Crown ethers as second-sphere ligands. The interactions of transition-metal ammines with 18-crown-6 and dibenzo-18-crown-6

Abstract

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)<inf>5</inf>(NH<inf>3</inf>)], trans-[PtCl<inf>2</inf>(PMe<inf>3</inf>)(NH<inf>3</inf>)], trans-[PtCl<inf>2</inf>(NH<inf>3</inf>)<inf>2</inf>], [Co(NH<inf>3</inf>)<inf>6</inf>][PF<inf>6</inf>]<inf>3</inf>, [Cu(NH<inf>3</inf>)<inf>4</inf>(H<inf>2</inf>O)][PF<inf>6</inf>]<inf>2</inf>, and [Pt(H<inf>2</inf>NCH<inf>2</inf>CH<inf>2</inf>NH<inf>2</inf>) <inf>2</inf>][PF<inf>6</inf>]<inf>2</inf>. 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.