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]

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)F-2] (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(B-Me2Cl)] (4bd) [B-Cl = 1.969(2)], the inverse chelate Co[C5H4(BiPr2)](2)(mu-F) (4ac) with the very rare feature of fluorine bridging two boron centers [C-2 symmetric; B-F 1.641(4) Angstrom, B-F-B' = 148.4(3)degrees], and the doubly quaternized di-adduct NMe4[CO{C5H4(BiPr2F)}2] (5) [exactly centrosymmetric; B-F = 1.477(4) Angstrom] 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)(mu-NH2) (4aa) (static in variable temperature NMR spectra, with diastereotopic Me groups), Co[C5H4(BR2)](2)(mu-OH) [4ab: R = iPr; 4bb: R = Me; dynamic; for 4ab T-c = 95 +/- 5 degreesC, DeltaG(368)(not equal) = 75(1) kJ.mol(-1) for interchange of the diastereotopic Me groups], and Co[C5H4(BMe2)](2)(mu-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 (DeltaG(368)(not equal) approximate to 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 (Delta(R)H = 2.5 +/- 1.1 kJ.mol(-1), Delta(R)S = 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. 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)(mu-NH2) (4aa) (static in variable temperature NMR spectra, with diastereotopic Me groups), Co[C5H4(BR2)](2)( mu-OH) [4ab: R = iPr; 4bb: R = Me; dynamic; for 4ab T-c = 95 +/- 5 degreesC, DeltaG(368)(not equal) = 75(1) 36 kJ(.)mol(-1) for interchange of the diastereotopic Me groups], and Co[C5H4(BMe2)](2)(mu-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 (DeltaG(368)(not equal) approximate to 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 (Delta(R)H = 2.5 +/- 1.1 kJ(.)mol(-1), Delta(R)S = 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.