Formation of aqua and tetraammine Cu(II) complexes inside the cavities of cucurbit[6,8]urils: A DFT forecast

Abstract

Results of DFT calculations of the structure and thermodynamics of formation of aqua and tetraammine Cu(II) complexes inside CB[n] (n = 6,8) are presented in this study. Formation thermodynamics of the complexes in the cavitands was evaluated by taking into account the most probable number of water molecules inside CB[n]. In this methodology, the complexation was first considered as a substitution reaction in which the guest complex displaces partially or completely the water molecules that are located inside the cavity. The water molecules present in the cavitand were shown to play an important role in the fixation of the guest complex inside the cavity due to the hydrogen bonds with the oxygen portals. The hydration of Cu(II) ion inside CB[6] leads to the formation of an inclusion compound with the formula {[Cu(H2O)4]2+·2H2O}@CB[6] while in CB[8] {[Cu(H2O)6]2+·4H2O}@CB[8] is formed. For the binding of tetraammine Cu(II) complex, CB[8] was determined to be a significantly more suitable “container” than CB[6]. Both a direct embedding of this complex into the CB[8] and another mechanism in which ammonia molecules replace the water molecules in the Cu(II) aqua complex, preexisting in CB[8] were determined to be thermodynamically possible. Both these lead to the formation of the resultant inclusion compound described by the formula {[Cu(NH3)4(H2O)2]2+·4H2O}@CB[8].

Standard Redox Potentials of Fe(III) Aqua Complexes Included into the Cavities of Cucurbit[n]urils (n = 6–8): A DFT Forecast

“…As shown earlier in refs , , and the fixation of metal complexes in CB­[ n ] with volumes larger than CB[6], for example, CB[7] or CB[8], is supported by additional water molecules that form bridge hydrogen bonds with ligands of the inner coordination sphere and oxygen atoms of the CB­[ n ] portals. In ref , when simulating the incorporation of Cu­(II) aqua-ions into the CB[8] cavity, it was established that four H 2 O molecules of the second coordination sphere of the Cu­(II) aqua complex perform this function. In other words, the inclusion compounds in this case have the composition [Cu­(H 2 O) 6 ·4H 2 O] 2+ @CB­[8].…”

“…It is quite obvious that the number of water molecules in the second coordination sphere of the Fe­(III) and Fe­(II) aqua-ions inside CB­[ n ] depend on the size of the CB­[ n ] cavity. Recently, our work showed that in the case of the incorporation of the Cu­(II) aqua-ion into the CB[6] cavity its first coordination sphere contains six water molecules with a tetragonally distorted octahedral environment of the central atom (i.e., [Cu­(H 2 O) 6 ] 2+ aqua complex). These molecules form hydrogen bonds directly with the oxygen atoms of the CB[6] portals and firmly fix the position of the aqua complex in the cavity of cavitand.…”