An indenide-tethered N-heterocyclic stannylene

Analysis of the coordination of the Sn to the indenyl ring shows that the Sn interacts in an η2 fashion. A database survey showed that whilst this coordination mode is unusual for Ge and Pb compounds, Sn displays a wider range of coordination modes to cyclopentadienyl ligands and their derivatives.


Chemical context
N-heterocyclic stannylenes (NHSns) are the tin analogues of N-heterocyclic carbenes (NHCs). With an unsaturated backbone, they have been found to be thermally unstable (Gans-Eichler et al., 2002, Gans-Eichler et al. 2006), but with a saturated backbone they are thermally robust (Mansell et al., 2008) and show interesting binding properties including a higher propensity for bridging coordination modes (Mansell et al., 2011).
We have sought to install NHSns into a tethered ligand system using a fluorenyl group linked to the NHSn with a C 2 H 4 linker, but this resulted in dimeric species with Sn-N dative bonding, even upon addition of suitable Rh salts (Roselló -Merino & Mansell, 2016). In this contribution we analyse the crystal structure of a monomeric NHSn with an indenyl donor group.

Structural commentary
The crystal structure of the title compound 2 shows a deprotonated indenide moiety connected to a diamidostannylene unit via a C 2 H 4 linker. The lithium cation is bound to the less sterically hindered N atom [Li-N = 2.043 (7) Å ], as well as to the 12-crown-4 tetradentate ether ligand (Fig. 1) (Atwood et al., 1981). The formation of 2 shows that the soft NHSn lone pair does not ISSN 2056-9890 interact with the relatively hard Li cation, unlike the situation in the lithium complexes of tethered NHCs previously published .
By surveying the coordination of cyclopentadienyl ligands to main group atoms using the CSD (Version 5.40, update of August 2019; Groom et al., 2016), we can clearly see the flexible coordination modes of tin compared to other group 14 metals. The position of the metal was projected onto the plane of the Cp ring and these datapoints were expanded according to C 5v symmetry (i.e. there are ten symmetry-equivalent data points for each crystal structure). The results are shown in Fig. 2a-c for germanium, tin and lead, respectively. Germanium and lead are almost always projected near the centre of the Cp ring; however, tin shows a wide range of projection points. The datapoints for this structure are displayed in red in Fig. 2b, showing the distinct interaction with two carbon centres, a unique coordination mode for group 14 metals.

Synthesis and crystallization
Synthesis of [Sn{(N,N 0 0 0 -j 2 -(C 9 H 7 )C 2 H 4 NC 2 H 4 N(2,6-i Pr 2 - To a solution of (C 9 H 7 )C 2 H 4 N(H)C 2 H 4 N(H)(2,6-i Pr 2 C 6 H 3 ) (Roselló -Merino & Mansell, 2016) (330 mg, 0.91 mmol) in THF (5 ml), Sn[N(SiMe 3 ) 2 ] 2 (400 mg, 0.91 mmol) dissolved in THF (2 ml) was added slowly at room temperature under nitrogen in a two-necked-flask in a glovebox. After 2 h, the solvent was removed by pipette and the precipitate was washed five times with 5 ml of petroleum ether by dispersing it and pipetting off the solvent after the residue had settled. Evaporation of the remaining solvent under high vacuum yielded the desired product as a light-yellow solid (348 mg, 0.73 mmol, 80%).  Displacement ellipsoid plot of 2 (shown at the 50% probability level) with all H atoms removed for clarity.

Synthesis of indenide-tethered N-heterocyclic stannylene 2
To 1 (10 mg, 0.03 mmol) in a glass vial under nitrogen in a glovebox was added Li[N(SiMe 3 ) 2 ] (5 mg, 0.03 mmol) in THF (0.5 mL) then 12-crown-4 (11 mg, 0.6 mmol) in THF (0.2 ml). This vial was placed in a freezer, producing a small number of single crystals. Reactions on larger scales led to concentrations that were too high, leading to decomposition processes. The material that was produced was not soluble in d 8 -THF.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. H atoms were positioned geometrically (C-H = 095-1.00 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl).

Funding information
Funding for this research was provided by: Engineering and Physical Sciences Research Council (PhD scholarship to K. J. Evans); Daphne Jackson Trust (award to M. F. Haddow).

Computing details
Data collection: APEX2 (Bruker, 2013); cell refinement: SAINT (Bruker, 2013); data reduction: SAINT (Bruker, 2013); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). 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.