Part 1: Molybdenum Amidophenolates and Catecholates for Nonclassical Oxygen Activation and Atom Transfer Reactions / Part 2: Silicon-Carbon Bond Activation in Aryloxy-Iminoquinones and Enhanced Reactivity Over Tin Analogues

Part 1: The tetradentate ligand tBuClipH4 (tBuClipH4 = 4,4'-di-tert-butyl-N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-2,2'-diaminobiphenyl) forms molybdenum(VI) complexes with a varying number (0-2) of terminal oxo groups. The doubly deprotonated cis-dioxo complex (tBuClipH2)MoO2 readily loses water to form a series of monooxo complexes (tBuClip)MoO(L) in the presence of donor ligands. (tBuClipH2)MoO2 also reacts with 3,5-di-tert-butylcatechol to form oxo-free (tBuClip)Mo(3,5-tBu2Cat). The facile formation and interconversion of these species highlights the ability of the amidophenoxide ligand to stabilize the high-valent molybdenum metal center by strong π-donation. (tBuClip)Mo(3,5-tBu2Cat) has lower Lewis acidicity compared to oxo and catecholate analogues (tBuClip)MoO, Mo(3,5-tBu2Cat)3, and (3,5-tBu2Cat)2MoO, which is apparent from rates of pyridine dissociation. These amidophenolate ligands enable oxygen atom abstraction by the molybdenum(VI) in a ?nonclassical? fashion, as the required reducing equivalents for oxygen reduction are drawn from ligand oxidation and not the molybdenum center. This generates a reactive intermediate, tentatively identified as (tBuClipSQ)MoO2, with oxidizing equivalents stored in a bis-semiquinone ligand. (tBuClipSQ)MoO2 can be deoxygenated by dimethylphenylphosphine, establishing a closed loop cycle for catalytic oxygen atom transfer.

Part 2: Silylation of the oxidized ligand in lead(II) bis(3,5-di-tert-butyl-1,2-quinone-(3,5-di-tert-butyl-2-oxy-1-phenyl)imine), Pb(ONOQ)2, with chlorosilanes RSiX2Cl (R = Me, Ph; X = Me, Ph, Cl) results in tetracyclic, pentacoordinate silicon compounds X(Y)Si(ON[R]O) in which the aryloxyiminoquinone ligand is irreversibly reduced. The trigonal bipyramidal silicon products of migration in some cases form kinetic isomers, R(Cl)Si(ON[R]O) with an equatorial chlorine substituent, which isomerize to their thermodynamically stable stereoisomers, Cl(R)Si(ON[R]O), in which the chlorine is axial. The kinetic stereoselectivity in these reactions is determined not only by relative barriers of migration from isomeric octahedral silicon intermediates, but also from the reversibility in isomerization of κ3-R(X)(Y)Si(ONOQ) isomers via the tetrahedral κ1-R(X)(Y)Si(ONOQ). In reaction of Me3MCl (M = Si or Sn) with H(DOPOQ) (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate), tetrahedral intermediates κ1-Me3M(ONOQ) can be isolated or spectroscopically identified. The rates of methyl migration from these intermediates indicates that migration from tetrahedral tin is 10^4 times faster than migration from silicon. In stark contrast, methyl migration from silicon in the octahedral intermediate κ3-MeCl2Si(DOPOQ) is at least 10^7 time faster than migration from tin. The enhanced reactivity of the silicon-carbon bond in monomethyl compounds results from the increased stability of the κ3 intermediate realtive to κ1. This inference can used to the drive the desired reactivity of silicon and tin.