Combining metal-metal cooperativity, metal-ligand cooperativity and chemical non-innocence in diiron carbonyl complexes

• Published in: Chemical science (Cambridge) Vol. 13; no. 7; pp. 2094 - 2104
• Main Authors: van Beek, Cody B, van Leest, Nicolaas P, Lutz, Martin, de Vos, Sander D, Klein Gebbink, Robertus J M, de Bruin, Bas, Broere, Daniël L J
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 16.02.2022; The Royal Society of Chemistry
• Subjects: Bimetals; Carbonyls; Catalytic activity; Chemistry; Electron transfer; Energy of dissociation; Free energy; Hydrogen evolution reactions; Hydrogenase; Iron; Ligands
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d1sc05473b

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(sp )-H bonds by ∼25 kcal mol . H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic ( = ) 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.