An unusually flexible expanded hexaamine cage and its Cu^II complexes: Variable coordination modes and incomplete encapsulation

The bicyclic hexaamine "cage" ligand Me ₈tricosaneN₆ (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 Cu^II the lability of the metal results in a dynamic equilibrium in solution between hexadentate- and pentadentate- coordinated complexes of Me₈tricosaneN₆. Both [Cu(Me ₈tricosaneN₆)](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 Cu^II/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) Cu^II complexes which are invariably hexadentate coordinated in neutral solution and cannot stabilize a Cu^I form.