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1. Synthesis, structural characterization and NMR studies of group 10 metal complexes with macrocyclic amine N-heterocyclic carbene ligands

   https://doi.org/10.1039/C7DT04666A  

   Lu, Taotao ; Liu, Zhiming ; Steren, Carlos A. ; Fei, Fan ; Cook, Tabitha M. ; Chen, Xue-Tai ; Xue, Zi-Ling ( January 2018 , Dalton Transactions)

   A series of Ni( ii ), Pd( ii ) and Pt( ii ) complexes [ML][PF 6 ] 2 [L = L 1 , M = Ni ( 1 ), Pd ( 2 ), Pt ( 3 ); L = L 2 , M = Ni ( 4 ), Pd ( 5 ), Pt ( 6 )] and [Pt(L 2 )(acac)] ( 7 ) have been prepared by the reactions of two tetradentate macrocyclic amine-NHC ligand precursors, [H 2 L 1 ][PF 6 ] 2 and [H 2 L 2 ][PF 6 ] 2 , with Ni(OAc) 2 ·4H 2 O, Pd(OAc) 2 and Pt(acac) 2 in the presence of NaOAc. Complex 7 is isolated along with 6 from the same reaction between [H 2 L 2 ][PF 6 ] 2 and Pt(acac) 2 . There are two atropisomers in 1–3 and two achiral conformers in 4–6 . The crystal structures of 1–3 and one conformer of 4–6 ( 4a–6a ) have been determined by single-crystal X-ray diffraction studies. The metal ion is found to reside in the cavity of the macrocyclic ring and adopts a square-planar configuration. Detailed NMR studies including variable-temperature NMR spectroscopy reveal a dynamic interconverting process between two atropisomers of 1–3 in the solutions via a ring twisting mechanism. Two conformers in the equilibrated solution of 4–6 , probably arising from the orientation of two amine N–H bonds with respect to the coordination plane, exchange slowly. Time-dependent 1 H NMR spectra show that one conformer ( 4a–6a ) in solution converts into the other ( 4b–6b ) via the inversion of the nitrogen atom. 

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2. The curious cases of tetrahydrosalen-type ligands interacting with Ni( ii ): structures and ligand-based oxidation reactions

   https://doi.org/10.1039/d3dt02023a  

   Xu, Vivian ; Pandey, Bedraj ; Jayawardhena, J. P. ; Krause, Jeanette A. ; Guan, Hairong ( August 2023 , Dalton Transactions)

   This work centers around the nickel complexes derived from two tetrahydrosalen-type proligands: N , N ′-bis(2-hydroxybenzyl)- o -phenylenediamine (H 2 salophan) and N , N ′-bis(2-hydroxy-3-methylbenzyl)- o -phenylenediamine (H 2 salophan_Me). The reaction of H 2 salophan with Ni(OAc) 2 ·4H 2 O generates a dinuclear complex Ni 2 (Hsalophan) 2 (OAc) 2 or Na[Ni 2 (salophan) 2 (OAc)] when NaOH is added to assist ligand deprotonation. The reaction of H 2 salophan_Me with Ni(OAc) 2 ·4H 2 O, however, yields a mononuclear complex Ni(Hsalophan_Me) 2 . Unlike the corresponding salen-type nickel complexes, these tetrahydrosalen-type complexes are paramagentic and air sensitive (in solution). Oxidation by O 2 or peroxides results in dehydrogenation of the ligand backbone to form the salen-type complexes. 

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3. Reaction Mechanism Underlying Pd(II)-Catalyzed Oxidative Coupling of Ethylene and Benzene to Form Styrene: Identification of a Cyclic Mono-Pd II Bis-Cu II Complex as the Active Catalyst

   https://doi.org/10.1021/acs.organomet.2c00183  

   Musgrave, Charles B. ; Bennett, Marc T. ; Ellena, Jeffrey F. ; Dickie, Diane A. ; Gunnoe, T. Brent ; Goddard, William A. ( July 2022 , Organometallics)

   A recent advance in the synthesis of alkenylated arenes was the demonstration that the Pd(OAc)2 catalyst precursor gives >95% selectivity toward styrene from ethylene and benzene under optimized conditions using excess Cu(II) carboxylate as the in situ oxidant [ Organometallics 2019, 38(19), 3532−3541]. To understand the mechanism underlying this catalysis, we applied density functional theory (DFT) calculations in combination with experimental studies. From DFT calculations, we determined the lowest-energy multimetallic Pd and Pd–Cu mixed metal species as possible catalyst precursors. From the various structures, we determined the cyclic heterotrinuclear complex PdCu2(μ-OAc)6 to be the global minimum in Gibbs free energy under conditions of excess Cu(II). For cyclic PdCu2(μ-OAc)6 and the parent [Pd(μ-OAc)2]3, we evaluated the barriers for benzene C–H activation through concerted metalation deprotonation (CMD). The PdCu2(μ-OAc)6 cyclic trimer leads to a CMD barrier of 33.5 kcal/mol, while the [Pd(μ-OAc)2]3 species leads to a larger CMD barrier at >35 kcal/mol. This decrease in the CMD barrier arises from the insertion of Cu(II) into the trimetallic species. Because cyclic PdCu2(μ-OAc)6 is likely the predominant species under experimental conditions (the Cu to Pd ratio is 480:1 at the start of catalysis) with a predicted CMD barrier within the range of the experimentally determined activation barrier, we propose that cyclic PdCu2(μ-OAc)6 is the Pd species responsible for catalysis and report a full reaction mechanism based on DFT calculations. For catalytic conversion of benzene and ethylene to styrene at 120 °C using Pd(OAc)2 as the catalyst precursor and Cu(OPiv)2 (OPiv = pivalate) as the oxidant, an induction period of ∼1 h was observed, followed by catalysis with a turnover frequency of ∼2.3 × 10–3 s–1. In situ1H NMR spectroscopy experiments indicate that during the induction period, Pd(OAc)2 is likely converted to cyclic PdCu2(η2-C2H4)3(μ-OPiv)6, which is consistent with the calculations and consistent with the proposal that the active catalyst is the ethylene-coordinated heterotrinuclear complex cyclic PdCu2(η2-C2H4)3(μ-OPiv)6. 

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4. Purinyl N -directed aroylation of 6-arylpurine ribo- and 2′-deoxyribonucleosides, and mechanistic insights

   https://doi.org/10.1039/D4OB00689E  

   Lakshman, Mahesh K ; Malinchak, Casina T ; Shank, Nathaniel ; Neary, Michelle C ; Stahl, Lothar ( August 2024 , Organic & Biomolecular Chemistry)

   The purinyl nitrogen atom is an effective metalation director, which in the presence of Pd(OAc)₂,t-BuOOH, and aryl aldehydes, leads to acylation of the aryl ring at the C6 position of the purine. 

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5. Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc) 2 -Catalyzed Aerobic Allylic Acetoxylation in the Absence and Presence of a Co(salophen) Cocatalyst

   https://doi.org/10.1021/acscatal.1c01074  

   Kozack, Caitlin V. ; Tereniak, Stephen J. ; Jaworski, Jonathan N. ; Li, Bao ; Bruns, David L. ; Knapp, Spring M. ; Landis, Clark R. ; Stahl, Shannon S. ( April 2021 , ACS Catalysis)

   null (Ed.)

   Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O2) as the terminal oxidant typically benefit from the use of benzoquinone (BQ) and a transition-metal (TM) cocatalyst, such as Co(salophen), to support oxidation of Pd0 during catalytic turnover. We previously showed that Pd(OAc)2 and 4,5-diazafluoren-9-one (DAF) as an ancillary ligand catalyze allylic oxidation with O2 in the absence of cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)2 catalytic activity, nearly matching the performance of reactions that include both BQ and Co(salophen). These observations are complemented by mechanistic studies of DAF/Pd(OAc)2 catalyst systems under three different oxidation conditions: (1) O2 alone, (2) O2 with cocatalytic BQ, and (3) O2 with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to the synergistic roles of O2 and BQ, both of which are capable of oxidizing Pd0 to PdII. The reaction of O2 generates H2O2 as a byproduct, which can oxidize hydroquinone to quinone in the presence of PdII. NMR spectroscopic studies, however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while inclusion of Co(salophen) maintains oxidized quinone throughout the reaction, resulting in better reaction performance. 

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