Nanoparticles of novel organotin(IV) complexes bearing phosphoric triamide ligands

Summary Four novel organotin(IV) complexes containing phosphoric triamide ligands were synthesized and characterized by multinuclear (1H, 31P, 13C) NMR, infrared, ultraviolet and fluorescence spectroscopy as well as elemental analysis. The 1H NMR spectra of complexes 1–4 proved that the Sn atoms adopt octahedral configurations. The nanoparticles of the complexes were also prepared by ultrasonication, and their SEM micrographs indicated identical spherical morphologies with particles sizes about 20–25 nm. The fluorescence spectra exhibited blue shifts for the maximum wavelength of emission upon complexation.


Introduction
In recent years, the increasing progress in the preparation of nanomaterials has led to characterization of a great number of nanostructures [1]. Nanoscale materials are of significance owing to their small sizes and large specific surface areas indicating novel properties that differ considerably from those of the corresponding bulk materials [2,3]. The coordination chemistry of organotin(IV) complexes has become of great interest due to the wide applications of these coordination compounds [4][5][6]. For example, they can act as potential antitumor agents [7,8], wood preservatives, agrochemical fungicides and biocides [9,10], as well as catalysts [11]. The organotin(IV) complexes with phosphorus-based ligands bearing the P(E) group (E = O, S, or Se) are especially important because of their various coordination numbers [12][13][14]. The central tin atom presents a diverse coordination environment depending on the different nature of substituents [15]. Hypervalent octahedral geometries were observed for the Sn(IV) atoms in organotin(IV) complexes of O-donor ligands, such as phosphoramidates [16][17][18][19][20][21][22], imidodiphosphonic acids [23] and bis(diphenylphosphino) pyridine [24], while a trigonal bipyramidal coordination was found for Sn atoms in thiophosphinates complexes [25].
Ultrasonic vibration has the potential to be a simple and effective process to produce homogenous nanomaterials. Some investigations have proved that ultrasonication can be applied to prepare nanosized coordination compounds [26][27][28][29]. The synthesis of nanoplates of a cadmium(II) coordination polymer by a sonochemical process was reported [30]. It is notable that, as far as we know, there are no reports about the preparation of nanosized organotin(IV) complexes bearing phosphoric triamide ligands.
Comparing the phosphorus chemical shift, δ( 31 P), of compounds 1-4 demonstrates that it is the most deshielded atom in 4 (containing three N-phenylpiperazinyl substituents on the P atom with δ( 31 P) = 17.33 ppm). The 31 P NMR of complexes 1-3, each containing identical phosphoric triamide ligands, show that the phosphorus atom is at the most upfield region in 1 with M = SnCl 2 (CH 3 ) 2 . Further, the δ( 31 P) shifts downfield from 1 to 3. The complexes 1 and 4 both contain SnCl 2 Me 2 , and they differ in the phosphoric triamide ligands. The 31 P NMR reveals that the phosphorus atom in 1 appears at a much more upfield region than that of 4, which is due to the presence of more electron-donating phosphoric triamide ligands in 1. The δ( 31 P) in complex 1 and its corresponding ligand 5 are observed at about 9.00 ppm, while those of complex 4 and its ligand 6 are at about 18.00 ppm. These downfield shifts in com-  (13) 18.38 c -88.9/-1138 - [21] a M = SnCl 2 (CH 3 ) 2 , L 1 = compound 5, L 2 = compound 6. b (DMSO), c (CHCl 3 ), d This work. It can be deduced from the 1 H NMR spectra of complexes 1-4 that the Sn atoms adopt octahedral configurations ( Figure 1 and Figure 2). The coordination number of the central Sn atom with  Table 1.
The IR spectra indicate that the ν(P=O) of complexes 1-4 are weaker than their corresponding ligands 5 and 6 due to the formation of Sn-O-P bonds. Also, the ν(C=O) of complexes 1-3 are nearly equal to (or slightly weaker than) the ν(C=O) value in ligand 5 (1675 cm −1 ). The bands at about 560 and 520 cm −1 correspond to asymmetric and symmetric stretching frequencies of Sn-C bonds. Also, the bands at ≈450 cm −1 are for the stretching frequencies of Sn-O bonds.

SEM and fluorescence studies
The nanoparticles of complexes 1-4 were prepared by ultrasonication. In this way, the synthesis of these compounds was performed in an ultrasonic bath at 30 °C for about 1-2 h. The SEM micrographs of the nanoparticles are shown in Figures 3-6 indicating that the particle sizes are about 20-25 nm with identical spherical morphologies of the nanoparticles. It is notable that SEM micrographs could be obtained from both dissolved and powdered samples. For the dissolved samples, a few droplets were placed on a small piece of foil, whereas the powdered compounds were directly placed on the sample holder. Here, the complexes were dissolved in methanol, and after evaporation of the solvent, the SEM images were obtained from the nanoparticles prepared on aluminum foil. The fluorescence spectra of complexes 1-4 and their related phosphoric triamide ligands 5 and 6 are represented in Figures 7-12 (see below), respectively. Also, a summary of the UV-vis and fluorescence spectra of compounds 1-6 is given in Table 2. The results reveal that the UV absorption wavelengths in these compounds vary from 280 to 330 nm, which are related to the interligand π→π* and n→π* electronic transitions.
The fluorescence spectra of each compound were repeated at various wavelengths from about 220 to 360 nm to find the maximum emission intensity. The emission signals with maximum intensities are shown in Figures 7-12. It can be seen that the λ(max) of emission appears at 360 nm for both ligands

and 6 while it decreases in their corresponding complexes (blue shift). This observation shows that the band gap (ΔE)
increases upon complexation. Moreover, the peak intensities are very much smaller in complexes 1-3 relative to that of their corresponding ligand 5. However, the λ(max) of emission and the peak intensity are very close to each other in compound 4 and its phosphoric triamide ligand 6.

Experimental Spectroscopic measurements
The 1 H, 13

Conclusion
Four new organotin(IV) complexes with phosphoric triamide ligands were synthesized and characterized by 1 H, 31 P, 13 C NMR, IR, UV, fluorescence spectroscopy and elemental analysis. According to the 1 H NMR spectra, it was concluded that the Sn atoms adopt octahedral conformations. The geminal Sn, H coupling constants 2 J( 119 Sn,H) = 55.1, 83.4 Hz, were measured in the 1 H NMR spectra of complexes 1 and 4. Using ultrasonication, spherical nanoparticles of complexes 1-4 were prepared, and their SEM micrographs indicate that the nanoparticle sizes are about 20-25 nm. The fluorescence spectra illustrate blue shifts for the λ(max) of emission and a decrease in the peak intensities upon complexation.