2.3.1 Mononuclear Lanthanide Complexes with β -Diketones
2.3.1.3 Eight-Coordinated Lanthanide Complexes with β-Diketones
The family of eight-coordinateβ-diketonate lanthanides are the most widely studied, as the complexes are coordinately saturated, or almost coordinately saturated, with the emissive properties being optimized if the appropriate ligands are chosen. Li and Huanget al. [48]
reported the synthesis, characterization, and photophysical properties of Eu(L38−40)3(H2O)2
and Eu(L38−40)3(TPPO)(H2O), where TPPO=triphenylphosphine oxide. An ORTEP molecu- lar structure diagram determined by single-crystal X-ray diffraction for the asymmetric unit of Eu(L40)3(TPPO)(H2O) is shown in Figure 2.14. The coordination geometry of the metal center is best described as a distorted bicapped trigonal prism with the trigonal prism being composed of six oxygen atoms (O1, O2, O3, O5, O6, O8). Of these, O1, O2 and O5, and O6 are from twoβ-diketones, and O3 and O8 are from the third diketone and a water molecule, respec- tively. Another two oxygen atoms (O4, O7) cap the two quadrilateral faces O3–O5–O8–O1 and O2–O6–O3–O1, respectively.
Wang and Huang and coworkers [70] reported the synthesis, photoluminescent, and electroluminescent properties of two Eu(III) mixed-ligand complexes, [Eu(L5)3(PBO)]
and [Eu(L5)3(PBT)] [PBO=2-(2-pyridyl)-1,3-benzoxazole, and PBT=2-(2-pyridyl)-1,3- benzothiazole]. Single-crystal X-ray diffraction analysis of [Eu(L5)3(PBO)] (see Figure 2.15) showed that it is eight-coordinated by three bidentate L5anions and one bidentate N,O-chelated PBO molecule. This is a rare example of a preference for N–O coordination rather than N–N coordination by a lanthanide ion.
C47 O6
O3
O5
O4 O7
O2 O1
O8
P1 Eu1
C25 C43
C29
C11 C7
Figure 2.14 ORTEP diagram of Eu(L40)3(TPPO)(H2O) with the thermal ellipsoids drawn at the 30%
probability level and the H atoms removed for clarity [48]. (Reprinted with permission from M. Shi, F. Li, T. Yi, D. Zhang, H. Hu and C. Huang, “Tuning the triplet energy levels of pyrazolone ligands to match the5D0level of europium(III),’’Inorganic Chemistry,44, 8929–8936, 2005. © 2005 American Chemical Society.)
Tanase et al. [40] reported mononuclear lanthanoid complexes with the general for- mula of [Ln(HL27)3(CH3OH)2]·CH3OH·2H2O (Ln3+=Pr, Nd, Eu, Gd) by using a new β-diketone H2L27. As shown in Figure 2.16, the single-crystal X-ray structure studies on [Eu(HL27)3(CH3OH)2]·CH3OH·2H2O indicate that the Eu(III) ion is eight-coordinated by six oxygen atoms from three monodeprotonated HL27 and by two oxygen atoms from two coordinated methanol molecules, and that the coordination geometry is based on a distorted square-antiprism. A three-dimensional network is formed by the intramolecular hydrogen bonding (OH· · ·O), intermolecular hydrogen-bonding interactions between the coordinated methanol molecule and the non-coordinated methanol molecule.
By using polyfluorinated β-diketones of HL3 and HL4, and the polyfluorinated phos- phine oxide of tris(pentafluorophenyl)phosphine oxide [OP(C6F5)3] as ligands, Monguzzi et al. [23c] synthesized and structurally characterized the NIR emissive Er3+ complexes, [Er(L3)3(OP(C6F5)3)2], and [Er(L4)3(OP(C6F5)3)2] (see Figures 2.17 and 2.18). Very recently, by using perfluorinated nitrosopyrazolone 3-trifluoromethyl-4-hydroxyimino-1- perfluorophenyl-1H-pyrazol-5-one and OP(C6F5)3as ligands, they have reported new types
C3
C4 C6
F2 S2 O1
O2 O3
C8F1
C9
F3 O7 C5 S1
C27 C28
N2 C35
C34
C33 C32
C36C30 C31
C25 C26 C29
O7 N1
C21 C17 C20
C18
C19
C22 C24
C23
C15 C16
C14 C11 C10 C12
Eu1 C13
O6 O4 S3 O5
F8
F9 F7
F5
F6 F4
Figure 2.15 Asymmetric unit of [Eu(L5)3(PBO)] with atom numbering scheme and thermal ellipsoids (30%) [70]. (Reproduced with permission from L.H. Gao, M. Guan, K.Z. Wang, L.P. Jin and C.H. Huang,
“A comparative study of the optical and electroluminescent properties of EuIIIcomplexes with TTA and 2- (2-pyridyl)azoles: the crystal structure of [Eu(TTA)3(PBO)],’’European Journal of Inorganic Chemistry, 2006,2006, 3731–3737. © Wiley-VCH Verlag GmbH & Co. KGaA.)
of NIR emissive Er3+ complexes that possess an NIR emission with lifetimes as long as 16µs.[71]. These chelates have the advantageous characteristics of nonhygroscopic, solution processable, high solubility providing processability, low optical gap enabling visible region pumping by commercially available LEDs, and the long NIR emission lifetimes.
Zhanget al.have recently xerogel-bonded Ln complex (Ln = Er, Nd, Yb, Sm) materials and structurally characterized the NIR luminescent model complexes Ln(L8)3phen (Ln=Er, Nd, Yb, Sm, and phen=1,10-phenanthroline) (see Figure 2.19 for the molecular structure of the Nd complex) [35].
2-Phenyl-4-aroyl-5-isoxazolones are an interesting family ofβ-diketone ligands for prepar- ing promising Eu(III) and Tb(III) complex-based light-conversion molecular devices [51a,b].
Reddyet al.[51a,b] reported crystal structures for such types of ligand HL42based lanthanide complexes Tb(L42)3(H2O)2(Figure 2.20) and Eu(L42)3·phen (Figure 2.21).
O1a
O3a O23 O5a
O1c O3c
O5c O5b O3b O21 O1b Eu1
Figure 2.16 View of the molecular structure of [Eu(HL27)3(CH3OH)2]·CH3OH·2H2O. The non- coordinated methanol molecule and hydrogen atoms were omitted for clarity [40]. (Reprinted from Polyhedron, 28, S. Tanase, M. Viciano-Chumillas, J.M.M. Smits, R. de Gelder and J. Reedijk, “Cop- per(II) and lanthanoid(III) complexes of a newβ-diketonate ligand with an appended non-coordinating phenol group,’’ 457–460, 2009, with permission from Elsevier.)
This group have also recently reported the crystal structure of Ln(L42)3(DPEPO) [Ln=Eu, Tb; DPEPO=bis(2-(diphenylphosphino)phenyl) ether oxide]) [51c]. As shown in the crys- tal structure of Eu(L42)3(DPEPO in Figure 2.22, the central Eu3+ion is coordinated by six oxygen atoms furnished by three bidentateβ-diketonate ligands and two oxygen atoms from the bidentate DPEPO ligand. The overall molecular geometry is distorted square prismatic.
Interestingly, there are molecular ladder structures that are held together byπ· · ·πand inter- molecular hydrogen-bonding interactions (see Figure 2.23). The replacement of the solvent molecules in Eu(L42)3(C2H5OH)(H2O) by a chelating phosphine oxide leads to an impressive enhancement in both the overall quantum yield (from 2 to 30%) and the5D0lifetime (from 250 to 1060◦µs). Furthermore, the substantial contribution of the ancillary ligand to the overall sensitization process for Eu3+-centered luminescence in Eu(L42)3(DPEPO) is confirmed by an increase in the intrinsic quantum yield from 26 to 59% and the substantial enhancement of sensitization yield (sen) from 8 to 45%.
By using a polyfluorinated alkyl group containing β-diketone HL17 as the ligands, and chelate phosphine oxide ligands of 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene oxide (DDXPO) and bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO) as co-ligands,
P1 O7
O6 O2 Er O1
O5 O3
O8 P2 O4
Figure 2.17 ORTEP view of the [Er(L3)3(OP(C6F5)3)2] molecule [23c]. (Reprinted with permission from A. Monguzzi, R. Tubino, F. Meinardiet al., “Novel Er3+ perfluorinated complexes for broad- band sensitized near infrared emission,’’Chemistry of Materials,21, 128–135, 2009. © 2009 American Chemical Society.)
O2 O1
Er P2
O8 O4 O5
O3 P1
O6 O7
Figure 2.18 ORTEP view of the [Er(L4)3(OP(C6F5)3)2] molecule [23c]. (Reprinted with permission from A. Monguzzi, R. Tubino, F. Meinardiet al., “Novel Er3+ perfluorinated complexes for broad- band sensitized near infrared emission,’’Chemistry of Materials,21, 128–135, 2009. © 2009 American Chemical Society.)
Eu(L17)3(DDXPO) and Eu(L17)3(DPEPO) were recently synthesized and structurally charac- terized (see Figures 2.24 and 2.25), with the coordination polyhedra being a distorted square antiprism [34]. The former complex has a solid-state photoluminescence quantum yield of 48%, about two times higher than that of the latter (28%).
F4 F1
F2
F5 F3
F10
F8 O3 O1
O5
O4 O2 O6
N1
N2 Nd1
F9 F6
F7
F13 F14 F15
F11 F12
Figure 2.19 ORTEP plot for Nd(L8)3phen with ellipsoids drawn at the 30% probability level. Hydrogen atoms omitted for clarity [35]. (Reproduced from J. Feng, J.B. Yu, S.Y. Song, L.N. Sun, W.Q. Fan, XM.
Guo, S. Dang and H.J. Zhang, “Near-infrared luminescent xerogel materials covalently bonded with ternary lanthanide [Er(III), Nd(III), Yb(III), Sm(III)] complexes,’’Dalton Transactions,13, 2406–2414, 2009, by permission of the Royal Society of Chemistry.)
Bunzliet al. reported the crystal structure of Nd(L34)3(phen) (see Figure 2.26), which is eight-coordinated with a square antiprism coordination polyhedron [44]. They demonstrated that the 1,3-diketone ligands HL34containing push–pull chromophores are suitable for visible light excitation of NIR emitting lanthanide ions. The main advantage of their reported ligand is its lowest-energy absorption transition, which extends into the visible range and allows excitation of lanthanide luminescence with wavelengths up to 550◦nm.
Pettinariet al. [49] reported a Zundel ion H5O+2 stabilized tetrakis(β-diketonate) europium complex, H5O+2[Eu(L41)4]. The complex anion in this ion-pair complex is charge balanced by the Zundel cations H5O+2, which is stabilized by strong H bonding with the N atoms of the anionic heterocyclic ligand L41. The crystal structure study revealed that different [Eu(L41)4]−
O4 O2
O3
O6 O1 O11
O5 O10 Tb1
Figure 2.20 Asymmetric unit of complexes Tb(L42)3(H2O)2, thermal ellipsoids drawn with 30%
probability and hydrogen atoms omitted for clarity [51a]. (Reproduced from S. Iju, M.L.P. Reddy, A.H. Cowley and K.V. Vasudevan, “3-Phenyl-4-acyl-5-isoxazolonate complex of Tb3+doped into poly- β-hydroxybutyrate matrix as a promising light-conversion molecular device,’’ Journal of Materials Chemistry,19, 5179–5187, 2009, by permission of the Royal Society of Chemistry.)
O5
O1 O8
O7 O9 O6
O2 O4
N2
N4
N1 O3 N5
Eu1 N3
Figure 2.21 Asymmetric unit of complex Eu(L42)3·phen [51b]. (Reprinted with permission from S. Biju, D.B.A. Raj, M.L.P. Reddy and B.M. Kariuki, “Synthesis, crystal structure, and luminescent properties of novel Eu3+heterocyclicβ-diketonate complexes with bidentate nitrogen donors,’’Inorganic Chemistry, 45, 10651–10660, 2006. © 2006 American Chemical Society.)
O8
O12 O11 O2 O7
O5
O4 O1 Eu1
Figure 2.22 Asymmetric unit of complex Eu(L42)3(DPEPO). Thermal ellipsoids are shown at the 30%
probability level and all hydrogen atoms have been omitted for clarity [51c]. (Reprinted with permission from S. Biju, M.L.P. Reddy, A.H. Cowley and K.V. Vasudevan, “Molecular ladders of lanthanide-3- phenyl-4-benzoyl-5-isoxazolonate and bis(2-(diphenylphosphino)phenyl) ether oxide complexes: the role of the ancillary ligand in the sensitization of Eu3+and Tb3+luminescence,’’Crystal Growth and Design,9, 3562–3569, 2009. © 2009 American Chemical Society.)
anions in the crystal are connected by [H5O2]+ bridges. In [Eu(L41)4]−, the Eu is eight- coordinated by four bidentate acylpyrazolonate moieties, the metal center geometry being well described as a square antiprism with square planes. This family of complexes may have both luminescent and proton conductive properties.
By using a bis(β-diketone) of 4-sebacoylbis(1-phenyl-3-methyl-5-pyrazolone (H2L58) as a ligand, and sodium dibenzo-18-crown-6 [Na(DB18C6)] as the counter cation, Remyaet al. syn- thesized and structurally characterized [Tb(L58)2][Na(DB18C6)H2O] (see Figure 2.27) [59].
The crystal structure of [Tb(L58)2][Na(DB18C6)H2O] is a one-dimensional molecular ladder structure based on C−H/π, intra- and intermolecular hydrogen-bonding interactions featur- ing a Tb3+center surrounded by two tetradentate bis-pyrazolone L58in a somewhat distorted square-antiprismatic geometry. The Na+coordination environment is distorted hexagonal pyra- midal and involves six oxygen atoms furnished by DB18C6 and one oxygen atom from a water molecule. The quantum yields and5D4lifetimes for [Tb(L58)2][Na(DB18C6)H2O] in the solid state were found to be 18.13% and and 2.82◦ms, respectively.
Li and Huanget al. reported a series of dendrimerβ-diketonate lanthanide complexes and structurally characterized Tb(L59)3(H2O)2[62]. They confirmed that dendrimerβ-diketonate lanthanide complexes exhibited enhanced emission due to the light-harvesting antenna and shell effects.
H59 C59 C60 H60 N3 O9
Figure 2.23 Molecular ladder of complex Eu(L42)3(DPEPO) involvingπ· · ·πinteractions (C4-C9) and intermolecular hydrogen bonding interactions (C59-H59· · ·O9, C60 – H60· · ·N3), when viewed along the direction of thec-axis [51c]. (Reprinted with permission from S. Biju, M.L.P. Reddy, A.H. Cowley and K.V. Vasudevan, “Molecular ladders of lanthanide-3-phenyl-4-benzoyl-5- isoxazolonate and bis(2- (diphenylphosphino)phenyl) ether oxide complexes: the role of the ancillary ligand in the sensitization of Eu3+and Tb3+luminescence,’’Crystal Growth and Design,9, 3562–3569, 2009. © 2009 American Chemical Society.)