Crystal structure of the triphenyltin(IV) chloride dimethyl N-cyanodithioiminocarbonate adduct

In the past, there were relatively few investigations in the literature on the main group metal complexes with cyanamide adducts (N≡C-N-). Early studies on this domain date back to the 1970s (Seltzer, 1968; Jain and Rivest, 1970). More recently, organoheterobimetallic cyanodithioimidocarbonates involving tin-based complexes have been also reported for electrical conducting properties (Singh and Kumar, 2008). To the best of our knowledge and up to now, only three examples, especially of tin(IV) derivatives, have been crystallographically resolved, exhibiting essentially polymeric structures: (CH₃)₂Sn[N(CN)₂]₂ and (CH₃)₂Sn[N(CN)₂] (Chow, 1971), and (CH₃)₂Sn[NCNNO₂] (Jäger et al., 1997).

In our ongoing studies on N-ligands coordinated to organotin(IV) moieties (Sow et al., 2012) we focused on the reactivity of dimethyl N-cyanodithioiminocarbonate, NCNC(SCH₃)₂ (L), a precursor for the synthesis of quinazolinone derivatives (Al-Salahi et al., 2014), with Ph₃SnCl. The resulting complex 1 has been characterized by infrared and multinuclear nuclear magnetic resonance (NMR) spectroscopy (¹H, ¹³C{¹H}, and ¹¹⁹Sn{¹H}), and its structure was finally determined by an X-ray crystallographic analysis. While the cyanodithioimidocarbonate, [NCNCS₂]^2-, dianion commonly adopts the S,S-bidentate bridging coordination mode (Burchell et al., 2004), L in which both sulfur atoms are methylated was found in 1 to be a monodentate N-ligand (Scheme 1). To date, such a structure is rather uncommon. The only examples are described for transition metal complexes. The first one was the copper complex, CuCl₂[NCNC(SCH₃)₂]₂ (Kojic-Prodic et al., 1992). Recently, some of us have isolated two new specimens, reporting the crystal structure of dichloridobis(dimethyl N-cyanodithioiminocarbonate)cobalt(II) (Diop et al., 2016a) and then dichloridobis(dimethyl N-cyanodithioiminocarbonate)zinc (Diop et al., 2016b).

Scheme 1:

Molecular representations of (A) L and (B) 1.

Compound 1 was first studied by middle and far infrared spectroscopy [attenuated total reflectance (ATR) mode], and its spectrum was compared to those of the free L ligand and Ph₃SnCl (Figures S1 and S2, respectively). The shift of the ν(C≡N) band to a higher frequency by 20 cm^-1 constitutes the most notable observation which can be explained by the coordination of L to Sn atom through the nitrogen atom of the cyano group. Additional vibration bands, characteristic of phenyl ligands (Ph, C₆H₅), are also observed at 732 and 696 cm^-1 corresponding to δ(C_sp2-H) and δ(C=C) elongations, respectively. The far infrared spectrum of 1 combines the fingerprints of Ph₃SnCl and L and evidences in the region of 330 cm^-1 the absence of the ν(Sn-Cl) band (Tudela and Calleja, 1993), as already observed before for such Ph₃SnCl adducts (Sall et al., 1995). The CHNS elemental analysis performed on crystals also supports the proposed formula. Furthermore, compound 1 is well soluble in chlorinated and aromatic solvents. ¹H and ¹³C{¹H} NMR spectra in CDCl₃ and toluene-d8 highlight the characteristic signals of L and phenyl groups and confirm the 1:3 ratio (Figures S2 and S3). The ¹¹⁹Sn{¹H} NMR spectrum in CDCl₃ exhibits only one resonance at -47.8 ppm (SnMe₄ as external reference, Figure S4) showing a very slight shift to higher field with respect to SnPh₃Cl used as organotin precursor (δ=44.7 ppm in CDCl₃, Carcelli et al., 1992). A comparable observation was made in toluene-d8 with δ=50.9 ppm. A concentration dependence study was also investigated in CD₂Cl₂ solution at 300 K involving a limited effect on the values of δ(¹¹⁹Sn{¹H}) [0.5 m, δ=-49.9 ppm; 1 m, δ=-51.2 ppm; 2 m, δ=-52.1 ppm] (Figure S6). However, the spectra recorded for the 0.5 and 1 m solutions exhibit broad signals. Such ¹¹⁹Sn{¹H} NMR chemical shift values are in line with the presence of four-coordinate species (Holeček et al., 1983). On the basis of the five-coordination of 1 (Scheme 1b), values in the range of -180 to -260 ppm would rather be expected (Holeček et al., 1983). Thus, the probable dissociation of L occurs in solution. Indeed, triphenyltin(IV) adducts are known to be unstable in solution releasing Ph₃SnCl. To date, several examples have been already reported in the literature (Momeni et al., 2010; Gholivand et al., 2014). Similar conclusions are also supported by middle infrared measurements (transmission mode) in solution (CH₂Cl₂). The frequency of the ν(CN) band recorded for 1 fits nearly with that of the free ligand L measured under the same conditions (Figure S7).

The coordination of L to an organotin(IV) moiety was definitively verified in the solid state by an X-ray crystallographic analysis performed on suitable crystals. Crystallographic data and refinement details are summarized in Table 1. An Ortep view with selected bond lengths and angles (Å, °) is shown in Figure 1. The dimethyl N-cyanodithioiminocarbonate ligand was found disordered over two positions with occupation factors equal to 0.5, due to the steric hindrance between the two closer molecules. The structure of 1 corresponds to a SnPh₃ moiety trans-coordinated by one L ligand, N-coordinated, and one chlorine atom, terminally bonded. The environment around the tin(IV) atom can be viewed as trigonal bipyramidal (tbp). Thus, the three phenyl groups occupy the equatorial planes [Sn-C1 2.1198(17), Sn-C7 2.1345(17), and Sn-C13 2.1345(17) Å]. The sum of the angles at tin [C1-Sn-C7 121.00(6), C1-Sn-C13 114.49(6), and C13-Sn-C7 121.86(6)] is equal to 357.35°. The Sn-C distances are in the range of those found in tbp SnPh₃ moieties (Pelizzi and Pelizzi, 1983). The apical positions are occupied by chlorine and nitrogen atoms [Sn-Cl 2.4717(5) and Sn-N1 2.5360(14) Å]. The Cl-Sn-N angle [177.13(4)°] reveals a slight deviation from linearity. Compared to the two existing examples of chlorotriphenyltin complexes with a cyano adduct, already described in the literature (Carini et al., 1990; Carcelli et al., 1992), some structural disparities are observed with 1 and are reported in Table 2. Thus, the Sn-N distance of 1, longer than in the two examples given, reflects the weak coordination of L and can explain the facile dissociation observed in solution, leading to the release of the starting tin(IV) complex.

Figure 1:

Molecular structure of [ClPh₃SnNCNC(SCH₃)₂] (1) showing 30% probability ellipsoids and the crystallographic numbering scheme [Ortep views. A) Showing the two positions of the dimethyl N-cyanodithioiminocarbonate ligand, B) showing the positioning between two molecules].

Selected bond lengths and angles (Å, °): Sn-Cl 2.4717(5), Sn-N1 2.5360(14), Sn-C1 2.1198(17), C1-Sn-Nl 83.30(5), Sn-C7 2.1345(17), Sn-C13 2.1345(17), N1-C19 1.148(2), C19-N2 1.342(14), N2-C20 1.290(13), C20-S1 1.740(3), C20-S2 1.724(3), S1-C21 1.799(10), S2-C22 1.748(2); Cl-Sn-N1 177.13(4), C1-Sn-Cl 95.03(4), C1-Sn-N1 83.30(5), C1-Sn-C7 121.00(6), C1-Sn-C13 114.49(6), C7-Sn-Cl 94.97(5), C7-Sn-N1 83.93(5), C13-Sn-Cl 96.24(5), C13-Sn-N1 86.58(6), C13-Sn-C7 121.86(6), C19-N1-Sn 175.75(14), C20-N2-C19 125.0(11), N1-C19-N2 172.0(6), N2-C20-S1 121.6(7), N2-C20-S2 120.3(7), S1-C20-S2 118.1(2). Symmetry transformation: (i) 1-x, y, 0.5-z.

As mentioned before, X-ray structures of triorganotin(IV) complexes with cyanamide adducts were rather rare and until now exclusively describe polymeric networks. Thus and to the best of our knowledge, the isolation of 1 in the solid state is the first example of a monomeric organotin complex bearing a terminal dimethyl N-cyanodithioiminocarbonate ligand. Further work is in progress with the aim to isolate such new derivatives.