Acknowledgments
N- Based Rare Earth Complexes
4.3 Rare Earth Complexes with N-Heterocyclic Type Ligands
4.3.1 Rare Earth Complexes with Pyridine Type Ligands
(a) Si1C
N1A Si1A
Si1B
Ce1A
Si1E Si1D
Si1 N1
N2 N4
N3 Ce1
Si2
Si3 N1D I1
N2A N1 N1E
N2
Si1
Ce1 Cl1 NlB
N1C
(b)
Figure 4.10 The structure of complex (a) [{Ce(8)}2(µ-Cl)] and (b) [Ce(8)I] [14]. (Reproduced with permission from C. Morton et al., ‘Stabilization of cerium(IV) in the presence of an iodide ligand:
remarkable effects of Lewis acidity on valence state,’’Journal of the American Chemical Society,121, 11255, 1999. © 1999 American Chemical Society.)
and magnetic resonances for the triamidoamine ligands between 220 and 300 K. Different to [{Ce(8)}2(µ-Cl)], [Ce(8)I] crystallizes in the monoclinic space groupP21/n.
N N
N N
11 9
N N
COOH 10
N
N N
12
N
N N
NBu2
13
N
N N
14
N N
15
N N
16
Figure 4.11 Eight representative pyridine type ligands.
Sm) based onβ-diketonates with bipyridine (bipy) were studied [17]. Even more recently, a series of chiral rare earth complexes with modified 2,2-bpy type ligand10were also investi- gated [18]. Research results revealed that a solvent-adaptive crystallization process exists in this system. When the complex was prepared from a mixture of Pr(III) ion and ligand 10 in the same metal-to-ligand ratio (1 : 2.25), two distinct self-assembly pathways led to two diastereoselective enantiopure architectures, a 2D-trinuclear array (in methanol) [Pr3{(+)- (10)}(µ3-OH)(H2O)3](ClO4)2 and a 3D-tetranuclear pyramidal polyhedron (in acetonitrile) [Pr4{(+)-(10)}9(µ3-OH)](ClO4)2. Interestingly, the mixture containing compound [Pr4{(+)- (10)}9(µ3-OH)](ClO4)2 and the minor species in CD3CN could be recovered back into compound [Pr3{(+)-(10)}(µ3-OH)(H2O)3](ClO4)2in the presence of water. The crystal struc- tures of [Pr4{(+)-(10)}9(µ3-OH)](ClO4)2(a) and [Pr3{(+)-(10)}(µ3-OH)(H2O)3](ClO4)2(b) are shown in Figure 4.12. For [Pr4{(+)-(10)}9(µ3-OH)](ClO4)2, the coordination polyhedron is a pseudo-trigonal-pyramidal structure and the three tridentated ligands10coordinated to the praseodymium ion through two nitrogen atoms from bipyridine and one oxygen atom from the carboxylate wrap helically around this stereogenic metal center, forming a chiral complex.
The basis of this metallic framework is a pseudoequilateral triangle, which is held together by six bridging ligands that are divided into two sets of three, and aµ3-OH group, whose oxygen atom is situated on the pyramidal pseudo-C3axis. The fourth metal cation is situated above the triangular base on the pseudo-C3axis on the same side as theµ3-OH group.
(a) (b)
Figure 4.12 The structure of (a) [Pr4{(+)-(10)}9(µ3-OH)](ClO4)2 and (b) [Pr3{(+)-(10)} (µ3- OH)(H2O)3] (ClO4)2[18b]. (Reproduced with permission from O. Mamula, M. Lama, H.S. Evans and S. Shova, “Switchable chiral architectures containing PrIII ions: an example of solvent-induced adaptive behavior,’’Angewandte Chemie International Edition, 2006,45, 4940. © Wiley-VCH Verlag GmbH &
Co. KgaA.)
4.3.1.2 Rare Earth Complexes with 4,4-Bipyridine (11) Type Ligands
Complexes of rare erath salts with 4,4-bipyridine for nitrate [19], picrate [20], and chloride [21] counteranions, have been reported. Using the mixed solvent of ethanol and water, these complexes can be easily synthesized and complexes of mononuclearity, binuclearity, or higher nuclearity structures were obtained. Among them, only the nitrate complexes have been thor- oughly investigated. The complexation properties of these compounds are sensitive to the solvent of crystallization. Sometimes, any slight variation in the nature of the solvent, acidity, basicity, or the ratio of the mixed solvent leads to a drastic change in the complexes formed.
According to the careful examination over more than 20 RE(NO3)3complexes in the Cambridge Structural Database with11, and 4,4-bpy ligands, bpy may be classified into three structurally distinct series: 4,4-bpy, [4,4-bpyH]+obtained from 4,4-bpy and hydrogen ion, and 4,4-bpy cation–nitrate anion pairs [22]. Usually, the nitrate anions bind in a chelating manner to the rare earth cation in all cases. However, 4,4-bpy has three modes, the neutral 4,4-bpy without coordination, [4,4-bpyH]+, and the coordination complexes between the nitrogen of pyridine and the rare earth ion. In some case, all these three modes can be included in the same complex, for example, the compound of [4,4-bpyH]2[(µ2-4,4-bpy)Nd2(NO3)8(H2O)4]·3(4,4-bpy), Figure 4.13 [23]. The complex belongs to a monoclinic space groupP21/cwitha=1.8723(10) (nm),b=1.0720(6) (nm),c=1.8027(10) (nm),β=94.43(5)◦, Z=2. The neodymium ion is ten-coordinate with one nitrogen atom from 4,4-bpy, six oxygens from the bidentate nitrate, one oxygen from the unidentate nitrate, and two oxygens from water. The bond length of Nd–N is 0.2701 (6) nm. The Nd–O (w) distances are 0.2429 (7) and 0.2460 (6) nm, respectively. The bond length of Nd–O formed from the unidentate nitrate is 0.2531 (6) nm and of the others formed from bidentate nitrate amounts 0.2529 (6), 0.2611 (6), with an average of 0.2566 nm.
O10A
O8 O1
O2 O4
O5
O13 O14
O10
O13A O5A
O4A O8A O14A
N2A
N3A O7A O1A N1A N5A Nd1A
O2A N5
Nd1 N2
N3 N1
Figure 4.13 The structure of complex [(µ2-11)Nd2(NO3)8(H2O)4]2− [23]. (Reproduced fromInor- ganica Chimica Acta, 95, T.J.R. Weakley, “The crystal structures of 4,4-bipyridinium µ-(4,4- bipyridine)bis[diaquatetranitratoneodymate(III)]-tris(4,4-bipyridine) and a second monoclinic form of triaquatrinitratoholmium(III) – bis (4,4-bipyridine),’’ 317, 1984, with permission from Elsevier.)
Ligand11can act as a linear bridging hydrogen-bond acceptor, so different types of com- plexes have been reported with structures with higher nuclearity, including interpenetrating 2D-networks [23], 3D-networks with small nitrate containing cavities [24], 3D-networks with- out significant cavities [25], and unusual self-catenating 3D-hydrogen-bonded arrays [26].
Figure 4.14 shows a full packing diagram of a 3D-hydrogen-bonded network of the complex [Yb(H2O)8]4(11)9.5C11·24.5(H2O) [27]. This complex possesses a square antiprismatic geo- metry and contains four [Yb(H2O)8]3+cations and 12 Cl anions. Each [Yb(H2O)8]3+cation hydrogen bonds to four bpy molecules, while each bpy molecule is a hydrogen-bond acceptor for two [Yb(H2O)8]3+cations, creating a 3D-network. Either bpy and water guests or just water guests occupy the channels of the network.
4.3.1.3 Rare Earth Complexes with 2,2,2-Bipyridine (12) Type Ligands
Terpyridine (tpy) type compounds are versatile ligands with three nitrogen donor atoms, which allow them to act as tridentate ligands. The complexes formed can be formulated as RE(12)nX3. The number of 12 was determined by the rare earth salts used for preparation of the cor- responding complexes. For example, for LnCl3 and LnBr3, the number of tpy is one and two, respectively. However, the number of tpy changes to three and one, respectively, when Ln(ClO4)3and Ln(NO3)3are used. In order to improve the varieties of12coordination mod- els and to endow the complexes with particular structure and properties, terpyridine was often modified with functional groups such as carbonyl or larger steric groups. For example, a more rigid terpyridine type ligand13was synthesized by introducing ap-dibutylamino-phenyl
Figure 4.14 Full packing diagram shows a 3D hydrogen-bonded network with channels containing either bpy and water guests or water guest only [27]. (Reprinted with permission from L. Cunha-Silva, A.
Westcott, N. Whitford, and M.J. Hardie, “Hydrogen-bonded 3-D network structures of lanthanide aquo ions and 4,4-bipyridine with carbaborane anions,’’Crystal Growth and Design,6, no. 3, 726–735, 2006.
© 2006 American Chemical Society.)
moiety into the 4-position of the central pyridinic ring, which can induce an intraligand charge- transfer transition from the amino donor to the pyridine acceptor group [28]. Interestingly, in order to force a cisoid conformation, a dimethylene annelation between the central pyri- dine ring and the distal quinoline moieties was introduced into the molecule. This is also helpful to stabilize their rare earth complexes by prohibiting the coordination of the solvent molecule. On treatment of ligand13 with 1 equiv of RE(NO3)3·xH2O (x=6 for RE=La, Gd, andx=5 for RE=Dy, Yb, and Y) in the mixed solvent of dichloromethane and acetoni- trile, brown-orange complexes of RE(13)(NO3)3were obtained in good yield after repeated crystallization from hot aectonitrile and a dichloromethan–pentane mixture, respectively. The complex [Gd(13)(NO3)3·½CH3CN·½H2O belongs to a centrosymmetric space group with a=4.8415 (nm), c=1.0628 (nm), γ=120◦, V=7.1913 nm3, and Z=18 (Figure 4.15).
Because of the intermolecular hydrogen bonding between non-coordinated oxygen atoms of the nitrato and protonated NR2-phenyl unit, the molecule adopts a head-to-tail configuration.
The central metal is nine-coordinate with three nitrogens from ligand13and six oxygens from nitrato ligands. The bite angle and distance between the two distal phenyl rings are=87.9◦ andd=0.685 nm, respectively, which are significantly smaller than in other complexes with tpy type ligands, for instance12Lu(NO3)3 (=105◦,d=0.823 nm) [29]. The Gd–Ncentral
bond length, 0.2445(9) nm, is remarkably shorter than that for the complex formed with the tpy ligand14 [30]. It is worth noting that no additional solvent molecule is coordinated to the metal in [Gd(13)(NO3)3·½CH3CN·½H2O. This seems unusual in the complexes formed
N2 N1
N3 N5
N8 0.603
0.4640.632 0.634 Y1 N7
(a) (b)
Figure 4.15 The structure of (a) complex [Gd(13)(NO3)3·1/2CH3CN·1/2H2O and (b) the head-to-tail stacking [29]. (Reprinted with permission from E. Terazzi,et al., “Molecular control of macroscopic cubic, columnar, and lamellar organizations in luminescent lanthanide-containing thermotropic liquid crystals,’’Journal of the American Chemical Society, 127, no. 3, 888–903, 2005. © 2005 American Chemical Society.)
from rare earth metals with other tpy type ligands, whose coordination number is generally 10 or even 11, in which the rare earth metal forms additional coordination bond(s) with solvent molecules such as water, acetonitrile, and methanol. This might be ascribed to the largerr steric hindrance of the ligand13. It has also been revealed that the metal f electrons also contribute to the NLO activity of Ln(13)(NO3)3.
4.3.1.4 Rare Earth Complexes with 1,10-Phenanthroline (15) Type Ligands
When 1,10-phenanthroline (phen) was reacted with rare earth salts of nitrate, acetate, thio- cyanate, and chloride, rare earth complexes with an RE : phen ratio of 1 : 2 can be obtained.
Because of the weaker coordination ability of the perchlorate, the RE : phen ratio of coordi- nation compounds for perchlorate (RE=Dy, Er, Yb) will increase to 1 : 3 or even 1 : 4 in the case of RE=La, Pr, Nd. Similar to the 2,2-bipyridine type ligands, phen often has two modes in the complexes, one as the neutral phen without coordination and the other coordinating with the rare earth ion. For instance, in the complex of [Nd(15)3(NCS)3]·EtOH (Figure 4.16a) [31], the rare earth ion is nine-coordinate and bound to six nitrogen atoms from the three bidentate 1,10-phenanthroline ligands and three nitrogen atoms from the three monodentate thiocyanate groups. In addition, no coordinated water molecule or ionic thiocyanate group is present. If the system has a bridging group, binuclear or multinuclear complexes can be obtained. Li and coworkers reported the synthesis and crystal structure of the dinuclear com- plex [Ho2(15)4(H2O)4(OH)2](15)2(NO3)4 (Figure 4.16b) [32]. This complex can be easily synthesized from the reaction of nitrated Ho2O3with 1,10-phenanthroline in CH3OH–H2O.
Single crystal X-ray diffraction analysis shows that [Ho2(15)4(H2O)4(OH)2](15)2(NO3)4crys- tallizes in the triclinic space groupP1 (No. 2) with the cell dimensions:a=1. 1241 (1) nm, b=1. 1439 (1) nm,c=1. 4058 (1) nm,α=93.989 (7)◦,β=98.173 (7)◦, γ=108.19 (1)◦,
N6 N4
N5 Nd1
N9
S3 S2
N8 N7
N3 S1
N1
N2 O2A
Ho1A
Ho1 N1
N4A N3A N2
N1A N2A
O1A O3A N3
N4 O2
O3 O1
(a) (b)
Figure 4.16 The structures of (a) Nd(15)3(NCS) and (b) [Ho(15)4(H2O)(OH)]4+[31, 32]. (Reproduced fromPolyhedron, 22, S.A. Cottonet al., “Synthesis of complexes of 2,2:6,2-terpyridine and 1,10- phenanthroline with lanthanide thiocyanates; the molecular structures of [Ln(terpy)2(NCS)3] (Ln=Pr, Nd), [Nd(terpy)2(NCS)3]·2EtOH and [Ln(phen)3(NCS)3]·EtOH (Ln=Pr, Nd),’’ 1489, 2003, with per- mission from Elsevier; and redrawn from D.Y. Wei, Y.Q. Zheng and J.L. Lin, “Synthesis, crystal structure and magnetic property of [Ho2(phen)4(H2O)4(OH)2](phen)2(NO3)4,’’Acta Chimica Sinica, 7, 1248, 2002.)
V=1.6874 (4) nm3, andZ=1. The compound is a square antiprism, consisting of the cen- trosymmetric dinuclear [Ho2(15)4(H2O)4(OH)2]+4 cation, uncoordinated 15molecules, and nitrate anions. The holmium atom is eight-coordinate with four nitrogens from the15ligands, two oxygens from H2O molecules, and two oxygens from hydroxo groups.
4.3.1.5 Rare Earth Complexes with 1,8-Naphthylridine (16) Type Ligands
Rare earth complexes with 1,8-naphthylridine (ntd) ligands can be formulated as RE(ntd)nX3
(H2O)x. When the complexes were synthesized with perchlorate [X=(ClO4)−], the coordi- nation number ofn changes along with the rare earth species. For RE=La, Ce, and Pr, n amounts to 6. Along with the decrease in the rare earth ionic radius to RE=Nd, Sm, and, Eu, the coordination number of the16ligand changes to 5. Owing to the comparatively stronger coordination ability, when the rare earth complexes were prepared with nitrate, the coordina- tion number of ligands16in RE(16)nX3(H2O)x[X=(NO3)−] becomes even smaller. This is exemplified by the coordination number of two for RE(16)nX3(H2O)x[X=(NO3)−, RE=Y, Sm–Yb]. However, as expected, the coordination number of ligands16increases to three when the rare earth metal with larger ionic radius such as La and Nd was taking part in the reaction.
Figure 4.17 shows the crystal structure of complex [Pr(16)6](ClO4)3[33]. The crystal is mon- oclinic with a space group ofP21/c. The cell dimensions area=1.3748 (3) nm,b=1.6979 (6) nm,c=2.2949 (8) nm,β=107.34 (1)◦,V=5.11314 nm3, andZ=4. Each16ring acts as a bidentate ligand, making the praseodymium atom 12-coordinate. The coordination polyhedron
N5 N11
N8 N6 N9
N1 N2
N10
N4 N3 N7
Pr1 N12
Figure 4.17 The structure of [Pr(16)6]3+ [33]. (Reproduced with permission from A. Clearfield, R. Gopal and R.W. Olsen, “Crystal structure of hexakis(1,8-naphthyridine)praseodymium(III) perchlo- rate,’’Inorganic Chemistry,16, 911, 1977. © 1977 American Chemical Society.)
is a distorted icosahedron, which results principally from the unequal nitrogen–nitrogen inter- atomic distance. The one appearing within individual ntd ring amounts to 0.2257 (12) nm, while that between adjacent nitrogen atoms in two different ntd rings ranges from 0.2890 (16) to 0.3195 (16) nm.