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-<i>tert</i>-butyl-<i>N</i>,<i>N</i>′-bis­(3,5-di-<i>tert</i>-butyl-2-hydroxyphenyl)-2,2′-diaminobiphenyl (^<i>t</i>BuClipH₄) reacts with MoO₂(acac)₂ to form (^<i>t</i>BuClipH₂)­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 (^<i>t</i>BuClip)­MoO­(L), which exhibit predominantly a cis-β 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 (^<i>t</i>BuClip)­Mo­(OR)₂ are formed from (^<i>t</i>BuClipH₂)­MoO₂ and ROH. Treatment of NMo­(O^<i>t</i>Bu)₃ with ^<i>t</i>BuClipH₄ results in complete deprotonation of the bis­(aminophenol) and formation of a dimolybdenum complex (^<i>t</i>BuClip)­Mo­(μ-N)­(μ-NH₂)­Mo­(^<i>t</i>BuClip) containing both a bridging nitride (Mo–N = 1.848 Å, Mo–N–Mo = 109.49°) and a bridging amide group. The strong π bonding of this bis­(amidophenoxide) ligand allows the molybdenum center to interconvert readily among species forming three, two, one, or zero π bonds from multiply bonded ligands