Computational Study of Methane CâH Activation ... - ACS Publications

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Computational Study of Methane C−H Activation by Earth-Abundant Metal Amide/Aminyl Complexes Bruce M. Prince*,† and Thomas R. Cundari*,‡ †

Center for Catalysis Computational Research (3CR), Department of Chemistry, Texas Southern University, 3100 Cleburne Street, Houston, Texas 77004, United States ‡ Department of Chemistry, CASCaM, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States S Supporting Information *

ABSTRACT: Density functional theory, augmented by multiconfiguration SCF (MCSCF) simulations, was used to understand the factors that control methane C−H activation by Earth-abundant, 3d metal (Cr - Ni) [(κ3-CNC)M(NH2)] complexes via hydrogen atom abstraction (HAA) and [2 + 2] pathways. Calculations suggest a significant amide/aminyl, i.e., [(κ3-CNC)2−M3+(NH2)−] ⇔ [(κ3-CNC)2−M2+(NH2)•], admixture in the electronic ground states of these complexes and thus significant unpaired electron density (radical character) on the NH2 ligand. The spin coupling between the aminyl radical and spin density on the central metal ion is interesting, particularly for the cobalt aminyl complex, in which both ferromagnetic and antiferromagnetic triplet states are found to be close in energy via both DFT and MCSCF methods. Modeled complexes are computed to have reasonable barriers to methane activation, with ΔG⧧ values being in approximately the upper 20s to mid 30s kcal/mol, generally decreasing toward the right in the 3d series, which loosely tracks with spin density (radical character) on the aminyl nitrogen, a switch from [2 + 2] to HAA activation pathways, and more favorable thermodynamics for C−H scission.

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INTRODUCTION The properties of classical coordination complexes are typically the result of interactions of a Lewis acidic central metal ion with Lewis basic and π-acidic/basic ligands. In many traditional catalyst applications, supporting ligands play a spectator role in the sense of maintaining their formal oxidation state/redox poise, while redox activity is limited to the metal. Chemists have, however, begun to exploit redox noninnocent ligands (NILs) in a variety of research areas,1,2 including photochemistry, sensors, and catalysis. While typical NILs are often elaborately constructed, conjugated heterocycles, the properties of simpler noninnocent ligands, such as those of the aminyl type (R2N•), are of growing interest. Aminyl complexes are much rarer and have been less investigated in relation to closedshell amide (R2N−) counterparts.3−10 Coordinatively and electronically unsaturated metal centers have been successful in C−H activation under mild conditions and are important intermediates in many catalytic cycles.11−14 Experimental and computational research on C−H activation reactions have revealed, among a myriad of activation pathways, metal-, ligand-, and metal−ligand-centered pathways. Hydrogen atom abstraction (HAA) is an example of a ligand-centered pathway, while oxidative addition is representative of metalcentered activation pathways. Metal−ligand-centered pathways include 1,2-addition, concerted metalation−deprotonation (CMD), oxidative hydrogen migration (OHM), and σ-bond metathesis (σBM)15−18 (Scheme 1). © XXXX American Chemical Society

Scheme 1. Reaction Pathways for Methane Activation by a Transition-Metal Aminyl Speciesa

a

Abbreviations: HAA, hydrogen atom abstraction; OHM, oxidative hydrogen migration; σBM, σ-bond metathesis.

Alkylamines are major industrial intermediates.19−22 The current process to convert CH3OH plus NH3 to monomethylamine involves two steps. The first entails the reaction CH3OH(g) + NH3(g) → TMA(g) (trimethylamine) + H2O(g) and is thermodynamically favored; the second is an Received: August 4, 2017

A

DOI: 10.1021/acs.organomet.7b00600 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Figure 1. Computed ground state geometries of (pincer)M amide/aminyl complexes and short-hand designations. Leading superscripts in these and other compounds denote the multiplicity of the system. Bond lengths are in Ångström units (Å); bond angles are in degrees (°). NP = pincer nitrogen; Nam = amide/aminyl nitrogen.

between the aminyl radical center and spin density on the central metal ion.

endothermic process that converts TMA into monomethylamine.23 Researchers have investigated amination at various temperatures and pressures with an assortment of homogeneous and heterogeneous catalysts to synthesize monomethylamine.23−27 Since the seminal work of Grützmacher and co-workers,3 aminyl complexes have received increased attention in the organometallic and inorganic communities.2,28−31 A RuII aminyl complex with bulky 2-(bis(2-pyridylmethyl)aminomethyl)anilido (NPh-bpa2−) ligands, [RuII(N•Ph-bpa)(tBu2cat)]−, was reported by Miyazato et al.31 Adhikari et al. reported a squareplanar NiII aminyl complex; the aminyl radical was part of a PNP pincer ligand.30 Penkert et al. characterized anilino radical complexes of Co and Mn.32 Mankad et al. reported a novel trigonal-planar CuI aminyl (LnCuI−NAr2) complex, which was characterized via experiment and theory.9 The activation of H atom donors such as tributylstannane, thiophenol, and 9,10dihydroanthracene was reported by these researchers. de Bruin and collaborators have done comprehensive research on nitrogen-based NILs.33−37 Zhang and co-workers have shown that Co imidyl complexes are key intermediates in Co(porphyrin)-catalyzed amination of C−H bonds; presumably, H atom transfer to these imidyl complexes would result in products with significant aminyl character.38 Grützmacher et al. reported the unexpected stability of an aminyl radical complex, [RhI(trop2N•)(bipy)]+OTf−, where trop is 5-H-dibenzo[a,d]cycloheptene-5-yl. Surprisingly, the Rh−aminyl complex was prepared directly from an amide derivative [RhI(trop2N)(bipy)] in DMSO solvent. The RhI aminyl complex is stable in the solid state for an extended period in organic solvents.3 Wiese et al. reported a CuII amide/CuI aminyl system that was capable of catalytically aminating sp3 C−H bonds.39 As with the aforementioned report of Mankad et al.,9 a combination of theory and experiment suggested that a key ingredient in C−H activating ability was the radical character (unpaired spin density) residing on the primary amide/aminyl nitrogen (NHAd, Ad = 1-adamantyl). This complex was capable of catalytically aminating ethylbenzene and indane (bond enthalpies ∼84−87 kcal/mol). Given the aforementioned recent exciting experimental precedents, herein is reported research that focuses on modeling C−H activation of methane gas by four-coordinate complexes of middle−late 3d metals (Cr, Mn, Fe, Co, Ni). These Earth-abundant metals are usually adept at one-electronredox activity. The simple model LnM(NR2) is used (R = hydrogen) for computational efficiency (Figure 1), which makes feasible the utilization of multiconfiguration SCF techniques to augment DFT studies of the spin coupling

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COMPUTATIONAL METHODS

Density functional theory (DFT) calculations utilized the Gaussian 09 suite of programs40 to probe the stationary points and energetics for methane activation by metal (M) amide/aminyl species supported by a κ3-CNC pincer ligand: (M = Cr, Mn, Fe, Co, Ni). The M0641 functional was used in concert with all-electron double-ζ and triple-ζ basis sets, 6-31+G(d) and 6-311++G(d,p),42,43 respectively. For example, a comparison of the 6-31+G(d) and a larger 6-311++G(d,p) basis set to compute the BDE (bond dissociation enthalpy) of methane42,43 suggested a ±0.1 kcal/mol difference; geometries used for BDE44 and all other calculations are collected in the Supporting Information. To conserve computational resources, the 6-31+G(d) allelectron double-ζ basis set was used to computed the geometries in this investigation. All quoted energetics are Gibbs free energies calculated at 298.15 K and 1 atm and utilize the M06/6-311++G(d,p)/SMD-DMSO//M06/ 6-31+G(d)/gas level of theory. The M06/6-31+G(d) level of theory was employed to optimize the geometries and obtain vibrational frequencies of complexes along the reaction coordinates. Single points used a continuum solvation model and larger basis set: M06/6-311+ +G(d,p)/SMD-DMSO (DMSO, ε = 46.83); the species investigated are all neutral. The DFT geometries were checked to ensure stability of the wave function by means of the stable = opt keyword. Tight convergence criteria and ultrafine/superfine grids for numerical integration were used. Using the superfine grid for numerical integration suggested an average absolute difference of only ±0.5 kcal/mol in relative free energies in comparison to the ultrafine grid. The stationary points were characterized as minima or transition states (TSs) via calculation of the energy Hessian and the observation of the correct number of imaginary frequencies, zero (0) or one (1), respectively. The TSs were authenticated using intrinsic reaction coordinate (IRC) methods. Optimization and single-point calculations were performed without symmetry restraint and utilized the (un)restricted Kohn−Sham formalism as appropriate.

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RESULTS AND DISCUSSION [(κ3-CNC)M(NH2)] Reactants. The x[(κ3-CNC)M(NH2)] (M = Cr, Mn, Fe, Co, Ni) complexes x1-M (where x = spin multiplicity) studied here are four-coordinate complexes with a tridentate CNC44 pincer supporting ligand and an amide/ aminyl occupying the final coordination site. The κ3-CNC pincer of Bartholomew et al.45 was selected for the present studies to try and enforce square-planar coordination geometries, thus providing access to the metal amide/aminyl reactive site for the methane substrate. Additionally, given the 2− formal charge expected for κ3-CNC, it was deemed that a formally trivalent 3d metal ion would provide the best opportunity (particularly among the later members of the B


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Organometallics

Table 1. M06/6-311++G(d,p)/SMD-DMSO//M06/6-31+G(d)/gas Gibbs Free Energies (kcal/mol) of the Low-, Intermediate-, and High-Spin States of Amide/Aminyl Reactants Relative to Their Lowest Energy Spin Statea multiplicity free energy low spin intermediate spin high spin

d3-CrIII

d4-MnIII

d5-FeIII

d6-CoIII

d7-NiIII

2

1

2

1

2

3

4

3

33.4 N/A 4 0.0

77.0 45.2 5 0.0

30.2 5.0 6 0.0

25.4 0.0 5 11.5

0.0 N/A 4 25.4

a M = Cr−Ni. The superscript prefixes denote multiplicities. N/A denotes not applicable. The d counts assume all ligands are in typical formal oxidation states.

pentad of metals studied) to display significant MIII(amide) ⇔ MII(aminyl) resonance. The lowest free energy multiplicities for the reactant complexes studied are quartet, quintet, sextet, triplet, and doublet (Table 1) for Cr through Ni, respectively. High-energy geometries/spin states are collected in the Supporting Information. Their C−M−C bond angles are 144.0, 159.2, 127.8, 165.8, and 167.2° for the Cr, Mn, Fe, Co, and Ni complexes, respectively; the N−M−N bond angles are 165.9, 178.8, 118.3, 178.9, and 179.0°, respectively. Hence, among the reactant complexes studied, the Fe example is quite distorted from a square-planar arrangement. The optimized M−Nam (amide/aminyl) bond lengths (Å) are 1.82 (41-Cr and 51-Mn), 1.85 (61-Fe), 1.77 (31-Co), and 1.76 (21-Ni) (Figure 1). Frazier et al. have reported a (κ3NNN)CrII-NTMS2 complex with a nearly square planar geometry and a Cr−Nam bond length of 2.03 Å. The NTMS2 plane is perpendicular to the pincer plane as calculated for the present Cr amide/aminyl model, and the CrII center is high spin per the Evans method.46 The ∼0.18 Å difference in Cr− Nam bond lengths between theory and experiment is commensurate with reported ionic radii differences between high-spin CrII and CrIII.47 Morris et al. have reported the structure of a four-coordinate (κ3-NNN)FeII-NTMS2 complex that is high spin, is distorted as per 61-Fe, and has a Fe−Nam distance of 1.967 Å.46 Caulton and co-workers have structurally characterized a Co amide, albeit with a nonconjugated PNP pincer ligand, with Co−Nam = 1.92 Å.48 A CCDC search (version 1.1949) yielded 17 pincer-Ni-NR2 complexes; the majority are best described as NiII, possess nearly square planar coordination about the central metal ion, and the Ni−Nam = 1.91 ± 0.03 Å, in agreement with 21-Ni. The complexes of Cr, Mn, and Fe are all high-spin dn configurations assuming a trivalent central metal ion. Intermediate and low-spin configurations are seen for the Co and Ni complexes (Figure 1). In each ground state reactant complex, the NH2 ligand is planar (sp2 N) and (except for Cr) is coplanar with the plane defined by the pincer ligand. It is notable that the rotational free energy barrier around the Cr−NH2 bond is very small, being calculated to be only 5 kcal/mol. This low barrier and the orientation of the other amide moieties suggest that these pincer metal amide/aminyl complexes are structurally prepared for C−H activation by an approach of the substrate normal to the pincer molecular plane. Electronic structure analyses signify that [(κ3-CNC)M(NH2)] have significant aminyl character and thus substantial spin density (>0.3 e−) resides on the NH2 nitrogen (Table 2), with the exception of the Mn complex. For the Mn complex, 5 1-Mn, only 0.01 e− spin density resides on the amide/aminyl nitrogen with the majority of the spin density on the metal per a Mulliken analysis. A modest amount of spin density resides on the pincer ligand, 0.2 e− or less, in the reactant complexes.

Table 2. Mulliken Spin Densities (e−) Computed at the M06/6-31+G(d)//M06/6-311++G(d,p)/SMD-DMSO/spin Level of Theory for (κ3-CNC)M(NH2) Complexes spin density complex 4

1-Cr 1-Mn 6 1-Fe 3 1−Co-F 3 1-Co-AF 2 1-Ni 5

M

Nam

3.36 4.02 4.16 1.50 2.50 0.59

−0.34 0.01 0.33 0.35 −0.55 0.58

The latest 3d metals within the series studied here, cobalt and nickel, have the most spin density on the aminyl nitrogen, −0.55 and +0.58 e−, respectively, perchance reflecting the greater stability of the 2+ (aminyl) versus 3+ (amide) formal oxidation state for these metals. As such, the calculations suggest a significant “amide” [(κ3-CNC)2−M3+(NH2)−] ⇔ “aminyl” [(κ3-CNC)2−M2+(NH2)•] admixture and thus significant unpaired electron density on the NH2 ligand. In cognizance of the work on copper amide/aminyl complexes by Mankad et al.9 and Warren and co-workers,39 the electronic structures of complexes 1 suggest that these systems are of interest for possible activation of strong aliphatic C−H bonds. For the cobalt amide/aminyl complex an antiferromagnetically coupled triplet state (i.e., ααααβ) was isolated; the DFToptimized geometries of these states, 31-Co-F and 31-Co-AF, are quite similar (Figure 2). There is a calculated 2 kcal/mol difference between these two electronic states in favor of 31Co-F using DFT methods. Using similar approaches, the antiferromagnetically coupled states of the other amide/aminyl complexes were investigated with no success in isolating antiferromagnetically coupled states. The DFT results are corroborated by CASSCF (complete active space SCF) calculations on 1-M with active spaces up to 14 orbitals and 14 electrons, which did not indicate a low-energy electronic excited state of the same multiplicity as the ground state for complexes other than 31-Co. For 31-Co, the two lowest energy triplet states are ∼7.5 kcal/mol apart in energy at the CAS(10,10)/6-31G(d) level of theory. C−H Activation of Methane by [(κ3-CNC)M(NH2)]. The computed free energy data for methane C−H activation involving [(κ3-CNC)MIII(NH2)] complexes are collected in Table 3, and the relevant computed transition state geometries are given in Figure 3; stationary point geometries are collected in the Supporting Information. Methane binding to 1-M is computed to be mildly endergonic for all metals, and the methane fragment is quite far from the metal center. For the C−H activation of methane by [(κ3-CNC)M(NH2)] complexes, hydrogen atom abstraction (HAA) and [2 + 2] pathways were the focus of investigation, given experimental C


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Organometallics

computed thermodynamics of (κ3-CNC)M(NH2) + CH4 → (κ3-CNC)M(CH3)(NH3) (Table 3) as a function of the metal. The calculated HAA transition state for the nickel complex, 2 (HAA-Ni)⧧, is computed to have a reasonable kinetic barrier, ΔG⧧ = 27.2 kcal/mol (Table 3), for methane activation. The nickel complex thus has ΔΔG⧧ = 8.7 kcal/mol and is lower in comparison to the corresponding cobalt HAA transition state, 3 (HAA-Co)⧧. The computed barrier for methane activation by [(κ3-CNC)NiII/III(NH2)•/‑] compares favorably to the experimentally determined barrier of 23(1) kcal/mol and the computed ΔG⧧ value of 25 kcal/mol for C−H activation of indane by (β-diketiminate)CuII/I amide/aminyl.39 The 2(HAA-Ni)⧧ transition state, which is the only transition state isolated for the Ni-aminyl complex, has an enormous imaginary frequency of 2183i cm−1 in comparison to 1543i (4([2 + 2]-Cr)⧧), 1156i (5([2 + 2]-Mn)⧧), 1478i (6([2 + 2]Fe)⧧), 1541i (3([2 + 2]-Co)⧧), and 1922i (3(HAA-Co)⧧) cm−1, respectively (Table 4). The magnitude of the imaginary modes is consistent with the main motion in the TS being transfer of the active H atom between the CMe and the amide/ aminyl N of the LnM(NH2) complex. It is notable that the computed C−H activation barriers as a f unction of the metal generally decrease lef t to right in the 3d series, akin to the trend in spin density, which increases on the aminyl N in a similar direction.9,51,52 Repeated calculations to locate HAA methane activation transition states for the earlier metal (Cr, Mn, and Fe) complexes were unsuccessful and led to [2 + 2] transition states: 4([2 + 2]-Cr)⧧, 5([2 + 2]-Mn)⧧, and 6([2 + 2]-Fe)⧧, with ΔΔG⧧ = 8.4, 15.3, and 16.3 kcal/mol above 2(HAA-Ni)⧧, respectively (Table 3). Likewise, repeated attempts to find a four-center methane activation TS for nickel led back to 2 (HAA-Ni)⧧. It is especially notable that for the Co complex a [2 + 2] TS with a Co−H bond length of 1.76 Å could also be isolated in addition to an HAA transition state and that it is energetically competitive, being only 3.2 kcal/mol above 3 (HAA-Co)⧧ (Table 3). Therefore, the calculations reveal a change in mechanism of activation as the metal traverses from left ([2 + 2]) to right (HAA) in the 3d series. Electronic Structure Analysis of Transition States. Investigation of the Mulliken-derived spin densities of the transition states revealed no spin density on the active hydrogen (Table 4). The highest unpaired spin density is typically located at the metal ion. The exception to the latter generalization is the transition state 2(HAA-Ni)⧧, in which there is 0.11 e− unaired spin density at the nickel, while the majority of the spin density is roughly split between the carbon of the methane substrate and the aminyl nitrogen (Table 4).

Figure 2. M06/6-31+G(d) optimized triplet states of [(κ3-CNC)CoIII(NH2)] (bond lengths are in Å, and bond angles are in deg): (top) spin density (1.50 e− on Co; 0.35 e− on Nam); (Bottom) spin density (2.50 e− on Co; −0.55 e− on Nam). Red denotes positive spin density and blue negative spin density. The contour value is 0.020.

precedents.9,50 The [2 + 2] pathway occurs without a change in metal formal oxidation state, assuming that the redox poise of the ligands does not change during the course of the bond activation. The HAA mechanism is a radical pathway and involves formal 1e− reduction of the metal. Methane C−H activation via HAA would produce a free methyl radical, which may then subsequently rebound onto another amide/aminyl complex to produce a monomethylamine complex,39 but this is not discussed, as the focus of this paper is the C−H activation reaction via amide/aminyl ligands. The reaction (κ3-CNC)M(NH2) + CH4 → (κ3-CNC)M(NH3) + •CH3 becomes decidedly more exergonic upon going from the chromium to the nickel complex (Table 3). There is less variation in the

Table 3. M06/6-311++G(d,p)/SMD-DMSO//M06/6-31+G(d)/gas Relative Free Energies (kcal/mol) for Methane C−H Bond Activation by (κ3-CNC)M(NH2) (M = Cr−Ni) Aminyla species (κ3-CNC)M(NH2) CH4 adduct C−H TS⧧ HAA [2 + 2] (κ3-CNC)M(NH2) (κ3-CNC)M(NH2) (κ3-CNC)M(NH2) a

+ MeH

+ CH4 → (κ3-CNC)M(NH3) + •CH3 + CH4 → (κ3-CNC)M-CH3 + NH3 + CH4 → (κ3-CNC)M(CH3)(NH3)

Cr

Mn

Fe

Co

Ni

0.0 3.4

0.0 5.3

0.0 5.9

0.0 5.3

0.0 6.0

b 35.6 22.9 16.3 2.8

b 43.1 42.5 21.1 11.0

b 43.5 26.7 14.2 11.3

35.9 39.1 17.0 12.2 5.5

27.2 b 6.0 9.3 4.9

HAA denotes hydrogen atom abstraction. bRepeated attempts to locate the transition state led to a different TS. D


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Organometallics

Figure 3. M06/6-31+G(d) transtion state geometies for methane activation by 3d metal amide/aminyl complexes. Bond lengths are in Ångström units (Å), and bond angles are in degrees. The superscript prefix denotes the spin multiplicity. The spin density is plotted (contour value 0.020).

Table 4. Calculated M06/6-311++G(d,p)/SMD-DMSO//M06/6-31+G(d)/gas Spin Densities in e− for (κ3-CNC)M(NH2)(- H−CH3) Transition States Derived from Mulliken Density Analysesa ([2 + 2]-Cr)⧧

4 −

metal spin TS (e ) aminyl N TS (e−) methyl group in TS (e−) substrate “H” in TS (e−) imaginary frequency (cm−1) a

3.24 −0.13 −0.08 −0.00 1543i

5

([2 + 2]-Mn)⧧

([2 + 2]-Fe)⧧

6

4.03 −0.12 0.06 0.02 1156i

4.13 0.11 0.15 0.01 1478i

3

([2 + 2]-Co)⧧ 1.70 0.18 0.19 0.01 1541i

(HAA-Co)⧧

3

0.82 0.39 0.55 −0.03 1922i

2

(HAA-Ni)⧧ 0.11 0.46 0.56 −0.03 2183i

The superscript prefix denotes spin multiplicity. HAA denotes hydrogen atom abstraction.

The transition state 3(HAA-Co)⧧ is similar to 2(HAA-Ni)⧧, except the former, being a triplet rather than a doublet, has greater unpaired e− on the cobalt. Interestingly, the isomeric 3 ([2 + 2]-Co)⧧ the metal has approximately double the spin density at cobalt (0.82 versus 1.70 e−, Table 4) and 0.2 e− at both the aminyl N and methyl C. For the later metals, cobalt and nickel, limited spin density exists on the supporting pincer ligand. Analysis of the bonding of the earliest metal complex studied, 4([2 + 2]-Cr)⧧, suggests a redox innocent pincer supporting ligand, with the majority of spin density on the metal and smaller, negative spin density contributions from the methyl C and aminyl N (Table 4). There are small individual spin density contributions (on the order of ±0.1−0.2 e−) from the pincer in 4([2 + 2]-Cr)⧧, presumably via spin polarization in these unrestricted DFT simulations.

H activation via the amide/aminyl ligand of these complexes. The mechanisms delineated involve hydrogen atom abstraction and [2 + 2] activation pathways. There are several conclusions from this investigation that may provide insight into C−H activation via complexes with potentially redox active actor and spectator ligands and that could inform experimental efforts to identify novel hydrocarbon functionalization catalysts based on Earth-abundant 3d metal complexes. (1) All 1-M geometries were optimized at the M06/631+G(d) level of theory for Cr through Ni, respectively. The lowest free energy states are square planar or nearly so (Table 1), except for iron. In each ground state reactant, the NH2 ligand is planar at the nitrogen and coplanar with the pincer ligand, except for Cr, which is perpendicular to the plane defined by the pincer ligand, but with a calculated rotational barrier about the Cr−NH2 bond of only 5 kcal/mol. As such, these pincer metal amide/aminyl complexes are structurally prepared for methane C−H activation via HAA or [2 + 2] (either σ-bond metathesis or the related oxidative hydrogen migration) pathways. (2) Electronic structure analyses suggest that, among the [(κ3-CNC)M(NH2)] complexes researched, Mn and Ni complexes have the least and the most spin density resident

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SUMMARY AND CONCLUSIONS The x[(κ3-CNC)M(NH2)] (M = Cr, Mn, Fe, Co, Ni) complexes x1-M (where x = spin multiplicity) studied here are four-coordinate complexes with a tridentate CNC45 pincer ligand and an amide/aminyl ligand occupying the final coordination site. The present research explored methane C− E


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Organometallics on the NH2 nitrogen atom, respectively (Table 2). Thus, the present investigation suggests significant [(κ3CNC)2−M3+(NH2)−] ⇔ [(κ3-CNC)2−M2+(NH2)•] admixture in the ground states of these complexes, apart from manganese, which leads to unpaired electron density (radical character) on the NH2 ligand. Antiferromagnetically coupled states were explored for all investigated aminyl complexes, but only for the Co examples were (anti)ferromagnetically coupled states isolated (Figure 2). The DFT results are corroborated by multiconfiguration SCF computations, which indicate that ferromagnetic and antiferromagnetic states of [(κ3-CNC)Co(NH2)] are close in energy, with similar optimized geometries. DFT and MCSCF computations did not reveal similar spin coupling in the other aminyl complexes studied. (3) Methane activation by the nickel complex, 2(HAA-Ni)⧧, is computed to have the lowest kinetic barrier of all the complexes investigated, ΔG⧧ = 27.2 kcal/mol relative to separate reactants (Table 3). For the Co-NH2 system, both HAA and the [2 + 2] transition states could be isolated, with the latter being 3.2 kcal/mol higher in free energy. This is notable in the context of earlier research53 on late 3d metal imide complexes (also supported by a potentially redox active pincer), which indicated a morphing from a preferred [2 + 2] to an HAA pathway for methane C−H activation as one traversed from earlier to later 3d metals. It is noted with great interest that the computed barrier for methane activation by [(κ3-CNC)NiII/III(NH2)•/−] of ∼27 kcal/ mol compares favorably to the experimental barrier of 23(1) kcal/mol and the computed ΔG⧧ value of 25 kcal/mol reported for benzylic C−H activation by an experimentally characterized (β-diketiminate)CuII/I amide/aminyl.38 The latter studies focused on benzylic sp3 C−H substrates, which have bond enthalpies ∼15−20 kcal/mol lower than that of methane, which is the substrate of interest in the current research. Recent experimental research9,38 on copper complexes clearly indicates that modification of the metal can enhance spin density on the amide/aminyl N through manipulation of their fragment orbital energies. It is reasonable to assume that the same applies to these late-metal amide/aminyl complexes and may thus be exploited to identify feasible catalysts for the functionalization of the strongest, most inert Csp3−H bonds: i.e., those of light alkanes. One expected challenge in identifying synthetically feasible aminyl complexes is to identify supporting ligands that can stabilize a formally M3+ ion for the later 3d metals. The majority of (pincer)M(NR2) motifs in the Cambridge Database are best viewed as Ni2+, while examples with cobalt are very rare and the best example is best described as a Co2+ amide.47 It was our presumption that the [(pincer)2−M3+(NH2)−] ⇔ [(pincer)2−M2+(NH2)•] admixture is more greatly weighted to the right than would be the analogous [(pincer)−M2+(NH2)−] ⇔ [(pincer)−M+(NH2)•] resonance. Studies are underway in our laboratories to quantify these issues in order to identify novel, synthetically feasible complexes that can serve as the basis for catalyst systems to activate and functionalize light alkanes.

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Cartesian coordinates of all calculated species along with their spin multiplicities (XYZ) Cartesian coordinates of all calculated species along with their spin multiplicities (XYZ) Cartesian coordinates of all calculated species along with their spin multiplicities (XYZ) Computational details and the full ref 40.

AUTHOR INFORMATION

Corresponding Authors

*E-mail for B.M.P.: [email protected]. *E-mail for T.R.C.: [email protected]. ORCID

Bruce M. Prince: 0000-0003-1333-9321 Thomas R. Cundari: 0000-0003-1822-6473 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by Texas Southern University High Performance Computing Center (http:/hpcc.tsu.edu/; Grant PHY-1126251), the Texas Southern University Department of Chemistry, the Center for Computational Catalysis Research (3CR), and CASCaM, University of North Texas. T.R.C. thanks the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences for support of this research via grant DE-FG0203ER15387 and the NSF for their support of computing facilities at UNT through MRI grant CHE-1531468.

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REFERENCES

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