Gallium "Shears" for C=N and C=O Bonds of Isocyanates

Digallane [L1 Ga-GaL1 ] (1, L1 =dpp-bian=1,2-[(2,6-iPr2 C6 H3 )NC]2 C12 H6 ) reacts with RN=C=O (R=Ph or Tos) by [2+4] cycloaddition of the isocyanate C=N bonds across both of its C=C-N-Ga fragments to afford [L1 (O=C-NR)Ga-Ga(RN-C=O)L1 ] (R=Ph, 3; R=Tos, 4). The reactions with both isocyanates result in new C-C and N-Ga single bonds. In the case of allyl isocyanate, the [2+4] cycloaddition across one C=C-N-Ga fragment of 1 is accompanied by insertion of a second allyl isocyanate molecule into the Ga-N bond of the same fragment to afford compound [L1 Ga-Ga(AllN- C=O)2 L1 ] (5) (All=allyl). In the presence of Na metal, the related digallane [L2 Ga-GaL2 ] (2; L2 =dpp-dad=[(2,6-iPr2 C6 H3 )NC(CH3 )]2 ) is converted into the gallium(I) carbene analogue [L2 Ga:]- (2 A), which undergoes a variety of reactions with isocyanate substrates. These include the cycloaddition of ethyl isocyanate to 2 A affording [Na2 (THF)5 ]{L2 Ga[EtN-C(O)]2 GaL2 } (6), cleavage of the N=C bond with release of 1 equiv. of CO to give [Na(THF)2 ]2 [L2 Ga(p-MeC6 H4 )(N-C(O))2 -N(p-MeC6 H4 )]2 (7), cleavage of the C=O bond to yield the di-O-bridged digallium compound [Na(THF)3 ]2 [L2 Ga-(μ-O)2 -GaL2 ] (8), and generation of the further addition product [Na2 (THF)5 ][L2 Ga(CyNCO2 )]2 (9). Complexes 3-9 have been characterized by NMR (1 H, 13 C), IR spectroscopy, elemental analysis, and X-ray diffraction analysis. Their electronic structures have been examined by DFT calculations.