New Iron–Phosphine Macrocycle Complexes for Use in the Pressure-Swing Purification of Natural Gas

Abstract

Two new synthetic routes to macrocyclic tetradentate phosphine complexes of Fe(II) were investigated. The interest in these materials stems from their potential application in the pressure-swing purification of natural gas contaminated by N2. Both synthetic routes used Fe(II) as a templating metal in order to avoid using Ni, Pd, or Pt, which can be difficult to remove from the macrocyclic phosphine ligand once it has formed. The first synthetic route involved the alkylation of a complex with two bidentate secondary phosphine ligands, specifically trans-[Fe(P2)2(CH3CN)2](PF6)2, where P2 is a bidentate secondary phosphine. Three electrophilic alkylating reagents (1,3-dibromopropane, 1,4-dibromobutane, and o-dibromoxylene) with the potential to bridge were investigated under a variety of reaction conditions, but all failed to yield a macrocycle complex. The second route involved the alkylation of an Fe complex that had a linear tetradentate secondary phosphine ligand. Again, a number of reactions conditions were investigated but all failed to give a macrocycle complex. A number of factors likely contribute to the inability of Fe(II) to act as a template in these reactions. It is proposed that the absence of reactivity can be attributed to the decreased electron density in the d6 iron(II) atom in comparison to the electron-rich d8 and d10 metals normally used as templates. After deprotonation of the coordinated secondary phosphines, the decreased electron density reduces the nucleophilicity of the resulting phosphido ligands and prevents the alkylation reaction from occurring. To aid in the structural characterization of the Fe(II) complexes with secondary phosphine ligands, the X-ray crystal structures of the cis-Fe(MPPP)2Cl2 and trans-[Fe(MPPP)2(CH3CN)2](PF6)2 complexes (MPPP = 1,3-bis(phenylphosphino)propane) were determined.