Synthesis, Characterization, and Reactivity of Fe Complexes Containing Cyclic Diazadiphosphine Ligands: The Role of the Pendant Base in Heterolytic Cleavage of H2

The iron complexes CpFe((P2N2Bn)-N-Ph)CI (1-CI), CpFe((P2N2Ph)-N-Ph)CI (2-CI), and CpFe((P2C5)-C-Ph)CI (3-CI) (where (P2N2Bn)-N-Ph is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, (P2N2Ph)-N-Ph is 1,3,5,7-tetrapheny1-1,5-diaza-3,7-diphosphacyclooctane, and (P2C5)-C-Ph is 1,4-diphenyl-1,4-diphosphacycloheptane) have been synthesized and characterized by NMR spectroscopy, electrochemical studies, and X-ray diffraction. These chloride derivatives are readily converted to the corresponding hydride complexes [CpFe-((P2N2Bn)-N-Ph)H (1-H), CpFe((P2N2Ph)-N-Ph)H (2-H), CpFe((P2C5)-C-Ph)H (3-H)] and H-2 complexes [CpFe((P2N2Bn)-N-Ph)(H-2)]BAr4F, [1-H-2]BAr4F, (where BAr4F is B[(3,5-(CF3)(2)C6H3)(4)](-)), [CpFe((P2N2Ph)-N-Ph)(H-2)]BAr4F, [2-H-2]BAr4F, and [CpFe((P2C5)-C-Ph)(H-2)]-BAr4F, [3-H-2]BAr4F, as well as [CpFe((P2N2Bn)-N-Ph)(CO)]BAr4F, [1-CO]CI. Structural studies are reported for [1-H-2]BAr4F, 1-H, 2-H, and [1-CO]CI. The conformations adopted by the chelate rings of the (P2N2Bn)-N-Ph ligand in the different complexes are determined by attractive or repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of the pendant amine and the C atom of the coordinated CO ligand for [1-CO]BAr4F is 2.848 angstrom, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes [1-H-2](+), [2-H-2](+), and [3-H-2](+) carried out using H-2 and D-2 indicate that the relatively rapid H/D exchange observed for [1-H-2](+) and [2-H-2](+) compared to [3-H-2](+) is consistent with intramolecular heterolytic cleavage of H-2 mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H-2. These mononudear Fe-II dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the distal Fe site of the active site of the [FeFe]-hydrogenase required for H-H bond formation and cleavage.