Electronic structure and reactivity of three-coordinate iron complexes

Abstract

[Reaction: see text]. The identity and oxidation state of the metal in a coordination compound are typically thought to be the most important determinants of its reactivity. However, the coordination number (the number of bonds to the metal) can be equally influential. This Account describes iron complexes with a coordination number of only three, which differ greatly from iron complexes with octahedral (six-coordinate) geometries with respect to their magnetism, electronic structure, preference for ligands, and reactivity. Three-coordinate complexes with a trigonal-planar geometry are accessible using bulky, anionic, bidentate ligands (beta-diketiminates) that steer a monodentate ligand into the plane of their two nitrogen donors. This strategy has led to a variety of three-coordinate iron complexes in which iron is in the +1, +2, and +3 oxidation states. Systematic studies on the electronic structures of these complexes have been useful in interpreting their properties. The iron ions are generally high spin, with singly occupied orbitals available for pi interactions with ligands. Trends in sigma-bonding show that iron(II) complexes favor electronegative ligands (O, N donors) over electropositive ligands (hydride). The combination of electrostatic sigma-bonding and the availability of pi-interactions stabilizes iron(II) fluoride and oxo complexes. The same factors destabilize iron(II) hydride complexes, which are reactive enough to add the hydrogen atom to unsaturated organic molecules and to take part in radical reactions. Iron(I) complexes use strong pi-backbonding to transfer charge from iron into coordinated alkynes and N 2, whereas iron(III) accepts charge from a pi-donating imido ligand. Though the imidoiron(III) complex is stabilized by pi-bonding in the trigonal-planar geometry, addition of pyridine as a fourth donor weakens the pi-bonding, which enables abstraction of H atoms from hydrocarbons. The unusual bonding and reactivity patterns of three-coordinate iron compounds may lead to new catalysts for oxidation and reduction reactions and may be used by nature in transient intermediates of nitrogenase enzymes.

Figure 1

Bulky β-diketiminate ligands, which stabilize three-coordinate complexes of iron. L^Me has R = Me, and L^tBu has R = ^tBu.

Figure 2

The trigonal planar geometry allows the isolation of iron(II) complexes containing “hard” ligands. (a) Thermal-ellipsoid plot of L^tBuFeF. (b) Thermal-ellipsoid plot of L^tBuFeOFeL^tBu.

Figure 3

Changes in d orbital energies that result from the 95° bite angle of the β-diketiminate ligand. The yz and z² orbitals are very close in energy, giving unusual magnetic properties. The xy orbital, which points directly at the N atoms of the β-diketiminate ligand, becomes highest in energy. (In this picture, the z axis is chosen to be out of the plane, rather than the standard axis choice in the C_2v point group.)

Figure 4

¹H NMR spectrum of L^MeFe(n-butyl) at room temperature, showing peak assignments and integrations.

Figure 5

Ligand-field splitting in some common geometries, compared to the C_3v symmetry of pseudotetrahedral complexes of tripodal ligands, and the C_2v symmetry of trigonal-planar β-diketiminate complexes. In each complex, the z axis is vertical on the page; note that the change in orientation causes the orbital labels for β-diketiminate complexes to be different than in Figures 3 and 6. The bold red labels highlight the two d orbitals that have the correct symmetry for π-interactions with the ligand labeled X.

Figure 6

Effect of π-acceptor and π-donor ligands on the energies of the ligand-field orbitals of three-coordinate iron compounds, as shown by computational studies.,,, Interestingly, iron(I), iron(II), and iron(III) complexes can each have three electrons in the nearly degenerate yz/z² orbitals. As in Figure 3, a non-standard choice of axes is used.

Figure 7

Resonance structures for L^RFeNNFeL^R. The coupling model on the right is more consistent with Mössbauer data and computations.

Figure 8

Thermal-ellipsoid plot of L^MeFeSFeL^Me, which features two low-coordinate iron(II) ions bridged by sulfide.

Scheme 1

An extreme bond-breaking reaction of three-coordinate molybdenum.

Scheme 2

The coordination number in iron(II) chloride complexes depends on the size of R, as evidenced by changes in the C-N-C angle (highlighted with bold bonds).

Scheme 3

Products from reaction of an iron(I) source with 1-azidoadamantane. tert-Butyl pyridine catalyzes N₂ loss to give an imidoiron(III) complex that abstracts hydrogen atoms from the ligand or added substrates.

Scheme 4

Insertion and bond cleavage reactions of a low-coordinate iron(II) hydride complex. The hydride dimer is in equilibrium with a three-coordinate monomer.

Chart 1