om7b00613_si_001.pdf (8.23 MB)
Experimental and Computational Studies of High-Valent Nickel and Palladium Complexes
journal contribution
posted on 2017-10-10, 17:43 authored by Nicole
M. Camasso, Allan J. Canty, Alireza Ariafard, Melanie S. SanfordThis article describes
a detailed comparison of the organometallic
chemistry of high-valent nickel and palladium model complexes supported
by tris(pyrazolyl)borate and cycloneophyl ligands. The accessibility
of the MIII and MIV oxidation states with each
metal is investigated through electrochemical and chemical oxidation
of the MII precursors. These studies show that the NiII precursor readily undergoes both one- and two-electron oxidations
to generate stable NiIII and NiIV products.
In contrast, under the conditions examined, the PdII analogue
undergoes exclusively two-electron-oxidation reactions to form PdIV. Reactivity studies of isolated NiIV and PdIV complexes show that both participate in C(sp3)–heteroatom coupling reactions and that the reactions at
NiIV are approximately 2 orders of magnitude faster than
those at PdIV. Experimental and computational mechanistic
studies implicate outer-sphere SN2-type pathways for these
processes. With most nucleophiles (e.g., phenoxide, acetate, thiophenoxide),
the C(sp3)–heteroatom coupling reaction yields a
TpMII(σ-aryl) product. However, with azide as the
nucleophile, the NiII product of initial C(sp3)–N3 coupling undergoes a subsequent C(sp2)–N insertion reaction. Computations implicate an anionic
NiIII–nitrene intermediate in this process and show
that the Pd analogue of this species is a much higher energy species.
Overall, the combined experimental and computational studies demonstrate
remarkable similarities in the chemistry of NiIV and PdIV but an enhanced role for NiIII in enabling reactivity
which is distinct from that of palladium.