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
(tBuClipH4) reacts with MoO2(acac)2 to form (tBuClipH2)­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 (tBuClip)­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 (tBuClip)­Mo­(OR)2 are formed from (tBuClipH2)­MoO2 and ROH. Treatment of NMo­(OtBu)3 with tBuClipH4 results in complete deprotonation of the
bis­(aminophenol) and formation of a dimolybdenum complex (tBuClip)­Mo­(μ-N)­(μ-NH2)­Mo­(tBuClip) 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.