Generation of trans-dioxoruthenium(VI) porphyrins: A photochemical approach

trans-Dioxoruthenium(VI) porphyrin complexes have been developed as one of the best-characterized model systems for heme-containing enzymes. Traditionally, this type of compounds can be prepared by oxidation of ruthenium(II) precursors with peroxyacids and other terminal oxidants under different conditions, depending on the porphyrin ligands. In this work, a new photochemical generation of trans-dioxoruthenium(VI) porphyrins has been developed by extension of the known photo-induced ligand cleavage reactions. Refluxing ruthenium(II) carbonyl porphyrins [Ru^II(Por)(CO)] in carbon tetrachloride afforded dichlororuthenium(IV) complexes [Ru^IV(Por)Cl₂]. Facile exchange of the counterions in [Ru^IV(Por)Cl₂] with Ag(ClO₃) or Ag(BrO₃) gave the corresponding dichlorate [Ru^IV(Por)(ClO₃)₂] or dibromate [Ru^IV(Por)(BrO₃)₂] salts. Visible-light photolysis of the photo-labile porphyrin-ruthenium(IV) dichlorates or dibromates resulted in homolytic cleavage of the two O-Cl or O-Br bonds in the axial ligands to produce trans-dioxoruthenium(IV) species [Ru^VI(Por)O₂] bearing different porphyrin ligands.     

A bicarbonate bridged diruthenium(I) complex: Key evidence for the decarboxylation step in the base-assisted reduction of Ru2Cl4(CO)6

Base-assisted reduction of [Ru(CO)₃Cl₂]₂ in the presence of NP-Me₂ (2,7-dimethyl-1,8-naphthyridine) in thf provides an unsupported diruthenium(I) complex [Ru₂(CO)₄Cl₂(NP-Me₂)₂] (1). Two NP-Me₂ and four carbonyls bind at equatorial positions and two chlorides occupy sites trans to the Ru-Ru single bond. Reaction of [Ru(CO)₃Cl₂]₂, TlOTf, KOH and NP-Me₂ in acetonitrile, in a sealed container, affords a bicarbonate bridged diruthenium(I) complex [Ru₂(CO)₂(μ-CO)₂(μ-O₂COH)(NP-Me₂)₂](OTf) (2). The in situ generated CO₂ is the source for bicarbonate under basic reaction medium. Isolation of 2 validates the decarboxylation step in the base-assisted reduction of [Ru^II(CO)₃Cl₂]₂ → [Ru^I₂(CO)₄]²⁺.     

A comparative DFT study on aquation and nucleobase binding of ruthenium (II) and osmium (II) arene complexes

The potential energy surfaces of the reactions of organometallic arene complexes of the type [(η ⁶-arene)M^II(pic)Cl] (where pic = 2-picolinic acid, M = Ru or Os) were examined by a DFT computational study. Among the seven density functional methods, hybrid exchange functional B3LYP outperforms the others to explain the aquation of the complexes. The reactions and binding energies of Ru^II and Os^II arene complexes with both 9EtG and 9EtA were studied to gain insight into the reactivity of these types of organometallic complexes with DNA. The obtained data rationalize experimental observation, contributing to partly understanding the potential biological and medical applications of organometallic complexes.

Novel [Ru(polypyridine)(CO)2Cl2] and [Ru(polypyridine)2(CO)Cl]-type complexes: Characterizing the effects of introducing azopyridyl ligands by electrochemical, spectroscopic and crystallographic measurements

The reaction of ruthenium carbonyl polymer ([Ru(CO)₂Cl₂]_n) with azopyridyl compounds (2,2′-azobispyridine; apy or 2-phenylazopyridine; pap) generated new complexes, [Ru(azo)(CO)₂Cl₂] (azo = apy, pap). [Ru(apy)(CO)₂Cl₂] underwent photodecarbonylation to give a chloro-bridged dimer complex, whereas the corresponding pap complex ([Ru(pap)(CO)₂Cl₂]) was not converted to a dimer. The reactions of the chloro-bridged dimer containing the bpy ligand (bpy = 2,2′-bipyridine) with either apy or pap resulted in the formation of mixed polypyridyl complexes, [Ru(azo)(bpy)(CO)Cl]⁺. The novel complexes containing azo ligands were characterized by various spectroscopic measurements including the determination of X-ray crystallographic structures. Both [Ru(azo)(CO)₂Cl₂] complexes have two CO groups in a cis position to each other and two chlorides in a trans position. The azo groups are situated cis to the CO ligand in [Ru(azo)(bpy)(CO)Cl]⁺. All complexes have azo N-N bond lengths of 1.26-1.29 Å. The complexes exhibited azo-based two-electron reduction processes in electrochemical measurements. The effects of introducing azopyridyl ligands to the ruthenium carbonyl complexes were examined by ligand-based redox potentials, stretching frequencies and force constants of CO groups and bond parameters around Ru-CO moieties.     

Synthesis and properties of trinuclear polypyridyl complexes Ru(II)-Co(II)-Ru(II) and Ru(II)-Co(III)-Ru(II): Their photoinduced interconversion

The trinuclear [{Ru^II(bpy)₂(bpy-terpy)}₂Co^II]⁶⁺ complex (1⁶⁺) in which a Co(II)-bis-terpyridine-like centre is covalently linked to two Ru(II)-tris-bipyridine-like moieties by a bridging bipyridine-terpyridine ligand has been synthesised and characterised. Its electrochemical, photophysical and photochemical properties have been investigated in CH₃CN. The cyclic voltammetry exhibits two successive reversible oxidation processes, corresponding to the Co^III/Co^II and Ru^III/Ru^II redox couples at E_1/2 = −0.06 and 0.91 V vs Ag/Ag⁺ 10 mM, respectively. The one-electron oxidized form of the complex, [{Ru^II(bpy)₂(bpy-terpy)}₂Co^III]⁷⁺ (1⁷⁺) obtained after exhaustive electrolysis carried out at 0.2 V is fully stable. 1⁶⁺ and 1⁷⁺ are only poorly luminescent, indicating that the covalent linkage of the Ru(II)-tris-bipyridine centre to the cobalt subunit leads to a strong quenching of the Ru^II excited state by an intramolecular process. Luminescence lifetime experiments carried out at different temperatures indicate that the transfer is more efficient for 1⁷⁺ compare to 1⁶⁺ due to lower activation energy. Continuous irradiation of 1⁷⁺ performed at 405 nm in the presence of P(Ph)₃ acting as sacrificial electron donor leads to its quantitative reduction into 1⁶⁺, whereas similar experiment starting from 1⁶⁺ with a sulfonium salt as sacrificial electron acceptor converts 1⁶⁺ into 1⁷⁺ with a slower rate and a maximum yield of 80%. These photoinduced electron transfers were followed by UV-Visible spectroscopy and compared with those obtained with a simple mixture of both mononuclear parent complexes i.e. [Ru^II(bpy)₃]²⁺ and [Co^II(tolyl-terpy)₂]²⁺ or [Co^III(tolyl-terpy)₂]³⁺ (tolyl-terpy = 4′-(4-methylphenyl)-2,2′:6′,2′′-terpyridine).     

Ruthenium(II) arene complexes containing four- and five-membered monoanionic O,O-chelate rings

Optimization of the design of half-sandwich organometallic Ru^II arene complexes as anticancer agents depends on control of ligand exchange reactions. We have studied the aqueous chemistry of complexes containing O,O-chelate rings. The presence of the four-membered O,O-chelate ring from acetate (AcO) in [η⁶-p-cymene)Ru(AcO)Cl] was confirmed by X-ray crystallography, but in solution the acetate ligand was labile and the hydroxo-bridged dimer [((η⁶-p-cymene)Ru)₂(μ-OH)₃]⁺ readily formed. The dimer was relatively unreactive towards 9-ethyl guanine. The tropolonato (trop) complex [(η⁶-p-cymene)Ru(trop)Cl] was stable in aqueous media and the X-ray crystal structure of the aqua adduct [(η⁶-p-cymene)Ru(trop)(H₂O)]CF₃SO₃, containing a five-membered O,O-chelate ring from trop, was determined. [(η⁶-p-cymene)Ru(trop)Cl] reacted with guanosine to form N7 adducts and with adenosine to form both N7 and N1 adducts. Competitive reactions with guanosine and adenosine gave rise to guanosine:adenosine adducts in a ca. 1.3:1 mol ratio.     

Complexes of the imine/thioether mixed-donor ligand 8-methylthioquinoline with d-configurated transition metal centers: synthesis, structures and comparison with complexes of 1-methyl-2-(methylthiomethyl)-1H-benzimidazole

Coordination compounds of chelating 8-methylthioquinoline (MTQ) with the complex fragments Re^I(CO)₃Cl, [Ru^II(bpy)₂]²⁺, [Rh^III(C₅Me₅)Cl]⁺, [Ir^III(C₅Me₅)Cl]⁺, and Pt^IVMe₄ were synthesized and structurally characterized. Whereas the ruthenium(II) complex displays the strongest preference of bonding to N versus S, the compound (MTQ)PtMe₄ shows the most balanced metal-donor bonding within the chelate ring due to a relatively short bond to S (2.319 Å) versus N (2.150 Å). The complex fac-(MTQ)Re(CO)₃Cl exhibits a particularly long metal-sulfur bond at 2.472 Å. Cyclic voltammetry of [(MTQ)Ru(bpy)₂](PF₆)₂ reveals one reversible oxidation to Ru^III and three closely spaced reduction waves for the coordinated ligands. In comparison with the imine/thioether chelate ligand 1-methyl-2-(methylthiomethyl)-1H-benzimidazole (mmb) the MTQ ligand with its more rigid chelate setting N(sp²)-C(sp²)-C(sp²)-S forms generally shorter M-S bonds and displays stronger π acceptor behaviour.     

Spectroscopic and electrochemical investigation of coordination complexes of octakis(benzylthio)tetraazaporphyrincobalt(II)

Synthesis of new bichromophoric di- and pentanuclear complexes 2-7 by datively binding (bpy)₂Ru^II, (phen)₂Ru^II and Cp (PPh₃)Ru^II units to the periphery of [Co(OBTTAP)], 1, and their spectroscopic properties are described. IR, ¹H NMR, UV-Vis, and mass spectral data were used for their characterization. Relative intensities and positions of the Soret and Q-bands absorptions in the di- and pentanuclear complexes were observed shifted vis-à-vis that in the precursor complex [Co(OBTTAP)], 1. These complexes particularly, those possessing [Co(OBTTAP)] and (bpy)₂Ru^II/(phen)₂Ru^II units, exhibited efficient inter-component electronic excitation energy transfer in their fluorescence excitation-emission spectra, that are suggestive of a high degree of inter-component electronic interaction in them. Also, the electrode activity of the complexes improved upon binding of the peripheral units and they exhibited multiple one-electron reversible oxidation waves in the cyclic voltammograms. These effects have been explained in terms of dπ(S)-dπ(Ru) interactions.     

Nickel(II) and iron(II) mononuclear complexes with 1-methylimidazole-2-aldoximate: New building units for molecule-assembled magnetic materials

A new class of mononuclear metal complexes with 1-methylimidazole-2-aldoximate (miao) has been synthesized and characterized: trans-Ni^II(Cl)₂(Hmiao)₂ (1), trans-Ni^II(miao)₂(py)₂ (2), NO-trans-Ni^II(miao)₂(phen) (3), and NO-trans-Fe^II(miao)₂(phen) (4). The crystal structures of 2, 3, and 4 have been determined by single-crystal X-ray crystallography. Compound 1 having the protonated miao ligand (i.e., Hmiao) is a precursor for synthesizing 2 and 3. Compound 2 is an octahedral Ni^II complex surrounded by two miao bidentate ligands and two monodentate ligands of pyridine in a trans-arrangement. Compound 3 is a cis-type octahedral Ni^II complex with two miao ligands and a bidentate ligand of 1,10-phenanthroline, in which the ligand arrangement around Ni^II center is found in an NO-trans form. Compound 4 is an isostructural Fe^II derivative of 3. Compounds 1, 2, and 3 exhibit paramagnetic nature with an S = 1 spin and a positive zero-field splitting, among which it for 3 is overlapped with intermolecular ferromagnetic interaction (zJ/k_B = +0.16 K). Compound 4 is diamagnetic due to the existence of low-spin Fe^II ion.     

Syntheses and properties of new metal-carbon bonded heteroleptic complexes of ruthenium(II) containing terpyridine coligand

Reactions of 2-(arylazo)aniline, HL [H represents the dissociable protons upon orthometallation and HL is p-RC₆H₄N = NC₆H₄-NH₂; R = H for HL¹; CH₃ for HL² and Cl for HL³] with Ru(R₁-tpy)Cl₃ (where R₁-tpy is 4′-(R₁)-2,2′,6′′,2′′-terpyridine and R₁ = H or 4-N,N-dimethylaminophenyl or 4-methylphenyl) afford a group of complexes of type [Ru(L)(R₁-tpy)]·ClO₄ each of which contains C,N,N coordinated L⁻ as a tridentate ligand along with a terpyridine. Structure of one such complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, display characteristic ¹H NMR signals and intense dπ(Ru^II) → π∗(tpy) MLCT transitions in the visible region. Cyclic voltammetric studies on [Ru(L)(R₁-tpy)]·ClO₄ complexes show Ru(II)-Ru(III) oxidation within 0.63-0.67 V versus SCE.     

Electrospray mass spectrometry of cis-[Ru(NO)Cl(bpy)2] (bpy=2,2-bipyridine): fragmentation from desolvated {[Ru(NO)Cl(bpy)2], Cl} ion pairs by electron transfer and internal Lewis base pathways

The electrospray mass spectrum (ESI-MS) of cis-[Ru(NO)Cl(bpy)₂]Cl₂ (bpy=2,2^′-bipyridine), obtained from 50% CH₃OH/50% H₂O as the mobile solvent, exhibited ruthenium-containing ions derived from a {[Ru^II(NO⁺)Cl(bpy)₂]²⁺, Cl⁻}⁺ ion pair (m/z=514) and [Ru^II(NO⁺)Cl(bpy)₂]²⁺ (m/z=239.5). [Ru^IIICl(bpy)₂]²⁺, from the loss of NO from the 239.5 ion, is detected at m/z=224.5. Only the m/z 514 ion pair is detected when 100% CH₃OH mobile solvent is used, but the presence of even small amounts of water prompted the additional detection of the m/z 239.5 and m/z 224.5 ions under tandem MS-MS conditions. Ruthenium-chloro-containing ions appear as a characteristic collection of eight main, and four lesser, intense ions created from combinations ¹⁰⁴Ru, ¹⁰²Ru, ¹⁰¹Ru, ⁹⁹Ru, ⁹⁸Ru, ⁹⁶Ru, ³⁵Cl and ³⁷Cl isotopes with minor contributions from ¹³C, etc. For convenience of discussion, only the most abundant m/z species are mentioned herein as representative of all the isotopically distributed ions.Four fragmentation channels are detectable from the m/z=514 chloride ion pair: (1) the loss of HCl (main channel; ca. 50% of fragmentation events), (2) the loss of NO (ca. 12% ), (3) the loss of bpy (minor pathway), and (4) the loss of Cl atom (ca. 38% ).Loss of NO from ion m/z 514 yields ion m/z 484, which is the precursor of ions m/z 448 (by loss of HCl), m/z 328 (by loss of bpy) and m/z 292 (by loss of HCl and bpy). Loss of HCl from ion m/z 514 generates ion m/z 478, [Ru^II(NO⁺)Cl(bpyH)(bpy-H)]⁺, deprotonated at the ortho C-H of one bpy ligand. In MS-MS experiments, the m/z 478 ion was established to undergo loss of NO, producing ion m/z 448, rejoining further fragmentation process for ion m/z 448 at this point. Loss of neutral bipyridine from m/z 514 in low yield produces ion m/z 358, which undergoes further loss of NO to form [RuCl₂(bpy)]⁺ ion (m/z=328). MS-MS “neutral loss of 30” spectra confirmed the NO loss events as part of the fragmentation sequence for all four pathways.A fourth species of m/z=479 from the “514” ion is obtained by an internal electron transfer from Cl⁻ of the ion pair, and loss of the resultant neutral Cl atom. The product [Ru^II(NO^·)Cl(bpy)₂]⁺ “479” fragment undergoes facile loss of NO to generate [Ru^IICl(bpy)₂]⁺ (m/z=449). Ion m/z 449 gives rise to ions m/z 413 (loss of HCl) and m/z 257(loss of HCl and bpy). MS-MS experiments confirm the neutral loss of Cl from the m/z 514 ion, and the formation of the m/z 449 ion via m/z 479 and m/z 514 parents. This pathway was not observed in a prior study for the related complex, [Ru(NO)Cl(dpaH)(dpa⁻)]⁺ (dpaH=2,2^′-dipyridylamine), which does not have an external Cl⁻ in an ion pair.     

Synthesis and characterization of ruthenium(II) complexes bearing the bis(2-pyridylmethyl)amine ligand

The reaction of [Ru(CO)₂Cl₂]_n with bis(2-pyridylmethyl)amine (bpma) in refluxing ethanol followed by anion exchange yields two products: cis,fac-[Ru(bpma)(CO)₂Cl]PF₆ (1a, 71%) and trans,fac-[Ru(bpma)(CO)₂Cl]PF₆ (1b, 29%). Reaction of 1a with AgBF₄ in acetone, followed by acetonitrile and then anion exchange gave cis,fac-[Ru(bpma)(CO)₂(CH₃CN)](PF₆)₂ (2a). In the same way, 1b afforded trans,fac-[Ru(bpma)(CO)₂(CH₃CN)](PF₆)₂ (2b). Reaction of depolymerized [Ru(CO)₂Cl₂]_n with bpma in ethanol at room temperature afforded cis,cis-[Ru(η²-bpma)(CO)₂Cl₂] (3). In refluxing ethanol, 3 was converted to cis,fac-[Ru(bpma)(CO)₂Cl]Cl (1a-Cl). Heating 3 in chlorobenzene afforded 1b-Cl, exclusively; heating 3 in ethylene glycol gave mainly 1a-Cl. Heating 1a-Cl in ethanol resulted in no isomerization, but heating in chlorobenzene gave a mixture of 3 and 1b-Cl. Anion exchange for PF₆ with 1a-Cl and 1b-Cl afforded 1a and 1b, respectively, whereas anion exchange for BPh₄ afforded 1a-BPh₄. Compounds 1a, 1b, 2a and 3 have been structurally characterized.     

Bis(acetylacetonato)ruthenium(II) complexes containing bulky tertiary phosphines. Formation and redox behaviour of Ru(acac)2 (PR3) (R = Pr, Cy) complexes with ethene, carbon monoxide, and bridging dinitrogen

Reaction of cis-[Ru(acac)₂(η²-C₈H₁₄)₂] (1) (acac = acetylacetonato) with two equivalents of PⁱPr₃ in THF at −25 °C gives trans-[Ru(acac)₂(PⁱPr₃)₂], trans-3, which rapidly isomerizes to cis-3 at room temperature. The poorly soluble complex [Ru(acac)₂(PCy₃)₂] (4), which is isolated similarly from cis-[Ru(acac)₂(η²-C₂H₄)₂] (2) and PCy₃, appears to exist in the cis-configuration in solution according to NMR data, although an X-ray diffraction study of a single crystal shows the presence of trans-4. In benzene or toluene 2 reacts with PⁱPr₃ or PCy₃ to give exclusively cis-[Ru(acac)₂(η²-C₂H₄)(L)] [L = PⁱPr₃ (5), PCy₃ (6)], whereas in THF species believed to be either square pyramidal [Ru(acac)₂L], with apical L, or the corresponding THF adducts, can be detected by ³¹P NMR spectroscopy. Complexes 3-6 react with CO (1 bar) giving trans-[Ru(acac)₂(CO)(L)] [L = PⁱPr₃ (trans-8), PCy₃ (trans-9)], which are converted irreversibly into the cis-isomers in refluxing benzene. Complex 5 scavenges traces of dinitrogen from industrial grade dihydrogen giving a bridging dinitrogen complex, cis-[{Ru(acac)₂(PⁱPr₃)} ₂(μ-N₂)] (10). The structures of cis-3, trans-4, 5, 6 and 10 · C₆H₁₄ have been determined by single-crystal X-ray diffraction. Complexes trans- and cis-3, 5, 6, cis-8, and trans- and cis-9 each show fully reversible one-electron oxidation by cyclic voltammetry in CH₂Cl₂ at −50 °C with E_1/2(Ru^3+/2+) values spanning −0.14 to +0.92 V (versus Ag/AgCl), whereas for the vinylidene complexes [Ru(acac)₂ (CCHR)(PⁱPr₃)] [R = SiMe₃ (11), Ph (12)] the process is irreversible at potentials of +0.75 and +0.62 V, respectively. The trend in potentials reflects the order of expected π-acceptor ability of the ligands: PⁱPr₃, PCy₃ <C ₂H₄ < CCHR < CO. The UV-Vis spectrum of the thermally unstable, electrogenerated Ru^III-ethene cation 6⁺ has been observed at −50 °C. Cyclic voltammetry of the μ-dinitrogen complex 10 shows two, fully reversible processes in CH₂Cl₂ at −50 °C at +0.30 and +0.90 V (versus Ag/AgCl) corresponding to the formation of 10⁺ (Ru^II,III) and 10²⁺ (Ru^III,III). The former, generated electrochemically at −50 °C, shows a band in the near IR at ca. 8900 cm⁻¹ (w_1/2 ca. 3700 cm⁻¹) consistent with the presence of a valence delocalized system. The comproportionation constant for the equilibrium 10 + 10²⁺ ? 2 10⁺ at 223 K is estimated as 10^13.6.     

Interfacial electron transfer on TiO2 sensitized with an axially anchored trans tetradentate Ru(II) compound

The ruthenium complexes, trans-[Ru(phen-NH-phen)(eina)₂](PF₆)₂ and trans-[Ru(phen-NH-phen)(ina)₂](PF₆)₂ where phen-NH-phen = N,N-bis(1,10-phenanthroline-2-yl)amine, ina = isonicotinic acid and eina = ethyl isonicotinate, have been synthesized and characterized by ¹H NMR, elemental analysis, and IR spectroscopy. The compounds were non-emissive at room temperature, but displayed intense photoluminescence in 4:1 ethanol/methanol glasses at 77 K with corrected emission maximum at 570-580 nm. A quasi-reversible wave observed in cyclic voltammetry experiments was assigned to the Ru^III/II couple, E° (trans-[Ru(phen-NH-phen)(eina)₂)^3+/2+ = +1.22 V versus Ag/AgCl. The trans-[Ru(phen-NH-phen)(ina)₂](PF₆)₂ compound was found to bind to nanocrystalline TiO₂ thin films from acetonitrile solution. Pulsed 532 nm excitation of trans-[Ru(phen-NH-phen)(ina)₂](PF₆)₂ anchored to mesoporous nanocrystalline TiO₂ thin films resulted in an absorption difference spectra consistent with the formation of an interfacial charge separated state trans-[Ru^III (phen-NH-phen)(ina)₂]⁺/TiO₂ (e⁻). The formation of this state could not be time resolved, consistent with rapid excited state injection into the TiO₂, k_inj > 10⁸ s⁻¹. Comparative measurements with a thin film actinometer yielded an injection quantum yield (?_inj) of 0.8. Charge recombination required milliseconds for completion and followed a bi-second-order equal concentration kinetic model with k₁ = 1.0 × 10⁸ s⁻¹, and k₂ = 3.0 × 10⁵ s⁻¹. In regenerative solar cells with 0.5 M LiI and 0.005 M I₂ in acetonitrile, incident photon-to-current efficiencies were typically less than 10%.     

Ditopic tris(2-mercaptoimidazol-1-yl)borate ligands and their coordination behavior toward [Ru(p-cymene)]

The ditopic tris(2-mercaptoimidazol-1-yl)borate ligand K₂[(mt^Et)₃B-B(mt^Et)₃] cannot be prepared from B₂(NMe₂)₄/4 Hmt^Et/2 Kmt^Et, because the stable intramolecular diadduct (mt^Et)B(μ-mt^Et)₂B(mt^Et) is generated instead (Hmt^Et = 2-mercapto-1-ethylimidazole). Introduction of a meta- or para-phenylene spacer between the two boron atoms precludes the 2-mercaptoimidazol-1-yl groups from adopting a bridging position so that the potassium salts K₂[(mt^Et)₃B-(m-C₆H₄)-B(mt^Et)₃] and K₂[(mt^Et)₃B-(p-C₆H₄)-B(mt^Et)₃] become readily accessible. These ligands react with [(p-cym)RuCl₂]₂ to give the dinuclear Ru^II complexes [(p-cym)Ru(mt^Et)₃B-(m-C₆H₄)-B(mt^Et)₃Ru(p-cym)]Cl₂ and [(p-cym)Ru(mt^Et)₃B-(p-C₆H₄)-B(mt^Et)₃Ru(p-cym)]Cl₂ (p-cym = p-cymene). After the exchange of the Cl⁻ counterions for [PF₆]⁻, both complexes have been crystallized and structurally characterized by X-ray diffraction.     

trans-[Ru(N-N)2Cl2] species, where N-N is bis(3,5-dimethylpyrazol-1-yl)methane

Using bis(3,5-dimethylpyrazol-1-yl)methane as an N-N donor ligand, a trans-[Ru^III(N-N)₂Cl₂]⁺ core has been isolated from the direct reaction of the ligand with RuCl₃ · xH₂O and characterized structurally for the first time. The core displays a rhombic EPR spectrum and a quasireversible Ru(II/III) couple with an E_1/2 of −0.34 V versus NHE.     

Luminescent α-diimine complexes of ruthenium(II) containing scorpionate ligands

We describe the synthesis, characterization, and reactivity of several Ru(II) complexes of the type cis-L₂Ru(Z)ⁿ⁺, where L is an α-diimine [e.g. 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen)] ligand and Z is a bis-coordinated scorpionate ligand such as tris-(1-pyrazolyl)methane (HC(pz)₃, PZ=1-pyrazolyl; n=2) or tetrakis-(1-pyrazolyl)borate anion (B(pz)₄ ⁻; n=1). The complexes each exhibit strong visible absorption assigned as a π*(L)←dπ(Ru) metal-to-ligand charge-transfer (MLCT) transition characteristic of the cis-L₂Ru²⁺ kernel. A corresponding MLCT excited state emission is observed in room temperature CH₃CN solution, although emission energies, lifetimes, and quantum yields are reduced relative to Ru(bpy)₃ ²⁺. Electronic spectra and cyclic voltammetry measurements indicate that the relative π-acceptor abilities of the coordinated Z are: Z=(1H-pyrazolyl)₂(pz)₂B(pz)₂<(pyridine)₂<(pz)₂CH(pz). Uncoordinated pz groups of cis-(bpy)₂Ru(pz)₂B(pz)₂ ⁺ can be reacted to form a sterically hindered, localized-valence (K_com33 l mol⁻¹) cis,cis-(bpy)₂Ru^II(pz)₂B(pz)₂Ru^II(bpy)₂ ³⁺ dimer. The dimer properties are interpreted by comparison to the known cis-(bpy)₂Ru^II(pz)₂Ru^II(bpy)₂ ²⁺ analog. The dimer is photoreactive and undergoes an asymmetrical photocleavage in CH₃CN (yielding cis-(bpy)₂Ru^III(pz)₂B(pz)₂ ²⁺ and cis-(bpy)₂Ru^II(CH₃CN)₂ ²⁺), similar to the corresponding thermal reaction observed for the mixed-valence cis-(bpy)₂Ru^II(pz)₂Ru^III(bpy)₂ ³⁺ system.     

Enhanced DNA photocleavage properties of Ru(II) terpyridine complexes upon incorporation of methylphenyl substituted terpyridine and/or the polyazine bridging ligand dpp (2,3-bis(2-pyridyl)pyrazine)

The heteroleptic complexes, [(MePhtpy)RuCl(dpp)](PF₆) and [(tpy)RuCl(dpp)](PF₆), have been synthesized, characterized, and investigated with respect to their photophysical, redox, and DNA photocleavage properties (where MePhtpy = 4′-(4-methylphenyl)-2,2′:6′,2′′-terpyridine and dpp = 2,3-bis(2-pyridyl)pyrazine, tpy = 2,2′:6′,2′′-terpyridine). The X-ray crystal structure confirms the identity of the new [(MePhtpy)RuCl(dpp)](PF₆) complex. These heteroleptic complexes were found to photocleave DNA in the presence of oxygen, unlike the previously studied complex, [Ru(tpy)₂](PF₆)₂. The photophysical, redox, and DNA photocleavage properties of the heteroleptic complexes were compared with those of the homoleptic complexes, [Ru(MePhtpy)₂](PF₆)₂ and [Ru(tpy)₂](PF₆)₂. The heteroleptic complexes showed intense metal to ligand charge transfer (MLCT) transition at lower energy ([(MePhtpy)RuCl(dpp)](PF₆), 522 nm; [(tpy)RuCl(dpp)](PF₆), 516 nm) and longer excited state lifetimes as compared to the homoleptic complexes. The [Ru(MePhtpy)₂]²⁺ complex was found to photocleave DNA in contrast to [Ru(tpy)₂]²⁺. The introduction of a methylphenyl group on the tepyridine ligand not only enhances the ³MLCT excited state lifetime but also increases the lipophilicity and thereby the DNA binding ability of the molecule. An increase in lipophilicity upon addition of a methylphenyl group on the 2,2′:6′,2′′-terpyridine ligand was confirmed by determination of the partition coefficient ([(MePhtpy)RuCl(dpp)](PF₆), log P = +1.16; [(tpy)RuCl(dpp)](PF₆), log P = −1.27). The heteroleptic complexes photocleave DNA more efficiently than the homoleptic complexes, with the greatest activity being observed for the newly prepared [(MePhtpy)RuCl(dpp)](PF₆) complex.