A Genuine Stannylone with a Monoatomic Two‐Coordinate Tin(0) Atom Supported by a Bis(silylene) Ligand

Abstract The monoatomic zero‐valent tin complex (stannylone) {[SiII(Xant)SiII]Sn0} 5 stabilized by a bis(silylene)xanthene ligand, [SiII(Xant)SiII=PhC(NtBu)2Si(Xant)Si(NtBu)2CPh], and its bis‐tetracarbonyliron complex {[SiII(Xant)SiII]Sn0[Fe(CO)4]2} 4 are reported. The stannylone 5 bearing a two‐coordinate zero‐valent tin atom is synthesized by reduction of the precursor 4 with potassium graphite. Compound 4 results from the SnII halide precursor {[SiII(Xant)SiII]SnIICl}Cl 2 or {[SiII(Xant)SiII]SnBr2} 3 through reductive salt‐metathesis reaction with K2Fe(CO)4. According to density functional theory (DFT) calculations, the highest occupied molecular orbital (HOMO) and HOMO‐1 of 5 correspond to a π‐type lone pair with delocalization into both adjacent vacant orbitals of the SiII atoms and a σ‐type lone pair at the Sn0 center, respectively, indicating genuine stannylone character.


A1. General Considerations
All experiments were carried out under dry oxygen-free nitrogen using standard Schlenk techniques or MBraun glove box fitted with a gas purification and recirculation unit. Solvents were dried by standard methods and freshly distilled prior to use. Potassium graphite (KC8) was prepared by reacting potassium with previously dried graphite in a 1:8 molar ratio at 160 °C for 2 h under dried nitrogen. Bis(NHSi)xanthene Si II (Xant)Si II 1 [1] [Si II (Xant)Si II = PhC(NtBu)2Si(Xant)Si(NtBu)2CPh] and K2Fe(CO)4 [2] were synthesized according to reported procedures. The solution NMR spectra were recorded on Bruker Spectrometers AV 200, 400 or 500 with residual solvent signals as internal reference ( 1 H NMR: D6-Benzene, 7.16 ppm; 13 C{ 1 H} NMR: D6-Benzene, 128.06 ppm) or external standards ( 29 Si{ 1 H} NMR: SiMe4, 0.0 ppm; 119 Sn{ 1 H} NMR: SnMe4, 0.0 ppm). The following abbreviations were used to describe peak patterns when appropriate: br = broad, s = singlet, d = doublet, t = triplet, dd = doublet of doublets, m = multiplet. Elemental analyses were performed by the analytical labor service in the Institute of Chemistry, Technical University of Berlin, Germany. High-resolution ESI-MS were measured on a Thermo Scientific LTQ orbitrap XL. UV/Vis spectra were recorded on an Analytik Jena Specord S600 diode array spectrometer. IR spectra were measured with a Nicolet iS5 FT-IR-Spectrometer from the company Thermo.

A2. Single-Crystal X-ray Structure Determination
Crystals were each mounted on a glass capillary in perfluorinated oil and measured in a cold N2 flow. The data of all compounds were collected on an Oxford Diffraction SuperNova, Single source at offset, Atlas at 150 K (Cu-Kα radiation, λ = 1.54184 Å). The structures were solved by direct methods and refined on F 2 with the SHELX-97 software package. [3] For the crystal of compound 4, the strongly disordered C6H6 molecules are treated using Solvent Masking in Olex2. In the molecular structure of compound 5 which contains two independent molecules a and b, one of the tert-butyl groups in molecule a is disordered over two orientations with an approximate occupancy ratio of 0.75:0.25; two of the tert-butyl groups in molecule b are disordered over two orientations with an approximate occupancy ratio of 0.75:0.25 and 0.72:0.28, respectively. CCDC: 2110196 (2), 2110197 (3), 2110198 (4) and 2110199(5) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures/

Synthesis of Compound 2.
To a mixture of bis(NHSi)xanthene 1 (728 mg, 1 mmol) and SnCl2(dioxane) (277 mg, 1 mmol) in a 50 mL Schlenk flask was added 25 mL Et2O at room temperature under stirring. A bright yellow precipitate formed slowly. After stirring overnight, the bright yellow precipitate was separated by filtration and dried under vacuum affording compound 2 as a yellow solid (780 mg, 85% isolated yields). Yellow crystals suitable for X-ray analysis were obtained from a concentrated toluene solution at -30 o C.

Synthesis of Compound 3.
To a mixture of bis(NHSi)xanthene 1 (728 mg, 1 mmol) and SnBr2(dioxane) (467 mg, 1 mmol) in a 50 mL Schlenk flask was added 25 mL Et2O at room temperature under stirring. A bright yellow precipitate formed slowly. After stirring overnight, the bright yellow precipitate was separated by filtration and dried under vacuum affording compound 3 as yellow solid (734mg, 72% isolated yields). Yellow crystals suitable for X-ray analysis were obtained from a concentrated Et2O solution at -30 o C.

Synthesis of Compound 4. Reduction of 2:
To a mixture of compound 2 (917 mg, 1 mmol) and K2Fe(CO)4 (295.2mg, 1.2 mmol) in a 100 mL Schlenk flask was added 60 mL THF at room temperature under stirring. The color of the mixture changed to red immediately. After stirring overnight, the red mixture was filtered and the residue was washed with THF (10 mL x 2). Volatiles were removed under vacuum and the residue was washed with Et2O (10 mL) to afford compound 4 as a red powder after dried under vacuum (496 mg, 42% isolated yield). Red crystals suitable for X-ray analysis were obtained from a concentrated benzene solution at room temperature.

Reduction of 3:
To a mixture of compound 3 (100.7 mg, 0.1 mmol) and K2Fe(CO)4 (29.52mg, 0.12 mmol) in a 25 mL Schlenk flask was added 10 mL THF at room temperature under stirring. The color of the mixture changed to red immediately. After stirring overnight, the red mixture was filtered and the residue was washed with THF (2 mL x 2). Volatiles were removed under vacuum and the residue was washed with Et2O (3 mL) to afford compound 4 as a red powder after dried under vacuum (47 mg, 40% isolated yield).

Synthesis of Compound 5.
To a mixture of compound 4 (591.5 mg, 0.5 mmol) and KC8 (276.75 mg, 2.05 mmol) in a 100 mL Schlenk flask was added 50 mL THF at room temperature under stirring. The color of the mixture from red changed to dark blue slowly. After stirring 6 h, the dark blue mixture was filtered and the black residue of graphite was washed with THF (10 mL x 3). The volatiles were removed under vacuum from the combined filtrate to give blue residue. Then hexane (80 mL) was introduced to the residue, the resulting solution was separated from the precipitates by filtration. The volatiles were removed under vacuum to afford almost pure compound 5 as a dark blue powder (474.32mg, 56% isolated yields). Dark blue crystal suitable for X-ray analysis was obtained from a saturated isopropyl ether solution at -20 o C more than 5 days. Reaction of 5 with Fe2(CO)9. To a mixture of compound 5 (42.4 mg, 0.05 mmol) and Fe2(CO)9 (18.2mg, 0.05 mmol) was added 20 mL THF at room temperature under stirring. The color of the mixture changed to red immediately. After stirring overnight, volatiles were removed under vacuum and the residue was washed with Et2O (10 mL) to afford compound 4 (47.3 mg, 80% isolated yields).                  Absolute structure parameter -0.026 (7) Extinction coefficient n/a Largest diff. peak and hole 1.059 and -1.553 e.Å -3 Figure S23. Molecular structure of compound 2. Thermal ellipsoids are drawn at the 50% probability level. H atoms and solvent (toluene) molecules are omitted for clarity. Table S2. Selected interatomic distances and angles of compound 2.

B. DFT Calculations
Computational details. All the DFT calculations were performed with Gaussian 16 (Revision A.03) program. [4] All structures were optimized at the PEB0 [5] -D3BJ [6] /Def2-SVP [7] ∼ma-TZVP [8] level of theory in the gas phase due to the smallest relative mean deviation (RD) of structural parameters in comparison with the experimental structure. No imaginary frequency was obtained at the same level, confirming a local minima. All presented principal interacting orbital (PIO) analyses [9] were performed by NBO 7.0 program [10] at the same level based on the optimized structure. Natural adaptive orbital (NAdO) [11] analyses were carried out by the Multiwfn program. [12] All orbitals were plotted with the help of Multiwfn and VMD programs. [13] Gauge-Independent Atomic Orbital (GIAO) calculation. The B97-2 [14] /Def2-TZVP (for all toms except Sn atom) [7] ∼Sapporo-DKH3-DZP-2012-diffuse (for Sn atom) [15] method is used to calculate the 119

C. TD-DFT Calculations
Computational details. Time dependent density functional theory (TD-DFT) were performed to calculate UV-vis spectra, under TD-B3LYP-D3BJ/Def2-SVP∼ma-TZVP level including solvent effect in a SMD continuum model (solvent= toluene), [16] due to the good agreement with experimental UV-vis spectra.
Results of UV-vis calculation：According to the TD-DFT calculation, the observed peak (ex.=674 nm, TD-DFT.=672 nm, Figure S31) is mainly assigned to the S1 state (Table S13), which corresponds to the -* excitation from HOMO to LUMO orbital (Table S13). Figure S31. TD-DFT absorption spectra for 5.