A computational DFT study of methane CH and ammine NH activations by group 9 N-pyrrolyl complexes

Computational and Theoretical Chemistry 1162 (2019) 112503

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A computational DFT study of methane CeH and ammine NeH activations by group 9 N-pyrrolyl complexes

T

Bruce M. Prince Center for Catalysis Computational Research (3CR), Department of Chemistry Texas Southern University, 3100 Cleburne Street, Houston, TX 77004, United States

A R T I C LE I N FO

A B S T R A C T

Keywords: Oxy-insertion Methane CeH activation Ammine NeH activation MTM Methane-to-methanol Oxo Oxy Hydrogen atom transfer HAT

A density functional theory with solvation model density analysis of methane CeH and ammine NeH activations with Group 9 N-pyrrolyl phosphine complexes (Co, Rh, Ir) is presented. Analysis of the reaction, [{(pyr)3P}M (NH2)]q+, (where M signify Co, Rh, Ir; pyr = N–pyrrolyl and q = +1 and + for d8 and d6 Group 9) showed that Ir system occurs with a lower free energy barrier. The computed complexes all have reasonable CeH activation barriers, ΔG‡ ∼ 18–32 kcal/mol, which tracks with the electron density on the metal center of the transition state complexes. Ammine NeH activation barriers are discreetly low, which emphasize the square planar, [{(pyr)3P} M(Py)(NH3)(OCH3)], complexes are uniquely ready to aid the reaction cycle within a single step. Thus, such ligands are worthy of experimental study due to their ability of strong π-backbonding, which may meet the strong metal-to-ligand coordination demands of an acidic environment of methanol production.

1. Introduction The homolytic bond dissociation enthalpy (BDE) is defined as the enthalpy change (ΔH) of a gas-phase reaction R − H → R% + H% [1]. Methane CeH activation remains one of the most difficult reactions because of its high BDE of 105 kcal/mol [2–4]. Hydrocarbons, explicitly methane from natural gas, are an important petroleum feedstock source for chemical functionalization [5–7]. The United States (U.S.) has remained the world’s largest producer of natural gas since surpassing Russia in 2011 under President Obama Administration [8]. However, methane gas is not easily functionalized in a selective manner because its BDE [2,4] is ∼9 kcal/mol higher in comparison to the CeH BDE of a desirable product such as methanol, CH3OH (CeH), NH3 (NeH), and CH3OH (OeH) where their experimental bond dissociation enthalpies in kcal/mol are 96, 108, and 105, respectively [2,4]. As outlined in a previous DFT investigation of amide/aminyl complexes [9], Scheme 1, CeH activation barrier as a function of the metal decreases as the spin density of the aminyl-nitrogen atom increases [9]. DFT investigations of methane CeH activation show that approximately 46% of the total spin density was localized on the N-atom and 11% on the metal atom, specifically nickel [9]. The fundamental theme of the present investigation is the functionalization step in a CH4 (gas) to CH3OH (liquid) catalytic cycle via NeH activation. Transition metal complexes are known to encourage NeH activation pathways [10,11], Scheme 2. There has been increased attention in the organometallic and

inorganic communities concerning non-innocent ligand systems [12–16]. For example, [CoI(trop2N%)(bpy)]+ with bulky, [bis(5-H-dibenzo[a,d]cyclohepten-5-yl)-amine] and 2,2′-bipyridine ligands, was reported by Rodríguez-Lugo and coworkers [14]. The results for [CoI(trop2N%(bpy)]+ completed the metalloradical series of Group 9 complexes by the Grützmacher group, which documented the first stable rhodium and cobalt aminyl complexes in DMSO [14,17,18]. Wang et al. reported a bulky 1,3,6,8-tetra-tert-butyl carbazyl (TTBC) aminyl radical species, which is stable at ambient conditions [19]. The TTBC complex showed a low kinetic barrier for hydrogen atom transfer (HAT) during its HeN reaction cycle. Given the above-mentioned experimental precedents, herein is reported a DFT investigation that focuses on modeling CeH activation of methane by Group 9 transition metal complexes with (pyr)3P ligands, (pyr = N–pyrrolyl). Experimental as well as computational investigations [6,20,21] have shown that (pyr)3P ligands generally form stronger π-bonds to a metal because of the substantial contribution of the resonance forms (A) through (E), Fig. 1. It is notable, that phosphine complexes have proven more prone to oxidative degradation in acidic environment hence this work focused on the non-saturated Group 9 complexes for a facile CeH activation cycle [22–24]. Structure “C” shows a possible strong π-acceptor character because the lone pair on the sp2 N-atom can be delocalized over the N-pyrrolyl carbons, which is the best resonance conformer of structures A, B, D, and E, Fig. 1 [25]. Thus, N-pyrrolyl ligand may have a favorable soft basicity for the low-oxidation state MI metal centers [26,27], which may increase the nucleophilicity on the metals. As a

E-mail address: [email protected]. https://doi.org/10.1016/j.comptc.2019.112503 Received 23 April 2019; Received in revised form 5 June 2019; Accepted 6 June 2019 Available online 07 June 2019 2210-271X/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 2. Computed ground state geometries of (M = Co, Rh, Ir) complexes. The leading superscripts in these and other metal-complex denotes triplet, singlet, singlet, multiplicities for the system of interest with the lower energy. Bond angles are in degrees (°) and bond lengths are in Ångström units (Å). The principal frontier orbital of d8-1IrI exemplifies the strong π-acceptor ability of {(pyr)3P}, where the positive and negative phases are the red and blue, respectively. The contour value is 0.020. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

compounds [30,31]. The proposed pathways used {(pyr)3P}M (where M = Co, Rh, Ir; pyr = N-pyrrolyl) complexes for the investigation of the proposed catalysis cycle. The M06 [28] and MN15-L [32] (Gaussian 16) functionals were utilized in concert with CEP-31G(d) and the CEP-121 G(d) pseudopotentials and valence basis sets for CeH activation for comparison [33–35]. The computation suggested a ± 3 kcal/mol difference between the functionals; thus, the reported results will be those gained with the M06 level of theory, which is the working functional for the work within. The CEP-31G(d) pseudopotential/valence basis set is triple-ζ for transition metals and double-ζ (with d polarization functions added) for main group elements [33,34]. In comparison, CEP-31G(d) and the larger CEP-121G(d) basis sets with M06 yield the BDE (bond dissociation enthalpy) of methane [36,37] within ± 0.1 kcal/mol of experiment; geometries used for BDE [2] and all other calculations are collected in Supporting Information. To conserve computational resources and cost in this investigation, CEP-31G(d) pseudopotentials/ valence basis set with M06 were used to compute the geometries and the thermal correction to Gibbs free energies, with the larger basis set reserved for the SMD (solvation model density) single point calculations [38]. All optimized energetics are quoted as Gibbs free energies at STP, 298.15 K and 1 atm, and use the M06/CEP-121G(d)/SMD-DMSO// M06/ CEP-31G(d) gas phase level of theory. The M06/ CEP-31G(d) level of theory was employed to identify the correct number of imaginary frequencies of the complexes along the reaction coordinates. Calculations used an analytic Hessian protocol. Single point energies used a continuum solvation model and larger basis set M06/ CEP-121 G(d)/SMD-DMSO (DMSO, ε = 46.83); the complexes computed are all neutral complexes. The DFT geometries were checked to ensure stability of the wavefunction by means of the stable = opt keyword. Tight convergence criteria, and superfine grids for numerical integration were used. The TSs were authenticated using intrinsic reaction coordinate (IRC) methods [39]. Optimization and single point calculations were performed without symmetry restraint and used the (un)restricted Kohn-Sham formalism as appropriate. Singlet spin states are expected for 4d and 5d complexes, and higher spin states for the 3d complex. The 4d and 5d complexes are consistently predicted to be singlets throughout the reaction cycle.

Scheme 1. Depicted amide/aminyl [{(pyr)3P}IrI(NH2)] complex where NH2 ligand is trans to the open coordinate site, and where M signify cobalt, rhodium, and iridium transition metals.

Scheme 2. Closing a catalytic cycle for the methane-to-methanol conversion via NeH activation. M = Group 9 transition metal ions (CoI, RhI, IrI).

Fig. 1. Five resonance structures of N-pyrrolyl phosphine ligand, where the δ+ is neighboring the phosphorus atom in (A, B, D, and E) resonance, while “C” illustrate the aromatic resonance. Where B and D as well as A and E are chemically equivalent species.

result, this work probes the impact of low coordination numbers upon methane CeH activation for Group 9 transition metal species. The model employed [{(pyr)3P}M(NH2)] (where M = Co, Rh Ir; pyr = N–pyrrolyl) was chosen for computational efficiency (Fig. 2). This research seeks to use computational chemistry to evaluate the feasibility of NH2 complexes for methane CeH activation, oxygen atom transfer, oxy-insertion, and ammine NeH activation. This work will (1) compare Group 9 CeH activation transition states (TSs), and its potential energy surface (PES), (2) investigate competing pathways such as oxidative addition/reductive elimination (OA/RE), [2σ + 2π] activation, and (3) assess the impact of electronic structure throughout the reaction cycle, Scheme 3.

3. Results and discussion 3.1. Geometry of [{(pyr)3P}M(NH2)] complex

2. Computational methods The x[{(pyr)3P}M(NH2)] complexes, x1M, investigated are two-coordinate with a (N-pyrrolyl)3phosphine supporting ligand and the NH2 ligand occupying the second coordination site (where M = Co, Rh, Ir; pyr = N-pyrrolyl, and x = spin multiplicity). The {(pyr)3P} ligand set was selected for the present studies to help stabilize lower coordination

All calculations were accomplished using the density functional theory (DFT) hybrid meta exchange-correlation functional M06 [28] as implemented in Gaussian 09 suite of programs [29], which is known to produce accurate reaction profiles for transition metal-containing 2

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its

Scheme 3. Proposed pathway for MTM (methane-to-methanol) catalysis.

3.2. Methane CeH activation via [{(pyr)3P}M(NH2)] complex

geometries and increase the oxidation potential of the metal [25,40], and as a result, methane gas substrate provides access to the N atom of the NH2 ligand. Table 1 collects the studied multiplicities of x1M reactant complexes. Based on previous work, a quintet spin state was also explored for cobalt, 51-Co, and found to be 9.5 kcal/mol higher than 11-Co [41,42]. The Supporting Information collects the high-energy geometries. The computed NNH2-MeP bond angles are 165.7, 101.8, and 115.3° for Co, Rh, and Ir complexes, respectively. The optimized MeP(pyr)3 bond lengths of 31-Co, 11-Rh, and 11-Ir are 2.22, 2.13 and 2.13 Å, respectively, Fig. 2. It is notable that the experimental geometries of {(pyr)3P}M (M = Co, Rh, Ir) complexes are in the range where Co-P is 2.13–2.16 Å, and Rh, IreP bond lengths are 2.13–2.32 Å in the CCDC (version 1.20 [43]) [44–50]. The computed MeNNH2 bond lengths are 1.80, 1.93, and 1.90 Å, Fig. 2, respectively. The shorter bond length of 31-Co is consistent with it being a smaller atom than its congeners. Grützmacher et al. have reported the complete series of Group 9 metal aminyl radical complexes bearing the [{2,2′bpy} M(trop2NH)] with MeNam bond lengths of 1.84, 1.93 and 1.99 Å for Co, Rh and Ir complexes, respectively [14,17,51]. The complexes of Rh and Ir are low-spin d8 while Co is high spin d8 electron configuration, Table 1, respectively Fig. 2. Thus, the longer and shorter bond lengths CoeP(pyr)3 and CoeNH2 over the experimental species may attributed to the bond angle 165.7° and therefore increased trans influence in comparison to both NH2NeMeP (where M = Rh, Ir) bond angles of 101.8 and 115.3°, respectively, Fig. 2.

The computed free energies of [{(pyr)3P}M(NH2)] and CH4 reaction has been investigated via computational chemistry (where M = Co, Rh, Ir). The relevant geometries are collected in Fig. 4 and the resulting data are collected in Scheme 4. The x2M methane adduct of 3Co is computed to have a relative free energy of 4.4 kcal/mol, which is lower by 3.0 and 4.3 kcal/mol in comparison to Rh and Ir 2 M, respectively, Scheme 4. The MeCH4 bond lengths are 2.84, 2.61 and 2.60 Å for Co, Rh, and Ir, respectively. The CH4 is computed to be ligated to the metal trans to an open coordinate site for each adduct, Fig. 3. The pertinent geometries are contained in the Supporting Information. For methane CeH activation by [{(pyr)3P}M(NH2)] complexes, oxidative addition/reductive elimination [OA/RE]‡ and [2σ + 2π]‡ mechanisms were the focus of this research given previous computational and experimental investigations [9,52,53]. The reaction [{(pyr)3P}M(NH2)] + CH4 → [{(pyr)3P}M(NH3) (CH3)] is more exergonic from cobalt to iridium complex, Scheme 4. The computed OA transition state for the iridium complex, 1[OA-Ir]‡, is optimized to have a reaction barrier where ΔG‡ = 17.8 kcal/mol, Scheme 4, for MeH activation. The iridium OA CeH activation barrier is lowered by 22.2 and 6.6 kcal/mol in comparison to cobalt and rhodium OA transition states, 3[OA-Co]‡, and 1[OA-Rh]‡, respectively. Similarly, the 1[OA-Ir]‡ CeH activation barrier in comparison to the ΔΔG‡ of [2σ + 2π-Co]‡, [2σ + 2π-Rh]‡, and [2σ + 2π-Ir]‡ is 19.3, 18.1, and 21.6 kcal/mol lower, respectively. The 1[OA-Ir]‡ computed barrier of methane by [{(pyr)3P}IrI(NH2)] with respect to the separated reactants is only 4.0 kcal/mol higher than the kinetic barrier of 13.8 kcal/mol by [TpRu {P(pyr)3}(η2-CHC6H6)CH3] for benzene CeH activation [54], Scheme 4. The geometries of the higher energy transition states are collected in the Supporting Information. Also, the imaginary frequencies of the CeH activation transition states are collected. It is notable that the computed CeH activation as a function of metal is favored because the Ir-metal is more electron rich with a natural bond orbital (NBO) population charge of −0.34 e− in comparison +0.26 e− for Co and where Rh electron (e−) density is lowered by 0.11e−. Thus, it could infer that the CeH kinetics barriers are akin to the trend in Mulliken charge within Group 9 congener series. It is particularly noteworthy that for the Co complex 3[OA-Co]‡ TS with a Co-H bond length of 1.51 Å, could also be isolated in addition to the 3 [2σ + 2π-Co]‡ transition state, and that it’s ΔΔG‡ is energetically

Table 1 M06/CEP-121G(d)/SMD-DMSO//M06/CEP-31G(d)/gas Gibbs free energies, STP, (kcal/mol) of the low and high spin states of N-pyrrolyl reactants respect to their lowest energy spin states. The metals are Co, Rh, and Ir triad. The superscript prefixes denote multiplicities (xM). The d counts assume all ligands are in typical formal oxidation states of (MI). Multiplicity

d8-CoI

d8-RhI

d8-IrI

Low Spin High Spin Quintet (S = 2): S = ∑ ms

1

1

1

3

3

3

23.0 0.0 5 32.5

0.0 10.2 N/A

0.0 8.2 N/A

N/A denotes not calculated. 3

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Fig. 4. The computed M06/CEP-31G(d) transition state geometries for methane activation by complex [{(pyr)3P}M(NH2)]. Bond lengths are in Ångström units (Å), and bond angles are in degrees (°). Co is the only triplet species, while the others are singlet. The contour value of Co spin density plot is 0.020.

reasonable, being only 2.9 kcal/mol above 3[2σ + 2π-Co]‡, which is unlike 1[2σ + 2π-Rh]‡ and 1[2σ + 2π-Ir]‡ with ΔΔG‡ above 1[OARh]‡ and 1[OA-Ir]‡ by 11.5 and 21.6 kcal/mol, Scheme 4. Thus, the computations disclose a transformation in the pathway of CeH activation within the congener of Group 9 moving from top to bottom, which favors the OA transition state for iridium complex. It is noteworthy that several attempted calculations to isolate hydrogen atom transfer (HAT) mechanism of methane activation transition state for Co, Rh, and Ir systems were unsuccessful and led to [2σ + 2π] transition state barriers. Scheme 4. M06/CEP-121G(d)/SMD-DMSO//M06/CEP-31G(d) Reaction Pathway for CeH and NeH Activation by Group 9 N-Pyrrolyl Complexes. Gibbs free energies are reported in kcal/mol for 3triplet (blue-Co), 1singlet (green-Rh), and 1singlet (red-Ir) states. The free energies are quoted in comparison to separated reactants of x[{(pyr)3P}M(NH2)] and CH4 gas. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.3. Reductive elimination from [{(pyr)3P}M(NH2)(H)(CH3)] The reductive elimination [RE], Fig. 5, from the hydride intermediate [{(pyr)3P}Ir(NH2)(CH3)(H)] → RE‡ → [{(pyr)3P}Ir(NH3) (CH3)], where the methyl group and hydride are trans to the open coordinate site. While, the methyl group and hydride are cis to the NH2 species, which is structurally organized and need not submit to ligand alignment in the transition state for the oxidation reduction of the metal center. The other isomers of [{(pyr)3P}Ir(NH2)(CH3)(H)] are collected in the Supporting Information. The calculated [RE]‡ of Ir is only ΔG‡ = 26.7 kcal/mol respect to separated reactants (Scheme 4), which computed to have a ΔΔG‡ of 19.7 and 7.4 kcal/mol lower in comparison to Co and Rh reductive elimination transition states, respectively. The formation of the T-shaped three-coordinate Ir intermediate

Fig. 3. The computed M06/CEP-31G(d) ground state geometry of CH4 ligated to [{(pyr)3P}Ir(NH2)]. Bond lengths are in Ångström units (Å), and bond angles are in degrees (°).

Fig. 5. Reductive elimination transition states of Co, Rh, and Ir, respectively. The computed M06/CEP-31G(d) transition state geometries of [{(pyr)3P}M (NH2)(CH3)(H)] complexes. Bond lengths are in Ångström units (Å), and bond angles are in degrees (°). Co is the only triplet multiplicity, while the others are singlet. 4

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[{(pyr)3P}Ir(NH3)(CH3)] is exergonic by −10.9 kcal/mol in comparison to the reactants. The Gibbs free energies of [{(pyr)3P}M(NH3) (CH3)] (where M = Co and Rh) are 7.8 and −7.4 kcal/mol, respectively, which are higher in respect to the Ir three-coordinate complex, Scheme 4. On the basis of the present calculations, the [OA/RE] pathway is favored for both Rh and Ir, with respect to the [2σ + 2π] pathway of Co for the three systems investigated, Scheme 4. It is noteworthy that calculations show that the ground state of [{(pyr)3P}M(NH3) (CH3)] (where M = Rh, Ir) complexes are computed to be more stable over its separate reactants, which suggests that the reverse reaction maybe less likely. 3.4. Oxy-Insertion pathway via [{(pyr)3P}M(NH3)(CH3)(PyO)] Pyridine-N-oxide (PyO) is utilized as the oxygen atom transfer reagent. Pyridine-N-oxide coordinates to [{(pyr)3P}M(NH3)(CH3)] complex (where M = Co, Rh, Ir), may result in a three- or a four-coordinate complex. The resulted geometries may undergo oxygen atom transfer (OAT) to form a four- or a five-coordinate metal-oxo intermediate species. The metal-oxo species are complexes that have metal–oxygen multiple bonds [55]. While d8 electronic configuration of terminal oxo complexes are unusual and rare, there has been an increase computational and experimental investigations in the study of these oxo complexes [41,56–59]. Nevertheless, displacement of the NH3 by PyO leading to formation of [{(pyr)3P}M(CH3)(PyO)] is computed to have consistently higher Gibbs free energy than [{(pyr)3P}M(NH3)(CH3) (PyO)] complexes. The Supporting Information collects [{(pyr)3P}M (CH3)(PyO] geometries. The five coordinate oxo intermediate undergoes oxy-insertion where the methyl ligand migrate to form a square planar methoxide complex. The reaction completes the methane-tomethanol (MTM) catalytic cycle via ammine NeH activation transition state across the MeOCH3 bond to produce methanol, and regenerate [{(pyr)3P}M(NH2)] after the dissociation of the pyridine ligand, which is exergonic for the three systems investigated of the proposed catalytic cycle, Scheme 3. The computed free energy surface of oxygen atom transfer (OAT) involving cobalt, rhodium, and iridium complexes are shown in Scheme 4. In respect to the separated reactants, the coordinate of pyridine-Noxide (PyO) leading to the complex of [{(pyr)3P}Ir(NH3)(CH3)(PyO)], is investigated to be the lower energy congener with, ΔG = −12.2 kcal/mol, by 15.9 and 0.8 kcal/mol in comparison to the Co and Rh PyO adduct species, respectively, Scheme 4. The Rh and Ir lower free energy difference from the oxidant of PyO coordinate trans to {(pyr)3P} ligand in a square planar fashion, and cis to both NH3 and CH3 species. For the Co complex, Fig. 6, PyO is weakly bound in a distorted tetrahedral geometry in respect to the separated reactants. The binding energetics of PyO on complexes [{(pyr)3P}M(NH3)(CH3)] to [{(pyr)3P}M(NH3)(CH3)(PyO)] (where = Co, Rh, Ir), are consistently exergonic, suggesting that the latter species are the preferred geometries for the investigation of MTM (methane-to-methanol) mechanism. The barrier for O-atom transfer from [{(pyr)3P}Ir(NH3)(CH3)(PyO)] via oxygen atom transfer [OAT]‡ is ΔG‡ = 18.7 kcal/mol in respect to separated reactants, Scheme 4 and Fig. 7. The OAT of cobalt and rhodium ΔΔG‡ barriers are 20.5 and 6.4 kcal/mol above that of Ir OAT free energy barrier, respectively. The formation of the trigonal bipyramidal five-coordinate IrIII-oxo-intermediate has the oxo-oxygen in the equatorial position of the complex [{(pyr)3P}Ir(NH3)(CH3)(O)(Py)], which for all three metals is computed to be exergonic, Scheme 4. On the potential energy surface (PES), the oxy-insertion (OxI) continue along the methyl migration pathway via transition state [OxI]‡, Fig. 8, which resulted in the square planar IrI-methoxide complex [{(pyr)3P}Ir(NH3)(OCH3)(Py)] where the methoxy and ammine ligands are geometrically cis to each other. The Gibbs free kinetic barrier of Ir, [OxI]‡, is found to be only 7.0 kcal/mol respect to separated reactants, Scheme 4. The OxI ΔΔG‡ barriers of Co and Rh are lower and higher by 4.7 and 3.8 kcal/mol in comparison to the Ir-

Fig. 6. The computed M06/CEP-31G(d) ground triplet state of [{(pyr)3P}Co (NH3)(CH3)(PyO)]. Bond lengths are in Ångström units (Å), and bond angles are in degrees (°).

Fig. 7. The computed oxygen atom transfer (OAT) transition state geometries by complex [{(pyr)3P}M(NH3)(CH3)(PyO)]. Bond lengths are in Ångström units (Å), and bond angles are in degrees (°). 3Co is the only triplet species, while the others are singlet.

[OxI]‡ transition state. It is noteworthy that oxy-insertion is computed to have reasonable barriers of ∼11 kcal/mol. The computed natural bond orbital (NBO) population on the 5

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Fig. 9. The computed M06/CEP-31G(d) transition state geometry of ammine NeH activation by [{(pyr)3P}Ir(NH3)(OCH3)(Py)] complexes. Bond lengths are in Ångström units (Å), and bond angles are in degrees (°). The principal frontier orbital of d8-1IrI during the TS cycle, where the positive and negative phases are the red and blue, respectively. The contour value is 0.020. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

decidedly lower. Interestingly, the other two barriers are modestly low with the highest ΔG = 31.6 kcal/mol for the OxI transition state, Scheme 4. The Ir CeH activation is less exergonic for the {(pyr)3P}Ir complex than the Co and Rh congener, ΔΔG‡ ∼ 7 and 19 kcal/mol, respectively. It is notable, that the potential energy surfaces (PESs) of the three systems investigated are consistently thermodynamically accessible, Scheme 4, for the Rh and Ir complexes. Overall, the {(pyr)3P}Ir system is superior to the {(pyr)3P}Co and {(pyr)3P}Rh systems, Scheme 4, in terms of calculated kinetics and thermodynamics for MTM of the Group 9 N-Pyrrolyl Complexes.

Fig. 8. The computed M06/CEP-31G(d) transition state geometry of a Oxy-insertion by [{(pyr)3P}Ir(NH3)(CH3)(O)(Py)] with the methoxy ground state species, respectively. Bond lengths are in Ångström units (Å), and bond angles are in degrees (°).

3.6. NBO electronic structure analysis of the transition states methoxy complexes resulted in Co more electrophilic (the calculated NBO charge on Co is +0.23 electron (e−), and Rh and Ir are nucleophilic by −0.11 and −0.13 e−, respectively). The oxygen-atom of the methoxy group NBOs are −0.89, −0.84, and −0.85 e−, respectively. Thus, the square planar methoxy complexes are structurally prepared for the more acidic hydrogen atom of the ammine ligand to migrate onto the oxygen-atom of the methoxy group, Fig. 8.

Investigation of the natural bond orbital (NBO) analysis was performed and tabulated to give further information on the transition state structures of CeH and NeH activation, Table 2. Analogous calculations with Effective Core Protentional (ECP) level of theory (M06/def2TZVPP/SMD-DMSO//M06/def2-SVP gas phase level of theory) show similar NBO e− density on the selective transition states structures with the cobalt metal centers as the exception. Thus, the reported results in Table 2 are those obtained with the working level of theory. The Supporting Information collects the (M06/def2-TZVPP/SMD-DMSO// M06/def2-SVP) geometries and the NBO e− densities. The cobalt metal is more electrophilic for both CeH and NeH activation pathways. The less nucleophilic metal center may have prevented an oxidative addition pathway and subscribe to the [2α + 2π] mechanism in which the NBO analysis is +0.26 electron (e−) density at the metal center. The NBO analyses for the Rh and Ir complexes show the metals to be nucleophilic, which computed to have the lower CeH activation OA transition states, Table 2. Computational investigation shows that the majority of the NBO e− density is divided between the nitrogen of NH2 and NH3, and the carbon of methyl and methoxy groups, Table 2. The CeH oxidative addition transition state of 1(OARh)‡ is similar to 1(OA-Ir)‡, except that the latter ΔΔG = 6.6 kcal/mol lower than the former, and the latter has a greater NBO density by 0.11e− on the iridium, which may suggest a lower kinetic barrier. Interestingly, the highest NBO e− density remain localized on the Natoms of NH2 and more so on the ammine substrate within the TS, respectively, which may suggest ammine NeH activation transition

3.5. NeH activation via [{(pyr)3P}M(Py)(NH3)(OCH3)] complex The final step of the methanol catalytic cycle utilized [{(pyr)3P}Ir (Py)(NH3)(OCH3)], square planar complex via ammine NeH activation via hydrogen atom transfer (HAT) transition states, Fig. 9 [11,60–63]. In comparison to HAT, Grützmacher et al. and de Bruin et al. have reported the unexpected stability of MI (Group 9: Co, Rh, Ir) and IrI noninnocent ligand complexes via NeH deprotonation reactions, respectively [11,14,15,19,64,65]. The barrier of NeH [HAT]‡ to [{(pyr)3P}Ir (Py)(NH2)(HOCH3)], is only 16.4 kcal/mol with respect to the Irmethoxy complex, Scheme 4. The NeH activation barriers of Co and Rh with respect to Ir (ΔΔG‡ = 4.0 and 3.8 kcal/mol) higher and lower, respectively, which suggest that all NeH activation kinetic barriers are modest, Scheme 4. As a consequence, in each ground state square planar complex, [{(pyr)3P}M(Py)(NH2)(HOCH3)] (where = Co, Rh, Ir), the final Gibbs free energies of reaction to the preferred product are exergonic by ∼36 kcal/mol, Scheme 4, for the three systems investigated. For the Ircomplex, three of the five free energy barriers investigated are 6

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Table 2 Calculated M06/CEP-121G(d)/SMD-DMSO//M06/CEP-31G(d) gas; Natural Bond Orbitals (NBO) analysis of [{(pyr)3P}M(NH2)](–H-CH3) and [{(pyr)3P}M(NH3) (CH3)(–PyO)] transition states derived from NBO population analysis. The superscript prefix denotes spin multiplicity. OA = oxidative addition. Co‡

Rh‡

3

Transition States

Metals NBO TS (e−) N-atom TS (e−) of NH2 Methyl Group in TS (e−) Moving “Hδ+” in TS (e−) Imaginary Frequency (cm−1)

N-atom TS (e−) of H2N–Hδ+ Methoxy Group in TS (e−)

1

1

Ir

3 [2 α + 2π] CeH

3 NeH NeH

1 OA CeH

1 NeH NeH

1 OA CeH

1 NeH NeH

+0.26 −1.22 −1.02 +0.32 1231i

+0.30 −1.28 −0.86 +0.49 461i

−0.23 −1.03 −0.72 +0.14 425i

−0.14 −1.11 −0.81 +0.49 580i

−0.34 −1.06 −0.73 +0.22 556i

−0.14 −1.13 −0.81 +0.51 519i

The superscript prefix denotes spin multiplicity. NeH activation.

state to the preferred mechanism, Table 2. Analysis of the NBO e− density of cobalt through iridium metals suggest a constant increased e− density on the methoxy O-atom in comparison to the methane methyl group. The exception to the earlier oversimplification is the transition state of 3[2α + 2π]‡, in which there is −1.02 versus −0.86 electron (e−) density decreased, respectively. The NBO analysis show that the acidity of the transfer hydrogen atom increased in comparison to the CeH activation, which may suggest that the TS is analogous to hydrogen atom transfer of the H-atom of NH3 to the methoxy substrate, which increased in e− density on N-atom of H2N–H species within the transition state.

ΔG‡ = 17.9 kcal/mol in comparison to separated reactants, Scheme 4; with Co and Rh higher by 19.3 and 6.6 kcal/mol. For the Co-NH2 system, both OA and the [2 α + 2π] transition states could be isolated with the latter being 2.9 kcal/mol lower in free energy. This is distinguishable in the context of earlier research [9,71] on late 3d metal complexes (also supported by a possibly non-innocent active ligand), which suggest a transforming from a preferred [2 α + 2π] to OA pathway for methane CeH activation as one work within the Group 9 triad, which could attributed to the increase in the NBO electron density charge of both Rh and Ir metals, Table 2. Reductive elimination Gibbs free kinetic barrier of iridium 1(RE-Ir)‡ is only 26.7 kcal/mol, which is 19.7 and 7.4 kcal/mol lower in comparison to Co and Rh on the potential energy surface. The pyridine-N-oxide ligand was introduced to be the internal oxygen carrying agent for the current investigation, and the iridium species, 1(OAT-Ir)‡, is computed to have the lowest barrier of all the species computed, ΔG‡ = 18.7 kcal/mol in comparisons to separated reactants, Scheme 4. The Co- and Rh-system OAT transition states are higher by 20.5 and 6.4 kcal/mol, respectively. It is possible that pyridine as a σ-donor approve the higher oxidation states of IrIII and oxo ligand favor stronger π-acceptor on the metal. Thus, the lower OAT barrier of IrIII may favored the increase electron density at IrIII metal center. The kinetic barrier to the formation of methoxy (Ir-OMe) square planar complex is moderately low (ΔG‡ = 7.0 kcal/mol), which is 4.7 and 3.8 kcal/mol higher and lower in respect to Co and Rh [OxI]‡ transition states. Thus, for the three systems, the investigation may suggest that the pathway to methane-to-methanol is via oxo-intermediate mechanisms because of the favored lower OxI transition states to the square planar methoxy complexes. Ammine activation by [{(pyr)3P}Rh(NH3)(OCH3)(Py)], RhI-complex, via hydrogen atom transfer, 1(HAT-Rh)‡, is computed to have the lowest kinetic barrier of the species researched, ΔG‡ =12.6 kcal/mol respect to methoxy square planar complex, Scheme 4. The Co- and IrNeH hydrogen transfer transition states are higher by only 7.8 and 3.8 kcal/mol, respectively. It is interesting that the computed ammine NeH barriers are all moderately low for the three systems investigated, and the desired product are all exergonic. The DFT results for the Ir complex seem most promising among the systems investigated at this juncture. The ammine NeH activation barrier of Ir computed to be reasonable at 16.4 kcal/mol. However, the methane CeH activation barrier of Rh is higher by ∼7 kcal/mol in comparison to the Ir-system. The present results further suggest that higher nucleophilicity at the metal-center may change the methane CeH activation barrier selectivity for the formation of the desired methanol product. Thus, an ongoing investigation in our laboratories seeks to make these Co through Ir complexes softer acids, and as a result, increasing the electron density on the metal of [{(pyr)3P}M]: pyr = N-pyrrolyl; M = Co, Rh, Ir] complexes, which the present calculations suggest would make for a more facile reaction because of the increase NBO

4. Summary, conclusions and prospectus The widely used process to produce methanol utilizes high temperature/pressure steam reforming technology that produces syngas (CO/H2), and the final process in methanol production utilizes the CO/ H2 from the first step, after adjusting the CO:H2 ratio closer to 1:2, which produces the methanol from copper/zinc oxide base catalyst [66–69]. However, the overall reaction of ΔΔrG° for the high temperature/pressure steam reforming syngas process is endergonic by ∼27 kcal/mol [70]. This computational work focused on the catalytic mechanism of Eq. (1), which is an exergonic oxy-insertion process, and where ΔS (entropy) and K (reaction constant) are both greater than zero (0) and one (1), respectively. CH4(g) + PyO(s) → Py(l) + CH3OH(l) PyO = pyridine-N-oxide Py = Pyridine

calΔG

calΔS =

= −26.9 kcal/mol

(1)

+2.91 cal/mol-K

19

K = 5.24 × 10 (STP)

The current model system explores the potential of (N-pyrrolyl)3phosphine {(pyr)3P} ligand in conjunction with Group 9 metals for CeH activation, oxygen atom transfer (OAT), oxy-insertion (OxI), followed by ammine NeH activation. The modeled investigation suggest that the overall reaction is exergonic with ΔG ≈ −27 kcal/mol respect to the separated reactants, Scheme 4. It is remarkable, that the energetics of the overall reaction cycle is similar to that of Eq. (1) for the three transition metals catalytic reactions investigated. The results of M06/CEP-121G(d)/SMD-DMSO//M06/CEP-31G(d) data support the scheme that (N-pyrrolyl)3phosphine of Group 9 complexes may assist the desired oxy-insertion (OxI) reaction for metalmethyl bonds by the association of pyridine-N-oxide (PyO) ligand to the metal and dissociation of Py ligand during the final stage of the cycle to regenerate the starting complex, [{(pyr)3P}M(NH2)] (where M = Co, Rh, Ir) transition metals. Several important conclusions have resulted that can influence experimental attempts to identify Group 9 transition metal complexes with N-pyrrolyl, {(pyr)3P}, supporting ligands for which methane CeH activation, OAT, OxI, and ammine NeH activation are possible. Methane activation by the iridium complex, 1(OA-Ir)‡, is computed to have the lowest kinetic barrier of all the complexes investigated, 7

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electron density on the metal. Therefore, this approach may be synthetically feasible that may allow the complexes to serve as the basis for catalyst systems to activate and functionalize small non-aromatic species. It is notable among the d8 model complexes studied that the Ir of Group 9 entities appear to be the most promising for experimental investigation. As a final point, on the potential energy surfaces of this mechanism, the three systems studied suggest that the reaction profile is exergonic with increased electron density on the metal from Co to Ir complexes, which tracked well with Eq. (1) and may suggest the lower methane CeH activation barrier of iridium complex system.

[18] N. Donati, D. Stein, T. Büttner, H. Schönberg, J. Harmer, S. Anadaram, H. Grützmacher, Rhodium and iridium amino, amido, and aminyl radical complexes, Eur. J. Inorg. Chem. 2008 (2008) 4691–4703. [19] Y. Wang, A. Olankitwanit, S. Rajca, A. Rajca, Intramolecular hydrogen atom transfer in aminyl radical at room temperature with large kinetic isotope effect, J. Am. Chem. Soc. 139 (2017) 7144–7147. [20] M. Muehlhofer, T. Strassner, W.A. Herrmann, New catalyst systems for the catalytic conversion of methane into methanol, Angew. Chem., Int. Ed. 41 (2002) 1745–1747. [21] T.R. Cundari, B.M. Prince, DFT study of the reactivity of methane and dioxygen with d10–L2M complexes, J. Organomet. Chem. 696 (2011) 3982–3986. [22] Y. Mizuhata, T. Sasamori, N. Tokitoh, Stable heavier carbene analogues, Chem. Rev. 109 (2009) 3479–3511. [23] M.S. Viciu, R.A. Kelly, E.D. Stevens, F. Naud, M. Studer, S.P. Nolan, Synthesis, Characterization, and catalytic activity of N-heterocyclic carbene (NHC) palladacycle complexes, Org. Lett. 5 (2003) 1479–1482. [24] M. Muehlhofer, T. Strassner, W.A. Herrmann, New catalyst systems for the catalytic conversion of methane into methanol, J. Mol. Catal. A: Chem. 41 (2002) 1745–1747. [25] K.G. Moloy, J.L. Petersen, N-pyrrolyl phosphines: an unexploited class of phosphine ligands with exceptional π-acceptor character, J. Am. Chem. Soc. 117 (1995) 7696–7710. [26] R.G. Pearson, Hard and soft acids and bases, J. Am. Chem. Soc. 85 (1963) 3533–3539. [27] S. Fischer, J. Hoyano, I. Johnson, L.K. Peterson, Syntheses and properties of phosphino- and phosphinatopyrroles, Can. J. Chem. 54 (1976) 2706–2709. [28] Y. Zhao, D.G. Truhlar, The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06class functionals and 12 other functionals, Theor. Chem. Acc. 120 (2008) 215–241. [29] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G. A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, GAUSSIAN 09, Revision D.01, Gaussian Inc., Wallingford, CT, 2013, 2009. [30] B.B. Laird, R.B. Ross, T. Ziegler, Chemical Applications of Density-Functional Theory, American Chemical Society, 1996. [31] S. Niu, M.B. Hall, Theoretical studies on reactions of transition-metal complexes, Chem. Rev. 100 (2000) 353–406. [32] H.S. Yu, X. He, D.G. Truhlar, MN15-L: a new local exchange-correlation functional for Kohn-Sham density functional theory with broad accuracy for atoms, molecules, and solids, J. Chem. Theory Comput. 12 (2016) 1280–1293. [33] W.J. Stevens, H. Basch, M. Krauss, Relativistic compact effective potentials and efficient, shared-exponent basis sets for the third-, fourth-, and fifth-row atoms, Can. J. Chem. 70 (1992) 612–630. [34] T.R. Cundari, W.J. Stevens, Effective core potential methods for the lanthanides, J. Chem. Phys. 98 (1993) 5555–5565. [35] W.J. Stevens, H. Basch, M. Krauss, Compact effective potentials and efficient shared-exponent basis sets for the first- and second-row atoms, Can. J. Chem. 81 (1984) 6026–6033. [36] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, Selfconsistent molecular orbital methods. XX. A basis set for correlated wave functions, J. Chem. Phys. 72 (1980) 650–654. [37] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, Self-consistent molecular-orbital methods. IX. An extended gaussian-type basis for molecular-orbital studies of organic molecules, J. Chem. Phys. 54 (1971) 724–728. [38] A.V. Marenich, C.J. Cramer, D.G. Truhlar, Universal solvation model based on solute electron density and on a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions, J. Phys. Chem. B 113 (2009) 6378–6396. [39] E. Bosch, M. Moreno, J.M. Lluch, J. Bertrán, Intrinsic reaction coordinate calculations for reaction paths possessing branching points, Chem. Phys. Lett. 160 (1989) 543–548. [40] M.J. Stark, M.J. Shaw, N.P. Rath, E.B. Bauer, Synthesis, structural characterization, and catalytic activity of indenyl tris(N-pyrrolyl)phosphine complexes of ruthenium, Eur. J. Inorg. Chem. 2016 (2016) 1093–1102. [41] B.M. Prince, T.R. Cundari, C.J. Tymczak, DFT study of the reaction of a two-coordinate iron(II) dialkyl complex with molecular oxygen, J. Phys. Chem. A 118 (2014) 11056–11061. [42] H. Chen, K. Cho, W. Lai, W. Nam, S. Shaik, Dioxygen activation by a non-heme iron (II) complex: theoretical study toward understanding ferric-superoxo complexes, J. Chem. Theory. Comput. 8 (2012) 915–926. [43] C.R. Groom, J. Bruno, M.P. Lightfoot, S.C. Ward, Case: How should I reference the CSD- The Cambridge Crystallographic Data Centre (CCDC), B72 (2016) 171–179. [44] A.D. Burrows, M.F. Mahon, M. Varrone, The synthesis and late transition metal chemistry of 7-aza-N-indolyl phosphines and the activity of their palladium complexes in CO–ethene co-polymerisation, Dalton Trans. 4718–4730 (2003). [45] A.D. Burrows, R.W. Harrington, M.F. Mahon, M.T. Palmer, F. Senia, M. Varrone,

Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgement This work was supported in part by Texas Southern University (TSU) High Performance Computing Center (http://hpcc.tsu.edu/; Grant PHY-1126251). Texas Southern University Seed Grant (TSU Seed Grant in 2018). The Center for Catalysis Computational Research (3CR). We would like thank Professor Daniel Vrinceanu for his valuable service on the High-Performance Computing Center (HPC) at TSU. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.comptc.2019.112503. References [1] J.A. Kerr, Bond dissociation energies by kinetic methods, Chem. Rev. 66 (1966) 465–500. [2] S.J. Blanksby, G.B. Ellison, Bond dissociation energies of organic molecules, Acc. Chem. Res. 36 (2003) 255–263. [3] Bond dissociation energy, 2018. [4] W. Zheng, Y. Fu, Q. Guo, G3//BMK and its application to calculation of bond dissociation enthalpies, J. Chem. Theory Comput. 4 (2008) 1324–1331. [5] J.R. Webb, T. Bolaño, T.B. Gunnoe, Cover picture: catalytic oxy-functionalization of methane and other hydrocarbons: fundamental advancements and new strategies (ChemSusChem 1/2011), ChemSusChem 4 (2011) 37–49. [6] B.M. Prince, T.R. Cundari, C-H bond activation of methane by PtII–N-heterocyclic carbene complexes. The importance of having the ligands in the right place at the right time, Organometallics 31 (2012) 1042–1048. [7] N.J. Gunsalus, A. Koppaka, S.H. Park, S.M. Bischof, B.G. Hashiguchi, R.A. Periana, Homogeneous functionalization of methane, Chem. Rev. 117 (2017) 8521–8573. [8] U.S. remained world's largest producer of petroleum and natural gas hydrocarbons in 2014 – Today in Energy - U.S. Energy Information Administration (EIA), < https://www.eia.gov/todayinenergy/detail.php?id=20692 > . [9] B.M. Prince, T.R. Cundari, Computational study of methane C-H activation by earthabundant metal amide/aminyl complexes, Organometallics 36 (2017) 3987–3994. [10] G.L. Hillhouse, J.E. Bercaw, Reactions of water and ammonia with bis(pentamethylcyclopentadienyl) complexes of zirconium and hafnium, J. Am. Chem. Soc. 106 (1984) 5472–5478. [11] M.G. Scheibel, J. Abbenseth, M. Kinauer, F.W. Heinemann, C. Würtele, B. de Bruin, S. Schneider, Homolytic N-H activation of ammonia: hydrogen transfer of parent iridium ammine, amide, imide, and nitride species, Inorg. Chem. 54 (2015) 9290–9302. [12] K. Yamamoto, J. Li, J.A.O. Garber, J.D. Rolfes, G.B. Boursalian, J.C. Borghs, C. Genicot, J. Jacq, M. van Gastel, F. Neese, T. Ritter, Palladium-catalysed electrophilic aromatic C-H fluorination, Nature 554 (2018) 511–514. [13] Z. Wangdong, H. Yongseok, R.M. Samara, K. Jinseok, L. Zafra José, P. Hoa, T.Y. Gopalakrishna, T.S. Herng, D. Jun, C. Juan, K. Dongho, W. Jishan, Stable nitrogen-centered bis(imino)rylene diradicaloids, Chem. Eur. J. 24 (2018) 4944–4951. [14] R. Rodriguez-Lugo, B. de Bruin, M. Trincado, H. Grützmacher, A stable aminyl radical coordinated to cobalt, Chem. Eur. J. 23 (2017) 6795–6802. [15] S. Daiki, F. Ko, O. Atsuhiro, Stable subporphyrin meso-aminyl radicals without resonance stabilization by a neighboring heteroatom, Angew. Chem., Int. Ed. 56 (2017) 7435–7439. [16] K. Ehud, The 82nd annual meeting of the israel chemical society, February 13–14, 2017, David Intercontinental Hotel, Tel-Aviv, Israel, Isr. J. Chem. 58 (2018) 151–175. [17] T. Büttner, J. Geier, G. Frison, J. Harmer, C. Calle, A. Schweiger, H. Schönberg, H. Grützmacher, A stable aminyl radical metal complex, Science 307 (2005) 235–238.

8

Computational and Theoretical Chemistry 1162 (2019) 112503

B.M. Prince

[46]

[47]

[48]

[49]

[50]

[51]

[52] [53]

[54]

[55] [56] [57] [58]

Synthesis and reactivity of rhodium(I) complexes containing keto-functionalised Npyrrolyl phosphine ligands, Dalton Trans. 3717–3726 (2003). K.N. Gavrilov, V.N. Tsarev, S.I. Konkin, S.E. Lyubimov, A.A. Korlyukov, M.Y. Antipin, V.A. Davankov, Imino- and oxazolino-functionalised pyrrolylphosphanes and pyrrolylphosphinites: an unexploited class of chiral P, N-bidentate ligands with unusual electronic properties, Eur. J. Inorg. Chem. 2005 (2005) 3311–3319. E. Kossoy, B. Rybtchinski, Y. Diskin-Posner, L.J.W. Shimon, G. Leitus, D. Milstein, Structure and reactivity of rhodium(I) complexes based on electron-withdrawing pyrrolyl-PCP-pincer ligands, Organometallics 28 (2009) 523–533. J. Castro, A. Moyano, M.A. Pericàs, A. Riera, M.A. Maestro, J. Mahía, Tris(pyrrolyl) phosphine-substituted acetylene−dicobaltcarbonyl complexes: syntheses, structural characterization, and reactivity studies, Organometallics 19 (2000) 1704–1712. C. Babij, C.S. Browning, D.H. Farrar, I.O. Koshevoy, I.S. Podkorytov, A.J. Poë, S.P. Tunik, Tripyrrolylphosphine as a unique bridging ligand in the Rh6(CO)14(μ2-P (NC4H4)3) cluster: structure, bonding, fluxionality, thermodynamics, and kinetics studies, J. Am. Chem. Soc. 124 (2002) 8922–8931. J. Huang, C.M. Haar, S.P. Nolan, W.J. Marshall, K.G. Moloy, Thermodynamics of phosphine coordination to the [PNP]RhI fragment: an example of the importance of reorganization energies in the assessment of metal−ligand “bond strengths”, J. Am. Chem. Soc. 120 (1998) 7806–7815. M. Königsmann, N. Donati, D. Stein, H. Schönberg, J. Harmer, A. Sreekanth, H. Grützmacher, Metalloenzyme-inspired catalysis: selective oxidation of primary alcohols with an iridium–aminyl-radical complex, Angew. Chem., Int. Ed. 46 (2007) 3567–3570. N.P. Mankad, W.E. Antholine, R.K. Szilagyi, J.C. Peters, Three-coordinate copper(I) amido and aminyl radical complexes, J. Am. Chem. Soc. 131 (2009) 3878–3880. S. Wiese, Y.M. Badiei, R.T. Gephart, S. Mossin, M.S. Varonka, M.M. Melzer, K. Meyer, T.R. Cundari, T.H. Warren, Catalytic C-H amination with unactivated amines through copper(II) amides, Angew. Chem., Int. Ed. 49 (2010) 8850–8855. N.A. Foley, M. Lail, T.B. Gunnoe, T.R. Cundari, P.D. Boyle, J.L. Petersen, Combined experimental and computational study of TpRu{P(pyr)3}(NCMe)Me (pyr = Npyrrolyl): inter- and intramolecular activation of C-H bonds and the impact of sterics on catalytic hydroarylation of olefins, Organometallics 26 (2007) 5507–5516. Y. Liu, T. Lau, Activation of metal oxo and nitrido complexes by lewis acids, J. Am. Chem. Soc. 141 (2019) 3755–3766. D. Munz, How to tame a palladium terminal oxo, Chem. Sci. 9 (2018) 1155–1167. A. Grünwald, D. Munz, How to tame a palladium terminal imido, J. Organomet. Chem. 864 (2018) 26–36. H. Kotani, T. Sugiyama, T. Ishizuka, Y. Shiota, K. Yoshizawa, T. Kojima, Redox-

[59]

[60]

[61]

[62]

[63]

[64] [65]

[66] [67] [68] [69] [70]

[71]

9

noninnocent behavior of tris(2-pyridylmethyl)amine bound to a lewis acidic Rh(III) ion induced by C-H deprotonation, J. Am. Chem. Soc. 137 (2015) 11222–11225. T.M. Figg, G. Schoendorff, B. Chilukuri, T.R. Cundari, Structure and bonding of palladium oxos as possible intermediates in metal-carbon oxy insertion reactions, Organometallics 32 (2013) 4993–4996. H. Fallah, F. Horng, T.R. Cundari, Theoretical study of two possible side reactions for reductive functionalization of 3d metal-methyl complexes by hydroxide ion: deprotonation and metal-methyl bond dissociation, Organometallics 35 (2016) 950–958. E. Morgan, D.F. MacLean, R. McDonald, L. Turculet, Rhodium and iridium amido complexes supported by silyl pincer ligation: ammonia N-H bond activation by a PSiP]Ir complex, J. Am. Chem. Soc. 131 (2009) 14234–14236. B. Koşar, Ç Albayrak, M. Odabaşoǧlu, O. Büyükgüngör, Theoretical and experimental studies on electronic structure, cocrystallization, and intramolecular proton transfer of two tautomers: (E)‐2‐{[2‐(hydroxymethyl)phenylimino]methyl}‐5‐methoxyphenol and (Z)‐6‐{[2‐(hydroxymethyl)phenylamino] methylene}‐3‐methoxy‐cyclohexa‐2, 4‐dienone, Int. J. Quantum Chem. 111 (11) 3654-3663. G. Espino, A. Caballero, B.R. Manzano, L. Santos, M. Péerez-Manrique, M. Moreno, Félix A. Jalón, Experimental and computational evidence for the participation of nonclassical dihydrogen species in proton transfer processes on Ru-Arene complexes with uncoordinated N centers. Efficient catalytic deuterium labeling of H2 with CD3OD, Organometallics 31 (2012) 3087–3100. K. Hindson, B. de Bruin, Cooperative & redox non-innocent ligands in directing organometallic reactivity, Eur. J. Inorg. Chem. 2012 (2012) 340. R. Sikari, S. Sinha, U. Jash, S. Das, P. Brandão, B. de Bruin, N.D. Paul, Deprotonation induced ligand oxidation in a NiII complex of a redox noninnocent N1-(2-Aminophenyl)benzene-1,2-diamine and its use in catalytic alcohol oxidation, Inorg. Chem. 55 (2016) 6114–6123. E. Despagnet-Ayoub, R.H. Grubbs, A stable four-membered N-heterocyclic carbene, J. Am. Chem. Soc. 126 (2004) 10198–10199. V. Havran, M.P. Duduković, C.S. Lo, Conversion of methane and carbon dioxide to higher value products, Ind Eng Chem Res 50 (2011) 7089–7100. E.M. Wilcox, G.W. Roberts, J.J. Spivey, Direct catalytic formation of acetic acid from CO2 and methane, Catal. Today Environ. Catal. React. Eng. 88 (2003) 83–90. B.M. Prince, A DFT investigation of substituent effects on carbon dioxide fixation: by a low-coordinate cobalt (I) complex, Comput. Theor. Chem. 1122 (2017) 1–8. G.A. Olah, A. Goeppert, G.K.S. Prakash, Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons, J. Org. Chem. 74 (2009) 487–498. O. Olatunji-Ojo, T.R. Cundari, C-H activation by multiply bonded complexes with potentially noninnocent ligands: a computational study, Inorg. Chem. 52 (2013) 8106–8113.