Abstract
Sulfur/carbon/sulfur pincer ligands have an interesting combination of strong-field and weak-field donors, a coordination environment that is also present in the nitrogenase active site. Here, we explore the electronic structures of iron(II) and iron(III) complexes with such a pincer ligand, bearing a monodentate phosphine, thiolate S donor, amide N donor, ammonia, or CO. The ligand scaffold features a proton-responsive thioamide site, and the protonation state of the ligand greatly influences the reduction potential of iron in the phosphine complex. The N-H bond dissociation free energy, derived from the Bordwell equation, is 56 ± 2 kcal/mol. Electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device (SQUID) magnetometry measurements show that the iron(III) complexes with S and N as the fourth donors have an intermediate spin (S = 3/2) ground state with a large zero field splitting, and X-ray absorption spectra show a high Fe-S covalency. The Mössbauer spectrum changes drastically with the position of a nearby alkali metal cation in the iron(III) amido complex, and density functional theory calculations explain this phenomenon through a change between having the doubly occupied orbital as dz2 or dyz, as the former is more influenced by the nearby positive charge.
Highlights
The organometallic chemistry of iron has been dominated by strong-field supporting ligands such as CO, CN, and phosphines, and by macrocyclic N ligands like porphyrins.[1,2,3,4,5,6] The active sites of hydrogenase enzymes incorporate S-based ligands, and these are low-spin due to the influence of carbonyl and cyanide donors.[7]
This unusual combination of potentially strong-field C donors and weak-field S donors could lead to changes in spin states during catalysis, which has been linked to changes in barriers and selectivity.[15,16,17,18]
Using a procedure modified from the literature,[55] isophthalic acid was treated with thionyl chloride followed by 2,6-diisopropylaniline, which provided the diamide (Scheme 1)
Summary
The organometallic chemistry of iron has been dominated by strong-field supporting ligands such as CO, CN, and phosphines, and by macrocyclic N ligands like porphyrins.[1,2,3,4,5,6] The active sites of hydrogenase enzymes incorporate S-based ligands, and these are low-spin due to the influence of carbonyl and cyanide donors.[7] the interesting reactions of nitrogenase enzymes, and a new generation of low-valent iron catalysts, have predominantly weak-field ligands and have led to increasing interest in organometallic iron complexes with higher spin states.[8,9,10,11] We focus here on iron coordination environments that result from a mixture of C and S donors – choices that are compelling since the six “belt” iron atoms in the ironmolybdenum cofactor (FeMoco) of nitrogenase have a mixed C/S coordination sphere.[12,13,14] This unusual combination of potentially strong-field C donors and weak-field S donors could lead to changes in spin states during catalysis, which has been linked to changes in barriers and selectivity.[15,16,17,18] the study of C- and S-ligated iron has relevance for both fundamental coordination chemistry and bioinorganic mechanisms.[19,20,21] CO-free iron complexes with supporting ligands that coordinate through only carbon and sulfur donors are rare.[22,23,24,25,26,27,28] Of these examples, only one multidentate C/S ligand is known to support N2 binding.[26] In addition, Qu has provided important studies on Cp*-supported iron dimers bridged by dithiolates, and this C/S ligand sphere can bind and facilitate the reduction of nitrogenase-relevant NxHy substrates.[23,24,25, 29]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
