Crown ethers as second-sphere ligands. The interactions of transition-metal ammines with 18-crown-6 and dibenzo-18-crown-6
Neutral and cationic transition-metal ammines interact with crown ethers in aqueous and non-aqueous solution via(N–H ⋯ O) hydrogen-bond formation to give discrete or polymeric adducts which are readily isolated by crystallisation. Dibenzo-18-crown-6 generally yields discrete 1 : 1 (metal complex : crown) adducts in which only one face of the macrocycle interacts with an ammine ligand, whereas 18-crown-6 often binds two such ligands, one to each face of the crown ether. The resulting adducts are then either polymeric or of 2 : 1 (metal complex : crown) stoicheiometry. Ammines which yield isolable adducts include [W(CO)5(NH3)], trans-[PtCl2(PMe3)(NH3)], trans-[PtCl2(NH3)2], [Co(NH3)6][PF6]3, [Cu(NH3)4(H2O)][PF6]2, and [Pt(H2NCH2CH2NH2)2][PF6]2. Such adduct formation provides the basis for a novel method of separating different transition metals; for example, 18-crown-6 selectively precipitates the copper(II) complex (as a polymeric 1:1 adduct) in the presence of an equimolar amount of the cobalt(III) ammine.