Mononuclear nickel(II) and nickel(III) complexes of a
bisamidate-bisalkoxide
ligand, (NMe4)2[NiII(HMPAB)] (1) and (NMe4)[NiIII(HMPAB)] (2), respectively, have been synthesized and characterized by various
spectroscopic techniques including X-ray crystallography. The reaction
of redox-inactive metal ions (M
n+ = Ca2+, Mg2+, Zn2+, Y3+, and Sc3+) with 2 resulted in 2-M
n+ adducts, which was assessed by an array of spectroscopic
techniques including X-ray absorption spectroscopy (XAS), electron
paramagnetic resonance (EPR), and reactivity studies. The X-ray structure
of Ca2+ coordinated to Ni(III) complexes, 2-Ca2+T, was determined and exhibited an average Ni–Ca
distance of 3.1253 Å, close to the metal ions’ covalent
radius. XAS analysis of 2-Ca2+ and 2-Y3+ in solution further revealed an additional coordination
to Ca and Y in the 2-M
n+ adducts
with shortened Ni–M distances of 2.15 and 2.11 Å, respectively,
implying direct bonding interactions between Ni and Lewis acids (LAs).
Such a short interatomic distance between Ni(III) and M is unprecedented
and was not observed before. EPR analysis of 2 and 2-M
n+ species, moreover, displayed
rhombic signals with g
av > 2.12 for
all
complexes, supporting the +III oxidation state of Ni. The NiIII/NiII redox potential of 2 and 2-M
n+ species was determined, and a plot
of E
1/2 of 2-M
n+ versus pK
a of [M(H2O)
n
]
m+ exhibited a linear relationship, implying that the NiIII/NiII potential of 2 can be tuned with different
redox-inactive metal ions. Reactivity studies of 2 and 2-M
n+ with different 4-X-2,6-ditert-butylphenol (4-X-DTBP) and other phenol derivatives
were performed, and based on kinetic studies, we propose the involvement
of a proton-coupled electron transfer (PCET) pathway. Analysis of
the reaction products after the reaction of 2 with 4-OMe-DTBP
showed the formation of a Ni(II) complex (1a) where one
of the alkoxide arms of the ligand is protonated. A pK
a value of 24.2 was estimated for 1a. The
reaction of 2-M
n+ species
was examined with 4-OMe-DTBP, and it was observed that the k
2 values of 2-M
n+ species increase by increasing the Lewis acidity of redox-inactive
metal ions. However, the obtained k
2 values
for 2-M
n+ species are much
lower compared to the k
2 value for 2. Such a variation of PCET reactivity between 2 and 2-M
n+ species may be
attributed to the interactions between Ni(III) and LAs. Our findings
show the significance of the secondary coordination sphere effect
on the PCET reactivity of Ni(III) complexes and furnish important
insights into the reaction mechanism involving high-valent nickel
species, which are frequently invoked as key intermediates in Ni-mediated
enzymatic reactions, solar-fuel catalysis, and biomimetic/synthetic
transformation reactions.
