Organotin(IV) scorpionates – X-ray structure and crystal packing of TpSn(Cl)₂(n-Bu) [Tp=hydrotris(pyrazol-1-yl)borate]

Introduction

During the 1990s, organotin(IV) derivatives of polypyrazolylborates [also called scorpionates by Trofimenko (1999)] were intensively studied. The group of Gioia Lobbia was particularly productive and active in this field describing the synthesis and the solid-state characterization of many examples of organotin(IV) polypyrazolylborates (Gioia Lobbia et al., 1991, 1992, 1995a,b, 1996; Calogero et al., 1996). In addition to structural considerations relevant in coordination chemistry, these compound aroused a great interest owing to their potential biological activity, qualified of antimutagenic properties (Zaidi et al., 1988). Thereafter and in connection with organometallic chemistry considerations, Tani and Mashima showed that Tp*Sn(Cl)_nBu_3-n complexes could be efficiently employed as reagents for the preparation of group 4 (Zr) and group 5 (Nb, Ta) hydrotris(3,5-dimethylpyrazolyl)borates (Mashima et al., 1997; Oshiki et al., 2000). Other research groups have then applied this useful method for the preparation of new transition metal (Oulié et al., 2005; Tang et al., 2011; Yuan and Chen, 2011) and rare earth pyrazolate derivatives (Beaini et al., 2008).

During our ongoing studies in the organotin(IV) chemistry field (Plasseraud and Cattey, 2013; Diallo et al., 2014), we reported recently the reactivity of Tp*Sn(Cl)₂Bu [Tp*=hydrido-tris(3,5-dimethylpyrazolyl)borate] with NaOH in a mixture of acetone/water leading to the unusual dimeric complex [(Me₂CO)₃(NaTp*)₂] (Plasseraud and Cattey, 2014). By seeking to extend this reaction to other organotin precursors, we prepared the complex TpSn(Cl)₂(n-Bu) (1) for which single crystals, suitable for an X-ray crystallographic analysis, were obtained in a mixture of toluene/dichloromethane by slow vapor phase diffusion at room temperature. Although the synthesis of 1 was described in the past by Gioia Lobbia et al. (1991), it seemed interesting to complete the characterization by describing its hitherto unknown crystal structure. Indeed, although several methyl and phenyl derivatives are known and identified as CCDC references [Sn(Cl)₂Me moiety=YIVVOO (Gioia Lobbia et al., 1995a), ROPGEI (Calogero et al., 1996); Sn(Cl)₂Ph moiety=PARPON (Gioia Lobbia et al., 1992), ZOFDON (Gioia Lobbia et al., 1995b), TOGKEF (Gioia Lobbia et al., 1996)], only one n-butyl example of dichloro tin(IV) scorpionate has been crystallographically characterized to date (CCDC reference=QEYHOR; Oshiki et al., 2000). In 1990, Jung and coworkers reported also the X-ray crystal structures of two tris(pyrazolyl)borate complexes of 3-methoxy-3-oxopropyltin(IV), CH₃OOCCH₂Sn((pz)₃BH)X₂ [X=Cl (CCDC reference=JIBRAN), NCS (CCDC reference=JIBRER)] (Jung et al., 1990). Experimentally, the title compound was obtained by reacting potassium hydrotris(pyrazol-1-yl)borate (KTp) with n-BuSnCl₃ at room temperature, in dichloromethane solution (Scheme 1). Crystallographic data and crystal packing of 1 are reported and discussed herein.

Scheme 1:

Synthetic pathway showing the molecular representations of KTp and 1.

Crystals of 1 were first investigated by infrared spectroscopy (ATR mode, Figure S1), and the spectrum was compared with that of the KTp precursor. The significant shift of the ν(B-N) band to higher frequency compared with KTp (2512 vs. 2400 cm^-1) constitutes the most notable change, which can be attributed to the coordination of Tp to Sn atom. Additional vibration bands are also observed in the range of 2850–3000 cm^-1 corresponding to ν(C-H) absorptions characteristic of a n-butyl chain bonded to Sn. The structure of 1 was then confirmed in the solid state by a single-crystal X-ray diffraction study. Selected crystallographic data and refinement details are reported in experimental section. An Ortep view with selected bond lengths and angles [Å, °] is shown in Figure 1. In the unit cell, one molecule of toluene co-crystallizes with two molecules of 1. A Sn(Cl)₂(n-Bu) moiety is linked to one hydrotris(pyrazol-1-yl)borate ligand, which adopts a classic tridentate facial coordination mode [Sn-N1 2.2638(13), Sn-N3 2.2457(13), and Sn-N5 2.1958(13) Å]. Thus, the tin atom is six-coordinated in a distorted octahedral environment [N5-Sn-C10 172.99(5)°]. The coordination sphere around the tin atom is completed by two chlorine atoms located in the equatorial plane [Sn-Cl1 2.4353(4) and Sn-Cl2 2.4406(4) Å] and one carbon atom of the n-C₄H₉ chain occupying an axial position [Sn-C10 2.1442(15) Å]. The resulting structure described a piano stool conformation, classically observed for tris(pyrazolyl)borate complexes. Furthermore, the CHN elemental analysis performed on crystals also supports the formula of 1.

Figure 1:

Molecular structure of TpSn(Cl)₂(n-Bu)·toluene (1) showing 40% probability ellipsoids and the crystallographic numbering scheme (Ortep view in Olex2).

Atom color code: B, yellow; N, blue; Cl, green; C, gray; H, white; Sn, turquoise. Selected bond lengths and angles [Å, °]: Sn-Cl1=2.4353(4), Sn-Cl2=2.4406(4), Sn-N1=2.2638(13), Sn-N3=2.2457(13), Sn-N5=2.1958(13), Sn-C10=2.1442(15), N1-N2=1.3677(18), N3-N4=1.3642(18), N5-N6=1.3661(18), N2-B=1.546(2), N4-B=1.543(2), N6-B 1.545(2); Cl1-Sn-Cl2=96.008(15), N1-Sn-Cl1=91.64(4), N3-Sn-N1=80.83(5), N3-Sn-Cl2=88.91(3), N5-Sn-C10=172.99(5), N5-Sn-Cl1=86.62(4), N5-Sn-Cl2=87.53(3), N5-Sn-N1=79.37(5), N5-Sn-N3=81.17(5), C10-Sn-Cl1=97.34(5), C10-Sn-Cl2=97.77(5), C10-Sn-N1=94.68(6), C10-Sn-N3=94.29(6), N2-N1-Sn=121.75(9), N4-N3-Sn=122.45(10), N6-N5-Sn=122.35(9), N1-N2-B=120.41(12), N3-N4-B=120.20(12), N5-N6-B=121.48(12).

Moreover, the examination of the crystal packing highlights that molecules of 1 are associated by pairs in the crystal. The resulting supramolecular organization shown in Figure 2 can be viewed as a V-shaped assembly, based on intermolecular interactions implicating two of the three pyrazolyl rings of each molecule. Thus, the two pyrazolyl rings formed by N5, N6, C7, C8, and C9 atoms (Ct2 and Ct2ⁱ in Figure 2) of two symmetric molecules are implicated in a parallel displaced π-π contacts. The interplanar distance between the rings is 3.383 Å. The pyrazolyl rings are slipped by 0.928 Å (slip angle 15.34°), and the centroid-centroid distance is 3.508(1) Å, which is consistent with the range of parameters observed previously for such π-π stacking involving aromatic nitrogen-containing ligands (Janiak, 2000). In addition, the resulting V-shaped assembly is also stabilized by two intermolecular C-H···π contacts oriented in antiparallel directions and involving again the pyrazolyl rings, respectively, between C8 and the centroid formed by N3, N4, C4, C5, and C6 atoms (Ct1) and between C6 and the centroid formed by the toluene molecule (Ct3). The C-H-centroid distances are respectively equal to 3.4733(17) and 3.9157(20)° Å with the corresponding C-H-centroid angle equal to 145.88° and 148.80°. These values are in the range of those typically identified for this type of weak interactions (Nishio, 2004). In the past, such comparable supramolecular architectures have been already reported in the literature, for example, by Marjo et al. (1994) and Reger et al. (2003), for dibromide quinoline and tris(pyrazolyl)methane derivatives, respectively. However, and to our knowledge, they remain relatively rare. Interestingly and beyond this first stage of supramolecular organization, both chlorine ligands linked to the tin atom are also engaged in intermolecular C-H···Cl interactions with pyrazolyl rings of neighboring pairs [C4-H4···Cl1=3.5362(17) Å, C3-H3···Cl2=3.7351(17) Å, and C9-H9···Cl2=3.6589(16) Å] (Thallapally and Nangia, 2001). Thus, the combination of all of these intermolecular interactions leads to the propagation of a three-dimensional network depicted in Figure S2.

Figure 2:

The V-shaped organization of 1 stabilized by π-π (

), C-H···π (

), and C-H···Cl (

) intermolecular interactions (Olex2 View).

Symmetry translation: (i) x, 1+y, z; (ii) 1-x, 1-y, 1-z; (iii) 1+x, y, z.

In solution, compound 1 is well soluble in chlorinated organic solvents and poorly in aromatics. ¹H and ¹³C{¹H} NMR spectra in CD₂Cl₂ show the characteristic signals of pyrazolyl and n-C₄H₉ groups, confirming the respective 1:1 ratio groups (Figures S3 and S4, respectively). The ¹¹⁹Sn{¹H} NMR spectrum in CD₂Cl₂ exhibits only one resonance at -485.6 ppm (SnMe₄ as external reference, Figure S5), corroborating the six-coordinate tin complexes, and shows that the structure determined at the solid state is maintained in solution. Even after an extended stay (several days), no alteration or modification is observed, which expresses a good stability of the coordination complex.

As already mentioned and to the best of our knowledge, 1 represents to date only the second example of a crystallographically characterized n-butyl dichloro tin(IV) scorpionate. Some structural data of 1 and its Tp* analogue [Tp*=Tris(3,5-dimethylpyrazol-1-yl)hydroborate] are compared in Table 1.

A marked difference was observed in particular for the Cl-Sn-Cl angle, which is significantly more open in the case of 1. The extension of this comparison to Cl₂RSn pyrazolylborate complexes shows that such angle value is unusual, which is generally <95° (Gioia Lobbia et al., 1996) and could be directly related to the close crystal packing involving the both chlorine atoms of 1.

Experimental