Cycloaddition versus Cleavage of the C=S Bond of Isothiocyanates Promoted by Digallane Compounds with Noninnocent α-Diimine Ligands.

Whereas the chemistry of single-bond activation by compounds of the main group elements has undergone some development in recent years, the cleavage of multiple bonds remains underexplored. Herein, the reactions of two digallanes bearing α-diimine ligands, namely, [L ¹ Ga-GaL ¹ ] (1, L ¹ =dpp-dad=[(2,6-iPr ₂ C ₆ H ₃ )NC(CH ₃ )] ₂ ) and [L ² Ga-GaL ² ] (2, L ² =dpp-bian=1,2-[(2,6-iPr ₂ C ₆ H ₃ )NC] ₂ C ₁₀ H ₆ ), with isothiocyanates are reported. Reactions of 1 or 2 with isothiocyanates in 1:2 molar ratio proceeded with [2+4] cycloaddition of the C=S bond across the C ₂ N ₂ Ga metallacycle with formation of C-C and S-Ga single bonds to afford [L ¹ (RN=C-S)Ga-Ga(S-C=NR)L ¹ ] (3, R=Me; 4, R=Ph) and [L ² (RN=C-S)Ga-Ga(S-C=NR)L ² ] (8, R=allyl; 9, R=Ph). In the cases of 8 and 9, this cycloaddition is reversible. The digallanes reacted with 2 equiv of PhNCS in the presence of Na metal or at high temperatures through a unique reductive cleavage of the C=S bond to yield the disulfide-bridged digallium species [Na(THF) ₃ ] ₂ [L ¹ Ga(μ-S) ₂ GaL ¹ ] (5), [L ² Ga(μ-S) ₂ GaL ² ] (10), and [Na(DME) ₃ ][L ² Ga(μ-S) ₂ GaL ² ] (11). Moreover, products 4 and 5 can further react with an excess of isothiocyanate, through cleavage of the C=S bond or cycloaddition, to give the bis- or mono-S-bridged complexes [Na(THF) ₂ ] ₂ [L ¹ (PhN=C-S)Ga(μ-S) ₂ Ga(S-C=NPh)L ¹ ] (6) and [L ¹ (PhN=C-S)Ga(μ-S)Ga(S-C=NPh)L ¹ ] (7). All the newly prepared compounds were characterized by elemental analysis, single-crystal X-ray diffraction, IR spectroscopy, NMR (3-9) or ESR spectroscopy (11), and DFT calculations.
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