Low-Coordinate Iron
Figure 10. Limiting resonance structures for metal-alkyne and metal-
alkene complexes. Though these descriptions are exaggerated, they help to
rationalize the properties of the iron-alkene and iron-alkyne complexes
described here.
Figure 9. Relative binding energies for different ligands, with the most
strongly bound ligands at the bottom. In some cases, only a lower limit on
the energy difference could be determined. The precision of the values is
better than 1 kJ mol-1 (0.2 kcal mol-1).
of other N4 macrocyclic ligands are typically formulated as
FeI.35 There is a single example of an iron(I) hydride
complex.36 A more recent approach to isolating formally FeI
compounds uses sterically hindered ligands. Examples of this
type of complex include PhTptBuFe(CO), [PhBP3]Fe(PPh3),15
and (iPrPDI)FeX (X ) Cl, Me).37 However, the fundamental
coordination chemistry of low-coordinate FeI complexes is
still under development.
In this work, we show that displacement of N2 in LMe-
FeNNFeLMe gives a variety of complexes of the type LMe-
Fe(ligand). This substitution reaction provides a general route
to low-coordinate, formally FeI complexes with various
Lewis bases as the second binding ligand. We previously
described the addition of PPh3, CO, and benzene to LMe-
FeNNFeLMe to give LMeFePPh3, LMeFe(CO)3, and LMe-
(benzene).14 Here, we focus on alkene and alkyne complexes
and use measurements of Keq to compare the binding of these
ligands to the phosphine and arene ligands.
Discussion
Synthesis and Isolation of Stable Mononuclear Fe
Complexes with a Formal Oxidation State of 1+. The
number of mononuclear FeI complexes is much fewer than
FeII and FeIII species. Most work on FeI has focused on gas-
phase Fe+ ions, which are capable of C-C and C-H bond
activation.29 However, these are not isolable, leaving the
further characterization of FeI as a challenge for synthetic
chemists. Cyclopentadienyl- and arene-containing organo-
metallic complexes account for a large fraction of known
FeI complexes.30 Strong back-bonding ligands stabilize FeI
in [Fe(CO)x(PR)y]+ 31 and nitrosyl complexes.32 Several
examples of formally FeI centers supported by porphyrin
have been reported, although there is controversy over
whether the best description of the complexes is FeII(porph•-)
or FeI(porph).33 This ambiguity in the oxidation states is also
evident in complexes of some S-based ligands.34 Complexes
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Evaluation of C-C Bonding. Alkene and alkyne com-
plexes can be described in terms of the two limiting
resonance structures shown in Figure 10.38 (a) In one
extreme, the metal has a primarily electrostatic interaction
with the ligand (best described through the dative interaction
at the left of Figure 10). For example, in alkyne complexes
of Ag, the C-C distance and C-C stretching frequency are
similar to those in the free ligand.39 Because the primary
metal-ligand interaction is donation of ligand electrons to
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