Probing the competition between acetate and 2,2′-bipyridine ligands to bind to d-block group 12 metals

Abstract

Herein, we were interested in probing the competition between 2,2′-bipyridine (2,2′-bipy) and acetate ligands in binding to Zn²⁺, Cd²⁺ and Hg²⁺. We have obtained eight new supramolecular architectures through tuning the proportion of these two ligands. On doubling the acetate availability compared to 2,2′-bipy, complexes with either Zn²⁺, Cd²⁺ or Hg²⁺ were formed with one 2,2′-bipy and two acetate ligands coordinated to the metal center. One water molecule is also coordinated to Zn²⁺ and Cd²⁺ in these two complexes, which are reported here for the first time. One 2,2′-bipy is still coordinated to the three metal ions with an acetate excess of 10-times, but another trinuclear Zn²⁺ complex is formed with two 2,2′-bipy and six acetate ligands (1 : 3 2,2′-bipy : acetate stoichiometry). Upon setting an equimolar ratio of the ligands, the complex [Zn(CH₃CO₂)(2,2′-bipy)₂]⁺ is formed, while two 2,2′-bipy and two acetate ligands are coordinated to Cd²⁺, giving rise to a [Cd(CH₃CO₂)₂(2,2′bipy)₂] complex. On doubling the 2,2′-bipy availability compared to acetate, the former does not coordinate to Zn²⁺ and Cd²⁺, as observed in the acetate salt form of [Zn(2,2′-bipy)₃]²⁺ and in [Cd(2,2′-bipy)₃]²⁺. This last Cd²⁺ complex did not crystallize, revealing its unfavorable crystallization as an acetate salt form. However, under this last ligand ratio, the persistence of at least one coordinated acetate was observed in the Hg²⁺ complex with 2 : 1 2,2′-bipy : acetate stoichiometry. Furthermore, there is a cocrystallized 2,2′-bipy in the acetate salt form of [Hg(CH₃CO₂)(2,2′-bipy)₂]⁺, which is not able to win the competition with acetate for the third coordination site to Hg²⁺. Even if the 2,2′-bipy amount is 10-times higher than that of acetate in the reaction batch, one acetate remains coordinated to Hg²⁺. Our crystal form of [Zn(CH₃CO₂)(2,2′-bipy)₂]⁺ is strongly photoluminescent, with highly efficient emission centered at 356 nm (external and internal quantum yields of 14.2(1)% and 41.3(1)%), whose optical efficiency was rationalized on the basis of time-dependent DFT calculations.