Discovery and mechanistic investigation of photoinduced sp3 C–H activation of hydrocarbons by the simple anion hexachlorotitanate

Abstract

The selective transformation of hydrocarbons into more chemically complex materials is an evergreen challenge in chemistry and catalysis. This report finds that the dianion hexachlorotitanate (TiCl 6 2− ) catalyzes the C–H activation of saturated hydrocarbons under 390 nm light irradiation. Investigations into the mechanism of this reaction are detailed. The photolysis event affords the formation of a chlorine radical and a mixture of Ti III Cl 4 (NCMe) 2 − and Ti III Cl 5 (NCMe) 2− . The Ti III Cl x species were characterized by spectrophotometry, electrochemistry, and X-ray crystallography. Alkyl radicals generated by these means were trapped by a range of alkene acceptors. Notably, the Ti III Cl x species were shown to be more reducing (Δ E pa = 0.72 V) than related Ce III Cl x species, enabling access to more electron-rich acceptors by facilitating the reduction of the alkyl radical trapped intermediate. Density functional theory calculations correctly identified the reactivity of alkene substrates using the CeCl 6 2− and TiCl 6 2− photoredox catalysts. The results herein demonstrate the impact of metal identity on C–H activation. • Identification of a simple TiCl 6 2− dianion that photolytically produces chlorine radicals • Identification of the titanium species produced after Ti–Cl bond homolysis • Reactivity studies with the gaseous ethane and methane • Increased scope of radical acceptors relative to the previously reported CeCl 6 2− dianion Recent reports on photochemical C–H functionalization of methane and other hydrocarbons via chlorine radical intermediates have renewed interest in metal photocatalysts that efficiently deliver such reactive, electrophilic radicals. Moreover, little mechanistic work has been performed to understand the impact of metal identity in the context of photocatalysis. This paper reports on the discovery of hexachlorotitanate (TiCl 6 2− ) as an efficient photocatalyst for C–H functionalization and examines reactivity differences between isostructural TiCl 6 2− and hexachlorocerate (CeCl 6 2− ) anions. The breadth of radical acceptors is notably increased as a result of the superior performance characteristics of TiCl 6 2− . Mechanistic studies implicate titanium species produced by Ti–Cl bond homolysis and indicate that subsequent reduction of radical intermediates is essential to the increased reactivity scope. We expect this work to have broad implications for photolytic C–H functionalization chemistry. The functionalization of simple hydrocarbons into more complex compounds is an important chemical goal. Photolysis of hexachlorotitanate (TiCl 6 2− ) produces highly reactive chlorine radicals that can react with hydrocarbon C–H bonds, including the strong C–H bonds of methane, allowing for the capture of methyl radicals. An exploration of the mechanism of hexachlorotitanate’s reactivity with alkene radical acceptors is explored via a combination of spectrophotometry, electrochemistry, X-ray crystallography, and DFT calculations.