Influence of donor-acceptor interactions on MLCT hole reconfiguration in {Ru(bpy)} chromophores.

In MLCT chromophores, internal conversion (IC) in the form of hole reconfiguration pathways (HR) is a major source of dissipation of the absorbed photon energy. Therefore, it is desirable to minimize their impact in energy conversion schemes by slowing them down. According to previous findings on {Ru(bpy)} chromophores, donor-acceptor interactions between the Ru ion and the ligand scaffold might allow to control HR/IC rates. Here, a series of [Ru(tpm)(bpy)(R-py)]2+ chromophores, where tpm is tris(1-pyrazolyl)methane, bpy is 2,2'-bipyridine and R-py is a 4-substituted pyridine, were prepared and fully characterized employing electrochemistry, spectroelectrochemistry, steady-state absorption/emission spectroscopy and electronic structure computations based on DFT/TD-DFT. Their excited-state decay was monitored using nanosecond and femtosecond transient absorption spectroscopy. HR/IC lifetimes as slow as 568 ps were obtained in DMSO at room temperature, twice as slow as in the reference species [Ru(tpm)(bpy)(NCS)]+.

Anti-Dissipative Strategies toward More Efficient Solar Energy Conversion.

In natural and artificial photosynthesis, light absorption and catalysis are separate processes linked together by exergonic electron transfer. This leads to free energy losses between the initial excited state, formed after light absorption, and the active catalyst formed after the electron transfer cascade. Additional deleterious processes, such as internal conversion (IC) and vibrational relaxation (VR), also dissipate as much as 20-30% of the absorbed photon energy. Minimization of these energy losses, a holy grail in solar energy conversion and solar fuel production, is a challenging task because excited states are usually strongly coupled which results in negligible kinetic barriers and very fast dissipation. Here, we show that topological control of oligomeric {Ru(bpy)3} chromophores resulted in small excited-state electronic couplings, leading to activation barriers for IC by means of inter-ligand electron transfer of around 2000 cm-1 and effectively slowing down dissipation. Two types of excited states are populated upon visible light excitation, that is, a bridging-ligand centered metal-to-ligand charge transfer [MLCT(Lm)], and a 2,2'-bipyridine-centered MLCT [MLCT(bpy)], which lies 800-1400 cm-1 higher in energy. As a proof-of-concept, bimolecular electron transfer with tri-tolylamine (TTA) as electron donor was performed, which mimics catalyst activation by sacrificial electron donors in typical photocatalytic schemes. Both excited states were efficiently quenched by TTA. Hence, this novel strategy allows to trap higher energy excited states before IC and VR set in, saving between 100 and 170 meV. Furthermore, transient absorption spectroscopy suggests that electron transfer reactions with TTA produced the corresponding Lm•--centered and bpy•--centered reduced photosensitizers, which involve different reducing abilities, that is, -0.79 and -0.93 V versus NHE for Lm•- and bpy•-, respectively. Thus, this approach probably leads in fine to a 140 meV more potent reductant for energy conversion schemes and solar fuel production. These results lay the first stone for anti-dissipative energy conversion schemes which, in bimolecular electron transfer reactions, harness the excess energy saved by controlling dissipative conversion pathways.

A photoinduced mixed valence photoswitch.

The ground state and photoinduced mixed valence states (GSMV and PIMV, respectively) of a dinuclear (Dp4+) ruthenium(II) complex bearing 2,2'-bipyridine ancillary ligands and a 2,2':4',4'':2'',2'''-quaterpyridine (Lp) bridging ligand were investigated using femtosecond and nanosecond transient absorption spectroscopy, electrochemistry and density functional theory. It was shown that the electronic coupling between the transiently light-generated Ru(II) and Ru(III) centers is HDA ∼ 450 cm-1 in the PIMV state, whereas the electrochemically generated GSMV state showed HDA ∼ 0 cm-1, despite virtually identical Ru-Ru distances. This stemmed from the changes in dihedral angles between the two bpy moieties of Lp, estimated at 30° and 4° for the GSMV and PIMV states, respectively, consistent with a through-bond rather than a through-space mechanism. Electronic coupling can be turned on by using visible light excitation, making Dp4+ a competitive candidate for photoswitching applications. A novel strategy to design photoinduced charge transfer molecular switches is proposed.

Helpful correlations to estimate the pKa of coordinated HNO: a potential-pH exploration in a pendant-arm cyclam-based ruthenium nitroxyl.

The acid-base speciation of coordinated azanone (HNO) remains a highly relevant topic in bioinorganic chemistry. Ruthenium nitroxyl complexes with sufficient robustness towards ligand loss have gained significance as operating platforms to delve into such studies. In this work, we revisit an octahedral {RuNO}6 complex containing the cyclam-based pentadentate ligand Lpy = 1-(pyridine-2-ylmethyl)-1,4,8,11-tetraazacyclotetradecane and explore the thermodynamic and spectroscopic aspects of its reduced states in aqueous media. Upon in situ electro-generation of the bound HNO moiety, we have undertaken different strategies to determine both its acidity and electrochemical properties. This robust HNO complex does not undergo deprotonation in a wide pH range. We have found pKa ([Ru(Lpy)(HNO)]2+) = 13.0 ± 0.1 and . There are indications that pKa (HNO) values in several ruthenium-based species correlate with the redox potential associated with the {RuNO}6,7 and {RuNO}7,8 couples. The present pKa extends the range of acidity of bound HNO to more than five pH units, confirming a remarkable sensitivity to the nature of the coordination sphere. This result lays new foundations to continue rational ligand design that may contribute to a better understanding of the different biological roles of both HNO and NO- by investigating key chemical aspects of model complexes.

Synthesis and crystallographic, spectroscopic and computational characterization of 3,3',4,4'-substituted biphenyls: effects of OR substituents on the intra-ring torsion angle.

Presented here are the synthesis, characterization and study (using single crystal X-ray diffraction, Raman scattering, quantum mechanics calculations) of the structures of a series of biphenyls substituted in positions 3, 3', 4 and 4' with a variety of R (R = methyl, acetyl, hexyl) groups connected to the biphenyl core through oxygen atoms. The molecular conformation, particularly the torsion angle between aromatic rings has been extensively studied both in the solid as well as in the liquid state. The results show that the compounds appearing as rigorously planar in the solid present instead a twisted conformation in the melt. The solid versus melt issue strongly suggests that the reasons for planarity are to be found in the packing restraints. A `rule of thumb' is suggested for the design of biphenyls with different molecular conformations, based on the selection of the OR substituent.

Cis-Trans Interconversion in Ruthenium(II) Bipyridine Complexes.

Most studies of ruthenium polypyridine complexes are devoted to their cis isomers. The fact that cis isomers are thermally more stable and thus easier to synthesize has prevented researchers from investigating the properties and applications of trans complexes. We present a study of thermal and photochemical cis-trans interconversion of the key complex [Ru(bpy)2(PMe3)(H2O)]2+ (bpy = 2,2'-bipyridine, PMe3 = trimethylphosphine), which results in specific synthetic applications of the trans species, potentially useful as a platform for designing highly efficient visible light activated caged compounds. We show, as a proof of concept, some examples of trans complexes bearing N-donor and P-donor ligands and their comparison with the cis isomers.

Remarkable Changes of the Acidity of Bound Nitroxyl (HNO) in the [Ru(Me3[9]aneN3)(L2)(NO)] n+ Family ( n = 1-3). Systematic Structural and Chemical Exploration and Bioinorganic Chemistry Implications.

This work demonstrates that the acidity of nitroxyl (HNO) coordinated to a metal core is significantly influenced by its coordination environment. The possibility that NO- complexes may be the predominant species in physiological environments has implications in bioinorganic chemistry and biochemistry. This (apparently simple) result pushed us to delve into the basic aspects of HNO coordination chemistry. A series of three closely related {RuNO}6,7 complexes have been prepared and structurally characterized, namely [Ru(Me3[9]aneN3)(L2)(NO)]3+/2+, with L2 = 2,2'-bipyridine, 4,4'-dimethoxy-2,2'-bipyridine, and 2,2'-bipyrimidine. These species have also been thoroughly studied in solution, allowing for a systematic exploration of their electrochemical properties in a wide pH range, thus granting access and characterization of the elusive {RuNO}8 systems. Modulation of the electronic density in the {RuNO} fragment introduced by changing the bidentate coligand L2 produced only subtle structural modifications but affected dramatically other properties, most noticeably the redox potentials of the {RuNO}6,7 couples and the acidity of bound HNO, which spans over a range of almost three pH units. Controlling the acidity of coordinated HNO by the rational design of coordination compounds is of fundamental relevancy in the field of inorganic chemistry and also fuels the growing interest of the community in understanding the role that different HNO-derived species can play in biological systems.

Targeting the mitochondrial VDAC in hepatocellular carcinoma using a polyclonal antibody-conjugated to a nitrosyl ruthenium complex.

The rational design of anti-cancer agents includes a new approach based on ruthenium complexes that can act as nitric oxide (NO) donor agents against specific cellular targets. One of the most studied classes of those compounds is based on bis(bipyridine) ruthenium fragment and its derivative species. In this work, we present the chemical and cytotoxicity properties against the liver hepatocellular carcinoma cell line HepG2 of cis-[RuII(NO+)Cl(dcbpy)2]2- conjugated to a polyclonal antibody IgG (anti-VDAC) recognizing a cell surface marker. UV-visible bands of the ruthenium complex were assigned with the aid of density functional theory, which also allowed estimation of the structures that explain the biological effects of the ruthenium complex-IgG conjugate. The interaction of cis-[RuII(NO+)Cl(dcbpy)2]3- with mitochondria was evaluated due to the potential of these organelles as anti-cancer targets, and considering they interact with the anti-VDAC antibody. The cytotoxicity of cis-[RuII(NO+)Cl(dcbpy)2]3--anti-VDAC antibody was up to 80% greater in comparison to the free cis-[RuII(NO+)Cl(dcbpy)2]3- complex. We suggest that this effect is due to site-specific interaction of the complex followed by NO release.

Pushing the photodelivery of nitric oxide to the visible: are {FeNO}7 complexes good candidates?

Photodelivery of NO requires stable compounds which can be made reactive by irradiation with (visible) light. Traditional {MNO}6 complexes require a substantial ligand design to shift their absorption spectra to the appropriate region of the electromagnetic spectrum. [Fe((CH2Py2)2Me[9]aneN3)(NO)](BF4)2 is a new {FeNO}7 octahedral coordination compound, which is thermally and air-stable in solution. Illumination with a 450 nm light source induces significant photodetachment of the coordinated NO (ϕNO = 0.52 mol einstein-1), suggesting that {FeNO}7 compounds can be in fact suitable compounds for therapeutic NO-photorelease.

A chemometric approach for determining the reaction quantum yields in consecutive photochemical processes.

A chemometric procedure to deal with spectroscopically monitored processes involving photochemical steps is fully described. The methodology makes it possible to work with reactions that involve several components with unknown (and eventually overlapping) spectra and provides a tool for the simultaneous determination of both the quantum yields of the reaction and the spectra of all the species present in a multi-step photochemical process. As a benchmark, we apply these ideas to extract the quantum yields of photodetachment of coordinated ligands employing data recorded over the course of the decomposition of [Ru(tpm)(bpy)(CH3CN)]2+ and cis-[Ru(bpy)2(CH3CN)2]2+ under stationary photolysis conditions. The approach is fast and robust and it is easily implemented in scientific programming languages.

Photoactivation of Anticancer Ru Complexes in Deep Tissue: How Deep Can We Go?

Activation of anticancer therapeutics such as ruthenium (Ru) complexes is currently a topic of intense investigation. The success of phototherapy relies on photoactivation of therapeutics after the light passes through skin and tissue. In this paper, the photoactivation of anticancer Ru complexes with 671-nm red light through tissue of different thicknesses was studied. Four photoactivatable Ru complexes with different absorption wavelengths were synthesized. Two of them (Ru3 and Ru4) were responsive to wavelengths in the "therapeutic window" (650-900 nm) and could be activated using 671-nm red light after passing through tissue up to 16-mm-thick. The other two (Ru1 and Ru2) could not be activated using red light. Additionally, activated Ru4 caused inhibition of cancer cells. These results suggest that photoactivatable Ru complexes are promising for applications in deep-tissue phototherapy.

Structural, Spectroscopic, and Photochemical Investigation of an Octahedral NO-Releasing {RuNO}(7) Species.

[Ru(Me3[9]aneN3)(bpy)(NO)](BF4)2 ([1](BF4)2) was explored by single-crystal X-ray diffractometry, leading to the first crystal structure of an octahedral {RuNO}(7) complex. The metal resides on the center of a distorted octahedron, with dN-O and ∠Ru-N-O at 1.177(3) Å and 141.6(2)°, respectively. [1](BF4)2 can be stored indefinitely under argon. Solutions of [1](2+) show no signs of decomposition when protected from air and light. The electron paramagnetic resonance X-band spectrum at 85 K in vitrified acetonitrile (MeCN) shows signals consistent with an S = (1)/2 spin state, better described as Ru(II)NO(•) (g = [2.030, 1.993, 1.880] and A = [11.0, 30.4, 3.9]/10(-4) cm(-1)). In water, the {RuNO}(7) species reacts with O2 in a 1:4 stoichiometry. The reaction is first-order in both reactants with k = (1.9 ± 0.2) M(-1) s(-1) at 25 °C (ΔH(⧧) = 11.5 ± 0.3 kJ mol(-1); ΔS(⧧) = -189 ± 1 J K(-1) mol(-1)). Solutions of [1](2+) evolve NO when irradiated a 365 nm with ϕNO = 0.024 and 0.090 mol einstein(-1) in H2O and MeCN, respectively.

Fluorescent ligands and energy transfer in photoactive ruthenium-bipyridine complexes.

Ruthenium bis(bipyridine) complexes have proved to be useful as phototriggers for visible and IR-light photodelivery of molecules. They usually expel one ligand heterolytically upon absorption of blue or green light. However, their absorption capabilities at wavelengths longer than 500 nm are poor. Through coordination of fluorescent ligands to the Ru center, it is possible to establish an energy transfer pathway that allows these kinds of complexes to extend the range of photoactivation up to yellow wavelengths. We introduce a study of this effect in several complexes of the family using a modified Rhodamine as fluorescent ligand with different coordinated linkers. The observed trends show that a rational design of fluorophore-enhanced Ru-bpy phototriggers is possible and that photolysis efficiency can be increased by choosing the right combination of ligands.

Nitrosyl-centered redox and acid-base interconversions in [Ru(Me3[9]aneN3)(bpy)(NO)](3,2,1+). The pKa of HNO for its nitroxyl derivative in aqueous solution.

This work reports the preparation of a new 6-coordinated nitrosyl compound and its use as a model to explore the redox and acid-base properties of the three redox states of bound nitrosyl (formally NO(+), NO(•), NO(-)/HNO) in {RuNO}(6,7,8) species. We prepared the octahedral {RuNO}(6) complex [Ru(Me3[9]aneN3)(bpy)(NO)](3+) (Me3[9]aneN3: 1,4,7-trimethyl-1,4,7-triazacyclononane; bpy = 2,2'-bipyridine), and the related [Ru(Me3[9]aneN3)(bpy)(NO2)](+) nitro derivative. The compounds were characterized by chemical analysis, X-ray diffraction, NMR, IR, and UV-vis spectroscopies, cyclic voltammetry (CV), UV-vis/IR spectroelectrochemistry, and theoretical calculations (DFT, (TD)DFT). The reaction kinetics between the {RuNO}(6) complex and the nucleophile OH(-) is also presented. The incorporation of tridentate and bidentate ligands in the coordination sphere prevents labilization issues associated with the trans effect when attaining the reduced states of the nitrosyl group. This allows for a consistent interpretation of the changes in the main geometrical parameters: Ru-N and N-O distances, Ru-N-O angle, and the νNO frequency and electronic transitions. We explore the redox properties in acetonitrile and aqueous solutions, and provide a potential (E1/2) - pH (Pourbaix) diagram for the three diatomic nitrosyl-bound species, as well as for HNO and NO2(-), including the report of the pKa of the [Ru(Me3[9]aneN3)(bpy)(HNO)](2+) ion, 9.78 ± 0.15 at 25.0 °C.

Class III delocalization in a cyanide-bridged trimetallic mixed-valence complex.

The NIR and IR spectroscopic properties of the cyanide-bridged complex, trans-[Ru(dmap)4 {(µ-CN)Ru(py)4 Cl}2 ](3+) (py=pyridine, dmap=4-dimethylaminopyridine) provide strong evidence that this trimetallic ion behaves as a Class III mixed-valence species, the first example reported of a cyanide-bridged system. This has been accomplished by tuning the energy of the fragments in the trimetallic complex to compensate for the intrinsic asymmetry of the cyanide bridge. Moreover, (TD)DFT calculations accurately predict the spectra of the trans-[Ru(dmap)4 {(µ-CN)Ru(py)4 Cl}2 ](3+) ion and confirms its delocalized nature.

Communication between remote moieties in linear Ru-Ru-Ru trimetallic cyanide-bridged complexes.

In this article, we report the structural, spectroscopic, and electrochemical properties of the cyanide-bridged complex salts trans-[(NC)Ru(II)(L)4(µ-CN)Ru(II)(py)4Cl]PF6 and trans-[Ru(II)(L)4{(µ-CN)Ru(II)(py)4Cl}2](PF6)2 (L = pyridine or 4-methoxypyridine). The mixed-valence forms of these compounds show a variety of metal-to-metal charge-transfer bands, including one arising from charge transfer between the remote ruthenium units. The latter is more intense when L = 4-methoxypyridine and points to the role of the bridging ruthenium unit in promoting mixing between the dπ orbitals of the terminal fragments.

Four Ni(II) complexes with the new cyclam-methylimidazole ligand 1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane.

Although it has not proved possible to crystallize the newly prepared cyclam-methylimidazole ligand 1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane (L(Im1)), the trans and cis isomers of an Ni(II) complex, namely trans-aqua{1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) bis(perchlorate) monohydrate, [Ni(C(15)H(30)N(6))(H(2)O)](ClO(4))(2)·H(2)O, (1), and cis-aqua{1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) bis(perchlorate), [Ni(C(15)H(30)N(6))(H(2)O)](ClO(4))(2), (2), have been prepared and structurally characterized. At different stages of the crystallization and thermal treatment from which (1) and (2) were obtained, a further two compounds were isolated in crystalline form and their structures also analysed, namely trans-{1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}(perchlorato)nickel(II) perchlorate, [Ni(ClO(4))(C(15)H(30)N(6))]ClO(4), (3), and cis-{1,8-bis[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) bis(perchlorate) 0.24-hydrate, [Ni(C(20)H(36)N(6))](ClO(4))(2)·0.24H(2)O, (4); the 1,8-bis[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane ligand is a minor side product, probably formed in trace amounts in the synthesis of L(Im1). The configurations of the cyclam macrocycles in the complexes have been analysed and the structures are compared with analogues from the literature.

Chlorido(4,4'-dimethoxy-2,2'-bipyridine)(1,4,7-trimethyl-1,4,7-triazacyclononane)ruthenium(II) perchlorate acetonitrile disolvate and aqua(4,4'-dimethoxy-2,2'-bipyridine)(1,4,7-trimethyl-1,4,7-triazacyclononane)ruthenium(II) bis(perchlorate) dihydrate.

The title complexes, [RuCl(C(9)H(21)N(3))(C(12)H(12)N(2)O(2))]ClO(4)·2C(2)H(3)N, (I), and [Ru(C(9)H(21)N(3))(C(12)H(12)N(2)O(2))(H(2)O)](ClO(4))(2)·2H(2)O, (II), display similar structures with the Ru atom in a distorted octahedral environment. In the crystal packing of the chloride complex, (I), the Ru complex molecules are held together in pairs through C-H···Cl interactions of the 4,4'-dimethoxy-2,2'-bipyridine and chloride ligands. In the case of the aqua complex, (II), hydrogen bonding affords a tetrameric hydrogen-bonded network. These two structures are the first examples of complexes with the {Ru(1,4,7-trimethyl-1,4,7-triazacyclononane)} motif and an electron-rich substituted 2,2'-bipyridine ligand.

Widely differing photochemical behavior in related octahedral {Ru-NO}6 compounds: intramolecular redox isomerism of the excited state controlling the photodelivery of NO.

trans-[(NC)Ru(py)(4)(mu-CN)Ru(py)(4)(NO)](3+) (py = pyridine) is a stable species in aqueous solution. It displays an intense absorption in the visible region of the spectrum (lambda(max) = 518 nm; epsilon(max) = 6100 M(-1) cm(-1)), which turns this compound into a promising agent for the photodelivery of NO. The quantum yield for the photodelivery process resulting from irradiation with 455 nm visible light was found experimentally to be (0.06 +/- 0.01) x 10(-3) mol einstein(-1), almost 3 orders of magnitude smaller than that in the closely related cis-[RuL(NH(3))(4)(mu-pz)Ru(bpy)(2)(NO)](5+) species (L = NH(3) or pyridine, pz = pyrazine, bpy = 2,2'-bipyridine; phi(NO) = 0.02-0.04 mol einstein(-1) depending on L) and also much smaller than the one in the mononuclear compound trans-[ClRu(py)(4)(NO)](2+) (phi(NO) = (1.63 +/- 0.04) x 10(-3) mol einstein(-1)). DFT computations provide an electronic structure picture of the photoactive excited states that helps to understand this apparently abnormal behavior.

From monomers to geometry-constrained molecules: one step further toward cyanide bridged wires.

We report on the synthesis and properties of a family of linear cyanide bridged mixed-valence heptanuclear complexes with the formula: trans-[L(4)Ru(II){(mu-NC)Fe(III)(NC)(4)(mu-CN)Ru(II)L'(4)(mu-NC)Fe(III)(CN)(5)}(2)](6-) (with L and L' a para substituted pyridine). We also report on the properties of a related pentanuclear complex. These oligomers were purified by size exclusion chromatography, characterized by electrospray ionization (ESI) mass spectrometry and elemental analysis, and their linear shape was confirmed by scanning tunneling microscopy (STM). These complexes present a rich electrochemistry associated with the seven redox active centers. The redox potential split of identical fragments indicates that there is considerable communication along the cyanide bridged backbone of the compounds, even for centers more than 3 nm apart. This small attenuation of the interaction at long distances make these cyanide bridged compounds good candidates for molecular wires. Interestingly, the extent of the communication depends on the relative energy of the fragments, as evaluated by their redox potentials, providing a guide for improvement of this interesting property.