Oxidative Addition of a Mechanically Entrapped C(sp)-C(sp) Bond to a Rhodium(I) Pincer Complex

Active template synthesis

The active template synthesis of mechanically interlocked molecular architectures exploits the dual ability of various structural elements (metals or, in the case of metal-free active template synthesis, particular arrangements of functional groups) to serve as both a template for the organisation of building blocks and as a catalyst to facilitate the formation of covalent bonds between them. This enables the entwined or threaded intermediate structure to be covalently captured under kinetic control. Unlike classical passive template synthesis, the intercomponent interactions transiently used to promote the assembly typically do not 'live on' in the interlocked product, meaning that active template synthesis can be traceless and used for constructing mechanically interlocked molecules that do not feature strong binding interactions between the components. Since its introduction in 2006, active template synthesis has been used to prepare a variety of rotaxanes, catenanes and knots. Amongst the metal-ion-mediated versions of the strategy, the copper(I)-catalysed alkyne-azide cycloaddition (CuAAC) remains the most extensively used transformation, although a broad range of other catalytic reactions and transition metals also provide effective manifolds. In metal-free active template synthesis, the recent discovery of the acceleration of the reaction of primary amines with electrophiles through the cavity of crown ethers has proved effective for forming an array of rotaxanes without recognition elements, including compact rotaxane superbases, dissipatively assembled rotaxanes and molecular pumps. This Review details the active template concept, outlines its advantages and limitations for the synthesis of interlocked molecules, and charts the diverse set of reactions that have been used with this strategy to date. The application of active template synthesis in various domains is discussed, including molecular machinery, mechanical chirality, catalysis, molecular recognition and various aspects of materials science.

A Platform Approach to Cleavable Macrocycles for the Controlled Disassembly of Mechanically Caged Molecules

Inspired by interlocked oligonucleotides, peptides and knotted proteins, synthetic systems where a macrocycle cages a bioactive species that is "switched on" by breaking the mechanical bond have been reported. However, to date, each example uses a bespoke chemical design. Here we present a platform approach to mechanically caged structures wherein a single macrocycle precursor is diversified at a late stage to include a range of trigger units that control ring opening in response to enzymatic, chemical, or photochemical stimuli. We also demonstrate that our approach is applicable to other classes of macrocycles suitable for rotaxane and catenane formation.

Kinetically controlled synthesis of rotaxane geometric isomers

Geometric isomerism in mechanically interlocked systems-which arises when the axle of a mechanically interlocked molecule is oriented, and the macrocyclic component is facially dissymmetric-can provide enhanced functionality for directional transport and polymerization catalysis. We now introduce a kinetically controlled strategy to control geometric isomerism in [2]rotaxanes. Our synthesis provides the major geometric isomer with high selectivity, broadening synthetic access to such interlocked structures. Starting from a readily accessible [2]rotaxane with a symmetrical axle, one of the two stoppers is activated selectively for stopper exchange by the substituents on the ring component. High selectivities are achieved in these reactions, based on coupling the selective formation reactions leading to the major products with inversely selective depletion reactions for the minor products. Specifically, in our reaction system, the desired (major) product forms faster in the first step, while the undesired (minor) product subsequently reacts away faster in the second step. Quantitative 1H NMR data, fit to a detailed kinetic model, demonstrates that this effect (which is conceptually closely related to minor enantiomer recycling and related processes) can significantly improve the intrinsic selectivity of the reactions. Our results serve as proof of principle for how multiple selective reaction steps can work together to enhance the stereoselectivity of synthetic processes forming complex mechanically interlocked molecules.

Tuning the Catalytic Activity of a Pincer Complex of Rhodium(I) by Supramolecular and Redox Stimuli

We report the rhodium(I) complex [Rh(CNC-NDI)(CO)]+ , in which CNC-NDI refers to a pincer-CNC ligand decorated with a naphthalenediimide moiety. Due to the presence of the planar CNC ligand and the naphthalenediimide moiety, the electronic nature of the complex can be modulated by means of supramolecular and redox stimuli, respectively. The metal complex shows a strong π-π-stacking interaction with coronene. This interaction has an impact on the electron-richness of the metal, as demonstrated by the shifting of the ν(CO) stretching band to a lower frequency. The addition of tetrabutylammonium fluoride facilitates the sequential one- and two-electron reduction of the NDI moiety of the ligand, thus resulting in a situation in which the ligand can increase its electron-donor strength in two levels. The nature of the interaction with the fluoride anion was studied computationally. The catalytic activity of the [Rh(CNC-NDI)(CO)]+ complex was tested in the cycloisomerization of alkynoic acids, where it is observed that the activity of the catalyst can be modulated between four levels of activity, which correspond to i) the use of the unmodified catalyst, ii) catalyst+coronene, iii) catalyst+2 equivalents of fluoride, and iv) catalyst+5 equivalents of fluoride.

Supramolecular Control of the Oxidative Addition as a Way To Improve the Catalytic Efficiency of Pincer-Rhodium (I) Complexes

1 H NMR studies using a cationic complex with a pyridine-di-imidazolylidene pincer ligand of formula [Rh(CNC)(CO)]+ revealed that this compound showed high binding affinity with coronene in CH2 Cl2 . The interaction between coronene and the planar RhI complex is established by means of π-stacking interactions. This interaction has a strong impact on the electron-donating strength of the pincer CNC ligand, which is increased significantly, as demonstrated by the shifting of the ν(CO) stretching bands to lower frequencies. The addition of coronene increases the reaction rate of the nucleophilic attack of methyl iodide on the rhodium (I) pincer complex, and also has a positive effect on the performance of the complex as a catalyst in the cycloisomerization of 4-pentynoic acid. These findings highlight the importance of supramolecular interactions for tuning the reactivity and catalytic activity of square-planar metal complexes.

C-H activation in bimetallic rhodium complexes to afford N-heterocyclic carbene pincer complexes

The pro-ligands 1,8-bis(di-R-phosphinomethyl)-2,3-dihydroperimidine (RH₂Pm, R = phenyl, cyclohexyl) react with [RhCl(CE)(PPh₃)₂] (E = O, S) to afford the bimetallic complexes [RhCl(CE)(μ-RH₂Pm)]₂ (E = O, S). Upon heating, these species undergo double C-H activation to afford the N-heterocyclic carbene (NHC) pincer complexes [RhCl(RPm)]. Reduction of [RhCl(CO)(μ-PhH₂Pm)]₂ with KC₈ results in the bimetallic rhodium(0) complex, [Rh(μ-CO)(PhH₂Pm)]₂, with a formal Rh-Rh bond and a hydrogen-bonding interaction between rhodium and the central methylene group (C-H⋯Rh = 2.802 Å). Upon treatment with tritylium, ferrocenium or triphenylcyclopropenium tetrafluoroborates this species undergoes double C-H activation to afford a mononuclear NHC pincer complex salt, [Rh(CO)(PhPm)]BF₄. Treatment of [RhCl(CO)(PhH₂Pm)]₂ with lithium (trimethylsilyl)acetylide provides another bimetallic species, [Rh(CCSiMe₃)(CO)(PhH₂Pm)]₂, however heating this species does not proceed cleanly to the monomeric NHC complex, [Rh(CCSiMe₃)(CO)(PhPm)] which may however be obtained from [RhCl(RPm)] and LiCCSiMe₃.

Modulation of Metal Carbonyl Stretching Frequencies in the Second Coordination Sphere through the Internal Stark Effect

Spectroscopic and computational examination of a homologous series of rhodium(I) pybox carbonyl complexes has revealed a correlation between the conformation of the flanking aryl-substituted oxazoline donors and the carbonyl stretching frequency. This relationship is also observed experimentally for octahedral rhodium(III) and ruthenium(II) variants and cannot be explained through the classical, Dewar-Chatt-Duncanson, interpretation of metal-carbonyl bonding. Instead, these findings are reconciled by local changes in the magnitude of the electric field that is projected along the metal-carbonyl vector: the internal Stark effect.

Pincer and Macrocyclic Pyridylidene Amide (PYA) AuIII Complexes

Gold-based homogeneous catalysis is dominated by redox neutral Au^I systems. Redox-active gold-based catalysts are less common, principally because of redox cycles between Au^I and Au^III being hampered by unfavorable potentials. We report gold(III) complexes containing pincer-based, donor-flexible pyridylidene amide (PYA) ligands to address these issues. These complexes act as electron reservoirs through two limiting resonance structures consisting of either soft, imine coordination sites or harder, zwitterionic amide donors. We further alter the donor properties by using the ortho-, meta-, and para-pyridylidene amide variants of the PYA pincer arms. These bis-PYA pincer ligands exhibited a high contribution of amide coordination in the solid-state of the gold(III) complexes; however, the solution data suggests a high contribution from the neutral L-type resonance forms. This L-type contribution, primarily shown through cyclic voltammetry studies, prevents reversible gold(III) reduction and also disfavors abstraction of the ancillary chloride ligand. Furthermore, a novel macrocyclic-PYA ligand is introduced, which shows secondary metal-ligand interactions.

C-C σ-Bond Oxidative Addition and Hydrofunctionalization by a Macrocycle-Supported Diiron Complex

This report describes the first examples of unassisted C(sp)-C(sp2) and C(sp)-C(sp3) bond oxidative addition reactions to give thermodynamically favorable products. Treatment of a diiron complex supported by a geometrically and electronically flexible macrocyclic ligand, (3PDI2)Fe2(μ-N2)(PPh3)2 ([Fe2N2]0), with stoichiometric amounts of various 4,4'-disubstituted diphenylacetylenes (ArX-C≡C-ArX; X = OMe, H, F, CF3) yielded C(sp)-C(sp2) bond oxidative addition products. When Ph-C≡C-R substrates were used as substrates (R = Me, Et, iPr, tBu), products of either C(sp)-C(sp2) or C(sp)-C(sp3) bond activation were obtained, with the less sterically encumbering alkynes exclusively undergoing C(sp)-C(sp3) bond activation. Treatment of the C-C activation species with either H2 or HBpin was found to form products of C-C σ-bond hydrofunctionalization. In both the hydrogenation and hydroboration schemes, the diiron species was observed to return to [Fe2N2]0, thereby completing synthetic cycles for C-C σ-bond functionalization.

Anion-π Catalysis Enabled by the Mechanical Bond

We report a series of rotaxane-based anion-π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in >60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.

Controlling catalyst activity, chemoselectivity and stereoselectivity with the mechanical bond

Mechanically interlocked molecules, such as rotaxanes and catenanes, are receiving increased attention as scaffolds for the development of new catalysts, driven by both their increasing accessibility and high-profile examples of the mechanical bond delivering desirable behaviours and properties. In this Review, we survey recent advances in the catalytic applications of mechanically interlocked molecules organized by the effect of the mechanical bond on key catalytic properties, namely, activity, chemoselectivity and stereoselectivity, and focus on how the mechanically bonded structure leads to the observed behaviour. Our aim is to inspire future investigations of mechanically interlocked catalysts, including those outside of the supramolecular community.

Rotaxane CoII Complexes as Field-Induced Single-Ion Magnets

Mechanically chelating ligands have untapped potential for the engineering of metal ion properties. Here we demonstrate this principle in the context of CoII -based single-ion magnets. Using multi-frequency EPR, susceptibility and magnetization measurements we found that these complexes show some of the highest zero field splittings reported for five-coordinate CoII complexes to date. The predictable coordination behaviour of the interlocked ligands allowed the magnetic properties of their CoII complexes to be evaluated computationally a priori and our combined experimental and theoretical approach enabled us to rationalize the observed trends. The predictable magnetic behaviour of the rotaxane CoII complexes demonstrates that interlocked ligands offer a new strategy to design metal complexes with interesting functionality.

Spin-labelled mechanically interlocked molecules as models for the interpretation of biradical EPR spectra

Biradical spin probes can provide detailed information about the distances between molecules/regions of molecules because the through-space coupling of radical centres, characterised by J, is strongly distance dependent. However, if the system can adopt multiple configurations, as is common in supramolecular complexes, the shape of the EPR spectrum is influenced not only by J but also the rate of exchange between different states. In practice, it is often hard to separate these variables and as a result, the effect of the latter is sometimes overlooked. To demonstrate this challenge unequivocally we synthesised rotaxane biradicals containing nitronyl nitroxide units at the termini of their axles. The rotaxanes exchange between the available biradical conformations more slowly than the corresponding non-interlocked axles but, despite this, in some cases, the EPR spectra of the axle and rotaxane remain remarkably similar. Detailed analysis allowed us to demonstrate that the similar EPR spectral shapes result from different combinations of J and rates of conformational interconversion, a phenomenon suggested theoretically more than 50 years ago. This work reinforces the idea that thorough analysis must be performed when interpreting the spectra of biradicals employed as spin probes in solution.

Recent advances in organophosphorus-chalcogen and organophosphorus-pincer based macrocyclic compounds and their metal complexes

The design and development of phosphorus based macrocycles containing one or more other heteroatoms is of crucial importance for the enhancement of modern synthetic chemistry. In recent years focus on phosphorus based macromolecules has led to intriguing and innovative structures with a variety of applications, including photophysical and host-guest properties, and in organic synthesis. This article summarizes the recent advancements in the synthesis of macrocycles that consist of organophosphorus-chalcogen (P-E, P[double bond, length as m-dash]E; E = O, S, Se) and organophosphorus-pincer based macrocyclic ligands and their transition metal complexes with emphasis given to synthetic methodologies. The reactions involve the modification of simple macrocycles with phosphorus sources or phosphorus-based chalcogenating reagents. Transition metal complexes of phosphine-based macrocyclic pincer ligands and their reactivity are also included.

The Fascinating Flexibility and Coordination Modes of a Pentamethylene Connected Macrocyclic CNC Pincer Ligand

The coordination chemistry of an electron-rich macrocyclic CNC pincer-ligand consisting of two pentamethylene tethered N-heterocyclic carbene moieties on a carbazole backbone (bimca^C5) is investigated by mainly NMR spectroscopy and X-ray crystal structure analysis. A bridging coordination mode is found for the lithium complex. With the larger and softer potassium ion, the ligand adopts a facial coordination mode and a polymeric structure by intermolecular potassium nitrogen interactions. The facial coordination is also confirmed at a Cp*Ru fragment, while C-H activation under dehydrogenation at the alkyl chain is observed upon reaction with [Ru(PPh₃)₃Cl₂]. In contrast, Pd(OAc)₂ reacts under C-H activation at the central carbon atom of the pentamethylene tether to an alkyl-pincer macrocycle.

Synthesis and reactivity of iridium complexes of a macrocyclic PNP pincer ligand

Having recently reported on the synthesis and rhodium complexes of the novel macrocyclic pincer ligand PNP-14, which is derived from lutidine and features terminal phosphine donors trans-substituted with a tetradecamethylene linker (Dalton Trans., 2020, 49, 2077-2086 and Dalton Trans., 2020, 49, 16649-16652), we herein describe our findings critically examining the chemistry of iridium homologues. The five-coordinate iridium(i) and iridium(iii) complexes [Ir(PNP-14)(η2:η2-cyclooctadiene)][BArF4] and [Ir(PNP-14)(2,2'-biphenyl)][BArF4] are readily prepared and shown to be effective precursors for the generation of iridium(iii) dihydride dihydrogen, iridium(i) bis(ethylene), and iridium(i) carbonyl derivatives that highlight important periodic trends by comparison to rhodium counterparts. Reaction of [Ir(PNP-14)H2(H2)][BArF4] with 3,3-dimethylbutene induced triple C-H bond activation of the methylene chain, yielding an iridium(iii) allyl hydride derivative [Ir(PNP-14*)H][BArF4], whilst catalytic homocoupling of 3,3-dimethylbutyne into Z-tBuC[triple bond, length as m-dash]CCHCHtBu could be promoted at RT by [Ir(PNP-14)(η2:η2-cyclooctadiene)][BArF4] (TOFinitial = 28 h-1). The mechanism of the latter is proposed to involve formation and direct reaction of a vinylidene derivative with HC[triple bond, length as m-dash]CtBu outside of the macrocyclic ring and this suggestion is supported experimentally by isolation and crystallographic characterisation of a catalyst deactivation product.

η2-Alkene Complexes of [Rh(PONOP-iPr)(L)]+ Cations (L = COD, NBD, Ethene). Intramolecular Alkene-Assisted Hydrogenation and Dihydrogen Complex [Rh(PONOP-iPr)(η-H2)]. Inorg Chem 2021;60:13903-13912. [PMID: 33570930 PMCID: PMC8456414 DOI: 10.1021/acs.inorgchem.0c03687]

Rhodium-alkene complexes of the pincer ligand κ³-C₅H₃N-2,6-(OPⁱPr₂)₂ (PONOP-ⁱPr) have been prepared and structurally characterized: [Rh(PONOP-ⁱPr)(η²-alkene)][BAr^F₄] [alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; Ar^F = 3,5-(CF₃)₂C₆H₃]. Only one of these, alkene = COD, undergoes a reaction with H₂ (1 bar), to form [Rh(PONOP-ⁱPr)(η²-COE)][BAr^F₄] (COE = cyclooctene), while the others show no significant reactivity. This COE complex does not undergo further hydrogenation. This difference in reactivity between COD and the other alkenes is proposed to be due to intramolecular alkene-assisted reductive elimination in the COD complex, in which the η²-bound diene can engage in bonding with its additional alkene unit. H/D exchange experiments on the ethene complex show that reductive elimination from a reversibly formed alkyl hydride intermediate is likely rate-limiting and with a high barrier. The proposed final product of alkene hydrogenation would be the dihydrogen complex [Rh(PONOP-ⁱPr)(η²-H₂)][BAr^F₄], which has been independently synthesized and undergoes exchange with free H₂ on the NMR time scale, as well as with D₂ to form free HD. When the H₂ addition to [Rh(PONOP-ⁱPr)(η²-ethene)][BAr^F₄] is interrogated using pH₂ at higher pressure (3 bar), this produces the dihydrogen complex as a transient product, for which enhancements in the ¹H NMR signal for the bound H₂ ligand, as well as that for free H₂, are observed. This is a unique example of the partially negative line-shape effect, with the enhanced signals that are observed for the dihydrogen complex being explained by the exchange processes already noted.

Oxidative Addition of a Mechanically Entrapped C(sp)-C(sp) Bond to a Rhodium(I) Pincer Complex

By use of a macrocyclic phosphinite pincer ligand and bulky substrate substituents, we demonstrate how the mechanical bond can be leveraged to promote the oxidative addition of an interlocked 1,3-diyne to a rhodium(I) center. The resulting rhodium(III) bis(alkynyl) product can be trapped out by reaction with carbon monoxide or intercepted through irreversible reaction with dihydrogen, resulting in selective hydrogenolysis of the C-C σ-bond.

Reactions of Rh(PNP) pincer complexes with terminal alkynes: homocoupling through a ring or not at all

“Switching on” a metal's capacity to promote terminal alkyne coupling reactions using a macrocyclic pincer ligand.