A series of new pyrazolyl-1,2,3-triazolyl N–N′ bidentate donor ligands (2a–c, 3a–d) were prepared via Cu(I)-catalyzed
Huisgen cycloaddition reactions between 1-propargylpyrazoles and 4-substituted
phenyl azides. The electron-withdrawing ability of the substituents
follows the trend PhCH2 < p-CH3Ph < Ph < p-CF3Ph < p-NO2Ph, as illustrated in the gradual downfield
shift of the 1,2,3-triazolyl-C4′ 13C NMR resonances. A series of Rh and Ir complexes containing these
pyrazolyl-1,2,3-triazolyl or bis(pyrazol-1-yl)methane donor ligands
of general formulae [Ir(N–N′)Cp*Cl]X (X = BArF
4, BPh4; 5–8), [Rh(N–N′)Cp*Cl]X (X = BArF
4, BPh4; 9–11), and [Rh(N–N′)(CO)2]BArF
4 (13–16) (BArF
4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)
were synthesized and fully characterized. The solid-state structures
of 5, 6a′, 6b, 7b, 8, 9, 10a, 10a′, 11, and 15c were determined
by X-ray diffraction studies. As the electron-withdrawing strength
of the phenylene substituent on the triazolyl ring is increased, the
M–N3′(triazole) bond length becomes longer. The efficiency
of these Rh and Ir complexes as catalysts for the synthesis of tricyclic
indoles via tandem C–N and C–C bond formation reactions
from 2-(hydroxyalk-1-ynyl)anilines (17S–20S) was assessed. The Ir(III) catalysts were the most efficient
for the C–C bond formation step, and the Rh(I) complexes 13–16 were the most efficient catalysts
for C–N bond formation, where TOFs >1000 h–1 were reached. However, the Ir(III) complexes 5–8 were found to be the only active catalysts for the tandem
C–N and C–C bond formation, as the Rh(I) complexes were
not active catalysts for the C–C bond formation step. The C–N
bond formation leading to the formation of indoles was found to proceed
via two reaction pathways with 2-(hydroxyalk-1-ynyl)aniline substrates:
(a) hydroamination and (b) hydroalkoxylation–Lewis acid mediated
isomerization. Pathway (b) is likely to be the main pathway in the
formation of indoles starting with 2-(hydroxyalk-1-ynyl)aniline substrates 17S, 18S, and 20S.
