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

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 BDFE_N-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 Zr^IV 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 Zr^IV/Co^-I aniline adduct PhH₂N-Zr(MesNP^<i>i</i>Pr₂)₃CoCN^<i>t</i>Bu (<b>2</b>). 2,4,6-tris-<i>tert</i>-butylphenoxyl radical (^<i>t</i>Bu₃ArO^•) was used to abstract one or two H atoms and produce the amido and imido complexes PhHN-Zr(MesNP^<i>i</i>Pr₂)₃CoCN^<i>t</i>Bu (<b>3</b>) and PhN≡Zr(MesNP^<i>i</i>Pr₂)₃CoCN^<i>t</i>Bu (<b>4</b>), respectively. Using open-circuit potential measurements, the BDFE_N-H within <b>2</b> and <b>3</b> were determined to be 37 kcal/mol (<b>2</b>) and 55 kcal/mol (<b>3</b>). Cyclic voltammetry measurements were conducted to determine the Co^I/0 and Co^0/-I redox potentials. The p<i>K</i>_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.