Synthesis, structures, and properties of luminescent (C∧N∧C)gold(III) alkyl complexes: Correlation between photoemission energies and C–H acidity

The reaction of (C(<^>)z(pz<^>)C)AuCI with C-H acidic compounds H,(CRR2)-R-1 in the presence of base readily affords thecorresponding alkyl complexes ((CNC)-N-<^>-C-pz <^>)(AuCHRR2)-R-1 (2a, R-1 = R-2 = CN; 3a, R-1 = R-2 = C(O)Me; 4, = R-2 = CO2Et; 5, R-1=R-2 = C(O)Ph; 6a, R-1 = H, RZ = C(O)Me). The analogouspyridine-pincer complexes 2b, 3b, and 6b were obtained similarly (C(<^>)NPZ(<^>)C = 2,6-bis(4'-tBuC(6)H(3))(2)pyrazine dianion; CANPYAC =2,6-bis(4'-BuC6H3)(2)pyridine dianion). The reactions of(CANP'AC)AuOAcF (7a) and (CANPYAC)AuOAcF (7b) with MeMgI gave the methyl complexes (C(<^>)N(pz<^>)c)AuMe (8a) and (CANPYAC)AuMe (8b), respectively. The crystal structures of 2a,b, 3a, 6a, and 7a have been determined. The photophysical properties of the new complexes and those of the previously reported gold hydride (C(boolean AND)Npz(boolean AND)C )AUH (AuH) are reported. The lower-energy absorption and the emission maxima follow the energy sequence 2a < 3a < 4 < 5 < 6a < AuH 8a for the pyrazine series and 2b < 3b < 6b 8b for the pyridine series. These values provide a correlation with the pK(a), values of the corresponding ancillary ligand precursors. In agreement with DFT calculations, the emissions are assigned to 3IL(c(<^>)NA<^>C)/(ILCT)-I-3 ((C(6)H(4)tBu-4') -> pz/py*) transitions, dominated by the HOMO and the LUMO orbitals. The LUMOis located in the heterocycle (py/pz) in a position trans to the ancillary ligand, which makes this orbital more sensitive to the electronic nature of the ancillary ligand than the HOMO. The calculations establish that the charge transfer from the tBuC(6)H(3) ligand fragments to the central heterocycle ring in the dominant transition explains the modulation of the properties with the sigma-donor characteristics of the alkyl or hydride ligands.