Versatility and adaptative behaviour of the P^N chelating ligand MeDalphos within gold( i ) π complexes
Résumé
The hemilabile P^N ligand MeDalphos enables access to a wide range of stable gold(I) -complexes with unbiased alkenes and alkynes, as well as electron-rich alkenes and for the first time electron-poor ones. All complexes have been characterized by multi-nuclear NMR spectroscopy and whenever possible, by X-ray diffraction analyses. They all adopt rare tricoordinate environment around gold(I), with chelation of the P^N ligand and side-on coordination of the alkene, including the electron-rich one, 3,4-dihydro-2H-pyrane. The strength of the N→Au coordination varies significantly in the series. It is the way the P^N ligand accommodates the electronic demand at gold, depending on the alkene. Comparatively, when the chelating P^P ligand (ortho-carboranyl)(PPh2)2 is used, gold(I) -complexes are only isolable with unbiased alkenes. The bonding situation within the gold(I) P^N -complexes has been thoroughly analyzed by DFT calculations supplemented by Charge Decomposition Analyses (CDA), Natural Bond Orbital (NBO) and Atoms-in-Molecules (AIM) analyses. Noticeable variations in the donation / back-donation ratio, C=C weakening, alkene to gold charge transfer and magnitude of the N→Au coordination were observed. Detailed examination of the descriptors for the Au/alkene interaction and the N→Au coordination actually revealed intimate correlation between the two, with linear response of the MeDalphos ligand to the alkene electronics. The P^N ligand displays non-innocent and adaptative character. The isolated P^N gold(I) -complexes are reactive and catalytically relevant, as substantiated by the chemo and regio-selective alkylation of indoles. 2
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