The coordination of pyridonates and a ketophenolate to metallocene based bispalladacycles is reported. Using the phenolate ligand in a catalytic asymmetric 1,4-addition, higher activity than with the benchmark system from literature was found, while the pyridonate decreased the activity. The different μ²- and κ²-coordination modes are discussed as reason.

Abstract

Planar chiral bispalladacycles based on ferrocene have previously been shown to be excellent catalysts for asymmetric 1,4-additions of α-cyanoacetates to enones. For a bimetallic reaction pathway, it was found that product decomplexation is probably rate-determining as a result of a bimetallic two-point binding. We hypothesized that the use of hemilabile chelating ligands might accelerate this step to improve the catalytic activity. Here we report the use of pyridin-2-olates (pyridonates) as potentially hemilabile 1,3-N,O-chelating ligands in this catalytic application. In this article, we describe the first coordination of pyridonate ligands to planar chiral, metallocene based palladacycles. Four of the resulting ferrocene and ruthenocene bis-palladium complexes were characterized by X-ray single crystal structure analysis revealing a μ²-(O,N) coordination mode. We suggest that an observed lower catalytic efficiency of the new complexes has its origin in this particular coordination mode. For that reason, a related trifluoromethylketophenolate ligand was installed for which κ²-(O,N) coordination was expected and also experimentally found. Indeed, in that case catalytic activity was improved compared to the benchmark system.

Lytic polysaccharide monoxygenase are Cu-dependent metalloenzymes that catalyze the hydroxylation of strong C−H bonds using O₂ and/or H₂O₂ as oxidants. In this article, we analyze the oxidation chemistry of an unusual mononuclear Cu complex bound by a podal ligand that can act as a H-bond/proton donor. Our results suggest that LPMOs react with O₂ to produce H₂O₂ (oxidase-like chemistry), which is used to promote C−H hydroxylation via a Fenton-like mechanism.

Abstract

Lytic polysaccharide monooxygenases (LPMOs) are Cu-dependent metalloenzymes that catalyze the hydroxylation of strong C−H bonds in polysaccharides using O₂ or H₂O₂ as oxidants (monooxygenase/peroxygenase). In the absence of C−H substrate, LPMOs reduce O₂ to H₂O₂ (oxidase) and H₂O₂ to H₂O (peroxidase) using proton/electron donors. This rich oxidative reactivity is promoted by a mononuclear Cu center in which some of the amino acid residues surrounding the metal might accept and donate protons and/or electrons during O₂ and H₂O₂ reduction. Herein, we utilize a podal ligand containing H-bond/proton donors (LH₂) to analyze the reactivity of mononuclear Cu species towards O₂ and H₂O₂. [(LH₂)Cu^I]¹⁺ (1), [(LH₂)Cu^II]²⁺ (2), [(LH⁻)Cu^II]¹⁺ (3), [(LH₂)Cu^II(OH)]¹⁺ (4), and [(LH₂)Cu^II(OOH)]¹⁺ (5) were synthesized and characterized by structural and spectroscopic means. Complex 1 reacts with O₂ to produce 5, which releases H₂O₂ to generate 3, suggesting that O₂ is used by LPMOs to generate H₂O₂. The reaction of 1 with H₂O₂ produces 4 and hydroxyl radical, which reacts with C−H substrates in a Fenton-like fashion. Complex 3, which can generate 1 via a reversible protonation/reduction, binds H₂O and H₂O₂ to produce 4 and 5, respectively, a mechanism that could be used by LPMOs to control oxidative reactivity.

Lanthanide complexes of DO3A-derivative ligands bearing a pyridine-carbamate or pyridine-amine pendant have potential interest in the design of enzymatically activated imaging probes. They are stable and inert, with an exceptionally high kinetic inertness for the carbamate analogue (estimated dissociation half-life ~10⁸ h at pH 7.4).

Abstract

Lanthanide complexes of DO3A-derivative ligands bearing a pyridine-carbamate (L1) or pyridine-amine (L2) arm have potential interest in the design of enzymatically activated imaging probes. Solid-state X-ray structures for CeL1 and YbL2 both demonstrate twisted square antiprismatic geometry, with the metal ion in a nine- or an eight-coordinate environment, respectively. As assessed by pH-potentiometry, in solution lanthanide ions form more stable complexes with the nonadentate L1 than with the octadentate L2 ligand (logK _ML=18.7–21.1 vs. 16.7–18.6, respectively), while stability constants are similar for L1 and L2 chelates of Mg²⁺, Ca²⁺, Zn²⁺ or Cu²⁺. The kinetic inertness of GdL1 is exceptionally high, with an estimated dissociation half-life of ~10⁸ h at pH 7.4, while LnL2 (Ln=Ce, Gd, Yb) complexes have 3–4 orders of magnitude faster dissociation, related to the presence of the protonatable, non-coordinating amine function. The water exchange rate determined for the monohydrated GdL2 (k _ex ²⁹⁸=1.3×10⁶ s⁻¹) shows a threefold decrease with respect to GdDOTA, as a consequence of a reduction in the negative charge and in the steric crowding around the water binding site, both important in dissociatively activated water exchange processes.

The Cover Feature shows a common idiom and pattern in Chinese traditional culture - Shuanglong Xizhu, which are both ancient totem symbols and profound cultural representations. At the same time, this picture also celebrates the coming of the Chinese New Year, “Year of the Dragon”, where the dragon is the symbol of China, representing auspiciousness, majesty and strength, and also expressing people's good wishes for the new year. The two dragons in the picture represent the two nucleophilic centers on the P and O atoms, respectively, which play and compete around the “pearl” evolved from the substrates. The molecular structure at the bottom of the picture is the simplified molecule of dialkyl-2H-1,2-phosphasiliren-3-olate. More information can be found in the Research Article by L. Wang, Z. Li, and co-workers.

The Front Cover shows the various “flavours” of α-functionalized phosphine chalcogenides that can be generated from the hydrophosphorylation of C=O/N bonds. Even with multiple potential operative mechanisms such as a concerted [2+2] hydrophosphorylation, or a stepwise P(V)−P(III) tautomerization and subsequent nucleophilic attack, the resultant α-functionalized phosphine chalcogenide remains the same. Additionally, the separation and organization of each α-functionalized phosphine chalcogenide displays that each is distinctly unique and is treated as such throughout the course of the Review. More information can be found in the Review by J.-W. Lamberink-Ilupeju, P. J. Ragogna, and J. M. Blacquiere.

A series of triamidoamine complexes of uranium and thorium with N-SiPh₂Bu^t substituents are described, including chlorides, azides, cyclometallates.

Abstract

We report the synthesis and characterisation of thorium(IV), uranium(III), and uranium(IV) complexes supported by a sterically demanding triamidoamine ligand with N-diphenyl-tert-butyl-silyl substituents. Treatment of ThCl₄(THF)_3.5 or UCl₄ with [Li₃(Tren^DPBS)] (Tren^DPBS={N(CH₂CH₂NSiPh₂Bu^t)₃}³⁻) afforded [An(Tren^DPBS)Cl] (An=Th, 1Th; U, 1U). Complexes 1An react with benzyl potassium to afford the cyclometallates (Tren^DPBS _cyclomet) [An{N(CH₂CH₂NSiPh₂Bu^t)₂(CH₂CH₂NSiPhBu^tC₆H₄)}] (An=Th, 2Th; U, 2U). Treatment of 1An with sodium azide affords [An(Tren^DPBS)N₃] (An=Th, 3Th; U, 3U). Reaction of 3Th with potassium graphite affords 2Th. In contrast, 3Th reacts with cesium graphite to afford the doubly-cyclometallated (Tren^DPBS _d-cyclomet) ate complex [Th{N(CH₂CH₂NSiPh₂Bu^t)(CH₂CH₂NSiPhBu^tC₆H₄)}₂Cs(THF)₃] (4). In contrast to 3Th, reaction of 3U with potassium graphite produces the uranium(III) complex [U(Tren^DPBS)] (5), and 5 can also be prepared by reaction of potassium graphite with 1U. The loss of azide instead of conversion to nitrides contrasts to prior work when the silyl group is iso-propyl silyl, underscoring how ligand substituents profoundly drive the reaction chemistry. Several complexes exhibit T-shaped meta-C−H⋅⋅⋅phenyl and staggered parallel π–π-stacking interactions, demonstrating subtle weak interactions that drive ancillary ligand geometries. Compounds 1An–3An, 4, and 5 have been variously characterised by single crystal X-ray diffraction, multi-nuclear NMR spectroscopy, infrared spectroscopy, UV/Vis/NIR spectroscopy, and elemental analyses.

NIR luminescent materials are widely available for biological applications, and the NIR emitting of Ho³⁺ ions has received widespread attention in the second biological window. The NIR emitting of Ho³⁺ ion sensitized by Mn⁴⁺ has achieved in Ca₁₄Zn₆Ga_9.88O₃₅ : Mn⁴⁺,Ho³⁺ upon UV-vis light excited, and the energy transfer from Mn⁴⁺ to Ho³⁺ ions is mainly dominated by dipole-dipole interaction. The sample displays NIR emitting spectra ranging from 1100 to 1300 nm with two emission centers including Mn⁵⁺ and Ho³⁺ emission, respectively, which holds promising potential for applications in the second biological window.

Abstract

NIR luminescent materials are widely available for biological applications, and the NIR emitting of Ho³⁺ ions has received widespread attention in the second biological window. In this work, Ca₁₄Zn₆Ga₁₀O₃₅ : Mn⁴⁺,Ho³⁺ luminescent materials were synthesized using a high temperature solid state method. Optical properties and energy transfer mechanisms have studied in detail. Upon UV-vis light excitation, the Mn single doped Ca₁₄Zn₆Ga₁₀O₃₅ phosphors exhibit deep red and NIR emission centered at 712 and 1151 nm assigned to Mn⁴⁺ and Mn⁵⁺, respectively. Another stronger NIR emitting peaked at 1195 nm occurs when the Ho³⁺ ion is co-doped into Ca₁₄Zn₆Ga₁₀O₃₅ : Mn⁴⁺. The energy transfer from Mn⁴⁺ to Ho³⁺ ion is performed through a resonant type by a dipole-dipole interaction mechanism. In addition, the thermal stability of Ca₁₄Zn₆Ga₁₀O₃₅ : Mn⁴⁺,Ho³⁺ phosphor has been investigated, and the NIR emission of Ho³⁺ ion maintains more than half the strength at 423 K. The findings indicate that the as-prepared Ca₁₄Zn₆Ga₁₀O₃₅ : Mn⁴⁺,Ho³⁺ luminescent materials hold promise for potential applications in the second biological window.

We briefly summarized the recent advances in the preparation of main group analogs of dioxirane, dithiirane, diselenirane and ditellurirane stable in the solid state. The unique structures were characterized by X-ray diffraction analysis. Dechalcogenation of thus obtained main group dichalcogeniranes afforded the corresponding double-bonded compounds. Other reactivity and outlook are also described.

Abstract

This Mini Review highlights recent advances in the preparation of main group analogs of dichalcogeniranes. The three–membered ring compounds are an intriguing class of compounds in terms of their strained structures and high reactivity. As for three–membered rings composed of a carbon and two group 16 atoms, dioxiranes have long been utilized as oxidants, while some dithiiranes have been synthesized. In contrast, diselenirane and ditellurirane remain elusive. On the other hand, the chemistry of three–membered rings consisting of a non–carbon main group atom and two group 16 atoms has recently gained significant track. These emerging three–membered ring compounds were characterized by X-ray diffraction analysis on many occasions, revealing unique molecular structures with very long chalcogen–chalcogen single–bond distances in some cases. A common reactivity is dechalcogenation reactions using phosphine reagents, which provide access to the corresponding double-bonded compounds. This minireview covers recent advances in the synthesis of main group analogs of dioxirane, dithiirane, diselenirane, and even ditellurirane.

Reactions of an Fe(-II) carbonyl dianion bearing an amino appendant with diverse electrophiles generated a range of unprecedented products, most notably a diphenylstannylene complex formed via chelate-assisted oxidative addition of Sn−Ph bond. The amino appendant is proposed to play a pivotal role in differentiating the reactivity of this dianion to the basic [Fe(CO)₄]²⁻ species.

Abstract

Studies toward transition metals in negative oxidation states are much less explored compared to those in zero or positive oxidation states. In this study, we present the synthesis and reactivity studies of an Fe(-II) carbonyl dianion (3) featuring an appended Lewis base, [(L)Fe(CO)₃]²⁻ (L=Ph₂PCH₂CH₂NMe₂). Unlike the well-known reactivity of [Fe(CO)₄]²⁻ with common electrophiles (E ⁺) which typically forms [(E)₂Fe(CO)₄], 3 reacted with 2 equiv. of Ph₃SnCl to afford a mixture of two products: one being an Fe(II) bis(triphenylstannyl) product (4), and the other an Fe stannylene product (5). Further insights into the reactivity of 3 was elucidated through its reactions with 2 equiv. of Cy₃SnCl or Me₃SiCl, producing an Fe(II) bis(tricyclohexylstannyl) product (8) and a zwitterionic complex (11), respectively, the latter emerging via THF ring-opening. Intermediates generated from reactions of 3 with 1 equiv. of each electrophile were isolated to shed light on the reaction mechanisms, highlighting the influence of appended Lewis base on the reactivity of metal carbonyl dianions, especially the generation of the novel stannylene complex 5. The electronic structure of this paramagnetic stannylene complex was also investigated by computational studies.

Facile synthesis of β-brominated manganese porphyrins via self-catalytic reaction and their catalytic potentials for the oxidative-bromination via haloperoxidases-like activity have been explored.

Abstract

A novel and sustainable approach has been developed for the synthesis of β-brominated Mn porphyrins, [Mn^III(Br)(TPPBr₄)] (2), [Mn^III(Br)(TPPBr₆)] (3) and [Mn^IV(Br)₂(TPPBr₈)] (4) by self catalytic haloperoxidase mimicking activity of [Mn^III(Br)(TPP)] [bromo(meso-tetraphenylporphyrinato)manganese(III)] (1) in aqueous medium under different mild and controlled reaction conditions. By precisely tweaking important parameters (e. g. H₂O_2, HClO₄ and KBr), these polybrominated porphyrin complexes have been synthesized. This method is safer and applicable under milder reaction conditions than the conventional procedures for β-bromination of porphyrins. These complexes were characterized by various spectroscopic techniques, including UV–Vis spectroscopy, elemental analysis, MALDI-TOF mass spectrometry, cyclic voltammetry, DFT calculations and single crystal X-ray diffraction analysis. Bromination of various phenol derivatives via haloperoxidase-catalyzed reaction using these manganese complexes has been explored. Carrying out the catalytic reaction at room temperature in the presence of H₂O₂ as an oxidant and KBr as a brominating agent in a mild aqueous acidic condition results in good to excellent yield of the brominated product(s). Extra stability of 4 compared to other catalysts due to trans-[Br−Mn^IV−Br] structure possibly prevents the interaction of Mn with oxidant which makes it less potential catalyst compared to 1, 2 and 3. Suitable catalytic reaction mechanism has been proposed for the bromination of substrates after identifying the reaction intermediates using mass spectrometry.