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Anion Binding of 1,1′‐Bis(dialkylboryl)cobaltocenium Complexes − Structures of [Co{C 5 H 4 (BMe 2 )} 2 ]PF 6 , Co[C 5 H 4 (BMe 2 )][C 5 H 4 (BMe 2 Cl)], Co[C 5 H 4 (B i Pr 2 )] 2 (µ‐F) and NMe 4 [Co{C 5 H 4 (B i Pr 2 F)} 2 ]
Author(s) -
Herberich Gerhard E.,
Englert Ulli,
Fischer Andreas,
Wiebelhaus Dag
Publication year - 2004
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200400269
Subject(s) - adduct , chemistry , pyridine , crystal structure , medicinal chemistry , lewis acids and bases , nuclear magnetic resonance spectroscopy , chloride , stereochemistry , crystallography , catalysis , organic chemistry
The cobaltocenium salts [Co{C 5 H 4 (B i Pr 2 )} 2 ]PF 6 [( 1a )PF 6 ] and [Co{C 5 H 4 (BMe 2 )} 2 ]PF 6 [( 1b )PF 6 ] are strong Lewis acids. With pyridine ( 1a )PF 6 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 PPh 4 Cl 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 NH 2 ) and ( 1b )X (X = F, Cl, and OH) as well as the di‐adduct salts NMe 4 [( 1a )F 2 ] ( 5 ) and K[( 1a )(OH) 2 ] ( 6 ) are made by treating the salts ( 1a , b )PF 6 in CH 2 Cl 2 or MeNO 2 with salts PPh 4 X, NMe 4 F, or powders of NaNH 2 or KOH in the appropriate ratios. X‐ray single‐crystal structures of the salt [Co{C 5 H 4 (BMe) 2 } 2 ]PF 6 [( 1b )PF 6 ], the semi‐quaternized mono‐adduct Co[C 5 H 4 (BMe 2 )][C 5 H 4 (BMe 2 Cl)] ( 4bd ) [B−Cl = 1.969(2) Å], the inverse chelate Co[C 5 H 4 (B i Pr 2 )] 2 (µ‐F) ( 4ac ) with the very rare feature of fluorine bridging two boron centers [ C 2 symmetric; B−F = 1.641(4) Å, B−F−B′ = 148.4(3)°], and the doubly quaternized di‐adduct NMe 4 [Co{C 5 H 4 (B i Pr 2 F)} 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 )PF 6 and ( 1b )PF 6 , 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[C 5 H 4 (B i Pr 2 )] 2 (µ‐NH 2 ) ( 4aa ) (static in variable temperature NMR spectra, with diastereotopic Me groups), Co[C 5 H 4 (BR 2 )] 2 (µ‐OH) [ 4ab : R = i Pr; 4bb : R = Me; dynamic; for 4ab T c = 95 ± 5 °C, Δ G ≠ 368 = 75(1) kJ·mol −1 for interchange of the diastereotopic Me groups], and Co[C 5 H 4 (BMe 2 )] 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 BMe 2 > B i Pr 2 (specifically 4bc > 4ac , and 4bd > 4ad ). Variable temperature NMR spectra of solutions of 4bd (CD 2 Cl 2 , 173−243 K) show that i) the ring‐opening of the chelated chloride (Δ G ≠ 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 R H = 2.5 ± 1.1 kJ·mol −1 , Δ 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)