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

High Resolution Image Download MS PowerPoint Slide 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 d 0 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 PhH 2 N–Zr(MesNP i Pr 2 ) 3 CoCN t Bu ( 2 ). 2,4,6-tris- tert -butylphenoxyl radical ( t Bu 3 ArO • ) was used to abstract one or two H atoms and produce the amido and imido complexes PhHN-Zr(MesNP i Pr 2 ) 3 CoCN t Bu ( 3 ) and PhN≡Zr(MesNP i Pr 2 ) 3 CoCN t Bu ( 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 p K a s were then estimated using the Bordwell equation to provide further insight into the thermochemical aspects of the observed proton coupled electron transfer (PCET) reactions.