Quantification of the Steric Properties of 1,8-Naphthyridine Based Ligands in Dinuclear Complexes

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

E-selective Semi-hydrogenation of Alkynes Under Mild Conditions by a Diruthenium Hydride Complex

Reacting the tBuPNNP expanded pincer ligand with two equiv of RuHCl(PPh3)3(CO) at high temperatures produces an insoluble air-stable complex, which was structurally characterized as [Ru2(tBuPNNP)H(μ-H)Cl(μ-Cl)(CO)2] 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 a soluble complex 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 produces an active catalyst for the E-selective semi-hydrogenation of alkynes under mild conditions (ambient temperature and pressure) with good functional group tolerance. Monitoring this reaction using 1H NMR spectroscopy for the hydrogenation of diphenylacetylene 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

Combining Metal-Metal Cooperativity, Metal-Ligand Cooperativity and Chemical Non-Innocence in Diiron Carbonyl Complexes

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand’s naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)–H bonds by ~25 kcal mol-1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = 1/2) complex containing a mixed-valent iron(0)-iron(I) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity