Acknowledgments
3.2 Rare Earth Complexes with Carboxylic Acids
3.2.2 Structural Chemistry of Rare Earth Complexes with Carboxylic Acids
3.2.2.3 Structures of the Rare Earth Complexes with Monocarboxylic Acids
water molecules, resulting in a CN=9 [40].
is monomeric, where the carboxylate/RE ratio is six, the RE(III) is coordinated by three unidentate (η1) acetates and three chelating (η2) acetates with CN=9 [35].
With phen as the auxiliary ligand, monomeric complexes with the formula [REL3(phen)2
(sol)n] (sol=solvent molecule) can be obtained. When the three carboxylates are all in the chelating modes (η2), there is no coordination solvent in the structure, and the CN=10. When one or two of the coordination carboxylates are unidentate, there will be one coordinating solvent. Examples can be found in Section 3.2.2.2.
Dimeric Complexes
The structures of dimeric complexes are characterized by the number of carboxylate bridges between the two metal centers and also the ways of bridging. So far, the bridge numbers found for such complexes are either two or four, and single- or triple-bridged dimers have not been reported, although bridge numbers from one to four are common for polymeric structures.
Most of the dimeric structures are centrosymmetric. Thus, only a half of the dimer is unique crystallographically, meaning that the two RE(III) ions have exactly the same coordination environment. Based on the bridging type, the dimeric complexes can be grouped into four types: (I) double bidentate bridging (µ2-η1η1)2; (II) double tridentate bridging (µ2-η2η1)2; (III) quadruple bidentate bridging (µ2-η1η1)4, and (IV) quadruple mixed bridging (=“double bidentate bridging+double tridentate bridging’’) “(µ2-η2η1)2+(µ2-η1η1)2.’’
Only a small number of ligands have been found to form dimers with double-bridging (acetic acid, propionic acid, benzoic acid and somepara-substituted derivatives of benzoic acid (p-RC6H4COOH) (R=–NH2, –OCH3, –CN, and so on for type I and acetic acid, methacrylic acid, 3-(2-hydroxyphenyl) acrylic acid, 2-thiophene carboxylic acid, 2-methoxybenozoic acid, 3-hydoxybenzoic acid, 4-hydroxy-3-methoxybenzoic acid, and 2,6-dichlorobenzoic acid for type II), but most of the monocarboxylic acid can form quadruply bridged dimers, that is, types III and IV. The structure of [TbL3(H2O)2]2·2H2O (HL=p-aminobenzoic acid) is shown in Figure 3.7a, the two Tb(III) are bridged by two bidentate (µ2-η1η1) carboxylates, and each of them is coordinated further by two chelating (η2) carboxylates and two water, CN=8 [44]. In complexes [RE2L6(H2O)4]·4H2O (RE=Sm–Lu; HL=acetic acid) [45], the two RE(III) centers are bridged by two tridentate (µ2-η2η1) carboxylates. Each of the RE(III) ions is further coordinated by two chelating (η2) carboxylates and four terminal water, CN=9 (Figure 3.7b).
Only trifluoroacetic acid is found to form quadruple simple bridging [(µ2-η1η1)4] dimers without any auxiliary ligands: [RE2L6(H2O)6] (HL=trifluoroacetic acid; RE=Pr, Gd, and Lu) [25, 46]. Other carboxylic acids can only form this type of complex with the help of auxiliary ligands, such as phen, terp, bipy, DMSO (dimethyl sulfoxide), DMF (N,N-dimethylformamide), ethanol, methanol, NO−3, and even the carboxylate (L−) or the carboxylic acid (HL). The structures of [GdL3(H2O)3]2 (HL=trifluoroacetic acid) and [EuL3(phen)(H2O)]2(HL=p-cynobenzoic acid) [47] are shown in Figure 3.8.
More than one third of the reported dimeric complexes are with quadruple chelating bridging interactions, and almost all of the monocarboxylic acids can form dimers with this type 2 of connectivity with or without auxiliary ligands. In a typical structure, the two RE(III) ions are bridged by two tridentate bridging [(µ2-η2η1)2] carboxylates and two bidentate bridging [(µ2-η1η1)2] carboxylates. Each of the two metal ions is further coordinated by one chelating (η2) and one auxiliary ligand. The structures of [CeL3(phen)]2(HL=acetic acid) [37] and [EuL3(phen)]2(HL=2-furancarboxylic acid) [48] are shown in Figure 3.9.
(a) Tb1
Sm1
(b)
Figure 3.7 Structures of (a) [TbL3(H2O)2]2 (HL=p-aminobenzonic acid) and (b) [SmL3(H2O)2]2 (HL=acetic acid) [RE, black (large balls); O, grey; N, black(small balls); C, white; H, omitted]. (Redrawn from the CIF files of L. Oyanget al., “Crystal structure and luminescence property of ternary terbium p-aminobenzoic acid complexes with different second ligands,’’Journal of Molecular Structure, 740, 175–180, 2005 [44]; and R. Vadura and J. Kvapil, “Growth and lattice parameters of the lanthanide carboxylates I. Tetrahydrated lanthanide acetates,’’Materials Research Bulletin,6, 865–873, 1971 [45].)
(a) Gd1
Eu1
(b)
Figure 3.8 Structures of (a) [GdL3(H2O)3]2(HL=trifluoroacetic acid) and (b) [EuL3(phen)(H2O)]2 (HL=p-cynobenzonic acid) [RE, black (large balls); O, grey; N and F, black (small balls); C, white; H, omitted]. (Redrawn from the CIF files of D. John, A. Rohde and W. Urland, “Synthesis, crystal structure and magnetic behaviour of dimeric and polymeric gadolinium trifluoroacetate complexes,’’Zeitschrift für Naturforschung, B: Chemical Sciences, 61 (6), 699–707, 2006 [46]; and Y. Li et al., “Crystal structures and magnetic and luminescent properties of a series of homodinuclear lanthanide complexes with 4-cyanobenzoic ligand,’’Inorganic Chemistry,45(16), 6308–6316, 2006 [47].)
By comparing the structures of the four types of dimeric complexes, we can see some trends:
(i) large RE(III) ions prefer to form type II and IV complexes, while smaller RE(III) ions prefer type I and III; (ii) without any auxiliary ligands, RE(III) ions tend to form double-bridged complexes, that is, type I and type II, whereas with an auxiliary ligand, such as phen, bipy, and even carboxylate anions, the complexes will adopt the modes of type III or IV, where the auxiliary ligands push one non-bridging carboxylate from each end to form two new bridges.
Tetrameric Complexes
Only four tetrameric complexes in two types have been reported so far, and they are all the complexes of small RE(III) (RE=Y, Dy, Tm, and Lu) ions. [YL3(H2O)2]4 (HL=p- hydoxybenzoic acid) [49] is of a linear structure. Only half of the tetramer is unique. The four
(a)
Ce1 Eu1 Eu2
(b)
Figure 3.9 Structures of (a) [CeL3(phen)]2 (HL=acetic acid) and (b) [EuL3(phen)]2 (HL=2- furancarboxylic acid) [RE, black (large balls); O, grey; N, black (small balls); C, white; H, omitted].
(Redrawn from the CIF files of A. Panagiotopouloset al., “Molecular structure and magnetic properties of acetato-bridged lanthanide(III) dimers,’’Inorganic Chemistry,34, 4918–4920, 1995 [37]; and X. Liet al., “Synthesis, structure and luminescence property of the ternary and quaternary europium complexes with furoic acid,’’Journal of Molecular Structure,604, 65–71, 2002 [48].)
Y(III) ions are linked together by simple double bridges [(µ2-η1η1)2]. Each of the two ter- minal Y(III) are then coordinated by two chelating (η2) ligands and two water, and each of the two internal Y(III) are coordinated by one chelating (η2) ligand and two water, CN=8 (Figure 3.10a). Similar structures are found for threep-nitrobenzoic acid complexes, [REL3(H2O)2]4(RE=Dy [50], Tm, and Y [51]; L=p-nitrobenzoic acid).
There is only one example for the second type of tetramer. Cs4[LuL4]4(HL=acetic acid), is a closed square, and only half of the structure is unique [52]. Lu1 and Lu2 or Lu1a and Lu2a are linked together by a mixed triple bridge [(µ2-η2η1)2+(µ2-η1η1)], while Lu1 and Lu2a or Lu2 and Lu1a, on the other hand, are bridged by a bidentate bridging ligand, resulting a square with four Lu(III) at the corners. Lu1 is then coordinated by two chelating (η2) carboxylates, CN=9. Lu2 has very a similar coordination environment, except that only one carboxylate is in the chelating (η2) mode, and the other one is unidentate (η1), CN=8 (Figure 3.10b). The formation mechanism of the tetramers is still not clear, although the sizes of the metals may play important roles here.
Polymeric Complexes with One Bridging Mode
In these complexes, the same bridging modes repeat in between the two neighboring metal ions.
So far, seven bridging modes have been observed in the complexes: (i) single bidentate bridging µ2-η1η1; (ii) double bidentate bridging (µ2-η1η1)2; (iii) double tridentate bridging (µ2-η2η1)2; (iv) triple bidentate bridging (µ2-η1η1)3; (v) triple mixed bridgingA[(µ2-η1η1)2+(µ2-η2η1)];
(vi) triple mixed bridging B [(µ2-η1η1)+(µ2-η2η1)2]; and (vii) triple tridentate bridging, (µ2-η2η1)3[53].
In [YbL3(H2O)2]n(HL=formic acid), the two neighboring Yb(III) ions are linked together by a single bidentate bridging (µ2-η1η1) formate, and each of the Yb(III) ions are then coordi- nated by two chelating (η2) formates and two aqua ligands, CN=8 [54]. The same connections are also observed in [YbL3(H2O)2]n(HL=methylthioacetic acid) [55].
There are two double bridging modes, (µ2-η1η1)2and (µ2-η2η1)2. Of these, (µ2-η1η1)2is only found in the complexes of benzoic acid and some of its derivatives, while (µ2-η2η1)2is popular in the complexes with aliphatic acids, such as acetates and propionates. This is because
Y1 Y2 Y2a Y1a
(a)
Lu2 Lu1a
Lu1 Lu2a
(b)
Figure 3.10 Structures of (a) [YL3(H2O)2]4 (HL=p-hydoxybenzoic acid) and (b) [LuL4]4−4 (HL=acetic acid) [RE, black; O, grey; C, white; H, omitted]. (Redrawn from the CIF files of M.S.
Khiyalov et al., “Crystalline and molecular structure of (p-hydroxybenzoato)yttrium(III),’’Koordi- natsionnaya Khimiya (Coordination Chemistry) (in Russian), 7(8), 1255–1261, 1981 [49]; and A.
Lossin and G. Meyer, “Ternary acetates of the lanthanides with cesium: dimers in CsLu(CH3COO)4and trimers in Cs2[Lu3(CH3COO)10(OH)(H2O)]. Synthesis, crystal structures, thermolysis,’’Zeitschrift für Anorganische und Allgemeine Chemie,619(8), 1465–1473, 1993 [52].)
aliphatates, with smaller steric hindrance than aromatates, can approach RE(III) centers more facilely to accommodate tridentate chelating. In [REL3(MeOH)2]n(RE=Sm [56], Eu, Gd, and Tb [20]; HL=benzoic acid), the two adjacent RE(III) ions are joined together by two bidentate benzoates, and each of the RE(III) centers is coordinated further by one chelating (η2) benzoate and two methanol, CN=8 (Figure 3.11a). In [Pr2L6(H2O)3]n·3nH2O (HL=propionic acid) [57], the adjacent Pr(III) ions are bridged together by two tridentate chelating [(µ2-η2η1)2] acetates. Each of them is then coordinated by a chelating (η2) propionate and three water molecules, CN=9 (Figure 3.11b).
Three of the four types of triple bridging polymeric structures are found with the RE(III) complexes with acetates. For Sc(III), the smallest RE(III), its anhydrous complex with acetic acid, [ScL3]n, has a triple bidentate bridging mode (µ2-η1η1)3, where the two adjacent Sc(III) ions are bridged together through three acetates in modeµ2-η1η1. Each of the Sc(III) ions is thus coordinated by six oxygen atoms from six different acetates, CN=6 [58]. The late rare earth analogs, [REL3]n(RE=Tm-Lu), on the other hand, are in the bridging mode, [(µ2-η2η1)+ (µ2-η1η1)2], with CN=7 [59], while the triple bridging mode [(µ2-η1η1)+(µ2-η2η1)2], is found with the anhydrous complex with larger RE(III) ions, [HoL3]n, and in the hydrated complexes with early RE(III) ions, [RE2L6(H2O)]n (RE=Sm and Eu) [60]. A few other ligands, such asβ-phenylacrylic acid [61],p-methylbenzoic acid [62],m-methylbenzoic acid [63], and o-aminobenzoic acid [64] are reported to form anhydrous complexes with early RE(III) ions, [REL3]n, in a triple tridentate bridging mode (µ2-η2η1)3.
Polymeric Structures with Two Bridging Modes Alternated
The three most often observed structure types with two bridging modes are the alternating double bidentate bridging (µ2-η1η1)2 and double tridentate bridging (µ2-η2η1)2, referred to as (µ2-η1η1)2//(µ2-η2η1)2, alternating double bridging and triple bridging, referred to as
Sm2 Sm1 Sm2a
(a)
Pr2
Pr1 Pr1a
(b)
Figure 3.11 Structures of (a) [Sm2L6(MeOH)4]n (HL=benzoic acid) and (b) [Pr2L6(H2O)3]n (HL=acetic acid) [RE, black; O, grey; C, white; H, omitted]. (Redrawn from the CIF files of U.P.
Singh, R. Kumar and S. Upreti, “Synthesis, structural, photophysical and thermal studies of ben- zoate bridged Sm(III) complexes,’’Journal of Molecular Structure, 831 (1–3), 97–105, 2007 [56];
and D. Deiters and G. Meyer, “Synthesis and crystal structure of praseodymium propionate trihydrate, Pr(CH3CH2COO)3(H2O)3,’’Zeitschrift für Anorganische und Allgemeine Chemie, 622(2), 325–328, 1996 [57].)
(µ2-η1η1)2//(µ2-η1η1)(µ2-η2η1)2, and also alternating double bridging and quadruple bridging, referred to as (µ2-η1η1)2//(µ2-η1η1)4or (µ2-η1η1)2//(µ2-η1η1)2(µ2-η2η1)2.
The bridging mode (µ2-η1η1)2//(µ2-η2η1)2is found in [La(L)3(CH3OH)2(H2O)]n·nCH3OH [L=E-3-(4-hydroxyl-phenyl)-acrylic acid] [65]. Two La(III) are linked together through the double tridentate bridging (µ2-η2η1)2, and each of the two La(III) then connects to its neigh- boring La(III) through the double bidentate bridging. Each of the La(III) is also coordinated by a monodentate (η1) carboxylate, two methanol molecules, and one water, CN=10.
Threep-nitrobenzoic acid complexes, [RE2L6(H2O)4]n·2nH2O (RE=La [53], Eu [66], and Tb [67]; HL=p-nitrobenzoic acid), are found to have the alternating double bridging and triple bridging structure: (µ2-η1η1)2//(µ2-η1η1)(µ2-η2η1)2. As shown in Figure 3.12a, Eu2 and Eu1a are linked by the double bridging (µ2-η1η1)2, and Eu1 and Eu2 are bridged through a triple bridging (µ2-η1η1)(µ2-η2η1)2with the two pairs ofη2oxygen atoms coordinating to Eu1 and Eu2, respectively. Eu1 is then coordinated by three water molecules, while Eu2 is coordinated further by a ligand in theη2mode and a water molecule. Both Eu1 and Eu2 are nine-coordinated.
The bridging mode, (µ2-η1η1)2//(µ2-η1η1)4, is found in [RE2L6(H2O)3]n·nH2O (RE=Er, Dy; HL=trichloroacetic acid) (Figure 3.12b) [68]. The two independent RE(III) ions are joined together by the quadruple bridge, that is, (µ2-η1η1)4, and each of them are then linked to the neighboring RE(III) ions with a double bridge (µ2-η1η1)2. In addition, the two metal ions are also bound to two and one water, CN=8 or 7.
The second form of alternating double bridging and quadruple bridging is (µ2-η1η1)2//
(µ2-η1η1)2(µ2-η2η1)2, where the quadruple bridging consists of a double bidentate bridge and double tridentate bridge. The terbium complex withm-nitrobenzoic acid obtained from DMF, [Tb2L6(DMF)2]n, exhibits this type of bridging [69], while its lanthanum analog is of a bridging mode, (µ2-η1η1)2//(µ2-η1η1)4[70].
Eu1a Eu2
(a)
Er2 Er2a Er1 Er1a
(b)
Er1b
Figure 3.12 Structures of (a) [Eu2L6(H2O)4]n(HL=p-nitrobenzoic acid) and (b) [Er2L6(H2O)3]n(HL= trichloroacetic acid) [RE, black (large balls); N and Cl black (small balls); O, grey; C, white; H, omitted].
(Redrawn from the CIF files of Ad. Bettencourt-Dias and S. Viswanathan, “Nitro-functionalization and luminescence quantum yield of Eu(III) and Tb(III) benzoic acid complexes,’’ Dalton Transac- tions, 4093–4103, 2006 [66]; and T. Imai and A. Ouchi, “The structure ofµ-aquabis(µ-trichloroacetato) bis[aquabis(trichloroacetato)erbium(III)] hydrate, [[Er(CCl3CO2)2(H2O)]2(CCl3CO2)2(H2O)]n·nH2O,’’
Bulletin of the Chemical Society of Japan,60(1), 408–410, 1987 [68].)