Revisiting the structure and properties of mid-valent monopentamethylcyclopentadienylchromium complexes

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 Cr^II/Cr^I 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.