Abstract
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.
Highlights
The studies reported in this paper address the energetics of hydrogen bonding to metal fluoride complexes, placing them on commonly used comparative scales of H-bond acceptor strength
We have shown that group 10 metal fluorides exhibit exceptionally strong H-bond acceptor character, with the nickel fluoride 1a and the palladium fluoride 2 of the PCy3 series having the highest association constants
We have measured the strength of halogen bonds from C6F5I with the same H-bond acceptors and find that these halogen bonds are comparable in strength to the weakest of the hydrogen bonds that we have examined, such as to pyrrole
Summary
The studies reported in this paper address the energetics of hydrogen bonding to metal fluoride complexes, placing them on commonly used comparative scales of H-bond acceptor strength. We probe the polarity of fluoride complexes down the triads of groups 4 and 10 of the Periodic. We demonstrate that the H-bond ability of group 10 fluoride complexes can be used in supramolecular chain structures. Hydrogen bonding involving metal-bound ligands is crucial to many fields, as is demonstrated in a review including applications in bioinorganic chemistry, photochemistry, organometallic chemistry, and host−guest interactions.[1] Here we are concerned with hydrogen bonding of ligands very close to the metal center, the “ligand domain” as described by Brammer,[2] in which metal and ligand atoms are in strong communication
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