Small cycloalkane (CN)2C-C(CN)2 structures are highly directional non-covalent carbon-bond donors

High-level calculations (RI-MP2/def2-TZVP) disclosed that the σ-hole in between two C atoms of cycloalkane X2 CCX2 structures (X=F, CN) is increasingly exposed with decreasing ring size. The interacting energy of complexes of F(-) , HO(-) , N≡C(-) , and H2 CO with cyclopropane and cyclobutane X2 CCX2 derivatives was calculated. For X=F, these energies are small to positive, while for X=CN they are all negative, ranging from -6.8 to -42.3 kcal mol(-1) . These finding are corroborated by a thorough statistical survey of the Cambridge Structural Database (CSD). No clear evidence could be found in support of non-covalent carbon bonding between electron-rich atoms (El.R.) and F2 CCF2 structures. In marked contrast, El.R.⋅⋅⋅(CN)2 CC(CN)2 interactions are abundant and highly directional. Based on these findings, the hydrophobic electrophilic bowl formed by 1,1',2,2'-tetracyano cyclopropane or cyclobutane derivatives is proposed as a new and synthetically accessible supramolecular synthon.