Versatility and adaptative behaviour of the PN chelating ligand MeDalphos within gold(i) π complexes

The hemilabile P<^>N ligand MeDalphos enables access to a wide range of stable gold(i) pi-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 a 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) pi-complexes are only isolable with unbiased alkenes. The bonding situation within the gold(i) P<^>N pi-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 & xe001;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) pi-complexes are reactive and catalytically relevant, as substantiated by the chemo and regio-selective alkylation of indoles.