Silicon(i) chemistry: the NHC-stabilised silicon(i) halides Si2X2(Idipp)2 (X = Br, I) and the disilicon(i)-iodido cation [Si2(I)(Idipp)2]+

An efficient method for the synthesis of the NHC-stabilised Si(i) halides Si2X2(Idipp)2 (2-X, X = Cl, Br, I) was developed, which involves the oxidation of Si2(Idipp)2 (1) with 1,2-dihaloethanes. Iodide abstraction from 2-I afforded the unprecedented silicon(i) salt [Si2(I)(Idipp)2][B(C6F5)4] (3).

Static vacuum was applied and the reaction mixture was stirred for one hour at −45 °C, and then warmed to ambient temperature and stirred for another hour at ambient temperature.
The solution was concentrated under vacuum to approximately 5 mL, whereupon a red solid started to precipitate. The suspension was stored at −60 °C for five days to complete crystallisation of 2-Br. The red-orange, microcrystalline solid was collected by filtration at −60 °C and dried under vacuum for two hours at ambient temperature. Compound 2-Br was obtained as a red-orange, extremely air-sensitive, microcrystalline solid. Yield: 1.175 g (1.18 mmol, 98 %).
[S4] The elemental analysis of 2-Br was repeated several times with different samples of 2-Br, which were all tested before to be completely soluble in benzene-d 6 and pure by NMR spectroscopy (cf. Figures S1 -S3). All samples yielded consistently lower C values (by ca. 1 %) probably due to incomplete combustion.

Thermolysis of 2-Br
The thermal behaviour of compound 2-Br in solution was studied by dissolving 10 mg of 2-Br in 0.5 mL C 6 D 6 and recording the 1 H NMR spectra at 25 °C and after heating of the solution at 85 °C for 2 h and 5 h ( Figure S5). The NMR spectra after heating of the solution showed the thermal decomposition of 2-Br (a) to SiBr 2 (Idipp) (b) and Idipp (c) ( Figure S5). The decomposition was accompanied by precipitation of a colorless solid of unknown composition. Figure S5. Si{ 1 H} NMR (59.63 MHz, C 6 D 6 , 298 K, ppm, Figure S9):  Si = 18.7 (s, 2Si).

Determination of the standard Gibbs energy of activation for 3·(C 6 H 5 F)
The thermodynamic values (G ≠ , H ≠ , S ≠ ) of the dynamic process of 3·(C 6 H 5 F) were determined with variable temperature 1 H NMR spectroscopy from 203 K to 263 K ( Figure   S20).

S21
The rate constants (k) were obtained from full line-shape analysis of the C 4,5 -H signals using the NMR simulation program gNMR. [S7] The calculations were performed using standard methods of dynamic NMR spectroscopy. [S8] The rate constants (k) obtained from simulation are given in Table S1. The Eyring plot of ln(k/T) versus 1/T gave a linear fit with R 2 = 0.9966 ( Figure S21). [S9] The activation parameters were obtainedfrom the Eyring plot using the modified Eyring equation (1) and the equations H ≠ = −slope · R, S ≠ = R · (intercept -ln(k B /h)), with ln(k B /h) = 23.760, and the Gibbs-Helmholtz equation (2) [S7] The program gNMR was used for the simulation of the spectra: gNMR, Version 5.0.6.0, P. H. M.
[S9] The program Origin Pro 8G was used for the determination of the thermodynamic parameters and the

Correlation of the Si-Si bond length in base-stabilized Si(I) compounds to the sum of bond angles at the silicon atoms
[S14] [a]: The average value of the sums of angles of the two silicon atoms is given for each compound.

Electronic structure calculations
Structure optimizations were performed without symmetry restraints using the ORCA 3.0.0 programm package or with symmetry restraints using the Turbomole 6.6 programm package, with their internal standard convergence criteria. [S15,S16] The B97-D3 [S17] functionals, including the COSMO-solvation model [S18] for THF and RI-JCOSX approximations (ORCA) or RIJ approximations (Turbomole) [S19, S20] were employed in combination with the def2-TZVP basis set for the Si, N and carbene C atoms, and the def2-SVP basis sets for all peripherical carbon and all hydrogen atoms. [S21] Relativistic effects were approximated for iodine by the ZORA method [S22] in combination with the def2-ZORA-TZVP basis set. [S23] The level of theory employed using the ORCA program was abbreviated with B97-D3/I and that using the Turbomole program with B97-D3/II. The optimized geometries were verified as minima on the potential energy surface by two-sided numerical differentiation of the analytical gradients to obtain harmonic frequencies, which were also used to calculate the zero point vibrational energies (ZPVE). NBO and NRT analyses were performed using NBO6.0. [S24] The cartesian coordinates of the solid state structures of 2-Br and 3 were used as a starting point for the structure optimization. A relaxed potential energy surface scan was performed involving a decrease of the Si2-I distance from 445 to 239 pm in twelve steps to obtain a starting point for the search of the transition state and the other minimum structure 3' calc (-bonded isomer). The obtained transition state 3 TS calc reveals one imaginary frequency at −92 cm −1 , which corresponds to a rocking vibration of the iodine atom interconverting 3 calc and the C 2symmetric -bonded isomer 3' calc .    (3) 1.936 (3) 2.3602 (8) 2.3677 (9) 97.87 (9) 96.74 (9) 103.78 (4) 104.17 (4) 101.42 (9) 102.22 (9) 303 Geometry optimization of (S,S)-2-Br was carried out using the ORCA program at the B97-D3/I level of theory (vide supra) and gave a C 1 -symmetric structure with bonding parameters very close to those of a C 2 symmetric structure. The geometry optimization of (S,S)-2-Br was repeated using the Turbomole program package at the B97-D3/II level of theory with and without a symmetry restriction to C 2 to elucidate the difference in energy of the two structures. The two structures were found to be isoenergetic suggesting that the package. The C i -symmetric minimum structure was found to be less stable by 32.6 kJ mol −1 than the C 1 -symmetric structure. The calculated energy difference between the C 1 -symmetric minimum structures of the (S,S) and the (R,S) stereoisomers was found to be 57.4 kJ mol −1 (ORCA, B97-D3/I) and 48.8 kJ mol −1 (Turbomole, B97-D3/II), respectively.    (6) 2.5916 (6) 97.04 (7) 103.42 (7) 97.94 (7) 103.90 (9) (7) 112.83 (7) 104.56 (7) 96.61 (7) 142.27 (3) 96.69 (7) 95.78 (7 Table S7: Selected results of the natural bond orbital (NBO) and natural resonance theory (NRT) analyses of (S,S)-2-Br calc (B97-D3/I). Atom numbering of the experimental structure was taken over in the calculated structure. [a] NBO analysis NPA partial charges [b] NRT analysis [c] occ.  Table S8: Selected results of the natural bond orbital (NBO) and natural resonance theory (NRT) analyses of 2-I calc (B97-D3/I). Atom numbering of the experimental structure was taken over in the calculated structure 2-I calc .