Synthesis and Structures of Strained, Neutral [d7] and Cationic [d6] Hydrocarbon-Bridged [n]Cobaltocenophanes (n = 2, 3)

• Published in: Organometallics 2008-04, Vol.27 (7), p.1524-1533
• Main Authors: Mayer, Ulrich F. J, Charmant, Jonathan P. H, Rae, James, Manners, Ian
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om700941v

The first neutral and paramagnetic [n]cobaltocenophanes Co(η5-C5H4)2(CH2)2 (5) and Co(η5-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 α: 27.1(4)°) and therefore highly strained, whereas [3]cobaltocenophane 6 is only moderately distorted (α: 12.0(11)°). Both 5 and 6 were readily oxidized with NH4Cl in the presence of atmospheric oxygen to form the corresponding cationic [n]cobaltocenophane salts [Co(η5-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 α: 21.4(2)°) 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° appeared to vary as a function of the tilt angle.