Single or Paired? Structure and Reactivity of PNP-Chromium(II) Hydrides.

The preparation and reactivity of a range of novel paramagnetic chromium(II) complexes supported by a carbazole-based PNP pincer ligand is reported. Deprotonation of the ligand precursors (PNP)H () and subsequent reaction with chromium(II) chloride led to the formation of square-planar chlorido complexes (PNP)CrCl (). Further reaction with various alkylating agents resulted in the isolation of chromium alkyl complexes (PNP)CrR' () which were then hydrogenated to yield two rare examples of paramagnetic chromium(II) hydrides and . Both compounds were characterized by X-ray diffraction and paramagnetic NMR spectroscopy supported by a comprehensive DFT-supported assignment of the resonances. While the di(-butyl)phosphino PNP substituted complex was found to exhibit a monomeric square-planar molecular structure, its isopropyl-substituted analog forms a dimer, also indicated by a strong antiferromagnetic coupling of the chromium centers. The pronounced reactivity of these compounds toward C═X double bonds was demonstrated by reaction with benzophenone, ,'-dicyclohexylcarbodiimide, and carbon dioxide, which gave the corresponding insertion products. The alkoxido complex , the amidinato complex , and the formato compound were also characterized by X-ray diffraction.