Synthesis and coordination chemistry of (PNEt₂)₂-bridged [2]ferrocenophanes

The trivalent phosphorus-bridged [2]ferrocenophane complex 2 having NEt₂ groups on the respective phosphorus centers was prepared, and its reactions as a diphosphine ligand were examined for iron and chromium carbonyl complexes. Both the phosphorus centers of 2 coordinated to Fe(CO)₄ fragments to form (μ-2)-[Fe(CO)₄]₂, while the bulkier Cr(CO)₅ fragment formed only a monochromium complex [Cr(κ¹-2)(CO)₅]. Dissociation of CO from [Cr(κ¹-2)(CO)₅] changed the coordination mode of 2 from κ¹ to κ² to form [Cr(κ²-2)(CO)₄] having a three-membered ring. A similar approach for the monoiron complex [Fe(κ¹-2)(CO)₄] did not afford a κ² complex but instead an Et₂NPC(O)PNEt₂-bridged [3]ferrocenophane complex in which a CO fragment was inserted into the P-P bond of 2 and both the phosphorus centers coordinated to Fe(CO)₃ as a chelate diphosphine. The reaction of this product with an Fe(CO)₄ fragment gave μ-{Fe(C₅H₄PNEt₂)₂-κP:κP}-[Fe(CO)₃]₂ (8), in which one terminal CO and the CO group between the two phosphorus atoms were lost to give an [FeFe]hydrogenase mimic having a bis(phosphido)ferrocene chelate as a bridging unit. The two NEt₂ groups of the bridging unit were expected to work as protonation sites. The protonated NEt₂ groups contributed to an improvement in the reduction potential of the complex to a less negative area, i.e., -2.3 V for the free 8 to -1.0 V for the diprotonated 8. The catalytic reduction of the proton, however, required a more negative potential of -2.0 V, which is almost comparable to that of the phosphido-bridged [FeFe]hydrogenase model complex having no protonation site.