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

Studies of a Series of [Ni(P^R₂N^Ph₂)₂(CH₃CN)]²⁺ Complexes as Electrocatalysts for H₂ Production: Substituent Variation at the Phosphorus Atom of the P₂N₂ Ligand

A series of [Ni(P^R₂N^Ph₂)₂(CH₃CN)](BF₄)₂ complexes containing the cyclic diphosphine ligands [P^R₂N^Ph₂ = 1,5-diaza-3,7-diphosphacyclooctane; R = benzyl (Bn), <i>n</i>-butyl (<i>n-</i>Bu), 2-phenylethyl (PE), 2,4,4-trimethylpentyl (TP), and cyclohexyl (Cy)] have been synthesized and characterized. X-ray diffraction studies reveal that the cations of [Ni(P^Bn₂N^Ph₂)₂(CH₃CN)](BF₄)₂ and [Ni(P^{<i>n</i>‑Bu}₂N^Ph₂)₂(CH₃CN)](BF₄)₂ have distorted trigonal bipyramidal geometries. The Ni(0) complex [Ni(P^Bn₂N^Ph₂)₂] was also synthesized and characterized by X-ray diffraction studies and shown to have a distorted tetrahedral structure. These complexes, with the exception of [Ni(P^Cy₂N^Ph₂)₂(CH₃CN)](BF₄)₂, all exhibit reversible electron transfer processes for both the Ni(II/I) and Ni(I/0) couples and are electrocatalysts for the production of H₂ in acidic acetonitrile solutions. The heterolytic cleavage of H₂ by [Ni(P^R₂N^Ph₂)₂(CH₃CN)](BF₄)₂ complexes in the presence of <i>p</i>-anisidine or <i>p</i>-bromoaniline was used to determine the hydride donor abilities of the corresponding [HNi(P^R₂N^Ph₂)₂](BF₄) complexes. However, for the catalysts with the most bulky R groups, the turnover frequencies do not parallel the driving force for elimination of H₂, suggesting that steric interactions between the alkyl substituents on phosphorus and the nitrogen atom of the pendant amines play an important role in determining the overall catalytic rate