%0 Journal Article
%A Vabre, Boris
%A Lambert, Melinda
L.
%A Petit, Alban
%A Ess, Daniel H.
%A Zargarian, Davit
%D 2012
%T Nickelation of PCP- and
POCOP-Type Pincer Ligands:
Kinetics and Mechanism
%U https://acs.figshare.com/articles/journal_contribution/Nickelation_of_PCP_and_POCOP_Type_Pincer_Ligands_Kinetics_and_Mechanism/2488696
%R 10.1021/om3003784.s001
%2 https://acs.figshare.com/ndownloader/files/4131484
%K 2PE
%K 2PO
%K 2PECH
%K nonmetalated intermediates
%K 2PCH
%K 2P
%K POCOP
%K PCP
%K metalation process exergonic
%K metalation transition states
%K 2OP
%K ground state energy
%K 2POCH
%K phosphine moieties vs
%K CH
%K PR 2 moiety
%K donor halide ligand
%X This report describes the results of a combined experimental
and
computational investigation on the kinetics and mechanism of the C–H
metalation step involved in the formation of PCP- and POCOP-type complexes
of nickel. The kinetics of the C–H nickelation reaction was
probed through competition studies involving two ligands reacting
with a substoicheometric quantity of {(i-PrCN)NiBr2}n. These experiments have confirmed
that metalation is more facile for aromatic ligands 1,3-(i-Pr2PE)2C6H4 vs their
aliphatic counterparts 1,3-(i-Pr2PECH2)2CH2 (sp2 C–H >
sp3 C–H; E = O, CH2), ligands bearing
phosphine
moieties vs those with phosphinite moieties (PCP > POCOP), ligands
bearing P substituents i-Pr2P vs t-Bu2P and Ph2P, and POCsp2OP ligands 1,3-(i-Pr2PO)2C6RnH4–n bearing
electron-donating vs electron-withdrawing substituents (p-OMe ≈ m-OMe > p-Me > m-CO2Me > p-CO2Me
> m,m-Cl2). Among the latter, there
is
a 6-fold difference in C–H metalation rate between ligands
bearing p-OMe and p-COOMe, whereas
the most readily metalating ligand, 1,3-(i-Pr2PCH2)2C6H4, is
metalated ca. 270 times more readily relative to the least reactive
ligand, 1,3-(i-Pr2POCH2)2CH2. Density functional calculations indicate that
PCP- or POCOP-type pincer ligands react with NiBr2 to generate
nonmetalated intermediates that form the corresponding pincer complexes
via a two-step mechanism involving an ionic dissociation of the bromide
to give a tight ion pair intermediate, followed by bromide-assisted
deprotonation of the C–H bond. The type of structure adopted
by the nonmetalated intermediates (mono- or dinuclear; tetrahedral,
cis or trans square planar) and the energy barriers for the metalation
transition states depend on the steric properties of the PR2 moiety. The presence of a base that can neutralize the HBr generated
in the metalation step is crucial for rendering the metalation process
exergonic. One rationale for the more facile metalation of PCP ligands
in comparison to their POCOP counterparts is the greater donor character
of phosphine moieties, which allows a more effective stabilization
of the coordination and metalation transition states wherein the strongly
donor halide ligand is displaced by a much weaker C–H bond
donor. The aromatic ligands metalate more readily than their aliphatic
analogues for multiple reasons, including the higher ground state
energy of the nonmetalated intermediates formed with aromatic ligands,
the stronger Csp2–Ni bond formed via
metalation, and the more stabilized anionic charge on the C atom being
metalated.
%I ACS Publications