Elucidation of the key role of Pt···Pt interactions in the directional self-assembly of platinum(II) complexes.

Here, we report the use of an amphiphilic Pt(II) complex, K[Pt{(O3SCH2CH2CH2)2bzimpy}Cl] (PtB), as a model to elucidate the key role of Pt···Pt interactions in directing self-assembly by combining temperature-dependent ultraviolet-visible (UV-Vis) spectroscopy, stopped-flow kinetic experiments, quantum mechanics (QM) calculations, and molecular dynamics (MD) simulations. Interestingly, we found that the self-assembly mechanism of PtB in aqueous solution follows a nucleation-free isodesmic model, as revealed by the temperature-dependent UV-Vis experiments. In contrast, a cooperative growth is found for the self-assembly of PtB in acetone­water (7:1, vol/vol) solution, which is further verified by the stopped-flow experiments, which clearly indicates the existence of a nucleation phase in the acetone­water (7:1, vol/vol) solution. To reveal the underlying reasons and driving forces for these self-assembly processes, we performed QM calculations and show that the Pt···Pt interactions arising from the interaction between the pz and dz2 orbitals play a crucial role in determining the formation of ordered self-assembled structures. In subsequent oligomer MD simulations, we demonstrate that this directional Pt···Pt interaction can indeed facilitate the formation of linear structures packed in a helix-like fashion. Our results suggest that the self-assembly of PtB in acetone­water (7:1, vol/vol) solution is predominantly driven by the directional noncovalent Pt···Pt interaction, leading to the cooperative growth and the formation of fibrous nanostructures. On the contrary, the self-assembly in aqueous solution forms spherical nanostructures of PtB, which is primarily due to the predominant contribution from the less directional hydrophobic interactions over the directional Pt···Pt and π−π interactions that result in an isodesmic growth.

Visible Light-Induced Oxidation of Alcohols by a Luminescent Osmium(VI) Nitrido Complex: Evidence for the Generation of PhIO+ as a Highly Active Oxidant in the Presence of PhIO.

Although alcohols are readily oxidized by a variety of oxidants, their oxidation by metal nitrido complexes is yet to be studied. We report herein visible-light-induced oxidation of primary and secondary alcohols to carbonyl compounds by a strongly luminescent osmium(VI) nitrido complex (OsN). The proposed mechanism involves initial rate-limiting hydrogen-atom transfer (HAT) from the α-carbon of the alcohol to OsN*. Attempts to develop catalytic oxidation of alcohols by OsN* using PhIO as the terminal oxidant resulted in the formation of novel osmium(IV) iminato complexes in which the nitrido ligand is bonded to a δ-carbon of the alcohol. Experimental and theoretical studies suggest that OsN* is reductively quenched by PhIO to generate PhIO+, which is a highly active oxidant that readily undergoes α- and δ-C-H activation of alcohols.

Visible-Light-Driven CO2 Reduction with Homobimetallic Complexes. Cooperativity between Metals and Activation of Different Pathways.

Visible-light-driven reduction of CO2 to both CO and formate (HCOO-) was achieved in acetonitrile solutions using a homobimetallic Cu bisquaterpyridine complex. In the presence of a weak acid (water) as coreactant, the reaction rate was enhanced, and a total of ca. 766 TON (turnover number) was reached for the CO2 reduction, with 60% selectivity for formate and 28% selectivity for CO, using Ru(phen)32+ as a sensitizer and amines as sacrificial electron donors. Mechanistic studies revealed that with the help of cooperativity between two Cu centers, a bridging hydride is generated in the presence of a proton source (water) and further reacts with CO2 to give HCOO-. A second product, CO, was also produced in a parallel competitive pathway upon direct coordination of CO2 to the reduced complex. Mechanistic studies further allowed comparison of the observed reactivity to the monometallic Cu quaterpyridine complex, which only produced CO, and to the related homobimetallic Co bisquaterpyridine complex, that has been previously shown to generate formate following a mechanism not involving the formation of an intermediate hydride species.

Ring Nitrogenation of Aromatic Amines by the Excited State of an Osmium(VI) Nitrido Complex.

The reactivity of electrophilic OsVI and RuVI nitrides toward various aliphatic and aromatic amines have been previously reported; these reactions all go through an initial nucleophilic addition of the amine nitrogen to MVI≡N (M = Os, Ru) to generate a MIV hydrazido species. Herein, we report that the excited state of a luminescent osmium(VI) nitrido complex, [OsVI(N)(L)(CN)3]- (OsN, HL = 2-(2-hydroxy-5-nitrophenyl)benzoxazole), undergoes unprecedented ring nitrogenation of aromatic amines. Visible-light irradiation of OsN generates OsN*, which predominantly attacks the aromatic ring of 2,6-dimethylaniline (Me2PhNH2) to give an Os(II) benzoquinone diimine compound (PPh4)[OsII(L)(CN)3(p-NH═Me2Ph═NH2)] [(PPh4)2] in 60% yield, while nucleophilic addition of the amine N to OsN* also occurs to give the osmium(II) diazonium compound (PPh4)[OsII(L)(CN)3(N = N-Me2Ph)] [(PPh4)1] as a minor product (10% yield). On the other hand, OsN* undergoes exclusive ring nitrogenation of diphenylamine, indole, and carbazole to give the corresponding osmium(II) benzoquinone diimines. All products have been characterized by various spectroscopic techniques and by X-ray crystallography. The reaction between OsN* and Ar2N is proposed to proceed via an initial 1e- transfer (ET) followed by proton transfer (PT) to generate OsVNH and Ar2N• intermediates, which then further combine to give the product. The benzoquinone diimine ligands are susceptible to oxidation. Oxidation of 2 with H2O2 at ambient conditions affords [OsIV(L)(CN)3(N═PhMe2(O)═O)]-, which bears the previously unknown (epoxy)benzoquinone iminato ligand.

Structure and Reactivity of a Seven-Coordinate Ruthenium Iodosylbenzene Complex.

Seven-coordinate (CN7) ruthenium-oxo species have attracted much attention as highly reactive intermediates in both organic and water oxidation. Apart from metal-oxo, other metal-oxidant adducts, such as metal-iodosylarenes, have also recently emerged as active oxidants. We reported herein the first example of a CN7 Ru-iodosylbenzene complex, [RuIV(bdpm)(pic)2(O)I(Cl)Ph]+ (H2bdpm = [2,2'-bipyridine]-6,6'-diylbis(diphenylmethanol); pic = 4-picoline). The X-ray crystal structure of this complex shows that it adopts a distorted pentagonal bipyramidal geometry with Ru-O(I) and O-I distances of 2.0451(39) and 1.9946(40) Å, respectively. This complex is highly reactive, and it readily undergoes O-atom transfer (OAT) and C-H bond activation reactions with various organic substrates. This work should provide insights for the development of new highly reactive oxidizing agents based on CN7 geometry.

Structure and Reactivity of One- and Two-Electron Oxidized Manganese(V) Nitrido Complexes Bearing a Bulky Corrole Ligand.

As a strategy to design stable but highly reactive metal nitrido species, we have synthesized a manganese(V) nitrido complex bearing a bulky corrole ligand, [MnV(N)(TTPPC)]- (1, TTPPC is the trianion of 5,10,15-Tris(2,4,6-triphenylphenyl)corrole). Complex 1 is readily oxidized by 1 equiv of Cp2Fe+ to give the neutral complex 2, which can be further oxidized by 1 equiv of [(p-Br-C6H4)3N•+][B(C6F5)4] to afford the cationic complex 3. All three complexes are stable in the solid state and in CH2Cl2 solution, and their molecular structures have been determined by X-ray crystallography. Spectroscopic and theoretical studies indicate that complexes 2 and 3 are best formulated as Mn(V) nitrido π-cation corrole [MnV(N)(TTPPC+•)] and Mn(V) nitrido π-dication corrole [MnV(N)(TTPPC2+)]+, respectively. Complex 3 is the most reactive N atom transfer reagent among isolated nitrido complexes; it reacts with PPh3 and styrene with second-order rate constants of 2.12 × 105 and 1.95 × 10-2 M-1 s-1, respectively, which are >107 faster than that of 2.

Oxidation of Hypophosphorous Acid by a Ruthenium(VI) Nitrido Complex in Aqueous Acidic Solution. Evidence for a Proton-Coupled N-Atom Transfer Mechanism.

The oxidation of hypophosphorous acid (H3PO2) by a ruthenium(VI) nitrido complex, [(L)RuVI(N)(OH2)]+ (RuVIN; L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion), has been studied in aqueous acidic solutions at pH 0-2.50. The reaction has the following stoichiometry: 2[(L)RuVI(N)(OH2)]+ + 3H3PO2 + H2O → 2[(L)RuIII(NH2P(OH)2)(OH2)]+ + H3PO3. The pseudo-first-order rate constant, kobs, depends linearly on [H3PO2], and the second-order rate constant k2 depends on [H+] according to the relationship k2 = k[H+]/([H+] + Ka), where k is the rate constant for the oxidation of H3PO2 molecule and Ka is the dissociation constant of H3PO2. At 298.0 K and I = 1.0 M, k = (2.04 ± 0.19) × 10-2 M-1 s-1 and Ka = (6.38 ± 0.63) × 10-2 M. A kinetic isotope effect (KIE) of 2.9 ± 0.1 was obtained when kinetic studies were carried out with D3PO2 at pH 1.16, suggesting P-H bond cleavage in the rate-determining step. On the other hand, when the kinetics were determined in D2O, an inverse KIE of 0.21 ± 0.03 (H3PO2 in H2O vs H3PO2 in D2O) was found. On the basis of experimental results and DFT calculations, the proposed mechanism involves an acid-catalyzed tautomerization of H2P(O)(OH) to HP(OH)2; the latter molecule is the reacting species which reacts with RuVIN via a proton-coupled N-atom transfer pathway.

Highly Efficient Photocatalytic Reduction of CO2 to CO by In Situ Formation of a Hybrid Catalytic System Based on Molecular Iron Quaterpyridine Covalently Linked to Carbon Nitride.

Efficient and selective photocatalytic CO2 reduction was obtained within a hybrid system that is formed in situ via a Schiff base condensation between a molecular iron quaterpyridine complex bearing an aldehyde function and carbon nitride. Irradiation (blue LED) of an CH3 CN solution containing 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), triethylamine (TEA), Feqpy-BA (qpy-BA=4-([2,2':6',2'':6'',2'''-quaterpyridin]-4-yl)benzaldehyde) and C3 N4 resulted in CO evolution with a turnover number of 2554 and 95 % selectivity. This hybrid catalytic system unlocks covalent linkage of molecular catalysts with semiconductor photosensitizers via Schiff base reaction for high-efficiency photocatalytic reduction of CO2 , opening a pathway for diverse photocatalysis.

Room Temperature Aerobic Peroxidation of Organic Substrates Catalyzed by Cobalt(III) Alkylperoxo Complexes.

Room temperature aerobic oxidation of hydrocarbons is highly desirable and remains a great challenge. Here we report a series of highly electrophilic cobalt(III) alkylperoxo complexes, CoIII(qpy)OOR supported by a planar tetradentate quaterpyridine ligand that can directly abstract H atoms from hydrocarbons (R'H) at ambient conditions (CoIII(qpy)OOR + R'H → CoII(qpy) + R'• + ROOH). The resulting alkyl radical (R'•) reacts rapidly with O2 to form alkylperoxy radical (R'OO•), which is efficiently scavenged by CoII(qpy) to give CoIII(qpy)OOR' (CoII(qpy) + R'OO• → CoIII(qpy)OOR'). This unique reactivity enables CoIII(qpy)OOR to function as efficient catalysts for aerobic peroxidation of hydrocarbons (R'H + O2 → R'OOH) under 1 atm air and at room temperature.

Hybridization of Molecular and Graphene Materials for CO2 Photocatalytic Reduction with Selectivity Control.

In the quest for designing efficient and stable photocatalytic materials for CO2 reduction, hybridizing a selective noble-metal-free molecular catalyst and carbon-based light-absorbing materials has recently emerged as a fruitful approach. In this work, we report about Co quaterpyridine complexes covalently linked to graphene surfaces functionalized by carboxylic acid groups. The nanostructured materials were characterized by X-ray photoemission spectroscopy, X-ray absorption spectroscopy, IR and Raman spectroscopies, high-resolution transmission electron microscopy and proved to be highly active in the visible-light-driven CO2 catalytic conversion in acetonitrile solutions. Exceptional stabilities (over 200 h of irradiation) were obtained without compromising the selective conversion of CO2 to products (>97%). Most importantly, complete selectivity control could be obtained upon adjusting the experimental conditions: production of CO as the only product was achieved when using a weak acid (phenol or trifluoroethanol) as a co-substrate, while formate was exclusively obtained in solutions of mixed acetonitrile and triethanolamine.

Structure and Reactivity of a Manganese(VI) Nitrido Complex Bearing a Tetraamido Macrocyclic Ligand.

Manganese complexes in +6 oxidation state are rare. Although a number of Mn(VI) nitrido complexes have been generated in solution via one-electron oxidation of the corresponding Mn(V) nitrido species, they are too unstable to isolate. Herein we report the isolation and the X-ray structure of a Mn(VI) nitrido complex, [MnVI(N)(TAML)]- (2), which was obtained by one-electron oxidation of [MnV(N)(TAML)]2- (1). 2 undergoes N atom transfer to PPh3 and styrenes to give Ph3P═NH and aziridines, respectively. A Hammett study for various p-substituted styrenes gives a V-shaped plot; this is rationalized by the ability of 2 to function as either an electrophile or a nucleophile. 2 also undergoes hydride transfer reactions with NADH analogues, such as 10-methyl-9,10-dihydroacridine (AcrH2) and 1-benzyl-1,4-dihydronicotinamide (BNAH). A kinetic isotope effect of 7.3 was obtained when kinetic studies were carried out with AcrH2 and AcrD2. The reaction of 2 with NADH analogues results in the formation of [MnV(N)(TAML-H+)]- (3), which was characterized by ESI/MS, IR spectroscopy, and X-ray crystallography. These results indicate that this reaction occurs via an initial "separated CPET" (separated concerted proton-electron transfer) mechanism; that is, there is a concerted transfer of 1 e- + 1 H+ from AcrH2 (or BNAH) to 2, in which the electron is transferred to the MnVI center, while the proton is transferred to a carbonyl oxygen of TAML rather than to the nitrido ligand.

Electrocatalytic and Photocatalytic Reduction of Carbon Dioxide by Earth-Abundant Bimetallic Molecular Catalysts.

Converting CO2 into useful resources by electrocatalysis and photocatalysis is a promising strategy for recycling of the gas and electrification of industries. Numerous studies have shown that multinuclear metal catalysts have higher selectivity and catalytic activity than monometallic catalysts due to the synergistic effects between the metal sites. In this review, we summarize some of the recent progress on the electrocatalytic and photocatalytic reduction of CO2 by earth-abundant bimetallic molecular catalysts.

Molecular quaterpyridine-based metal complexes for small molecule activation: water splitting and CO2 reduction.

Artificial photosynthesis is considered as one of the most promising strategies for solar-to-fuel conversion through sunlight-driven water splitting and CO2 reduction. This tutorial describes recent developments in the use of metal quaterpyridine complexes as electrocatalyts and photocatalysts for artificial photosynthesis.

A Photocaged, Water-Oxidizing, and Nucleolus-Targeted Pt(IV) Complex with a Distinct Anticancer Mechanism.

Targeted anticancer prodrugs that can be controllably activated are highly desired for personalized precision medicine in cancer therapy. Such prodrugs with unique action modes are also promising to overcome drug resistance. Herein, we report coumaplatin, an oxaliplatin-based and photocaged Pt(IV) prodrug, to realize nuclear accumulation along with "on-demand" activation. This prodrug is based on a Pt(IV) complex that can be efficiently photoactivated via water oxidation without the requirement of a reducing agent. Coumaplatin accumulates very efficiently in the nucleoli, and upon photoactivation, this prodrug exhibits a level of photocytotoxicity up to 2 orders of magnitude higher than that of oxaliplatin. Unexpectedly, this prodrug presents strikingly enhanced tumor penetration ability and utilizes a distinct action mode to overcome drug resistance; i.e., coumaplatin but not oxaliplatin induces cell senescence, p53-independent cell death, and immunogenic cell death along with T cell activation. Our findings not only provide a novel strategy for the rational design of controllably activated and nucleolus-targeted Pt(IV) anticancer prodrugs but also demonstrate that accumulating conventional platinum drugs to the nucleus is a practical way to change its canonical mechanism of action and to achieve reduced resistance.

Efficient Visible-Light-Driven CO2 Reduction by a Cobalt Molecular Catalyst Covalently Linked to Mesoporous Carbon Nitride.

Achieving visible-light-driven carbon dioxide reduction with high selectivity control and durability while using only earth abundant elements requires new strategies. Hybrid catalytic material was prepared upon covalent grafting a Co-quaterpyridine molecular complex to semiconductive mesoporous graphitic carbon nitride (mpg-C3N4) through an amide linkage. The molecular material was characterized by various spectroscopic techniques, including XPS, IR, and impedance spectroscopy. It proved to be a selective catalyst for CO production in acetonitrile using a solar simulator with a high 98% selectivity, while being remarkably robust since no degradation was observed after 4 days of irradiation (ca. 500 catalytic cycles). This unique combination of a selective molecular catalyst with a simple and robust semiconductive material opens new pathways for CO2 catalytic light-driven reduction.

Tunable Luminescent Properties of Tricyanoosmium Nitrido Complexes Bearing a Chelating O

We have recently reported a strongly luminescent osmium(VI) nitrido complex [OsVI(N)(NO2-L)(CN)3]- [HNO2-L = 2-(2-hydroxy-5-nitrophenyl)benzoxazole]. The excited state of this complex readily activates the strong C-H bonds of alkanes and arenes (Commun. Chem. 2019, 2, 40). In this work, we attempted to tune the excited-state properties of this complex by introducing various substituents on the bidentate L ligand. The series of nitrido complexes were characterized by IR, UV/vis, 1H NMR, and electrospray ionization mass spectrometry. The molecular structures of five of the nitrido compounds have been determined by X-ray crystallography. The photophysical and electrochemical properties of these complexes have been investigated. The luminescence of these nitrido complexes in the solid state, in a CH2Cl2 solution, and in a CH2Cl2 solid matrix at 77 K glassy medium clearly shows that these emissions are due to 3LML'CT [L ligand to Os≡N] phosphorescence. The presence of strongly electron-withdrawing substituents in these complexes enhances the LML'CT emission. Our result demonstrates that the excited-state properties of this novel class of luminescent osmium(VI) nitrido complexes can be fine-tuned by introducing various substituents on the bidentate L ligand.

Activation of Metal Oxo and Nitrido Complexes by Lewis Acids.

Metal oxo species (M═O) play key roles as oxidants in chemical and biological systems. Although Brønsted acids have long been known to enhance the oxidizing power of metal oxo complexes, the use of Lewis acids (LAs), such as metal ions, to activate these complexes has received much less attention until recently. The report of the presence of a Mn4CaO5 cluster active site in the oxygen-evolving center of photosystem II in 2004 has stimulated intense interest in understanding the interaction of LAs with metal oxo species. This Perspective analyzes the various modes of activation of metal oxos by LAs and the pathways for the oxidation of various substrates by LA/M═O systems. The interaction of LAs with metal nitrides will also be discussed, although it is much less studied than that with metal oxo complexes.

Generation and Reactivity of a One-Electron-Oxidized Manganese(V) Imido Complex with a Tetraamido Macrocyclic Ligand.

The synthesis and X-ray structure of a new manganese(V) mesitylimido complex with a tetraamido macrocyclic ligand (TAML), [MnV (TAML)(N-Mes)]- (1), are reported. Compound 1 is oxidized by [(p-BrC6 H4 )3 N]+. [SbCl6 ]- and the resulting MnVI species readily undergoes H-atom transfer and nitrene transfer reactions.

A Carbon Nitride/Fe Quaterpyridine Catalytic System for Photostimulated CO2-to-CO Conversion with Visible Light.

Efficient and selective photostimulated CO2-to-CO reduction by a photocatalytic system consisting of an iron-complex catalyst and a mesoporous graphitic carbon nitride (mpg-C3N4) redox photosensitizer is reported for the first time. Irradiation in the visible region (λ ≥ 400 nm) of an CH3CN/triethanolamine (4:1, v/v) solution containing [Fe(qpy)(H2O)2]2+ (qpy = 2,2':6',2'':6'',2''-quaterpyridine) and mpg-C3N4 resulted in CO evolution with 97% selectivity, a turnover number of 155, and an apparent quantum yield of ca. 4.2%. This hybrid catalytic system, comprising only earth abundant elements, opens new perspectives for solar fuels production using CO2 as a renewable feedstock.