Homogeneous polynuclear metal clusters constitute a broad class of coordination compounds with important applications in catalysis. The current interest of synthetic chemistry in this field demands the exploration of new strategies to develop catalytic methods that work under mild conditions and maximize atom utilization. This review covers the application of polynuclear clusters of nuclearity ≥3 in homogeneous catalytic processes, with focus on providing an array of examples of various reaction types within cluster catalysis.

Electron transfer (ET) is broadly described by Marcus-type theories and plays a central role in many materials and catalytic systems and in biomolecules such as cytochromes. Classic ET processes are the self-exchange reactions between hydrated transition-metal ions such as Fe(aq) + Fe(aq) → Fe(aq) + Fe(aq). A well-known anomaly of Marcus theory is Co/Co exchange, which proceeds ∼10 times faster than predicted. Co(aq) is a complex and reactive system widely thought to feature low-spin Co. We studied the self-exchange process systematically for Cr/Cr, V/V, Fe/Fe, and Co/Co using six distinct density functionals. We identify directly the ∼10 anomaly of Co/Co from the electronic reorganization energies without the use of empirical cross-relations. Furthermore, when Co is modeled as high-spin, the anomaly disappears, bringing all four processes on a linear trend within the uncertainty of the experiments and theory. We studied both the acid-independent [Co(HO)] species that dominates at low pH and the acid-dependent [Co(OH)(HO)] species that becomes important at higher pH and used two distinct explicit second-sphere hydration models and models of perchlorate anion association. The high-spin state with weaker Co-O bonds is stabilized by vibrational energy and entropy by ∼11 and ∼12 kJ mol, correcting the gap estimates from absorption spectroscopy. High-spin Co(aq) explains the full experimental data series of the M(aq) systems. Low-spin Co and high-spin Co involve changes in the e occupation upon ET with associated M-O bond changes and increased reorganization energy. In contrast, with high-spin Co(aq), the redox-active electrons shuffle between t orbitals to minimize structural changes, producing a relative rate in excellent agreement with the experiments. This e occupation effect explains most of the experimental differences in the rate constants, with the remaining part explained by second-sphere hydration and anion effects. Our results consistently suggest that some high-spin Co(aq) is active during the experiments.

We herein present a broadly useful method for the chemoselective modification of a wide range of tryptophan-containing peptides. Exposing a tryptophan-containing peptide to 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) resulted in a selective cyclodehydration between the peptide backbone and the indole side chain of tryptophan to form a fully conjugated indolyl-oxazole moiety. The modified peptides show a characteristic and significant emission maximum at 425 nm, thus making the method a useful strategy for fluorescence labeling.