Solvent-Dependent Carbon-to-Metal Hydrogen Atom Transfer Reactivity of a Square Planar Rhodium(II) Alkynyl Complex

We report on the synthesis and characterization of the
square planar
rhodium(II) alkynyl complex [Rh(PNP-tBu)(CCtBu)]+ and its transformation into the vinylidene
derivative [Rh(PNP-tBu)(CCHtBu)]+ by reaction with 9,10-dihydroanthracene. Computational
analysis supports a mechanism involving carbon-to-metal hydrogen atom
transfer followed by 1,3-hydride migration, and intermediate formation
of the associated rhodium(III) alkynyl hydride [Rh(PNP-tBu)H(CCtBu)]+ has been substantiated
experimentally. Study of [Rh(PNP-tBu)(CCtBu)]+ in the solid state by EPR spectroscopy,
supplemented by multireference CAS(9,6)/NEVPT2 calculations, enabled
assignment as a metal-centered radical to be corroborated, while analysis
of frozen glass solutions revealed the presence of square pyramidal
solvent adducts of tetrahydrofuran, 2-methyltetrahydrofuran, 1,2-difluorobenzene,
fluorobenzene, and α,α,α-trifluorotoluene. Consistent
with a carbon-to-metal hydrogen atom transfer mechanism, the extent
of solvent coordination measured by EPR spectroscopy inversely correlates
with the rate at which the metalloradical reacts with 9,10-dihydroanthracene.