In this work, we developed a new catalytic strategy to synthesise an industrially relevant, fully renewable cyclic carbonate: 2,5-bis-dicarboxyl furan cyclic carbonate (BDFCC). This compound contains two terminal cyclic carbonate groups, which makes it attractive for application as a building block for green polymers. BDFCC was prepared from two renewable compounds: the methyl ester of 2,5-furandicarboxylic acid and glycerol carbonate, via a transesterification reaction at mild temperature (80 degrees C) using a novel, metal-free dual catalytic system consisting of commercially available ion-exchange resin beads (Amberlite IRA-900-Cl) and glycidol. Under conditions previously reported for homogeneous catalysis, involving n-hexane as solvent, only a moderate yield of BDFCC was achieved (42%). An improvement in both performance and sustainability of the process was achieved by implementing an equilibrium-shifting solvent-free strategy, which overcomes the thermodynamic limit of the reaction (Delta rG degrees = +25 kJ mol-1 based on DFT calculations). This approach, combined with an optimisation of the reaction conditions, allowed increasing the BDFCC yield to 86%. Furthermore, the heterogeneous resin bead catalyst was recyclable over multiple runs without experiencing loss of activity. Control experiments allowed to gain insight in the reaction mechanism and revealed the crucial role played by glycidol in promoting the catalytic activity of the Amberlite resin beads.

As a broad-scale energy storage solution, redox flow batteries (RFBs) offer high efficiency and tunable design. However, conventional RFBs rely on transition-metal ion couples, (e.g., vanadium or iron), whose implementation is limited by low energy densities, high cost, and environmental leaching. Main-group compounds, comprising earth-abundant, p-block elements, represent highly promising, yet underexplored candidates for RFBs. Herein, we evaluate three boron-formazanate complexes as negolyte and symmetric electrolytes in nonaqueous organic redox flow batteries (NAORFBs). Detailed electrochemical characterization of these complexes reveals two sequential reduction processes with the first being exceptionally stable (<3% capacity fade after charge/discharge cycling in a static H-cell for 3 days). In contrast, cycling that includes the two-electron reduced state results in rapid degradation (>59% capacity fade over 2.5 days in a static H-cell), most likely due to fluoride elimination from the BF2 moiety. Guided by these insights, a B(Ph)2 unit was introduced to mitigate this degradation pathway. The elimination of labile B─F bonds as well as steric protection conferred by two phenyl groups led to improved cycling performance (>85% capacity retention after charge/discharge cycling in a flow battery for 15 days). These findings guide the rational design of inexpensive main-group electrolytes for application in energy storage.

Re(I) tricarbonyl complexes of the type fac-[ReX(CO)3(L)]n (n = -1, 0, +1) (X = Br-, MeCN), furnished with the redox-active formazan (L = Ph-N(R)-N═CH-N═N-Ph; R = H, (H5Br); R = Me, (Me5X)) or formazanate (L = [Ph-N═N-C(-Ph-4-R1)═N-N-Ph]-; R1 = H [1Br]-, Me [2Br]-, MeO [3Br]-, F [4X]-; L = [Ph-N═N-C(-H)═N-N-Ph]-, Py = pyridine 5Py) ligands were prepared and characterized by spectroscopy and electrochemistry. In situ characterization of the reduced species by (spectroelectro)chemical and computational methods revealed that the redox-active scaffold behaves as a two-electron sink, allowing two consecutive one-electron reductions to take place at the ligand. The reactivity of the reduced formazan(ate) rhenium complexes toward CO2 was explored. (Spectroelectro)chemical experiments along with DFT calculations suggested that CO2 reacts with the reduced formazanate Re(I) complexes at low overpotentials forming a carbamate-type adduct. This ligand-based reactivity provides a thermodynamic sink for CO2 binding and hinders catalytic turnover via metal-centered CO2 activation. These findings provide new insights into the advantages and limitations of using catalysts with redox-active ligands to activate and convert small molecules.

Enzymes are attractive catalysts due to their high chemo-, regio-, and enantioselectivity. In recent years, the application of enzymes in organic synthesis has expanded dramatically, especially for the synthesis of chiral alcohols and amines, two very important functional groups found in many active pharmaceutical ingredients (APIs). Indeed, many elegant routes employing such compounds have been described by industry. Yet, for the synthesis of chiral thiols and thioethers, likewise found in APIs albeit less ubiquitous, only very few biocatalytic syntheses have been reported, and stereocontrol has proved challenging. Here, we apply ene-reductases (EREDs), whose ability to initiate and control chemically challenging radical chemistries has recently emerged, to the synthesis of chiral thioethers from α-bromoacetophenones and pro-chiral vinyl sulfides, without requiring light. Depending on the choice of ERED either enantiomer of the product could be accessed. The highest conversion and selectivity were achieved with GluER T36A using fluorinated substrates, reaching up to 82% conversion and >99.5% ee. With α-bromoacetophenone and α-(methylthio)styrene, the reaction could be performed on a 100 mg scale, affording the product in a 46% isolated yield with a 93% ee. Finally, mechanistic studies were carried out using stopped-flow spectroscopy and protein mass spectrometry, providing insight into the preference of the enzyme for the intermolecular reaction. This work paves the way for new routes for the synthesis of thioether-containing compounds.

The use of organic active materials in redox flow batteries (RFBs) presents a promising approach to sustainable large-scale energy storage. However, the stability of nonaqueous organic RFB electrolytes is generally limited by degradation reactions that cause capacity fade. These reactions are commonly thought to convert redox-active organics to products that are no longer electrochemically active. Here we uncover an additional pathway leading to capacity fade that involves the supporting electrolyte salt. Capacity fade in nonaqueous RFBs is studied in detail for the 1,2,4-benzotriazin-4-yl radical (1) as a model compound and extended to several other classes of representative redox-active organics. By using symmetrical batteries (1 0/- ∥1 0/+ ), we delineate that capacity fade occurs in a nonlinear (autocatalytic) fashion via acid-induced decomposition of the supporting salt anion PF6 - in the posolyte solution. This is shown to be a universal degradation reaction in the posolyte of nonaqueous RFBs. Although the acidic degradation products are not detrimental to the posolyte, the crossover of acid to the opposite compartment leads to capacity-limiting protonation of the negolyte active material. Replacement of PF6 - with other anions substantially improves the stability of these nonaqueous electrolytes, as demonstrated with a symmetrical RFB based on 0.38 M active material 1 that can be cycled for >69 days with very high capacity retention (fade rate of ≤0.1% per day). The improved understanding of factors determining the lifetime of nonaqueous electrolytes unlocks rational strategies to develop more durable electrochemical energy storage systems.

The influence of the spin state of the metal centre in spin crossover compounds on the aromaticity of the ligands has been investigated for iron(ii)tris-bipyridine (Fe(bpy)32+), and Fe(ii)(formazanate)2 (as a truncated model and the full phenyl substituted compound). It was found that the aromaticity of the bipyridine ligands is unaffected by changing the spin state of the central iron atom, but that of the formazanate ligands is reduced upon transition to the high-spin state. This change in aromaticity is rationalized using the symmetry selection rules for aromaticity in terms of virtual excitations from occupied to empty orbitals. A further consequence of this loss in aromaticity is a shift to higher energy in the ring vibrations of the formazanate compounds that can be observed in either its IR or Raman spectrum; this prediction has been confirmed here. This change in aromaticity as a consequence of change in spin state can be regarded as an indication for non-innocent ligands. Aromaticity of non-innocent ligands is affected when the spin state changes.

Pd-catalyzed C-H functionalization reactions of non-directed substrates have recently emerged as an attractive alternative to the use of directing groups. Key to the success of these transformations has been the discovery of new ligands capable of increasing both the reactivity of the inert C-H bond and the selectivity of the process. Among them, a new type of S,O-ligand has shown to be highly efficient in promoting a variety of Pd-catalyzed C-H olefination reactions of non-directed arenes. Despite the success of this type of S,O-ligand, its role in the C-H functionalization processes is unknown. Herein, we describe a detailed mechanistic study focused on elucidating the role of the S,O-ligand in the Pd-catalyzed C-H olefination of non-directed arenes. For this purpose, several mechanistic tools, including isolation and characterization of reactive intermediates, NMR and kinetic studies, isotope effects and DFT calculations have been employed. The data from these experiments suggest that the C-H activation is the rate-determining step in both cases with and without the S,O-ligand. Furthermore, the results indicate that the S,O-ligand triggers the formation of more reactive Pd cationic species, which explains the observed acceleration of the reaction. Together, these studies shed light on the role of the S,O-ligand in promoting Pd-catalyzed C-H functionalization reactions

Redox flow batteries based on organic electrolytes are promising energy storage devices, but stable long-term cycling is often difficult to achieve. Bipolar organic charge-storage materials allow the construction of symmetrical flow batteries (i. e., with identical electrolyte composition on both sides), which is a strategy to mitigate crossover-induced degradation. One such class of bipolar compounds are verdazyl radicals, but little is known on their stability/reactivity either as the neutral radical, or in the charged states. Here, we study the chemical properties of a Kuhn-type verdazyl radical (1) and the oxidized/reduced form (1(+/-)). Chemical synthesis of the three redox-states provides spectroscopic characterization data, which are used as reference for evaluating the composition of the electrolyte solutions of an H-cell battery during/after cycling. Our data suggest that, rather than the charged states, the decomposition of the parent verdazyl radical is responsible for capacity fade. Kinetic experiments and DFT calculations provide insight in the decomposition mechanism, which is shown to occur by bimolecular disproportionation to form two closed-shell products (leuco-verdazyl 1H and triazole derivative 2).

Redox-active organic molecules are promisingcharge-storage materials for redox-flow batteries (RFBs), butmaterial crossover between the posolyte and negolyte and chemicaldegradation are limiting factors in the performance of all-organicRFBs. We demonstrate that the bipolar electrochemistry of 1,2,4-benzotriazin-4-yl (Blatter) radicals allows the construction ofbatteries with symmetrical electrolyte composition. Cyclicvoltammetry shows that these radicals also retain reversible bipolarelectrochemistry in the presence of water. The redox potentials ofderivatives with a C(3)-CF3substituent are the least affected bywater, and moreover, these compounds show >90% capacityretention after charge/discharge cycling in a static H-cell for 7 days(ca. 100 cycles). Testing these materials in aflow regime at a 0.1 Mconcentration of the active material confirmed the high cycling stability under conditions relevant for RFB operation anddemonstrated that polarity inversion in a symmetricalflow battery may be used to rebalance the cell. Chemical synthesis providesinsight in the nature of the charged species by spectroscopy and (for the oxidized state) X-ray crystallography. The stability of thesecompounds in all three states of charge highlights their potential for application in symmetrical organic redox-flow batteries.

Redox-switching of a formazanate zinc catalyst in ring-opening polymerization (ROP) of lactide is described. Using a redox-active ligand bound to an inert metal ion (Zn2+) allows modulation of the catalytic activity by reversible reduction/oxidation chemistry at a purely organic fragment. A combination of kinetic and spectroscopic studies, together with mass spectrometry of the catalysis mixture, provides insight in the nature of the active species and the initiation of lactide ring-opening polymerization. The mechanistic data highlight the key role of the redox-active ligand and provide a rationale for the formation of cyclic polymer.

. Abstract: The selective α-deuteration of α,β-unsaturated nitriles using the strong base t BuOK or a metal-ligand cooperative Ru pincer catalyst is described. With D 2 O as deuterium source and glyme as solvent at 70 ° C, t BuOK is an efficient catalyst for deuteration at the α-C( sp 2 ) position of cinnamonitriles, providing access to a broad range of deuterated derivatives in good to excellent yields and with very high levels of deuterium incorporation. While the t BuOK-catalysed protocol does not tolerate base-sensitive functional groups, cinnamonitrile derivatives containing a benzylic bromide or ester moiety were deuterated in excellent yields using Milstein’s ruthenium PNN pincer catalyst. Moreover, the activity for H/D exchange of the metal-ligand cooperative Ru catalyst is found to be significantly higher than that of t BuOK, allowing reactions to proceed well even at room temperature. A mechanistic proposal is put forward that involves deprotonation of the cinnamonitrile α-CH position when using t BuOK as catalyst, whereas H/D exchange catalysis with the Ru PNN pincer likely proceeds via (reversible) oxa-Michael addition of D 2 O.

Metal complexes with ligands that coordinate via the nitrogen atom of azo (N═N) or imino (C═N) groups are of interest due to their π-acceptor properties and redox-active nature, which leads to interesting (opto)electronic properties and reactivity. Here, we describe the synthesis and characterization of rhenium(I) tricarbonyl complexes with neutral N,N-bidentate formazans, which possess both N═N and C═N fragments within the ligand backbone (Ar1-NH-N═C(R3)-N═N-Ar5). The compounds were synthesized by reacting equimolar amounts of [ReBr(CO)5] and the corresponding neutral formazan. X-ray crystallographic and spectroscopic (IR, NMR) characterization confirmed the generation of formazan-type species with the structure fac-[ReBr(CO)3(κ2-N2,N4(Ar1-N1H-N2═C(R3)-N3═N4-Ar5))]. The formazan ligand coordinates the metal center in the 'open' form, generating a five-membered chelate ring with a pendant NH arm. The electronic absorption and emission properties of these complexes are governed by the presence of low-lying π*-orbitals on the ligand as shown by DFT calculations. The high orbital mixing between the metal and ligand results in photophysical properties that contrast to those observed in fac-[ReBr(CO)3(L,L)] species with α-diimine ligands.

A multistable molecular switching system based on an anthracene-extended bis-thiaxanthylidene with three individually addressable states that can be interconverted by electrochemical, thermal, and photochemical reactions is reported. Besides reversible switching between an open-shell diradical- and a closed-shell electronic configuration, our findings include a third dicationic state and control by multiple actuators. This dicationic state with an orthogonal conformation can be switched electrochemically with the neutral open-shell triplet state with orthogonal conformation, which was characterized by EPR. The remarkably stable diradical shows kinetic stability as a result of a significant activation barrier for isomerization to a more stable neutral closed-shell folded geometry. We ascribe this activation barrier of ΔG⧧(293 K) = 25.7 kcal mol-1 to steric hindrance in the fjord region of the overcrowded alkene structure. The folded closed-shell state can be converted back to the diradical state by irradiation with 385 nm. The folded state can also be oxidized to the dicationic state. These types of molecules with multiple switchable states and in particular stable diradicals show great potential in the design of new functional materials such as memory devices, logic gates, and OFETs.

Bis(formazanate)iron(II) complexes undergo a thermally induced S = 0 to S = 2 spin transition in solution. Here we present a study of how steric effects and π-stacking interactions between the triarylformazanate ligands affect the spin-crossover behavior, in addition to electronic substituent effects. Moreover, the effect of increasing the denticity of the formazanate ligands is explored by including additional OMe donors in the ligand (7). In total, six new compounds (2–7) have been synthesized and characterized, both in solution and in the solid state, via spectroscopic, magnetic, and structural analyses. The series spans a broad range of spin-crossover temperatures (T1/2) for the LS ⇌ HS equilibrium in solution, with the exception of compound 6 which remains high-spin (S = 2) down to 210 K. In the solid state, 6 was shown to exist in two distinct forms: a tetrahedral high-spin complex (6a, S = 2) and a rare square-planar structure with an intermediate-spin state (6b, S = 1). SQUID measurements, 57Fe Mössbauer spectroscopy, and differential scanning calorimetry indicate that in the solid state the square-planar form 6b undergoes an incomplete spin-change-coupled isomerization to tetrahedral 6a. The complex that contains additional OMe donors (7) results in a six-coordinate (NNO)2Fe coordination geometry, which shifts the spin-crossover to significantly higher temperatures (T1/2 = 444 K). The available experimental and computational data for 7 suggest that the Fe···OMe interaction is retained upon spin-crossover. Despite the difference in coordination environment, the weak OMe donors do not significantly alter the electronic structure or ligand-field splitting, and the occurrence of spin-crossover (similar to the compounds lacking the OMe groups) originates from a large degree of metal–ligand π-covalency.

A series of heteroleptic three-coordinate mono(formazanate)zinc methyl complexes were synthesized, and the influence of the ligand on the structure as well as redox and optical properties of these complexes was investigated. The heteroleptic mono(formazanate)zinc methyl complexes were found to show ligand redistribution in solution, reminiscent of the Schlenk equilibrium, to generate an equilibrium mixture containing the corresponding homoleptic complexes as well. Monitoring the approach to equilibrium by NMR spectroscopy in benzene-d(6) allowed determination of the forward and backward rate constants. A correlation was found between the steric environment around the zinc center and equilibrium concentration of (formazanate)zinc methyl compounds, whereas the kinetics for approach to equilibrium are also dependent on the electronic properties.

Here we report that chiral Mn(I) complexes are capable of H-P bond activation. This activation mode enables a general method for the hydrophosphination of internal and terminal α,β-unsaturated nitriles. Metal-ligand cooperation, a strategy previously not considered for catalytic H-P bond activation, is at the base of the mechanistic action of the Mn(I)-based catalyst. Our computational studies support a stepwise mechanism for the hydrophosphination and provide insight into the origin of the enantioselectivity.

The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (ΔO) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T1/2 above room temperature (300-368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the π-acceptor properties of the formazanate ligand, resulting in an "inverted" ligand field, with an approximate "two-over-three" splitting of the d-orbitals and a high degree of metal-ligand covalency due to metal → ligand π-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C-Ar or N-Ar rings, which is ascribed to the opposing effect on metal-ligand σ- and π-bonding.