Effect of Bridgehead Steric Bulk on the Intramolecular C–H Heterolysis of [FeFe]-Hydrogenase Active Site Models Containing a P₂N₂ Pendant Amine Ligand

A series of pendant amine-containing [FeFe]-hydrogenase models, [X­(CH₂S-μ)₂{Fe­(CO)₃}­{Fe­(CO)­(P₂^PhN₂^Bn)}] (<b>1</b>_H, X = CH₂; <b>2</b>_Me, C­(CH₃)₂; <b>3</b>_Et, C­(CH₂CH₃)₂; and P₂^PhN₂^Bn = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) with different groups at the bridgehead carbon of the S-to-S linker were synthesized. The oxidations of these complexes as well as the reverse reduction reaction were studied by cyclic voltammetry and <i>in situ</i> IR spectroscopy. Regardless of the bridgehead steric bulk, all three complexes demonstrate intramolecular iron-mediated C­(sp³)–H bond heterolytic cleavage with the assistance of the pendant amine base within the chelating diphosphine ligand in the two-electron oxidation process. X-ray crystallographic analysis shows that the doubly oxidized products, [<b>1</b>′_H]⁺, [<b>2</b>′_Me]⁺, and [<b>3</b>′_Et]⁺, all have a rigid FeSC three-membered ring at the open apical site of the rotated iron center. The most noticeable difference in structures of the oxidized complexes is that the single CO ligand of the rotated Fe­(P₂^PhN₂^Bn)­(CO) unit in [<b>1</b>′_H]⁺ and [<b>2</b>′_Me]⁺ is found below the Fe···Fe vector, while in [<b>3</b>′_Et]⁺ an unusually rotated Fe­(P₂^PhN₂^Bn)­(CO) moiety positions one of the P donors within the bidentate ligand under the Fe···Fe vector. The starting Fe^IFe^I complexes can be recovered from their corresponding doubly oxidized complexes by reduction in the presence of Brönsted acid