Di- and tri-valent chlorochromium complexes bearing hydrotris(3,5-diisopropylpyrazolyl)borato ligand (TpiPr2), TpiPr2CrCln(L) (n = 1, 2) and [TpiPr2Cr(μ-Cl)]2: flexible coordination behavior of the TpiPr2 ligand and reduction features of Cr(II) species

Divalent chlorochromium complexes bearing the TpiPr2 ligand [hydrotris(3,5-diisopropylpyrazolyl)borato], TpiPr2CrIICl(L) [L = 3,5-diisopropylpyrazole (1a), py (1b), bipy (2)] and TpiPr2CrII(μ-Cl)]2 (3), were prepared by treatment of CrCl2 with a TpiPr2 salt in the presence (1,2)/absence (3) of a donor (L). Reaction of 1 with benzyl chloride results in quantitative conversion to the corresponding trivalent dichlorochromium species, TpiPr2CrIIICl2(L) (6), while reaction with CH2Cl2 gives the methylene-inserted product, TpiPr2CrIIICl(CH2-L) 7, via a carbene intermediate, which is supported by cyclopropanation of styrene with CH2Cl2 in the presence of 3. The Cr(II) complex 1b induces reductive coupling of benzaldehyde to give a 3,4-diphenyl-2,5-dioxachromacyclopent-3-ene, TpiPr2Cr[κ2-O–C(Ph)C(Ph)–O] 8. Steric tuning effects of the TpiPr2 ligand leads to a variety of coordination geometries including five-coordinate square-pyramidal and trigonal-bipyramidal structures and a six-coordinate octahedral structure as revealed by crystallographic characterization.