Self-assembly of a Pd-based molecular bowl as anion receptor featured by multiple C–H···anion hydrogen bonds

The self-assembly of (tmen)Pd(NO₃)₂ (where tmen = N¹, N¹, N², N²-tetramethylethane-1,2-diamine) and 4,7-phenanthroline rigid ligand (L) in aqueous solution affords positively charged [Pd₃L₃]⁶⁺ Pd-based molecular bowls (1a·6NO₃⁻ and 1b·6BF₄⁻). Their structures were characterized by ¹H NMR, electrospray ionization mass spectrometry and single-crystal X-ray diffraction analysis. Interestingly, X-ray diffraction analysis of 1a·6NO₃⁻ and 1b·6BF₄⁻ revealed that an anion was bound inside the cavity of the bowl-shaped structure by multiple C–H···O hydrogen bonds in 1a·6NO₃⁻, while multiple C–H···F hydrogen bonds in 1b·6BF₄⁻. The interactions between [Pd₃L₃]⁶⁺ molecular bowl and a variety of anions were investigated via NMR titration and fluorescent studies, respectively.