Formation of a Hexacarbonyl Diiron Complex Having a Naphthalene-1,8-bis(phenylphosphido) Bridge and the Electrochemical Behavior of Its Derivatives

The P–P bond of the cis-1 ligand in (μ-cis-1)­[Fe­(CO)₄]₂ (cis-1 = naphthalene-1,8-diphenyldiphosphine) was cleaved by the two iron centers after CO dissociation from the iron centers, although the P–P bond of cis-1 was stereochemically stabilized with a robust naphthalene group, unlike the usual diphosphines, which lack such support. The resulting (μ-nabip)­[Fe­(CO)₃]₂ (3; nabip = naphthalene-1,8-bis­(phenylphosphido)) had the diiron core linked by the bisphosphido bridge. Since the trans isomer (μ-trans-1)­[Fe­(CO)₄]₂ was stable under ambient conditions, the cis disposition of the two Fe­(CO)₄ fragments was responsible for the cleavage of the P–P bond. The one or two terminal CO ligands of 3 can be replaced by MeCN and a range of phosphine ligands: i.e., PMe₃, PPh₃, cis-1, and trans-1. Interestingly, it was found that the diphosphine cis-1 could coordinate the iron center in an unusual κ² 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 CH₂Cl₂ to form cationic complexes having a μ-H bridge between the two iron centers. The parent hexacarbonyl complex 3 could act as a proton reduction catalyst at −2.0 V in the presence of TsOH as the proton source in CH₂Cl₂. 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.