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 ^R(PNP)H (1) and subsequent reaction with chromium(II) chloride led to the formation of square-planar chlorido complexes ^R(PNP)CrCl (2). Further reaction with various alkylating agents resulted in the isolation of chromium alkyl complexes ^R(PNP)CrR' (3-R') which were then hydrogenated to yield two rare examples of paramagnetic chromium(II) hydrides 4 and 4. 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(tert-butyl)phosphino PNP substituted complex 4 was found to exhibit a monomeric square-planar molecular structure, its isopropyl-substituted analog 4 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, N,N'-dicyclohexylcarbodiimide, and carbon dioxide, which gave the corresponding insertion products. The alkoxido complex 5, the amidinato complex 6, and the formato compound 7 were also characterized by X-ray diffraction.