Ruthenium complexes containing 2-(2-nitrosoaryl)pyridine: structural, spectroscopic, and theoretical studies.
Ruthenium complexes containing 2-(2-nitrosoaryl)pyridine (ON(^)N) and tetradentate thioether 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4), [Ru(ON(^)N)([14]aneS4)](2+) [ON(^)N = 2-(2-nitrosophenyl)pyridine (2a), 10-nitrosobenzo[h]quinoline (2b), 2-(2-nitroso-4-methylphenyl)pyridine, (2c), 2-(2-nitrosophenyl)-5-(trifluoromethyl)pyridine (2d)] and analogues with the 1,4,7-trithiacyclononane ([9]aneS3)/tert-butylisocyanide ligand set, [Ru(ON(^)N)([9]aneS3)(C≡N(t)Bu)](2+) (4a and 4b), have been prepared by insertion of a nitrosonium ion (NO(+)) into the Ru-aryl bond of cyclometalated ruthenium(II) complexes. The molecular structures of the ON(^)N-ligated complexes 2a and 2b reveal that (i) the ON(^)N ligands behave as bidentate chelates via the two N atoms and the bite angles are 86.84(18)-87.83(16)° and (ii) the Ru-N(NO) and N-O distances are 1.942(5)-1.948(4) and 1.235(6)-1.244(5) Å, respectively. The Ru-N(NO) and N-O distances, together with ν(N═O), suggest that the coordinated ON(^)N ligands in this work are neutral moiety (ArNO)(0) rather than monoanionic radical (ArNO)(•-) or dianion (ArNO)(2-) species. The nitrosated complexes 2a-2d show moderately intense absorptions centered at 463-484 nm [ε(max) = (5-6) × 10(3) dm(3) mol(-1) cm(-1)] and a clearly discriminable absorption shoulder around 620 nm (ε(max) = (6-9) × 10(2) dm(3) mol(-1) cm(-1)), which tails up to 800 nm. These visible absorptions are assigned as a mixing of d(Ru) → ON(^)N metal-to-ligand charge-transfer and ON(^)N intraligand transitions on the basis of time-dependent density functional theory (TD-DFT) calculations. The first reduction couples of the nitrosated complexes range from -0.53 to -0.62 V vs Cp(2)Fe(+/0), which are 1.1-1.2 V less negative than that for [Ru(bpy)([14]aneS4)](2+) (bpy = 2,2'-bipyridine). Both electrochemical data and DFT calculations suggest that the lowest unoccupied molecular orbitals of the nitrosated complexes are ON(^)N-centered. Natural population analysis shows that the amount of positive charge on the Ru centers and the [Ru([14]aneS4)] moieties in 2a and 2b is larger than that in [Ru(bpy)([14]aneS4)](2+). According to the results of the structural, spectroscopic, electrochemical, and theoretical investigations, the ON(^)N ligands in this work have considerable π-acidic character and behave as better electron acceptors than bpy.
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, People's Republic of China.
Siu-Chung
SC
Chan
Ho-Yuen
HY
Cheung
Chun-Yuen
CY
Wong