Experimental and Computational Characterization of the Transition State for C–X Bimetallic Oxidative Addition at a Cu–Fe Reaction Center

The
heterobimetallic complex (IPr)­Cu-Fp (IPr = N,N′-bis­(2,6-diisopropylimidazol-2-ylidene,
Fp = FeCp­(CO)2) was identified previously as a nonprecious
metal catalyst for C–H borylation. To better understand the
nature of the bimetallic reaction pathways operative in this system,
we have conducted a thorough mechanistic study of alkyl halide activation
by the Cu–Fe heterobimetallic reaction center. Use of cyclopropylmethyl
halide substrates as radical clocks established that alkyl halide
activation occurs by a two-electron mechanism for alkyl bromides and
chlorides but not iodides. Eyring analysis of the activation of benzyl
chloride allowed for experimental determination of activation parameters,
including a large and negative entropy of activation (ΔS⧧ = −36 eu). A Hammett study with para-substituted benzyl chlorides revealed a reaction constant
of ρ = 1.6, indicating accumulation of negative charge in the
transition state on the alkyl halide carbon. The Ru analogue, (IPr)­Cu-Rp
(Rp = RuCp­(CO)2), was found to react approximately 17–25
times more slowly with selected benzyl chlorides than (IPr)­Cu-Fp,
indicating that the relative nucleophilicities of the free metal carbonyl
anions are predictive of the relative reactivities of their heterobimetallic
counterparts. Synthesis and characterization of the new Ag and Au
analogues, (IPr)­Ag-Fp and (IPr)­Au-Fp, are reported along with the
observation that these more covalent congeners are significantly less
reactive toward alkyl halides. DFT calculations were used to model
a transition state for the Cu–Fe reaction, which was identified
as stereoinvertive at the alkyl halide carbon. NBO calculations indicate
crucial roles played by the CO ligands within the Fp group: they both
act as redox noninnocent ligands and also provide structural templating
to stabilize the transition state as the metal–metal bond breaks.