Gold(I) cationic complexes of general formula [LAu+...X-] [L = phosphines or NHCs (N-Heterocyclic Carbenes), X- = weakly coordinating anion] are successfully employed as catalysts in a large variety of organic reactions involving the activation of unsaturated carbon-carbon bonds.[1] A key role in such reactions is played by the counterion [2], which strongly affects activity, regio- and stereo-selectivity. [3]
The linear phosphine gold(I) alkyne [4] complex [(PArF3)Au(2-hexyne)]BF4 (1BF4) [ArF=3,5-bis(trifluoromethyl) phenyl], its analogous [(NHC)Au(2-hexyne)]BF4 (2BF4) [NHC=1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene] and the alkenes complexes [5] [(PPh3)Au(4-Me-styrene)]BF4 (3BF4) and [(NHC)Au(4-Me-styrene)]BF4 [4BF4; NHC = 1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene] have been synthesized. Their intramolecular and interionic structures have been investigated by combining 1D and 2D multinuclear NMR spectroscopy and Density Functional Theory calculations.
The relative anion-cation orientation in [LAuS...X] [S= alkenes and alkynes] ,investigated by NMR spectroscopy and DFT, shows that the exact position of the counterion is critically determined by the nature of the ancillary ligand and substrate; this opens the way to a greater control over the properties and activity of these catalysts.
Also, It has been found that unsatured bonds in the phosphine complexes is depleted of electron density [6] to a greater extent than in NHC ones. In the case of alkynes the charge trasferred correlates with the (13C) NMR of the carbons triple bond. 2BF4 is much more “kinetically stable” than 1BF4. All these findings support the view that catalysts with phosphine ancillary ligands are more effective in activating alkyne substrates (higher TOF), while NHC catalysts are more robust (higher TON).
References
[1] Hashmi, A. S. K. Chem. Rev., 107, 3180. (2007)
[2] Macchioni, A. Chem. Rev., 105, 2039. (2005)
[3] Toste, F. D. et al. Science, 317, 496. 2007
[4] Zuccaccia, D. et al Inorg. Chem, , 49, 3080 (2010)
[5] Zuccaccia D. et al. J. Am. Chem. Soc.,131, 3170 (2009)
[6] Belpassi, L. et al. J. Am. Chem. Soc., 130, 1048. (2008)
