Synthesis and structures of strained, neutral [d7] and cationic [d6] hydrocarbon-bridged [n]cobaltocenophanes (n=2, 3)

The first neutral and paramagnetic [n]cobaltocenophanes Co(eta(5)-C5H4)(2)(CH2)(2) (5) and Co(eta(5)-C5H4)(2)(CH2)(3) (6) with sterically undemanding hydrocarbon bridges were prepared by the reaction of CoCl2 with the corresponding ligand precursor (LiC5H4)(2)(CH2)(n) (n = 2 for 5; n = 3 for 6) in THF. In order to analyze the ring strain present, the molecular structures of 5 and 6 were determined using single-crystal X-ray diffraction. The neutral, paramagnetic 19-valence-electron [2]cobaltocenophane 5 is substantially distorted from metallocene-type geometry (tilt angle alpha: 27.1(4)degrees) and therefore highly strained, whereas [3]cobaltocenophane 6 is only moderately distorted (alpha: 12.0(11)degrees). Both 5 and 6 were readily oxidized with NH4Cl in the presence of atmospheric oxygen to form the corresponding cationic [n]cobaltocenophane salts [Co(eta(5)-C5H4)(2)(CH2)(n)(+)][A(-)] 7 and 8 (n = 2) and 9 (n = 3). Compound 7 (A = Cl) was isolated by means of extraction into CH2Cl2, whereas 8 and 9 were isolated by phase precipitation from aqueous solutions of NaPF6. The X-ray analysis for cationic, diamagnetic, 18-valence-electron [2]cobaltoceniumphane 7 revealed substantially less distortion from metallocene-type geometry (tilt-angle alpha: 21.4(2)degrees) compared to paramagnetic 19-valence-electron compound 5. In order to examine the electronic structures of 5, 6, 8, and 9, solution UV-vis spectra were recorded, and these reflected the increasing distortion as a result of the introduction of progressively shorter ansa-bridges. Cyclic voltammetric analysis of 8 and 9 in CH2Cl2 revealed that each cobaltoceniumphane exhibits a one-electron reduction, whereby redox potentials E degrees appeared to vary as a function of the tilt angle.