Reactions of Iso(thio)cyanates with Dialanes: Cycloaddition, Reductive Coupling, or Cleavage of the CS or CO Bond

The dialanes [(dpp-Bian)­Al–Al­(dpp-Bian)] (1) and [(dpp-dad)­Al­(THF)–(THF)­Al­(dpp-dad)] (2) (dpp-Bian = 1,2-[(2,6-iPr2C6H3)­NC]2C12H6, dpp-dad = [(2,6-iPr2C6H3)­NC­(CH3)]2) react with some isothiocyanates, isocyanates, and diphenylketene via [2 + 4] cycloaddition of the CO or CS bond across the CCNAl fragment to afford complexes [L­(XCY)­AlAl­(XCY)­L] with an intact Al–Al single bond (3, L = dpp-Bian, X = PhN, Y = O; 4, L = dpp-Bian, X = Ph2C, Y = O; 6, L = dpp-dad, X = BnN, Y = S; 7, L = dpp-dad, X = tBuN, Y = O; 8, L = dpp-dad, X = iPrN, Y = S; and 9, L = dpp-dad, X = CyN, Y = S). A mixed CN and CO mode cycloadduct, [(dpp-Bian)­(TosNCO)­AlAl­(TosNCO)­(dpp-Bian)] 5, was obtained in the reaction of 1 with tosylisocyanate. Heating the solution of 3 resulted in a thermal transformation and a change of the cycloaddition mode from CO to CN to give the product [(dpp-Bian)­(PhNCO)­Al­(O)­Al­(PhNCO)­(dpp-Bian)] 10. The reduction of 7 and 8 with Na yielded the products [Na­(THF) n ]2[(dpp-dad–H)­(XCY)­Al]2 (12, X = iPrN, Y = S, n = 2 and 13, X = tBuN, Y = O, n = 3) in which one of the methyl groups of the backbone of the initial dpp-dad ligand was dehydrogenated. When 2 was reacted with the bulky adamantyl isocyanate AdNCO, the C–C coupling of two substrates occurred to form 14 [(dpp-dad)­Al­(OCNAd)2Al­(dpp-dad)] in which the coupled dianionic oxamide ligand bridged two Al atoms in a μ,η4-N,O/N,O mode. Moreover, in the presence of 2.0 equiv of Na metal, precursor 2 reacts with tBuNCS, p-TolylNCS, or Me3SiNCO, possibly through the reduced AlI intermediate, to yield the sulfur- or oxygen-bridged dimer [Na­(solv) n ]2[(dpp-dad)­Al­(μ-E)]2 (15, E = S, solv = THF, n = 3 and 16, E = O, solv = DME, n = 2) upon CS or CO bond cleavage. Dialane 1 reacts with dimethylsulfone to give a Lewis adduct [(dpp-Bian)­(Me2SO2)­Al]2 (17), which releases dimethylsulfone upon heating. The diamagnetic compounds 3–10 and 12–17 were characterized by NMR and IR spectroscopy. The molecular structures of 3–17 were established by single-crystal X-ray diffraction analysis. Electronic structures of the compounds and possible isomers have been examined by DFT calculations.