Acknowledgments
N- Based Rare Earth Complexes
4.4 Rare Earth Complexes with Schiff Base Type Ligands
4.4.1 Rare Earth Complexes with Imine Type Ligands
Imine type Schiff base ligands have been revealed to possess stronger coordination ability, resulting in an emerging interest in rare earth separations using this type of chelating ligand.
Thus far, many macrocyclic and macroacyclic imine type Schiff base ligands have been pre- pared and their single crystal structures reported [68]. Figure 4.42 shows some representative compounds.
It is well known that imine is not a very stable compound and is easy to hydrolyze. For macrocyclic imine type Schiff base ligands, synthesis in the metal-free form with high yield becomes more difficult due to their side polymerization reactions. One effective method to fix this problem involves anin situreaction for adding the rare earth metal in the cyclization process [69]. In other words, the rare earth ions act as a templating ions, directing the condensation preferentially to cyclic rather than polymeric products. Thus far, many macrocyclic imine type Schiff base rare earth complexes have been synthesized. For example, reaction of hydrated rare earth nitrate, 1,2-diaminoethane, and 2,6-diacetylpyridine in the ratio of 1 : 2 : 2 in refluxing methanol for 4–6 hours gave the macrocyclic complex [RE(24)(NO3)3] (RE=La, Ce) [70].
The compound [La(24)(NO3)3] crystallizes in a monoclinic system and space groupP21/cwith cell constantsa=1.6113(5) (nm),b=0.9782(2) (nm),c=1.7901(5) (nm),β=95.92(3)◦, and
N
N N
N N
N
H H
H H N
N N
N N
N
N
N N
N HN
N
H H
H
H OH
N
N N
N N
N
N N
N OH
N
HO NH NH
H3C NH2
24 25 26
27 28 29
Figure 4.42 Six representative imine type ligands.
Z=4. The crystal structure is shown in the Figure 4.43. The La coordination polyhedron can be depicted as a folded butterfly configuration. Viewed from the two flexible –CH2–CH2– lateral units, the whole molecule can be divided into two hemispheres, one above and one below the donor atom plane. This structure effectively relaxes the repulsion among the coordination atoms and allows the central metal ion to attain its highest possible coordination number, 12.
The donor atoms consist of six nitrogen atoms from one macrocyclic24and four nitrogen atoms from two bidentate nitrates on one side of the macrocycle and the other bidentate nitrate on the opposite site.
It must be pointed out that although the rare earth complexes with ligand24(RE=La, Ce) were successfully prepared, attempts at obtaining analogous complexes of heavier rare earth metals failed. To improve the validity of this method, De Cola andet al. optimized the reaction conditions by adopting a more appropriate counterion ClO4−instead of NO3−, strictly controlling dehydration of the rare earth perchlorate, and adding∼0.1 M Cl− [71].
As a total result, a series of rare earth complexes [RE(24)(ClO4)2(OH)·nH2O] (n=0 for RE=La, Ce, Pr, Nd, Sm, and Er;n=1 for Eu;n=2 for RE=Gd and Tb) were obtained.
The range of rare earth species was further extended to the whole series of rare earth metals except for Pm by using the rare earth acetate, providing RE(24)(CH3COO)2Cl·nH2O [72].
O3
O1 O2
N1 N7
N2
La1 O5
O4 O6
O8
O7
O9 N9
N6
N8 N3
N4 N5
Figure 4.43 The structure of [La(24)(NO3)3] [70]. (Reproduced from J.D.J Backers-Dirks et al.,
“Preparation and properties of complexes of lanthanides with a hexadentate nitrogen-donor macro- cycle: X-ray crystal structure of the complex [La(NO3)3l],’’Journal of the Chemical Society, Chemical Communications, 774, 1979, by permission of The Royal Society of Chemistry.)
The ease and good yield of this reaction was ascribed to the CH3COO− counterion, which favors the reaction more than Cl−or ClO4−. It is worth noting that in this series of complexes the IR absorptions for symmetric and antisymmetric stretching of CH3COO−, in particular the separation between these two peaks,ν, can be used to estimate the coordination mode of the CH3COO−ligands. The absorptions exhibiting largerνvalue contribute from the unidentate ionic acetate, whereas those with smallerνvalue are due to the bidentate chelating acetate.
In detail, a strong absorption broadly centered at 1540 cm−1together with shoulders at 1550 and 1530 cm−1, respectively, in the IR spectra of RE(24)(CH3COO)2Cl·nH2O is due to the antisymmetric COO−stretching, and a pair of strong bands observed at 1445 and 1430 cm−1 with shoulders at 1443 and 1460 cm−1due to the symmetric COO−stretching. Accordingly, the vibrations at 1550 and 1430 cm−1withν=120 cm−1were assigned to the ionic acetate, whereas those at 1540 and 1455 cm−1 withν=85 cm−1 were assigned to the bidentate chleating acetate. On the basis of this result, these complexes can be further formulated as [RE(24)(CH3COO)(H2O)](CH3COO)C1·nH20, in which the central rare earth ion achieves the usually observed nine-coordinated mode.
By using the template directed cyclization between 2,6-pyridinedicarbaldehyde and ethyl- diamine in the presence of rare earth nitrate salts, a series of corresponding complexes
N5 N4
N3
N2 N1 N6
N7
O1 O3 O2
O4 O5 Sm1
Figure 4.44 Structure of [Sm(26)(NO3)(OH)(H2O)]+ [73]. (Reproduced from Polyhedron, 23, F.B. Tambouraet al., “Structural studies of bis-(2,6-diacetylpyridine-bis-(phenylhydrazone)) and X-ray structure of its Y(III), Pr(III), Sm(III) and Er(III) complex,’’ 1191, 2004, with permission from Elsevier.)
formulated as [RE(26)(NO3)3]·nH2O were provided [73]. The IR spectra of the heavier rare earth (Nd–Lu except for Eu and Pm) complexes are different from those of the lighter rare earth counterparts for RE=La–Pr and Eu with respect to the former group exhibiting a distinctive sharp band atabout3220 cm−1, assigned to a secondary amine group. Previous research of the transition metal complexes with this ligand indicated that addition of a water molecule across the imine double bond led to the formation of a carbinolamine species, ligand 27.
This induces an increase in the flexibility of the macrocycle, making it capable of accom- modating smaller metal cations. Obviously, the IR spectral result for the heavier rare earth complexes proves the existence of ligand27in [RE(26)(NO3)3]·nH2O. It is worth noting that carbinolamine complexes might also be proved to be in the solution by13C and1H NMR spec- troscopy on the lutetium derivatives. However, significant change occurs in the IR spectrum of the samarium complex with ligand26after recrystallization from water. The sharp band appearing at 3210 cm−1assigned to the secondary amine group vanished and a new sharp band attributed to a Sm–OH group was observed at 3560 cm−1. The single crystal molecular structure of this compound shown in Figure 4.44 indicates that this complex consists of a discrete [Sm(26)(NO3)(OH)(H2O)]+cation, NO3−anions, and clathrate MeOH molecules.
The coordination polyhedron is an irregular antiprism capped on its “square’’ face by N(1) and N(4). The central samarium ion is ten-coordinate with six nitrogen atoms from one heterocycle, two nitrogen atoms from one bidentate nitrate group, one oxygen atom from the OH−ion, and one oxygen atom from a water molecule. The Sm–N (pyridine) bond length is 0.266(1) and 0.265(1) nm, significantly longer than that of Sm-N (imine), which is 0.262 nm.
According to the structure determination results, it is presumed that the reversion from the carbinolamine form to the tetraimine one exists probably because of the optimal cation-cavity criteria and the fact that the samarium ion can be accommodated by either form of the two macrocycles. Carbinolamine, acting as the intermediate of the tetraimine Schiff base27, is the kinetically favored product. In contrast, the latter species is the thermodynamically favored product. On dissolution and recrystallization in water, a higher temperature is reached than in the original reaction in alcohol, which facilitates completion of the reaction. Furthermore,
[Sm(OH)(H2O)n–1]2+ + H3O+ [Sm(H2O)n]3+
Figure 4.45 Schematic representation of the reaction to form [Sm(26)(NO3)(OH)(H2O)]NO3.
O6
O1 O4 O5 O3
O2 N4
N5 N6
N3 N2
N1
Y1
Figure 4.46 The structure of Y(28)(H2O)(NO3) [73]. (Reproduced fromPolyhedron,23, F.B. Tamboura et al., “Structural studies of bis-(2,6-diacetylpyridine-bis-(phenylhydrazone)) and X-ray structure of its Y(III), Pr(III), Sm(III) and Er(III) complex,’’ 1191, 2004, with permission from Elsevier.)
Sm(26)(NO3)3is also hydrolyzed during the recrystallization process to give the compound [Sm(26)(NO3)(OH)(H2O)]NO3. It is plausible that the sequence of the dissociation process for this complex shown in Figure 4.45 occurs.
Associated with the ramifying applications of rare earth elements, a series of rare earth com- plexes with macroacyclic imine type Schiff bases have been prepared, among which ligands 28and29are two representative compounds.
4.4.1.1 RE(28)(H2O)(NO3)
Reaction of 2,6-bis(phenylhydrazone)pyridine with Ln(NO3)3·6H2O in alcohol gave RE(28)(H2O)(NO3) (RE=Y and Er). The crystal structure of Y(28)(H2O)(NO3) is shown in Figure 4.46 [73], which belongs to the monoclinic system and crystallizes in a space group P21/n witha=0.8174(3) nm, b=1.0099(4) nm, c=2.9423(6) nm, β=9.023(2)◦, V=2.4287(9) nm3, and Z=4. The central yttrium ion is eight-coordinate and the donor atoms consist of two oxygen atoms from the iminolic of hydrazone, two nitrogen atoms from the imines of hydrazone, one nitrogen atom from pyridine, two oxygen atoms from the bidentate nitrate group, and one coordinated water molecule. The Y–O(hydrazonic) distance is 0.2265(4) and 0.2268(4) nm and that of Y–N(hydrazonic) is 0.2418(8) and 0.2450(8) nm. Figure 4.47 shows three isomeric forms of the ligand28in solution. Infrared and NMR data proved that in the molecule of 2,6-diacetylpyridine-bis-(benzoylhydrazone), the imine group (O=C−NH) is transformed into iminol group (HO−C(=N)) and the ligand acts with the rare earth ion in the iminol form.
N
N N
HN O
NH O
N
N N
N OH
N HO
N
N N
HN O
N HO
Figure 4.47 Schematic representation of the equilibrium of ligand28.
4.4.1.2 [{(HNdCMe)2MeCNH2}Dy(MeCN)6]I3
Reaction of DyI2with excess acetonitrile provided a yellow–brown solution [74]. Recrystal- lization of this solution resulted in the complex [{(HNdCMe)2MeCNH2}Dy(MeCN)6]I3. The crystal was revealed to contain a new type of tridentate ligand, 1,1-bis(iminoethyl)ethylamine (HN=CMe)2MeCNH2, which is prepared by the C–C coupling reaction of acetonitrile.
The complex belongs to the orthorhombic space group Pnma with the cell parameters of a=2.03218(3) (nm), b=1.38313(1) (nm), c=1.54992(1) (nm), V=4.35647(8) nm3, and Z=4. In the cell unit, four [{(HNdCMe)2MeCNH2}Dy(MeCN)6]+ cations, 12 I− anions, and four non-coordinating acetonitrile solvent molecules exist. The coordination polyhedron can be described as a distorted tricapped trigonal prism and the cation showsCs symmetry (Figure 4.48) with the Dy atom, two of the coordination MeCN molecules, and the central carbon and nitrogen atoms of the 1,1-bis(iminoethyl)ethylamine ligand located on a crystallo- graphic mirror plane. The central dysprosium ion is nine-coordinate with three nitrogen atoms from 1,1-bis(iminoethyl)ethylamine and six nitrogen atoms from acetonitrile. The Dy–Nimino
distance is 0.245 nm, shorter than that of Dy–Namino 0.251 nm. The Dy–Nacetonitrile bond length is in the range 0.248–0.253 nm. It is worth noting that due to the presence of heavy rare earth atoms, the hydrogen atoms were not localized experimentally. However, their existence is supported by the IR spectroscopy results, with the appearance of a strong absorption at 3150 cm−1.
N2 N2A
N4A N3A Dy1 N1
N4 N5 N3 N6
Figure 4.48 The structure of [{(HNdCMe)2MeCNH2}Dy(MeCN)6]3+[74]. (Reproduced with permis- sion from M.N. Bochkarev, G.V. Khoroshenkov, H. Schumann and S. Dechert, “A novel bis(imino)amine ligand as a result of acetonitrile coupling with the diiodides of Dy(II) and Tm(II),’’Journal of the American Chemical Society,125, 2894, 2003. © 2003 American Chemical Society.)