in neat (no solvent) on storing in air;

in ethanol; H2O soln. of 4,4'-bipyridine in EtOH slowly added to aq. soln. of Cu(BF4)2, slurry stirred at room temp. for 4 h; according to A. Baiker et al., J. Catal. 257 (2008) 390 and Chem. Eur. J. 15 (2009) 12255; solid filtered off, washed (H2O, EtOH), dried (room temp., air), treatedat 100°C and 50 Pa for 2 h; XRD;