Abstract
In order to synthesize new iron (II) complexes of 1,5-diaza-3,7-diphosphacyclooctanes with a wider variety of the substituents on ligands heteroatoms (including functionalized ones, namely, pyridyl groups) and co-ligands, it was found that these ligands with relatively small phenyl, benzyl, and pyridin-2-yl substituents on phosphorus atoms in acetonitrile formed bis-P,P-chelate cis-complexes [L2Fe(CH3CN)2]2+ (BF4)2−, whereas P-mesityl-substituted ligand formed only monoligand P,P-complex [LFe(CH3CN)4]2+ (BF4)2−. 3,7-dibenzyl-1,5-di(1′-(R)-phenylethyl)-1,5-diaza-3,7-diphosphacyclooctane reacted with hexahydrate of iron (II) tetrafluoroborate in acetone to give an unusual bis-ligand cationic complex of the composition [L2Fe(BF4)]+ BF4−, where two fluorine atoms of the tetrafluoroborate unit occupied two pseudo-equatorial positions at roughly octahedral iron ion, according to X-ray diffraction data. 1,5-diaza-3,7-diphosphacyclooctanes replaced tetrahydrofurane and one of the carbonyl ligands of cyclopentadienyldicarbonyl(tetrahydrofuran)iron (II) tetrafluoroborate to form heteroligand complexes [CpFeL(CO)]+BF4−. The structural studies in the solid phase and in solutions showed that diazadiphosphacyclooctane ligands of all complexes adopted chair-boat conformations so that their nitrogen atoms were in close proximity to the central iron ion. The redox properties of the obtained complexes were performed by the cyclic voltammetry method.
Highlights
The complexes of aminomethylphosphines with earth-abundant metals, such as nickel, cobalt, and iron, draw permanent attention as molecular catalysts and electrocatalysts for the activation of small molecules (H2, N2, CO2, etc.) [1,2,3,4,5,6]
The most studied objects are the nickel (II) complexes of 1,5-diaza-3,7-diphosphacyclooctanes, which show the high effectiveness in the electrocatalysis, both of the reduction of protons to generate H2 and of the oxidation of H2 [1,2,3,7], and have been successfully used as both the cathode and anode catalysts in fuel cells [4,8,9]
It has been shown that the nature of the central ion could essentially influence on the type of the complexes formed on the basis of 1,5-diaza-3,7-diphosphacyclooctanes
Summary
The complexes of aminomethylphosphines with earth-abundant metals, such as nickel, cobalt, and iron, draw permanent attention as molecular catalysts and electrocatalysts for the activation of small molecules (H2 , N2 , CO2 , etc.) [1,2,3,4,5,6]. It should be mentioned that the varying of the substituents on the nitrogen atoms of acyclic P2 N ligands, namely, bis(diethylphosphinomethyl)organylamines, has shown the essential influence of the nature of these substituents on the activity of the corresponding iron (II) complexes in the hydrogen electrooxidation [26]. These data indicate that the wider variety of the diazadiphosphacyclooctane ligands of iron complexes and the further study of their structures and properties is expedient for the search of new catalysts on the basis of these compounds. 1,5-diaza-3,7-diphosphacyclooctanes, including functionalized P-pyridyl substituted ligands, and the results of our study of their structures and redox properties were reported in this contribution
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