The Contrasting Character of Early and Late Transition Metal Fluorides as Hydrogen Bond Acceptors

The
association constants and enthalpies for the binding of hydrogen bond
donors to group 10 transition metal complexes featuring a single fluoride
ligand (trans-[Ni­(F)­(2-C5NF4)­(PR3)2], R = Et 1a, Cy 1b, trans-[Pd­(F)­(4-C5NF4)­(PCy3)2] 2, trans-[Pt­(F)­{2-C5NF2H­(CF3)}­(PCy3)2] 3 and of group 4 difluorides (Cp2MF2, M = Ti 4a, Zr 5a, Hf 6a; Cp*2MF2, M = Ti 4b, Zr 5b, Hf 6b) are reported.
These measurements allow placement of these fluoride ligands on the
scales of organic H-bond acceptor strength. The H-bond acceptor capability
β (Hunter scale) for the group 10 metal fluorides is far greater
(1a 12.1, 1b 9.7, 2 11.6, 3 11.0) than that for group 4 metal fluorides (4a 5.8, 5a 4.7, 6a 4.7, 4b 6.9, 5b 5.6, 6b 5.4), demonstrating that the group
10 fluorides are comparable to the strongest organic H-bond acceptors,
such as Me3NO, whereas group 4 fluorides fall in the same
range as N-bases aniline through pyridine. Additionally, the measurement
of the binding enthalpy of 4-fluorophenol to 1a in carbon
tetrachloride (−23.5 ± 0.3 kJ mol–1)
interlocks our study with Laurence’s scale of H-bond basicity
of organic molecules. The much greater polarity of group 10 metal
fluorides than that of the group 4 metal fluorides is consistent with
the importance of pπ–dπ bonding in the latter.
The polarity of the group 10 metal fluorides indicates their potential
as building blocks for hydrogen-bonded assemblies. The synthesis of trans-[Ni­(F)­{2-C5NF3(NH2)}­(PEt3)2], which exhibits an extended chain
structure assembled by hydrogen bonds between the amine and metal-fluoride
groups, confirms this hypothesis.