Ruthenium pincer complexes: Ligand design and complex synthesis

Tridentate κ3-P,P,C iridium complexes: influence of ligand saturation on intramolecular C-H bond activation

Whereas κ3-P,C,P-based donor ligands are ubiquitous across synthetic chemistry, related unsymmetric systems having κ3-P,P,C-coordination are comparatively rare. In this contribution, we expose a new κ3-P,P,C ligand system, bearing a C3-anionic linker and its coordination chemistry with iridium. The title ligand has been coordinated in saturated and unsaturated forms. The degree of ligand saturation affected an onward (and unusual) oxidative rearrangement reaction.

Double Intramolecular 1,2 C-H Addition of o-Methyl Groups To Form Ruthenium Pincer Double Tuck-In Complexes

Ruthenium pincer complexes have a rich history of coordination and reaction chemistries. In this work, we report our discoveries of previously unreported Ru pincer coordination geometries. We found that mono tuck-in κ4-ArPNHPRuLCl complexes react with NaN(SiMe3)2 producing double tuck-in mer-κ5-ArPNHPRuL complexes. Interestingly, when κ4-MesPNHPRuCl is dehydrohalogenated, the resulting double tuck-in complex binds N2, forming the nitrogen complex κ5-MesPNHPRuN2. The mer-κ5-ArPNHPRuL complexes thermally isomerize to the fac-κ5-ArPNHPRuL isomers, which is an uncommon reaction for pincer complexes. The mer-κ5-ArPNHPRuL complexes react with CO and CO2 to form amide κ4-ArPNHPRu(CO)L or carbamate κ5-ArPN(CO2)PRuL complexes, respectively, supporting the hypothesis that the κ4-ArPNPRuL amide intermediates are accessible and reactive.

Cinchona-Alkaloid-Derived NN Ligands and Achiral Phosphines for Iridium-Catalyzed Asymmetric Hydrogenation of Heteroaromatic and α-Chloroheteroaryl Ketones

A concise synthesis of cinchona-alkaloid-derived NN ligands bearing alkyl substituents on chiral nitrogen atoms was described. Iridium catalysts containing new chiral NN ligands and achiral phosphines were effective for the asymmetric hydrogenation of heteroaromatic ketones, which afforded corresponding alcohols in up to 99.9% ee. The same protocol was applicable to the asymmetric hydrogenation of α-chloroheteroaryl ketones. Most importantly, the gram-scale asymmetric hydrogenation of 2-acetylthiophene and 2-acetylfuran proceeded smoothly even under 1 MPa of H2.

Bimetallic complexes that merge metallocene and pincer-metal building blocks: synthesis, stereochemistry and catalytic reactivity

This perspective is to illustrate the synthesis and applications of bimetallic complexes by merging a metallocene and a (cyclopentadienyl/aryl) pincer metal complex. Four possible ways to merge metallocene and pincer-metal motifs are reported and representative examples are discussed in more detail. These bimetallic complexes have been employed in some important catalytic reactions such as cross-coupling, transfer hydrogenation or synthesis of ammonia. The metallocene fragment may tune the electronic properties of the pincer ligand, due to its redox reversible properties. Also, the presence of two metals in a single complex allows their electronic communication, which proved beneficial for, e.g., the catalytic activity of some species. The presence of the metallocene fragment provides an excellent opportunity to develop chiral catalysts, because the metallocene merger generally renders the two faces of the pincer-metal catalytic site diastereotopic. Besides, an extra chiral functionality may be added to the bimetallic species by using pincer motifs that are planar chiral, e.g. by using the different substituents of pincer ligand "arms" or non-symmetrical arene groupings. Post-functionalization of pre-formed pincer-metal complexes, via η⁶-coordination with an areneophile such as [CpRu]⁺ and [Cp*Ru]⁺ presents a striking strategy to obtain diastereomeric metallocene-pincer type derivatives, that actually involve half-sandwich metallocenes. This approach offers the possibility to create diastereomerically pure derivatives by using the chiral TRISPHAT anion. The authors hope that this report of the synthetic, physico-chemical properties and remarkable catalytic activities of metallocene-based pincer-metal complexes will inspire other researchers to continue exploring this realm.

Development of a ruthenium–aquo complex for utilization in synthesis and catalysis for selective hydration of nitriles and alkynes

A ruthenium(ii)–aquo complex serves as a precursor for the synthesis of new ternary complexes and also as an efficient catalyst for selective hydration of aryl nitriles to aryl amides and aryl alkynes to aryl aldehydes.

Chemically robust and readily available quinoline-based PNN iron complexes: application in C-H borylation of arenes

Iron catalysts have been used for over a century to produce ammonia industrially. However, the use of iron catalysts generally remained quite limited until relatively recently, when the abundance and low toxicity of iron spurred the development of a variety of iron catalysts. Despite the fact that iron catalysts are being developed as alternatives to precious metal catalysts, their reactivities and stabilities are quite different because of their unique electronic structures. In this context, our group previously developed a new family of quinoline-based PNN pincer-type ligands for low- to mid-valent iron catalysts. These chemically robust PNN ligands provide air- and moisture-tolerant iron complexes, which exhibit excellent catalytic performances in the C-H borylation of arenes. This feature article summarises our recent work on PNN iron complexes, including their conception and design, as well as related reports on iron pincer complexes and iron-catalysed C-H borylation reactions.

Designed pincer ligand supported Co(II)-based catalysts for dehydrogenative activation of alcohols: Studies on N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines

Base-metal catalysts Co1, Co2 and Co3 were synthesized from designed pincer ligands L1, L2 and L3 having NNN donor atoms respectively. Co1, Co2 and Co3 were characterized by IR, UV-Vis. and ESI-MS spectroscopic studies. Single crystal X-ray diffraction studies were investigated to authenticate the molecular structures of Co1 and Co3. Catalysts Co1, Co2 and Co3 were utilized to study the dehydrogenative activation of alcohols for N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines. Under optimized reaction conditions, a broad range of substrates including alcohols, anilines and ketones were exploited. A series of control experiments for N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines were examined to understand the reaction pathway. ESI-MS spectral studies were investigated to characterize cobalt-alkoxide and cobalt-hydride intermediates. Reduction of styrene by evolved hydrogen gas during the reaction was investigated to authenticate the dehydrogenative nature of the catalysts. Probable reaction pathways were proposed for N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines on the basis of control experiments and detection of reaction intermediates.

Calculations on the non-classical β-hydride elimination observed in trans-(H)(OMe)-Ir(Ph)(PMe3)3: possible production and reaction of methyl formate

The octahedral trans hydrido-alkoxide complex trans-(H)(OMe)-Ir(Ph)(PMe3)3 (2-OCH3) was prepared by Milstein and coworkers by addition of methanol to Ir(Ph)(PMe3)3 (1). 2-OCH3 was discovered to undergo a methanol catalyzed outer-sphere carbonyl de-insertion in which a vacant coordination site is not required. The reaction yields the octahedral trans dihydride complex trans-(H)2-Ir(Ph)(PMe3)3 (2-H) as a kinetic product along with formaldehyde derivatives reported as [CH2=O]x. We investigate the mechanism and products of this reaction using density functional theory. The de-insertion transition state has an ion-pair character leading to a high barrier in benzene continuum: ΔG ‡ = 27.9 kcal/mol. Adding one methanol molecule by H-bonding to the alkoxide of 2-OCH3 lowers the barrier to 22.7 kcal/mol. When the calculations are conducted in a methanol continuum, the barrier drops to 8.8 kcal/mol. However, the thermodynamics of de-insertion are endergonic by near 5 kcal/mol in both benzene and methanol. The calculations identify a low energy outer-sphere H/OMe metathesis pathway that transforms the formaldehyde and another 2-OCH3 molecule directly into a second 2-H complex and methyl formate. Likewise, a second H/OCH3 metathesis reaction interconverting methyl formate and 2-OCH3 into 2-H and dimethyl carbonate is computed to be exergonic and kinetically facile. These results imply that the production of methyl formate and dimethyl carbonate from 2-OCH3 is plausible in this system. The net transformation from the square planar 1 and methanol to 2-H and either methyl formate or dimethyl carbonate would represent a unique stoichiometric dehydrogenative coupling reaction taking place at room temperature by an outer-sphere mechanism.

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.

Asymmetric copper-catalyzed fluorination of cyclic β-keto esters in a continuous-flow microreactor

A highly enantioselective homogeneous fluorination of cyclic β-keto esters catalyzed by diphenylamine linked bis(oxazoline)-Cu(OTf)2 complexes has been established in a continuous flow microreactor. The microreactor allowed an efficient transformation with reaction times ranging from 0.5 to 20 min, and the desired products were afforded in high yields (up to 99%) with excellent enantioselectivities (up to 99% ee) at a low catalyst loading of 1 mol%.

Synthesis of a Redox-Active NNP-Type Pincer Ligand and Its Application to Electrocatalytic CO2 Reduction With First-Row Transition Metal Complexes

We report the synthesis of a rigid phosphine-substituted, redox-active pincer ligand and its application to electrocatalytic CO₂ reduction with first-row transition metal complexes. The tridentate ligand was prepared by Stille coupling of 2,8-dibromoquinoline and 2-(tributylstannyl)pyridine, followed by a palladium-catalyzed cross-coupling with HPPh₂. Complexes were synthesized from a variety of metal precursors and characterized by NMR, high-resolution mass spectrometry, elemental analysis, and cyclic voltammetry. Formation of bis-chelated metal complexes, rather than mono-chelated complexes, was favored in all synthetic conditions explored. The complexes were assessed for their ability to mediate electrocatalytic CO₂ reduction, where the cobalt complex was found to have the best activity for CO₂-to-CO conversion in the presence of water as an added proton source.

Effects of the number of cyclometalated rings and ancillary ligands on the rate of MeI oxidative addition to platinum(ii)–pincer complexes

The reactivity of new organoplatinum(ii)–pincer complexes in their oxidative addition reactions with MeI is related to the ancillary ligand and the number of cyclometalated rings present in the coordination sphere of the Pt centre.

Homogenous electrochemical water oxidation by a nickel(ii) complex based on a macrocyclic N-heterocyclic carbene/pyridine hybrid ligand

Electrochemical water oxidation catalyzed by a homogeneous Ni–NHC/pyridine complex demonstrated electrolyte-dependent catalytic performances. The catalyst displayed a stable catalytic current of oxygen evolution in long-term bulk electrolysis.

Hydrogen elimination reactivity of ruthenium pincer hydride complexes: a DFT study

The pincer effect is explained for various pincer hydride complexes, differing in the donor atoms, using activation barriers, and MESP parameters.

Cobalt-Pincer Complexes in Catalysis

Non-noble metal catalysts based on pincer type compounds are of special interest for organometallic chemistry and organic synthesis. Next to iron and manganese, currently cobalt-pincer type complexes are successfully applied in various catalytic reactions. In this review the recent progress in (de)hydrogenation, transfer hydrogenation, hydroboration and hydrosilylation as well as dehydrogenative coupling reactions using cobalt-pincer complexes is summarised.

Synthesis and characterization of TADDOL-based chiral group six PNP pincer tricarbonyl complexes

ABSTRACT

The new chiral PNP pincer ligand N 2,N 6-bis((3aR, 8aR)-2,2-dimethyl-4,4,8,8-tetraphenyltetrahydro[1,3]dioxolo[4,5-e][1,3,2]dioxaphosphepin-6-yl)pyridine-2,6-diamine (PNP-TADDOL) was synthesized in 80% isolated yield. Complexes of the type [M(PNP-TADDOL)(CO)3] (M = Cr, Mo, and W) were prepared via a solvothermal approach. This methodology constitutes a fast, simple, and practical synthetic method to obtain complexes of that type in high isolated yields. The X-ray structure of the molybdenum complex is presented.

GRAPHICAL ABSTRACT

Unsymmetrical NCN-pincer mononuclear and dinuclear nickel(ii) complexes of N-heterocyclic carbene (NHC): synthesis, structure and catalysis for Suzuki-Miyaura cross-coupling

This report describes the synthesis and characterization of a family of unsymmetrical NCN pincer Ni(ii) complexes 2-8 with NHC-triazole arms. All of these complexes have been fully characterized by X-ray single crystal analysis, NMR spectroscopy, and elemental analysis. Complexes 2 and 4-6 were square planar with a chloride trans to the carbene carbon atoms. Complex 3 was a paramagnetic octahedral complex with its central metal surrounded by two NCN pincer ligands. Complexes 7 and 8 contain [(NHC)₂Ni₂-OH] moieties bearing a OH bridge. Both the [(NHC)₂Ni₂-OH] complexes 7 and 8 and [(NC_NHCN)Ni-Cl] complexes 2 and 4-6 were synthesized similarly via the reactions of the in situ formed Ag-NHCs from the corresponding imidazolium salts with [NiCl₂(PPh₃)₂]. The catalytic activities of all complexes for Suzuki-Miyaura cross-coupling were examined. Under the optimized conditions, complex 4 was active in the Suzuki-Miyaura cross-coupling reactions of aryl iodides and aryl bromides at 110 °C. Aryl chlorides were successfully coupled in the presence of triphenylphosphine as an additive.