Syntheses and electrochemical properties of ruthenium(II) complexes with 4,4-bipyrimidine and 4,4-bipyrimidinium ligands

The syntheses and electrochemical properties of novel ruthenium(II) polypyridyl complexes with 4,4^′-bipyrimidine, [Ru(trpy)(bpm)Cl](X) ([1](X; X=PF₆⁻, BF₄⁻)) and with a quaternized 4,4^′-bipyrimidinium ligand, [Ru(trpy)(Me₂bpm)Cl](BF₄)₃ ([2](BF₄)₃) (trpy=2,2^′:6^′,2″-terpyridine, bpm=4,4^′-bipyrimidine, Me₂bpm=1,1^′-dimethyl-4,4^′-bipyrimidinium) are presented. The bpm complex [1]⁺ was prepared by the reaction of Ru(trpy)Cl₃ with 4,4^′-bipyrimidine in EtOH/H₂O. The structural characterization of [1]⁺ revealed, that the bpm ligand coordinated to the ruthenium atom with the bidentate fashion. Diquaternization of the non-coordinating nitrogen atoms on bpm of [1]⁺ by (CH₃)₃OBF₄ in CH₃CN gave [2](BF₄)₃. The electrochemical and spectroelectrochemical properties of the complexes are described.     

Chiral diruthenium(II) complexes containing a modified 4,4-bipyridine bridging ligand: syntheses, characterization and DNA binding

A pair of novel chiral dimeric ruthenium(II) complexes [ΔΔ-, ΛΛ-Ru(bpy)₂(btpb)Ru(bpy)₂]⁴⁺ (1; btpb=2,2^′-bis(1,2,4-triazino[5,6-f]phenanthren-3-yl)-4,4^′-bipyridine) have been synthesized and characterized by electrospray mass spectra, ¹H NMR, UV-Vis and circular dichroism spectra. Binding behaviors of the complexes with calf thymus DNA have been investigated by absorption spectra and viscosity measurements. The electronic absorption spectrum of ΔΔ-1 at 505.5 nm exhibits hypochromism of about 8.4% and bathochromism of 2.5 nm; ΛΛ-1 at 500.0 nm exhibits hypochromism of about 9.1% and bathochromism of 4.5 nm, respectively. The experiments suggest that ΔΔ-1 and ΛΛ-1 may be bound to DNA by non-intercalating binder.     

4′-(4-Pyridyl)-2,2′:6′,2″-terpyridine as ligand in the lead(II) complexes, [Pb(pyterpy)(MeOH)I2] · MeOH and [Pb(pyterpy)(μ-AcO)]2(ClO4)2

Two new lead(II) complexes with the ligand 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine (pyterpy), [Pb(pyterpy)(MeOH)I₂] · MeOH and [Pb(pyterpy)(μ-AcO)]₂(ClO₄)₂, have been synthesized and characterized by CHN elemental analysis, ¹H NMR-, ¹³C NMR-, IR spectroscopy and structurally analyzed by X-ray single-crystal diffraction. The thermal stabilities of these compounds were studied by thermal gravimetric (TG) and differential thermal analyses (DTA). The single crystal X-ray analyses show that the coordination number in these complexes is six with three “pyterpy” N-donor atoms and two or three of the anionic ligands. The arrangement of donor atoms in these complexes suggest a gap or hole in the coordination geometry of the lead atoms, possibly occupied by a stereoactive lone pair of electrons on lead(II) and the coordination sphere is hemidirected. The potentially tetradentate ligand 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine (pyterpy) acts as a tridentate donor to Pb(II). The noncoordinated pyridyl group interacts with hydrogen atoms of adjacent molecules and forms normal hydrogen bonds in [Pb(pyterpy)(MeOH)I₂] · MeOH and weak C-H?N interactions for [Pb(pyterpy)(μ-AcO)]₂(ClO₄)₂, thus extending the monomeric structures into one-dimensional networks.     

Mixed ligand complexes of osmium(II)-2,2-bipyridine: synthesis, spectral characterization and electrochemical properties of bis-chelated-arylazoimidazole-bipyridine-osmium(II) and X-ray crystal structure of [(2,2-bipyridine)-bis-{1-methyl-2-(p-tolylazo)imidazole}osmium(II) hexafluorophosphate

Reaction of ctc-OsBr₂(RaaiR^′)₂ [RaaiR^′=1-alkyl-2-(arylazo)imidazole, p-R-C₆H₄-NN-C₃H₂-NN-1-R^′, where R=H (a), Me (b), Cl (c) and R^′=Me (2), Et (3) and CH₂Ph (4)] with 2,2^′-bipyridine (bpy) in presence of AgNO₃ in EtOH followed by the addition of NH₄PF₆ afforded a mixed ligand complex [Os(bpy)(RaaiR^′)₂](PF₆)₂. The structure of the complex, in one case [Os(bpy)(MeaaiMe)₂](PF₆)₂ · 4H₂O, has been confirmed by X-ray crystallography. The complexes are diamagnetic (low spin d⁶, s=0) and they show intense MLCT transition in the visible region (480-525 nm) and a weak transition at longer wavelength (>850 nm) in CH₃CN solution. Cyclic voltammetry of the complexes show two metal oxidation, Os(II)/Os(III) at 0.72-0.76 V and Os(III)/Os(IV) at 1.34-1.42 V and three successive ligand reductions.     

Reactions of 6,6-dimethyl-2,2-bipyridyl with iron(II) in aqueous and non-aqueous media

Reaction of anhydrous FeCl₂ with 6,6^′-dimethyl-2,2^′-bipyridyl (dmby) in non-aqueous media gives the yellow, high spin, tetrahedral complex FeCl₂(dmby), which is characterized crystallographically, magnetically and by ¹H NMR spectroscopy. In contrast, reaction of FeCl₂ · 4H₂O with dmby in 0.1 M hydrochloric acid, the method of choice for preparing 3:1 and 2:1 iron(II) complexes of 2,2^′-bipyridyl, gives [H₂dmby][FeCl₄] and [Hdmby][FeCl₄], in which the dmby has been protonated. These complexes are also characterized crystallographically.     

Strong and weak coupling in mixed-valence complexes bridged by 4,4′-azo-di(phenylcyanamido)

The complexes [{Ru(tpy^∗)(bpy)}₂(μ-adpc)][PF₆]₂ where tpy^∗ is 4,4′,4″-tri-(tert-butyl)-2,2′:6′,2″-terpyridine, bpy is 2,2′-bipyridine, and adpc²⁻ is 4,4′-azo-diphenylcyanamide dianion and trans,trans-[{Ru(tpy^∗)(pc)}₂(μ-adpc)] where pc⁻ is 2-pyrazine-carboxylato were prepared and characterized by cyclic voltammetry and spectroelectrochemical methods. Intervalence band properties and IR spectroelectrochemistry of the mixed-valence complexes [{Ru(tpy^∗)(bpy)}₂(μ-adpc)]³⁺ and trans,trans-[{Ru(tpy^∗)(pc)}₂(μ-adpc)]⁺ are consistent with delocalized and valence-trapped mixed-valence properties respectively. The reduction in mixed-valence coupling upon substituting a bipyridine ligand with 2-pyrazine carboxylato strongly suggests that hole-transfer superexchange is the dominant mechanism for metal-metal coupling in these complexes.     

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.     

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.     

Anionic effects on the formation of tridentate 4′-chloro-2,2′:6′,2″-terpyridine copper(II) complexes having 1:1 or 1:2 ratio of metal and ligand and their crystal symmetry

A facile synthetic procedure has been used to prepare one five-coordinate and four six-coordinate copper(II) complexes of 4′-chloro-2,2′:6′,2″-terpyridine (tpyCl) ligand with different counterions (, , , , and ) in high yields. They are formulated as [Cu(tpyCl-κ³N,N′,N′′)(SO₄-κO)(H₂O-κO)] · 2H₂O (1), trans-[Cu(tpyCl-κ³N,N′,N″)(NO₃-κO)₂(H₂O-κO)] (2), [Cu(tpyCl-κ³N,N′,N″)₂](BF₄)₂ (3), [Cu(tpyCl-κ³N,N′,N″)₂](PF₆)₂ (4) and [Cu(tpyCl-κ³N,N′,N″)₂](ClO₄)₂ (5) and versatile interactions in supramolecular level including coordinative bonding, O-H?O, O-H?Cl, C-H?F, and C-H?Cl hydrogen bonding, π-π stacking play essential roles in forming different frameworks of 1-5. It is concluded that the difference of coordination abilities of the counterions used and the experimental conditions codominate the resulting complexes with 1:1 or 1:2 ratio of metal and ligand.     

pH- and DNA-induced dual molecular light switches based on a novel ruthenium(II) complex

A novel Ru(II) complex, [Ru(bpy)₂(btppz)]Cl₂, where bpy = 2,2′-bipyridine and btppz = benzo[h]tripyrido[3,2-a:2′,3′-c:2″,3″-j]phenazine, has been synthesized and characterized. The pH effects on UV-visible (UV-vis) absorption and emission spectra of the complex have been studied and ground- and excited-state ionization constants of the complex have been derived. The calf thymus DNA (ct-DNA) binding properties of the complex were investigated with UV-vis absorption and luminescence spectrophotometric titrations, steady-state emission quenching by [Fe(CN)₆]⁴⁻, DNA competitive binding with ethidium bromide, DNA melting experiments, reverse salt titrations and viscosity measurements. The complex was demonstrated to act as dual molecular switches: pH-induced “on-off” emission switch with an on-off intensity ratio of ∼54 which is favorably compared with those reported for structurally analogous Ru(II) complexes, and a DNA molecular light switch with a luminescence enhancement factor of 22 as it intercalatively bound to the DNA.     

The multichromophore approach: A case of temperature controlled switching between single and dual emission in Ru(II) polypyridyl complexes

A new series of ruthenium(II) complexes, based on 4′-(9-anthryl)-2,2′:6′,2″-terpyridine (an-tpy), have been synthesized from the Ru(III) precursor (an-tpy)RuCl₃ (4). These new Ru(II) complexes, [(an-tpy)Ru(tpy-pm-R)] (R = H, 3a; Cl, 3b; phenyl, 3c; p-bromophenyl, 3d), have extended π-conjugation through the 5′-substituted pyrimidyl group, and have been characterized by analytical and spectroscopic methods, and X-ray single crystal structure determination for 3b and 3d. Luminescence lifetime measurements have shown that the anthryl chromophore greatly increases room temperature (r.t.) excited-state lifetime, even though it is not directly connected to the ligand involved in the metal-to-ligand charge transfer (³MLCT) emitting state. This result demonstrates that an equilibrium is established between the anthryl triplet state (³an) and the ³MLCT state even though the two chromophores are physically separated by more than one nanometer. At low temperature in rigid matrix, such equilibrium does not take place and emission arises from both ³an and ³MLCT excited states. The temperature dependence of the excited-state equilibration induces a temperature switch from single (room temperature) to dual (77 K) emission behavior.     

DNA-binding and photocleavage studies of mixed polypyridyl ruthenium(II) complexes with calf thymus DNA

New mixed polypyridyl {NMIP = 2′-(2″-nitro-3″,4″-methylenedioxyphenyl)imidazo-[4′,5′-f][1,10]-phenanthroline, dmb = 4,4′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine} ruthenium(II) complexes [Ru(dmb)₂(NMIP)]²⁺ (1) and [Ru(bpy)₂(NMIP)]²⁺ (2) have been synthesized and characterized. The binding of these complexes to calf thymus DNA (CT-DNA) has been investigated with spectroscopic methods, viscosity and electrophoresis measurements. The experimental results indicate that both complexes could bind to DNA via partial intercalation from the minor/major groove. In addition, both complexes have been found to promote the single-stranded cleavage of plasmid pBR 322 DNA upon irradiation. Under comparable experimental conditions compared with [Ru(phen)₂(NMIP)]²⁺, during the course of the dialysis at intervals of time, the CD signals of both complexes started from none, increased to the maximum magnitude, then no longer changed, and the activity of effective DNA cleavage dependence upon concentration degree lies in the following order: [Ru(phen)₂NMIP]²⁺ > complex 2 > complex 1.     

Complexation of lead(II) with O,O-dialkyldithiophosphate ligands: P and C CP/MAS NMR and single-crystal X-ray diffraction studies

Polycrystalline lead(II) complexes with O,O^′-dipropyl- and O,O^′-di-cyclo-hexyldithiophosphate ions were prepared and studied by means of ³¹P, ³¹C CP/MAS NMR spectroscopy and single-crystal X-ray diffraction. Prepared complexes are characterised by polynuclear structures, in which pairs of dithiophosphate groups asymmetrically link neighbouring lead atoms, forming infinite linear zigzag chains. In spite of the same combined structural function, dithiophosphate ligands in both complexes display structural inequivalence. To characterise the combined structural state of the dialkyldithiophosphate ligands, ³¹P chemical shift anisotropy parameters, δ_aniso and η, were estimated from spinning sideband patterns in experimental CP/MAS NMR spectra for each of the two prepared complexes as well as the initial potassium O,O^′-dipropyl- and O,O^′-di-cyclo-hexyldithiophosphate salts.     

In situ hydrothermal syntheses, crystal structures and luminescent properties of two novel zinc(II) coordination polymers based on tetrapyridyl ligand

Two novel Zn(II) coordination polymers, [Zn(2-pytpy)(fum)]_n·nH₂O (1) and [Zn₆(4-pytpy)₃(mal)₄]_n·5n(H₂O) (2), (2-pytpy = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine, 4-pytpy = 4′-(4-pyridyl)-4,2′:6′,4″-terpyridine, H₂fum = fumaric acid and H₂mal = malic acid) have been hydrothermally synthesized and structurally characterized. Notably, in situ ligand reactions occur in the formation of complexes 1 and 2, in which maleic acid is converted into fumaric acid and malic acid, respectively. Complex 1 is a 1D infinite chain structure, which is extended into a supramolecular layer by intermolecular π…π stacking interactions. Complex 2 is a 3D network structure, in which the bidentate-bridging 4-pytpy ligands link the layers based on the tetranuclear Zn(II) subunits to form the (4,10)-connected network. The luminescent properties of 1 and 2 have been investigated with emission spectra and UV-Vis diffuse reflectance spectra in the solid state. Additionally, these two complexes possess great thermal stabilities.     

Preparation and properties of ruthenium (II) complexes of 2,2′:6′,2″-terpyridines substituted at the 4′-position with heterocyclic groups

Four ligands whose general formula is R-terpy with terpy = 2,2′:6′,2″ terpyridine bearing at the 4′-position a substituent R = 2-furyl, 2-pyrrolyl, 2-thienyl and 5-2,2′bithienyl were synthesised. The absorption spectra and the electrochemical behaviour of the corresponding homoleptic Ru(II) complexes were investigated and compared to those of the parent complex [Ru(terpy)]²⁺. Due to the donor effect of the grafted heterocyclic groups, the absorption and emission maxima are red-shifted and the energy levels of the HOMO Π(t_2g) metal orbitals are slightly higher. The incorporation of these heterocyclic moieties extends the electronic delocalisation over the corresponding ligands, leading to higher emission quantum yields. Cyclic voltammetric studies of pyrrolyl-, thienyl- and bithienyl-functionalised complexes show that an electroactive layer can be deposited on the electrode. Preliminary results point out that an electrodeposited film could be used as a photocathode in an aqueous electrolyte.     

Rapid synthesis of bis (2,2′-bipyridine) nitratocopper(II) nitrate using a hydrothermal method and its application to dye-sensitized solar cells

In this study we synthesized bis (2,2′-bipyridine) nitratocopper(II) nitrate in order to examine its the crystal structure, optical property and application to dye-sensitized solar cells (DSSCs). Single X-ray analysis results revealed that the acquired complex exhibited five-coordination with four nitrogen atoms of bipyridine and the oxygen bond of the ion. The reflectance UV-Vis absorptions showed three absorptions that were assigned to ligand-to-ligand at around 230-350 nm, metal-to-ligand charge transfer at around 350-600 nm, and d-d transfer at around ∼650 nm. Cyclic voltammetry in acetonitrile revealed a reversible Cu(I) → Cu(II) oxidation process at a highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO) levels of −4.692 and −4.071 eV, respectively. The photoelectric efficiency in DSSCs was approximately 0.032% with the nanometer-sized TiO₂ in the condition of an open-circuit voltage (V_oc) of 0.346 V, a short-circuit current density (J_sc) of 0.166 mA/cm² at an incident light intensity of 100 mW/cm².     

Pyrrole-substituted tridentate complexes of Ru(II): Spectroscopy, electrochemistry, photosensitization and the role of orbital mixing

One-step syntheses are reported of 4′-(pyrrol-2-yl)-2,2′;6′,2″-terpyridine, 4-(pyrrol-2-yl)-2,6-di(pyrazol-2-yl)pyridine and of their homoleptic Ru(II) complexes, in good to very good yields. DFT calculations confirmed that the pyrrole rings lay coplanar with the tridentate cores and constituted effective π-donors, but also showed that the properties of the Ru(II) complexes defied classical analyses based on localized metal- or ligand-centred orbitals. The low-potential electrochemical oxidations led to electropolymerization but were not purely pyrrole-centred. The low-energy electronic spectral absorptions were not purely metal-to-ligand charge transfer (MLCT) in character, but resulted from mixed metal-to-ligand and intraligand transitions. The complexes’ photosensitization abilities showed that the pyrrole groups were beneficial to the survival of the photoexcited states, albeit not as much as p-tolyl groups.     

Aromatic N-oxide bridged copper(II) coordination polymers: Synthesis, characterization and magnetic properties

The complexes [Cu₂(o-NO₂-C₆H₄COO)₄(PNO)₂] (1), [Cu₂(C₆H₅COO)₄(2,2′-BPNO)]_n (2), [Cu₂(C₆H₅COO)₄(4,4′-BPNO)]_n (3), [Cu(p-OH-C₆H₄COO)₂(4,4′-BPNO)₂·H₂O]_n (4), (where PNO = pyridine N-oxide, 2,2′-BPNO = 2,2′-bipyridyl-N,N′-dioxide, 4,4′-BPNO = 4,4′-bipyridyl-N,N′-dioxide) are prepared and characterized and their magnetic properties are studied as a function of temperature. Complex 1 is a discrete dinuclear complex while complexes 2-4 are polymeric of which 2 and 3 have paddle wheel repeating units. Magnetic susceptibility measurements from polycrystalline samples of 1-4 revealed strong antiferromagnetic interactions within the {Cu₂}⁴⁺ paddle wheel units and no discernible interactions between the units. The complex 5, [Cu(NicoNO)₂·2H₂O]_n·4nH₂O, in which the bridging ligand to the adjacent copper(II) ions is nicotinate N-oxide (NicoNO) the transmitted interaction is very weakly antiferromagnetic.     

The synthesis and characterization of mixed ligand Ru(II) complexes with dipyrido(2,3-a:3′,2′-j)phenazine (dpop′) and 2,3,5,6-tetra(2-pyridyl)-pyrazine (tppz)

The synthesis of the mixed ligand mono metallic [Ru(dpop′)(tppz)]²⁺ and bimetallic [(dpop′)Ru(tppz)Ru(dpop′)]⁴⁺ (dpop′ = dipyrido(2,3-a:3′,2′-j)phenazine; tppz = 2,3,5,6 tetra-(2-pyridyl)pyrazine) complexes is described. The [Ru(dpop′)(tppz)]²⁺ complex display an intense absorption at 518 nm which is assigned to a Ru(dπ) → dpop′ (π∗) MLCT transition, and at 447 nm which is assigned to a Ru(dπ) → tppz(π∗) MLCT transition. It undergoes emission at RT in CH₃CN with λ_em = 722 nm. The bimetallic [(dpop′)Ru(tppz)Ru(dpop′)]⁴⁺ complex shows a low energy absorption shoulder near 635 nm assigned to a Ru(dπ) → tppz(π∗) MLCT transition and an intense peak at 542 nm due to Ru(dπ) → dpop′ (π∗) MLCT transition. The bimetallic complex also emits at RT in CH₃CN with λ_em = 785 nm. Cyclic voltammetry shows reversible Ru^+2/+3 oxidations at 1.68 V for the monometallic complex and Ru^+2/+3 oxidation couples at +1.94 and +1.70 V for the bimetallic complex.