The controlled functionalization of a single fluorine in a CF₃ group is difficult and rare, but desirable due to both the high bioaccumulation and toxicity of many perfluorinated compounds and the utility of fluorinated pharmaceuticals. We have found that the light-driven C–F bond functionalization of the strong, inert C(sp³)-F bond in trifluorotoluene, PhCF₃, is achievable using photocatalysts made from earth-abundant, cheap and non-toxic lanthanides ((Cp^Me4)₂Ln(2-O-3,5-^tBu₂-C₆H₂(1-C{N(CH)₂N(ⁱPr)}) (Ln = La, Ce, Nd and Sm, Cp^Me4 = C₅Me₄H)), as well as complexes of highly abundant magnesium. Cerium, with similar earth-abundance to copper, is the most abundant lanthanide, and possesses both an accessible Ce^III/IV redox couple and an allowed photochemical excitation. Here, we show the Ce compound is the most effective at selectively mediating hydrodefluorination of PhCF₃to form PhCF₂H_,and catalyzes the defluoroalkylative coupling to form 1-difluoro-1,2-diphenylethane when a magnesium dialkyl coupling partner is used to enable turnover. Surprisingly, catalytic C–F bond cleavage and C-C bond formation is possible with all the early lanthanide complexes. This unusual photocatalytic activity is made possible by the light-absorbing ability of the phenoxy N-heterocyclic carbene (NHC) ligand, and the redox non-innocence of the ligand framework. We will present mechanistic experiments that confirm the essential role of the Lewis acidic metal center and support an inner-sphere mechanism of C–F activation, with computational studies agreeing that coordination of the C–F substrate is essential for C–F bond cleavage. We will discuss the importance of metal-ligand cooperativity, specifically the synergy between the metal and ligand in both light absorption and redox reactivity, in organometallic photocatalysis.
We will also show that simple lanthanide cyclopentadienyl (Cp^Me4) complexes can photocatalytically cleave the C(sp³)-Cl bond of unactivated chlorinated hydrocarbons including those in polyvinyl chloride (PVC). The mechanism involves the formation of an alkyl intermediate and because light absorption by the Cp^Me4 ligand is possible, photocatalytic reactivity is enhanced for cerium and made possible for neighboring, normally photoinactive, lanthanide congeners. We use a combination of experiment and theory to probe the and mechanism of this process.

Polly Arnold

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