Solution-State and Solid-State Structural Characterization of Complexes of a New Macrocyclic Ligand Containing the 1,5-Diazacyclooctane Subunit^†

The synthesis and characterization of a new constrained tetraazamacrocyclic ligand, 1,4,8,11-tetraazabicyclo[9.3.3]heptadecane (1,11-C₃-cyclam), is reported. Because of its basicity, this ligand (pK_a of the protonated form >13.5) requires aprotic solvents for its metalation reactions. Two complexes of this ligand, [Ni(1,11-C₃-cyclam](OTf)₂ and [Co(1,11-C₃-cyclam)(NCS)₂](OTf), have been characterized by single-crystal X-ray crystallography. For the Ni(II) complex, the 1,5-diazacyclooctane (daco) subunit of the ligand is in the chair−boat conformation, whereas that same subunit in the Co(III) complex is in the chair−chair conformation. For the Ni(II) complex, C(12) and H(12a) block one of the coordination sites. The ¹H and ¹³C NMR spectra of the Ni(II) complex in D₂O have very sharp resonances, indicative of low-spin Ni(II). The resonance for H(12a) appears at 4.5 ppm, suggesting an interaction with Ni(II). In acetonitrile, the ¹H and ¹³C spectra are broadened, indicative of a low-spin/high-spin equilibrium due to axial coordination by acetonitrile. C(12) experiences the greatest degree of broadening in the ¹³C NMR spectrum. Variable-temperature NMR spectroscopy from −70 to +80 °C shows no significant change as a function of temperature. The electronic spectrum of the Ni(II) complex (λ_max = 449.9 nm) is consistent with steric and electronic factors for this complex.