Acknowledgments
3.4 Rare Earth Complexes with Amino Acids
3.4.2 Structural Chemistry of Rare Earth Complexes with Amino Acids
At pH 1–4, amino acids are in their zwitterions form. The ligands can thus be considered as simple carboxylates with a positively charged side group –NH+3 at itsα-position. In fact, the
structures reported so far indicate that amino acids behave very similarly to monocarboxy- late in terms of their coordination mode with the –NH+3 group only participating in forming hydrogen bonded networks. However, no mononuclear RE(III)–amino acid complexes have been reported so far, and only five coordination modes are found in the structures:η1; η2; µ2-η1η1 ZZ;µ2-η1η1 ZE; and µ2-η2η1(Figure 3.1). The complexes are either dimeric or polymeric with RE(III) to ligand ratios of 1 : 1, 1 : 2 or 1 : 3. The coordination numbers of the RE(III) range from six to ten, with most of them being eight with a distorted square anti-prism geometry. Among the ligands, Gly and Pro are found to form three types of complexes with RE : L=1 : 3, 1 : 2, and 1 : 1, while both Ala and Ser can form two types of complexes with RE : L=1 : 2 and 1 : 1 for Ala, and RE : L=1 : 2 and 2 : 3 for Ser, respectively. All of the other ligands can only form either 1 : 2 or 1 : 1 complexes [9].
3.4.2.1 Structures of 1 : 3 (RE : L) Complexes
The 1 : 3 (RE : L) complexes are only found when Gly or Pro are the ligands, but with a wide range of metal ions from the smallest Sc(III) to the largest La(III). Correspondingly, CN=6, 8, 9, or 10 complexes are observed, all of which are of 1D-polymeric structures with either alternating double bridging and quadruple bridging, triple bridging, or double bridging connectivity.
Three types of alternating double bridging and quadruple bridging connections, that is, (I) (µ2-η2η1)2//(µ2-η1η1)2(µ2-η2η1)2, (II) (µ2-η1η1)2//(µ2-η1η1)2(µ2-η2η1)2, and (III) (µ2-η1η1)2//(µ2-η1η1)2(µ2-η1η1)2, two types of triple bridging connections, (IV) (µ2-η2η1)2(µ2-η1η1) and (V) (µ2-η2η1)2, and one type of double bridging connection, i.e. (VI) (µ2-η1η1)2, have been identified in the 1 : 3 complexes (Figure 3.30). Gly forms type (I) structures with light La(III), and type (II) structures with smaller RE(III), including Pr, Nd, Sm, Ho, Er, and Y. However, the 1 : 3 complexes of Pro with Pr(III), Nd(III), and Gd(III) are all of type (III) structures. Three representatives of structure type (I) to (III) are shown in Figure 3.30a, b, and c. The three complexes have the same formula: [RE2L6(H2O)4]6nn +, although their structures are different. In the [La2Gly6(H2O)4]6nn+, four of the six carboxy- lates around La(III) are in the tridentate bridging coordination mode (µ2-η2η1) [121], while there are only two and zero carboxylates in this mode for [Sm2(Gly)6(H2O)4]6nn+[122] and [Pr2(Pro)6(H2O)4]6nn+[123], respectively. Thus the coordination numbers of the RE(III) in the three structures are ten, nine, and eight. This is a result of the lanthanide contraction and steric effect: the La(III) ion, with large ionic radius, prefers theµ2-η2η1coordination mode to achieve high coordination numbers, and Pro is bulkier than Gly.
The structures of the two triple bridging complexes are shown in Figure 3.30d and e.
The coordination numbers for La(III) and Yb(III) are nine and eight, respectively [124, 125].
The double bridging structures are only found in the Pro complexes, [RE2(Pro)6(H2O)6]6nn+ [RE=Sm(III), Eu(III), Gd(III), Er(III), and Y(III)] (Figure 3.30f), where the two neighbor- ing metal ions are linked by a double bridge (µ2-η1η1)2, with each of them being further coordinated by a monodentate (η1) Pro and three water molecules, CN=8 [126].
3.4.2.2 Structures of 1 : 2(RE : L) Complexes
The 1 : 2(RE : L) complexes are inclined to form when Gly, Ala, Val, Phe, Met, Ile, His, Cys, and Pro are used as the ligands. Most of these complexes are dimeric, with the general formula
(a)
(d)
(b)
(e)
(c)
(f)
Figure 3.30 The structures of (a) [La2Gly6(H2O)4]6n+n ; (b) [Sm2Gly6(H2O)4]6n+n ; (c) [Pr2Pro6(H2O)4]6n+n ; (d) [La2Gly6(H2O)4]6n+n ; (e) [Yb2Gly6(H2O)4]6n+n ; and (f) Sm2(Pro)6(H2O)6]6n+n [RE, black (large balls); O, grey; N, black (small balls); C, white; H, omitted)]. (Redrawn from the CIF files of A. Maet al., “Crystal structure and infrared spectra of a lanthanum coordination compound with glycine, {[La(Gly)32H2O](ClO4)3}n,Journal of Coordination Chemistry, 33(1), 59–67, 1994 [121];
A. Maet al., “Synthesis and crystal structure of {[Sm2(Gly)6(H2O)4](ClO4)6(H2O)5}n,’’Wuji Huaxue Xuebao(Chinese Journal of Inorganic Chemistry),9(4), 401–406, 1993 [122]; Z. Wanget al., “Synthesis characterization and crystal structure of rare earth complexes with L-proline,’’Huaxue Xuebao(Acta Chimica Sinica),51(3), 257–264, 1993 [123]; S.L. Gaoet al., “Catena-Poly[[[diaqualanthanum(III)]- tri-µ-glycinato] trichloride monohydrate],’’Acta Crystallographica Section E,E58, m234–m236, 2002 [124]; F. Renet al., “Study on the regularity of crystal structures of complexes of rare earth perchlorate with glycine,’’Xibei Daxue Xuebao, Ziran Kexueban (Journal of Northwest University, Natural Science Edition), 31(2), 111–114, 2001 [125]; and J. Torreset al., “Sm(III) complexation with amino acids.
Crystal structures of [Sm2(Pro)6(H2O)6](ClO4)6and [Sm(Asp)(H2O)4]Cl2,’’Journal of the Chemical Society, Dalton Transactions, (21), 4035–4041, 2002 [126].)
[RE2L4(H2O)8]X6·nH2O (RE=Pr, Nd, Sm, Eu, Ho, Er, and Y; X=Cl−or ClO−4;n=0–4) with the exception of some Pro complexes, [RE(Pro)2(H2O)5]Cl3(RE=Er, Ho, Dy, and Yb), which are 1D-polymers.
In a typical dimeric 1 : 2 complex, the two RE(III) ions are linked through a quadruple bridge (µ2-η1η1)4. Each of the metal ions is further coordinated by four water molecules, CN=8. A representative structure, [Eu2(Ala)4(H2O)8]6+, is shown in Figure 3.31a. While most dimeric RE(III)–carboxylic acid complexes are centrosymmetric, with amino acids as ligands, similarly structured complexes are only obtainable when the racemic form of the ligands are used for the synthesis [127].
(a) (b)
Figure 3.31 The structures of (a) [Er2(Ala)4(H2O)8]6+ and (b) [Er(Pro)2(H2O)5]3n+n [RE, black (large balls); O, grey; N, black (small balls); C, white; H, omitted)]. (Redrawn from the CIF files of T. Glowiaket al., “Ligand chirality effect on the structure and its spectroscopic consequences in [Ln2(Ala)4(H2O)8] (ClO4)6 crystals,’’ Polyhedron, 15 (17), 2939–2947, 1996 [127]; and A.Z. Ma et al., “Structure of an erbium coordination compound with L-proline, {[Er(Pro)2(H2O)5]Cl3}n,’’Acta Crystallographica Section C,49(5), 865–867, 1993 [128].)
Figure 3.31b shows the structure of [Er(Pro)2(H2O)5]3nn+. The two adjacent Er(III) ions in the structure are joined together by a bridging carboxylate in the coordination modeµ2-η1η1, and each of the Er(III) are further coordinated by one monodentate (η1) ligand and five water molecules, CN=8 [128].
3.4.2.3 Structures of 1 : 1 and 2 : 3(RE : L) Complexes
A diverse range of structures have been revealed for the 1 : 1 complexes with the simple amino acids, Gly, Pro, and Ala, as well as the amino acids with side groups capable of metal coordination, such as Ser, Thr, Glu, and Asp. The structures can be dimeric, 1D-polymeric or 2D-layered.
The complexes of Gly and Pro are dimeric with the two RE(III) ions bridged by two carboxy- late groups in a bidentate bridging mode (µ2-η1η1)2. The structure of [Nd2(Pro)2(H2O)12]6nn+ is shown in Figure 3.32a [129]. However, the Ala complex, [Dy(Ala)(H2O)6]3nn+is an infinite chain. Each of the Dy(III) ions is coordinated by six water molecules, and linked to its adjacent metal centers through single carboxylate bridgesµ2-η1η1(Figure 3.32b) [130].
There are only two structures reported for Ser and Thr, [Ho2(Ser)2(H2O)12]nCl3n and [Ho2(Thr)2(H2O)12]nCl3n. The two complexes have very similar structures with an –OH group and one carboxylate O chelating to one Ho(III), and the other carboxylate O coordinating its neighboring Ho(III) to form a 1D-polymer (Figure 3.32c) [131].
The only known 2 : 3 complex with the only seven-coordinate RE(III)–amino acid complex reported so far is the dimeric Ser complex, [Er2(Ser)3(H2O)8](ClO4)6[132]. In the structure, the two Er(III) ions are bridged through three carboxylates in theirµ2-η1η1coordination mode, and the coordination sphere of Er(III) is completed by four water molecules, CN=7.
Both Glu and Asp have an extra carboxylic group, which deprotonates when coordinating RE(III) ions. Thus, Glu and Asp are negatively charged in the complexes. The coordination modes of Glu and Asp are shown in Figure 3.33. Several Glu complexes [RE=Pr, Nd(Y), Ho Er] have been reported, all displaying very similar 2D-layered structures. A typical Glu complex
(a) (b) (c)
Figure 3.32 The structures of (a) [Nd2(Pro)2(H2O)12]6+; (b) [Dy(Ala)(H2O)6]n; and (c) [Ho2(Ser)2(H2O)12]6n+n [RE, black (large balls); O, grey; N, black (small balls); C, white; H, omitted)]. (Redrawn from the CIF files of T. Glowiak et al., “Structures of neodymium(III) com- plexes with amino acids: (I) catena-poly{[triaquatrichloroneodymium(III)]-µ-(β-alanine-O,O)}; (II) pentaaquachloro-µ-(L-proline-O,O)-neodymium(III) hexaaqua-µ-(L-proline-O,O)-neodymium(III) pentachloride,’’ Acta Crystallographica Section C, 47 (1), 78–81, 1991 [129]; T. Glowiak et al.,
“Absorption, luminescence and crystal structure studies of dysprosium compound with L-α-Alanine:
[Dy(L-α-AlaH)(H2O)6]Cl3,’’ Journal of the Less Common Metals, 168 (2), 237–248, 1991 [130];
and T. Glowiak and C.N. Dao, “Structure of pentaaqua(L-serine)holmium(III) trichloride,’’ Acta Crystallographica Section C,49(6), 1171–1173, 1993 [131].)
O NH3 O
O
O RE RE
RE RE
O NH3
O
O O RE
RE
RE RE
O NH3 O
O
O RE RE
RE
Figure 3.33 Coordination modes of Glu (left) and Asp (middle and right).
can be formulated as [RE2(Glu)2(H2O)8]n(X)4(X=ClO−4, Cl−or NO−3) (Figure 3.34a). In the structure, two RE(III) ions are bridged to form a dimeric unit by two tridentate chelating(µ2- η2η1)γ-carboxylates and two bidentate bridging(µ2-η1η1)α-carboxylates from four ligands, and the dimeric units are linked to four other sets of dimeric units through the other ends of the ligands to form a layered structure. Each of the RE(III) ions is also coordinated by four water molecules, CN=9 [133].
Only two Asp complexes have been structurally characterized by X-ray diffraction, but they are structurally distinctly different. [Sm(Asp)(H2O)4]3nn+is a 2D-layered polymer [126], in which both the α- andβ-carboxylates are in theµ2-η1η1bridging coordination mode (Fig- ure 3.34b), and each ligand is coordinated to four Sm(III) centers. Each of the metal centers is surrounded by four oxygen atoms, one from each ligand. The ligands then coordinate another eight metal centers to form a layered structure. [Ho(Asp)(H2O)5]3nn+is an infinite chain [134].
Only theβ-carboxylates are in bridging coordination modeµ2-η1η1; theα-carboxylates are monodentateη1(Figure 3.34c). Each Ho(III) is coordinated by five water molecules and three oxygen from different ligands, CN=8.
As we have seen from the discussion above, theγ- and β-carboxylates are father away from the positively charged –NH+3 than theα-carboxylate, and they are expected to coordinate
(a) (b) (c)
Eu2
Eu1
Sm1
Ho1
Figure 3.34 The structures of (a) [Eu2(Glu)2(H2O)8]4n+n ; (b) [Sm(Asp)(H2O)4]2n+n ; and (c) [Ho(Asp)(H2O)5]2n+n [RE, black (large balls); O, grey; N, black (small balls); C, white; H, omit- ted)]. (Redrawn from the CIF files of B. Barjaet al., “Gadolinium(III) and europium(III) L-glutamates:
synthesis and characterization.Inorganica Chimica Acta,359(12), 3921–3926, 2006 [133]; J. Torres et al., “Sm(III) complexation with amino acids. Crystal structures of [Sm2(Pro)6(H2O)6](ClO4)6
and [Sm(Asp)(H2O)4]Cl2,’’ Journal of the Chemical Society, Dalton Transactions, (21), 4035–
4041, 2002 [126]; and I. Csoereghet al., “Crystal structure of holmium aspartate chloride hydrate Ho(L-Asp)Cl2·6H2O,’’Acta Chemica Scandinavica,43(7), 636–640, 1989 [134].)
RE(III) ions more effectively. This effect can help us to understand why they show two different coordination modesµ2-η2η1versusµ2-η1η1in Glu andµ2-η1η1versusη1in Asp (Figure 3.34a and c). However, the effect is not significant enough to generate a sizable difference for RE(III)–O(carboxylate) distances betweenα-,β-, andγ-carboxylates.