Synthesis, detailed geometric analysis and bond-valence method evaluation of the strength of π-arene bonding of two isotypic cationic prehnitene tin(II) complexes: [{1,2,3,4-(CH3)4C6H2}2Sn2Cl2][MCl4]2 (M = Al and Ga)

The first main-group-metal–prehnitene π complexes have been obtained in form of the isotypic pair {[{1,2,3,4-(CH3)4C6H2}2Sn2Cl2][MCl4]2}x (M = Al, Ga) and crystal structure determinations thereof give strong evidence that a distorted η6 coordination mode, characterized by a small but significant ring slippage of ca 0.4 Å as well as a pronounced tilt of the plane of the arene ligand against the plane of the central (Sn2Cl2)2+ four-membered ring species, is an intrinsic feature of this kind of arene complexed dimeric chloridostannylene cation. Application of the bond-valence method in a indirect manner yields empirical bond valences of 0.38 and 0.37, respectively, which allow for classifying the metal–π-arene bonding as a strong non-covalent interaction, which is in line with the expectation that [AlCl4]− is the slightly weaker coordinating anion as compared to [GaCl4]−.


Chemical context
Compounds that are known today to have arene (= benzenoid) molecules -bonded to main-group metal central atoms have been studied since the late 19 th century (Smith, 1879;Smith & Davis, 1882;Lecoq de Boisbaudran, 1881). The best recognized work of the early period seems to be the series of investigations by Menshutkin, exploring the composition of compounds in systems of the type EX 3 /arene (E = As, Sb; X = Cl, Br), subsequently often referred to as 'Menshutkin complexes' (e.g. Menshutkin, 1911). However, the nature of bonding in such compounds remained unclear until the first structure determinations of p-block-metal-arene complexes were published in the late 1960s (Lü th & Amma, 1969;Hulme & Szymanski, 1969). Although a significant number of cationic ISSN 2056-9890 main-group metal--arene complexes have been synthesized and structurally characterized since then (see review by Schmidbaur & Schier, 2008), the knowledge of isotypic pairs containing the same cation but different anions is so far limited to two couples of bis(arene) complexes, viz. {[(C 6 H 6 ) 2 Ga][GaX 4 ]} 2 [X = Cl (Schmidbaur et al. 1983 (Weininger et al., 1979;Frank, 1990a;Schmidbaur et al., 1990; for further information see Section 4), a detailed analysis of the structural parameters of the isotypic cationic tin(II)--arene title complexes allows for: (i) identification of the intrinsic features of the -bonding geometry of mono(arene) complexation in this class; (ii) investigation of the impact of anion change on the -bonding situation unaffected by more principal structural differences; (iii) the indirect estimation of an empirical bond valence for the -arene bonding as introduced to organometallic chemistry by one of us in the early 1990s (Frank, 1990a,b,c). The title compounds are the first main-group metal-prehnitene complexes. Strictly anhydrous conditions are needed for successful syntheses from the ternary halides SnMCl 5 (= (SnCl) [MCl 4 ]; M = Al, Ga; Schloots & Frank, 2016) and prehnitene (1,2,3,4-tetramethylbenzene) in the inert solvent chlorobenzene and for the subsequent crystallization.

Structural commentary
The asymmetric units of the isotypic compounds 1 and 2 consist of one half of a Sn 2 Cl 2 2+ moiety close to a centre of inversion, one [MCl 4 ] À moiety and one prehnitene molecule, all in general positions. As shown in Fig. 1, these components define one half of the centrosymmetric building block that represents the crystallographic repeating unit of a coordination-polymeric chain in which [{1,2,3,4-(CH 3 ) 4 C 6 H 2 } 2 Sn 2 Cl 2 ] 2+ cations are connected by two [MCl 4 ] À anions in a 1 2 Cl,Cl 2 :3Cl 3 -bridging mode. Bond lengths within the dimeric chloridostannylene cation (in direct comparison, Sn-Cl bond lengths and selected further geometric details of the bonding situation at the tin central atoms of 1 and 2 are given in Table 1) (Schmidbaur et al., 1987)] are as expected, taking into account the mode of association of these species in the polymeric chains. For 1, a section of this chain involving three repeating units is displayed in Fig. 2. Considering the dimensions of the repeating unit along the chain concatenation direction [010] and the orientation of the SnÁ Á ÁSn i connection line with respect to this direction, the secondary structure of 1 and 2 established by the mode of concatenation differs principally from all other related structures apart from that of the mesitylene derivative. However, for this derivative the tertiary structure established by the arrangement of columns is entirely different. A more detailed discussion of the packing is given in Section 3.
Two primary and three secondary bonded chlorine atoms of the dimeric cation and the metallate anions, respectively, as Asymmetric units of the crystal structures of 1 (top) and 2 (bottom) displaying the atom-labelling schemes, and in transparent mode the symmetry-related second half completing the dimeric building block that defines the repeating units of the coordination polymeric, secondary structure of the compounds[(symmetry code (i) 1 À x, Ày, 1 À z]. The direction of secondary bonding to atoms of the neighbouring moieties is indicated by thin sticks. Displacement ellipsoids are drawn at the 50% probability level, hydrogen atoms are drawn with an arbitrary radius. well as one -coordinating prehnitene molecule establish the coordination sphere around the Sn II central atom (Fig. 3).
Considering the arene -ligand as occupying one coordination site only, the coordination number is six. The range of cis-Cl-Sn-Cl angles [1: 66.360 (18)-120.01 (2) ; 2: 67.82 (2)-121.37 (3) ] is far from allowing the coordination to be described as octahedral, and in our feeling a description aspentagonal bipyramidal with the arene ligand and Cl2 in axial position [Cnt arene -Sn-Cl 160.856 (15) (1) and 161.137 (18) (2)] and with the probable equatorial position of the stereochemically active 5s 2 lone pair between Cl3 and Cl4 ii [Cl3-Sn-Cl4 ii 120.01 (2) (1) and 121.37 (3) (2)] is much more appropriate. This fits to the observation that the best plane of the arene atoms C1 to C6 at this side of the coordination sphere is more tilted away from this probable lone pair position in the plane defined by Sn1, Cl3 and Cl4 ii [27.50 (8) (1) and 26.98 (8) (2)] than from the equatorial ligands Cl1 and Cl1 i at the opposite side [1: 15.59 (11) , 2: 15.69 (9) ]. As documented in the two sections of Fig. 3, the tin--prehnitene bonding is characterized by the non-methyl-substituted arene C atoms positioned closest to the Sn II central atom, by a significant ring slippage (1 and 2: 0.37 Å ) also indicated by the dispersion of Sn-C distances [1: 2.881 (2)-3.216 (2) Å ; 2: 2.891 (3)-3.214 (3) Å ], and by the tilt of the plane of the arene ligand against the plane of the central planar (Sn 2 Cl 2 ) 2+ fourmembered ring species as mentioned above. Finally, in the absence of -the transition-metal-specific --arene backbonding, it is not unexpected that no significant influence of the Sn II coordination on the prehnitene six-membered ring geometry is found in comparison with the results of DFT calculations (Becke, 1993) for non-coordinating prehnitene ( Table 1).
The -bonding interaction in 1 and 2 is of medium strength on the overall scale including all types of arene -bonding, but strong on the scale of non-covalent main-group metal-arene bonding, as easily illustrated by the application of the bondvalence method according to the formalism of Brown (2009) in an indirect manner: defining the bond valence of the -arene bonding to the Sn II central atom as s(Sn II -arene) = 2 À AEs(Sn II -Cl) (Frank, 1990a), which gives s(Sn II -arene) = 0.37 and 0.38 valence units for the aluminate and the gallate, respectively. These values are in line with the expectation that [AlCl 4 ] À is the slightly weaker coordinating anion as  Table 1 Selected bond lengths and contact distances (Å ) in 1 and 2 and corresponding ring slippage values and bond valences, calculated using the Brown formalism (Brown, 2009) with r 0 = 2.42 and B = 0.39 (Frank, 1990a). Cnt arene = arene centre; Lsqpl arene = arene plane.. C . . . C bond lengths were calculated on the B3LYP/6-311++G(d,p) level of theory using the GAUSSIAN09 program package (Frisch et al., 2009). (7) 3.1499 (9) Sn1-C6 3.028 (2) 3.043 (3) Symmetry codes: (i) 1 À x, Ày, 1 À z; (ii) 1 À x, 1 À y, 1 À z.

Supramolecular features
As in all {[(arene) 2 Sn 2 Cl 2 ][AlCl 4 ] 2 } x structures described before [arene = benzene, toluene (two polymorphs), p-xylene, mesitylene (see Section 4 and for a detailed comparison; Frank, 1990a), in both 1 and 2 the chains (propagating along [010]) are aligned parallel to each other, resulting in a distorted hexagonal packing of rods. However, taking into account primary, secondary and tertiary bonding, the crystal structure of 1 and 2 is unique. Exemplarily, Fig. 4 shows the packing of 1, mainly characterized by the face-to-face orientation of the prehnitene ligands of neighbouring columns in direction [001]. The orientation of the arene molecules arranged parallel to each other suggests the presence ofinteractions. However, the distance between the best planes of the prehnitene ligands in discussion is greater than 3.6 Å and only 'conventional' van der Waals interactions have to be assumed in this direction. A Hirshfeld analysis of the [(1,2,3,4tetramethylbenzene) 2 Sn 2 Cl 2 ] 2+ moiety (Fig. 5) clearly shows three contact points between (Sn 2 Cl 2 ) 2+ cations and [MCl 4 ] À anions as described above. Additionally, it reveals a weak C-HÁ Á ÁCl interaction between the methyl groups in the 1-and 4positions of the prehnitene ligand and chlorine atoms of the [MCl 4 ] À anions (Tables 2 and 3), as shown in the corresponding fingerprint plot.  Symmetry code: (i) Àx þ 1; Ày þ 1; Àz þ 1.

Figure 4
Distorted hexagonal packing of chains in the crystal of 1 (view direction [010]). The most characteristic feature is the parallel orientation of the planes of neighbouring prehnitene ligands in the [001] direction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The positions of all hydrogen atoms were identified via subsequent difference-Fourier syntheses. In the refinement a riding model was applied using idealized C-H bond lengths [0.94 (CH) and 0.97 (CH 3 ) Å ] as well as H-C-H and C-C-H angles. In addition, the H atoms of the CH 3 groups were allowed to rotate around the neighbouring C-C bonds. The U iso values were set to 1.5U eq (C methyl ) and 1.2U eq (C ar ). program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2018); software used to prepare material for publication: publCIF (Westrip, 2010). 6 -1,2,3,4 6 -1,2,3,4

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Sn1 0.52308 (2) 0.16549 (2)  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.