Vacant Coordination Site Research Articles

Air-Stable Coordinatively Unsaturated Ruthenium(II) Complex for Ligand Binding and Catalytic Transfer Hydrogenation of Ketones from Ethanol.

Coordinatively unsaturated complexes are interesting from a fundamental level for their formally empty coordination site and, in particular, from a catalytic perspective as they provide opportunities for substrate binding and transformation. Here, we describe the synthesis of a novel underligated ruthenium complex [Ru(cym)(N,N')]+, 3, featuring an amide-functionalized pyridylidene amide (PYA) as the N,N'-bidentate coordinating ligand. In contrast to previously investigated underligated complexes, complex 3 offers potential for dynamic modifications, thanks to the flexible donor properties of the PYA ligand. Specifically, they allow both for stabilizing the formally underligated metal center in complex 3 through nitrogen π-donation and for facilitating through π-acidic bonding properties the coordination of a further ligand L to the ruthenium center to yield the formal 18 e- complexes [Ru(cym)(N,N')(L)]+ (4: L = P(OMe)3; 5: L = PPh3; 6: L = N-methylimidazole; 7: L = pyridine) and neutral complex [RuCl(cym)(N,N')] 8. Analysis by 1H NMR and UV-vis spectroscopies reveals an increasing Ru-L bond strength along the sequence pyridine <1-methylimidazole < PPh3 < P(OMe)3 with binding constants varying over 3 orders of magnitude with log(Keq) values between 2.8 and 5.7. The flexibility of the Ru(PYA) unit and the ensuing accessibility of saturated and unsaturated species with one and the same ligand are attractive from a fundamental point of view and also for catalytic applications, as catalytic transformations rely on the availability of transiently vacant coordination sites. Thus, while complex 3 does not form stable adducts with O-donors such as ketones or alcohols, it transiently binds these species, as evidenced by the considerable catalytic activity in the transfer hydrogenation of ketones. Notably, and as one of only a few catalysts, complex 3 is compatible with EtOH as a hydrogen source. Complex 3 shows excellent performance in the transfer hydrogenation of pyridyl-containing substrates, in agreement with the poor coordination strength of this functional group to the ruthenium center in 3.

Spectroscopic and Computational Evidence of Uranium Dihydrogen Complexes.

Dihydrogen complexation, a phenomenon with robust precedent in the transition metal series, is spectroscopically detected for a uranium(III) complex and thereby extended for the first time to the 5f series. The vacant coordination site and low valence of (C5H4SiMe3)3U prove to be key to the reversible formation of (C5H4SiMe3)3U-H2 (complex 1), and the paramagnetism of the f3 center facilitates the detection of complex 1 by NMR spectroscopy. Density functional theory calculations reveal that the delocalization of the 5f electron density from (C5H4SiMe3)3U onto the side-on dihydrogen ligand is crucial to complex formation, an unusual bonding situation for an actinide acid-base complex. The spectroscopic and computational results are compared to those reported for lanthanide metallocenes to yield insight into the nature of─and future possibilities for─f-element dihydrogen complexation.

Synthesis and Coordination Behavior of a Hybrid Phosphanylferrocene Amidine Ligand

AbstractThe combination of different donor moieties often results in unconventional coordination behavior. This contribution describes the synthesis of a new hybrid ligand, phosphanylferrocene amidine Ph2PfcN=CHNMe2 (1; fc=ferrocene‐1,1′‐diyl), and its coordination behavior toward the light platinum metals. Reacting compound 1 with [PdCl2(MeCN)2] produced the chelate complex [PdCl2(1‐κ2P,N)] and the bis (phosphane) complex trans‐[PdCl2(1‐κP)2], depending on the ligand‐to‐metal stoichiometry. Halide removal from [PdCl2(1‐κ2P,N)] with Na[BARF] afforded the chloride‐bridged dimer [Pd2(μ‐Cl)2(1‐κ2P,N)2][BARF]2, which was presumably in equilibrium with the mononuclear κ3‐complex [PdCl(1‐κ3Fe,P,N)][BARF] in solution (BARF=tetrakis [3,5‐bis(trifluoromethyl)phenyl]borate). The reactions of 1 with the isoelectronic precursors [(arene)MCl(μ‐Cl)]2 (M/arene=Ru(II)/p‐cymene, and Rh(III)/C5Me5) generated the respective bridge‐cleavage products [(arene)MCl2(1‐κP)], which transformed into the P,N‐chelate complexes [(arene)MCl(1‐κ2P,N)][BARF] when treated with Na[BARF] as a chloride scavenger. All this suggests preferential coordination the phosphane moiety to soft metal ions, while the coordination of the amidine moiety can be induced through changing the reaction stoichiometry or by generating a vacant coordination site at the metal ion. The compounds were characterized using a combination of standard spectroscopic methods (multinuclear NMR, IR, and ESI MS) and single‐crystal X‐ray crystallography, and compounds 1 and 3 were further studied by cyclic voltammetry.

A Different Perspective on Tuning the Photophysical and Photochemical Properties: The Influence of Constitutional Isomers in Group 6 Carbonyl Complexes with Pyridyl-Mesoionic Carbenes.

Exploring novel and existing design principles to tune the photochemical and photophysical properties of transition-metal complexes is an important goal in contemporary research. Here, we highlight the influence of constitutional isomers of pyridyl-1,2,3-triazolylidene mesoionic carbene (MIC) ligands on the photophysical and photochemical properties of the corresponding tetracarbonyl group 6 metal complexes (M = Cr, Mo, W). All new complexes [M(C-C)] presented herein incorporate a C-C linked pyridyl-MIC ligand and were fully characterized by X-ray diffraction analysis, elemental analysis, and 1H NMR and IR spectroscopy. Detailed photophysical investigations reveal a single emission in the VIS region, which extends into the NIR with lifetimes of up to 3.5 μs in the solid state at lower temperatures. The quantum yields were determined for all three complexes, and, in particular, the W0 complex shows an unusually high quantum yield of 29% compared to the values of 0.02% obtained for the [M(C-N)] isomers investigated in earlier works. Beyond this, the investigated W0 complex also exhibits an emission at 717 nm in a fluid solution. The combination of luminescence and FTIR-step scan spectroscopy with theoretical calculations reveals an emissive 3MLCT state. Irradiation of the presented complexes leads to a clean cleavage of one axial CO ligand. A metastable 16 VE species with a vacant axial coordination site was detected in the solid state at low temperatures. In solution, the respective solvato complexes are formed. A dark reverse reaction is observed, as previously described for the [M(C-N)] analogues. The increased electron density induced by the C-C linked pyridyl-MIC ligand leads to an increased kinetic rate constant for the reformation of the starting species and is also reflected in the lower photodissociation quantum yields.

Design of Heme Enzymes with a Tunable Substrate Binding Pocket Adjacent to an Open Metal Coordination Site.

The catalytic versatility of pentacoordinated iron is highlighted by the broad range of natural and engineered activities of heme enzymes such as cytochrome P450s, which position a porphyrin cofactor coordinating a central iron atom below an open substrate binding pocket. This catalytic prowess has inspired efforts to design de novo helical bundle scaffolds that bind porphyrin cofactors. However, such designs lack the large open substrate binding pocket of P450s, and hence, the range of chemical transformations accessible is limited. Here, with the goal of combining the advantages of the P450 catalytic site geometry with the almost unlimited customizability of de novo protein design, we design a high-affinity heme-binding protein, dnHEM1, with an axial histidine ligand, a vacant coordination site for generating reactive intermediates, and a tunable distal pocket for substrate binding. A 1.6 Å X-ray crystal structure of dnHEM1 reveals excellent agreement to the design model with key features programmed as intended. The incorporation of distal pocket substitutions converted dnHEM1 into a proficient peroxidase with a stable neutral ferryl intermediate. In parallel, dnHEM1 was redesigned to generate enantiocomplementary carbene transferases for styrene cyclopropanation (up to 93% isolated yield, 5000 turnovers, 97:3 e.r.) by reconfiguring the distal pocket to accommodate calculated transition state models. Our approach now enables the custom design of enzymes containing cofactors adjacent to binding pockets with an almost unlimited variety of shapes and functionalities.

Dioxygen Activation by a Bioinspired Tungsten(IV) Complex.

An increasing number of discovered tungstoenzymes raises interest in the biomimetic chemistry of tungsten complexes in oxidation states +IV, +V, and +VI. Bioinspired (sulfur-rich) tungsten(VI) dioxido complexes are relatively prevalent in literature. Still, their energetically demanding reduction directly correlates with a small number of known tungsten(IV) oxido complexes, whose chemistry is not well explored. In this paper, a reduction of the [WO2(6-MePyS)2] (6-MePyS = 6-methylpyridine-2-thiolate) complex with PMe3 to a phosphine-stabilized tungsten(IV) oxido complex [WO(6-MePyS)2(PMe3)2] is described. This tungsten(IV) complex partially releases one PMe3 ligand in solution, creating a vacant coordination site capable of activating dioxygen to form [WO2(6-MePyS)2] and OPMe3. Therefore, [WO2(6-MePyS)2] can be used as a catalyst for the aerobic oxidation of PMe3, rendering this complex a rare example of a tungsten system utilizing dioxygen in homogeneous catalysis. Additionally, the investigation of the reactivity of the tungsten(IV) oxido complex with acetylene, substrate of a tungstoenzyme acetylene hydratase (AH), revealed the formation of the tungsten(IV) acetylene adduct. Although this adduct was previously reported as an oxidation product of the tungsten(II) acetylene carbonyl complex, here it is obtained via substitution at the sulfur-rich tungsten(IV) center, mimicking the initial step of the first shell mechanism for AH as suggested by computational studies.

Agostic interaction versus small molecule binding in [RuH(CO)(PPhNiPrPPh)]BAr4F complex

Coordinatively unsaturated, 16 electron, cationic [RuH(CO)(PPhNiPrPPh)]BAr4F complex (2) (PPhNiPrPPh = [(CH3)2CHN(CH2CH2PPh2)2]) was prepared and characterized in solution using NMR spectroscopy. Complex 2 shows close contact of the iPr C−H bond with the vacant coordination site of the metal center and is involved in an agostic interaction. Reactivity of complex 2 with X−H (X = H, H2B·NMe3, SiEt3, CH3) bond of small molecules to obtain σ-complexes via displacement of the agostic interaction was explored. The σ-complexes, [RuH(η2-H2)(CO)(PPhNiPrPPh)]BAr4F (4) and [RuH(η1-HSiEt3)(CO)(PPhNiPrPPh)]BAr4F (6) have been characterized at low temperature (213 K - 183 K) using NMR spectroscopy.

Ruthenium Complexes of a Triphosphorus-Coordinating Pincer Ligand: Ru–P Ligand-Substituent Exchange Reactions Driven by Large Variations of Bond Energies

The reaction of [(p-cymene)RuCl2]2 with the triphosphine ligand bis(2-di-tert-butylphosphinophenyl)phosphine (tBuPHPP) results in an unusual exchange reaction in which a chloride ligand and a phosphorus-bound H atom are exchanged ("H-P/Ru-Cl exchange") to give the (chlorophosphine)ruthenium hydride complex (tBuPClPP)RuHCl [1Cl-HCl; tBuPClPP = bis(2-di-tert-butylphosphinophenyl)chlorophosphine]. Density functional theory calculations indicate that the presumed initial product of metalation, (tBuPHPP)RuCl2 (1H-Cl2), undergoes an H-P/Ru-Cl exchange via sequential P-to-Ru α-H migration to give the intermediate (tBuPPP)RuHCl2, followed by Ru-to-P α-Cl migration to give the observed product 1Cl-HCl (crystallographically characterized). Dehydrochlorination of 1Cl-HCl under a H2 atmosphere gives (tBuPClPP)RuH4 (1Cl-H4), which then can undergo a second dehydrochlorination and addition of H2 to give (tBuPHPP)RuH4 (1H-H4). This reaction may proceed via the reverse of the intramolecular exchange by 1H-Cl2, i.e., loss of H2 from 1Cl-H4 to give 1Cl-H2, which could undergo Cl-P/Ru-H exchange to give (tBuPHPP)RuHCl (1H-HCl). Accordingly, the thermodynamics of Cl-P/Ru-H exchange are found to be highly dependent on the nature of the ancillary anionic ligand (H or Cl), which is not directly involved in the exchange. The origin of this thermodynamic dependence can be explained in terms of the high stability of complexes (RPXPP)RuHCl (X = H, Cl; R = Me, tBu), in which the hydride is approximately trans to a vacant coordination site and the central phosphine group is approximately trans to the weak-trans-influence chloride ligand. This conclusion has general implications for five-coordinate d6 complexes, both pincer- and nonpincer-ligated.

Mechanism of external K+ sensitivity of KCNQ1 channels.

KCNQ1 voltage-gated K+ channels are involved in a wide variety of fundamental physiological processes and exhibit the unique feature of being markedly inhibited by external K+. Despite the potential role of this regulatory mechanism in distinct physiological and pathological processes, its exact underpinnings are not well understood. In this study, using extensive mutagenesis, molecular dynamics simulations, and single-channel recordings, we delineate the molecular mechanism of KCNQ1 modulation by external K+. First, we demonstrate the involvement of the selectivity filter in the external K+ sensitivity of the channel. Then, we show that external K+ binds to the vacant outermost ion coordination site of the selectivity filter inducing a diminution in the unitary conductance of the channel. The larger reduction in the unitary conductance compared to whole-cell currents suggests an additional modulatory effect of external K+ on the channel. Further, we show that the external K+ sensitivity of the heteromeric KCNQ1/KCNE complexes depends on the type of associated KCNE subunits.

Selective recognition of Hg2+ ions in aqueous solution by a CdII-based metal-organic framework with good stability and vacant coordination sites.

A novel water-stable CdII-based metal-organic framework, namely {[Cd(BIBT)(TDC)]·2H2O}n (JXUST-28, BIBT = 4,7-bi(1H-imidazol-1-yl)benzo-[2,1,3]thiadiazole and H2TDC = 2,5-thiophenedicarboxylic acid), was synthesized using a mixed-ligand strategy. Structural analysis demonstrates that JXUST-28 exhibits a two-dimensional layer structure with 4-connected sql topology. Intriguingly, JXUST-28 presents good stability in boiling water (at least 5 days), common organic solvents and aqueous solutions with different pH values of 2-12 (more than 24 hours). Furthermore, fluorescence experiments revealed that JXUST-28 could sense Hg2+ ions in aqueous solution via a quenching effect with a detection limit of 0.097 μM. Meanwhile, JXUST-28 can also be regenerated at least 5 times to detect Hg2+ ions. In addition, light-emitting diode lamps, luminescent films, and test papers of JXUST-28 have been successfully developed for practical applications.

Fluorine-based Zn salan complexes.

We synthesised and characterised the racemic and chiral versions of two Zn salan fluorine-based complexes from commercially available materials. The complexes are susceptible to absorbing H2O from the atmosphere. In solution (DMSO-H2O) and at the millimolar level, experimental and theoretical studies identify that these complexes exist in a dimeric-monomeric equilibrium. We also investigated their ability to sense amines via19F NMR. In CDCl3 or d6-DMSO, strongly coordinating molecules (H2O or DMSO) are the limiting factor in using these easy-to-make complexes as chemosensory platforms since their exchange with analytes requires an extreme excess of the latter.

Mechanism of Alkyne Hydroarylation Catalyzed by (P,C)-Cyclometalated Au(III) Complexes.

Over the last 5-10 years, gold(III) catalysis has developed rapidly. It often shows complementary if not unique features compared to gold(I) catalysis. While recent work has enabled major synthetic progress in terms of scope and efficiency, very little is yet known about the mechanism of Au(III)-catalyzed transformations and the relevant key intermediates have rarely been authenticated. Here, we report a detailed experimental/computational mechanistic study of the recently reported intermolecular hydroarylation of alkynes catalyzed by (P,C)-cyclometalated Au(III) complexes. The cationic (P,C)Au(OAcF)+ complex (OAcF = OCOCF3) was authenticated by mass spectrometry (MS) in the gas phase and multi-nuclear NMR spectroscopy in solution at low temperatures. According to density functional theory (DFT) calculations, the OAcF moiety is κ2-coordinated to gold in the ground state, but the corresponding κ1-forms featuring a vacant coordination site sit only slightly higher in energy. Side-on coordination of the alkyne to Au(III) then promotes nucleophilic addition of the arene. The energy profiles for the reaction between trimethoxybenzene (TMB) and diphenylacetylene (DPA) were computed by DFT. The activation barrier is significantly lower for the outer-sphere pathway than for the alternative inner-sphere mechanism involving C-H activation of the arene followed by migratory insertion. The π-complex of DPA was characterized by MS. An unprecedented σ-arene Au(III) complex with TMB was also authenticated both in the gas phase and in solution. The cationic complexes [(P,C)Au(OAcF)]+ and [(P,C)Au(OAcF)(σ-TMB)]+ stand as active species and off-cycle resting state during catalysis, respectively. This study provides a rational basis for the further development of Au(III) catalysis based on π-activation.

Linear Mn(II)4Ln(III)2 (Ln = Gd, Dy, Tb) heterometallic complexes from a ditopic hydrazone ligand: Slow magnetic relaxation in Mn4Dy2 complex

L4 (C13H12N6O2), a ditopic ligand containing a carbohydrazide with terminal pyridyl moieties, has been successfully used to prepare a new class of heterometallic hexanuclear Mn4Ln2 systems. The reaction of L4 with Ln(NO3)3∙6H2O produced three linear heterometallic compounds with the general formula [Mn4Ln2(L4)4(CH3O)2(H2O)6Cl6][Cl]∙2H2O (Ln = Gd (1), Dy (2), Tb (3)). Monodeprotonated L4 binds lanthanide ions via Ndiazine and Ohydrazone donors in these complexes, while CH3O–, water, and chloride ions occupy the remaining vacant coordination sites. dc magnetic studies have confirmed that ferromagnetic exchange interactions are present in all three complexes with Curie temperatures of 2.32 K, 1.2 K, and 2.21 K, respectively, for 1, 2, and 3. In complex 1, the exchange coupling values between the two Mn(II)-Gd(III) and Mn(II)-Mn(II) pairs were found to be +0.11 and +0.58 cm−1, respectively, with a Lande g-factor of 2.01 for Mn(II) in this complex. ac studies reveal the slow relaxation of magnetization in complex 2 with an energy barrier of 1.5 K, while complexes 1 and 3 have not shown this feature.

Ligand-Protecting Strategy for the Controlled Construction of Multinuclear Copper Cores within a Ring-Shaped Polyoxometalate.

The ring-shaped polyoxometalate (POM) [P8W48O184]40- contains a large cavity and is an attractive inorganic multidentate ligand for accumulating metal cations. Until now, several multinuclear metal cores are constructed within the {P8W48} framework in aqueous solvents. However, it is still challenging to control the number and arrangement of introduced metal cations because of the numerous coordination sites inside the {P8W48} framework. In this study, we developed a novel approach for the selective synthesis of several multinuclear copper-containing ring-shaped POMs in organic solvents using methoxy groups as organic protecting ligands for the reactive coordination sites. Reacting a tetra-n-butylammonium (TBA) salt of [P8W48O184]40- (P8W48) with 4 and 8 equiv of copper(II) acetate in the presence of methanol (MeOH), tetra- and octacopper cores were incorporated into the cavity to form TBA11H13[Cu4(H2O)4P8W48O176(OCH3)8]·28H2O·3CH3NO2 (Cu4) and TBA14H2[Cu8(H2O)12P8W48O176(OCH3)8]·24H2O·CH3CN (Cu8), respectively. For both structures, methoxy groups served as protecting ligands and temporarily inactivated vacant coordination sites. Without MeOH, dodeca- and hexadecacopper cores were constructed inside the cavity to form TBA14H2[Cu12(H2O)16P8W48O184]·4H2O (Cu12) and TBA16H8[Cu16(OH)16(H2O)4P8W48O184]·12H2O·C3H6O (Cu16), respectively. The arrangement of copper ions on the same {P2W12} units could be controlled by the input number of copper ions. Moreover, all four POMs could be synthesized from P8W48 by the stepwise addition of 4 equiv of copper(II) acetate, clarifying the introduction process.

Tetravalent Spiroselenurane Catalysts: Intramolecular Se···N Chalcogen Bond-Driven Catalytic Disproportionation of H2O2 to H2O and O2 and Activation of I2 and NBS.

Chalcogen-bonding interactions have recently gained considerable attention in the field of synthetic chemistry, structure, and bonding. Here, three organo-spiroselenuranes, having a Se(IV) center with a strong intramolecular Se···N chalcogen-bonded interaction, have been isolated by the oxidation of the respective bis(2-benzamide) selenides derived from an 8-aminoquinoline ligand. Further, the synthesized spiroselenuranes, when assayed for their antioxidant activity, show disproportionation of hydrogen peroxide into H2O and O2 with first-order kinetics with respect to H2O2 for the first time by any organoselenium molecules as monitored by 1H NMR spectroscopy. Electron-donating 5-methylthio-benzamide ring-substituted spiroselenurane disproportionates hydrogen peroxide at a high rate of 15.6 ± 0.4 × 103 μM min-1 with a rate constant of 8.57 ± 0.50 × 10-3 s-1, whereas 5-methoxy and unsubstituted-benzamide spiroselenuranes catalyzed the disproportionation of H2O2 at rates of 7.9 ± 0.3 × 103 and 2.9 ± 0.3 × 103 μM min-1 with rate constants of 1.16 ± 0.02 × 10-3 and 0.325 ± 0.025 × 10-3 s-1, respectively. The evolved oxygen gas from the spiroselenurane-catalyzed disproportion of H2O2 has also been confirmed by a gas chromatograph-thermal conductivity detector (GCTCD) and a portable digital polarographic dissolved O2 probe. Additionally, the synthesized spiroselenuranes exhibit thiol peroxidase antioxidant activities for the reduction of H2O2 by a benzenethiol co-reductant monitored by UV-visible spectroscopy. Next, the Se···N bonded spiroselenuranes have been explored as catalysts in synthetic oxidation iodolactonization and bromination of arenes. The synthesized spiroselenurane has activated I2 toward the iodolactonization of alkenoic acids under base-free conditions. Similarly, efficient chemo- and regioselective monobromination of various arenes with NBS catalyzed by chalcogen-bonded synthesized spiroselenuranes has been achieved. Mechanistic insight into the spiroselenuranes in oxidation reactions has been gained by 77Se NMR, mass spectrometry, UV-visible spectroscopy, single-crystal X-ray structure, and theoretical (DFT, NBO, and AIM) studies. It seems that the highly electrophilic nature of the selenium center is attributed to the presence of an intramolecular Se···N interaction and a vacant coordination site in spiroselenuranes is crucial for the activation of H2O2, I2, and NBS. The reaction of H2O2, I2, and NBS with tetravalent spiroselenurane would lead to an octahedral-Se(VI) intermediate, which is reduced back to Se(IV) due to thermodynamic instability of selenium in its highest oxidation state and the presence of a strong intramolecular N-donor atom.

Surface Anchoring and Active Sites of [Mo3S13]2- Clusters as Co-Catalysts for Photocatalytic Hydrogen Evolution.

Achieving light-driven splitting of water with high efficiency remains a challenging task on the way to solar fuel exploration. In this work, to combine the advantages of heterogeneous and homogeneous photosystems, we covalently anchor noble-metal- and carbon-free thiomolybdate [Mo3S13]2– clusters onto photoactive metal oxide supports to act as molecular co-catalysts for photocatalytic water splitting. We demonstrate that strong and surface-limited binding of the [Mo3S13]2– to the oxide surfaces takes place. The attachment involves the loss of the majority of the terminal S22– groups, upon which Mo–O–Ti bonds with the hydroxylated TiO2 surface are established. The heterogenized [Mo3S13]2– clusters are active and stable co-catalysts for the light-driven hydrogen evolution reaction (HER) with performance close to the level of the benchmark Pt. Optimal HER rates are achieved for 2 wt % cluster loadings, which we relate to the accessibility of the TiO2 surface required for efficient hole scavenging. We further elucidate the active HER sites by applying thermal post-treatments in air and N2. Our data demonstrate the importance of the trinuclear core of the [Mo3S13]2– cluster and suggest bridging S22– and vacant coordination sites at the Mo centers as likely HER active sites. This work provides a prime example for the successful heterogenization of an inorganic molecular cluster as a co-catalyst for light-driven HER and gives the incentive to explore other thio(oxo)metalates.

A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation.

Formed via aqueous carbonation of Mg2+ ions, the crystallization of magnesite (MgCO3) is a promising route to carbon capture and reuse, albeit limited by the slow precipitation of MgCO3. Although magnesite is naturally abundant, forming at low temperature conditions, its industrial production is an energy-intensive process due to the temperatures required to prevent the formation of hydrated phases. The principal difficulty in aqueous conditions arises from the very strong Mg2+···H2O interaction, with high barriers to Mg2+ dehydration. Using atomistic simulations, we have investigated the influence of 30 additive anions (Xn–, n = 1–3), ranging from simple halides to more complex molecules, on the first two steps of MgCO3 aggregation from solution, as follows: Mg2+ dehydration and subsequent prenucleative Mg2+···CO32– pairing. We have computed the thermodynamic stabilities of solvent shared ion pairs (Mg2+···H2O···Xn–) and contact ion pairs (Mg2+···Xn–) to reveal the propensity of solution additives to inhibit or promote Mg2+···CO32– formation. We have determined the stabilization of undercoordinated hydrated Mg2+ states with a vacant coordination site to which CO32– can bind, subsequently initiating MgCO3 nucleation or Mg2+ incorporation into the crystal lattice. Extensive molecular dynamics simulations of electrolyte solutions containing Na2CO3 with different sources of Mg2+ (i.e., MgCl2, MgSO4, and Mg(CH3COO)2) further show that the degree of dehydration of Mg2+ and the structure of prenucleation MgCO3 clusters change depending on the counterion identity. Through a fundamental understanding of the role of solution additives in the mechanism of Mg2+ dehydration, our results help to rationalize previously reported experimental observation of the effect of solvation environments on the growth of magnesite. This understanding may contribute to identifying the solution composition and conditions that could promote the low-temperature CO2 conversion into MgCO3 at industrially relevant scales.

First example of cationic titanium (III) complexes with crown ether as catalysts for ethylene polymerization

Complex cations with a vacant coordination site are considered as the most plausible catalytically active centers of metallocene and post-metallocene polymerization catalysts. For the first time we demonstrated that cationic titanium (III) complexes stabilized with crown ether can be used as pre-catalysts for ethylene polymerization. In the presence of alkyl aluminum chlorides and MgBu2 as cocatalysts, these complexes catalyze ethylene polymerization to produce Ultra High Molecular Weight Polyethylene (UHMWPE) with productivity up to 4650 kgPE/mol Ti h⋅atm and molecular weight up to 1.8·106 Da. UHMWPE powders were processed by the solid-phase method with subsequent orientation drawing into high-strength (1.4–2.1 GPa) and high-modulus (91–118 GPa) films.

3d Metal Complexes in T-shaped Geometry as a Gateway to Metalloradical Reactivity.

ConspectusLow-valent, low-coordinate 3d metal complexes represent a class of extraordinarily reactive compounds that can act as reagents and catalysts for challenging bond-activation reactions. The pursuit of these electron-deficient metal complexes in low oxidation states demands ancillary ligands capable of providing not only energetic stabilization but also sufficiently high steric bulk at the metal center. From this perspective, pincer ligands are particularly advantageous, as their prearranged, meridional coordination mode scaffolds the active center while the substituents of the peripheral donor atoms provide effective steric shielding for the coordination sphere. In a T-shaped geometry, the transition metal complexes possess a precisely defined vacant coordination site, which, combined with the often observed high-spin electron configuration, exhibits unusually high selectivity of these compounds with respect to one-electron redox chemistry. In light of the intractable reaction pathways typically observed with related electronically unsaturated 3d transition metal complexes, the pincer coordination mode enables the isolation of low-valent compounds with more controlled and unique reactivity. We have thus investigated a series of T-shaped metal(I) complexes using three different types of pincer ligands, which may be regarded as "metalloradicals" due to their selectively exposed unpaired electrons.These compounds display remarkably high thermal stability and represent rarely observed "naked" monovalent metal species featuring both monomeric and dimeric structures. Extensive reactivity studies using various organic substrates highlight a strong tendency of these paramagnetic compounds to undergo one-electron oxidation, leading to the isolation of a plethora of metal(II) species with reduced organic ligands as unusual structural elements. The exploration of C2 symmetric T-shaped Ni(I) complexes as asymmetric catalysts also shows success in enantioselective hydrodehalogenation of geminal dihalogenides. In addition, this specific class of low-valent, low-coordinate complexes can be further diversified by introducing redox-active pincer ligands or building homobimetallic systems with two T-shaped units.This Account focuses on the discussion of selected examples of iron, cobalt, and nickel pincer complexes bearing a [P,N,P] or [N,N,N] donor set; however, their electronic structure and radical-type reactivity can be broadly extended to other pincer systems. The availability of various types of pincer ligands should allow fine-tuning of the reactivity of the T-shaped complexes. Given the unprecedented reactivity observed with these compounds, we expect the studies of T-shaped 3d metal complexes to be a fertile field for advancing base metal catalysis.

Subcomponent self-assembly of circular helical Dy6(L)6 and bipyramid Dy12(L)8 architectures directed via second-order template effects.

In situ metal-templated (hydrazone) condensation also called subcomponent self-assembly of 4,6-dihydrazino-pyrimidine, o-vanillin and dysprosium ions resulted in the formation of discrete hexa- or dodecanuclear metallosupramolecular Dy6(L)6 or Dy12(L)8 aggregates resulting from second-order template effects of the base and the lanthanide counterions used in these processes. XRD analysis revealed unique circular helical or tetragonal bipyramid architectures in which the bis(hydrazone) ligand L adopts different conformations and shows remarkable differences in its mode of metal coordination. While a molecule of trimethylamine acts as a secondary template that fills the void of the Dy6(L)6 assembly, sodium ions take on this role for the formation of heterobimetallic Dy12(L)8 by occupying vacant coordination sites, thus demonstrating that these processes can be steered in different directions upon subtle changes of reaction conditions. Furthermore, Dy6(L)6 shows an interesting spin-relaxation energy barrier of 435 K, which is amongst the largest values within multinuclear lanthanide single-molecular magnets.