A paradigm shift in the construction of heterobimetallic complexes: Synthesis of group 2 & 4 metal-calix[6]arene complexes

Deprotonation of calix[6] arenes with barium in methanol followed by the addition of [Ti(OPri)(4)] or [Zr(OBun)(4)] is effective in the formation of novel dimeric 2:1 barium-titanium(IV)/zirconium(IV) calix[6] arene complexes. In these complexes a central Ti(IV)/Zr(IV) coordinated in the exo-position connects the two calix[6] arenes in the 1,3-alternate conformation, each with an endo-barium sharing common phenolate groups with the titanium/zirconium centre and participating in cation-pi interactions. A homometallic barium calix[6] arene dimer was also prepared wherein the calix[ 6] arenes are in the 1,3-alternate conformation with each coordinating one endo- and one exo-barium centre. The exo-barium cations connect the two calix[6] arenes through bridging methanol ligands. In this and the heterometallic complexes, cation-pi complexation of the Ba2+ ion within the 1,3 alternate conformation of calix[6] arene facilitates the formation of the dimeric complexes in methanol. In contrast, the smaller Sr2+ ion did not form similar complexes in methanol, and the formation of an analogous 2:1 strontium-titanium calixarene complex required the use of the more sterically demanding donor alcohol, isopropanol, the resulting complex being devoid of cation-pi interaction. The results show (i) that a subtle interplay of solvation strength, coordination array type and cavity/cation size influences the accessibility of heterobimetallic complexes based on calix[6] arenes, and (ii) a synergistic endo-exo binding behaviour.