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

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journal contribution
posted on 2003-01-03, 00:00 authored by Chuanjiang Hu, Thoi D. Nguyen, Paul S. Wagenknecht, Lawrence C. Nathan
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 (pKa 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 (λmax = 449.9 nm) is consistent with steric and electronic factors for this complex.