Synthesis, Structures, and Properties of Luminescent (C∧N∧C)gold(III) Alkyl Complexes: Correlation between Photoemission Energies and C–H Acidity

5 years ago

Synthesis, Structures, and Properties of Luminescent (C∧N∧C)gold(III) Alkyl Complexes: Correlation between Photoemission Energies and C–H Acidity

Manfred Bochmann, Anna Pintus, Benoı̂t Bertrand, Julio Fernandez-Cestau

The reaction of (C^∧N^pz^∧C)AuCl with C–H acidic compounds H₂CR¹R² in the presence of base readily affords the corresponding alkyl complexes (C^∧N^pz^∧C)AuCHR¹R² (2a, R¹ = R² = CN; 3a, R¹ = R² = C(O)Me; 4, R¹ = R² = CO₂Et; 5, R¹ = R² = C(O)Ph; 6a, R¹ = H, R² = C(O)Me). The analogous pyridine-pincer complexes 2b, 3b, and 6b were obtained similarly (C^∧N^pz^∧C = 2,6-bis(4′-^tBuC₆H₃)₂pyrazine dianion; C^∧N^py^∧C = 2,6-bis(4′-^tBuC₆H₃)₂pyridine dianion). The reactions of (C^∧N^pz^∧C)AuOAc^F (7a) and (C^∧N^py^∧C)AuOAc^F (7b) with MeMgI gave the methyl complexes (C^∧N^pz^∧C)AuMe (8a) and (C^∧N^py^∧C)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^∧N^pz^∧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^∧N^∧C)/³ILCT ((C₆H₄^tBu-4′) → pz/py*) transitions, dominated by the HOMO and the LUMO orbitals. The LUMO is 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 tBuC₆H₃ ligand fragments to the central heterocycle ring in the dominant transition explains the modulation of the properties with the σ-donor characteristics of the alkyl or hydride ligands.

Publisher URL: http://dx.doi.org/10.1021/acs.organomet.7b00439

DOI: 10.1021/acs.organomet.7b00439