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CdS Nanoparticles Fabricated from the Single-Source Precursor [Cd{Et2NC(S)NP(S)(OiPr)2}2]: In Depth Experimental and Theoretical Studies Maria G. Babashkina,† Damir A. Safin,*,† Mariusz P. Mitoraj,*,‡ Filip Sagan,‡ Michael Bolte,§ and Axel Klein*,† †

Department of Chemistry, Institute for Inorganic Chemistry, University of Cologne, Greinstrasse 6, D-50939 Cologne, Germany Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, R. Ingardena 3, 30-060 Cracow, Poland § Institut für Anorganische Chemie, J.-W.-Goethe-Universität, 60323 Frankfurt/Main, Germany ‡

S Supporting Information *

ABSTRACT: Reaction of the diethylammonium salt of N-thiophosphorylated thioureate [Et2NC(S)NP(S)(OiPr)2]− (L−) with CdCl2 in aqueous ethanol leads to the complex [CdL2]. The compound crystallizes in the triclinic space group P1̅ with Z = 2 and the metal cation is found in a tetrahedral S2S′2 coordination environment formed by the C−S and P−S sulfur atoms. The Hirshfeld surface analysis showed that the structure of [CdL2] is dominated by H···H, S···H, and O···H contacts. According to charge and energy decomposition scheme ETSNOCV, topological noncovalent index (NCI) and quantum theory of atoms in molecules (QTAIM) calculations, both inter- and intramolecular noncovalent C− H···S and C−H···H−C interactions are the main factors that stabilize [CdL2]. Calculated NMR data, based on the GIAO approach, are in very good agreement with experimental data. The complex [CdL2] is an efficient single-source precursor for the formation of TOPO-capped CdS nanoparticles of about 5 nm diameter with wurtzite structure (TOPO = tri-n-octylphosphine oxide). Their growth was monitored over a period of time by means of UV−vis spectroscopy. From ETS-NOCV modeling, the TOPO molecules were found to strongly adhere to the CdS nanoparticles through dative-covalent Cd−O bonds as well as through secondary noncovalent C−H···Cd and C−H···S interactions. The characteristic band edge luminescence was observed in the emission spectra of all samples. The TEM microscopy showed well-dispersed spherical CdS nanoparticles; the composition was supported by EDX.

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ets,7,8,15 thiosemicarbazide,16 dithioacetylacetonates,17 dithiophosphinates,7,8,18 and dithioimidodiphosphinates,7,8,19 have been used for the production of CdS nanoparticles passivated with tri-n-octylphosphine oxide (TOPO).7,8,14,16 However, no use of N-thiophosphorylated thioamidate and thioureate [RC(S)NP(S)R′2]− ligand-based SSPs, which are asymmetric derivatives of dithioimidodiphosphinates, for the formation of CdS nanoparticles has been reported so far. Recently, we have studied the structures of CdII complexes with [RC(S)NP(X)R′2]− (X = O, S) ligands. The majority of these structures corresponds to the phosphorylated [RC(S)NP(O)R′2]− anions.20−23 In contrast to this, only four structures of thiophosphorylated [RC(S)NP(S)R′2]− anions are known for CdII.24−27 This is surprising since it has been postulated for a long time that complexes of the dithio derivatives are much more stable compared with those ligands containing oxygen and sulfur atoms simultaneously. Thus, new coordination compounds of [RC(S)NP(S)R′2]− with CdII are

INTRODUCTION Nowadays, nanoparticles are of great importance and have received considerable attention due to innovative applications in cutting edge technologies, such as biomedicine,1 catalysis,2,3 data storage,4 light-emitting diodes,5 and solar cells.6 Nanoparticles possess intriguing properties, which are related to their size and are different from those of bulk materials. This unique feature allows, e.g., variation of particle size to tailor the bandgap. Metal chalcogenides nanoparticles, in particular, cadmium sulfide (CdS), have attracted considerable attention.7,8 The use of single-source precursors (SSPs) for the growth of cadmium chalcogenides has proven to be a highly efficient synthetic route to produce monodispersed nanoparticles and avoids the use of hazardous and volatile compounds, e.g., H2S, H2Se, and CdMe2.7,8 Another advantage of the use of SSPs is that this approach enables one to obtain a wide range of nanoparticles by varying both the structure of the precursor and its decomposition temperature and time. Varying the latter two parameters allows using the same SSP to produce nanoparticles with different composition and stoichiometry. Various SSPs, including dithiocarbamates,7−11 alkyl xanthates,7,8,12 N-alkyl thioureas,7,8,13 dithiobiurea,14 thiobiur© XXXX American Chemical Society

Received: February 19, 2016 Revised: April 1, 2016

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DOI: 10.1021/acs.cgd.6b00275 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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of great importance and value, especially when considering CdII complexes of [RC(S)NP(S)R′2]− as SSPs for CdS nanomaterials. Recently, we have demonstrated that complexes of [RC(S)NP(X)R′2]− with AgI and NiII are efficient precursors for nanoparticles and nanofilms.28−30 In this contribution we describe the synthesis of a new CdII complex [CdL2] with the diethylammonium salt of Nthiophosphorylated thioureate [Et2NC(S)NP(S)(OiPr)2]− (Et2NH2L),31 and describe a complete structural investigation of the obtained complex in both solution and solid state. We also report on the detailed synthesis and characterization of TOPO-capped CdS nanoparticles obtained from [CdL2]. In both parts, the experimental data is compared to results from in-depth quantum chemical calculations to gain insight into structural parameters of importance for the Cd nanoparticle formation and stability.

Figure 1. Molecular structure of [CdL2]. Thermal ellipsoids are drawn at the 50% probability level.

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lengthening of the C−S and P−S, and shortening of the C− N(P) and P−N bonds in the structure of [CdL2], compared with the values for the N-thiophosphorylated thioamides and thioureas, were observed.31−36 The relative high symmetry of the coordination surrounding of the metal atom in the complex is visible in the very similar Cd−S(C) and Cd−S(P) bonds (Table S1 in the Supporting Information). The bulk material of [CdL2] was studied by means of X-ray powder diffraction analysis (Figure 2). The experimental X-ray

RESULTS AND DISCUSSION The complex [CdL2] was prepared by reacting Et2NH2L with CdCl2 (Scheme 1). The obtained colorless solid material is soluble in most polar solvents. Scheme 1. Synthesis of [CdL2]

The IR spectrum of [CdL2] contains a band at about 582 cm−1 representing the P−S group of the anionic form of L− with a delocalized 6π electron S−C−N−P−S system (bond order ∼1.5).32,33 This band is shifted by 35 cm−1 to low frequencies compared with that in the spectrum of the parent Et2NH2L.31 Further bands at 971 and 1504 cm−1 correspond to the POC group and the conjugated SCN fragment, respectively. The 31P{1H} NMR spectrum of [CdL2] in CDCl3 exhibits a unique signal at 51.0 ppm, which indicates the exclusive presence of 1,5-S,S′-coordinated ligands in the complex.31−36 The 1H NMR spectrum in the same solvent contains one set of signals. The signals of the ethyl and isopropyl CH3 protons are observed as two triplets and one doublet, respectively, at 1.14− 1.33 ppm, while the ethyl CH2 protons are found as two quartet signals at 3.53−3.81 ppm. The isopropyl CH(O) protons appear as a doublet of septets at 4.74 ppm with the characteristic coupling constants 3JPOCH = 10.4 Hz and 3JH,H = 6.1 Hz. Crystals of [CdL2] were obtained by slow evaporation of the solvent from a CH2Cl2−n-hexane solution. The structure represents a spirocyclic bischelate complex and was refined in the triclinic space group P1.̅ The CdII cation is found in a tetrahedral S2S′2 coordination environment formed by the C−S and P−S sulfur atoms (Figure 1). The six-membered Cd−S− C−N−P−S metallacycles have an asymmetric boat form. The values of the endocyclic S−Cd−S angles are about 103.1°, while the values for the exocyclic S(C)−Cd−S(C), S(P)−Cd− S(P), and S(C)−Cd−S(P) angles fall in the range of 109.7− 116.1° (Table S1 in the Supporting Information). The

Figure 2. Calculated (black) and experimental (red) X-ray powder diffraction patterns of [CdL2].

powder pattern is in agreement with the calculated powder pattern obtained from a single crystal X-ray analysis, showing that the bulk material of [CdL2] is free from crystalline phase impurities. In order to examine the interactions in the crystal structure of [CdL2], a Hirshfeld surface analysis37 and 2D fingerprint plots38 were obtained using CrystalExplorer 3.1.39 We found that intermolecular H···H contacts, comprising 76.2%, are the major contributors to the crystal packing of [CdL2] (Figure 3). This is obviously due to the branched terminal aliphatic groups Et and iPr surrounding the [CdL2] molecules (Figure 1). The shortest H···H contacts are shown in the fingerprint plot at de + di ≈ 2.2 Å (Figure 3). A subtle feature is evident in the fingerprint plot as a distinct splitting of the short H···H contacts. This splitting occurs when the shortest contact occurs between three atoms, rather than between two atoms.38 The structure of [CdL2] is also governed by S···H and O···H contacts, comprising 15.6% and 5.5%, respectively, of total Hirshfeld surface areas (Figure 3). Very small contributions from C···H (1.2%), N···H (0.9%), and Cd···H (0.6%) contacts are also observed (Figure 3). Quantitative insight into factors determining the stability of [CdL2] in the crystal were provided from quantum chemical charge and energy calculations using the ETS-NOCV method40−42 as implemented in the ADF program.43,44 We B



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Figure 3. Decomposed 2D fingerprint plots of observed contacts for [CdL2].

applied the DFT/BLYP-D3/TZP protocol to calculate individual monomer−monomer interactions, as this protocol has recently provided accurate results for noncovalent interactions.45−47 The dispersion (ΔEdisp = −19.85 kcal/mol) is by far the most relevant stabilizing contribution to monomer−monomer binding in the crystal (Figure 4, part A). Less important are the electrostatic term (ΔEelstat = −8.08 kcal/mol) and the orbital interaction contribution (ΔEorb = −5.42 kcal/mol). It can be further seen that the major contributions to ΔEorb stem from the formation of typical hydrogen bonds C−H···S and nonpolar dihydrogen interactions C−H···H−C, which are depicted by the deformation density contributions as Δρ(C−H···S) and Δρ(C−H···H−C) in Figure 4, part B. The results from the ETS-NOCV calculations also allowed us to conclude that the formation of C−H···S bonds leads to the following chargetransfer stabilization: ΔEorb(C−H···S) = −1.94 kcal/mol and ΔEorb(C−H···H−C) = −0.65 kcal/mol. Using the Quantum Theory of Atoms in Molecules (QTAIM) method,48 we confirmed the existence of these intermolecular C−H···S and C−H···H−C interactions in [CdL2] (Figure S1 in the Supporting Information). It is noteworthy that recently more and more work has been reported in the literature, highlighting the important role of homopolar dihydrogen interactions.26,49−55 In the monomer of [CdL2], both heteropolar C−H···S, C− H···N, and C−H···O and homopolar C−H···H−C intramolecular interactions are clearly recognized by the reduced density gradient surfaces of the NCI method (Figure 5).56,57

They can also be concluded from the QTAIM results (Figure S2 in the Supporting Information). In order to confirm the existence of these intramolecular noncovalent interactions, we calculated the 1H NMR chemical shifts for [CdL2] using the GIAO approach58 as implemented in the ADF package.43,44 The calculated shifts (Figure 6) are in good agreement with the experimental values. The calculated signals of the CH(O) protons appear at 4.8−5 ppm (marked in red), whereas the ethyl and isopropyl CH3 protons are at 1−2 ppm (marked in blue). It is also evident from the calculated data that one of the protons of the methyl group (marked in green) that is engaged in the C−H···S interaction exhibits a larger chemical shift (by ∼1 ppm) as compared with the corresponding nonbonded protons marked in blue (Figure 6). A similar case is valid for the CH2 protons. In particular, the proton interacting with sulfur (marked in red) appears low-field shifted (4.8 ppm) with respect to the corresponding nonbonded protons (3.12 ppm) (Figure 6). These results further testify that the [CdL2] monomer is also stabilized by intramolecular noncovalent interactions. The thermal decomposition of [CdL2], dissolved in tri-noctylphosphine (TOP) and injected into a hot TOPO (TOP oxide) solution at 250 °C, resulted in a fast reaction of the complex and the growth of CdS nanoparticles (capped with TOPO). The reaction was monitored by recording the UV−vis absorption and emission spectra over time (Figure 7) allowing us to calculate the energy band gap from the Tauc plot with an R2 > 0.98 value for the fit. The following band edges for the obtained samples were found: 2.62, 2.60, 2.58, and 2.51 eV for t C



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Figure 5. Surfaces describing the reduced density gradient at an isovalue of 0.5 au for the [CdL2] monomer. The surfaces are colored in a blue-green-red scale according to values of sign(λ2)ρ, ranging from −0.05 to 0.02 au.

Figure 4. Results from ETS-NOCV energy decomposition calculations describing the interaction between two monomers in the crystal of [CdL2] (part A), and the most relevant NOCV-based deformation density channels, characterizing the C−H···S and C−H···H−C interactions (part B).

Figure 6. Calculated 1H NMR spectrum (top) for [CdL2]; the protons in the structure (bottom) were highlighted using the same color code as in the spectrum.

= 10, 30, and 60 min and 12 h, respectively. Macrocrystalline samples of CdS have a band gap of 2.42 eV. The band gap for all the obtained samples is blue-shifted compared with the bulk material, which is due to the CdS nanoparticles being smaller than the bulk exciton of CdS.59,60 On the other hand, a decrease of the band gap is observed during the increase of time. This is associated with the increase in particle size with time, which is consistent with an Ostwald ripening process. The photoluminescence spectra of the obtained samples of CdS nanoparticles are red-shifted compared with the corresponding absorption spectra, exhibiting emission maxima ranging from 512 to 523 nm (Figure 7). This shift, which is also affected by coupling of the organic TOPO moieties on the

surfaces of nanoparticles,61 has been ascribed to emission from the so-called “dark exciton”.62−65 Nanoparticles of CdS exist in either hexagonal or cubic phase.66−68 It was also established that the crystalline structures of nanocrystallites and corresponding bulk materials might be the same even if their electrooptical properties are different.69 According to the X-ray powder diffraction pattern (Figure 8), the CdS samples are largely crystalline and exhibit exclusively the hexagonal wurtzite phase. The broadening of the peaks in the pattern supports the nanoscale regime of the obtained particles. The average particle size was calculated from the most D



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particles in combination with our thiourea-based SSP. The clearly defined lattice fringe is visible in the HRTEM image (Figure 9), testifying to the crystallinity of the obtained material. The EDX spectrum of the sample after several washings shows the presence of Cd and S in the product (Figure 10) in a 1:1 ratio. Furthermore, the presence of P is also detected by EDX, presumably due to the strongly bound TOPO capping molecules.

Figure 7. UV−vis absorption (solid line) and emission (dashed line, λexc = 470 nm) spectra of CdS nanoparticles obtained at different times from the start of synthesis.

Figure 10. EDX spectrum of TOPO-capped CdS nanoparticles, obtained at t = 10 min from the synthesis start; measured on an aluminum substrate.

Importantly, no other phases were detected resulting, e.g., from the N or P content of the ligand. CdS nanoparticles have been previously prepared from several SSPs, but frequently the PXRD reveals impurities which can be traced back to the ligand. Recently, the dithioimidodiphosphinato complex [Cd[(SPiPr2)2N]2] was used to prepare thin films of hexagonal CdS and several phase impurities were detected, one of them being Cd6P7.7,8,19 On the other hand, this precursor allowed to prepare cadmium phosphides. Also, when the Cd dithiobiurea complex [Cd(NH2CSNHNHCSNH2)Cl2] was used to prepare CdS nanoparticles, phase impurities were detected.14 In contrast to this, the use of the thiosemicarbazide complex [Cd(NH2CSNHNH2)2Cl2] yielded phase pure nanorods when decomposed in TOPO.16 Rod-like CdS were also obtained from thiourea Cd complexes [Cd(CH3COO)2(SC(NHR)2)2] with R = c-hexyl or i-propyl,13 or from dithiocarbamato Cd complexes [Cd(S2C(NC5H10))2(NC5H5)].9−11 CdS nanoparticles in the cubic (sphalerite) phase were found when using cadmium bis(diphenyldithiophosphinate) [Cd(S2PPh2)2] together with Cd tetradecanoate.18 The formation of the cubic phase in this work is probably due to the preparation temperature of 250 °C. Cubic CdS shells have also been obtained for CdSe/CdS core/ shell nanoparticles from CdSe nanoparticles and the Cd dithiocarbamate complex [Cd(S2CNMe(nHex))2] as SSP in oleylamine at 200 °C.10 It is generally assumed that at T below 300° the cubic phase might be formed, whereas above 300° the thermodynamically more stable hexagonal phase is exclusively formed.66−68 It is therefore worth mentioning that our precursor yields phase pure hexagonal nanoparticles at only 250° in a solvent free process. Comparable results to ours have been obtained using xanthate Cd complexes such as [Cd(EtOCS2)2].12 In order to shed some light on the mechanism of interaction between TOPO and CdS nanoparticles from quantum chemical

Figure 8. Experimental (top) X-ray powder diffraction pattern of CdS nanoparticles obtained after 10 min from the synthesis start. Calculated for hexagonal (middle) and cubic (bottom) X-ray powder diffraction patterns of CdS.

prominent peaks (110), (103), and (112) (Figure 8) using the Scherrer equation70 and was found to be 6.3 nm, quite close to the size determined by TEM (see below). The transmission electron microscopy (TEM) image (Figure 9) shows that the TOPO-capped CdS spherical nanoparticles (t = 10 min) exhibit an average particle size of 5.2 ± 0.7 nm with a narrow size distribution. We conclude that a compact layer of TOPO is formed on the CdS nanoparticles preventing them from aggregation, confirming TOPO to be an effective surfactant for the production of well-dispersed CdS nano-

Figure 9. TEM (A) and HRTEM (B) images of CdS nanoparticles obtained at t = 10 min from the start of synthesis. The inset shows the particle size distribution. E



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tripropylphosphine oxide. We have used these simplifications in order to reduce computational time as well as to be able to apply molecular type of calculations instead of periodic ones. The molecular electrostatic potentials suggest that the negatively charged oxygen atoms of the oxide might interact with the positively charged Cd atoms of [Cd9S9] (Figure 11). Indeed, the lowest energy isomer of the complex, containing [Cd9S9] and three molecules of tripropylphosphine oxide, exhibits close Cd−O contacts (∼2.4 Å) (Figure 12, part A). Interestingly, in addition to the Cd−O connections, the propyl units adhere to the surface of [Cd9S9] (Figure 12, part A). In order to obtain a more detailed picture on the interaction between three tripropylphosphine oxide units and [Cd9S9], the ETS-NOCV method was applied. The results show that the oxide moieties very strongly adhere to the CdS nanoparticles with an overall binding energy of −108.2 kcal/mol. The results of energy decomposition calculations demonstrate that the major stabilization (52%) originates from the electrostatics, ΔEelstat = −157.32 kcal/mol. The orbital interaction term ΔEorb = −74.26 kcal/mol covers 24.5% of the overall stabilization energy (ΔEelstat + ΔEorb + ΔEdisp). The dispersion term (ΔEdisp = −70.94 kcal/mol) also appeared to be very important (23.4%), suggesting the existence of noncovalent interactions. Indeed, the ETS-NOCV results (Figure 12, part B) show that, apart from the primary stabilization that stems from the formation of Cd−O bonds, Δρ(Cd−O), one can identify

calculations on an optimized model of the TOPO capped CdS nanoparticles (Figure 11) were carried out. It has previously

Figure 11. Calculated models of [Cd9S9] and tripropylphosphine oxide (top), shown along with their corresponding molecular electrostatic potentials (bottom).

been indicated in the literature that surfaces of CdS nanoparticles are rather inhomogeneous.71 Accordingly, a [Cd9S9] cluster model was chosen (Figure 11) containing both S and Cd sites exposed and TOPO was modeled as

Figure 12. Lowest energy geometry of [Cd9S9] with three molecules of tripropylphosphine oxide along with the results of the energy decomposition (part A), the most relevant deformation density contributions for Cd−O bonds and example components describing the Cd···H−C and S···H−C interactions (part B). In ETS-NOCV calculations two fragments are applied, i.e., three molecules of tripropylphosphine oxide are treated as one fragment, whereas [Cd9S9] is considered as the second one. F



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formation of the secondary C−H···Cd, Δρ(C−H···Cd), and C−H···S, Δρ(C−H···S), interactions, that might enhance the stability of TOPO capped nanoparticles.

accounts for the repulsive Pauli interaction between occupied orbitals on the two fragments in the combined molecule. Finally, the last stabilizing term, ΔEorb, represents interactions between the occupied molecular orbitals of one fragment with the unoccupied molecular orbitals of the other fragment as well as mixing of occupied and virtual orbitals within the same fragment (inner-fragment polarization). This energy term may be linked to the electronic bonding effect coming from the formation of a chemical bond. In the combined ETS-NOCV scheme the orbital interaction term (ΔEorb) is expressed in terms of NOCV’s eigenvalues (vk) as

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EXPERIMENTAL SECTION Physical Measurements. Infrared spectra (Nujol) were recorded with a Thermo Nicolet 380 FT-IR spectrometer in the range 400−3600 cm−1. NMR spectra in CDCl3 were obtained on a Bruker Avance 300 MHz spectrometer at 25 °C. 1 H and 31P{1H} NMR spectra were recorded at 299.948 and 121.420 MHz, respectively. Chemical shifts are reported with reference to SiMe4 (1H) and 85% H3PO4 (31P{1H}). Electronic absorption spectra were recorded with a PerkinElmer Lambda 35 spectrophotometer using toluene as a reference. Photoluminescence spectra were recorded with a Varian Cary Eclipse Fluorescence Spectrophotometer. The electrospray ionization (ESI) mass spectra were measured with a Finnigan-Mat TCQ 700 mass spectrometer. The speed of sample submission was 2 μL/min. The ionization energy was 4.5 kV. The capillary temperature was 200 °C. Transmission electron microscopy (TEM) was performed using a Philips CM 20 FEG electron microscope. The composition analysis was done with an energy dispersive X-ray analyzer LEICA.S44Oi. Elemental analyses were performed on a Thermoquest Flash EA 1112 Analyzer from CE Instruments. DFT Calculations. Theoretical considerations are based on calculations performed within the DTF framework in the Amsterdam Density Functional (ADF) package (version 2012.01c).43,44 The BLYP correlation-exchange functional together with the D3 dispersion correction were used. Triplezeta frozen core STO basis set with one set of polarization functions was adopted. ETS-NOCV Bonding Analysis. The natural orbitals for chemical valence (NOCV)40−42 are eigenvectors that diagonalizes the deformation density matrix

M /2

ΔEorb =

k

Ψi =

2(3π )

∑ Ci ,jλj

where Ci is a vector of coefficients, expanding Ψi in the basis of fragment orbitals λj; N is a total number of fragment λj orbitals. It was shown that the natural orbitals for chemical valence pairs (ψ−k,ψk) decompose the differential density Δρ into NOCVcontributions (Δρk) M /2

M /2

∑ vk[−ψ−2k(r) + ψk2(r)] = ∑ Δρk (r) k=1

ρ

be a useful quantity for the description of noncovalent interactions.56,57 In order to obtain information about the type of bonding, plots of the reduced density gradient s versus molecular density ρ are examined. When a weak inter- or intramolecular interaction is present, a characteristic spike, lying at low values of both density ρ and reduced density gradient s, exists. To distinguish between attractive and repulsive interactions, the eigenvalues (λi) of the second derivative of density (Hessian, ∇2ρ) are used, ∇2ρ = λ1 + λ2 + λ3. Namely, bonding interactions are characterized by λ2 < 0, whereas λ2 > 0 indicates that atoms are in nonbonded contact. Therefore, within the NCI technique, one can draw information about noncovalent interactions from the plots of sign(λ2)ρ versus s. In such plots the low gradient spike, an indicator of the stabilizing interaction, is located within the region of negative values of density. On the contrary, the repulsive interaction is characterized by positive values of sign(λ2)ρ. The contour of s, colored by the value of sign(λ2)ρ, can also be plotted. This gives a pictorial representation of noncovalent interactions. Synthesis of [CdL2]. To a solution of Et2NH2L (4 mmol, 1.542 g) in aqueous EtOH (40 mL) an aqueous (40 mL) solution of CdCl2 (2.4 mmol, 0.440 g) was added dropwise under vigorous stirring. The mixture was stirred at room temperature for 1 h and left overnight. The resulting complex was extracted using CH2Cl2, the organic phase was washed with water, and dried with anhydrous MgSO4. The solvent was then removed in vacuo. Colorless crystals were isolated by recrystallization from a 1:4 mixture of CH2Cl2 and n-hexane. Yield: 1.324 g (90%). IR ν: 582 (P = S), 971 (POC), 1504 (SCN) cm−1. 1H NMR, δ: 1.14 (t, 3JH,H = 7.0 Hz, 6H, CH3, Et), 1.23 (t, 3JH,H = 7.0 Hz, 6H, CH3, Et), 1.33 (d, 3JH,H = 6.2 Hz, 24H, CH3, iPr), 3.53 (q, 3JH,H = 7.0 Hz, 4H, CH2, Et), 3.81 (q, 3JH,H = 7.1 Hz, 4H, CH2, Et), 4.74 (d. sept, 3JPOCH = 10.4 Hz, 3JH,H = 6.1 Hz, 4H, OCH) ppm. 31P{1H} NMR, δ: 51.0

j

Δρ(r ) =

k=1

where Fi,iTS are diagonal Kohn−Sham matrix elements defined over NOCV with respect to the transition state (TS) density at the midpoint between density of the molecule and the sum of fragment densities. The above components ΔEorb(k) provide the energetic estimation of Δρk that may be related to the importance of a particular electron flow channel for the bonding between the considered molecular fragments. The ETS-NOCV analysis was done based on the Amsterdam Density Functional (ADF) package43,44 in which this scheme was implemented. Non-Covalent Index (NCI) Technique. It has been shown |∇ ρ| 1 that the reduced density gradient, s = 2 1/3 4/3 , appeared to

N

ΔPCi = vC i i

∑ ΔEorb(k) = ∑ vk[−F −TSk ,−k + FkTS,k]

k=1

where νk and M stand for the NOCV eigenvalues and the number of basis functions, respectively. Visual inspection of deformation density plots (Δρk) helps to attribute symmetry and the direction of the charge flow. In addition, these pictures are enriched by providing the energetic estimations, ΔEorb(k), for each Δρk within the ETS-NOCV scheme.40−42 The exact formula, which links the ETS and NOCV methods, will be given in the next paragraph, after we briefly present the basic concept of the ETS scheme. In this method, the total bonding energy, ΔEtotal, between interacting fragments, exhibiting the geometry as in the combined complex, is divided into three components: ΔEtotal = ΔEelstat + ΔEPauli + ΔEorb. The first term, ΔEelstat, corresponds to the classical electrostatic interaction between the promoted fragments as they are brought to their positions in the final complex. The second term, ΔEPauli, G



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spectra, which was confirmed by quantum chemical calculations (Non-Covalent Index (NCI), QTAIM, and GIAO approach). The complex [CdL2] is an efficient single-source precursor for the formation of TOPO-capped CdS nanoparticles, those growth was monitored over time by means of UV−vis spectroscopy. Theoretical modeling (ETS-NOCV) allowed concluding that TOPO (tri-n-octylphosphine oxide) molecules strongly bind CdS nanoparticles through dative-covalent Cd− O bonds as well as through secondary noncovalent H−C···Cd and C−H···S interactions. The characteristic band edge luminescence was observed in the emission spectra of all samples. TEM microscopy showed spherical CdS nanoparticles of about 5 nm diameter with a quite narrow size distribution. EDX confirms the purity of the CdS nanoparticles. Finally, [CdL2] is the first CdII complex of N-thiophosphorylated thioamidate and thioureate [RC(S)NP(S)R′2]− ligand that was successfully used as single-source precursor for the formation of CdS nanoparticles. The remarkably low decomposition temperature and the high (phase) purity of the obtained material is probably due to a perfectly balanced charge distribution within the [CdL2] complex and multiple intramolecular interactions resulting in turn in rather weak intermolecular interactions and thus allowing a smooth decomposition and nanoparticle formation.

ppm. Anal. Calcd for C22H48CdN4O4P2S4 (735.25): C 35.94, H 6.58, N 7.62. Found: C 35.86, H 6.51, N 7.69%. Synthesis of TOPO-Capped CdS Nanoparticles. Complex [CdL2] (1.5 mmol, 1.103 g) was dissolved in tri-noctylphosphine (TOP) (30 mL) and injected into hot trioctylphosphine oxide (TOPO) at 250 °C. A decrease in temperature from 250 to about 225 °C was observed. The solution was then allowed to stabilize, and the reaction was continued for 30 min at 250 °C. After completion, the reaction mixture was cooled to 70 °C, and methanol was added to precipitate the nanoparticles. The solid was separated by centrifugation and washed five times with methanol. The resulting solid precipitates of TOPO-capped cadmium sulfide nanoparticles were dissolved in toluene for further analysis. 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 = 1.541 836 Å). X-ray powder diffraction studies of nanoparticles were performed with a Bruker AXS D8 diffractometer (λ = 1.5406 Å). Single Crystal X-ray Diffraction. The X-ray diffraction data for the crystal of [CdL2] were collected at 173(2) K on a STOE IPDS-II diffractometer with graphite-monochromatized 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.72 Data were corrected for absorption using the PLATON program.73 The structures were solved by direct methods using the SHELXS97 program74 and refined first isotropically and then anisotropically using SHELXL-97.74 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.75 C22H48CdN4O4P2S4, Mr = 735.22 g mol−1, triclinic, space group P1,̅ a = 8.8485(4), b = 13.4354(5), c = 15.2691(6) Å, α = 79.325(3), β = 79.563(3), γ = 77.591(3)°, V = 1723.15(12) Å3, Z = 2, ρ = 1.417 g cm−3, μ(Mo Kα) = 1.000 mm−1, reflections: 33745 collected, 8786 unique, Rint = 0.0535, R1(all) = 0.0304, wR2(all) = 0.0727.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00275. Further results from quantum chemical QTAIM calculations (two figures) and a table with selected bond lengths and angles for [CdL2] (PDF) Accession Codes

CCDC 1407623 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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CONCLUSIONS We have synthesized the CdII complex [CdL2] of the Nthiophosphorylated thioureate ligand [Et2 NC(S)NP(S)(OiPr)2]− (L−). The molecular structure of the complex was studied by IR and NMR spectroscopy revealing two deprotonated ligands with a delocalized 6π electron S−C− N−P−S system. In the solid, the structure was elucidated by single crystal X-ray diffraction analysis, revealing that the compound crystallized in the triclinic space group P1̅. The metal cation displays a tetrahedral S2S′2 coordination environment formed by the C−S and P−S sulfur atoms. According to the Hirshfeld surface analysis, the crystal structure of [CdL2] is mainly characterized by H···H, S···H, and O···H intermolecular contacts. A theoretical charge and energy decomposition study using the ETS-NOCV method allowed identifying attractive C−H···H−C and C−H···S intermolecular interactions as the main stabilizing factors. Furthermore, intramolecular C−H···S, C−H···N, and C−H···O and noncovalent C−H···H−C interactions were also noted in the monomers that constitute the crystal structure of [CdL2]. Such intramolecular noncovalent interactions appeared to influence also the 1H NMR

AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: +49 221

damir.a.safi[email protected]. [email protected]. [email protected]. Tel.: +49 221 4704006. Fax: 4705196.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Results presented in this work were partially obtained using PLGrid Infrastructure and resources provided by ACC Cyfronet AGH.

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DEDICATION Dedicated to Professor Wolfgang Kaim on the occasion of his 65th Birthday. REFERENCES

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