High-Valent Nonheme Iron. Two Distinct Iron(IV) Species Derived from a Common Iron(II) Precursor

Abstract

The reaction of [Fe(II)(beta-BPMCN)(OTf)2] (1, BPMCN = N,N'-bis(2-pyridylmethyl)-N,N'-dimethyl-trans-1,2-diaminocyclohexane) with tBuOOH at low-temperature yields alkylperoxoiron(III) intermediates 2 in CH2Cl2 and 2-NCMe in CH3CN. At -45 degrees C and above, 2-NCMe converts to a pale green species 3 (lambda(max) = 753 nm, epsilon = 280 M(-1) cm(-1)) in 90% yield, identified as [Fe(IV)(O)(BPMCN)(NCCH3)]2+ by comparison to other nonheme [Fe(IV)(O)(L)]2+ complexes. Below -55 degrees C in CH2Cl2, 2 decays instead to form deep turquoise 4 (lambda(max) = 656, 845 nm; epsilon = 4000, 3600 M(-1) cm(-1)), formulated to be an unprecedented alkylperoxoiron(IV) complex [Fe(IV)(BPMCN)(OH)(OOtBu)]2+ on the basis of Mössbauer, EXAFS, resonance Raman, NMR, and mass spectral evidence. The reactivity of 1 with tBuOOH in the two solvents reveals an unexpectedly rich iron(IV) chemistry that can be supported by the BPMCN ligand.

A synthetic precedent for the [Fe IV 2 (μ-O) 2 ] diamond core proposed for methane monooxygenase intermediate Q

“…Given that the iron sites of the Fe III Fe IV species 1a and 1b are low-spin (formally S 1 ϭ 1/2 and S 2 ϭ 1), it is reasonable to assume that the two local sites of 2b have S 1 ϭ S 2 ϭ 1. This assignment is in accord with observations by us and others that mononuclear and dinuclear Fe IV complexes with N 4 O 2 and N 3 O 3 ligation have S ϭ 1 ground states (12,13,16,18). To date, no mononuclear octahedral Fe IV complex with an S ϭ 0 ground state has been reported.…”

“…Indeed, the FeOFe distance of 2b is comparable to the value of 2.68 Å found in the crystal structure of 1c (14). Finally, as with previous EXAFS studies of Fe complexes with pyridine ligands (14,16), the fit also is improved by inclusion of an additional shell of six C atoms at 2.93 Å, accounting for the shoulder at rЈ ϭ 2.5 Å and corresponding to the pyridine ␣-carbon atoms. Thus, the XAS analysis provides strong support for the formulation of 2b as…”

“…In 2001, we claimed to have generated such a complex (15); however, additional data obtained in our laboratories strongly suggested that the compound should be reformulated as an Fe IV (OH)(OOR) species (16). In 2005, Collins and coworkers (17) reported the crystallographic and spectroscopic characterization of the first example of a ( -oxo)diiron(IV) complex; in this case, the high oxidation state was stabilized by the tetraanionic TAML ligand.…”

Contrasting cis and trans Effects on the Reactivity of Nonheme Oxoiron(IV) Complexes

“…[3,4] The formulation of 3 is established by its high-resolution electrospray ionization mass spectrum, in which the dominant ion cluster is observed at m/z 517.0818 with a mass and isotope distribution pattern corresponding to the [Fe À1 , which is consistent with the two-amine, two-pyridine ligand combination. [3,4,7,8] Its quadrupole splitting, in contrast, is the largest found thus far for this family of complexes; it is larger by 0.55 mm s À1 than that of 2 (DE Q = 1.24 mm s À1 ).…”

“…Another nonheme ligand with four N donor groups constrained to be in a plane is L 8 Py 2 (see 3), [6] which has two pyridine rings in place of two of the tertiary amine donors in tmc to eliminate the steric constraints that prevented binding of PyO to 2. (Figure 1 b, dashed line).…”