Ion-selective properties of 1,2-bis[2-(2-diphenylphosphorylmethylphenoxy)ethoxy]benzene (L1) and 1,2-bis[2-(2-diphenylphosphorylphenoxy)ethoxy]benzene (L2). Crystal structure of complexes [CdI2(μ-L1)]2, [CdBr2 (μ-L1)]2 · C2H5OH, and [Cd(L2)2(H2O)2][Cd2I6]