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

Digallane [L Ga-GaL ] (1, L =dpp-bian=1,2-[(2,6-iPr C H )NC] C H ) 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 [L (O=C-NR)Ga-Ga(RN-C=O)L ] (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 [L Ga-Ga(AllN- C=O) L ] (5) (All=allyl). In the presence of Na metal, the related digallane [L Ga-GaL ] (2; L =dpp-dad=[(2,6-iPr C H )NC(CH )] ) is converted into the gallium(I) carbene analogue [L Ga:] (2 A), which undergoes a variety of reactions with isocyanate substrates. These include the cycloaddition of ethyl isocyanate to 2 A affording [Na (THF) ]{L Ga[EtN-C(O)] GaL } (6), cleavage of the N=C bond with release of 1 equiv. of CO to give [Na(THF) ] [L Ga(p-MeC H )(N-C(O)) -N(p-MeC H )] (7), cleavage of the C=O bond to yield the di-O-bridged digallium compound [Na(THF) ] [L Ga-(μ-O) -GaL ] (8), and generation of the further addition product [Na (THF) ][L Ga(CyNCO )] (9). Complexes 3-9 have been characterized by NMR ( H, C), IR spectroscopy, elemental analysis, and X-ray diffraction analysis. Their electronic structures have been examined by DFT calculations.