Formation and ligand-based reductive chemistry of bridged bis-alkylidene scandium(iii) complexes

Bridged bis-alkylidene Sc(iii) complexes featuring a 2-butene-1,1,4,4-tetraanion are synthesized and show unexpected ligand-based two-electron or four-electron reduction reactivity towards different oxidants.


1) Experimental Details and Characterization Data S2
2) Copies of 1

H NMR and 13 C NMR Spectra of All New Compounds S7
3) X-ray Crystallographic Studies S17

4) Mechanistic Investigations for the Formation of 3a S23
In situ 1

H NMR Monitoring the Transformation from 2a to 3a and Alkyne S23
In situ 1

H NMR Monitoring the Transformation from 3a to 2a S23
In situ 1

H NMR Monitoring the Transformation from 3a to Alkyne S24
The Crossover Reaction among 2a and 2a-D 10 S25 Proposed Mechanisms for the Formation of 3a from 2a S26

6) References S43
Electronic Supplementary Material (ESI) for Chemical Science. This journal is © The Royal Society of Chemistry 2017 the reaction mixture was allowed to warm to room temperature. The solvents were removed under vacuum in the glovebox and 1,4-dilithio-1,3-butadienes 1a-c were used for next step without further purification. The pure 1a-D 10 was obtained by extracting the in situ-generated 1a-D 10 with hexane and recrystallization in hexane after filtration of LiI.

General Procedure for the Synthesis of 6
In the glovebox, a THF solution (~10 mL) of tetramethylthiuram disulfide (

3) X-ray Crystallographic Studies
The single crystals of 2a, 3a, 6 and 7 suitable for X-ray analysis were grown as described in experimental section. The crystals were wrapped in mineral oil and then were frozen in low temperature.
Data collections were performed on a SuperNova diffractometer, using graphite-monochromated Mo K radiation (λ = 0.71073 Å). The structures were solved with the shelxs-97 3 or Olex2 4 and refined with the XL refinement package using Least Squares minimization. Refinement was performed on F 2 anisotropically for all the non-hydrogen atoms by the full-matrix least-squares method. The hydrogen atoms were placed at the calculated positions and were included in the structure calculation without further refinement of the parameters. Disordered solvent in 2a was squeezed by using Platon. 5

4) Mechanistic Investigations for the Formation of 3a
In situ 1 H NMR Monitoring the Transformation from 2a to 3a and Alkyne. A J.Young valve NMR tube containing THF-d8 solution of pure 2a (38.0 mg, 0.05 mmol) was injected into NMR spectrum spectrometer at 25 o C, and the 1 H NMR spectra was recorded (SFigure 6 above). The 1 H NMR spectra showed no obvious change for 2 weeks at 25 o C. When the NMR tube was heated over 80 o C for 10 min or 45 o C for 3 h in an oil bath, the peak at -0.38 ppm completely disappeared and two new singlets integrated to the same numbers of protons appeared at -0.23 ppm and 0.20 ppm (SFigure 6 bottom). The singlet at 0.20 ppm was assigned to the TMS proton resonance of Ph-C≡C-TMS by comparing with its standard spectrum. The GC retention time and molecular ion peak (m/z = 174) detected by GC-MS are also consistent with the standard sample of Ph-C≡C-TMS. The other new singlet at -0.23 ppm was assigned to the TMS groups of complex 3a. SFigure 6. In situ 1 H NMR spectra of the transformation from 2a (above) to 3a + alkyne (bottom). The transformation was observed by the signals of TMS groups of each compound. injected into NMR spectrum spectrometer at 25 o C, and the 1 H NMR spectra was recorded (SFigure 7 above). When 2 equiv of hexachloroethane (9.5 mg, 0.04 mmol) were added into the NMR tube at 25 o C, the peak at -0.23 ppm completely disappeared and a new singlet appeared at -0.38 ppm which was assigned to the TMS signal of complex 2a (SFigure 7 bottom). The conversion ratio (70%) was based on the internal standard ferrocene. The formation of ScCl3 can be characterized as ScCl3(THF)3 determined by X-ray analysis after post processing, cell parameters: a = 8.2062(6)Å, b = 12.5280(9)Å, c = 16.6537Å, α = 90°, β = 93.040(8)°, γ = 90°, its crystal structure have been already reported 7 and thus its crystal structure is not illustrated. The formation of tetrachloroethylene can be identified by the 13 C NMR spectrum and GC-MS.

SFigure 7.
In situ 1 H NMR spectra of the transformation from 3a to 2a by adding hexachloroethane. The conversion ratio was based on the internal standard ferrocene.
In situ 1 H NMR Monitoring the Transformation from 3a to Alkyne. A J.Young valve NMR tube containing THF-d8 solution of pure 3a (36.5 mg, 0.02 mmol) with ferrocene as an internal standard was injected into NMR spectrum spectrometer at 25 o C, and the 1 H NMR spectra was recorded (SFigure Two molecules of 2a will undergo β,β′-C−C bond cleavage to generate scandacyclopropene by release of two equiv of alkyne, and then two molecules of scandacyclopropene undergo dimerization to generate 3a.

SScheme 2.
Proposed mechanisms for the formation of 3a from 2a.

5) Details of DFT Calculations
All calculations were carried out with the GAUSSIAN 09 program package. 8 The optimization structure and correction energy of all the minima and transition states were fully calculated at the B3LYP level 9 using the LANL2DZ (for Sc) and 6-31+G(d) (for other elements) in gas phase. Gibbs free energies in the text were given in Hartree. All distances were given in Å.