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.