Fluxional Behavior of cis-Fe(DMeOPrPE)2(H)2 (DMeOPrPE = 1,2-[bis(dimethoxypropyl)phosphino]ethane); Implications for the Pressure Swing Purification of Natural Gas

Abstract

Materials such as trans-Fe(DMeOPrPE)2(X)(H2O)+ (DMeOPrPE = 1,2-[bis(dimethoxypropyl)phosphino]ethane; X = Cl or H) show considerable promise as sorbents for the removal of N2 from natural gas in a pressure swing process. One problem, however, is that in long-term tests these sorbents slowly degrade over a period of many weeks. This study probed the degradation mechanism for the purpose of finding ways to stabilize the sorbents. To investigate the likelihood of degradation initiated by Fe–P bond dissociation, the fluxional behavior of the related cis-Fe(DMeOPrPE)2(H)2 complex was investigated by variable temperature 1P{1H} NMR spectroscopy. Analysis of the spectra gave the following activation parameters for the fluxional process: ΔH‡ = 11.6 kcal/mol and ΔS‡ = −9.2 cal/mol K. These results suggest that Fe–P bond dissociation (20–45 kcal/mol) is not occurring in the fluxional process and by implication that Fe–P bond dissociation is not initiating the decomposition of the trans-Fe(DMeOPrPE)2(X)(H2O)+ sorbents. Consequently, macrocycle ligands may not be the best approach for stabilizing the sorbents, as we have previously proposed. A “tetrahedral jump” mechanism is proposed for the fluxional process in cis-Fe(DMeOPrPE)2(H)2.