Formation of a Hexacarbonyl Diiron Complex Having
a Naphthalene-1,8-bis(phenylphosphido)
Bridge and the Electrochemical Behavior of Its Derivatives
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Abstract
The
P–P bond of the <i>cis</i>-<b>1</b> ligand
in (μ-<i>cis</i>-<b>1</b>)[Fe(CO)<sub>4</sub>]<sub>2</sub> (<i>cis</i>-<b>1</b> = naphthalene-1,8-diphenyldiphosphine)
was cleaved by the two iron centers after CO dissociation from the
iron centers, although the P–P bond of <i>cis</i>-<b>1</b> was stereochemically stabilized with a robust naphthalene
group, unlike the usual diphosphines, which lack such support. The
resulting (μ-nabip)[Fe(CO)<sub>3</sub>]<sub>2</sub> (<b>3</b>; nabip = naphthalene-1,8-bis(phenylphosphido)) had the diiron core
linked by the bisphosphido bridge. Since the trans isomer (μ-<i>trans</i>-<b>1</b>)[Fe(CO)<sub>4</sub>]<sub>2</sub> was
stable under ambient conditions, the <i>cis</i> disposition
of the two Fe(CO)<sub>4</sub> fragments was responsible for the cleavage
of the P–P bond. The one or two terminal CO ligands of <b>3</b> can be replaced by MeCN and a range of phosphine ligands:
i.e., PMe<sub>3</sub>, PPh<sub>3</sub>, <i>cis</i>-<b>1</b>, and <i>trans</i>-<b>1</b>. Interestingly,
it was found that the diphosphine <i>cis</i>-<b>1</b> could coordinate the iron center in an unusual κ<sup>2</sup> fashion to form a three-membered ring, which was confirmed by NMR
spectra as well as X-ray analysis. These diiron complexes can be protonated
with the strong acid TfOH in CH<sub>2</sub>Cl<sub>2</sub> to form
cationic complexes having a μ-H bridge between the two iron
centers. The parent hexacarbonyl complex <b>3</b> could act
as a proton reduction catalyst at −2.0 V in the presence of
TsOH as the proton source in CH<sub>2</sub>Cl<sub>2</sub>. When protonated
complexes having MeCN or phosphine ligands were used, the proton reduction
potentials catalyzed by these complexes were shifted to a more positive
range of around −1.77 to −1.37 V, depending on the terminal
ligand