Rare Earth Complexes with Imidazole Type Ligands

Acknowledgments

N- Based Rare Earth Complexes

4.3 Rare Earth Complexes with N-Heterocyclic Type Ligands

4.3.2 Rare Earth Complexes with Imidazole Type Ligands

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.

N N N N N

R³ R³

R² R²

N H N

N N

H N

19 18

17

Figure 4.18 Four representative imidazole type ligands.

structure of a series of +2 and+3 charged samarium complexes withN-methylimidazole ligand [34]. By reacting SmI2with 4 equiv of N-methylimidazole at room temperature, a divalent complex of SmI2(17)4was first isolated. Direct crystallization of SmI2(17)4from THF led to the formation of the dimer crystal [SmI(µ-I)(17)3]2. However, crystallization from17solvent over a long period led to the hydrolyzed and oxidized complexes {[(17)4Sm]

(µ-OH)}3(µ3-OH)2}I4and Sm(17)8I3. Meanwhile, hydroxide complex [(17)5Sm(µ-OH)]2I4

was also isolated from crystallization of the trivalent samarium complex Sm(17)8I3in17. The crystal structures and comparison between crystallographic data are shown in Figure 4.19.

Through the iodide anion bridging, in the crystal of [SmI(µ-I)(17)3]2, the two samarium ions form a dimer structure, with each of them exhibiting an octahedral coordination environ- ment. The donor atoms consist of three nitrogen atoms from three terminal 17, one iodide atom from terminal iodide, and two iodide atoms from two bridiging iodide ligands. The Sm–Ibridging bond lengths are 0.328(1) and 0.3307(1) nm, longer than that of Sm–Iterminal, 0.3237(1) nm. The Sm–N distance is in the range of 0.2621(7) to 0.2641(6) nm. As opposed to the divalent samarium-containing complex [SmI(µ-I)(17)3]2with a six-coordinate number, the trivalent samarium ion is completely surrounded by17with an eight-coordinate number in Sm(17)8I3, probably due to the soft nature of Sm²⁺in comparison with Sm³⁺, which prefers to coordinate with the softer iodide anion over the harder17donor atom. However, the Sm–N distance in Sm(17)8I3is revealed to be in the range of 0.2563(6)–0.2596(6) nm, similar to that found in [SmI(µ-I)(17)3]2. At the first glance, this result appears strange but can be rationalized by the conflicting trend in the ionic radius and coordination number of the samarium ion in these two complexes. In Sm(17)8I3, the trivalent samarium ion with a smaller ionic radius should lead to a smaller Sm–N distance for this complex. In fact, the larger coordinated number for the trivalent samarium ion in the same complex also results in an increase in the Sm–N distance.

As a total consequence, both complexes exhibit a similar Sm–N distance. In addition, as the hydrolyzed product of [SmI(µ-I)(17)3]2, {[(17)4Sm](µ-OH)}3(µ3-OH)2}I4can be depicted as a dodecahedron. The samarium ion is eight-coordinate with four nitrogen atoms from the terminal 17and four oxygen atoms from four bridging hydroxides. The Sm–O(µ-OH) bond length is in the range of 0.2323(11)–0.2358(10) nm. However, the Sm–N bond length ranges from 0.2538(16) to 0.2631(12) nm, overlapping the two distinct Sm–N distances in [SmI(µ-I) (17)3]2. Unlike Sm(17)8I3and {[(17)4Sm](µ-OH)}3(µ3-OH)2}I4, the hydrolyzed product of Sm(17)8I3is seven-coordinate. In the crystal, all the terminal ligands are17and all of the bridging ligands are hydroxides.

(a) N4A

N3A I2A

I1A I1

I2 N3

N4 N1 N5

N6

N2

N2A

N4B N1A N3B

N2B

N1BN3C N1C Sm1

N1 N3 N2

N4 I1 I2

N2C N4C N4A

N3A

N5A N6A

N12

N11 N8 N2O N23

N24

N19

N17 N7 N10

N9 N15 N16

N13

N14 N6

N3 N5

N18 N4

N2

N21 N22 N10 N9 N7 N8 N6

N5 N3 N4 N1

N2 O1

O1A N2A N1A

N3A N4A

N9A

N8A N7A N6A

N5A N10A N1

O3 O4O2

O1

O5 Sm2

Sm1 Sm3

Sm1 Sm1A

N2A N1A

Sm1A

Sm1

(b)

(c) (d)

Figure 4.19 The structures of four complexes [SmI(µ-I)(17)₃]₂, Sm(17)₈I₃, {[(17)₄Sm](µ-OH)}₃(µ3- OH)₂}I₄, and [(17)₅Sm(µ-OH)]₂I₄[34]. (Reproduced with permission from W.J. Evans, G.W. Rabe and J.W. Ziller, “Utility of N-methylimidazole in isolating crystalline lanthanide iodide and hydroxide com- plexes: crystallographic characterization of octasolvated [Sm(N-MeIm)8]I3 and polymetallic [SmI(µ^- I)(N-MeIm)₃]₂, [(N-MeIm)₅Sm(µ^-OH)]2I₄, and {[(N-MeIm)₄Sm(µ^-OH)]3(µ3-OH)₂}I₄,’’ Inorganic Chemistry,33, 3072, 1994. © 1994 American Chemical Society.)

4.3.2.2 Complex [Sm(N3C12H8)2(N3C12H9)2][Sm(N3C12H8)4](N3C12H9)2

By reaction of the rare earth ion (RE=Y, Tb, Yb, La, Sm, Eu) with 2-(2-pyridine)- benzimidazole,18, two types of complexes (NC12H8(NH)2)[RE(N3C12H8)4] (RE=Y, Tb, Yb) and [RE(N3C12H8)2(N3C12H9)2][RE(N3C12H8)4](N3C12H9)2 (RE=La, Sm, Eu) were obtained [35]. The reaction was carried out by melting the amine without any solvent and the type of the complex obtained was determined by the melting temperature and the

N6 N10A

N12A N11 N8 N9

N10 N12

N6A N11A

N8A N9A

N5

N7 N2

N4 N3A

N1A

N4A N2A

N5A

N1 N3

N7A Sm1

Sm2

Figure 4.20 The structure of [Sm(N₃C₁₂H₈)₂(N₃C₁₂H₉)₂][Sm(N₃C₁₂H₈)₄](N₃C₁₂H₉)₂ [35]. (Repro- duced with permission from K. Muller-Buschbaum and C.C. Quitmann, “Two new groups of homoleptic rare earth pyridylbenzimidazolates: (NC₁₂H₈(NH)₂)[Ln(N₃C₁₂H₈)₄] with Ln=Y, Tb,Yb, and [Ln(N₃C₁₂H₈)₂(N₃C₁₂H₉)₂][Ln(N₃C₁₂H₈)₄](N₃C₁₂H₉)₂with Ln=La, Sm, Eu,’’Inorganic Chemistry, 42, 2742, 2003. © 2003 American Chemical Society.)

rare earth ionic radius. The crystal structure of the complex [Sm(N3C12H8)2(N3C12H9)2] [Sm(N3C12H8)4](N3C12H9)2 is shown in Figure 4.20. The coordination polyhedron of the cation [Sm(N3C12H8)2(N3C12H9)2]⁺can be described as a stronger distorted square antiprism, which is similar to its anion [Sm(N3C12H8)4](N3C12H9)2]⁻. The complex crystallizes in an iso- typic tetragonal space group with the cell parameters,a=1.6901(2) (nm),c=3.7595(4) (nm), andZ=4. The central samarium ion is eight-coordinate with eight nitrogen atoms from four 2-(2-pyridine)benzimidazoles. The Sm–N distance is in the range of 0.244(2)–0.260(2) nm, with the shortest length from the pyridyl-N species. Despite the difference in the chemical for- mula from this complex, another type of complex [NC12H8(NH)2][RE(N3C12H8)4] (RE=Y, Tb, Yb) obtained from the same reaction also crystallizes in the same isotypic tetragonal space group. However, it must be pointed out that 2-(2-pyridine)benzimidazole was chosen as the counterion to satisfy the charge balance instead of [Sm(N3C12H8)2(N3C12H9)2]⁺in the complex [Sm(N3C12H8)2(N3C12H9)2][Sm(N3C12H8)4](N3C12H9)2.

4.3.2.3 Rare Earth Complexes with Bis(benzimidazole)pyridine (19) Type Ligands In recent years, scientific researchers have focused their attention on developing rare earth- containing materials for functional devices by taking advantage of the fascinating optical and magnetic properties of the rare earth metals to endow materials with enhanced physic- ochemical properties. One tactic to realize this purpose relies on the design and synthesis

N5

N3 N4

O1 N2 O2

N1

N9 N10 N8 N6

N7

Lu1

Figure 4.21 The structure of [Lu(19)₂](CH₃OH)(H₂O)]³⁺ [37]. (Reproduced with permission from C. Piguet, A.F. Williams, C. Bemardine and J.C.G. Bünzli, “Structural and photophysical properties of lanthanide complexes with planar aromatic tridentate nitrogen ligands as luminescent building blocks for triple-helical structures,’’Inorganic Chemistry,32, 4139, 1993. © 1993 American Chemical Society.)

of novel pre-organized ligands. Among these, a series of terdentate chelating ligands, bis(benzimidazole)pyridine derivatives, were prepared and their rare earth complexes for nitrate and perchlorate reported [36]. Research results indicate that this type of ligand can effectively encapsulate the rare earth ion and therefore provide a rigid and protective envi- ronment for this ion, leading to the formation of complexes with pre-determined structure and thermodynamic, magnetic, and spectroscopic characteristics. For example, by com- plexation with the rare earth ions, the shape of the ligands19transfers from an I-shape to a V-shape [36c], which effectively improves the liquid crystal property of this ligand. In 1993, Piguetet al. reported the preparation and crystal structure of a series of rare earth complexes with bis(benzimidazole)pyridine type ligands for perchlorate [37]. The synthesis process can be simply depicted as follows: Lu(C1O4)3·7H2O in methanol was slowly added to a solution of bis(benzimidazole)pyridine19in methanol at 70^◦C. After being cooled, the crude precipitate was filtered and dissolved in acetonitrile, then methanol was allowed to dif- fuse in for 8 days to give a transparent complex [Lu(19)2](CH3OH)(H2O)](ClO4)3·3CH3OH, Figure 4.21. The crystal belongs to a monoclinic system and the cell unit contains the cation [Lu(19)2](CH3OH)(H2O)]³⁺, three uncoordinated perchlorate anions, and three methanol molecules. The center lutetium ion is eight-coordinate with six nitrogen atoms from two tridentate bis(benzimidazole)pyridine ligands, one oxygen from a methanol molecule, and one oxygen from one water molecule, leading to a low-symmetry coordination sphere around the metal ion. The Lu–N distance ranges from 0.237(1) to 0.246(1) nm, with an average of 0.2415(1) nm. The Lu–O distance from the water-O and methanol-O species is 0.229(1) and 0.235(1) nm, respectively.

N

NH N

HN

2 8

10

12 13 15 17 18 20

Figure 4.22 The structure of porphyrin.

N

NH N

HN

X X

X X

R

R

R R

N NH

N HN

20 X = C, R = H, H₂TPP 21 X = C, R = CH₃, H₂TTP 22 X = N, TPyP

23 octaethylporphyrin, OEP

Figure 4.23 Four representative porphyrin type ligands.