The nature of the [TTF]˙+···[TTF]˙+ interactions in the [TTF]2(2+) dimers embedded in charged [3]catenanes: room-temperature multicenter long bonds.

The properties of tetrathiafulvalene dimers (TTF(2+)) and the functionalized ring-shaped bispropargyl (BPP)-functionalized TTF dimers, BPP-TTF(2+), found at room temperature in charged [3]catenanes, were evaluated by M06L calculations. The results showed that their isolated TTF(2+) and BPP-TTF(2+) dimers are energetically unstable towards dissociation. When enclosed in the 4(+)-charged central cyclophane ring of charged [3]catenanes (CBPQT(4+)), TTF(2+) and BPP-TTF(2+) dimers are also energetically unstable with respect to leaving the CBPQT(4+) ring; since the barrier for the exiting process is only about 3 kcal mol(-1), that is, within the reach of thermal energies at room temperature (neutral TTF(0) dimers are stable within the CBPQT(4+) ring). However, the BPP-TTF(2+) dimers in charged [3]catenanes cannot exit, because this would imply breaking the covalent bonds of the BPP-TTF(+) macrocycle. Finally, it was shown that the TTF(2+), BPP-TTF(2+) dimers, and charged [3]catenanes are energetically stable in solution and in crystals of their salts, in the first case due to the interactions with the solvent, and in the second case mostly due to cation-anion interactions. In these environmental conditions at room temperature the TTF units of the BPP-TTF(2+) dimers make short contacts, thus allowing their SOMO orbitals to overlap: a room-temperature multicenter long bond is formed, similar to those previously found in other TTF(2+) salts and their solutions.