(ARENE)CYCLOPENTADIENYLCHROMIUM CHEMISTRY - SYNTHESIS, EPR, NMR, AND CYCLIC VOLTAMMETRY OF NEUTRAL COMPOUNDS AND THEIR MONOCATIONS
Kohler, FH; Metz, B; Strauss, W
The unsymmetric sandwich compounds Cp(*)Cr(arene) (arene = benzene (3), benzene-d(6), (3d(6)), toluene (4), toluene-d(8), (4d(8)), ethylbenzene (5), mesitylene (6), ethylmesitylene (7), hexamethylbenzene (8)) were synthesized from [Cp(*)CrCl(2)](2) in yields up to 75% by applying three procedures. Two reaction pathways could be substantiated by NMR measurements and cyclic voltammetry. For comparison CpCr(C6H6) (1) and Cp'Cr(C6H6) (2) (Cp' = CH3C5H4) were also made in low yield. Well-resolved EPR spectra in liquid and solid solutions were obtained below -120 degrees C, while H-1 and C-13 NMR spectra could be recorded at 25 degrees C and above. From the spectroscopic results a (2)A(1) (e(2)(4), a(1)(l)) ground state was deduced; its bearing on the electron and nuclear relaxation was analyzed. Appreciable spin density was found on the ligands; in the ligand pi orbitals the spin is negative, more spin sits on the arene than on Cp((*))), and the pi orbitals dominating the delocalization are antisymmetric. Cyclic voltammetry showed 3, 6, and 8 to undergo quasi-reversible electron transfers to [Cp(*)Cr(arene)](-) and [Cp(*)Cr(arene)](+), while the oxidation to the dication was followed by chemical reaction. The hitherto unknown sandwich type [Cp(*)Cr(arene)](+) could be synthesized in the case of 3(+), 6(+), and 8(+) as [PF6](-) and partly as [B(C6H5)(4)](-), I-, [Cp(*)CrCl(3)](-), and [Cp(*)CrI(3)](-) salts. The H-1 and C-13 NMR spectra proved that the monocations are paramagnetic having most probably a (3)E(2) (e(2)(3), a(1)(l)) ground state. Negative spin density was found in the ligand pi system so that these cations are potential building blocks in ferromagnetic donor-acceptor stacks.