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

DFT calculations (B3PW91) predicted structures for hydrogen-bonded complexe s of type Ir(H)(L)(bq-G)(PH3)(2)(q+) (bq-H = benzo[h]quinoline-10-yl, L = e mpty site, FH or OH2; G = H or NH2, q = 1; L = F-, G = NH2, q = 0), which a re either too unstable for X-ray crystallography study, or for which the cr ystal 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 i s 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 t he aqua complex is fully interpreted by location of the appropriate TS. One such TS is strongly stabilized by hydrogen bonding leading to rapid exchan ge of NH2 positions even at -80 degreesC. An improved ligand is suggested f or stabilizing an HF complex.