Acknowledgments
3.2 Rare Earth Complexes with Carboxylic Acids
3.2.2 Structural Chemistry of Rare Earth Complexes with Carboxylic Acids
3.2.2.4 Structures of the Rare Earth Complexes with Polycarboxylic Acids
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].)
Therefore, RE(III)–polycarboxylic acid complexes can be considered as polymeric structures made by edge-sharing rare earth–oxygen polyhedra REOm (m=7–10) linked together by carbon chains [71].
Nevertheless, the ionic character of the RE–O bond, the high coordination number require- ment, and flexible coordination geometry of rare earth ions often makes the structures of com- plexes unpredictable. As a result, very few complexes obtained so far can be potentially used for gas storage, liquid absorption, magnetic materials, fluorescent probes or as Lewis acid catalysts.
Among them are [Tb2(bdc)3(H2O)4]n, [Tb(btc)(H2O)]n·0.5nH2O·nDMF [Tb(btc)(DMF)2]n· nH2O, and [Tb(bpdc)1.5(H2O)]n·0.5nDMF. It is worth noting that [Tb2(bdc)3(H2O)4]nreported by Yaghi and coworkers can be viewed as the first rare earth-based MOF material. The work was the very first attempt to seek open metal–organic framework materials beyond transi- tion metal compounds, and it is also the first time rare earth–carboxylic acid complexes were examined as porous materials [72].
[Tb2(bdc)3(H2O)4]nwas prepared by hydrothermal synthesis using an aqueous mixture of Tb(NO3)3·nH2O, 1,4-benzodicarboxylic acid (H2bdc), and triethylamine [72]. In the structure, each Tb(III) ion is coordinated by six oxygens of the bdc anions in a monodentate fashion and two water molecules, CN=8 (Figure 3.13). The overall structure can be described as a
Figure 3.13 The crystal structure of [Tb2(BDC)3·(H2O)4]nshown approximately down the crystallo- graphicb-axis, where aqua ligands are found to point toward the center of the 1D-channels (Tb, black;
O, grey; C, white; H, omitted). (Redrawn from the CIF file of T.M. Reinekeet al., “From condensed lanthanide coordination solids to microporous frameworks having accessible metal sites,’’Journal of the American Chemical Society,121, 1651–1657, 1999 [72].)
leaving a 1D-channel (5.1×6.1 Å2) running in thebdirection filled with coordination water.
Experiment showed that the aqua ligands were removed at 115◦C without the framework collapsing, and re-introduction of water to the dehydrated sample restored the original porous structure. The dehydrated porous solid with coordinatively unsaturated metal sites may be useful as a fluorescent probe and a Lewis acid catalyst.
[Tb(btc)(H2O)]n·0.5nH2O·nDMF (H3btc=1,3,5-benzenetricarboxylic acid), dubbed as MOF-76, was obtained by solvothermal synthesis [73]. The structure is shown in Figure 3.14.
Each Tb(III) ion is linked to each of its two Tb(III) neighbors through three carboxylates with the mode ofµ2-η1η1ZZto form an edge-sharing infinite chain of rare earth—oxygen polyedra REO7. They may be viewed as “rod-like’’ building units in the construction of the overall framework structure. Each rod is then connected to four neighboring rods through the ligand benzene ring. The rods pack in a tetragonal fashion, resulting in 6.6×6.6 Å2square channels in thecdirection, filled with solvent molecules.
[Tb(btc)(DMF)2]n·nH2O was obtained by heating a mixture of Tb(NO3)3·nH2O, 1,3,5- benzotricarboxylic acid (H3btc) and caprolactam (molar ratio 1 : 1 : 1) in a mixture of DMF and EtOH at 55◦C [74]. X-ray diffraction studies showed that each metal center is coordinated with six oxygen atoms from four carboxylate groups of four different ligands and two oxygen atoms from two terminal DMF, CN=8. Each of the four ligands is then connected to another four RE(III) to form a very complicated 3D-network (Figure 3.15). The most attractive feature
Figure 3.14 The structure of MOF-76 showing the 6.6×6.6 Å2square channels in thec-direction with the “rod-like’’ chains of rare earth–oxygen polyedra REO7linked together via the benzene ring of 1,3,5- benzenetricarboxylate (RE, black; O, grey; C, white; H, omitted; DMF and H2O guest molecules have been removed for clarity). (Redrawn from the CIF file of N.L. Rosiet al., “Rod packings and met- alorganic frameworks constructed from rod-shaped secondary building units,’’Journal of the American Chemical Society,127(5), 1504–1518, 2005 [73].)
(a) Tb1
(b)
Figure 3.15 The structure of [Tb(btc)(DMF)2]n·nH2O. (a) Each Tb center is connected to six other Tb centers through four ligands. (b) The 13.5×7.6 Å2rectangle channels viewed down thecdirection (Tb, black; O, grey; C, white; H, omitted; DMF and H2O guest molecules in (b) have been removed for clarity).
(Redrawn from the CIF file of Z. Liet al., “Synthesis, structure, and luminescent and magnetic properties of novel lanthanide metal-organic frameworks with zeolite-like topology,’’Inorganic Chemistry,46(13), 5174–5178, 2007 [74].)
(a) Tb1
(b)
Figure 3.16 The structure of [Tb(bpdc)1.5(H2O)]n·0.5nDMF. (a) The paddle-wheel building block.
(b) The 3D-framework showing the large rhombic channels (Tb, black; O, grey; C, white; H, omitted;
DMF guest molecules in (b) have been removed for clarity). (Redrawn from the CIF file of X. Guo et al., “Synthesis, structure and luminescent properties of rare earth coordination polymers constructed from paddle-wheel building blocks,’’Inorganic Chemistry,44(11), 3850–3855, 2005 [75].)
of the structure is the eight-membered channel consisting of four metal centers and four phenyl groups linked through carboxylic groups in the [110] direction. The size of the channel is around 7.6×13.5 Å2 and is filled with coordinating DMF and water molecules of crystallization.
Experiments suggested that the dehydrated sample could absorb up to 15 water molecules per unit cell.
[Tb(bpdc)1.5(H2O)]n·0.5nDMF was synthesized by diffusion of triethylamine into a mixture of Tb(NO3)3·nH2O and 4,4-biphenyldicarboxylic acid (H2bpdc) (molar ratio 2 : 1) in a mixture of DMF and EtOH at 4◦C then at 55◦C [75]. Its crystal structure is shown in Figure 3.16a.
The terbium atom is coordinated with six oxygen atoms from six bpdc2− and one oxygen atom from a terminal aqua ligand, CN=7. The crystallographically equivalent Tb(III) ions
oxygen polyedra REO7in the [001] direction. The 1D-chains are linked by biphenyl groups in the [110] and [110] directions to form a 3D-framework with remarkably large rhombic channels sized 25.2×17.1 Å2along the diagonals (calculated from the distances of metal ion centers) (Figure 3.16b).
3.2.2.5 Structures of Rare Earth Complexes with Carboxylic Acids Bearing Other