Coordination-Induced Weakening of N–H Bonds Driven by Bimetallic Cooperativity in Zr/Co Compounds

TL;DR

This paper explores how bimetallic coordination in Zr/Co compounds weakens N–H bonds, enabling insights into proton-coupled electron transfer reactions.

Contribution

The study introduces a bimetallic system with spatially separated redox and substrate binding sites to investigate N–H bond weakening.

Findings

The BDFEN–H of the aniline adduct was measured at 37 kcal/mol.

Amido complex BDFEN–H was found to be 55 kcal/mol.

Redox potentials of the Co center were determined via cyclic voltammetry.

Abstract

The bond dissociation free energy (BDFE) of the element-hydrogen
bonds of protic substrates have been found to decrease upon metal
coordination. Herein, an early/late heterobimetallic complex is used
to examine the impact on the BDFEN–H when the substrate
binding site and the redox-active site are two different metals that
are spatially separated. A tris­(phosphinoamide) framework is used
to link a d0 ZrIV center with an accessible
substrate binding site to a coordinatively saturated redox-active
Co center, which serves as an appended electron reservoir. A series
of aniline, amido, and imido Zr/Co model compounds were synthesized
starting from the ZrIV/Co–I aniline adduct
PhH2N–Zr­(MesNP
i
Pr2)3CoCN
t
Bu (2). 2,4,6-tris-tert-butylphenoxyl radical (
t
Bu3ArO•) was used to
abstract one or two H atoms and produce the amido and imido complexes
PhHN-Zr­(MesNP
i
Pr2)3CoCN
t
Bu (3) and PhNZr­(MesNP
i
Pr2)3CoCN
t
Bu (4), respectively. Using open-circuit
potential measurements, the BDFEN–H within 2 and 3 were determined to be 37 kcal/mol (2) and 55 kcal/mol (3). Cyclic voltammetry measurements
were conducted to determine the CoI/0 and Co0/–I redox potentials. The pK
as were then
estimated using the Bordwell equation to provide further insight into
the thermochemical aspects of the observed proton coupled electron
transfer (PCET) reactions.

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