Redox Activity of IrIII Complexes with Multidentate Ligands Based on Dipyrido-Annulated N-Heterocyclic Carbenes: Access to High Valent and High Spin State with Carbon Donors.

Synthetic strategies to access high-valent iridium complexes usually require use of π donating ligands bearing electronegative atoms (e. g. amide or oxide) or σ donating electropositive atoms (e. g. boryl or hydride). Besides the η5 -(methyl)cyclopentadienyl derivatives, high-valent η1 carbon-ligated iridium complexes are challenging to synthesize. To meet this challenge, this work reports the oxidation behavior of an all-carbon-ligated anionic bis(CCC-pincer) IrIII complex. Being both σ and π donating, the diaryl dipyrido-annulated N-heterocyclic carbene (dpa-NHC) IrIII complex allowed a stepwise 4e- oxidation sequence. The first 2e- oxidation led to an oxidative coupling of two adjacent aryl groups, resulting in formation of a cationic chiral IrIII complex bearing a CCCC-tetradentate ligand. A further 2e- oxidation allowed isolation of a high-valent tricationic complex with a triplet ground state. These results close a synthetic gap for carbon-ligated iridium complexes and demonstrate the electronic tuning potential of organic π ligands for unusual electronic properties.

Uniform wrapping of copper(I) oxide nanocubes by self-controlled copper-catalyzed azide-alkyne cycloaddition toward selective carbon dioxide electrocatalysis.

Copper(I) oxide nanocubes were wrapped with an extremely uniform organic layer grown by self-controlled, Cu-mediated catalysis. This layer aided in retaining the initial cubic structure of the copper nanocubes during their use as a CO2 reduction electrocatalyst, resulting in high CO2 reduction selectivity by strong suppression of hydrogen evolution because of exclusion of water from the surface.

The [Ag25Cu4H8Br6(CCPh)12(PPh3)12]3+ : Ag13H8 silver hydride core protected by [CuAg3(CCPh)3(PPh3)3]+ motifs.

Copper alkynyl complexes [CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3]+ (Ar = Ph, p-C6H4Me), in which three Ag(PPh3) units are bound among three C[triple bond, length as m-dash]CAr arms of trigonal-planar [Cu(C[triple bond, length as m-dash]CAr)3]2-, were selected as a protecting unit to cover the metal core of an atomically precise core-shell-type cluster. First, the formation of the protecting unit through the reaction of Cu(NCMe)4(PF6) with Ag(C[triple bond, length as m-dash]CAr) and PPh3 in a 1 : 3 : 3 ratio was confirmed. The reaction gave dimeric [CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3]22+, in which the two planar [CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3]+ units were stacked. Next, core-shell-type clusters were synthesized by adding NaBH4 and Et4NX (X = Cl, Br) to a solution similar to that used to prepare the protecting unit. The trigonal-planar protecting units nicely formed core-shell-type Ag nanoclusters formulated as [Ag13H8X6{CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3}4]3+ (X = Cl, Ar = p-C6H4Me; X = Br, Ar = p-C6H4Me; X = Br, Ar = Ph). Their crystal structures revealed that the four [CuAg3(C[triple bond, length as m-dash]CAr)3(PPh3)3]+ units are linked by six halogen ions to form a tetrahedral cage that accommodates a polyhydride-Ag cluster formulated as Ag13H85+. As a concrete proof of the existence of the polyhydride, deuterated analogs Ag13D85+ were synthesized and subsequently characterized by high-resolution electrospray-ionization mass spectrometry measurements.

Eudesmane-type sesquiterpene glycosides: sonneratiosides A-E and eudesmol ß-D-glucopyranoside from the leaves of Sonneratia alba.

Five eudesmane-type sesquiterpene glycosides, named sonneratiosides A-E (1-5), were isolated from the leaves of Sonneratia alba (Lythraceae). The aglycone of sonneratioside A was identified as cryptomeridiol also known as proximadiol. X-ray crystallographic analysis of sonneratioside A confirmed its structure and its absolute stereochemistry. Eudesmol ß-D-glucopyranoside (6) was also isolated from nature for the first time. The tyrosinase inhibitory activity was assayed for the new compounds together with seven known compounds. Among them, arbutin (12) showed the expected activity and luteolin 7-O-rutinoside (10) showed comparable activity to arbutin.

Protonation and electrochemical properties of a bisphosphide diiron hexacarbonyl complex bearing amino groups on the phosphide bridge.

A bisphosphide-bridged diiron hexacarbonyl complex 3 with NEt2 groups on the phosphide bridge was synthesized to examine a new proton relay system from the NEt2 group to the bridging hydride between the two iron centers. As a precursor of the bridging moiety, peri-Et2NP-PNEt2-bridged naphthylene 5 was synthesized by the reaction of 1,8-dilithionaphthylene with two equivalents of Cl2PNEt2 followed by reductive P-P bond formation by magnesium. The reaction of the diphosphine ligand 5 with Fe2(CO)9 gave the diiron hexacarbonyl complex 3, in which the P-P bond of the ligand was cleaved to form the bisphosphide-bridge. The molecular structure of 3 indicated that the trigonal plane of the NEt2 group was forced to face the Fe-Fe bond to avoid steric congestion with the naphthylene group linking the two phosphide groups. The NEt2 group could be protonated by p-toluenesulfonic acid. Density functional theory (DFT) calculations confirmed that the proton of the N(H)Et2 group adopted a position close to the bridging hydride. The DFT results for the ferrocene analogue 1, in which the 1,8-naphthylene group of 3 was replaced with the 1,1'-ferrocenylene group, also revealed that the most stable orientation of the protonated NHEt2 group was that in the protonated 3. As a result, electrochemical proton reduction reactions using complexes 1 and 3 proceeded with similar catalytic efficiencies. Unfortunately, the catalytic efficiencies (CEs) of these complexes were much lower than those of the complexes with a proton relay system of the terminal hydrogen, indicating that the reactive properties of the bridging hydride in the present proton relay system cannot exceed those of the terminal hydride.

On-Surface Modification of Copper Cathodes by Copper(I)-Catalyzed Azide Alkyne Cycloaddition and CO2 Reduction in Organic Environments.

In this study, organic structures were introduced onto copper cathodes to induce changes in their electrocatalytic CO2 reduction activity. Poorly soluble organic polymers were distributed onto the copper surface as a thin layer by polymerizing monomeric precursors via a copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) activated by anodization of the copper substrate. The resulting structure possesses copper surface atoms that are available to participate in the CO2 reduction reaction-comparable to close-contact organic structures-and stabilize the adsorption of organic layers through the CO2 reduction process. The CO2 reduction performance of the on-surface modified copper cathode exhibited improved CO2 reduction over H2 evolution compared with traditional cast modification systems. Preventing organic moieties from forming densely packed assemblies on the metal surface appears to be important to promote the CO2 reduction process on the copper atoms. The suppression of H2 evolution, a high methane/ethylene ratio, and the influence of stirring demonstrate that the improved CO2 reduction activity is not only a result of the copper atom reorganization accompanied by repeating anodization for modification; the organic layer also apparently plays an important role in proton transfer and CO2 accumulation onto the copper surface.

Novel chloride-centered Ag18 clusters featuring a cuboctahedral Ag12 skeleton.

Two novel chloride-centered Ag18 clusters with the same framework but different supporting phosphines are synthesized by the reaction of PhC[triple bond, length as m-dash]CAg, AgSbF6, PPh3 (or P(p-Tol)3), and NaBH4 in CH2Cl2, followed by the addition of PhC[triple bond, length as m-dash]CH and NEt3. The inner twelve Ag atoms of these two clusters are arranged in a rare cuboctahedral structure, which can be rationalized by considering a ligand effect. Through careful analysis, we find that the central chloride arises from a generally ignored but nonetheless existing reaction between CH2Cl2 and NEt3, which is well recognized as the Menshutkin reaction. This research provides insights into the dependence of the cluster structure on the nature of the ligand and into the cluster formation mechanism of the Menshutkin reaction.

An Alkynyl-Stabilized Pt5 Ag22 Cluster Featuring a Two-Dimensional Alkynyl-Platinum "Crucifix Motif".

The "staple motif" has been widely applied to depict and predict the structures of thiolate or alkynyl-protected gold nanoclusters. By contrast, the composition, dimensions, configuration, and functionality of the platinum-ligand motif has remained completely unknown. Herein, we report the synthesis and crystal structure of a novel luminescent Pt5 Ag22 (C≡CPh)32 (1) cluster, in which two-dimensional and two-functional alkynyl-platinum "crucifix motif" was observed. Such a crucifix motif with one Pt center and four alkynyl groups in the same plane acts as a two-dimensional unit in the cluster and functions as both a protective cover and an intermediate joint.

An Atomically Precise Alkynyl-Protected PtAg42 Superatom Nanocluster and Its Structural Implications.

The synthesis and structure determination of an alkynyl-protected Pt-doped Ag superatom nanocluster, [PtAg42 (C≡CC6 H4 CH3 )28 ](SbF6 )6  (1), are reported. The metallic core of this cluster can be viewed as a concentric three-shell Russian doll comprising Pt@Ag12 @Ag30 , in which the missing icosidodecahedral Ag30 shell and a new structural unit, M43 , have been observed. On the surface of 1, 28 alkynyl groups and 4 SbF6- anions were found co-protecting it. The protective role of SbF6- in nanoclusters is unprecedented. Moreover, of the 28 alkynyl ligands, 12 are connected not only with the outermost Ag30 shell, but also the inner Ag12 shell through the twelve pentagonal faces of the outermost icosidodecahedral Ag30 shell. Overall, by determining the crystal structure of 1, we discovered the missing icosidodecahedral Ag30 shell and the protective effect of SbF6- anions and demonstrated a novel M43 structural unit and the unique penetrability of pentagonal faces.

Synthesis and coordination chemistry of (PNEt2)2-bridged [2]ferrocenophanes.

The trivalent phosphorus-bridged [2]ferrocenophane complex 2 having NEt2 groups on the respective phosphorus centers was prepared, and its reactions as a diphosphine ligand were examined for iron and chromium carbonyl complexes. Both the phosphorus centers of 2 coordinated to Fe(CO)4 fragments to form (µ-2)-[Fe(CO)4]2, while the bulkier Cr(CO)5 fragment formed only a monochromium complex [Cr(κ1-2)(CO)5]. Dissociation of CO from [Cr(κ1-2)(CO)5] changed the coordination mode of 2 from κ1 to κ2 to form [Cr(κ2-2)(CO)4] having a three-membered ring. A similar approach for the monoiron complex [Fe(κ1-2)(CO)4] did not afford a κ2 complex but instead an Et2NPC(O)PNEt2-bridged [3]ferrocenophane complex in which a CO fragment was inserted into the P-P bond of 2 and both the phosphorus centers coordinated to Fe(CO)3 as a chelate diphosphine. The reaction of this product with an Fe(CO)4 fragment gave µ-{Fe(C5H4PNEt2)2-κP:κP}-[Fe(CO)3]2 (8), in which one terminal CO and the CO group between the two phosphorus atoms were lost to give an [FeFe]hydrogenase mimic having a bis(phosphido)ferrocene chelate as a bridging unit. The two NEt2 groups of the bridging unit were expected to work as protonation sites. The protonated NEt2 groups contributed to an improvement in the reduction potential of the complex to a less negative area, i.e., -2.3 V for the free 8 to -1.0 V for the diprotonated 8. The catalytic reduction of the proton, however, required a more negative potential of -2.0 V, which is almost comparable to that of the phosphido-bridged [FeFe]hydrogenase model complex having no protonation site.

Selection of two optional covalent bonds by electric stimuli: dual catalytic switching of redox-active copper.

Two types of redox functionality were selected for covalent immobilization on a carbon electrode, using an electric potential as the sole stimulus. A redox-active copper catalyst transformed a terminal alkyne in two ways with and without an oxidation process, to form a triazole or butadiyne.

Structure elucidation of secondary metabolites isolated from the leaves of Ixora undulate and their inhibitory activity toward advanced glycation end-products formation.

Three aromatic glycosides (1-3), two sulfur and nitrogen-containing compound glucosides (4, 5), and one flavonoid glycoside (6) were isolated from the leaves of Ixora undulata. Their structures were established by extensive 1D, 2D NMR, and HRESIMS experiments, and structure 4 was further confirmed by single crystal X-ray diffraction analysis. Of the assayed compounds, 7, 11 and 12 showed strong inhibitory activity toward advanced glycation end-products formation with IC50 values of 86.0 µM, 76.6 µM and 98.6 µM, respectively.

Preparation and redox studies of α1- and α2-isomers of mono-Ru-substituted Dawson-type phosphotungstates with a DMSO ligand: [α1/α2-P2W17O61Ru(II)(DMSO)](8-).

Both α1- and α2-isomers of mono-Ru-substituted Dawson-type heteropolytungstates with a DMSO ligand, [α1-P2W17O61Ru(II)(DMSO)](8-) and [α2-P2W17O61Ru(II)(DMSO)](8-), are prepared from the α2-isomer of a monolacunary derivative, [α2-P2W17O61](10-). Reaction of [α2-P2W17O61](10-) with Ru(DMSO)4Cl2 under hydrothermal conditions produces [α2-P2W17O61Ru(II)(DMSO)](8-) as a main product together with [α1-P2W17O61Ru(II)(DMSO)](8-), [PW11O39Ru(II)(DMSO)](5-), and [P2W18O62](6-) as byproducts. By addition of KCl to the reaction mixture, K8[α2-P2W17O61Ru(II)(DMSO)] is isolated in a moderate yield. On the other hand, reaction of [α2-P2W17O61](10-) with Ru2(benzene)2Cl4 under hydrothermal conditions produces an isomeric mixture of [P2W17O61Ru(III)(H2O)](7-) (α1-isomer/α2-isomer ratio: ca. 8/1) as a main product together with [PW11O39Ru(III)(H2O)](4-) and [P2W18O62](6-) as byproducts. By addition of acetone to the reaction mixture, K7[P2W17O61Ru(III)(H2O)] is isolated in a good yield. Reaction of [P2W17O61Ru(III)(H2O)](7-) with DMSO produces [α1-P2W17O61Ru(III)(DMSO)](7-) as a main product and [α2-P2W17O61Ru(III)(DMSO)](7-) as a minor product. By addition of KCl and acetone, the α1-isomer K8[α1-P2W17O61Ru(II)(DMSO)] is isolated in a good yield. Both compounds are fully analyzed by CV, NMR ((1)H, (13)C, (31)P, and (183)W), IR, UV-vis, elemental analysis, mass spectroscopy, and single-crystal structure analysis. Assuming that isomerization does not occur during the reaction of [P2W17O61Ru(III)(H2O)](7-) with DMSO, the isolated [P2W17O61Ru(III)(H2O)](7-) contains the α1-isomer as a main compound with the α2-isomer as a minor compound. Unusual transformation of the α2-isomer of [P2W17O61](10-) to the α1-isomer occurs. Redox behaviors of [α1-P2W17O61Ru(II)(DMSO)](8-) and [α2-P2W17O61Ru(II)(DMSO)](8-) are compared together with Ru(DMSO)-substituted α-Keggin-type heteropolytungstates, [α-XW11O39Ru(DMSO)](n-) (X = Si, Ge, and P).

Kinetic stabilization and reactivity of π single-bonded species: effect of the alkoxy group on the lifetime of singlet 2,2-dialkoxy-1,3-diphenyloctahydropentalene-1,3-diyls.

Kinetic stabilization and reactivity of π single-bonded species have been investigated in detail by generating a series of singlet 2,2-dialkoxy-1,3-diphenyloctahydropentalene-1,3-diyls (DRs). The lifetime at 293 K in benzene was found to increase when the carbon chain length of the alkoxy groups was increased; 292 ns (DRb; OR = OR' = OCH3) <880 ns (DRc; OR = OR' = OC2H5) <1899 ns (DRd; OR = OR' = OC3H7) ≈2292 ns (DRe; OR = OR' = OC6H13) ≈2146 ns (DRf; OR = OR' = OC10H21). DRh (OR = OC3H7, OR' = OCH3; 935 ns) with the mixed-acetal moiety is a longer-lived species than another diastereomer DRg (OR = OCH3, OR' = OC3H7; 516 ns). Activation parameters determined for the first-order decay process reveal that the enthalpy factor plays a crucial role in determining the energy barrier of the ring-closing reaction, that is, from the π-bonding to the σ-bonding compounds. Computational studies using density functional theory provided more insight into the structures of the singlet species with π single-bonded character and the transition states for the ring-closing reaction, thereby clarifying the role of the alkoxy group on the lifetime and the stereoselectivity of the ring-closing reaction.

Modified synthesis and supramolecular polymerization of rim-to-rim connected bisresorcinarenes.

The acid-catalyzed condensation reaction of resorcinol and bisdimethoxyacetals gave rise to rim-to-rim connected bisresorcinarenes in good yields. In the presence of ethanol, the homoditopic bisresorcinarenes assembled to form supramolecular polymers via hydrogen bonding interactions. The fibrous morphologies of the supramolecular polymers were confirmed by atomic force microscopy and scanning electron microscopy.

Formation of macrocyclic lactones in the Paternò-Büchi dimerization reaction.

Furan-2-ylmethyl 2-oxoacetates 1a,b, in which the furan ring and the carbonyl moiety were embedded intramolecularly, were synthesized from commercially available furan-2-ylmethanol and their photochemical reaction (hν > 290 nm) was investigated. Twelve-membered macrocyclic lactones 2a,b with C(i) symmetry including two oxetane-rings, which are the Paternò-Büchi dimerization products, were isolated in ca. 20% yield. The intramolecular cyclization products, such as 3-alkoxyoxetane and 2,7-dioxabicyclo[2.2.1]hept-5-ene derivatives, were not detected in the photolysate.

Synthesis of binuclear complexes bound in an enlarged tetraphosphamacrocycle: two diphosphine metal units linked in front-to-front style.

A large-hole tetraphosphamacrocycle 2, with four phosphorus centers separated at the corners of a 3.7 A wide and 9.7 A long rectangle, was synthesized by a stepwise cyclization reaction between PCl-bridged [1.1]ferrocenophane and bisphenol A in a 2:2 ratio. The macrocycle 2 could incorporate two Ag(+) or Pt(0) fragments in the hole to provide binuclear complexes, which were identified as mu-2-[Ag(NCMe)(2)](2)(BF(4))(2) (3) and mu-2-[Pt(PhCCPh)](2) (5), respectively, using X-ray and spectroscopic analysis. The X-ray structure of 3 demonstrates that the macrocycle 2 serves as a framework in which two diphosphine silver units are aligned in a front-to-front style, while that of 5 indicates that 2 can also bind two bulky Pt(PhCCPh) fragments by the flexible change of its conformation.

Cobalt-catalyzed trimethylsilylmethylmagnesium-promoted radical alkenylation of alkyl halides: a complement to the Heck reaction.

A cobalt complex, [CoCl2(dpph)] (DPPH = [1,6-bis(diphenylphosphino)hexane]), catalyzes an intermolecular styrylation reaction of alkyl halides in the presence of Me3SiCH2MgCl in ether to yield beta-alkylstyrenes. A variety of alkyl halides including alkyl chlorides can participate in the styrylation. A radical mechanism is strongly suggested for the styrylation reaction. The sequential isomerization/styrylation reactions of cyclopropylmethyl bromide and 6-bromo-1-hexene provide evidence of the radical mechanism. Crystallographic and spectroscopic investigations on cobalt complexes reveal that the reaction would begin with single electron transfer from an electron-rich (diphosphine)bis(trimethylsilylmethyl)cobalt(II) complex followed by reductive elimination to yield 1,2-bis(trimethylsilyl)ethane and a (diphosphine)cobalt(I) complex. The combination of [CoCl2(dppb)] (DPPB = [1,4-bis(diphenylphosphino)butane]) catalyst and Me3SiCH2MgCl induces intramolecular Heck-type cyclization reactions of 6-halo-1-hexenes via a radical process. On the other hand, the intramolecular cyclization of the prenyl ether of 2-iodophenol would proceed in a fashion similar to the conventional palladium-catalyzed transformation. The nonradical oxidative addition of carbon(sp2)-halogen bonds to cobalt is separately verified by a cobalt-catalyzed cross-coupling reaction of alkenyl halides with Me3SiCH2MgCl with retention of configuration of the starting vinyl halides. The cobalt-catalyzed intermolecular radical styrylation reaction of alkyl halides is applied to stereoselective variants. Styrylations of 1-alkoxy-2-bromocyclopentane derivatives provide trans-1-alkoxy-2-styrylcyclopentane skeletons, one of which is optically pure.

Ring-opening reaction of phosphorus-bridged [1]ferrocenophane via ring slippage from eta(5)- to eta(1)-Cp.

A reaction mechanism was investigated for a ring-opening reaction of RP(E)-bridged [1]ferrocenophane, where RP(E) = PhP(S) (3a), PhP (3b), and MesP (3c) (Mes = 2,4,6-trimethylphenyl). Irradiation of UV-vis light in the presence of an excess amount of P(OMe)(3) transformed 3a to [Fe(PhP(S)(eta(5)-C(5)H(4))(eta(1)-C(5)H(4)))(P(OMe)(3))(2)] (4a), in which one of the two cyclopentadienyl (Cp) rings of 3a changed its coordination mode from eta(5) to eta(1) and vacant coordination sites thus formed on the iron center were occupied by two P(OMe)(3) ligands. The molecular structure of 4a was determined by X-ray analysis, in which eta(1)-Cp adopted a 1-Fe-2-P-1,3-cyclopentadiene structure. Under the same reaction conditions, 3b and 3c also gave similar ring-slipped products 4b and 4c, respectively. Photolysis of 3a using more strongly coordinating PMe(3) in place of P(OMe)(3) led to complete dissociation of a Cp ligand from the iron center to form [Fe(PhP(S)(eta(5)-C(5)H(4))(C(5)H(4)))(PMe(3))(3)] (5). The formation of the ring-slipped and -dissociated products on the photolysis of 3 strongly supports the view that photolytic ring-opening polymerization of 3 proceeds via an unprecedented Fe-Cp bond cleavage mechanism.