Molybdenum(VI) Complexes of a 2,2′-Biphenyl-bridged Bis(amidophenoxide): Competition between Metal-Ligand and Metal-Amidophenoxide π Bonding

The 2,2'-biphenyl-bridged bis(2-aminophenol) ligand 4,4'-di-tert-butyl-N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-2, 2'-diaminobiphenyl ((t)BuClipH(4)) reacts with MoO2(acac)(2) to form ((t)BuClipH(2))MoO2, where the diarylamines remain protonated and bind trans to the terminal oxo groups. This complex readily loses water on treatment with pyridine or 3,5-lutidine to form mono-oxo complexes ((t)BuClip)MoO(L), which exhibit predominantly a cis-beta geometry with an aryloxide trans to the oxo group. Exchange of the pyridine ligands is rapid and takes place by a dissociative mechanism, which occurs with retention of stereochemistry at molybdenum. Oxo-free alkoxide complexes ((t)BuClip)Mo(OR)(2) are formed from ((t)BuClipH(2))MoO2 and ROH. Treatment of NMo((OBu)-Bu-t)(3) with (t)BuClipH(4) results in complete deprotonation of the bis(aminophenol) and formation of a dimolybdenum complex ((t)BuClip)Mo(mu-N)(mu-NH2)Mo((t)BuClip) containing both a bridging nitride (Mo-N = 1.848 angstrom, mo-N-Mo = 109.49 degrees) and a bridging amide group. The strong pi bonding of this bis(amidophenoxide) ligand allows the molybdenum center to interconvert readily among species forming three, two, one, or zero pi bonds from multiply bonded ligands.