Metal ion influences distortion of the ligand in the structure of $[M\{2-MeO(O)CC_6H_4NHC(S)NP(S)(OiPr)_2\}_2] (M = Zn^{II}, Cd^{II})$ complexes : a driving force for intermolecular aggregation

Reaction of the in situ deprotonated N-thiophosphorylated thiourea $2-MeO(O)CC_6H_4NHC(S)NHP(S)(OiPr)_2 (HL)$ with $MCl_2 (M = Zn^{II}, Cd^{II})$ in aqueous ethanol leads to complexes of the formula $[ML_2]$. Both compounds crystallise in the triclinic space group P[1 with combining macron] with Z = 2 and the metal cations are found in a tetrahedral S_2S′_2 coordination environment formed by the C–S and P–S sulfur atoms. The crystal structures reveal intramolecular N–H⋯O[double bond, length as m-dash]C hydrogen bonds formed within the $2-MeO(O)CC_6H_4NH$ fragments. Both structures are further stabilised by intermolecular π⋯π stacking interactions, which are more efficient in $[CdL_2]$. 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 $CdS_2S′_2$. The aggregation is also reflected in the positive electrospray ionisation (ESI) mass spectrum of the CdII complex, which exhibits peaks for the dimeric cations $[Cd_2L_3]+$, $[Cd_2L_4 + H]+$ and $[Cd_2L_4 + Na]+$, while for the ZnII analogue only monomeric species were observed. Quantum chemical ETS-NOCV (ADF) calculations confirm the higher stability of dimers in $[CdL_2]$ compared with $[ZnL_2]$. 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.