Synthesis, Characterization, and Reactivity of a High-Spin Iron(II) Hydrido Complex Supported by a PNP Pincer Ligand and Its Application as a Homogenous Catalyst for the Hydrogenation of Alkenes

Abstract

This study focused on the synthesis and characterization of a range of low-valent, high-spin iron(II) complexes supported by a carbazole-based PNP pincer-type ligand. The addition of the lithiated ligand (PNP)Li to FeCl(THF) yielded the chlorido complex (PNP)FeCl (1), which could be readily converted to the four-coordinate iron(II) alkyl complexes (PNP)FeR [R = CHSiMe (3a), Me (3b), CHPh (3c)]. These iron(II) complexes were fully characterized by X-ray analysis and a comprehensive, density-functional-theory-assisted study with complete assignment of their paramagnetic H andC NMR spectra. Treatment of 1 with KHBEt or the addition of molecular hydrogen to (PNP)FeR afforded a high-spin iron(II) PNP hydrido complex, which was identified as the dimer [(PNP)Fe(μ-H)] (4) with two bridging hydrido ligands between the iron centers. Exposing complexes 1 and 4 to carbon monoxide led to the corresponding six-coordinate, diamagnetic complexes (PNP)Fe(CO)Cl (2) and (PNP)Fe(CO)H (5), of which 2 was present as cis/trans isomers. Furthermore, 4 was found to be an active catalyst for the hydrogenation of alkenes.

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

“…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) ).…”

Phosphines and N‐Heterocycles Joining Forces: an Emerging Structural Motif in PNP‐Pincer Chemistry

“…The compound's relatively slow nuclear relaxation enabled the detection of its 1 H NMR hydride resonance at –3560 ppm (295 K) . For the P i Pr 2 ‐substituted ligand derivative, the dimeric µ ‐hydrido‐bridged high‐spin iron(II) complex 43 was isolated and employed as hydrogenation catalyst for the synthesis of alkanes from the corresponding alkenes and dihydrogen . A similar catalytic activity was also reported for the related mononuclear cobalt(II) pincer complex CbzC ( i PrPNP)Co(H) …”

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.…”

“…Um dieses Verhalten zu untersuchen, wurde 2 mit Benzophenon umgesetzt, wobei sich der thermodynamisch stabile Eisen‐Diphenylketyl‐Radikalkomplex 3 bildete (Schema ). Die Röntgenstrukturanalyse von 3 offenbarte eine verzerrt‐tetraedrische Koordinationssphäre am Eisenzentrum ( =0.70; Abbildung ), wie für tetrakoordinierte High‐Spin‐Eisen(II)‐Komplexe zu erwarten …”