Role of the secondary coordination sphere in metal-mediated dioxygen activation

Abstract

Alfred Werner proposed nearly 100 years ago that the secondary coordination sphere has a role in determining the physical properties of transition-metal complexes. We now know that the secondary coordination sphere impacts nearly all aspects of transition-metal chemistry, including the reactivity and selectivity in metal-mediated processes. These features are highlighted in the binding and activation of dioxygen by transition-metal complexes. There are clear connections between control of the secondary coordination sphere and the ability of metal complexes to (1) reversibly bind dioxygen or (2) bind and activate dioxygen to form highly reactive metal-oxo complexes. In this Forum Article, several biological and synthetic examples are presented and discussed in terms of structure-function relationships. Particular emphasis is given to systems with defined noncovalent interactions, such as intramolecular H-bonds involving dioxygen-derived ligands. To further illustrate these effects, the homolytic cleavage of C-H bonds by metal-oxo complexes with basic oxo ligands is described.

Figure 1

Two examples of secondary coordination sphere interactions with crystalline lattices: (A) 18-crown-6.[Cu^II(NH₃)₄] and (B) 18-crown-6.[Mn^II(H₂O)₄(Cl)₂]. Only ammine and aquo hydrogen atoms are shown for clarity.

Figure 2

Common structural motifs that arise from the activation of dioxygen by iron, manganese, and copper complexes.

Figure 3

Molecular structures of the homodimeric copper trafficking protein Cu(Hah)₂ (PDB, 1FEE) (A) and myoglobin (PDB, 1A6M) (B).

Figure 4

Molecular structure of the Fe-O₂ picket-fence porphyrin complex.

Figure 5

Molecular structure of a Fe₂(μ-1,2-peroxo) complex containing a bulky binucleating ligand.

Figure 6

Molecular structures of Cu-superoxo complex, [CuTp^t-Bu,iPr(η²-O₂)] (A) and μ-η²:η²-peroxo dicopper(II) complex, [Cu^IITp^iPr,iPr]₂(O₂) (B) showing the controlling effects of steric bulk.

Figure 7

Active site structures of the oxygenated forms of human hemoglobin (A) and nematode hemoglobin (B) illustrating the different H-bonding networks.

Figure 8

Active site structure of SyrB2 showing the H-bond network surrounding the chloride ion. The red spheres represent water molecules.

Figure 9

H-bonding porphyrin systems: urea-modified “picket fence” porphyrin—only the urea group is shown for clarity (A), the system developed by Change (B), the Fe^III–OH "Hangman" porphyrin complex (C).

Figure 10

Molecular structures of metal-peroxo complexes with intramolecular H-bonds: [Mn^III(O₂)] (A), [V^V(O₂)(O)]⁻ (B), and [Cu^II(OOH)]⁺ (C).

Figure 11

Synthetic details and molecular structure for [Mn^IIHbpaa]. Selected distance (Å): Mn1–N1, 2.287(1), Mn1–N2, 2.157(1), Mn1–N3, 2.279(1), Mn1–N4, 2.266(2), Mn1–O2, 2.247(1), Mn1–O3, 2.047(1)

Figure 12

Parallel-mode EPR spectrum (black) and simulation (blue) of [Mn^IIIH₂bpaa(O₂)] (2 mM in THF) recorded at 2.2 K (A) and absorbance spectra of [Mn^IIH₂bpaa] (---) and [Mn^IIIH₂bpaa(O₂)] (—) (10 mM in DMSO) measured at room temperature (B). EPR parameters: Microwave frequency and power, 9.26 GHz, 0.2 mW; modulation, 10 G EPR simulation parameters: S = 2, g = 2.0, D = −2 cm⁻¹, E/D = 0.13, A = 160 MHz.

Figure 13

The active sites of cytochrome P450 (A) and compound 1 of cytochrome c peroxidase (PDB, 1ZBZ) highlight the intramolecular H-bonding networks surrounding the iron centers.

Figure 14

Design criteria for complexes with the H-bonding ligand [H₃buea]³⁻.

Figure 15

Molecular structures of [Fe^IIIH₃buea(O)]²⁻ (A) and [Mn^IIIH₃buea(O)]²⁻ (B) determined by X-ray diffraction methods.

Figure 16

Qualitative orbital diagrams for metal-oxo complexes with C_4v (left) and C₃ symmetry (right) (adapted from Mayer and Thorn).

Figure 17

Bonding decomposition scheme for the Fe^III–O complex illustrating the effects of the intramolecular H-bonding network.

Figure 18

Summary of the dioxygen reactivity for a series of Co^II complexes with varied H-bonding networks.

Figure 19

Thermodynamic cycles used to evaluate the BDE_OH for [Mn^IIH₃buea(OH)]²⁻ and [Mn^IIIH₃buea(OH)]⁻.

Figure 20

Proposed mechanism for the reactions of [Mn^IVH₃buea(O)]⁻ and [Mn^IIIH₃buea(O)]²⁻ with DHA.

Figure 21

Proposed structure of Compound I in cytochrome P450 (A) and the relationship between redox potential and pK_a for a metal-oxo species in the cleavage of a C—H bond in methane with BDE_C–H = 104 kcal/mol (B).

Scheme 1

Scheme 2

Synthetic route to the hybrid ligand, H₃bpaa. Conditions: (a) C₇H₉N, Et₃N, THF, 67°C, 76%; (b) C₆H₁₀, 20% Pd(OH)₂/C, EtOH, 78°C, 69%; (c) C₈H₇BrFNO, Et₃N, THF, 67°C, 92%.

Scheme 3

Synthetic procedure for the isolation of [M^IIIH₃buea(O)]²⁻ (M^III = Fe, Mn). Conditions: (a) 4 KH, DMA, Ar, rt; (b) M(OAc)₂, DMA, Ar, rt; (c) ½ O₂, DMA, rt