mmol), CrCl3ꢃ6H2O (133 mg, 0.5 mmol) and granular
(Mesh size 20) zinc (0.10 g, 1.5 mmol) were put in flask, and meth-
anol (20 mL) was added. After an hour’s refluxing, ethanediamine
(en, 1 mL, 15.0 mmol) was added to the solution; the solution
was kept stirring and refluxing for 0.5 h, purple precipitate was
obtained. Filtered through the funnel, purple minicrystal and
aubergine filtrate were obtained. The purple minicrystal was redis-
solved by addition of ethanol: H2O = 1:1, then KBr was added to the
solution. The solution was filtered through the funnel, plenty of
block purple crystals (complex 1) were obtained in the filtrate after
a few days (yield > 70%). The aubergine filtrate was placed over
night, and another aubergine crystal 5 was obtained (yield < 10%).
Interestingly, the yield of complex 1 or 5 was affected by the
amount of added en solution: the more en was added, the more
1 was produced. Anal. Cacld. for C19H30Br0.3Cl0.7CrN4O3 (1): C,
49.50; H, 6.56; N, 12.15. Find: C, 49.38; H, 6.49; N, 12.19. Anal. Cac-
ld. for C36H52Cr2N4O9 (5): C, 54.81, H, 6.64, N, 7.10. Find: C, 54.63,
H, 6.42, N, 7.22. IR: 1: 3400, 3113, 1595, 1480, 1275, 1053, 762,
direct method and the structure was further expanded using Fou-
rier difference techniques with the SHELXTL-97 program package
[9]. Absorption corrections were carried out using the SADABS pro-
gram supplied by Bruker [10]. The anisotropic refinement was
applied for all non-hydrogen atoms while hydrogen atoms in all
samples were included in idealized positions and their Uiso values
were set to ride on the Ueq values of the parent carbon, nitrogen or
oxygen atoms. Details of the crystallographic data collection, struc-
tural determination and refinement are summarized in Table 1.
Selected bond lengths (Å) and angles (°) of 1, 3, 4, and 5 are shown
in Table 2. For crystallographic data in CIF format, see the ESI. CCDC
reference numbers: 712642, 913849, 1002820, 1002821.
Photochemical stability
1 ꢁ 10ꢂ3 M complex 1–4 in Tris–HCl buffer (0.01 M, pH = 7.4)
was exposed to light of xenon lamp (CEL-S500/350, 50 mW/cm2,
AM 1.5) in methanol, 293 K for hours. This course was monitored
by UV–Vis spectra.
628 cmꢂ1; 5: 3176, 2925, 1595, 1478, 1290, 1064, 758 cmꢂ1
.
Magnetic measurements
[Cr(L(1))(pr)]Cl (2)
H2L(1) (136 mg, 0.5 mmol), CrCl3ꢃ6H2O (133 mg, 0.5 mmol) and
Variable-temperature magnetic susceptibility on a crystalline
sample was performed on a Quantum Design MPMS-XL SQUID
granular (Mesh size 20) zinc (0.10 g, 1.5 mmol) were put in flask,

B. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 437–443
439
Table 1
Crystal data and refinement details for complex 1, 3, 4 and 5.
Complex
1
3
4
5
Formula
Formula weight
C
19H30Br0.30Cl0.70CrN4O3
C19H28ClCrN4O6
495.90
C21H34ClCrN4O3
477.97
C36H52Cr2N4O9
788.82
463.26
Crystal system space group
Orthorhombic 2(1)2(1)2(1)
Triclinic P ꢂ 1
Monoclinic, P2(1)/c
7.5886(8)
12.1186(13)
25.557(2)
90
97.441(2)
90
2330.5(4)
4
Monoclinic, P2(1)/c
7.6249(9)
13.7319(12)
19.087(2)
90
100.895(2)
90
1962.4(4)
2
a/Å
b/Å
c/Å
8.2150(5)
13.2027(8)
19.8414(13)
90
90
90
2152.0(2)
4
1.430
9.6977(8)
11.4352(10)
11.8037(12)
114.782(2)
104.2810(10)
99.5480(10)
1096.44(17)
2
a
(°)
b (°)
c
(°)
V/Å3
Z
Density/g cm3
1.502
1.362
1.335
F(000)
970
518
1012
832
h
2.57–25.02
ꢂ7/9, ꢂ9/15, ꢂ23/22
8094/805/0.0629
3805/255
0.998
3.02–25.01
ꢂ11/11, ꢂ13/13, 14/14
3850/3850/0.0000
3850/324
2.71–25.02
ꢂ5/9, ꢂ13/14, ꢂ30/30
18588/4092/0.1393
4092/271
1.102
1.84–25.01
h, k, l
ꢂ8/9, ꢂ16/13, ꢂ22/21
9620/3450/0.0522
3450/254
Tot. Uniq. Data R(int)
N
GOF on F2
reflections, Nparameter
1.126
1.043
R1[I > 2s (I)], wR2
0.0579, 0.0965
0.0802, 0.2294
0.1288, 0.2967
0.0612, 0.1605
Table 2
Selected bond lengths (Å) and angles (°) of 1, 3, 4, and 5.
Bond lengths
Bond angles
Bond angles
Bond angles
Compound 1
Cr1–O1
Cr1–O2
Cr1–N1
Cr1–N2
Cr1–N3
Cr1–N4
1.900(4)
1.927(4)
2.100(4)
2.070(4)
2.097(4)
2.105(4)
O1-Cr1-O2
O1-Cr1-N2
O2-Cr1-N2
O1-Cr1-N3
O2-Cr1-N3
90.45(17)
93.21(17)
91.62(16)
92.95(19)
87.67(17)
N2-Cr1-N3
O1-Cr1-N1
O2-Cr1-N1
N2-Cr1-N1
N3-Cr1-N1
173.80(18)
90.03(17)
173.27(18)
81.65(18)
99.00(18)
O1-Cr1-N4
O2-Cr1-N4
N2-Cr1-N4
N3-Cr1-N4
N1-Cr1-N4
173.55(18)
88.37(17)
93.16(17)
80.67(18)
91.89(17)
Compound 3
Cr1–O2
Cr1–O1
Cr1–N1
Cr1–N2
Cr1–N3
Cr1–N4
1.920(5)
1.931(5)
2.124(6)
2.090(6)
2.108(6)
2.112(6)
O2-Cr1-O1
O2-Cr1-N2
O1-Cr1-N2
O2-Cr1-N3
O1-Cr1-N3
92.7(2)
91.8(2)
87.2(2)
85.5(2)
95.5(2)
N2-Cr1-N3
O2-Cr1-N4)
O1-Cr1-N4
N2-Cr1-N4
N3-Cr1-N4
176.4(2)
86.3(2)
176.6(2)
96.1(2)
81.2(2)
O2-Cr1-N1
O1-Cr1-N1
N2-Cr1-N1
N3-Cr1-N1
N4-Cr1-N1
176.1(2)
89.3(2)
91.6(2)
91.0(2)
91.6(2)
Compound 4
Cr1–O2
Cr1–O1
Cr1–N1
Cr1–N2
Cr1–N3
Cr1–N4
1.930(8)
1.931(8)
2.128(9)
2.083(10)
2.075(11)
2.065(11)
O2-Cr1-O1
O2-Cr1-N4
O1-Cr1-N4
O2-Cr1-N3
O1-Cr1-N3
87.6(3)
88.2(4)
92.5(4)
91.6(4)
177.8(4)
N4-Cr1-N3
O2-Cr1-N2
O1-Cr1-N2
N4-Cr1-N2
N3-Cr1-N2
85.4(5)
89.9(4)
93.7(4)
173.4(4)
88.3(4)
O2-Cr1-N1
O1-Cr1-N1
N4-Cr1-N1
N3-Cr1-N1
N2-Cr1-N1
175.3(3)
89.7(3)
88.0(4)
91.0(4)
94.2(4)
Compound 5
Cr1–O2
Cr1–O1
Cr1–O3#1
Cr1–O3
Cr1–N1
1.943(4)
1.954(3)
1.958(4)
1.968(4)
2.068(5)
2.078(4)
O2-Cr1-O1
90.58(15)
94.39(16)
92.81(16)
172.64(16)
92.60(16)
O3#1-Cr1-O3
O2-Cr1-N1
O1-Cr1-N1
O3#1-Cr1-N1
O3-Cr1-N1
78.82(19)
92.70(17)
91.25(17)
171.78(17)
93.87(17)
O2-Cr1-N2
O1-Cr1-N2
O3#1-Cr1-N2
O3-Cr1-N2
N1-Cr1-N2
90.25(16)
174.15(19)
92.90(17)
87.27(16)
82.93(18)
O2-Cr1-O3#1
O1-Cr1-O3#1
O2-Cr1-O3
O1-Cr1-O3
Cr1–N2
magnetometer. The susceptibility measurement was performed in
the 300–1.8 K temperature range with an applied field of 1 kOe.
Data were corrected for the sample holder and diamagnetism
was estimated from Pascal constants. X-ray powder diffraction
was performed on bulk samples prior to magnetic studies.
the reaction of H2L(1) (1.0 equivalents), excess en with CrCl3ꢃ6H2O
in methanol affords the mononuclear complex 1 in 70% yield
(Scheme 1) and binuclear complex 5 (<10% yield); The yield of 1
or 5 was affected by the amount of added en solution. The reaction
of H2L(1) (1.0 equivalents), excess pr with CrCl3ꢃ6H2O in methanol
afforded the Cr(III) complex 2; Similarly the reaction of H2L(2)
,
excess en with CrCl3ꢃ6H2O in methanol afforded complex 3 and
the reaction of H2L(2), pr with CrCl3ꢃ6H2O in methanol afforded
complex 4. All the complexes could be isolated as single crystals
via a classical solvent (MeOH) evaporation process except 2.
The solid-state structures of the mononuclear and dinuclear
complexes 1, 3, 4 and 5 were determined by X-ray crystallographic
studies. Here after is a short structural description of these com-
pounds. Selected bond lengths and angles are given in Table 2.
1 crystallizes as a mononuclear species with the Cr(III) ion che-
lated by L(1)-tetradentate ligand and en bidentate ligand (Fig. 1).
Results and discussion
Synthetic and structural studies
Both ligands N,N0-bis(2-hydroxybenzyl)-1,2-ethanediamine
(H2L(1)
)
and N,N0-bis (2-hydroxybenzyl)-1,3-diaminopropane
(H2L(2)) were prepared as previously reported and were isolated
in good yields [1c]. The first step of the present work aimed at
studying the coordination of H2L(1) and H2L(2) toward Cr(III) salt
for the obtainment of well-defined Cr(III) metal complexes. Thus,

440
B. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 437–443
NH
NH
(CH2)n
N
N
O
(CH2)n
O
HO
Cr
OH
H2N
NH2
[H]
3
(CH2)n
1, 2, 3, 4
N
N
(CH2)n
H
H
HO
OH
C
n
NH
O
NH
O
r
C
n=0, 1
e
l
O
Cr
Cr
O
O
O
NH
NH
5
Fig. 2. ORTEP view of complex 3. The hydrogen atoms were omitted for clarity. The
Scheme 1. Synthesis of Cr(III) complexes 1–5 from the ligands H2L(1) (n = 0) and
H2L(2) (n = 1), 1: [Cr(L(1))(en)]Br0.3Cl0.7ꢃCH3OH, 2: [Cr(L(1))(pr)]Cl, 3: [Cr(L(2))(-
ellipsoids enclose 30% of the electronic density.
en)]ClO4, 4: [Cr(L(2))(pr)]ClꢃCH3OH, 5: [Cr(L(1))(
l-OMe)]2ꢃ2CH3OH.
conformation. The angle N3–Cr–N4 is 81.2°, which is not signifi-
cantly different from that in 1 (80.67°).
In complex 4, Cr(III) ion is chelated by L(2)-tetradentate ligand
and pr bidentate ligand forming a slightly distorted octahedral
geometry (Fig. 3). Although L(2) forms larger chelate rings than
L(1), the Cr(III) structure obtained remains an accommodation of
the preferred octahedral geometry of the metal cation coupled
with the stability of the chair conformation of the six member
Cr(L(2)) chelate. There are two relatively large torsion angles N1–
The metal center Cr(III) adopts a slightly distorted octahedral
geometry. Around Cr ions the two phenoxo atoms (O1, O2) or
nitrogen (N1, N2) of L(1) are in cis configuration [6b]. The tetrahy-
drogenated L(1) adopts a non-planar configuration: angle O1–Cr–
O2 = 90.43(17) and
a
large torsion angle C15–N1–C1–
C2 = 73.9(6)°. It is different to the original Schiff base complexes
such as [Cr(salen)(H2O)2]+ during which the Schiff base tends to
adopt a approximate square-planar ligand geometry [5b]. The
hydrogenation of Schiff bases not only reduces the covalent inter-
C1–C2–C3 = ꢂ58.7(14) and C15–N1–C1–C2 = ꢂ70.7(13)° in L(2)
.
O2–Cr–O1 angle is 87.6(3)°, which is smaller than that in complex
3 (92.7°). N2–Cr–N1 angle 94.2(4)° is a little bigger than that in 3
(91.6°). It should be mentioned that there are four six-member
rings around Cr(III) ions into 4 including two saturated cyclic ring
and two unsaturated ring.
action p–p effect in L but also increases the ligand flexibility for
free rotations of the azomethine carbon, which is beneficial to
coordination by metal ions tightly. The en adopts a k conformation.
It is not long enough for en to chelate with Cr along two axial
positions.
In 5, each chromium metal center is chelated by L(1)2ꢂ-tetraden-
tate ligand and connected to one another via two cis-methoxy
bridging groups (Fig. 4). The configuration of the dinuclear mole-
3 crystallizes as a mononuclear species with the Cr(III) chelated
by L(2)2ꢂ tetradentate ligand and en bidentate ligand (Fig. 2). The
two phenoxo atoms (O1, O2) or nitrogen (N1, N2) of L(2) are in
cis configuration. Two relatively large torsion angle N1–C4–C5–
C6 = 58.4(9) and N2–C11–C12–C13 = 60.0(9)° exist in L(2)2ꢂ. During
the original Schiff base complexes [Cr(salpr)Cl2]Cl the Schiff base
also tend to adopt a approximate square-planar geometry [7b].
The N1–C1–C2–C3–N2–Cr forms a six-member ring with the N1–
Cr–N2 angle 91.6° in 3, while in 1 the N1–Cr–N2 is in a five-mem-
ber ring with a smaller angle (81.65°). The average Cr–O bond and
Cr–N bond is 1.926 and 2.107 Å respectively, which is also longer
than that in complex 1 (1.913, 2.085 Å). The en ligand adopts a d
cule is cis,cis –O,O0–
two oxygen donors and
D
,
K
D
, where cis refers to the position of the
and refer to the configuration of the
K
two N, O chelates of the tetradentate L(1) at each of the two Cr(III)
centers. The same cis,cis –O,O0 configuration of L(1) has been
reported [6d]. L(1) adopts a similar configuration to that of 1, but
the Cr–O (phenoxo) bond (1.943–1.954 Å) is longer than that in 1
(1.900–1.920). The Cr–N distances differ only slightly (2.068–
2.078 Å) and the O1–Cr–O2 angle is 90.58(15)°. The (Cr–O)2 bridg-
ing unit is strictly planar with a crystallographic inversion center in
Fig. 1. ORTEP view of complex 1. The hydrogen atoms were omitted for clarity. The
Fig. 3. ORTEP view of complex 4. The hydrogen atoms were omitted for clarity. The
ellipsoids enclose 30% of the electronic density.
ellipsoids enclose 30% of the electronic density.

B. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 437–443
441
the middle of the dimer. The Cr–Cr# and Cr–O3 (methoxide) dis-
Kinetics studies
tances and the Cr–O3–Cr# angle of the bridging moiety are 3.033
and 1.968(4) Å and 101.18(19)°, respectively, which are compara-
ble to the similar bis(l-OMe)–bridged dichromium(III) complexes
[11]. As shown in Fig. S5, two intramolecular hydrogen bond
(N2–H2ꢃ ꢃ ꢃO1, 3.162 Å, ꢂx + 2, ꢂy + 1, ꢂz + 1) between L(1) and
L(1)# can be observed, which further strengthen the stability of
dinuclear structure. Intra- as well as intermolecular hydrogen
bonds of 1, 3, 4 and 5 are listed in Table S1–S4 (ESI).
In order to compare the stability of these different Cr(III) com-
plexes, EDTA was employed as a simple competitive ligand. To
examine the transfer of Cr(III) to EDTA, 100 equiv. of EDTA was
added to 1.0 mM complex (1–4) in 0.01 mM Tris–HCl, pH 7.4.
The reaction mixture was stored at 37 °C. The UV–vis spectra were
monitored as a function of time until the spectra became constant
with time. The UV–vis spectra were shown in Fig. 5. Complex 5 is
polymeric and neutral; it is not surprising that it is insoluble or
sparingly soluble in neutral soluble solvents. The low solubility
made it impossible to study its stability using the visible spectra.
All of the ligand field spectrum of these complexes showed one
broad absorption band at about 520 nm, which could be assigned
to the d ? d transitions (4A2g(F) ? 4T1g(F)) of Cr(III) ions (Fig. 5).
With time going, the electronic spectrum was altered in intensity
and shifted in position of the adsorption bands. For example, the
amount of red shift for 1 was 20 nm. According to the ligand field
theories and inorganic electronic spectroscopy, the coordination
environment of Cr(III) had been changed from CrN4O2 to lower
field [12]. It showed that the Cr(III) complex was destructed com-
pletely in the competition of excess EDTA. The kinetics analysis
data was shown in Fig. 5 inset. Fitting the experimental data with
first-order consecutive kinetic equations, the rate constants at
37 °C were estimated to be k(1) = (5.2 0.2) ꢁ 10ꢂ3 hꢂ1
<
k(2) = (6.7 0.3) ꢁ 10ꢂ3 hꢂ1 < k(3) = (8.0 0.1) ꢁ 10ꢂ3 hꢂ1 < k(4) =
(9.5 0.2) ꢁ 10ꢂ3 hꢂ1, respectively. The smaller rate constant is
due to the d3 electronic configuration of Cr(III) forming stable
and kinetically substitutional inert complex [13].
The above behavior shows a gradation along the series as the
chelate ring sizes in the respective complexes increase [14]. There
are a lot of rings formed around the Cr(III) iron, as shown in
Scheme 2. The stability is influenced by the size of chelate ring.
The five-member ligands have bigger chelate effect than the six-
member ones. But when the chelate ring contains a double bond
(unsaturated), the six-member metal complex is stable [15]. There
Fig. 4. ORTEP view of complex 5. The hydrogen atoms were omitted for clarity. The
ellipsoids enclose 30% of the electronic density.
0.3
2.0
3.0
2.5
2.0
1.5
1.0
.5
ln[a/(a-x)]=0.0052t
ln[a/(a-x)]=0.0067t
1.6
1
2
1.2
.8
0.2
.4
0.2
0.0
0
100 200 300
t/h
0.0
0
100 200 300 400
t/h
0.1
0.0
0.1
0.0
400
3
500
600
700
800
400 500 600 700 800
Wavelength/nm
Wavelength/nm
0.3
0.2
0.1
0.0
3.5
ln[a/(a-x)]=0.0080t
4
3.0
2.5
2.0
1.5
1.0
.5
ln[a/(a-x)]=0.0095t
4
0.3
0.2
0.1
0.0
3
2
1
0
0.0
0
100 200 300 400
t/h
0
100 200 300
t/h
400
500
600
700
800
400
500
600
700
800
Wavelength/nm
Wavelength/nm
Fig. 5. UV–vis spectra at different times for the reaction of 1.0 mM complex (1–4) with 100 equiv. EDTA at 37 °C in 0.01 M Tris–HCl at pH 7.4. Inset: The plot of ln[a/(a ꢂ x)]
vs. time, a = 1.0 mM, x = a(A ꢂ A1)/(A0 ꢂ A1)a.

442
B. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 437–443
undergo aquation thermally with loss of the en ligand in the
excited state. The vacant locations will be substituted by other
molecules such as H2O molecule.
NH
NH
NH
NH
O
Cr
O
Cr
O
O
NH2
The photochemical stability experiments were carried out by
exposing these complexes to the light of xenon lamp (CEL-S500/
350, 50 mW/cm2, AM 1.5) in methanol, 293 K. This reaction was
monitored by UV–Vis spectra. Revealed by Fig. 6, when light was
irradiated on the complex (1–4) in methanol solution respectively,
a considerable decrease and red shift (to 540 nm) in the d–d
absorption spectra were observed. The viole solution gradually
faded to pink. As a control experiment, these complexes are stable
under ultraviolet light such as k = 254 or 365 nm (data not shown).
The first-order rate constants were obtained according to the
absorbance at 520 nm, k0(1) = (4 0.1) ꢁ 10ꢂ4 sꢂ1 < k0(2) = (6
0.3) ꢁ 10ꢂ4 sꢂ1 < k0(3) = (1.1 0.2) ꢁ 10ꢂ3 sꢂ1 < k0(4) = (1.4 0.2) ꢁ
10ꢂ3 sꢂ1, as shown in Fig. 6 inset. This sequence was in accordance
with that in kinetics studies with EDTA. These experimental results
gave a further verification on the relationship between the chelate
ring sizes and structural stability.
H2N
NH2
H2N
1
2
NH
NH
NH
NH
Cr
O
O
O
Cr
O
NH2
H2N
H2N
NH2
4
3
Scheme 2. Molecular structure diagrams of 1–4, the pink ring represents five-
member ring, the blue ring represents six-member ring. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
are the same number of unsaturated six-member rings (black
rings) around center Cr(III) in these four complexes, but different
number of saturated five-member rings (pink ring) and six-mem-
ber rings (blue rings, shown in Scheme 2). The numbers of five-
member rings are 2, 1, 1 and 0 for complex 1–4, respectively. So,
complex 1 should be the stable one and 4 should be the instable
one relatively. This prediction is just confirmed herein by the EDTA
competitive experiments.
Magnetic properties
The temperature dependences of the v
mT and vꢂ1 product for 5
are shown in Fig. 7. At room temperature, the
vmT product is
3.19 cm3 K molꢂ1, slightly less than two isolated paramagnetic
Cr(III) metal ions 3.75 cm3 K molꢂ1 assuming g = 2.0. Upon a lower-
ing of the temperature, the
vmT value decreases persistently to a
minimum value 0.0389 cm3 K molꢂ1 at 1.8 K. Fitting of the experi-
mental data using a simple dinuclear S = 3/2 model in addition to a
Curie law, then reproduce well the magnetic behavior with
J = ꢂ10.8 cmꢂ1 and g = 1.98 [18]. The negative J value highlights
the antiferromagnetic nature of the interaction between dinuclear
Cr(III) metal ions through the methoxide bridge. Fitting of the
Photochemical stability
The photochemistry of Cr(III) complexes has been one of the
most studied aspects of modern inorganic photochemistry [16].
As typically visible light sensitive model complexes, [Cr(en)3]3+
and cis or trans-Cr(en)2X2 complexes (X = NH3, Clꢂ, Brꢂ, Fꢂ, Iꢂ,
SCNꢂ, H2O, N3ꢂ, OHꢂ, C2O42ꢂ, en = ethylenediamine), theirs photo-
chemistry have been fully studied [17]. Under visible light, en will
vmꢂ1–T experimental data among 75–300 K using
v
= C/(T ꢂ h),
reproduce C = 3.98 cm3 molꢂ1 and h = ꢂ72.6 K (Fig. 7 insert). The
negative h value demonstrates the prominent antiferromagnetic
0.8
0.8
.68
.64
.60
.56
.52
A=0.6376-0.1105*
(1-exp(-0.0004*t))
1
.76
2
A=0.7505-0.3090*
(1-exp(-0.0006*t))
.72
.68
.64
.60
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
0
6000
2000 t/4s000
0
2000 4000 6000
t/s
500
600
700
800
500
600
700
800
Wavelength/nm
Wavelength/nm
0.8
0.8
0.6
0.4
0.2
0.0
.48
4
.6
3
A=0.5321-0.1900*
A=0.4580-0.1651*
(1-exp(0.0014*t))
.44
.40
.36
.32
.28
(1-exp(-0.0011*t))
.5
.4
.3
0.6
0.4
0.2
0.0
0
2000 4000 6000
t/s
0
2000 4000 6000
t/s
500
600
700
800
500
600
700
800
Wavelength/nm
Wavelength/nm
Fig. 6. UV–vis spectra at different times for the solution of 1.0 mM complex (1–4) under xenon lamp in methanol. Inset: the changes of absorbance at 520 nm with time and a
plot of absorbance vs. time. Inset: a plot of A520nm vs. time for Cr(III) complexes.

B. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 437–443
443
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Four new mononuclear and one dinuclear octahedral Cr(III)
compounds based on tetrahydrogenated ligand [H4]salen2ꢂ or [H4-
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The hydrogenation of tetradentate Schiff bases increases the ligand
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Acknowledgments
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We are very grateful for financial support from the National
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Research Project Supported by Shanxi Scholarship Council of China
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CCDC 712642, 913849, 1002820 and 1002821 contain the sup-
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