Ruthenium nitrosyl complexes with NO release capability: the use of fluorene as an antenna

A ruthenium nitrosyl complex of formula [RuII(fluorene(C6)CH2O-terpy)(bipy)(NO)]3+ (AC) in which fluorene(C6) is the 9,9-dihexylfluorene, terpy the 2,2';6',2''-terpyridine, and bipy the 2,2'-bipyridine is presented with its related [RuII(MeO-terpy)(bipy)(NO)]3+ (C) and 9,9-dihexylfluorene 2-hydroxymethylfluorene (A) building blocks. The reference complex C undergoes NO release capabilities under irradiation at λ = 365 nm. The effect of the introduction of the fluorescent A antenna within the resulting AC complex is discussed both experimentally and theoretically. The importance of the encaging parameter defined as ϕAC·IAC, in which IAC is the quantity of light absorbed by AC and ϕAC the quantum yield of NO release is evidenced and found to be concentration dependent. The conditions of optimization of the antenna approach to maximize ϕAC·IAC are discussed. The crystal structure of [RuII(fluorene(C6)CH2O-terpy)(bipy)(NO2)](PF6), the last intermediate in the synthesis of AC is also presented.

Sterically attenuated electronic communication in cobalt complexes of meridional isoquinoline-derived ligands for applications in electrocatalysis

The ability to synthetically tune the ligand frameworks of redox-active molecules is of critical importance to the economy of solar fuels because manipulating their redox properties can afford control over the operating potentials of sustained electrocatalytic or photoelectrocatalytic processes. The electronic and steric properties of 2,2':6',2″-terpyridine (Terpy) ligand frameworks can be tuned by functional group substitution on ligand backbones, and these correlate strongly to their Hammett parameters. The synthesis of a new series of tridentate meridional ligands of 2,4,6-trisubstituted pyridines that engineers the ability to finely tune the redox potentials of cobalt complexes to more positive potentials than that of their Terpy analogs is achieved by aryl-functionalizing at the four-position and by including isoquinoline at the two- and six-positions of pyridine (Aryl-DiQ). Their cobalt complex syntheses, their electronic properties, and their catalytic activity for carbon dioxide (CO2) reduction are reported and compared to their Terpy analogs. The cobalt derivatives generally experience a positive shift in their redox features relative to the Terpy-based analogs, covering a complementary potential range. Although those evaluated fail to produce any quantifiable products for the reduction of CO2 and suffer from long-term instability, these results suggest possible alternate strategies for stabilizing these compounds during catalysis. We speculate that lower equilibrium association constants to the cobalt center are intrinsic to these ligands, which originate from a steric interaction between protons on the pyridine and isoquinoline moieties. Nevertheless, the new Aryl-DiQ ligand framework has been engineered to selectively tune homoleptic cobalt complexes' redox potentials.

Ligand-Field Spectroscopy of Co(III) Complexes and the Development of a Spectrochemical Series for Low-Spin d6 Charge-Transfer Chromophores

A study of a series of six-coordinate Co(III) complexes has been carried out to quantify spectroscopic parameters for a range of ligands that are commonly employed to realize strong charge-transfer absorptions in low-spin, d⁶ systems. Identification of any three ligand-field transitions allows for the determination of the splitting parameter (10 Dq) as well as the Racah B and C parameters for a given compound. The data revealed a relatively small spread in the magnitude of 10 Dq, ranging from ca. 23 000 cm^-1 in the case of [Co(pyrro-bpy)₃]³⁺ (where pyrro-bpy is 4,4'-dipyrrolidinyl-2,2'-bipyridine) to ca. 26 000 cm^-1 for [Co(terpy)₂]³⁺ (where terpy is 2,2':6',2″-terpyridine). Significantly, trends across the series suggest that polypyridyl ligands behave as net π-donors when interacting with Co(III), in contrast to the net π-accepting character they exhibit when bound to second- and third-row metals. The influence of strong σ donation associated with carbene-based ligands was evident from the data acquired for [Co(B^MeImPy)₂]³⁺ (where B^MeImPy is 3,3'-(pyridine-2,6-diyl)bis(1-methyl-1H-3-imidazolium)), where a 10 Dq value of ca. 30 000 cm^-1 was determined. Spectroscopic data were also analyzed for [Fe(bpy)₃]²⁺ using the results on [Co(bpy)₃]³⁺ as a reference point. A value for 10 Dq of 21 000 cm^-1 was estimated, indicating a reduction in the ligand-field strength of ca. 3000 cm^-1 upon replacing Co(III) with Fe(II). We suggest that this approach of taking advantage of the blueshift of the charge-transfer feature in Co(III) complexes to reveal otherwise obscured ligand-field bands can be a useful tool for the development of new ligand systems to expand the photofunctionality of first-row transition-metal-based chromophores.

Metal Ions Determined Self-Assembly Using Terpyridine Building Blocks

Due to the dynamic reversibility of coordination-driven force, the structures of metallo-cages are sensitive to many stimulus, including ligand geometry, temperature, concentration, anions, pH, light, and so on. Among them,...

Fine-Tuning Metal and Ligand-Centered Redox Potentials of Homoleptic Bis-Terpyridine Complexes with 4'-Aryl Substituents

Homoleptic transition-metal complexes of 2,2':6',2″-terpyridine (terpy) and substituted derivatives of the form [M(R-terpy)₂]²⁺ display a wide range of redox potentials that correlate well to the Hammett parameter of the terpy substituents. Less is known about the impact of incorporating a phenyl spacer between the functional group responsible for controlling the electron density of terpy and how that translates to metal complexes of the form [M(4'-aryl-terpy)₂]²⁺, where M = Mn, Fe, Co, Ni, and Zn. Herein, we report our studies on these complexes revealed a good correlation of redox potentials of both metal- and ligand-centered events with the Hammett parameters of the aryl substituents, regardless of aryl-substitution pattern (i.e., the presence of multiple functional groups, combinations of withdrawing and donating functional groups). The phenyl spacer results in 60-80% attenuation of electron density as compared to the 4'-substituted terpy analogue, depending on the metal and redox couple analyzed. Density functional theory calculations performed on a simple model system revealed a strong correlation between the Hammett parameters and lowest unoccupied molecular orbital energies of the corresponding substituted pyridine models, thus serving as an inexpensive predictive tool when coupled with electrochemical data. Overall, these data suggest that such ligand modifications may be used in combination with previous approaches to further fine-tune the redox potentials of homoleptic transition-metal complexes, which may have applications in photochemical and electrochemical catalytic processes.

Functionalised Terpyridines and Their Metal Complexes—Solid-State Interactions

Analysis of the weak interactions within the crystal structures of 33 complexes of various 4′-aromatic derivatives of 2,2′:6′,2″-terpyridine (tpy) shows that interactions that exceed dispersion are dominated, as expected, by cation⋯anion contacts but are associated with both ligand–ligand and ligand–solvent contacts, sometimes multicentred, in generally complicated arrays, probably largely determined by dispersion interactions between stacked aromatic units. With V(V) as the coordinating cation, there is evidence that the polarisation of the ligand results in an interaction exceeding dispersion at a carbon bound to nitrogen with oxygen or fluorine, an interaction unseen in the structures of M(II) (M = Fe, Co, Ni, Cu, Zn, Ru and Cd) complexes, except when 1,2,3-trimethoxyphenyl substituents are present in the 4′-tpy.

Clover leaf-shaped supramolecules assembled using a predesigned metallo-organic ligand

A series of clover-like supramolecular structures were designed and synthesized using a combination of Ru–Zn, Ru–Co, Ru–Mn or Ru–Ni metal ions. These structures with distinct redox properties may have further applications as functional materials.

Self-Assembly of Metallo-Supramolecules under Kinetic or Thermodynamic Control: Characterization of Positional Isomers Using Scanning Tunneling Spectroscopy

Coordination-driven self-assembly has been extensively employed to construct a variety of discrete structures as a bottom-up strategy. However, mechanistic understanding regarding whether self-assembly is under kinetic or thermodynamic control is less explored. To date, such mechanistic investigation has been limited to distinct, assembled structures. It still remains a formidable challenge to study the kinetic and thermodynamic behavior of self-assembly systems with multiple assembled isomers due to the lack of characterization methods. Herein, we use a stepwise strategy which combined self-recognition and self-assembly processes to construct giant metallo-supramolecules with 8 positional isomers in solution. With the help of ultrahigh-vacuum, low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy, we were able to unambiguously differentiate 14 isomers on the substrate which correspond to 8 isomers in solution. Through measurement of 162 structures, the experimental probability of each isomer was obtained and compared with the theoretical probability. Such a comparison along with density functional theory (DFT) calculation suggested that although both kinetic and thermodynamic control existed in this self-assembly, the increased experimental probabilities of isomers compared to theoretical probabilities should be attributed to thermodynamic control.

Introducing Seven Transition Metal Ions into Terpyridine-Based Supramolecules: Self-Assembly and Dynamic Ligand Exchange Study

In coordination-driven self-assembly, 2,2':6',2″-terpyridine (tpy) has gained extensive attention in constructing supramolecular architectures on the basis of ⟨tpy-M-tpy⟩ connectivity. In direct self-assembly of large discrete structures, however, the metal ions were mainly limited to Cd(II), Zn(II), and Fe(II) ions. Herein, we significantly broaden the spectrum of metal ions with seven divalent transition metal ions M(II) (M = Mn, Fe, Co, Ni, Cu, Zn, Cd) to assemble a series of supramolecular fractals. In particular, Mn(II), Co(II), Ni(II), and Cu(II) were reported for the first time to form such large and discrete structures with ⟨tpy-M-tpy⟩ connectivity. In addition, the structural stabilities of those supramolecules in the gas phase and the kinetics of the ligand exchange process in solution were investigated using mass spectrometry. Such a fundamental study gave the relative order of structural stability in the gas phase and revealed the inertness of coordination in solution depending on the metal ions. Those results would guide the future study in tpy-based supramolecular chemistry in terms of self-assembly, characterization, property, and application.

Systematic investigation of the antiproliferative activity of a series of ruthenium terpyridine complexes

Due to acquired resistance or limitations of the currently approved drugs against cancer, there is an urgent need for the development of new classes of compounds. Among others, there is an increasing attention towards the use of Ru(II) polypyridyl complexes. Most studies in the literature were made on complexes based on the coordination of N-donating bidentate ligands to the ruthenium core whereas studies on 2,2':6', 2″-terpyridine (terpy) coordinating ligands are relatively scare. However, several studies have shown that [Ru(terpy)2]²⁺ derivatives are able bind to DNA through various binding modes making these compounds potentially suitable as chemotherapeutic agents. Additionally, light irradiation of these compounds was shown to enable DNA cleavage, highlighting their potential use as photosensitizers (PSs) for photodynamic therapy (PDT). In this work, we present the systematic investigation of the potential of 7 complexes of the type [Ru(terpy)(terpy-X)]2+ (X = H (1), Cl (2), Br (3), OMe (4), COOH (5), COOMe (6), NMe2 (7)) as potential chemotherapeutic agents and PDT PSs. Importantly, six of the seven complexes were found to be stable in human plasma as well as photostable in acetonitrile upon continuous light irradiation (480 nm). The determination of the distribution coefficient logP values for the 7 complexes revealed their good water solubility. Complex 7 was found to be cytotoxic in the micromolar range in the dark as well as to have some phototoxicity upon light exposure at 480 nm in non-cancerous retinal pigment epithelium (RPE-1) and cancerous human cervical carcinoma (HeLa) cells. SYNOPSIS: The systematic investigation of the potential of 7 complexes of the type [Ru(terpy)(terpy-X)]²⁺ (terpy: 2,2':6', 2″-terpyridine; X = H (1), Cl (2), Br (3), OMe (4), COOH (5), COOMe (6), NMe2 (7)) as potential chemotherapeutic agents and photosensitizers for photodynamic therapy is presented.

Diverse structural assemblies of U-shaped hydrazinyl-sulfonamides: experimental and theoretical analysis of non-covalent interactions stabilizing solid state conformations

The preparation and structures of five new U-shaped hydrazinyl-sulfonamides are reported.

A solvent- and temperature-dependent intramolecular equilibrium of diamagnetic and paramagnetic states in Co complexes bearing triaryl amines

Complexes [Co(L)₂](ClO₄)₂ (L = o-substituted 2-(pyridine-2-yl)-1,10-phenanthrolines 1a-c) containing three redox active centres (a Co²⁺ ion and two triaryl amine (Tara) units) have been synthesised. The order of oxidation steps in [Co(L)₂](ClO₄)₂ (L = 1a-c) was determined using cyclic voltammetry and EPR/UV-vis-NIR spectroelectrochemistry. In acetonitrile solutions, at room temperature, the first oxidation is Co-centred followed by the Tara oxidation at more anodic potentials. The order of oxidation is inverted in solutions of the less polar solvent dichloromethane. The Co^3+/2+-centred redox event leads to a spin transition between the paramagnetic high-spin (HS) Co²⁺ and the diamagnetic low-spin (LS) Co³⁺ state, which was proven using ¹H NMR and EPR spectroscopy. After one-electron oxidation of [Co(L)₂](ClO₄)₂, an equilibrium between the diamagnetic [Co³⁺(L)]³⁺ and paramagnetic [Co²⁺(L)(L⁺)]³⁺ state in [Co(L)₂]³⁺ (L = 1a-c) was found. Cyclic voltammetry showed enhanced intermolecular electron transfer between the [Co²⁺(L)₂]²⁺ and [Co³⁺(L)₂]³⁺ redox states mediated by [Co²⁺(L)(L⁺)]³⁺. Variable temperature vis-NIR spectroscopy of in situ generated [Co(L)₂]³⁺ revealed a temperature-dependent redox equilibrium between the [Co³⁺(L)₂]³⁺ and the [Co²⁺(L⁺)(L)]³⁺ states (L = 1a-c). Magnetic coupling between the HS-Co²⁺ ion and the Tara⁺ radical in [HS-Co²⁺(L⁺)(L)]³⁺ (L = 1a,c) was deduced from broad and undetectable lines observed in the corresponding EPR spectra. Complete oxidation to [LS-Co³⁺(L⁺)₂]⁵⁺ (L = 1a,c) leads to characteristic EPR spectra of Tara biradicals with non-interacting spins.

Quantitative investigation of C–H⋯π and other intermolecular interactions in a series of crystalline N-(substituted phenyl)-2-naphthamide derivatives

In this study, we have investigated the nature and characteristics of different intermolecular interactions present in a series of sevenN-(substituted phenyl)-2-naphthamides.

Oxidatively Locked [Co₂L₃]6+ Cylinders Derived from Bis(bidentate) 2-Pyridyl-1,2,3-triazole "Click" Ligands: Synthesis, Stability, and Antimicrobial Studies

A small family of [Co₂(Lpytrz)₃]6+ cylinders was synthesised from bis(bidentate) 2-pyridyl-1,2,3-triazole "click" ligands (Lpytrz) through an "assembly-followed-by-oxidation" method. The cylinders were characterised using ¹H, 13C, and DOSY NMR, IR, and UV-Vis spectroscopies, along with electrospray ionisation mass spectrometry (ESMS). Stability studies were conducted in dimethyl sulfoxide (DMSO) and D₂O. In contrast to similar, previously studied, [Fe₂(Lpytrz)₃]4+ helicates the more kinetically inert [Co₂(Lpytrz)₃]6+ systems proved stable (over a period of days) when exposed to DMSO and were even more stable in D₂O. The triply stranded [Co₂(Lpytrz)₃]6+ systems and the corresponding "free" ligands were tested for antimicrobial activity in vitro against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) microorganisms. Agar-based disk diffusion and Mueller-Hinton broth micro-dilution assays showed that the [Co₂(Lpytrz)₃]6+ cylinders were not active against either strain of bacteria. It is presumed that a high charge of the [Co₂(Lpytrz)₃]6+ cylinders is preventing them from crossing the bacterial cell membranes, rendering the compounds biologically inactive.

The surprising lability of bis(2,2′:6′,2′′-terpyridine)chromium(iii) complexes

The first heteroleptic chromium(iii) complexes with 2,2′:6′,2′′-terpyridine ligands are reported; in basic aqueous conditions or in MeOH in the presence of F⁻ion, the complexes are unexpectedly labile.

2,2':6',2''-terpyridine substituted in the 4'-position by the solubilizing and sterically demanding tert-butyl group: a surprisingly new ligand

We describe the synthesis and characterization of 4'-tert-butyl-2,2':6',2''-terpyridine (4'-(t)Butpy, 1), a convergent tpy ligand that exhibits both a sterically demanding and solubilizing 4'-substituent. In the solid state, molecules of 1 pack with alternating tpy and tert-butyl domains, and the bulky alkyl substituents prevent the molecules from engaging in the face-to-face π-interactions which are typical of simple tpy ligands. Instead, the predominant packing forces involve CH···N hydrogen bonds and weak CH···π contacts. The syntheses of the homoleptic complexes [M(1)(2)][PF(6)](2) (M = Fe, Co, Zn and Ru) and the heteroleptic [Ru(tpy)(1)][PF(6)](2) are described. The complexes have been fully characterized in solution, including the (1)H NMR spectroscopic characterization of the paramagnetic [Co(1)(2)][PF(6)](2). Cyclic voltammetric data are consistent with the tert-butyl substituent being slightly electron releasing. The single crystal structures of [Zn(1)(2)][PF(6)](2) and [Ru(1)(2)][PF(6)](2) have been determined; the compounds are essentially isomorphous. The packing of the cations is such that the tert-butyl substituents are accommodated in pockets between the tpy domains of adjacent cations, and as a consequence, the {M(tpy)(2)}-embrace that is a ubiquitous feature of many related structures is not observed.

When is a metallopolymer not a metallopolymer? When it is a metallomacrocycle

We report reactions of cobalt(II) acetate with a series of ditopic bis(tpy) ligands (tpy = 2,2':6',2''-terpyridine) containing flexible polyethyleneoxy spacers (tpy-4'-O{(CH(2))(2)O}(n)-4'-tpy, n = 2, 3, 4 or 6 ligands 1-4, respectively) which result in the formation of complicated mixtures of species, presumed to be both open chain and cyclic species. Well resolved paramagnetically shifted (1)H NMR spectra are a powerful tool for the analysis of these solution systems. Upon equilibration, [n + n] metallomacrocycles are isolated as the dominant (thermodynamic) species in some cases, and the single crystal X-ray structures of [Co(2)(3)(2)][PF(6)](4)·6MeCN and [Co(2)(4)(2)][PF(6)](5)·2MeCN (a mixed cobalt(ii)/cobalt(iii) species) are presented. Oxidation of the equilibrated cobalt(II) mixtures to kinetically inert cobalt(III) species provides additional evidence for the formation of metallomacrocycles as the thermodynamic products. Single crystal structural data for [Co(3)(2)(3)][PF(6)](9)·2MeCN·3.5H(2)O, [Co(2)(3)(2)][PF(6)](6)·10MeCN and [Co(2)(4)(2)][PF(6)](6)·6MeCN confirm the assembly of [2 + 2] and [3 + 3] metallomacrocycles. PGSE NMR spectroscopy has been used to determine the hydrodynamic radii of the solution species.