Supramolecular Chemistry of Halogens: Complementary Features of Inorganic (M−X) and Organic (C−X‘) Halogens Applied to M−X···X‘−C Halogen Bond Formation

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Abstract
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···X‘−C halogen bonds. The synthesis and crystal structures of a family of compounds trans-[MCl2(NC5H4X-3)2] (M = Pd(II), Pt(II); X = F, Cl, Br, I; 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···X−C interactions show Cl···X separations shorter than van der Waals values, shorter distances being observed for heavier halogens (X). Geometries with near linear Cl···X−C angles (155−172°) and markedly bent M−Cl···X angles (92−137°) are consistently observed. DFT calculations on the model dimers {trans-[MCl2(NH3)(NC5H4X-3)]}2 show association through M−Cl···X−C (X ≠ 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, I) 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···F−C interactions. The M−Cl···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···X−C halogen bonds could provide a valuable new synthon for supramolecular chemists.

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