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

A series of complexes of the type [(TpR1,R2)M(X)] (Tp=trispyrazolylborato) with R1/R2 combinations Me/tBu, Ph/Me, iPr/iPr, Me/Me and for M=Mn or Fe coordinating [PzMe,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 O2 and tBuOOH. The [(TpR1,R2)M(PzMe,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 [(TpR1,R2)M(PzMe,tBu)] pre‐catalysts with O2 led to product distributions and TOFs that were very similar to those found in the absence of O2, so that it may be argued that tBuOOH and O2 both lead to the same active species. The results of EPR spectroscopy and ESI‐MS suggest that the initial product of the reaction of [(TpMe,Me)Mn(PzMe,tBu)] with O2 contains a MnIII(O)2MnIV core. Prolonged exposure to O2 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 O2 atmosphere. The active ingredient: Trispyrazolylborato(Tp)iron and ‐manganese complexes containing pyrazolato (Pz) co‐ligands were prepared and systematically investigated as pre‐catalysts for the oxidation of cyclohexene with tBuOOH (see scheme). Good activities were found, and the results of investigations concerning the active species suggested these to be dinuclear.