Ligand Steric Hindrances Switch Bridging (μ₂-I)···O,O to Two-Center I···O Halogen-Bonding Mode in the Assembly of Diketonate Copper(II) Species

The diketonate complex [Cu{OC(tBu)CC(H)O}2] (1) and its cocrystals with 1,4-diiodotetrafluorobenzene (1,4-FIB), 1,3,5-triiodotrifluorobenzene (1,3,5-FIB), and 1,4-dioxane were studied by X-ray diffraction. In the solid state, 1 exhibits the cis-geometry with syn-tBu groups; the geometry is stable in storage at 20-25 °C or even in heating in the solid state, solutions, or sublimation. The DFT modeling for isolated molecules of cis- and trans-1 revealed that the cis-form is only slightly more energetically favorable. However, cocrystals trans-1·C4H8O2, trans-1·1,4-FIB, and trans-1·1,3,5-FIB·C4H8O2 display the trans-geometry with the anti-tBu substituents; the comparison of the geometries revealed the facile crystal-packing-driven cis-to-trans isomerization of 1 upon the cocrystal growth. Compared to the cocrystals of the unsubstituted [Cu(acetylacetonate)2] complex with the FIBs, in the assembled architectures of trans-1·1,4-FIB and trans-1·1,3,5-FIB·C4H8O2, the switch from the bridging (μ2-I)···O,O to two-center I···O halogen bonding is demonstrated. The theoretical study revealed that the energetic features of the (μ2-I)···O,O halogen bonding in [Cu(acetylacetonate)2]·1,4-FIB are similar to those of the I···O halogen bonding in 1·1,4-FIB. Variable-temperature XRD data (at 100, 150, 200, 250, and 300 K) for cis-1 and trans-1·1,4-FIB indicate that the infinite chains of molecules stabilized by I···O halogen bonding remain the main packing motif at all applied temperatures.