Synthesis, spectroscopic and theoretical studies of Ruthenium complexes containing nitrosoarene-pyridine hybrid ligands

Abstract

Ruthenium complexes containing 2-(2-nitrosoaryl)pyridine (ON^N) and 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4), [Ru([14]aneS4)(ON^N)]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); 2-(2-nitroso-4-nitrophenyl)pyridine (2e); 2-(2-nitroso-5-nitrophenyl)pyridine (2f)] and analogues with the 1,4,7-trithiacyclononane ([9]aneS3)/L ligand set, [Ru([9]aneS3)(ON^N)L]2+ [ON^N = 2-(2-nitrosoaryl)pyridine, L = CH3CN (4a-CH3CN); L = tert-butylisocyanide (4a-BuNC); ON^N = 10-nitrosobenzo[h]quinoline, L = tert-butylisocyanide (4b-BuNC)] have been prepared by insertion of the nitrosonium ion (NO+) into the Ru-aryl bond of cyclometalated ruthenium(II) complexes [Ru([14]aneS4)(C^N)]+ (1a–1f) and [Ru([9]aneS3)(C^N)L]+ (3a-CH3CN, 3a-BuNC and 3b-CH3CN, C^N = 2-phenylpyridine anion-type cyclometalating ligands) respectively. Kinetic studies on the reaction between NO+ and 3a-BuNC reveal that the insertion reaction follows a second order rate law of the form k[NO+][3a-BuNC]. Theoretical calculations suggest that the insertion reaction may be rationalized as a frontier orbital interaction between the [Ru-aryl]-dominated highest occupied molecular orbital (HOMO) of the Ru(II) complexes and the lowest unoccupied molecular orbital (LUMO) of the NO+. The Ru-(ON^N) bonding interaction has been studied by structural, spectroscopic, electrochemical and theoretical methods. The crystal structures of five ON^N-ligated complexes 2a, 2b, 2e, 2f, and 4a-CH3CN have been obtained. Important structural findings include: (i) the ON^N ligands behave as bidentate chelates via the two N atoms, (ii) the Ru-NNO and N-O distances are 1.928(3)-1.948(4) and 1.235(6)-1.250(4) Å , respectively. The Ru-NNO and N-O distances, together with N=O (1353-1396 cm-1), suggest that the coordinated ON^N ligands in these complexes are neutral moieties rather than charged species. The nitrosated complexes feature moderately intense absorption bands covering the entire visible region. It is noted that the lowest-energy transition of the nitrosated complexes is red shifted by at least 3000 cm-1 compared with that of [Ru([14]aneS4)(bpy)]2+. Time dependent-density function theory (TD-DFT) calculations reveal that the lowest-energy transitions are a mixture of d(Ru) →ON^N MLCT and ON^N intra-ligand (IL) transitions. The first reduction potential of the nitrosated complexes are 1.1-1.4 V less negative than that of [Ru([14]aneS4)(bpy)]2+ (bpy = 2,2'-bipyridine). Both electrochemical data and DFT calculations suggest that the LUMO of the nitrosated complexes are ON^N-centered. Theoretically calculated charge distributions in 2a, 2b, and [Ru(bpy)([14]aneS4)]2+ show that the ON^N ligands are better electron acceptors than bpy. The one-electron reduced species of 2a, 2e, and 2f have been prepared by either chemical or electrochemical reduction in order to obtain the 2-(2-nitrosoaryl)pyridine monoanion radical (ON^N)•– -ligated complexes (5a, 5e, and 5f). The crystal structures of 5e and 5f reveal that (i) the Ru–NNO distances in 5e and 5f (2.049(3)–2.062(4) Å ) are notably longer than those in 2e and 2f (1.929(3)–1.934(4) Å ); (ii) the NNO-O bond distances of 5e and 5f (1.285(5)–1.308(4) Å ) are intermediate between a single and a double bond. All these three (ON^N)•– -ligated complexes exhibit electron paramagnetic resonance (EPR) spectra. The isotropic g-values together with hyperfine splitting parameters suggest that the spin density is localized in a molecular orbital with significant contribution from a single nitrogen atom closely associated with the Ru center. This is consistent with the DFT calculations which show that the calculated Mulliken spin densities for 5a, 5e and 5f are distributed mainly on the ON^N ligands (0.92-0.94 electrons). Apart from thioether-ligated systems, polypyridine-ligated Ru-(NO-PhPy) (NO-PhPy = 2-(2-nitrosophenyl)pyridine) complexes [Ru(bpy)2(NO-PhPy)]2+ (6a) and [Ru(terpy)(NO-PhPy)L]2+ [terpy = 2,2';6',2"-terpyridine; L = CH3CN (7a-CH3CN); L = NO2 (7a- NO2)] have also been prepared. The Ru-NNO and N-O distances for 6a and 7a-CH3CN have been determined to be 1.922(3)–1.933(4) and 1.253(4)–1.256(5) Å , respectively by X-ray crystallography, revealing that the coordinated NO-PhPy in these complexes is a neutral moiety. The successful synthesis of these Ru-(NO-PhPy) complexes suggests that ON^N is a type of versatile bidentate ligand which is non-innocent in nature.

Date of Award	14 Feb 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chun Yuen Alex WONG (Supervisor) & Hoi Lun KWONG (Co-supervisor)