Mechanistic Elucidation of the Stepwise Formation of a Tetranuclear Manganese Pinned Butterfly Cluster via N–N Bond Cleavage, Hydrogen Atom Transfer, and Cluster Rearrangement

A mechanistic
pathway for the formation of the structurally characterized
manganese-amide-hydrazide pinned butterfly complex, Mn4(μ3‑PhN-NPh-κ3N,N′)2­(μ‑PhN-NPh-κ2-N,N′)­(μ‑NHPh)2L4 (L = THF, py), is proposed and supported by
the use of labeling studies, kinetic measurements, kinetic competition
experiments, kinetic isotope effects, and hydrogen atom transfer reagent
substitution, and via the isolation and characterization of intermediates
using X-ray diffraction and electron paramagnetic resonance spectroscopy.
The data support a formation mechanism whereby bis­[bis­(trimethylsilyl)­amido]­manganese­(II)
(Mn­(NR2)2, where R = SiMe3) reacts
with N,N′-diphenyl­hydrazine
(PhNHNHPh) via initial proton transfer, followed by reductive N–N
bond cleavage to form a long-lived MnIV imido multinuclear
complex. Coordinating solvents activate this cluster for abstraction
of hydrogen atoms from an additional equivalent of PhNHNHPh resulting
in a Mn­(II)­phenylamido dimer, Mn2­(μ‑NHPh)2­(NR2)2L2. This dimeric
complex further assembles in fast steps with two additional equivalents
of PhNHNHPh replacing the terminal silylamido ligands with η1-hydrazine ligands to give a dimeric Mn2­(μ‑NHPh)2­(PhN-NHPh)2L4 intermediate, and
finally, the addition of two additional equivalents of Mn­(NR2)2 and PhNHNHPh gives the pinned butterfly cluster.