Bifurcated μ₂‑I···(N,O) Halogen Bonding: The Case of (Nitrosoguanidinate)Ni^II Cocrystals with Iodine(I)-Based σ‑Hole Donors

Cocrystallization of the nitrosoguanidinate complexes [Ni­{NHC­(NR₂)­NN­(O)}₂] (R₂ = Me₂ (1), Me/Ph (2), C₅H₁₀ (3)) with 1,2-diiodotetrafluorobenzene (1,2-FIB), 1,4-diiodotetrafluorobenzene (1,4-FIB), 1,3,5-triiodotrifluorobenzene (1,3,5-FIB), and tetraiodoethylene (C₂I₄) gave new cocrystals (1–3)·2­(1,2-FIB), 2·(1,4-FIB), 3·2­(1,3,5-FIB), 1·C₂I₄, and 2·2C₂I₄; all of these solids were studied by single-crystal X-ray diffraction. Each structure exhibits the asymmetric bifurcated halogen bond μ₂-I···(N,O) as a characteristic feature and a structure-directing contact. DFT calculations were used to evaluate the energetic features of the bifurcated halogen bonds that range from −6.7 to −8.3 kcal/mol; in general, the I···O contact is stronger that the I···N contact. Molecular electrostatic potential (MEP) surfaces along with the quantum theory of “atoms in molecules” (QTAIM) and the noncovalent interaction plot index (NCIPlot) computational tools were used to characterize the bifurcated halogen bonds in the cocrystals of 1–3. Various types of bifurcated halogen bonds involving nonmetal atoms as halogen-bond-accepting centers were systematized and categorized.