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(P^Ph₂N^Bn₂)Cl (<b>1-Cl</b>), CpFe­(P^Ph₂N^Ph₂)Cl (<b>2-Cl</b>), and CpFe­(P^Ph₂C₅)Cl (<b>3-Cl</b>) (where P^Ph₂N^Bn₂ is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, P^Ph₂N^Ph₂ is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane, and P^Ph₂C₅ 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(P^Ph₂N^Bn₂)H (<b>1-H</b>), CpFe­(P^Ph₂N^Ph₂)H (<b>2-H</b>), CpFe­(P^Ph₂C₅)H (<b>3-H</b>)] and H₂ complexes [CpFe­(P^Ph₂N^Bn₂)­(H₂)]­BAr^F₄, <b>[1-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, (where BAr^F₄ is B­[(3,5-(CF₃)₂C₆H₃)₄]⁻), [CpFe­(P^Ph₂N^Ph₂)­(H₂)]­BAr^F₄, <b>[2-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, and [CpFe­(P^Ph₂C₅)­(H₂)]­BAr^F₄, <b>[3-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, as well as [CpFe­(P^Ph₂N^Bn₂)­(CO)]­BAr^F₄, <b>[1-CO]­Cl</b>. Structural studies are reported for <b>[1-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, <b>1-H</b>, <b>2-H</b>, and <b>[1-CO]­Cl</b>. The conformations adopted by the chelate rings of the P^Ph₂N^Bn₂ 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 <b>[1-CO]­BAr</b>^<b>F</b>_<b>4</b> is 2.848 Å, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes <b>[1-H</b>_<b>2</b><b>]</b>^<b>+</b>, <b>[2-H</b>_<b>2</b><b>]</b>^<b>+</b>, and <b>[3-H</b>_<b>2</b><b>]</b>^<b>+</b> carried out using H₂ and D₂ indicate that the relatively rapid H/D exchange observed for <b>[1-H</b>_<b>2</b><b>]</b>^<b>+</b> and <b>[2-H</b>_<b>2</b><b>]</b>^<b>+</b> compared to <b>[3-H</b>_<b>2</b><b>]</b>^<b>+</b> is consistent with intramolecular heterolytic cleavage of H₂ mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H₂. These mononuclear 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

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(P^Ph₂N^Bn₂)Cl (<b>1-Cl</b>), CpFe­(P^Ph₂N^Ph₂)Cl (<b>2-Cl</b>), and CpFe­(P^Ph₂C₅)Cl (<b>3-Cl</b>) (where P^Ph₂N^Bn₂ is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, P^Ph₂N^Ph₂ is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane, and P^Ph₂C₅ 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(P^Ph₂N^Bn₂)H (<b>1-H</b>), CpFe­(P^Ph₂N^Ph₂)H (<b>2-H</b>), CpFe­(P^Ph₂C₅)H (<b>3-H</b>)] and H₂ complexes [CpFe­(P^Ph₂N^Bn₂)­(H₂)]­BAr^F₄, <b>[1-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, (where BAr^F₄ is B­[(3,5-(CF₃)₂C₆H₃)₄]⁻), [CpFe­(P^Ph₂N^Ph₂)­(H₂)]­BAr^F₄, <b>[2-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, and [CpFe­(P^Ph₂C₅)­(H₂)]­BAr^F₄, <b>[3-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, as well as [CpFe­(P^Ph₂N^Bn₂)­(CO)]­BAr^F₄, <b>[1-CO]­Cl</b>. Structural studies are reported for <b>[1-H</b>_<b>2</b><b>]­BAr</b>^<b>F</b>_<b>4</b>, <b>1-H</b>, <b>2-H</b>, and <b>[1-CO]­Cl</b>. The conformations adopted by the chelate rings of the P^Ph₂N^Bn₂ 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 <b>[1-CO]­BAr</b>^<b>F</b>_<b>4</b> is 2.848 Å, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes <b>[1-H</b>_<b>2</b><b>]</b>^<b>+</b>, <b>[2-H</b>_<b>2</b><b>]</b>^<b>+</b>, and <b>[3-H</b>_<b>2</b><b>]</b>^<b>+</b> carried out using H₂ and D₂ indicate that the relatively rapid H/D exchange observed for <b>[1-H</b>_<b>2</b><b>]</b>^<b>+</b> and <b>[2-H</b>_<b>2</b><b>]</b>^<b>+</b> compared to <b>[3-H</b>_<b>2</b><b>]</b>^<b>+</b> is consistent with intramolecular heterolytic cleavage of H₂ mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H₂. These mononuclear 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