Counterion-Controlled Formation of an Octanuclear Uranyl Cage with cis-1,2-Cyclohexanedicarboxylate Ligands

Woven, Polycatenated, or Cage Structures: Effect of Modulation of Ligand Curvature in Heteroleptic Uranyl Ion Complexes

Combining the flexible zwitterionic dicarboxylate 4,4'-bis(2-carboxylatoethyl)-4,4'-bipyridinium (L) and the anionic dicarboxylate ligands isophthalate (ipht^2-) and 1,2-, 1,3-, or 1,4-phenylenediacetate (1,2-, 1,3-, and 1,4-pda^2-), of varying shape and curvature, has allowed isolation of five uranyl ion complexes by synthesis under solvo-hydrothermal conditions. [(UO₂)₂(L)(ipht)₂] (1) and [(UO₂)₂(L)(1,2-pda)₂]·2H₂O (2) have the same stoichiometry, and both crystallize as monoperiodic coordination polymers containing two uranyl-(anionic carboxylate) strands united by L linkers into a wide ribbon, all ligands being in the divergent conformation. Complex 3, [(UO₂)₂(L)(1,3-pda)₂]·0.5CH₃CN, with the same stoichiometry but ligands in a convergent conformation, is a discrete, binuclear species which is the first example of a heteroleptic uranyl carboxylate coordination cage. With all ligands in a divergent conformation, [(UO₂)₂(L)(1,4-pda)(1,4-pdaH)₂] (4) crystallizes as a sinuous and thread-like monoperiodic polymer; two families of chains run along different directions and are woven into diperiodic layers. Modification of the synthetic conditions leads to [(UO₂)₄(LH)₂(1,4-pda)₅]·H₂O·2CH₃CN (5), a monoperiodic polymer based on tetranuclear (UO₂)₄(1,4-pda)₄ rings; intrachain hydrogen bonding of the terminal LH⁺ ligands results in diperiodic network formation through parallel polycatenation involving the tetranuclear rings and the LH⁺ rods. Complexes 1-3 and 5 are emissive, with complex 2 having the highest photoluminescence quantum yield (19%), and their spectra show the maxima positions usual for tris-κ²O,O'-chelated uranyl cations.

Ni(2,2':6',2″-Terpyridine-4'-carboxylate)2 Zwitterions and Carboxylate Polyanions in Mixed-Ligand Uranyl Ion Complexes with a Wide Range of Topologies

The zwitterionic complex formed by Ni^II and 2,2':6',2″-terpyridine-4'-carboxylate, Ni(tpyc)₂, has been used as a coligand with a diverse group of polycarboxylates in uranyl ion complexes synthesized under solvo-hydrothermal conditions, thus giving a series of 14 mixed ligand, heterometallic compounds. Both [(UO₂)₂(c-1,2-chdc)Ni(tpyc)₂(NO₃)₂]₂·4CH₃CN (1) and [(UO₂)₂(tdc)Ni(tpyc)₂(NO₃)₂]₂ (2), where c-1,2-chdc^2- is cis-1,2-cyclohexanedicarboxylate and tdc^2- is 2,5-thiophenedicarboxylate, display discrete U₄Ni₂ dinickelatetrauranacycles, a motif which is also found as part of a daisychain coordination polymer in [(UO₂)₄(bdc)₃Ni₂(tpyc)₄(NO₃)₂]·2CH₃CN·2H₂O (3), where bdc^2- is 1,4-benzenedicarboxylate. Similar U₄Ni₂ rings associate to form a nanotubular polymer in [(UO₂)₂(tca)Ni(tpyc)₂(NO₃)]·2CH₃CN·2H₂O (4), where tca^3- is tricarballylate. [(UO₂)₂(1,2-pda) (1,2-pdaH)Ni(tpyc)₂(NO₃)]·CH₃CN (5), where 1,2-pda^2- is 1,2-phenylenediacetate, crystallizes as a meander-like chain in which each bent section can be seen as an open, semi-U₄Ni₂ ring. Oxalate (ox^2-) gives [(UO₂)₂(ox)₂Ni(tpyc)₂] (6), a monoperiodic polymer containing smaller U₄Ni rings, while 1,2,3-benzenetricarboxylate (1,2,3-btc^3-) and citrate (citH^3-) give [Ni(tpycH)(H₂O)₃][UO₂(1,2,3-btc)]₂·2H₂O (7) and [UO₂Ni₂(tpyc)₄][UO₂(citH)]₂ (8), two complexes with charge separation, the latter displaying one-periodic + two-periodic semi-interpenetration. [(UO₂)₂(btcH)Ni(tpyc)₂(NO₃)] (9) and [(UO₂)₂(cbtcH)Ni(tpyc)₂(NO₃)] (10), where btc^4- and cbtc^4- are 1,2,3,4-butanetetracarboxylate and cis,trans,cis-1,2,3,4-cyclobutanetetracarboxylate, respectively, are diperiodic networks with hcb topology, and [(UO₂)₂(ndc)Ni(tpyc)₂(OH)(NO₃)] (11), where ndc^2- is 2,6-naphthalenedicarboxylate, is a sql network containing dinuclear nodes and involving 100-membered U₁₀Ni₄ metallacyclic units. U₄Ni₂ rings are found in the diperiodic polymer formed in [(UO₂)₄(t-R-1,2-chdc)₄Ni₂(tpyc)₄] (12), where t-R-1,2-chdc^2- is trans-R,R-1,2-cyclohexanedicarboxylate, the heavily puckered sheets being interlocked. 1,3-Phenylenediacetate (1,3-pda^2-) gives a very thick diperiodic polymer with KIa topology, [(UO₂)₄(1,3-pda)₄Ni₂(tpyc)₄]·CH₃CN·2H₂O (13). A triperiodic framework is formed with nitrilotriacetate (nta^3-) in [(UO₂)₂(nta)₂Ni₂(tpyc)₂] (14), where Ni^II is found in Ni(tpyc)₂ units as well as in Ni(nta)₂^4- moieties which both act as 4-coordinated nodes.

Enhanced luminescence of tris(carboxylato)uranyl(VI) complexes and energy transfer to Eu(III): a combined spectroscopic and theoretical investigation

Complex formation between uranyl and carboxylate ligands (benzoate, nicotinate and isonicotinate) has been studied extensively by absorption and luminescence spectroscopy in acetonitrile medium. Experimental data had indicated the existence of stable and enhanced luminescent tris(carboxylato) uranyl(VI) complexes i.e. [UO₂(L)₃]^- with D_3h symmetry. The high luminescence of these complexes was due to the sensitization of the O_yl → U ligand to metal charge transfer (LMCT) emission by extremely intense equatorial (carboxylate ligands) LMCT bands. The variation in the experimentally observed parameters such as intensity of equatorial LMCT bands, luminescence lifetimes, quantum yields and structural parameters among tris(carboxylato) uranyl(VI) complexes are affirmed by quantum chemical calculations using density functional theory and the computational results are found to be in good agreement with experimental findings. Interestingly, in a very dilute mixture of [UO₂(L)₃]^- and Eu(III), energy transfer from uranyl to Eu(III) is observed and it leads to the detection of europium at trace levels. This is an intriguing observation as none of the previous studies have reported such a low level of detection limit of Eu(III) by means of energy transfer from any metal.

Recent advances in structures and applications of coordination polymers based on cyclohexanepolycarboxylate ligands

Coordination polymers (CPs) are emerging crystalline materials constructed by metal entities and organic ligands through coordination bonds, containing infinite coordination units in one, two, or three dimensions. Here an overview...

Plumbing the uncertainties of solvothermal synthesis involving uranyl ion carboxylate complexes

Uranyl ion complexes with long-chain, saturated or unsaturated aliphatic dicarboxylate ligands illustrate how solvo-hydrothermal synthetic conditions sometimes result in the formation of species different from those hoped for.

3D Uranyl Organic Frameworks Supported by Rigid Octadentate Carboxylate Ligand: Synthesis, Structure Diversity, and Luminescence Properties

Seven three dimensional (3D) uranyl organic frameworks (UOFs), formulated as [NH₄ ][(UO₂ )₃ (HTTDS)(H₂ O)] (1), [(UO₂ )₄ (HTTDS)₂ ](HIM)₆ (2, IM=imidazole), [(UO₂ )₄ (TTDS)(H₂ O)₂ (Phen)₂ ] (3, Phen=1,10-phenanthroline), [Zn(H₂ O)₄ ]_0.5 [(UO₂ )₃ (HTTDS)(H₂ O)₄ ] (4), and {(UO₂ )₂ [Zn(H₂ O)₃ ]₂ (TTDS)} (5), {Zn(UO₂ )₂ (H₂ O)(Dib)_0.5 (HDib)(HTTDS)} (6, Dib=1,4-di(1H-imidazol-1-yl)benzene) and [Na]{(UO₂ )₄ [Cu₃ (u₃ -OH)(H₂ O)₇ ](TTDS)₂ } (7) have been hydrothermally prepared using a rigid octadentate carboxylate ligand, tetrakis(3,5-dicarboxyphenyl)silicon(H₈ TTDS). These UOFs have different 3D self-assembled structures as a function of co-ligands, structure-directing agents and transition metals. The structure of 1 has an infinite ribbon formed by the UO₇ pentagonal bipyramid bridged by carboxylate groups. With further introduction of auxiliary N-donor ligands, different structure of 2 and 3 are formed, in 2 the imidazole serves as space filler, while in 3 the Phen are bound to [UO₂ ]²⁺ units as co-ligands. The second metal centers were introduced in the syntheses of 4-7, and in all cases, they are part of the final structures, either as a counterion (4) or as a component of framework (5-7). Interesting, in 7, a rare polyoxometalate [Cu₃ (μ₃ -OH)O₇ (O₂ CR)₄ ] cluster was found in the structure. It acts as an inorganic building unit together with the dimer [(UO₂ )₂ (O₂ CR)₄ ] unit. Those uranyl carboxylates were sufficiently determined by single crystal X-ray diffraction, and their topological structures and luminescence properties were analyzed in detail.

Cavity Formation in Uranyl Ion Complexes with Kemp's Tricarboxylate: Grooved Diperiodic Nets and Polynuclear Cages

Kemp's triacid (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid, H₃kta) was reacted with uranyl nitrate under solvo-hydrothermal conditions in the presence of diverse counterions or additional metal cations to give eight zero- or diperiodic complexes. All the coordination polymers in the series, [PPh₃Me][UO₂(kta)]·0.5H₂O (1), [PPh₄][UO₂(kta)] (2), [C(NH₂)₃][UO₂(kta)] (3), [Cd(bipy)₃][UO₂(kta)]₂ (4), and [Zn(phen)₃][UO₂(kta)]₂·2H₂O (5) (bipy = 2,2'-bipyridine, phen = 1,10-phenanthroline) crystallize as networks with the hcb topology, the ligand being in the chair conformation with the three carboxylate groups equatorial, except in 3, in which the axial/diequatorial boat conformation is present. Various degrees of corrugation and different arrangements of neighboring layers are observed depending on the counterion, with complexes 4 and 5, in particular, displaying cavities containing the bulky cations. [Co(en)₃]₂[(UO₂)₂(kta)(Hkta)₂]₂·2NMP·10H₂O (6) (en = 1,2-ethanediamine; NMP = N-methyl-2-pyrrolidone) contains a metallatricyclic, tetranuclear anionic species, displaying two clefts in which the cations are held by extensive hydrogen bonding, and with the ligands in both triaxial chair and axial/diequatorial boat conformations. [(UO₂)₃Pb(kta)₂(Hkta)(H₂O)]₂·1.5THF (7) (THF = tetrahydrofuran) and [(UO₂)₂Pb₂(kta)₂(Hkta)(NMP)]₂ (8) are two heterometallic cage compounds containing only the convergent, triaxial chair form of the ligand, which have the same topology in spite of the different U/Pb ratio. These complexes are compared to previous ones also involving Kemp's triacid anions, and the roles of ligand conformation and of counterions in the formation of cavities, either in cage-like species or as grooves in diperiodic networks, is discussed.

Uranyl Tricarballylate Triperiodic and Nanotubular Species. Counterion Control of Nanotube Diameter

Tricarballylic acid (propane-1,2,3-tricarboxylic acid, H₃ tca) was reacted with uranyl nitrate hexahydrate under solvo-hydrothermal conditions and in the presence of different additional cations, yielding four complexes which have been crystallographically characterized. [(UO₂)₂Ba(tca)₂(H₂O)₄] (1), isomorphous to the Pb^II analogue previously reported, crystallizes as a triperiodic framework in which diperiodic uranyl-tca^3- subunits with the hcb (honeycomb) topology are linked by carboxylate-bound Ba^II cations. Triperiodic polymerization is also found in [(UO₂)₂(tca)₂Ni(cyclam)] (2) and [(UO₂)₂(tca)₂Cu(R,S-Me₆cyclam)] (3), but here the diperiodic uranyl-tca^3- subunits have the sql (square lattice) topology, and the frameworks formed through bridging by Ni^II or Cu^II cations have different topologies, tcs in 2 and xww in 3. [Co(en)₃][UO₂(tca)]₃·2H₂O (4) crystallizes as a monoperiodic coordination polymer with the hcb topology and a nanotubular geometry. In contrast to the square-section nanotubules previously found in [NH₄][(UO₂)₂Pb(tca)₂(NO₃)(bipy)] (bipy = 2,2'-bipyridine), those in 4 have a hexagonal section with a width of ∼7 Å. The structure-directing role of the hydrogen bonded counterions in these nanotubular species, either NH₄⁺ located within the nanotubule cavity or [Co(en)₃]³⁺ located outside, is discussed. Emission spectra in the solid state display the usual vibronic fine structure for 1 and 4, while uranyl emission is quenched in 3.

Uranyl Organic Framework as a Highly Selective and Sensitive Turn-on and Turn-off Luminescent Sensor for Dual Functional Detection Arginine and MnO4. Inorg Chem 2020;59:5004-5017. [PMID: 32207299 DOI: 10.1021/acs.inorgchem.0c00236]

Five new uranyl coordination polymers were prepared by the hydrothermal method based on 5-nitroisophthalic acid (H₂nip) as (UO₂)(nip)(2,2'-bpy) (1), (H₂4,4'-bpy)·[(UO₂)₃(nip)₄]·(4,4'-bpy) (2), (H₂bpe)·[(UO₂)_0.5(nip)] (3), (H₂ bpp)·[(UO₂)₂-(nip)₃]·H₂O (4), and (H₂tmp)·[(UO₂)(nip)₂](5) [2,2'-bpy = 2,2'-bipyridine, 4,4'-bpy = 4,4'-bipyridine, bpe = 4,4'-vinylenedipyridine, bpp = 4,4' -trimethylenedipyridine, tmp = tetramethylpyrazine]. All of these synthesized complexes have been characterized by single crystal and powder X-ray diffraction, IR spectra, thermogravimetric analysis, elemental analysis, and luminescent properties. In particular, it is found that compounds 1 and 4 can be used as a luminescent sensor to efficiently detect arginine in aqueous solution by means of "turn-on"; the detection limits were 1.06 × 10^-6 and 6.42 × 10^-6 mol/L, respectively. Moreover, 4 can also be used as a bifunctional sensor for selective sensing of MnO₄^- anion by "turn-off". The detection limit of MnO₄^- in water was 1.79 × 10^-6 mol/L; the K_sv was 1.88 × 10⁴. The sensing effect of arginine in simulated grape juice samples and MnO₄^- in simulated river water samples was also investigated by this sensing system with high recovery. In addition, the possible mechanism of sensing arginine and MnO₄^- in the aqueous solution was discussed.

Uranyl Ion Complexes of Polycarboxylates: Steps towards Isolated Photoactive Cavities

Consideration of the extensive family of known uranyl ion complexes of polycarboxylate ligands shows that there are quite numerous examples of crystalline solids containing capsular, closed oligomeric species with the potential for use as selective heterogeneous photo-oxidation catalysts. None of them have yet been assessed for this purpose, and some have obvious deficiencies, although related framework species have been shown to have the necessary luminescence, porosity and, to some degree, selectivity. Aspects of ligand design and complex composition necessary for the synthesis of uranyl ion cages with appropriate luminescence and chemical properties for use in selective photo-oxidation catalysis have been analysed in relation to the characteristics of known capsules.

Occurrence of polyoxouranium motifs in uranyl organic networks constructed by using silicon-centered carboxylate linkers: structures, spectroscopy and computation

Four polyoxouranium-based uranyl carboxylates have been synthesized based on silicon-centered carboxylate linkers. Oligomerization of the uranyl units from tetrameric unit, to octameric motif and ultimately infinite polyoxouranium chain was observed.

Zero-, mono- and diperiodic uranyl ion complexes with the diphenate dianion: influences of transition metal ion coordination and differential UVI chelation

Diphenate complexes with uranyl cations are generally of low periodicity (0 or 1), but for one 2-periodic uranyl–Cu^II species.

The sulfonate group as a ligand: a fine balance between hydrogen bonding and metal ion coordination in uranyl ion complexes

Nine uranyl ion complexes have been synthesized using two kinds of sulfonate-containing ligands, i.e. 2-, 3- and 4-sulfobenzoic acids (2-, 3- and 4-SBH₂), which include additional carboxylic donors, and p-sulfonatocalix[4]arene (H₈C4S), with additional phenolic groups, and [Ni(cyclam)]²⁺, [Cu(R,S-Me₆cyclam)]²⁺ or PPh₄⁺ as counterions. [Ni(cyclam)][UO₂(4-SB)₂(H₂O)₂]·2CH₃CN (1) and [Ni(cyclam)][UO₂(3-SB)₂(H₂O)₂] (2) are molecular species in which only the carboxylate groups are coordinated to uranyl, the sulfonate groups being essentially hydrogen bond acceptors. In contrast, uranyl κ¹-O(S);κ¹-O(C)-chelation is found in the four complexes involving 2-SB^2-, different bridging interactions producing diverse geometries. [UO₂(2-SB)₂Ni(cyclam)]·H₂O (3) crystallizes as a two-dimensional (2D) assembly with fes topology, in which uranyl ion dimeric subunits are bridged by six-coordinate Ni^II cations. Complexes [UO₂(2-SB)₂Cu(R,S-Me₆cyclam)]₂·2H₂O (4) and [(UO₂)₂(2-SB)₂(C₂O₄)Cu(R,S-Me₆cyclam)] (5), obtained together from the same solution, are a molecular tetranuclear complex and a 2D species with fes topology, respectively, depending on the coordination number, 5 or 6, of the Cu^II cation. The complex [PPh₄]₂[(UO₂)₂(2-SB)₃(H₂O)]·H₂O (6) is a one-dimensional (1D), ribbon-like coordination polymer with a layered packing of alternate cationic and anionic sheets. No heterometallic complex was obtained with H₈C4S, but the copper-only compound [{Cu(R,S-Me₆cyclam)}₅(H₃C4S)₂]·17H₂O (7) displays mixed coordination/hydrogen bonding association of the copper azamacrocycle complex with the phenolic groups. The complexes [PPh₄]₅[UO₂(H₄C4S)(H₂O)₄][UO₂(H₃C4S)(H₂O)₄]·14H₂O (8) and [PPh₄]₃[UO₂(H₃C4S)(H₂O)₃]·9H₂O (9) were crystallized from the same solution and are a molecular complex and a 1D polymer, respectively, with monodentate sulfonate coordination to uranyl, while [PPh₄]₂[UO₂(H₄C4S)(H₂O)₃]·11H₂O (10) is also a 1D polymer. The anionic complexes in the last three complexes form layers (9) or double layers (8 and 10) separated from one another by hydrophobic layers of PPh₄⁺ cations. The balance between coordination and hydrogen bonding interactions with the macrocyclic ligands provides an indication of the energy of the sulfonate coordinate bond. Complex 6 is the only luminescent species in this series, albeit with a low quantum yield of 3%, and its emission spectrum is typical of a uranyl complex with five equatorial donors.

Tubelike Uranyl-Phenylenediacetate Assemblies from Screening of Ligand Isomers and Structure-Directing Counterions

Reaction of 1,2-, 1,3-, or 1,4-phenylenediacetic acids (1,2-, 1,3-, or 1,4-H₂PDA) with uranyl ions under solvo-hydrothermal conditions and in the presence of [M(L) _n] ^q+ cations, in which M = transition metal cation, L = 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen), n = 2 or 3, and q = 1 or 2, gave 10 complexes which have been crystallographically characterized. The diacetate ligands are bis-chelating and the uranyl cations are tris-chelated in all cases. [UO₂(1,2-PDA)₂Zn(phen)₂]·2H₂O (1) and [UO₂(1,4-PDA)₂Mn(bipy)₂]·H₂O (2) are heterometallic, neutral one-dimensional (1D) coordination polymers in which the carboxylate-coordinated 3d block metal cation is either decorating only (1) or participates in polymer building (2). [Zn(phen)₃][(UO₂)₂(1,3-PDA)₃] (3) and [Ni(phen)₃][(UO₂)₂(1,4-PDA)₃]·H₂O (4), with separate counterions, crystallize as anionic two-dimensional (2D) networks, as does [Cu(bipy)₂][H₂NMe₂][(UO₂)₂(1,4-PDA)₃] (5), which displays parallel 2D interpenetration. The complex [Zn(phen)₃][(UO₂)₂(1,2-PDA)₃]·7H₂O (6) crystallizes as a ladderlike, slightly inflated ribbon. The same topology is found in [Zn(bipy)₃][(UO₂)₂(1,3-PDA)₃] (7), but the larger separation between coordination sites and the coexistence of curved and divergent ligand conformations produce a tubelike assembly. An analogous but more regular and spacious tubular geometry is found in [M(bipy)₃][(UO₂)₂(1,4-PDA)₃], with M = Co (8) or Ni (9), and {Λ-[Ru(bipy)₃]}[(UO₂)₂(1,4-PDA)₃] (10). The disordered counterions in 8 and 9 are replaced by well-ordered, enantiomerically pure chiral counterions in 10. The tubular assemblies formed in 7-10 are characterized by an oblong section and the presence of gaps in the walls, which enable the inclusion of two rows of counterions in the cavity.

Chiral Discrete and Polymeric Uranyl Ion Complexes with (1 R,3 S)-(+)-Camphorate Ligands: Counterion-Dependent Formation of a Hexanuclear Cage

Reaction of (1 R,3 S)-(+)-camphoric acid (H₂cam) with uranyl ions under solvo-hydrothermal conditions and in the presence of bulky countercations gave five chiral complexes of varying dimensionality. [Cu( R,S-Me₆cyclam)][UO₂(Hcam)₂(HCOO)₂] (1) and [Ni( R,S-Me₆cyclam)][UO₂(cam)(HCOO)₂] (2), in which the formate coligand is formed in situ, involve very similar countercations, but 1 is a discrete, mononuclear complex, whereas 2 crystallizes as a one-dimensional (1D) coordination polymer, and NH-bond donation by the macrocyclic ligand of the countercation complexes is present in both. [Co(en)₃][(UO₂)₄(cam)( R,R-tart)₂(OH)]·3H₂O (3), in which en is ethylenediamine and H₄ R,R-tart is R,R-tartaric acid, contains three enantiomerically pure chiral species, and it displays a two-dimensional (2D) arrangement, with the countercation again involved in NH-bond donation. While [PPh₄][UO₂(cam)(NO₃)] (4) is a 1D polymer, [PPh₃Me]₃[NH₄]₃[(UO₂)₆(cam)₉] (5) is a discrete, homochiral, and homoleptic hexanuclear cage with C₃ point symmetry and a trigonal prismatic arrangement of the uranium atoms. This cage differs from the octanuclear, pseudocubic uranyl camphorate species previously described, thus providing an example of modulation of the cage size through variation of the structure-directing counterions. The cage in 5 is closely associated with three PPh₃Me⁺ cations, two of them outside and with their methyl group directed toward the prism basis center, and one inside the cage cavity. While complex 5 is nonluminescent, complexes 1 and 4 have emission spectra in the solid state typical of equatorially hexacoordinated uranyl complexes. Solid-state photoluminescence quantum yields of 2 and 23% have been measured for complexes 1 and 4, respectively.

Structure-Directing Effects of Counterions in Uranyl Ion Complexes with Long-Chain Aliphatic α,ω-Dicarboxylates: 1D to Polycatenated 3D Species

Nine uranyl ion complexes were synthesized under (solvo-)hydrothermal conditions using α,ω-dicarboxylic acids HOOC-(CH₂) _n-2-COOH (H₂C n, n = 6-9) and diverse counterions. Complexes [PPh₄][UO₂(C6)(NO₃)] (1) and [PPh₄][UO₂(C8)(NO₃)] (2) contain zigzag one-dimensional (1D) chains, with further polymerization being prevented by the terminal nitrate ligands. [PPh₃Me][UO₂(C7)(HC7)] (3) crystallizes as a 1D polymer with a curved section, with hydrogen bonding of the uncomplexed carboxylic groups giving rise to formation of 3-fold interpenetrated two-dimensional (2D) networks. [PPh₄][H₂NMe₂][(UO₂)₂(C7)₃] (4) and [PPh₃Me]₂[(UO₂)₂(C8)₃] (5) contain 1D chains, either ladder-like or containing doubly bridged dimers, while [PPh₃Me]₂[(UO₂)₂(C9)₃]·2H₂O (6) displays interdigitated, strongly corrugated honeycomb 2D nets. Ladder-like 1D polymers in [Cu( R,S-Me₆cyclam)][(UO₂)₂(C7)₂(C₂O₄)]·4H₂O (7) are associated into layers by the hydrogen bonded counterions, whereas the [Ni(cyclam)]²⁺ moieties are part of the 2D polymeric arrangement in [(UO₂)₂(C7)₂(HC7)₂Ni(cyclam)]·2H₂O (8) because of axial coordination of the nickel(II) center, with hydrogen bonding mediated by water molecules generating a three-dimensional (3D) net. [(UO₂)₂K₂(C7)₃(H₂O)]·0.5H₂O (9) contains convoluted uranyl dicarboxylate 2D subunits, which generate a 3D framework through 2D → 3D parallel polycatenation similar to that previously found in [NH₄]₂[(UO₂)₂(C7)₃]·2H₂O; further linking of these subunits is provided by bonding of the potassium cations to carboxylate and uranyl oxido groups. The solid-state emission spectra of complexes 1-6 and 9 display maxima positions typical of hexacoordinated uranyl carboxylate complexes, but uranyl luminescence is quenched in 7. A solid-state photoluminescence quantum yield of 11.5% has been measured for complex 1, while those for compounds 3-6 and 9 are in the range of 2.0-3.5%.

Closed Uranyl-Dicarboxylate Oligomers: A Tetranuclear Metallatricycle with Uranyl Bridgeheads and 1,3-Adamantanediacetate Linkers

In the presence of NH₄⁺ and either PPh₄⁺ or PPh₃Me⁺ cations, 1,3-adamantanediacetic acid (H₂ADA) reacts with uranyl ions under solvo-hydrothermal conditions to give the complexes [NH₄]₂[PPh₄]₂[(UO₂)₄(ADA)₆] (1) and [NH₄]₂[PPh₃Me]₂[(UO₂)₄(ADA)₆] (2), both of which contain a tetranuclear metallatricycle built from two 2:2 rings including convergent ligands, linked by two additional ligands in an extended conformation defining a third, larger ring. While the ammonium cations are closely associated with the 2:2 rings through triple hydrogen bonding, the large PPh₄⁺ or PPh₃Me⁺ cations are more loosely bound to each of the two faces of the larger ring. In contrast, the complex [H₂NMe₂][PPh₃Me][(UO₂)₂(ADA)₃]·H₂O (3), in which dimethylammonium replaces ammonium cations, crystallizes as a two-dimensional network with honeycomb {6³} topology, albeit with very distorted, elongated hexagonal cells. These and previous results show that both NH₄⁺ and PPh₄⁺ or PPh₃Me⁺ cations are essential to the formation of the metallatricycle. The role of the flexibility imparted to ADA^2- by the acetate arms, in comparison to the more rigid 1,3-adamantanedicarboxylate (ADC^2-), is also discussed. All three complexes are luminescent, with quantum yields of 0.06, 0.06, and 0.09 for 1-3, respectively. The vibronic fine structure apparent on the emission spectra gives peak positions typical of species in which the uranyl ion is chelated by three carboxylate groups.