Diimine Complexes Research Articles

Photochemistry of Heteroleptic 1,4,5,8-Tetraazaphenanthrene- and Bi-1,2,3-triazolyl-Containing Ruthenium(II) Complexes.

Diimine metal complexes have significant relevance in the development of photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) applications. In particular, complexes of the TAP ligand (1,4,5,8-tetraazaphenanthrene) are known to lead to photoinduced oxidation of DNA, while TAP- and triazole-based complexes are also known to undergo photochemical ligand release processes relevant to PACT. The photophysical and photochemical properties of heteroleptic complexes [Ru(TAP)n(btz)3-n]2+ (btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, n = 1 (1), 2 (2)) have been explored. Upon irradiation in acetonitrile, 1 displays analogous photochemistry to that previously observed for [Ru(bpy)(btz)2]2+ (bpy = 2,2'-bipyridyl) and generates trans-[Ru(TAP)(btz)(NCMe)2]2+ (5), which has been crystallographically characterized, with the observation of the ligand-loss intermediate trans-[Ru(TAP)(κ2-btz)(κ1-btz)(NCMe)]2+ (4). Complex 2 displays more complicated photochemical behavior with not only preferential photorelease of btz to form cis-[Ru(TAP)2(NCMe)2]2+ (6) but also competitive photorelease of TAP to form 5. Free TAP is then taken up by 6 to form [Ru(TAP)3]2+ (3) with the proportion of 5 and 3 observed to progressively increase during prolonged photolysis. Data suggest a complex set of reversible photochemical ligand scrambling processes in which 2 and 3 are interconverted. Computational DFT calculations have enabled optimization of geometries of the pro-trans 3MCcis states with repelled btz or TAP ligands crucial for the formation of 5 from 1 and 2, respectively, lending weight to recent evidence that such 3MCcis states play an important mechanistic role in the rich photoreactivity of Ru(II) diimine complexes.

Structural characterization, DFT studies and luminescent properties of dinuclear copper(I)-diimine complexes with the S-shape configurations

Three dinuclear Cu(I) complexes, [Cu2(pnd)(PPh3)4](BF4)2(1), [Cu2(pnd)(dppp)2] (BF4)2(2), and [Cu2(pnd)(DPEphos)2](BF4)2(3) (PPh3 = triphenylphosphine, dppp = 1, 3-bis (diphenylphosphino)propane, DPEphos = bis(2-diphenylphosphinophenyl)ether, pnd = N, N’-bis (pyridin-2-ylmethylene)naphthyl-1, 5-diimine), have been prepared and characterized by IR, 1H NMR, 31P NMR, 13C NMR, XRD, elemental analysis and X-ray crystal structure analysis. The structural analysis reveals that the flexible pnd in 1–3 acts in a trans bridging coordination mode to connect two Cu+ ions, forming a S-shape conformation, while each of Cu (I) ion is four-coordinate N2P2, adopting a distorted-tetrahedral geometry. DFT calculations show that the HOMOs of complexes 1–3 are primarily located on the copper d-orbital and the phosphine ligand, together with a specific contribution of pnd, while LUMOs are mostly composed of pnd with admixed Cu(I) d-orbital character. Corresponding calculations of atomic charges indicate that the charge of the copper atom in complexes 1–3 is below + 1, while the total charges of the coordinated atoms are 0.743–0.786. Moreover, the solid-state emission spectra of complexes 1–3 show the existence of broad emission bands at 350–550 nm.

Combatting AMR: A molecular approach to the discovery of potent and non-toxic rhenium complexes active against C. albicans-MRSA co-infection

Antimicrobial resistance (AMR) is a major emerging threat to public health, causing serious issues in the successful prevention and treatment of persistent diseases. While the problem escalates, lack of financial incentive has lead major pharmaceutical companies to interrupt their antibiotic drug discovery programs. The World Health Organisation (WHO) has called for novel solutions outside the traditional development pathway, with emphasis on new classes of active compounds with non-classical mechanisms of action. Metal complexes are an untapped source of antibiotic potential owing to unique modes of action and a wider range of three-dimensional geometries as compared to purely organic compounds. In this study, we present the antimicrobial and antifungal efficacy of a family of rhenium tricarbonyl diimine complexes with varying ligands, charge and lipophilicity. Our study allowed the identification of potent and non-toxic complexes active in vivo against S. aureus infections at MIC doses as low as 300 ng/mL, as well as against C. albicans-MRSA mixed co-infection. The compounds are capable of suppressing the C. albicans morphogenetic yeast-to-hyphal transition, eradicating fungal-S. aureus co-infection, while showing no sign of cardio-, hepato-, hematotoxiciy or teratogenicity.

Luminescent Platinum(II) Complexes with Bidentate Diacetylide Ligands: Structures, Photophysical Properties and Application Studies

A series of platinum(II) complexes supported by terphenyl diacetylide as well as diimine or bis-N-heterocyclic carbene (NHC) ligands have been prepared. The diacetylide ligands adopt a cis coordination mode featuring non-planar terphenyl moieties as revealed by X-ray crystallographic analyses. The electrochemical, photophysical and photochemical properties of these platinum(II) complexes have been investigated. These platinum(II) diimine complexes show broad emission with peak maxima from 566 nm to 706 nm, with two of them having emission quantum yields >60% and lifetimes <2 μs in solutions at room temperature, whereas the platinum(II) diacetylide complexes having bis-N-heterocyclic carbene instead of diimine ligand display photoluminescence with quantum yields of up to 28% in solutions and excited state lifetimes of up to 62 μs at room temperature. Application studies revealed that one of the complexes can catalyze photoinduced aerobic dehydrogenation of alcohols and alkenes, and a relatively non-toxic water-soluble Pt(II) complex displays anti-angiogenic activity.

Functionalizing Collagen with Vessel-Penetrating Two-Photon Phosphorescence Probes: A New In Vivo Strategy to Map Oxygen Concentration in Tumor Microenvironment and Tissue Ischemia.

The encapsulation and/or surface modification can stabilize and protect the phosphorescence bio‐probes but impede their intravenous delivery across biological barriers. Here, a new class of biocompatible rhenium (ReI) diimine carbonyl complexes is developed, which can efficaciously permeate normal vessel walls and then functionalize the extravascular collagen matrixes as in situ oxygen sensor. Without protective agents, ReI‐diimine complex already exhibits excellent emission yield (34%, λ em = 583 nm) and large two‐photon absorption cross‐sections (σ 2 = 300 GM @ 800 nm) in water (pH 7.4). After extravasation, remarkably, the collagen‐bound probes further enhanced their excitation efficiency by increasing the deoxygenated lifetime from 4.0 to 7.5 µs, paving a way to visualize tumor hypoxia and tissue ischemia in vivo. The post‐extravasation functionalization of extracellular matrixes demonstrates a new methodology for biomaterial‐empowered phosphorescence sensing and imaging.

Synthesis, structures, DFT studies and luminescent properties of copper(I)-diimine complexes and application in yellow-green light-emitting diode

A series of copper(I)-diimine complexes, [Cu(bdpp)(bpy)]BF4 (1), [Cu(bdpp) (pen)]BF4 (2), [Cu(bdpp)(damp)]BF4 (3), [Cu(bdpp)(pen)]BF4 (4) (bdpp = 4,6-bis-(biphenyl phosphino) phenoxazine, bpy = 2,2′-bipyridine, phen = 1,10-Phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, bphen = 4,7-diphenyl-1,10-phenanthroline), have been synthesized and characterized by IR, 1H NMR, 31P NMR, XRD and X-ray crystal structure analysis. Structural analysis reveals that each Cu+ in 1–4 is four-coordinated by two N atoms and two P atoms and adopts a distorted tetrahedral geometry. DFT calculations show that the HOMOs in 1–4 are mainly comprised of bdpp and copper d–orbitals, while the LUMOs are mainly dominated by diimine ligands. Calculation of atomic charges indicates that the charges of the central Cu atoms in 1–4 are less than + 1, and the diimine ligands may be the key to the metal charge transfer to the coordinated atom. In 1–4, the maximum phosphorescent lifetimes and quantum yields are up to 349 μs and 22%, respectively. Moreover, complex 3 is used for fabricating the LED device, showing a decent performance with the maximum EQE of 11%.

Next Generation Cuprous Phenanthroline MLCT Photosensitizer Featuring Cyclohexyl Substituents.

A new long-lived, visible-light-absorbing homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizer, [Cu(dchtmp)2]PF6 (dchtmp = 2,9-dicyclohexyl-3,4,7,8-tetramethyl-1,10-phenanthroline), has been synthesized, structurally characterized, and evaluated in terms of its molecular photophysics, electrochemistry, and electronic structure. Static and time-resolved transient absorption (TA) and photoluminescence (PL) spectroscopy measured on the title compound in CH2Cl2 (τ = 2.6 μs, ΦPL = 5.5%), CH3CN (τ = 1.5 μs, ΦPL = 2.6%), and THF (τ = 2.0 μs, ΦPL = 3.7%) yielded impressive photophysical metrics even when dissolved in Lewis basic solvents. The combined static spectroscopic data along with ultrafast TA experiments revealed that the pseudo-Jahn-Teller distortion and intersystem crossing dynamics in the MLCT excited state displayed characteristics of being sterically arrested throughout its evolution. Electrochemical and static PL data illustrate that [Cu(dchtmp)2]PF6 is a potent photoreductant (-1.77 V vs Fc+/0 in CH3CN) equal to or greater than all previously investigated homoleptic Cu(I) diimine complexes. Although we successfully prepared the cyclopentyl analog dcptmp (2,9-dicyclopentyl-3,4,7,8-tetramethyl-1,10-phenanthroline) using the same C-C radical coupling photochemistry as dchtmp, the corresponding Cu(I) complex could not be isolated due to the steric hindrance presented at the metal center. Ultimately, the successful preparation of [Cu(dchtmp)2]+ represents a major step forward for the design and discovery of novel earth-abundant photosensitizers made possible through a newly conceived ligand synthetic strategy.

A valproic acid-modified platinum diimine complex as potential photosensitizer for photodynamic therapy

Histone deacetylase inhibitors have often been used in combination treatment of various types of cancers due to their non-genotoxic epigenetic potential. Valproic acid (VPA) is a well-known histone deacetylase inhibitor. Conjugate of VPA with a phtoactive platinum diimine complex through an ester bond has been fabricated to potentiate the photocytotoxicity of the photosensitizer. Its capability to generate singlet oxygen, behavior in the presence of esterase, and photocytotoxicity in tumor cells have also been studied. The results revealed that the novel VPA-modified platinum diimine complex could produce singlet oxygen efficiently and release VPA in the presence of porcine liver esterase. The results also suggested that incorporation of VPA moiety into the platinum diimine complex might significantly enhance the cytotoxicity of the complex.

Determining Excited-State Structures and Photophysical Properties in Phenylphosphine Rhenium(I) Diimine Biscarbonyl Complexes Using Time-Resolved Infrared and X-ray Absorption Spectroscopies.

We have explored the structural factors on the photophysical properties in two rhenium(I) diimine complexes in acetonitrile solution, cis,trans-[Re(dmb)(CO)2(PPh2Et)2]+ (Et(2,2)) and cis,trans-[Re(dmb)(CO)2(PPh3)2]+ ((3,3)) (dmb = 4,4'-dimethyl-2,2'-bipyridine, Ph = phenyl, Et = ethyl) using the combination method of time-resolved infrared spectroscopy, time-resolved extended X-ray absorption fine structure, and quantum chemical calculations. The difference between these complexes is the number of phenyl groups in the phosphine ligand, and this only indirectly affects the central Re(I). Despite this minor difference, the complexes exhibit large differences in emission wavelength and excited-state lifetime. Upon photoexcitation, the bond length of Re-P and angle of P-Re-P are significantly changed in both complexes, while the phenyl groups are largely rotated by ∼20° only in (3,3). In contrast, there is little change in charge distribution on the phenyl groups when Re to dmb charge transfer occurs upon photoexcitation. We concluded that the instability from steric effects of phenyl groups and diimine leads to a smaller Stokes shift of the lowest excited triplet state (T1) in (3,3). The large structural change between the ground and excited states causes the longer lifetime of T1 in (3,3).

Synthesis, Spectroscopic Characterization, and Biological Activities of New Binuclear Co(II), Ni(II), Cu(II), and Zn(II) Diimine Complexes

Metal-ligand complexes have attracted major interest due to their potential medical applications as anticancer agents. The work described in the current article aimed to synthesize, spectroscopic, thermal, and biological studies of some metal-diimine complexes. A diimine ligand, namely 2-{[2-(4-chlorophenyl)-2-hydroxyvinyl]-hydrazonomethyl}phenol (diim) was prepared via the reaction of p-chlorophenacyl bromide with hydrazine hydrate in ethanol, then condensation was completed with 2-hydroxybenzaldehyde in acetic acid. The Co(II), Ni(II), Cu(II), and Zn(II) complexes were prepared with a metal:ligand stoichiometric ratio of (2:1). 1H-NMR, UV-Vis, FTIR spectroscopic data, molar conductivity measurements, and microanalytical data (carbon, hydrogen, nitrogen, and halogen) were used for characterization of the formed ligand and its metal complexes. It was found that the diimine ligand act as tetradentate fashion. The non-electrolytic character for all the complexes was proved by molar conductivity. The first metal atom of the synthesized binuclear diimine complexes coordinates with the nitrogen of hydrazine group and oxygen of OH group. While, the second metal atom coordinates with the other nitrogen atom of the hydrazine group and oxygen of phenolic group. All the synthesized metal complexes have a six-coordinated except for Zn(II) has four-coordinated. Thermogravimetric analysis and its differential analysis were done to discuss the thermal degradation of the free ligand and its metal complexes. Molecular docking calculation showed that the diimine ligand is a good inhibitor for breast cancer 3hb5 and 4o1v kidney cancer proteins. Additionally, these compounds were evaluated as antibacterial and antifungal agents.

Triphenylphosphonium-functionalized nanocomposites as carriers of a platinum diimine complex for photodynamic therapy

The photodamage induced by PDT drugs usually occurs at the site where the photosensitizers accumulate in the tumor cells, thus the modulation of intrinsic apoptosis by mitochondria-targeting PDT drugs might be a promising way to enhance the therapeutic efficacy of PDT drugs against tumor cells. Novel triphenylphosphonium-functionalized nanocomposites employed as carriers of a photoactive platinum diimine complex have been fabricated and characterized. Upon irradiation, the IC50 value of photosensitizer-loaded triphenylphosphonium-functionalized nanocomposites was found to be 17.4 or 14.4 times lower than that of the photosensitizer studied alone against HCT116 cells or A549 cells, respectively. The results suggested that the triphenylphosphonium- functionalized nanocomposites as drug delivery vehicles could significantly enhance the photodynamic therapy efficacy of the photosensitizer.

Role of Axial Ligation in Gating the Reactivity of Dimethylplatinum(III) Diimine Radical Cations

Electrochemical methods are coupled with chemical oxidant-based analytical strategies to evaluate the oxidative reactivity of the platinum(II) diimine complex (bpy)PtII(CH3)2 (bpy = 2,2′-bipyridine...

Photo-initiated oxidation of C-H bonds by diimine complexes of vanadium(V).

The photochemical activation of carbon-hydrogen bonds by vanadium(v)-dioxo and vanadium(v)-oxo-peroxo diimine complexes is described. Reactions were carried out using a selection of organic substrates with C-H bond dissociation free energy values between 70 and 97 kcal mol-1. The ability to activate C-H bonds using vanadium(v)-dioxo and vanadium(v)-oxo-peroxo diimine complexes varies with different bond dissociation free energy. Compounds with weaker C-H bonds are oxidized in minutes, rather than in days for thermal oxidations by the corresponding complexes. Dioxygen is necessary for substrate consumption, which suggests that the electronically excited V complexes are radical reaction initiators via H-atom abstraction from the organic substrate.

Copolymerisation of 1-alkenes with bulky oxygen-containing olefins for dual-stage functionalisation of polyolefins

Pd–Diimine complexes catalyse the copolymerisation of 1-alkenes with olefins having a bulky oxygen-containing substituents, such as cyclic acetal or bicycloorthoester groups, affording the copolymers containing the comonomer unit up to 34%.

A systematic study of the influence of ligand field on the slow magnetic dynamics of Co(ii)-diimine compounds.

Herein we report heteroleptic Co(ii) diimine complexes [Co(H2bip)2Cl2] (1), [Co(H2bip)2Br2] (2), [Co(H2bip)3]Br2·1MeOH (3) and [Co(H2bip)2(Me2bpy)]Br2·(MeCN)0.5·(H2O)0.25 (4) (H2bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, bpy = 2,2'-dipyridyl, Me2bpy = 4,4'-Me-2,2'-dipyridyl), purposefully prepared to enable a systematic study of magnetic property changes arising from the increase of overall ligand field from σ/π-donor chlorido (1) to π-acceptor 4,4'Me-2,2'bpy (4). The presence of axial and rhombic anisotropy (D and E) of these compounds is sufficient to allow 1-4 to show field-induced slow relaxation of magnetization. Interestingly, we found as the effective ligand field is increased in the series, rhombicity (E/D) decreases, and the magnetic relaxation profile changes significantly, where relaxation of magnetization at a specific temperature becomes gradually faster. We performed mechanistic analyses of the temperature dependence of magnetic relaxation times considering Orbach relaxation processes, Raman-like relaxation and quantum tunnelling of magnetization (QTM). The effective energy barrier of the Orbach relaxation process (Ueff) is largest in compound 1 (19.2 cm-1) and gradually decreases in the order 1 > 2 > 3 > 4 giving a minimum value in compound 4 (8.3 cm-1), where the Raman-like mechanism showed the possibility of different types of phonon activity below and above ∼2.5 K. As a precursor of 1, the tetrahedral complex [Co(H2bip)Cl2] (1a) was also synthesized and structurally and magnetically characterized: this compound exhibits slow relaxation of magnetization under an applied dc field (1800 Oe) with a record slow relaxation time of 3.39 s at 1.8 K.

Reactivity and Mechanism of Photo- and Electrocatalytic Hydrogen Evolution by a Diimine Copper(I) Complex

The tetrahedral copper(I) diimine complex [Cu(pq)2]BF4 displays high photocatalytic activity for the H2 evolution reaction with a turnover number of 3564, thus representing the first type of a Cu(I) quinoxaline complex capable of catalyzing proton reduction. Electrochemical experiments indicate that molecular mechanisms prevail and DFT calculations provide in-depth insight into the catalytic pathway, suggesting that the coordinating nitrogens play crucial roles in proton exchange and hydrogen formation.

Switching the Reactivity of Palladium Diimines with “Ancillary” Ligand to Select between Olefin Polymerization, Branching Regulation, or Olefin Isomerization

AbstractCoordinating solvents are commonly employed as ancillary ligands to stabilize late transition metal complexes and are conventionally considered to have little effect on the reaction products. Our work identifies that the presence of ancillary ligand in Pd‐diimine catalyzed polymerizations of α‐olefins can drastically alter reactivity. The addition of different amounts of acetonitrile allows for switching between distinct reaction modes: isomerization–polymerization with high branching (0 equiv.), regular chain‐walking polymerization (1 equiv.), and alkene isomerization with no polymerization (&gt;20 equiv.). Optimization of the isomerization reaction mode led to a general set of conditions to switch a wide variety of diimine complexes into efficient alkene isomerization catalysts, with catalyst loading as low as 0.005 mol %.

Switching the Reactivity of Palladium Diimines with "Ancillary" Ligand to Select between Olefin Polymerization, Branching Regulation, or Olefin Isomerization.

Coordinating solvents are commonly employed as ancillary ligands to stabilize late transition metal complexes and are conventionally considered to have little effect on the reaction products. Our work identifies that the presence of ancillary ligand in Pd-diimine catalyzed polymerizations of α-olefins can drastically alter reactivity. The addition of different amounts of acetonitrile allows for switching between distinct reaction modes: isomerization-polymerization with high branching (0 equiv.), regular chain-walking polymerization (1 equiv.), and alkene isomerization with no polymerization (>20 equiv.). Optimization of the isomerization reaction mode led to a general set of conditions to switch a wide variety of diimine complexes into efficient alkene isomerization catalysts, with catalyst loading as low as 0.005 mol %.

Targeted Synthesis of NIR Luminescent Rhenium Diimine cis,trans-[Re( )(CO)2 (L)2 ]n+ Complexes Containing N-Donor Axial Ligands: Photophysical, Electrochemical, and Theoretical Studies.

The combined action of ultraviolet irradiation and microwave heating onto acetonitrile solution of [Re( )(CO)3 (NCMe)]OTf ( =phenantroline and neocuproine) afforded cis,trans-Re( )(CO)2 (NCMe)2 ]+ acetonitrile derivatives. Substitution of relatively labile NCMe with a series of aromatic N-donor ligands (pyridine, pyrazine, 4,4'-bipyridine, N-methyl-4,4'-bipyridine) gave a novel family of the diimine cis,trans-[Re( )(CO)2 (L)2 ]+ complexes. Photophysical studies of the obtained compounds in solution revealed unusually high absorption across the visible region and NIR phosphorescence with emission band maxima ranging from 711 to 805 nm. The nature of emissive excited states was studied using DFT calculations to show dominant contribution of 3 MLCT (dπ(Re)→π*( )) character. Electrochemical (CV and DPV) studies of the monocationic diimine complexes revealed one reduction and one oxidation wave assigned to reduction of the diimine moiety and oxidation of the rhenium center, respectively.

Octahedral Iron Complexes of Pyrazine(diimine) Pincers: Ligand Electronic Effects and Protonation.

Redox noninnocent ligands are known to be involved in altering the overall electronic nature of organometallic complexes by serving as an electron reservoir. Pyrazine(diimine) backbones in these complexes introduce enhanced π acidity over their more well-studied pyridine(diimine) analogues and open up the opportunity for functionalization of the nitrogen at the 4-position of the ring. Herein we report the characterization of bis-chelated pyrazine(diimine) [(PzDI)2Fe]n+ (n = 0, 1, and 2) complexes for electronic and structural comparison to pyridine(diimine) complexes (PDI) with similar architectures. Cyclic voltammetry studies show three reductions, two of which are ligand-based and reversible. Reduction of [(PzDI)2Fe]2+ (1) to [(PzDI)2Fe]+ (2) and (PzDI)2Fe (3) gives rise to characteristic structural changes, such as imine C═N bond lengthening, indicating the formation of a ligand radical, a conclusion which is further supported by electron paramagnetic resonance (EPR) and electronic structure calculations. Comparisons between the PzDI and PDI systems are highlighted. Complex 1 can be protonated at the uncoordinated pyrazine nitrogen, resulting in changes to its spectroscopic and redox properties; efforts to further functionalize the ligand are discussed.