Ethanol oxidation by imidorhenium(V) complexes: formation of amidorhenium(III) complexes

The reaction of Re(NC6H4R)Cl3(PPh3)2 (R = H, 4-Cl, 4-OMe) with 1,2-bis(diphenylphosphino)ethane (dppe) is investigated in refluxing ethanol. The reaction produces two major products, Re(NC6H4R)Cl(dppe)(2)2+ (R = H, 1-H; R = Cl, 1-Cl; R = OMe, 1-OMe) and the rhenium(III) species Re(NHC6H4R)Cl(dppe)2+ (R = H, 2-H; R = Cl, 2-Cl). Complexes 1-H (orthorhombic, Pcab, a = 22.3075(10) A, b = 23.1271(10) A, c = 23.3584(10) A, Z = 8), 1-Cl (triclinic, P1, a = 11.9403(6) A, b = 14.6673(8) A, c = 17.2664(9) A, alpha = 92.019(1) degrees, beta = 97.379(1) degrees, gamma = 90.134(1) degrees, Z = 2), and 1-OMe (triclinic, P1, a = 11.340(3) A, b = 13.134(4) A, c = 13.3796(25) A, alpha = 102.370(20) degrees, beta = 107.688(17) degrees, gamma = 114.408(20) degrees, Z = 1) are crystallographically characterized and show an average Re-N bond length (1.71 A) typical of imidorhenium(V) complexes. There is a small systematic decrease in the Re-N bond length on going from Cl to H to OMe. Complex 2-Cl (monoclinic, Cc, a = 24.2381(11) A, b = 13.4504(6) A, c = 17.466(8) A, beta = 97.06900(0) degrees, Z = 4) is also crystallographically characterized and shows a Re-N bond length (1.98 A) suggestive of amidorhenium(III). The rhenium(III) complexes exhibit unusual proton NMR spectra where all of the resonances are found at expected locations except those for the amido protons, which are at 37.8 ppm for 2-Cl and 37.3 ppm for 1-H. The phosphorus resonances are also unremarkable, but the 13C spectrum of 2-Cl shows a significantly shifted resonance at 177.3 ppm, which is assigned to the ipso carbon of the phenylamido ligand. The extraordinary shifts of the amido hydrogen and ipso carbon are attributed to second-order magnetism that is strongly focused along the axially compressed amido axis. The reducing equivalents for the formation of the Re(III) product are provided by oxidation of the ethanol solvent, which produces acetal and acetaldehyde in amounts as much as 30 equiv based on the quantity of rhenium starting material. Equal amounts of hydrogen gas are produced, suggesting that the catalyzed reaction is the dehydrogenation of ethanol to produce acetaldehyde and hydrogen gas. Metal hydrides are detected in the reaction solution, suggesting a mechanism involving beta-elimination of ethanol at the metal center. Formation of the amidorhenium(III) product possibly arises from migration of a metal hydride in the imidorhenium(V) complex.