Three situations are observed when the new cobaltocene derivative Co(C5H4BPri 2)2 (2) is oxidized. The cobaltocenium salt [2]PF6 (3) is obtained with [FeCp2]PF6 as oxidant. With Cu(OH)2 the inverse chelate [2]OH (4) with a μ-OH group bridging the two boron centers is formed. Oxidation with C2Cl6 gives a zwitterionic chloride (5) with a trigonal and a quaternized boron center; in solution 5 is fluxional.

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 4ab Tc = 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 (ΔRH = 2.5 ± 1.1 kJ·mol−1, ΔRS = 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)

Bis(boryl)cobaltocenes Co(C5H4BR2)2 (1) can be made from CoBr2(DME) and alkali metal borylcyclopentadienides M(C5H4BR2) (M = Li, Na) (2). The two dialkylamino compounds 1c (R = NMe2) and 1d (R = NEt2) can be obtained in this way. Oxidation with C2Cl6 provides the ionic cobaltocenium chlorides (1c)Cl and (1d)Cl. Further cobaltocenium compounds can be synthesized by modification of the substituents at boron. Treatment of (1d)Cl with excess BCl3 affords the highly reactive chloride Co(C5H4BCl2)(C5H4BCl3) (5). Pinacolysis of 5 then affords the monosubstitution product Co[C5H4B(OCMe2)2](C5H4BCl3) (9) and the disubstitution product [Co{C5H4B(OCMe2)2}2]Cl [(1h)Cl], respectively, depending on stoichiometry and reaction conditions. Reaction of 5 with tetramethyltin replaces two chlorine atoms with methyl groups to give Co(C5H4BMe2)(C5H4BCl3) (10), while the more reactive trimethylaluminum replaces four chlorine substituents to give the salt [Co(C5H4BMe2)2]AlCl4 [(1b)AlCl4] and, after metathesis with NBu4PF6 in CH2Cl2, the more convenient hexafluorophosphate (1b)PF6. The corresponding cobaltocene 1b is then accessible via conventional amalgam reduction of (1b)AlCl4. Reaction of 5 with commercial AsF3 affords the robust inverse chelate Co(C5H4BF2)2(μ-OH) (11). Three structural types are encountered for the cobaltocenium derivatives:  (i) ionic compounds (type A) such as (1c,d,h)Cl, (1b)AlCl4, and (1b)PF6; (ii) zwitterionic or semiquaternized compounds (type B) with one trigonal and one tetrahedral boron center such as 5, 9, and 10; of these, 5 is fluxional in solution with two effectively equivalent ligands while 9 and 10 display static structures; and (iii) the inverse chelate structure of 11 (type C) which is found in the crystal and in solution.

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