How hydrogen bonding affects ligand binding and fluxionality in transition metal complexes:: a DFT study on interligand hydrogen bonds involving HF and H2O

DFT calculations (B3PW91) predicted structures for hydrogen-bonded complexes of type Ir(H)(L)(bq-G)(PH3)(2)(q+) (bq-H = benzo[h]quinoline-10-yl, L = empty site, FH or OH2; G = H or NH2, q = 1; L = F-, G = NH2, q = 0), which are either too unstable for X-ray crystallography study, or for which the crystal structure does not allow H atom positions reliably to be located. The work shows how the two-point binding site provided by the bq-NH2 complex is ideal for HF but not for H2O binding, thus stabilizing the former to the extent that it can be observed by NMR at low temperature. Fluxionality in the aqua complex is fully interpreted by location of the appropriate TS. One such TS is strongly stabilized by hydrogen bonding leading to rapid exchange of NH2 positions even at -80 degreesC. An improved ligand is suggested for stabilizing an HF complex.