Photochemical Formation and Electronic Structure of an Alkane 𝜎-Complex from Time-Resolved Optical and X-ray Absorption Spectroscopy

C-H bond activation reactions with transition metals typically proceed via the formation of alkane 𝜎-complexes, where an alkane C-H 𝜎-bond binds to the metal. Due to the weak nature of metal-alkane bonds, 𝜎-complexes are challenging to characterize experimentally. Here, we photochemically prepare the model 𝜎-complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal-ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photo-induced CO dissociation from Cr(CO)6 to occur within the 100 fs time-resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane 𝜎-complex both from the metal and ligand perspectives. We find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C-H bond to the undercoordinated Cr center in the Cr(CO)5-alkane 𝜎-complex, accompanied with rehybridization between metal and ligand orbitals. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the properties of alkane 𝜎-complexes as the decisive intermediates in C-H bond activation reactions.