Apparent or real water exchange reactions on [Zn(H2O)4(L)]2+·2H2O (L = sp-nitrogen donor ligands)? A quantum chemical investigation

Abstract

The exchange of a second coordination sphere water molecule in [Zn(H₂O)₄(L)]²⁺·2H₂O (L = HN₃, HCN, FCN, ClCN, BrCN, CH₃CN, (C₄H₃)CN, PhCN, (CH₃)₃CCN, CF₃CN, CCl₃CN, CHCl₂CN, and CH₂ClCN) against a coordinated water molecule was studied by quantum chemical calculations (RB3LYP/6-311+G**). The complete reaction consists of an associative binding of one H₂O from the second coordination sphere leading to a six-coordinate intermediate [Zn(H₂O)₅(L)]²⁺·H₂O, followed by the dissociation of a water molecule to reach the product state [Zn(H₂O)₄(L)]²⁺·2H₂O. For a real water exchange reaction to occur two different transition states have to be included, otherwise only an apparent water exchange reaction takes place. For the water exchange reaction in [Zn(H₂O)₄(L)]²⁺·2H₂O, nearly iso-energetic cis- and trans-orientated transition states are crossed. The gas-phase proton affinity of L shows instructive correlations with structural parameters and energy gaps for the investigated reactions.