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(MesNPⁱPr₂)₃CoCN^tBu (2). 2,4,6-tris-tert-butylphenoxyl radical (^tBu₃ArO^•) was used to abstract one or two H atoms and produce the amido and imido complexes PhHN-Zr(MesNPⁱPr₂)₃CoCN^tBu (3) and PhN≡Zr(MesNPⁱPr₂)₃CoCN^tBu (4), respectively. Using open-circuit potential measurements, the BDFE_N-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 Co^I/0 and Co^0/-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.