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, [L1 Ga-GaL1 ] (1, L1 =dpp-dad=[(2,6-iPr2 C6 H3 )NC(CH3 )]2 ) and [L2 Ga-GaL2 ] (2, L2 =dpp-bian=1,2-[(2,6-iPr2 C6 H3 )NC]2 C10 H6 ), 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 C2 N2 Ga metallacycle with formation of C-C and S-Ga single bonds to afford [L1 (RN=C-S)Ga-Ga(S-C=NR)L1 ] (3, R=Me; 4, R=Ph) and [L2 (RN=C-S)Ga-Ga(S-C=NR)L2 ] (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 [L1 Ga(μ-S)2 GaL1 ] (5), [L2 Ga(μ-S)2 GaL2 ] (10), and [Na(DME)3 ][L2 Ga(μ-S)2 GaL2 ] (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 [L1 (PhN=C-S)Ga(μ-S)2 Ga(S-C=NPh)L1 ] (6) and [L1 (PhN=C-S)Ga(μ-S)Ga(S-C=NPh)L1 ] (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.