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

Computing Free Energy Landscapes: Application to Ni-based Electrocatalysts with Pendant Amines for H₂ Production and Oxidation

A general strategy is reported for the computational exploration of catalytic pathways of molecular catalysts. Our results are based on a set of linear free energy relationships derived from extensive electronic structure calculations that permit predicting the thermodynamics of intermediates, with accuracy comparable to experimental data. The approach is exemplified with the catalytic oxidation and production of H₂ by [Ni­(diphosphine)₂]²⁺ electrocatalysts with pendant amines incorporated in the second coordination sphere of the metal center. The analysis focuses upon prediction of thermodynamic properties including reduction potentials, hydride donor abilities, and p<i>K</i>_a values of both the protonated Ni center and the pendant amine. It is shown that all of these chemical properties can be estimated from the knowledge of only the two redox potentials for the Ni­(II)/Ni­(I) and Ni­(I)/Ni(0) couples of the nonprotonated complex, and the p<i>K</i>_a of the parent primary aminium ion. These three quantities are easily accessible either experimentally or theoretically. The proposed correlations reveal intimate details about the nature of the catalytic mechanism and its dependence on chemical structure and thermodynamic conditions such as applied external voltage and species concentration. This computational methodology is applied to the exploration of possible catalytic pathways, identifying low and high-energy intermediates and, consequently, possibly avoiding bottlenecks associated with undesirable intermediates in the catalytic reactions. We discuss how to optimize some of the critical reaction steps to favor catalytically more efficient intermediates. The results of this study highlight the substantial interplay between the various parameters characterizing the catalytic activity, and form the basis needed to optimize the performance of this class of catalysts

Protonation Studies of a Tungsten Dinitrogen Complex Supported by a Diphosphine Ligand Containing a Pendant Amine

Treatment of <i>trans</i>-[W­(N₂)₂(dppe)­(P^EtN^MeP^Et)] (dppe = Ph₂PCH₂CH₂PPh₂; P^EtN^MeP^Et = Et₂PCH₂N­(Me)­CH₂PEt₂) with 3 equiv of tetrafluoroboric acid (HBF₄·Et₂O) at −78 °C generated the seven-coordinate tungsten hydride <i>trans</i>-[W­(N₂)₂(H)­(dppe)­(P^EtN^MeP^Et)]­[BF₄]. At higher temperatures, protonation of a pendant amine is also observed, affording <i>trans</i>-[W­(N₂)₂(H)­(dppe)­(P^EtN^Me(H)­P^Et)]­[BF₄]₂, with formation of the hydrazido complex [W­(NNH₂)­(dppe)­(P^EtN^Me(H)­P^Et)]­[BF₄]₂ as a minor product. A similar product mixture was obtained using triflic acid (HOTf). The protonated products are thermally sensitive and do not persist at ambient temperature. Upon acid addition to the carbonyl analogue <i>cis</i>-[W­(CO)₂(dppe)­(P^EtN^MeP^Et)], the seven-coordinate carbonyl hydride complex <i>trans</i>-[W­(CO)₂(H)­(dppe)­(P^EtN^Me(H)­P^Et)]­[OTf]₂ was generated. A mixed diphosphine complex without the pendant amine in the ligand backbone, <i>trans</i>-[W­(N₂)₂(dppe)­(depp)] (depp = Et₂P­(CH₂)₃PEt₂), was synthesized and treated with HOTf, selectively generating a hydrazido complex, [W­(NNH₂)­(OTf)­(dppe)­(depp)]­[OTf]. Computational analysis probed the proton affinity of three sites of protonation in these complexes: the metal, pendant amine, and N₂ ligand. Room-temperature reactions with 100 equiv of HOTf produced NH₄⁺ from reduction of the N₂ ligand (electrons come from W). The addition of 100 equiv of HOTf to <i>trans</i>-[W­(N₂)₂(dppe)­(P^EtN^MeP^Et)] afforded 0.81 equiv of NH₄⁺, while 0.40 equiv of NH₄⁺ was formed upon treatment of <i>trans</i>-[W­(N₂)₂(dppe)­(depp)] with HOTf, showing that the complexes containing proton relays produce more products of reduction of N₂

Dinitrogen Reduction by a Chromium(0) Complex Supported by a 16-Membered Phosphorus Macrocycle

We report a rare example of a Cr–N₂ complex supported by a 16-membered phosphorus macrocycle containing pendant amine bases. Reactivity with acid afforded hydrazinium and ammonium, representing the first example of N₂ reduction by a Cr–N₂ complex. Computational analysis examined the thermodynamically favored protonation steps of N₂ reduction with Cr leading to the formation of hydrazine

Protonation of Ferrous Dinitrogen Complexes Containing a Diphosphine Ligand with a Pendent Amine

The addition of acids to ferrous dinitrogen complexes [FeX­(N₂)­(P^EtN^MeP^Et)­(dmpm)]⁺ (X = H, Cl, or Br; P^EtN^MeP^Et = Et₂PCH₂N­(Me)­CH₂PEt₂; and dmpm = Me₂PCH₂PMe₂) gives protonation at the pendent amine of the diphosphine ligand rather than at the dinitrogen ligand. This protonation increased the ν_N2 band of the complex by 25 cm^–1 and shifted the Fe­(II/I) couple by 0.33 V to a more positive potential. A similar IR shift and a slightly smaller shift of the Fe­(II/I) couple (0.23 V) was observed for the related carbonyl complex [FeH­(CO)­(P^EtN^MeP^Et)­(dmpm)]⁺. [FeH­(P^EtN^MeP^Et)­(dmpm)]⁺ was found to bind N₂ about three times more strongly than NH₃. Computational analysis showed that coordination of N₂ to Fe­(II) centers increases the basicity of N₂ (vs free N₂) by 13 and 20 p<i>K</i>_a units for the trans halides and hydrides, respectively. Although the iron center increases the basicity of the bound N₂ ligand, the coordinated N₂ is not sufficiently basic to be protonated. In the case of ferrous dinitrogen complexes containing a pendent methylamine, the amine site was determined to be the most basic site by 30 p<i>K</i>_a units compared to the N₂ ligand. The chemical reduction of these ferrous dinitrogen complexes was performed in an attempt to increase the basicity of the N₂ ligand enough to promote proton transfer from the pendent amine to the N₂ ligand. Instead of isolating a reduced Fe(0)–N₂ complex, the reduction resulted in isolation and characterization of HFe­(Et₂PC­(H)­N­(Me)­CH₂PEt₂)­(P^EtN^MeP^Et), the product of oxidative addition of the methylene C–H bond of the P^EtN^MeP^Et ligand to Fe

Dinitrogen Reduction by a Chromium(0) Complex Supported by a 16-Membered Phosphorus Macrocycle

We report a rare example of a Cr–N₂ complex supported by a 16-membered phosphorus macrocycle containing pendant amine bases. Reactivity with acid afforded hydrazinium and ammonium, representing the first example of N₂ reduction by a Cr–N₂ complex. Computational analysis examined the thermodynamically favored protonation steps of N₂ reduction with Cr leading to the formation of hydrazine

Proton and Electron Additions to Iron(II) Dinitrogen Complexes Containing Pendant Amines

Protonation of an iron C–H activated complex containing pendant amines in the presence of N₂ generated a <i>cis</i>-(H)­Fe^II–N₂ complex. Addition of acid protonates the pendant amines. Reduction of the protonated complex results in N₂ loss and H₂ formation, followed by N₂ binding. The origin of H₂ formation in this Fe system is compared to proposed mechanisms for H₂ loss and N₂ coordination in the E₄ state of nitrogenase

Incorporating Amino Acid Esters into Catalysts for Hydrogen Oxidation: Steric and Electronic Effects and the Role of Water as a Base

Four derivatives of a hydrogen oxidation catalyst, [Ni­(P^Cy₂N^Bn‑R₂)₂]²⁺ (Cy = cyclohexyl, Bn = benzyl, R = OMe, COOMe, CO-alanine-methyl ester, CO-phenylalanine-methyl ester), have been prepared to investigate steric and electronic effects on catalysis. Each complex was characterized spectroscopically and electrochemically, and thermodynamic data were determined. Crystal structures are also reported for the −OMe and −COOMe derivatives. All four catalysts were found to be active for H₂ oxidation. The methyl ester (R = COOMe) and amino acid ester containing complexes (R = CO-alanine-methyl ester or CO-phenylalanine-methyl ester) had rates slower (4 s^–1) than that of the parent complex (10 s^–1), in which R = H, which is consistent with the lower amine p<i>K</i>_a's and less favorable Δ<i>G</i>_H₂'s found for these electron-withdrawing substituents. Dynamic processes for the amino acid ester containing complexes were also investigated and found not to hinder catalysis. The electron-donating methyl ether derivative (R = OMe) was prepared to compare electronic effects and has a catalytic rate similar to that of the parent complex. In the course of these studies, it was found that water could act as a weak base for H₂ oxidation, although catalytic turnover requires a higher potential and utilizes a different sequence of catalytic steps than when using a base with a higher p<i>K</i>_a. Importantly, these catalysts provide a foundation upon which larger peptides can be attached to [Ni­(P^Cy₂N^Bn₂)₂]²⁺ hydrogen oxidation catalysts in order to more fully investigate and implement the effects of the outer coordination sphere

Incorporating Amino Acid Esters into Catalysts for Hydrogen Oxidation: Steric and Electronic Effects and the Role of Water as a Base

Four derivatives of a hydrogen oxidation catalyst, [Ni­(P^Cy₂N^Bn‑R₂)₂]²⁺ (Cy = cyclohexyl, Bn = benzyl, R = OMe, COOMe, CO-alanine-methyl ester, CO-phenylalanine-methyl ester), have been prepared to investigate steric and electronic effects on catalysis. Each complex was characterized spectroscopically and electrochemically, and thermodynamic data were determined. Crystal structures are also reported for the −OMe and −COOMe derivatives. All four catalysts were found to be active for H₂ oxidation. The methyl ester (R = COOMe) and amino acid ester containing complexes (R = CO-alanine-methyl ester or CO-phenylalanine-methyl ester) had rates slower (4 s^–1) than that of the parent complex (10 s^–1), in which R = H, which is consistent with the lower amine p<i>K</i>_a's and less favorable Δ<i>G</i>_H₂'s found for these electron-withdrawing substituents. Dynamic processes for the amino acid ester containing complexes were also investigated and found not to hinder catalysis. The electron-donating methyl ether derivative (R = OMe) was prepared to compare electronic effects and has a catalytic rate similar to that of the parent complex. In the course of these studies, it was found that water could act as a weak base for H₂ oxidation, although catalytic turnover requires a higher potential and utilizes a different sequence of catalytic steps than when using a base with a higher p<i>K</i>_a. Importantly, these catalysts provide a foundation upon which larger peptides can be attached to [Ni­(P^Cy₂N^Bn₂)₂]²⁺ hydrogen oxidation catalysts in order to more fully investigate and implement the effects of the outer coordination sphere