Metal ion in ﬂ uences distortion of the ligand in the structure of [M{2-MeO(O)CC 6 H 4 NHC(S)NP(S)-(OiPr) 2 } 2 ] (M = Zn II , Cd II ) complexes: a driving force for intermolecular aggregation †‡

a Reaction of the in situ deprotonated N -thiophosphorylated thiourea 2-MeO(O)CC 6 H 4 NHC(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 Z = 2 and the metal cations are found in a tetrahedral S 2 S ’ 2 coordination environment formed by the C – S and P – S sulfur atoms. The crystal structures reveal intramolecular N – H ⋯ O v C hydrogen bonds formed within the 2-MeO(O)CC 6 H 4 NH fragments. Both structures are further stabilised by intermolecular π ⋯ π stacking interactions, which are more e ﬃ cient 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 signi ﬁ cantly distorted coordination tetrahedron CdS 2 S ’ 2 . The aggregation is also re ﬂ ected in the positive electrospray ionisation (ESI) mass spectrum of the Cd II complex, which exhibits peaks for the dimeric cations [Cd 2 L 3 ] + , [Cd 2 L 4 + H] + and [Cd 2 L 4 + Na] + , while for the Zn II analogue only monomeric species were observed. Quantum chemical ETS-NOCV (ADF) calculations con ﬁ rm 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.


Introduction
The complexation properties of imidodiphosphinate [R 2 P(X)-NP(Y)R′ 2 ] − (X, Y = O, S, Se, Te) 1 and aroylthioureate [R 2 NC(X)-NC(Y)R′] − (X, Y = O, S, Se) 2 ligands towards Zn II and Cd II have previously been reported. This is reflected in a total of 33 struc-tures found in the Cambridge Structural Database. 3 Recently, we have also studied the structures of Zn II and Cd II complexes with N-(thio)phosphorylated thioamidate and thioureate [RC-(S)NP(X)R′ 2 ] − (X = O, S) ligands, which are asymmetric derivatives of imidodiphosphinate and aroylthioureate anions. The overwhelming majority of these structures corresponds to the phosphorylated [RC(S)NP(O)R′ 2 ] − anions. 4 In contrast to this, only three structures of thiophosphorylated [RC(S)NP(S)R′ 2 ] − anions are known for each Zn II and Cd II . 5 This is surprising since it has been postulated for a long time that complexes of the dithioderivatives are much more stable compared with those ligands containing oxygen and sulfur atoms simultaneously. Thus, every new structure of coordination compounds of [RC(S)NP(S)R′ 2 ] − with Zn II and Cd II are of great importance and value. This becomes relevant when considering that Zn II and Cd II complexes with imidodiphosphinate and aroylthioureate have been extensively used as single source precursors for nanomaterials. 6 Furthermore, we have recently demonstrated that complexes of [RC(S)NP(X)R′ 2 ] − with Ag I and Ni II are efficient precursors for nanoparticles and nanofilms. 7 In addition, the coordination chemistry of Zn II vs. Cd II towards † Electronic supplementary information (ESI) available: Fig. S1, showing the crystal packing of the complexes, Table S1 with selected structural data, and thiolate ligands receives great interest from the binding of these metals in metallothioneins. 8 In this contribution we describe the synthesis of new Zn II and Cd II complexes with the N-thiophosphorylated thiourea 2-MeO(O)CC 6 H 4 NHC(S)NHP(S)(OiPr) 2 (HL). 9 We also describe a complete structural investigation of the obtained complexes [ZnL 2 ] and [CdL 2 ] both in solution and solid state together with their thermal properties. The experimental results were supported by detailed quantum chemical calculations.

Results and discussion
The complexes [ZnL 2 ] and [CdL 2 ] were prepared by reacting the in situ deprotonated ligand, using KOH, with MCl 2 (M = Zn II , Cd II ) (Scheme 1). The obtained colourless solid materials are soluble in most polar solvents.
The IR spectra of complexes are very similar and contain a band at about 560 cm −1 representing the P-S group of the anionic form L − with a delocalised 6 π electron S-C-N-P-S system (bond order ∼1.5). 10 This band is shifted by ∼90 cm −1 to low frequencies compared with that in the spectrum of the parent HL. 9 Further bands at 1530 and 1695 cm −1 correspond to the conjugated SCN fragment and CvO group, respectively. In addition, there is a broad intense absorption arising from the POC group at about 970-990 cm −1 . Also, the characteristic band for the arylNH group is found at about 3215 cm −1 .
The 31 P{ 1 H} NMR spectra of [ZnL 2 ] and [CdL 2 ] in CDCl 3 exhibit a unique signal at 55.7 and 55.9 ppm, respectively, which indicates the exclusive presence of 1,5-S,S′-coordinated ligands in the complexes. [9][10][11] The 1 H NMR spectra of complexes in CDCl 3 each contain one set of signals. The signals of the isopropyl CH 3 protons are observed at about 1.40 ppm, while the aryl CH 3 protons are at 3.90 ppm. The isopropyl CH(O) protons appear as a doublet of septets at 4.81 ppm with the characteristic coupling constants 3 J POCH = 10.4 Hz and 3 J H,H = 6.1 Hz. The phenylene protons are observed as four multiplet peaks at about 7.05, 7.45, 8.00 and 8.65 ppm with the typical coupling constants 3 J H,H ranging from 7.9 to 8.3 Hz. The spectra also contain a doublet for the arylNH protons, observed at 11.25 ppm with a coupling constant 4 J PNCNH of 7.5 Hz. The latter is exclusively observed when the structure of the H-N-C-N-P fragment meets the so-called "W-criterion" (marked by red in Scheme 1). 12 The low-field shift of the signal for the arylNH protons is due to the formation of intramolecular hydrogen bonds of the type arylN-H⋯OvC.
Crystals of the complexes were obtained by slow evaporation of the solvent from CH 2 Cl 2 -n-hexane solutions. Both structures represent spirocyclic chelates and were refined in the triclinic space group P1, each containing one independent molecule in the unit cell. In the crystal, both Δ and Λ enantiomers of the complexes are present due to the inversion centre in the nonchiral space group P1 (Fig. S1 in the ESI †). Such pairs of enantiomers can be expected for nonplanar bis-chelate complexes. 13 Both metal cations are found in a tetrahedral S 2 S′ 2 coordination environment formed by the C-S and P-S sulfur atoms (Fig. 1)   the Zn II complex, while the corresponding values of about 116. 3, 112.0, 107.5 and 115.6 were observed in the Cd II analog.
The two values for the N-P-S, Zn-S-C and Zn-S-P angles are very similar in [ZnL 2 ] representing a quite symmetric surrounding. This is not the case for [CdL 2 ] where the corresponding pairs show significant deviations (Table 1). The lengthening of the C-S and P-S and shortening of the C-N(P) and P-N bonds in both complexes, compared with the values for the parent ligand HL, 9 are in agreement with the IR data. The relative higher symmetry of the coordination surrounding of the metal atoms in [ZnL 2 ] compared with [CdL 2 ] is also visible in the M-S(C) and M-S(P) bonds. While the each two values for the M-S(C) and M-S(P) bonds in [ZnL 2 ] are very similar and range from 2.31 to 2.35 Å, the two M-S(C) and M-S(P) bonds in the structure of [CdL 2 ] are rather dissimilar and range from 2.43 to 2.62 Å.
The arylNH protons are involved in intramolecular hydrogen bonds of the type arylN-H⋯OvC ( Fig. 1 and Table 2) in line with the structure concluded from NMR (Scheme 1). A closer inspection of the crystal structures revealed further H⋯X hydrogen bonds (Table S1 in the ESI †), however, based on established criteria 14 we consider them not to be determining the crystal or molecular structures.
Both structures are further stabilised by an intermolecular π⋯π stacking interaction leading to dimers for both [ZnL 2 ] and [CdL 2 ] (Fig. 2, Table 3). The π⋯π stacking seems to be much more efficient in [CdL 2 ] leading to an almost coplanar arrangement (interplanar angle α = 0.03°) and a quite short interplanar distance of 3.756(3) Å with a displacement angle β of 24.12°. 15 For [ZnL 2 ] a second, only slightly longer, π⋯π stacking interaction (Table 3) finally leads to a 1D polymeric chain ( Fig. 3), which is not observed for the Cd derivative. This marked difference seems to be caused by the pronounced distortion of the chelate [(S)CNP(S)] − fragment of the ligands upon coordination in the structure of [CdL 2 ] as well as the significantly distorted coordination tetrahedron CdS 2 S′ 2 ( Fig. 4) discussed already above. Thus, the nature and especially the size of the metal cation (Zn II : 74 pm vs. Cd II : 92 pm) 16 drive the supramolecular aggregation of molecules in the structures of [ZnL 2 ] and [CdL 2 ]. A similar influence of Zn II vs. Cd II was found for the formation of supramolecular coordination complexes of the N-thiophosphorylated 2,5-dithiobiurea [NHC(S)-NHP(S)(OiPr) 2 ] 2 . 5e The dinuclear mesocate structure was formed upon reacting with Zn II , while the tetranuclear nanoscaled aggregate was isolated in the reaction with Cd II . Furthermore, the idea to use the different sizes of these two d 10 configured so-called "spherical ions" 17 to control the supramolecular aggregation has been worked out recently. 17,18 However, in most cases the observed supramolecular aggregate is the product of several interactions and carefuly ligand design is necessary.
The bulk samples of [ZnL 2 ] and [CdL 2 ] were studied by means of X-ray powder diffraction analysis (Fig. 5). The experimental X-ray powder patterns are in agreement with the calculated powder patterns obtained from a single crystal X-ray analysis, showing that the bulk materials of [ZnL 2 ] and [CdL 2 ] are free from phase impurities.
The formation of a rather stable dimeric aggregate via efficient intermolecular π⋯π stacking interactions is also reflected in the positive electrospray ionisation (ESI) mass spectrum of the Cd II complex, which exhibits intense peaks for the dimeric cations [Cd 2 L 3 ] + , [Cd 2 L 4 + H] + and [Cd 2 L 4 + Na] + (Fig. 6). For the Zn II analogue only monomeric species were found in the positive ion ESI mass spectrum.
The thermal properties of [ZnL 2 ] and [CdL 2 ] in air atmosphere were studied by means of TG analyses in order to deter-mine their respective stabilities (Fig. 7). The compound [ZnL 2 ] is stable up to about 100°C and decomposed in two steps.
[CdL 2 ] is stable up to 140°C and decomposed in three steps with the third step to be poorly defined. The observed final    residues of 11.6 and 16.4% are in excellent agreement with the calculated 11.54 and 16.21% for ZnS and CdS, respectively. The formation of these sulfides, each exhibiting a hexagonal form (wurtzite for ZnS and greenockite for CdS, respectively), was proved based on the powder X-ray diffraction analysis with full correspondence with data calculated from a corresponding single crystal analysis (Fig. 8). 21 Thus, the complexes [ZnL 2 ] and [CdL 2 ], being easily obtained, and air/mosture stable might be very suitable single source precursors for the formation of ZnS and CdS, respectively.
In order to shed some light on the nature of bonding between the monomers in [ZnL 2 ] and [CdL 2 ], we have applied the charge and energy decomposition method (ETS-NOCV) 22 as implemented in the ADF2012.01 program suite. 23 We have used DFT/BLYP-D3 when performing the ETS-NOCV calculations as it was shown that such computational details provides satisfactory results for the noncovalent interactions as compared with the accurate CCSD(T) results. 24 The coordinates of the dimers were extracted from the crystal structures.
The total interaction energy (ΔE total ) in the dimer of [CdL 2 ] is significantly more pronounced compared to that of [ZnL 2 ] (Table 4). These results are in line with the ESI-mass spectra (Fig. 6), which also suggest a higher stability of the dimer of [CdL 2 ]. Decomposition of the π⋯π stacking interaction energies shows that the dispersion contribution (ΔE dispersion ) is the most important for the overall stabilisation and it contributes as much as −31.6 kcal mol −1 for [CdL 2 ] and −40.3 kcal mol −1 for [ZnL 2 ] (Table 4). Quantitatively less important is the electrostatic term (ΔE elstat ), which appeared to be quite similar for both dimers and of about −10.0 kcal mol −1 (Table 4). Finally, the least contribution is the orbital interaction (ΔE orb ), which is approximately twice more important for the cadmium-containing system. In order to shed further light on the ΔE orb term we have plotted the overall deformation density (Δρ orb ) upon formation of dimers (Fig. 9). The formation of both dimers leads rather to internal polarisations within the monomers, no typical charge transfer between the stacking rings is observed. It is noticeable that changes in the electron density are not only within the stacking rings but they also cover further regions including the OiPr groups as well as the sulphur atoms. The domination of the dispersion contribution in such stacking interactions is in line with the literature. 25 We have further performed the geometry optimisation of the monomers of [ZnL 2 ] and [CdL 2 ] in the gas phase based on DFT/BLYP-D3/TZP (Fig. 10), and the calculated parameters are in qualitative agreement with the experimental values (Tables  1 and 2 and Table S2 in the ESI †).
We have finally performed a preliminary study of bonding in the monomers of [ZnL 2 ] and [CdL 2 ] based on the Quantum Theory of Atoms in Molecules (QTAIM) 26 method as implemented in the ADF program. 23 It was found that in both cases not only the strongest coordinating bonds Zn-S and Cd-S are observed, but also less important secondary intramolecular non-covalent interactions of the types O⋯H-N, N⋯H-C and S⋯H-C. From density values at the bond critical points as well as distances one can further infer that the strength of these interactions decreases in the following order O⋯H-N > N⋯H-C > S⋯H-C. Finally, given the fact that the   bond critical points implies that stabilisation exists from the electronic exchange channel between atoms, further studies are required to more deeply describe each contribution of intramolecular close contacts. Neverthless, we believe that these preliminary QTAIM based results indentify important factors that might influence the overall stability of the [ZnL 2 ] and [CdL 2 ] monomers (Fig. 11).

Conclusions
We have synthesised the Zn II and Cd II complexes [ZnL 2 ] and [CdL 2 ] of the deprotonated N-thiophosphorylated thiourea 2-MeO(O)CC 6 H 4 NHC(S)NHP(S)(OiPr) 2 (HL). The molecular structures of the complexes were studied by IR and NMR spectroscopy revealing two deprotonated ligands with a delocalised 6 π electron S-C-N-P-S system. Intramolecular N-H⋯OvC hydrogen bonds favour the all-trans arrangement of the ligands H-N-C-N-P fragment (the so-called "W-criterion"). In the solid, the structure was elucidated by single crystal X-ray diffraction analysis, revealing that both compounds crystallised isostructural in the triclinic space group P1, each containing one independent molecule in the unit cell. The metal cations display a tetrahedral S 2 S′ 2 coordination environment formed by the C-S and P-S sulfur atoms and strong intramolecular N-H⋯OvC hydrogen bonds were observed within the 2-MeO(O)CC 6 H 4 NH fragments. Both structures are further stabilised by intermolecular π⋯π stacking interactions, which are more efficient in [CdL 2 ] leading to isolated stongly connected dimers. In contrast to this, a second, weaker π⋯π stacking is observed in the Zn analogue which leads to a 1D polymeric structure in the solid. This difference between the two complexes is also reflected in the positive electrospray ionisation (ESI) mass spectrum of the Cd II complex, which exhibits peaks for the dimeric cations [Cd 2 L 3 ] + , [Cd 2 L 4 + H] + and [Cd 2 L 4 + Na] + . Only peaks for the monomeric species were found in the ESI mass spectrum of the Zn II analogue. Theoretical calculations confirm a higher stability of [CdL 2 ] compared with [ZnL 2 ]. Furthermore, the π⋯π stacking interactions are prodominantly due to dispersion contributions, though the electrostatic and orbital interaction components are also important. Thus, there is strong evidence from experiment and theory that formation of the dimeric intermolecular aggregate in the structure of [CdL 2 ] is driven by the pronounced distortion of the chelate [(S)CNP(S)] − backbones of ligands upon coordination to Cd II as well as by a significantly distorted

DFT calculations
We have used the ADF2012.01 program 23 based on DFT/ BLYP-D3/TZP. For topological description of electron density in the monomers the Quantum Theory of Atoms in Molecules (QTAIM) 26 were applied.

Synthesis of [ZnL 2 ] and [CdL 2 ]
A solution of HL (1 mmol, 0.390 g) in aqueous EtOH (10 mL) was mixed with KOH (1.1 mmol, 0.062 g). An aqueous (10 mL X-Ray powder diffraction X-Ray powder diffraction for bulk samples was carried out using a Rigaku Ultima IV X-ray powder diffractometer. The Parallel Beam mode was used to collect the data (λ = 1.541836 Å).

Single crystal X-ray diffraction
The X-ray diffraction data for the crystals of [ZnL 2 ] and [CdL 2 ] were collected at 173(2) K on a STOE IPDS-II diffractometer with graphite-monochromatised Mo-Kα radiation generated by a fine-focus X-ray tube operated at 50 kV and 40 mA. The reflections of the images were indexed, integrated and scaled using the X-Area data reduction package. 27 Data were corrected for absorption using the PLATON program. 28 The structures were solved by direct methods using the SHELXS97 program 29 and refined first isotropically and then anisotropically using SHELXL-97. 29 Hydrogen atoms were revealed from Δρ maps and those bonded to carbon atoms were refined using appropriate riding models. Hydrogen atoms bonded to nitrogen atoms were freely refined. All figures were generated using the program Mercury. 30 Crystal data for [ZnL 2 ]. C 30 H 44 N 4 O 8 P 2 S 4 Zn, M r = 844.24 g mol −1 , triclinic, space group P1, a = 10.4280(5), b = 11.4053 (6)