NHC-based pincer ligands: carbenes with a bite

Synthesis and comparative study of (NHCF)PdCl2Py and (NHCF)Ni(Cp)Cl complexes: investigation of the electronic properties of NHC ligands and complex characteristics

The electron-donating and electron-accepting properties of N-heterocyclic carbene (NHC) ligands play a pivotal role in governing their interactions with transition metals, thereby influencing the selectivity and reactivity in catalytic processes. Herein, we report the synthesis of Pd/NHCF and Ni/NHCF complexes, wherein the electronic parameters of the NHC ligands were systematically varied. By performing a series of controlled structure modifications, we elucidated the influence of the σ-donor and π-acceptor properties of NHC ligands on interactions with the transition metals Pd and Ni and, consequently, the catalytic behavior of Pd and Ni complexes. The present study deepens our understanding of NHC-metal interactions and provides novel information for the rational design of efficient catalysts for organic synthesis.

Controllable synthesis of hollow mesoporous organosilica nanoparticles with pyridine-2,6-bis-imidazolium frameworks for CO2 conversion

A series of hard-template-derived hollow mesoporous organosilica nanoparticles (HMONs) with pyridine-2,6-bis-imidazolium frameworks have been described for the first time. As a part of the investigation, to evaluate the effects of the hard template nature, the Si/CTAB and organosilica/TEOS molar ratios, and the stepwise addition of precursors, four reaction conditions denoted as methods A-D were designed. In the presence of polystyrene latex as a hard template, the HMONs that we wished to synthesize were not yielded with a Si/CTAB molar ratio of 3 (method A), but we could synthesize the desired HMONs with a Si/CTAB molar ratio of 9 and an organosilica : TEOS ratio of 1 : 99 (method B). The ratio of organosilica to TEOS could be improved up to 2.5 : 97.5 if the precursor additions are made in a stepwise manner rather than by simultaneous additions (method C). Using sSiO2 as a hard template, a yolk-shell morphology was observed by adopting a Si/CTAB molar ratio of 3 (method D). The HMONs were modified by iodide ions and their activity was explored toward the coupling of CO2 with epoxides. Among the catalysts, I-HMON-L-C-2.5 exhibited excellent results under mild reaction conditions. Well-oriented pore sizes and short channel length facilitated easy mass transfer from one side and the integration of the interior hollow regions of the catalyst particles from the other side improved the CO2 retention time around pores where the imidazolium organocatalysts were located, which made I-HMON-L-C-2.5 an effective catalyst for title CO2 utilization. The catalyst was reused at least six times without exhibiting any changes in its activity. HMONs can also be used as solid CNC ligands for the preparation of copper catalysts for the click reaction between phenyl acetylene and benzyl azide.

An N-heterocyclic carbene-based pincer system of palladium and its versatile reactivity under oxidizing conditions

NHC-based pincers (NHC = N-heterocyclic carbene) have been broadly employed as supporting platforms, and their palladium complexes have found many synthetic applications. However, previous studies mainly focused on the NHC pincers of palladium featuring an oxidation number of +II. In contrast, oxidation of these well-defined Pd(II) species and the study of their fundamental high-valent Pd chemistry remain largely undeveloped. In addition, from a perspective of PdII/PdIV catalysis, the reactivity and degradation of NHC pincers in catalytically relevant reactions have not been well understood. In this work, a series of Pd(II) complexes supported by a well-known NHC^Aryl^NHC pincer platform have been prepared. Their reactivity towards various oxidizing reagents, including halogen surrogates, electrophilic fluorine reagents, and alkyl/aryl halides, has been examined. In some cases, ambient-characterizable high-valent Pd NHCs, which have been scarcely reported, were obtained. The carbenes incorporated into the pincer framework proved to be effective spectator donors. In contrast, the central aryl moiety exhibits versatile reactivity and collapse pathways, allowing it to function either as a spectator or a non-innocent actor.

New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes

The pincer complexes [NiIIBr(CNC)]Br (4), [CrIIIBr3(CNC)] (5 a) and [CrIIIBr2.3Cl0.7(CNC)] (5 b), where CNC=3,3'-(pyridine-2,6-diyl)bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene), were obtained from the novel ligand CNC, generated in situ from the precursor (CHNCH)Br2 and [NiIIBr2(PPh3)2] or from [CrII{N(SiMe3)2}2(THF)2] and (CHNCH)Br2 by aminolysis, respectively. The tetrahedrally distorted square planar (τ4≅0.30) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the CNC backbone. Computational methods highlighted the dependence of the coordination geometry and the singlet-triplet energy difference on the size of the N-substituent of the tetrahydropyrimidine wingtips and contrasted it to the situation in 5-membered imidazolin-2-ylidene pincer analogues. The octahedral CrIII metal center in 5 a and 5 b is presumably formed after one electron oxidation from CH2Cl2. 4/MAO and 5 a/MAO were catalysts of moderate activity for the oligomerization and polymerization of ethylene, respectively. The analogous (CH^N^CH)Br2 precursor, where (CH^N^CH)=3,3'-(pyridine-2,6-diylbis(methylene))bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-1-ium), was also prepared, however its coordination chemistry was not studied due to the inherent instability of the resulting free C^N^C ligand.

Diverse structural reactivity patterns of a POCOP ligand with coinage metals

We have utilised the 4,6-di-tert-butyl resorcinol bis(diphenylphosphinite) (POCOP) ligand for exploring its coordination ability towards group 11 metal centres. The treatment of the bidentate ligand 1 with various coinage metal precursors afforded a wide range of structurally diverse complexes 2-12, depending upon the metal precursors used. This furnishes several multinuclear Cu(I) complexes with dimeric (2) and tetrameric cores (3, 4, and 5). The tetrameric stairstep complex 4 shows thermochromic behaviour, whereas the dimeric complex 2 and tetrameric complex 3 show luminescence properties at cryogenic temperatures. Interestingly, the halide substitution reaction of the dimeric complex 2 with KPPh2 produces a unique mixed phosphine-based tetrameric Cu(I) complex, 5. Treatment of the POCOP ligand with [CuBF4(CH3CN)4] in the presence of 2,2'-bipyridine afforded heteroleptic complex 6, consisting of tri- and tetra-coordinated cationic Cu(I) centres. Furthermore, we could also isolate cubane (8) and stairstep (9) complexes of Ag(I). The cationic Au(I) complex (12) was obtained from the dinuclear Au(I) complex of POCOP, 11. Complex 12 revealed the presence of a strong intramolecular aurophilic interaction with an Au⋯Au bond distance of 3.1143(9) Å. Subsequently, the photophysical properties of these complexes have been studied. All the complexes were characterised by single-crystal X-ray diffraction studies, routine NMR techniques, and mass spectroscopy.

Iron complexes supported by silyl-NHC chelate ligands: synthesis and use for double hydroboration of nitriles

Iron complexes bearing new silyl-NHC bidentate ligands were synthesised by treating Fe3(CO)12 with a mixture of N-(hydrosilyl)methyl imidazolium salts and a base. These complexes showed high performance in the catalytic double hydroboration of nitrile with pinacolborane (HBpin) to produce N,N-bis(boryl)amine by a combination of UV irradiation and mild heating (60 °C). The product yields for the hydroboration of aromatic and aliphatic nitriles reached 85%-95% (NMR) using an iron complex (5 mol%). Reducing the loading amount of the iron complex to 0.5 mol% still afforded the products in high yields. An analogous ruthenium complex, which was similarly synthesised using Ru3(CO)12, showed lower activity. Stoichiometric reactions of the iron complex with nitriles afforded Fe(0)-N-silylimine complexes, which may be dormant states in nitrile hydroboration. A catalytic mechanism including Fe(0) N-silylimine species is proposed.

Bis-tridentate Ir(III) Phosphors and Blue Hyperphosphorescence with Suppressed Efficiency Roll-Off at High Brightness

Narrowband blue emitters are indispensable in achieving ultrahigh-definition OLED displays that satisfy the stringent BT 2020 standard. Hereby, a series of bis-tridentate Ir(III) complexes bearing electron-deficient imidazo[4,5-b]pyridin-2-ylidene carbene coordination fragments and 2,6-diaryloxy pyridine ancillary groups were designed and synthesized. They exhibited deep blue emission with quantum yields of up to 89% and a radiative lifetime of 0.71 μs in the DPEPO host matrix, indicating both the high efficiency and excellent energy transfer process from the host to dopant. The OLED based on Irtb1 showed an emission at 468 nm with a maximum external quantum efficiency (EQE) of 22.7%. Moreover, the hyper-OLED with Irtb1 as a sensitizer for transferring energy to terminal emitter v-DABNA exhibited a narrowband blue emission at 472 nm and full width at half-maximum (FWHM) of 24 nm, a maximum EQE of 23.5%, and EQEs of 19.7, 16.1, and 12.9% at a practical brightness of 100, 1000, and 5000 cd/m2, respectively.

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.

Phosphine-NHC-Phosphonium Ylide Pincer Ligand: Complexation with Pd(II) and Unconventional P-Coordination of the Ylide Moiety

An efficient synthesis of two pincer preligands [Ph2PCH(R)ImCH2CH2CH2PPh3]X2 (R = H, X = OTf; R = Ph, X = BF4) was developed. Subsequent reactions with PdCl2 and an excess of Cs2CO3 led to the formation of highly stable cationic ortho-metalated Pd(II) complexes [(P,C,C,C)Pd]X exhibiting phosphine, NHC, phosphonium ylide, and σ-aryl donor extremities. The protonation of one of the latter complexes with R = H affords the Pd(II) complex [(P,C,C)Pd(MeCN)](OTf)2 bearing an unprecedented nonsymmetrical NHC core pincer scaffold with a 5,6-chelating framework. The overall donor properties of this phosphine-NHC-phosphonium ylide ligand were estimated using the experimental νCN stretching frequency in the corresponding [(P,C,C)Pd(CNtBu](OTf)2 derivative and were shown to be competitive with the related bis(NHC)-phosphonium ylide and phenoxy-NHC-phosphonium ylide pincers. The presence of a phenyl substituent in the bridge between phosphine and NHC moieties in the ortho-metalated complex [(P,C,C,C)Pd](BF4) makes possible the deprotonation of this position using LDA to provide a persistent zwitterionic complex [(P,C,C,C)Pd] featuring a rare P-coordinated phosphonium ylide moiety in addition to a conventional C-coordinated one. The comparison of the 31P and 13C NMR data for these C- and P-bound phosphonium ylide fragments within the same molecule was performed for the first time, and the bonding situation in both cases was studied in detail by QTAIM and ELF topological analyses.

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.

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.

Terminal Alkyne Coupling Reactions Through a Ring: Effect of Ring Size on Rate and Regioselectivity

Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincer ligands-which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC-n)(C2 H4 )][BArF 4 ] (n=12, 14, 16; ArF =3,5-(CF3 )2 C6 H3 )-have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr' (Ar'=3,5-tBu2 C6 H3 ) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3-enynes by C-C bond reductive elimination through the annulus of the ancillary ligand. The intermediates are formed with orthogonal regioselectivity, with E-alkenyl complexes derived from HC≡CtBu and gem-alkenyl complexes derived from HC≡CAr', and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E-tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar'C≡CC(=CH2 )Ar' is encumbered relative to head-to-head coupling of HC≡CAr' and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C2 H4 )][BArF 4 ] and solvent effects.

Oxidative Addition of Biphenylene and Chlorobenzene to a Rh(CNC) Complex

The synthesis and organometallic chemistry of rhodium(I) complex [Rh(CNC-Me)(SOMe2)][BArF 4], featuring NHC-based pincer and labile dimethyl sulfoxide ligands, is reported. This complex reacts with biphenylene and chlorobenzene to afford products resulting from selective C-C and C-Cl bond activation, [Rh(CNC-Me)(2,2'-biphenyl)(OSMe2)][BArF 4] and [Rh(CNC-Me)(Ph)Cl(OSMe2)][BArF 4], respectively. A detailed DFT-based computational analysis indicates that C-H bond oxidative addition of these substrates is kinetically competitive, but in all cases endergonic: contrasting the large thermodynamic driving force calculated for insertion of the metal into the C-C and C-Cl bonds, respectively. Under equivalent conditions the substrates are not activated by the phosphine-based pincer complex [Rh(PNP-iPr)(SOMe2)][BArF 4].

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

A shape changing tandem Rh(CNC) catalyst: preparation of bicyclo[4.2.0]octa-1,5,7-trienes from terminal aryl alkynes

The preparation of a range of tetraaryl-substituted bicyclo[4.2.0]octa-1,5,7-trienes using a one-pot procedure starting from terminal aryl alkynes and catalysed by a rhodium(i) complex is reported. This synthesis proceeds by a reaction sequence involving head-to-tail homocoupling of the terminal alkyne and zipper annulation of the resulting gem-enyne. The rhodium catalyst employed is notable for the incorporation of a flexible NHC-based pincer ligand, which is suggested to interconvert between mer- and fac-coordination modes to fulfil the orthogonal mechanistic demands of the two transformations. Evidence for this interesting auto-tandem action of the catalyst is provided by reactions of the precatalyst with model substrates, corroborating proposed intermediates in both component cycles, and norbornadiene, which reversibly captures the change in pincer ligand coordination mode, along with a DFT-based computational analysis.

Synthesis and group 9 complexes of macrocyclic PCP and POCOP pincer ligands

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer (pro)ligands derived from meta-xylene (PCP-14) and resorcinol (POCOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. The former was accomplished using a seven-step asymmetric procedure involving (-)-cis-1-amino-2-indanol as a chiral auxiliary and ring-closing olefin metathesis. A related, but non-diastereoselective route was employed for the latter, which consequently necessitated chromatographic separation from the cis-substituted by-product. The proligands are readily metalated and homologous series of M^I(CO) and M^IIICl₂(CO) derivatives (M = Rh, Ir) have been isolated and fully characterised in solution and the solid state. Metal hydride complexes are generated during the synthesis of the former and have been characterised in situ using NMR spectroscopy.

Synthesis and rhodium complexes of macrocyclic PNP and PONOP pincer ligands

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer ligands derived from lutidine (PNP-14) and 2,6-dihydroxypyridine (PONOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. This was accomplished using an eight-step procedure involving borane protection, ring-closing olefin metathesis, chromatographic separation from the cis-substituted diastereomers, and borane deprotection. The rhodium coordination chemistry of these ligands has been explored, aided by the facile synthesis of 2,2'-biphenyl (biph) adducts [Rh(PNP-14)(biph)][BAr^F₄] and [Rh(PONOP-14)(biph)][BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃). Subsequent hydrogenolysis enabled generation of dihydrogen, ethylene and carbonyl derivatives; notably the ν(CO) bands of the carbonyl complexes provide a means to compare the donor properties of the new pincer ligands with established acyclic congeners.

Transition metal complexes of a bis(carbene) ligand featuring 1,2,4-triazolin-5-ylidene donors: structural diversity and catalytic applications

Dialkylation of the 1,3-bis(1,2,4-triazol-1-yl)benzene with ethyl bromide results in the formation of [L-H2]Br2 which, upon salt metathesis with NH4PF6, readily yields the bis(triazolium) salt [L-H2](PF6)2 with non-coordinating counterions. [L-H2](PF6)2 and Ag2O react in a 1 : 1 ratio to yield a binuclear AgI-tetracarbene complex of the composition [(L)2Ag2](PF6)2 which undergoes a facile transmetalation reaction with [Cu(SMe2)Br] to deliver the corresponding CuI-NHC complex [(L)2Cu2](PF6)2. In contrast, the [L-H2]Br2 reacts with [Ir(Cp*)Cl2]2 to generate a doubly C-H activated IrIII-NHC complex 5. Similarly, the triazolinylidene donor supported diorthometalated RuII-complex 6 is also obtained. Complexes 5 and 6 represent the first examples of a stable diorthometalated binuclear IrIII/RuII-complex supported by 1,2,4-triazolin-5-ylidene donors. The synthesized IrIII-NHC complex 5 is found to be more effective than its RuII-analogue (6) for the reduction of a range of alkenes/alkynes via the transfer hydrogenation strategy. Conversely, RuII-complex 6 is identified as an efficient catalyst (0.01 mol% loading) for the β-alkylation of a wide range of secondary alcohols using primary alcohols as alkylating partners via a borrowing hydrogen strategy.

Efficient and Practical Transfer Hydrogenation of Ketones Catalyzed by a Simple Bidentate Mn-NHC Complex

Catalytic reductions of carbonyl-containing compounds are highly important for the safe, sustainable, and economical production of alcohols. Herein, we report on the efficient transfer hydrogenation of ketones catalyzed by a highly potent Mn(I)-NHC complex. Mn-NHC 1 is practical at metal concentrations as low as 75 ppm, thus approaching loadings more conventionally reserved for noble metal based systems. With these low Mn concentrations, catalyst deactivation is found to be highly temperature dependent and becomes especially prominent at increased reaction temperature. Ultimately, understanding of deactivation pathways could help close the activity/stability-gap with Ru and Ir catalysts towards the practical implementation of sustainable earth-abundant Mn-complexes.