Exploring New Assembly Modes of Uranyl Terephthalate: Templated Syntheses and Structural Regulation of a Series of Rare 2D → 3D Polycatenated Frameworks

Abstract

The reaction of uranyl nitrate with terephthalic acid (HTP) under hydrothermal conditions in the presence of an organic base, 1,3-(4,4'-bispyridyl)propane (BPP) or 4,4'-bipyridine (BPY), provided four uranyl terephthalate compounds with different entangled structures by a pH-tuning method. [UO(TP)](HBPP)·2HO (1) obtained in a relatively acidic solution (final aqueous pH, 4.28) crystallizes in the form of a noninterpenetrated honeycomb-like two-dimensional network structure. An elevation of the solution pH (final pH, 5.21) promotes the formation of a dimeric uranyl-mediated polycatenated framework, [(UO)(μ-OH)(TP)](HBPP)·4.5HO (2). Another new polycatenated framework with a monomeric uranyl unit, [(UO)(TP)](HBPP) (3), begins to emerge as a minor accompanying product of 2 when the pH is increased up to 6.61, and turns out to be a significant product at pH 7.00. When more rigid but small-size BPY molecules replace BPP molecules, [UO(TP)](HBPP) (4) with a polycatenated framework similar to 3 was obtained in a relatively acidic solution (final pH, 4.81). The successful preparation of 2-4 represents the first report of uranyl-organic polycatenated frameworks derived from a simple HTP linker. A direct comparison between these polycatenated frameworks and previously reported uranyl terephthalate compounds suggests that the template and cavity-filling effects of organic bases (such as BPP or BPY), in combination with specific hydrothermal conditions, promote the formation of uranyl terephthalate polycatenated frameworks.

Structural Consequences of 1,4-Cyclohexanedicarboxylate Cis/Trans Isomerism in Uranyl Ion Complexes: From Molecular Species to 2D and 3D Entangled Nets

“…9 Obviously, even if a general trend appears here, within a family of complexes all involving carboxylate ligands, it is nevertheless important to stress that the coordination number is only one factor at play, the ligand strength in particular exerting an influence. 74,75 Some carboxylate complexes studied by other groups fit well in the present series, 15,16,18,19,26,[86][87][88] but a different evolution of peak positions with respect to number of donors has been found in a family of pyrazine-or pyridine-containing carboxylates. 89 It is known that a red-shift of uranyl emission peaks positions is related to an increase of donor strength in the equatorial plane, inducing a decrease in uranyl oxo bond order, 74,90 which would suggest that the reduced crowding in the equatorial plane, and the associated shorter bond lengths, in O5 species with respect to O6 ones must result in an overall donor strength increase in the former.…”

“…34 A 2-fold 2D → 2D parallel interpenetrated geometry with terephthalate as only ligand has been reported, 8 in which the individual networks have the same {6 3 } topology as found here, with much more regular hexagonal rings (∼23 Å) due to the presence of linear ligands only, and examples of 2D → 3D polycatenation, two of them involving honeycomb nets, have lately been described. 19 More generally, the 2D honeycomb geometry, which is frequently found in uranyl ion complexes, is particularly conducive to the formation of entangled species (and makes for a substantial part of them 53 ), provided the ligands are of sufficient length, such as is also found, for example, with 2,5-thiophenedicarboxylate, 30,35 4,4ʹ-biphenyldicarboxylate, 32 or undecane-1,11dicarboxylate, 34 in the case of uranyl complexes. In comparison with complex 6, it appears that the different cis/trans ratio found in 7 has a direct bearing on the topology: the tetranuclear rings found in the former are replaced by hexanuclear rings in the latter due to the higher content in the more linear t-1,4-chdc 2ligand which brings about a larger separation between the metal centres (∼11.4 Å versus ∼8.6 Å) and is better adapted to the hexagonal ring geometry .5H2O (8) has a formula very similar to that of 7, and the cis/trans ratio is probably the same.…”

“…4,5 Although our studies of complexes of cis-1,2-cyclohexanedicarboxylate have to date been limited, they do indicate significant differences to be associated with this isomeric form, providing reason for an extension to examination of the complexes of cis-and trans-1,4-cyclohexanedicarboxylic acid, for which but a single uranyl molecular complex of the trans isomer is known. 6 This acid is the alicyclic counterpart of terephthalic acid, which has been used in the synthesis of several coordination polymers, [7][8][9][10][11][12][13][14][15][16][17][18][19] most often one-or two-dimensional (1D or 2D), with only two cases of threedimensional (3D) frameworks, 12,16 and also some examples of interpenetrated nets. 8,14,19 Some of these complexes are heterometallic, 7,11,13,18 and display photocatalytic activity.…”

“…6 This acid is the alicyclic counterpart of terephthalic acid, which has been used in the synthesis of several coordination polymers, [7][8][9][10][11][12][13][14][15][16][17][18][19] most often one-or two-dimensional (1D or 2D), with only two cases of threedimensional (3D) frameworks, 12,16 and also some examples of interpenetrated nets. 8,14,19 Some of these complexes are heterometallic, 7,11,13,18 and display photocatalytic activity. 7,13,18 We used two commercially available forms of 1,4-cyclohexanedicarboxylic acid, the pure trans form (t-1,4-chdcH2), which, when the two acid groups are in equatorial position, is geometrically close to terephthalic acid, and a mixture of cis and trans isomers (c,t-1,4-chdcH2).…”

“…Although they are not extremely common, the list of entangled nets [20][21][22][23][24][25] in uranyl coordination chemistry is steadily growing and covers a very diverse set of topologies. 8,14,19,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]…”