Metal ion influences distortion of the ligand in the structure of [M{2-MeO(O)CC6H4NHC(S)NP(S)(OiPr)2}2] (M = ZnII, CdII) complexes: a driving force for intermolecular aggregation

Abstract

Reaction of the in situ deprotonated N-thiophosphorylated thiourea 2-MeO(O)CC₆H₄NHC(S)NHP(S)(OiPr)₂ (HL) with MCl₂ (M = Zn^II, Cd^II) in aqueous ethanol leads to complexes of the formula [ML₂]. Both compounds crystallise in the triclinic space group P with Z = 2 and the metal cations are found in a tetrahedral S₂S^′₂ coordination environment formed by the C–S and P–S sulfur atoms. The crystal structures reveal intramolecular N–H⋯OC hydrogen bonds formed within the 2-MeO(O)CC₆H₄NH fragments. Both structures are further stabilised by intermolecular π⋯π stacking interactions, which are more efficient in [CdL₂]. Here, a pronounced dimeric intermolecular aggregate is observed which goes along with a pronounced distortion of the chelate [(S)CNP(S)]⁻ backbone of the ligand upon coordination to Cd^II as well as a significantly distorted coordination tetrahedron CdS₂S^′₂. The aggregation is also reflected in the positive electrospray ionisation (ESI) mass spectrum of the Cd^II complex, which exhibits peaks for the dimeric cations [Cd₂L₃]⁺, [Cd₂L₄ + H]⁺ and [Cd₂L₄ + Na]⁺, while for the Zn^II analogue only monomeric species were observed. Quantum chemical ETS-NOCV (ADF) calculations confirm the higher stability of dimers in [CdL₂] compared with [ZnL₂]. The π⋯π stacking interactions are prodominantly due to dispersion contributions, though the electrostatic and orbital interaction components are also important. QTAIM (ADF) type calculations additionally quantify the covalent and non-covalent interactions in the momomers.