High Symmetry Dirhodium(II) Paddlewheel Complexes as Chiral Catalysts

D4-Symmetric Dirhodium Tetrakis(binaphthylphosphate) Catalysts for Enantioselective Functionalization of Unactivated C-H Bonds

Dirhodium tetrakis(2,2'-binaphthylphosphate) catalysts were successfully developed for asymmetric C-H functionalization with trichloroethyl aryldiazoacetates as the carbene precursors. The 2,2'-binaphthylphosphate (BNP) ligands were modified by introduction of aryl and/or chloro functionality at the 4,4',6,6' positions. As the BNP ligands are C2-symmetric, the resulting dirhodium tetrakis(2,2'-binaphthylphosphate) complexes were expected to be D4-symmetric, but X-ray crystallographic and computational studies revealed this is not always the case because of internal T-shaped CH-π and aryl-aryl interactions between the ligands. The optimum catalyst is Rh2(S-megaBNP)4, with 3,5-di(tert-butyl)phenyl substituents at the 4,4' positions and chloro substituents at the 6,6' positions. This catalyst adopts a D4-symmetric arrangement and is ideally suited for site-selective C-H functionalization at unactivated tertiary sites with high levels of enantioselectivity, outperforming the best dirhodium tetracarboxylate catalyst developed for this reaction. The standard reactions were conducted with a catalyst loading of 1 mol % but lower catalyst loadings can be used if desired, as illustrated in the C-H functionalization of cyclohexane in 91% ee with 0.0025 mol % catalyst loading (29,400 turnover numbers). These studies further illustrate the effectiveness of donor/acceptor carbenes in site-selective intermolecular C-H functionalization and expand the toolbox of catalysts available for catalyst-controlled C-H functionalization.

Quantum Chemical Interrogation of Reactions Promoted by Dirhodium Tetracarboxylate Catalysts─Mechanism, Selectivity, and Nonstatistical Dynamic Effects

ConspectusRh2L4 catalysts have risen in popularity in the world of organic synthesis, being used to accomplish a variety of reactions, including C-H insertion and cyclopropanation, and often doing so with high levels of stereocontrol. While the mechanisms and origins of selectivity for such reactions have been examined with computational quantum chemistry for decades, only recently have detailed pictures of the dynamic behavior of reacting Rh2L4-complexed molecules become accessible. Our computational studies on Rh2L4 catalyzed reactions are described here, with a focus on C-H insertion reactions of Rh2L4-carbenes. Several issues complicate the modeling of these reactions, each providing an opportunity for greater understanding and each revealing issues that should be incorporated into future rational design efforts. First, the fundamental mechanism of C-H insertion is discussed. While early quantum chemical studies pointed to transition structures with 3-center [C-H-C] substructures and asynchronous hydride transfer/C-C bond formation, recent examples of reactions with particularly flat potential energy surfaces and even discrete zwitterionic intermediates have been found. These reactions are associated with systems bearing π-donating groups at the site of hydride transfer, allowing for an intermediate with a carbocation substructure at that site to be selectively stabilized. Second, the possible importance of solvent coordination at the Rh atom distal to the carbene is discussed. While effects on reactivity and selectivity were found to be small, they turn out not to be negligible in some cases. Third, it is shown that, in contrast to many other transition metal promoted reactions, many Rh2L4 catalyzed reactions likely involve dissociation of the Rh2L4 catalyst before key chemical steps leading to products. When to expect dissociation is associated with specific features of substrates and the product-forming reactions in question. Often, dissociation precedes transition structures for pericyclic reactions that involve electrons that would otherwise bind to Rh2L4. Finally, the importance of nonstatistical dynamic effects, characterized through ab initio molecular dynamics studies, in some Rh2L4 catalyzed reactions is discussed. These are reactions where transition structures are shown to be followed by flat regions, very shallow minima, and/or pathways that bifurcate, all allowing for trajectories from a single transition state to form multiple different products. The likelihood of encountering such a situation is shown to be associated again with the likelihood of formation of zwitterionic structures along reaction paths, but ones for which pathways to multiple products are expected to be associated with very low or no barriers. The connection between these features and reduced yields of desired products are highlighted, as are the means by which some Rh2L4 catalysts modulate dynamic behavior to produce particular products in high yield.

Unlocking catalytic potential: a rhodium(II)-based coordination polymer for efficient carbene transfer reactions with donor/acceptor diazoalkanes

Herein, we report the use of a molecular-defined rhodium(II) coordination polymer (Rh-CP) as a heterogeneous, recyclable catalyst in carbene transfer reactions. We showcase the application of this heterogeneous catalyst in a range of carbene transfer reactions and conclude with the functionalization of natural products and drug molecules.

Running Wild through Dirhodium Tetracarboxylate-Catalyzed Combined CH(C)-Functionalization/Cope Rearrangement Landscapes: Does Post-Transition-State Dynamic Mismatching Influence Product Distributions?

A special type of C-H functionalization can be achieved through C-H insertion combined with Cope rearrangement (CHCR) in the presence of dirhodium catalysts. This type of reaction was studied using density functional theory and ab initio molecular dynamics simulations, the results of which pointed to the dynamic origins of low yields observed in some experiments. These studies not only reveal intimate details of the complex reaction network underpinning CHCR reactions but also further cement the generality of the importance of nonstatistical dynamic effects in controlling Rh2L4-promoted reactions.

Theoretical Investigation of Linear Relationships between the Dihedral Torsion Angles and Diosmium Bond Distances in Diosmium Sawhorse Complexes

Strong linear relationships between their Ceq-Os-Os-Ceq dihedral angles and their Os-Os bond distances in diosmium sawhorse complexes Os2(u-O2CR)2(CO)4L2 (L = CO and/or PR3) form two trendlines depending upon the presence or absence of terminal phosphines. These trends appear unrelated to the basicity of the bridging ligand or the number of phosphines. The mathematical derivation of the relationship between the O-Os-Os-O dihedral angle and the Os-Os bond distance shows how the other geometric parameters affect this relationship. Optimized density functional theory (DFT) structures reveal a similar strong linear correlation, where more electron-donating ligands render shorter Os-Os bond distances and larger dihedral angles, but these results form a single trendline. Computational scans of individual parameters show that the Os-Os bond responds strongly to changes in the dihedral angles, but the dihedral angles only respond weakly to changes in the Os-Os bond distance because the Os-Os-O bond angle links and modifies their direct coupling. Solid-state analysis of their structures, including DFT geometry optimizations, shows that phosphines protect the Os-Os bond distance from packing influences along the Os-Os axis, while in complexes without phosphines, packing compresses the Os-Os bond and the weak dihedral responses create the second trendline.

Elucidating the Electronic Nature of Rh-based Paddlewheel Catalysts from 103 Rh NMR Chemical Shifts: Insights from Quantum Mechanical Calculations

The tremendous importance of dirhodium paddlewheel complexes for asymmetric catalysis is largely the result of an empirical optimization of the chiral ligand sphere about the bimetallic core. It was only recently that a H(C)Rh triple resonance 103 Rh NMR experiment provided the long-awaited opportunity to examine - with previously inconceivable accuracy - how variation of the ligands impacts on the electronic structure of such catalysts. The recorded effects are dramatic: formal replacement of only one out of eight O-atoms surrounding the metal centers in a dirhodium tetracarboxylate by an N-atom results in a shielding of the corresponding Rh-site of no less than 1000 ppm. The current paper provides the theoretical framework that allows this and related experimental observations made with a set of 19 representative rhodium complexes to be interpreted. In line with symmetry considerations, it is shown that the shielding tensor responds only to the donor ability of the equatorial ligands along the perpendicular principal axis. Axial ligands, in contrast, have no direct effect on shielding but may come into play via the electronicc i s ${cis}$ -effect that they exert onto the neighboring equatorial sites. On top of these fundamental interactions, charge redistribution within the core as well as the electronict r a n s ${trans}$ -effect of ligands of different donor strengths is reflected in the recorded 103 Rh NMR shifts.

NMR spectroscopy of a 18O-labeled rhodium paddlewheel complex: Isotope shifts, 103Rh-103Rh spin-spin coupling, and 103Rh singlet NMR

Despite the importance of rhodium complexes in catalysis, and the favorable 100% natural abundance of the spin-1/2 103Rh nucleus, there are few reports of 103Rh nuclear magnetic resonance (NMR) parameters in the literature. In part, this is the consequence of the very low gyromagnetic ratio of 103Rh and its dismal NMR sensitivity. In a previous paper [Harbor-Collins et al., J. Chem. Phys. 159, 104 307 (2023)], we demonstrated an NMR methodology for 1H-enhanced 103Rh NMR and demonstrated an application to the 103Rh NMR of the dirhodium formate paddlewheel complex. In this paper, we employ selective 18O labeling to break the magnetic equivalence of the 103Rh spin pair of dirhodium formate. This allows the estimation of the 103Rh-103Rh spin-spin coupling and provides access to the 103Rh singlet state. We present the first measurement of a 18O-induced 103Rh secondary isotope shift as well as the first instance of singlet order generated in a 103Rh spin pair. The field-dependence of 103Rh singlet relaxation is measured by field-cycling NMR experiments.

Deconvoluting Nonlinear Catalyst-Substrate Effects in the Intramolecular Dirhodium-Catalyzed C-H Insertion of Donor/Donor Carbenes Using Data Science Tools

Interactions between catalysts and substrates can be highly complex and dynamic, often complicating the development of models to either predict or understand such processes. A dirhodium(II)-catalyzed C-H insertion of donor/donor carbenes into 2-alkoxybenzophenone substrates to form benzodihydrofurans was selected as a model system to explore nonlinear methods to achieve a mechanistic understanding. We found that the application of traditional methods of multivariate linear regression (MLR) correlating DFT-derived descriptors of catalysts and substrates leads to poorly performing models. This inspired the introduction of nonlinear descriptor relationships into modeling by applying the sure independence screening and sparsifying operator (SISSO) algorithm. Based on SISSO-generated descriptors, a high-performing MLR model was identified that predicts external validation points well. Mechanistic interpretation was aided by the deconstruction of feature relationships using chemical space maps, decision trees, and linear descriptors. Substrates were found to have a strong dependence on steric effects for determining their innate cyclization selectivity preferences. Catalyst reactive site features can then be matched to product features to tune or override the resultant diastereoselectivity within the substrate-dictated ranges. This case study presents a method for understanding complex interactions often encountered in catalysis by using nonlinear modeling methods and linear deconvolution by pattern recognition.

Hydroxypyridinate-bridged paddlewheel-type dirhodium complex as a catalyst for photochemical and electrochemical hydrogen evolution

Electrochemical and photochemical hydrogen (H2) evolution activities of a 6-fluoro-2-hydroxypyridinate (fhp-)-bridged paddlewheel-type dirhodium (Rh2) complex, [Rh2(fhp)4], were investigated through experimental and theoretical approaches. In DMF, the [Rh2(fhp)4] underwent a one-electron reduction (assigned to Rh24+/3+) at -1.31 V vs SCE in the cathodic region. Adding trifluoroacetic acid as a proton source to the electrochemical cell containing [Rh2(fhp)4], the significant catalytic current, i.e., electrochemical H2 evolution, was observed; the turnover frequency and overpotential of electrochemical H2 evolution were 18 244 s-1 and 732 mV, respectively. The reaction mechanism of electrochemical H2 evolution catalyzed by [Rh2(fhp)4] in DMF was examined in detail by theoretically predicting the redox potentials and pKa values of the reaction intermediates using density functional theory calculations. The calculations revealed that (i) the formation of a one-electron reduced species, [Rh2(fhp)4]-, triggered for H2 evolution and (ii) the protonation and reduction processes of [Rh2(fhp)4]- to further reduced hydride intermediates proceeded directly via a concerted proton-electron transfer mechanism. Moreover, [Rh2(fhp)4] was shown to be a highly efficient H2 evolution catalyst (HEC) for photochemical proton reduction reactions when combined with an artificial photosynthetic (AP) system containing [Ir(ppy)2(dtbbpy)]PF6 and triethylamine, which served as a photosensitizer and a sacrificial electron donor, respectively. Under visible light irradiation, the total amount of H2 evolved and its turnover number (per Rh ion) were 1361.0 µmol and 13 610, respectively, which are superior to those of previously reported AP systems with rhodium complexes as HEC.

Catalytic, asymmetric carbon-nitrogen bond formation using metal nitrenoids: from metal-ligand complexes via metalloporphyrins to enzymes

The introduction of nitrogen atoms into small molecules is of fundamental importance and it is vital that ever more efficient and selective methods for achieving this are developed. With this aim, the potential of nitrene chemistry has long been appreciated but its application has been constrained by the extreme reactivity of these labile species. This liability however can be attenuated by complexation with a transition metal and the resulting metal nitrenoids have unique and highly versatile reactivity which includes the amination of certain types of aliphatic C-H bonds as well as reactions with alkenes to afford aziridines. At least one new chiral centre is typically formed in these processes and the development of catalysts to exert control over enantioselectivity in nitrenoid-mediated amination has become a growing area of research, particularly over the past two decades. Compared with some synthetic methods, metal nitrenoid chemistry is notable in that chemists can draw from a diverse array of metals and catalysts , ranging from metal-ligand complexes, bearing a variety of ligand types, via bio-inspired metalloporphyrins, all the way through to, very recently, engineered enzymes themselves. In the latter category in particular, rapid progress is being made, the rate of which suggests that this approach may be instrumental in addressing some of the outstanding challenges in the field. This review covers key developments and strategies that have shaped the field, in addition to the latest advances, up until September 2023.

Aspartyl β-Turn-Based Dirhodium(II) Metallopeptides for Benzylic C(sp3)-H Amination: Enantioselectivity and X-ray Structural Analysis

Amination of C(sp3)-H bonds is a powerful tool to introduce nitrogen into complex organic frameworks in a direct manner. Despite significant advances in catalyst design, full site- and enantiocontrol in complex molecular regimes remain elusive using established catalyst systems. To address these challenges, we herein describe a new class of peptide-based dirhodium(II) complexes derived from aspartic acid-containing β-turn-forming tetramers. This highly modular system can serve as a platform for the rapid generation of new chiral dirhodium(II) catalyst libraries, as illustrated by the facile synthesis of a series of 38 catalysts. Critically, we present the first crystal structure of a dirhodium(II) tetra-aspartate complex, which unveils retention of the β-turn conformation of the peptidyl ligand; a well-defined hydrogen-bonding network is evident, along with a near-C4 symmetry that renders the rhodium centers inequivalent. The utility of this catalyst platform is illustrated by the enantioselective amination of benzylic C(sp3)-H bonds, in which state-of-the-art levels of enantioselectivity up to 95.5:4.5 er are obtained, even for substrates that present challenges with previously reported catalyst systems. Additionally, we found these complexes to be competent catalysts for the intermolecular amination of N-alkylamides via insertion into the C(sp3)-H bond α to the amide nitrogen, yielding differentially protected 1,1-diamines. Of note, this type of insertion was also observed to occur on the amide functionalities of the catalyst itself in the absence of the substrate but did not appear to be detrimental to reaction outcomes when the substrate was present.

Catalytic Enantioselective Amination of Enol Silyl Ethers Using a Chiral Paddle-Wheel Diruthenium Complex

A chiral paddle-wheel dinuclear ruthenium catalyst was applied to a catalytic asymmetric nitrene-transfer reaction with enol silyl ethers. The ruthenium catalyst was applicable to aliphatic enol silyl ethers as well as aryl-containing enol silyl ethers. The substrate scope of the ruthenium catalyst was superior to that of analogous chiral paddle-wheel rhodium catalysts. α-Amino ketones derived from aliphatic substrates were obtained in up to 97% ee with the ruthenium catalyst, while analogous rhodium catalysts resulted in only moderate enantioselectivity.

C-H Insertion in Dirhodium Tetracarboxylate-Catalyzed Reactions despite Dynamical Tendencies toward Fragmentation: Implications for Reaction Efficiency and Catalyst Design

Rh-catalyzed C-H insertion reactions to form β-lactones suffer from post-transition state bifurcations, with the same transition states leading to ketones and ketenes via fragmentation in addition to β-lactones. In such a circumstance, traditional transition state theory cannot predict product selectivity, so we employed ab initio molecular dynamics simulations to do so and provide a framework for rationalizing the origins of said selectivity. Weak interactions between the catalyst and substrate were studied using energy decomposition and noncovalent interaction analyses, which unmasked an important role of the 2-bromophenyl substituent that has been used in multiple β-lactone-forming C-H insertion reactions. Small and large catalysts were shown to behave differently, with the latter providing a means of overcoming dynamically preferred fragmentation by lowering the barrier for the recombination of the product fragments in the grip of the large catalyst active site cavity.

Self-Adaptive Dirhodium Complexes in a Metal-Organic Framework for Synthesis of N-H Aziridines

Conformational dynamics of active sites in enzymes enable great control over the catalytic process. Herein, we constructed a metal-organic framework with conformationally dynamic active sites (Rh₂-ZIF-8). The active sites in Rh₂-ZIF-8 were composed of the imidazolate-bridged bimetallic center with a catalytic dirhodium moiety and structural zinc site. Even though the coordination sphere of the dirhodium species was saturated with two circularly arranged esp groups and two axial 2-MeIm ligands, it could still effectively catalyze the direct synthesis of N-H aziridines from olefins with high activity. We found that such a self-adaptive catalytic process was based on the dynamic breakage and reformation of the rhodium-zinc imidazolate bridges. Interestingly, the in situ generated dirhodium site with a unique Rh₂(esp)₂(2-MeIm)₁ configuration was able to exhibit obviously enhanced selectivity compared to homogeneous catalyst Rh₂(esp)₂. Furthermore, the surrounding zinc imidazolate groups could effectively protect the dirhodium moieties from harsh environments, and this ultimately endowed it with high stability.

Recent advances in transition-metal-catalyzed carbene insertion to C-H bonds

C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp³)-H, aryl (aromatic) C(sp²)-H, heteroaryl (heteroaromatic) C(sp²)-H bonds, alkenyl C(sp²)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Heteroleptic dirhodium(II,II) paddlewheel complexes as carbene transfer catalysts

This review highlights the applications of dirhodium(II,II) paddlewheel complexes with a heteroleptic scaffold. Dirhodium(II,II) paddlewheel complexes are well known as highly efficient and selective carbene transfer catalysts. While the majority of described complexes are homoleptic, comparatively fewer studies have concerned heteroleptic complexes. Here, we emphasise the use of heteroleptic complexes in order to highlight their benefits as carbene transfer catalysts and spur future research. Methods to synthesise heteroleptic dirhodium(II,II) paddlewheel complexes are discussed as well as a categorical review of their types of heteroleptic complexes and the carbene reactions in which they have been used.

Catalyst-Controlled Stereoselective Barton-Kellogg Olefination

Overcrowded alkenes are expeditiously prepared by the versatile Barton-Kellogg olefination and have remarkable applications as functional molecules owing to their unique stereochemical features. The induced stereodynamics thereby enable the controlled motion of molecular switches and motors, while the high configurational stability prevents undesired isomeric scrambling. Bistricyclic aromatic enes are prototypical overcrowded alkenes with outstanding stereochemical properties, but their stereocontrolled preparation was thus far only feasible in stereospecific reactions and with chiral auxiliaries. Herein we report that direct catalyst control is achieved by a stereoselective Barton-Kellogg olefination with enantio- and diastereocontrol for various bistricyclic aromatic enes. Using Rh2 (S-PTAD)4 as catalyst, several diazo compounds were selectively coupled with a thioketone to give one of the four anti-folded overcrowded alkene stereoisomers upon reduction. Complete stereodivergence was reached by catalyst control in combination with distinct thiirane reductions to provide all four stereoisomers with e.r. values of up to 99:1. We envision that this strategy will enable the synthesis of topologically unique overcrowded alkenes for functional materials, catalysis, energy- and electron transfer, and bioactive compounds.

Dirhodium Carboxylate Catalysts from 2-Fenchyloxy or 2-Menthyloxy Arylacetic Acids: Enantioselective C-H Insertion, Aromatic Addition and Oxonium Ylide Formation/Rearrangement

A new class of dirhodium carboxylate catalysts have been designed and synthesized from 2-fenchyloxy or 2-menthyloxy arylacetic acids which display excellent enantioselectivity across a range of transformations of α-diazocarbonyl compounds. The catalysts were successfully applied to enantioselective C-H insertion reactions of aryldiazoacetates and α-diazo-β-oxosulfones affording the respective products in up to 93 % ee with excellent trans diastereoselectivity in most cases. Furthermore, efficient desymmetrization in an intramolecular C-H insertion was achieved. In addition, these catalysts prove highly enantioselective for intramolecular aromatic addition with up to 88 % ee, and oxonium ylide formation and rearrangement with up to 74 % ee.

Enantioselective Rh(II)-Catalyzed Desymmetric Cycloisomerization of Diynes: Constructing Furan-Fused Dihydropiperidines with an Alkyne-Substituted Aza-Quaternary Stereocenter

Described herein is an enantioselective dirhodium(II)-catalyzed cycloisomerization of diynes achieved by the strategy of desymmetrization, which not only represents a new cycloisomerization reaction of diynes but also constitutes the first Rh(II)-catalyzed asymmetric intramolecular cycloisomerization of 1,6-diynes. This protocol provides a range of valuable furan-fused dihydropiperidine derivatives with an enantiomerically enriched alkynyl-substituted aza-quaternary stereocenter in high efficiency, complete atom economy, and excellent enantioselectivity (up to 98% ee). Besides, the highly functionalized products could be easily transformed into various synthetically useful building blocks and conjugated with a series of pharmaceutical molecules. The mechanism involving a concerted [3+2] cycloaddition/[1,2]-H shift of the Rh(II) carbenoid intermediate was elucidated by DFT calculations and mechanistic studies. More importantly, the first single crystal of alkyne-dirhodium(II) was obtained to show that a η²-coordinating activation of alkynal by dirhodium(II) was involved. Weak hydrogen bondings between the carboxylate ligands and alkynal were found, which probably made the well-defined paddlewheel-like dirhodium(II) distinctive from other metal complexes in catalyzing this transformation. Furthermore, the origin of the enantioselectivity was elucidated by a Rh₂(R-PTAD)₄-alkyne complex and additional calculational studies.

Enantioselective Intermolecular C-H Amination Directed by a Chiral Cation

The enantioselective amination of C(sp³)-H bonds is a powerful synthetic transformation yet highly challenging to achieve in an intermolecular sense. We have developed a family of anionic variants of the best-in-class catalyst for Rh-catalyzed C-H amination, Rh₂(esp)₂, with which we have associated chiral cations derived from quaternized cinchona alkaloids. These ion-paired catalysts enable high levels of enantioselectivity to be achieved in the benzylic C-H amination of substrates bearing pendant hydroxyl groups. Additionally, the quinoline of the chiral cation appears to engage in axial ligation to the rhodium complex, providing improved yields of product versus Rh₂(esp)₂ and highlighting the dual role that the cation is playing. These results underline the potential of using chiral cations to control enantioselectivity in challenging transition-metal-catalyzed transformations.

Chiral Dirhodium Tetraphosphate-Catalyzed Enantioselective Si-H Bond Insertion of α-Aryldiazoacetates

A highly enantioselective Si-H bond insertion reaction of α-aryldiazoacetates catalyzed by chiral spiro dirhodium tetraphosphate was developed. Various chiral α-silyl esters were prepared with high yield (up to 92%) and excellent enantioselectivity (up to >99% ee) through this protocol. It is noteworthy that the 2-substituted aryl diazoacetates, which are challenging substrates for other chiral dirhodium catalysts, also exhibited good results in this reaction. This work represents one of the few successful applications of chiral dirhodium phosphates in asymmetric catalysis.

Vapochromism and Magnetochemical Switching of a Nickel(II) Paddlewheel Complex by Reversible NH3 Uptake and Release

Reaction of [NiCl2 (PnH)4 ] (1) (PnH=6-tert-butyl-pyridazine-3-thione) with NiCl2 affords the binuclear paddlewheel (PW) complex [Ni2 (Pn)4 ] (2). Diamagnetic complex 2 is the first example of a PW complex capable of reversibly binding and releasing NH3 . The NH3 ligand in [Ni2 (Pn)4 (NH3 )] (2⋅NH3 ) enforces major spectroscopic and magnetic susceptibility changes, thus displaying vapochromic properties (λmax (2)=532 nm, λmax (2⋅NH3 )=518 nm) and magnetochemical switching (2: S=0; 2⋅NH3 : S=1). Upon repeated adsorption/desorption cycles of NH3 the PW core remains intact. Compound 2 can be embedded into thin polyurethane films (2P ) under retention of its sensing abilities. Therefore, 2 qualifies as reversible optical probe for ammonia. The magnetochemical switching of 2 and 2⋅NH3 was studied in detail by SQUID measurements showing that in 2⋅NH3 , solely the Ni atom coordinated the NH3 molecule is responsible for the paramagnetic behavior.

Transition Metal Catalyzed Insertion Reactions with Donor/Donor Carbenes

Donor/donor carbenes are relatively new in the field of carbene chemistry; although applications in C-H and X-H insertion reactions are few in number, they demonstrate exquisite chemo- and stereo-selectivity. Recent reports have shown that C-H, N-H, B-H, O-H, S-H, Si-H, Ge-H, Sn-H and P-H insertion reactions are feasible with a variety of transition metal catalysts, both inter- and intramolecularly. Furthermore, high degrees of diastereo- and enantioselectivity have been observed in several cases. Methods typically involve the formation of a diazo-based carbene precursor, but procedures using diazo-free metal carbenes have been developed with significant success. This Minireview covers transition-metal catalyzed insertion reactions with donor/donor and donor carbenes, providing context for future developments in this emerging field.

Tuning metal-metal interactions for cooperative small molecule activation

Cluster complexes have attracted interest for decades due to their promise of drawing analogies to metallic surfaces and metalloenzyme active sites, but only recently have chemists started to develop ligand scaffolds that are specifically designed to support multinuclear transition metal cores. Such ligands not only hold multiple metal centers in close proximity but also allow for fine-tuning of their electronic structures and surrounding steric environments. This Feature Article highlights ligand designs that allow for cooperative small molecule activation at cluster complexes, with a particular focus on complexes that contain metal-metal bonds. Two useful ligand-design elements have emerged from this work: a degree of geometric flexibility, which allows for novel small molecule activation modes, and the use of redox-active ligands to provide electronic flexibility to the cluster core. The authors have incorporated these factors into a unique class of dinucleating macrocycles (nPDI2). Redox-active fragments in nPDI2 mimic the weak-overlap covalent bonding that is characteristic of M-M interactions, and aliphatic linkers in the ligand backbone provide geometric flexibility, allowing for interconversion between a range of geometries as the dinuclear core responds to the requirements of various small molecule substrates. The union of these design elements appears to be a powerful combination for analogizing critical aspects of heterogeneous and metalloenzyme catalysts.

(4+1)-Cycloadditions Exploiting the Biphilicity of Oxyphosphonium Enolates and RhII/PdII-Stabilized Metallocarbenes for the Construction of Five-Membered Frameworks

(4+1)-Cyclizations are an underutilized disconnect for the formation of five-membered heterocyclic and carbocyclic frameworks. Herein we analyze methods employing oxyphosphonium enolates and RhII/PdII-metallocarbenes as C1 synthons in the presence of several four-atom components for the synthesis of 2,3-dihydrobenzofurans, 2,3-dihydroindoles, oxazolones, cyclopentenones, and pyrrolones.

1 Introduction

2 (4+1)-Cyclizations Employing Kukhtin–Ramirez-Like Reactivity

3 (4+1)-Cyclizations Employing a Cyclopropanation/Ring-Expansion Sequence

4 Pd-Catalyzed (4+1)-Cyclizations through Carbene Migratory Insertion/Reductive Elimination Processes

5 Summary

Iron-catalysed chemo- and ortho-selective C–H bond functionalization of phenols with α-aryl-α-diazoacetates

We present a novel chemo- and ortho-selective C–H bond functionalization of phenols with α-aryl-α-diazoacetates catalysed by a new iron porphyrin.

Heteronuclear Dirhodium-Gold Anionic Complexes: Polymeric Chains and Discrete Units

In this article, we report on the synthesis and characterization of the tetracarboxylatodirhodium(II) complexes [Rh2(μ-O2CCH2OMe)4(THF)2] (1) and [Rh2(μ-O2CC6H4-p-CMe3)4(OH2)2] (2) by metathesis reaction of [Rh2(μ-O2CMe)4] with the corresponding ligand acting also as the reaction solvent. The reaction of the corresponding tetracarboxylato precursor, [Rh2(μ-O2CR)4], with PPh4[Au(CN)2] at room temperature, yielded the one-dimensional polymers (PPh4)n[Rh2(μ-O2CR)4Au(CN)2]n (R = Me (3), CH2OMe (4), CH2OEt (5)) and the non-polymeric compounds (PPh4)2{Rh2(μ-O2CR)4[Au(CN)2]2} (R = CMe3 (6), C6H4-p-CMe3 (7)). The structural characterization of 1, 3·2CH2Cl2, 4·3CH2Cl2, 5, 6, and 7·2OCMe2 is also provided with a detailed description of their crystal structures and intermolecular interactions. The polymeric compounds 3·2CH2Cl2, 4·3CH2Cl2, and 5 show wavy chains with Rh-Au-Rh and Rh-N-C angles in the ranges 177.18°-178.69° and 163.0°-170.4°, respectively. A comparative study with related rhodium-silver complexes previously reported indicates no significant influence of the gold or silver atoms in the solid-state arrangement of these kinds of complexes.

Rhodium(I)-NHC Complexes Bearing Bidentate Bis-Heteroatomic Acidato Ligands as gem-Selective Catalysts for Alkyne Dimerization

A series of Rh(κ² -BHetA)(η² -coe)(IPr) complexes bearing 1,3-bis-hetereoatomic acidato ligands (BHetA) including carboxylato (O,O), thioacetato (O,S), amidato (O,N), thioamidato (N,S), and amidinato (N,N), have been prepared by reaction of the dinuclear precursor [Rh(μ-Cl)(IPr)(η² -coe)]₂ with the corresponding anionic BHetA species. The Rh^I -NHC-BHetA compounds catalyze the dimerization of aryl alkynes, showing excellent selectivity for the head-to-tail enynes. Among them, the acetanilidato-based catalyst has shown an outstanding catalytic performance reaching unprecedented TOF levels of 2500 h^-1 with complete selectivity for the gem-isomer. Investigation of the reaction mechanism supports a non-oxidative pathway in which the BHetA ligand behaves as proton shuttle through intermediate κ¹ -HBHetA species. However, in the presence of pyridine as additive, the identification of the common Rh^III H(C≡CPh)₂ (IPr)(py)₂ intermediate gives support for an alternative oxidative route.

Coordination-Induced Self-Assembly of a Heteroleptic Paddlewheel-Type Dirhodium Complex

A novel heteroleptic paddlewheel-type dirhodium (Rh2) complex [Rh2(O2CCH3)3(PABC)] (1; PABC = para-aminobenzenecarboxylate), which has an amino group as a potential donor site for coordination with the metal ion, was synthesized and characterized by 1H NMR, ESI-TOF-MS, infrared spectra, and elemental analysis. The slow evaporation of N,N-dimethylformamide (DMF)-dissolved 1 produces the purple-colored crystalline polymeric species [Rh2(O2CCH3)3 (PABC)(DMF)]n (1P). Single-crystal and powder X-ray diffraction analyses, as well as thermo-gravimetric analysis, clarified that 1P formed a one-dimensional polymeric structure, in which the two axial sites of the Rh2 ion in 1P are coordinated by a DMF molecule and an amino group of the PABC ligand of the neighboring molecule 1, by coordination-induced self-assembly (polymerization) with an Rh-amino bond. The reversible structural change (self-assembly and disassembly transformations) between the discrete species [Rh2(O2CCH3)3(PABC)(DMF)2] (1D; green solution) and the polymeric species 1P (purple solid) was accompanied by a color change, which easily occurred by the dissolution and evaporation procedures with DMF.

Paddlewheel Complexes with Azulenes: Electronic Interaction between Metal Centers and Equatorial Ligands

Rhodium dinuclear complexes (1-3) with azulene moieties as equatorial ligands were obtained by reacting Rh₂ (OAc)₄ with guaiazulene-2-carboxylic acid, azulene-2-carboxylic acid, and azulene-1-carboxylic acid, respectively. The molecular structures in their crystalline states were determined by X-ray diffraction to be 1 ⋅ (H₂ O)₂ , 1 ⋅ (MeCN)₂ , 2 ⋅ (MeCN)₂ , and 3 ⋅ (DMF)₂ , which were coordinated with the crystallization solvent at the axial positions. Among these, the crystal packing of 1 ⋅ (H₂ O)₂ , 1 ⋅ (MeCN)₂ , and 3 ⋅ (DMF)₂ revealed the formation of one-dimensional stacked chains nearly along the axial direction and of two-dimensional stacked sheets along the equatorial direction. In addition, it was found that 1 ⋅ (H₂ O)₂ contained cavities that could adsorb CO₂ , thereby inducing structural changes. Furthermore, redox measurements revealed the stepwise one-electron redox behaviors of these complexes, indicating the intramolecular interactions between the azulene units. In addition, transient absorption measurements suggested the presence of an ultrafast intersystem crossing caused by the heavy-atom effect of rhodium, and an extended lifetime of the triplet state due to the energy migration among the azulene ligands.

Discrete and polymeric ensembles based on dinuclear molybdenum(vi) building blocks with adaptive carbohydrazide ligands: from the design to catalytic epoxidation

Discrete and polymeric ensembles based on dimolybdenum(vi) units with adaptive carbohydrazide ligands are described. The polymeric complexes are efficient catalysts for cyclooctene epoxidation under eco-friendly conditions.

Finding Opportunities from Surprises and Failures. Development of Rhodium-Stabilized Donor/Acceptor Carbenes and Their Application to Catalyst-Controlled C-H Functionalization

Catalyst-controlled C-H functionalization by means of the C-H insertion chemistry of rhodium carbenes has become a powerful synthetic method. The key requirements for the development of this chemistry are donor/acceptor carbenes and the chiral dirhodium tetracarboxylate catalysts. This perspective will describe the stages involved in developing this chemistry and illustrate the scope of the donor/acceptor carbene C-H functionalization.