NIR luminescence and catalysis of multifarious salen type ytterbium complexes modulated by anions

Article abstract of DOI:10.1039/c3dt51556g

A series of five N,N′-bis(2-hydroxy-3-methoxybenzylidene)-1,3- propanediamine (H₂L) ytterbium complexes, namely, [Yb(H ₂L)₂(CH₃OH)](ClO₄)₃ (1), [Yb(H₂L)(NO₃)₃

Full text of DOI:10.1039/c3dt51556g

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NIR luminescence and catalysis of multifarious salen
type ytterbium complexes modulated by anions†
Cite this: Dalton Trans., 2013, 42, 13190
Xiaoyan Zou, Pengfei Yan,* Juwen Zhang, Fengming Zhang, Guangfeng Hou and
Guangming Li*
A series of five N,N’-bis(2-hydroxy-3-methoxybenzylidene)-1,3-propanediamine (H2L) ytterbium com-
plexes, namely, [Yb(H2L)2(CH3OH)](ClO4)3 (1), [Yb(H2L)(NO3)3]2·CH2Cl2 (2), [Yb(H2L)2(NO3)]2(PF6)4·
4H2O·2CH2Cl2 (3), [Yb(H2L)(OAc)2]2(PF6)2·4CH2Cl2 (4) and [Yb3L’3(OH)2Cl(H2O)5]Cl3·4H2O (5) (HL’ =
2-hydroxy-3-methoxybenzaldehyde), have been synthesized by reactions of H2L with multifarious Yb(III)
salts. X-ray diffraction analysis reveals that complex 1 shows a unique mononuclear structure constructed
from two chelating H2L ligands in crossover mode. Complex 2 exhibits a dinuclear structure bridged by
two H2L ligands. Complexes 3 and 4 possess two novel dinuclear structures linked by NO3 and OAc−
−
anions, respectively. Complex 5 displays a trinuclear structure supported by three L’ and two OH− anions.
−
Noticeable, complex 3 can be transformed from 2 by introducing (NH4)(PF6). The PF6 counterion plays
Received 12th June 2013,
Accepted 21st June 2013
an essential role in steering the structural transformation. The anions dominate the final structures of
1–5. All complexes 1–5 exhibit NIR luminescence, which can be rationalized on the basis of different
structural effects. Preliminary catalytic studies reveal that all complexes 1–5 are able to catalyze effectively
a typical Henry reaction with good yields.
DOI: 10.1039/c3dt51556g
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which shows NIR luminescence of the Yb(III) ion.8 In 2008,
Jones et al. presented a salen type pentanuclear tetradecker
Introduction
Lanthanide complexes constructed from multidentate ligands Nd5 complex which exhibits NIR luminescence of the Nd(III)
are of considerable interest because of their unusual lumine- ion.9 In 2012, Weixu Feng et al. presented a series of tetra-
scent,1 magnetic2 and catalytic properties.3 One of the well- nuclear lanthanide complexes [Ln4(L)2(HL)2(NO3)2(OH)2](NO3)2
known types of multidentate ligand, salen, is able to stabilize (Ln = Nd, Yb and Er) (H2L = N,N′-bis(salicylidene)cyclohexane-
different metals in various coordination environments.4 Exten- 1,2-diamine) which display characteristic NIR luminescence
sive studies have focused on employing a variety of salen type of the Nd(III) and Yb(III) ions, respectively.10 Nevertheless, the
ligands to stabilize lanthanide ions and provide the antenna structures of salen type lanthanide complexes are often influ-
roles for lanthanide luminescence.5 Previously, much attention enced by a variety of factors such as the radii of lanthanide
has been focused on the structure and UV-visible lumine- ions,11 the structure of the ligand12 and the anions.13 The
scence of salen type Sm(III), Eu(III), Tb(III) and Dy(III) complexes design and synthesis of novel salen type lanthanide complexes
on account of their potential applications in photochemical with unique luminescence still remains a challenge.
reactions and photoswitches.6 In contrast, NIR luminescence
On the other hand, a wide variety of lanthanide complexes
of salen type lanthanide complexes containing Nd(III), Er(III) have been used as efficient catalysts for organic reactions.14
and Yb(III) ions, is rarely reported, especially for salen type However, salen type lanthanide complexes as catalysts for
homo-multinuclear lanthanide complexes.7 In 2006, Jones organic reactions have been rarely known15 although salen
et al. reported the first salen type (N,N′-bis(5-bromo-3-methoxy- type transition metal complexes as catalysts for organic reac-
salicylidene)phenylene-1,2-diamine) sandwich Yb3 complex tions have been reported.16
In view of the recent important progress on the structure,
NIR luminescence, magnetism and catalysis of salen type
lanthanide complexes17 as well as our long-standing research
Key Laboratory of Functional Inorganic Material Chemistry (MOE), School of
on this domain,18 we selected the flexible salen type ligand
Chemistry and Materials Science, Heilongjiang University, Harbin 150080,
P. R. China. E-mail: gmli_2000@163.com; Fax: +86-451-86673647;
N,N′-bis(2-hydroxy-3-methoxybenzylidene)-1,3-propanediamine
(H2L) and different Yb(III) salts with potential NIR lumine-
scence to construct novel lanthanide complexes to explore the
Tel: +86-451-86608458
†Electronic supplementary information (ESI) available: Additional structural
information. CCDC 872855–872859. For ESI and crystallographic data in CIF or
effect of the anions and the flexible salen type ligand on the
other electronic format see DOI: 10.1039/c3dt51556g
13190 | Dalton Trans., 2013, 42, 13190–13199
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structures of lanthanide complexes as well as their lumine- RAPID imaging plate diffractometer with graphite-monochro-
scence and catalytic performance. As a result, a series of five mated Mo Kα (λ = 0.71073 Å) at 293 K.20 The structure was
new salen type complexes have been isolated and structurally solved by direct methods and all non-hydrogen atoms are ani-
characterized. The NIR luminescence and catalysis of com- sotropically refined by full matrix least-squares on F2 using the
plexes 1–5 on a typical Henry reaction have been investigated.
SHELXTL program.21 The experimental details for the struc-
tural determination are presented in Table 1.
Experimental section
Materials and measurements
Synthesis of complexes 1–5
Complexes 1–5 were prepared by similar procedures
(Scheme S1†). Crystals of 1, 4, 5 were grown by solvent
diffusion and crystals of 2 by solution diffusion. Complex 3
was isolated from the reaction of 2 with (NH4)(PF6). In a typical
synthesis of 1, methanol solution (5 mL) of Yb(ClO4)3·6H2O
(0.058 g, 0.1 mmol) was added to a dichloromethane solution
(20 mL) of H2L (0.068 g, 0.2 mmol). The mixed solution was
allowed to stir for 10 h at ambient temperature. The solution
was filtered and hexane was diffused slowly into the filtrate.
The resulting solution was stored in the dark at ambient temp-
erature. Yellow crystals were obtained in about two weeks.
[Yb(H2L)2(CH3OH)](ClO4)3 (1). Yield: 0.061 g (51%). Anal. (%)
Calcd for C39H48Cl3N4O21Yb (1188.20): C, 39.42; H, 4.07;
N, 4.72; Found: C, 39.30; H, 4.12; N, 4.70. IR (KBr pellet,
cm−1): 3420 (w), 2928 (w), 1650 (vs), 1455 (s), 1223 (s),
1082 (m), 956 (s), 725 (m), 612 (s). UV-vis [MeOH, λ]: 335, 264,
227 nm.
All chemicals except H2L and lanthanide salts were commer-
cially available and used without further purification. H2L was
prepared according to the literature.19 YbCl3·6H2O, Yb(NO3)3·
6H2O, Yb(ClO4)3·6H2O and Yb(OAc)3·6H2O were prepared by
the reactions of Yb2O3 with hydrochloric acid, nitric acid, per-
chloric acid and acetic acid in aqueous solution, respectively.
Elemental (C, H and N) analyses were performed on a Perkin-
Elmer 2400 analyzer. FT-IR data were collected on a Perkin-
Elmer 100 spectrophotometer by using a KBr pellet in the
range of 4000–400 cm−1. UV spectra (in methanol) were
recorded on a Perkin-Elmer Lambda 35 spectrometer. Thermal
analyses were conducted on a Perkin-Elmer STA 6000 with a
heating rate of 10 °C min−1 in a temperature range from 30 °C
to 800 °C under atmosphere. Powder X-ray diffraction (PXRD)
data were recorded on a Rigaku D/Max-3B X-ray diffractometer
with CuKα as the radiation source (λ = 0.15406 nm) in the
angular range θ = 5–50° at room temperature. NIR lumine-
scence of complexes 1–5 were conducted with a FLSP920 Com-
bined Steady State Fluorescence and Phosphorescence
Lifetime Spectrometer.
[Yb(H2L)(NO3)3]2·CH2Cl2 (2). Yield: 0.098 g (66%). Anal. (%)
Calcd for C39H46Cl2N10O26Yb2 (1487.82): C, 31.48; H, 3.12; N,
9.41; Found: C, 32.62; H, 4.18; N, 8.63. IR (KBr pellet, cm−1):
3418 (w), 3104 (w), 1654 (vs), 1453 (s), 1285 (s), 1228 (s), 1099 (m),
1034 (m), 813 (m). UV-vis [MeOH, λ]: 340, 265, 222 nm.
[Yb(H2L)2(NO3)]2(PF6)4·4H2O·2CH2Cl2 (3). Yield: 0.060 (45%).
X-ray crystallography
Suitable single crystals of 1–5 were selected for X-ray diffrac-
tion analysis. All data were collected on a Rigaku R-AXIS Anal. (%) Calcd for C78H100Cl4F24N10O26P4Yb2 (2661.43):
Table 1 Crystal data and structure refinement for complexes 1–5
Complexes
1
2
3
4
5
Formula
Formula weight
Crystal system
Space group
C
39H48Cl3N4O21Yb C39H46Cl2N10O26Yb2 C78H100Cl4F24N10O26P4Yb2 C50H64Cl8F12N4O16P2Yb2 C24H41Cl4O20Yb3
1188.20
Triclinic
ˉ
P1
1487.82
Monoclinic
P21/c
2661.43
Triclinic
P1
1896.67
Monoclinic
P21/c
1310.49
Monoclinic
ˉ
P1
ˉ
a/Å
b/Å
c/Å
13.373(5)
14.457(5)
14.548(6)
61.970(12)
71.040(14)
82.000(12)
2347.7(14)
2
12.111(4)
14.824(5)
15.133(6)
90
98.985(2)
90
2683.5(16)
2
1.841
12.472(4)
17.080(4)
28.223(7)
101.100(8)
91.450(11)
109.730(9)
5527(2)
11.757(9)
26.210 (16)
15.000(8)
90
127.101(4)
90
3686.7(4)
2
1.709
33.078(2)
17.937(12)
17.314(12)
90
115.197(3)
90
9295.2(12)
8
1.873
α (°)
β (°)
γ (°)
V/Å3
Z
2
D (g−1 cm−3
)
1.681
2.247
1.599
1.946
μ/mm−1
3.657
2.944
6.280
F(000)
θ range (°)
Reflection collected/
unique
1198
3.05–27.48
22 844
1464
3.05–27.47
24 630
2658
3.03–25.00
43 218
1868
1.55–25.00
20 192
4984
1.64–26.03
29 325
10 544
6108
19 231
6225
9128
R(int)
R [I > 2σ(I)]
0.0499
0.0448
0.0499
0.0444
0.0300
R1 = 0.0600
wR2 = 0.1532
R1 = 0.0819
wR2 = 0.1670
1.036
R1 = 0.0404
wR2 = 0.0853
R1 = 0.0598
wR2 = 0.0924
1.077
R1 = 0.0836
wR2 = 0.2433
R1 = 0.1221
wR2 = 0.2809
1.031
R1 = 0.1535
wR2 = 0.4344
R1 = 0.1700
wR2 = 0.4419
1.153
R1 = 0.0394
wR2 = 0.1095
R1 = 0.0506
wR2 = 0.1161
1.075
R (all data)
GOF
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C, 35.20; H, 3.79; N, 5.47; Found: C, 35.20; H, 3.79; N, 5.26. IR
(KBr pellet, cm−1): 3420 (w), 3055 (w), 1650 (vs), 1503 (s),
1287 (s), 1215 (s), 843 (m), 731 (m), 555 (m). UV-vis [MeOH, λ]:
370, 265, 222 nm.
[Yb(H2L)(OAc)2]2(PF6)2·4CH2Cl2 (4). Yield: 0.072 g (76%).
Anal. (%) Calcd for C50H64Cl8F12N4O16P2Yb2 (1896.67): C,
31.66; H, 3.40; N, 2.93; Found: C, 34.12; H, 3.30; N, 2.97. IR
(KBr pellet, cm−1): 3420 (w), 3084 (w), 1644 (vs), 1482 (s),
1285 (s), 1202 (s), 1026 (m), 843 (m), 752 (m). UV-vis [MeOH,
λ]: 332, 261, 226 nm.
[Yb3L′3(OH)2Cl(H2O)5]Cl3·4H2O (5). Yield: 0.029 g (66%).
Anal. (%) Calcd for C24H41Cl4O20Yb3 (1310.49): C, 22.00; H,
3.15; Found: C, 22.2; H, 3.30. IR (KBr pellet, cm−1): 3392 (w),
2963 (w), 1650 (vs), 1377 (s), 1230 (s), 1033 (m), 850 (s), 563 (s).
UV-vis [MeOH, λ]: 368, 271, 226 nm.
Fig. 1 Cationic structure of 1. (All hydrogen atoms are omitted for clarity).
Selected bond distances (Å) and angles (°): Yb(1)–O(1) 2.371(4), Yb(1)–O(2)
2.588(4), Yb(1)–O(3) 2.418(4), Yb(1)–O(4) 2.633(5), Yb(1)–O(5) 2.331(4), Yb(1)–
O(6) 2.607(4), Yb(1)–O(7) 2.409(4), Yb(1)–O(8) 2.553(5), Yb(1)–O(9) 2.445(5);
O(1)–Yb(1)–O(2) 63.18(14), O(3)–Yb(1)–O(4) 62.60(16), O(5)–Yb(1)–O(6)
63.82(15), O(7)–Yb(1)–O(8) 63.18(15).
Results and discussion
Spectral analysis
Infrared spectra of complexes 1–5 are similar (Fig. S1†).
The strong v(CvN) bands occurring in the range of
1648–1654 cm−1 for these complexes shift to higher wavenum-
bers in comparison with that for free H2L (1636 cm−1), which
are attributed to the coordination of the deprotonated groups
reducing the strength of CvN groups. The two additional
strong characteristic absorptions at 1452–1476 cm−1 and
1283–1291 cm−1 for complexes 2 and 3 are tentatively attri-
buted to the v(NO3−). In addition, the absorption bands
at 843 cm−1 for complexes 3 and 4 are attributed to the
PF6− stretching vibrations.22
Structural description of [Yb(H2L)2(CH3OH)](ClO4)3 (1)
Fig. 2 Packing structure of 1 formed by the π–π interactions between the aryl
rings.
X-ray crystallographic analysis reveals that complex 1 crystal-
ˉ
lizes in a triclinic space group P1, possessing a discrete ionic
mononuclear structure. The cation is the [Yb(H2L)2(CH3OH)]3+
moiety and the anions are three ClO4−. In a typical cationic
unit of 1 (Fig. 1), the Yb(III) ion is surrounded by nine oxygen
atoms from two ligands and one methanol, forming a tri-
capped trigonal prism (Fig. S2†). The Yb–O bond lengths
depend on the nature of the oxygen atoms, varying from 2.331(4)
to 2.633(5) Å. The Yb–O bond lengths (2.331(4)–2.419(4) Å)
from phenoxo are distinctively shorter than those from the
methoxy groups (2.553(5)–2.633(5) Å) which is in accordance
with the report.23 Notably, the mononuclear complex 1 is a
unique ionic salen type lanthanide complex in which the two
ethylenediamine]26 although the ratio is H2L : Ln = 2 : 1. The
complex is neutral and the Ce(VI) ion is in the +4 oxidation
state in which the Ce(VI) ion is coordinated by N atoms from
the ligand. Further, two adjacent mononuclear molecules of 1
are connected by the π–π interactions between the aryl rings
with a center-to-center distance of 3.6945(12) Å (Fig. 2).
Structural description of [Yb(H2L)(NO3)3]2·CH2Cl2 (2)
ligands are crossover coordinating to the Yb(III) ion with the X-ray crystallographic analysis reveals that complex 2 crystal-
ratio of H2L : Ln = 2 : 1. It differs from the reported mono- lizes in a monoclinic space group of P21/c, possessing a
nuclear [(Nd(H2L1)(NO3)3] [H2L1 = N,N′-bis(3-methoxysalicyli- neutral dimer structure (Fig. 3). The two crystallographically
dene)propane-1,2-diamine)]24 and [(Nd(H2L2)(NO3)3] [H2L2
=
equivalent Yb(III) ions are bridged by a pair of chelating oxygen
N,N′-ethylene-bis(3-methoxysalicylideneimine)]25 in which atoms from phenoxo and methoxy groups of ligands with
both complexes are neutral and the Nd(III) ions are shielded the Yb1–Yb1A separation of 9.986 Å. The Yb(III) ions are
in the outer O2O2 site of the single hexadentate salen 10-coordinated with six oxygen atoms from three nitrate
type ligand forming a crab-like structure with the ratio of groups and four oxygen atoms from two ligands forming a dis-
H2L : Yb = 1 : 1. It’s also different from the reported analogous torted hexadecahedron coordination polyhedron (Fig. S2†).
complex Ce(VI)(L3)2 [H2L3 = N,N′-bis(3,5-di-tert-butylsalicylidene)- The Yb–O bond lengths are in the range of 2.193(4)–2.853(4) Å.
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Fig. 3 Molecular structure of 2. (All hydrogen atoms are omitted for clarity).
Selected bond distances (Å) and angles (°): Yb(1)–O(1) 2.196(3), Yb(1)–O(2)
2.853(4), Yb(1)–O(3A) 2.193(4), Yb(1)–O(4A) 2.591(4), Yb(1)–O(13) 2.799(5),
Yb(1)–O(11) 2.397(5), Yb(1)–O(10) 2.397(4), Yb(1)–O(8) 2.397(4), Yb(1)–O(7)
2.462(4), Yb(1)–O(5) 2.372(4); O(1)–Yb(1)–O(2) 60.42(11), O(5)–Yb(1)–O(3A)
101.25(14).
Fig. 5 Cationic structure of 3. (All hydrogen atoms are omitted for clarity).
Selected bond distances (Å) and angles (°): Yb(1)–O(3) 2.417(6), Yb(1)–O(7)
2.424(6), Yb(1)–O(5) 2.461(7), Yb(1)–O(1) 2.511(7), Yb(1)–O(20) 2.663(7),
Yb(1)–O(17) 2.705(7), Yb(1)–O(8) 2.809(6), Yb(1)–O(6) 2.827(7), Yb(1)–O(4)
2.861(8), Yb(1)–O(2) 2.884(8); O(20)–Yb(1)–O(17) 58.1(2), O(17)–Yb(2)–O(20)
57.4(2), Yb(1)–O(17)–Yb(2) 120.7(2), Yb(1)–O(20)–Yb(2) 123.1(3).
Similarly, the Yb–O bond lengths from the phenoxo groups
(2.196(3), 2.193(4) Å) are distinctively shorter than those from
the methoxy groups (2.853(4), 2.591(4) Å), which are com-
parable to those for complex 1. The unique dimer structure is
mainly attributed to the incorporation of the flexible ligand of
N,N′-bis(2-hydroxy-3-methoxybenzylidene)-1,3-propanediamine.
Such a salen type dinuclear structure is unique among the
known salen type lanthanide complexes. It is distinctively
different from the reported tetra-dentate salen type dinuclear
ytterbium complex of [Yb2(L4)3(CH3OH)] (H2L4 = N,N′-bis(sali-
cylidene)-1,3-phenylene-ediamine)27 with a sandwich structure
in which the Yb1⋯Yb2 distance is 3.79 Å. It also differs from
the salen type dinuclear complex [Yb2L5(acac)2(H2O)]·2CH2Cl2
(H2L5 = N,N′-bis(salicylidene)-1,2-cyclohexanediamine, acac =
acetylacetone)18b in which the two Yb(III) ions are bridged by
two phenol oxygen atoms with a distance of 3.79 Å. Further,
the adjacent dimer molecules are extended into a two-dimen-
sional (2D) supramolecular network (Fig. 4) by the hydrogen
bonding interactions (N2–H2⋯O6 3.0696 Å).
Structural description of [Yb(H2L)2(NO3)]2(PF6)4·4H2O·
2CH2Cl2 (3)
X-ray crystallographic analysis reveals that 3 crystallizes in a tri-
ˉ
clinic space group P1, possessing a discrete ionic dinuclear
4+
structure (Fig. 5). The cation is the [Yb(H2L)2(NO3)]2 moiety
and the anions are four PF6−. In a typical cationic unit of 3,
there are two crystallographically unequivalent Yb(III) ions,
four ligands, two NO3− anions with a ratio of 1 : 2 : 1. The Yb1(III)
ion is ten-coordinated to eight O atoms from two ligands
−
and two O atoms (O20 and O17) from two bridging NO3 ions
forming a di-capped square antiprism coordination poly-
hedron (Fig. S2†) in which the two ligands are crossover coordi-
nating to the Yb1(III) ion. While the Yb2(III) ion is nine-
coordinated to seven O atoms from the two ligands and two
−
O atoms (O20 and O17) from two bridging NO3 ions forming
a mono-capped square antiprism geometry (Fig. S2†) in which
one O atom from the methoxy does not participate in the
coordination in contrast to the Yb1(III) ion. The four-mem-
bered core of Yb1O17O20Yb2 is almost coplanar forming a
rhombus with a dihedral angle of 5.70° between the planes of
Yb1O17O20 and Yb2O17O20. The angles of ∠O20Yb1O17 and
∠Yb1O17Yb2 are 58.1(2) and 120.7(2), respectively. The Yb–O
bond lengths vary from 2.417(4) to 2.884 (8) Å in which the
bond lengths (2.417(4)–2.663(7) Å) from phenoxo are shorter
than those from the methoxy groups (2.705(7)–2.884(8) Å),
which are similar to those for 1 and 2. The two Yb(III) ions are
−
bridged by two NO3 ions with a distance of 4.721 Å, which is
in between those for complex 2 and the reported complexes
[Yb2(L4)3(CH3OH)]27 and [Yb2L5(acac)2(H2O)]·2CH2Cl2.18b Notice-
ably, the p–π conjugations between the aryl ring and CvN bond
are observed in complex 3.
Structural description of [Yb(H2L)(OAc)2]2(PF6)2·4CH2Cl2 (4)
X-ray crystallographic analysis reveals that complex 4 crystal-
lizes in a monoclinic space group P21/c, possessing a discrete
Fig. 4 2D supramolecular network of 2 formed by the intermolecular hydro-
gen bonds.
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Fig. 6 Cationic structure of 4. (All hydrogen atoms are omitted for clarity).
Selected bond distances (Å) and angles (°): Yb(1)–O(1) 2.343(5), Yb(1)–O(3)
2.505(6), Yb(1)–O(4) 2.486(6), Yb(1)–O(5) 2.541(6), Yb(1)–O(6A) 2.533(6),
Yb(1)–O(7) 2.454(5), Yb(1)–O(7A) 2.367(5), Yb(1)–O(8) 2.389(6); O(7)–Yb(1)–
O(7A) 71.2(2), O(5)–Yb(1)–O(6A) 68.9(18), Yb(1)–O(7)–Yb(1A) 108.8(2), O(7)–
Yb(1)–O(8) 54.16(17).
Fig. 8 Cationic structure of 5. (All hydrogen atoms are omitted for clarity).
Selected bond distances (Å) and angles (°): Yb(1)–O(8) 2.298(5), Yb(1)–O(7)
2.35(5), Yb(2)–O(7) 2.297(5), Yb(2)–O(8) 2.344(5), Yb(3)–O(8) 2.314(5), Yb(3)–
O(7) 2.351(5), Yb(1)–Yb(2) 3.4484(5), Yb(1)–Yb(3) 3.4803(5), Yb(2)–Yb(3)
3.4597(5), Yb(1)–O(4)–Yb(2) 98.30(19), Yb(1)–O(2)–Yb(3) 99.85(19), Yb(2)–
O(6)–Yb(3) 99.0(2).
Structural description of [Yb3L′3(OH)2Cl(H2O)5]Cl3·4H2O (5)
X-ray crystallographic analysis reveals that 5 crystallizes in a
ˉ
monoclinic space group of P1, possessing a discrete ionic tri-
nuclear structure (Fig. 8). The cation is the [Yb3L′3(OH)2Cl-
(H2O)5]3+ moiety and the anions are three Cl−. In a typical
cationic unit of 5, there are three Yb(III) ions, three ligands,
two hydroxy anions, one chloride anion and five water mole-
cules. The Yb1(III) ion and Yb2(III) ion are both eight-co-
ordinated to four O atoms from the phenoxo and methoxy of
two ligands, two O atoms from hydroxo ions and two O atoms
from two water, forming a bicapped trigonal prism coordi-
nation polyhedron (Fig. S2†). The Yb3(III) ions are eight-
coordinated to four O atoms from the phenoxo and methoxy
of two ligands, two O atoms from hydroxo ions, one O atom
from a water molecule and one Cl− anion, forming a bicapped
Fig. 7 2D supramolecular network of 4 constructed from the π–π interactions.
2+
ionic dinuclear structure. The cation is the [Yb(H2L)(OAc)2]2
moiety and the anions are the two PF6−. In a typical cationic trigonal prism coordination polyhedron (Fig. S2†). In the
unit of 4 (Fig. 6), there are two crystallographically equivalent Yb3O5 cores, the three Yb(III) ions are held together by the mul-
Yb(III) ions, two ligands, four OAc− ions with a ratio of 1 : 1 : 2. tiple bridging O atoms, e.g. O(2), O(4), O(6), O(7) and O(8). The
The Yb(III) ion is eight-coordinated to three oxygen atoms from three Yb⋯Yb distances are different at 3.448 Å for Yb1⋯Yb2,
the ligand (two from phenoxo and one from methoxy) and five 3.480 Å for Yb1⋯Yb3 and 3.460 Å for Yb2⋯Yb3, respectively.
oxygen atoms from two bidentate and two tridentate acetate These Yb⋯Yb distances are the shortest among those reported
groups, forming a trigondodecahedron geometry (Fig. S2†). Yb⋯Yb distances for salen type ytterbium complexes.8–10,18b,27
Notably, one O atom from methoxy does not participate in the Notably, complex 5 is analogous to previously reported gado-
coordination. The Yb–O bond lengths vary from 2.367(5) to linium28 and dysprosium triangles,29 and the ligand (HL′ =
2.541(6) Å in which the bond length of Yb(1)–O(3) from one 2-hydroxy-3-methoxybenzaldehyde) is supposed to result from
phenoxo (2.505 Å) is unexpectedly longer than that of Yb(1)– the decomposition of H2L. We suggest that the Cl− anions lead
O(4) methoxy (2.486 Å), which are different from those of 1, 2 to the decomposition of H2L on the basis of the reactions in
−
and 3. Noticeably, the two Yb(III) ions are multi-bridged by two comparison with other anions, e.g. ClO4−, NO3 and OAc−. It
types of OAc− anions, e.g. bidentate and two tridentate with has been confirmed by the experiments when other salen type
the Yb1⋯Yb1A separation of 3.921 Å, which is shorter than ligands, such as (H2L6 = N,N′-bis(2-oxy-3-methoxybenzylidene)-
that of 2 and 3 resulting from the multi-bridging bonds 1,2-phenylenediamine) and (H2L7
= N,N′-bis(2-hydroxy-3-
between the two Yb(III) ions. Further, the adjacent binuclear methoxybenzylidene)-1,2-diaminocyclohexane), react with LnCl3·
molecules are linked by three kinds of π–π interactions 6H2O in which the ligands also decomposed to HL′.30 Notice-
(C1⋯N1 3.446 Å, C14⋯C15 3.636 Å and C2⋯C8 3.425 Å) ably, the trinuclear units are connected by the hydrogen bonding
forming a 2D supramolecular network (Fig. 7).
interactions forming a one-dimensional supramolecular chain
13194 | Dalton Trans., 2013, 42, 13190–13199
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distinct structures. Hence, it would be interesting and essen-
tial to discuss the effects of the anions on the synthesis and
structure for complexes 1–5 (Table 2). It is obvious that the
coordination ability of the anions plays an important role on
the structures of the resultant complexes. For example, the
−
ClO4 anion is not involved in the coordination to lanthanide
ions, and it plays a charge-balancing role. Therefore, it
provides an opportunity for the ligand to meet the high co-
ordination requirement for the lanthanide ions when
Yb(ClO4)3·6H2O is employed, which leads to the ionic mono-
nuclear structure of 1 with H2L : Yb = 2 : 1. In contrast, the
−
NO3 anion is of strong coordination ability so that it is hard
to be displaced by other anions, especially for a neutral ligand.
Thus, when the ligand reacts with Yb(NO3)3·6H2O, the ligand
replaces only the neutral water molecules in Yb(NO3)3·6H2O
forming the neutral dinuclear complex 2 containing three
NO3− anion for each Yb(III) ion. However, when excessive (NH4)-
Fig. 9 2D supramolecular network of
5 constructed from the hydrogen
bonding interactions and the π–π interactions.
−
(PF6) was added to complex 2, the NO3 anion was partially
substituted by the ligand resulting in a structural conversion
forming the ionic dinuclear complex 3 under the inducement
of the deionized agent of (NH4)(PF6). Once the dimer 3 is
formed, where the two metal centers are held in close proxi-
−
mity to one another, the bridging NO3 will be quite difficult
to be replaced by other anions because of the steric and charge
effects.31 Since the coordination ability of the OAc− anion is in
−
between ClO4 and NO3−, the ionic dinuclear complex 4 in-
volving the multi-bridging OAc− bonds is isolated when the
ligand reacts with Yb(OAc)3·6H2O and excessive (NH4)(PF6).
However, when the Cl− anion was employed, the salen type
ligand was decomposed by the strong acidity of YbCl3·6H2O so
that the ionic trinuclear complex 5 is unexpectedly harvested.
−
It should be noted that the PF6 counterion plays uncanny
roles in constructing novel salen type lanthanide complexes
with diverse structures.22
Fig. 10 3D supramolecular structure of 5 constructed from hydrogen bonding
PXRD and TG-DSC analysis
interactions.
All powder X-ray diffraction (PXRD) patterns of complexes 1–5
are in agreement with the simulated ones from the respective
crystal data (Fig. S3–S7†). This suggests that the crystal struc-
tures are truly representative of the bulk materials. The differ-
ences in intensity are due to the preferred orientation of the
powder samples.
The thermal stabilities of complexes 1–5 were examined by
TG-DSC analysis (Fig. S8–S12†). Complex 1 exhibits a gradual
weight loss of 2.93% in the temperature range of 33–180 °C,
(Fig. 9), which are further linked through the hydrogen bonding
(Table S1†) and π–π interactions (3.711 Å) to generate a 2D supra-
molecular network (Fig. 9). The adjacent 2D networks are
packed into a three-dimensional supramolecular architecture by
the hydrogen bonding interactions (Fig. 10, Table S1†).
Structural diversity and the effect of anions
On the basis of the above descriptions, it is suggested that the which corresponds to the loss of the coordinated methanol
diversiform anions result in a series of five complexes 1–5 with molecules (calculated 2.69%). The weight loss in the range of
Table 2 A characteristic parameter comparison for complexes 1–5
Complexes
Anions
Nuclearity
Molecular type
Distances of Yb⋯Yb
Mole ratio of Yb : H2L
−
1
2
3
4
5
ClO4
NO3
Mononuclear
Dinuclear
Dinuclear
Dinuclear
Trinuclear
Ionic
Neutral
Ionic
Ionic
Ionic
—
9.986 Å
4.721 Å
3.921 Å
1 : 2
1 : 1
1 : 2
1 : 1
1 : 1
−
NO3−, PF6−
OAc−, PF6
Cl−
−
3.480 Å, 3.448 Å, 3.460 Å
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30–267 °C for 2 can be attributed to the loss of four crystalline
dichloromethane (observed 5.74%, calculated 5.71%).
Complex 3 began to lose four crystalline water and two dichloro-
methane molecules in the range of 33–118 °C (observed
9.92%, calculated 9.09%). The weight loss in the range of
33–151 °C for 4 can be attributed to the loss of crystalline
dichloromethane (observed 14.90%, calculated 17.78%).
The loss of four crystalline water molecules (observed 5.49%,
calculated 5.43%) was observed in the TGA curve of 5 in the
range of 33–118 °C.
NIR luminescence
The UV-vis spectra of complexes 1–5 in CH3CN (Fig. 11) reveal
similar ligand-centered bands around 223–227, 262–271, and
330–374 nm which are red-shifted in contrast to that for the
free ligand at 219, 261 and 298 nm. Notably, the absorption
bands of complex 3 are distinguishably red-shifted about 9 nm
more than those of the other four complexes, which is attri-
buted to the p–π conjugations between the aryl ring and CvN
bond which lower the singlet state (1ππ*) energy level of the
ligand leading to the red-shift. In general, the emission
spectra of complex [Gd(H2L)(OAc)2]2(PF6)2·6H2O at 77 K was
used to estimate the energy level of H2L. The single state (1ππ*)
energy level of H2L was estimated by referencing its absor-
bance edge, which was 25 000 cm−1 (400 nm). While, the emis-
sion spectra of complex Gd(H2L)2(NO3)]2(PF6)4·4H2O·2CH2Cl2
at 77 K shows that the single state (1ππ*) energy level of H2L
was 22 989 cm−1 (435 nm) (Fig. S13†). Obviously, the singlet
state (1ππ*) energy level of the ligand is reduced because of the
p–π conjugations between the aryl ring and CvN bond in
complex 3.
Fig. 12 The NIR spectra of 1–5 in CH3CN in isoabsorptive solutions.
ions and forbid the attachment of the solvent molecules and
−
NO3 anions which may partially quench the lanthanide
luminescence under the inducement of the deionized agent of
PF6−. And it is in accordance with p–π conjugation lowering
the triplet energy level of the ligand in complex 3, which then
better matches the excited state level of NIR-emissive Yb(III)
ions (Fig. S13†). The triplet (T1) energy levels were calculated
by referring to the lower wavelength emission peaks of the
corresponding phosphorescence spectra of complex [Gd(H2L)-
(OAc)2]2(PF6)2·6H2O, which was 21 141 cm−1 (473 nm). The
emission spectra of complex Gd(H2L)2(NO3)]2(PF6)4·4H2O·
2CH2Cl2 at 77 K show that the triplet (T1) energy levels were
20 661 cm−1 (484 nm). Therefore, the triplet energy level of the
ligand is reduced, which better matches the excited state level
of NIR-emissive Yb(III) ions. Complex 4 shows the second
strongest NIR-emission intensity among complexes 1–5, which
is attributed to that the two salen type ligands coordinate to
The NIR luminescent spectra (Fig. 12) reveal that all com-
plexes 1–5 exhibit a NIR emission around 975 nm, which can
2
be assigned to the F5/2
→
2F7/2 transition of the Yb(III) ion.
two NIR-emissive Yb(III) ions forming a rigid dinuclear struc-
ture under the inducement of the deionized agent of PF6
The relative intensity of the NIR emission follows the order of
3 > 4 > 1 > 2 > 5 in isoabsorptive solutions. The NIR lumine-
scence intensity of the Yb(III) ion for 3 is the strongest among
the complexes 1–5. This observation supports that the four
ligands in complex 3 provide an improved shield on Yb(III)
−
,
which may prompt the NIR luminescence. In general, the NIR-
emission for the dimers is more intense than those for the
monomers with the same number of ligands as there are two
emissive metal centers in the dimmers instead of only one in
the monomers.32 Although complex 1 is of a mononuclear
structure, the NIR-emissive Yb(III) ion is completely encapsu-
lated by two salen type ligands providing improved shielding
of Yb(III) ions. Therefore, the luminescent intensity of mono-
nuclear complex 1 is stronger than that of dinuclear complex 2
with six nitrate groups which enhance the quenching of the
luminescence of Yb(III) ions. Nevertheless, although complex 5
has three emissive centers of Yb(III) ions in the trinuclear
entity, the nine coordinated solvent H2O molecules and four
coordinated Cl− anions will quench the NIR luminescence of
the Yb(III) ions which lack encapsulation.10 Thus, complex 5
exhibits the weakest NIR luminescent intensity among com-
plexes 1–5.
In addition to the steady-state emission, we also perform
time-resolved measurements of complexes 1–5 in the NIR
region by using the time-correlated single photon counting
Fig. 11 The UV-vis spectra of 1–5 in CH3CN at the concentration of 1.0 × 10−5 M.
13196 | Dalton Trans., 2013, 42, 13190–13199
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Table 3 The NIR luminescent life times and the intrinsic luminescence
quantum yields of Yb(III) ions for complexes 1–5
Table 5 Nitroaldol reactions catalyzed by complexes 1–5a
Complexes
τ (μs)
ΦLn (%)
3
4
1
5
18.31
15.73
13.87
9.08
5.10
13.22
0.916
0.787
0.693
0.454
0.255
0.661
Entry
Catalyst (10 mmol%)
Time (h)
Yieldb (%)
2
10
1
2
3
4
5
3
1
4
2
5
30
18
13
13
13
70
74
81
89
96
[Ln4(L)2(HL)2(NO3)2(OH)2](NO3)2
Table 4 Nitroaldol reactions catalyzed by complex 4 under different solventsa
a Reagents and conditions: 4-nitrobenzaldehyde (0.2 mmol),
nitromethane (2.0 mmol), water (3.0 mL), complex catalysts (10 mol
%), room temperature. b Isolated yield after chromatographic column
purification.
Entry
Solvents
Time (h)
Yieldb (%)
catalysts to catalyze the Henry reaction in water. The results
suggest that this series of complexes are able to catalyze the
Henry reaction with the active order as 5 > 2 > 4 > 1 > 3
(Table 5). This supports that the three active centers of Yb(III)
ions and nine H2O molecules in complex 5 effectively accele-
rate the reaction. While, complex 3 exhibits the weakest cata-
lytic activity, since the four ligands effective shield the two Yb(III)
ions which obstruct the active centers. Thus it can be con-
cluded that the more completely Yb(III) ions are encapsulated
by the salen type moiety, the lower the catalyst activity. Inter-
estingly, this primary conclusion seems contrary to the struc-
tural effect on the NIR luminescence in terms of the ligand
shielding for the Yb(III) ions. Noticeably, since the effects of
the structure of the complexes on the catalytic activity are com-
plicated and cooperative, it is not possible to suggest the exact
relationship between the catalytic activity and the structure of
the complexes at the current stage. Further study is being con-
ducted in our laboratory.
1
2
3
4
5
6
Toluene
CH2Cl2
DMSO
EtOH
MeOH
H2O
24
24
24
24
24
24
Trace
22
35
58
70
90
a Reagents and conditions: 4-nitrobenzaldehyde (0.25 mmol),
nitromethane (2.5 mmol), water (3.0 mL), complex catalysts (10 mol
%), room temperature. b Isolated yield after chromatographic column
purification.
(TCSPC) technique. The life-time gives a satisfactory fit to a
monoexponential lifetime for 1–5 (Table 3). The lifetime of 3 is
the longest among complexes 1–5. Since the intrinsic lumines-
cence quantum yield of the Ln(III) ions may be estimated by
ΦLn = τobs/τ0, where τobs is the observed emission lifetime and
τ0 is the “nature lifetime”, viz. 2.0 ms for the Yb(III) ions.32 The
NIR intrinsic quantum yields of complexes 1, 3 and 4 are
higher than that for the reported salen type tetranuclear Yb(III)
complexes10 resulting from the improved shielding for Yb(III)
ions (Table 3).
Conclusion
A series of five salen type ytterbium complexes with distinct
structure have been isolated by the reactions of N,N′-bis-
(2-hydroxy-3-methoxybenzylidene)-1,3-propanediamine (H2L)
Catalysis
In order to explore the catalytic activity of the salen type with diverse ytterbium salts. Isolation of complexes 1–5 un-
lanthanide complexes on organic reactions, complex 4 was ambiguously reveals that the anions and the flexible ligand
selected as a catalyst to catalyze the Henry reaction under a play essential roles on modulating the structures. NIR lumine-
variety of solvents to explore the influence of solvents on the scent investigations show that the NIR emission intensity and
reaction rate and the yield (Table 4). Under homogenous con- the intrinsic quantum yields are dominated mainly by the
ditions, polar protic solvents accelerate the reaction with structural rigidity, the shielding of the ligand on Yb(III) ions
observed reaction yield in the following order: H2O > MeOH > and the numbers of the coordinated solvents. Preliminary cata-
EtOH > DMSO > CH2Cl2 > toluene. This trend suggests that lytic studies for complexes 1–5 suggest that the more comple-
hydrogen bonding, charge stabilization, and dipolar effects tely encapsulated the Yb(III) ions are by the salen moiety, the
play important roles on rate acceleration.33
lower the expected catalyst activity is, which seems contrary to
The Henry reaction doesn’t occur without using any cata- the effect on NIR luminescence. Further study on the cor-
lyst. In order to further explore the influence of the structure relation between the structure and luminescence as well as
on the catalytic activity, complexes 1–5 were employed as catalytic activity is being conducted in our group.
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11 T. Gao, P.-F. Yan, G.-M. Li, G.-F. Hou and J.-S. Gao, Poly-
hedron, 2007, 26, 5382.
Acknowledgements
This work is financially supported by the National Natural 12 F.-B. Han, Q.-Q. Teng, Y. Zhang, Y.-R. Wang and Q. Shen,
Science Foundation of China (no. 51272069, 21072049 and Inorg. Chem., 2011, 50, 2634.
21072050), Heilongjiang Province (no. 2010td03) and Heilong- 13 (a) R. B. Zhang, Z. J. Li, J. K. Cheng, Y. Y. Qin, J. Zhang
jiang University.
and Y. G. Yao, Cryst. Growth Des., 2008, 7, 2562;
(b) Q. Shi, Y. T. Sun, L. Z. Sheng, K. F. Ma, M. L. Hu,
X. G. Hu and S. M. Huang, Cryst. Growth Des., 2008, 8,
3401; (c) D.-P. Li, X.-H. Zhou, X.-Q. Liang, C.-H. Li,
C. Chen, J. Liu and X.-Z. You, Cryst. Growth Des., 2010,
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