Mixed-ligand molecular paneling: Dodecanuclear cuboctahedral coordination cages based on a combination of edge-bridging and face-capping ligands

Reaction of a tris-bidentate ligand L¹ (which can cap one triangular face of a metal polyhedron), a bis-bidentate ligand L² (which can span one edge of a metal polyhedron), and a range of M²⁺ ions (M = Co, Cu, Cd), which all have a preference for six coordination geometry, results in assembly of the mixed-ligand polyhedral cages [M ₁₂(μ³-L¹)₄(μ-L ²)₁₂]²⁴⁺. When the components are combined in the correct proportions [M²⁺:L¹:L² = 3:1:3] in MeNO₂, this is the sole product. The array of 12 M²⁺ cations has a cuboctahedral geometry, containing six square and eight triangular faces around a substantial central cavity; four of the eight M₃ triangular faces (every alternate one) are capped by a ligand L¹, with the remaining four M₃ faces having a bridging ligand L ² along each edge in a cyclic helical array. Thus, four homochiral triangular {M₃(L²)₃}⁶⁺ helical units are connected by four additional L¹ ligands to give the mixed-ligand cuboctahedral array, a topology which could not be formed in any homoleptic complex of this type but requires the cooperation of two different types of ligand. The complex [Cd₃(L²)₃(ClO ₄)₄(MeCN)₂(H₂O)₂] (ClO₄)₂, a trinuclear triple helicate in which two sites at each Cd(II) are occupied by monodentate ligands (solvent or counterions), was also characterized and constitutes an incomplete fragment of the dodecanuclear cage comprising one triangular {M₃(L²)₃} ⁶⁺ face which has not yet reacted with the ligands L¹. ¹H NMR and electrospray mass spectrometric studies show that the dodecanuclear cages remain intact in solution; the NMR studies show that the Cd₁₂ cage has four-fold (D₂) symmetry, such that there are three independent Cd(II) environments, as confirmed by a ¹¹³Cd NMR spectrum. These mixed-ligand cuboctahedral complexes reveal the potential of using combinations of face-capping and edge-bridging ligands to extend the range of accessible topologies of polyhedral coordination cages.