Concerted Halogen Bonding and Orthogonal Metal-Halogen Interactions in Dimers of Lithium Formamidinate and Halogenated Formamidines: An ab Initio Study

Dimers of lithium formamidinate, CH(NH)(2)Li, and halogenated formamidines, HN=CHNHX, (X=Cl, Br, or I) are used as model systems to investigate simultaneous N-X center dot center dot center dot N and N-Li center dot center dot center dot N interactions, in tandem with orthogonal Li center dot center dot center dot X interactions. Geometry optimizations and energy calculations for the dimers are examined with the MP2 method and the M06-2X hybrid functional and the aug-cc-pVTZ basis set (the aug-cc-pVTZ-PP basis set is used for the iodine atom). Both methods predict the formation of a planar structure of C-2v symmetry, regardless of the identity of the halogen atom. In this structure, the identities of the constituent monomers are essentially lost. Accordingly, the N-X center dot center dot center dot N interactions emerge as a rather symmetric quasi-linear N center dot center dot center dot X center dot center dot center dot N, where the covalent N-X bond in the halogenated formamidine is replaced by a partly covalent N center dot center dot center dot X interaction. Formation of the C-2v structure is also driven by a fairly linear N center dot center dot center dot Li center dot center dot center dot N interaction parallel to the N center dot center dot center dot X center dot center dot center dot N interaction, and a Li center dot center dot center dot X interaction orthogonal to both the N center dot center dot center dot X center dot center dot center dot N and N center dot center dot center dot Li center dot center dot center dot N interactions. The strength of the interactions increases with the size of the halogen. The robustness of the interactions suggests that the dimers studied here or suitable analogues may find diverse applications including their use as novel polymeric synthons.