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Scrutinizing the Noninnocence of Quinone Ligands in Ruthenium Complexes: Insights from Structural, Electronic, Energy, and Effective Oxidation State Analyses Gabriella Skara,† Marti Gimferrer,‡ Frank De Proft,† Pedro Salvador,*,‡ and Balazs Pinter*,† †

Eenheid Algemene Chemie (ALGC), Member of the QCMM VUB-UGent Alliance Research Group, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium ‡ Institut de Química Computacional i Catàlisi (IQCC) i Department de Química, Universitat de Girona, 17071 Girona, Spain S Supporting Information *

ABSTRACT: The most relevant manifestations of ligand noninnocence of quinone and bipyridine derivatives are thoroughly scrutinized and discussed through an extensive and systematic set of octahedral ruthenium complexes, [(en)2RuL]z, in four oxidation states (z = +3, +2, +1, and 0). The characteristic structural deformation of ligands upon coordination/noninnocence is put into context with the underlying electronic structure of the complexes and its change upon reduction. In addition, by means of decomposing the corresponding reductions into electron transfer and structural relaxation subprocesses, the energetic contribution of these structural deformations to the redox energetics is revealed. The change of molecular electron density upon metal- and ligand-centered reductions is also visualized and shown to provide novel insights into the corresponding redox processes. Moreover, the charge distribution of the π-subspace is straightforwardly examined and used as indicator of ligand noninnocence in the distinct oxidation states of the complexes. The aromatization/dearomatization processes of ligand backbones are also monitored using magnetic (NICS) and electronic (PDI) indicators of aromaticity, and the consequences to noninnocent behavior are discussed. Finally, the recently developed effective oxidation state (EOS) analysis is utilized, on the one hand, to test its applicability for complexes containing noninnocent ligands, and, on the other hand, to provide new insights into the magnitude of state mixings in the investigated complexes. The effect of ligand substitution, nature of donor atom, ligand frame modification on these manifestations, and measures is discussed in an intuitive and pedagogical manner.

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INTRODUCTION The concept of ligand “noninnocence” in chemistry was introduced by C. K. Jørgensen in the 1960s regarding the apparent ambiguity of metal and ligand oxidation states in dithiolene nickel and cobalt complexes.1 By now, the definitions of ligand noninnocence and ligand redox activity have blended together, but nonetheless they are conceptually different phenomena, where the latter describes the active participation of the ligand in the redox process of the corresponding transition metal complex. Both descriptions however clearly indicate that these behaviors are not exclusive ligand properties, but also depend on the metal to which the ligand is coordinated. For a long time, redox noninnocence of ligands was considered a rare spectroscopic curiosity, a uniquely electronic phenomenon, but over the past decades its critical role in enzymatic transformations was evidenced.2−4 For example, in the active form of the active site of galactose oxidase enzyme,5,6 an extra electron is added to the tyrosine ligand bound to the CuII center when facilitating the overall two-electron oxidation of alcohols to aldehydes.7 The great © XXXX American Chemical Society

potential in this cooperative mechanism has mostly been recognized in the ongoing intense efforts to achieve sustainable chemical energy conversion covering fields of inert molecule activation, artificial photosynthesis, multielectron reactivity, etc.8−10 Conceptually it is the most intuitive strategy of cooperative catalysis: use redox-noninnocent ligands as electron reservoirs for metals that cannot store enough electrons for the required multielectron transformations. The emerging research activity in the field of ligand redox noninnocence follows two major trends, namely, (i) triggering redox noninnocence for yielding reactive ligand radicals to establish ligand-based catalysis11 and (ii) the application of nonreactive redox active groups as electron buffers to promote two-electron processes in redox catalysts where the metal center alone cannot do it. Figure 1 exemplifies two spectacular examples for the latter strategy: both complex (PDI)Fe(N2)12 (Figure 1a) and [Co(LNO)2]−1 (Figure 1b)13 facilitate twoReceived: November 9, 2015

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DOI: 10.1021/acs.inorgchem.5b02543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

geometries, and the versatile reactivity, including catalysis, that these ligands can facilitate when bound to metal.30−35 Especially the groups Storr, Stack, and Thomas provided critical insights and solid understanding for the colorful chemistry of these metallosalen complexes, including the electronic structure variance with the metal center, effect of substitution at various positions, length and nature of tethering fragments, protonation, effect of exogenous ligands, and the shift of oxidation locus upon these chemical triggers.35−57 Most importantly, depending on the relative ordering of metal and ligand frontier orbitals (vide infra), oxidation of metallosalens often takes place at the ligand and, accordingly, leads to metal−ligand−radical species, sometimes with mixed valence (vide infra) electronic structure, clearly demonstrating the redox active and noninnocent nature of salen-type ligands.16,17,23,29,38,39,41,42,55−64 A strongly related, if not inseparable, field is understanding, characterizing, and taking explicit control over the phenoxide to noninnocent phenoxyl ligand radical transitions54,65,66,42,47,67 in order to stimulate transition metal catalyst development incorporating redox active ligands. There are no clear structural criteria for the occurrence of ligand noninnocence, and, accordingly, numerous ligand scaffolds without any structural similarity behave noninnocently.3 Nevertheless, α-dicarbonyl ligands, such as α-diimine and their benzo-fused orthoquinoid derivatives, such as benzoquinone, (LOO), aminophenol (LNO), bezoquinonediimine (LNN), and dithiolene (LSS) (Figure 2a), strikingly often exhibit redox noninnocent behavior in their respective transition metal complexes. This behavior can be attributed to the energetically accessible redox states of ligands, analogous to quinone(0)/semiquinonate(−1)/catecholate(−2) (Figure 2a) when bound to a positive metal center. This salient feature promoted quinone derivatives to become the guinea pig of fundamental research of ligand noninnocence and related phenomena, such as valence tautomerism68 (Scheme 1). For example, Pierpont and co-workers69−79 synthesized numerous quinone complexes (LOO) of different composition and analyzed the geometries, magnetic and redox properties, and NMR, EPR, and electronic spectra leading to observations on intramolecular electron transfer between close energy redox isomers, described as an example for valence tautomerism (Scheme 1).77,80,81 Also, the long-standing research of Wieghardt and Neese and co-workers on homoleptic bis(dithiolene) complexes82−89 (Figure 2b), [M(LSS)2]z (z = −2, −1, 0, M = Fe, Co, Ni, Pd, Pt, Cu, Au), as well as on oaminothiophenolate (LNS) ligand systems86,90−92 analyzed and clarified the complicated electronic structures by means of, among others, X-ray absorption spectroscopy, resonance Raman, EPR, magnetic circular dichroism, and also computational techniques. Considerable further insights were gained in the thorough and systematic studies of Lever on substituted benzoquinonediimine (LNN), benzoquinone (LOO), and aminophenol (LNO) ruthenium complexes of general formula [(acac)2RuLXY]+z and [(bpy)2RuLXY]+z.93−102 From these comprehensive studies, it appears that noninnocence of quinone ligands, i.e., the ambiguity in metal vs ligand oxidation states, originates from the strong mixing of the redox active LUMO π* orbital of the ligand with the appropriate metal d orbital (Figure 2c).94 Valence tautomerism, which is an equilibrium between redox isomers differing in charge distribution, arises when the resulting orbital mixing is small and the electronic levels of electroactive ligand and metal lie close.68,78,80,81 In contrast to the delocalized charge distribution

Figure 1. Electron borrowing strategy exemplified by (a) (PDI)Fe(N2) and (b) [Co(LNO)2]−1 to facilitate two-electron transformations.

electron transformations by utilizing electron(s) originating also from the ligand(s). A clean oxidative addition process leading to C−C bond activation materialized when reacting the former iron complex with biphenylene (Figure 1a),14 in which one electron originates from both the FeII/III and PDI−2/−1 couples to facilitate the net two-electron oxidative addition step. The ligand electron borrowing strategy has been also exemplified with cobalt when supported by two strongly oxidizing redox active amidophenolate (LNO) ligands8 in [Co(LNO)2]−1 (Figure 1b). Because each LNO ligand supplies one electron for the net two-electron C−X activation process, the oxidation state of the reactive metal center remains the same. Another landmark example is the oxidative addition of Cl2 and O2 to d0 ZrIV complexes15 that contain redox active ligands to provide the two electrons needed for the process. The high demand for economic and sustainable solutions for similar redox transitions intensifies academic and industrial research for finding/developing new ligand scaffolds and metal−ligand combinations and also the fundamental understanding of the working mechanism of these systems. Particularly inspirational are the developments achieved since the establishment of the mechanism of galactose oxidase (GOase) to promote the two-electron oxidation of alcohols16−19 with the aid of a redox active tyrosine cofactor6,20 in the active site, in which progress the work of Whittaker on the understanding of GOase must be emphasized.21,22 Soon after, several groups succeeded to implement approaches toward models of structure, electronic structure, redox behavior, and even reactivity of the active site of GOase. For example, Whittaker, Pierre and co-workers, and others reported tripodal ligands containing two distinct phenolate moieties able to form biomimetic complexes with copper,23−28 including species with CuII antiferromegnetically coupled to a phenoxyl ligand radical, which promotes the oxidation of benzyl alcohol in the presence of O2.25 Access to nonsymmetrical salen ligands opened a completely new horizon in implementing this cooperativity into model complexes with greater structural and electronic fidelity to GOase.29 The main advantages of these tetradentate bis(Schiff base)-bis(phenolate) ligands are the relative ease of modular synthesis and functionalization, flexibility, the ability to bind to various metals in different oxidation states and B


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To derive a quantitative measure of noninnocence of ligands, Lever thoroughly analyzed the spatial distribution of the most relevant molecular orbitals describing σ-, π-, and δ-interactions in different [(acac)2RuLNN]0 and [(bpy)2RuLNN]+2 complexes.94,95,99,101 The contributions of metal and ligand to the M−L π-interactions were in good agreement with various measures including bond orders of ligand bonds, net charge of ligand, and π-backdonation, and could even be correlated with redox properties of the central metal and the ligand as well (vide infra). The chemically intuitive concept that the relative energy of interacting metal and ligand orbitals significantly influences the level of orbital mixing was clearly illustrated by Neese and Wieghardt for the isoelectronic [Fe(LSS)2]−2 and [Co(LSS)2]−1 systems.82 The higher effective nuclear charge of cobalt in the valence region lowers the energy of metal orbitals and brings them closer to the deeper lying ligand π-orbitals, resulting in stronger mixing. For copper, with an even higher effective charge, the metal orbitals drop in energy below the ligand orbitals, resulting in an inverted bonding situation104 shown in Figure 2d. The oxidation locus was also demonstrated to depend on the relative ordering of metal and ligand frontier orbitals in metallosalen complexes. In a recent contribution105 on the redox activity of quinone derivatives we introduced a novel concept that accounts for the high electron accepting ability of the common XC−CY structural motif (X, Y = O, S, and NH, red in Figure 2a) especially when fused with a C6benzo ring. It was shown that the thermodynamic driving force for ligand-based reduction is significantly influenced by the change in M−L interactions upon ligand-centered reductions, which critically stabilizes the M−L−1/−2 configurations. In line, we revealed that electron density increases mostly on the contact atoms of the ligand upon ligand-centered reductions, whereas delocalization to the backbone of the ligand is much less apparent than could perhaps be expected by the shape of the LUMO. By means of a systematic redox-potential investigation on an extensive set of ruthenium complexes, we also investigated how and why various factors, such as ring fusion, nature of contact atoms, substitution, etc., influence the redox energetics of these complexes. Most importantly, we demonstrated that the benzo-C6 ring of these ligands undergoes a formal antiaromatic (4 π electrons)-to-aromatic(6 π electrons) transition upon two-electron reduction that significantly eases ligand-centered reductions. Moreover, reduction of the aromatic bipyridine ligand to bpy−1 and to bpy−2 induces a loss of aromatic stabilization in the ligand which manifests in a penalty as much as 0.7 V for both bpy/ bpy−1 and bpy−1/−2 transitions.105 These simple rules might be applicable in efficient redox-leveling (moderating redox potentials) to avoid high overpotentials in multielectron transitions. In this study, we aim to follow up on the latter study on redox properties of quinone ligands ligands105 focusing on the most important aspects of the apparent noninnocent behavior of ligands in these species. Accordingly, this large-scale systematic computational study on octahedral ruthenium complexes provides in-depth insight into the structural deformations of ligands due to noninnocence and redox activity as well as into the stabilization contribution of this geometry change. Moreover, we deploy an arsenal of state-ofthe-art computational techniques to scrutinize π-density distribution, antiaromatic-to-aromatic transitions, effective

Figure 2. (a) Two-electron redox series of quinone to cathecolate, definitions of LOO, LNO, LNN, and LSS, and the common XC−CY structural motif in red. (b) General structure of square-planar bisdithiolene complexes ([M(LSS)2]−1) and the assigned formal oxidation state of the metal and ligands for Fe, Co, Ni, and Cu derivatives. (c) The most important π-type interactions between the metal dπ-orbital (dxz) and the redox active orbital of quinone ligands. (d) Normal and inverted bonding for a two-orbital-three-electron case, e.g., orbitals in panel c with three electrons. Partially reproduced from ref 105 with the permission from the Royal Society of Chemistry.

Scheme 1

in noninnocence (mixing of states), the charge distributions of isomers involved in valence tautomerism are typically localized. The latter phenomenon is characteristic to first row transition metal complexes with benzoquinone (LOO) ligands,78 but it might manifest also in other metal−ligand (e.g., LNS) combinations.103 A rule of thumb is that the mixing of metal and ligand orbitals is large in complexes of second and third row metals as well as complexes of LNO, LNN, LNS, and LSS so that these systems typically fall under the umbrella of the noninnocence regime. C


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Figure 3. (a) The four-state redox series investigated for octahedral ruthenium complexes containing one noninnocent ligand, L, and two spectator 1,2-diaminoethane (en) ligands. (b) Redox active ligands investigated.

Table 1. Computed C−X and Central C−C Bond Distances and Their Relative Values to Free Neutral Ligand (in Parentheses) in Å in the Various Oxidation States of the Investigated Complexes +3 [(en)2RuL]n LNN LNO LOO LNO2 LOMe LCl LMe i CH2

L

bpy bpyMe i

bpyCH2

bdp

+2

+1

+0

d(C−X)

d(C−C)

d(C−X)

d(C−C)

d(C−X)

d(C−C)

d(C−X)

d(C−C)

1.337 (0.045) 1.328/1.293 (0.038/0.069) 1.280 (0.059) 1.340 (0.049) 1.360 (0.065) 1.349 (0.058) 1.345 (0.053) 1.306 (0.028) 1.376 (0.028) 1.375 (0.028) 1.339 (0.051) 1.409 (−0.005)

1.476 (−0.041) 1.491 (−0.043) 1.513 (−0.045) 1.474 (−0.042) 1.472 (−0.044) 1.474 (−0.042) 1.475 (−0.040) 1.475 (−0.034) 1.468 (−0.025) 1.462 (−0.029) 1.478 (−0.027) 1.328 (−0.033)

1.333 (0.041) 1.338/1.294 (0.048/0.070) 1.292 (0.071) 1.334 (0.043) 1.336 (0.041) 1.335 (0.044) 1.334 (0.042) 1.312 (0.034) 1.373 (0.025) 1.370 (0.023) 1.393 (0.105) 1.410 (−0.004)

1.466 (−0.051) 1.467 (−0.067) 1.476 (−0.082) 1.464 (−0.052) 1.470 (−0.046) 1.465 (−0.051) 1.466 (−0.049) 1.465 (−0.044) 1.470 (−0.023) 1.467 (−0.024) 1.470 (−0.035) 1.334 (−0.027)

1.362 (0.070) 1.371/1.330 (0.081/0.106) 1.331 (0.110) 1.364 (0.073) 1.361 (0.066) 1.361 (0.070) 1.363 (0.071) 1.345 (0.067) 1.404 (0.056) 1.401 (0.054) 1.360 (0.072) 1.386 (−0.028)

1.446 (−0.071) 1.440 (−0.094) 1.440 (−0.118) 1.448 (−0.068) 1.446 (−0.070) 1.446 (−0.070) 1.444 (−0.071) 1.420 (−0.089) 1.424 (−0.069) 1.425 (−0.066) 1.434 (−0.071) 1.360 (−0.001)

1.394 (0.102) 1.402/1.351 (0.112/0.127) 1.353 (0.132) 1.396 (0.105) 1.393 (0.098) 1.393 (0.102) 1.398 (0.106) 1.381 (0.103) 1.444 (0.096) 1.438 (0.091) 1.328 (0.040) 1.367 (−0.047)

1.436 (−0.081) 1.436 (−0.098) 1.435 (−0.123) 1.448 (−0.068) 1.438 (−0.078) 1.439 (−0.077) 1.437 (−0.078) 1.384 (−0.125) 1.386 (−0.107) 1.390 (−0.101) 1.411 (−0.094) 1.388 (0.027)

oxidation states of complexes (+3, +2, +1, and 0), we expect to reveal important aspects of noninnocence in RuIII (+3) and RuII (+2, +1, and 0) complexes in combination with ligands having formal charges of 0, −1, and −2. I. Structural Deformations of Ligands. One of the clearest manifestations of redox noninnocence of α-diimine and quinone-related ligands is the characteristic geometry change of the ligand framework upon reduction, according to the spatial topology of the π* LUMO orbital. This LUMO orbital is bonding along the central C−C bond and antibonding along the C−X and C−Y ones (X and Y represent the donor atoms of the ligand in general, see plotted LUMOs in Figure 5). Accordingly, the shortening of the C−C bond and the elongation of the C−X and C−Y ones of the central XC−

oxidation states, and the quantification of noninnocence and its dependence on various factors.

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RESULTS AND DISCUSSION Figure 3a depicts the general formula of ruthenium complexes and redox series [(en)2RuLNN]+3/+2/+1/0 selected for this systematic computational investigation. These octahedral complexes contain two spectator bidentate en, 1,2-diaminoethane, ligands and a bidentate noninnocent ligand L. The set of noninnocent ligands is shown in Figure 3b, covering a demonstrative chemical space, with significant variation in functionalization, donor atom substitution, frame perturbations, etc. As formally represented by Lewis structures for the four D


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distinguishing elongation of the C−X bonds and shortening of the central C−C bond in accordance with the antibonding and bonding characteristics of LUMO along these bonds, respectively. C−N bonds of quinoid derivatives elongate about equally (by ∼0.03 Å) in each redox step. The central C−C bond typically undergoes a more pronounced shortening upon the first electron reduction than with the second electron addition, except for iLCH2. Bipyridine derivatives (bpy and bpyMe) behave differently in that there is only very weak mixing of ligand π-orbitals with metal d orbitals, or in other words, the parent bpy frame is a very weak π-acid. For example, Gorelsky and Lever found that the LUMO of [(bpy)2Ru(LNN)]+2 has a Ru/LNN composition of 73/20% whereas the LUMO of [(bpy)2Ru(bpy)]+2 is completely metal based, i.e., Ru/bpy composition is 100/0%.116 Also, the metal−ligand mixing of HOMO of these species shows similar differences. Accordingly, the reduction of [(en)2Ru(bpy)]+3 takes place exclusively on the ruthenium center and the geometry of the ligand is only affected by the change of σ-donation along the +3 to +2 transition. This change results in a slight shortening of the C−N bonds, and elongation of the central C−C bond in bpy and bpyMe. The subsequent one- and two-electron reduction of the bpy ligand induces an analogous geometry change as discussed for quinone ligands, according to the bonding/antibonding characteristics of the LUMO of bpy Figure 5. Bond distances of bdp, as well as their alteration upon reduction, are in striking contrast with the other investigated quinone or bpy derivatives. We designed105 this ligand frame to improve the redox activity of the parent LNN ligand by utilizing the simultaneous antiaromatic-to-aromatic transitions of the auxiliary pyrrole rings upon reduction. Bond distances indicate that neutral bdp has a dominant Lewis structure and alternation upon reduction as depicted in Scheme 2, rather than as

CY motif is a commonly recognized and investigated feature of noninnocence and ligand-centered reductions, as a result of the population of the LUMO orbital. Moreover, the corresponding structural metrics, such as C−C and C−N bond lengths, are often used as reliable indicators for the oxidation state of benzoquinone derivatives in various complexes and the magnitude of noninnocence.82,106−110 Structural metrics work best as indicators in complexes with localized charge distribution, e.g., first row metal complexes of LOO. This simple approach fails in highly delocalized systems,79,111,112 especially for dithiolene83,84,86,113 and second and third row metal complexes. The ligand oxidation state has been a matter of debate101,111,114 especially for prototypical ruthenium complexes due to some inconsistencies in the structural and spectral predictions; Remenyi and Kaupp115 however convincingly demonstrated the presence of pure RuIII/ L0 and mixed RuII/L0 ↔ RuIII/L−1 and RuII/L−1 ↔ RuIII/L−2 states in the different oxidation states of the complex. For the investigated molecules, Table 1 shows the equilibrium distances of C−X bonds (X = N, O) and the central C−C bond of ligands, as well as their relative values (difference, in parentheses) to neutral ligands. First, positive relative values for C−X distances as well as negative relative C− C distances indicate that the former bonds elongate whereas the latter ones shorten upon coordination of ligands to the metal. Note that this change already takes place to some extent in the +3 state, i.e., when the ligand binds to a formal ruthenium(III) center without notable π-backdonating ability. Obviously, in these cases (+3 states) most of this deformation with respect to free ligands originates from the σ-donation of the ligand to the metal; the MO representing the interacting lone pairs of the donor atoms has a bonding/(antibonding) character along the C−X/(C−C) bonds, and, accordingly, removal of electron density from this orbital results in the observed deformation of the ligand. π-Donation from the ligand to the metal is also possible between π-symmetry orbitals in the +3 state and would generate a similar change in the ligand. Most plausibly this mechanism is also in operation for LOMe with a π-donating methoxy substituent. Nevertheless, π density analysis (vide infra) as well as the very small change of geometry of the ligand backbone upon coordination (C4−C5 bond 1.531 Å in [(en)2RuLOO]+3 vs 1.558 Å in free LOO) confirms that bond alterations within the ligand in the +3 state originate from the ligand-to-metal donation in the σ-subspace, which leaves the ligand backbone structure unaltered. Going from the +3 to the +2 state of the complex, the C−X bonds and the central C−C bond vary only slightly, in most cases by about 0.01 Å. The slight shortening of the central C− C bond (as well as the slight elongation of C−O bonds in LNO and LOO) can be rationalized with the π-backdonation from RuII in the +2 state. Interestingly, the C−N bonds do not elongate as expected on this enhanced π-backdonation, but even shorten very slightly (∼0.004 Å) but systematically. This finding implies that, going from electron poor RuIII to electron richer RuII, the σ-donation from the ligand to the metal is reduced and it overcompensates the effect of π-backdonation on the geometry, resulting in very minor C−N bond shortenings for the +3 to +2 transitions. Gradual reduction of the complex to +1 and to 0 oxidation states prompts the expected and well-understood alteration of ligand geometry characteristic for ligand-centered electron transfers. That is, populating the redox active LUMO orbital (see later, in Figure 5) of the quinoid ligands induces a

Scheme 2

indicated in Figure 3b. In accordance with a δ-symmetry LUMO (see Figure 5d), bdp undergoes an opposite structural change as quinone or bpy derivatives, namely, shortening of C− N bonds and lengthening of the central C−C bond. The magnitude of these bond alterations upon reduction is similar to that revealed and discussed for quinone and bpy ligands. II. Manifestation of Structural Change in the Energetics of Reduction. To quantify the energy contribution of the structural change upon reductions, we have computed so-called theoretical square schemes for each redox step, which are abbreviated in Figure 4 as I ([(en)2RuL]+3 → [(en)2RuL]+2), II ([(en)2RuL]+2 → [(en)2RuL]+1), and III ([(en)2RuL]+1 → [(en)2RuL]0). The method of theoretical square schemes has been introduced by Baik, Schauer, and Ziegler117 to reveal the origin of potential-inversion phenomena in organic and inorganic species.118,119 Figure 4 depicts a general theoretical square scheme, which separates the reduction process (diagonal) into vertical electron attachment E


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Table 2. Solvent Phase Energy Contribution (ΔEsol, eV) of Vertical Electron Attachment (a) and Structural Relaxation Steps (b) of Redox Processes I, II, and III I (+3 → +2) [(en)2RuL] LNN LNO LOO LNO2 LOMe LCl LMe i CH2 L bpy bpyMe i bpyCH2 bdp

Figure 4. Theoretical square schemes separating redox events I, II, and III into electron attachment and structural relaxation steps.

(horizontal) and structural relaxation (vertical) energies. Although this method is constrained to the analysis of gas or solvent phase electronic energy changes during reductions, it provides crucial information in return for negligible computational effort. The energy contribution of electron attachment and structural relaxation upon reduction can be determined by calculating single point energy and solvent correction for a nonequilibrium structure for each redox event, e.g., [(en)2RuL*a]+2 for I. Accordingly, in the overall reduction process (I = Ia + Ib), Ia represents an electron attachment process going from [(en)2RuL]+3 to [(en)2RuL*a]+2 whereas step Ib corresponds to the following structural change to form [(en)2RuL]+2. Thus, the effect of one of the clearest manifestations of ligand noninnocence and redox active behavior, the above-discussed structural distortion of the ligand upon reduction, can be evaluated through this analysis. To our best knowledge, the energy contribution of this effect to redox activity of ligands and the importance of structural flexibility in redox active behavior have not been assessed so far in this detail. Actually, theoretical square schemes can be applied for analyzing oxidation processes as well. In these cases one has to compute the lower “triangle” of the “square” and has to move from the right to the left along this “triangle”, which separates the oxidation process to an initial electron removal followed by structural relaxation (for further comments see the Supporting Information). Since the two “directions” provide the same conceptual answer in our case we discuss the results of this energy separation technique only for reduction processes. Table 2 summarizes the solvent-phase energy change (ΔEsol) upon vertical electron attachment (a), and following structural relaxation (b) processes for redox events I, II, and III of all investigated species. This analysis reveals, in general, a negligible structural relaxation contribution ( LNO > LOO series in accord with the increasing noninnocent nature of the ligand, as well as in line with other noninnocence indicating measures as well as aromaticity, discussed above. Moreover, substituent effects can also be clearly rationalized with the R values. For example, the electron withdrawing and donating effects in π-modulated LNO2 and LOMe ligands manifest in lower and higher R values than for LNN, respectively, which is line with a different contribution of the alternative oxidation state formalism, e.g., RuIII/L−2. The lowest value (R = 65%) is obtained for LOO in [(en)2Ru(LOO)]+1, which is indeed the ligand with the highest electron accepting power. In this and similar complexes with LNO and LNO2 ligands, the electronic structure is not clearly described by a single “pure” oxidation state assignment, but the contribution of RuIII/L−2 becomes more relevant, as clearly demonstrated by this EOS analysis in form of lower reliability values. Nevertheless, for the rest of the ligands, e.g., bpy, bpyMe, etc., including LNN, the contribution of contribution of the RuIII/L−2 state is insignificant. The latter conclusion might seem to be in contrast to the widespread belief of very apparent ligand noninnocence of all quinone derivatives, i.e., more significant ambiguity in oxidation state assignment. However, as has been pinpointed recently also by Das et al.,120 noninnocence in similar complexes strongly depends on the augmenting spectator ligands. In this respect, diaminoethane (en) is a weak spectator ligand and, accordingly, the ruthenium center is more electronegative than in analogous complexes of stronger bpy or acac− spectator ligands, resulting in smaller noninnocence in the +2 and +1 states of the investigated complexes. Finally, the EOS analysis clearly reveals significantly pure states for the +2 state of complexes, i.e., [(en)2Ru(L)]+2 molecules. Reliability indices above 80% indicate negligible noninnocence of ligands in this oxidation state and a very small mixing of RuIII/L−1 formal assignment into the dominant RuII/ L0 state (the strongest mixing is implied for LOO with R = 80%, which is still minor). Accordingly, this EOS analysis shows that the relatively high ΔNπ(L) values computed for the [(en)2Ru(L)]+2 states (up to 0.52 e, Table 3) originate from the πbackdonation rather than from the mixing of states.

In this study, the systematic set of ruthenium complexes in Figure 3 provides a natural and appropriate platform to monitor the performance and insightfulness of the recently introduced EOS analysis on these rather complex systems. On the other hand, if providing reliable and convincing oxidation states for the metal center and noninnocent ligands in complexes, the EOS analysis might become a general protocol in the field of computational organometallic chemistry, significantly enhancing the formal communication between experimentalists and theoretical chemists. Driven by these motivations we carried out an EOS analysis on the studied complexes. The EOS analysis identified that ruthenium is in oxidation state +3, +2, +2, and +2 along the [(en)2Ru(L)]+3, [(en)2Ru(L)]+2, [(en)2Ru(L)]+1, and [(en)2Ru(L)]0 series in all complexes, whereas the formal charge on the (en)2 ligands is always 0, as expected, in all cases. The oxidation state of the L ligand is 0, 0, −1, and −2 along the same series. The R values obtained in the EOS analysis are very high in general, and, accordingly, this EOS study clearly reveals significantly dominant RuIII/L0, RuII/L0, RuII/L−1, and RuII/ L−2 assignments in the +3, +2, +1, and 0 oxidation states of the complexes, respectively, independently of the redox active ligand. Table 5 summarizes the values of the reliability index R corresponding to these dominant states. Table 5. Metal and Ligand Oxidation State Assignations and Associated Reliability Indices R (%) for the Species [(en)2Ru(L)]+3, [(en)2Ru(L)]+2, [(en)2Ru(L)]+1, and [(en)2Ru(L)]0a

a

[(en)2RuL]n

+3: RuIII/L0

+2: RuII/L0

+1: RuII/L−1

0: RuII/L−2

LNN LNO LOO LNO2 LOMe LCl LMe i CH2 L bpy bpyMe i bpyCH2 bdp

90 93 95 86 80 91 92 96 96 95 96 86

86 82 80 83 92 87 89 97 100 100 95 99

85 75 65 77 91 84 88 99 100 100 93 100

100 100 100 100 100 100 100 98 100 100 100 100

The predicted oxidation state of spectator ligands (en)2 was always 0.

As mentioned above, the oxidation state assignation is quite unquestionable for the most reduced (+3) and most oxidized (0) forms of complexes, having prevailing RuIII/L0 and RuII/L−2 states, respectively. The EOS analysis fully confirms this expectation by yielding R = 100% for ruthenium(+2) and L(−2), i.e., RuII/L−2, for essentially all complexes in the 0 oxidation state of the complex (last column in Table 5). Similarly, the oxidation state assignment for the +3 state of the molecules is also straightforward and unambiguous with R values somewhat smaller than 100%. Indeed, one should expect somewhat lower R values in general for systems with highvalent fragments, as the formal picture of integer (and large) charges deviates more from the actual electronic structure (this can also be seen from standard population analysis). This is a known feature of the EOS method, which is reproduced here in the case of higher oxidation state of the Ru moiety (+3). Nevertheless, the R values are still around 85% or higher, high enough to unambiguously assign these species as RuIII/L0. The J


Article

Inorganic Chemistry

consistent conditions. The computed metrics imply a significant deformation of the (free) ligand upon coordination to a RuIII center. Due to the weak backdonation ability of RuIII we interpreted the change in C−X and C−C distances as the manifestation of σ-donation from the ligand to the metal in this coordination process. Backdonation from RuII to the ligand in [(en)2RuL]+2 states induces the expected, and often discussed, shortening of central C−C bond and lengthening of C−Odonor bonds in line with the bonding characteristics of the redox active π* LUMO. Interestingly, in contrast to C−O bonds, we revealed a very slight shortening of the C−Ndonor bonds for benzoquinone derivatives upon +3 to +2 transition. As expected, more pronounced bond length changes are observed upon ligand-centered reductions, i.e., going from [(en)2RuL]+2 to [(en)2RuL]+1 and to [(en)2RuL]0. bpy derivatives behave slightly differently in that their structural change is negligible upon RuIII/RuII due to the insignificant mixing of metal d and ligand π* orbitals. Also, characteristically different trends in structural alteration have been revealed for bdp ligand, which has a δ-symmetry LUMO, in contrast to other ligands with πsymmetry LUMO. Utilizing theoretical square schemes we decomposed the investigated reductions into vertical electron transfer and structural relaxation subprocesses to quantify the energetic contribution of the scrutinized structural changes to the various redox steps. For parent and σ-functionalized benzoquinonediimine ligands (LNN) as well as for bpy and bdp we revealed an insignificant stabilization associated with the structural relaxation processes. In contrast, the more characteristic structural change of, e.g., LOO and LNO2 eases the reduction process by as much as −0.39 eV (+2 → +1) and −0.4 eV (+1 → 0), respectively, which can be termed as significant for the overall reduction. By plotting the electron density change upon vertical electron transfer processes we visualized the regions where the incoming electron accumulates provide intuitive insights into the redox active and noninnocent behavior of ligands in the various transitions. These 3D plots show a characteristic accumulation of electron density on the metal center in formal RuIII/II transitions, whereas mixed ligand−metal accumulations in formal ligand-centered redox processes of quinone derivatives. Moreover, we can further differentiate that donor atoms play a more important role in hosting the incoming electrons than the backbone of the ligand, which is a critical aspect in easing ligand-centered reductions. The different behaviors of bpy and bdp, including the larger participation of the extended delocalized backbone in the redox process as well as the δ-symmetry LUMO of bdp can be straightforwardly monitored in the introduced density change analysis. Also, we quantified the delocalization of π-charge distribution between metal and noninnocent ligand and its change upon reduction by separating σ- and π-electron densities using a straightforward NPA-based approach. The excess of π-electron density located on the ligand was found to be an insightful indicator, however not a direct measure of noninnocence, providing support for different intuitive expectations. Most importantly, this quantity indicates a detectable mixing of RuII/ L−1 and RuIII/L−2 states for [(en)2RuL]+1 species. Also, chemically important trends could be monitored through the computed data, for example the effect of ligand functionalization and NH to O donor atom substitution. For [(en)2RuL]+2 complexes, strong π-backdonation also interferes with the

Lastly, we compare the trends of noninnocence as described by different criteria, such as structural, π-density distribution and EOS analysis, for the most affected [(en)2Ru(L)]+1 state (see Supporting Information for the other states). As it is the case for other multidimensional phenomena like aromaticity, the different criteria of noninnocence should also expectedly converge to some extent. In the effort to clearly present the essence of noninnocence manifested in these criteria we had to “normalize” the reliability value, R (%), of the EOS analysis (as 1 − R/100) as well as the excess of π-electron density located on the ligand (as ΔNπ(L) − 1) for [(en)2RuL]+1 species and, also, the relative Ndonor−C distances (as Δd(C−N)/0.17, where 0.17 Å is the difference between the averages of carbon− nitrogen single and double bonds). Accordingly, these normalized descriptors vary between 0 and 1 and can be plotted on a unified scale, as in Figure 6. We have to note,

Figure 6. Correlation of “normalized” indicators of ligand noninnocence in [(en)2RuL]+1 species based on three criteria, such as the excess of π-electron density located on the ligand (ΔNπ(L) − 1), reliability index of EOS analysis (1 − R/100), and structural metrics (Δd(C−N)/0.17).

however, that, beyond expecting correlations of any kind, one should not expect numerical parity between these normalized indicators. They originate from completely different aspects of noninnocence, and only in the case of EOS the values can be interpreted as direct measures of noninnocence because the atomic/fragment populations are internally analyzed in terms of individual effective orbital contributions. As reflected in Figure 6, all three criteria indeed show very similar variations among the different ligands. Also, the derived structural criterion has the lowest sensitivity toward electronic changes, whereas EOS analysis responds most pronouncedly to the designed chemical triggers. As discussed above, both the analysis of π-density distribution and EOS analysis pinpoint LNO, LOO, and LNO2 ligands to be the most noninnocent, where state mixing has to be considered.

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CONCLUSIONS In this contribution we computationally investigated various appearances of noninnocence of quinone and bipyridine derivatives in an extensive and systematic set of octahedral ruthenium complexes of general formulas [(en)2RuL]z in various oxidation states (z = +3, +2, +1, and 0). Thereby, we aimed also to provide links between the delocalized charge distribution, structural distortion, the effective oxidation state of fragments and aromaticity of these complexes, and changes of these features upon redox transitions and their stabilization effect as well. First we compared the characteristic structural deformation of ligands with each other and also to free ligands under K


Article

Inorganic Chemistry mixing of states (RuII/L0 and RuIII/L−1) and it manifests in the computed π-electron density values located on the ligand. Using magnetic (NICS) and electronic (PDI) indicators of aromaticity we unambiguously demonstrated gradual antiaromatic to nonaromatic and to aromatic transitions in the C6benzo ring of quinone derivatives upon subsequent reductions of the corresponding complexes. On the contrary, reduction of bpy derivatives induces the dearomatization of pyridine subunits, which has a clear manifestation in the monitored descriptors as well as in the energetics of ligand-centered reductions. The hypothesized simultaneous antiaromatic to aromatic transitions of pyrrole and C6-benzo rings of bdp have been also unambiguously confirmed. The recently introduced effective oxidation state (EOS) analysis revealed unambiguous RuIII/L0 oxidation states for [(en) 2 RuL] +3 , whereas Ru II /L −2 oxidation states for [(en)2RuL]0 with very high reliability index. In contrast, the values for the reliability index obtained in this analysis imply, in line with the other investigated metrics as depicted in Figure 6, a partial mixing of states in [(en)2RuL]+1 complexes. In the cases of LNO, LOO, and LNO2 the mixing of RuII/L−1 and RuII/ L−2 states is the most characteristic one, nevertheless, even in these cases it is clear that the dominant state is RuII/L−1. The oxidation state assignations are rather unquestionable for the rest of the ligands, implying insignificant noninnocence. Also, reliability indices above 80% are discovered for [(en)2RuL]+2 species, indicating only minor mixing of the RuIII/L−1 state into the RuII/L0 configuration. This analysis was also found to respond to the chemical triggers applied in the ligand space (e.g., functionalization, ligand frame modulations) according to established expectations. Lastly, we found convincing correlation between the change of EOS, π-density, and structure-based criteria for the noninnocence of different ligands. Finally, in addition to these specific conclusions, with this study we also aimed to showcase how trend analyses on largescale, systematic, and consistent computational data can provide new and often intuitive insights into complex chemical phenomena, such as noninnocence of ligands. Moreover, none of the applied techniques is system specific or constrained to DFT and, accordingly, these insightful analyzing tools can be straightforwardly used for any other chemical problem or system, even in combination with WF-based computational methods.

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general,140−142 we found, in line with a recent review,139 that TPSSh is one of the most accurate functionals for computing the energetics, including high-spin−low-spin relative energies, of these first row analogues of the investigated complexes. Solvent effects were simulated at the TPSSh/cc-pVTZ(-PP) level of theory using the SMD implicit solvation model143 as implemented in Gaussian09. As is the case for all continuum models, the solvation energies are subject to empirical parametrization of the atomic radii that are used to generate the solute surface. In our calculations the solvent accessible surface (SAS) method was used to create the molecular surface representing the solute−solvent boundary. We employed radii for H (1.400 Å), O (2.000 Å), N (2.000 Å), C (2.300 Å), Cl (2.400 Å), and Ru (1.800 Å) and a solvent radius of 1.8 Å for acetonitrile. The spin-resolved effective atomic orbitals (eff-AOs) and effective oxidation state (EOS) analysis have been obtained with the APOST3D program,144 using a 70 × 434 atomic grid for the numerical integrations. Due to technical limitations single-point calculations at the TPSSh/cc-pVTZ(-PP) level of theory but without the g-type orbitals for the Ru atoms where used. The topological fuzzy Voronoi cells (TFVC) atomic definition127 was used throughout with a threshold on the eff-AO occupations of 0.01 (i.e., eff-AOs with occupation numbers below the threshold were ignored). PDI were also calculated with APOST-3D.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02543. Cartesian coordinates of optimized structures, aspects of theoretical square schemes, Table S1, and Figures S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.D.P. wishes to acknowledge the Research Foundation Flanders (FWO) and Vrije Universiteit Brussel (VUB) for continuous support, among others through a Strategic Research Program. G.S. thanks FWO for the travel grant (V444315N) whereas B.P. acknowledges FWO for financial support through a postdoctoral grant (1279414N). M.G. and P.S. are thankful for financial support from the FEDER grant UNGI10-4E-801 and MINECO grant CTQ2014-59212-P/BQU.

EXPERIMENTAL SECTION

Methodology and Computational Details. Geometry optimizations were carried out using the TPSSh129 density functional coupled with the relativistic core potential containing cc-pVDZPP130,131 basis set for Ru and the cc-pVDZ basis132,133 for light atoms. The energies of the optimized structures were reevaluated using the triple-ζ basis set cc-pVTZ(-PP)134 (-PP applies for Ru). Analytical vibrational frequency calculations within the harmonic approximation were computed with the cc-pVDZ(-PP) basis to confirm minima on the potential energy surface. All of these calculations were carried out with Gaussian09.135 Wave functions evaluated at the TPSSh/ccpVTZ(-PP) level of theory were used to analyze the electronic structures with various methods and techniques including MO analysis, NBO analysis,136−138 charges, and spin densities. When coupled to a balanced basis set, such as cc-pVDZ-(PP), the hybrid TPSSh functional is known to perform well to describe various properties of transition metal complexes,139 including their equilibrium geometries and, as we recently demonstrated, redox energetics as well.105 Also, when we benchmarked and tested our method toward iron and cobalt complexes as well, for which computing spin-state energetics represents one of the weaknesses of DFT methods in

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DEDICATION Dedicated to Professor Paul Geerlings on the occasion of his 65th birthday. REFERENCES

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