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NiII cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) is an efficient catalyst for the selective reduction of CO2 to CO. A crucial elementary step in the proposed catalytic cycle is the coordination of CO2 to a NiI cyclam intermediate. Isolation and spectroscopic characterization of this labile NiI species without solvent has proven to be challenging, however, and only partial IR spectra have previously been reported using multiple photon fragmentation of ions generated by gas-phase electron transfer to the NiII cyclam dication at 300 K. Here, we report a chemical reduction method that efficiently prepares NiI cyclam in solution. This enables the NiI complex to be transferred into a cryogenic photofragmentation mass spectrometer using inert-gas-mediated electrospray ionization. The vibrational spectra of the 30 K ion using both H2 and N2 messenger tagging over the range 800-4000 cm-1 were then measured. The resulting spectra were analyzed with the aid of electronic structure calculations, which show strong method dependence in predicted band positions and small molecule activation. The conformational changes of the cyclam ligand induced by binding of the open shell NiI cation were compared with those caused by the spherical, closed-shell LiI cation, which has a similar ionic radius. We also report the vibrational spectrum of a NiI cyclam complex with a strongly bound O2 ligand. The cyclam ligand supporting this species exhibits a large conformational change compared to the complexes with weakly bound N2 and H2, which is likely due to significant charge transfer from Ni to the coordinated O2.

The kinetics of hydride transfer from Re(

The synthesis of triarylmethanes via Pd-catalyzed Suzuki-Miyaura reactions between diarylmethyl 2,3,4,5,6-pentafluorobenzoates and aryl boronic acids is described. The system operates at mild conditions and has a broad substrate scope, including the coupling of diphenylmethanol derivatives that do not contain extended aromatic substituents. This is significant as these substrates, which result in the types of triarylmethane products that are prevalent in pharmaceuticals, have not previously been compatible with systems for diarylmethyl ester coupling. Further, the reaction can be performed stereospecifically to generate stereo-inverted products. On the basis of DFT calculations, it is proposed that the oxidative addition of the diarylmethyl 2,3,4,5,6-pentafluorobenzoate substrate occurs via an S(N)2 pathway, which results in the inverted products. Mechanistic studies indicate that oxidative addition of the diarylmethyl 2,3,4,5,6-pentafluorobenzoate substrates to (IPr)Pd(0) results in the selective cleavage of the O–C(benzyl) bond in part because of a stabilizing η(3)-interaction between the benzyl ligand and Pd. This is in contrast to previously described Pd-catalyzed Suzuki-Miyaura reactions involving phenyl esters, which involve selective cleavage of the C(acyl)–O bond, because there is no stabilizing η(3)-interaction. It is anticipated that this fundamental knowledge will aid the development of new catalytic systems, which use esters as electrophiles in cross-coupling reactions.

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (RPBP)M(CH3) (RPBP = B(NCH2PR2)2C6H4−; R = Cy or tBu; M = Ni or Pd) to generate κ1-acetate complexes of the form (RPBP)M{OC(O)CH3} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual RPBP ligand, which features a central boryl donor that exerts a strong trans-influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into (RPBP)M(CH3) is facile and occurs at room temperature because of the high trans-influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using (tBuPBP)Pd(CH3). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into (tBuPBP)M(CH3) (M = Ni or Pd) proceeds via an SE2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.

The syntheses of four new tunable homogeneous organic reductants based on a tetraaminoethylene scaffold are reported. The new reductants have enhanced air-stability compared to current homogeneous reductants for metal mediated reductive transformations, such as cross-electrophile coupling (XEC), and are solids at room temperature. In particular, the weakest reductant is indefinitely stable in air and has a reduction potential of −0.85 V versus ferrocene, which is significantly milder than conventional reductants used in XEC. All of the new reductants are able to facilitate C(sp(2))–C(sp(3)) Ni-catalyzed XEC reactions and are compatible with complex substrates that are relevant to medicinal chemistry. The reductants span a range of nearly 0.5 V in reduction potential, which allows for control over the rate of electron transfer events in XEC. Specifically, we report a new strategy for controlled alkyl radical generation in Ni-catalyzed C(sp(2))–C(sp(3)) XEC. The key to our approach is to tune the rate of alkyl radical generation from Katritzky salts, which liberate alkyl radicals upon single electron reduction, by varying the redox potentials of the reductant and Katritzky salt utilized in catalysis. Using our method, we perform XEC reactions between benzylic Katritzky salts and aryl halides. The method tolerates a variety of functional groups, some of which are particularly challenging for most XEC transformations. Overall, we expect that our new reductants will both replace conventional homogeneous reductants in current reductive transformations due to their stability and relatively facile synthesis, and lead to the development of novel synthetic methods due to their tunability.

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (

Ni(II) cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) is an efficient catalyst for the selective reduction of CO(2) to CO. A crucial elementary step in the proposed catalytic cycle is the coordination of CO(2) to a Ni(I) cyclam intermediate. Isolation and spectroscopic characterization of this labile Ni(I) species without solvent has proven to be challenging, however, and only partial IR spectra have previously been reported using multiple photon fragmentation of ions generated by gas phase electron transfer to the Ni(II) cyclam dication at 300 K. Here, we report a chemical reduction method that efficiently prepares Ni(I) cyclam in solution. This enables the Ni(I) complex to be transferred into a cryogenic photofragmentation mass spectrometer using inert gas-mediated electrospray ionization. The vibrational spectra of the 30 K ion using both H(2) and N(2) messenger tagging over the range 800 – 4000 cm(−1) were then measured. The resulting spectra were analyzed with the aid of electronic structure calculations, which show strong method dependence in predicted band positions and small molecule activation. The conformational changes of the cyclam ligand induced by binding of the open shell Ni(I) cation were compared with those caused by the spherical, closed-shell Li(I) cation, which has a similar ionic radius. We also report the vibrational spectrum of a Ni(I) cyclam complex with a strongly bound O(2) ligand. The cyclam ligand supporting this species exhibits a large conformational change compared to the complexes with weakly bound N(2) and H(2), which is likely due to significant charge transfer from Ni to the coordinated O(2).

The reversible activation of H2 via a pathway involving metal–ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Ne...

Aryl carbonates, a common protecting group in synthetic organic chemistry, are potentially valuable electrophiles in cross-coupling reactions. Here, after performing a thorough evaluation of different precatalysts, we demonstrate that (dcypf)Ni(2-ethylphenyl)(Br) (dcypf = 1,1-bis-(dicyclohexylphosphino)ferrocene) is an efficient precatalyst for Suzuki-Miyaura reactions using a variety of aryl carbonates as substrates. Mechanistic studies indicate that (dcypf)Ni(2-ethylphenyl)(Br), which contains a bidentate phosphine that binds in a trans geometry, is an effective precatalyst for these reactions for two reasons: (i) it rapidly forms the Ni(O) active species and (ii) it minimizes comproportionation reactions between the Ni(O) active species and both the unactivated Ni(II) precatalyst and on-cycle Ni(II) complexes to form catalytically inactive Ni(I) species. In contrast, the state of the art precatalyst (dppf)Ni(o-tolyl)(Cl) (dppf = 1,1-bis(diphenylphosphino)ferrocene), which contains a bidentate phosphine that binds in a cis geometry, forms Ni(I) species during activation and is essentially inactive for aryl carbonate couplings. Although the exact reasons on a molecular level why the dcypf system is more active than the dppf system are unclear, our results indicate that in general Ni catalysts supported by the dcypf ligand will give better performance for catalytic reactions involving substrates which undergo relatively slow oxidative addition, such as aryl carbonates.

Methanol is a crucial precursor for the preparation of many high-value commercial chemicals and synthesizing it from an inexpensive and renewable carbon source such as carbon dioxide (CO2) could ha...

The environmental and societal consequences of the increasing levels of carbon dioxide in our atmosphere are among the most significant challenges society currently faces. Carbon dioxide utilizatio...

Transition-metal-catalyzed transformations of amino acids and peptides could provide a powerful method for their site-selective modification. Here, we report non-decarbonylative Pd-catalyzed Suzuki-Miyaura reactions of phenyl ester derivatives of aspartic acid to form aryl-amino ketones. These products are potentially important in the synthesis of pharmaceuticals, and our methodology represents a new route to access molecules of this type.

The insertion of CO2 into a metal hydride bond to form a metal formate is a key elementary step in many catalytic cycles for CO2 conversion. Similarly, the microscopic reverse reaction, the decarboxylation of a metal formate to form a metal hydride and CO2, is important in both organic synthesis and strategies for hydrogen storage using organic liquids. There are however few experimental studies probing the mechanism of these reactions and identifying the effects of specific variables such as Lewis acid (LA) additives or solvent, which have been shown to significantly impact catalytic performance. In this study, we use a rapid mixing stopped-flow instrument to study the kinetics of CO2 insertion into the cationic ruthenium hydride [Ru(tpy)bpy)H]PF6 (tpy = 2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine) in various solvents, both in the presence and in the absence of a LA. We show that LAs can increase the observed rate of this reaction and determine the first quantitative trends for the rate enhancement observed for CO2 insertion in the presence of cationic LAs, Li+ ≫ Na+ > K+ > Rb+. Furthermore, we show that the rate enhancement observed with LAs is solvent dependent. Specifically, as the acceptor number (AN) of the solvent increases, the effect of the LA becomes smaller. Last, we demonstrate that there is a significant solvent effect on CO2 insertion in the absence of a LA. Although the AN of the solvent has been previously used to predict the rate of CO2 insertion, this work shows that the best model for the rate of insertion is based on the Dimroth-Reichardt ET(30) value of the solvent, a parameter that better accounts for specific solute/solvent interactions.

A number of new transition metal catalyzed methods for the formation of C(sp2 )-C(sp3 ) bonds have recently been described. These reactions often utilize bidentate polypyridyl-ligated Ni catalysts, and paramagnetic NiI halide or aryl species are proposed in the catalytic cycles. However, there is little knowledge about complexes of this type. Here, we report the synthesis of paramagnetic bidentate polypyridyl-ligated Ni halide and aryl complexes through elementary reactions proposed in catalytic cycles for C(sp2 )-C(sp3 ) bond formation. We investigate the ability of these complexes to undergo organometallic reactions that are relevant to C(sp2 )-C(sp3 ) coupling through stoichiometric studies and also explore their catalytic activity.

The nickel-catalyzed carboxylation of organic halides or pseudohalides using carbon dioxide is an emerging method to prepare synthetically valuable carboxylic acids. Here, we report a detailed mechanistic investigation of these reactions using the carboxylation of aryl halides with (PPh(3))(2)Ni(II)Cl(2) as a model reaction. Our studies allow us to understand several general features of nickel-catalyzed carboxylation reactions. For example, we demonstrate that both a Lewis acid and halide source are beneficial for catalysis. To this end, we establish that heterogeneous Mn(0) and Zn(0) reductants are multifaceted reagents that generate noninnocent Mn(II) or Zn(II) Lewis acids upon oxidation. In a key result, a rare example of a well-defined nickel(I) aryl complex is isolated, and it is demonstrated that its reaction with carbon dioxide results in the formation of a carboxylic acid in high yield (after workup). The carbon dioxide insertion product undergoes rapid decomposition, which ca These three oxidation states correspond to the onbe circumvented by a ligand metathesis reaction with a halide source. Our studies have led to both a revised mechanism and the development of a broadly applicable strategy to improve reductive carboxylation reactions. A critical component of this strategy is that we have replaced the heterogeneous Mn(0) reductant typically used in catalysis with a well-defined homogeneous organic reductant. Through its use, we have increased the range of ancillary ligands, additives, and substrates that are compatible with the reaction. This has enabled us to perform reductive carboxylations at low catalyst loadings. Additionally, we demonstrate that reductive carboxylations of organic (pseudo)halides can be achieved in high yields in more practically useful, non-amide solvents. Our results describe a mechanistically guided strategy to improve reductive carboxylations through the use of a homogeneous organic reductant, which may be broadly translatable to a wide range of cross-electrophile coupling reactions.

A dual catalytic system for cross-electrophile coupling reactions between aryl halides and alkyl halides that features a Ni catalyst, a Co cocatalyst, and a mild homogeneous reductant is described. Mechanistic studies indicate that the Ni catalyst activates the aryl halide, while the Co cocatalyst activates the alkyl halide. This allows the system to be rationally optimized for a variety of substrate classes by simply modifying the loadings of the Ni and Co catalysts based on the reaction product profile. For example, the coupling of aryl bromides and aryl iodides with alkyl bromides, alkyl iodides, and benzyl chlorides is demonstrated using the same Ni and Co catalysts under similar reaction conditions but with different optimal catalyst loadings in each case. Our system is tolerant of numerous functional groups and is capable of coupling heteroaryl halides, di-ortho-substituted aryl halides, pharmaceutically relevant druglike aryl halides, and a diverse range of alkyl halides. Additionally, the dual catalytic platform facilitates a series of selective one-pot three-component cross-electrophile coupling reactions of bromo(iodo)arenes with two distinct alkyl halides. This demonstrates the unique level of control that the platform provides and enables the rapid generation of molecular complexity. The system can be readily utilized for a wide range of applications as all reaction components are commercially available, the reaction is scalable, and toxic amide-based solvents are not required. It is anticipated that this strategy, as well as the underlying mechanistic framework, will be generalizable to other cross-electrophile coupling reactions.

The prevalence of transition metal-mediated hydride transfer reactions in chemical synthesis, catalysis, and biology has inspired the development of methods for characterizing the reactivity of transition metal hydride complexes. Thermodynamic hydricity represents the free energy required for heterolytic cleavage of the metal–hydride bond to release a free hydride ion, H−, as determined through equilibrium measurements and thermochemical cycles. Kinetic hydricity represents the rate of hydride transfer from one species to another, as measured through kinetic analysis. This tutorial review describes the common methods for experimental and computational determination of thermodynamic and kinetic hydricity, including advice on best practices and precautions to help avoid pitfalls. The influence of solvation on hydricity is emphasized, including opportunities and challenges arising from comparisons across several different solvents. Connections between thermodynamic and kinetic hydricity are discussed, and opportunities for utilizing these connections to rationally improve catalytic processes involving hydride transfer are highlighted.

A new dual catalytic system for cross-electrophile coupling reactions between aryl and alkyl halides that features a Ni catalyst, a Co co-catalyst, and a mild homogeneous reductant, is described. This is a unique combination of reagents for cross-electrophile coupling reactions, which results in one of the most versatile systems reported to date. For example, the coupling of aryl bromides and aryl iodides with alkyl bromides, alkyl iodides, alkyl mesylates, and benzyl chlorides is demonstrated under similar reaction conditions. The system is tolerant of numerous functional groups and is capable of coupling heteroaryl halides, di-ortho-substituted aryl halides, pharmaceutically relevant drug-like aryl halides, and a diverse range of alkyl halides. Additionally, the dual catalytic platform facilitates a series of novel one-pot three-component cross-electrophile coupling reactions of bromo(iodo)arenes with two distinct alkyl halides. Mechanistic studies indicate that the Ni catalyst activates the aryl halide electrophile, while the Co catalyst activates the alkyl electrophile.

Although the catalytic carboxylation of unactivated alkyl electrophiles has reached remarkable levels of sophistication, the intermediacy of (phenanthroline)Ni(I)–alkyl species—complexes proposed in numerous Ni-catalyzed reductive cross-coupling reactions—has been subject to speculation. Herein we report the synthesis of such elusive (phenanthroline)Ni(I) species and their reactivity with CO2, allowing us to address a long-standing question related to Ni-catalyzed carboxylation reactions.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Organometallics, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.organomet.0c00229. In photosynthesis, carbon dioxide is used as the carbon source; indeed, most carbon atoms in the structure of a massive tree—the trunk, the branches, the leaves—originate from CO2. This insight has profound implications for chemical synthesis: complex molecular structures are built from something as simple and inert as CO2. Scientists have long been fascinated by this concept, and chemists have tried to reproduce it by generating artificial systems for the transformation of CO2 into more valuable products. Over the past two decades, chemical conversion of CO2 has developed into a major research field. Several comprehensive reviews have summarized advances on coupling CO2 with nucleophiles to form carboxylic acids, carbonates, or carbamates or on reducing CO2 to C1 species such as formate and methanol. The reviews of this field also emphasize an important point: chemical utilization of CO2 is not a strategy to mitigate climate change. CO2 is, however, a renewable carbon feedstock that can replace nonrenewable fossil-fuel-based starting materials. Therefore, methods for the efficient conversion of CO2 should be viewed as an integral part in the development of sustainable chemical processes. The 22 articles in this Special Issue highlight the many ways in which organometallic chemistry can help solve challenges related to CO2 utilization: organometallic complexes can be used in thermal, electrochemical, or photochemical conversion of CO2 to various products such as formate, carbon monoxide, carboxylic acids, acrylates, and polycarbonates. Remarkably, the work presented here involves the activation of CO2 with 15 different metals, including the first-row transition metals titanium, manganese, iron, cobalt, nickel, and copper, the second-row elements ruthenium and rhodium, the third-row species rhenium, platinum, and iridium, the actinide uranium, and the main-group metals cesium, magnesium, and aluminum. One may think, there are many roads to Rome; however, an important point is that the various metals have distinct strengths in terms of selectivity and activity for chemical CO2 utilization. The behavior of these different metals in CO2 conversion can further be modulated through introduction of different ligands.

Theory and experiments were used to rationally design co-catalysts for the deaminative hydrogenation of amides. TBD was found to be the optimal catalyst, assisting the C–N bond cleavage while preventing the formation of adducts with the catalyst., The catalytic hydrogenation of amides is an atom economical method to synthesize amines. Previously, it was serendipitously discovered that the combination of a secondary amide co-catalyst with (iPrPNP)Fe(H)(CO) (iPrPNP = N[CH2CH2(PiPr2)]2–), results in a highly active base metal system for deaminative amide hydrogenation. Here, we use DFT to develop an improved co-catalyst for amide hydrogenation. Initially, we computationally evaluated the ability of a series of co-catalysts to accelerate the turnover-limiting proton transfer during C–N bond cleavage and poison the (iPrPNP)Fe(H)(CO) catalyst through a side reaction. TBD (triazabicyclodecene) was identified as the leading co-catalyst. It was experimentally confirmed that when TBD is combined with (iPrPNP)Fe(H)(CO) a remarkably active system for amide hydrogenation is generated. TBD also enhances the activity of other catalysts for amide hydrogenation and our results provide guidelines for the rational design of future co-catalysts.

Surface functionalization with organic electron donors (OEDs) is an effective doping strategy for two-dimensional (2D) materials, which can achieve doping levels beyond those possible with conventional electric field gating. While the effectiveness of surface functionalization has been demonstrated in many 2D systems, the doping efficiencies of OEDs have largely been unmeasured, which is in stark contrast to their precision syntheses and tailored redox potentials. Here, using monolayer MoS2 as a model system and an organic reductant based on 4,4-bipyridine (DMAP-OED) as a strong organic dopant, we establish that the doping efficiency of DMAP-OED to MoS2 is in the range of 0.63 to 1.26 electrons per molecule. We also achieve the highest doping level to date in monolayer MoS2 by surface functionalization and demonstrate that DMAP-OED is a stronger dopant than benzyl viologen, which was the previous best OED dopant. The measured range of the doping efficiency is in good agreement with the values predicted from first-principles calculations. Our work provides a basis for the rational design of OEDs for high-level doping of 2D materials., Comment: 19 pages, 4 figures, 1 table

Although the catalytic carboxylation of unactivated alkyl electrophiles has reached remarkable levels of sophistication, the intermediacy of (phenanthroline)Ni(I)-alkyl species – complexes proposed in numerous Ni-catalyzed reductive cross-coupling reactions – has been subject to speculation. Herein, we report the synthesis of such elusive (phenanthroline)Ni(I) species and their reactivity with CO2, allowing us to address a long-standing challenge related to metal-catalyzed carboxylation reactions.

The factors that influence the selectivity of pincer supported group 10 transition metal hydride catalysts for CO2 hydroboration are investigated. We demonstrate that selective CO2 reduction to eit...

The insertion of CO2 into metal–element σ bonds such as M–H, M–OH, and M–NH2 is often proposed to be a key step in catalytic CO2 conversion. Nevertheless, there are few experimental studies measuring the kinetics of CO2 insertion or comparing insertion into LnM–E (E = H, OH, NH2) complexes with the same ancillary ligands. Here, we use a rapid-mixing stopped-flow instrument to measure the rate of CO2 insertion into (tBuPCP)Ni(OH) (tBuPCP = 2,6-C6H3(CH2PtBu2)2) and show that it proceeds at a faster rate in comparison to insertion into the corresponding metal hydride. We also demonstrate that the rate of CO2 insertion into (tBuPCP)Ni(OH) can generally be increased by using solvents with higher acceptor numbers or by modifying the ligand scaffold to reduce steric bulk or increase electron donation to the metal center. We also attempt to measure the rate of CO2 insertion into (tBuPCP)Ni(NH2); however, this reaction is too fast to measure with our methodology. This work enables the direct comparison of CO2 inse...

It has previously been demonstrated that complexes of the form (iPrPNP)Fe(H)(C≡NR) (iPrPNP = N(CH2CH2P(iPr)2)2–, R = 2,6-dimethylphenyl or 4-methoxyphenyl), which contain a pincer ligand capable of...

International audience; Amides functionalities are among the most widely found groups in biologically active molecules, and their selective catalytic reduction is an important target for synthetic methods. Recent advances in base metal catalysis have identified efficient systems for selective hydrogenolysis of the amide C–N linkage. This study examines in detail the mechanism for deaminative hydrogenation of formanilide and dimethylformamide (DMF) to the corresponding amines (aniline and dimethylamine, respectively) and methanol catalyzed by (iPrPNHP)Fe(H)2(CO) (iPrPNHP = HN{CH2CH2(PiPr2)}2) using density functional theory (DFT) calculations and microkinetic modeling. Following an initial hydrogenation of the amide carbonyl group, protonolysis of the C–N bond of the hemiaminal intermediate produces amine and formaldehyde, which is further hydrogenated to methanol. Remarkably, protonolysis of the C–N bond of the hemiaminal intermediate follows different pathways, depending on the nature of the substrate and the experimental conditions (presence or absence of cocatalyst) to yield the same products. In particular, cleavage of the C–N bond can be facilitated by either the metal catalyst or one of the organic species present (the amide substrate itself or an amide cocatalyst), with interesting consequences on the kinetics as evidenced by simulations using DFT calculated energy profiles for the entire pathway. This study reveals the subtle interplay between the nature of the substrate and the need for additives and changes the long-established principle that the hydrogenation of electron rich carbonyl substrates is governed by their carbonyl hydrogenation step and, therefore, only metal-hydride hydricity is relevant for catalyst design.

A family of iron(II) carbonyl hydride species supported by PNP pincer ligands was identified as highly productive catalysts for the N-formylation of amines via CO2 hydrogenation. Specifically, iron complexes supported by two different types of PNP ligands were examined for formamide production. Complexes containing a PNP ligand with a tertiary amine afforded superior turnover numbers in comparison to complexes containing a bifunctional PNP ligand with a secondary amine, indicating that bifunctional motifs are not required for catalysis. Systems incorporating a tertiary amine containing a PNP ligand were active for the N-formylation of a variety of amine substrates, achieving TONs up to 8900 and conversions as high as 92%. Mechanistic experiments suggest that N-formylation occurs via an initial, reversible reduction of CO2 to ammonium formate followed by dehydration to produce formamide. Several intermediates relevant to this reaction pathway, as well as iron-containing deactivation species, were isolated ...

The insertion of CO2 into metal hydrides and the microscopic reverse decarboxylation of metal formates are important elementary steps in catalytic cycles for both CO2 hydrogenation to formic acid and methanol as well as formic acid and methanol dehydrogenation. Here, we use rapid mixing stopped-flow techniques to study the kinetics and mechanism of CO2 insertion into transition metal hydrides. The investigation finds that the most effective method to accelerate the rate of CO2 insertion into a metal hydride can be dependent on the nature of the rate-determining transition state (TS). We demonstrate that for an innersphere CO2 insertion reaction, which is proposed to have a direct interaction between CO2 and the metal in the rate-determining TS, the rate of insertion increases as the ancillary ligand becomes more electron rich or less sterically bulky. There is, however, no rate enhancement from Lewis acids (LA). In comparison, we establish that for an outersphere CO2 insertion, proposed to proceed with no interaction between CO2 and the metal in the rate-determining TS, there is a dramatic LA effect. Furthermore, for both inner- and outersphere reactions, we show that there is a small solvent effect on the rate of CO2 insertion. Solvents that have higher acceptor numbers generally lead to faster CO2 insertion. Our results provide an experimental method to determine the pathway for CO2 insertion and offer guidance for rate enhancement in CO2 reduction catalysis.

Non-fullerene organic solar cells (OSCs) have attracted tremendous interest because of their potential to replace traditional expensive fullerene-based OSCs. To further increase the power conversion efficiency (PCE), it is necessary to offset the narrow absorption of the non-fullerene materials, which is often achieved by adding an additive (>10 wt%) to form a ternary blend. However, a high ratio of the third component can often be detrimental to the active layer morphology and can increase the complexity in understanding the device physics toward rationally designed improvements. In this work, we introduce 2,4-bis-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio) squaraine (ASSQ) in the poly [(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo [1,2-b:4,5-b′] dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl) benzo [1,2-c:4,5-c′] dithiophene-4,8-dione)] (PBDB-T): 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno [2,3-d:2′,3′-d′]-s-indaceno [1,2-b:5,6-b′] dithiophene (ITIC) as an active layer “crystallizing-agent”. Through detailed morphology characterization, we find that the addition of 4 wt% ASSQ assists ITIC organization order and promotes PDBD-T:ITIC aggregation in the preferential face-on orientation. In addition, we demonstrate that the ASSQ and PBDB-T show efficient exciton dissociation in the ternary blend over Forster resonance energy transfer (FRET). We reveal using surface potential and solubility measurements that a ASSQ-ITIC co-crystalline structure forms which facilitates a significant improvement in the device PCE, from 8.98% to 10.86%.

Substituted ureas have numerous applications but their synthesis typically requires the use of highly toxic starting materials. Herein we describe the first base-metal catalyst for the selective synthesis of symmetric ureas via the dehydrogenative coupling of methanol with primary amines. Using a pincer supported iron catalyst, a range of ureas was generated with isolated yields of up to 80% (corresponding to a catalytic turnover of up to 160) and with H2 as the sole byproduct. Mechanistic studies indicate a stepwise pathway beginning with methanol dehydrogenation to give formaldehyde, which is trapped by amine to afford a formamide. The formamide is then dehydrogenated to produce a transient isocyanate, which reacts with another equivalent of amine to form a urea. These mechanistic insights enabled the development of an iron-catalyzed method for the synthesis of unsymmetric ureas from amides and amines.

Pincer-supported iron complexes of the form [(iPrPNHP)Fe(H)(HBH3)(CO)] (iPrPNHP = HN(CH2CH2PiPr2)2), [(iPrPNHP)Fe(H){OC(O)H}(CO)], [(iPrPNP)Fe(H)(CO)]; iPrPNP = N(CH2CH2PiPr2)2–) have proven to be a privileged class of catalysts for hydrogenation and dehydrogenation reactions. Here, the synthesis, characterization, and reactivity of a family of complexes related to these species, in which the ancillary carbonyl ligand has been replaced with an aryl isonitrile ligand, is described. The complexes [(iPrPNHP)Fe(Cl)2(C≡NR)] (R = 2,6-dimethylphenyl (1a), 4-methoxyphenyl (1b)) were prepared in good yield through the dropwise addition of the aryl isonitrile to an in situ generated solution of [(iPrPNHP)Fe(Cl)2]. If the aryl isonitrile was added too quickly, significant amounts of the cationic bis(isonitrile) complexes [(iPrPNHP)Fe(Cl)(C≡NR)2][Cl] (R = 2,6-dimethylphenyl (2a); 4-methoxyphenyl (2b)) were also formed. Treatment of 1a with excess NaBH4 generated [(iPrPNHP)Fe(H)(HBH3)(C≡NR)] (3a), but the correspondin...

Over the past decade, there has been significant improvement in the performance of polymer solar cells (PSCs). Devices with power conversion efficiency (PCE) well over 10% have been developed and in some cases the estimated device lifetime is approaching 10 years. In contrast, there has been limited research into the aesthetics of PSCs, which may be crucial to successful commercialization. Here, we report a variety of colored polymer solar cells by employing different amounts of an energy transfer dye molecule in a polymer-polymer binary photoactive system. In fact, by varying the amount of the dye molecule in the device, PSCs covering one third of the Munsell standard color chart (in this case green to red) are observed. Additionally, the dye molecule serves as an energy transfer donor to the host photoactive system which leads to 10% enhancement in PCE. Our approach may broaden the appeal of PSCs in applications where aesthetics are important.

The five-coordinate iron(II) hydride complex (iPrPNP)Fe(H)(CO) (iPrPNP = N[CH2CH2(PiPr2)]2) selectively catalyzes the dehydrogenative intermolecular coupling of alcohols and secondary amines to form tertiary amides. This is the most productive base-metal catalyst for dehydrogenative amidation reported to date, in some cases achieving up to 600 turnovers. The catalyst works well for sterically undemanding amines and alcohols or cyclic substrates and is particularly effective in the synthesis of formamides from methanol. However, the catalyst performance declines rapidly with the incorporation of large substituents on the amine or alcohol substrate. Variable-temperature NMR spectroscopic studies suggest that the catalyst resting state is an off-cycle iron(II) methoxide species, (iPrPN(H)P)Fe(H)(OCH3)(CO), resulting from addition of methanol across the Fe–N bond of (iPrPNP)Fe(H)(CO). This reversibly formed iron(II) methoxide complex is favored at mild temperatures but eliminates methanol upon heating.