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^R) and subsequent reaction with chromium­(II) chloride led to the formation of square-planar chlorido complexes ^R(PNP)­CrCl (2^R). Further reaction with various alkylating agents resulted in the isolation of chromium alkyl complexes ^R(PNP)­CrR′ (3^R-R′) which were then hydrogenated to yield two rare examples of paramagnetic chromium­(II) hydrides 4^iPr and 4^tBu. 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^tBu was found to exhibit a monomeric square-planar molecular structure, its isopropyl-substituted analog 4^iPr 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^iPr, the amidinato complex 6^iPr, and the formato compound 7^tBu were also characterized by X-ray diffraction.