Metal Fluorides Form Strong Hydrogen Bonds and Halogen Bonds: Measuring Interaction Enthalpies and Entropies in Solution

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Abstract
The organometallic compound trans-(tetrafluoropyrid-2-yl)bis(triethylphosphine)-fluoronickel(II) (NiF) is shown to serve as a strong hydrogen bond and halogen bond acceptor in solution via intermolecular interactions with the fluoride ligand. The nature of the interactions has been confirmed by multinuclear NMR spectroscopy. Experimental binding constants, enthalpies, and entropies of interaction with hydrogen-bond-donor indole and halogen-bond-donor iodopentafluorobenzene have been determined by 19F NMR titration. In toluene-d8 solution indole forms a 1:1 and 2:1 complex with NiF (K1 = 57.9(3), K2 = 0.58(4)). Interaction enthalpies and entropies are −23.4(2) kJ mol−1 and −44.5(8) J mol−1 K−1, respectively, for the 1:1 complex; −14.8(8) kJ mol−1 and −53(3) J mol−1 K−1, respectively, for the 2:1 complex. In toluene-d8 solution iodopentafluorobenzene forms only a 1:1 complex (K1 = 3.41(9)) with enthalpy and entropy of interaction of −16(1) kJ mol−1 and −42(4) J mol−1 K−1, respectively. A marked solvent effect was observed for the halogen bond interaction. NMR titrations in heptane solution indicated formation of both 1:1 and 2:1 complexes of iodopentafluorobenzene with NiF (K1 = 21.8(2), K2 = 0.22(4)). Interaction enthalpies and entropies are −26(1) kJ mol−1 and −63(4) J mol−1 K−1, respectively, for the 1:1 complex; −21(1) kJ mol−1 and −83(5) J mol−1 K−1, respectively, for the 2:1 complex. There is a paucity of such experimental energetic data particularly for halogen bonds despite substantial structural data. These measurements demonstrate that halogen bonds are competitive with hydrogen bonds as intermolecular interactions and provide a suitable benchmark for theoretical calculations and quantitative input into design efforts in supramolecular chemistry and crystal engineering.