Revisiting the structure and properties of mid-valent monopentamethylcyclopentadienylchromium complexes

Abstract

The structure and properties of half-sandwich chromium complexes derived from the dinuclear chloride compound [{CrCp*(μ-Cl)}2] (1) (Cp* = η5-C5Me5) are revisited. Complex 1 does not react with H2 and N2 but cleaves the nitrogen–nitrogen bonds of azobenzene and 1,2-diphenylhydrazine at room temperature to give dinuclear chromium(IV) bis(imido) [{CrCp*Cl(μ-NPh)}2] (2) and chromium(III) bis(amido) [{CrCp*Cl(μ-NHPh)}2] (3) derivatives, respectively. Reactions of 1 with cyclopentyllithium [Li(C5H9)] in hexane or toluene under reflux conditions afford the previously reported tetranuclear chromium(II) hydride complex [{CrCp*(μ3-H)}4] (4) and the unsymmetrical chromium(I) sandwich compound [CrCp*(η6-C6H5Me)] (5) as crystals suitable for X-ray diffraction studies. While the treatment of 1 with excess LiBH4 leads to an analogous dinuclear complex [{CrCp*(μ-κ3-BH4)}2] (6), the reaction of the chromium(III) compound [{CrCp*Cl(μ-Cl)}2] with LiBH4 gives the mononuclear species [CrCp*(κ2-BH4)2] (7). Complex 6 cleanly reacts with the 2,6-lutidinium salt (LutH)(BPh4) to form the zwitterionic sandwich derivative [CrCp*(η6-C6H5-BPh3)] (8). Compounds 1 and 6 react with LiAlH4 to give a diamagnetic tetrachromium aggregate [(Al{(μ-H)4CrCp*})4] (9), which can be described as low-spin chromate(II) {CrCp*H4}3− units stabilizing the Al3+ ions primarily through Cr–H → Al interactions and weaker Cr → Al donation according to density functional theory (DFT) calculations. The thermal decomposition of 9 in benzene at 90 °C affords a mixed-valence CrII/CrI hexachromium species [(Al2{(μ-H)4CrCp*}{(μ3-H)4Cr2Cp*2})2] (10) with analogous interactions of {CrCp*H4}3− and {Cr2Cp*2H4}3− units with Al3+ ions.