Irreversible and reversible formation of a [2]rotaxane containing platinum(ii) complex with an N-alkyl bipyridinium ligand as the axis component

An N-Alkyl bipyridinium having a polymethylene chain and a bulky aryl group at the end, [4,4'-bpy-N-(CH2)10OC6H(3)-3,5-tBu2]Cl (Cl), reacts with K[PtCl3(dmso)] to produce the Pt complex with the N-alkyl bipyridinium ligand [Cl2(dmso)Pt{4,4'-bpy-N-(CH2)10OC6H(3)-3,5-tBu2}][PtCl3(dmso)] as a 6:1 mixture of trans and cis isomers ([trans-][PtCl3(dmso)] and [cis-][PtCl3(dmso)]). Addition of alpha-cyclodextrin (alpha-CD) to a solution of Cl in dmso-d6/D2O (3:1) forms [2]pseudorotaxane [{4,4'-bpy-N-(CH2)10OC6H(3)-3,5-tBu2}.(alpha-CD)]Cl (Cl) which is equilibrated with Cl and alpha-CD in solution. The reaction of K[PtCl3(dmso)] with Cl affords the [2]rotaxane [trans-Cl2(dmso)Pt{4,4'-bpy-N-(CH2)10OC6H(3)-3,5-tBu2}.(alpha-CD)][PtCl3(dmso)] ([trans-][PtCl3(dmso)]) which contains alpha-CD and [trans-][PtCl3(dmso)] as the cyclic and axis components, respectively. Dissolution of a mixture of [trans-][PtCl3(dmso)], [cis-][PtCl3(dmso)] and alpha-CD in dmso-d6/D2O (3:1) forms a mixture of the rotaxanes containing [trans--d6][PtCl3(dmso)] and [cis--d6][PtCl3(dmso)]. The reaction involves partial dissociation of the bipyridinium from Pt of [trans-][PtCl3(dmso)] or [cis-][PtCl3(dmso)] to yield [PtCl3(dmso)] and formation of pseudorotaxane with alpha-CD, followed by recoordination of the bipyridinium to the Pt. The reversible formation of the Pt-N coordination bond is studied in a dmso solution of the N-butyl compounds [trans-Cl2(dmso)Pt{4,4'-bpy-N-nBu}][PtCl3(dmso)] ([trans-][PtCl3(dmso)]).