Steric properties of ligands are an important parameter for tuning the reactivity of the corresponding complexes. For various ligands used in mononuclear complexes, methods have been developed to quantify their steric bulk. In this work, we present an expansion of the buried volume and the G-parameter to quantify the steric properties of 1,8-napthyridine-based dinuclear complexes. Using this methodology, we explored the tunability of the steric properties associated with these ligands and complexes.

Understanding how multicopper oxidases (MCOs) reduce oxygen in the trinuclear copper cluster (TNC) is of great importance for development of catalysts for the oxygen reduction reaction (ORR). Herein, we report a mechanistic investigation into the ORR activity of the dinuclear copper complex [Cu 2 L(μ-OH)] 3+ (L = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine). This complex is inspired by the dinuclear T3 site found in the MCO active site and confines the Cu centers in a rigid scaffold. We show that the electrochemical reduction of [Cu 2 L(μ-OH)] 3+ follows a proton-coupled electron transfer pathway and requires a larger overpotential due to the presence of the Cu-OH-Cu motif. In addition, we provide evidence that metal−metal cooperativity takes place during catalysis that is facilitated by the constraints of the rigid ligand framework, by identification of key intermediates along the catalytic cycle of [Cu 2 L(μ-OH)] 3+ . Electrochemical studies show that the mechanisms of the ORR and hydrogen peroxide reduction reaction found for [Cu 2 L(μ-OH)] 3+ differ from the ones found for analogous mononuclear copper catalysts. In addition, the metal−metal cooperativity results in an improved selectivity for the four-electron ORR of more than 70% because reaction intermediates can be stabilized better between both copper centers. Overall, the mechanism of the [Cu 2 L(μ-OH)] 3+ -catalyzed ORR in this work contributes to the understanding of how the cooperative function of multiple metals in close proximity can affect ORR activity and selectivity.

The synthesis, characterization and catalytic activity of a new class of diruthenium hydrido carbonyl complexes bound to the tBuPNNP expanded pincer ligand is described. Reacting tBuPNNP with two equiv of RuHCl(PPh3)3(CO) at 140 °C produces an insoluble air‐stable complex, which was structurally characterized as [Ru2(tBuPNNP)H(μ‐H)Cl(μ‐Cl)(CO)2] (1) using solid‐state NMR, IR and X‐ray absorption spectroscopies and follow‐up reactivity. A reaction with KOtBu results in deprotonation of a methylene linker to produce [Ru2(tBuPNNP*)H(μ‐H)(μ‐OtBu)(CO)2] (3) featuring a partially dearomatized naphthyridine core. This enables metal‐ligand cooperative activation of H2 analogous to the mononuclear analogue, [Ru(tBuPNP*)H(CO)]. In contrast to the mononuclear system, the bimetallic analogue 3 catalyzes the E‐selective semi‐hydrogenation of alkynes at ambient temperature and atmospheric H2 pressure with good functional group tolerance. Monitoring the semi‐hydrogenation of diphenylacetylene by 1H NMR spectroscopy shows the intermediacy of Z‐stilbene, which is subsequently isomerized to the E‐isomer. Initial findings into the mode of action of this system are provided, including the spectroscopic characterization of a polyhydride intermediate and the isolation of a deactivated species with a partially hydrogenated naphthyridine backbone.

Steric properties of ligands are an important parameter for tuning the reactivity of the corresponding complexes. For various ligands used in mononuclear complexes, methods have been developed to quantify their steric bulk. In this work we present an expansion of the buried volume and G-parameter to quantify the steric properties of 1,8-napthyridine based dinuclear complexes. Using this methodology, we explored the tuneability of the steric properties associated with these ligands and complexes.