Molecular structures of Sn(II) and Sn(IV) compounds with di-, tri- and tetramethylene bridged salen* type ligands

Introduction

The coordination chemistry of dianionic salen-type tetradentate chelators [salen=ethylene-N,N′-bis(salicylideneimine)] has been explored with group 14 elements in the past decades. An early report of a crystal structure of (salen)SnMe₂ (Calligaris et al., 1972) indicated trans-configuration of the monodentate substituents and mer-mer-configuration of the tetradentate chelator in such tin complexes. In the course of our investigations of mainly silicon complexes and, to a lesser extent, germanium and tin complexes of salen-type chelators, we noticed some remarkable features that distinguish such silicon complexes from related tin compounds, as highlighted in Scheme 1. Whereas crystallographic evidence for hexacoordinate Si salen complexes with two monodentate substituents (Mucha et al., 1998, 1999; Wagler et al., 2004, 2005a,b,c, 2009; González-García et al., 2009) points at the exclusive formation of complexes with trans-disposed substituents (like in I and the germanium analog II, Wagler et al., 2005a, 2009), the related tin compound III was found to prefer cis-arrangement of the monodentate substituents (Wagler et al., 2009). In the latter case, the salen-type ligand is mer-fac-arranged, and the same arrangement of the tetradentate ligand had been reported for a salen-tin(IV) complex with cis-forced substituents within a stannacycle (Agustin et al., 1999a). In a related reaction with a ligand that bears imine-α-hydrogen atoms, the Sn complex VI (Wagler et al., 2005b) even resists the formation of an enamine like in Si and Ge compounds IV and V (Wagler et al., 2002, 2005b) in favor of the formation of a mer-fac-configured hexacoordinate Sn complex. In Si(IV) complexes this mer-fac-arrangement of the tetradentate ligand can be enforced by the steric constraints of silacycles, e.g., in VII (Wagler et al., 2005a) and related compounds (Wagler et al., 2006; Wagler and Roewer, 2007). These results and literature reports of mer-mer-configured hexacoordinate Sn(IV) salen complexes, such as group VIII of complexes (Jing et al., 2004; Darensbourg et al., 2005) indicate enhanced flexibility of the salen ligand in the tin coordination sphere. This flexibility of the tetradentate ligand appears to be founded on the ethylene bridge, because hexacoordinate silicon (Lippe et al., 2009) and tin complexes (Teoh et al., 1997; Dey et al., 1999a,b; Yearwood et al., 2002; Jing et al., 2004; Darensbourg et al., 2005; Wang et al., 2010) with two monodentate substituents and related but more rigid ligand backbones (e.g., 1,2-phenylene or 1,2-cyclohexanediyl bridged Schiff base ligands) have been found in trans form exclusively in terms of crystallographically proven isomers. Furthermore, salen-tin complexes already have been reported for Sn(II), like complex IX (Kuchta et al., 1999; Westerhausen et al., 2003; Jing et al., 2004), the latter of which also have been utilized as ligands in transition metal coordination chemistry (Agustin et al., 2000a,b), silylene analogs (i.e., Si(II) complexes) are still unknown.

Scheme 1

Selected Si, Ge, and Sn complexes with salen-type ligands.

A search in the Cambridge Structural Database, CSD (ConQuest 1.16, 2013) revealed a noticeable lack of crystallographically characterized tin complexes with salen-type (ONNO) ligands, which bear more than two methylene groups in the central (N-Sn-N) chelate. The convenient availability of a series of such salen-type ligands, H₂L¹, H₂L², H₂L³, which has been used in one of our previous studies of silicon complexes (Wagler et al., 2005b), the general feasibility of syntheses of tin(II) and tin(IV) complexes with salen-type ligands, and the enhanced coordinative flexibility of tin (cf. silicon) served as motivation for the herein presented study.

Results and discussion

As shown in Scheme 2, the reaction of bis(bis(trimethylsilyl)amino)stannylene with the tetradentate Schiff base ligand (H₂L¹, H₂L² or H₂L³) affords the corresponding tin(II) complex (L¹Sn, L²Sn or L³Sn). Interestingly, compounds L¹Sn and L²Sn were soluble in chloroform and could be characterized by NMR spectroscopy in solution state, whereas compound L³Sn already precipitated from the reaction mixture (in THF) and could not be dissolved in chloroform or DMSO. Thus, we have not succeeded in crystallizing L³Sn, and this complex was characterized by solid state ¹¹⁹Sn NMR only. The ¹¹⁹Sn NMR shifts (L¹Sn, CDCl₃: -571 ppm; L²Sn, CDCl₃: -620 ppm; L³Sn, solid: -524 ppm) are in the expected range for tin(II) complexes with salen-type ligands [e.g., -536 ppm (Westerhausen et al., 2003), -501, -517, -522, -527 ppm (Jing et al., 2004)] and other Sn(II) complexes with four donor atoms, such as aryloxy-O or sp² (amide, imine) N atoms; for comparison with ¹¹⁹Sn NMR shifts of other Sn(II) complexes with such donor spheres, see Scheme 3 (IX: van den Bergen et al., 1990; X: Brendler et al., 2011). The ¹H and ¹³C NMR data of L¹Sn and L²Sn suggest symmetric monomeric complexes in chloroform solution. In both cases the spectra show a single set of signals in accord with a C_s symmetric molecule. In the solid state structure (as determined by single-crystal X-ray diffraction, Figure 1, Table 1) compound L¹Sn is monomeric, and the Sn atom is located at the apex of an idealized square pyramid with ONNO base, thus this structure accords well with the proposed C_s symmetric monomers of L¹Sn in solution and similar to the molecular structures of previously published tetracoordinate Sn(II) complexes with related (ONNO) ligand systems (Atwood et al., 1995; Agustin et al., 1999b; Kuchta et al., 1999; Westerhausen et al., 2003; Jing et al., 2004) and monomeric Sn(II) compounds with other (ONNO) ligand systems (Zöller et al., 2011). The molecular structure of L²Sn in the solid state is surprisingly different (Figure 1). The tetradentate ligand furnishes a see-saw-type coordination sphere about the Sn atom (with O1 and N2 in apical positions and O2 and N1 on equatorial sites). Furthermore, each of the apical O1 atoms establishes a contact to the tin atom of a neighboring molecule, thus forming dimers with 4+1 coordinated tin atoms (Scheme 4). The highly unsymmetric Sn1-O1···Sn1* bridges (Sn-O separations are 2.15 and 3.04 Å, respectively) already hint at weak bridging and possible dissociation into monomers in solution. In the solid state structure of L²Sn the tetradentate ligand adopts a mer-fac configuration (mer-O1,N1,N2; fac-N1,N2,O2) with respect to an idealized octahedral coordination sphere (the additional two coordination sites being occupied by the O1* donor and the Sn-located lone pair). This feature relates the molecular conformation of L²Sn to that of the tin(IV) complex L²SnCl₂ (vide infra). As both the formation of dimers (Berends et al., 2009; Iovkova-Berends et al., 2011, 2012a,b) and polymers (Zöller et al., 2013) via Sn₂ O₂ four-membered cycles has been encountered with stanna(II)-ocanes, we attribute the poor solubility of L³Sn to a similar kind of formation of polymeric networks.

Scheme 2

Generic routes of the syntheses of compounds L^1-3Sn and L^1-3SnCl₂.

Scheme 3

Examples of tetracoordinate Sn(II) complexes and their ¹¹⁹Sn NMR shifts.

Figure 1

Molecular structures of (from top) L¹Sn and [L²Sn]₂ in the solid state. ORTEP/POV-RAY diagrams (Farrugia, 1997; POV-RAY 3.6, 2004) with thermal displacement ellipsoids are shown at the 50% probability level. Selected atoms are labeled and hydrogen atoms are omitted for clarity. For L¹Sn, only the predominant position (ca. 70% occupancy) of the cross-wise disordered ethylene bridge is depicted. The dimer [L²Sn]₂ is located around a center of inversion (operation 1-x, 1-y, -z), which generates the symmetry equivalent atoms (asterisked labels) from the asymmetric unit. Selected bond lengths (Å) and angles (°) for L¹Sn: Sn1-O1 2.1093(14), Sn1-O2 2.1032(14), Sn1-N1 2.3554(18), Sn1-N2 2.3511(16), O1-C1 1.331(2), O2-C10 1.321(2), N1-C7 1.295(3), N2-C16 1.293(3), O1-Sn1-O2 76.43(5), O1-Sn1-N1 76.19(6), O2-Sn1-N2 77.43(5), N1-Sn1-N2 70.74(6), O1-Sn1-N2 113.67(6), O2-Sn1-N1 124.20(6); for [L²Sn]₂: Sn1-O1 2.1524(8), Sn1-O2 2.0944(8), Sn1···O1* 3.035(1), Sn1-N1 2.2906(9), Sn1-N2 2.5316(9), O1-C1 1.324(1), O2-C10 1.325(1), N1-C7 1.304(1), N2-C16 1.290(1), O1-Sn1-O2 82.86(3), O1-Sn1-N1 79.59(3), O2-Sn1-N2 74.78(3), N1-Sn1-N2 74.24(3), O1-Sn1-N2 143.70(3), O2-Sn1-N1 94.65(3).

Scheme 4

Steric representation of the conformations of L¹Sn (monomeric, square pyramidal Sn coordination sphere) and L²Sn (dimeric, see-saw Sn coordination of each monomeric unit).

As outlined in Scheme 2, the tin(IV) complexes L¹SnCl₂, L²SnCl₂ and L³SnCl₂ were synthesized from the respective ligand and tin(IV) chloride by base supported HCl elimination (using triethylamine as the sacrificial base). Their ¹¹⁹Sn NMR shifts in DMSO (L¹SnCl₂, -610 ppm; L²SnCl₂, -623 ppm; L³SnCl₂, -617 ppm) are characteristic for hexacoordinate (salen)SnCl₂ complexes (shifts of -594 and -596 ppm have been reported by Jing et al., 2004, for CD₂ Cl₂ solutions of related complexes), and they appear in a narrow range in spite of the different chelate sizes and ligand conformations. According to ¹H and ¹³C NMR spectroscopy compound L¹SnCl₂ exhibits two chemically equivalent halves of the salen ligand in solution, and this is in accord with the molecular configuration observed in the solid state (Figure 2). Compounds L²SnCl₂ and L³SnCl₂ each produce ¹H and ¹³C NMR spectra characteristic of salen ligands with non-equivalent half-sides. Apparently, in solution these complexes retain the molecular conformation that is found in their solid state structures (Figure 2). In both cases the tetradentate ligand adopts mer-fac configuration. Retention of this configuration in solution is in contrast to the observation made with compounds III and VI (Wagler et al., 2005b, 2009), which produce solution state ¹H and ¹³C NMR spectra characteristic of complexes with chemically equivalent ligand halves and thus indicate formation of the mer-mer configuration or rapid configurational exchange in solution.

Figure 2

Molecular structures of (from top) L¹SnCl₂, L²SnCl₂ and L³SnCl₂ in the solid state. ORTEP/POV-RAY diagrams (Farrugia, 1997; POV-RAY 3.6, 2004) with thermal displacement ellipsoids are shown at the 50% probability level. Selected atoms are labeled, and hydrogen atoms are omitted for clarity. For L¹SnCl₂ and L²SnCl₂, which contain two molecules of the same isomer in the asymmetric unit each, only one molecule is depicted as a representative example. Selected bond lengths (Å) for L¹SnCl₂: Sn1-Cl1 2.4518(4), Sn1-Cl2 2.4179(4), Sn1-O1 2.0026(11), Sn1-O2 2.0054(11), Sn1-N1 2.1525(13), Sn1-N2 2.1676(12), O1-C1 1.345(2), O2-C10 1.348(2), N1-C7 1.298(2), N2-C16 1.303(2), O1-Sn1-O2 103.70(5), O1-Sn1-N1 87.69(5), O2-Sn1-N2 88.57(5), N1-Sn1-N2 80.12(5), O1-Sn1-N2 167.71(5), O2-Sn1-N1 167.99(5), Cl1-Sn1-Cl2 175.32(2); for L²SnCl₂: Sn1-Cl1 2.389(2), Sn1-Cl2 2.402(2), Sn1-O1 1.995(5), Sn1-O2 2.021(6), Sn1-N1 2.211(7), Sn1-N2 2.196(7), O1-C1 1.362(10), O2-C9 1.364(10), N1-C7 1.296(10), N2-C15 1.302(10), O1-Sn1-O2 89.4(2), O1-Sn1-N1 83.2(2), O2-Sn1-N2 82.5(2), N1-Sn1-N2 82.5(2), O1-Sn1-N2 163.8(2), O2-Sn1-N1 88.1(2), Cl1-Sn1-Cl2 91.27(8), Cl1-Sn1-O2 174.93(17), Cl2-Sn1-N1 178.54(18); for L³SnCl₂: Sn1-Cl1 2.4187(5), Sn1-Cl2 2.4083(5), Sn1-O1 2.0228(14), Sn1-O2 2.0205(15), Sn1-N1 2.1990(17), Sn1-N2 2.2073(18), O1-C1 1.340(2), O2-C9 1.338(2), N1-C7 1.311(3), N2-C15 1.307(3), O1-Sn1-O2 91.49(8), O1-Sn1-N1 82.90(8), O2-Sn1-N2 82.87(7), N1-Sn1-N2 86.95(7), O1-Sn1-N2 87.96(6), O2-Sn1-N1 168.55(7), Cl1-Sn1-Cl2 90.09(2), Cl1-Sn1-O1 174.15(7), Cl2-Sn1-N2 174.66(5).

From the structural and spectroscopic observations made with compounds L¹SnCl₂, L²SnCl₂, and L³SnCl₂, we conclude that upon transition from the ethylene bridged ligand L¹ to the more flexible propylene bridged ligand L², the mer-fac configuration of the tetradentate chelator is favored over the mer-mer configuration. To prove this hypothesis, we have performed quantum chemical analyses (with Gaussian 09, Frisch et al., 2009) of the relative energies of the mer-mer, mer-fac, and fac-fac configurational isomers of L¹SnCl₂ and L²SnCl₂ (Scheme 5, Figure 3). In addition to the evaluation of the relative energies obtained by optimization of the molecular structures at the DFT-B3LYP (CHNOCl: 6-31G(d); Sn: SDD) level the trends were confirmed by single-point energy analyses at the RHF-MP2 (CHNOCl: 6-31G(d); Sn: SDD) level. In case of L¹SnCl₂ the molecule with mer-mer configuration should indeed represent the stable isomer, but the mer-fac isomer is less favorable by only 1–2 kcal/mol, whereas formation of the fac-fac isomer would require significantly more energetic effort (7–10 kcal/mol). These results are in agreement with the occasional observation of mer-fac configured (salen)Sn(IV) complexes (compounds III and VI) in the solid state (most likely driven by packing effects) and their transformation into the mer-mer isomers in solution. Noteworthy, for related silicon complexes with ethylene bridged tetradentate ONNO ligand, the formation of the mer-fac and the fac-fac isomer out of the mer-mer isomer would require 5.6 and 18.7 kcal/mol, respectively (Seiler et al., 2005). Thus, the tin(IV) complexes appear to be more flexible than their Si(IV) congeners, which is in agreement with the exclusive observation of mer-mer isomers of (salen)Si(IV) complexes with two monodentate substituents as pointed out earlier. For L²SnCl₂ (Figure 3) the mer-mer isomer is indeed less stable than the mer-fac isomer and, to our surprise, the mer-fac and fac-fac isomer exhibit similar energetics. The lower stability of the mer-mer isomer is in accord with our NMR spectroscopic observation of the retention of the mer-fac configuration in solution. Furthermore, the ¹H and ¹³C NMR data do not hint at transformation into the fac-fac isomer in solution (which would have to give rise to a set of signals characteristic of chemically equivalent ligand halves because of their relation via a two-fold axis).

Scheme 5

Principle isomers of complexes of the type LⁿSnCl₂ with hexacoordinate Sn atom. The notations mer-mer, mer-fac, fac-fac refer to the relative positions of three donor atoms of the tetradentate (ONNO) ligand, i.e., (ONN) sequence-(NNO) sequence.

Figure 3

Molecular structures of (from left) the mer-mer, mer-fac and fac-fac isomer of L¹SnCl₂ (top row) and L²SnCl₂ (bottom row), optimized with Gaussian09 (Frisch et al., 2009) at the B3LYP (CHNOCl: 6-31G(d); Sn: SDD) level. For each row, the relative energies of the isomers (in kcal/mol) are given below the molecular structure. The relative energies in parentheses (also in kcal/mol) were obtained from single-point analyses at the MP2 (CHNOCl: 6-31G(d); Sn: SDD) level.

Experimental

Ligands H₂L¹, H₂L² and H₂L³ were prepared according to a method published previously (Wagler et al., 2005b). Bis(bis(trimethylsilyl)amino)stannylene, which also is known from the literature (Fjeldberg et al., 1983) was prepared from commercially available sodium bis(trimethylsilyl)amide solution (2M in THF, Sigma-Aldrich, Steinheim, Germany) and SnCl₂(dioxane) (Morrison et al., 1967). All other chemicals used were commercially available and used without further purification. Tetrahydrofuran (THF) and triethylamine were distilled from sodium/benzophenone and stored under dry argon. Dichloromethane (DCM) was distilled from calcium hydride and stored under argon. Acetonitrile, dimethyl sulfoxide (DMSO) and chloroform (stabilized with amylenes) were stored over molecular sieves 3 Å. Elemental microanalyses (C, H, N) were performed on a ‘Vario Micro Cube’ analyzer (Elementar, Hanau, Germany). NMR spectra of solutions were recorded on an Avance 500 spectrometer (Bruker Biospin, Rheinstetten, Germany) and internally referenced to tetramethylsilane for ¹H and ¹³C and externally referenced to tetramethylstannane for ¹¹⁹Sn. The ¹¹⁹Sn solid state NMR spectrum of L³Sn was recorded on an Avance 400 WB spectrometer (Bruker Biospin, Rheinstetten, Germany) using a 4-mm zirconia (ZrO₂) spinner. Single-crystal X-ray diffraction data sets were collected on an IPDS-2(T) diffractometer (Stoe, Darmstadt, Germany) using Mo Kα-radiation. Structures were solved by direct methods with ShelXS (Sheldrick, 1997b), and all nonhydrogen atoms were anisotropically refined in full-matrix least-squares cycles against ∣F²∣ (ShelXL) (Sheldrick, 1997a). Hydrogen atoms were placed in idealized positions and refined isotropically (riding model). The structure of L¹Sn bears a cross-wise disorder of the ligand ethylene bridge with site occupancies of 0.702(7) and 0.298(7). The structure of L₂SnCl₂ was refined as a racemic twin with twin populations of 0.54(3) and 0.46(3), and the Sn atoms of this structure were refined in two alternative positions with site occupancies of 0.949(1) and 0.051(1), the latter probably arising from stacking faults in the crystal lattice, thus simulating disorder positions. The remaining atoms (C,H,N,O,Cl) of those phantom molecules have not been detected because of their relatively low occupancy, and therefore the ligand atoms have been refined without disorder (with full site occupancy). Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary material publication no. CCDC-990379 (L¹Sn), CCDC-990380 (L²Sn), CCDC-990381 (L¹SnCl₂), CCDC-990382 (L²SnCl₂), and CCDC-990383 (L³SnCl₂). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: + 44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk).