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Steric Switching From Photochemical to Thermal N₂ Splitting: A Computational Analysis of the Isomerization Reaction {(Cp^)(Am)Mo}₂(?-?¹:?¹-N₂) ? {(Cp^)(Am)Mo}₂(?-N)₂.

ABSTRACT: A ?-?¹:?¹-N₂-bridged Mo dimer, {(?⁵-C₅Me₅)[N(Et)C(Ph)N(Et)]Mo}₂(?-N₂), cleaves dinitrogen thermally resulting in a crystallographically characterized bis-?-N-bridged dimer, {(?⁵-C₅Me₅)[N(Et)C(Ph)N(Et)]Mo}₂(?-N)₂. A structurally related Mo dimer with a bulkier amidinate ligand, ([N(ⁱPr)C(Me)N(ⁱPr)]), is only capable of photochemical dinitrogen activation. These opposing reactivities were rationalized as steric switching between the thermally and photochemically active species. A computational analysis of the geometric and electronic structures of intermediates along the isomerization pathway from Mo₂(?-?¹:?¹-N₂) to Mo₂(?-?²:?¹-N₂) and Mo₂(?-?²:?²-N₂), and finally Mo₂(?-N)₂, is presented here. The extent to which dispersion affects the thermodynamics of the isomers is evaluated, and it is found that dispersion interactions play a significant role in stabilizing the product and making the reaction exergonic. The concept of steric switching is further explored with theoretical models with sterically even less demanding ligands, indicating that systematic ligand modifications could be used to rationally design the N₂ activation energy landscape. An analysis of electronic excitations in the computed UV-vis spectra of the two complexes shows that a particular type of asymmetric excitations is only present in the photoactive complex.

SUBMITTER: Krewald V

PROVIDER: S-EPMC6535493 | biostudies-literature | 2019

REPOSITORIES: biostudies-literature

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Steric Switching From Photochemical to Thermal N2 Splitting: A Computational Analysis of the Isomerization Reaction {(Cp)(Am)Mo}2(μ-η1:η1-N2) → {(Cp)(Am)Mo}2(μ-N)2.

Krewald Vera V

Frontiers in chemistry 20190516

A μ-η1:η1-N2-bridged Mo dimer, {(η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo}2(μ-N2), cleaves dinitrogen thermally resulting in a crystallographically characterized bis-μ-N-bridged dimer, {(η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo}2(μ-N)2. A structurally related Mo dimer with a bulkier amidinate ligand, ([N(iPr)C(Me)N(iPr)]), is only capable of photochemical d ...[more]

PMID: 31165063

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Project description:The mechanisms of photoextrusion reactions are determined theoretically for the model system of six-coordinated M(CO)5(CS) (M = Cr, Mo, and W), using both CASSCF and MP2-CAS methods and the Def2-SVPD basis set. Three types of elimination reaction pathways (i.e., path I, path II, and path III for axial CO extrusion, equatorial CO extrusion, and CS ligand extrusion, respectively) are considered in this study. Theoretical findings show that the photoextrusion mechanism for Cr and Mo complexes proceeds as follows: M-S0-Rea + hν → M-S1-FC → M-CI → M-Pro + CO. This study shows that when the reactant, M(CO)5(CS) (M-S0-Rea), is photoirradiated by UV light, it is excited vertically to many low-lying singlet excited states. It then relaxes to the first singlet excited state from the Franck-Condon point (M-S1-FC). After passing through a conical intersection point (M-CI), this species eliminates a CO group to yield a five-coordinated product, M(CO)4(CS) (M-S0-Pro). However, for the W analogue, the photolysis mechanism is represented as W-S0-Rea + hν → W-T1-Min → W-T1-TS → W-T1/S0 → W-S0-Pro + CO. That is to say, when the reactant, W(CO)5(CS) (W-S0-Rea), absorbs UV light, it is excited to its several low-lying excited states by a vertical excitation. This species may then return to an intermediate at the first triplet excited state (W-T1-Min) by means of intersystem crossings or conical intersections. After passing through a triplet transition state (W-T1-TS) and a subsequent intersystem crossing (W-T1/S0), this molecule finally loses a CO ligand to produce a photoproduct at the ground singlet state (W-S0-Pro). In other words, conical intersections and intersystem crossings play a decisive role in these photoextrusion reactions for M(CO)5(CS). Theoretical evidence from a kinetic viewpoint strongly supports the theory that the photolysis of M(CO)5(CS) only produces CO-loss photoproducts rather than the CS-loss photoproduct. Theoretical analysis using the results of this study allows a good interpretation of the available experimental observations.

Project description:Neutral {CpFeII(CO)2[SnII(Pc•3-)]} {Cp is cyclopentadienyl (1, 2) or Cp* is pentamethylcyclopentadienyl (3); Pc: phthalocyanine}, {Cp*FeII(CO)2[SnII(Nc•3-)]} (4, Nc: naphthalocyanine), and {CpFeII(CO)2[SnII(TPP•3-)]} (5, TPP: tetraphenylporphyrin) complexes in which CpFeII(CO)2 fragments (Cp: Cp or Cp*) are coordinated to SnII(macrocycle•3-) have been obtained. The product complexes were obtained at the reaction of charge transfer from CpFeI(CO)2 (Cp: Cp or Cp*) to [SnII(macrocycle2-)] to form the diamagnetic FeII and paramagnetic radical trianionic macrocycles. As a result, these formally neutral complexes contain S = 1/2 spins delocalized over the macrocycles. This provides alternation of the C-Nimine or C-Cmeso bonds in the macrocycles, the appearance of new bands in the near-infrared spectra of the complexes, and blue shift of both Soret and Q-bands. The {CpFeII(CO)2SnII(macrocycle•3-)} units (Cp: Cp or Cp*, macrocycle: Pc or Nc) form closely packed π-stacking dimers in 1 and 3 or one-dimensional chains in 2 and 4 with effective π-π interaction between the macrocycles. Such packing allows strong antiferromagnetic coupling between S = 1/2 spins. Magnetic interaction can be described well by the Heisenberg model for the isolated dimers in 1 and 3 with exchange interaction J/k B = -78 and -85 K, respectively. Magnetic behavior of 2 and 4 is described well by the model that includes contributions from an antiferromagnetically coupled S = 1/2 dimer (J intra) and a Heisenberg S = 1/2 chain with alternating antiferromagnetic spin exchange between the neighbors (J inter). Compound 2 demonstrates large intradimer interaction of J intra/k B = -54 K and essentially weaker interdimer exchange interactions of J inter/k B = -6 K, whereas compound 4 shows strong magnetic coupling of spins within the dimers (J intra/k B = -170 K) as well as between the dimers (J inter/k B = -40 K). Compound {CpFeII(CO)2[SnII(TPP•3-)]} (5) shows no π-π interactions between the porphyrin macrocycles, and magnetic coupling is weak in this case (Weiss temperature is -5 K). Preparation of a similar complex with indium(III) chloride phthalocyanine yields {CpFe(CO)2[In(Pc2-)]} (6). In this complex, indium(III) atoms are reduced instead of the phthalocyanine macrocycles that explains electron paramagnetic resonance silence of 6 in the 4-295 K range.

Dataset Information

Steric Switching From Photochemical to Thermal N₂ Splitting: A Computational Analysis of the Isomerization Reaction {(Cp^)(Am)Mo}₂(?-?¹:?¹-N₂) ? {(Cp^)(Am)Mo}₂(?-N)₂.

Publications

Steric Switching From Photochemical to Thermal N<sub>2</sub> Splitting: A Computational Analysis of the Isomerization Reaction {(Cp<sup></sup>)(Am)Mo}<sub>2</sub>(μ-η<sup>1</sup>:η<sup>1</sup>-N<sub>2</sub>) → {(Cp<sup></sup>)(Am)Mo}<sub>2</sub>(μ-N)<sub>2</sub>.

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Dataset Information

Steric Switching From Photochemical to Thermal N2 Splitting: A Computational Analysis of the Isomerization Reaction {(Cp*)(Am)Mo}2(?-?1:?1-N2) ? {(Cp*)(Am)Mo}2(?-N)2.

Publications

Steric Switching From Photochemical to Thermal N<sub>2</sub> Splitting: A Computational Analysis of the Isomerization Reaction {(Cp<sup>*</sup>)(Am)Mo}<sub>2</sub>(μ-η<sup>1</sup>:η<sup>1</sup>-N<sub>2</sub>) → {(Cp<sup>*</sup>)(Am)Mo}<sub>2</sub>(μ-N)<sub>2</sub>.

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OmicsDI is part of the ELIXIR infrastructure

Tweets

Steric Switching From Photochemical to Thermal N₂ Splitting: A Computational Analysis of the Isomerization Reaction {(Cp^)(Am)Mo}₂(?-?¹:?¹-N₂) ? {(Cp^)(Am)Mo}₂(?-N)₂.

Steric Switching From Photochemical to Thermal N<sub>2</sub> Splitting: A Computational Analysis of the Isomerization Reaction {(Cp<sup></sup>)(Am)Mo}<sub>2</sub>(μ-η<sup>1</sup>:η<sup>1</sup>-N<sub>2</sub>) → {(Cp<sup></sup>)(Am)Mo}<sub>2</sub>(μ-N)<sub>2</sub>.