Benchmarking of Halogen Bond Strength in Solution with Nickel Fluorides: Bromine versus Iodine and Perfluoroaryl versus Perfluoroalkyl Donors

The energetics of halogen bond formation in solution have been investigated for a series of nickel fluoride halogen bond acceptors; trans-NiF(2-C₅NF₄)(PEt₃)₂] ( A1 ), trans-NiF{2-C₅NF₃(4-H)}(PEt₃)₂] ( A2 ), trans-NiF{2-C₅NF₃(4-NMe₂)}(PEt₃)₂] ( A3 ) and trans-NiF{2-C₅NF₂H(4-CF₃)}(PCy₃)₂] ( A4 ) with neutral organic halogen bond donors, iodopentafluorobenzene ( D1 ), 1-iodononafluorobutane ( D2 ) and bromopentafluorobenzene ( D3 ), in order to establish the significance of changes from perfluoroaryl to perfluoroalkyl donors and from iodine to bromine donors. ¹⁹F NMR titration experiments have been employed to obtain the association constants, enthalpy, and entropy for the halogen bond formed between these donor-acceptor partners in protiotoluene. For A2 – A4 , association constants of the halogen bonds formed with iodoperfluoroalkane ( D2 ) are consistently larger than those obtained for analogous complexes with the iodoperfluoroarene ( D1 ). For complexes formed with A2 – A4 , the strength of the halogen bond is significantly lowered upon modification of the halogen donor atom from I (in D1 ) to Br (in D3 ) (for D1 : 5≤K₂₈₅≤12 m ^?1, for D3 : 1.0≤K₁₉₃≤1.6 m ^?1). The presence of the electron donating NMe₂ substituent on the pyridyl ring of acceptor A3 led to an increase in ?ΔH, and the association constants of the halogen bond complexes formed with D1 – D3 , compared to those formed by A1 , A2 and A4 with the same donors.