Nonheme FeIV═O Complexes Supported by Four Pentadentate Ligands: Reactivity toward H- and O- Atom Transfer Processes

The [(Bn-tpen)FeII]2+ Complex as a Catalyst for the Oxidation of Cyclohexene and Limonene with Dioxygen

[(Bn-tpen)FeII(MeCN)](ClO4)2, containing the pentadentate Bn-tpen-N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane ligand, was studied in the oxygenation of cyclohexene and limonene using low-pressure dioxygen (0.2 atm air or 1 atm pure O2) in acetonitrile. 2-Cyclohexen-1-one and 2-cyclohexen-1-ol are the main products of cyclohexene oxidations, with cyclohexene oxide as a minor product. Limonene is oxidized to limonene oxide, carvone, and carveol. Other oxidation products such as perillaldehyde and perillyl alcohol are found in trace amounts. This catalyst is slightly less active than the previously reported [(N4Py)FeII(MeCN)](ClO4)2 (N4Py-N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine). Based on cyclic voltammetry experiments, it is postulated that [(Bn-tpen)FeIV=O]2+ is the active species. The induction period of approx. 3 h during cyclohexene oxygenation is probably caused by deactivation of the reactive Fe(IV)=O species by the parent Fe(II) complex. Equimolar mixtures of Fe(II) salt and the ligand (in situ-formed catalyst) gave catalytic performance similar to that of the synthesized catalyst.

NMR and Mössbauer Studies Reveal a Temperature-Dependent Switch from S = 1 to 2 in a Nonheme Oxoiron(IV) Complex with Faster C-H Bond Cleavage Rates

S = 2 FeIV═O centers generated in the active sites of nonheme iron oxygenases cleave substrate C-H bonds at rates significantly faster than most known synthetic FeIV═O complexes. Unlike the majority of the latter, which are S = 1 complexes, [FeIV(O)(tris(2-quinolylmethyl)amine)(MeCN)]2+ (3) is a rare example of a synthetic S = 2 FeIV═O complex that cleaves C-H bonds 1000-fold faster than the related [FeIV(O)(tris(pyridyl-2-methyl)amine)(MeCN)]2+ complex (0). To rationalize this significant difference, a systematic comparison of properties has been carried out on 0 and 3 as well as related complexes 1 and 2 with mixed pyridine (Py)/quinoline (Q) ligation. Interestingly, 2 with a 2-Q-1-Py donor combination cleaves C-H bonds at 233 K with rates approaching those of 3, even though Mössbauer analysis reveals 2 to be S = 1 at 4 K. At 233 K however, 2 becomes S = 2, as shown by its 1H NMR spectrum. These results demonstrate a unique temperature-dependent spin-state transition from triplet to quintet in oxoiron(IV) chemistry that gives rise to the high C-H bond cleaving reactivity observed for 2.