Chromium complexes of an isomeric N-donor ligand, 2-[(N-arylamino)phenylazo]pyridine: amination reactions,X-ray structure,and redox properties

The chromium chemistry of two positional isomers of the ligand 2-[(N-arylamino)phenylazo]pyridine (HL(1)and HL(2)) are described. While the ligand HL(1) coordinates as a bischelating tridentate N,N,N-donor, [L(1)](-), with deprotonation of the amine nitrogen, its isomer HL(2) coordinates as a neutral bidentate N,N-donor. The amine nitrogen in this case remains protonated. Thus the reaction of CrCl(3).nH(2)O with HL(1) produced the brown cationic complex, [Cr(L(1))(2)](+), [1](+). The representative X-ray structure of [1a](ClO(4)) is reported. The two azo nitrogens of the anioinc tridentate ligand approach the metal center closest with Cr(1)-N(azo) av 1.862(6) A. There is a significant degree of ligand backbone conjugation in the coordinated ligands, which resulted in shortening of the C-N distances and also in lengthening of the diazo (N=N) distances. Two synthetic approaches for the synthesis of chromium complexes of HL(2) are investigated. The first approach is based on the substitution reaction, wherein all the coordinated CO ligands of Cr(CO)(6) were completely substituted by the three bidentate HL(2) ligands to produce a violet complex [Cr(HL(2))(3)]. The second approach is based on para-amination reaction of coordinated 2-(phenylazo)pyridine (pap). Thus the reaction of an inert complex, [CrCl(2)(pap)(2)], with ArNH(2) yields a mixed ligand complex, [CrCl(2)(pap)(HL(2))], 3. In this reaction one of the two coordinated pap ligands in [CrCl(2)(pap)(2)] undergoes amination at the para carbon (with respect to the diazo function) to yield HL(2) in situ. This metal-promoted transformation is authenticated by the X-ray structure determination of a representative complex, [CrCl(2)(pap)(HL(2a))], 3a. Notable differences in bond distances along the ligand backbones of the two coordinated ligands in 3a indicate different levels of metal-ligand overlap in this complex. All the chromium complexes of HL(2) are characterized by their intense blue-violet color. The frequencies of the visible range transitions in these complexes linearly correlate with the Hammett's substitution constant. Intraligand charge-transfer transitions in the visible region are believed to be responsible for the intense color. Redox properties of all these complexes are reported.     

Synthesis, structure and spectral and redox properties of new mixed ligand monomeric and dimeric Ru(II) complexes: predominant formation of the "cis-alpha" diastereoisomer and unusual 3MC emission by dimeric complexes

The tetradentate ligands 1,8-bis(pyrid-2-yl)-3,6-dithiaoctane (pdto) and 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (bbdo) form the complexes [Ru(pdto)(mu-Cl)](2)(ClO(4))(2) 1 and [Ru(bbdo)(mu-Cl)](2)(ClO(4))(2) 2 respectively. The new di-mu-chloro dimers 1 and 2 undergo facile symmetrical bridge cleavage reactions with the diimine ligands 2,2'-bipyridine (bpy) and dipyridylamine (dpa) to form the six-coordinate complexes [Ru(pdto)(bpy)](ClO(4))(2) 3, [Ru(bbdo)(bpy)](ClO(4))(2) 4, [Ru(pdto)(dpa)](ClO(4))(2) 5 and [Ru(bbdo)(dpa)](ClO(4))(2) 6 and with the triimine ligand 2,2':6,2'-terpyridine (terpy) to form the unusual seven-coordinate complexes [Ru(pdto)(terpy)](ClO(4))(2) 7 and [Ru(bbdo)(terpy)](ClO(4))(2) 8. In 1 the dimeric cation [Ru(pdto)(mu-Cl)](2)(2+) is made up of two approximately octahedrally coordinated Ru(II) centers bridged by two chloride ions, which constitute a common edge between the two Ru(II) octahedra. Each ruthenium is coordinated also to two pyridine nitrogen and two thioether sulfur atoms of the tetradentate ligand. The ligand pdto is folded around Ru(II) as a result of the cis-dichloro coordination, which corresponds to a "cis-alpha" configuration [DeltaDelta/LambdaLambda(rac) diastereoisomer] supporting the possibility of some attractive pi-stacking interactions between the parallel py rings at each ruthenium atom. The ruthenium atom in the complex cations 3a and 4 exhibit a distorted octahedral coordination geometry composed of two nitrogen atoms of the bpy and the two thioether sulfur and two py/bzim nitrogen atoms of the pdto/bbdo ligand, which is actually folded around Ru(II) to give a "cis-alpha" isomer. The molecule of complex 5 contains a six-coordinated ruthenium atom chelated by pdto and dpa ligands in the expected distorted octahedral fashion. The (1)H and (13)C NMR spectral data of the complexes throw light on the nature of metal-ligand bonding and the conformations of the chelate rings, which indicates that the dithioether ligands maintain their tendency to fold themselves even in solution. The bis-mu-chloro dimers 1 and 2 show a spin-allowed but Laporte-forbidden t(2g)(6)((1)A(1g))--> t(2g)(5) e(g)(1)((1)T(1g), (1)T(2g)) d-d transition. They also display an intense Ru(II) dpi--> py/bzim (pi*) metal-to-ligand charge transfer (MLCT) transition. The mononuclear complexes 3-8 exhibit dpi-->pi* MLCT transitions in the range 340-450 nm. The binuclear complexes 1 and 2 exhibit a ligand field ((3)MC) luminescence even at room temperature, whereas the mononuclear complexes 3 and 4 show a ligand based radical anion ((3)MLCT) luminescence. The binuclear complexes 1 and 2 undergo two successive oxidation processes corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru(II)Ru(III) state (K(c): 1, 3.97 x 10(6); 2, 1.10 x 10(6)). The mononuclear complexes 3-7 exhibit only one while 8 shows two quasi-reversible metal-based oxidative processes. The coordinated 'soft' thioether raises the redox potentials significantly by stabilising the 'soft' Ru(II) oxidation state. One or two ligand-based reduction processes were also observed for the mononuclear complexes.     

The synthesis and structure of heteroleptic tris(diimine)ruthenium(II) complexes

The reactions of bidentate diimine ligands (L2) with cationic bis(diimine)[Ru(L)(L1)(CO)Cl]+ complexes (L, L1, L2 are dissimilar diimine ligands), in the presence of trimethylamine-N-oxide (Me3NO) as a decarbonylation reagent, lead to the formation of heteroleptic tris(diimine) ruthenium(II) complexes, [Ru(L)(L1)(L2)]2+. Typically isolated as hexafluorophosphate or perchlorate salts, these complexes were characterised by UV-visible, infrared and mass spectroscopy, cyclic voltammetry, microanalyses and NMR spectroscopy. Single crystal X-ray studies have elucidated the structures of K[Ru(bpy)(phen)(4,4'-Me(2)bpy)](PF(6))(3).1/2H(2)O, [Ru(bpy)(5,6-Me(2)phen)(Hdpa)](ClO(4))(2), [Ru(bpy)(phen)(5,6-Me(2)phen)](ClO(4))(2), [Ru(bpy)(5,6'-Me(2)phen)(4,4'-Me(2)bpy)](PF(6))(2).EtOH, [Ru(4,4'-Me(2)bpy)(phen)(Hdpa)](PF(6))(2).MeOH and [Ru(bpy)(4,4'-Me(2)bpy)(Hdpa)](ClO(4))(2).1/2Hdpa (where Hdpa is di(2-pyridyl)amine). A novel feature of the first complex is the presence of a dinuclear anionic adduct, [K(2)(PF(6))(6)](4-), in which the two potassium centres are bridged by two fluorides from different hexafluorophosphate ions forming a K(2)F(2) bridging unit and by two KFPFK bridging moieties.     

Diruthenium complexes [((acac)2RuIII)2(mu-OC2H5)2], [((acac)(2RuIII)2(mu-L)](ClO4)2, and [((bpy)(2RuII)2(mu-L)](ClO4)4 [L = (NC5H4)2-N-C6H4-N-(NC5H4)2, acac = acetylacetonate, and bpy = 2,2'-bipyridine]. synthesis, structure, magnetic, spectral, and photophysical aspects

Paramagnetic diruthenium(III) complexes (acac)(2)Ru(III)(mu-OC(2)H(5))(2)Ru(III)(acac)(2) (6) and [(acac)(2)Ru(III)(mu-L)Ru(III)(acac)(2)](ClO(4))(2), [7](ClO(4))(2), were obtained via the reaction of binucleating bridging ligand, N,N,N',N'-tetra(2-pyridyl)-1,4-phenylenediamine [(NC(5)H(4))(2)-N-C(6)H(4)-N-(NC(5)H(4))(2), L] with the monomeric metal precursor unit (acac)(2)Ru(II)(CH(3)CN)(2) in ethanol under aerobic conditions. However, the reaction of L with the metal fragment Ru(II)(bpy)(2)(EtOH)(2)(2+) resulted in the corresponding [(bpy)(2)Ru(II) (mu-L) Ru(II)(bpy)(2)](ClO(4))(4), [8](ClO(4))(4). Crystal structures of L and 6 show that, in each case, the asymmetric unit consists of two independent half-molecules. The Ru-Ru distances in the two crystallographically independent molecules (F and G) of 6 are found to be 2.6448(8) and 2.6515(8) A, respectively. Variable-temperature magnetic studies suggest that the ruthenium(III) centers in 6 and [7](ClO(4))(2) are very weakly antiferromagnetically coupled, having J = -0.45 and -0.63 cm(-)(1), respectively. The g value calculated for 6 by using the van Vleck equation turned out to be only 1.11, whereas for [7](ClO(4))(2), the g value is 2.4, as expected for paramagnetic Ru(III) complexes. The paramagnetic complexes 6 and [7](2+) exhibit rhombic EPR spectra at 77 K in CHCl(3) (g(1) = 2.420, g(2) = 2.192, g(3) = 1.710 for 6 and g(1) = 2.385, g(2) = 2.177, g(3) = 1.753 for [7](2+)). This indicates that 6 must have an intermolecular magnetic interaction, in fact, an antiferromagnetic interaction, along at least one of the crystal axes. This conclusion was supported by ZINDO/1-level calculations. The complexes 6, [7](2+), and [8](4+) display closely spaced Ru(III)/Ru(II) couples with 70, 110, and 80 mV separations in potentials between the successive couples, respectively, implying weak intermetallic electrochemical coupling in their mixed-valent states. The electrochemical stability of the Ru(II) state follows the order: [7](2+) < 6 < [8](4+). The bipyridine derivative [8](4+) exhibits a strong luminescence [quantum yield (phi) = 0.18] at 600 nm in EtOH/MeOH (4:1) glass (at 77 K), with an estimated excited-state lifetime of approximately 10 micros.     

Reactions of acetonitrile coordinated to a nitrosylruthenium complex with H2O or CH(3)OH under mild conditions: structural characterization of imido-type complexes

The reaction of cis-[Ru(NO)(CH(3)CN)(bpy)(2)](3+) (bpy = 2,2'-bipyridine) in H(2)O at room temperature proceeded to afford two new nitrosylruthenium complexes. These complexes have been identified as nitrosylruthenium complexes containing the N-bound methylcarboxyimidato ligand, cis-[Ru(NO)(NH=C(O)CH(3))(bpy)(2)](2+), and methylcarboxyimido acid ligand, cis-[Ru(NO)(NH=C(OH)CH(3))(bpy)(2)](3+), formed by an electrophilic reaction at the nitrile carbon of the acetonitrile coordinated to the ruthenium ion. The X-ray structure analysis on a single crystal obtained from CH(3)CN-H(2)O solution of cis-[Ru(NO)(NH=C(O)CH(3))(bpy)(2)](PF(6))(3) has been performed: C(22)H(20.5)N(6)O(2)P(2.5)F(15)Ru, orthorhombic, Pccn, a = 15.966(1) A, b = 31.839(1) A, c = 11.707(1) A, V = 5950.8(4) A(3), and Z = 8. The structural results revealed that the single crystal consisted of 1:1 mixture of cis-[Ru(NO)(NH=C(O)CH(3))(bpy)(2)](2+) and cis-[Ru(NO)(NH=C(OH)CH(3))(bpy)(2)](3+) and the structural formula of this single crystal was thus [Ru(NO)(NH=C(OH(0.5))CH(3))(bpy)(2)](PF(6))(2.5). The reaction of cis-[Ru(NO)(CH(3)CN)(bpy)(2)](3+) in dry CH(3)OH-CH(3)CN at room temperature afforded a nitrosylruthenium complex containing the methyl methylcarboxyimidate ligand, cis-[Ru(NO)(NH=C(OCH(3))CH(3))(bpy)(2)](3+). The structure has been determined by X-ray structure analysis: C(25)H(29)N(8)O(18)Cl(3)Ru, monoclinic, P2(1)/c, a = 13.129(1) A, b = 17.053(1) A, c = 15.711(1) A, beta = 90.876(5) degrees, V = 3517.3(4) A(3), and Z = 4.     

Valence and spin situations in isomeric [(bpy)Ru(Q')2]n (Q' = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine). An experimental and DFT analysis

The article deals with the ruthenium complexes, [(bpy)Ru(Q')(2)] (1-3) incorporating two unsymmetrical redox-noninnocent iminoquinone moieties [bpy = 2,2'-bipyridine; Q' = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine, aryl = C(6)H(5) (Q'(1)), 1; m-Cl(2)C(6)H(3) (Q'(2)), 2; m-(OCH(3))(2)C(6)H(3) (Q'(3)), 3]. 1 and 3 have been preferentially stabilised in the cc-isomeric form while both the ct- and cc-isomeric forms of 2 are isolated [ct: cis and trans and cc: cis and cis with respect to the mutual orientations of O and N donors of two Q']. The isomeric identities of 1-3 have been authenticated by their single-crystal X-ray structures. The collective consideration of crystallographic and DFT data along with other analytical events reveals that 1-3 exhibit the valence configuration of [(bpy)Ru(II)(Q'(Sq))(2)]. The magnetization studies reveal a ferromagnetic response at 300 K and virtual diamagnetic behaviour at 2 K. DFT calculations on representative 2a and 2b predict that the excited triplet (S = 1) state is lying close to the singlet (S = 0) ground state with singlet-triplet separation of 0.038 eV and 0.075 eV, respectively. In corroboration with the paramagnetic features the complexes exhibit free radical EPR signals with g～2 and (1)HNMR spectra with broad aromatic proton signals associated with the Q' at 300 K. Experimental results in conjunction with the DFT (for representative 2a and 2b) reveal iminoquinone based preferential electron-transfer processes leaving the ruthenium(ii) ion mostly as a redox insensitive entity: [(bpy)Ru(II)(Q'(Q))(2)](2+) (1(2+)-3(2+)) ? [(bpy)Ru(II)(Q(')(Sq))(Q(')(Q))](+) (1(+)-3(+)) ? [(bpy)Ru(II)(Q(')(Sq))(2)] (1-3) ? [(bpy)Ru(II)(Q(')(Sq))(Q(')(Cat))](-)/[(bpy)Ru(III)(Q(')(Cat))(2)](-) (1(-)-3(-)). The diamagnetic doubly oxidised state, [(bpy)Ru(II)(Q'(Q))(2)](2+) in 1(2+)-3(2+) has been authenticated further by the crystal structure determination of the representative [(bpy)Ru(II)(Q'(3))(2)](ClO(4))(2) [3](ClO(4))(2) as well as by its sharp (1)H NMR spectrum. The key electronic transitions in each redox state of 1(n)-3(n) have been assigned by TD-DFT calculations on representative 2a and 2b.     

Synthesis, anticancer and antibacterial activity of some novel mononuclear Ru(II) complexes

In search of potential anticancer drug candidates in ruthenium complexes, a series of mononuclear ruthenium complexes of the type [Ru(phen)(2)(nmit)]Cl(2) (Ru1), [Ru(bpy)(2)(nmit)]Cl(2) (Ru2), [Ru(phen)(2)(icpl)]Cl(2) (Ru3), Ru(bpy)(2)(icpl)]Cl(2) (Ru4) (phen=1,10-phenanthroline; bpy=2,2'-bipyridine; nmit=N-methyl-isatin-3-thiosemicarbazone, icpl=isatin-3-(4-Cl-phenyl)thiosemicarbazone) and [Ru(phen)(2)(aze)]Cl(2) (Ru5), [Ru(bpy)(2)(aze)]Cl(2) (Ru6) (aze=acetazolamide) and [Ru(phen)(2)(R-tsc)](ClO(4))(2) (R=methyl (Ru7), ethyl (Ru8), cyclohexyl (Ru9), 4-Cl-phenyl (10), 4-Br-phenyl (Ru11), and 4-EtO-phenyl (Ru12), tsc=thiosemicarbazone) were prepared and characterized by elemental analysis, FTIR, (1)H-NMR and FAB-MS. Effect of these complexes on the growth of a transplantable murine tumor cell line (Ehrlich Ascites Carcinoma) and their antibacterial activity were studied. In cancer study the effect of hematological profile of the tumor hosts have also been studied. In the cancer study, the complexes Ru1-Ru4, Ru10 and Ru11 have remarkably decreased the tumor volume and viable ascitic cell count as indicated by trypan blue dye exclusion test (p<0.05). Treatment with the ruthenium complexes prolonged the lifespan of Ehrlich Ascites Carcinoma (EAC) bearing mice. Tumor inhibition by the ruthenium chelates was followed by improvements in hemoglobin, RBC and WBC values. All the complexes showed antibacterial activity, except Ru5 and Ru6. Thus, the results suggest that these ruthenium complexes have significant antitumor property and antibacterial activity. The results also reflect that the drug does not adversely affect the hematological profiles as compared to that of cisplatin on the host.     

Synthesis and cytotoxicity of dinuclear complexes containing ruthenium(II) bipyridyl units linked by a bis(pyridylimine) ligand

Enantiopure dinuclear ruthenium polypyridyl complexes of the form [Ru(2)(LL)(4)L(1)](PF(6))(4) (LL = 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen); L(1)= C(25)H(20)N(4) a bis(pyridylimine) ligand containing a diphenylmethane spacer) have been synthesized using the chiral building blocks cis-[Ru(bpy)(2)(py)(2)](2+) and cis-[Ru(phen)(2)(py)(2)](2+). These dinuclear ruthenium complexes have been characterised using NMR, mass spectrometry, UV-visible absorbance, circular dichroism and linear dichroism. The compounds exhibit good photo and thermal stability. The extinction coefficient for the bpy complex at 478 nm is epsilon(478) = 15,700 mol(-1) cm(-1) dm(3) and for the phen complex is epsilon(478) = 24,900 mol(-1) cm(-1) dm(3). Both complexes have their longest wavelength (metal to ligand charge transfer) transition predominantly x/y (short axis)-polarised while the transitions at shorter wavelength are a mixture of x/y and z-polarisations, similar to both the copper helicate and iron triple helicate studied previously. Cytotoxicity studies reveal that the compounds are dramatically less active against cancer cell lines than the recently reported supramolecular cylinders prepared from the same bis(pyridylimine) ligand.     

Synthesis, structure, spectroscopic properties, and electrochemical oxidation of ruthenium(II) complexes Incorporating monocarboxylate bipyridine ligands

[Ru(bpy)(2)(Mebpy-COOH)](PF(6))(2).3H(2)O (1), [Ru(phen)(2)(Mebpy-COOH)](ClO(4))(2).5H(2)O (2), [Ru(dppz)(2)(Mebpy-COOH)]Cl(2).9H(2)O (3), and [Ru(bpy)(dppz)(Mebpy-COOH)](PF(6))(2).5H(2)O (4) (bpy = 2,2'-bipyridine, Mebpy-COOH = 4'-methyl-2,2'-bipyridine-4-carboxylic acid, phen = 1,10-phenanthroline, dppz = dipyrido[3,2,-a;2',3-c]phenazine) have been synthesized and characterized spectroscopically and by microanalysis. The [Ru(Mebpy-COOH)(CO)(2)Cl(2)].H(2)O intermediate was prepared by reaction of the monocarboxylic acid ligand, Mebpy-COOH, with [Ru(CO)(2)Cl(2)](n), and the product was then reacted with either bpy, phen, or dppz in the presence of an excess of trimethylamine-N-oxide (Me(3)NO), as the decarbonylation agent, to generate 1, 2, and 3, respectively. For compound 4, [Ru(bpy)(CO)Cl(2)](2) was reacted with Mebpy-COOH to yield [Ru(bpy)(Mebpy-COOH)(CO)Cl](PF(6)).H(2)O as a mixture of two main geometric isomers. Chemical decarbonylation in the presence of dppz gave 4 also as a mixture of two isomers. Electrochemical and spectrophotometric studies indicated that complexes 1 and 2 were present as a mixture of protonated and deprotonated forms in acetonitrile solution because of water of solvation in the isolated solid products. The X-ray crystal structure determination on crystals of [Ru(bpy)2(MebpyCOO)][Ru(bpy)(2)(MebpyCOOH)](3)(PF(6))(7), 1a, and [Ru(phen)(2)(MebpyCOO)](ClO(4)).6H(2)O, 2a, obtained from solutions of 1 and 2, respectively, revealed that 1a consisted of a mixture of protonated and deprotonated forms of the complex in a 1:3 ratio and that 2a consisted of the deprotonated derivative of 2. A distorted octahedral geometry for the Ru(II) centers was found for both complexes. Upon excitation at 450 nm, MeCN solutions of the protonated complexes 1-4 were found to exhibit emission bands in the 635-655 nm range, whereas the corresponding emission maxima of their deprotonated forms were observed at lower wavelengths. Protonation/deprotonation effects were also observed in the luminescence and electrochemical behavior of complexes 1-4. Comprehensive electrochemical studies in acetonitrile show that the ruthenium centers on 1, 2, 3, and 4 are oxidized from Ru(II) to Ru(III) with reversible potentials at 917, 929, 1052, and 1005 mV vs Fc(0/+) (Fc = ferrocene), respectively. Complexes 1 and 2 also exhibit an irreversible oxidation process in acetonitrile, and all compounds undergo ligand-based reduction processes.     

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) ?, 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.     

Synthesis and structural,electrochemical, and optical properties of Ru(II) complexes with azobis(2,2'-bipyridine)s

Two new ditopic ligands, 5,5"-azobis(2,2'-bipyridine) (5,5"-azo) and 5,5"-azoxybis(2,2'-bipyridine) (5,5"-azoxy), were prepared by the reduction of nitro precursors. Mononuclear and dinuclear Ru(II) complexes having one of these bridging ligands and 2,2'-bipyridine terminal ligands were also prepared, and their properties were compared with previously reported Ru(II) complexes having 4,4"-azobis(2,2'-bipyridine) (4,4"-azo). The X-ray crystal structure showed that 5,5"-azo adopts the trans conformation and a planar rodlike shape. The X-ray crystal structure of [(bpy)(2)Ru(5,5"-azo)Ru(bpy)(2)](PF(6))(4) (Ru(5,5"-azo)Ru) showed that the bridging ligand is in the trans conformation and nearly planar also in the complex and the metal-to-metal distance is 10.0 A. The azo or azoxy ligand in these complexes exhibits reduction processes at less negative potentials than the terminal bpy's due to the low-lying pi level. The electronic absorption spectra for the complexes having 5,5"-azo or 5,5"-azoxy exhibit an extended low-energy metal-to-ligand charge-transfer absorption. The ligands, 5,5"-azo and 5,5"-azoxy, and the mononuclear complex, [(bpy)(2)Ru(5,5"-azo)](2+), isomerize reversibly upon light irradiation. The low-energy MLCT state sensitizes the isomerization of the azo moiety in this complex. While [(bpy)(2)Ru(4,4"-azo)Ru(bpy)(2)](PF(6))(4) exhibits light switch properties, namely, significant electrochromism and a large luminescence enhancement, upon reduction, Ru(5,5"-azo)Ru does not show these properties. The radical anion formation upon reduction of these complexes has been confirmed by ESR spectroscopy.     

Photocatalytic generation of a non-heme oxoiron(IV) complex with water as an oxygen source

The photocatalytic formation of a non-heme oxoiron(IV) complex, [(N4Py)Fe(IV)(O)](2+) [N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine], efficiently proceeds via electron transfer from the excited state of a ruthenium complex, [Ru(II)(bpy)(3)](2+)* (bpy = 2,2'-bipyridine) to [Co(III)(NH(3))(5)Cl](2+) and stepwise electron-transfer oxidation of [(N4Py)Fe(II)](2+) with 2 equiv of [Ru(III)(bpy)(3)](3+) and H(2)O as an oxygen source. The oxoiron(IV) complex was independently generated by both chemical oxidation of [(N4Py)Fe(II)](2+) with [Ru(III)(bpy)(3)](3+) and electrochemical oxidation of [(N4Py)Fe(II)](2+).     

A platinum-ruthenium dinuclear complex bridged by bis(terpyridyl)xanthene

4,5-Bis(terpyridyl)-2,7-di-tert-butyl-9,9-dimethylxanthene (btpyxa) was prepared to serve as a new bridging ligand via Suzuki coupling of terpyridin-4'-yl triflate and 2,7-di-tert-butyl-9,9-dimethylxanthene-4,5-diboronic acid. The reaction of btpyxa with either 1 equiv or an excess of PtCl(2)(cod) (cod = 1,5-cyclooctadiene) followed by anion exchange afforded mono- and dinuclear platinum complexes [(PtCl)(btpyxa)](PF(6)) ([1](PF(6))) and [(PtCl)(2)(btpyxa)](PF(6))(2) ([2](PF(6))(2)), respectively. The X-ray crystallography of [1](PF(6)).CHCl(3) revealed that the two terpyridine units in the ligand are nearly parallel to each other. The heterodinuclear complex [(PtCl)[Ru((t)Bu(2)SQ)(dmso)](btpyxa)](PF(6))(2) ([4](PF(6))(2)) (dmso = dimethyl sulfoxide; (t)Bu(2)SQ = 3,5-di-tert-butyl-1,2-benzosemiquinone) and the monoruthenium complex [Ru((t)Bu(2)SQ)(dmso)(trpy)](PF(6)) ([5](PF(6))) (trpy = 2,2':6',2' '-terpyridine) were also synthesized. The CV of [2](2+) suggests possible electronic interaction between the two Pt(trpy) groups, whereas such an electronic interaction was not suggested by the CV of [4](2+) between Pt(trpy) and Ru((t)Bu(2)SQ) frameworks.     

Designed synthesis of a multimetallic system having Ru(4)Cu(2) core using trimetallic coordination of 2,2'-biimidazolate ion

Synthesis of a novel hexametallic compound, [[(bpy)(2)Ru(biim)](4)Cu(2)](ClO(4))(4), (bpy = 2,2'-bipyridine and biim = 2,2'-biimidazolate dianion) from a monometallic complex, [(bpy)(2)Ru(biim)] and hydrated Cu(ClO(4))(2) is described. The X-ray structure consists of four octahedral ruthenium(II) centers arranged around a bimetallic Cu(2)-core. The four octahedral [(bpy)(2)Ru(biim)] units arranged around the Cu(2)-moiety that resulted in a propeller shape arrangement. The results of cryomagnetic measurements on the di-copper complex indicate that the two Cu(II) ions are coupled antiferromagnetically through the two bridging biim ligands. The EPR spectrum of the complex showed a typical axial spectrum. Optical spectra and redox properties of are reported. An intense absorption at 525 nm is assigned as Ru(dpi) --> pi(*)(bpy) transition.     

3,6-bis(2'-pyridyl)pyridazine (L) and its deprotonated form (L - H+)- as ligands for {(acac)2Ru(n+)} or {(bpy)2Ru(m+)}: investigation of mixed valency in [{(acac)2Ru}2(mu-L - H+)]0 and [{(bpy)2Ru}2(mu-L - H+)]4+ by spectroelectrochemistry and EPR

Crystallographically characterised 3,6-bis(2'-pyridyl)pyridazine (L) forms complexes with {(acac)2Ru} or {(bpy)2Ru2+}via one pyridyl-N/pyridazyl-N chelate site in mononuclear Ru(II) complexes (acac)2Ru(L), 1, and [(bpy)2Ru(L)](ClO4)2, [3](ClO4)2. Coordination of a second metal complex fragment is accompanied by deprotonation at the pyridazyl-C5 carbon {L --> (L - H+)-} to yield cyclometallated, asymmetrically bridged dinuclear complexes [(acac)2Ru(III)(mu-L - H+)Ru(III)(acac)2](ClO4), [2](ClO4), and [(bpy)2Ru(II)(mu-L - H+)Ru(II)(bpy)2](ClO4)3, [4](ClO4)3. The different electronic characteristics of the co-ligands, sigma donating acac- and pi accepting bpy, cause a wide variation in metal redox potentials which facilitates the isolation of the diruthenium(III) form in [2](ClO4) with antiferromagnetically coupled Ru(III) centres (J = -11.5 cm(-1)) and of a luminescent diruthenium(II) species in [4](ClO4)3. The electrogenerated mixed-valent Ru(II)Ru(III) states 2 and [4]4+ with comproportionation constants Kc > 10(8) are assumed to be localised with the Ru(III) ion bonded via the negatively charged pyridyl-N/pyridazyl-C5 chelate site of the bridging (L - H+)- ligand. In spectroelectrochemical experiments they show similar intervalence charge transfer bands of moderate intensity around 1300 nm and comparable g anisotropies (g1-g3 approximatly 0.5) in the EPR spectra. However, the individual g tensor components are distinctly higher for the pi acceptor ligated system [4]4+, signifying stabilised metal d orbitals.     

cis-Bis(bipyridine)ruthenium Imidazole Derivatives: A Spectroscopic, Kinetic, and Structural Study

The ruthenium bis(bipyridine) complexes cis-[Ru(bpy)(2)Im(OH(2))](2+), cis-[Ru(bpy)(2)(Im)(2)](2+), cis-[Ru(bpy)(2)(N-Im)(2)](2+), cis-[Ru(dmbpy)(2)Im(OH(2))](2+), cis-[Ru(dmbpy)(2)(N-Im)(OH(2))](2+)(bpy = 2,2'-bipyridine, dmbpy = 4,4'-dimethyl-2,2'-bipyridine, Im = imidazole, N-Im = N-methylimidazole), have been synthesized under ambient conditions in aqueous solution (pH 7). Their electrochemical and spectroscopic properties, absorption, emission, and lifetimes were determined and compared. The substitution kinetics of the cis-[Ru(bpy)(2)Im(OH(2))](2+) complexes show slower rates and have lower affinities for imidazole ligands than the corresponding cis-[Ru(NH(3))(4)Im(OH(2))](2+) complexes. The crystal structures of the monoclinic cis-[Ru(bpy)(2)(Im)(2)](BF(4))(2), space group = P2(1)/a, Z = 4, a = 11.344(1) ?, b = 17.499(3) ?, c = 15.114(3) ?, and beta = 100.17(1) degrees, and triclinic cis-[Ru(bpy)(2)(N-Im)(H(2)O)](CF(3)COO)(2).H(2)O, space group = P&onemacr;, Z = 2, a = 10.432(4) ?, b = 11.995(3) ?, c = 13.912(5) ?, alpha = 87.03(3) degrees, beta = 70.28(3) degrees, and gamma = 71.57(2) degrees, complexes show that these molecules crystallize as complexes of octahedral Ru(II) to two bidentate bipyridine ligands with two imidazole ligands or a water and an N-methylimidazole ligand cis to each other. The importance of these molecules is associated with their frequent use in the modification of proteins at histidine residues and in comparisons of the modified protein derivatives with these small molecule analogs.     

Ruthenium nitrosyl complexes with 1,4,7-trithiacyclononane and 2,2'-bipyridine (bpy) or 2-phenylazopyridine (pap) coligands. Electronic structure and reactivity aspects

The present article describes ruthenium nitrosyl complexes with the {RuNO}(6) and {RuNO}(7) notations in the selective molecular frameworks of [Ru(II)([9]aneS(3))(bpy)(NO(+))](3+) (4(3+)), [Ru(II)([9]aneS(3))(pap) (NO(+))](3+) (8(3+)) and [Ru(II)([9]aneS(3))(bpy)(NO˙)](2+) (4(2+)), [Ru(II)([9]aneS(3))(pap)(NO˙)](2+) (8(2+)) ([9]aneS(3) = 1,4,7-trithiacyclononane, bpy = 2,2'-bipyridine, pap = 2-phenylazopyridine), respectively. The nitrosyl complexes have been synthesized by following a stepwise synthetic procedure: {Ru(II)-Cl} → {Ru(II)-CH(3)CN} → {Ru(II)-NO(2)} → {Ru(II)-NO(+)} → {Ru(II)-NO˙}. The single-crystal X-ray structure of 4(3+) and DFT optimised structures of 4(3+), 8(3+) and 4(2+), 8(2+) establish the localised linear and bent geometries for {Ru-NO(+)} and {Ru-NO˙} complexes, respectively. The crystal structures and (1)H/(13)C NMR suggest the [333] conformation of the coordinated macrocyclic ligand ([9]aneS(3)) in the complexes. The difference in π-accepting strength of the co-ligands, bpy in 4(3+) and pap in 8(3+) (bpy < pap) has been reflected in the ν(NO) frequencies of 1945 cm(-1) (DFT: 1943 cm(-1)) and 1964 cm(-1) (DFT: 1966 cm(-1)) and E°({Ru(II)-NO(+)}/{Ru(II)-NO˙}) of 0.49 and 0.67 V versus SCE, respectively. The ν(NO) frequency of the reduced {Ru-NO˙} state in 4(2+) or 8(2+) however decreases to 1632 cm(-1) (DFT: 1637 cm(-1)) or 1634 cm(-1) (DFT: 1632 cm(-1)), respectively, with the change of the linear {Ru(II)-NO(+)} geometry in 4(3+), 8(3+) to bent {Ru(II)-NO˙} geometry in 4(2+), 8(2+). The preferential stabilisation of the eclipsed conformation of the bent NO in 4(2+) and 8(2+) has been supported by the DFT calculations. The reduced {Ru(II)-NO˙} exhibits free-radical EPR with partial metal contribution revealing the resonance formulation of {Ru(II)-NO˙}(major)?{Ru(I)-NO(+)}(minor). The electronic transitions of the complexes have been assigned based on the TD-DFT calculations on their DFT optimised structures. The estimated second-order rate constant (k, M(-1) s(-1)) of the reaction of the nucleophile, OH(-) with the electrophilic {Ru(II)-NO(+)} for the bpy derivative (4(3+)) of 1.39 × 10(-1) is half of that determined for the pap derivative (8(3+)), 2.84 × 10(-1) in CH(3)CN at 298 K. The Ru-NO bond in 4(3+) or 8(3+) undergoes facile photolytic cleavage to form the corresponding solvent species {Ru(II)-CH(3)CN}, 2(2+) or 6(2+) with widely varying rate constant values, (k(NO), s(-1)) of 1.12 × 10(-1) (t(1/2) = 6.2 s) and 7.67 × 10(-3) (t(1/2) = 90.3 s), respectively. The photo-released NO can bind to the reduced myoglobin to yield the Mb-NO adduct.     

Dinitrogen formation by oxidative intramolecular N---N coupling in cis,cis-[(bpy)2(NH3)RuORu(NH3)(bpy)2]4+

The (15)N-labeled diammine(mu-oxo)ruthenium complex cis,cis-[(bpy)(2)(H(3)(15)N)Ru(III)ORu(III)((15)NH(3))(bpy)(2)](4+) ((2-(15)N)(4+)) was synthesized from cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(H(2)O)(bpy)(2)](4+) by using ((15)NH(4))(2)SO(4) and isolated as its perchlorate salt in 17% yield. A 1:1 mixture of (2-(15)N)(4+) and nonlabeled cis,cis-[(bpy)(2)(H(3)(14)N)Ru(III)ORu(III)((14)NH(3))(bpy)(2)](4+) were electrochemically oxidized in aqueous solution. The gaseous products (14)N(2) and (15)N(2) were formed in equimolar amounts with only a small amount of (14)N(15)N detected. This demonstrates that dinitrogen formation by oxidation of the diammine complex proceeds by intramolecular N---N coupling.     

Ruthenium(II) coordination chemistry of a fused donor-acceptor ligand: synthesis, characterization, and photoinduced electron-transfer reactions of [{Ru(bpy)2}(n)(TTF-ppb)](PF6)(2n) (n = 1, 2)

A pi-extended, redox-active bridging ligand 4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[2,3-a:3',2'-c]phenazine (L) was prepared via direct Schiff-base condensation of the corresponding diamine-tetrathiafulvalene (TTF) precursor with 4,7-phenanthroline-5,6-dione. Reactions of L with [Ru(bpy)(2)Cl(2)] afforded its stable mono- and dinuclear ruthenium(II) complexes 1 and 2. They have been fully characterized, and their photophysical and electrochemical properties are reported together with those of [Ru(bpy)(2)(ppb)](2+) and [Ru(bpy)(2)(mu-ppb)Ru(bpy)(2)](4+) (ppb = dipyrido[2,3-a:3',2'-c]phenazine) for comparison. In all cases, the first excited state corresponds to an intramolecular TTF --> ppb charge-transfer state. Both ruthenium(II) complexes show two strong and well-separated metal-to-ligand charge-transfer (MLCT) absorption bands, whereas the (3)MLCT luminescence is strongly quenched via electron transfer from the TTF subunit. Clearly, the transient absorption spectra illustrate the role of the TTF fragment as an electron donor, which induces a triplet intraligand charge-transfer state ((3)ILCT) with lifetimes of approximately 200 and 50 ns for mono- and dinuclear ruthenium(II) complexes, respectively.     

9-Oxidophenalenone: a noninnocent β-diketonate ligand?

The redox systems [Ru(L)(bpy)(2)](k), [Ru(L)(2)(bpy)](m), and [Ru(L)(3)](n) containing the potentially redox-active ligand 9-oxidophenalenone = L(-) were investigated by spectroelectrochemistry (UV-vis-near-IR and electron paramagnetic resonance) in conjunction with density functional theory (DFT) calculations. Compounds [Ru(L(-))(bpy)(2)]ClO(4) ([1]ClO(4)) and [Ru(L(-))(2)(bpy)]ClO(4) ([2]ClO(4)) were structurally characterized. In addition to establishing electron-transfer processes involving the Ru(II)/Ru(III)/Ru(IV) and bpy(0)/bpy(?-) couples, evidence for the noninnocent behavior of L(-) was obtained from [Ru(IV)(L(?))(L(-))(bpy)](3+), which exhibits strong near-IR absorption due to ligand-to-ligand charge transfer. In contrast, the lability of the electrogenerated anion [Ru(L)(2)(bpy)](-) is attributed to a resonance situation [Ru(II)(L(?2-))(L(-))(bpy)](-)/[Ru(II)(L(-))(2) (bpy(?-))](-), as suggested by DFT calculations.