Anion-induced self-assembly of luminescent and magnetic homoleptic cyclic tetranuclear Ln4(Salen)4 and Ln4(Salen) 2 complexes (Ln = Nd, Yb, Er, or Gd)

Article abstract of DOI:10.1021/ic300918c

Unique homoleptic cyclic tetranuclear Ln₄(Salen)₄ complexes [Ln₄(L)₂(HL)₂(μ₃-OH) ₂Cl₂]·2Cl (Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3; Ln = Gd, 4) or Ln₄(Salen)

Full text of DOI:10.1021/ic300918c

Article
pubs.acs.org/IC
Anion-Induced Self-Assembly of Luminescent and Magnetic
Homoleptic Cyclic Tetranuclear Ln (Salen) and Ln (Salen)
4
4
4
2
Complexes (Ln = Nd, Yb, Er, or Gd)
†
†
†
,*,†,‡ Han Liu, Guoxiang Shi, Dan Zou,
†
†
†
Weixu Feng, Yao Zhang, Zhao Zhang, Xingqiang Lu
̈
†
†
,§
∥
Jirong Song, Daidi Fan, Wai-Kwok Wong,* and Richard A. Jones
†Shaanxi Key Laboratory of Degradable Medical Material, Shaanxi Key Laboratory of Physico-inorganic Chemistry, Northwest
University, Xi’an 710069, Shaanxi, China
‡
Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, Fujian, China
§
Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, China
∥
Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, Texas
7
8712-0165, United States
*
S Supporting Information
ABSTRACT: Unique homoleptic cyclic tetranuclear Ln (Salen) complexes
4
4
[
Ln (L) (HL) (μ -OH) Cl ]·2Cl (Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3; Ln =
4 2 2 3 2 2
Gd, 4) or Ln (Salen) complexes [Ln (L) (μ -OH) (OAc) ] (Ln = Nd, 5;
4
2
4
2
3
2
6
Ln = Yb, 6; Ln = Er, 7; Ln = Gd, 8) have been self-assembled from the
reaction of the hexadentate Salen-type Schiff-base ligand H L with
2
LnCl ·6H O or Ln(OAc) ·6H O (Ln = Nd, Yb, Er, or Gd), respectively
3
2
6
2
(
H L: N,N′-bis(salicylidene)cyclohexane-1,2-diamine). The result of their
2
photophysical properties shows that the strong and characteristic NIR
luminescence for complexes 1−2 and 5−6 with emissive lifetimes in
microsecond ranges are observed, and the sensitization arises from the
excited state (both LC and LC) of the hexadentate Salen-type Schiff-base
ligand with the flexible linker. Temperature dependence (1.8−300 K)
magnetic susceptibility studies of the eight complexes suggest the presence of
an antiferromagnetic interaction between the Ln3+ ions.
1
3
1
0
INTRODUCTION
single-molecule magnets (SMMs), the promotion of
magnetic-exchange interactions between Ln3+ ions through
the overlap of bridging ligands orbitals with their “contracted”
■
Polynuclear Ln3 complexes with distinct luminescent and
+
magnetic properties are currently of interest because of their
11
1
4f orbitals is also a difficult task.
Compared to the amount of efforts on the photophysical or
magnetic behavior of 3d−4f heteronuclear complexes from
the compartmental Salen-type Schiff-base ligands, the research
on luminescent and magnetic polynuclear Ln Salen
complexes has not been researched nearly as extensively,
especially the limited single-crystal X-ray diffraction studies
have been reported for those classic complexes. Nonetheless,
for the typical quadridentate Salen-type Schiff-base ligands,
potential applications in the preparation of new optical or
12
2
3
magnetic materials and ideal probes in biology. However, the
13
3
+
control of structures of polynuclear Ln complexes is often
problematic due to the small energy differences, the high
coordination numbers, and the flexible coordination geometries
3
+
3
+
4
adopted by the Ln ions. In fact, the challenge to resolve the
problem is strengthened because that construction of the
14
polynuclear Ln3+ complexes is distinctively affected by other
5
factors, such as the character of organic ligands, the nature of
3
+
6
7
binuclear triple-decker and trinuclear triple-decker Ln
counterions, and the reaction conditions. Moreover, from the
viewpoint of the enhancement of their photophysical proper-
ties, it is required that the strong light-harvesting of the organic
chromophores, the effective energy transfer from the
15
complexes are obtained, in which the Salen-type Schiff-base
ligand with the rigid linker has been used, their photophysical
properties should be further enhanced due to the mismatch of
energy levels, despite the chromophores with rigid linkers
absorbing at longer wavelength. As to the pure quadridentate
Salen-type Schiff-base ligands with flexible linkers, anion-
3
+
chromophores to the Ln ion, and the minimization of
3
+
8
nonradiative processes of the Ln ion are achieved, besides
complete avoiding or decreasing the luminescent quenching
effect arising from OH-, CH-, or NH-oscillators around the
3
+
9
3+
Ln ion. On the other hand, as to the discrete Ln -based
clusters as the promising compounds for the development of
Received: May 5, 2012
Published: October 8, 2012
©
2012 American Chemical Society
11377
dx.doi.org/10.1021/ic300918c | Inorg. Chem. 2012, 51, 11377−11386

Inorganic Chemistry
Article
3
+
dependent discrete binuclear or tetranuclear homoleptic Ln
scan spectrofluorometer, and visible decay spectra on a pico-N
system (PTI Time Master). The quantum yield of the visible
luminescence for each sample was determined by the relative
comparison procedure, using a reference of a known quantum yield
2
laser
1
6
3+
17
complexes and polymeric Ln complexes have been
reported, while the self-assembly process is complicated and
strictly relative to the detailed reaction conditions, besides the
diverse quadridentate and bidentate coordination codes
adopted. Moreover, the self-assembly from the rigid hex-
adentate Salen-type Schiff-base ligand with the outer O O
moiety gives anion-induced trinuclear triple-decker, trinuclear
(
quinine sulfate in dilute H SO solution, Φ = 0.546). NIR emission
2
4
em
and excitation in solution were recorded by PTI QM4 spectro-
fluorometer with a PTI QM4 near-infrared InGaAs detector. The data
of magnetic susceptibility were collected using the Quantum Design
SQUID MPMS-XL magnetometer from polycrystalline samples at an
eternal field of 1000 Oe with the temperature range from 1.8 to 300 K.
2
2
18
1
9
20
3+
tetra-decker, or pentanuclear tetra-decker Ln complexes,
in which the luminescent quenching effect that arises from
X-ray Crystallography. Single crystals of [Nd
4
(L)
2
(HL)
(μ
3
2
(μ
3
-
-
3
+
coordinated MeOH or H O around the Ln ions inevitably
OH) Cl ]·2Cl·6EtOH (1·6EtOH), [Gd (L)
2
2
4
2
(HL)
2
2
OH) Cl ]·2Cl·2EtOH·2H O (4·2EtOH·2H O), [Yb (L) (μ -
exists. To the best of our knowledge, few reports of the self-
assembly of polynuclear lanthanide complexes from the flexible
hexadentate Salen-type Schiff-base ligand with the outer O O
2
2
2
2
4
2
3
OH) (OAc) ] (6), and [Er (L) (μ -OH) (OAc) ] (7) of suitable
2
6
4
2
3
2
6
dimensions were mounted onto thin glass fibers. All the intensity data
were collected on a Bruker SMART CCD diffractometer (Mo Kα
radiation and λ = 0.710 73 Å) in Φ and ω scan modes. Structures were
solved by direct methods followed by difference Fourier syntheses, and
2
2
21
moiety have been documented. Moreover, compared with
polynuclear Dy3 families, examples of other Ln -containing
+
3+
22
SMMs are still scarce. Herein, starting from the hexadentate
2
then refined by full-matrix least-squares techniques against F using
2
3
Salen-type Schiff-base ligand H L (H L = N,N′-bis(3-methoxy-
SHELXL-97. All other non-hydrogen atoms were refined with
2
2
salicylidene)cyclohexane-1,2-diamine) with the flexible linker,
anisotropic thermal parameters. Absorption corrections were applied
2
‑
‑
24
the richness of its coordination codes ((L) and (HL) modes,
using SADABS. Hydrogen atoms were placed in calculated positions
and refined isotropically using a riding model. CCDC reference
numbers 879715 for 1·6EtOH, and 879617−879619 for 4·2EtOH·2-
as shown in Scheme 1) endows the formation of two series of
H O and 6−7, respectively.
2
Scheme 1. Molecular Structure and Bonding Modes of the
Synthesis of the Salen-Type Schiff-Base Ligand H L (H L =
2
2
Salen-Type Schiff-Base Ligand H L for Polynuclear
2
N,N′-Bis(3-methoxy-salicylidene)cyclohexane-1,2-diamine). To
a stirred solution of an equimolar mixture of cis- and trans-1,2-
diaminocyclohexane (6.0 mL, 50 mmol) in absolute EtOH (20 mL), o-
vanillin (15.0 g, 100 mmol) was added, and the resulting mixture was
refluxed for 5 h. After cooling to room temperature, the insoluble
yellow precipitate was filtered and was recrystallized using absolute
EtOH to give a pale yellow polycrystalline solid. Yield: 13.6 g, 71%.
Anal. Found: C, 69.01; H, 6.94; N, 7.26. Calcd for C H N O : C,
Lanthanide Complexes 1−8
2
2
26
2
4
−1
6
9.09; H, 6.85; N 7.32. IR (KBr, cm ): 3455 (b), 3058 (w), 2933 (w),
2
862 (w), 2597 (w), 1619 (s), 1588 (w), 1470 (s), 1418 (m), 1345
(w), 1251 (vs), 1196 (w), 1168 (w), 1144 (w), 1085 (m), 1036 (w),
9
84 (m), 953 (w), 894 (w), 849 (m), 776 (w), 731 (m), 668 (w), 616
1
(w), 595 (w), 568 (w), 516 (w), 467 (w), 422 (w). H NMR (400
MHz, CD CN): δ (ppm) 13.84 (s, 2H, OH), 8.24 (s, 2H, CH=N),
3
6
.85 (d, 2H, Ph), 6.78 (d, 2H, Ph), 6.72 (t, 2H, Ph), 3.86 (s, 6H,
MeO), 3.32 (m, 2H, Ch), 1.92 (m, 4H, Ch), 1.58 (m, 4H, Ch).
Synthesis of [Nd (L) (HL) (μ -OH) Cl ]·2Cl (1). To a stirred
4
2
2
3
2
2
solution of H L (0.115 g, 0.3 mmol) in absolute EtOH (8 mL) were
2
added Et N (100 μL) and a solution of NdCl ·6H O (0.3 mmol, 0.107
3
3
2
g) in absolute EtOH (8 mL), respectively. The resultant mixture was
refluxed for 2 h, and the clear pale yellow solution was then cooled to
room temperature and filtered. Diethyl ether was allowed to diffuse
slowly into the filtrate at room temperature, and the pale yellow
microcrystal products of 1 were obtained in a few weeks. For 1: yield
anion-induced homoleptic cyclic tetranuclear
[
Ln (L) (HL) (μ -OH) Cl ]·2Cl (Ln = Nd, 1; Ln = Yb, 2;
4 2 2 3 2 2
Ln = Er, 3; Ln = Gd, 4) and [Ln (L) (μ -OH) (OAc) ] (Ln =
4
2
3
2
6
0
.080 g, 47%. Anal. Found: C, 46.35; H, 4.49; N, 4.85. Calcd for
88 100 8 18 4 4
Nd, 5; Ln = Yb, 6; Ln = Er, 7; Ln = Gd, 8). The sensitization
−1
3
+
C H N O Cl Nd : C, 46.43; H, 4.43; N, 4.92. IR (KBr, cm ):
for the NIR luminescence of the Ln ions and the magnetic
properties of the homoleptic cyclic tetranuclear Ln (Salen) or
2
(
1
(
933 (w), 2862 (w), 1651 (s), 1618 (s), 1553 (w), 1507 (m), 1467
s), 1451 (s), 1407 (w), 1372 (w), 1344 (w), 1300 (w), 1284 (w),
236 (m), 1222 (s), 1170 (m), 1143 (w), 1080 (vs), 1069 (s), 984
m), 946 (w), 902 (w), 857 (w), 784 (w), 743 (m), 657 (w), 576 (w),
4
4
Ln (Salen) complexes are discussed.
4
2
EXPERIMENTAL SECTION
1
■
534 (w), 496 (w), 473 (w), 442 (w). H NMR (400 MHz, CD CN): δ
3
General Methods. All chemicals were commercial products of
reagent grade and were used without further purification. Elemental
analyses were performed on a Perkin-Elmer 240C elemental analyzer.
Infrared spectra were recorded on a Nicolet Nagna-IR 550
spectrophotometer in the region 4000−400 cm using KBr pellets.
H NMR spectra were recorded on a Bruker Plus 400 spectrometer
(ppm) 14.61 (s, 2H), 14.18 (s, 2H), 10.21 (t, 4H), 9.51 (t, 4H), 8.63
(d, 4H), 8.32 (d, 4H), 7.63 (m, 4H), 6.51 (m, 4H), 5.42 (m, 6H), 4.63
(m, 6H), 4.01 (m, 4H), 3.85 (m, 4H), 1.73 (s, 6H), 1.33 (s, 6H), 1.18
(m, 4H), 1.06 (m, 4H), −0.13 (m, 4H), −0.47 (m, 4H), −0.87 (m,
4H), −1.27(m, 4H), −1.85 (m, 4H), −2.67 (m, 4H), −2.93 (m, 4H),
−5.63 (m, 4H). ESI-MS (in MeCN) m/z: 1102.14 (100%), [M −
−1
1
2
+
+
with SiMe as internal standard in CD CN at room temperature. ESI-
(Cl)
Synthesis of [Yb
prepared in the same way as 1 except that YbCl ·6H O (0.3 mmol,
2
] ; 2241.26 (19%), [M − Cl] .
4
3
DECA
MS was performed on a Finnigan LCQ
XP HPLC-MS mass
4
(L) (HL) (μ -OH)
2
2
3
Cl ]·2Cl (2). Complex 2 was
2 2
n
spectrometer with a mass to charge (m/z) range of 4000 using a
standard electrospray ion source and MeCN as solvent. Electronic
absorption spectra in the UV−vis region were recorded with a Cary
3
2
0.117 g) was used instead of NdCl ·6H O (0.3 mmol, 0.107 g). For 2:
yield 0.074 g, 41%. Anal. Found: C, 44.05; H, 4.29; N, 4.54. Calcd for
C H N O Cl Yb : C, 44.19; H, 4.21; N, 4.68. IR (KBr, cm ):
88 100 8 18 4 4
3
2
−1
300 UV spectrophotometer, and steady-state visible fluorescence, PL
excitation spectra on a Photon Technology International (PTI) Alpha
2937 (w), 2862 (w), 1652 (s), 1622 (s), 1561 (w), 1506 (m), 1475
1
1378
dx.doi.org/10.1021/ic300918c | Inorg. Chem. 2012, 51, 11377−11386

Inorganic Chemistry
Article
Scheme 2. Anion-Induced Formation of Homoleptic Cyclic Tetranuclear Ln (Salen) and Ln (Salen) Complexes
4
4
4
2
1
(
1
(
5
s), 1456 (s), 1408 (w), 1367 (w), 1345 (w), 1307 (w), 1291 (w),
230 (m), 1223 (s), 1169 (m), 1144 (w), 1088 (vs), 1071 (s), 983
m), 952 (w), 904 (w), 858 (w), 783 (w), 742 (m), 662 (w), 578 (w),
(w), 548 (m), 471 (w), 463 (w). H NMR (400 MHz, CD CN): δ
3
(ppm) 15.03 (s, 2H), 14.38 (s, 2H), 12.10 (t, 4H), 11.29 (t, 4H), 9.80
(d, 2H), 9.07 (d, 2H), 8.43 (m, 2H), 8.22 (m, 2H), 7.33 (m, 6H), 7.00
(m, 6H), 6.37 (m, 2H), 6.03 (m, 2H), 4.68 (s, 6H), 3.85 (s, 6H), 3.67
(m, 3H), 3.44 (m, 3H), 3.09 (m, 2H), 2.65 (m, 2H), 1.37 (m, 2H),
0.61 (m, 2H), −0.92 (m, 1H), −1.58 (m, 1H), −2.90 (m, 2H), −3.10
39 (w), 483 (w), 461 (w), 435 (w). ESI-MS (in MeCN) m/z:
2
+
+
1
160.43 (100%), [M − (Cl) ] ; 2356.64 (23%), [M − Cl] .
2
Synthesis of [Er (L) (HL) (μ -OH) Cl ]·2Cl (3). Complex 3 was
4
2
2
3
2
2
+
prepared in the same way as 1 except that ErCl ·6H O (0.3 mmol,
0
yield 0.071 g, 40%. Anal. Found: C, 44.56; H, 4.32; N, 4.67. Calcd for
C H N O Cl Er : C, 44.62; H, 4.26; N, 4.73. IR (KBr, cm ): 2938
(
1
(
(
3
2
(m, 2H). ESI-MS (in MeCN) m/z: 1727.14 (100%), [M + H] ;
2+
.115 g) was used instead of NdCl ·6H O (0.3 mmol, 0.107 g). For 3:
3 2
804.02 (15%), [M − (OAc) ] .
2
Synthesis of [Yb (L) (μ -OH) (OAc) ] (6). Complex 6 was
prepared in the same way as 5 except that Yb(OAc) ·6H O (0.3
mmol, 0.138 g) was used instead of Nd(OAc) ·6H O (0.3 mmol,
.129 g). For 6: yield 0.146 g, 53%. Anal. Found: C, 36.47; H, 3.77; N,
4
2
3
2
6
−1
88
100
8
18
4
4
3 2
w), 2858 (w), 1650 (s), 1622 (s), 1559 (w), 1505 (m), 1473 (s),
454 (s), 1408 (w), 1366 (w), 1345 (w), 1307 (w), 1291 (w), 1238
m), 1226 (s), 1171 (m), 1143 (w), 1078 (vs), 1065 (s), 985 (m), 947
w), 900 (w), 861 (w), 782 (w), 742 (m), 660 (w), 572 (w), 530 (w),
3
2
0
3
.02. Calcd for C H N O Yb : C, 36.53; H, 3.72; N, 3.04. IR (KBr,
56
68
4
22
4
−1
cm ): 2934 (w), 2862 (w), 2352 (w), 2068 (w), 1660 (m), 1640 (s),
601 (s), 1561 (s), 1468 (vs), 1420 (s), 1361 (w), 1307 (w), 1239
w), 1220 (m), 1207 (w), 1171 (w), 1105 (m), 1080 (s), 1027 (w),
81 (m), 859 (m), 782 (w), 758 (w), 744 (m), 652 (m), 616 (w), 590
w), 542 (m), 475 (w), 453 (w). ESI-MS (in MeCN) m/z: 1842.34
4
80 (w), 470 (w), 455 (w). ESI-MS (in MeCN) m/z: 1148.87
1
(
9
2+
+
(
100%), [M − (Cl) ] ; 2333.40 (18%), [M − Cl] .
2
Synthesis of [Gd (L) (HL) (μ -OH) Cl ]·2Cl (4). Complex 4 was
4
2
2
3
2
2
prepared in the same way as 1 except that GdCl ·6H O (0.3 mmol,
0
yield 0.086 g, 49%. Anal. Found: C, 45.32; H, 4.38; N, 4.74. Calcd for
C H N O Cl Gd : C, 45.39; H, 4.33; N, 4.81. IR (KBr, cm ):
3
2
(
(
.112 g) was used instead of NdCl ·6H O (0.3 mmol, 0.107 g). For 4:
+
2+
3
2
100%), [M + H] ; 861.62 (11%), [M − (OAc) ] .
2
Synthesis of [Er (L) (μ -OH) (OAc) ] (7). Complex 7 was
−1
4
2
3
2
6
88
100
8
18
4
4
prepared in the same way as 5 except that Er(OAc) ·6H O (0.3
mmol, 0.136 g) was used instead of Nd(OAc) ·6H O (0.3 mmol,
.129 g). For 7: yield 0.147 g, 54%. Anal. Found: C, 36.93; H, 3.83; N,
.04. Calcd for C H N O Er : C, 36.99; H, 3.77; N, 3.08. IR (KBr,
cm ): 2976 (w), 2936 (w), 2390 (w), 2053 (w), 1662 (m), 1635 (s),
1600 (s), 1556 (s), 1460 (vs), 1416 (s), 1348 (w), 1310 (w), 1244
w), 1224 (m), 1206 (w), 1170 (w), 1101 (m), 1088 (s), 1027 (w),
95 (m), 864 (m), 785 (w), 757 (w), 742 (m), 644 (m), 616 (w), 588
w), 540 (m), 469 (w), 457 (w). ESI-MS (in MeCN) m/z: 1819.22
100%), [M + H] ; 850.06 (9%), [M − (OAc) ] .
Synthesis of [Gd (L) (μ -OH) (OAc) ] (8). Complex 8 was
prepared in the same way as 5 except that Gd(OAc) ·6H O (0.3
mmol, 0.133 g) was used instead of Nd(OAc) ·6H O (0.3 mmol,
.129 g). For 8: yield 0.128 g, 48%. Anal. Found: C, 37.76; H, 3.90; N,
.12. Calcd for C H N O Gd : C, 37.83; H, 3.85; N, 3.15. IR (KBr,
3
2
2
(
1
(
930 (w), 2860 (w), 1654 (s), 1619 (s), 1550 (w), 1509 (m), 1468
s), 1454 (s), 1403 (w), 1370 (w), 1348 (w), 1302 (w), 1287 (w),
232 (m), 1224 (s), 1172 (m), 1148 (w), 1081 (vs), 1071 (s), 983
m), 941 (w), 900 (w), 859 (w), 781 (w), 742 (m), 659 (w), 573 (w),
3
2
0
3
56
68
4
22
4
−
1
5
1
31 (w), 491 (w), 472 (w), 452 (w). ESI-MS (in MeCN) m/z:
2
+
+
128.85 (100%), [M − (Cl) ] ; 2293.32 (15%), [M − Cl] .
2
(
9
Synthesis of [Nd (L) (μ -OH) (OAc) ] (5). To a stirred solution
4
2
3
2
6
of H L (0.115 g, 0.3 mmol) in absolute MeOH (5 mL) were added
Et N (100 μL) and a solution of Nd(OAc) ·6H O (0.3 mmol, 0.129
2
(
(
3
3
2
+
2+
g) in absolute MeOH (5 mL), respectively. The resultant mixture was
refluxed for 2 h, then 5 mL of absolute MeCN was added, and the
clear pale yellow solution was refluxed another 2 h. After cooling to
room temperature, the solution was filtered, and diethyl ether was
allowed to diffuse slowly into the filtrate at room temperature. The
pale yellow microcrystal products of 5 were obtained in a few weeks.
For 5: yield 0.132 g, 51%. Anal. Found: C, 38.90; H, 4.06; N, 3.18.
Calcd for C H N O Nd : C, 38.97; H, 3.97; N, 3.25. IR (KBr,
2
4
2
3
2
6
3
2
3
2
0
3
56 68
4
22
4
−
1
cm ): 2935 (w), 2860 (w), 2355 (w), 2061 (w), 1669 (m), 1642 (s),
1600 (s), 1559 (s), 1463 (vs), 1417 (s), 1352 (w), 1302 (w), 1231
(w), 1221 (m), 1201 (w), 1170 (w), 1105 (m), 1088 (s), 1025 (w),
963 (m), 861 (m), 780 (w), 757 (w), 742 (m), 657 (m), 619 (w), 588
(w), 542 (m), 477 (w), 465 (w), 437 (w). ESI-MS (in MeCN) m/z:
56
68
4
22
4
−1
cm ): 2930 (w), 2862 (w), 2344 (w), 2061 (w), 1662 (m), 1638 (s),
600 (s), 1563 (s), 1471 (vs), 1422 (s), 1369 (w), 1300 (w), 1244
w), 1223 (m), 1200 (w), 1178 (w), 1100 (m), 1083 (s), 1020 (w),
87 (m), 863 (m), 789 (w), 761 (w), 747 (m), 656 (m), 609 (w), 595
1
(
9
+
2+
1779.18 (100%), [M + H] ; 830.04 (12%), [M − (OAc) ] .
2
1
1379
dx.doi.org/10.1021/ic300918c | Inorg. Chem. 2012, 51, 11377−11386

Inorganic Chemistry
Article
Table 1. Crystal Data and Structure Refinement for Complexes 1·6EtOH, 4·2EtOH·2H O, and 6−7
2
compd
formula
1·6EtOH
4·2EtOH·2H O
6
7
2
C100H136N O Cl Nd
C H N O Cl Gd
C H N O Yb
C H N O Er
8
24
4
4
92 116
8
22
4
4
56 68
4
22
4
56 68
4
22
4
fw
2552.93
2456.73
1841.30
1818.18
cryst syst
space group
a, Å
monoclinic
monoclinic
triclinic
triclinic
P2 /n
1
P2 /n
1
P1
̅
P1
̅
18.5179(14)
16.1970(12)
19.1175(14)
90
18.506(3)
16.011(2)
18.964(3)
90
11.833(2)
12.133(2)
12.696(3)
104.541(4)
91.386(4)
95.173(3)
1755.2(6)
1
11.824(3)
12.126(3)
12.717(3)
104.693(5)
91.462(5)
95.205(5)
1754.3(8)
1
b,Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
95.4820(10)
90
95.885(2)
90
5707.8(7)
2
5589.5(15)
2
Z
Dcalcd, g cm−
3
1.485
1.460
1.742
1.721
cryst size, mm3
0.30 × 0.25 × 0.22
273(2)
0.29 × 0.24 × 0.19
273(2)
0.27 × 0.24 × 0.20
296(2)
0.28 × 0.23 × 0.21
296(2)
temp, K
F(000)
2584
2448
888
880
−
1
μ, mm
1.951
2.502
5.349
4.804
θ range, deg
reflns measd
reflns used
params
1.46−32.22
56 090
1.62−31.28
57 530
1.66−26.09
9401
1.66−26.24
9404
18 813
17 824
6734
6784
632
586
389
388
R (I > 2σ(I))
R1 = 0.0518
wR2 = 0.1413
R1 = 0.0723
wR2 = 0.1634
1.110
R1 = 0.0675
wR2 = 0.2007
R1 = 0.0964
wR2 = 0.2292
1.078
R1 = 0.0766
wR2 = 0.2116
R1 = 0.1224
wR2 = 0.2523
1.031
R1 = 0.0832
wR2 = 0.2157
R1 = 0.1414
wR2 = 0.2638
1.023
R (all data)
S
RESULTS AND DISCUSSION
spectra in CD CN of complexes 1 and 5, large shifts (δ from
3
■
1
4.61 to −5.63 ppm for 1 and 15.03 to −3.10 ppm for 5) of
As shown in Scheme 2, reaction of equimolar amounts of the
2‑
2
‑
two sets of the proton resonances of the L ligands endowed
from the observed (L) and/or (HL) modes in the molecular
structures are observed, due to the Nd -induced shift,
deprotonated L ligand and LnCl ·6H O (Ln = Nd, Yb, Er or
3
2
2‑
‑
Gd) in refluxing absolute EtOH produced the yellow solution,
3+
26
from which the series of tetranuclear Ln (Salen) complexes
4
4
significantly spread in relative to those of the free H L ligand (δ
2
[
=
Ln (L) (HL) (μ -OH) Cl ]·2Cl (Ln = Nd, 1; Ln = Yb, 2; Ln
4 2 2 3 2 2
from 13.84 to 1.58 ppm). The ESI-MS spectra of the two series
of complexes (1−4 and 5−8) in MeCN display the respective
similar patterns and exhibit the strong mass peak at m/z
Er, 3 or Ln = Gd, 4) were isolated as yellow microcrystalline
solids, respectively. Reaction of equimolar amounts of the
2
‑
deprotonated L ligand and Ln(OAc) ·6H O (Ln = Nd, Yb, Er
3
2
1
1
102.14 (1), 1160.43 (2), 1148.87 (3), or 1128.85 (4), and
727.14 (5), 1842.34 (6), 1819.22 (7), or 1779.18 (8),
or Gd) in refluxing absolute MeOH−MeCN resulted in the
formation of the series of tetranuclear Ln (Salen) complexes
4
2
2
+
assigned to the major species [Ln (L) (HL) (μ -OH) Cl ] of
[
Ln (L) (μ -OH) (OAc) ] (Ln = Nd, 5; Ln = Yb, 6; Ln = Er, 7
4
2
2
3
2
2
4
2
3
2
6
+
complexes 1−4 and [Ln (L) (μ -OH) (OAc) -H] of com-
or Ln = Gd, 8), respectively. Similar to the good solubility of
the Salen-type Schiff-base ligand H L in common organic
solvents except for water, complexes 5−8 are also soluble in
absolute MeCN due to the use of the Salen-type Schiff-base
4
2
3
2
6
plexes 5−8, respectively. These observations further indicate
2
that the respective discrete homoleptic Ln (Salen) or
4
4
Ln (Salen) unit is retained in the respective dilute MeCN
4
2
solution.
ligand H L with the flexible linker, while the better solubility for
2
The solid state structure of 1·6EtOH or 4·2EtOH·2H O as
the series of complexes 1−4 should be further assigned to the
2
the representative of 1−4 and 6 or 7 as the representative of 5−
charge of the two components (the cationic [Ln (L) (HL) (μ -
4
2
2
3
2+
−
8
was determined by X-ray single-crystal diffraction analysis.
OH) Cl ] part and two Cl anions) in each of the four
2
2
Crystallographic data for the four complexes are presented in
Table 1, and selected bond lengths and angles are given in
Table 1S in the Supporting Information.
complexes.
The ligand H L and the two series of eight complexes 1−8
2
1
were well characterized by EA, FT-IR, H NMR, and ESI-MS.
In the FT-IR spectra, the characteristic strong absorptions of
Complex 1·6EtOH crystallizes in the monoclinic space group
−
1
the ν(CN) vibration at 1650−1654 cm for complexes 1−4,
P2
of one cation [Nd
anions, and six solvates EtOH. As shown in Figure 1, for the
1
/n. For complex 1·6EtOH, the structural unit is composed
−1
2+
−
or 1635−1642 cm for complexes 5−8, are slightly blue-
(L) (HL) (μ -OH) Cl ] , two free Cl
4 2 2 3 2 2
−
1
shifted by the range 31−35 or 16−23 cm relative to that of
−
1
2+
the free Salen-type Schiff-base ligand H L (1619 cm ) upon
cationic [Nd
(L)
4
2
(HL)
2
(μ
3
-OH)
Cl
2
2
]
part lying about an
2
3
+
the coordination of the Ln ions. For complexes 5−8, two
inversion center, two equivalent Nd
(L)(HL) moieties are
2
additional strong characteristic absorptions at 1600−1601 and
bridged by two μ-O phenoxide atoms (O7 and O7a) of two
−
1
‑
1
417−1422 cm
were observed, which are tentatively
Salen-type Schiff-base (HL) ligands with O tetradentate mode
4
−
−
attributed to the ν vibration and the ν vibration of OAc
anions, respectively. As to the room temperature H NMR
((HL) mode, as shown in Scheme 1) and two O atoms (O9
and O9a) of two coordinated μ -OH groups, resulting in the
as
s
25
1
−
3
1
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with the host structure. The slight variation of the detailed
structures in the complexes 1·6EtOH and 4·2EtOH·2H O
2
isolated under the same reaction conditions should be due to
27
the effect of lanthanide contraction.
Complex 6 crystallizes in the triclinic space group P1
of the crystal structure of complex 6 is shown in Figure 2, and
̅
. A view
Figure 1. Perspective drawing of the cationic part in complex
1·6EtOH. Free anions, H atoms, and solvates are omitted for clarity.
formation of a homoleptic cyclic tetranuclear (Nd (Salen) )
4
4
host structure. In each of two equivalent Nd (L)(HL) moieties,
2
two Nd3 (Nd1 and Nd2) ions with different coordination
+
environments also linked by two μ-O phenoxide atoms (O2
Figure 2. Perspective drawing of complex 6. H atoms are omitted for
clarity.
2
‑
and O3) of one Salen-type Schiff-base (L) ligand with N O
2
4
2
‑
hexadentate mode ((L) mode) and one O atom (O9) of the
−
3+
coordinated μ -OH group. The unique inner Nd ion (Nd1)
3
is eight-coordinate and bound by the N O core of the Salen-
reveals a tetranuclear centrosymmetric core with two equivalent
2
2
2
‑
−
type Schiff-base (L) ligand in addition to two O atoms (O8a
of MeO group and O7a of μ-O phenoxide atom) from the
Yb L moieties linked by two μ -OH groups, two bridged
2
3
−
OAc anions, and two μ-O phenoxide atoms (O2 and O2a) of
the Salen-type Schiff-base (L) ligand. In each of two
equivalent Yb L moieties, two Yb (Yb1 and Yb2) ions with
‑
2‑
Salen-type Schiff-base (HL) ligand and two O atoms (O9 and
−
3+
O9a) of two coordinated μ -OH groups. However, the outer
3
2
3+
Nd ion (Nd2) is nine-coordinate: in addition to the seven
oxygen atoms from the two outer O O moieties of the two
different coordination environments also linked by the μ-O
2
‑
phenoxide atom (O3) of the Salen-type Schiff-base (L) ligand
2
2
2
‑
Salen-type Schiff-base ligands, where four O atoms (two of
with N O hexadentate mode ((L) mode), another bridged
2 4
−
−
MeO groups and two of phenoxide atoms) are from the Salen-
OAc anion, and one O atom (O5) of the coordinated μ -OH
3
group. The unique inner Yb ion (Yb1) is eight-coordinate and
bound by the N O core of the Salen-type Schiff-base (L)
2
‑
3+
type Schiff-base (L) ligand and three O atoms (one of MeO
2
‑
groups and two of phenoxide atoms) from the Salen-type
2 2
‑
Schiff-base (HL) ligand, it saturates its coordination environ-
ligand in addition to one O atoms (O5) of the coordinated μ -
3
−
−
ment with one O atom (O9) from the coordinated μ -OH
OH groups and three O atoms (O6, O8, and O10) from two
3
−
bridged OAc− anions and one monodentate OAc anion,
−
group and one Cl anion (Cl1). Three unique Nd···Nd
3+
distances are different at 3.6624(4), 3.8384(5), and 3.9892(4)
Å for Nd1···Nd2, Nd1···Nd1a, and Nd1···Nd2a, respectively, in
which each of the Nd1···Nd2 separations in the equivalent
respectively. For the outer Yb ion (Yb2), although it also has
the eight-coordinate, its coordination environment is saturated
by eight O atoms, with four O atoms (O1a, O2a, O3, and O4)
from the two outer O O moieties of the two Salen-type Schiff-
Nd L(HL) moieties is slightly shorter than that (Nd1···Nd1a or
2
2
2
Nd1···Nd2a separation) between two equivalent Nd L(HL)
base ligands, two O atoms (O5 and O5a) from two coordinated
2
−
moieties. It is interesting to notice, as to the charge balance to
μ -OH groups, and two O atoms (O7 and O9a) from two
3
2
+
−
the cationic [Nd (L) (HL) (μ -OH) Cl ] part, it should be
bridged OAc anions. Three unique Yb···Yb distances are
4
2
2
3
2
2
balanced by the protonation of one (N4 or N4a) of the imino
nitrogen atoms for two of the four deprotonated Salen-type
different at 3.5761(11), 3.6826(12), and 3.7103(12) Å for
Yb1···Yb2, Yb1···Yb2a, and Yb2···Yb2a, respectively, in which
each of the Yb1···Yb2 separations in the equivalent Yb2L
moieties is slightly shorter than that (Yb1···Yb2a or Yb2···Yb2a
2
‑
Schiff-base (L) ligands, which endows the formation of two
strong intramolecular N3−H3···O6 (2.603(5) Å and
3
+
1
35.6(3)°) H-bond interactions shown in Figure 1S. The two
separation) between two equivalent Yb L moieties. When Yb
2
−
3+
free Cl anions and the six solvate EtOH molecules of complex
ion was replaced by Er ion, the isomorphous complex 7 was
obtained. As shown in Figure 4S and Table 1S, the slight
variation of the detailed structures in complexes 6−7, isolated
under the same reaction conditions, should also be due to the
1
·6EtOH are not bound to the framework, and they exhibit no
observed interactions with the host structure. X-ray structural
analysis indicated that complex 4·2EtOH·2H O is isomorphous
2
27
with complex 1·6EtOH, as shown in Figure 2S, and the similar
strong intramolecular H-bond interactions with the short N3−
H3···O6 distances (2.610(10) Å) and the reasonable N3−
H3···O6 bond angle (134.7(5)°) in the cationic
effect of lanthanide contraction.
In a comparison of complexes 1·6EtOH and 4·2EtOH·2H O
2
−
−
with complexes 6−7, the use of different anions (Cl or OAc )
is critical to the formation of the homoleptic cyclic
tetetranuclear Ln (Salen)4 or Ln (Salen)2 complexes. The
2
+
[
Gd (L) (HL) (μ -OH) Cl ] host structure are observed in
4 2 2 3 2 2
4
4
−
Figure 3S. The two free Cl anions and solvates EtOH and
common feature in both structures is the retention of one
−
−
3+
H O molecules of complex 4·2EtOH·2H O are not bound to
Cl or monodentate OAc per inner Ln ion which is bound
2
2
the framework, and they also exhibit no observed interactions
to the central N O core of one Salen-type Schiff-base ligand.
2
2
1
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Two equivalent Ln (L)(HL) or Ln L moieties are also bridged
Table 2. Photophysical Properties of the H L and
Complexes 1−8 at 1 × 10 M in Absolute MeCN Solution
at Room Temperature or 77 K
2
2
2
−
−5
by two coordinated μ -OH groups to endow the homoleptic
3
cyclic tetranuclear host frameworks, and no solvent molecules
3
+
found in the host structures are bound to the Ln center. In
absorption λ /nm
ab
the formation of Ln (Salen) complexes 6−7, due to the hard
3
4
2
[log(ε/dm
excitation
λex/nm
emission λem/3nm (τ, Φ ×
3
+
−
−1
−1
Lewis acidity of Ln ions, the excess OAc anions are able to
coordinate effectively to the Ln ions and prevent the further
compd
mol cm )]
10 )
3
+
H2L 220(0.84), 260(0.40),
303, 321,
370
362(1.22 ns, 0.23), 478(1.57
ns, 0.25)
coordination of the ligands. Moreover, the structure formation
332(0.10)
for complexes 1·6EtOH and 4·2EtOH·2H O also resulted from
1
228(2.89), 268(1.31),
342(0.45)
374(sh),
401
525(w); 888(1.53 μs),
1068(1.57 μs), 1361
2
−
2‑
the flexibility of the ligand, where both (HL) and (L)
(
1.55 μs)
coordination modes are incorporated for the stability of the
Ln (Salen) complexes. It is worth noting that the homoleptic
2
3
4
230(2.63), 268(1.14),
342(0.44)
354,
527(w); 979(14.77 μs)
4
4
395(sh)
cyclic tetranuclear Ln (Salen) or Ln (Salen) host structure is
228(2.53), 269(1.15),
403
406
524(w); a
4
4
4
2
3
38(0.43)
distinctively different from the reported structures of binuclear
1
5
15,18
3+
226(2.88), 268(1.32),
340(0.45)
526(0.77 ns, 0.67)
triple-decker, trinuclear triple-decker,
trinuclear tetra-
1
9
20
decker, or pentanuclear tetra-decker Ln complexes based
on the Salen-type Schiff-base ligands with the rigid linkers,
which should be due to the use of the flexible Salen-type Schiff-
base ligand H L with the outer O O moiety. On the other
5
07(3.2 ns, 77 K), 565(7.5
ms, 77 K)
5
234(1.05), 276(0.48),
346(0.21)
383
490(w); 900(1.44 μs),
1063(1.47 μs), 1332
2
2
2
(
1.46 μs)
hand, the formation of homoleptic cyclic tetranuclear
6
7
8
228(1.31), 270(0.57),
387
385
384
483(w); 978(13.82 μs)
(
Ln (Salen) ) framework of complexes 1−4 appears to be
4
4
3
46(0.21)
−
−
Cl anion-dependent, which is comparable to NO3 anion-
233(1.05), 275(0.48),
345(0.21)
486(w); a
dependent Ln (Salen) complexes from the same ligand in our
4
4
2
1
−
recent report, while the OAc inducement should dominate
227(1.20), 269(0.52),
487(0.64 ns, 0.58)
3
43(0.19)
the Ln (Salen) strucutres in complexes 5−8. It is of special
4
2
4
73(2.5 ns, 77 K), 552(6.4
ms, 77 K)
interest to compare the self-assembly of the two series of
Ln (Salen)4 and Ln (Salen) complexes with that of the
4
4
2
a
1
6
The emission is too weak to be detected.
reported Tb (Salen) complex. Although the similar
4
6
tetranuclear framework is obtained necessarily from the
Salen-type Schiff-base ligands with flexible linkers, the character
of the Salen-type Schiff-base ligand H L with both the inner
2
N O core and the outer O O moiety in complexes 1−8,
2
2
2
2
instead of the pure Salen-type Schiff-base ligand without the
16
outer O O moiety in the reported Tb (Salen) complex,
2
2
4
6
endows the unusual formation of anion-dependent homoleptic
cyclic tetranuclear frameworks.
Photophysical Properties of Lanthanide Complexes.
The photophysical properties of the ligand H L and complexes
2
1
−4 have been examined in dilute MeCN solution at room
temperature or 77 K, and summarized in Table 2 and Figures
3
−5. As shown in Figure 3, the similar ligand-centered solution
absorption spectra (226−230, 268−269, and 338−342 nm) of
complexes 1−4 in the UV−vis region are observed, red-shifted
3
+
upon the coordination of the Ln ions as compared to that
Figure 3. UV−vis absorption spectra of the ligand H L and complexes
(
220, 260, and 332 nm) of the ligand H L in MeCN. The
2
2
−5
1
−4 in MeCN solution at 1 × 10 M at room temperature.
molar absorption coefficients of complexes 1−4 in all the
lowest energy bands (338−342 nm) are almost 4 orders of
magnitude larger than that (332 nm) of the ligand H L due to
for complex 3 cannot be observed. The free ligand H L or
2
2
the involvement of four chromophores. For complexes 1−3,
the similar weak visible emissions (ca. λ = 526 nm and τ < 1
ns) with low quantum yields (Φ < 10 ) are observed in
dilute MeCN solution at room temperature. In addition to the
residual weak visible emission, as shown in Figure 4, photo
excitation of the chromophore in the range 200−480 nm (λ =
4
ligand-field splitting emissions of the Nd ion ( F3 → I , J
=
complex 4 also does not exhibit the NIR emission under the
same condition, and just displays the typical strong
luminescence of the Salen-type Schiff-base ligand in the visible
range, as shown in Figure 5. The excitation spectra of
complexes 1−2, monitored at the respective NIR emission
peak (1068 nm for 1 or 979 nm for 2), are similar to those
monitored at their respective visible emission peak, which
clearly demonstrates that both the NIR and visible emissions
for complexes 1−2 originated from the same π−π* transitions
em
−5
em
ex
01 nm for 1 or 354 nm for 2) gives rise to the characteristic
3
+
4
4
/2
J/2
3
+
2
2
9, 11, 13) or the Yb ion ( F → F7/2) in the NIR range,
5/2
respectively. For complex 1, the emissions at 888, 1068, and
of the ligand H L, and the energy transfer from the
2
4
4
4
4
3+
28
1
361 nm can be assigned to F → I , F → I11/2, and
chromophore to the Ln ions takes place efficiently. As a
reference compound, complex 4 endows the further study of
the chromophore luminescence in the absence of energy
transfer, because the Gd ion has no energy levels below 32
000 cm , and thus cannot accept any energy from the excited
3
/2
9/2
3/2
4
4
3+
F3/2 → I
transitions of the Nd ion, respectively, and the
13/2
2
2
emission at 979 nm can be attributed to F → F transition
5
/2
7/2
of the Yb3 ion for complex 2. Moreover, unlike that for
+
3+
3+
−1
complex 1 or 2, the characteristic NIR emission of the Er ion
1
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unobservable luminescence in the range of 800−1800 nm for
31
complex 3.
Moreover, for complexes 1−2, the respective NIR
luminescent decay curves obtained from time-resolved
luminescent experiments can be fitted monoexponentially
with time constant of microseconds (1.57 μs for 1 at 1068
nm and 14.77 μs for 2 at 979 nm), and the intrinsic quantum
3
+
yield Φ (0.62% for 1 or 0.79% for 2) of the Ln emission
Ln
may be estimated by Φ = τ /τ , where τ is the observed
Ln
obs
0
obs
emission lifetime and τ is the “natural lifetime”, viz. 0.25 and
0
3+
32
2
.0 ms for the Nd3+ and Yb ions, respectively. As to the
relatively higher quantum efficiency of 2 (0.79%) than that of 1
2
(
1
0.62%), although there is a slightly larger energy gap ( F5/2,
−1
−1
3+
0215 cm , ΔE = 7484 cm ) of Yb ion in complex 2 than
4 −1 −1 3+
that ( F , 9363 cm , ΔE = 7336 cm ) of the Nd ion in
3/2
3+
complex 1, the excited state of the Nd ion in complex 1 is
Figure 4. NIR emission and excitation spectra of complexes 1−2 in
−5
more sensitive to quenching by the O−H oscillators of the
MeCN solution at 1 × 10 M at room temperature.
−
coordinated μ -OH groups and the distant C−H or N−H
3
oscillators of the Salen-type Schiff-base ligand H L around the
2
3+
3+
Nd ions, besides the quantity of accepting levels of the Nd
ion while only one for the Yb ion.
The change of use of Ln(OAc) instead of LnCl results in
3+
3
3
structure changes from Ln (Salen) to Ln (Salen) , and the
4
4
4
2
different photophysical properties of complexes 5−8 are
presented in Table 2 and Figures 5−7. As shown in Figure 6,
Figure 5. Visible emission and excitation spectra of the ligand H L and
2
−5
complexes 4 and 8 in MeCN solution at 1 × 10 M at room
temperature.
29
state of the chromophores. In dilute MeCN solution at 77 K,
compared with that (λ = 526 nm, τ = 0.77 ns and ϕ = 0.67 ×
em
−3
1
0 ) at room temperature on the same condition, complex 4
exhibits the strengthened fluorescence, which shows the higher
luminescent intensity (λem = 507 and 565 nm) and the
distinctively longer luminescence lifetimes (3.2 ns and 7.5 ms).
This result demonstrates that the sensitization of the NIR
luminescence for complexes 1−2 should arise from both the
Figure 6. UV−vis absorption spectra of the ligand H L and complexes
2
−5
5−8 in MeCN solution at 1 × 10 M at room temperature.
1
−1
3
−1
LC (19 734 cm ) and the LC (17 699 cm ) excited state of
21
the Schiff-base ligand H L at low temperature. If the antennae
2
luminescence lifetime of complex 4 is to represent the excited-
state lifetime in the absence of the energy transfer, the energy
transfer rate (k ) in complexes 1−3 can thus be calculated
ET
30
from k = 1/τ − 1/τ , where τ is the residual lifetime of the
luminescent emission undergoing quenching by the respective
ET
q
u
q
3+
Ln ion, and τ is the unquenched lifetime in the reference
u
3
+
complex 4, so the energy transfer rates for the Ln ions in
8
−1
complexes 1−3 may all be estimated to be above 5 × 10 s ,
which could well imply the reason for the effective energy
transfer for complexes 1−3. Furthermore, from the viewpoint
of the energy level match, in spite of the effective energy
transfer also taking place in complex 3, the larger energy gap
3
−1
between the energy-donating LC level (17 699 cm ) and the
4
3+
emitting level ( I13/2) of Er ion than those of complexes 1−2
results in the great nonradiative energy loss during the energy
transfer, which should be the reason to the weak and
Figure 7. NIR emission and excitation spectra of complexes 5−6 in
−5
MeCN solution at 1 × 10 M at room temperature.
1
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Article
although the absorption spectra of complexes 5−8 are also red-
shifted relative to that of the free ligand H L, the molar
2
absorption coefficients of complexes 5−8 in all the lowest
energy bands (343−346 nm) are almost 2 not 4 orders of
magnitude larger than that (332 nm) of the ligand H L due to
2
the involvement of two chromophores. For complexes 5−6, the
very weak residual visible emissions (λ = 483−490 nm, τ < 1
em
−
5
ns, and Φ < 10 ) and the characteristic NIR emission of the
Nd ion ( F3 → I , J = 9, 11, 13) or the Yb ion ( F →
F ) also suggests that the sensitization from the chromo-
phores to these Ln ions takes place efficiently. Through the
further investigation on the emission of the reference
compound 8, especially at 77 K, the strong fluorescence (λem
em
3
+
4
4
3+
2
/2
J/2
5/2
2
7
/2
3
+
=
473 nm, τ = 2.5 ns and 552 nm, τ = 6.5 ms) demonstrates
that the sensitization of the NIR luminescence for complexes
1
−1
5
−6 should also arise from both the LC (21 142 cm ) and
Figure 8. Temperature dependence of the χ
m
T values for four
3
−1
Ln (Salen) complexes 1−4 with an applied field of 1000 Oe.
the LC (18 116 cm ) excited state of the Schiff-base ligand
4
4
H L. Similar to that of complex 3, no characteristic NIR
2
3
+
emission of the Er ion for complex 7 is observed due to the
larger energy gap between the energy-donating LC level (18
16 cm ) and the emitting level ( I13/2) of Er ion. As to the
3
−1
4
3+
1
relatively lower NIR intrinsic quantum yields of Ln (Salen)2
4
complexes 5−6 (0.58% for 5 and 0.69% for 6) than those of the
corresponding Ln (Salen) complexes 1−2 (0.62% for 1 and
4
4
0
.79% for 2), one of the reasons should be due to the larger
−1
−1
energy gaps (ΔE = 8709 cm for 5 and ΔE = 7891 cm for 6)
−1
in complexes 5−6 than those (ΔE = 7336 cm for 1 and ΔE =
484 cm− for 2) of the Ln ion in complexes 1−2, and the
1
3+
7
other should arise from additional quenching effects by the
nearby C−H oscillators of the monodentate and bidentate
−
3+
coordinated OAc groups around the Ln ions. It is worth
noting that the relatively larger NIR intrinsic quantum yields for
both complexes 1−2 and complexes 5−6 than those of
binuclear triple-decker or trinuclear triple-decker lanthanide
Figure 9. Temperature dependence of the χ T values for four
m
3+
3+
15
Ln (Salen) complexes 5−8 with an applied field of 1000 Oe.
4
2
(
Nd or Yb ) complexes based on the typical Salen-type
Schiff-base ligand with the rigid linker should result from the
3
−1
1
K for 1, C = 8.98 cm mol K and θ = −9.72 K for 2, C = 7.71
3 −1 3 −1
sensitization of the NIR luminescence from both the LC and
3
cm mol K and θ = −18.17 K for 5, and C = 8.62 cm mol
K and θ = −10.23 K for 6, respectively (Figures 5−6S).
the LC excited state of the flexible Schiff-base ligand H L for
2
complexes 1−2 and 5−6, and the decrease of the luminescent
As to complexes 3−4 and 7−8, the observed χ T values of
m
quenching effect by the coordinated MeOH solvates around the
3
−1
3+
3+
3+
45.92 and 45.97 cm K mol for the two Er complexes 3 and
Ln ions for the triple-decker Ln complexes.
3
−1
3+
7
, 32.89 and 32.88 cm K mol for the two Gd complexes 4
and 8, respectively, are close to the theoretical value for four
noninteracting Er (45.92 cm K mol , I15/2, S = 3/2, L = 6, g
Magnetic Properties of Lanthanide Complexes. The
direct-current (dc) magnetic measurements were performed on
polycrystalline samples of Ln (Salen) complexes 1−4 and
3+
3
−1 4
4
4
3+
3
−1 8
=
6/5) or Gd (31.52 cm K mol , S7/2, S = 7/2, L = 0, g =
33
Ln (Salen) complexes 5−8 between 1.8 and 300 K under an
4
2
2
) ions. Upon decreasing the temperature the χ T values
m
external field of 1000 Oe, shown in Figures 8 and 9,
remain fairly constant down to ∼60 K for 4 and 8 before
dropping rapidly down to ca. 17.70 cm K mol at 1.8 K. The
respectively, where the observed paramagnetic behaviors of
3
−1
3
+
the two series of eight complexes arise from the Ln (Ln = Nd,
−1
χm versus T for complex 4 or 8 also obeys the Curie−Weiss
Yb, Er, or Gd) ions. For complexes 1−2 or 5−6, the observed
3
−1
law, with C = 33.57 cm mol K and θ = −4.47 K for 4 and C
3
values of χ T at 300 K are 4.79 and 8.72 or 7.50 and 8.55 cm
3
−1
m
=
33.43 cm mol K and θ = −3.18 K for 8, respectively
3+
−1
K mol , respectively, slightly smaller than the respective
(
Figures 5−6S). Due to the isotropic nature of Gd ions it is
3
+
3
−1
expected value for four noninteracting Nd (6.56 cm K mol ,
reasonable to assume the latter behavior is indicative of
4
2
3+
3
−1
I , S = 3/2, L = 6, g = 8/11) or Yb (10.28 cm K mol ,
35
9
/2
intramolecular antiferromagnetic interactions. In the case of
33
the Er3+ complexes 3 and 7, although the χm 3 versus T also
−1
F , S = 1/2, L = 3, g = 8/7) ions. On cooling, each χmT
7
/2
−1
value gradually decreases, and the experimental values at 1.8 K
obeys the Curie−Weiss law, with C = 46.82 cm mol K and θ
3
−1
3
−1
of 1.72, 5.39, 2.94, and 5.23 cm K mol are obtained for 1, 2,
, and 6, respectively, which is mostly attributed to the
progressive thermal depopulation of the excited-state Stark
= −7.15 K for 3 and C = 46.51 cm mol K and θ = −5.29 K
5
for 7, respectively (Figures 5−6S), the negative deviation of
χ T values starts to occur at slightly higher temperature (∼65
m
3
+
3+
34
3
−1
sublevels due to the crystal-field effects of Nd or Yb ions.
K) before reaching 21.52 and 26.22 cm K mol , respectively,
at 1.8 K. This is most likely due to the combination of
antiferromagnetic interactions, thermal depopulation of Stark
The χm− versus T data for complexes 1−2 and 5−6 in the
1
range 1.8−300 K obey the Curie−Weiss law, with Curie
3
−1
36
constant C = 4.89 cm mol K and Weiss constant θ = −8.59
sublevels, and the presence of significant magnetic anisotropy.
1
1384
dx.doi.org/10.1021/ic300918c | Inorg. Chem. 2012, 51, 11377−11386

Inorganic Chemistry
Article
CONCLUSIONS
the Petroleum Research Fund, administered by the American
Chemical Society (47014-AC5).
■
Through the self-assembly of the flexible hexadentate Salen-
type Schiff-base ligand H L with LnCl ·6H O or Ln-
2
3
2
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ASSOCIATED CONTENT
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2
*
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AUTHOR INFORMATION
■
(12) (a) Wong, W. K.; Liang, H. Z.; Wong, W. Y.; Cai, Z. W.; Li, K.
F.; Cheag, K. W. New J. Chem. 2002, 26, 275−278. (b) Lo, W. K.;
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Corresponding Author
*
E-mail: lvxq@nwu.edu.cn (X.L.), wkwong@hkbu.edu.hk (W.-
K.W.). Phone: 86-29-88302312(o) (X.L.), 862-34117348(o)
W.-K.W.).
(
Notes
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Chai, W. L.; Song, J. R.; Meng, J. X.; Wong, W. Y.; Wong, W. K.;
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, X. Q.; Bi, W. Y.;
The authors declare no competing financial interest.
̈
ACKNOWLEDGMENTS
■
This work is funded by the National Natural Science
Foundation (21173165, 20871098), the Program for New
Century Excellent Talents in University from the Ministry of
Education of China (NCET-10-0936), the research fund for the
Doctoral Program (20116101110003) of Higher Education of
China, the State Key Laboratory of Structure Chemistry
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