Electronic and Spin-State Effects on Dinitrogen Splitting to Nitrides in a Rhenium Pincer System

Nitrogen Fixation by Manganese Complexes - Waiting for the Rush?

Manganese is currently experiencing a great deal of attention in homogeneous catalysis as a sustainable alternative to platinum group metals due to its abundance, affordable price and low toxicity. While homogeneous nitrogen fixation employing well-defined transition metal complexes has been an important part of coordination chemistry, manganese derivatives have been only sporadically used in this research area. In this contribution, the authors systematically cover manganese organometallic chemistry related to N2 activation spanning almost 60 years, identify apparent pitfalls and outline encouraging perspectives for its future development.

(Electro)chemical N2 Splitting by a Molybdenum Complex with an Anionic PNP Pincer-Type Ligand

Molybdenum(III) complexes bearing pincer-type ligands are well-known catalysts for N2-to-NH3 reduction. We investigated herein the impact of an anionic PNP pincer-type ligand in a Mo(III) complex on the (electro)chemical N2 splitting ([LMoCl3]-, 1 -, LH = 2,6-bis((di-tert-butylphosphaneyl)methyl)-pyridin-4-one). The increased electron-donating properties of the anionic ligand should lead to a stronger degree of N2 activation. The catalyst is indeed active in N2-to-NH3 conversion utilizing the proton-coupled electron transfer reagent SmI2/ethylene glycol. The corresponding Mo(V) nitrido complex 2H exhibits similar catalytic activity as 1H and thus could represent a viable intermediate. The Mo(IV) nitrido complex 3 - is also accessible by electrochemical reduction of 1 - under a N2 atmosphere. IR- and UV/vis-SEC measurements suggest that N2 splitting occurs via formation of an "overreduced" but more stable [(L(N2)2Mo0)2μ-N2]2- dimer. In line with this, the yield in the nitrido complex increases with lower applied potentials.

To Split or Not to Split: [AsCCAs]-Coordinated Mo, W, and Re Complexes and Their Reactivity toward Molecular Dinitrogen

Molybdenum, tungsten, and rhenium halides bearing a 2,2'-(iPr2As)2-substituted diphenylacetylene ([AsCCAs], 1-As) were prepared and reduced under an atmosphere of dinitrogen in order to activate the latter substrate. In the case of molybdenum, a diiodo (2-As) and a triiodo molybdenum precursor (5) were equally suited for reductive N2 splitting, which led to the isolation of [AsCCAs]Mo≡N(I) (3-As) in each case. For tungsten, [AsCCAs]WCl3 (6) was reduced under N2 to afford {[AsCCAs]WCl2}2(N2) (7), which is best described as a dinuclear π8δ4-configured μ-(η1: η1)-N2-bridged dimer. Attempts to reductively cleave the N2 unit in 7 did not lead to the expected tungsten nitride (8), which had to be prepared independently via the treatment of 7 with sodium azide. To arrive at a π10δ4-configured N2-bridged dimer in a tetragonally distorted ligand environment, [AsCCAs]ReCl3 (9) was reduced in the presence of N2. As expected, a μ-(η1: η1)-N2-bridged dirhenium species, namely, {[AsCCAs]ReCl2}2(N2) (10), was formed, but found to very quickly decompose (presumably via loss of N2), not only under reduced pressure, but also upon irradiation or heating. Hence, an alternative synthetic route to the originally envisioned nitride, [AsCCAs]Re≡N(Cl)2 (11), was developed. While all the aforementioned nitrides (3-As, 8, and 11) were found to be fairly robust, significantly different stabilities were noticed for {[AsCCAs]MCl2}2(N2) (7 for M = W, 10 for M = Re), which is ascribed to the electronically different MN2M cores (π8δ4 for 7 vs π10δ4 for 10) in these μ-(η1: η1)-N2-bridged dimers.

High-Pressure Coupling Reactions to Produce a Spherical Bulk RexN/Fe3N Composite

We report a novel high-pressure coupling (HPC) reaction that couples the nitridation of Re with high-pressure solid-state metathesis (HPSSM) of Fe₃N to produce a spherical bulk Re_xN/Fe₃N composite. Compared with conventional methods, upon coupling of the HPSSM reactions, the synthetic pressure for Re nitridation was successfully reduced from 13 to 10 GPa (for Re₃N) and from 20 to 15 GPa (for Re₂N). The product Re_xN species would be surrounded by product Fe₃N, resulting in a spherical bulk Re_xN/Fe₃N composite (x = 2 or 3). The composite exhibits a soft magnetic behavior, and the content of nitrogen in Re_xN (x = 2 or 3) was controlled by adjusting the P-T conditions. The HPC reaction establishes a new approach for the bulk synthesis of 5d transition metal nitride.

Lewis Structures and the Bonding Classification of End-on Bridging Dinitrogen Transition Metal Complexes

The activation of dinitrogen by coordination to transition metal ions is a widely used and promising approach to the utilization of Earth's most abundant nitrogen source for chemical synthesis. End-on bridging N₂ complexes (μ-η¹:η¹-N₂) are key species in nitrogen fixation chemistry, but a lack of consensus on the seemingly simple task of assigning a Lewis structure for such complexes has prevented application of valence electron counting and other tools for understanding and predicting reactivity trends. The Lewis structures of bridging N₂ complexes have traditionally been determined by comparing the experimentally observed NN distance to the bond lengths of free N₂, diazene, and hydrazine. We introduce an alternative approach here and argue that the Lewis structure should be assigned based on the total π-bond order in the MNNM core (number of π-bonds), which derives from the character (bonding or antibonding) and occupancy of the delocalized π-symmetry molecular orbitals (π-MOs) in MNNM. To illustrate this approach, the complexes cis,cis-[(^iPr4PONOP)MCl₂]₂(μ-N₂) (M = W, Re, and Os) are examined in detail. Each complex is shown to have a different number of nitrogen-nitrogen and metal-nitrogen π-bonds, indicated as, respectively: W≡N-N≡W, Re═N═N═Re, and Os-N≡N-Os. It follows that each of these Lewis structures represents a distinct class of complexes (diazanyl, diazenyl, and dinitrogen, respectively), in which the μ-N₂ ligand has a different electron donor number (total of 8e^-, 6e^-, or 4e^-, respectively). We show how this classification can greatly aid in understanding and predicting the properties and reactivity patterns of μ-N₂ complexes.

Heterobimetallic-Mediated Dinitrogen Functionalization: N-C Bond Formation at Rhenium-Group 9 Diazenido Complexes

We report the synthesis and characterization of rhenium-group 9 heterobimetallic diazenido species (η⁵-Cp)Re(μ-BDI)(μ-N₂)M(η⁴-COD) (1-M, M = Ir or Rh, Cp = cyclopentadienide, BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate, COD = 1,5-cyclooctadiene), formed from salt elimination reactions between Na[(η⁵-Cp)Re(BDI)] and [MCl(η⁴-COD)]₂. Additionally, we find that these same reagents react under an argon atmosphere to instead produce bridging hydride complexes (BDI)Re(μ-η⁵:η¹-C₅H₄)(μ-H)M(η⁴-COD) (2-M), which undergo rearrangements upon protonation to form the alternative bridging hydrides [(η⁵-Cp)Re(μ-BDI)(μ-H)M(η⁴-COD)][(B(m-C₆H₃(CF₃)₂)₄)] (3-M). Further, we demonstrate the first example of N-C bond formation at a heterobimetallic dinitrogen complex through reactions of 1-M and methyl triflate, which produces the alkylated species [(η⁵-Cp)Re(μ-N(Me)N)(μ-BDI)M(η⁴-COD)][OTf] (4-M, OTf = trifluoromethanesulfonate). A combination of spectroscopic studies, X-ray structural analysis, and computational investigations is discussed as an aid to understanding the modes of dinitrogen activation within these unique heterobimetallic complexes.

Halide Effects in Reductive Splitting of Dinitrogen with Rhenium Pincer Complexes

Transition metal halide complexes are used as precursors for reductive N₂ activation up to full splitting into nitride complexes. Distinct halide effects on the redox properties and yields are frequently observed yet not well understood. Here, an electrochemical and computational examination of reductive N₂ splitting with the rhenium(III) complexes [ReX₂(PNP)] (PNP = N(CH₂CH₂PtBu₂)₂ and X = Cl, Br, I) is presented. As previously reported for the chloride precursor ( J. Am. Chem. Soc. 2018, 140, 7922), the heavier halides give rhenium(V) nitrides upon (electro-)chemical reduction in good yields yet with significantly anodically shifted electrolysis potentials along the halide series. Dinuclear, end-on N₂-bridged complexes, [{ReX(PNP)}₂(μ-N₂)], were identified as key intermediates in all cases. However, while the chloride complex is exclusively formed via 2-electron reduction and Re^III/Re^I comproportionation, the iodide system also reacts via an alternative Re^II/Re^II-dimerization mechanism at less negative potentials. This alternative pathway relies on the absence of the potential inversion after reduction and N₂ activation that was observed for the chloride precursor. Computational analysis of the relevant Re^III/II and Re^II/I redox couples by energy decomposition analysis attributes the halide-induced trends of the potentials to the dominating electrostatic Re-X bonding interactions over contributions from charge transfer.

N2 Cleavage on d4/d4 Molybdenum Centers and Its Further Conversion into Iminophosphorane under Mild Conditions

The synthesis of N-containing organophosphine compounds using N₂ as the nitrogen source under mild conditions has attracted much attention. Herein, the conversion of N₂ into iminophosphorane was reported. By visible light irradiation, N₂ was split on a Mo^II complex bearing a PNCNP ligand, directly forming the Mo^V nitride. After the N-P bond formation on the terminal nitride, the N atom from N₂ was ultimately transferred into iminophosphorane. Key intermediates were characterized.