Four‐Coordinate Trispyrazolylboratomanganese and ‐iron Complexes with a Pyrazolato Co‐ligand: Syntheses and Properties as Oxidation Catalysts

Abstract

A series of complexes of the type [(Tp(R1,R2))M(X)] (Tp = trispyrazolylborato) with R(1)/R(2) combinations Me/tBu, Ph/Me, iPr/iPr, Me/Me and for M = Mn or Fe coordinating [Pz(Me,tBu)](-) (Pz = pyrazolato) or Cl(-) as co-ligand X has been synthesised. Although the chloride complexes were very unreactive and stable in air, the pyrazolato series was far more reactive in contact with oxidants like O(2) and tBuOOH. The [(Tp(R1,R2))M(Pz(Me,tBu))] complexes proved to be active pre-catalysts for the oxidation of cyclohexene with tBuOOH, reaching turnover frequencies (TOFs) ranging between moderate and good in comparison to other manganese catalysts. Cyclohexene-3-one and cyclohexene-3-ol were always found to represent the main products, with cyclohexene oxide occasionally formed as a side product. The ratios of the different oxidation products varied with the reaction conditions: in the case of a peroxide/alkene ratio of 4:1, considerably more ketone than alcohol was obtained and cyclohexene oxide formation was almost negligible, whereas a ratio of 1:10 led to a significant increase of the alcohol proportion and to the formation of at least small amounts of the epoxide. Pre-treatment of the dissolved [(Tp(R1,R2))M(Pz(Me,tBu))] pre-catalysts with O(2) led to product distributions and TOFs that were very similar to those found in the absence of O(2), so that it may be argued that tBuOOH and O(2) both lead to the same active species. The results of EPR spectroscopy and ESI-MS suggest that the initial product of the reaction of [(Tp(Me,Me))Mn(Pz(Me,tBu))] with O(2) contains a Mn(III)(O)(2)Mn(IV) core. Prolonged exposure to O(2) leads to a different dinuclear complex containing three O-bridges and resulting in different TOFs/product distributions. Analogous findings were made for other complexes and formation of these overoxidised products may explain the deviation of the catalytic performances if the reactions are carried out in an O(2) atmosphere.

Enhancing Tris(pyrazolyl)borate‐Based Models of Cysteine/Cysteamine Dioxygenases through Steric Effects: Increased Reactivities, Full Product Characterization and Hints to Initial Superoxide Formation

“…Symmetric and asymmetrics tretching vibrations of the SO 2 units bound in the oxygenated products 2b,[ Tp Mes Fe(O 2 S-CH 2 -CH-NH 2 )(CO 2 Et)], 5b,[ Tp Mes Fe( 18 O 2 S-CH 2 -CH 2 -NH 2 )], 5b( 18 O), and [Tp Mes *Fe(O 2 S-CH 2 -CH 2 -NH 2 )], 5'b. eaction with18 O 2 (99.9 %) generated the labeled complexes 5a ( 18 O) and 5b ( 18 O), as confirmed by electrospray ionization mass spectrometry (ESI-MS), which revealed the expected shifts by four units for the [(Tp Mes Fe(O 2 S-CH 2 -CH-NH 2 )(CO 2 Et))+ +K] + peak from m/z 842.290 to 846.291 and from m/z 770.263 to 774.267f or the [(Tp Mes Fe(O 2 S-CH 2 -CH 2 -NH 2 ))+ +K] + peak (seeFigure 3andSupporting Information, Figure S50). In the IR spectra18 O-labeling led to as hift of the S=O and FeÀOv ibrations to lower wavenumbers for 5a( 18 O) and 5b( 18 O).…”

“…eaction with18 O 2 (99.9 %) generated the labeled complexes 5a ( 18 O) and 5b ( 18 O), as confirmed by electrospray ionization mass spectrometry (ESI-MS), which revealed the expected shifts by four units for the [(Tp Mes Fe(O 2 S-CH 2 -CH-NH 2 )(CO 2 Et))+ +K] + peak from m/z 842.290 to 846.291 and from m/z 770.263 to 774.267f or the [(Tp Mes Fe(O 2 S-CH 2 -CH 2 -NH 2 ))+ +K] + peak (seeFigure 3andSupporting Information, Figure S50). In the IR spectra18 O-labeling led to as hift of the S=O and FeÀOv ibrations to lower wavenumbers for 5a( 18 O) and 5b( 18 O). Exemplarily,i nc ase of complex 5b( 18 O) the two asymmetric vibrations shifted by 23 and 19 cm À1…”

A Trispyrazolylborato Iron Cysteinato Complex as a Functional Model for the Cysteine Dioxygenase

“…[8] To create a shielding reaction pocket approaching the situation in enzymes and also to prevent the formation of FeÀOÀFe units, phenyl groups at the 3-positions of the pyrazolate units seemed advantageous. A suitable starting material was therefore the complex [Tp Me,Ph FeCl] (Tp Me,Phe = hydridotris(3-phenyl-5-methylpyrazol-1-yl)borato), [9] which, after dissolution in dichloromethane, was reacted with lcysteine ethyl ester hydrochloride (l-HCysOEt·HCl) in the presence of triethylamine. After workup of the reaction mixture, the cysteinato complex [Tp Me,Ph FeCysOEt] (1) could be isolated enantiomerically pure in a yield of 65 % (Scheme 2).Compound 1 crystallizes from CH 2 Cl 2 in the chiral space group P2 1 , and the molecular structure is shown in Figure 1.…”

A Structural and Functional Model for the 1‐Aminocyclopropane‐1‐carboxylic Acid Oxidase

“…[10] To mimic mononuclear non-heme iron dioxygenases based on a3 -His structural motif,w eh ave in recent years successfully employed the tris(pyrazolyl)borato (Tp) ligand system, [11] which in the past has also been employed to simulate representatives based on the 2-His-1-carboxylate structural motif. Hence,w ec hose phenyl residues at the 3-position of the pyrazole donors within Tp to create as hielding reaction pocket approaching the situation in the enzyme.Asuitable starting material was therefore the complex [Tp Me,Ph FeCl] [13] (Tp Me,Phe = 3-Phenyl-5-methylhydrido-trispyrazol-1-ylborato), which after dissolution in dichloromethane was reacted with the potassium salt of the amino acid 1-amino-cyclopropane-1-carboxylic acid (Scheme 2). Hence,w ec hose phenyl residues at the 3-position of the pyrazole donors within Tp to create as hielding reaction pocket approaching the situation in the enzyme.Asuitable starting material was therefore the complex [Tp Me,Ph FeCl] [13] (Tp Me,Phe = 3-Phenyl-5-methylhydrido-trispyrazol-1-ylborato), which after dissolution in dichloromethane was reacted with the potassium salt of the amino acid 1-amino-cyclopropane-1-carboxylic acid (Scheme 2).…”