Speciation and Reactivity of Mono- and Binuclear Ni Intermediates in Aminoquinoline-Directed C-H Arylation and Benzylation.

This paper describes detailed organometallic studies of the aminoquinoline-directed Ni-catalyzed C-H functionalization of 2,3,4,5-tetrafluoro-N-(quinolin-8-yl)benzamide with diaryliodonium reagents. A combination of 19F NMR spectroscopy and X-ray crystallography is used to track and characterize diamagnetic and paramagnetic intermediates throughout this transformation. These provide key insights into both the cyclometalation and oxidative functionalization steps of the catalytic cycle. The reaction conditions (solvent, ligands, base, and stoichiometry) play a central role in the observation of a NiII precyclometalation intermediate as well as in the speciation of the NiII products of C-H activation. Both mono- and binuclear cyclometalated NiII species are observed and interconvert, depending on the reaction conditions. Cyclic voltammetry reveals that the NiII/III redox potentials for the cyclometalated intermediates vary by more than 700 mV depending on their coordination environments, and these differences are reflected in their relative reactivity with diaryliodonium oxidants. The oxidative functionalization reaction affords a mixture of arylated and solvent functionalization organic products, depending on the conditions and solvent. For example, conducting oxidation in toluene leads to the preferential formation of the benzylated product. A series of experiments implicate a NiII/III/IV pathway for this transformation.

Bioinspired Herbicides-Ionic Liquids or Liquid Cocrystals?

This research provides information about combinations of several amino acids, including l-proline (Pro), l-arginine (Arg), and l-histidine (His), with phenoxyacetic acid herbicides (MCPA and 2,4-D). Five amino acid ionic liquids (AAILs), one amino acid higher-melting salt (AAHMS), and two amino acid liquid cocrystals (AALCs) were obtained in high yields (>90%). The ionization of the six new structures was confirmed by NMR, IR, and molecular modeling. X-ray crystallography was used to definitively confirm the binding location of the mobile hydrogen. Furthermore, we propose a computational method for estimating the energy of specific hydrogen bond(s) in AAIL crystals based on the NBO and QTAIM hydrogen bond parameters obtained by model calculations. An in-depth analysis of the structures allowed to answer the question posed in the title, ionic liquids or liquid cocrystals? AAILs based on arginine and histidine were obtained. In contrast, combining proline with MCPA and 2,4-D led to AALCs. Finally, the compounds were analyzed to measure their herbicidal activity. These studies proved that the novel form of MCPA or 2,4-D improved its ability to control weeds compared to commercial formulations containing the same active ingredients.

Peptide-mediated targeted delivery of SOX9 nanoparticles into astrocytes ameliorates ischemic brain injury.

Astrocytes are highly activated following brain injuries, and their activation influences neuronal survival. Additionally, SOX9 expression is known to increase in reactive astrocytes. However, the role of SOX9 in activated astrocytes following ischemic brain damage has not been clearly elucidated yet. Therefore, in the present study, we investigated the role of SOX9 in reactive astrocytes using a poly-lactic-co-glycolic acid (PLGA) nanoparticle plasmid delivery system in a photothrombotic stroke animal model. We designed PLGA nanoparticles to exclusively enhance SOX9 gene expression in glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes. Our observations indicate that PLGA nanoparticles encapsulated with GFAP:SOX9:tdTOM reduce ischemia-induced neurological deficits and infarct volume through the prostaglandin D2 pathway. Thus, the astrocyte-targeting PLGA nanoparticle plasmid delivery system provides a potential opportunity for stroke treatment. Since the only effective treatment currently available is reinstating the blood supply, cell-specific gene therapy using PLGA nanoparticles will open a new therapeutic paradigm for brain injury patients in the future.

Practical and Scalable Two-Step Process for 6-(2-Fluoro-4-nitrophenyl)-2-oxa-6-azaspiro[3.3]heptane: A Key Intermediate of the Potent Antibiotic Drug Candidate TBI-223.

A low-cost, protecting group-free route to 6-(2-fluoro-4-nitrophenyl)-2-oxa-6-azaspiro[3.3]heptane (1), the starting material for the in-development tuberculosis treatment TBI-223, is described. The key bond forming step in this route is the creation of the azetidine ring through a hydroxide-facilitated alkylation of 2-fluoro-4-nitroaniline (2) with 3,3-bis(bromomethyl)oxetane (BBMO, 3). After optimization, this ring formation reaction was demonstrated at 100 g scale with isolated yield of 87% and final product purity of >99%. The alkylating agent 3 was synthesized using an optimized procedure that starts from tribromoneopentyl alcohol (TBNPA, 4), a commercially available flame retardant. Treatment of 4 with sodium hydroxide under Schotten-Baumann conditions closed the oxetane ring, and after distillation, 3 was recovered in 72% yield and >95% purity. This new approach to compound 1 avoids the previous drawbacks associated with the synthesis of 2-oxa-6-azaspiro[3,3]heptane (5), the major cost driver used in previous routes to TBI-223. The optimization and multigram scale-up results for this new route are reported herein.

Ruthenium Complexes with Protic Ligands: Influence of the Position of OH Groups and π Expansion on Luminescence and Photocytotoxicity.

Protic ruthenium complexes using the dihydroxybipyridine (dhbp) ligand combined with a spectator ligand (N,N = bpy, phen, dop, Bphen) have been studied for their potential activity vs. cancer cells and their photophysical luminescent properties. These complexes vary in the extent of π expansion and the use of proximal (6,6'-dhbp) or distal (4,4'-dhbp) hydroxy groups. Eight complexes are studied herein as the acidic (OH bearing) form, [(N,N)2Ru(n,n'-dhbp)]Cl2, or as the doubly deprotonated (O- bearing) form. Thus, the presence of these two protonation states gives 16 complexes that have been isolated and studied. Complex 7A, [(dop)2Ru(4,4'-dhbp)]Cl2, has been recently synthesized and characterized spectroscopically and by X-ray crystallography. The deprotonated forms of three complexes are also reported herein for the first time. The other complexes studied have been synthesized previously. Three complexes are light-activated and exhibit photocytotoxicity. The log(Do/w) values of the complexes are used herein to correlate photocytotoxicity with improved cellular uptake. For Ru complexes 1-4 bearing the 6,6'-dhbp ligand, photoluminescence studies (all in deaerated acetonitrile) have revealed that steric strain leads to photodissociation which tends to reduce photoluminescent lifetimes and quantum yields in both protonation states. For Ru complexes 5-8 bearing the 4,4'-dhbp ligand, the deprotonated Ru complexes (5B-8B) have low photoluminescent lifetimes and quantum yields due to quenching that is proposed to involve the 3LLCT excited state and charge transfer from the [O2-bpy]2- ligand to the N,N spectator ligand. The protonated OH bearing 4,4'-dhbp Ru complexes (5A-8A) have long luminescence lifetimes which increase with increasing π expansion on the N,N spectator ligand. The Bphen complex, 8A, has the longest lifetime of the series at 3.45 µs and a photoluminescence quantum yield of 18.7%. This Ru complex also exhibits the best photocytotoxicity of the series. A long luminescence lifetime is correlated with greater singlet oxygen quantum yields because the triplet excited state is presumably long-lived enough to interact with 3O2 to yield 1O2.

Ruthenium pincer complexes for light activated toxicity: Lipophilic groups enhance toxicity.

Nine ruthenium CNC pincer complexes (1-9) were tested for anticancer activity in cell culture under both dark and light conditions. These complexes included varied CNC pincer ligands including OH, OMe, or Me substituents on the pyridyl ring and wingtip N-heterocyclic carbene (NHC) groups which varied as methyl (Me), phenyl (Ph), mesityl (Mes), and 2,6-diisopropylphenyl (Dipp). The supporting ligands included acetonitrile, Cl, and 2,2'-bipyridine (bpy) donors. The synthesis of complexes 8 and 9 is described herein and are fully characterized by spectroscopic (1H NMR, IR, UV-Vis, MS) and analytical techniques. Single crystal X-ray diffraction results are reported herein for 8 and 9. The other complexes (1-7) are reported elsewhere. The four most lipophilic ruthenium complexes (6, 7, 8, and 9) showed the best activity vs. MCF7 cancer cells with complexes 6 and 9 showing cytotoxicity and complex 7 and 8 showing light activated photocytotoxicity. The distribution of these compounds between octanol and water is reported as log(Do/w) values, and increasing log(Do/w) values correlate roughly with improved activity vs. cancer cells. Overall, lipophilic wingtip groups (e.g. Ph, Mes, Dipp) on the NHC ring and a lower cationic charge (1+ vs. 2+) appears to be beneficial for improved anticancer activity.

CO2 capture from ambient air via crystallization with tetraalkylammonium hydroxides.

Aqueous solutions of a series of short carbon chain tetra(n-alkyl)ammonium hydroxides, [Nnnnn][OH] with n = 2: n-ethyl, 3: n-propyl, 4: n-butyl, have been serendipitously found to be potential candidates for direct air carbon capture (DAC) when being used as reagents in more complicated reactions. Aqueous solutions of [N3333][OH], [N2222][OH], or [N3333][OH] with UO2SO4·3H2O and 1,4-diamidoximylbenzene, and [N4444][OH] with cytosine (HCyt) directly absorb CO2 from the atmosphere upon mild heating in the open atmosphere crystallizing in complexes reaching up to 2 : 1 CO2/[Nnnnn]OH ratio. [N2222][HCO3]·3H2O (1), [N2222]2[H(HCO3)3]·5H2O (2), [N3333][HCO3]·0.5H2O (3), [N3333][H(HCO3)2] (4), [N3333]2[(tpa)(H2CO3)2] (5; tpa = terephthalate), [N4444][H(Cyt)(HCO3)]·H2O (6) and [N4444][H2(Cyt)2(HCO3)]·H2O (7) have been isolated in crystalline form and structurally characterized by single crystal X-ray diffraction. The compounds are characterized by complex polyanionic formations from bicarbonate dimers ([(HCO3)2·(H2O)]24-) or chains ([H(HCO3)2]nn- or [H2(tpa)(HCO3)2]n2n-) to water-bicarbonate associates ([(HCO3)2·6H2O]2- and [(H2CO3·(HCO3)2)2·6H2O·2H2O]2-) and three-component anionic layers ([H(Cyt)(HCO3)·H2O]nn- and [H2(Cyt)2(HCO3)·H2O]nn-) frequently showing proton sharing. While some hydroxides themselves can maintain a high CO2/[Nnnnn][OH] ratio, particularly 2 and 4, the presence of secondary hydrogen bond donors/acceptors may increase the sorption efficiency through decreased solubility and enhanced crystallization.

Correction: Solvent-driven azide-induced mononuclear discrete versus one-dimensional polymeric aromatic Möbius cadmium(II) complexes of an N6 tetradentate helical ligand.

Correction for 'Solvent-driven azide-induced mononuclear discrete versus one-dimensional polymeric aromatic Möbius cadmium(II) complexes of an N6 tetradentate helical ligand' by Farhad Akbari Afkhami et al., Dalton Trans., 2017, 46, 14888-14896, https://doi.org/10.1039/C7DT02952G.

Light-responsive and Protic Ruthenium Compounds Bearing Bathophenanthroline and Dihydroxybipyridine Ligands Achieve Nanomolar Toxicity towards Breast Cancer Cells.

We report new ruthenium complexes bearing the lipophilic bathophenanthroline (BPhen) ligand and dihydroxybipyridine (dhbp) ligands which differ in the placement of the OH groups ([(BPhen)2 Ru(n,n'-dhbp)]Cl2 with n = 6 and 4 in 1A and 2A , respectively). Full characterization data are reported for 1A and 2A and single crystal X-ray diffraction for 1A . Both 1A and 2A are diprotic acids. We have studied 1A , 1B , 2A , and 2B (B = deprotonated forms) by UV-vis spectroscopy and 1 photodissociates, but 2 is light stable. Luminescence studies reveal that the basic forms have lower energy 3 MLCT states relative to the acidic forms. Complexes 1A and 2A produce singlet oxygen with quantum yields of 0.05 and 0.68, respectively, in acetonitrile. Complexes 1 and 2 are both photocytotoxic toward breast cancer cells, with complex 2 showing EC50 light values as low as 0.50 µM with PI values as high as >200 vs. MCF7. Computational studies were used to predict the energies of the 3 MLCT and 3 MC states. An inaccessible 3 MC state for 2B suggests a rationale for why photodissociation does not occur with the 4,4'-dhbp ligand. Low dark toxicity combined with an accessible 3 MLCT state for 1 O2 generation explains the excellent photocytotoxicity of 2.

Singlet Oxygen Formation vs Photodissociation for Light-Responsive Protic Ruthenium Anticancer Compounds: The Oxygenated Substituent Determines Which Pathway Dominates.

Ruthenium complexes bearing protic diimine ligands are cytotoxic to certain cancer cells upon irradiation with blue light. Previously reported complexes of the type [(N,N)2Ru(6,6'-dhbp)]Cl2 with 6,6'-dhbp = 6,6'-dihydroxybipyridine and N,N = 2,2'-bipyridine (bipy) (1A), 1,10-phenanthroline (phen) (2A), and 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (dop) (3A) show EC50 values as low as 4 µM (for 3A) vs breast cancer cells upon blue light irradiation ( Inorg. Chem. 2017, 56, 7519). Herein, subscript A denotes the acidic form of the complex bearing OH groups, and B denotes the basic form bearing O- groups. This photocytotoxicity was originally attributed to photodissociation, but recent results suggest that singlet oxygen formation is a more plausible cause of photocytotoxicity. In particular, bulky methoxy substituents enhance photodissociation but these complexes are nontoxic ( Dalton Trans 2018, 47, 15685). Cellular studies are presented herein that show the formation of reactive oxygen species (ROS) and apoptosis indicators upon treatment of cells with complex 3A and blue light. Singlet oxygen sensor green (SOSG) shows the formation of 1O2 in cell culture for cells treated with 3A and blue light. At physiological pH, complexes 1A-3A are deprotonated to form 1B-3B in situ. Quantum yields for 1O2 (ϕΔ) are 0.87 and 0.48 for 2B and 3B, respectively, and these are an order of magnitude higher than the quantum yields for 2A and 3A. The values for Ï•Δ show an increase with 6,6'-dhbp derived substituents as follows: OMe < OH < O-. TD-DFT studies show that the presence of a low lying triplet metal-centered (3MC) state favors photodissociation and disfavors 1O2 formation for 2A and 3A (OH groups). However, upon deprotonation (O- groups), the 3MLCT state is accessible and can readily lead to 1O2 formation, but the dissociative 3MC state is energetically inaccessible. The changes to the energy of the 3MLCT state upon deprotonation have been confirmed by steady state luminescence experiments on 1A-3A and their basic analogs, 1B-3B. This energy landscape favors 1O2 formation for 2B and 3B and leads to enhanced toxicity for these complexes under physiological conditions. The ability to convert readily from OH to O- groups allowed us to investigate an electronic change that is not accompanied by steric changes in this fundamental study.

Experimental and Computational Study of the Structure, Steric Properties, and Binding Equilibria of Neopentylphosphine Palladium Complexes.

Steric properties of crystallographically and computationally determined structures of linear palladium(0) and square planar palladium(II) complexes of di(tert-butyl)neopentylphosphine (P(t-Bu)2Np), tert-butyldineopentylphosphine (P(t-Bu)Np2), and trineopentylphosphine (PNp3) have been determined. Structures of linear palladium(0) complexes show that steric demand increases as tert-butyl groups are replaced with neopentyl groups (P(t-Bu)2Np < P(t-Bu)Np2 < PNp3). In square planar palladium(II) complexes, PNp3 gives the smallest steric parameters, whereas P(t-Bu)Np2 has the largest steric demand. The change in the steric demand of PNp3 compared to P(t-Bu)2Np and P(t-Bu)Np2 results from a significant conformational change in PNp3 depending on the coordination number of the metal. The steric properties of these ligands were also probed by measuring the equilibrium constant for coordination of free phosphine to dimeric [(R3P)Pd(µ-Cl)Cl]2 complexes. Binding equilibria follow the same trend as the steric parameters for square planar complexes with PNp3 having the highest binding constant. In contrast to the normal trend, the neopentylphosphines show increased pyramidalization at phosphorus with increasing steric demand. We hypothesize that this unusual dependence reflects the low back side strain of the neopentyl group, which allows the ligand to be more pyramidalized while still exerting a significant front side steric demand.

Synthesis, Structural Characterization, and Coordination Chemistry of (Trineopentylphosphine)palladium(aryl)bromide Dimer Complexes ([(Np3P)Pd(Ar)Br]2).

A series of [(PNp3)Pd(Ar)Br]2 complexes (PNp3 = trineopentylphosphine, Ar = 4-tolyl, 4-tert-butylphenyl, 2-tolyl, 4-methoxy-2-methylphenyl, 2-isopropylphenyl, and 2,6-dimethylphenyl) were synthesized and structurally characterized by X-ray crystallography and density functional theory optimized structures. The trineopentylphosphine ligand is able to accommodate coordination of other sterically demanding ligands through changes in its conformation. These conformational changes can be seen in changes in percent buried volume of the PNp3 ligand. The binding equilibria of the [(PNp3)Pd(Ar)Br]2 complexes with pyridine derivatives were determined experimentally and analyzed computationally. The binding equilibria are sensitive to the steric demand of the pyridine ligand and less sensitive to the steric demand of the aryl ligand on palladium. In contrast to previous studies, the binding equilibria do not correlate with pyridine basicity. The binding equilibria results are relevant to fundamental ligand coordination steps in cross-coupling reactions, such as Buchwald-Hartwig aminations.

Highly Active Ruthenium CNC Pincer Photocatalysts for Visible-Light-Driven Carbon Dioxide Reduction.

Five ruthenium catalysts described herein facilitate self-sensitized carbon dioxide reduction to form carbon monoxide with a ruthenium catalytic center. These catalysts include four new and one previously reported CNC pincer complexes featuring a pyridinol derived N-donor and N-heterocyclic carbene (NHC) C-donors derived from imidazole or benzimidazole. The complexes have been characterized fully by spectroscopic and analytic methods, including X-ray crystallography. Introduction of a 2,2'-bipyridine (bipy) coligand and phenyl groups on the NHC ligand was necessary for rapid catalysis. [(CNC)Ru(bipy)(CH3CN)](OTf)2 is among the most active and durable photocatalysts in the literature for CO2 reduction without an external photosensitizer. The role of the structure of this complex in catalysis is discussed, including the importance of the pincer's phenyl wingtips, the bipyridyl ligand, and a weakly coordinating monodentate ligand.

Sterically demanding methoxy and methyl groups in ruthenium complexes lead to enhanced quantum yields for blue light triggered photodissociation.

Ruthenium complexes containing a sterically congested metal center can serve as light activated prodrugs through photo-activated chemotherapy (PACT). In this work, we modified PACT agents containing 6,6'-dihydroxybipyridine (6,6'-dhbp) (Papish et al., Inorg. Chem., 2017, 56, 7519) by replacing it with a sterically bulky isoelectronic ligand, 6,6'-dimethoxybipyridine (6,6'-dmbp). The resulting complexes, [(phen)2Ru(6,6'-dmbp)]Cl2 (2OMe, phen = 1,10-phenanthroline) and [(dop)2Ru(6,6'-dmbp)]Cl2 (3OMe, dop = 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline), have been fully characterized and display enhanced quantum yields for blue light triggered photodissociation of 0.024(6) and 0.0030(2), respectively. We have also synthesized 4OH = [(dmphen)2Ru(4,4'-dhbp)]Cl2 wherein dmphen = 2,9-dimethyl-1,10-phenanthroline and 4,4'-dhbp = 4,4'-dihydroxybipyridine. These ligands enhance steric bulk near the metal center and move the hydroxy groups further from the metal center, respectively. Complex 4OH displays a relatively low quantum yield of 0.0014(2). All of the new complexes (2OMe, 3OMe, 4OH) were tested in breast cancer cells (MDA-MB-231) and were non-toxic (IC50 > 100 µM). This has been interpreted in terms of unfavorable log(Do/w) values and furthermore photodissociation alone is insufficient for cytotoxicity. We also report the crystal structures of 4OH and 2OMe, the thermodynamic acidity of complex 4OH, and the redox potentials for all new complexes.

Solvent-driven azide-induced mononuclear discrete versus one-dimensional polymeric aromatic Möbius cadmium(ii) complexes of an N6 tetradentate helical ligand.

We report the synthesis and structural characterization of a heteroleptic mononuclear discrete complex [Cd(N3)2(L)(MeOH)]·MeOH (1·MeOH) and a one-dimensional coordination polymer of the composition [Cd3(N3)6(L)]n (2), fabricated from Cd(NO3)2·4H2O and the helical organic ligand benzilbis((pyridin-2-yl)methylidenehydrazone) (L) in the presence of two equivalents of NaN3. The formation of different structures is driven by the solvent. The former complex is formed in the presence of MeOH, while the latter complex is formed in EtOH. The CdII centre in 1·MeOH is trapped by the two pyridyl-imine units of the tetradentate ligand L, two azide ligands and one oxygen atom of one methanol ligand with the CdN6O coordination polyhedron yielding a square face monocapped trigonal prism. The asymmetric unit of 2 consists of three symmetrically independent atoms of CdII, six azide anions and one L. The polymeric structure of 2 is realized through chains of the Cd(N3)2 units which are decorated with Cd(N3)2L units. The CdII atoms from the backbone of the coordination polymer have a distorted octahedral coordination, while the remaining CdII atom forms a trigonal prism with two basal planes nearly parallel to each other. In both complexes, the 12π electron chelate ring of the CdL fragment is shown to be aromatic by establishing it as a Möbius object. Hirshfeld surface analysis of 1 in 1·MeOH and L in 2 showed that the structures of both species are highly dominated by HX (X = H, C and N) contacts, of which the latter two are highly favoured, as well as some contribution from highly enriched CC contacts is clearly observed.

Ruthenium(ii) complexes of pyridinol and N-heterocyclic carbene derived pincers as robust catalysts for selective carbon dioxide reduction.

A new pincer ligand with N-heterocyclic carbene (NHC) and 4-pyridinol-derived rings supports ruthenium complexes for photocatalytic CO2 reduction. The methoxy group on the pyridine ring offers unique catalysis advantages not seen with the unsubstituted analog. Our best catalyst offers selective CO formation, ∼250 turnover cycles, and a 40 h lifetime.

Ruthenium Complexes are pH-Activated Metallo Prodrugs (pHAMPs) with Light-Triggered Selective Toxicity Toward Cancer Cells.

Metallo prodrugs that take advantage of the inherent acidity surrounding cancer cells have yet to be developed. We report a new class of pH-activated metallo prodrugs (pHAMPs) that are activated by light- and pH-triggered ligand dissociation. These ruthenium complexes take advantage of a key characteristic of cancer cells and hypoxic solid tumors (acidity) that can be exploited to lessen the side effects of chemotherapy. Five ruthenium complexes of the type [(N,N)2Ru(PL)]2+ were synthesized, fully characterized, and tested for cytotoxicity in cell culture (1A: N,N = 2,2'-bipyridine (bipy) and PL, the photolabile ligand, = 6,6'-dihydroxybipyridine (6,6'-dhbp); 2A: N,N = 1,10-phenanthroline (phen) and PL = 6,6'-dhbp; 3A: N,N = 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (dop) and PL = 6,6'-dhbp; 4A: N,N = bipy and PL = 4,4'-dimethyl-6,6'-dihydroxybipyridine (dmdhbp); 5A: N,N = 1,10-phenanthroline (phen) and PL = 4,4'-dihydroxybipyridine (4,4'-dhbp). The thermodynamic acidity of these complexes was measured in terms of two pKa values for conversion from the acidic form (XA) to the basic form (XB) by removal of two protons. Single-crystal X-ray diffraction data is discussed for 2A, 2B, 3A, 4B, and 5A. All complexes except 5A showed measurable photodissociation with blue light (λ = 450 nm). For complexes 1A-4A and their deprotonated analogues (1B-4B), the protonated form (at pH 5) consistently gave faster rates of photodissociation and larger quantum yields for the photoproduct, [(N,N)2Ru(H2O)2]2+. This shows that low pH can lead to greater rates of photodissociation. Cytotoxicity studies with 1A-5A showed that complex 3A is the most cytotoxic complex of this series with IC50 values as low as 4 µM (with blue light) versus two breast cancer cell lines. Complex 3A is also selectively cytotoxic, with sevenfold higher toxicity toward cancerous versus normal breast cells. Phototoxicity indices with 3A were as high as 120, which shows that dark toxicity is avoided. The key difference between complex 3A and the other complexes tested appears to be higher uptake of the complex as measured by inductively coupled plasma mass spectrometry, and a more hydrophobic complex as compared to 1A, which may enhance uptake. These complexes demonstrate proof of concept for dual activation by both low pH and blue light, thus establishing that a pHAMP approach can be used for selective targeting of cancer cells.

Iridium and Ruthenium Complexes of N-Heterocyclic Carbene- and Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation: The Role of the Alkali Metal.

Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO2 hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6'-dihydroxybipyridine (6,6'-dhbp)) for both their proton-responsive features and for metal-ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir(NHC-pyOR)Cl]OTf complexes where R = t Bu (1), H (2), or Me (3). For comparison, we tested analogous bipy-derived iridium complexes as catalysts, specifically [Cp*Ir(6,6'-dxbp)Cl]OTf, where x = hydroxy (4Ir ) or methoxy (5Ir ); 4Ir was reported previously, but 5Ir is new. The analogous ruthenium complexes were also tested using [(η6-cymene)Ru(6,6'-dxbp)Cl]OTf, where x = hydroxy (4Ru ) or methoxy (5Ru ); 4Ru and 5Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5Ir , and for two [Ag(NHC-pyOR)2]OTf complexes 6 (R = t Bu) and 7 (R = Me). The aqueous catalytic studies of both CO2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1-5. In general, NHC-pyOR complexes 1-3 were modest precatalysts for both reactions. NHC complexes 1-3 all underwent transformations under basic CO2 hydrogenation conditions, and for 3, we trapped a product of its transformation, 3SP , which we characterized crystallographically. For CO2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4Ir ) is 5-8 times more active than x = methoxy (5Ir ). Notably, ruthenium complex 4Ru showed 95% of the activity of 4Ir . For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4Ir ≫ 4Ru and 4Ir ≈ 5Ir . Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6'-dhbp are deprotonated and alkali metals can bind and help to activate CO2. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO2 hydrogenation and formic acid dehydrogenation.

Dioxygen activation by an organometallic Pd(II) precursor: formation of a Pd(IV)-OH complex and its C-O bond formation reactivity.

The complex (Me3tacn)Pd(II)(CH2CMe2C6H4) is readily oxidized by O2 or H2O2 to yield the Pd(IV)-OH complex [(Me3tacn)Pd(IV)(OH)(CH2CMe2C6H4)](+). Thermolysis of this product leads to the selective C(sp(2))-O reductive elimination of 2-tert-butylphenol, no C(sp(3))-O elimination product being detected. This system represents a rare example of selective C(sp(2))-O bond formation that is relevant to Pd-catalyzed aerobic C-H hydroxylation reactions.

Structural and reactivity comparison of analogous organometallic Pd(III) and Pd(IV) complexes.

The tetradentate ligands (R)N4 ((R)N4 = N,N'-di-alkyl-2,11-diaza[3,3](2,6)pyridinophane, R = Me or iPr) were found to stabilize cationic ((R)N4)PdMe(2) and ((R)N4)PdMeCl complexes in both Pd(III) and Pd(IV) oxidation states. This allows for the first time a direct structural and reactivity comparison of the two Pd oxidation states in an identical ligand environment. The Pd(III) complexes exhibit a distorted octahedral geometry, as expected for a d(7) metal center, and display unselective C-C and C-Cl bond formation reactivity. By contrast, the Pd(IV) complexes have a pseudo-octahedral geometry and undergo selective non-radical C-C or C-Cl bond formation that is controlled by the ability of the complex to access a five-coordinate intermediate.