Static versus dynamic Jahn–Teller distortions in octahedral-like copper(II) complexes of 1,4,7-triazacyclononane and 1-oxa-4,7-diazacyclononane: implications for hydrolytic reactivity

The syntheses and characterizations by X-ray crystallography, EPR, and electronic absorption spectroscopy are reported for [Cu([9]aneN3)2][CF3SO3]2·H2O (1) [Cu([9]aneN2O)2][BF4]2 (2), and [Cu([9]aneN2O)2][NO3]2 (3). The dynamic Jahn–Teller distortion present in the Cu([9]aneN3)2 2+ cation is removed upon replacement of one nitrogen donor with oxygen. The pseudo-octahedral structures of 2 and 3, determined crystallographically, exhibit a strong tetragonal distortion with the two oxygen atoms in the elongated axial positions. In contrast, crystallographic characterizations of a number of Cu([9]aneN3)2 2+ salts reveal that the octahedral-like coordination about the Cu(II) is dependent on crystal packing forces, and the degree of tetragonal distortion varies with the identity of the counter ion. A room temperature powder EPR spectrum of [Cu([9]aneN2O)2]Br2 exhibits a static Jahn–Teller distortion while that of [Cu([9]aneN3)2][CF3SO3]2 is dynamically averaged. Solution EPR and electronic absorption spectra of [Cu([9]aneN2O)2][CF3SO3]2 indicate a strong interaction between the equatorial nitrogen atoms and the Cu(II) as evidenced by the observation of nitrogen hyperfine splitting in gx and a shift of the ligand-field absorption to higher energy. The absence of a dynamic Jahn–Teller effect in the presence of the [9]aneN2O ligand correlates with a lowered reactivity for phosphodiester hydrolysis, suggesting that ligand fluxionality may be important in accessing the transition state in copper-promoted hydrolysis.