There is considerable interest in [Ru(tpy)(L)Cl] n+ type complexes (where tpy = 2,2′:6′,2″-terpyridine, L = bidentate ligand, n = 0 or 1), because they are used as precursors of aquo complexes, which can finally be converted to oxo compounds. Oxo-ruthenium complexes are useful as catalysts in different reactions, particularly in the catalytic oxidation of water to dioxygen.1,2 As a part of serial studies on the properties of oxoruthenium complexes, we synthesized the title complex, [Ru(tpy)(L)Cl]Cl (where L = 2-(phenylazo)pyridine = azpy).2-(Phenylazo)pyridine (azpy) and [Ru(tpy)Cl3] were prepared by methods reported in the literatures. 3,4 Ru(tpy)Cl3 (0.2 g) and azpy (0.09 g) were refluxed for 3 h in 25 mL of ethanol-water containing LiCl (0.07 g) and triethylamine (0.06 mL). A hot solution was filtered and the filtrate was allowed to stand for 10 days. Crystals suitable for an X-ray technique were selected from the obtained solid. Details concerning the X-ray structure determination are given in Tables 1 -3. Ru(II) is in a distorted octahedral environment bonded to three nitrogen atoms of the tpy ligand, two nitrogen atoms of the azpy ligand and one chlorine atom. The four nitrogen atoms (N3, N4, N5, N6) define the equatorial plane, whereas the Cl1 and N1 atoms occupy the trans-axial sites. The longest bond, Ru-Cl1, is comparable to those in cis-Ru(azpy)2Cl2. 5 It is noted that the N(6)-Ru-N(3) angle is 104.82(16)˚, greater than that (99.4(3)˚) in [Ru(tpy)(azpy)(CH3CN)](ClO4)2.

The Ru-N bond distances in the title complex, [Ru(NO(2))(C(11)H(9)N(3))(C(15)H(11)N(3))]BF(4) or [Ru(NO(2))(tpy)(azpy)]BF(4), [tpy is 2,2':6',2"-terpyridine and azpy is 2-(phenylazo)pyridine], are Ru-N(py) 2.063 (4), Ru-N(azo) 2.036 (4), Ru-N(nitro) 2.066 (3) A, and Ru-N(tpy) 2.082 (4), 1.982 (3) and 2.074 (4) A. The azo N atom is trans to the nitro group. The azo N=N bond length is 1.265 (5) A, which is the shortest found in such complexes to date. This indicates a multiple bond between Ru and the N atom of the nitro group, and pi-backbonding [dpi(Ru) --> pi*(azo)] is decreased.

The polypyridine complexes of a type [Ru(tpy)(L)X] n+ (where tpy = 2,2′:6′,2″-terpyridine, L = a bidentate ligand such as 2,2′-bipyridine (bpy), X = a monodentate ligand and n = 0 or 1) have generated considerable interest in several areas, for example, used as effective DNA cleavage agents 1 and as catalysts in oxidation of water to dioxygen 2 based on the corresponding high-valent ruthenium oxo complexes. It is our continuing interest to study structures of ruthenium(II) complexes with a bidentate ligand such as 2-(phenylazo)pyridine (azpy) according to its property as a strong π-acceptor. 3 Azpy and [Ru(tpy)Cl3] were synthesized by procedures reported in the literatures, 3,4 [Ru(tpy)(azpy)Cl]Cl was prepared using a similar method to that described before. 5[Ru(tpy)(azpy)Cl]Cl (52 mg) and AgNO3 (30 mg) were refluxed in the mixture of acetone and water for 1 h and the solution was filtered while hot. LiBr (42 mg) was added to the filtrate, and the mixture was heated and refluxed for 1 h. NH4BF4 (45 mg) was added to the solution. After 5 days, the solid was collected and crystals suitable for X-ray techniques were selected from the obtained solid. Details of the X-ray structure determination of the title compound are given in Tables 1 -3. The ruthenium center is in a distorted octahedral environment with three nitrogens (N4, N5, N6) of the tpy ligand and the N1 (py) atom of azpy ligand in the equatorial plane. The trans axial positions are occupied by the N3 (azo) atom of azpy and the Br atom. The bite angle of azpy ligand (76.17 (13) This phenomenon can be ascribed to the π-interaction between metal ion and ligands. Since azpy is a stronger π-acceptor than bpy and bpz, the ruthenium ion becomes an electron-rich center and lengthens the distances to tpy ligand in order to stabilize the structure of complex ion. The Ru-Br bond length in the title compound (2.547(5)Å) is slightly longer than those observed in [Ru(C5H5)(CO)2Br] (2.536(2)Å) 11 but shorter than those in [Ru(Br)2(Me2SO)4] (2.601(1), 2.578(1)Å). 12 The Ru-N(azo) distance (1.960(3)Å) is shorter than the Ru-N(py) distance (2.061(3)Å) and becomes shortest among the related complexes. 2,5,6 This indicates that there is strong π-backbonding in the title compound, greater The title compound, C26H20N6BrRu(BF4), crystallizes in the centrosymmetric space group P21/n and consists of discrete complex units. The Ru(II) ion is octahedrally coordinated to one 2,2′:6′,2″-terpyridine (tpy), one 2-(phenylazo)pyridine (azpy) and a Br atom in trans-axial position at a distance of 2.547(5)Å. The shorter Ru-N (azo) distance (1.960(3)Å) than the Ru-N(py) distance (2.061(3)Å) signifies a strong π-backbonding, which leads to a longer, N=N (azo) bond (1.304(4)Å). † To whom correspondence should be addressed. E-mail: thlu@phys.nthu.edu.tw than in other complexes, and that bromide has a weaker π-bond interaction with the metal ion compared to chloride, acetonitrile and the nitro group. In addition, the N=N bond distance (1.304(4)Å), which is sensitive to the extent of π-backbo...