Mixed amidophenolate-catecholates of molybdenum(VI).

The dioxomolybdenum(vi) complex ((t)BuClipH2)MoO2 ((t)BuClipH4 = 4,4'-di-tert-butyl-N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-2,2'-diaminobiphenyl) reacts with 3,5-di-tert-butylcatechol to form oxo-free ((t)BuClip)Mo(3,5-(t)Bu2Cat). The bis(amidophenoxide)-monocatecholate complex is monomeric and exhibits a cis-β geometry in the solid state. Variable-temperature NMR data are consistent with two fluxional processes, one that interconverts several geometric isomers at low temperature, and a second that interchanges the ends of the (t)BuClip ligand at ambient temperatures. The high-temperature fluxional process can be explained by a single Bailar trigonal twist coupled with atropisomerization of the chiral diaminobiaryl backbone. Addition of excess catechol to ((t)BuClipH2)MoO2 results in formation of a dimolybdenum mono-oxo complex ((t)BuClip)Mo(μ-3,5-(t)Bu2Cat)2Mo(O)(3,5-(t)Bu2Cat). This complex, which contains a seven-coordinate bis(amidophenoxide)molybdenum center and a six-coordinate oxomolybdenum center, represents a structural hybrid between dimeric oxomolybdenumbis(catecholate) and molybdenum tris(catecholate) complexes. Both mono- and dimolybdenum complexes are best formulated as containing Mo(vi), but there is structural evidence for significant π donation from the amidophenolates. ((t)BuClip)Mo(3,5-(t)Bu2Cat) binds pyridine to form a mixture of isomeric seven-coordinate adducts. The Lewis acidity of the mixed amidophenoxide-catecholate appears to be lower than its tris(catecholate) or oxobis(amidophenoxide) analogues, which manifests itself principally in relatively slow binding of pyridine to the six-coordinate complex (k = 8 × 10(4) L mol(-1) s(-1) at 0 °C) rather than in the rate of dissociation of pyridine from the seven-coordinate adduct.