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 TpiPr2ligand and reduction features of Cr(ii) species

Abstract

Divalent chlorochromium complexes bearing the Tp^iPr2 ligand [hydrotris(3,5-diisopropylpyrazolyl)borato], Tp^iPr2Cr^IICl(L) [L = 3,5-diisopropylpyrazole (1a), py (1b), bipy (2)] and Tp^iPr2Cr^II(μ-Cl)]₂ (3), were prepared by treatment of CrCl₂ with a Tp^iPr2 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, Tp^iPr2Cr^IIICl₂(L) (6), while reaction with CH₂Cl₂ gives the methylene-inserted product, Tp^iPr2Cr^IIICl(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, Tp^iPr2Cr[κ²-O–C(Ph)C(Ph)–O] 8. Steric tuning effects of the Tp^iPr2 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.