a theoretical investigation into the first-row transition metal–o2...

INORGANIC CHEMISTRYFRONTIERS

RESEARCH ARTICLE

Cite this: Inorg. Chem. Front., 2019,6, 2071

Received 12th April 2019,Accepted 17th June 2019

DOI: 10.1039/c9qi00407f

rsc.li/frontiers-inorganic

A theoretical investigation into the first-rowtransition metal–O2 adducts†

Xiao-Xi Li,a Kyung-Bin Cho *a,b and Wonwoo Nam *a

An extensive investigation into various M–O2 species (M = CrI, MnI, FeI, CoI, NiI, CuI) has been conducted

using a Density Functional Theory (DFT) approach, generating MI–O2, MII–superoxo or MIII–peroxo

species. Two different ligands, 12-TMC and 14-TMC, are used to gauge the effects of the ligand ring-size.

In general, theory reproduces the experimental results (where available) well enough to give confidence

in the calculations. In addition to the usual calculated features of the individual metal complexes, a stat-

istical analysis has been done by comparing the M–O2 species across the periodical system. It is found

that the O2 binding energy diminishes with higher metal atomic number, while an end-on structure

becomes gradually favored. Also, multi-spin state reactivity becomes more likely for metals above Fe. The

spin density on O2 (and with it the formal oxidation state of the metal) is more dependent on the prevail-

ing spin state of the compound rather than the metal type per se, and the higher flexibility of the larger

14-TMC ring has also been verified. The theoretical methods used are also evaluated regarding their

accuracy.

Introduction

In biochemical systems, evolution has taken advantage of thefact that air (and with it, O2) is easily accessible.1 Not only isthe O–O bond energy rich which can be utilized to drive thereaction, but the oxygen atoms can be attached to substrates tomake them solvable in in vivo systems. The O–O bond break isfrequently catalysed by metal centres; thus, an initial metal–dioxygen species is expected to form.2 Depending on the exter-nal electron and proton donor sources, one could expect thepresence of one or more type(s) of such intermediates, e.g.metal–superoxo, –peroxo and –hydroperoxo species.3–5 It isevident that the initial oxidation state and type of the metal isalso a key factor, as for instance both metal(II)- and metal(III)–peroxo species could be plausibly formed.6,7 In addition to thefact that these species may be en route to form high-valentmetal-oxo species8 (which are usually the prime candidates tobe the oxygenating species9,10), there are suggestions thatthese species can act as the oxygenating species themselves aswell.11 Thus, the literature is full of studies on both synthetic

and biologically occurring metal–dioxygen species, in order tounderstand the inherent features of these species.12–16

Our current study is an attempt to contribute to this fieldby comparing the effects and features of different metals toeach other, in an otherwise identical setting. The last con-dition however, is not easy to achieve. From already publishedstudies, it is sometimes difficult to compare similar speciesside-by-side due to differences in methods, ligands, metals,solvents etc. used in experiments. This heterogeneity of thestudies is probably necessary, as the individual species maynot be stable for studies in different experimental conditions.However, much of this problem can be circumvented intheoretical studies, where the stability is not an issue. Eventhough theoretical methods also have methodological vari-ations between the studies, at least over-the-board homogen-eity is possible. The added advantage is that species not yetexperimentally isolated could also be studied and compared.Thus, we opted for a theoretical approach in our current study.

In this study, we have calculated [(L)MO2]+ species (Fig. 1),

where M is one of the six transition metals (Cr, Mn, Fe, Co, Niand Cu) with an oxidation state of +1 and O2 is a neutralspecies. Hence, any re-distribution of electrons within thecomplex could result in MII–superoxo or MIII–peroxo speciesdepending on the precise circumstances. The ligands (L)of both 12-TMC (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclo-dodecane) and 14-TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane) were used, enabling us to compare theeffects of the ligand ring size. The particular choice of theformal metal(III)-peroxide and TMC ligands for this study is

†Electronic supplementary information (ESI) available: Energies, NBO results,Mulliken spin density distributions, geometries and coordinates. See DOI:10.1039/c9qi00407f

aDepartment of Chemistry and Nano Science, Ewha Womans University,

Seoul 03760, Korea. E-mail: [email protected] of Chemistry, Chonbuk National University, Jeonju 54896, Korea.

E-mail: [email protected]

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due to the existence of several experimental results for thisset of compounds.17 The existing experimental studies arenot a complete set, but enough to allow us to assess the accu-racy of our calculations. Therefore, the present work is also astudy in how well theoretical methods can match up to andpredict experiments. For each of the metal species, we havecalculated all the probable spin states, without the O2 adductas well as with it in both side-on and end-on fashions. Thecalculated structures presented here include 97 gas-phaseoptimized structures and 105 solvent-phase optimized ones.We will denote each species as 1M for [(12-TMC)MO2]

+ speciesand 2M for [(14-TMC)MO2]

+ species (M = Cr, Mn, Fe, Co, Nior Cu).

MethodsComputational details

Calculations were performed using density functional theory(DFT)18 as implemented in ORCA19 package. Two functionals,B3LYP20–24 and BP86,21,25,26 were used in this study to estimatethe energy accuracy of hybrid vs. pure functionals for thesesystems. The basis set used was CP(PPP)27,28 for the metal, andDef2-TZVPP29 for the rest. The optimizations were done at theBP86/[CP(PPP)+Def2-TZVPP]/GAS level (where GAS denotesgas-phase), whereby a single-point frequency calculation wascarried out at the same level. This also enabled us to calculatethe free energy (ΔG) at this level. Also, a single-point B3LYP//BP86/[CP(PPP)+Def2-TZVPP]/GAS calculation was done on thatgeometry for energy comparisons. The effect of the functionalfor the geometry optimization itself was investigated only in

few cases where it could possibly matter (as detailed in rele-vant sections). The effects of solvent (acetonitrile) were dealtwith separately as a new optimization at the BP86/[CP(PPP)+Def2-TZVPP]/COSMO level, with the solvent beingincluded as a dielectric medium using the COSMO30 schemeas implemented in ORCA. A subsequent single-point B3LYP/[CP(PPP)+Def2-TZVPP]/COSMO calculation was done for theBP86/[CP(PPP)+Def2-TZVPP]/COSMO optimized geometry aswell. No frequency calculation has been carried out due toquestions of the validity of such calculation on the solventoptimized structures.31 Dispersion calculations, where it issuspected to matter, was done by the DFT-D3 program tocorrect the energies.32 To test its effect on the geometries aswell, the representative species (vide infra) were reoptimized atthe BP86-D level. The RMSDs were found to be on average at0.07 Å for 1M and 0.05 Å for 2M (see ESI, Table S9†) Thesevalues are much smaller than the RMSDs found when compar-ing the BP86 structures (without dispersion) to the X-ray struc-tures (Table 1); hence the dispersion effect on geometryoptimizations were deemed to be of little consequence to ourstudy and not used. To test the sensitivity of the calculationsto basis sets, the entire set of calculations were also conductedwith CP(PPP) + TZVP28,33 basis set, without no significantbearing for the current results (data not shown). The dataquoted in the text are taken from the BP86/[CP(PPP)+Def2-TZVPP]/COSMO calculations, unless explicitly stated otherwise.A more in-depth look at the electronic structures was donewith Natural Bond Orbital (NBO) 6.0 program.34 All the datacan be found in the ESI.†

Models used

When it comes to geometry, TMC is notorious regarding poss-ible variations in the structure. For instance, there may or maynot be an axial (solvent) ligand bound to the opposite side ofthe molecular oxygen. Also, the methyl groups of TMC can beoriented all syn or trans to the bound oxygen, or a mix thereof.The ring structure symmetry is also subject to variations: givena particular C–C bond in the ligand, its corresponding bondon the other side could be either parallel or crossing to it. Afull investigation into all these and more possible geometriesin combination with all possible spin states would requiresome resources, with little gain in our understanding of thecore metal system. Instead, we have opted to start our calcu-lations from the known X-ray structure for NiIII–peroxo speciesfor the 12-TMC ligand (1Ni)35 and FeIII–peroxo species for the14-TMC ligand (2Fe).36 In these structures, the molecularoxygen is bound syn to the methyl groups without any otheraxial ligands. Also in 2Fe, for a given C–C bond, the counter-part at the opposite side of the ring is not parallel but cross-ing. Using the same ligand conformation, we only replace themetal atom. While the ligand conformation may or may notdiffer for other metal species (not all the species have X-raystructures solved), this gives us the advantage to be able tocompare the different metal species with a consistent constantinfluence from the ligand.

Fig. 1 The models used in this study. [(12-TMC)MO2]+ (upper row) and

[(14-TMC)MO2]+ (lower row) with O2 binding in either a side-on (left

column) or an end-on (right column) fashion. M = Cr, Mn, Fe, Co, Ni orCu. H-Atoms were omitted for clarity.

Research Article Inorganic Chemistry Frontiers

2072 | Inorg. Chem. Front., 2019, 6, 2071–2081 This journal is © the Partner Organisations 2019

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ResultsCorrelation to experiments

Before discussing new conclusions drawn from the calcu-lations, it is necessary to assess the reliability of the calcu-lations. This is done by comparing two experimentally avail-able criteria: geometry and ground spin state.

Having decided on the ligand conformation (see Methodssection), the most obvious geometric feature is whether thedioxygen molecule (O2) is bound in an end-on or a side-onfashion. There are seven cases in total, where the bindingmode of O2 to the metal is experimentally known: a side-onmode for 1M (M = Mn, Co, Ni) and 2M (M = Mn, Fe, Co), andan end-on mode for 2Ni (Table 1). The calculations are in goodagreement with all these seven cases, regardless of the DFTfunctional or solvent phase used. Moreover, the calculationsgive the same unanimous answers to the potential structuresof the remaining unknown structures. Consequently, the pre-dictive value of the calculations for the binding geometries ishigh, and should reflect the preferred O2 binding mode cor-rectly for the unknown structures as well.

To date, there are only five X-ray structures available amongthe investigated species. For the 1Ni species, the Ni–O and O–Obond lengths differ by less than 0.01 Å between the calculatedand X-ray structures (Table 1). Overlaying only the core section(defined as the metal, O and N atoms), the root-mean squaredeviation (RMSD) of the atom positions is 0.03 Å, while over-laying the whole atoms results in a RMSD of 0.11 Å. Thus, thecalculation seems to be able to reproduce the X-ray structurequite excellently. The RMSD is only marginally larger for 1Co

species (0.36 Å), but this is mostly due to the outlying part ofthe ligand as the core has the same RMSD as in the Ni case(0.03 Å). Similarly, the calculations on 2Fe and 2Mn speciesshow good agreement with experiments as well (Table 1). Forthe 1Mn species, the agreement is a little bit worse, with aRMSD of 0.34 Å at the core. In this case, the discrepancy

indeed comes from the different C–C bond configurations ofthe TMC ligand; our use of the 12-TMC ligand configurationfrom Ni differs slightly from what is experimentally obtainedin the Mn case. However, we have shown in our earlier workthat a small RMSD of 0.03 Å could be obtained if we were tocalculate the 12-TMC configuration corresponding to what hasbeen obtained experimentally for the Mn case.37 As our statedgoal is to compare the change of the metal but not the ligand,we keep the 12-TMC structure as calculated in this study,noting that the experimental and theoretical Mn–O and O–Odistance differences are still within 0.03 Å (Table 1).

Regarding the spin states, there are six cases where theground states are experimentally determined. Theory repro-duces the experiments unambiguously only in two cases: 2Mn

and 2Fe (Table 1). If the “right” functional or energy treatmentis chosen, theory can match the experiments in three othercases: 1Co, 1Ni, and 2Co. For 1Co, the electronic energies of bothfunctionals, with or without solvent, predict an S = 0 groundstate, in agreement with experiments. Using Gibb’s freeenergy, S = 0 is indeed the ground state at T = 10 K (at whichthe experiments were done), but not at 298 K. For the 1Ni case,BP86 prefers the S = 1/2 state, while B3LYP prefers S = 3/2instead. For 2Co, BP86 prefers S = 0, while B3LYP prefers S = 2.There are conflicting reports about the preferred spin state ofthis species,38,39 but the latest NMR spectra taken at 233 Kpoints to an S = 1 state for this species.39 Indeed, inclusion ofthermal effects at 233 K (i.e. obtaining Gibb’s free energy) cor-rects the BP86 energy to an S = 1 preference (see ESI,Table S1†). Only in one case do the current DFT method-ologies not match the experimentally determined spin state atall: 2Ni. The erroneous results on Ni from DFT calculationshave been observed before,40–43 and is discussed in the rele-vant section below.

The overall conclusion drawn from the above describedcomparisons of geometry and spin state between the calcu-lated and experimental structures indicates that the accuracy

Table 1 Comparison to experimentsa

Metal

12-TMC 14-TMC

End-on orSide-on

Bond lengthsM–O(avg)/O–O (Å)

RMSDb

core/all (Å)

Spin SEnd-on orSide-on

Bond lengthsM–O(avg)/O–O (Å)

RMSDb

core/all (Å)

Spin S

DFT EXP DFT(solv) EXP DFT EXP DFT EXP DFT(solv) EXP DFT EXP

Cr S 1.88/1.47 3/2 S 1.89/1.45 3/2Mn37,47 S S 1.86/1.44 1.85/1.41 0.34/0.98 2 2 S S 1.86/1.43 1.88/1.40 0.04/0.13 2 2Fe36,49 S 1.94/1.44 ?c S S 1.95/1.43 1.91/1.46 0.05/0.13 ?c 5/2Co38,39,65 S S 1.88/1.43 1.87/1.44 0.03/0.36 0 0 S S 1.87/1.38 1.88d/— N/A ?c 1Ni35,40 S S 1.88/1.40 1.89/1.39 0.03/0.11 ?c 1/2 E E 1.93e/1.31 1.98d/— N/A 3/2 1/2Cu E 2.42/1.30 1 E 2.06/1.29 1

a The theoretical values presented in this table correspond to the lowest energy spin states. In case there are ambiguities in the theoreticalresults, the spin state matching experiments are chosen. For the theoretical distances, solvent optimized geometries are given. Values for all spinstates are given in ESI.† b Root mean square deviation of the atomic positions between the calculated and the X-ray structures. Core is defined asconsisting of the metal, O and N atoms. c The results are ambiguous depending on different functionals as well as the solution phase and/or freeenergy corrections, that can be chosen to match the experimental values. d EXAFS value. e The distance to the bonding oxygen only, in the spinstate matching the DFT column.

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can be fairly good, if some special attention is given to certaincases.

Orbitals

We begin this section by presenting the orbitals involved andthe principle of orbital mixings that defines all the species inthis study. We define the z-axis as along the normal of theligand plane (in the case of end-on, this would be along theM–O bond) and the x-axis in the direction of the O–O bond,projected perpendicular to the z-axis (Fig. 2). The five atomic3d-orbitals on the metal are usually defined as dxy, dxz, dyz, dz2and dx2−y2. As the last orbital mixes with the ligand nitrogenpx/y orbitals in a head-to-head fashion, we denote the gener-ated antibonding molecular orbital as σ*z2−y2. On the peroxideside, each of the oxygen atoms contains three p orbitals thatcan mix with the corresponding p orbitals on the other oxygenatom to form the six classical O2 bonding π- and anti-bondingπ*-orbitals. Of these, only the valence orbitals named π*y and

π*z are of interest in our study. These π*y/z orbitals mix withthe metal d orbitals in different fashion depending on thestructure, and understanding this mixing enables us to cor-rectly describe the electronic configurations of the metal–peroxo species.

For the side-on bound general M–O2 species, the five d orbi-tals of the metal are in Fig. 2a shown to overlap symmetricallywith the O2 π*-orbitals. The lowest lying d orbital, dxy, mixeswith π*y to form two new orbitals, a bonding δ-orbital and ananti-bonding δ*-orbital. Similarly, dxz is found to mix with π*zresulting in a low-energy bonding (π) and a high-energy anti-bonding (π*) orbitals as well. Although dyz shows some signsof mixing with the bonding πy-orbital of the oxygen, thismixing is relatively small and not significant for our discussionbelow; hence, we regard this as a pure dyz orbital. In theabsence of an axial ligand, dz2 does not really mix with otherorbitals, and we retain its designation as dz2. Thus, the sevenresulting orbitals of the side-on mode species are π, δ, δ*, dyz,dz2, σ*x2–y2 and π*. Hence, while the metals are usuallydescribed to have “five d orbitals” in colloquial language,strictly speaking this may not be the case outside of the iso-lated metal atom. In our case, molecular interaction createsseven molecular orbitals, which have metal atomic d orbitalcomponents. However, usually (and fortunately) only the fivevalence orbitals are energetically high enough to matter inreactions, thereby supporting the “five d orbitals” notion.

For the end-on species (Fig. 2b), the orbital mixing occursslightly different. As expected, the O2 π* orbitals now overlapthe metal orbitals asymmetrically, with only the proximaloxygen atom part interacting with the metal. π*y now mixesheavily with dyz to form the bonding and anti-bonding pair oforbitals (π and π*). The π*z orbital on the other hand now over-laps with the dz2 orbital in a head-to-head fashion to create σ/σ* orbital. The seven generated orbitals here are thus denotedas σ, dxy, π, dxz, σ*x2–y2, π* and σ*.

Depending on the exact nature of the metal and spin state,the degree of mixing can vary and other alternative descrip-tions of the involved orbitals are possible. However, we foundthat significant deviations from the above descriptions onlyoccur in case of high energy spin configurations, where thereare some room for alternative orbital mixings. The abovedescription was general and systematic enough for us to applyit to our systems of interest, which are the ground states ofeach species. Henceforth, we will use the above defined orbitalterminology. Table 2 shows the energetically lowest valenceorbital occupations found in this study using the abovedescribed orbital nomenclature.

However, an alternate view is available for the end-on12-TMC species. In the 14-TMC case, the metal–N ligandsform a fairly planar plane, which is not the case for 12-TMC.Due to the small ring size, the “plane” is heavily distorted inthe 12-TMC case. In fact, it is so distorted that one of theligand nitrogen atoms forms almost a linear N–M–O direction,which can serve as the x-axis. Together with a perpendicularM–N direction, we can define a new “plane”. Looking at thefive d orbitals of the metals, they fit remarkably well to this

Fig. 2 The five metal valence orbitals (dxy, dxz, dyz, dz2 and σ*x2–y2)combine with two O2 valence orbitals (π*y and π*z) to create a sevenvalence orbital system of an M–O2 complex. (a) Side-on system wherethe dxy orbital on the metal combines with the π*y orbital on O2 to formδ and δ* orbitals. Similarly, the dxz orbital combines with the π*z orbitalon O2 to form π and π* orbitals. The actual electron occupation shown isfor the S = 5/2 side-on FeI + O2 species. (b) End-on system where thedyz orbital combines with the π*y orbital to form π and π* orbitals. Thedz2 orbital combines with the π*z orbital to form δ and δ* orbitals. Theelectron occupation shown is for the S = 1 end-on CuI + O2 species.



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coordinate system, where the metal orbitals simply switch des-ignations (σ*x2–y2 → σ*z2, dxy → dyz, etc.) due to the change ofthe coordinate axis (x → z, y → y, z → −x). However, in theinterest of comparability to the 14-TMC species, we approxi-mate the heavily distorted 12-TMC plane as planar, and desig-

nate the orbitals with the same nomenclature as close as poss-ible to the 14-TMC orbitals.

Individual metal–O2 species

In this section, we detail the individual results obtained for allthe metal–O2 species separately. A statistical analysis over allthe species will be presented afterwards based on the desig-nated representative species in each of the subsections below.All the energy values for all the species and computational pro-tocols are presented in the ESI, Table S1.†

CrO2. The “CrIII–peroxo” species has not yet been experi-mentally synthesized, although the experimental data of bothCrIII–superoxo44,45 and CrIV–peroxo46 species are available.Nevertheless, the calculations on our CrO2 species are uni-formly suggesting a side-on structure. This is in contrast to theCrIII–superoxo species where an end-on crystal structure wasobtained with 14-TMC.44,45 With 12-TMC, however, a side-onCrIV–peroxo species was seen instead.46 The calculated groundstate for the CrO2 species in our study is a tri-radical side-onS = 3/2 state with singly occupied orbitals of δ*, dyz and dz2 inboth 1Cr and 2Cr. This state is around 20 kcal mol−1 more pre-ferable than the next available spin state (Fig. 3), making thistheoretical spin state assignment a fairly certain prediction.Based on the low energies, we designate the side-on S = 3/2states as representative states for 1Cr and 2Cr.

The O2 moiety shows almost zero in Mulliken spin densitydistribution, a fact that curiously enough occurs in many ofthe other MO2 species studied here as well (Table 3). A closerlook at the orbitals shows that the relevant electrons arelocated in the δ, δ* and π orbitals, which are partially deloca-lized on the O2 moiety (Fig. 2). Examining δ(β), one finds thatthis orbital is of significantly less mixed character than δ(α).δ(β) is centred more on O2 rather than Cr, hence it’s β-electronspin density is roughly 1 on the O2 part and 0 on the Cr part.This cancels out the combined 1 of α-spin from both δ(α) and

Table 2 Valence orbital electronic configuration of the MO2 species

Side-on π δ δ* dyz dz2 σ*x2–y2 π*

1Cr, 2Cr ↑↓ ↑↓ ↑ ↑ ↑1Mn, 2Mn ↑↓ ↑↓ ↑ ↑ ↑ ↑1Fe, 2Fe ↑↓ ↑↓ ↑ ↑ ↑ ↑ ↑1Co ↑↓ ↑↓ ↑↓ ↑↓ ↑↓2Co ↑↓ ↑↓ ↑(↓)a ↑↓ ↑(↓)b ↑1Ni ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑

End-on σ dxy π dxz σ*x2–y2 π* σ*2Ni ↑↓ ↑↓ ↑↓ ↑↓ ↑ ↑ ↑1Cu, 2Cu ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑ ↑

a According to B3LYP, there is a β-electron in this orbital. b Accordingto BP86, there is a β-electron in this orbital.

Fig. 3 Energy difference between the ground state and the secondlowest state, without changing end-on/side-on geometry for (a)12-TMC and (b) 14-TMC ligand.

Table 3 Electron count obtained from NBO calculations at the BP86/Def2-TZVPP/COSMO level

Total electron count Oxidation state assignment based on

Metal O2

Total electron count Valence orbital analysisα β Δα–β α β Δα–β

1Cr 12.93 10.06 2.86 8.55 8.39 0.16 4CrI + 1O24[CrIII–peroxo]

1Mn 13.64 9.89 3.75 8.51 8.44 0.07 5MnI + 1O25[MnIII–peroxo]

1Fe 14.09 10.32 3.78 8.95 8.07 0.88 5FeII + 2O2•− 6[FeII–superoxo]

1Co 12.96 12.96 0.00 8.50 8.50 0.00 1CoI + 1O21[CoIII–peroxo]

1Ni 13.73 12.97 0.76 8.41 8.50 –0.09 2NiI + 1O22[NiIII–peroxo]

1Cu 14.09 13.70 0.39 8.96 7.59 1.37 1CuI + 2O2•− a 3[CuII–superoxo]

2Cr 12.92 10.02 2.90 8.53 8.41 0.12 4CrI + 1O24[CrIII–peroxo]

2Mn 13.65 9.86 3.79 8.48 8.46 0.03 5MnI + 1O25[MnIII–peroxo]

2Fe 14.09 10.33 3.76 8.95 8.05 0.91 5FeII + 2O2•− 6[FeII–superoxo]

2Co 13.66 12.06 1.59 8.48 8.30 0.18 3CoI + 1O23[CoIII–peroxo]

2Ni 14.05 12.71 1.34 8.96 7.66 1.30 2NiI + 2O2•− b 4[NiII –superoxo]

2Cu 14.08 13.71 0.37 8.96 7.58 1.38 1CuI + 2O2•− a 3[CuII–superoxo]

a Round-off error implicates this S = 1 species to appear to be in an S = 1/2 state. Adding spin from the ligand to Cu spin count will imply CuII.b Round-off error implicates this S = 1/2 or 3/2 species to appear to be in an S = 1 state. Adding spin from the ligand to Ni spin count will implyNiII.



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δ*(α) orbitals; i.e. the contribution of spin density on the O2

moiety from both δ and δ* orbitals is zero. The π orbital on theother hand is more evenly delocalized between Cr and O2.Therefore, it contributes partially with its α (0.5) and β (0.4)electrons to the electron count, but largely cancel each otherout in terms of spin.

To investigate the electron distribution in more detail, wehave performed NBO calculations, where the default Lewisstructure determined by the program was used on all the repre-sentative species in this study. NBO analysis on 1Cr and 2Cr

yielded a Mulliken natural population of 13 α-electrons and 10β-electrons on the Cr atom (Table 3). One arrives therefore tothe conclusion that the Cr atom has 23 electrons and 3 in spindensity, i.e. an S = 3/2 CrI species. At the same time, the O2

moiety has 8.5 α-electrons and 8.4 β-electrons. This wouldimply almost 17 electrons on O2 (i.e. superoxo), but as the frac-tional 0.5 α- and 0.4 β-electrons almost cancel each other outin spin density distribution, these decimals may be besttreated here as round-off errors. This would imply 16 electronson O2, and the resulting designation should be a singlet O2

species. Taken together, the spatial distribution of the elec-trons is thus more consistent with a 4CrI–1O2 designation (themultiplicity number M = 2S + 1 is used as the left superscript),which would not be compatible with the conventional oxi-dation state designation of these kinds of species. Given thatthere are some error margins to consider when dealing withMulliken analysis, and in order to confirm the existing spectro-scopic conventions, a better designation would be S = 3/2CrIII–peroxo (see Discussion section). This species wouldproduce the same spin density distribution and the samenumber of electrons in the valence orbitals as 4CrI–1O2.

MnO2. Experimentally, the “MnIII–peroxo” species has beenfound to be in its side-on S = 2 state for 14-TMC47 (2Mn) and13-TMC48 ligands through Evan’s method, while the spin statefor 1Mn was determined only indirectly by geometrical com-parisons to DFT results.37 The calculations indeed uniformlygive an energetically very stable side-on S = 2 state as theground state for the MnO2 species (Fig. 3), with singly occu-pied electrons at δ*, dyz, dz2 and σ*x2–y2 orbitals (Table 2).Despite its high-spin ground state, no unpaired electron isseen on the O2 group. This is because the orbital occupation isidentical to that of CrO2 (Table 2), but with an extra electron inthe σ*x2−y2 orbital, which does not affect the spin on O2. Likethe CrO2 case described above, the designated representativestate is deduced to be a side-on S = 2 MnIII–peroxo species,based on the conventional valence electron analysis.

FeO2. Different from the two previous cases, the energeticdifferences between different spin states are within 5 kcalmol−1 (Fig. 3). Therefore, one could expect that spin statesother than the ground state could be observed or utilizedduring any substrate reactions. Reflecting these close energies,the ground state designation depends on the calculation proto-col. For all B3LYP based calculations, the ground state is anS = 5/2 state for both 1Fe and 2Fe, consistent with known biasfor the high-spin state preference of the B3LYP functional.BP86 reproduces this result in the case of 2Fe, except in the

gas-phase, which supports an S = 3/2 state by less than 0.4 kcalmol−1. Since more calculations implicate an S = 5/2 ratherthan S = 3/2 ground state for 2Fe, we deem the representativestructure to be a side-on S = 5/2 structure for 2Fe, consistentwith experimental evidence.36,49 For 1Fe, BP86 results in S =3/2, in total contrast to the B3LYP calculations (includingB3LYP optimized structures). In the absence of any experi-mental results for 1Fe, we rely on the B3LYP results (i.e. a side-on S = 5/2 species), which in our experience usually rendersreliable results for Fe species.50–52

Assuming the S = 5/2 ground state, Table 3 shows that theFe structures have one radical on the O2 moiety and four onFe, seemingly implicating a FeII–superoxo. This stems from thesingle occupations of the π* and σ* orbitals, which spin-polarizes the O2 moiety to its superoxo character. As men-tioned above, the obtained bond lengths indicate that the cal-culated geometries match experiments (Table 1) and therefore,the calculation results should warrant a serious consideration.However, a FeII–superoxo structure is not (yet) supportedexperimentally. Based on the Raman spectroscopy and X-raystructure data, this structure has been determined to be aFeIII–peroxo species.36,49 As mentioned above, the X-ray geome-tries are well reproduced by the current calculations, while theRaman vibrations are reasonably close (BP86 gas-phase Fe–O/O–O stretching frequencies: 431/957 cm−1, experimentalvalues: 487/825 cm−1).36 Noteworthy is that if S = 3/2 isassumed in 2Fe, B3LYP gives that the δ* orbital is doubly occu-pied (while BP86 reveals that dz2 is doubly occupied instead).The configuration in this case is more akin to FeIII–peroxo, ina similar pattern to the other MO2 structures. Hence, dependingon the spin state, the O2 peroxo/superoxo character fluctuates.

CoO2. Co represents the metal where its ground stateremains most difficult to be determined, albeit pure end-onstructures can be ruled out. For 1Co, the calculations generallyresult in a side-on S = 0 ground state. On the free energy scalehowever, the preferred spin state is found to be temperaturedependent. At our default calculation temperature (298 K), theside-on S = 1 is the ground state. However, the experimentswere done at 10 K, where an S = 0 ground state was found.Indeed, changing the calculation settings to 10 K do yield anS = 0 ground state. This state is also in excellent agreementwith the crystal structure regarding Co–O and O–O bondlengths. Therefore, the calculations predict temperature effectsto be of some importance in experiments. We assign the side-on S = 0 structure as the representative state for 1Co.

For 2Co, BP86 gas- and solvent-phase calculations favour aside-on S = 0 state, while B3LYP yields an S = 2 spin state. TheBP86 preference for S = 0 was found in earlier calculations aswell.39 Again, the temperature effect seems to matter here aswell, as the calculated BP86 free energies (at 233 K) yield an S =1 preference. Indeed, the most recent NMR experiments at233 K have determined that this structure is in an S = 1 state.39

Examining our computationally obtained S = 1 structure, wefind that it exhibits a “half side-on/end-on” structure, whereone Co–O bond is longer than the other (1.80 Å/1.94 Å, average1.87 Å). This is somewhat in contradiction to earlier calcu-



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lations, where symmetric Co–O2 distances were obtained,despite a very similar calculation setup (albeit a slightly largerbasis sets was applied here).39 However, the average Co–O(1.87 Å) and O–O distances are similar to earlier theoreticalresults,39 and match the EXAFS Co–O bond distance of1.88 Å.39 We therefore assign the half side-on/end-on S = 1structure as the representative state for 2Co. The earlier calcu-lations assigned this structure as a CoIII–peroxo structurebased on the Mulliken spin density distribution analysis forthe S = 1 spin state,39 which is about the same as in thecurrent study. An interesting note however is that the elec-tronic structure is different between the functionals: BP86finds a β-electron in the dz2 orbital, while B3LYP finds this elec-tron in the δ* orbital. The former electron occupation makesthe structure slightly ferromagnetically coupled between Coand O2, while the latter spin distribution makes the structureslightly anti-ferromagnetically coupled (see ESI, Tables S2 vs.S3†).

NiO2. The spin state preference of the 1Ni structure isdivided along the functional lines. BP86 prefers the S = 1/2state in accordance with experiments,35 while B3LYP prefersS = 3/2. Both however prefer the side-on species to the end-onone. We have earlier advocated the use of BP86 when calculat-ing Ni species partly because of this and other issues.43 Thegeometry also supports an S = 1/2 species, as the S = 3/2 Ni–Obond length of 2.05 Å is too long compared to the experi-mental result of 1.89 Å.35 The side-on S = 1/2 species show vir-tually no spin density on O2, and we therefore designate 1Ni asa side-on S = 1/2 NiIII–peroxo species, in accordance withexperimental findings.

For 2Ni, the present and previous40,41 DFT calculations uni-formly prefer the S = 3/2 state over S = 1/2, irrespective of func-tional, solvent or thermal additions.53 Noteworthy is that evenab initio CASSCF calculations yielded an S = 3/2 state for thisstructure.42 While an end-on species is experimentally sup-ported by EXAFS,35,40 EPR suggest that this species is in an S =1/2 state.40 The reason for this last discrepancy to theoreticalresults is not known. Both spin states exhibit calculated O–Obond vibrations close to the experimentally obtained values tobe of no help in identifying the ground state. Also, in three outof five calculation protocols, the end-on S = 1/2 structure is noteven the second favoured structure. In fact, the B3LYP side-onS = 3/2 structure is slightly preferred over even the end-on S =3/2 structure, let alone the S = 1/2 species (optimizing the geo-metries with B3LYP did not change this fact). The only supportthe calculations give to this being an S = 1/2 species is the cal-culated Ni–O bond distance. This bond was determined byEXAFS to be 1.98 Å,40 which is in fact close to the calculatedS = 1/2 value (1.96 Å, Table S4 in ESI†). However, the calculatedS = 3/2 value is also close (1.93 Å) and therefore also plausible.In terms of spin density distribution, there is 1.2 α-spin densityon Ni and 0.5 β-spin density on O2 in the end-on S = 1/2 state.Rounding off both these values strictly to 1 introduces a roundoff error, as it will yield an S = 1 2NiIII–2O2

•− species. There is,however, partial significant spin on the 14-TMC ligand (0.3). Ifwe reason that this spin also belongs to Ni as much of it comes

from the singly occupied σ*x2−y2 orbital, the round offs yield a3NiII–2O2

•− species. Thus, the spin distribution seems to imply aborderline case between anti-ferromagnetic coupled 3NiII–2O2

•−

and 2NiIII–1O22−. In the S = 3/2 state, the spin density distri-

bution is higher on both Ni (1.3) and O2 (1.3), which can alsobe rounded off to 3NiII–2O2

•− with addition of 0.4 spin from theligand to Ni. Both spin states could thus fit into the descriptionof “NiII–superoxo” as described by XAS.40

Since the spin state determination was done by EPR com-parison to known metal–superoxo S = 3/2 species (and not toS = 3/2 Ni species specifically as this has not been reportedbefore, see footnote 16 of ref. 40), it leaves some room for poss-ible experimental errors. We chose in this publication to desig-nate 2Ni as an end-on S = 3/2 species since the theoreticalvalues overwhelmingly support this structure, but a finalreconciliation between theory and experiment in this issue ispending.

CuO2. While no TMC CuO2 structures exist, there are by nowa number of literature examples of biological54 and syntheticCuII–superoxo55–58 or CuIII–peroxo59,60 species. Theoreticaltreatment of these species have suggested that the CuII–super-oxo/CuIII–peroxo assignment is on a sliding scale,5 and thatDFT may not treat the antiferromagnetically coupled S = 0CuII–superoxo species well in terms of energy, but its geometri-cal features have been still reproduced satisfactorily.61 Giventhe lack of a specific TMC Cu species, the last point cannot beverified for the current species, and we present our DFT resultshere as obtained.

Both 1Cu and 2Cu yield similar results from the calculations.The preferred spin state is S = 1, as was earlier found in a DFTstudy on 2Cu with a Cl axial ligand.62 The preferred structuresobtained here are the end-on species. Hence, these are ourrepresentative structures for both 1Cu and 2Cu. The O–O bonddistances are short, 1.30 Å for 1Cu and 1.29 Å for 2Cu. The spindensity distribution is ambiguous 1.4 on the O2 moiety and, inconjunction with the short O–O bond length, would probablybe designated as a superoxo species (i.e. CuII–superoxo). TheNBO calculation reveals 0.3 spin on Cu, which can be raised to0.6 by adding spin from the ligand to it, being closer to theexpected 1 for an S = 1/2 CuII species. However, given the weakCu–O bond strength (under 20 kcal mol−1 depending on thecalculation protocol) and the long Cu–O bond lengths of1.96 Å and 2.06 Å for 1Cu and 2Cu, respectively, a borderlinecase between 2CuII–2O2

•− and a weakly interacting 1CuI–3O2

species seems to be plausible in this case.

Statistical analysis of the MO2 structures

In the below section, we aim to utilize our extensive data onthe individual MO2 structures to gain some insights into thetrends in metal variations.

Spin state stability. In transition metal systems, multi-spinstate reactivity is invoked in many cases, where the reactivespin state is not necessarily the same as the stable ground spinstate.63 We have therefore looked into the energy gap betweenthe energetically lowest spin state structure and the second-lowest spin state for each of the species (without changing the



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side-on/end-on geometry). Here, the energetically lowest struc-ture is not necessarily the representative structure that we haveestablished earlier. We have strictly followed the energeticorder given within each of the computational protocol used,enabling us to compare the performance of the computationalprotocols as well.

As Fig. 3 shows, there is a considerable energy gap (around20 kcal mol−1) for 1Cr and 2Cr to switch its spin state.Therefore, any multi-spin state reactivity here can be ruled out.The massive preference for the ground spin state is clear forany of the computational protocol given; indicating that thespin state preference is very reliable. For 1Mn and 2Mn, theenergy gap is in the 15 kcal mol−1 range that varies dependingon the methods and ligand. This makes multi-spin state reac-tivity not easy, but still within the theoretically possible rangeand cannot be ignored. For all the other metals, the gap is typi-cally within 5 kcal mol−1 and thus spin transitions are real andpossible, which must be considered in any studies of thesespecies.

Stability of the peroxo adduct. One way of measuring thestability of the MO2 species is to consider the O2 bindingenergy with respect to MI + O2 → MIIIOO. As the energy refer-ence point, the lowest energy MI state + triplet O2 within eachcomputational protocol was chosen, and compared to theenergetically lowest 1M and 2M structure. As can be seen fromFig. 4, there is a clear trend that the binding energy becomesweaker with higher metal atomic number. With a bindingenergy of 60 kcal mol−1 or more, the 1Cr and 2Cr complexescan be seen as “stable” with respect to O2 dissociation, while1Cu and 2Cu would feature a binding energy of no more than14 kcal mol−1 (depending on the calculation method). Partialexperimental support for this can be inferred as 1Cu and 2Cu

species has yet to be synthesized, and the current data predictthat it would be a somewhat challenging task compared toother metals, at least with respect to O2 uncoupling. By thislogic, 1Cr and 2Cr species should be obtained easily.

Side-on vs. end-on. A related observation in this study is theenergy gap between the energetically lowest side-on structureversus the lowest end-on structure. Fig. 5 shows a trend wherethis energy gap diminishes with higher metal atomic number.The trend however continues beyond the 0 kcal mol−1 linewhere the end-on structure is out-right preferred in 1Cu, 2Ni

and 2Cu. This has an implication for the reaction mechanisms.As the reorganization energy from a side-on to an end-on struc-ture is high for the lower atomic number MO2 species (M = Cr,Mn and Fe), it is predicted that any O–O bond cleaving reac-tion would generally not proceed through an end-on MIII–

peroxo species containing those metals, although it remains aweak possibility for 1Fe, 2Fe and 2Mn. Indeed, in a theoreticalstudy on cyclohexadiene C–H activation by (14-TMC)FeIII–superoxo species,52 we found that the product of this step, the(14-TMC)FeIII–hydroperoxo species, essentially kept its side-onconfiguration in its energetically lowest state despite the proto-nation. This state was within 1 kcal mol−1 of the corres-ponding end-on species, showing that in a putative FeIII–peroxo proton abstraction reaction, protonation with a follow-ing reorganization is preferred rather than reorganization fol-lowed by protonation. The same side-on structure was alsofound even for 12-TMC CoIII–hydroperoxo species,64 indicatingthat the reorganization to an end-on MIII–peroxo species maynot be all that common, even transiently during reactions.

Spin on O2. A common notion is that the spin density distri-bution on the O2 group may be proportional to its reactivity.Plotting the spin density distribution on the O2 group versusmetals in the representative state determined above reveals aninteresting feature. As can be seen from Fig. 6, 1Cu, 2Cu and 2Ni

give the largest spin density distribution on the O2 moiety,indicating a high reactivity (hence instability) of these species.

Fig. 4 The M–O2 binding energies for (a) 12-TMC and (b) 14-TMCligand.

Fig. 5 Energy differences (Eend-on − Eside-on) for various M–O2 com-plexes with (a) 12-TMC and (b) 14-TMC ligand.

Fig. 6 Spin density distribution on the O2 moiety for various M–O2

complexes with (a) 12-TMC and (b) 14-TMC ligand. The spin statechosen for each of the metal structure is corresponding to the represen-tative state for each species, as discussed in the text.



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On the other end of the spectra, Cr and Mn based complexesare unanimously shown to have very small spin on the O2

moiety, indicating stability. For Fe, due to its unique five-elec-tron high spin preference, large spin density distribution onO2 is found, and therefore implying a high reactivity. Worthnoting is that 1Ni due to its side-on conformation has lowerspin density on O2 than the end-on 2Ni, hence lower reactivityis expected and confirmed experimentally.35 However, itshould be kept in mind that spin density distribution on O2 isnot necessarily the sole determining factor for the reactivity.

DiscussionRing size effect

In term of differences between 12-TMC and 14-TMC ligands,the general trends on spin state stability, O2 binding energiesand spin density distribution are similar (Fig. 3, 4 and 6).Looking at the average M–N bond lengths, it is clear that thesebonds in 2M are longer than those in 1M, due to the larger ringsize (Fig. 7a). As a result, this probably causes the σ*x2−y2orbital energy to be lower and closer to the dz2 orbital in 2M

compared to the corresponding case in 1M. This may be thecause to why 1Co prefers to avoid occupying the σ*x2−y2 orbitalby switching to an S = 0 state (Table 2), while the more accessi-ble σ*x2−y2 orbital in 2Co is occupied to form an S = 1 groundstate.

The flexibility of the ligand binding can also be illustratedwith the (smallest) N–M–N angle, where the two N atomsoppose each other, and therefore indicate the planarity of theM–(4 × N) plane. In 2M, this angle deviates from 125° to 149°within the representative species (a span of 24°, Fig. 7b). In 1M

however, this angle is markedly more constrained, rangingfrom 105° to 114° (a span of 9°). The larger ring size thus gen-erates more flexibility. The smaller N–M–N angle also meansthat the O2 binding site is wider and more accessible to O2.This can be seen from the O2 binding energies, where thebinding is few kcal mol−1 stronger for 1M than 2M (Fig. 4). Thecalculations thus corroborate experimental results,35,65 thatexplain the preference of a side-on binding mode for 1Ni vs. an

end-on binding mode for 2Ni as a result of the spatial open-ness at the O2 binding site.

Metal oxidation states

The availability of the exact electronic structure enables us toinvestigate the question of whether a certain structure is infact an MI, MII or MIII state, coupled to O2, superoxo or peroxomoiety, respectively. This is, however, a challenging task froma computational point of view. One might, naturally, assumethat the electron count would have something to do with it. Auseful case was discussed in some length above for 1Cr and2Cr, vide supra. Thus, the electron count by NBO generally sup-ports a MMI–1O2 designation in our current study (Table 3).However, as MI–O2 species are clearly not acceptable in light ofcurrent conventions, most computational chemists look at thenumber of electrons in the valence orbitals only, combinedwith the spin density distribution. In the current study, mostof the species were found to have no spin on the O2 part andwith the number of electrons in the five highest valence orbi-tals matching the isolated MIII, consistent with a MIII–peroxodesignation. Hence, this is the oxidation state we generallypresent above (with the exception of Fe and Cu based struc-tures, vide supra). Noteworthy is that the excess radical distri-bution would be the same for MMIII–peroxo as for MMI–1O.There is also a good agreement with the geometries and spinstates obtained in comparison to experiments; implying thatthe calculations and experiments in fact agree but that the oxi-dation states interpretations may need some further reconcilia-tions between theoreticians and experimentalists.

Comparing with experiments, our calculated oxidationstates differ from the experimentally determined ones in thecase of Fe. We determine these species to be an FeII–superoxospecies because of the high-spin S = 5/2 configuration that isonly attainable for Fe, yielding a total spin of just below 1 onthe O2 moiety (Table 3). The experiments indicate an FeIII–peroxo species instead based on both the O–O distance andresonance Raman frequencies, which are both reproduced byour calculations. For Cu based structures, no experimentalstructure exists, but if attainable, we predict it to be a border-line case to a CuI–O2 species.

Computational protocol dependence

In this study, we have deliberately made use ofBP86 geometries only as this functional is generally known tocompete with (and even excel over) B3LYP when it comes tothe quality of geometries, while being faster to compute.66 Wehave shown here that the obtained BP86 geometries areindeed in agreement with experiments, where available(Table 1). B3LYP is generally thought of as being more accuratein energy evaluations, while BP86 seemed to be a better fit forNi species specifically.43 This prompted us to consider bothenergies in our evaluations. We consider both the SCF gas-phase energy and the Gibb’s free energy, and we also incorpor-ate acetonitrile solvent effects on both geometries and energiesfor comparison. There are individual cases where the choice ofcomputational protocol matter (vide supra), but if looking at

Fig. 7 (a) The average M–N distance, showing that the 14-TMC ligandaffords larger M–N distances and (b) the smallest N–M–N angle,showing that 14-TMC fluctuates more and therefore is more flexible.



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trends over multiple species (as in Fig. 3–6), the conclusionswill most likely be the same. Hence, we do not see any exces-sive method dependence in our calculations, but there areenough deviations to imply that methodology checks arealways warranted in any individual calculation.

Conclusions

We have used five different computational protocols (BP86//BP86/GAS ΔE, B3LYP//BP86/GAS ΔE, BP86//BP86/GAS ΔG,BP86//BP86/COSMO ΔE and B3LYP//BP86/COSMO ΔE) to evalu-ate the characteristics of six MI + O2 compounds (M = Cr, Mn,Fe, Co, Ni and Cu) with two different closely related ligands,12-TMC and 14-TMC. The geometries, valence orbital electronoccupations and oxidation states are either predicted or con-firmed (where experiments are available). A statistical compari-son over all the metals reveals that as the metal atomicnumber goes up, (1) O2 binding energy decreases, (2) spinstate preference becomes less clear, (3) end-on structure isgradually being stabilized, and becomes preferred at Ni or Cu.In addition, the spin density distributions on the O2 group areseen to be more dependent on the individual spin state ratherthan the metal atomic number, and experimental observationsabout 14-TMC having a more flexible ring wrapping than12-TMC have also been confirmed. Determining the mostlikely structure and spin state, we find that 1Cr and 2Cr areside-on S = 3/2 CrIII–peroxo species, 1Mn and 2Mn are side-onS = 2 MnIII–peroxo species, 1Fe and 2Fe are side-on S = 5/2FeII–superoxo species, 1Co is a side-on S = 0 CoIII–peroxospecies, 2Co is a half side-on/end-on S = 1 CoIII–peroxo species,1Ni is a side-on S = 1/2 NiIII–peroxo species, 2Ni is an end-onS = 3/2 NiII–superoxo species, and finally 1Cu and 2Cu are end-on S = 1 CuII–superoxo species.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was gratefully supported by the NRF of Koreathrough CRI (NRF-2012R1A3A2048842 to W. N.), GRL(NRF-2010-00353 to W. N.), and MSIP (NRF-2013R1A1A2062737to K.-B. C.). Professors Joan S. Valentine and Jaeheung Cho areacknowledged for ideas and consultations.

Notes and references

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a theoretical investigation into the first-row transition metal–o2...

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