Ruthenium Carbene and Allenylidene Complexes Supported by the Tertiary Amine-Aromatic Diimine Ligand Set: Structural, Spectroscopic, and Theoretical Studies

Abstract

Ruthenium−methoxycarbene and −allenylidene complexes bearing 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3Tacn) and 1,10-phenanthroline (phen), [(Me3Tacn)(phen)Ru═C(OMe)R]2+ (R = CH2Ph (1), CH═CPh2 (2), CH═C(C6H4Cl-4)2 (3), CH═C(C6H4Me-4)2 (4)), and [(Me3Tacn)(phen)Ru═C═C═CR2]2+ (R = Ph (5), C6H4OMe-4 (6)) have been prepared. The molecular structures of 1(PF6)2 and 2(PF6)2 reveal Ru−C distances of 1.917(3) and 1.906(4) Å, respectively. The lowest-energy dipole-allowed absorptions for complexes 1−4 (λmax ≈ 435 nm) are assigned as dπ(RuII) → π*(phen) metal-to-ligand charge transfer (MLCT) transitions, while those for complexes 5 and 6 (λmax = 530 and 585 nm, respectively) are assigned as metal-perturbed π−π* [Ru═C═C═CR2] intraligand transitions. Complexes 1−4 are emissive in glassy MeOH/EtOH at 77 K: excitation at λ = 430 nm produces emission at λmax = 570−620 nm, which are tentatively assigned as dπ(RuII) → π*(phen) 3MLCT in nature. Density functional theory (DFT) calculations, charge decomposition analysis (CDA), and natural bond orbital (NBO) analysis on complexes 1, 2, 5, and 6 suggest that allenylidene ligands are better electron donors and poorer acceptors compared with methoxycarbene ligands, and the Ru−C interactions in ruthenium−allenylidene and −methoxycarbene complexes can be depicted by the polarized formulation Ruδ+═Cδ− and nonpolarized formulation Ru═C, respectively. The methoxycarbene/allenylidene rotational barriers on 1, 2, and 5 are calculated to be 8.3, 6.3, and 1.5 kcal mol−1, respectively.