Four Soft Donors and a Hard Centre: Rhodium Complexes of a Novel Tetrakis(NHC)-Encapsulated Crown Ether Ligand

2,3,13,14-Tetrakis(bromomethyl)dibenzo-18-crown-6 (1) has been demonstrated to be a versatile synthetic intermediate for constructing multinucleating ligands with a hard-donor, crown ether centre. By using this intermediate, novel salts of tetrakis(imidazoliomethyl) dibenzo-18-crown-6, [H4L1]X-4 (2: X- = Br-; 4: X- = PF6-), and tetrakis(benzimidazoliomethyl)-dibenzo-18-crown-6, [H4L2]X-4 (3: X- = Br-; 5: X- = PF6-), were prepared. These salts are precursors to novel tetrakis(N-heterocyclic carbene) complexes spanned by a crown ether bridge capable of binding various hard metal ions. The tetrakis(imidazoliomethyl) salt 4 was used to prepare the bimetallic rhodium complexes [{(cod) Rh}(2)(mu-L-1)][PF6](2) (6) and [{(CO)(2)Rh}(2)(mu-L-1)][PF6](2) (7). These complexes selectively bind alkali metal ions.