Mononuclear metal-O2 complexes bearing macrocyclic N-tetramethylated cyclam ligands

Abstract

Metalloenzymes activate dioxygen to carry out a variety of biological reactions, including the biotransformation of naturally occurring molecules, oxidative metabolism of xenobiotics, and oxidative phosphorylation. The dioxygen activation at the catalytic sites of the enzymes occurs through several steps, such as the binding of O(2) at a reduced metal center, the generation of metal-superoxo and -peroxo species, and the O-O bond cleavage of metal-hydroperoxo complexes to form high-valent metal-oxo oxidants. Because these mononuclear metal-dioxygen (M-O(2)) adducts are implicated as key intermediates in dioxygen activation reactions catalyzed by metalloenzymes, studies of the structural and spectroscopic properties and reactivities of synthetic biomimetic analogues of these species have aided our understanding of their biological chemistry. One particularly versatile class of biomimetic coordination complexes for studying dioxygen activation by metal complexes is M-O(2) complexes bearing the macrocyclic N-tetramethylated cyclam (TMC) ligand. This Account describes the synthesis, structural and spectroscopic characterization, and reactivity studies of M-O(2) complexes bearing tetraazamacrocyclic n-TMC ligands, where M ═ Cr, Mn, Fe, Co, and Ni and n = 12, 13, and 14, based on recent results from our laboratory. We have used various spectroscopic techniques, including resonance Raman and X-ray absorption spectroscopy, and density functional theory (DFT) calculations to characterize several novel metal-O(2) complexes. Notably, X-ray crystal structures had shown that these complexes are end-on metal-superoxo and side-on metal-peroxo species. The metal ions and the ring size of the macrocyclic TMC ligands control the geometric and electronic structures of the metal-O(2) complexes, resulting in the end-on metal-superoxo versus side-on metal-peroxo structures. Reactivity studies performed with the isolated metal-superoxo complexes reveal that they can conduct electrophilic reactions such as oxygen atom transfer and C-H bond activation of organic substrates. The metal-peroxo complexes are active oxidants in nucleophilic reactions, such as aldehyde deformylation. We also demonstrate a complete intermolecular O(2)-transfer from metal(III)-peroxo complexes to a Mn(II) complex. The results presented in this Account show the significance of metal ions and supporting ligands in tuning the geometric and electronic structures and reactivities of the metal-O(2) intermediates that are relevant in biology and in biomimetic reactions.

FIGURE 1

Crystal structures of M-O₂ complexes: (A) [Cr^III(14-TMC)(O₂)(Cl)]⁺ (1); (B) [Mn^III(14-TMC)(O₂)]⁺ (2); (C) [Mn^III(13-TMC)(O₂)]⁺ (3); (D) [Fe^III(14-TMC)-(O₂)]⁺ (4); (E) [Co^III(13-TMC)(O₂)]⁺ (6); (F) [Co^III(12-TMC)(O₂)]⁺ (7); (G) [Ni^III(12-TMC)(O₂)]⁺ (9).

FIGURE 2

Plot of O–O stretching frequency (cm⁻¹) versus O–O bond distance (Å) for side-on metal–O₂ complexes. Circles represent experimental data points, and squares represent theoretical ones. The solid line represents a least-squares linear fit of the experimental and theoretical data. Data points for [Fe^III(14-TMC)(O₂)]⁺ (4) (red right triangle), [Co^III(13-TMC)(O₂)]⁺ (6) (magenta up triangle), [Co^III(12-TMC)- (O₂)]⁺ (7) (blue down triangle), and [Ni^III(12-TMC)(O₂)]⁺ (9) (green tilted square) are included in the diagram.

FIGURE 3

Plot of M-O versus O–O bond distances (Å) of M–O₂ complexes. The dashed line represents a least-squares linear fit (y = −0.52x + 2.61, R² = 0.794) of the experimental data except 4, although the data point of 4 is included in the diagram.

FIGURE 4

Plot of log k₂ of [Mn(13-TMC)(O₂)(X)]⁺ (3-X; X = N₃⁻, CF₃CO₂⁻, NCS⁻, CN⁻) at 0 °C against E_p,a values of 3-X.

FIGURE 5

UV–vis spectral changes showing the formation of [Mn(14-TMC)(O₂)]⁺ (2) (red) and the disappearance of [Ni(12-TMC)(O₂)]⁺ (9) (green) by addition of [Mn(14-TMC)]²⁺ to a solution of 9. Inset shows the spectroscopic titration at 453 nm for the formation of 2 as a function of the equivalents of [Mn(14-TMC)]²⁺ added to a solution of 9.

SCHEME 1

End-on and Side-on Metal–O₂ Complexes

SCHEME 2

TMC Ligands Used in the Synthesis of M–O₂ Complexes

SCHEME 3

Reactions Showing the Synthesis of Metal–Superoxo and –Peroxo Complexes

SCHEME 4

Preferred Geometry of M–O₂ Complex Depending on the Oxidation State of Metal Ion

SCHEME 5

Reactivities of End-on Metal–Superoxo and Side-on Metal–Peroxo Complexes

SCHEME 6

Proposed Mechanism for the Deformylation of CCA by [Mn^III(14-TMC)(O₂)]⁺ (2)

SCHEME 7

Axial Ligand Effect of Metal–Peroxo Complexes in Nucleophilic Reactions

SCHEME 8

Reaction Scheme Showing an Intermolecular O₂-Transfer between Metal Complexes