SYNTHESIS AND CHARACTERIZATION OF PERMETHYLINDENYL COMPLEXES OF COBALT AND CHROMIUM - CRYSTAL-STRUCTURES OF [CR(ETA-5-C9ME7)2], [CO(ETA-5-C9ME7)2][PF6] AND [CR(ETA-5-C9ME7)2][PF6]

Treatment of lithium heptamethylindenide with either anhydrous [Co(acac)2] (acac = acetylacetonate) or CrCl2 in tetrahydrofuran (thf) gave [Co(eta5-C9Me7)2] and [Cr(eta5-C9Me7)2] respectively. Single-crystal X-ray studies showed that [Cr(eta5-C9Me7)2] belongs to the centrosymmetric space group Pbca with a = 19.333(10), b = 14.586(9), c = 9.241 (2) angstrom, Z = 4, R = 0.043 and R' = 0.047. The metal centre lies on a crystallographic inversion centre and the permethylindenyl rings adopt a staggered conformation with a M-C9Me7 (C5-centroid) distance of 1.81 (2) angstrom. Gas-phase UV photoelectron spectroscopy experiments on [Cr(eta5-C9Me7)2] yield a first ionisation potential of 4.67 eV. The complexes [Co(eta5-C9Me7)2] [PF6] and [Cr(eta5-C9Me7)2] [PF6] were isolated by treatment of the neutral complexes with NH4PF6 in thf. Single-crystal X-ray studies show that both of these complexes belong to the centrosymmetric space group P1BAR. For [Co(eta5-C9Me7)2] [PF6], a = 14.332(15), b = 16.661 (11), c = 1 4.426 (15) angstrom, alpha = 109.55 (10), beta = 89.99 (10), gamma = 112.75(9), Z = 2, R = 0.064 and R' = 0.071, average M-C9Me7 (C5-centroid) distance = 1.68(2) angstrom. For [Cr(eta5-C9Me7)2] [PF6], a = 14.297(8), b = 16.842(9), c = 14.169(9) angstrom, alpha = 109.10(5), beta = 90.06(5), gamma = 111.49(4)degrees, Z = 2, R = 0.075, R' = 0.095, average M-C9Me7 (C5-centroid) distance = 1.84(1) angstrom. The cyclic voltammogram of [Cr(eta5-C9Me7)2][PF6] in MeCN indicates that this complex is redox active with a one-electron reduction at E1/2 = -1.15 V vs. saturated calomel electrode (SCE). The solid-state magnetic susceptibility data for [Co(eta5-C9Me7)2], [Cr(eta5-C9Me7)2] and [Cr(eta5-C9Me7)2] [PF6] can be modelled by the Curie law (chi(M) = C/T) in the temperature range 4-300 K with mu(eff) = 1.65, 2.67 and 3.47 for the three complexes respectively. The latter exhibits an axially symmetric EPR signal at 6 K with g(parallel-to) = 1.99 (m(s) = -3/2 --> m(s) = -1/2) and the double quantum transition g(perpendicular-to) = 4.33 (m(s) = -3/2 --> m(s) = 1/2), [Cr(eta5-C9Me7)2] also exhibits an axially symmetric powder line-shape at 11 K with g(parallel-to) = 2.04 and g(perpendicular-to) = 2.06.