Photoinduced Ligand-to-Metal Charge Transfer of Cobaltocene: Radical Release and Catalytic Cyclotrimerization

Photophysical Characterization and Excited State Dynamics of Decamethylruthenocenium

Understanding the landscape of molecular photocatalysis is vital to enable efficient conversion of feedstock molecules to targeted products and inhibit off-cycle reactivity. In this study, the light-promoted reactivity of [RuCp*2]+ was explored via electronic structure, photophysical, and photostability studies and the reactivity of [RuCp*2]+ within a photocatalytic hydrogen evolution cycle was assessed. TD-DFT calculations support the assignment of a low-energy ligand-to-metal charge transfer transition (LMCT) centered at 500 nm, where an electron from a ligand-based orbital delocalized across both Cp* ligands is promoted to a dx2-y2-based β-LUMO orbital. Upon irradiating the LMCT absorption feature, ultrafast transient absorption spectroscopy measurements show that an initial excited state (τ1 = 1.3 ± 0.1 ps) is populated, which undergoes fast relaxation to a longer-lived state (τ2 = 12.0 ± 0.9 ps), either via internal conversion or vibrational relaxation. Despite the short-lived nature of these excited states, bulk photolysis of [RuCp*2]+ demonstrates that photochemical conversion to decomposition products is possible upon prolonged illumination. Collectively, these studies reveal that [RuCp*2]+ undergoes light-driven decomposition, highlighting the necessity to construct molecular photocatalytic systems resistant to off-cycle reactivity in both the ground and excited states.

Ru-catalyzed activation of free phenols in a one-step Suzuki-Miyaura cross-coupling under mechanochemical conditions

Activation of phenols by a Ru-catalyst allows for the resulting η5-phenoxo complex to selectively react with a variety of nucleophiles under mechanochemical conditions. Conversion of phenolic hydroxy groups without derivatization is important for late-stage modifications of pharmaceuticals and in the context of lignin-material processing. We present a one-step, Ru-catalyzed cross-coupling of phenols with boronic acids, aryl trialkoxysilanes and potassium benzoyltrifluoroborates under mechano-chemical conditions. The protocol accepts a wide scope of starting materials and allows for gram-scale synthesis in excellent yields. The developed approach constitutes a very interesting and waste-limiting alternative to the known methods.

A new era of LMCT: leveraging ligand-to-metal charge transfer excited states for photochemical reactions

Ligand-to-metal charge transfer (LMCT) excited states are capable of undergoing a wide array of photochemical reactions, yet receive minimal attention compared to other charge transfer excited states. This work provides general criteria for designing transition metal complexes that exhibit low energy LMCT excited states and routes to drive photochemistry from these excited states. General design principles regarding metal identity, oxidation state, geometry, and ligand sets are summarized. Fundamental photoreactions from these states including visible light-induced homolysis, excited state electron transfer, and other photoinduced chemical transformations are discussed and key design principles for enabling these photochemical reactions are further highlighted. Guided by these fundamentals, this review outlines critical considerations for the future design and application of coordination complexes with LMCT excited states.

Electronic Structure and Photophysics of Low Spin d5 Metallocenes

The electronic structure and photophysics of two low spin metallocenes, decamethylmanganocene (MnCp*2) and decamethylrhenocene (ReCp*2), were investigated to probe their promise as photoredox reagents. Computational studies support the assignment of 2E2 ground state configurations and low energy ligand-to-metal charge transfer transitions for both complexes. Weak emission is observed at room temperature for ReCp*2 with τ = 1.8 ns in pentane, whereas MnCp*2 is not emissive. Calculation of the excited state reduction potentials for both metallocenes reveal their potential potency as excited state reductants (E°'([MnCp*2]+/0*) = -3.38 V and E°'([ReCp*2]+/0*) = -2.61 V vs Fc+/0). Comparatively, both complexes exhibit mild potentials for photo-oxidative processes (E°'([MnCp*2]0*/-) = -0.18 V and E°'([ReCp*2]0*/-) = -0.20 V vs Fc+/0). These results showcase the rich electronic structure of low spin d5 metallocenes and their promise as excited state reductants.