(ARENE)CYCLOPENTADIENYLCHROMIUM CHEMISTRY - SYNTHESIS, EPR, NMR, AND CYCLIC VOLTAMMETRY OF NEUTRAL COMPOUNDS AND THEIR MONOCATIONS

The unsymmetric sandwich compounds Cp()Cr(arene) (arene = benzene (3) , benzene-d(6), (3d(6)), toluene (4), toluene-d(8), (4d(8)), ethylbenz ene (5), mesitylene (6), ethylmesitylene (7), hexamethylbenzene (8)) w ere 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) an d Cp'Cr(C6H6) (2) (Cp' = CH3C5H4) were also made in low yield. Well-re solved 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 n uclear 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 th e delocalization are antisymmetric. Cyclic voltammetry showed 3, 6, an d 8 to undergo quasi-reversible electron transfers to [Cp()Cr(arene)] (-) and [Cp()Cr(arene)](+), while the oxidation to the dication was f ollowed 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)](-), an d [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 ferro magnetic donor-acceptor stacks.