During the course of our work we have investigated the synthesis of numerous organometallics based on iron and cobalt. Many of these complexes are chiral, and the control of regio, relative and absolute stereochemistry has resulted in structures for incorporation into novel ligands, catalysts and molecular devices.
Cobalt sandwich complexes such as 1 are synthesised readily from the formal 2 + 2 dimerisation of diphenylacetylene when this is heated with a source of Co(I) and a cyclopentadienyl anion.1 On extension, non-symmetrical diarylacetylenes containing a single ortho-substituent give 2 with good trans selectivity, and the steric and conformational properties of these may be used to control highly regioselective Friedel-Crafts acylation.2 In addition to providing a starting point for much of our work on metallacycles, these complexes have also been incorporated to give molecular gears, as illustrated by X-ray structure 3 [a 4:3 gear derived from 2 (R = Me) and an acetylene linked tryptycene (in green)].3
Similarly, chiral linked diynes were anticipated to give planar chiral complexes 4. Instead this reaction results in the highly diastereoselective synthesis of chiral-at-metal cobaltacycle 5.4 When CpCo(CO)2 is used as the cobalt source a result similar to the original objective is achieved, with the moderately diastereoselective synthesis of metallocenes 6.5
Related to bulky cobalt complexes such as 1 is the pentaphenylferrocene building block 7 obtained by Friedel-Crafts acylation of the parent metallocene followed by carbonyl group manipulation.6 Other ferrocene derivatives synthesised include C2-symmetric ferrocenophane 8, for which a key reaction is the stereospecific double alkylation of a C2-symmetric diether.7 Ring closing metathesis may also be used for the synthesis of ferroceneophanes, and we were the first to report this reaction with the synthesis of complexes 9.8
Predictive NMR spectroscopy
The numerous ferrocene and cobalt sandwich complexes synthesised have enabled the construction of tables for the prediction of cyclopentadienyl proton chemical shifts. These may be summed to estimate the chemical shifts of cyclopentadienyl groups containing two or more substituents. Software for predictive 1H NMR spectroscopy of organic compounds is now routinely available, but this has yet to be extended to ferrocene and related complexes. These tables may be used to assist with both signal and compound identification in this area of chemistry.1,9
1. H. V. Nguyen, M. R. Yeamine, J. Amin, M. Motevalli and C. J. Richards, J. Organomet. Chem. 2008, 693, 3668.
2. D. Cassar, E. Nagaradja, D. C. D. Butler, D. Villemin and C. J. Richards, Org. Lett. 2012, 14, 894.
3. A. M. Stevens and C. J. Richards, Tetrahedron Lett. 1997, 38, 7805.
4. J. Amin and C. J. Richards, Chem. Commun. 2012, 48, 10192. [open access]
5. C. J. Taylor, M. Motevalli and C. J. Richards, Organometallics 2006, 25, 2899.
6. D. C. D. Butler and C. J. Richards, Organometallics 2002, 21, 5433.
7. A. J. Locke and C. J. Richards, Organometallics 1999, 18, 3750.
8. A. J. Locke, C. Jones and C. J. Richards, J. Organomet. Chem. 2001, 637 – 639, 669.
9. T. E. Pickett and C. J. Richards, Tetrahedron Lett. 1999, 40, 5251.