Anion Binding of 1,1′-Bis(dialkylboryl)cobaltocenium Complexes − Structures of [Co{C₅H₄(BMe₂)}₂]PF₆, Co[C₅H₄(BMe₂)][C₅H₄(BMe₂Cl)], Co[C₅H₄(BiPr₂)]₂(µ-F) and NMe₄[Co{C₅H₄(BiPr₂F)}₂]

Graphical Abstract

Abstract

The cobaltocenium salts [Co{C₅H₄(BiPr₂)}₂]PF₆ [(1a)PF₆] and [Co{C₅H₄(BMe₂)}₂]PF₆ [(1b)PF₆] are strong Lewis acids. With pyridine (1a)PF₆ forms a mono-adduct 2 and a di-adduct 3 which show NMR spectra in the low-temperature regime of pyridine exchange. With chloride from PPh₄Cl the adduct formed is dynamic; quite remarkably, the formation of an anionic di-adduct is also observed. The mono-adducts (1a)X (X = F, Cl, Br, I, OH, and NH₂) and (1b)X (X = F, Cl, and OH) as well as the di-adduct salts NMe₄[(1a)F₂] (5) and K[(1a)(OH)₂] (6) are made by treating the salts (1a,b)PF₆ in CH₂Cl₂ or MeNO₂ with salts PPh₄X, NMe₄F, or powders of NaNH₂ or KOH in the appropriate ratios. X-ray single-crystal structures of the salt [Co{C₅H₄(BMe)₂}₂]PF₆ [(1b)PF₆], the semi-quaternized mono-adduct Co[C₅H₄(BMe₂)][C₅H₄(BMe₂Cl)] (4bd) [B−Cl = 1.969(2) Å], the inverse chelate Co[C₅H₄(BiPr₂)]₂(µ-F) (4ac) with the very rare feature of fluorine bridging two boron centers [C₂ symmetric; B−F = 1.641(4) Å, B−F−B′ = 148.4(3)°], and the doubly quaternized di-adduct NMe₄[Co{C₅H₄(BiPr₂F)}₂] (5) [exactly centrosymmetric; B−F = 1.477(4) Å] are given. Solution structures of the 1:1 products greatly depend on the nature of the anion, displaying i) exclusively ionic structures for (1a)PF₆ and (1b)PF₆, ii) semi-quaternized structures for the heavier halides (1a)Br (4ae) and (1a)I (4af) with some noticeable ionic dissociation, iii) semi-quaternized structures in equilibrium with minor amounts of inverse chelate isomers for (1a)F (4ac), (1b)Cl (4bd), and very likely for (1a)Cl (4ad), and iv) stable inverse chelate structures for Co[C₅H₄(BiPr₂)]₂(µ-NH₂) (4aa) (static in variable temperature NMR spectra, with diastereotopic Me groups), Co[C₅H₄(BR₂)]₂(µ-OH) [4ab: R = iPr; 4bb: R = Me; dynamic; for 4abT_c = 95 ± 5 °C, ΔG^≠₃₆₈ = 75(1) kJ·mol⁻¹ for interchange of the diastereotopic Me groups], and Co[C₅H₄(BMe₂)]₂(µ-F) (4bc). The stability of the inverse chelates decreases in the order amide (4aa) > hydroxides (4ab and 4bb) > fluorides (4ac and 4bc) > chlorides (4ad and 4bd), and also in the order BMe₂ > BiPr₂ (specifically 4bc > 4ac, and 4bd > 4ad). Variable temperature NMR spectra of solutions of 4bd (CD₂Cl₂, 173−243 K) show that i) the ring-opening of the chelated chloride (ΔG^≠₃₆₈ ≈ 45 kJ·mol⁻¹) is energetically easier than for the chelated hydroxide 4ab, ii) the predominance of the semi-quaternized isomer over the inverse chelate (Δ_RH = 2.5 ± 1.1 kJ·mol⁻¹, Δ_RS = 37.6 ± 5.4 kJ·mol⁻¹·K⁻¹) in the equilibrium is entropic in nature, and iii) the semi-quaternized isomer still undergoes fast chloride-exchange in the low-temperature regime of this equilibrium, proving the existence of an independent, intermolecular chloride-exchange mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)