Synthesis of single crystal samples. Solid Br4catH2 (0.15 g, 0.36 mmol) and tpa (35 mg, 0.12 mmol) were dissolved in MeCN (15 mL) followed by the addition of equimolar triethylamine (NEt3) (0.36 mg, 0.36 mmol). This solution was added dropwise to a solution of RE(NO3)3 (0.12 mmol) in MeCN (10 mL) without stirring. Single block-shaped crystals suitable for structure determination formed overnight and were kept in contact with the mother liquor. All compounds were colourless, with the exception of 1-Eu which was light orange.