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

The trivalent phosphorus-bridged [2]ferrocenophane complex 2 having NEt 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) fragments to form (μ-2)-[Fe(CO) ] , while the bulkier Cr(CO) fragment formed only a monochromium complex [Cr(κ -2)(CO) ]. Dissociation of CO from [Cr(κ -2)(CO) ] changed the coordination mode of 2 from κ to κ to form [Cr(κ -2)(CO) ] having a three-membered ring. A similar approach for the monoiron complex [Fe(κ -2)(CO) ] did not afford a κ complex but instead an Et NPC(O)PNEt -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) as a chelate diphosphine. The reaction of this product with an Fe(CO) fragment gave μ-{Fe(C H PNEt ) -κP:κP}-[Fe(CO) ] (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 NEt groups of the bridging unit were expected to work as protonation sites. The protonated NEt 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.