The Role of Electron Transfer in Copper-Mediated C(sp)-H Trifluoromethylation.

We report copper(II) and copper(III) trifluoromethyl complexes supported by a pyridinedicarboxamide ligand (L) as a platform for investigating the role of electron transfer in C(sp)-H trifluoromethylation. While the copper(II) trifluoromethyl complex is unreactive towards (hetero)arenes, the formal copper(III) trifluoromethyl complex performs C(sp)-H trifluoromethylation of a wide range of (hetero)arenes. Mechanistic studies using the copper(III) trifluoromethyl complex suggest that the mechanism of arene trifluoromethylation is substrate-dependent.

Tuning Secondary-Sphere Lewis Acidity via Late-Stage Modification of an Appended Borane.

To develop synthetic strategies to construct ligands containing secondary-sphere acids, we demonstrate that an appended borane of low Lewis acidity (-BPin) can be upgraded to a strong Lewis acid (-BF). Using a pyridine-pyrazole ligand coordinated to Mo(CO), we show that a pendent -BPin group undergoes exhaustive fluorination to -BFK, a precursor to a highly acidic -BF unit (acceptor number ∼15× greater than that of -BPin).

Synthesis of heteroleptic bis-phosphine bis-NHC iron (0) complexes: a strategy to enhance small molecule activation.

We report the synthesis of heteroleptic iron complexes supported by both a bis-phosphine ligand (depe) and a bis-NHC ligand. The mixed ligand sets provide access to iron (0) adducts of N and CO that are highly activated, in comparison to homoleptic ( Fe(depe)L) variants. Computational and experimental studies revealed the mixed ligand set distorts the geometric and electronic structure to yield an unusually basic iron.

Additive Effects in Metal/Lewis Acid Cooperativity Assessed in a Tetrahedral Copper Hydrazine Complex Featuring an Appended Borane.

Within metal/ligand cooperative systems employing acidic groups, studies that empirically assess distance relationships are needed to maximize cooperative interactions with substrates. We report the formation of two Cu(I)-NH complexes using 1,4,7-triazacyclononane ligand frameworks bearing two -butyl groups and either a Lewis acidic trialkylborane or an inert alkyl group. Metal/Lewis acid cooperativity imparts heightened acidification of the hydrazine substrate and plays a key role in the release of substrate to a competitive Lewis acidic group.

Lewis Acid-Tethered (cAAC)-Copper Complexes: Reactivity for Hydride Transfer and Catalytic CO Hydrogenation.

We present a series of borane-tethered cyclic (alkyl)(amino)carbene (cAAC)-copper complexes, including a borane-capped Cu(I) hydride. This hydride is unusually hydridic and reacts rapidly with both CO and 2,6-dimethylphenol at room temperature. Its reactivity is distinct from variants without a tethered borane, and the underlying principles governing the enhanced hydricity were evaluated experimentally and theoretically.