Two different hydrogen bond donor ligands together: a selectivity improvement in organometallic {Re(CO)3} anion hosts

Rhenium(I) compounds [Re(CO)(3)(Hdmpz)(2)(ampy)]BAr'(4) and [Re(CO)(3)(N-MeIm)(2)(ampy)]BAr'(4) (Hdmpz = 3,5-dimethylpyrazole, N-MeIm = N-methylimidazole, ampy = 2-aminopyridine or 3-aminopyridine) have been prepared stepwise as the sole reaction products in good yields. The cationic complexes feature two different types of hydrogen bond donor ligands, and their anion binding behavior has been studied both in solution and in the solid state. Compounds with 2-ampy ligands are labile in the presence of nearly all of the anions tested. The X-ray structure of the complex [Re(CO)(3)(Hdmpz)(2)(ampy)](+) (2) shows that the 2-ampy ligand is metal-coordinated through the amino group, a fact that can be responsible for its labile character. The 3-ampy derivatives (coordinated through the pyridinic nitrogen atom) are stable toward the addition of several anions and are more selective anion hosts than their tris(pyrazole) or tris(imidazole) counterparts. This selectivity is higher for compound [Re(CO)(3)(N-MeIm)(2)(MeNA)]BAr'(4) (5·BAr'(4), MeNA = N-methylnicotinamide) that features an amido moiety, which is a better hydrogen bond donor than the amino group. Some of the receptor-anion adducts have been characterized in the solid state by X-ray diffraction, showing that both types of hydrogen bond donor ligands of the cationic receptor participate in the interaction with the anion hosts. DFT calculations suggest that coordination of the ampy ligands is more favorable through the amino group only for the cationic complex 2, as a consequence of the existence of a strong intramolecular hydrogen bond. In all other cases, the pyridinic coordination is clearly favored.