Interaction between a transition metal fluoride and a transition metal hydride: water-mediated HF evolution following fluoride solvation

The reaction between the nickel(II) PCP pincer fluoride complex (tBuPCP)Ni(F) [tBuPCP = 2,6-C6H3(CH2PtBu2)2] and the tungsten(II) carbonyl hydride CpW(H)(CO)3 (Cp = h5-C5H5-) leads to hydrogen fluoride evolution and formation of the bimetallic isocarbonylic species [CpW(CO)2(m-k,C:k,O-CO)---Ni(tBuPCP)]. The process has been monitored through multinuclear (19F, 31P{1H}, 1H) variable-temperature NMR spectroscopy, collecting 19F T1 data values for a fluoride ligand bound to a transition metal. The extremely short relaxation time (minimum value of 13 msec at 193 K) is ascribed to the large chemical shift anisotropy of the Ni-F bond (688 ppm). The in-depth NMR analysis has revealed that the fluoride-hydride interaction is not direct but water-mediated, at odds with what was previously observed for the "hydride-hydride" case (tBuPCP)Ni(H)/ CpW(H)(CO)3. Kinetic measurements have unveiled that the first step of the overall mechanism is thought to be the solvation of the fluoride ligand (as a result of a Ni-FoooH2O hydrogen bonding), while further reaction of the solvated fluoride with CpW(H)(CO)3 is extremely slow, and competes with the side-reaction of fluoride replacement by a water molecule on the Ni centre to form the [(tBuPCP)Ni(H2O)]+ aquo species. Finally, a DFT analysis of the solvation process through a discrete + continuum model has been accomplished, at the M06//6-31+G(d,p) level of theory, to support the mechanistic hypothesis.