Iron Complexes of a Proton-Responsive SCS Pincer Ligand with a Sensitive Electronic Structure

Abstract

SCS pincer ligands have an interesting combination of strong-field and weak-field donors 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 protonresponsive 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 can be quantitated as 56 ± 2 kcal/mol. EPR spectroscopy and 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 large zero field splitting, and X-ray absorption spectra show 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 DFT 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.

Sulfur-Ligated [2Fe-2C] Clusters as Synthetic Model Systems for Nitrogenase

“…Crystals were grown from saturated Et 2 O solution in 44% yield revealing 4 to be a monometallic Fe complex in which one of the S ligands has been displaced from the metal and a new C–C bond has formed between the isocyanide C atom and C of the yldiide (Figure ). Thus, this donor goes beyond weakening the Fe–S bond to completely breaking it, which may be relevant to current proposals of the nitrogenase mechanism in which Fe–S cleavage is crucial - though some of the structural support for this idea is under debate. − The insertion of isocyanide into a Fe–C bonds is frequently encountered in organometallic chemistry. , The Mössbauer spectrum of 4 (Figure D) displays a doublet (δ = 0.12 mm s –1 and Δ E Q = 1.89 mm s –1 ), which is comparable to that of previously reported octahedral, low-spin Fe II complexes. , …”

“… 68 , 69 The Mössbauer spectrum of 4 ( Figure 3 D) displays a doublet (δ = 0.12 mm s –1 and Δ E Q = 1.89 mm s –1 ), which is comparable to that of previously reported octahedral, low-spin Fe II complexes. 70 , 71 …”

Investigating Metal–Metal Bond Polarization in a Heteroleptic Tris-Ylide Diiron System

“…The poor quantitative agreement with the experimental data may be explained in part by the use of the dicationic unit of 2 in the computational experiments. Holland and co-workers have recently demonstrated the sensitivity of 57 Fe Mössbauer features to the electrostatic environment created by the specific locations of counterions . Furthermore, the low quadrupole splitting that is apparent from the experimental data is consistent with the presence of a minimal electric field gradient about the Fe nucleus that commonly results from a high-spin S = 5/2 electron configuration.…”

Oriented internal electrostatic fields: an emerging design element in coordination chemistry and catalysis

“…8). 59 Most notable for the present discussion was their observation of an interesting result when trying to computationally reproduce the Mössbauer parameters for a bis(trimethylsilyl)amido complex that featured an outersphere [K(18-crown-6)(thf) 2 ] + counterion (Fig. 8).…”

“…8 Valence manifold tuning at an Fe III amido complex supported by an SCS pincer ligand. 59 The full [K(crown)(thf) 2 ] + ion was found to be critical for reproducing experimental observables.…”