Protodemetalation of (Bipyridyl)Ni(II)–Aryl Complexes Shows Evidence for Five‑, Six‑, and Seven-Membered Cyclic Pathways

Protonation
of C–M bonds and its microscopic reverse,
metalation
of C–H bonds, are fundamental steps in a variety of metal-catalyzed
processes. As such, studies on protonation of C–M bonds can
shed light on C–H activation. We present here studies on the
rate of protodemetalation (PDM) of a suite of arylnickel­(II) complexes
with various acids that provide evidence for a concerted, cyclic transition
state for the PDM of C–Ni bonds and demonstrate that five-,
six-, and seven-membered transition states are particularly favorable.
Our data show that while the rate of protodemetalation of arylnickel­(II)
complexes scales with acidity for many acids, several are faster than
predicted by pKa. For example, while acetic
acid and acetohydroxamic acid are much less acidic than HCl, they
both protodemetalate arylnickel­(II) complexes significantly faster
than HCl. Our data also show how in the case of acetohydroxamic acid,
a seven-membered cyclic transition state (CH3C­(O)­NHOH) can be more favorable than a six-membered transition
state (CH3C­(O)­NHOH). Similarly,
five-membered transition states, such as for pyrazole, are highly
favorable as well. Comparison of transition state polarization (from
density functional theory) compares these new nickel transition states
to better-studied precious-metal systems and demonstrates how the
base can change the polarization of the transition state giving rise
to opposing electronic preferences. Collectively, these studies suggest
several new avenues for study in C–H activation as well as
approaches to accelerate or slow protodemetalation in nickel catalysis.