An Unusually Flexible Expanded Hexaamine Cage and Its CuII Complexes: Variable Coordination Modes and Incomplete Encapsulation

The bicyclic hexaamine “cage” ligand Me8tricosaneN6 (1,5,5,9,13,13,20,20-octamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane) is capable of encapsulating octahedral metal ions, yet its expanded cavity allows the complexed metal to adopt a variety of geometries comprising either hexadentate or pentadentate coordination of the ligand. When complexed to CuII the lability of the metal results in a dynamic equilibrium in solution between hexadentate- and pentadentate-coordinated complexes of Me8tricosaneN6. Both [Cu(Me8tricosaneN6)](ClO4)2 (6-coordinate) and [Cu(Me8tricosaneN6)](S2O6) (5-coordinate) have been characterized structurally. In weak acid (pH 1) a singly protonated complex [Cu(HMe8tricosaneN6)]3+ has been isolated that finds the ligand binding as a pentadentate with the uncoordinated amine being protonated. vis–NIR and electron paramagnetic resonance (EPR) spectroscopy show that the predominant solution structure of [Cu(Me8tricosaneN6)]2+ at neutral pH comprises a five-coordinate, square pyramidal complex. Cyclic voltammetry of the square pyramidal [Cu(Me8tricosaneN6)]2+ complex reveals a reversible CuII/I couple. All of these structural, spectroscopic, and electrochemical features contrast with the smaller cavity and well studied “sarcophagine” (sar, 3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane) CuII complexes which are invariably hexadentate coordinated in neutral solution and cannot stabilize a CuI form.