We present the first BICAAC adducts of ECl₃ (E=P, Sb) and a phosphaalkene, (BICAAC)=P−Ph stabilized by BICAAC (BICAAC=bicyclic (alkyl)(amino)carbene)). Further, strong π-acceptor character of BICAAC has been exploited in three electron reduction of the BICAAC:ECl₃ adducts and afforded the bis(BICAAC)P₂ and bis(BICAAC)Sb₂ dinuclear compounds with the pnictogen elements in low valent state.

Abstract

The stoichiometric reaction of bicyclic (alkyl)(amino)carbene (BICAAC) with group 15 chlorides, ECl₃ (E=P, Sb) to form the Lewis adducts BICAAC:ECl₃ (E=P (1), Sb (2)) has been investigated in the present work. The BICAAC smoothly reacts with PPhCl₂ to form [BICAAC:PPhCl₂] which on in-situ two electron reduction with 2 equivalents of KC₈ afforded the phosphinidene complex, BICAAC=P-Ph (3). Complete dechlorination of the BICAAC-ECl₃ adducts 1 and 2 with 3 equivalents of KC₈ leads to three-electron reduction to afford low-valent trans bent bis(BICAAC)E₂ complexes (E=P (4) and Sb (5)). These complexes are the first examples of BICAAC adducts with pnictogens and all the complexes have been characterized by different spectroscopic techniques and their solid-state structure have been elucidated by single crystal X-ray diffraction.

meso-Functionalized vanadyl porphyrins were employed as efficient catalysts for oxidizing benzoin to benzil via oxygen atom transfer using DMSO or aq. H₂O₂ as an oxidant.

Abstract

Systematic analysis of the effect of para-substituents (H, Cl, Br and OMe) on the meso-phenyl group in vanadyl meso-tetraphenylporphyrins ([V^IVO(TPP)] (R=H, 1), [V^IVO(TCPP)] (R=Cl, 2), [V^IVO(TBPP)] (R=Br, 3) and [V^IVO(TMPP)] (R=OMe, 4)) on their properties and catalytic oxygen atom transfer (OAT) for oxidation of benzoin to benzil using DMSO as well as 30 % aqueous H₂O₂ as the sacrificial oxygen source have been studied. Electrochemical and theoretical (density functional theory) studies are in good agreement with the influence of these substituents on the catalytic property of these complexes. Complex [V^IVO(TCPP)] (2) displayed the best catalytic activity for the conversion (92 %) of benzoin to benzil in 30 h with >99 % product selectivity when DMSO was used as an oxygen source, whereas excellent conversion (~100 %) of benzoin to benzil was noticed in 18 h with 95 % product selectivity when 30 % aqueous H₂O₂ was used as a source of oxygen. Furthermore, among these complexes, the electron-withdrawing nature of the chloro substituent at the p-position of meso-phenyl group significantly influences the oxygen atom transfer. Experimental and simulated EPR studies confirmed the +4 oxidation of vanadium in these complexes. The structure of 2, 3 and 4, confirmed by single crystal X-ray diffraction method, are domed in shape, and the displacement of V(IV) ion from the mean porphyrin plane follows the order: 2 (0.458 Å) <3 (0.459 Å) <4 (0.479 Å). We observed that the electron-withdrawing nature of chloro substituent at the p-position of meso-phenyl group influence the oxygen atom transfer from vanadyl porphyrin to dimethyl sulfide much.

The Front Cover shows the evolution of a dibismuthane with olefin functional groups to give a set of potential hybrid ligands featuring chalcogen and olefin coordination sites. In the picture, a dibismuthane caterpillar is moving along a branch towards a delicious meal of chalcogen leaves. The facile insertion of elemental chalcogens into the Bi−Bi bond transforms the caterpillars into colorful butterflies, with a central Bi−E−Bi motif, fluttering around a mountain meadow. DFT calculations were used to investigate the potential of these bismuth compounds to act as tridentate chalcogen/olefin ligands towards a variety of transition metals. These metal atoms are represented by blue flowers in the picture, attracting the butterflies to form a range of coordination entities. More information can be found in the Research Article by C. Lichtenberg and co-workers.

The multi-functional discrete Eu³⁺−pyrene assembly shows both color tunable emission (blue through to red including white-emission) and molecular oxygen sensing properties.

Abstract

We report the synthesis of a new pyrene, dipicolinic acid-based ligand (L₁H) and its corresponding multi-emissive and multifunctional europium complex [Eu(L₁ )₃] that is capable of single component color switchable emission from red to blue and also white. At high concentration (10 mM) the single component system results in near pure white emission (CIE coordinates x,y=0.329, 0.324). Furthermore, the system showed ratiometric oxygen sensing with oxygen significantly quenching the pyrene centered emission but not the Eu³⁺ emission, resulting in an overall emission color change from blue to red on increasing oxygen content.

A first example of a tellurium(II) compound with three different bonding modes to iodine featuring covalent and non-covalent bonds such as two different σ-hole interactions is introduced: [MesTe(I)(I₂)(I₃)]⁻. The character of chalcogen and halogen bonds are evaluated by the combination of crystallographic data and computational methods.

Abstract

A first example of an aryltellurium(II) compound with three different bonding modes to iodine featuring covalent and non-covalent bonds such as two orthogonal, ambiphilic σ-hole interactions is introduced: [MesTe(I)(I₂)(I₃)]⁻. It is a member of a series of mesityltellurenyl anions, which are formed during reactions of (MesTe)₂ with ZnI₂, phenanthroline (phen) and iodine. [Zn(phen)₃][MesTe(I)₂] (1), [Zn(phen)₃][{MesTe(I)-(I)…Te(I)Mes}{MesTeI₂}] (2) and [Zn(phen)₃][MesTe(I)(I₂)(I₃)][MesTeI₂] (3) are isolated depending on the amount of iodine used. The products contain tellurium atoms bonded to a variety of iodine species (I⁻, μ₂-I⁻, I₂ and I₃ ⁻) and are, thus, perfectly suitable to explore the amphiphilic behavior of tellurium(II) and its relevance for the formation of non-covalent bonds, where tellurium acts as both donor and acceptor simultaneously. The character of chalcogen and halogen bonds are evaluated by the combination of crystallographic data and computational methods.

Copper(II) bis-oxamidate(L1) complex undergoes selective water oxidation while the analogous nickel(II) complex demonstrate both homogeneous water oxidation and catalyst deactivation due to the 2e⁻ oxidized anionic intermediate [(L1⋅)Ni^III(OH)]¹⁻ which promote both nucleophilic (OH⁻) and electrophilic (H⁺) attack while the corresponding copper intermediate, [(L1⋅)Cu^II(OH⋅)]¹⁻, display radical character on the hydroxyl ligand with greater electrophilicity and avoids catalyst deactivation.

Abstract

Water splitting is a potential pathway for hydrogen gas evolution and thereby realization of a carbon-neutral sustainable energy scheme. However, oxidation of water to dioxygen is the major impediment in conversion of solar energy to fuel. Herein, density functional studies are conducted to explore the reactivity conduits of two molecular electro-catalysts consisting of nickel and copper tetra-anionic tetradentate amide ligand complexes of the type [(L1)M^II]²⁻, where L1=o-phenylenebis(oxamidate), and their substitutionally modified analogues. While nickel complexes demonstrate complex borderline chemistry between homogeneous and heterogeneous pathways, showing competition between water oxidation and molecular species degradation, copper complexes display robust and efficient molecular water oxidation behavior. Our analysis predict that this disparity is primarily due to the reversible O−O bond formation in nickel complexes, which provide the platform necessary for a direct attack of OH⁻/H⁺ on the metal and terminally accessible amidate groups of the 2e⁻ oxidized anionic intermediate, [(L1⋅)Ni^III(OH)]¹⁻, respectively. This intermediate streamline ligand deactivation with a comparatively higher driving force for nickel complexes in acidic medium. Contrarily, the copper complexes display radical character on the hydroxyl ligand in the corresponding intermediate, [(L1⋅)Cu^II(OH⋅)]¹⁻, that expedite O−O interaction, leading to predominant homogeneous water oxidation under all conditions.

The cobalt(II) complex Co{i-N₄} comprising the macrocyclic ligand isocyclam (1,4,7,11-tetraazacyclotetradecane) was tested for its catalytic potential in the oxygen reduction reaction revealing the generation of hydrogen peroxide. In the reaction of Co{i-N₄} with O₂, a superoxo Co(III) and a dimeric μ-peroxo Co(III) species were identified as reactive Co−O₂ intermediates, which can be employed in HAT and OAT reactions as well.

Abstract

Cobalt complexes are extensively studied as bioinspired models for non-heme oxygenases as they facilitate both the stabilization and characterization of metal-oxygen intermediates. As an analog to the well-known Co(cyclam) complex Co{N₄} (cyclam=1,4,8,11-tetraazacyclotetradecane), the Co^II complex Co{i-N₄} with the isomeric isocyclam ligand (isocyclam=1,4,7,11-tetraazacyclotetradecane) was synthesized and characterized. Despite the identical N₄ donor set of both complexes, Co{i-N₄} enables the 2e⁻/2H⁺ reduction of O₂ with a lower overpotential (η _eff of 385 mV vs. 540 mV for Co{N₄}), albeit with a diminished turnover frequency. Characterization of the intermediates formed upon O₂ activation of Co{i-N₄} reveals a structurally identified stable μ-peroxo Co^III dimer as the main product. A superoxo Co^III species is also formed as a minor product, as indicated by EPR spectroscopy. In further reactivity studies, the electrophilicity of these in situ generated Co−O₂ species was demonstrated by the oxidation of the O−H bond of TEMPO−H (2,2,6,6-tetramethylpiperidin-1-ol) via a H atom abstraction process. Unlike the known Co(cyclam), Co{i-N₄} can be employed in oxygen atom transfer reactions oxidizing triphenylphosphine to the corresponding phosphine oxide highlighting the impact of geometrical modifications of the ligand while preserving the ring size and donor atom set on the reactivity of biomimetic oxygen activating complexes.

Molybdenum tricarbonyl complexes supported by PN^PhP ligands with terminal alkyl substituents were synthesized. Surprisingly, all of these complexes are found in the fac configuration, irrespective of the steric demand of the alkyl substituents. This is due to the fact that mer-fac isomerization is hindered.

Abstract

Series of linear tridentate PN^PhP^R-ligands (R=Me, Et, Pln, Ph, Cyp, ⁱPr, Cy, ^tBu) and molybdenum tricarbonyl complexes [Mo(CO)₃PN^PhP^R] (R=Ph, Et, Cyp, ⁱPr, Cy,) were synthesized and characterized using NMR-, IR-, and Raman spectroscopy as well as X-ray crystallography. The influence of the different phosphine donor groups of the PN^PhP^R ligands on the bonding and activation of CO ligands is investigated. Importantly, all complexes are found to adopt a fac geometry, both in solution and in the solid state. This is in contrast to analogous complexes supported by PN^HP ligands. DFT calculations reveal that the phenyl ring at the central amine function is the cause of the preferred geometry, hindering isomerization to a mer geometry.

Alkoxyphosphoranes, Ph₂P(OR)(O₂C₆Cl₄) and the metal chlorides generate corresponding phosphate-catecholate chelated Nd(III), Zr(IV) and Al(III) chlorides via ethyl chloride elimination. These monometallic and bimetallic metal complexes are stabilized by chelating P−O and catecholate-O donors.

Abstract

Metal phosphates are important catalysts and materials in synthesis chemistry. Herein, we describe the synthesis and characterization of phosphate-catecholate chelated Nd(III), Zr(IV) and Al(III) chlorides (2–5). These species are achieved via ethyl chloride elimination reaction of oxophosphoranes with corresponding metal chlorides. The product 2–5 represent a new serial of monometallic and bimetallic phosphate-catecholate chelated metal complexes stabilized by both P−O and catecholate-O donors. These findings pave the way for future explorations of such species in catalysis.

Photoinduced oxygen atom transfer to α-pinene and R-carvone using a dioxo-molybdenum (VI) complex incorporated within a modified UiO-67 (Zr/Ti) MOF was studied. Organometallic frameworks (MOFs) are an alternative support to heterogenize the molybdenum dioxo complex [MoO₂Ln₂], which catalyzes the Oxygen Atom Transfer (OAT). UiO-67 is a microporous material that allows the formation of the dioxo-Mo complex anchored to the 5,5′-dicarboxylate-2,2′-bipyridine (bpydc) ligand to achieve a variable number of dioxo-Mo units in the network. The MOF UiO-67 allows a post-synthetic ion exchange of Zr by Ti, modifying the optical properties that facilitate the use of UV light in the OAT reaction. The materials prepared are highly selective (100 %) for the epoxidation of α-pinene and R-carvone using O₂ as an oxidant through a photoinduced oxygen atom transfer (OAT) process. The dioxo-Mo complex anchored on the UiO-67 (Zr/Ti) MOF during the photoinduced TAO process towards the monoterpenes forms the Mo^IV reduced unit, which interacts with O₂ to regenerate of the active catalytic Mo^VI unit, and to continue the catalytic cycle of the oxygen atom transfer.

Abstract

We report a highly selective (100 %) epoxidation of α-pinene and R-carvone using an oxygen atom transfer (OAT) reaction facilitated by a dioxo-Mo complex (Mo^(VI)O₂Cl₂Ln) incorporated into the ligand 5,5’-dicarboxylate-2,2’-bipyridine (bpydc) within a Metal-Organic Framework (MOF) type UiO-67. Photo-stimulated (350 nm) OAT reaction was carried out with oxygen molecular used as the oxidant for 10 h. UiO-67 was synthesized with a mixture of the ligands 2,2′-biphenyl-5,5′-dicarboxylate (bpdc) and 2,2-bipyridine-5,5-dicarboxylate (bpydc) in different molar ratios (67 : 33, 50 : 50, 70 : 30, 0 : 100 bpdc : bpydc) to promote a higher presence of catalytic sites, i. e., the dioxo-Mo complex units. Furthermore, a post-synthetic exchange of Zr for Ti, between 64 : 36 to 78 : 22 Ti : Zr molar ratio, was performed to improve the optical properties of the MOF and promote the photoinduced OAT reaction. The Catalytic system was characterized by FTIR, XRD, ¹H NMR, XPS, TGA, N₂ adsorption/desorption and UV-Vis-DR. The amount of the epoxide monoterpene is proportional to the number of the dioxomolybdenum^(VI) units (MoO₂) incorporated in the UiO-67 (Zr/Ti), and the OAT reaction selectivity is due to the absence of the oxygen radicals in the medium of reaction. Besides, The Mo complex exhibited excellent stability after five cycles of use.