The sulfonate group as a ligand: a fine balance between hydrogen bonding and metal ion coordination in uranyl ion complexes
Résumé
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 κ 1-O(S);κ 1-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 to 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.
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