Reaction of a polydentate cysteine-based ligand and its nickel(ii) complex with electrophilic and nucleophilic methyl-transfer reagents - from S-methylation to acetyl coenzyme A synthase reactivity

Nickel(ii)dibenzotetramethyltetraaza[14]annulene complex immobilized on amino-functionalized TUD-1: an efficient catalyst for immediate and quantitative epoxidation of cyclohexene under ambient conditions

Nickel(ii)dibenzotetramethyltetraaza[14]annulene complex immobilized on amino-functionalized TUD-1 as a new nanocatalyst for spontaneous and quantitative epoxidation of cyclohexene under ambient conditions.

Synthesis, Characterization, and Efficient Catalytic Activities of a Nickel(II) Porphyrin: Remarkable Solvent and Substrate Effects on Participation of Multiple Active Oxidants

A new nickel(II) porphyrin complex, [Ni^II (porp)] (1), has been synthesized and characterized by ¹ H NMR, ¹³ C NMR and mass spectrometry analysis. This Ni^II porphyrin complex 1 quantitatively catalyzed the epoxidation reaction of a wide range of olefins with meta-chloroperoxybenzoic acid (m-CPBA) under mild conditions. Reactivity and Hammett studies, H₂¹⁸ O-exchange experiments, and the use of PPAA (peroxyphenylacetic acid) as a mechanistic probe suggested that participation of multiple active oxidants Ni^II -OOC(O)R 2, Ni^IV -Oxo 3, and Ni^III -Oxo 4 within olefin epoxidation reactions by the nickel porphyrin complex is markedly affected by solvent polarity, concentration, and type of substrate. In aprotic solvent systems, such as toluene, CH₂ Cl₂ , and CH₃ CN, multiple oxidants, Ni^II -(O)R 2, Ni^IV -Oxo 3, and Ni^III -Oxo 4, operate simultaneously as the key active intermediates responsible for epoxidation reactions of easy-to-oxidize substrate cyclohexene, whereas Ni^IV -Oxo 3 and Ni^III -Oxo 4 species become the common reactive oxidant for the difficult-to-oxidize substrate 1-octene. In a protic solvent system, a mixture of CH₃ CN and H₂ O (95:5), the Ni^II -OOC(O)R 2 undergoes heterolytic or homolytic O-O bond cleavage to afford Ni^IV -Oxo 3 and Ni^III -Oxo 4 species by general acid catalysis prior to direct interaction between 2 and olefin, regardless of the type of substrate. In this case, only Ni^IV -Oxo 3 and Ni^III -Oxo 4 species were the common reactive oxidant responsible for olefin epoxidation reactions.

Half-sandwich complexes of iridium and ruthenium containing cysteine-derived ligands

The dimers [{(ηⁿ-ring)MCl}₂(μ-Cl)₂] ((ηⁿ-ring)M = (η⁵-C₅Me₅)Ir, (η⁶-p-MeC₆H₄iPr)Ru) react with the modified cysteines S-benzyl-l-cysteine (HL1) or S-benzyl-α-methyl-l-cysteine (HL2) affording cationic complexes of the formula [(ηⁿ-ring)MCl(κ²N,S-HL)]Cl (1, 2) in good yield. Addition of NaHCO₃ to complexes 1 and 2 gave equilibrium mixtures of neutral [(ηⁿ-ring)MCl(κ²N,O-L)] (3, 4) and cationic [(ηⁿ-ring)M(κ³N,O,S-L)]Cl (6Cl, 7Cl) complexes. Similar mixtures were obtained in one-pot reaction by successive addition of the modified cysteine and NaHCO₃ to the above formulated dimers. Addition of the N-Boc substituted cysteine derivative S-benzyl-N-Boc-l-cysteine (HL3) and NaHCO₃ to the dimers [{(ηⁿ-ring)MCl}₂(μ-Cl)₂] affords the neutral compounds [(ηⁿ-ring)MCl(κ²O,S-L3)] ((ηⁿ-ring)M = (η⁵-C₅Me₅)Ir (5a), (η⁶-p-MeC₆H₄iPr)Ru (5b)). Complexes of the formula [(ηⁿ-ring)MCl(κ³N,O,S-L)][SbF₆] (6Sb-8Sb), in which the cysteine derivative acts as a tridentate chelate ligand, can be prepared by adding one equivalent of AgSbF₆ to the solutions of compounds 5 or to the mixtures of complexes 3/6Cl and 4/7Cl. The amide proton of compounds 8aSb and 8bSb can be removed by addition of NaHCO₃ affording the neutral complexes [(ηⁿ-ring)M(κ³N,O,S-L3-H)] ((ηⁿ-ring)M = (η⁵-C₅Me₅)Ir (9a), (η⁶-p-MeC₆H₄iPr)Ru (9b)). Complexes 9a and 9b can also be prepared by reacting the dimers [{(ηⁿ-ring)MCl}₂(μ-Cl)₂] with HL3 and two equivalents of NaHCO₃. The absolute configuration of the complexes has been established by spectroscopic and diffractometric means including the crystal structure determination of (R_Ir,R_C,R_S)-[(η⁵-C₅Me₅)Ir(κ³N,O,S-L1)][SbF₆] (6aSb). The thermodynamic parameters associated with the epimerization at sulphur that the iridium compound [(η⁵-C₅Me₅)Ir(κ³N,O,S-L3-H)] (9a) undergoes have been determined through variable temperature ¹H NMR studies.