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

This study focused on the synthesis and characterization of a range of low-valent, <i>high-spin</i> iron­(II) complexes supported by a carbazole-based PNP pincer-type ligand. The addition of the lithiated ligand (PNP)­Li to FeCl₂(THF)_1.5 yielded the chlorido complex (PNP)­FeCl (<b>1</b>), which could be readily converted to the four-coordinate iron­(II) alkyl complexes (PNP)­FeR [R = CH₂SiMe₃ (<b>3a</b>), Me (<b>3b</b>), CH₂Ph (<b>3c</b>)]. 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 ¹H and ¹³C NMR spectra. Treatment of <b>1</b> with KHBEt₃ or the addition of molecular hydrogen to (PNP)­FeR afforded a <i>high-spin</i> iron­(II) PNP hydrido complex, which was identified as the dimer [(PNP)­Fe­(μ-H)]₂ (<b>4</b>) with two bridging hydrido ligands between the iron centers. Exposing complexes <b>1</b> and <b>4</b> to carbon monoxide led to the corresponding six-coordinate, diamagnetic complexes (PNP)­Fe­(CO)₂Cl (<b>2</b>) and (PNP)­Fe­(CO)₂H (<b>5</b>), of which <b>2</b> was present as cis/trans isomers. Furthermore, <b>4</b> was found to be an active catalyst for the hydrogenation of alkenes