Metal Centers as Nucleophiles: Oxymoron of Halogen Bond-Involving Crystal Engineering.

This review highlights recent studies discovering unconventional halogen bonding (HaB) that involves positively charged metal centers. These centers provide their filled d-orbitals for HaB, and thus behave as nucleophilic components toward the noncovalent interaction. This role of some electron-rich transition metal centers can be considered an oxymoron in the sense that the metal is, in most cases, formally cationic; consequently, its electron donor function is unexpected. The importance of Ha⋅⋅⋅d-[M] (Ha=halogen; M is Group 9 (Rh, Ir), 10 (Ni, Pd, Pt), or 11 (Cu, Au)) interactions in crystal engineering is emphasized by showing remarkable examples (reported and uncovered by our processing of the Cambridge Structural Database), where this Ha⋅⋅⋅d-[M] directional interaction guides the formation of solid supramolecular assemblies of different dimensionalities.

Four-Center Nodes: Supramolecular Synthons Based on Cyclic Halogen Bonding.

The isocyanide trans-[PdBr2 (CNC6 H4 -4-X')2 ] (X'=Br, I) and nitrile trans-[PtX2 (NCC6 H4 -4-X')2 ] (X/X'=Cl/Cl, Cl/Br, Br/Cl, Br/Br) complexes exhibit similar structural motif in the solid state, which is determined by hitherto unreported four-center nodes formed by cyclic halogen bonding. Each node is built up by four Type II C-X'⋅⋅⋅X-M halogen-bonding contacts and include one Type I M-X⋅⋅⋅X-M interaction, thus giving the rhombic-like structure. These nodes serve as supramolecular synthons to form 2D layers or double chains of molecules linked by a halogen bond. Results of DFT calculations indicate that all contacts within the nodes are typical noncovalent interactions with the estimated strengths in the range 0.6-2.9 kcal mol-1 .