Electronic and Steric Influences of Pendant Amine Groups on the Protonation of Molybdenum Bis(dinitrogen) Complexes

The synthesis of a series of P^EtP^{NRR^′} (P^EtP^{NRR^′} = Et₂PCH₂CH₂P­(CH₂NRR′)₂, R = H, R′ = Ph or 2,4-difluorophenyl; R = R′ = Ph or ^<i>i</i>Pr) diphosphine ligands containing mono- and disubstituted pendant amine groups and the preparation of their corresponding molybdenum bis­(dinitrogen) complexes <i>trans</i>-Mo­(N₂)₂­(PMePh₂)₂­(P^EtP^{NRR^′}) is described. In situ IR and multinuclear NMR spectroscopic studies monitoring the stepwise addition of triflic acid (HOTf) to <i>trans</i>-Mo­(N₂)₂­(PMePh₂)₂­(P^EtP^{NRR^′}) complexes in tetrahydrofuran at −40 °C show that the electronic and steric properties of the R and R′ groups of the pendant amines influence whether the complexes are protonated at Mo, a pendant amine, a coordinated N₂ ligand, or a combination of these sites. For example, complexes containing monoaryl-substituted pendant amines are protonated at Mo and the pendant amine site to generate mono- and dicationic Mo–H species. Protonation of the complex containing less basic diphenyl-substituted pendant amines exclusively generates a monocationic hydrazido (Mo­(NNH₂)) product, indicating preferential protonation of an N₂ ligand. Addition of HOTf to the complex featuring more basic diisopropyl amines primarily produces a monocationic product protonated at a pendant amine site, as well as a trace amount of dicationic Mo­(NNH₂) product that is additionally protonated at a pendant amine site. In addition, <i>trans</i>-Mo­(N₂)₂­(PMePh₂)₂­(depe) (depe = Et₂PCH₂CH₂PEt₂) was synthesized to serve as a counterpart lacking pendant amines. Treatment of this complex with HOTf generated a monocationic Mo­(NNH₂) product. Protonolysis experiments conducted on several complexes in this study afforded trace amounts of NH₄⁺. Computational analysis of <i>trans</i>-Mo­(N₂)₂­(PMePh₂)₂­(P^EtP^{NRR^′}) complexes provides further insight into the proton affinity values of the metal center, N₂ ligand, and pendant amine sites to rationalize differences in their reactivity profiles

Catalytic Silylation of N₂ and Synthesis of NH₃ and N₂H₄ by Net Hydrogen Atom Transfer Reactions Using a Chromium P₄ Macrocycle

We report the first discrete molecular Cr-based catalysts for the reduction of N₂. This study is focused on the reactivity of the Cr-N₂ complex, <i>trans</i>-[Cr­(N₂)₂­(P^Ph₄N^Bn₄)] (<b>P</b>_<b>4</b><b>Cr­(N</b>_<b>2</b><b>)</b>_<b>2</b>), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-P^Ph₄N^Bn₄ ligand is critical to preserve the structural integrity of the catalyst. <b>P</b>_<b>4</b><b>Cr­(N</b>_<b>2</b><b>)</b>_<b>2</b> was found to mediate the reduction of N₂ at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N₂ to N­(SiMe₃)₃ by Na and Me₃SiCl, affording up to 34 equiv N­(SiMe₃)₃; (2) stoichiometric reduction of N₂ by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH₃, or at −78 °C afforded a mixture of NH₃ and N₂H₄); and (3) the first example of NH₃ formation from the reaction of a terminally bound N₂ ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that <i>trans</i>-[Cr­(¹⁵N₂)₂­(P^Ph₄N^Bn₄)] reacts with excess TEMPOH to afford 1.4 equiv of ¹⁵NH₃. Isotopic labeling studies using TEMPOD afforded ND₃ as the product of N₂ reduction, confirming that the H atoms are provided by TEMPOH

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

A Cobalt Hydride Catalyst for the Hydrogenation of CO₂: Pathways for Catalysis and Deactivation

The complex Co­(dmpe)₂H catalyzes the hydrogenation of CO₂ at 1 atm and 21 °C with significant improvement in turnover frequency relative to previously reported second- and third-row transition-metal complexes. New studies are presented to elucidate the catalytic mechanism as well as pathways for catalyst deactivation. The catalytic rate was optimized through the choice of the base to match the p<i>K</i>_a of the [Co­(dmpe)₂(H)₂]⁺ intermediate. With a strong enough base, the catalytic rate has a zeroth-order dependence on the base concentration and the pressure of hydrogen and a first-order dependence on the pressure of CO₂. However, for CO₂:H₂ ratios greater than 1, the catalytically inactive species [(μ-dmpe)­(Co­(dmpe)₂)₂]²⁺ and [Co­(dmpe)₂CO]⁺ were observed

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

Understanding the Relationship Between Kinetics and Thermodynamics in CO₂ Hydrogenation Catalysis

Catalysts that are able to reduce carbon dioxide under mild conditions are highly sought after for storage of renewable energy in the form of a chemical fuel. This study describes a systematic kinetic and thermodynamic study of a series of cobalt and rhodium bis­(diphosphine) complexes that are capable of hydrogenating carbon dioxide to formate under ambient temperature and pressure. Catalytic CO₂ hydrogenation was studied under 1.8 and 20 atm of pressure (1:1 CO₂/H₂) at room temperature in tetrahydrofuran with turnover frequencies (TOF) ranging from 20 to 74 000 h^–1. The catalysis was followed by ¹H and ³¹P NMR spectroscopy in real time under all conditions to yield information about the rate-determining step. The cobalt catalysts displayed a rate-determining step of hydride transfer to CO₂, while the hydrogen addition and/or deprotonation steps were rate limiting for the rhodium catalysts. Thermodynamic analysis of the complexes provided information on the driving force for each step of catalysis in terms of the catalyst hydricity (Δ<i>G</i>°_H^–), acidity (p<i>K</i>_a), and free energy for H₂ addition (Δ<i>G</i>°_H₂). Linear free-energy relationships were identified that link the kinetic activity for catalytic hydrogenation of CO₂ to formate with the thermodynamic driving force for the rate-limiting steps of catalysis. The catalyst exhibiting the highest activity, Co­(dmpe)₂H, was found to have hydride transfer and hydrogen addition steps that were each downhill by approximately 6 to 7 kcal mol^–1, and the deprotonation step was thermoneutral. This indicates the fastest catalysts are the ones that most efficiently balance the free energy relationships of every step in the catalytic cycle

Ammonia Oxidation by Abstraction of Three Hydrogen Atoms from a Mo–NH₃ Complex

We report ammonia oxidation by homolytic cleavage of all three H atoms from a [MoNH₃]⁺ complex using the 2,4,6-tri-<i>tert</i>-butylphenoxyl radical to yield a Mo-alkylimido ([MoNR]⁺) complex (R = 2,4,6-tri-<i>tert</i>-butylcyclohexa-2,5-dien-1-one). Chemical reduction of [MoNR]⁺ generates a terminal MoN nitride complex upon NC bond cleavage, and a [MoNH]⁺ complex is formed by protonation of the nitride. Computational analysis describes the energetic profile for the stepwise removal of three H atoms from [MoNH₃]⁺ and formation of [MoNR]⁺