A linear CuII–GdIII–CuII–GdIII complex derived from the assembly reaction of [NaCuIIH3Ldpen(meso)] and [GdIII(thd)3(H2O)2] (H3L = meso-1,2-diphenyl-1-(2-oxybenzamido)-2-(2-oxy-3-ethoxybenzylideneamino)ethane and Hthd = 2,2,6,6-tetra-methyl-3,5-heptanedione)