Supramolecular chemistry of halogens:: Complementary features of inorganic (M-X) and organic (C-X′) halogens applied to M-X•••X′-C halogen bond formation

Electronic differences between inorganic (M-X) and organic (C-X) halogens in conjunction with the anisotropic charge distribution associated with terminal halogens have been exploited in supramolecular synthesis based upon intermolecular M-X center dot center dot center dot X'-C halogen bonds. The synthesis and crystal structures of a family of compounds trans-[MCl2(NC5H4X-3)(2)] (M = Pd(l]), Pt(II); X = F, Cl, Br, 1; NC5H4X-3 = 3-halopyridine) are reported. With the exception of the fluoropyridine compounds, network structures propagated by M-Cl... X-C halogen bonds are adopted and involve all M-Cl and all C-X groups. M-Cl center dot center dot center dot X-C interactions show Cl center dot center dot center dot X separations shorter than van der Waals values, shorter distances being observed for heavier halogens (X). Geometries with near linear Cl center dot center dot center dot X-C angles (155-172 degrees) and markedly bent M-Cl center dot center dot center dot X angles (92-137 degrees) are consistently observed. DFT calculations on the model dimers {trans-[MCl2(NH3)(NC5H4X-3)]}(2) show association through M-Cl center dot center dot center dot X-C (X not equal F) interactions with geometries similar to experimental values. DFT calculations of the electrostatic potential distributions for the compounds trans-[PdCl2(NC5H4X-3)(2)] (X = F, Cl, Br, 1) demonstrate the effectiveness of the strategy to activate C-X groups toward halogen bond formation by enhancing their electrophilicity, and explain the absence of M-Cl center dot center dot center dot F-C interactions. The M-Cl center dot center dot center dot X-C halogen bonds described here can be viewed unambiguously as nucleophile-electrophile interactions that involve an attractive electrostatic contribution. This contrasts with some types of halogen-halogen interactions previously described and suggests that M-Cl center dot center dot center dot X-C halogen bonds could provide a valuable new synthon for supramolecular chemists.