Molybdenum(VI) cis-Dioxo Complexes with Chiral Schiff Base Ligands: Synthesis, Characterization, and Catalytic Applications

Zhou, Xiangge; Zhao, Jin; Santos, Ana Clara; Kühn, Fritz E.

doi:10.1515/znb-2004-11-1240

Cited by 48 publications

(6 citation statements)

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“…Transition metal complexes with chiral N^O-chelate Schiff base ligands are of continued interest − and have been used efficiently as chiral catalysts. , We have recently paid attention to the syntheses, characterization, diastereoselectivity, and molecular structures of four-coordinated nonplanar chiral M( R - or S -N^O) 2 complexes (N^O = chiral deprotonated Schiff base ligands, M = Rh/Cu/Ni/Zn). − In the formation of tetrahedral or distorted tetrahedral/square-planar complexes containing two chiral N^O-chelate units, diastereoselectivity plays a major role because the resulting C 2 -symmetrical molecules may show preferential chirality at metal center (Λ- or Δ-configuration) induced by the ligand chirality. Thus, use of the enantiopure R - or S -N^O chelates can lead to the formation of diastereomeric pairs of Λ-M- R -N^O and Δ-M- R -N^O (or Δ-M- S -N^O and Λ-M- S -N^O) in a selective way, while the racemic R / S -N^O ligands give all four isomers.…”

Section: Introductionmentioning

confidence: 99%

Synthesis, X-ray, and Spectroscopic Study of Dissymmetric Tetrahedral Zinc(II) Complexes from Chiral Schiff Base Naphthaldiminate Ligands with Apparent Exception to the ECD Exciton Chirality

et al. 2016

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Bidentate enantiopure Schiff base ligands, (R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminate (R- or S-N^O), diastereoselectively provide Λ- or Δ-chiral-at-metal four-coordinated Zn(R- or S-N^O)2 {Ar = C6H5; Zn-1R or Zn-1S and p-C6H4OMe; Zn-2R or Zn-2S}. Two R- or S-N^O-chelate ligands coordinate to the zinc(II) in a tetrahedral mode and induce Λ- or Δ-configuration at the zinc metal center. In the solid state, the R- or S-ligand diastereoselectively gives Λ- or Δ-Zn configuration, respectively, and forms enantiopure crystals. Single crystal structure determinations show two symmetry-independent molecules (A and B) in each asymmetric unit to give Z' = 2 structures. Electronic circular dichroism (ECD) spectra show the expected mirror image relationship resulting from diastereomeric excess toward the Λ-Zn for R-ligands and Δ-Zn for S-ligands in solution. ECD spectra are well reproduced by TDDFT calculations, while the application of the exciton chirality method, in the common point-dipole approximation, predicts the wrong sign for the long-wavelength couplet. A dynamic diastereomeric equilibrium (Λ vs Δ) prevails for both R- and S-ligand-metal complexes in solution, respectively, evidenced by (1)H NMR spectroscopy. Variable temperature (1)H NMR spectra show a temperature-dependent shift of the diastereomeric equilibrium and confirm Δ-Zn configuration (for S-ligand) to be the most stable one and favored at low temperature. DSC analyses provide quantitative diastereomeric excess in the solid state for Zn-2R and Zn-2S, which is comparable to the results of solution studies.

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Section: Introductionmentioning

confidence: 99%

Synthesis, X-ray, and Spectroscopic Study of Dissymmetric Tetrahedral Zinc(II) Complexes from Chiral Schiff Base Naphthaldiminate Ligands with Apparent Exception to the ECD Exciton Chirality

et al. 2016

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“…The ligands (1a-20a) and complexes 1b, 2b, 8b, 10b, 14b-17b, and 20b used in this study were synthesized by modifications of literature protocols. [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] Representative procedure for the synthesis of salan ligands https://doi.org/10.26434/chemrxiv-2024-k94kx ORCID: https://orcid.org/0000-0002-0466-3042 Content not peer-reviewed by ChemRxiv. License: CC BY-NC-ND 4.0…”

Section: Methodsmentioning

confidence: 99%

“…H,H L4MoO2 (15b) 59 : Yield (0.310 g, 75%). 3, 129.4, 128.7, 120.8, 120.4, 118.9, 57.8, 50.1, 29.6, 24.3.…”

Section: Representative Procedures For the Synthesis Of Molybdenum Co...mentioning

confidence: 99%

Molybdenum Catalysts based on Salan Ligands for the Deoxydehydration Reaction

Wagner,

Tang,

Wagner

et al. 2024

Preprint

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Dioxomolybdenum complexes based on salan ligands have been evaluated for their potential in catalyzing the deoxydehydration (DODH) reaction. The DODH reaction is a formal reduction that converts vicinal diols into olefins using an oxometal catalyst and a sacrificial reductant. The reaction holds enormous potential in transforming biomass-derived molecules into platform chemicals. This study evaluated 20 molybdenum complexes supported by salan ligands in the DODH reaction with the goal of establishing structure-activity relationships. Catalyst screenings were performed using styrene glycol as a model substrate and 1-10 mol% loading of the molybdenum complexes at 170 oC producing styrene in up to 54% yield. Aliphatic diols and meso-/R,R-hydrobenzoin were also converted to the corresponding alkenes in moderate to good yields that are comparable to previously reported molybdenum catalysts. A bio-derived glycol, (+)−diethyltartrate, could be converted to the alkene product (diethyl fumarate) in >98% yield using 10 mol% catalyst. A high yield of diethyl fumarate (78%) was also obtained with Na2SO3 (cheap, readily available, and benign) as reductant. Quite significantly, a 42% yield of diethyl fumarate was also obtained at a 1 mol% catalyst loading which represents a turnover number (TON) of 42; this is one of highest activity in a DODH reaction observed with molybdenum catalysts. The catalytic studies along with preliminary kinetic investigations reveal significant ligand effects: sterically bulky ortho-substituents and electron-withdrawing para-substituents on the phenol arms were found to enhance catalytic activity while a rigid phenyl as well as an ethylene backbone featuring a tertiary amine were observed to impede catalysis.

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“…The ligands ( 1a–20a ) and complexes 1b , 2b , 8b , 10b , 14b–17b , and 20b used in this study were synthesized by modifications of literature protocols. 46,49–62…”

Section: Methodsmentioning

confidence: 99%

“…The ligands (1a-20a) and complexes 1b, 2b, 8b, 10b, 14b-17b, and 20b used in this study were synthesized by modifications of literature protocols. 46,[49][50][51][52][53][54][55][56][57][58][59][60][61][62] Representative procedure for the synthesis of salan ligands A round bottom flask was charged with 1 mmol of diamine (1 equivalent), 10 mL methanol, and salicylaldehyde (2 equivalents). The solution was stirred overnight at room temperature.…”

Section: Catalysis Science and Technology Papermentioning

confidence: 99%