Copper complexes supported by N-heterocyclic carbene (NHC) ligands are used widely for numerous catalytic applications including hydrosilylation of carbonyl compounds, conjugate reduction of α,β-unsaturated carbonyl compounds, cyclopropanation of alkenes, cross coupling of alkyl electrophiles, hydrofunctionalization of alkynes, and many others. A key feature of this privileged scaffold is the ability to harness the reactivity of nominally 2-coordinate, 14-electron (NHC)CuX species. Recent work by Sollogoub has demonstrated that, when placed inside supramolecular cavities, these (NHC)CuX species participate in anagostic interactions with surrounding C-H bonds that could be probed by spectroscopic and crystallographic methods due to the system’s structural rigidity. Those findings raise a question: do traditional (NHC)CuX complex feature anagostic interactions that stabilize the low-coordinate Cu center fluxionally? In this presentation, meta-analysis of structural data from the Cambridge Structural Database will be presented to demonstrate that, in the vast majority of cases of the nominally 2-coordinate (NHC)CuX motif, there is ≥1 C-H bond within anagostic bonding range of the Cu center. Using canonical complexes (IPr)CuCl and (I<i>t</i>Bu)CuCl as case studies, DFT modeling and QTAIM calculations will be presented to further elucidate the presence of such interactions and estimate their energetic value toward stabilizing the low-coordinate Cu sites. Effects on copper hydride aggregation will also be discussed. Cumulatively, the results are expected to cause a re-evaluation of how (NHC)CuX complexes are formulated and inform the design of future (NHC)CuX architectures.

Are 2-coordinate (NHC)CuX complexes really 2-coordinate?

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ACS Fall 2022

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