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Luminescent 4f and d-4f polynuclear complexes and coordination polymers with flexible salen-type ligands
Accepted Manuscript Title: Luminescent 4f and d-4f Polynuclear Complexes and Coordination Polymers with Flexible Salen-type Ligands Author: Xiaoping Yang Richard A. Jones Shaoming Huang PII: DOI: Reference:
S0010-8545(13)00263-4 http://dx.doi.org/doi:10.1016/j.ccr.2013.11.012 CCR 111800
To appear in:
Coordination Chemistry Reviews
Received date: Revised date: Accepted date:
4-9-2013 12-11-2013 12-11-2013
Please cite this article as:http://dx.doi.org/10.1016/j.ccr.2013.11.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Luminescent 4f and d-4f Polynuclear Complexes and Coordination Polymers with Flexible Salen-type Ligands
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, China. E-mail: [email protected]; [email protected]
Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University
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b
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a
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Xiaoping Yang,a,b* Richard A. Jonesb * and Shaoming Huanga *
Station A5300, Austin, Texas, 78712, USA. E-mail: [email protected]
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* Corresponding author.
Abstract: The synthesis, crystal structures and photophysical properties of thirty-one 4f and d-4f
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polynuclear complexes and coordination polymers based on nine flexible salen-type ligands are described in this review. Most of these lanthanide complexes exhibit either interesting “twisted”, drum-like or polymeric structures. In these complexes, the multidentate salen-type ligands can
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efficiently sensitize lanthanide emissions by serving as antennas that absorb excitation light and
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transfer the energy to the lanthanide centers. With the lanthanide ions encapsulated by chromophoric salen-type ligands and shielded from solvent molecules which can quench the
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emissions from lanthanide ions, those lanthanide complexes which have “twisted” and drum-like structures show impressive luminescence properties.
Keywords: 4f and d-4f complexes / flexible salen-type ligands / synthesis / crystal structures / luminescence properties /
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Content Overview 1. Introduction
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2. Luminescent Polynuclear 4f Complexes with Flexible Salen-type Ligands 2.1 Polynuclear 4f Complexes with H2salen ligand
2.2 Hexanuclear 4f Complexes with salen-type ligands bearing backbone hydroxyl groups
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3. Luminescent Polynuclear d-4f Complexes with Flexible Salen-type Ligands
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3.1 Polynuclear Zn/Ni/Cu-4f Salen Complexes 3.2 High-nuclearity Cd-4f Salen Complexes
4. Luminescent 4f and d-4f Coordination Polymers with Flexible Salen-type Ligands
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4.1 1-D 4f coordination polymers 4.2 1-D Ni-4f coordination polymers
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5. Conclusions 6. Acknowledgements
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7. References
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Overview The synthesis, structures and luminescence properties of thirty-one 4f and d-4f polynuclear complexes and coordination polymers which are formed from nine flexible salentype Schiff base ligands are described in this review.
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1. Introduction Lanthanide (4f) homometallic and d-block transition metal-lanthanide (d-4f) heterometallic
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polynuclear complexes are currently of great interest for the potential to develop new optical, electrical and magnetic materials [1,2]. Self-assembly by metal-ligand coordination is one of the
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most efficient processes that organize individual molecular components into higher nuclearity or polymeric species. However, compared to coordination frameworks of the d-block transition
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metals, the construction of polynuclear lanthanide complexes, as well as lanthanide coordination polymers, is more challenging. This may be due to the difficulty in controlling the coordination
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environment of Ln (III) ions which often display high and variable coordination numbers [3]. Many lanthanide systems reported have involved the use of polydentate rigid ligands, such as multicarboxylic acids, iminodiacetic acids, pyridinecarboxylate and carbonyl ligands [4,5].
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Flexible ligands may provide more potential for the construction of unique frameworks
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because of their freedom of conformation. For example, some flexible ligands featuring S, N or O atom donors have been employed in the design of d-block transition metal frameworks
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[6]. One of the best known multidentate ligands is the Schiff-base "H 2salen" (N,N'-ethylene bis(salicylideneimine), H2L1, Scheme 1). Tetradentate H2salen and salen derivatives are versatile ligands, and their lanthanide complexes can exhibit various structures [7-12]. This may due to the fact that flexible salen-type ligands can show different coordination modes with metal ions in the construction of supramolecular frameworks. Recent studies in our laboratories have focused on the construction of luminescent polynuclear lanthanide complexes with various Schiff base ligands. For example, it has been found that rigid conjugated Schiff base ligands with phenylene backbones, such as N,N'bis(3-methoxysalicylidene)phenylene-1,2-diamine, tended to form “multi-decker” 4f and 3d4f complexes [13-16], while the use of Schiff base ligands with flexible carbon-carbon backbones, for example, H2L1-9 (Scheme 1), resulted in various 4f and d-4f polynuclear complexes and coordination polymers (d = Ni 2+, Cu2+, Zn2+ and Cd2+) [17-27]. In this review,
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we describe the synthesis, crystal structures and luminescence properties of 24 polynuclear 4f and d-4f complexes, as well as 7 coordination polymers, based on flexible salen-type ligands H2L1-9. In these complexes, the flexible salen-type ligands not only stabilize lanthanide centers in the formation of polynuclear assemblies but also act as antennas that
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sensitize the luminescence of lanthanide ions.
H3CO
OCH3
R
R
H2L2: R = H; H2L3: R = OCH3
OH
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H2L5
OCH3
H3CO
H2L8 OCH3
OCH3
HO
N
HO
O
HO
OCH 3
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N
N
H2salen (H2L1)
Ac ce p N
H3CO
OH
d
H2L6
HO
HO
OH
H3CO
OCH 3
OCH3
N
N
N
HO
OH
OH
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H2L4
N
N
HO
OH
HO
N
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OH
N
N
N
N
cr
OH
N OH
N
OCH3
H2L7
H2L9
Scheme 1. Flexible salen-type Schiff base ligands H2L1-9.
2. Luminescent Polynuclear 4f Complexes with Flexible Salen-type Ligands Luminescent polynuclear lanthanide complexes are of considerable interest due to their potential use in materials science and as probes in biology [28]. For efficient lanthanide-induced emissions, chromophoric ligands are often employed to transfer absorbed energy efficiently to the lanthanide ions. The photophysical properties of Ln(III) ions also depend markedly on their
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coordination environments which can be designed to protect the metal center by chromophoric ligands (sensitizers) from solvent molecules (i.e. H2O and CH3OH) which can quench emissions [29,30]. The control over the stoichiometries and structures of polynuclear lanthanide complexes is synthetically challenging due to the difficulty in controlling the variable coordination
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environment of the ions. In this section, we describe the formation and properties of ten polynuclear lanthanide complexes (1-10) with H2salen (H2L1) and H2L2-3 ligands.
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2.1 Polynuclear 4f Complexes with the H2salen ligand
Complexes formed by Ln(III) with H2salen were reported as early as 1968 [31]. For
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example, some mononuclear, dinuclear and tetranuclear lanthanide complexes with formations of Ln/(ligand)1~3 [32], Ln2/(ligand)2~3 [33-35], and Ln4/(ligand)6 [36], (ligand = H2L1 or L1) have been reported.
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The structures of lanthanide salen complexes are often influenced by a variety of factors such as lanthanide ionic radius, the nature of counter ions and pH value of environment. Thus, [Yb6(L1)9(H2L1)2]
complexes 1
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the reactions of H2salen with Yb(CF3SO3)3, Yb(OAc)3·4H2O and Yb(NO3)3·6H2O resulted in [Yb3(L1)3(HL1)(OH)2]
(1),
1
(2)
and
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[Yb2(L )2(H2L )2(NO3)(MeOH)2]·NO3 (3), respectively [17]. To the best of our knowledge, the hexanuclear complex 1 is the highest nuclearity lanthanide complex with H2salen to date. A view
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of the crystal structure of 1 is shown in Figure 1, and reveals a centrosymmetric core with two equivalent Yb3L5 moieties linked by a salen ligand. The complex 1 has a “twisted” structure, in
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which no MeOH or H2O molecules are bound to the metal centers. Meanwhile, there are no CF3SO3- anions coordinated to the Yb3+ ions, probably due to steric factors.
Figure 1. A view of the crystal structure of 1.
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The complex 2 displays a trinuclear structure which is similar to the Yb3L5 moiety in 1, with two monodentate OH- anions replacing two coordinated salen ligands of 1 (Figure 2). The counter OAc- anions may play a key role in the formation of 2. Acetate can serve as a weak base in the reaction system to deprotonate H2O (introduced in Yb(OAc)3·4H2O), giving OH- ions
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which lead to the formation of 2. The Yb-Yb distances in 1 and 2 are approximately 3.7 Å.
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Figure 2. A view of the crystal structure of 2. The geometry of 3 is somewhat different from those of 1 and 2 in having a more flattened
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structure (Figure 3). In 3, two Yb3+ ions are bridged by one salen ligand with a separation of
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11.166 Å, significantly longer than the Yb-Yb distances in 1 and 2. The “twisted” structures of 1
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and 2 feature intramolecular ! -! stacking (3.870 to 3.999 Å) and C-H···! interactions (2.566 to 2.782 Å) between salen ligands, while these interactions are not found in the flattened structure of 3.
Figure 3. A view of the crystal structure of 3.
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Upon excitation of the ligand centered absorption band either at 275 or 330-360 nm complexes 1-3 show the typical NIR emission bands of Yb3+ assigned to the 2F5/2→2F7/2 transitions (Figure 4). The relative NIR emission intensity at 979 nm in CH3CN was estimated to be 5.1:1.8:1 for
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1:2:3, indicating that 1 has superior luminescence properties compared to 2 and 3. This may due to the fact that the coordinated CH3OH molecules and OH- anions in 2 and 3 can efficiently
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quench the luminescence of lanthanide ions through non-radiative exchange of electronic energy of Ln3+ to the high vibrational modes of OH-groups (ν = 3700 cm-1) [37,38]. Meanwhile, the
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influence of the structural differences in 1-3 on the photophysical properties was investigated by comparing their NIR emission intensities in CH3CN and CH3OH. All three complexes display weaker NIR emissions in CH3OH than in CH3CN. However, the intensities of 1 and 2 are
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reduced much less than that of 3, indicating that the “twisted” structures of complexes 1 and 2 may help to shield the metal centers from the outside solvent environment and improve their
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luminescent properties [17].
1.2
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0.8 0.6
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Intensity
1.0
1 2 3
d
1.4
0.4 0.2
0.0 800
900
1000
1100
1200
1300
! / nm
Figure 4. The NIR luminescence of complexes 1-3 in CH3CN.
The homoleptic complex [Tb4(L1)6] (4) was synthesized from the reaction of Tb(OAc)3·4H2O and H2salen [18]. A view of the crystal structure of 4 is shown in Figure 5, and reveals a tetranuclear centrosymmetric core with two equivalent Tb2L3 moieties linked by two μ-O phenoxide atoms. None of the MeOH or H2O molecules found in the structure are bound to a
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metal center. The molecular structure of 4 also features ! -! stacking (3.805 to 3.994 Å) between
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aryl groups of H2salen ligands.
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Figure 5. A view of the crystal structure of 4. The construction of 4 appears to be anion dependent, since the use of Tb(NO3)3·6H2O and
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TbCl3·6H2O under similar reaction conditions results in the formation of [Tb2(L1)2(μH2L1)(NO3)2(MeOH)2] (5) and [Tb2(L1)2(μ-H2L1)Cl2(MeOH)2] (6). As shown in Figure 6 and 7,
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the overall structures of 5 and 6 are similar. The common feature in both structures is the retention of one Cl- or NO3- per Tb3+ ion which is bound to the central N2O2 core of a salen
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ligand. Two Tb3+ centers are also bridged by a single neutral molecule of H2salen. In 5 and 6, one MeOH is directly coordinated to the Tb3+ ion.
Figure 6. A view of the crystal structure of 5.
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Figure 7. A view of the crystal structure of 6.
Excitation of the ligand centered absorption bands in complexes 4-6 result in typical visible emission bands for the Tb3+ ion (5D4 → 7Fn transitions, n = 6, 5, 4 and 3), while the ligand
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centered 1! -! * emissions were not detected (Figure 8). Compared with those in 5 and 6, the Tb3+ ions in 4 are completely encapsulated by the Schiff base ligands and protected from solvent
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molecules. As a result, 4 shows higher fluorescence quantum yield (Φem = 0.230) than 5 (0.127)
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Intensity
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and 6 (0.096) [18].
400
500
600 ! / nm
Em (Free ligand) Em (Tb complex)
700
800
Figure 8. Emission spectra of free H2L1 (---) and Tb(III) complex 4 (—).
In 1-6, the salen ligands exhibit four bonding modes with Yb3+ ions: (I) monodentate bonding to one Yb3+ ion; (II) tetradentate bonding to one Yb3+ ion; (III) bidentate bonding to two Yb3+ ions; (IV) pentadentate bonding to two Yb3+ ions (Scheme 2).
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OH
N
N
N
N Ln
O
OH
O
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Ln
Ln
Ln O
N
cr
HO N
N
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OH Ln
N
Ln
O Ln
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Scheme 2. Four bonding modes for H2salen with Ln3+ ions.
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2.2 Hexanuclear 4f Complexes with salen-type ligands bearing backbone hydroxyl groups The modification of a given salen-type ligand may affect the formation of polynuclear
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assemblies. To investigate the influence of the groups introduced in the H2salen ligand on the construction of coordination frameworks, N,N'-bis(salicylidene)(propylene-2-ol)-1,3-diamine
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(H2L2) and N,N'-bis(3-methoxysalicylidene)(propylene-2-ol)-1,3-diamine (H2L3) were used in the synthesis of lanthanide complexes. Differing from the H2salen ligand, H2L2 and H2L3 feature
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a backbone hydroxyl group. The introduction of the backbone hydroxyl groups in these two ligands adds one more coordination position than the H2salen ligand possesses, and may reduce the flexibility of the ligands when they bond to lanthanide ions. Two hexanuclear lanthanide complexes [Eu6(L2)4(OH)4(MeOH)2(EtOH)2(H2O)2]·2Cl (7) and [Er6(L2)4(OH)4(EtOH)2(H2O)2]·2Cl (8) were synthesized from reactions of H2L2 with LnCl3·6H2O [19]. Complexes 7 and 8 have similar hexanuclear structures with six lanthanide ions enclosed by four Schiff base ligands. A view of the cationic complex 7 is shown in Figure 9. The X-ray structure of 7 reveals a centrosymmetric core with two equivalent Eu3(L2)2 moieties linked by two μ3-OH- anions, in which two Eu3+ ions are coordinated with three Schiff base ligands, while one Eu3+ is coordinated with two Schiff base ligands. In 7, each μ3-OH- anion links three Eu3+ ions.
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Figure 9. A view of the crystal structure of 7.
Two hexanuclear lanthanide complexes [Ln6(L3)4(OH)4(MeOH)4]·2Cl (Ln = Nd (9), Tb (10))
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were obtained from reactions of H2L3 with LnCl3·6H2O [19]. Although H2L3 has two more methoxy groups than H2L2, complexes 9 and 10 have hexanuclear structures which are similar to
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those observed in 7 and 8. Complexes 9 and 10 are isomorphous and a view of the crystal structure of 9 is shown in Figure 10. It also reveals a centrosymmetric core with two equivalent Nd3(L2)2 moieties that are bridged by two μ3-OH- anions. For each Nd3L2 moiety, the Nd3+ ions
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have coordination environments similar to the Eu3+ ions in 7. One methoxy group of the Schiff
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base ligand bonds to one Nd3+ ion in the Nd3L2 moiety. The Schiff base ligands in 7-10 have a formal -3 charge resulting from the deprotonation of two phenolic hydroxyls and one backbone
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hydroxyl group.
Figure 10. A view of the crystal structure of 9.
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In CH3CN, excitation of the absorption bands in the free ligands H2L2 and H2L3 produce broad emission bands at ! max = 493 nm and 518 nm, respectively (Figures 11 and 12). For lanthanide complexes, excitations of the ligand centered absorption bands in 7 and 10 result in visible emission bands for the Eu3+ ion (5D0→7Fj transitions, j = 1, 2, 3 and 4) and Tb3+ ion (5D4
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→ 7Fn transitions, n = 6, 5, 4 and 3), respectively. Compound 9 shows typical NIR emission bands of Nd3+ assigned to the 4F3/2→4Ij/2 (j = 9, 11, 13) transitions upon excitation of the ligand
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centered absorption band (Figure 13). For complex 8, no NIR emission due to Er3+ ion was
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observed under the experimental conditions employed.
2
Ex: H2L Ex: 7
2
400
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300
d
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Intensity
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Em: H2L Em: 7
500 600 ! / nm
700
800
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Figure 11. Excitation spectra and visible emission spectra of the free ligand H2L2 (----- and blue line) and the Eu(III) complex 7 (-.-.-.- and red line) in CH3CN. Ex: H2L Ex: 10
Em: H2L Em: 10
Intensity
300
3
400
500
600
3
700
! / nm
Figure 12. Excitation spectra and visible emission spectra of the free ligand H2L3 (----- and blue line) and the Tb(III) complex 10 (-.-.-.- and red line) in CH3CN.
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cr
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Intensity
Em: 9
! / nm
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800 1000 1200 1400 1600
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Figure 13. The NIR luminescence of the Nd(III) complex 9 in CH3CN.
3. Luminescent Polynuclear d-4f Complexes with Flexible Salen-type Ligands The design and construction of polynuclear d-f clusters has received extensive attention due
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to the remarkable physical and chemical properties associated with this class of materials [39-44]. For example, research on polynuclear d-f complexes of Yb(III), Nd(III) and Er(III) with near-
d
infrared (NIR) emission in the 900-1600 nm range has become a hot topic due to potential applications in bioassays and laser systems [45]. Light-absorbing d-block metal chromophores
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(i.e. PtII [46,47], RuII [48], ZnII [49], and CdII [50,51]) have been used as sensitizers for NIR
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luminescence from Ln(III) centers following ligand→f and d→f energy-transfers. The Schiff base ligands described in this review have two dissimilar metal-binding sites, one with a higher affinity for the d metal ion and another for the f metal ion. A number of bimetallic 3d-4f Schiff base complexes have been prepared in order to study their magnetic properties in the past few years [52]. In this section, we describe the construction of 14 polynuclear d-f clusters with H2L4-9 ligands. The d-block metal ions introduced into these lanthanide complexes can play two different roles in the luminescence properties of Ln3+ ions. They may enhance the luminescence via d→f energy transfer, or they may quench the luminescence via f→d energy transfer. 3.1 Polynuclear Zn/Ni/Cu-4f Salen Complexes As shown in Scheme 1, H2L5 (N,N'-bis(5-bromo-3-methoxysalicylidene)propylene-1,3diamine) has one more methylene (-CH2-) group in its backbone compared to H2L4 (N,N׀-bis(3methoxysalicylidene)ethylene-1,2-diamine). In contrast to simple bi-metallic ZnLn salen
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complexes, use of these ligands resulted in the formation of a trinuclear complex [YbZn2(L4)2(OAc)2]·(OAc) (11) and tetrametallic [Zn2Yb2(L5)2(OH)2Cl4] (12) [20]. In 11, the Zn2+ ions are bound in the O2N2 cavities of each L4 group while the Yb3+ ion is bound by the
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and phenolic oxygen atoms, and the average Zn-Yb distance is 3.453 Å.
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outer O2O2 sets of both L4 groups (Figure 14). Yb3+ and Zn2+ ions are bridged by OAc- anions
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Figure 14. A view of the crystal structure of 11.
d
In 12, each Yb3+ ion is located in the O2O2 cavity of one L group. Two Yb3+ ions are bridged by two hydroxide anions, with a separation of 3.588 Å. The two central bridging
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hydroxides give the molecule an overall slipped sandwich configuration (Figure 15). A series of Cu-Ln complexes [LnCu2(L5)2(NO3)(H2O)2]·(NO3)2 (Ln = Ce, Pr, Nd and La) which show
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similar trinuclear structures as 11 have been reported by Cristóvão et al. [26], in order to study their magnetic properties. Both 11 and 12 show typical emission bands of Yb3+ assigned to the 2
F5/2→2F7/2 transition upon excitation of the ligand centered absorption bands (Figure 16).
Figure 15. A view of the crystal structure of 12.
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Intensity
Em: 12
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800 1000 1200 1400 1600 1800 ! / nm
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Figure 16. The NIR luminescence of 12 in CH3CN.
The nuclearity of 3d-4f Schiff base complexes can be manipulated by the introduction of
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different linkers which can bridge 3d-4f moieties. Thus, 1,4-benzenedicarboxylate (BDC) is a convenient bidentate linker used for the construction of polynuclear complexes. In our hands a
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hexanuclear Zn-Nd complex [Zn4Nd2(L5)4(1,4-BDC)2]·[Nd(NO3)5(H2O)] (13) was formed which contained two linking BDC groups [21]. As shown in Figure 17, two BDC groups bridge the
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Zn2Nd moieties such that each carboxylate group spans a Zn-Nd set. The 1H NMR spectrum of
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13 shows that this kind of bridged structure is stable in solution at room temperature (Figure 18). A Cu-Sm complex [Cu4Sm2(L5)4(fum)2]·(OH)2 which has a similar hexanuclear structure as 13 was constructed by using fumaric acids as linkers. [27].
Figure 17. A view of the crystal structure of 13.
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Figure 18. The 1H NMR spectrum of 13 in CD3OD at room temperature. The complex 13 shows typical NIR emission bands of NdIII (4F3/2→4Ij/2 transition, j = 9, 11,
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13) upon excitation of the ligand centered absorption bands both in the solid state and MeCN solution (Figure 19). It is noticeable that, with the central metal ion encapsulated by four
d
chromophoric ligands and shielded from solvent interactions, this complex shows superior luminescence properties compared to the related simple dinuclear Zn-Nd complex
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[ZnNdL5(OAc)(NO3)2] [21]. With the same absorbance value at the excitation wavelength (275 nm), the emission intensity at 1068 nm in 13 is 5.5 times as strong as that in
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[ZnNdL5(OAc)(NO3)2].
Intensity
Em: 13
800 1000 1200 1400 1600 1800 ! / nm
Figure 19. The NIR luminescence of 13 in CH3CN.
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Schiff base ligands with four methylene (CH2) units in the bridge, such as H2L6 (H2L = N,N'-bis(3-methoxysalicylidene)butane-1,4-diamine) have also been employed to prepare high nuclearity
Ln
complexes.
Two
hexanuclear
3d-4f
Ni-Eu
and
Cu-Eu
complexes
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[Eu4Ni2(L6)2(OAc)12(EtOH)2] (14) and [Eu4Cu2(L6)2(OAc)12]·2H2O (15) were formed from the reactions of H2L6 with M(OAc)2·4H2O (M = Ni(II) and Cu(II)) and Eu(OAc)3·4H2O [22]. As
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shown in Figures 20 and 21, complexes 14 and 15 have similar hexanuclear structures, containing two equivalent NiEu2L and CuEu2L moieties, respectively. In the MEu2L (M = Ni
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and Cu) moiety, the 3d metal ion and Eu(1) are bridged by two bidentate OAc- anions in addition to the phenolic oxygen atoms of the Schiff base ligand. Ni(1)-Eu(1) and Cu(1)-Eu(1) distances
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d
M
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are 3.346 Å and 3.467 Å, respectively.
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Figure 20. A view of the crystal structure of 14.
Figure 21. A view of the crystal structure of 15.
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The OAc- anions introduced form the metal salts act as linking groups in these hexanuclear structures and clearly play a key role in the formation of 14 and 15. In contrast, two more simple binuclear complexes [EuNiL6Cl3(H2O)4] and [EuCuL6(NO3)3] were formed from the reactions of
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no OAc- anions were present in the reaction mixtures (Figures 22 and 23).
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H2L6 with NiCl2·6H2O/EuCl3·6H2O and Cu(NO3)2·3H2O/Eu(NO3)3·6H2O, respectively, in which
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d
Figure 22. A view of the crystal structure of binuclear complex [EuNiL6Cl3(H2O)4].
Figure 23. A view of the crystal structure of binuclear complex [EuCuL6(NO3)3].
The photophysical properties of 14 and 15 were studied in CD3OD (Figure 24). The free ligand H2L6 shows two broad emission bands at ! max = 319 and 496 nm upon excitation of the absorption band at 275 nm. Both 14 and 15 show weak emission bands of the Eu3+ ion (5D0→7Fj transitions, j = 0, 1, 2, 3 and 4) upon excitation of the ligand-centered absorption bands in CD3OD. The quantum yields of 14 and 15 in CD3OD are 0.0011 and <10-4, respectively, which
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are more than ten times less than that of the polymeric complex [Eu2(H2L6)(OAc)6]n (Φem = 0.015) that does not bear 3d metal ions [22]. For both 14 and 15, the weak luminescence of Eu3+ ions maybe due to lanthanide to transition metal (4f→3d) energy transfer which can efficiently
6
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Intensity
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H2L 14 H2L 14
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quench lanthanide ion emission (Scheme 3).
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300 400 500 600 700 800 ! / nm
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d
Figure 24. Excitation and emission spectra of free H2L6 and Eu(III) complex 14.
E, cm-1/1000
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20
5
D0
15
d ET
10
5
0
7F
6 5 4 3 2 1 0
d
Eu(III)
Ni2+/Cu2+
Scheme 3. Lanthanide to transition metal (4f→3d) energy transfer.
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As noted previously the ligand H2L3 has an additional backbone hydroxyl (-OH) group compared to H2L5. The trinuclear complex [NdCu2(L3)2Cl2(H2O)2]·Cl (16) was formed from the reaction of H2L3 with Cu(OAc)2·4H2O and NdCl3·6H2O. As shown in Figure 25, the Nd3+ ion is
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groups of two L3 ligands do not bond to the metal ions in 16.
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centered between two CuL moieties. Unlike those in complexes 9 and 10, the backbone hydroxyl
M
Figure 25. A view of the crystal structure of 16. Replacing Cu(OAc)2·4H2O with Ni(OAc)2·4H2O in above reaction produces the simple
Ac ce p
te
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binuclear complex [NdNiL3Cl2(H2O)3(MeOH)2] (Figure 26).
Figure 26. A view of the crystal structure of [NdNiL3Cl2(H2O)2(MeOH)2].
Another variation in the Schiff base ligand is to introduce ether linkages into the backbone as in H2L7 (6,6'-((1E,11E)5,8-dioxa-2,11-diazadodea-1,11-diene-1,12-diyl)bis(2-methoxyphenol)) which has two oxy (-O-) groups in its backbone and is even more flexible than H2L1-6. The heptanuclear complex [Ni3Nd4(L7)3(NO3)3(OAc)6(OH)3] (17) was synthesized from the reaction of H2L7 with Ni(OAc)2·4H2O and Nd(NO3)3·6H2O. As shown in Figure 27, three Ni2+ and four
20
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Nd3+ ions are surrounded by three Schiff base ligands. In 17, the two oxy (-O-) groups of each
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an
us
cr
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ligand do not bind to the metal ions.
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Figure 27. A view of the crystal structure of 17.
3.2 High-nuclearity Cd-4f Salen Complexes
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As with Ln(III) ions, the Cd(II) ion can have high coordination numbers (6-8) and display various coordination geometries, and high-nuclearity Cd-Ln clusters are possible. Two flexible
Ac ce p
salen-type ligands H2L8 (N,N'-bis(3-methoxysalicylidene)hexane-1,6-diamine) and H2L9 (6,6'((1E,1'E)-(dodecane-1,12-diylbis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol)), which have six and twelve methylene (-CH2-) groups respectively in their backbones, were employed to construct high nuclearity Cd-4f complexes. Reactions of H2L8 with Cd(OAc)2·4H2O and Ln(OAc)3·4H2O produced a series of Cd-Ln heterometallic clusters [Ln8Cd24(L8)12(OAc)48] (Ln = Nd (18), Er (19) and Yb (20)) [23]. These complexes are isomorphous, and possess interesting 32-metal drum-like structures. Two views of the crystal structure of 18 are shown in Figure 28. The top view is essentially a side-on view while the lower one is looking down into the top of the drum. The complex is of nanoscale proportions (19 × 26 × 26 Å). The ends of the drum are created by two rings of 16 metals (4 Nd(III) and 12 Cd(II)). The sides of the drum are formed by the -(CH2)6- linkers of twelve Schiff base ligands. Thus each Schiff base ligand is coordinated to metals at both ends of the drum.
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Figure 28. A view of the crystal structure of 18. Viewed along the a-axis (top) and c-axis (lower). (Nd3+: green; Cd3+: brown; unit cell: a = 54.7766 Ǻ, b = 62.2436 Ǻ, c = 79.9350 Ǻ, α = β = γ = 90º).
The self-assembly process of the drum-like structures appears to be anion dependant. Thus, if Cl- anions are introduced into the reactions with the use of LnCl3·6H2O, smaller 24-metal drum-like complexes [Nd6Cd18(L8)9Cl8(OAc)28] (21) and [Ln6Cd18(L8)9Cl10(OAc)26] (Ln = Er (22) and Yb (23)) are produced [23]. Complexes 21-23 show similar 24-metal drum-like structures. As shown in Figure 29, the drum-like architecture of 21 is formed by two 12-metal rings (Er3Cd9Cl4(OAc)14) linked by nine Schiff base ligands. The molecular dimensions of 21 (16 × 21 × 21 Å) are smaller than those of 18.
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Figure 29. A view of the crystal structure of 21. Viewed along the a-axis (top) and c-axis (lower). (Nd3+: green; Cd3+: brown)
Cd(II) chromophores with visible emissions can be used to sensitize the luminescence of Ln(III) ions [23,24]. The luminescence properties of these complexes were studied in CH3CN. Upon excitation of the ligand-centered absorption bands, complexes 18 and 21 show the NIR luminescence of Nd3+ (4F3/2→4Ij/2 transitions, j = 9, 11 and 13)), and complexes 20 and 23 show that of Yb3+ (2F5/2→2F7/2 transition) (Figure 30). For Er(III) complexes 19 and 22 no obvious or very weak NIR emission attributable to the Er3+ ion was observed.
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Page 23 of 34
200
400
600
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Intensity
Ex - 21 Ex - 23 Em -21 Em -23
800 1000 1200 1400
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! / nm
Figure 30. Excitation and emission spectra of complexes 21 and 23 in CH3CN.
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The differences in NIR luminescence properties of these complexes may be explained by considering the energy levels of the various lanthanide ions. As shown in Scheme 4, in Nd(III) 4
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complexes, the Nd3+ ion has many possible energy-accepting levels lying above the emissive F3/2 state. In Yb(III) complexes, although the Yb3+ ion has only a single excited state 2F5/2, the
d
efficient energy-transfer to Yb3+ ions in these Yb(III) complexes may take place via an electron transfer mechanism and/or phonon-assisted energy-transfer mechanisms [53,54]. For Er(III)
te
complexes, the lanthanide-induced NIR emission may be quenched by C-H vibrations from the
Ac ce p
ligand.
E, cm-1/1000
20
Cd/L excited states a
25
4G
ET
15
10
2H
11/2 4S 3/2
7/2
4
G 5/2
2
H 11/2 4 F9/2 4 S3/2 , 4 F7/2
4F
4
2
H 9/2 , 4F 5/2 4F 3/2
I 9/2
2
F5/2
4I
4I 4I
5
9/2
11/2
v=2 13/2
15/2
v=1
4
I13/2
4I
11/2
4I
0 Cd/L-center
v=3
2F 7/2
9/2
Nd(III)
Yb(III)
4I
15/2
Er(III)
C-H
Scheme 4. The energy levels in lanthanide complexes 18-23.
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Page 24 of 34
A 32-metal complex [Nd8Cd24(L9)12(OAc)48] (24) was prepared using the more flexible ligand H2L9 which contains 12 bridging methylene (CH2) units. As shown in Figure 31, the complex 24 has a drum-like structure similar to those found in complexes 18-20. The molecular
Ac ce p
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an
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size of 24 is 32 × 26 × 26 Å, which is longer than that of the Nd(III) complex 18.
Figure 31. A view of the crystal structure of 24. Viewed along the a-axis (top) and c-axis (lower). (unit cell: a = 56.3430 Ǻ, b = 61.7861 Ǻ, c = 108.7203 Ǻ, α = β = γ = 90º).
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Page 25 of 34
From the structures of these drum-like architectures, we can conclude that the key to the construction of these 24- and 32-metal Ln/Cd complexes is that both Ln(III) and Cd(II) ions have the potential for high coordination numbers (6-8) with variable coordination geometries. The adaptability of flexible ligands enables them to form different molecular systems and extended
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network materials.
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4. Luminescent 4f and 3d-4f Coordination Polymers with Flexible Salen-type Ligands
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Compared with the growth of studies on 3d metal-organic-framework (MOF) materials, reports of 4f or 3d-4f coordination polymers have been far fewer [55-58]. Lanthanide ions behave as hard acids and they prefer oxygen to nitrogen donors, while d-block metal ions can
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coordinate to both N and O donors. One approach for the construction 4f and 3d-4f frameworks is self-assembly of selected metal ions using ligands containing a mixed-donor atom set. Flexible
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salen type ligands, in combination with anions such as acetate (OAc-) acting as a bridging ligand
4.1 1-D 4f coordination polymers
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between metal centers, results in the formation of 1-D coordination polymers.
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Although the first X-ray crystal structure of a lanthanide coordination polymer, from the reaction of the neutral H2salen ligand (H2L1) with La(NO3)3 hydrate, was reported by Xie in type
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1999, there are relatively few reports of lanthanide coordination polymers which employ salen Schiff-base
ligands
[59-61].
Three
lanthanide
1-D
coordination
polymers
{[Ln2(L1)2(CF3SO3)(H2L1)4(MeOH)]·CF3SO3}n (Ln = Eu (25), Nd (26) and Er (27)) were prepared from the reactions of H2L1 and Ln(CF3SO3)3 [24]. As shown in Figure 32, for 25 Eu(1) and Eu(2) are bridged by a neutral salen ligand with a separation of 10.171 Å. The 1-D polymeric structure is formed by a zig-zag chain of alternating Eu3+ ions and H2L1 ligands which bridge as neutral ligands between the metal centers.
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Figure 32. A view of the crystal structure of 25.
The formation of 1-D lanthanide coordination polymers from H2salen ligand can be affected
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by the anions present in the reactions. Thus, the reaction of Yb(OAc)3·4H2O with H2L1 produced a 1-D polymeric material [Yb2(L1)2(OAc)2(MeOH)2]n (28), in which the 1-D polymeric structure is formed by the linking of OAc- anions. As shown in Figure 33, in the central Yb2 unit, each
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Yb3+ ion is bound by the N2O2 cavity of a deprotonated salen ligand. These Yb2 units are bridged
Ac ce p
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by OAc- anions, forming the 1-D coordination polymeric structure.
Figure 33. Acetate (OAc-) units bridging a Yb2 unit giving a 1-D polymer structure in 28. The 1-D polymeric complex {[Tb3(L4)2(OAc)5]·Et2O·(MeOH)0.5}n (29) was obtained from the reaction of Tb(OAc)3·4H2O and H2L4 [24]. As shown in Figure 34, the central Tb3 unit contains
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Page 27 of 34
two (L4)2- groups bound to the three Tb3+ ions. The 1-D polymeric framework is completed by
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OAc- units which link Tb3 units.
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Figure 34. Acetate (OAc-) units bridging a Tb3 unit giving a 1-D polymer structure in 29. A lanthanide 1-D coordination polymer ({[Eu2(H2L6)(OAc)6]·EtOH·2H2O}n (30) was prepared from the reaction of H2L6 with Eu(OAc)3·4H2O [25]. As shown in Figure 35, two Eu
d
atoms are bridged by four OAc- anions with a Eu-Eu separation of 3.985 Å, forming an Eu2 unit.
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The 1-D polymeric framework is completed by neutral Schiff base ligands (H2L6) which link the
Ac ce p
Eu2 units via phenolic and methoxy oxygen atoms.
Figure 35. A view of the crystal structure of 30.
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The photophysical properties of the Eu(III) complexes 25 and 30, and Tb(III) complex 29 were studied in solution (Figures 36 and 37). Upon excitations of ligand-centered absorption bands, complexes 25, 30 and 29 show visible emission bands of Eu3+ ion (5D0→7Fj transitions, j = 1, 2, 3 and 4) and Tb3+ ion (5D4 → 7Fn transitions, n = 6, 5, 4 and 3), respectively. For the
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Eu(III) complexes, the fluorescence quantum yield of 30 (0.015) is higher than that of 25 (less
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than 10-3).
200
300
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an
Intensity
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Ex Em
400
500
600
700
800
d
! / nm
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Figure 36. Excitation and emission spectra of 29 in CH3CN.
Intensity
Ex Em
200
300
400
500
600
700
800
! / nm
Figure 37. Excitation and emission spectra of 30 in CH3CN.
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4.2 1-D Ni-4f coordination polymers The reaction of H2L6 with YbCl3·6H2O and Ni(OAc)3·4H2O in refluxing MeCN/EtOH produced the 1-D polymeric material {YbNiL6Cl(OAc)2(H2O)}n (31) [25]. As shown in Figure 38, the Ni2+ and Yb3+ ions in 31 are located in the inner N2O2 and O2O2 cavities of the Schiff
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base ligand, respectively. Within each Yb-Ni-L moiety the Ni2+ and Yb3+ ions are bridged by a single bidentate OAc- group in addition to the phenolic oxygen atoms of the Schiff base ligand.
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The Ni2+ ion has bi-pyramidal geometry with one chlorine anion and one oxygen atom from the bidentate OAc- anion occupying the axial positions. The 1-D polymeric framework is formed by
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OAc- units which link the Yb-Ni-L6 moieties (Figure 38).
Ac ce p
Figure 38. A view of the crystal structure of 31. It appears that the use of Ni(OAc)2·4H2O for the preparation of 31 is critical to the formation of the 1-D polymeric complex, since the acetate (OAc –) is employed as a linker in the formation of 1-D polymeric structure. In contrast, the reaction of H2L6 with YbCl3·6H2O and NiCl2·6H2O under similar conditions produced a simple heterobinuclear complex [YbNiL6Cl3(H2O)3] [25]. For 31, upon excitation of ligand-centered absorption bands, emission in near-infrared (NIR) range due to the Yb3+ ion was not observed.
5. Conclusions Nine flexible salen-type Schiff base ligands have been used in the synthesis of 4f and d-4f polynuclear complexes and coordination polymers. The stoichiometry and structures of these
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complexes are dependent on the Schiff base ligands employed in their syntheses as well as lanthanide ionic radius, counter ions and reaction conditions. These multi-metallic lanthanide complexes have interesting “twisted”, drum-like, or polymeric structures. Of the complexes described in this review, one hexanuclear Zn-Nd assembly has been described which is formed
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by the use of two different ligands (one a Schiff base ligand and the other a bridging ligand 1,4BDC).
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In these lanthanide complexes, the flexible salen-type ligands can stabilize LnIII centers and act as antenna sensors for lanthanide luminescence. In d-4f complexes, Zn2+ and Cd2+ may
us
enhance the luminescence via d→f energy transfer, while in contrast, Cu2+ and Ni2+ may quench the luminescence via f→d energy transfer. With the Ln3+ centers protected by the Schiff base ligands from solvent and water molecules, those lanthanide complexes with “twisted” and drum-
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like structures show impressive luminescence properties.
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6. Acknowledgements
We thank the Welch Foundation (Grant F-816, RAJ) and the National Natural Science
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Foundation of China (No. 51025207, SMH) for financial support. Single crystal X-ray data were
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collected using instrumentation purchased with funds provided by the National Science
Ac ce p
Foundation (CHE-0741973), USA.
7. References
[1] P. Zanello, S. Tamburini, P. A. Vigato, G. A. Mazzocchin, Coord. Chem. Rev. 77 (1987) 165-273.
[2] A. Müller, Nature 447 (2007) 1034-1035. [3] C. Piguet, J.-C. G. Bünzli, Chem. Soc. Rev. 28 (1999) 347-358. [4] N. Sabbatini, M. Guardigli, J. M. Lehn, Coord. Chem. Rev. 123 (1993) 201-228. [5] P. Guerriero, S. Tamburini, P. A. Vigato, Coord. Chem. Rev. 139 (1995) 17-243. [6] A. Y. Robin, K. M. Fromm, Coord. Chem. Rev. 250 (2006) 2127-2157. [7] M. Sakamoto, K. Manseki, H. Okawa, Coord. Chem. Rev. 219 (2001) 379-414. [8] R. E. P. Winpenny, Chem. Soc. Rev. 27 (1998) 447-452.
31
Page 31 of 34
[9] C. Camp, V. Guidal, B. Biswas, J. Pecaut, L. Dubois, M. Mazzanti, Chem. Sci. 3 (2012) 2433-2438. [10] W. J. Evans, C. H. Jujimoto, J. W. Ziller, Chem. Commun. 35 (1999) 311-313. Mallah, Chem. Commun. 48 (2012) 9071-9073. [12] L. Zhao, S. Xue, J. Tang, Inorg. Chem. 51 (2012) 5994-5996.
cr
[13] X.-P. Yang, R. A. Jones, J. Am. Chem. Soc. 127 (2005) 7686-7687.
ip t
[11] G. Magadur, F. Bouanis, E. Norman, R. Guillot, J.-S. Lauret, V. Huc, C.-S. Cojocaru, T.
[14] X.-P. Yang, R. A. Jones, W.-K. Wong, Dalton Trans. (2008) 1676-1678.
us
[15] X.-P. Yang, R. A. Jones, M. M. Oye, A. Holmes, W.-K. Wong, Crystal Growth & Design 6 (2006) 2122-2125.
[16] W.-K. Wong, X.-P. Yang, R. A. Jones, J. H. Rivers, V. Lynch, W.-K. Lo, D. Xiao, M. M.
an
Oye, A. L. Holmes, Inorg. Chem. 45 (2006) 4340-4345.
[17] X.-P. Yang, M. O. Michael, R. A. Jones, S.-M. Huang, Chem. Commun. 49 (2013) 9579-
M
9581.
[18] X.-P. Yang, R. A. Jones, W.-K. Wong, Chem. Commun. (2008) 3266-3268.
d
[19] S. Liao, X.-P. Yang, R. A. Jones, Crystal Growth & Design 12 (2012) 970-974. [20] X.-P. Yang, R. A. Jones, V. Lynch, M. M. Oye, A. Holmes, Dalton Trans. 5 (2005) 849-851. (2006) 1836-1838.
te
[21] X.-P. Yang, R. A. Jones, W.-K. Wong, V. Lynch, M. M. Oye, A. Holmes, Chem. Commun.
Ac ce p
[22] X.-P. Yang, C. Chan, D. Lam, D. Schipper, J. M. Stanley, X. Chen, R. A. Jones, B. J. Holliday, W.-K. Wong, S. Chen, Q. Chen, Dalton Trans. 41 (2012) 11449-11453. [23] X.-P. Yang, D. Schipper, R. A. Jones, L. A. Lytwak, B. J. Holliday, S.-M. Huang, J. Am. Chem. Soc. 135 (2013) 8468-8471. [24] X.-P. Yang, R. A. Jones, J. Rivers, W.-K. Wong, Dalton Trans. 47 (2009) 10505-10510. [25] X.-P. Yang, D. Lam, C. Chan, J. M. Stanley, R. A. Jones, B. J. Holliday, W.-K. Wong, Dalton Trans. 40 (2011) 9795-9801. [26] B. Cristóvão, B. Miroslaw, J. Klak, Polyhedron 34 (2012) 121-127. [27] R. Gheorghe, P. Cucos, M. Andruh, J.-P. Costes, B. Donnadieu, S. Shova, Chem. Eur. J. 12 (2006) 187-183. [28] C. Piguet, Chimia 50 (1996) 144-153. [29] F. S. Richardson, Chem. Rev. 82 (1982) 541-552.
32
Page 32 of 34
[30] S. Yanagida, Y. Hasegawa, K. Murakoshi, Y. Wada, N. Nakashima, T. Yamanaka, Coord. Chem. Rev. 171 (1998) 461-480. [31] N. K. Dutt, K. Nag, J. Inorg. Nucl. Chem. 30 (1968) 2493-2496. [32] J. I. Bullock, H. A. Tajmir-Rianhi, Dalton Trans. (1978) 36-39. (2011) 10993-10995.
cr
[34] H. Chen, R. D. Archer, Inorg. Chem. 33 (1994) 5195-5202.
ip t
[33] P.-H. Lin, W.-B. Sun, M.-F. Yu, G.-M. Li, P.-F. Yan, M. Murugesu, Chem. Commun. 47
[35] R. D. Archer, H. Chen, L. C. Thompson, Inorg. Chem. 37 (1998) 2089-2095.
us
[36] B. H. Koo, K. S. Lim, D. W. Ryu, W. R. Lee, E. K. Koh, C. S. Hong, Chem. Commun. 48 (2012) 2519-2521.
[37] F. Artizzu, M. L. Mercuri, A. Serpe, P. Deplano, Coord. Chem. Rev. 255 (2011) 2514-2529.
an
[38] Y. Wada, T. Okubo, M. Ryo, T. Nakazawa, Y. Hasegawa, S. Yanagida, J. Am. Chem. Soc. 122 (2000) 8583-8584.
M
[39] A. Müller, P. Kögerler, A. W. M. Dress, Coord. Chem. Rev. 222 (2001) 193-218. [40] X.-J. Kong, Y.-P. Ren, W.-X. Chen, L.-S. Long, Z. Zheng, R.-B. Huang, L.-S. Zheng,
d
Angew. Chem., Int. Ed. 47 (2008) 2398-2401.
[41] E. G. Mednikov, M. C. Jewell, L. F. Dahl, J. Am. Chem. Soc. 129 (2007) 11619-11630. 37 (1998) 3359-3363.
te
[42] A. Müller, E. Krickemeyer, H. Bögge, M. Schmidtmann, F. Peters, Angew. Chem., Int. Ed.
Ac ce p
[43] D. Fenske, C. Persau, S. Dehnen, C. E. Anson, Angew. Chem., Int. Ed. 43 (2004) 305-309. [44] D. Fenske, C. E. Anson, A. Eichhöfer, O. Fuhr, A. Ingendoh, C. Persau, C. Richert, Angew. Chem., Int. Ed. 44 (2005) 5242-5246. [45] J. W. Stouwdam, G. A. Hebbink, J. Huskens, F. C. J. M. Van Veggel, Chem. Mater. 15 (2003) 4604-4616.
[46] X.-L. Li, L.-X. Shi, L.-Y. Zhang, H.-M. Wen, Z.-N. Chen, Inorg. Chem. 46 (2007) 1089210900.
[47] H.-B. Xu, L.-X. Shi, E. Ma, L.-Y. Zhang, Q.-H. Wei, Z.-N. Chen, Chem. Commun. (2006) 1601-1603. [48] S. G. Baca, H. Adams, C. S. Grange, A. P. Smith, I. Sazanovich, M. D. Ward, Inorg. Chem. 46 (2007) 9779-9789.
33
Page 33 of 34
[49] J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini, M. Bettinelli, J. Appl. Phys. 93 (2003) 9460-9465. [50] Y.-X. Chi, S.-Y. Niu, J. Jin, R. Wang, Y. Li, Dalton Trans. 47 (2009) 7653-7659. [52] J-P Cortes, F. Dahan, A. Dupuis, Inorg. Chem. 39 (2000) 165-168.
ip t
[51] Y.-X. Chi, S.-Y. Niu, Z.-L.Wang, J. Jin, Eur. J. Inorg. Chem. (2008) 2336-2343. [53] W. D. Horrocks, J. P. Bolender, W. D. Smith, R. M. Supkowski, J. Am. Chem. Soc. 119 [54] C. Reinhard, H. U. Gudel, Inorg. Chem. 41 (2002) 1048-1055.
cr
(1997) 5972-5973.
us
[55] T. K. Prasad, M. V. Rajasekharan, J. P. Costes, Angew. Chem., Int. Ed. 46 (2007) 28512855.
[56] J. W. Cheng, J. Zhang, S. T. Zheng, M. B. Zhang, G. Y. Yang, Angew. Chem., Int. Ed. 45
an
(2006) 73-76.
[57] C. E. Plecnik, S. M. Liu, X. N. Chen, E. A. Meyers, S. G. Shore, J. Am. Chem. Soc. 126
M
(2004) 204-207.
[58] X. Q. Zhao, B. Zhao, Y. Ma, W. Shi, P. Cheng, Z. H. Jiang, D. Z. Liao, S. P. Yan, Inorg.
d
Chem. 46 (2007) 5832-5834.
[59] W. Xie, M. J. Heeg, P. G. Wang, Inorg. Chem. 38 (1999) 2541-2543.
te
[60] M. P. Hogerheide, J. Boersma, G. V. Konten, Coord. Chem. Rev. 155 (1996) 87-126.
Ac ce p
[61] D. E. Featon, P. A. Vigato, Chem. Soc. Rev. 17 (1988) 69-90.
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S0010-8545(13)00263-4 http://dx.doi.org/doi:10.1016/j.ccr.2013.11.012 CCR 111800
To appear in:
Coordination Chemistry Reviews
Received date: Revised date: Accepted date:
4-9-2013 12-11-2013 12-11-2013
Please cite this article as:
Luminescent 4f and d-4f Polynuclear Complexes and Coordination Polymers with Flexible Salen-type Ligands
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, China. E-mail: [email protected]; [email protected]
Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University
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b
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a
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Xiaoping Yang,a,b* Richard A. Jonesb * and Shaoming Huanga *
Station A5300, Austin, Texas, 78712, USA. E-mail: [email protected]
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* Corresponding author.
Abstract: The synthesis, crystal structures and photophysical properties of thirty-one 4f and d-4f
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polynuclear complexes and coordination polymers based on nine flexible salen-type ligands are described in this review. Most of these lanthanide complexes exhibit either interesting “twisted”, drum-like or polymeric structures. In these complexes, the multidentate salen-type ligands can
d
efficiently sensitize lanthanide emissions by serving as antennas that absorb excitation light and
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transfer the energy to the lanthanide centers. With the lanthanide ions encapsulated by chromophoric salen-type ligands and shielded from solvent molecules which can quench the
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emissions from lanthanide ions, those lanthanide complexes which have “twisted” and drum-like structures show impressive luminescence properties.
Keywords: 4f and d-4f complexes / flexible salen-type ligands / synthesis / crystal structures / luminescence properties /
1
Page 1 of 34
Content Overview 1. Introduction
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2. Luminescent Polynuclear 4f Complexes with Flexible Salen-type Ligands 2.1 Polynuclear 4f Complexes with H2salen ligand
2.2 Hexanuclear 4f Complexes with salen-type ligands bearing backbone hydroxyl groups
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3. Luminescent Polynuclear d-4f Complexes with Flexible Salen-type Ligands
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3.1 Polynuclear Zn/Ni/Cu-4f Salen Complexes 3.2 High-nuclearity Cd-4f Salen Complexes
4. Luminescent 4f and d-4f Coordination Polymers with Flexible Salen-type Ligands
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4.1 1-D 4f coordination polymers 4.2 1-D Ni-4f coordination polymers
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5. Conclusions 6. Acknowledgements
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7. References
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Overview The synthesis, structures and luminescence properties of thirty-one 4f and d-4f polynuclear complexes and coordination polymers which are formed from nine flexible salentype Schiff base ligands are described in this review.
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1. Introduction Lanthanide (4f) homometallic and d-block transition metal-lanthanide (d-4f) heterometallic
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polynuclear complexes are currently of great interest for the potential to develop new optical, electrical and magnetic materials [1,2]. Self-assembly by metal-ligand coordination is one of the
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most efficient processes that organize individual molecular components into higher nuclearity or polymeric species. However, compared to coordination frameworks of the d-block transition
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metals, the construction of polynuclear lanthanide complexes, as well as lanthanide coordination polymers, is more challenging. This may be due to the difficulty in controlling the coordination
M
environment of Ln (III) ions which often display high and variable coordination numbers [3]. Many lanthanide systems reported have involved the use of polydentate rigid ligands, such as multicarboxylic acids, iminodiacetic acids, pyridinecarboxylate and carbonyl ligands [4,5].
d
Flexible ligands may provide more potential for the construction of unique frameworks
te
because of their freedom of conformation. For example, some flexible ligands featuring S, N or O atom donors have been employed in the design of d-block transition metal frameworks
Ac ce p
[6]. One of the best known multidentate ligands is the Schiff-base "H 2salen" (N,N'-ethylene bis(salicylideneimine), H2L1, Scheme 1). Tetradentate H2salen and salen derivatives are versatile ligands, and their lanthanide complexes can exhibit various structures [7-12]. This may due to the fact that flexible salen-type ligands can show different coordination modes with metal ions in the construction of supramolecular frameworks. Recent studies in our laboratories have focused on the construction of luminescent polynuclear lanthanide complexes with various Schiff base ligands. For example, it has been found that rigid conjugated Schiff base ligands with phenylene backbones, such as N,N'bis(3-methoxysalicylidene)phenylene-1,2-diamine, tended to form “multi-decker” 4f and 3d4f complexes [13-16], while the use of Schiff base ligands with flexible carbon-carbon backbones, for example, H2L1-9 (Scheme 1), resulted in various 4f and d-4f polynuclear complexes and coordination polymers (d = Ni 2+, Cu2+, Zn2+ and Cd2+) [17-27]. In this review,
3
Page 3 of 34
we describe the synthesis, crystal structures and luminescence properties of 24 polynuclear 4f and d-4f complexes, as well as 7 coordination polymers, based on flexible salen-type ligands H2L1-9. In these complexes, the flexible salen-type ligands not only stabilize lanthanide centers in the formation of polynuclear assemblies but also act as antennas that
ip t
sensitize the luminescence of lanthanide ions.
H3CO
OCH3
R
R
H2L2: R = H; H2L3: R = OCH3
OH
M
H2L5
OCH3
H3CO
H2L8 OCH3
OCH3
HO
N
HO
O
HO
OCH 3
te O
N
N
H2salen (H2L1)
Ac ce p N
H3CO
OH
d
H2L6
HO
HO
OH
H3CO
OCH 3
OCH3
N
N
N
HO
OH
OH
an
H2L4
N
N
HO
OH
HO
N
us
OH
N
N
N
N
cr
OH
N OH
N
OCH3
H2L7
H2L9
Scheme 1. Flexible salen-type Schiff base ligands H2L1-9.
2. Luminescent Polynuclear 4f Complexes with Flexible Salen-type Ligands Luminescent polynuclear lanthanide complexes are of considerable interest due to their potential use in materials science and as probes in biology [28]. For efficient lanthanide-induced emissions, chromophoric ligands are often employed to transfer absorbed energy efficiently to the lanthanide ions. The photophysical properties of Ln(III) ions also depend markedly on their
4
Page 4 of 34
coordination environments which can be designed to protect the metal center by chromophoric ligands (sensitizers) from solvent molecules (i.e. H2O and CH3OH) which can quench emissions [29,30]. The control over the stoichiometries and structures of polynuclear lanthanide complexes is synthetically challenging due to the difficulty in controlling the variable coordination
ip t
environment of the ions. In this section, we describe the formation and properties of ten polynuclear lanthanide complexes (1-10) with H2salen (H2L1) and H2L2-3 ligands.
cr
2.1 Polynuclear 4f Complexes with the H2salen ligand
Complexes formed by Ln(III) with H2salen were reported as early as 1968 [31]. For
us
example, some mononuclear, dinuclear and tetranuclear lanthanide complexes with formations of Ln/(ligand)1~3 [32], Ln2/(ligand)2~3 [33-35], and Ln4/(ligand)6 [36], (ligand = H2L1 or L1) have been reported.
an
The structures of lanthanide salen complexes are often influenced by a variety of factors such as lanthanide ionic radius, the nature of counter ions and pH value of environment. Thus, [Yb6(L1)9(H2L1)2]
complexes 1
M
the reactions of H2salen with Yb(CF3SO3)3, Yb(OAc)3·4H2O and Yb(NO3)3·6H2O resulted in [Yb3(L1)3(HL1)(OH)2]
(1),
1
(2)
and
d
[Yb2(L )2(H2L )2(NO3)(MeOH)2]·NO3 (3), respectively [17]. To the best of our knowledge, the hexanuclear complex 1 is the highest nuclearity lanthanide complex with H2salen to date. A view
te
of the crystal structure of 1 is shown in Figure 1, and reveals a centrosymmetric core with two equivalent Yb3L5 moieties linked by a salen ligand. The complex 1 has a “twisted” structure, in
Ac ce p
which no MeOH or H2O molecules are bound to the metal centers. Meanwhile, there are no CF3SO3- anions coordinated to the Yb3+ ions, probably due to steric factors.
Figure 1. A view of the crystal structure of 1.
5
Page 5 of 34
The complex 2 displays a trinuclear structure which is similar to the Yb3L5 moiety in 1, with two monodentate OH- anions replacing two coordinated salen ligands of 1 (Figure 2). The counter OAc- anions may play a key role in the formation of 2. Acetate can serve as a weak base in the reaction system to deprotonate H2O (introduced in Yb(OAc)3·4H2O), giving OH- ions
an
us
cr
ip t
which lead to the formation of 2. The Yb-Yb distances in 1 and 2 are approximately 3.7 Å.
M
Figure 2. A view of the crystal structure of 2. The geometry of 3 is somewhat different from those of 1 and 2 in having a more flattened
d
structure (Figure 3). In 3, two Yb3+ ions are bridged by one salen ligand with a separation of
te
11.166 Å, significantly longer than the Yb-Yb distances in 1 and 2. The “twisted” structures of 1
Ac ce p
and 2 feature intramolecular ! -! stacking (3.870 to 3.999 Å) and C-H···! interactions (2.566 to 2.782 Å) between salen ligands, while these interactions are not found in the flattened structure of 3.
Figure 3. A view of the crystal structure of 3.
6
Page 6 of 34
Upon excitation of the ligand centered absorption band either at 275 or 330-360 nm complexes 1-3 show the typical NIR emission bands of Yb3+ assigned to the 2F5/2→2F7/2 transitions (Figure 4). The relative NIR emission intensity at 979 nm in CH3CN was estimated to be 5.1:1.8:1 for
ip t
1:2:3, indicating that 1 has superior luminescence properties compared to 2 and 3. This may due to the fact that the coordinated CH3OH molecules and OH- anions in 2 and 3 can efficiently
cr
quench the luminescence of lanthanide ions through non-radiative exchange of electronic energy of Ln3+ to the high vibrational modes of OH-groups (ν = 3700 cm-1) [37,38]. Meanwhile, the
us
influence of the structural differences in 1-3 on the photophysical properties was investigated by comparing their NIR emission intensities in CH3CN and CH3OH. All three complexes display weaker NIR emissions in CH3OH than in CH3CN. However, the intensities of 1 and 2 are
an
reduced much less than that of 3, indicating that the “twisted” structures of complexes 1 and 2 may help to shield the metal centers from the outside solvent environment and improve their
M
luminescent properties [17].
1.2
te
0.8 0.6
Ac ce p
Intensity
1.0
1 2 3
d
1.4
0.4 0.2
0.0 800
900
1000
1100
1200
1300
! / nm
Figure 4. The NIR luminescence of complexes 1-3 in CH3CN.
The homoleptic complex [Tb4(L1)6] (4) was synthesized from the reaction of Tb(OAc)3·4H2O and H2salen [18]. A view of the crystal structure of 4 is shown in Figure 5, and reveals a tetranuclear centrosymmetric core with two equivalent Tb2L3 moieties linked by two μ-O phenoxide atoms. None of the MeOH or H2O molecules found in the structure are bound to a
7
Page 7 of 34
metal center. The molecular structure of 4 also features ! -! stacking (3.805 to 3.994 Å) between
an
us
cr
ip t
aryl groups of H2salen ligands.
M
Figure 5. A view of the crystal structure of 4. The construction of 4 appears to be anion dependent, since the use of Tb(NO3)3·6H2O and
d
TbCl3·6H2O under similar reaction conditions results in the formation of [Tb2(L1)2(μH2L1)(NO3)2(MeOH)2] (5) and [Tb2(L1)2(μ-H2L1)Cl2(MeOH)2] (6). As shown in Figure 6 and 7,
te
the overall structures of 5 and 6 are similar. The common feature in both structures is the retention of one Cl- or NO3- per Tb3+ ion which is bound to the central N2O2 core of a salen
Ac ce p
ligand. Two Tb3+ centers are also bridged by a single neutral molecule of H2salen. In 5 and 6, one MeOH is directly coordinated to the Tb3+ ion.
Figure 6. A view of the crystal structure of 5.
8
Page 8 of 34
ip t cr
us
Figure 7. A view of the crystal structure of 6.
Excitation of the ligand centered absorption bands in complexes 4-6 result in typical visible emission bands for the Tb3+ ion (5D4 → 7Fn transitions, n = 6, 5, 4 and 3), while the ligand
an
centered 1! -! * emissions were not detected (Figure 8). Compared with those in 5 and 6, the Tb3+ ions in 4 are completely encapsulated by the Schiff base ligands and protected from solvent
M
molecules. As a result, 4 shows higher fluorescence quantum yield (Φem = 0.230) than 5 (0.127)
te
Ac ce p
Intensity
d
and 6 (0.096) [18].
400
500
600 ! / nm
Em (Free ligand) Em (Tb complex)
700
800
Figure 8. Emission spectra of free H2L1 (---) and Tb(III) complex 4 (—).
In 1-6, the salen ligands exhibit four bonding modes with Yb3+ ions: (I) monodentate bonding to one Yb3+ ion; (II) tetradentate bonding to one Yb3+ ion; (III) bidentate bonding to two Yb3+ ions; (IV) pentadentate bonding to two Yb3+ ions (Scheme 2).
9
Page 9 of 34
OH
N
N
N
N Ln
O
OH
O
ip t
Ln
Ln
Ln O
N
cr
HO N
N
us
OH Ln
N
Ln
O Ln
an
Scheme 2. Four bonding modes for H2salen with Ln3+ ions.
M
2.2 Hexanuclear 4f Complexes with salen-type ligands bearing backbone hydroxyl groups The modification of a given salen-type ligand may affect the formation of polynuclear
d
assemblies. To investigate the influence of the groups introduced in the H2salen ligand on the construction of coordination frameworks, N,N'-bis(salicylidene)(propylene-2-ol)-1,3-diamine
te
(H2L2) and N,N'-bis(3-methoxysalicylidene)(propylene-2-ol)-1,3-diamine (H2L3) were used in the synthesis of lanthanide complexes. Differing from the H2salen ligand, H2L2 and H2L3 feature
Ac ce p
a backbone hydroxyl group. The introduction of the backbone hydroxyl groups in these two ligands adds one more coordination position than the H2salen ligand possesses, and may reduce the flexibility of the ligands when they bond to lanthanide ions. Two hexanuclear lanthanide complexes [Eu6(L2)4(OH)4(MeOH)2(EtOH)2(H2O)2]·2Cl (7) and [Er6(L2)4(OH)4(EtOH)2(H2O)2]·2Cl (8) were synthesized from reactions of H2L2 with LnCl3·6H2O [19]. Complexes 7 and 8 have similar hexanuclear structures with six lanthanide ions enclosed by four Schiff base ligands. A view of the cationic complex 7 is shown in Figure 9. The X-ray structure of 7 reveals a centrosymmetric core with two equivalent Eu3(L2)2 moieties linked by two μ3-OH- anions, in which two Eu3+ ions are coordinated with three Schiff base ligands, while one Eu3+ is coordinated with two Schiff base ligands. In 7, each μ3-OH- anion links three Eu3+ ions.
10
Page 10 of 34
ip t cr
us
Figure 9. A view of the crystal structure of 7.
Two hexanuclear lanthanide complexes [Ln6(L3)4(OH)4(MeOH)4]·2Cl (Ln = Nd (9), Tb (10))
an
were obtained from reactions of H2L3 with LnCl3·6H2O [19]. Although H2L3 has two more methoxy groups than H2L2, complexes 9 and 10 have hexanuclear structures which are similar to
M
those observed in 7 and 8. Complexes 9 and 10 are isomorphous and a view of the crystal structure of 9 is shown in Figure 10. It also reveals a centrosymmetric core with two equivalent Nd3(L2)2 moieties that are bridged by two μ3-OH- anions. For each Nd3L2 moiety, the Nd3+ ions
d
have coordination environments similar to the Eu3+ ions in 7. One methoxy group of the Schiff
te
base ligand bonds to one Nd3+ ion in the Nd3L2 moiety. The Schiff base ligands in 7-10 have a formal -3 charge resulting from the deprotonation of two phenolic hydroxyls and one backbone
Ac ce p
hydroxyl group.
Figure 10. A view of the crystal structure of 9.
11
Page 11 of 34
In CH3CN, excitation of the absorption bands in the free ligands H2L2 and H2L3 produce broad emission bands at ! max = 493 nm and 518 nm, respectively (Figures 11 and 12). For lanthanide complexes, excitations of the ligand centered absorption bands in 7 and 10 result in visible emission bands for the Eu3+ ion (5D0→7Fj transitions, j = 1, 2, 3 and 4) and Tb3+ ion (5D4
ip t
→ 7Fn transitions, n = 6, 5, 4 and 3), respectively. Compound 9 shows typical NIR emission bands of Nd3+ assigned to the 4F3/2→4Ij/2 (j = 9, 11, 13) transitions upon excitation of the ligand
cr
centered absorption band (Figure 13). For complex 8, no NIR emission due to Er3+ ion was
us
observed under the experimental conditions employed.
2
Ex: H2L Ex: 7
2
400
te
300
d
M
Intensity
an
Em: H2L Em: 7
500 600 ! / nm
700
800
Ac ce p
Figure 11. Excitation spectra and visible emission spectra of the free ligand H2L2 (----- and blue line) and the Eu(III) complex 7 (-.-.-.- and red line) in CH3CN. Ex: H2L Ex: 10
Em: H2L Em: 10
Intensity
300
3
400
500
600
3
700
! / nm
Figure 12. Excitation spectra and visible emission spectra of the free ligand H2L3 (----- and blue line) and the Tb(III) complex 10 (-.-.-.- and red line) in CH3CN.
12
Page 12 of 34
cr
ip t
Intensity
Em: 9
! / nm
us
800 1000 1200 1400 1600
an
Figure 13. The NIR luminescence of the Nd(III) complex 9 in CH3CN.
3. Luminescent Polynuclear d-4f Complexes with Flexible Salen-type Ligands The design and construction of polynuclear d-f clusters has received extensive attention due
M
to the remarkable physical and chemical properties associated with this class of materials [39-44]. For example, research on polynuclear d-f complexes of Yb(III), Nd(III) and Er(III) with near-
d
infrared (NIR) emission in the 900-1600 nm range has become a hot topic due to potential applications in bioassays and laser systems [45]. Light-absorbing d-block metal chromophores
te
(i.e. PtII [46,47], RuII [48], ZnII [49], and CdII [50,51]) have been used as sensitizers for NIR
Ac ce p
luminescence from Ln(III) centers following ligand→f and d→f energy-transfers. The Schiff base ligands described in this review have two dissimilar metal-binding sites, one with a higher affinity for the d metal ion and another for the f metal ion. A number of bimetallic 3d-4f Schiff base complexes have been prepared in order to study their magnetic properties in the past few years [52]. In this section, we describe the construction of 14 polynuclear d-f clusters with H2L4-9 ligands. The d-block metal ions introduced into these lanthanide complexes can play two different roles in the luminescence properties of Ln3+ ions. They may enhance the luminescence via d→f energy transfer, or they may quench the luminescence via f→d energy transfer. 3.1 Polynuclear Zn/Ni/Cu-4f Salen Complexes As shown in Scheme 1, H2L5 (N,N'-bis(5-bromo-3-methoxysalicylidene)propylene-1,3diamine) has one more methylene (-CH2-) group in its backbone compared to H2L4 (N,N׀-bis(3methoxysalicylidene)ethylene-1,2-diamine). In contrast to simple bi-metallic ZnLn salen
13
Page 13 of 34
complexes, use of these ligands resulted in the formation of a trinuclear complex [YbZn2(L4)2(OAc)2]·(OAc) (11) and tetrametallic [Zn2Yb2(L5)2(OH)2Cl4] (12) [20]. In 11, the Zn2+ ions are bound in the O2N2 cavities of each L4 group while the Yb3+ ion is bound by the
an
us
cr
and phenolic oxygen atoms, and the average Zn-Yb distance is 3.453 Å.
ip t
outer O2O2 sets of both L4 groups (Figure 14). Yb3+ and Zn2+ ions are bridged by OAc- anions
M
Figure 14. A view of the crystal structure of 11.
d
In 12, each Yb3+ ion is located in the O2O2 cavity of one L group. Two Yb3+ ions are bridged by two hydroxide anions, with a separation of 3.588 Å. The two central bridging
te
hydroxides give the molecule an overall slipped sandwich configuration (Figure 15). A series of Cu-Ln complexes [LnCu2(L5)2(NO3)(H2O)2]·(NO3)2 (Ln = Ce, Pr, Nd and La) which show
Ac ce p
similar trinuclear structures as 11 have been reported by Cristóvão et al. [26], in order to study their magnetic properties. Both 11 and 12 show typical emission bands of Yb3+ assigned to the 2
F5/2→2F7/2 transition upon excitation of the ligand centered absorption bands (Figure 16).
Figure 15. A view of the crystal structure of 12.
14
Page 14 of 34
cr
ip t
Intensity
Em: 12
us
800 1000 1200 1400 1600 1800 ! / nm
an
Figure 16. The NIR luminescence of 12 in CH3CN.
The nuclearity of 3d-4f Schiff base complexes can be manipulated by the introduction of
M
different linkers which can bridge 3d-4f moieties. Thus, 1,4-benzenedicarboxylate (BDC) is a convenient bidentate linker used for the construction of polynuclear complexes. In our hands a
d
hexanuclear Zn-Nd complex [Zn4Nd2(L5)4(1,4-BDC)2]·[Nd(NO3)5(H2O)] (13) was formed which contained two linking BDC groups [21]. As shown in Figure 17, two BDC groups bridge the
te
Zn2Nd moieties such that each carboxylate group spans a Zn-Nd set. The 1H NMR spectrum of
Ac ce p
13 shows that this kind of bridged structure is stable in solution at room temperature (Figure 18). A Cu-Sm complex [Cu4Sm2(L5)4(fum)2]·(OH)2 which has a similar hexanuclear structure as 13 was constructed by using fumaric acids as linkers. [27].
Figure 17. A view of the crystal structure of 13.
15
Page 15 of 34
ip t cr us
an
Figure 18. The 1H NMR spectrum of 13 in CD3OD at room temperature. The complex 13 shows typical NIR emission bands of NdIII (4F3/2→4Ij/2 transition, j = 9, 11,
M
13) upon excitation of the ligand centered absorption bands both in the solid state and MeCN solution (Figure 19). It is noticeable that, with the central metal ion encapsulated by four
d
chromophoric ligands and shielded from solvent interactions, this complex shows superior luminescence properties compared to the related simple dinuclear Zn-Nd complex
te
[ZnNdL5(OAc)(NO3)2] [21]. With the same absorbance value at the excitation wavelength (275 nm), the emission intensity at 1068 nm in 13 is 5.5 times as strong as that in
Ac ce p
[ZnNdL5(OAc)(NO3)2].
Intensity
Em: 13
800 1000 1200 1400 1600 1800 ! / nm
Figure 19. The NIR luminescence of 13 in CH3CN.
16
Page 16 of 34
Schiff base ligands with four methylene (CH2) units in the bridge, such as H2L6 (H2L = N,N'-bis(3-methoxysalicylidene)butane-1,4-diamine) have also been employed to prepare high nuclearity
Ln
complexes.
Two
hexanuclear
3d-4f
Ni-Eu
and
Cu-Eu
complexes
ip t
[Eu4Ni2(L6)2(OAc)12(EtOH)2] (14) and [Eu4Cu2(L6)2(OAc)12]·2H2O (15) were formed from the reactions of H2L6 with M(OAc)2·4H2O (M = Ni(II) and Cu(II)) and Eu(OAc)3·4H2O [22]. As
cr
shown in Figures 20 and 21, complexes 14 and 15 have similar hexanuclear structures, containing two equivalent NiEu2L and CuEu2L moieties, respectively. In the MEu2L (M = Ni
us
and Cu) moiety, the 3d metal ion and Eu(1) are bridged by two bidentate OAc- anions in addition to the phenolic oxygen atoms of the Schiff base ligand. Ni(1)-Eu(1) and Cu(1)-Eu(1) distances
te
d
M
an
are 3.346 Å and 3.467 Å, respectively.
Ac ce p
Figure 20. A view of the crystal structure of 14.
Figure 21. A view of the crystal structure of 15.
17
Page 17 of 34
The OAc- anions introduced form the metal salts act as linking groups in these hexanuclear structures and clearly play a key role in the formation of 14 and 15. In contrast, two more simple binuclear complexes [EuNiL6Cl3(H2O)4] and [EuCuL6(NO3)3] were formed from the reactions of
M
an
us
cr
no OAc- anions were present in the reaction mixtures (Figures 22 and 23).
ip t
H2L6 with NiCl2·6H2O/EuCl3·6H2O and Cu(NO3)2·3H2O/Eu(NO3)3·6H2O, respectively, in which
Ac ce p
te
d
Figure 22. A view of the crystal structure of binuclear complex [EuNiL6Cl3(H2O)4].
Figure 23. A view of the crystal structure of binuclear complex [EuCuL6(NO3)3].
The photophysical properties of 14 and 15 were studied in CD3OD (Figure 24). The free ligand H2L6 shows two broad emission bands at ! max = 319 and 496 nm upon excitation of the absorption band at 275 nm. Both 14 and 15 show weak emission bands of the Eu3+ ion (5D0→7Fj transitions, j = 0, 1, 2, 3 and 4) upon excitation of the ligand-centered absorption bands in CD3OD. The quantum yields of 14 and 15 in CD3OD are 0.0011 and <10-4, respectively, which
18
Page 18 of 34
are more than ten times less than that of the polymeric complex [Eu2(H2L6)(OAc)6]n (Φem = 0.015) that does not bear 3d metal ions [22]. For both 14 and 15, the weak luminescence of Eu3+ ions maybe due to lanthanide to transition metal (4f→3d) energy transfer which can efficiently
6
an
us
Intensity
cr
H2L 14 H2L 14
ip t
quench lanthanide ion emission (Scheme 3).
M
300 400 500 600 700 800 ! / nm
te
d
Figure 24. Excitation and emission spectra of free H2L6 and Eu(III) complex 14.
E, cm-1/1000
Ac ce p
20
5
D0
15
d ET
10
5
0
7F
6 5 4 3 2 1 0
d
Eu(III)
Ni2+/Cu2+
Scheme 3. Lanthanide to transition metal (4f→3d) energy transfer.
19
Page 19 of 34
As noted previously the ligand H2L3 has an additional backbone hydroxyl (-OH) group compared to H2L5. The trinuclear complex [NdCu2(L3)2Cl2(H2O)2]·Cl (16) was formed from the reaction of H2L3 with Cu(OAc)2·4H2O and NdCl3·6H2O. As shown in Figure 25, the Nd3+ ion is
an
us
cr
groups of two L3 ligands do not bond to the metal ions in 16.
ip t
centered between two CuL moieties. Unlike those in complexes 9 and 10, the backbone hydroxyl
M
Figure 25. A view of the crystal structure of 16. Replacing Cu(OAc)2·4H2O with Ni(OAc)2·4H2O in above reaction produces the simple
Ac ce p
te
d
binuclear complex [NdNiL3Cl2(H2O)3(MeOH)2] (Figure 26).
Figure 26. A view of the crystal structure of [NdNiL3Cl2(H2O)2(MeOH)2].
Another variation in the Schiff base ligand is to introduce ether linkages into the backbone as in H2L7 (6,6'-((1E,11E)5,8-dioxa-2,11-diazadodea-1,11-diene-1,12-diyl)bis(2-methoxyphenol)) which has two oxy (-O-) groups in its backbone and is even more flexible than H2L1-6. The heptanuclear complex [Ni3Nd4(L7)3(NO3)3(OAc)6(OH)3] (17) was synthesized from the reaction of H2L7 with Ni(OAc)2·4H2O and Nd(NO3)3·6H2O. As shown in Figure 27, three Ni2+ and four
20
Page 20 of 34
Nd3+ ions are surrounded by three Schiff base ligands. In 17, the two oxy (-O-) groups of each
M
an
us
cr
ip t
ligand do not bind to the metal ions.
d
Figure 27. A view of the crystal structure of 17.
3.2 High-nuclearity Cd-4f Salen Complexes
te
As with Ln(III) ions, the Cd(II) ion can have high coordination numbers (6-8) and display various coordination geometries, and high-nuclearity Cd-Ln clusters are possible. Two flexible
Ac ce p
salen-type ligands H2L8 (N,N'-bis(3-methoxysalicylidene)hexane-1,6-diamine) and H2L9 (6,6'((1E,1'E)-(dodecane-1,12-diylbis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol)), which have six and twelve methylene (-CH2-) groups respectively in their backbones, were employed to construct high nuclearity Cd-4f complexes. Reactions of H2L8 with Cd(OAc)2·4H2O and Ln(OAc)3·4H2O produced a series of Cd-Ln heterometallic clusters [Ln8Cd24(L8)12(OAc)48] (Ln = Nd (18), Er (19) and Yb (20)) [23]. These complexes are isomorphous, and possess interesting 32-metal drum-like structures. Two views of the crystal structure of 18 are shown in Figure 28. The top view is essentially a side-on view while the lower one is looking down into the top of the drum. The complex is of nanoscale proportions (19 × 26 × 26 Å). The ends of the drum are created by two rings of 16 metals (4 Nd(III) and 12 Cd(II)). The sides of the drum are formed by the -(CH2)6- linkers of twelve Schiff base ligands. Thus each Schiff base ligand is coordinated to metals at both ends of the drum.
21
Page 21 of 34
ip t cr us an M d te Ac ce p
Figure 28. A view of the crystal structure of 18. Viewed along the a-axis (top) and c-axis (lower). (Nd3+: green; Cd3+: brown; unit cell: a = 54.7766 Ǻ, b = 62.2436 Ǻ, c = 79.9350 Ǻ, α = β = γ = 90º).
The self-assembly process of the drum-like structures appears to be anion dependant. Thus, if Cl- anions are introduced into the reactions with the use of LnCl3·6H2O, smaller 24-metal drum-like complexes [Nd6Cd18(L8)9Cl8(OAc)28] (21) and [Ln6Cd18(L8)9Cl10(OAc)26] (Ln = Er (22) and Yb (23)) are produced [23]. Complexes 21-23 show similar 24-metal drum-like structures. As shown in Figure 29, the drum-like architecture of 21 is formed by two 12-metal rings (Er3Cd9Cl4(OAc)14) linked by nine Schiff base ligands. The molecular dimensions of 21 (16 × 21 × 21 Å) are smaller than those of 18.
22
Page 22 of 34
ip t cr us an M d te Ac ce p
Figure 29. A view of the crystal structure of 21. Viewed along the a-axis (top) and c-axis (lower). (Nd3+: green; Cd3+: brown)
Cd(II) chromophores with visible emissions can be used to sensitize the luminescence of Ln(III) ions [23,24]. The luminescence properties of these complexes were studied in CH3CN. Upon excitation of the ligand-centered absorption bands, complexes 18 and 21 show the NIR luminescence of Nd3+ (4F3/2→4Ij/2 transitions, j = 9, 11 and 13)), and complexes 20 and 23 show that of Yb3+ (2F5/2→2F7/2 transition) (Figure 30). For Er(III) complexes 19 and 22 no obvious or very weak NIR emission attributable to the Er3+ ion was observed.
23
Page 23 of 34
200
400
600
cr
ip t
Intensity
Ex - 21 Ex - 23 Em -21 Em -23
800 1000 1200 1400
us
! / nm
Figure 30. Excitation and emission spectra of complexes 21 and 23 in CH3CN.
an
The differences in NIR luminescence properties of these complexes may be explained by considering the energy levels of the various lanthanide ions. As shown in Scheme 4, in Nd(III) 4
M
complexes, the Nd3+ ion has many possible energy-accepting levels lying above the emissive F3/2 state. In Yb(III) complexes, although the Yb3+ ion has only a single excited state 2F5/2, the
d
efficient energy-transfer to Yb3+ ions in these Yb(III) complexes may take place via an electron transfer mechanism and/or phonon-assisted energy-transfer mechanisms [53,54]. For Er(III)
te
complexes, the lanthanide-induced NIR emission may be quenched by C-H vibrations from the
Ac ce p
ligand.
E, cm-1/1000
20
Cd/L excited states a
25
4G
ET
15
10
2H
11/2 4S 3/2
7/2
4
G 5/2
2
H 11/2 4 F9/2 4 S3/2 , 4 F7/2
4F
4
2
H 9/2 , 4F 5/2 4F 3/2
I 9/2
2
F5/2
4I
4I 4I
5
9/2
11/2
v=2 13/2
15/2
v=1
4
I13/2
4I
11/2
4I
0 Cd/L-center
v=3
2F 7/2
9/2
Nd(III)
Yb(III)
4I
15/2
Er(III)
C-H
Scheme 4. The energy levels in lanthanide complexes 18-23.
24
Page 24 of 34
A 32-metal complex [Nd8Cd24(L9)12(OAc)48] (24) was prepared using the more flexible ligand H2L9 which contains 12 bridging methylene (CH2) units. As shown in Figure 31, the complex 24 has a drum-like structure similar to those found in complexes 18-20. The molecular
Ac ce p
te
d
M
an
us
cr
ip t
size of 24 is 32 × 26 × 26 Å, which is longer than that of the Nd(III) complex 18.
Figure 31. A view of the crystal structure of 24. Viewed along the a-axis (top) and c-axis (lower). (unit cell: a = 56.3430 Ǻ, b = 61.7861 Ǻ, c = 108.7203 Ǻ, α = β = γ = 90º).
25
Page 25 of 34
From the structures of these drum-like architectures, we can conclude that the key to the construction of these 24- and 32-metal Ln/Cd complexes is that both Ln(III) and Cd(II) ions have the potential for high coordination numbers (6-8) with variable coordination geometries. The adaptability of flexible ligands enables them to form different molecular systems and extended
ip t
network materials.
cr
4. Luminescent 4f and 3d-4f Coordination Polymers with Flexible Salen-type Ligands
us
Compared with the growth of studies on 3d metal-organic-framework (MOF) materials, reports of 4f or 3d-4f coordination polymers have been far fewer [55-58]. Lanthanide ions behave as hard acids and they prefer oxygen to nitrogen donors, while d-block metal ions can
an
coordinate to both N and O donors. One approach for the construction 4f and 3d-4f frameworks is self-assembly of selected metal ions using ligands containing a mixed-donor atom set. Flexible
M
salen type ligands, in combination with anions such as acetate (OAc-) acting as a bridging ligand
4.1 1-D 4f coordination polymers
d
between metal centers, results in the formation of 1-D coordination polymers.
te
Although the first X-ray crystal structure of a lanthanide coordination polymer, from the reaction of the neutral H2salen ligand (H2L1) with La(NO3)3 hydrate, was reported by Xie in type
Ac ce p
1999, there are relatively few reports of lanthanide coordination polymers which employ salen Schiff-base
ligands
[59-61].
Three
lanthanide
1-D
coordination
polymers
{[Ln2(L1)2(CF3SO3)(H2L1)4(MeOH)]·CF3SO3}n (Ln = Eu (25), Nd (26) and Er (27)) were prepared from the reactions of H2L1 and Ln(CF3SO3)3 [24]. As shown in Figure 32, for 25 Eu(1) and Eu(2) are bridged by a neutral salen ligand with a separation of 10.171 Å. The 1-D polymeric structure is formed by a zig-zag chain of alternating Eu3+ ions and H2L1 ligands which bridge as neutral ligands between the metal centers.
26
Page 26 of 34
ip t cr us
an
Figure 32. A view of the crystal structure of 25.
The formation of 1-D lanthanide coordination polymers from H2salen ligand can be affected
M
by the anions present in the reactions. Thus, the reaction of Yb(OAc)3·4H2O with H2L1 produced a 1-D polymeric material [Yb2(L1)2(OAc)2(MeOH)2]n (28), in which the 1-D polymeric structure is formed by the linking of OAc- anions. As shown in Figure 33, in the central Yb2 unit, each
d
Yb3+ ion is bound by the N2O2 cavity of a deprotonated salen ligand. These Yb2 units are bridged
Ac ce p
te
by OAc- anions, forming the 1-D coordination polymeric structure.
Figure 33. Acetate (OAc-) units bridging a Yb2 unit giving a 1-D polymer structure in 28. The 1-D polymeric complex {[Tb3(L4)2(OAc)5]·Et2O·(MeOH)0.5}n (29) was obtained from the reaction of Tb(OAc)3·4H2O and H2L4 [24]. As shown in Figure 34, the central Tb3 unit contains
27
Page 27 of 34
two (L4)2- groups bound to the three Tb3+ ions. The 1-D polymeric framework is completed by
an
us
cr
ip t
OAc- units which link Tb3 units.
M
Figure 34. Acetate (OAc-) units bridging a Tb3 unit giving a 1-D polymer structure in 29. A lanthanide 1-D coordination polymer ({[Eu2(H2L6)(OAc)6]·EtOH·2H2O}n (30) was prepared from the reaction of H2L6 with Eu(OAc)3·4H2O [25]. As shown in Figure 35, two Eu
d
atoms are bridged by four OAc- anions with a Eu-Eu separation of 3.985 Å, forming an Eu2 unit.
te
The 1-D polymeric framework is completed by neutral Schiff base ligands (H2L6) which link the
Ac ce p
Eu2 units via phenolic and methoxy oxygen atoms.
Figure 35. A view of the crystal structure of 30.
28
Page 28 of 34
The photophysical properties of the Eu(III) complexes 25 and 30, and Tb(III) complex 29 were studied in solution (Figures 36 and 37). Upon excitations of ligand-centered absorption bands, complexes 25, 30 and 29 show visible emission bands of Eu3+ ion (5D0→7Fj transitions, j = 1, 2, 3 and 4) and Tb3+ ion (5D4 → 7Fn transitions, n = 6, 5, 4 and 3), respectively. For the
ip t
Eu(III) complexes, the fluorescence quantum yield of 30 (0.015) is higher than that of 25 (less
cr
than 10-3).
200
300
M
an
Intensity
us
Ex Em
400
500
600
700
800
d
! / nm
Ac ce p
te
Figure 36. Excitation and emission spectra of 29 in CH3CN.
Intensity
Ex Em
200
300
400
500
600
700
800
! / nm
Figure 37. Excitation and emission spectra of 30 in CH3CN.
29
Page 29 of 34
4.2 1-D Ni-4f coordination polymers The reaction of H2L6 with YbCl3·6H2O and Ni(OAc)3·4H2O in refluxing MeCN/EtOH produced the 1-D polymeric material {YbNiL6Cl(OAc)2(H2O)}n (31) [25]. As shown in Figure 38, the Ni2+ and Yb3+ ions in 31 are located in the inner N2O2 and O2O2 cavities of the Schiff
ip t
base ligand, respectively. Within each Yb-Ni-L moiety the Ni2+ and Yb3+ ions are bridged by a single bidentate OAc- group in addition to the phenolic oxygen atoms of the Schiff base ligand.
cr
The Ni2+ ion has bi-pyramidal geometry with one chlorine anion and one oxygen atom from the bidentate OAc- anion occupying the axial positions. The 1-D polymeric framework is formed by
te
d
M
an
us
OAc- units which link the Yb-Ni-L6 moieties (Figure 38).
Ac ce p
Figure 38. A view of the crystal structure of 31. It appears that the use of Ni(OAc)2·4H2O for the preparation of 31 is critical to the formation of the 1-D polymeric complex, since the acetate (OAc –) is employed as a linker in the formation of 1-D polymeric structure. In contrast, the reaction of H2L6 with YbCl3·6H2O and NiCl2·6H2O under similar conditions produced a simple heterobinuclear complex [YbNiL6Cl3(H2O)3] [25]. For 31, upon excitation of ligand-centered absorption bands, emission in near-infrared (NIR) range due to the Yb3+ ion was not observed.
5. Conclusions Nine flexible salen-type Schiff base ligands have been used in the synthesis of 4f and d-4f polynuclear complexes and coordination polymers. The stoichiometry and structures of these
30
Page 30 of 34
complexes are dependent on the Schiff base ligands employed in their syntheses as well as lanthanide ionic radius, counter ions and reaction conditions. These multi-metallic lanthanide complexes have interesting “twisted”, drum-like, or polymeric structures. Of the complexes described in this review, one hexanuclear Zn-Nd assembly has been described which is formed
ip t
by the use of two different ligands (one a Schiff base ligand and the other a bridging ligand 1,4BDC).
cr
In these lanthanide complexes, the flexible salen-type ligands can stabilize LnIII centers and act as antenna sensors for lanthanide luminescence. In d-4f complexes, Zn2+ and Cd2+ may
us
enhance the luminescence via d→f energy transfer, while in contrast, Cu2+ and Ni2+ may quench the luminescence via f→d energy transfer. With the Ln3+ centers protected by the Schiff base ligands from solvent and water molecules, those lanthanide complexes with “twisted” and drum-
an
like structures show impressive luminescence properties.
M
6. Acknowledgements
We thank the Welch Foundation (Grant F-816, RAJ) and the National Natural Science
d
Foundation of China (No. 51025207, SMH) for financial support. Single crystal X-ray data were
te
collected using instrumentation purchased with funds provided by the National Science
Ac ce p
Foundation (CHE-0741973), USA.
7. References
[1] P. Zanello, S. Tamburini, P. A. Vigato, G. A. Mazzocchin, Coord. Chem. Rev. 77 (1987) 165-273.
[2] A. Müller, Nature 447 (2007) 1034-1035. [3] C. Piguet, J.-C. G. Bünzli, Chem. Soc. Rev. 28 (1999) 347-358. [4] N. Sabbatini, M. Guardigli, J. M. Lehn, Coord. Chem. Rev. 123 (1993) 201-228. [5] P. Guerriero, S. Tamburini, P. A. Vigato, Coord. Chem. Rev. 139 (1995) 17-243. [6] A. Y. Robin, K. M. Fromm, Coord. Chem. Rev. 250 (2006) 2127-2157. [7] M. Sakamoto, K. Manseki, H. Okawa, Coord. Chem. Rev. 219 (2001) 379-414. [8] R. E. P. Winpenny, Chem. Soc. Rev. 27 (1998) 447-452.
31
Page 31 of 34
[9] C. Camp, V. Guidal, B. Biswas, J. Pecaut, L. Dubois, M. Mazzanti, Chem. Sci. 3 (2012) 2433-2438. [10] W. J. Evans, C. H. Jujimoto, J. W. Ziller, Chem. Commun. 35 (1999) 311-313. Mallah, Chem. Commun. 48 (2012) 9071-9073. [12] L. Zhao, S. Xue, J. Tang, Inorg. Chem. 51 (2012) 5994-5996.
cr
[13] X.-P. Yang, R. A. Jones, J. Am. Chem. Soc. 127 (2005) 7686-7687.
ip t
[11] G. Magadur, F. Bouanis, E. Norman, R. Guillot, J.-S. Lauret, V. Huc, C.-S. Cojocaru, T.
[14] X.-P. Yang, R. A. Jones, W.-K. Wong, Dalton Trans. (2008) 1676-1678.
us
[15] X.-P. Yang, R. A. Jones, M. M. Oye, A. Holmes, W.-K. Wong, Crystal Growth & Design 6 (2006) 2122-2125.
[16] W.-K. Wong, X.-P. Yang, R. A. Jones, J. H. Rivers, V. Lynch, W.-K. Lo, D. Xiao, M. M.
an
Oye, A. L. Holmes, Inorg. Chem. 45 (2006) 4340-4345.
[17] X.-P. Yang, M. O. Michael, R. A. Jones, S.-M. Huang, Chem. Commun. 49 (2013) 9579-
M
9581.
[18] X.-P. Yang, R. A. Jones, W.-K. Wong, Chem. Commun. (2008) 3266-3268.
d
[19] S. Liao, X.-P. Yang, R. A. Jones, Crystal Growth & Design 12 (2012) 970-974. [20] X.-P. Yang, R. A. Jones, V. Lynch, M. M. Oye, A. Holmes, Dalton Trans. 5 (2005) 849-851. (2006) 1836-1838.
te
[21] X.-P. Yang, R. A. Jones, W.-K. Wong, V. Lynch, M. M. Oye, A. Holmes, Chem. Commun.
Ac ce p
[22] X.-P. Yang, C. Chan, D. Lam, D. Schipper, J. M. Stanley, X. Chen, R. A. Jones, B. J. Holliday, W.-K. Wong, S. Chen, Q. Chen, Dalton Trans. 41 (2012) 11449-11453. [23] X.-P. Yang, D. Schipper, R. A. Jones, L. A. Lytwak, B. J. Holliday, S.-M. Huang, J. Am. Chem. Soc. 135 (2013) 8468-8471. [24] X.-P. Yang, R. A. Jones, J. Rivers, W.-K. Wong, Dalton Trans. 47 (2009) 10505-10510. [25] X.-P. Yang, D. Lam, C. Chan, J. M. Stanley, R. A. Jones, B. J. Holliday, W.-K. Wong, Dalton Trans. 40 (2011) 9795-9801. [26] B. Cristóvão, B. Miroslaw, J. Klak, Polyhedron 34 (2012) 121-127. [27] R. Gheorghe, P. Cucos, M. Andruh, J.-P. Costes, B. Donnadieu, S. Shova, Chem. Eur. J. 12 (2006) 187-183. [28] C. Piguet, Chimia 50 (1996) 144-153. [29] F. S. Richardson, Chem. Rev. 82 (1982) 541-552.
32
Page 32 of 34
[30] S. Yanagida, Y. Hasegawa, K. Murakoshi, Y. Wada, N. Nakashima, T. Yamanaka, Coord. Chem. Rev. 171 (1998) 461-480. [31] N. K. Dutt, K. Nag, J. Inorg. Nucl. Chem. 30 (1968) 2493-2496. [32] J. I. Bullock, H. A. Tajmir-Rianhi, Dalton Trans. (1978) 36-39. (2011) 10993-10995.
cr
[34] H. Chen, R. D. Archer, Inorg. Chem. 33 (1994) 5195-5202.
ip t
[33] P.-H. Lin, W.-B. Sun, M.-F. Yu, G.-M. Li, P.-F. Yan, M. Murugesu, Chem. Commun. 47
[35] R. D. Archer, H. Chen, L. C. Thompson, Inorg. Chem. 37 (1998) 2089-2095.
us
[36] B. H. Koo, K. S. Lim, D. W. Ryu, W. R. Lee, E. K. Koh, C. S. Hong, Chem. Commun. 48 (2012) 2519-2521.
[37] F. Artizzu, M. L. Mercuri, A. Serpe, P. Deplano, Coord. Chem. Rev. 255 (2011) 2514-2529.
an
[38] Y. Wada, T. Okubo, M. Ryo, T. Nakazawa, Y. Hasegawa, S. Yanagida, J. Am. Chem. Soc. 122 (2000) 8583-8584.
M
[39] A. Müller, P. Kögerler, A. W. M. Dress, Coord. Chem. Rev. 222 (2001) 193-218. [40] X.-J. Kong, Y.-P. Ren, W.-X. Chen, L.-S. Long, Z. Zheng, R.-B. Huang, L.-S. Zheng,
d
Angew. Chem., Int. Ed. 47 (2008) 2398-2401.
[41] E. G. Mednikov, M. C. Jewell, L. F. Dahl, J. Am. Chem. Soc. 129 (2007) 11619-11630. 37 (1998) 3359-3363.
te
[42] A. Müller, E. Krickemeyer, H. Bögge, M. Schmidtmann, F. Peters, Angew. Chem., Int. Ed.
Ac ce p
[43] D. Fenske, C. Persau, S. Dehnen, C. E. Anson, Angew. Chem., Int. Ed. 43 (2004) 305-309. [44] D. Fenske, C. E. Anson, A. Eichhöfer, O. Fuhr, A. Ingendoh, C. Persau, C. Richert, Angew. Chem., Int. Ed. 44 (2005) 5242-5246. [45] J. W. Stouwdam, G. A. Hebbink, J. Huskens, F. C. J. M. Van Veggel, Chem. Mater. 15 (2003) 4604-4616.
[46] X.-L. Li, L.-X. Shi, L.-Y. Zhang, H.-M. Wen, Z.-N. Chen, Inorg. Chem. 46 (2007) 1089210900.
[47] H.-B. Xu, L.-X. Shi, E. Ma, L.-Y. Zhang, Q.-H. Wei, Z.-N. Chen, Chem. Commun. (2006) 1601-1603. [48] S. G. Baca, H. Adams, C. S. Grange, A. P. Smith, I. Sazanovich, M. D. Ward, Inorg. Chem. 46 (2007) 9779-9789.
33
Page 33 of 34
[49] J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini, M. Bettinelli, J. Appl. Phys. 93 (2003) 9460-9465. [50] Y.-X. Chi, S.-Y. Niu, J. Jin, R. Wang, Y. Li, Dalton Trans. 47 (2009) 7653-7659. [52] J-P Cortes, F. Dahan, A. Dupuis, Inorg. Chem. 39 (2000) 165-168.
ip t
[51] Y.-X. Chi, S.-Y. Niu, Z.-L.Wang, J. Jin, Eur. J. Inorg. Chem. (2008) 2336-2343. [53] W. D. Horrocks, J. P. Bolender, W. D. Smith, R. M. Supkowski, J. Am. Chem. Soc. 119 [54] C. Reinhard, H. U. Gudel, Inorg. Chem. 41 (2002) 1048-1055.
cr
(1997) 5972-5973.
us
[55] T. K. Prasad, M. V. Rajasekharan, J. P. Costes, Angew. Chem., Int. Ed. 46 (2007) 28512855.
[56] J. W. Cheng, J. Zhang, S. T. Zheng, M. B. Zhang, G. Y. Yang, Angew. Chem., Int. Ed. 45
an
(2006) 73-76.
[57] C. E. Plecnik, S. M. Liu, X. N. Chen, E. A. Meyers, S. G. Shore, J. Am. Chem. Soc. 126
M
(2004) 204-207.
[58] X. Q. Zhao, B. Zhao, Y. Ma, W. Shi, P. Cheng, Z. H. Jiang, D. Z. Liao, S. P. Yan, Inorg.
d
Chem. 46 (2007) 5832-5834.
[59] W. Xie, M. J. Heeg, P. G. Wang, Inorg. Chem. 38 (1999) 2541-2543.
te
[60] M. P. Hogerheide, J. Boersma, G. V. Konten, Coord. Chem. Rev. 155 (1996) 87-126.
Ac ce p
[61] D. E. Featon, P. A. Vigato, Chem. Soc. Rev. 17 (1988) 69-90.
34
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