Synthesis and characterisation of permethylindenyl complexes of cobalt and chromium: crystal structures of [Cr(η5-C9Me7)2], [Co(η5-C9Me7)2][PF6] and [Cr(η5-C9Me7)2][PF6]

Abstract

Treatment of lithium heptamethylindenide with either anhydrous [Co(acac)₂](acac = acetylacetonate) or CrCl₂ in tetrahydrofuran (thf) gave [Co(η⁵-C₉Me₇)₂] and [Cr(η⁵-C₉Me₇)₂] respectively. Single-crystal X-ray studies showed that [Cr(η⁵-C₉Me₇)₂] belongs to the centrosymmetric space group Pbca with a= 19.333(10), b= 14.586(9), c= 9.241(2)Å, 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)Å. Gas-phase UV photoelectron spectroscopy experiments on [Cr(η⁵-C₉Me₇)₂] yield a first ionisation potential of 4.67 eV. The complexes [Co(η⁵-C₉Me₇)₂][PF₆] and [Cr(η⁵-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 P. For [Co(η⁵-C₉Me₇)₂][PF₆], a= 14.332(15), b= 16.661(11), c= 14.426(15)Å, α= 109.55(10), β= 89.99(10), γ= 112.75(9), Z= 2, R= 0.064 and R′= 0.071, average M–C₉Me₇(C₅-centroid) distance = 1.68(2)Å. For [Cr(η⁵-C₉Me₇)₂][PF₆], a= 14.297(8), b= 16.842(9), c= 14.169(9)Å, α= 109.10(5), β= 90.06(5), γ= 111.49(4)°, Z= 2, R= 0.075, R′= 0.095, average M–C₉Me₇(C₅-centroid) distance = 1.84(1)Å. The cyclic voltammogram of [Cr(η⁵-C₉Me₇)₂][PF₆] in MeCN indicates that this complex is redox active with a one-electron reduction at E_½=–1.15 V vs. saturated calomel electrode (SCE). The solid-state magnetic susceptibility data for [Co(η⁵-C₉Me₇)₂], [Cr(η⁵-C₉Me₇)₂] and [Cr(η⁵-C₉Me₇)₂][PF₆] can be modelled by the Curie law (χ_M=C/T) in the temperature range 4–300 K with µ_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_∥= 1.99 (m_s=→m_s=–½) and the double quantum transition g⊥= 4.33 (m_s=–→m_s=½), [Cr(η⁵–C₉Me₇)₂] also exhibits an axially symmetric powder line-shape at 11 K with g_∥= 2.04 and g⊥= 2.06.