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1. Pyridinol-based CNC Pincer catalysts for carbon dioxide reduction: The big impact of one small remote group

   Papish, Elizabeth T. ; Das, Sanjit ; Boudreaux, Chance M. ; Burks, Dalton Bodine ; Rodrigues, Roberta R. ; Webster, Charles Edwin ; Delcamp, Jared H. ( April 2019 , ACS National Meeting Abstracts)

   The first examples of a CNC pincer ligands with a central pyridinol derived ring were recently reported.  The differences in catalytic reactivity between CNC ligands with a central pyridine ring vs. a pyridinol derived ring are substantial and highly active and robust catalysts have been synthesized and studied.  In these pincer ligands, the 4-substituent can be OMe, OH, or O , and these latter two options allow for altered catalyst properties as a function of proton concn.  Catalytic studies have used ruthenium(II), nickel(II), and other transition metals.  We have made metal complexes that can be protonated or deprotonated reversibly in situ to switch on or off the photocatalytic performance towards CO redn.  Furthermore, the methoxy group on the pyridine ring offers unique catalysis advantages not seen with the unsubstituted analog.  Our best catalysts offer selective CO formation, >300 turnover cycles, and a 40 h lifetime.  Highly active self-sensitized catalysts have recently been developed.  Steric and electronic ligand effects are being studied with these catalysts by exptl. and computational methods.
2. Stable abnormal N-heterocyclic carbenes and their applications

   https://doi.org/10.1039/c9cs00866G  

   Sau, Samaresh Chandra ; Hota, Pradip Kumar ; Mandal, Swadhin K. ; Soleilhavoup, Michele ; Bertrand, Guy ( February 2020 , Chemical Society Reviews)

   Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd( ii ), Cu( i ), Ni( ii ), Fe(0), Zn( ii ), Ag( i ), and Au( i / iii ) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki–Miyaura cross-coupling reaction, C–H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K( i ), Al( iii ), Zn( ii ), Sn( ii ), Ge( ii ), and Si( ii / iv ). Among them, K( i ), Al( iii ), and Zn( ii ) complexesmore »were used for the polymerization of caprolactone and rac -lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO 2 ), nitrous oxide (N 2 O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO 2 and the catalytic reduction of carbon dioxide, including atmospheric CO 2 , into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.« less
3. Ligand and solvent effects on CO ₂ insertion into group 10 metal alkyl bonds

   https://doi.org/10.1039/D1SC06346D  

   Deziel, Anthony P. ; Espinosa, Matthew R. ; Pavlovic, Ljiljana ; Charboneau, David J. ; Hazari, Nilay ; Hopmann, Kathrin H. ; Mercado, Brandon Q. ( February 2022 , Chemical Science)

   The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type ( R PBP)M(CH 3 ) ( R PBP = B(NCH 2 PR 2 ) 2 C 6 H 4 − ; R = Cy or t Bu; M = Ni or Pd) to generate κ 1 -acetate complexes of the form ( R PBP)M{OC(O)CH 3 } is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual R PBP ligand, which features a central boryl donor that exerts a strong trans -influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into ( R PBP)M(CH 3 ) is facile and occurs at room temperature because of the high trans -influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using ( t Bu PBP)Pd(CH 3more »). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into ( t Bu PBP)M(CH 3 ) (M = Ni or Pd) proceeds via an S E 2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.« less
4. Palladium(0) complexes of diferrocenylmercury diphosphines: synthesis, X-ray structure analyses, catalytic isomerization, and C–Cl bond activation

   https://doi.org/10.1039/d1dt00641j  

   Tagne Kuate, Alain C. ; Lalancette, Roger. A. ; Bockfeld, Dirk ; Tamm, Matthias ; Jäkle, Frieder ( April 2021 , Dalton Transactions)

   Palladium(0) phosphine complexes are of great importance as catalysts in numerous bond formation reactions that involve oxidative addition of substrates. Highly active catalysts with labile ligands are of particular interest but can be challenging to isolate and structurally characterize. We investigate here the synthesis and chemical reactivity of Pd 0 complexes that contain geometrically adaptable diferrocenylmercury-bridged diphosphine chelate ligands (L) in combination with a labile dibenzylideneacetone (dba) ligand. The diastereomeric diphosphines 1a (p S p R , meso -isomer) and 1b (p S p S -isomer) differ in the orientation of the ferrocene moieties relative to the central Ph 2 PC 5 H 3 –Hg–C 5 H 3 PPh 2 bridging entity. The structurally distinct trigonal LPd 0 (dba) complexes 2a ( meso ) and 2b (p S p S ) are obtained upon treatment with Pd(dba) 2 . A competition reaction reveals that 1b reacts faster than 1a with Pd(dba) 2 . Unexpectedly, catalytic interconversion of 1a ( meso ) into 1b ( rac ) is observed at room temperature in the presence of only catalytic amounts of Pd(dba) 2 . Both Pd 0 complexes, 2a and 2b , readily undergo oxidative addition into the C–Cl bond of CHmore »2 Cl 2 at moderate temperatures with formation of the square-planar trans -chelate complexes LPd II Cl(CH 2 Cl) ( 3a , 3b ). Kinetic studies reveal a significantly higher reaction rate for the meso -isomer 2a in comparison to (p S p S )- 2b .« less
5. Copper(I) SNS pincer complexes: Impact of ligand design and solvent coordination on conformer interconversion from spectroscopic and computational studies

   Lynn, M.A. ; Miecznikowski, J.R. ; Jasinski, J.P. ; Kaur, M. ; Mercado, B.Q. ; Reinheimer, E. ; Almanza, E. ; Kharbouch, R.M. ; Smith, M.R. ; Zygmont, S.E. ; et al ( September 2019 , Inorganica Chimica Acta)

   The syntheses and detailed characterizations (X-ray crystallography, NMR spectroscopy, cyclic voltammetry, infrared spectroscopy, electrospray mass spectrometry, and elemental analyses) of two new Cu(I) pincer complexes are reported. The pincer ligand coordinates through one nitrogen and two sulfur donor atoms and is based on bis-imidazole or bis-triazole precursors. These tridentate SNS ligands incorporate pyridine and thione-substituted imidazole or triazole functionalities with connecting methylene units that provide flexibility to the ligand backbone and enable high bite-angle binding. Variable temperature 1H NMR analysis of these complexes and of a similar zinc(II) SNS system shows that all are fluxional in solution and permits the determination of delta G double dagger and delta S double dagger. DFT calculations are used to model the fluxionality of these complexes and indicate that a coordinating solvent molecule can promote hemilability of the SNS ligand by lowering the energy barrier involved in the partial rotation of the methylene units.