Metal Hydrides Form Halogen Bonds: Measurement of Energetics of Binding

The formation of halogen bonds from iodopentafluorobenzene and 1-iodoperfluorohexane to a series of bis(eta(5)-cyclopentadienyl)metal hydrides (Cp2TaH3, 1; Cp2MH2, M = Mo, 2, M = W, 3; Cp2ReH, 4; Cp2Ta(H)CO, 5; Cp = eta(5)-cyclopentadienyl) is demonstrated by H-1 NMR spectroscopy. Interaction enthalpies and entropies for complex 1 with C6F5I and C6F13I are reported (Delta H degrees = -10.9 +/- 0.4 and -11.8 +/- 0.3 kJ/mol; Delta S degrees = -38 +/- 2 and -34 +/- 2 J/(mol.K), respectively) and found to be stronger than those for 1 with the hydrogen-bond donor indole (Delta H degrees = -7.3 +/- 0.1 kJ/mol, Delta S degrees = -24 +/- 1 J/(rnol.K)). For the more reactive complexes 2-5, measurements are limited to determination of their low-temperature (212 K) association constants with C6F5I as 2.9 +/- 0.2, 2.5 +/- 0.1, <1.5, and 12.5 +/- 0.3 M-1, respectively.