An unusually flexible expanded hexaamine cage and its Cu(II) complexes: variable coordination modes and incomplete encapsulation.

The bicyclic hexaamine "cage" ligand Me(8)tricosaneN(6) (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 Me(8)tricosaneN(6). Both Cu(Me(8)tricosaneN(6))(2) (6-coordinate) and Cu(Me(8)tricosaneN(6)) (5-coordinate) have been characterized structurally. In weak acid (pH 1) a singly protonated complex Cu(HMe(8)tricosaneN(6)) 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(Me(8)tricosaneN(6)) at neutral pH comprises a five-coordinate, square pyramidal complex. Cyclic voltammetry of the square pyramidal Cu(Me(8)tricosaneN(6)) 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.