Synthesis of bulk samples. Solid RE(NO3)3 hydrate (RE = Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; 0.12 mmol) and tpa (35 mg, 0.12 mmol) were dissolved in acetonitrile (MeCN) (10 mL) with rapid stirring, forming a colourless suspension after 1 minute. Equimolar NEt3 (36 mg, 0.36 mmol) was added to a solution of Br4catH2 (0.15 g, 0.36 mmol) in MeCN (5 mL) and the resulting solution added dropwise to the RE solution. The suspension disappeared after the complete addition of Et3NH + Br4catH -. A microcrystalline suspension of [RE(Br4catH)3(tpa)]•0.5 MeCN formed after 5 min, which was collected by filtration and washed with MeCN and diethyl ether (Et2O). The bulk samples were used for all measurements except for the single crystal X-ray structural analyses.