Supporting coordination through hydrogen bonding in lanthanide complexes of 7-azaindole-N-oxide

Abstract

Ligands capable of simultaneous metal coordination and hydrogen bond donation provide useful structural features to enhance cooperativity and favour specific geometries within the coordination sphere. Here we present the first structurally characterised examples of coordination compounds containing protonated 7-azaindole-N-oxide HL, bearing a neutral oxo donor capable of terminal or bridging coordination modes adjacent to a convergent hydrogen bond donor. The ligand itself shows a strong tendency for dimeric assembly in the solid state, but is easily deprotonated to give the chelate complex [CuL₂] 1. In the presence of lanthanide ions, however, four new complexes [Eu(NO₃)₃(HL)₃] 2, [Gd(NO₃)₃(HL)₃] 3, [Eu₂(μ₂-HL)₂(HL)₄Cl₆] 4 and [YbCl₃(HL)₃][YbCl(HL)₅OH₂]2Cl 5 were prepared and crystallographically characterised. All four species show strong tendencies for hydrogen bonding from the ligand to impact their overall structures, including a C₃ propellor-like macrocyclic motif in 2 and 3 and a combination of intramolecular N–H⋯Cl contacts, and intermolecular tridentate anion binding in 5. Solution studies including HRMS and phosphorescence emission spectroscopy reveal persistence of the europium complex 2 in solution, despite the multiple possible binding modes of this ligand, hinting at a degree of cooperativity in these systems. These results show the utility of hydrogen bonding within the coordination sphere for influencing structural outcomes, relevant to the construction of stable higher-order crystalline assemblies.