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

Abstract

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-SB²⁻, 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.