Taking Solution Proton NMR to Its Extreme: Prediction and Detection of a Hydride Resonance in an Intermediate-Spin Iron Complex

Abstract

Guided by DFT based modeling the chemical shift range of a hydride resonance in the proton nuclear magnetic resonance (NMR) spectrum of the intermediate-spin, square planar iron complex tBu (PNP)-Fe−H was predicted and detected as a broad resonance at −3560 ppm (295 K) with a temperature dependent shift of approximately 2000 ppm between 223 and 383 K. The first detection of a metal-bonded hydrogen atom by solution NMR in a complex with a paramagnetic ground state illustrates the interplay of theory and experiment for the characterization of key components in paramagnetic base metal catalysis.

Opening up the Valence Shell: A T‐Shaped Iron(I) Metalloradical and Its Potential for Atom Abstraction

“…Computational modeling of both contact and pseudocontact contributions to the paramagnetic shifts allowed the full assignment of all proton resonances of 2 as well as the assignment of the corresponding 13 C NMR resonances (see the Supporting Information). However, in contrast to the complete set of ligand resonances observed for [ tBu (PNP)FeH], [10] only those resonances of carbon nuclei were observed in the 13 C NMR spectrum of the d 7 high-spin system 2 with a position more than two bonds away from the paramagnetic center, indicating faster nuclear relaxation rates for the latter compared to the d 6 intermediate-spin Fe II hydrido complex.…”

“…Notably, the Fe À N bond length of 2.0369 (16) is longer than that of previously reported Fe II complexes of this ligand, as would be expected for a lower oxidation state of the central metal atom. [10] Interestingly, no solvent molecule occupies the vacant coordination site of the compound, which was generally found to be relatively inert towards the coordination of pure donor ligands such as ethers or amines. To obtain insight into the electronic structure of this remarkable complex, a density functional theory (DFT) study was carried out, using the B3LYP [14] hybrid density functional with the def2-TZVP [15] basis set for the iron atom and the 6-311G(d,p) [16] basis set for all other atoms.…”

“…The X-ray structure analysis of 3 revealed a distorted tetrahedral coordination sphere at the iron center (" t 0 4 = 0.70, Figure 4), [17] which is common for tetracoordinate, high-spin iron(II) complexes. [10,11] Whereas Holland et al recently described an iron h 2 -benzophenone complex, the end-on coordination mode of the ketyl ligand in complex 3 has not been reported in iron chemistry. [18] Analysis of the bond metrics revealed an elongation of the CÀO bond length to 1.2989 (19) [compared to free benzophenone (1.2233 (17) )], [19] indicating a decreased bond order, which is in line with a comparable coordinated benzophenone ketyl radical at uranium (1.334 (6) ).…”

Öffnung der Valenzschale: Ein T‐förmiges Eisen(I)‐Metalloradikal und sein Potential als Atomabstraktor

“…Beispiele von T‐förmigen Eisenkomplexen sind extrem selten, und unseres Wissens nach wurde das bisher einzige Beispiel eines Eisenkomplexes dieses Typs von der Caulton‐Gruppe veröffenlicht . Wir haben über die Fähigkeit des Carbazol‐basierten Liganden (PNP)H (mit (PNP)H=3,6‐Di‐ tert ‐butyl‐1,8‐bis((di‐ tert ‐butylphosphanyl)methyl)‐9 H ‐carbazol) berichtet, eine Reihe von niedrig‐koordinierten 3d‐Metallverbindungen zu stabilisieren und eine bemerkenswert langsame Kernrelaxation zu induzieren, was zu einzigartigen spektroskopischen Eigenschaften führt . Hier berichten wir über die Synthese einer “nackten” (PNP)Fe‐Spezies, ihre elektronischen Eigenschaften und ihre Reaktivität als effektiver Sauerstoffatom‐Abstraktor.…”

“…Die Struktur weicht leicht von der idealisierten Koordinationsgeometrie ab, was sich in den N‐Fe‐P‐Winkeln von 96° widerspiegelte. Bemerkenswert ist, dass die Fe‐N‐Bindung von 2.0369(16) Å länger ist als in den Fe II ‐Komplexen dieses Liganden, wie aufgrund der niedrigeren Oxidationsstufe des Metallatoms zu erwarten war . Interessant ist die Abwesenheit eines Lösungsmittelmoleküls an der freien Koordinationsstelle der Verbindung, die sich im Übrigen als relativ inert gegen Koordination von reinen Donorliganden wie Ethern oder Aminen erwiesen hat.…”