Assessment of the applicability of DFT methods to [Cp*Rh]-catalyzed hydrogen evolution processes.

The present computational study provides a benchmark of density functional theory (DFT) methods in describing hydrogen evolution processes catalyzed by [Cp*Rh]-containing organometallic complexes. A test set was composed of 26 elementary reactions featuring chemical transformations and bonding situations essential for the field, including the emerging concept of non-innocent Cp* behavior. Reference values were obtained from a highly accurate 3/4 complete basis set and 6/7 complete PNO space extrapolated DLPNO-CCSD(T) energies. The performance of lower-level extrapolation procedures was also assessed. We considered 84 density functionals (DF) (including 13 generalized gradient approximations (GGA), nine meta-GGAs, 33 hybrids, and 29 double-hybrids) and three composite methods (HF-3c, PBEh-3c, and r2SCAN-3c), combined with different types of dispersion corrections (D3(0), D3BJ, D4, and VV10). The most accurate approach is the PBE0-DH-D3BJ (MAD of 1.36 kcal mol-1) followed by TPSS0-D3BJ (MAD of 1.60 kcal mol-1). Low-cost r2SCAN-3c composite provides a less accurate but much faster alternative (MAD of 2.39 kcal mol-1). The widely used Minnesota-family M06-L, M06, and M06-2X DFs should be avoided (MADs of 3.70, 3.94, and 4.01 kcal mol-1, respectively).

Conformational Locking Induced Enantioselective Diarylcarbene Insertion into B-H and O-H Bonds Using a Cationic Rh(I)/Diene Catalyst.

Transition-metal-catalyzed enantioselective transformations of aryl/aryl carbene are inherently challenging due to the difficulty in distinguishing between two arene rings in the reaction process thus remain largely less explored. The few successful examples reported so far, without exception, have all been catalyzed by Rh(II)-complexes. Herein, we describe our successful development of a novel cationic Rh(I)/chiral diene catalytic system capable of efficient enantioselective B-H and O-H insertions with diaryl diazomethanes, allowing the access to a broad range of gem-diarylmethine boranes and gem-diarylmethine ethers in good yields with high enantioselectivities. Notably, previously unattainable asymmetric diarylcarbene insertion into the O-H bond was achieved for the first time. A remarkable feature of this newly designed Rh(I)/diene catalyst bearing two ortho-amidophenyl substitutents is that it can distinguish between two arene rings of the diaryl carbene through a stereochemically selective control of π-π stacking interactions. DFT calculations indicate that the rotation-restricted conformation of Rh(I)/diene complex played an important role in the highly enantioselective carbene transformations. This work provides an interesting and unprecedented stereocontrol mode in diaryl metal carbene transformations.

Rhodium-Catalyzed Pyridylation of Alkynamides with Pyridylboronic Acids: A Route to Functionalized Enamides.

The catalytic direct hydroarylation of alkynamides is a highly efficient approach for accessing functionalized trisubstituted arylalkenes with amide groups. Herein, we report a rhodium-catalyzed pyridylation of alkynamides with pyridylboronic acids, yielding diverse primary, secondary, and tertiary enamides in good to excellent yields (up to 94%). This reaction demonstrates broad tolerance towards various alkyl and aryl functional groups, providing convenient access to a diverse array of alkenylpyridine derivatives. To demonstrate potential applications in late-stage hydropyridylation, we synthesized α,ß-unsaturated ketones, aldehydes, and esters with high yields from the pyridylation product of Weinreb amides. This indirect expansion of the substrate scope enhances the practicality of this strategy. Additionally, the α,ß-unsaturated ketone obtained can be further reduced to yield a chiral alcohol with a 99% ee, further demonstrating the versatility and potential utility of this approach.

Influence of chemical structures on reduction rates and defluorination of fluoroarenes during catalytic reduction using a rhodium-based catalyst.

Continuous growth in fluoroarene production has led to environmental pollution and health concerns owing to their persistence, which is attributed to the stable C-F bond in their structures. Herein, we investigated fluoroarene decomposition via hydrodefluorination using a rhodium-based catalyst, focusing on the effects of the chemical structure and functional group on the defluorination yield. Most compounds, except (pentafluoroethyl)benzene, exhibited full or partial reduction with pseudo-first-order rate constants in the range of 0.002-0.396 min-1 and defluorination yields of 0%-100%. Fluoroarenes with hydroxyl, methyl, and carboxylate groups were selected to elucidate how hydrocarbon and oxygen-containing functional groups influence the reaction rate and defluorination. Inhibition of the reaction rate and defluorination yield based on functional groups increased in the order of hydroxyl < methyl < carboxylate, which was identical to the order of the electron-withdrawing effect. Fluoroarenes with polyfluoro groups were also assessed; polyfluoro groups demonstrated a different influence on catalyst activity than non-fluorine functional groups because of fluorine atoms in the substituents undergoing defluorination. The reaction kinetics of (difluoromethyl)fluorobenzenes and their intermediates suggested that hydrogenation and defluorination occurred during degradation. Finally, the effects of the type and position of functional groups on the reaction rate and defluorination yield were investigated via multivariable linear regression analysis. Notably, the electron-withdrawing nature of functional groups appeared to have a greater impact on the defluorination yield of fluoroarenes than the calculated C-F bond dissociation energy.

Stereocontrol of metal-centred chirality in rhodium(III) and ruthenium(II) complexes with N2N'P ligand.

Rh(III) and Ru(II) complexes, [RhCl2(κ4-N2N'P-L)][SbF6] (1) and [RuCl2(κ4-N2N'P-L)] (2), were synthesised using the tetradentate ligand L (L = N,N-bis[(pyridin-2-yl)methyl]-[2-(diphenylphosphino)phenyl]methanamine). The chloride ligand trans to pyridine can be selectively abstracted by AgSbF6, with the ruthenium complex (2) reacting more readily at room temperature compared to the rhodium complex (1) which requires elevated temperatures. Rhodium complexes avoid the second chloride abstraction, whereas ruthenium complexes can form the chiral bisacetonitrile complex [Ru(κ4-N2N'P-L)(NCMe)2][SbF6]2 (5) upon corresponding treatment with AgSbF6. The complex [RhCl2(κ4-N2N'P-L)][SbF6] (1) has also been used to synthesise polymetallic species, such as the tetrametallic complex [{RhCl2(κ4-N2N'P-L)}2(µ-Ag)2][SbF6]4 (6) which was formed with complete diastereoselectivity and chiral molecular self-recognition. In addition, a stable bimetallic mixed-valence complex [{Rh(κ4-N2N'P-L)}{Rh(COD)}(µ-Cl)2][SbF6]2 (7) (COD = cyclooctadiene) was synthesised. These results highlight the significant differences in chloride lability between Rh3+ and Ru2+ complexes and demonstrate the potential for complexes to act as catalyst precursors and ligands in further chemistry applications.

Total Synthesis of (+)-Kalmanol.

Kalmanol, the flagship member of the kalmane diterpene family, possesses a complex and highly oxidized 5/5/8/5 tetracyclic skeleton with nine contiguous stereocenters and exhibits significant analgesic effects and cardiotoxic properties. We have achieved the efficient total synthesis of (+)-kalmanol in 22 steps with 2.3% yield. The synthesis featured a Rh-catalyzed [5+2+1] cycloaddition reaction to construct 5/5/8 tricyclic skeleton, and a meticulously designed sequence of stereoselective oxidations of the 5/5/8/5 tetracyclic skeleton.

Highly efficient Rh-doped MnO2 nanocatalysts for complete elimination of CO at room temperature.

Obliteration of carbon monoxide is significant due to its hazardous effect on human health and potential application in different fields. Catalytic CO oxidation at lower temperature is the most convenient method to diminish the toxicity of CO. The low-cost catalysts which are exhibiting higher activity at lower temperature with good stability are in demand. The nanosized Rh-doped MnO2 catalysts have been prepared by dextrose-assisted co-precipitation method. Catalytic CO oxidation reaction was carried out over these prepared nanocatalysts under environmentally suitable conditions. XRD confirms the phase formation of prepared catalysts. These samples exhibit rod-like morphology with thickness of rods of less than 10 nm which is substantiated from electron microscopy images. XPS data reveals the oxidation state of Mn (+ 4) and Rh (+ 3). These catalysts are highly active for CO oxidation reaction at lower temperature, and one showed complete CO conversion at room temperature. The time-on-stream studies revealed that these catalysts are highly stable for CO oxidation for several hours. These catalysts are decidedly stable in moist condition and also showed higher activity in the presence of moisture, indicating participation of moisture in the oxidation reaction at above room temperatures.

Reticular Imine-linked Coordination Polymers Based on Paddlewheel Diruthenium/Dirhodium Nodes: Synthesis and Metal-site Dependent Photocatalytic Reduction of CO2.

The paddlewheel-type dimetal core ([M2]) is a ubiquitous motif in the nodes in coordination polymers (CPs) and metal-organic frameworks (MOFs). However, their preparation has relied on ligand-substitution-labile metal ions owing to challenges associated with crystallization. Consequently, examples featuring ligand-substitution-inert metal ions, such as Ru or Rh, are scarce. This study presents the synthesis of novel reticular imine-linked CPs incorporating the paddlewheel-type diruthenium(II, II) ([Ru2II,II]; 1-Ru) or dirhodium(II, II) ([Rh2II,II]; 1-Rh) subunits. The synthetic approach involved a Schiff base dehydration condensation reaction between p-formylbenzoate-bridged [Ru2II,II] or [Rh2II,II] precursors (i.e., CHO-Ru and CHO-Rh, respectively) and 2,5-dimethyl-1,4-phenylenediamine in a 1:2 ratio. The catalytic activities of 1-Ru and 1-Rh for the photochemical reduction of CO2 in a heterogeneous system depended on the metal site. The 1-Rusystem exhibited exceptional selectivity, generating 3.0 ´ 104 mmol g-1 of CO after 24 h of irradiation, whereas the 1-Rhsystem generated a lower amount of CO (3.2 ´ 103 mmol g-1). The catalytic activity of 1-Ru ranked with that of all relevant catalytic systems. This study paves the way for the exploration of [Ru2II,II]- or [Rh2II,II]-based polymers with open metal site-dependent functional properties.

Rhodium-catalyzed homo-coupling reaction of aryl Grignard reagents and its application for the synthesis of an integrin inhibitor.

A novel Rh-catalyzed one-pot homo-coupling reaction of aryl Grignard reagents was achieved. The reaction with bromobenzenes having an electron-donating group or a halogen substituent gave the corresponding homo-coupling products in good yields, although the reaction using heterocyclic or aliphatic bromides scarcely proceeded. A Rh(III)-bis(aryl) complex, which might be formed from RhCl(PPh3)3 and the aryl Grignard reagents, plays an important role in giving the homo-coupling products in this reaction. Furthermore, we applied the reaction to the synthesis of a novel inhibitor for integrins which is critical for several diseases.

Understanding Ligand Effects on Bielectronic Transitions: Chemo- and Electroreduction of Rhodium Bis(Diphosphine) Complexes to Low Oxidation States.

Rhodium complexes in the -I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two-electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)2]NTf2 (x = P-substituent, y = alkanediyl bridging P atoms). Through (electro)reductions from the RhI parents, Rh-I d10-complexes were obtained and characterized spectroscopically, including 103Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E0(RhI/0) and second E0(Rh0/-I) reduction potentials are normally ordered, leading to monoelectronic stepwise events, or inverted, giving bielectronic transitions. Reductionist approaches based on Hammett parameters or the P-Rh-P bite angles provide only partial correlations with the redox potentials. However, we identified the C-O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two-electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions.

Modular synthesis of pentacyclic-fused pyranoquinoliziniums as organelle-selective fluorescent probes.

On basis of their unique chemical and photophysical properties, and excellent biological activities, quinoliziniums have been widely used in various research fields. Herein, modular synthetic strategies for efficient synthesis of novel fluorescent quinoliziniums by using one-pot and stepwise rhodium(III)-catalyzed C-H annulations were developed. In the one-pot synthesis, the reaction between 2-aryl-4-quinolones (1) and 1,2-diarylalkynes (2) proceeded in a chemo- and regioselective manner to give quinolinone-fused isoquinolines (3) and pentacyclic-fused pyranoquinoliziniums (4). The structural diversity of pentacyclic-fused pyranoquinoliziniums (4) was expanded by the stepwise synthesis from 3 and 2, allowing the strategic incorporation of electron-donating (OMe and OH) and electron-withdrawing (Cl) substituents on the top and bottom parts of the pyranoquinoliziniums (4). These newly synthesized pyranoquinoliziniums (4) exhibited tunable absorptions (455-532 nm), emissions (520-610 nm), fluorescence lifetime (0.3-5.6 ns), large Stokes shifts (up to 120 nm), and excellent fluorescence quantum yields (up to 0.73) upon adjusting the different substituents. The the unique arrangement of N and O atoms and extended π-conjugation of 4 could cause the relocation of HOMO comparing with our previous quinoliziniums. Importantly, pyranoquinoliziniums (4a-4g and 4i) targeted the mitochondria, while 4h was localized in lysosome. Due to the remarkable photophysical properties and the potential for organelle targeting of the novel class of quinoliziniums, they could be further applied for biological, chemical and material applications.

Implications for the Hydrogenation of Propyne and Propene with Parahydrogen due to the in†situ Transformation of Rh2C to Rh0/C.

NMR spectroscopy studies using parahydrogen-induced polarization have previously established the existence of the pairwise hydrogen addition route in the hydrogenation of unsaturated hydrocarbons over heterogeneous catalysts, including those based on rhodium (Rh0). This pathway requires the incorporation of both hydrogen atoms from one hydrogen molecule to the same product molecule. However, the underlying mechanism for such pairwise hydrogen addition must be better understood. The involvement of carbon, either in the form of carbonaceous deposits on the surface of a catalyst or as a metal carbide phase, is known to modify catalytic properties significantly and thus could also affect the pairwise hydrogen addition route. Here, we explored carbon's role by studying the hydrogenation of propene and propyne with parahydrogen on a Rh2C catalyst and comparing the results with those for a Rh0/C catalyst obtained from Rh2C via H2 pretreatment. While the catalysts Rh2C and Rh0/C differ notably in the rate of conversion of parahydrogen to normal hydrogen as well as in terms of hydrogenation activity, our findings suggest that the carbide phase does not play a significant role in the pairwise H2 addition route on rhodium catalysts.

Enantioselective Construction of Tetrahydroindole Skeletons by Rh-Catalyzed [2+2+2] Cycloaddition of Homopropargyl Enamides with Alkynes.

We have developed the Rh-catalyzed enantioselective [2+2+2] cycloaddition of homopropargyl enamides (tosylamide-tethered 1,6-enynes) with alkynes to construct tetrahydroindole skeletons found in natural alkaloids and pharmaceuticals. This cycloaddition proceeds at room temperature in high yields and regio- and enantioselectivity with a broad substrate scope. The preparative scale reaction followed by substituent conversion on the nitrogen atom and the diastereoselective [4+2] cycloaddition with singlet O2 affords hexahydroindole-diols bearing three stereogenic centers and variable substituents on the nitrogen. Mechanistic studies have revealed that the substituents of the enynes change the ratio of intramolecular and intermolecular rhodacycle formation when using terminal alkynes, varying the ee values of the cycloadducts.

Probing Basal and Prismatic Planes of Graphitic Materials for Metal Single Atom and Subnanometer Cluster Stabilization.

Supported metal single atom catalysis is a dynamic research area in catalysis science combining the advantages of homogeneous and heterogeneous catalysis. Understanding the interactions between metal single atoms and the support constitutes a challenge facing the development of such catalysts, since these interactions are essential in optimizing the catalytic performance. For conventional carbon supports, two types of surfaces can contribute to single atom stabilization: the basal planes and the prismatic surface; both of which can be decorated by defects and surface oxygen groups. To date, most studies on carbon-supported single atom catalysts focused on nitrogen-doped carbons, which, unlike classic carbon materials, have a fairly well-defined chemical environment. Herein we report the synthesis, characterization and modeling of rhodium single atom catalysts supported on carbon materials presenting distinct concentrations of surface oxygen groups and basal/prismatic surface area. The influence of these parameters on the speciation of the Rh species, their coordination and ultimately on their catalytic performance in hydrogenation and hydroformylation reactions is analyzed. The results obtained show that catalysis itself is an interesting tool for the fine characterization of these materials, for which the detection of small quantities of metal clusters remains a challenge, even when combining several cutting-edge analytical methods.

Rhodium-Catalyzed Enantioselective Intramolecular Hydroalkoxylation of Allenes and Application in the Total Synthesis of (R,R,R)-α-Tocopherol.

We report herein of a novel, enantioselective and rhodium- catalyzed cyclisation of allenyl alcohols towards chiral α-vinylic, cyclic ethers employing a rhodium/(R,R)-Me-ferrocelane catalyst. The corresponding chiral cyclic products were obtained in general high yields and enantioselectivities. The synthetic value of our obtained products was further exemplified by transformations of the allylic ether function. Furthermore, applying our newly developed method in our previously reported route towards the total synthesis of (R,R,R)-α-tocopherol, we accomplished a significantly improved 2nd generation synthesis of the chromane building providing a total number of 13 steps and an overall total yield of 27 %.

Activation of SO2F2 at a Rhodium PNP Pincer Complex: Ligand Supported S-F Bond Cleavage to Generate NSO2F Derivatives.

The κ2-(P,N)-phosphine ligand precursor NH(CH2CH2PCy2)2 can be used for the synthesis of the rhodium(I) complex [Rh(CO){ĸ3-(P,N,P)-Cy2PC2H4NHC2H4PCy2}][Cl] (1). The deprotonated complex [Rh(CO){ĸ3-(P,N,P)-Cy2PC2H4NC2H4PCy2}] (2) shows a cooperative reactivity of the PNP ligand in the activation reaction of SO2F2 to yield the rhodium fluorido complex trans-[Rh(F)(CO){ĸ2-(P,P)-Cy2PC2H4N(SO2F)C2H4PCy2}]2 (3) by S-F bond cleavage. It is remarkable that no reaction was observed when 3 was treated with hydrogen sources e. g. dihydrogen, organosilicon compounds such as triethylsilane or TMS-CF3 and different fluorine sources such as SF4 or Selectfluor®. However, the treatment of complex 3 with XeF2 in the presence of CsF resulted in the formation of the unique fluorido rhodium(III) complex cis,trans-[Rh(F)3(CO){ĸ2-(P,P)-Cy2PC2H4N(SO2F)C2H4PCy2}]2 (4). In the presence of pyridine(HF)X or BF3 the fluorido complex 3 converted into the dicationic complexes [Rh(CO){ĸ2-(P,P)-Cy2PC2H4N(SO2F)C2H4PCy2}]2[XF]2, X=HF (5) or BF3 (6), respectively.

[2.2]Benzoindenophane-Based Chiral Indenyl Ligands: Design, Synthesis, and Applications in Asymmetric C-H Activation.

Development of chiral indenyl ligands for asymmetric C-H activation is a longstanding challenge, and extremely few successes have been achieved. In this paper, we describe a class of readily accessible, facilely tunable and user-friendly chiral indenyl ligands featuring a [2.2]benzoindenophane skeleton via a divergent synthesis strategy. The corresponding chiral indenyl rhodium catalysts were successfully applied in the asymmetric C-H activation reaction of O-Boc hydroxybenzamide with alkenes to give various chiral dihydroisoquinolone products (up to 97 % yield, up to 98 % ee). Moreover, the asymmetric C-H activation reaction of carboxylic acids with alkynes was also successfully accomplished, providing a range of axially chiral isocoumarins (up to 99 % yield, up to 94 % ee). Notably, this represents the first example of enantioselective transition metal catalyzed C(sp2)-H activation/oxidative coupling of benzoic acids with internal alkynes to construct isocoumarins. Given many attractive features of this class of indenyl ligands, such as convenient synthesis, high tunability and exclusive face-selectivity of coordination, its applications in more catalytic asymmetric C-H activation and in other asymmetric catalysis are foreseen.

In situ Low-Energy Electron Microscopy of Chemical Waves on a Composite V-oxide/Rh(110) Surface.

Chemical wave patterns and V-oxide redistribution in catalytic methanol oxidation on a VOx/Rh(110) surface have been investigated in the 10-4 mbar range with low-energy electron microscopy (LEEM) and micro spot low-energy electron diffraction (micro-LEED) as in situ methods. V coverages of θV=0.2 and 0.4 MLE (monolayer equivalents) were studied. Pulses display a c(2×2) pattern in the reduced part and (1×2) and c(2×8) structures in the oxidized part of the surface. At θV=0.4 MLE (1×2)/(1×4) patterns with streaks along the [001]-direction at the 1/8 positions are present on the oxidized part of the surface. This phase can be assigned to V-oxide. On a tentative basis, an excitation mechanism for pulses is presented, Annealing the surface to 990 K under reaction conditions results in a macroscopic hole pattern in which holes of low VOx coverage are surrounded by a V-oxide layer. Chemical waves propagate inside the holes as well as on the VOx covered parts of the surface. The results demonstrate for the first time that also in supported oxidic overlayers selforganization processes can take place leading to chemical waves and a large scale redistribution of the oxide.

Understanding the Activation Mechanism of RhCrOx Cocatalysts for Hydrogen Evolution with Nanoparticulate Electrodes.

Mixed oxides of Rh-Cr (RhCrOx), containing Rh3+ and Cr3+ cations, are commonly used as cocatalysts for the hydrogen evolution reaction (HER) on particulate photocatalysts. The precise physicochemical mechanisms of the HER at the catalytic sites of these oxides are not well understood. In this study, model cocatalyst electrodes, composed of nanoparticulate RhCrOx, were fabricated to investigate the physicochemical mechanisms of the HER. Electroanalytical and X-ray photoelectron spectroscopic measurements revealed that nanoparticulate RhCrOx produces reduced Rh (Rh0) species by maintaining an electrode potential more negative than 0.03 V versus the reversible hydrogen electrode (VRHE). This results in significant enhancement of the HER activity. The catalytic activity for the HER stems from the reduced Rh species, and the inclusion of Cr3+ (CrOx) aided in the electron transfer process at the solid/liquid interface, resulting in a higher current density during the HER. To achieve a solar-to-hydrogen efficiency of over 3%, the conduction band minimum of the particulate photocatalyst should be positioned more negatively than -0.10 VRHE. Moreover, the formation of electron trap states at potentials more positive than 0.03 VRHE should be avoided. This study highlights the importance of understanding the catalytic sites on metal oxide cocatalysts. Moreover, it offers a design strategy for enhancing the efficiency of photocatalytic water splitting.