Acknowledgments
3.2 Rare Earth Complexes with Carboxylic Acids
3.2.2 Structural Chemistry of Rare Earth Complexes with Carboxylic Acids
3.2.2.2 Control of the Polymerization of the Complexes
Owing to the high positive charge, large ionic radii of RE(III) ions and the ionic nature of the RE(III)–oxygen bonds, RE(III) ions tend to share the carboxylato-groups to form polymeric
O O
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Figure 3.1 Coordination modes observed for rare earth–carboxylic acid complexes: (a)η1; (b) η2; (c)µ2-η1η1(O, O); (d)µ2-η1η1ZZ; (e)µ2-η1η1EE; (f)µ2-η1η1ZE; (g)µ2-η2η1; (h)µ3-η2η1; and (i)µ3-η2η2.
complexes. In theory, this tendency can be prevented either by supplying “extra’’ donor atoms or by increasing the steric hindrance of the ligands. In fact, monomeric or dimeric RE(III) complexes are obtainable by using bulky ligands, raising the molar ratio of carboxylate to RE(III), or introducing auxiliary ligands, such as phen, bipy or terp, to the complexes.
Steric Hindrance of the Ligand
When a carboxylate is bulky enough relative to the sizes of the RE(III) ions, the carboxylate itself can prevent the complex from polymerizing. As such, dimeric or monomeric complexes can be obtained. Formate has the smallest steric hindrance. Its complexes with RE(III) are not surprisingly all polymeric. At least two series of formate–RE(III) complexes have been reported, one with the general formula [REL3]n(RE=La, Ce, Gd, Tb, Tm, and Gd) [22] and the other [REL3(H2O)2]n(RE=Gd, Tb, Dy, Ho Er, Tm, and Y) [23, 24]. For acetic acid, its complexes with large RE(III) ions (RE=La–Nd) are polymeric, but the complexes with smaller RE(III) ions (RE=Gd–Lu) are dimeric, and its mid-rare earth ions (RE=Sm and Eu) complexes are either dimeric [25] or polymeric dimeric [26]. Only a handful of propionic acid complexes with RE(III) have been structurally characterized, but the trend is very similar to the complexes with acetic acid, that is, small RE(III) ions form dimers while the large ones form polymers [27–29]. Similar trends are also found for benzoic acid complexes: polymeric structures for the complexes of large RE(III) ions [RE(III)=La-Tb], and dimeric structures for the complexes of small RE(III) ions, for example, Y(III) and Yb(III).
Figure 3.3 shows several bulky carboxylic acid ligands. Pivalic acid forms monomeric complexes with small RE(III) ions. Its complex with Dy(III), [DyL3(H2O)3]·(HL) is a monomer with Dy(III) coordinated by three chelating pivalates [30], while its complexes with large RE(III) (RE=La–Eu) are dimeric: [RE2L6(HL)6] (HL=pivalic acid), where the two RE(III)
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Figure 3.2 Bridging connectivities observed in dimeric and polymeric rare earth complexes with carboxylic acids.
COOH COOH COOH
(a) (b) (c)
Figure 3.3 Structures of (a) pivalic acid; (b) 2,2-dimethylbutyric acid; and (c) 1-adamantane carboxylic acid.
(a) (b)
La1 Er1
Figure 3.4 Structures of (a) [LaL6]3+ and (b) [ErL3L3] (HL=acetic acid, L=urea) [RE, black (large balls); O, grey; N, black (small balls); C, white; H, omitted]. (Redrawn from the CIF files of G. Meyer and D. Gieseke-Vollmer, “Anhydrous lanthanum acetate, La(CH3COO)3, and its precursor, ammonium hexaacetatolanthanate hemihydrate (NH4)3[La(CH3COO)6]·1/2H2O: synthesis, structures, thermal behaviour,’’Zeitschrift für Anorganische und Allgemeine Chemie,619, 1603–1608, 1993 [35];
and G.V. Romanenkoet al., “Crystal structure of tris(acetato)tris(urea)erbium(III) monourea,’’Zhurnal Strukturnoi Khimii,26(5), 103–108, 1985 [36].)
ions are bridged by four bidentate (µ2-η1η1ZZ) pivalates. Each of the two RE(III) ions are coordinated by one unidentate pivalate and three unidentate pivalic acid molecules with CN=8 [31]. The complexes of two bulkier monocarboxylates (Figure 3.3b and c) with large RE(III) (RE=La, Nd) ions have also been found to be dimeric [32, 33]. So far, no structural data are available for their complexes with smaller RE(III) ions.
Molar Ratio of Coordinating Carboxylate to RE(III)
The molar ratio of coordinating carboxylate to RE(III) is usually noted as “carboxylate/RE.’’
From the discussion above, we have seen that formic acid forms coordination polymers with the whole series of RE(III) when the carboxylate/RE=3. However, when carboxylate/RE=6 or 8, the complexes become monomeric, where the metal centers are eight-coordinated either by four unidentate (η1) and two bidentate (η2) formates, or by eight unidentate (η1) formates [34]. Similarly, the acetate complexes, (NH4)2[LaL6]·0.5H2O, are also of monomeric structure, where the carboxylate/RE ratio is 6, with the RE(III) being coordinated by three unidentate (η1) acetates and three chelating (η2) acetates with CN=9 [35] (Figure 3.4a). Complex [ErL3L3]·L (HL=acetic acid; L=urea) stands out as a unique example of this category (Figure 3.4b).
The carboxylate/RE ratio of the complex is 3, but the structure of [ErL3L3] is very much the same as [LaL6]3−(HL=acetic acid). The three acetates coordinate to the Er(III) ion in a chelating mode (η2), while the three urea molecules act as another three unidentate acetates [36]. So far, this type of monomeric complex has only been found in the formatate and acetate complexes.
Use of Auxiliary Ligands
Some chelating ligands, for example, 1,10-phenanthroline (phen), 2,2-bipyridine (bippy) or 2,2:6,2-terpyridine (terp), can prevent the complexes from polymerizing. It has been found that ternary complexes, [RE2L6(phen)2] (HL=acetic acid; RE=Ce, Ho, and Lu) is dimeric [37–39], where the two RE(III) ions are bridged by four acetates, and each of the two RE(III) is chelated by an acetate and a phen. With 4-aminobenzonic acid, a bulkier ligand, three types of ternary complexes with monomeric structures are isolated: (I) [LaL3(HL)(phen)2(H2O)]·H2O) [40], (II) [REL3(phen)(H2O)]·2H2O (RE=Eu, Tb) [40], and (III) [TbL2(phen)2(H2O)2]·(L)(phen)·4H2O [41]. In type I complexes, the metal center is coor- dinated by three carboxylates [(two unidentate (η1) and one chelating (η2)], one protonated carboxylate [unidentate (η1)], and two chelating phen, with its coordination number as ten (Figure 3.5a). In type II complexes, however, the RE(III) ions are coordinated by three chelating (η2) carboxylates, one chelating phen, and one water with CN=9 (Figure 3.5b).
Type III complexes were obtained when the molar ratio of phen in the structure is high (RE : L: phen=1 : 3 : 2). One of the 4-aminobenzoates is pushed out of the coordination sphere by the second phen. The Tb(III) center is coordinated by two unidentated (η1) carboxy- lates, two chelating phen, and two water molecules, and the coordination number is thus eight, which shows the strong coordination ability of phen to RE(III).
The use of bipy or terp as the auxiliary ligand can also lead to the formation of the monomeric or dimeric structures. In [PrL3(bipy)2] (HL=trichloroacetic acid) [42], the three carboxy- lates chelate (η2) to Pr(III), whereas the two bipy coordinate to the metal center with their two nitrogen atoms. [TbL3(terp)(H2O)]2 (L=4-aminobenzonic acid) is a dimer, where the
(a)
La1 Eu1
(b)
Figure 3.5 Structures of (a) [LaL3(HL)(phen)2(H2O)] and (b) [EuL3(phen)(H2O)] (HL=4- aminobenzonic acid) [RE, black (large balls); O, grey; N, black (small balls); C, white; H, omitted].
(Redrawn from the CIF files of T. Fiedleret al., “Synthesis, structural and spectroscopic studies on the lanthanoid p-aminobenzoates and derived optically functional polyurethane composites,’’European Journal of Inorganic Chemistry,2007, 291–301, 2006 [40].)
water molecules, resulting in a CN=9 [40].