1,5-Diamido-9,10-anthraquinone, a Centrosymmetric Redox-Active

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1,5-Diamido-9,10-anthraquinone, a Centrosymmetric Redox-Active Bridge with Two Coupled β‑Ketiminato Chelate Functions: Symmetric and Asymmetric Diruthenium Complexes Mohd. Asif Ansari,§ Abhishek Mandal,§ Alexa Paretzki,† Katharina Beyer,† Jan Fiedler,‡ Wolfgang Kaim,*,† and Goutam Kumar Lahiri*,§ §

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany ‡ J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-18223 Prague, Czech Republic †

S Supporting Information *

ABSTRACT: The dinuclear complexes {(μ-H2L)[Ru(bpy)2]2}(ClO4)2 ([3](ClO4)2), {(μH2L)[Ru(pap)2]2}(ClO4)2 ([4](ClO4)2), and the asymmetric [(bpy)2Ru(μ-H2L)Ru(pap)2](ClO4)2 ([5](ClO4)2) were synthesized via the mononuclear species [Ru(H3L)(bpy)2]ClO4 ([1]ClO4) and [Ru(H3L)(pap)2]ClO4 ([2]ClO4), where H4L is the centrosymmetric 1,5diamino-9,10-anthraquinone, bpy is 2,2′-bipyridine, and pap is 2-phenylazopyridine. Electrochemistry of the structurally characterized [1]ClO4, [2]ClO4, [3](ClO4)2, [4](ClO4)2, and [5](ClO4)2 reveals multistep oxidation and reduction processes, which were analyzed by electron paramagnetic resonance (EPR) of paramagnetic intermediates and by UV−vis−NIR spectroelectrochemistry. With support by time-dependent density functional theory (DFT) calculations the redox processes could be assigned. Significant results include the dimetal/bridging ligand mixed spin distribution in 33+ versus largely bridge-centered spin in 43+a result of the presence of RuII-stabilizig pap coligands. In addition to the metal/ligand alternative for electron transfer and spin location, the dinuclear systems allow for the observation of ligand/ligand and metal/metal site differentiation within the multistep redox series. DFT-supported EPR and NIR absorption spectroscopy of the latter case revealed class II mixed-valence behavior of the oxidized asymmetric system 53+ with about equal contributions from a radical bridge formulation. In comparison to the analogues with the deprotonated 1,4-diaminoanthraquinone isomer the centrosymmetric H2L2− bridge shows anodically shifted redox potentials and weaker electronic coupling between the chelate sites.

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INTRODUCTION 9,10-Anthraquinone (AQ) and its substituted derivatives such as 1,2-dihydroxy-AQ (alizarin) or 1,4-dihydroxy-AQ (quinizarin) can occur as natural products1a and have found many applications since their establishment ∼150 years ago.1b They constitute a major class of dyes,2 are pharmaceutically active,3a serve in industrial catalysis,3b can act as sensors,4a form supramolecular structures,4b−d and have been studied recently in connection with energy research (flow batteries5). Organic catalysis also makes use of (anthra)quinones.6 AQ and quinones in general behave as two-step reversible redox systems with nonaromatic oxidized forms and typically stable semiquinone intermediates.7 Whereas the ortho-quinone/ catecholate redox system involves a “natural” chelate ligand situation,7 the para-quinones, including AQ, require additional coordinating groups if chelation is desired. Diorganophosphine substituents have been employed for that purpose,8 but the most frequently used substituents supporting p-quinone O atoms for metal chelation are deprotonated OH and also NHR groups.9 Taking quinizarin as a dihydroxo-substituted model, we have previously used the doubly deprotonated form of 1,4-diamino9,10-anthraquinone (4-DAAQ = H4L′) with the more basic © XXXX American Chemical Society

amine/amide functions as a bis-chelating bridging ligand for ruthenium complex fragments RuL2, L = bpy,10 pap,11 acac−.10 Like the bridging ligand, the metals can undergo redox reactions with RuII, RuIII, and RuIV as accessible oxidation states,12,13 and ruthenium complexes of other anthraquinone derivatives have been reported.9 Within those studies the question of electron distribution in symmetric versus asymmetric configurations has emerged.14 We therefore set out to synthesize and study diruthenium compounds derived from the coordinatively less utilized 1,5diamino-9,10-anthraquinone (5-DAAQ = H4L), both experimentally (structure determination, voltammetry, spectroelectrochemistry) and via density functional theory (DFT) Received: March 23, 2016

A

DOI: 10.1021/acs.inorgchem.6b00726 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Representations of the complexes.

of the mononuclear complex [1]ClO 4 with ctc-[Ru(pap)2(EtOH)2]2+ in refluxing EtOH in the presence of NEt3. The complexes were purified by column chromatography using neutral alumina (see the Experimental Section), which consistently led to the formation of only one diasteromeric form for 32+, 42+, or 52+. The complexes are highly soluble in CH3CN, CH2Cl2, and EtOH, while the free ligand (H4L) is fully and partially soluble in CH2Cl2 and CH3CN or EtOH, respectively. Satisfactory microanalytical and mass spectrometric data established the compositions of 1:1 (1+, 2+) and 1:2 conducting (32+, 42+, and 52+) complexes (see the Experimental Section and Figure S1 in the Supporting Information). 1 H NMR spectra of the diamagnetic mononuclear (1+, 2+) and dinuclear (32+, 42+) complexes in (CD3)2SO exhibited the calculated number of overlapping aromatic proton resonances within the chemical shift region δ, 6−9 ppm. The D2O exchangeable NH protons associated with coordinated H3L− or H2L2− could also be identified (see the Experimental Section and Figure S2 in the Supporting Information). Although the symmetric dinuclear complexes 32+ and 42+ can exist in rac: ΔΔ/ΛΛ and meso: ΔΛ forms, the observed 40 (4 bpy: 32 + H2L2−: 8) and 44 (4 pap: 36 + H2L2−: 8) proton resonances corresponding to the full molecules supports the former. The 1 H NMR spectrum of the diamagnetic asymmetric complex 52+ also showed partially overlapping 42 proton resonances corresponding to the full molecule (bpy: 16, pap: 18, H2L2−: 8 resonances). Crystal Structure Determination. Molecular structures from single-crystal X-ray diffraction could be obtained for the perchlorates of 1+, 2+, 32+, and 42+. Efforts to obtain suitable crystals for [5](ClO4)2 remained unsuccessful. The crystal and refinement data are summarized in Table S1 in the Supporting Information, and the essential structure parameters discussed

calculations. In contrast to 4-DAAQ the 5-DAAQ isomer is distinguished by centrosymmetry, which invited our studies of homodinuclear (3n, 4n) and heterodinuclear complexes (5n) with the doubly deprotonated 5-DAAQ, that is, H2L2−. Moderately π-accepting 2,2′-bipyridine (bpy) and strongly πaccepting11,15 2-phenylazopyridine (pap) were chosen as related but significantly different ancillary ligands. The mononuclear precursors 1n and 2n containing singly deprotonated 5-DAAQ (H3L−) were also investigated structurally and (spectro)electrochemically (Figure 1). Figure 1 already suggests to recognize the compounds as ligands with one or two β-ketiminate16 chelate functions, leaving only partial conjugation for the C6 ring of the tricyclic ion. The following presentation of analytical results and structure determination ([1]ClO4, [2]ClO4, [3](ClO4)2, [4](ClO4)2), of voltammetric (CV, DPV) and spectro-electrochemical investigations (EPR, UV−vis−NIR), and of quantumchemical calculations (DFT, TD-DFT) was undertaken to probe the suitability and characteristics of deprotonated 5DAAQ as a ligand.

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RESULTS AND DISCUSSION Synthesis and Identification. The complexes [1]ClO4, [3](ClO4)2, [2]ClO4, and [4](ClO4)2 were obtained via reactions of the in situ generated precursors [Ru(bpy)2(EtOH)2]2+ (bpy = 2,2′-bipyridine) and ctc-[Ru(pap)2(EtOH)2]2+ (pap = 2-phenylazopyridine, ctc = cis−trans−cis configuration with respect to EtOH, pyridine, and azo nitrogens of pap, respectively) with H4L (H4L = 1,5-diamino-9,10anthraquinone) in refluxing EtOH and in the presence of NEt3 under a dinitrogen atmosphere. The mononuclear and the dinuclear counterparts were always obtained simultaneously irrespective of the precursor metal−ligand ratio. The asymmetric dinuclear complex [(bpy)2RuII(μ-H2L2−)RuII(pap)2](ClO4)2 ([5](ClO4)2) was prepared from the reaction B


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Inorganic Chemistry

Table 1. Selected Experimentala Bond Lengths (Å) for Cationic Ions in [1]ClO4·CH3CN, [2]ClO4, [3](ClO4)2, and [4](ClO4)2 bond length of 1+ Ru(1)−N(1) Ru(1)−O(1) Ru(1)−N(3) Ru(1)−N(4) Ru(1)−N(5) Ru(1)−N(6) C(1)−N(1) C(13)−O(1) C(8)−N(2) C(6)−O(2) C(1)−C(2) C(1)−C(14) C(2)−C(3) C(3)−C(4) C(4)−C(5) C(5)−C(6) C(5)−C(14) C(6)−C(7) C(7)−C(8) C(7)−C(12) C(8)−C(9) C(9)−C(10) C(10)−C(11) C(11)−C(12) C(12)−C(13) C(13)−C(14)

a

2.014(2) 2.040(2) 2.063(2) 2.075(3) 2.042(3) 2.053(3) 1.330(4) 1.288(4) 1.355(4) 1.243(4) 1.441(4) 1.451(4) 1.361(4) 1.410(5) 1.369(4) 1.486(4) 1.442(4) 1.458(4) 1.429(4) 1.417(4) 1.410(4) 1.373(5) 1.395(4) 1.385(4) 1.491(4) 1.408(4)

bond length of 2+ Ru(1)−N(1) Ru(1)−O(1) Ru(1)−N(3) Ru(1)−N(4) Ru(1)−N(6) Ru(1)−N(7) C(1)−N(1) C(13)−O(1) C(8)−N(2) C(6)−O(2) N(5)−N(4) N(8)−N(7) C(1)−C(2) C(1)−C(14) C(2)−C(3) C(3)−C(4) C(4)−C(5) C(5)−C(6) C(5)−C(14) C(6)−C(7) C(7)−C(8) C(7)−C(12) C(8)−C(9) C(9)−C(10) C(10)−C(11) C(11)−C(12) C(12)−C(13) C(13)−C(14)

bond length of 32+ /

2.050(5) 2.053(4) 2.054(5) 2.024(5) 2.030(5) 2.004(5) 1.340(8) 1.291(7) 1.395(8) 1.270(7) 1.311(7) 1.319(7) 1.461(8) 1.445(8) 1.359(9) 1.439(9) 1.369(8) 1.477(8) 1.471(8) 1.461(8) 1.416(8) 1.449(8) 1.410(9) 1.378(9) 1.403(8) 1.385(7) 1.483(8) 1.429(7)

Ru(1)−O(1 ) Ru(1)−N(1) Ru(1)−N(2) Ru(1)−N(3) Ru(1)−N(4) Ru(1)−N(5) C(1)−N(1) C(1)-O(1/) C(1)−C(2) C(1)-C(7/) C(2)−C(3) C(3)−C(4) C(4)−C(5) C(5)−C(6) C(5)-C(7/) C(6)−C(7)

2.055(4) 2.009(5) 2.052(6) 2.044(6) 2.045(6) 2.060(6) 1.327(8) 1.287(7) 1.435(10) 1.467(9) 1.352(10) 1.415(10) 1.373(9) 1.477(9) 1.458(8) 1.397(8)

bond length of 42+ Ru(1)−O(1) Ru(1)−N(1) Ru(1)−N(3) Ru(1)−N(5) Ru(1)−N(6) Ru(1)−N(8) Ru(2)−O(2) Ru(2)−N(2) Ru(2)−N(9) Ru(2)−N(11) Ru(2)−N(13) Ru(2)−N(14) N(3)−N(4) N(6)−N(7) N(9)−N(10) N(12)−N(13) C(1)−N(1) C(13)−O(1) C(8)−N(2) C(6)−O(2) C(1)−C(2) C(1)−C(14) C(2)−C(3) C(3)−C(4) C(4)−C(5) C(5)−C(6) C(5)−C(14) C(6)−C(7) C(7)−C(8) C(7)−C(12) C(8)−C(9) C(9)−C(10) C(10)−C(11) C(11)−C(12) C(12)−C(13) C(13)−C(14)

2.035(4) 2.025(5) 1.981(5) 2.019(5) 2.029(5) 2.042(5) 2.041(4) 2.028(5) 1.967(5) 2.050(5) 2.045(5) 2.043(5) 1.297(7) 1.291(7) 1.302(7) 1.301(7) 1.352(8) 1.297(7) 1.333(7) 1.285(7) 1.446(9) 1.437(8) 1.350(10) 1.402(9) 1.365(8) 1.471(8) 1.442(8) 1.405(8) 1.448(8) 1.456(7) 1.425(8) 1.379(8) 1.408(8) 1.373(8) 1.455(8) 1.415(8)

From single-crystal X-ray diffraction.

A corresponding configuration is also observed for 2+ (see Figure 2). The pap coligands show only slightly lengthened N−N bonds in [3](ClO4)2 and [4](ClO4)2 due to π back-donation from the metal;17 a radical anion formation can thus be ruled out. The same is true for the potentially redox-active bridging ligand H2L2−. The intramolecular metal−metal distances (8.765 Å for [3](ClO4)2 and 8.787 Å for [4](ClO4)2) are ∼0.5 Å longer than those found for related compounds with the deprotonated 1,4-diaminoanthraquinone bridge;10 intermolecular Ru···Ru distances are longer than 9.2 Å. The binding of the metals occurs in a β-ketiminato chelate fashion with correspondingly averaged C−O, C−C, and C−N bond lengths.16 The Ru−N and Ru−O bond lengths are also as expected,10−12,16 with shorter distances observed for Ru− N(imine) and Ru−N(azo) bonds. The remaining C−C bond lengths show a typical alternancy (Table 1), reflecting the double/single bond situation as indicated by the formulas in Figure 1. While the double bonds C2−C3 and C4−C5 are therefore short at ∼1.36 Å, the two single bonds (C5−C6) connecting the two benzo-β-ketiminato chelates are rather long at ∼1.47 Å. Accordingly, the central and centrosymmetric

below are listed in Table 1. Figures 2 and 3 show the molecular structures of the cations. The presence of N−H functions in the ligands H3L− and H2L2− leads to hydrogen bonding to oxygen atoms of the ClO4− anions and of the quinone function, which is depicted and summarized in Figures S3−S6 in the Supporting Information. As in the free ligand,4d there are intra- and intermolecular NH···O interactions. The symmetrically bridged bis(tris-chelate) compounds with only slightly distorted octahedral configurations at RuII the homodinuclear species [3](ClO4)2 and [4](ClO4)2 were isolated in the respective rac forms (see NMR, Figure S2 in the Supporting Information). Compound [4](ClO4)2 was crystallized in the rac form (Figure 3b), while the crystallization of [3](ClO4)2 in the meso form with the centrosymmetric structure of the H2Ln− ligand bridge (Figure 3a) can be attributed to a rac → meso transformation during the prolonged crystallization from a 1:1 MeOH−toluene solution. Relative to the approximately planar central part of 42+ the pyridine rings of coordinated pap adopt the axial positions, whereas the more π-accepting N(azo) atoms lie in the equatorial positions, that is, trans to the donating NH and O atoms of the bridge (Figure 3). C


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Inorganic Chemistry

DFT calculations reproduce the experimental structures (Tables S2−S11 and Figure S7 in the Supporting Information) and were performed for other charge forms as well, in support of the oxidation state assignments given below. Cyclic and Differential Pulse Voltammetry. The redox activity of the metals and of anthraquinone in the bridging ligands should give rise to several redox transitions.9−13 However, the presence of potentially labile hydrogen centers in NH could affect the reversibility of these redox processes.18 Both voltammetric (CV, DPV) and spectro-electrochemical studies (see below) indicated that such effects do indeed occur for the NH2 containing mononuclear compounds, especially for system 2n, which limits the significance of corresponding data. Another potential reactivity of 1,5-diaminoanthraquinones is anodic electropolymerization,19 which, however, was not observed here. Nevertheless, the redox potentials in the accessible range of the acetonitrile solvent were measured (Figure 4) and are listed in Table 2.

Figure 2. Perspective views of the cationic parts of the compounds (a) [1]ClO4·CH3CN and (b) [2]ClO4. Ellipsoids are drawn at 40% probability level. Hydrogen atoms (C−H) and solvent molecule (CH3CN) are removed for clarity.

Figure 4. Cyclic (black) and differential pulse (green) voltammograms of (a) [1]ClO4, (b) [2]ClO4, (c) [3](ClO4)2, (d) [4](ClO4)2, and (e) [5](ClO4)2 in CH3CN/0.1 M Et4NClO4 versus SCE; scan rate 100 mV s−1. Figure 3. Perspective views of the cationic parts of the compounds (a) meso-[3](ClO4)2 and (b) rac-[4](ClO4)2. Ellipsoids are drawn at 25% probability level. Hydrogen atoms (C−H) are removed for clarity.

In comparison with the earlier reported bpy/pap mixed dinuclear complex of doubly deprotonated 1,4-diamidoanthraquinone14 the system 5n from this study reveals invariable more positive oxidation and less negative reduction potentials. The same holds for corresponding compounds with bpy10 or pap ligands11 in the periphery. This difference, that is, the general anodic shift for the systems with H2Ln− relative to complexes

ligand H2L2− can be described as a bridge with coupled bis(βketiminato) chelating f unctions. D


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Inorganic Chemistry Table 2. Electrochemical Dataa E°298[V] (ΔE[mV])b redox states Ox2 Ox1 Red1 Red2 Red3 Red4 Red5 Kcc,d Kc1 Kc2 Kc3 Kc4 Kc5 Kc6

[1]ClO4

[2]ClO4

0.24(70) −0.89(90) −1.17(90) −1.53(70) −1.78(90)

0.79(70) −0.44(70) −1.00(70) −1.29(70) −1.59(130)

1.4 5.3 1.3 1.7

× × × ×

1019 104 106 104

5.8 2.9 7.9 1.1

× × × ×

[3](ClO4)2

[4](ClO4)2

[5](ClO4)2

0.39(90) 0.13(60) −0.89(90) −1.19(80) −1.52(90)

0.98(80) 0.73(90) −0.46(90) −1.04(90) −1.41(60)

0.93(80) 0.27(80) −0.42(60) −0.99(80) −1.22(180) −1.43(110) −1.67(140)

2.4 1.8 4.8 1.3

1020 109 104 105

× × × ×

104 1017 105 105

1.4 1.5 6.4 1.8

× × × ×

104 1020 109 106

1.5 5.9 5.5 7.1 3.2 1.1

× × × × × ×

1011 1011 109 103 103 104

From cyclic voltammetry in CH3CN/0.1 M Et4NClO4 at 100 mV s−1. bPotentials in volts versus SCE; peak potential differences ΔE [mV] (in parentheses). cComproportionation constant from RT ln Kc = nF(ΔE). dKc1 between Ox1 and Ox2, Kc2 between Ox1 and Red1, Kc3 between Red1 and Red2, Kc4 between Red2 and Red3, Kc5 between Red3 and Red4, Kc6 between Red4 and Red5. a

with twofold deprotonated 4-DAAQ reflects a more electronrich situation in the latter system with the para-positioned amide groups. As an important consequence of this anodic shift the compounds were isolated as diamagnetic unoxidized species such as [3](ClO4)2, whereas the corresponding [Ru(bpy)2]2 containing dinuclear system with deprotonated 4-DAAQ was obtained as the tris-perchlorate, that is, as the oxidized radical compound due to the very low (negative) oxidation potential of −0.06 V versus saturated calomel electrode (SCE).10 In comparison to mononuclear precursors (1n, 2n) the corresponding dinuclear analogues 3n and 4n exhibit only little shifted oxidation and reduction potentials, suggesting a rather weak electronic interaction of the two benzo-β-ketiminato chelates. This result is in agreement with the structural situation as noted above. In accordance with better π-accepting capacity, the papcontaining systems 2n and 4n exhibit distinctly higher potentials than the bpy-containing analogues 1n and 3n, respectively. The asymmetric system 5n with mixed bpy and pap coligation shows the pap-involving processes with very similar potentials as 4n; only the first oxidation, centered on bpy-coordinated ruthenium (see below), shows a considerably lower potential, closer to that of 3n. As a consequence, the comproportionation constant Kc for the one-electron oxidized intermediate 53+ is much larger (∼1 × 1011) than that of 33+ or 43+ (ca. 1 × 104), whereas the potential range for the isolated 52+ form is relatively small at 0.69 V (Kc = 5.9 × 1011; Table 2). However, the expected small highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap is not reflected by optical absorptions because of incompatible frontier MO characteristics as evident from EPR (see the following). Electron Paramagnetic Resonance. EPR information was obtained for the S = 1/2 species resulting from oneelectron oxidation or reduction of the diamagnetic precursor compounds. While hyperfine splitting could not be observed due to the relatively large line widths, the presence of a heavy metal with rather large spin−orbit coupling contribution affects the anisotropy of the g tensor as observable at low temperatures in glassy frozen solutions of electrogenerated intermediates. By that measure one can conveniently20 and fairly reliably21

estimate the amount of metal participation at the singly occupied molecular orbital (SOMO). Figures 5−7, S8, and S9 in the Supporting Information show the spectra with respective computer simulations, while the data

Figure 5. EPR spectra of [3](ClO4)2 after in situ oxidation (top, no signal observed at 298 K) and reduction (bottom) in CH2Cl2/0.1 M Bu4NPF6.

are listed in Table 3. The measurements were routinely performed in CH2Cl2/0.1 M Bu4NPF6 for better formation of glassy frozen solutions at low temperatures; redox potentials are comparable in dichloromethane and acetonitrile solutions.11 DFT-calculated spin densities based on geometry optimizations (Tables S12−S16 in the Supporting Information) are also provided here in Table 4 and depicted in Figures 8−11 and S10 in the Supporting Information. Starting with the mononuclear system 1n the oxidation of 1+ produces an isotropic single-line EPR signal at room temperature, which splits into three components in the glassy frozen state (Figure S8 in the Supporting Information). The isotropic g factor giso of 2.052 shows significant deviation from the free electron value of g(electron) = 2.0023 and from typical g factors of organic radicals;22 together with the sizable g anisotropy Δg = g1 − g3 = 0.248, this result signifies20,21 considerable E


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Inorganic Chemistry

Table 4. Density Functional Theory Calculated (UB3LYP/ LanL2DZ/6-31G*) Mulliken Spin Densities Ru1

2+

0.408

0.595

−0.003

−0.029

0.008

1.021

−0.065 0.055

0.682 0.972

1.387 −0.028

0.248

0.320

1.427

0.303

0.709

(S = 1 /2) 1 (S = 1 /2) 1− (S = 1) 12− (S = 1 /2) 13− (S = 1) 22+ (S = 1 /2) 2 (S = 1 /2) 2− (S = 1) 22− (S = 3 /2) 23− (S = 1) 34+ (S = 1) 33+ (S = 1 /2) 3+ (S = 1 /2) 3 (S = 1) 31− (S = 3 /2) 44+ (S = 1) 43+ (S = 1 /2) 4+ (S = 1 /2) 4 (S = 1) 4− (S = 3 /2) 54+ (S = 1) 53+ (S = 1 /2) 5+ (S = 1 /2) 5 (S = 1) 5− (S = 3 /2) 52− (S = 2) 53− (S = 3 /2) 1

Figure 6. EPR spectra of [4](ClO4)2 after in situ oxidation (top) and reduction (bottom, no signal observed at 298 K) in CH2Cl2/0.1 M Bu4NPF6.

Figure 7. EPR spectra of [5](ClO4)2 after in situ oxidation (top) and reduction (bottom, no signal observed at 298 K) in CH2Cl2/0.1 M Bu4NPF6.

Table 3. Electron Paramagnetic Resonance Data of Complexesa complex

T/K

g

12+

298 125 298 125 125 125 298 125 298 125 125 298 125 125

2.052

1 2 33+ 3+ 43+ 4+ 53+ 5+

g1

g2

g3

⟨g⟩b

Δgc

2.188

2.007

1.940

2.048

0.248

2.009 2.01 2.164

1.99 1.98 2.056

1.974 1.98 1.947

1.991 1.99 2.058

0.035 0.03 0.217

2.02

2.01

2.00

2.01

0.02

2.05 2.01

2.02 1.99

1.99 1.97

2.02 1.99

0.06 0.04

2.176 2.01

2.049 1.99

1.939 1.98

2.057 1.99

0.237 0.03

2.008

Ru2

H3L−/H2L2−

complex

bpy

pap

−0.011

−0.147

−0.005

1.152

0.253 0.434

−0.004 1.017

1.755 1.547

0.429

0.978

0.596

0.583

0.583

0.828

−0.008

0.238

0.238

−0.018

−0.009

−0.009

0.540 (α = 1.34, β = −0.80) 0.738

−0.032 −0.049

−0.032 −0.049

0.008 0.524

0.543

0.543

0.918

−0.001

0.183

0.179

0.675

−0.038

−0.060

−0.060

−0.009

1.132

−0.0149 0.135

−0.151 0.135

−0.014 0.002

2.310 2.724

0.282 2.058 2.574

0.523

0.594

0.879

0.004

−0.001

0.026

0.429

0.555

−0.005

−0.003

−0.144

−0.000

−0.002

0.000

1.147

−0.149 0.262

−0.030 −0.040

0.013 −0.001

1.007 1.036

1.163 1.737

0.322

0.059

0.113

1.829

1.671

0.030

0.157

0.648

1.849

0.311

this metal−ligand mixed-spin situation, yielding a 40:60% distribution for Ru and H2L (Table 4, Figure 8a). In contrast, the reduction of 1+ to 1 is calculated to take place exclusively at the bpy ligands (Table 4, Figure 8b) in confirmation of the EPR experiment, which shows a giso = 2.008 close to g(electron) and a very small Δg = 0.035 (Table 3, Figure S8 in the Supporting Information). The analogous 2+ ion is not reversibly oxidized but shows a reversible first reduction. Although the experimental (Table 3, Figure S9 in the Supporting Information) and computational results (Table 4, Figure S10 in the Supporting Information) confirm that this reduction takes place at the pap ligands (Δg = 0.03, g(average) = 1.99, spin density on pap), the EPR signal was not observed at room temperature, only at 125 K. As in previous studies23 we assume facile electron hopping between equivalent (degenerate) azo-ligand sites24 for spin accommo-

2.011 2.020

2.055

From in situ electrolysis in CH2Cl2/0.1 M Bu4NPF6. b⟨g⟩ = {(1/ 3)(g12 + g22 + g32)}1/2. cΔg = g1 − g3.

a

contributions from the heavy element Ru to the spin distribution. However, the observed giso and Δg values are still lower than those found for cases with predominantly metalbased spin (as in RuIII).20 DFT-calculated spin densities reflect F


Article

Inorganic Chemistry

Figure 8. Spin-density representations of (a) 12+ (S = 1/2), (b) 1 (S = 1/2), (c) 1− (S = 1), and (d) 12− (S = 1/2).

Figure 9. Spin-density representations of (a) 34+ (S = 1), (b) 33+ (S = 1/2), (c) 3+ (S = 1/2), (d) 3 (S = 1), and (e) 3− (S = 3/2).

Figure 10. Spin-density representations of (a) 44+ (S = 1), (b) 43+ (S = 1/2), (c) 4+ (S = 1/2), (d) 4 (S = 1), and (e) 4− (S = 3/2).

dation in 2 = [RuII(H2L2−)(pap•−) (pap)], which can result in severe EPR line broadening.25 Apparently, the barrier for such intramolecular electron transfer is very different in 1 and 2. The symmetrically dinuclear 32+ is oxidized to a (3+) ion with well-mixed dimetal−ligand spin. The division of spin density between both metals (1/4 each) and the redox-active bridge (1/2) is suggested by the spin-density calculations (Table 4, Figure 9b) and by the experiment (Δg = 0.217, g(average) = 2.058; Figure 5, Table 3). The absence of an EPR signal at 298 K is attributed to rapid relaxation and also indicates considerable spin on the metals. We therefore invoke a delocalized mixed-valent formulation (see Scheme 2) as significantly contributing to the ground state of 33+. The α and β spins (α = 1.34, β = −0.80, Table 4, Figure 9) on the bridge in 33+ reflect the effect of spin polarization as well.

In notable contrast to 33+, the 43+ ion with ruthenium(II) stabilizing pap ligands undergoes the oxidation of the (2+) precursor on the central bridge, as evident from rather low giso = 2.020 and the small parameter Δg = 0.06 (Figure 6, Table 3). DFT spin-density calculations do not quite reflect this stark contrast, although the trend to lesser spin on Ru and increased spin density on H2L is reproduced (Table 4, Figure 10b). Although both reductions to 3+ and 4+, respectively, are ligand-based, there are remarkable differences: As for compound 2 discussed above, the Ru(pap)2-containing system does not show an EPR signal at room temperature, attributed to strong line broadening due to intramolecular spin-hopping. The small g anisotropies Δg with g > 2 (3+) or g < 2 (4+), reflecting the different sites of reduction (Figures 9c and 10c and Table 4), 3+ has the extra electron added mainly to the G


Article

Inorganic Chemistry

Figure 11. Spin-density representations of (a) 54+ (S = 1), (b) 53+ (S = 1/2), (c) 5+ (S = 1/2), (d) 5 (S = 1), (e) 5− (S = 3/2), and (f) 52− (S = 2).

bridge, whereas the spin in 4+ resides at the superior π-acceptor pap (see Schemes 2 and 3). Like 32+ the asymmetric complex 52+ undergoes a metal− ligand mixed oxidation (Table 4, Figure 11b) to 53+ with corresponding spectral consequences (Δg > 0.2, giso = 2.055, Figure 7, Table 3). The asymmetry, however, produces a localized metal oxidation, occurring mainly at the more electron-rich metal, specifically, in the Ru(bpy)2 fragment. Such a situation corresponds to a Class II mixed-valency according to Robin and Day,26,27 which will be further analyzed below. Reduction of 52+ to 5+ is pap-centered, with corresponding EPR silence at room temperature and small g(average) and Δg values (Table 3). Table 4 contains information on spin densities in further oxidized or reduced states of the complexes, as long as they can be generated reversibly by optical absorption spectro-electrochemistry (see the following). Species with S > 1/2 could not be observed by the conventional EPR methods employed here. Nevertheless, the successful confirmation of the EPR-detected spin distribution by the calculated spin densities justifies a discussion of the predicted electron-transfer sequence, as summarized in Schemes 1−4. According to computed and experimental results the oxidation of complex cation 1+ leads to a mixed spin arrangement with comparable contributions from a RuIII and a H3L• ligand radical formulation (Scheme 1). The first reduction to 1 is localized at the bipyridine ligands (similarly at pap for 2), whereas the DFT calculation predicts the second

reduction for the anthraquinone bridge and the third reduction again at bpy. Deprotonated hydroxy- or amino-anthraquinones can undergo both two-step oxidations and reductions.28 Like 1+ the centrosymmetric dication 32+ undergoes a first oxidation to produce a mixed spin arrangement between two equivalent metals and the redox active bridge (Scheme 2). This Scheme 2. Oxidation State Formulations of Redox Series 3n

special situation29 is supported by experiment and theory. The second reversible oxidation is calculated to yield a triplet system involving both delocalized mixed-valent metals connected by an oxidized (radical anion) bridge. Such configurations have been detected by us previously albeit in singlet ground states.30 The first reduction of 32+ occurs at the bridge before further electron additions take place at the peripheral bpy coligands. In contrast to 33+ the 43+ analogue with RuII-stabilizing pap acceptor ligands is oxidized mainly at the bridge before the second oxidation affects the metals. This difference is especially reflected experimentally through the g-factor behavior. The reduction sequence involves successive electron uptake by the individual pap coligands (Scheme 3). As pointed out above, the first oxidation of asymmetric 52+ proceeds also in a mixed metal−ligand fashion, however, with only the electron-richer (bpy-coordinated) metal involved. This Class II mixed-valent situation will be analyzed further below using the classical Hush approach. As for 34+ and 44+ the 54+ ion is calculated as radical anion-bridged mixed-valent species.

Scheme 1. Oxidation State Formulations of Redox Series 1n and 2n

H


Article

Inorganic Chemistry Scheme 3. Oxidation State Formulations of Redox Series 4n

(LLCT) transitions between different donor and acceptor ligands, depending on and changing with the oxidation states. The radical forms of bpy33 and pap34 alone as well as the various implied oxidation states of H2Ln− are also expected to produce characteristic absorption bands in the visible or nearinfrared (NIR) region (intraligand (IL) transitions) because of transitions to or from the SOMO.35 Because of the large number of expected transitions (Tables 5 and 6 and Tables S17−S19 in the Supporting Information) in the long-wavelength (NIR and vis) regions and the corresponding band overlap in the absorption spectra we restrict the discussion to selected features. The precursor compounds exhibit long-wavelength transitions between the frontier orbitals as ascertained by EPR characterization of the one-electron oxidized and reduced forms. In the more interesting series of dinuclear systems, the 32+ ion displays an absorption around 800 nm due to excitation from a metal/bridge-mixed HOMO to a bridge-centered LUMO (MLCT, IL transition; Table S18 and Figure S12 in the Supporting Information). At longer wavelengths (around 950 nm) the system 42+ shows a different frontier orbital transition from a bridge-centered HOMO to a π*(pap)centered LUMO (LLCT, Table S19 in the Supporting Information). The asymmetric 52+ exhibits a less well-defined shoulder around 1000 nm (Figure 13); the corresponding MLCT/LLCT transitions above 1000 nm are calculated to have very low intensities (Table 6). The mononuclear precursor compounds show corresponding MLCT/LLCT (1+) and LLCT (2+) absorptions at slightly higher energies (Table 5 and Table S17 in the Supporting Information). For most of the electrogenerated intermediates NIR absorptions were observed, caused by transitions between metal/ligand-mixed frontier orbitals (12+, 2, 34+, 43+, 44+, 5−), by IL transitions (1−, 4−, 5), or by LLCT processes (3+, 4+, 4, 54+). The long wavelength absorptions of 33+ and 53+ are of special interest because of the potential for ruthenium-based mixed valency and the resulting metal-to-metal charge transfer (MMCT) transitions. Molecule-bridged diruthenium compounds have had a long and extensive history of investigations regarding their mixed-valent behavior.20b,36−38 Although MMCT (or intervalence charge transfer, IVCT) bands often occur in the NIR region, it should be pointed out that coordination compounds of radical ligands can also absorb in the NIR due to transitions involving the SOMO,35 requiring supporting experimental information (EPR) or theoretical assignment (TD-DFT) to decide between these alternatives. The symmetric 33+ species, characterized by EPR with ∼50% metal contribution to the SOMO and calculated to have a delocalized electronic structure (Figure 9b), exhibits only a less well-defined shoulder for the MMCT/MLCT absorption around 1200 nm (TD-DFT: 1271 nm; Table S18 and Figure S12 in the Supporting Information). In contrast, the asymmetric 53+ shows a separate broad NIR band at λmax = 1740 nm with εmax = 900 M−1 cm−1 and a bandwidth at half height, Δν1/2, of ∼3100 cm−1. Comparing the latter value with the 3675 cm−1 calculated from the Hush formula (Δν1/2 = (2310νmax)1/2) for weakly coupled mixed-valent systems36,37,39 reveals a reasonable agreement with a typically smaller experimental value, confirming a Class II situation with localized valences due to “chemical” asymmetry but still possible metal−metal interaction. Note that this Class II case also involves a sizable contribution, ∼50%, from the potentially radical bridging ligand to the SOMO (Table 4 and Figure

Stepwise reduction of 52+ takes place first on pap then bpy, then on the second pap coligand (Scheme 4). Scheme 4. Oxidation State Formulations of Redox Series 5n

Ultraviolet−Visible−Near-Infrared Spectro-Electrochemistry and Time-Dependent Density Functional Theory Calculations. Using an optically transparent thinlayer electrolysis (OTTLE) cell,31 we investigated the absorption changes of the precursor compounds in acetonitrile solution. Some seemingly reversible redox processes in cyclic voltammetry or DPV did not produce clean spectro-electrochemical response, either due to reactivity involving labile hydrogen from NH2 groups (e.g., oxidation and second reduction of 2+) or because of adsorption at the platinum electrode (second reduction of 32+). The spectra are shown in Figures 12, 13, and S11−S13 in the Supporting Information, and the absorption data are summarized together with results from TD-DFT calculations (including assignments) in Tables 5 and 6, Tables S17−S51, and Figures S14−S18 in the Supporting Information. A discussion of the most conspicuous absorption features must include the recognition of the expected metal-to-ligand charge transfer (MLCT) transitions dπ(Ru) → π* to the various ligands9,32 and the ligand-to-ligand charge transfer I


Article

Inorganic Chemistry

Figure 12. UV−vis−NIR spectro-electrochemistry of 1n in CH3CN/0.1 M Bu4NPF6.

Figure 13. UV−vis−NIR spectro-electrochemistry of 5n in CH3CN/0.1 M Bu4NPF6.

11b)a rather uncommon situation giving rise to a mixed MMCT/IL character of the NIR transition. Because of its asymmetry this example 53+ has a large comproportionation constant Kc = 1 × 1011 but reveals rather weak metal−metal interaction (low absorption energy and

intensity), the broad band suggesting extensive structural reorganization.

■

CONCLUSION AND OUTLOOK The hitherto untapped coordination chemistry of ligands based on deprotonated forms of 1,5-diamino-9,10-anthraquinone H4L J


Article

Inorganic Chemistry Table 5. Experimental and TD-DFT ((U)B3LYP/CPCM/CH3CN) Calculated Electronic Transitions for 1n λmax, nm (expt) (ε, M−1 cm−1) 800(3000 br) 567(15 750)

λ, nm (DFT) ( f) 728(0.07) 632(0.02) 517(0.25)

480(14 900) 420 sh 360(9700)

454(0.14) 406(0.03) 344(0.05) 320(0.03)

295(48 800)

271(0.31)

1450(1300)

1212(0.01)

700−900 sh,br

924(0.01) 756(0.13)

573 sh 500(12 100) 448(10 770)

676(0.04) 519(0.15) 480(0.01)

400(11 970)

401(0.10) 362(0.03)

284(39 400)

279(0.09)

990 sh 820(7280) 740(8350) 560(18 430) 509(15 650) 384(11 520) 290 (44 400)

947(0.01) 705(0.07)

1700 br

472(0.12)

281(0.38)

583(16 800)

2175(0.02) 1005(0.01) 890(0.02) 844(0.01) 745(0.02) 676(0.03)

493(25 200) 400 sh

510(0.16) 457(0.06)

290(43 500)

350(0.05) 295(0.12)

830 sh

transitions 1+ (S = 0) HOMO → LUMO(0.69) HOMO → LUMO+2(0.51) HOMO−2 → LUMO(0.57) HOMO−1 → LUMO(0.27) HOMO−3 → LUMO(0.59) HOMO−4 → LUMO(0.64) HOMO−1 → LUMO+5(0.42) HOMO−1 → LUMO+7(0.29) HOMO−2 → LUMO+5(0.29) HOMO−4 → LUMO+5(0.42) HOMO−8 → LUMO+1(0.32) 12+ (S = 1/2) HOMO−3(β) → LUMO(β)(0.69) HOMO(β) → LUMO(β)(0.62) HOMO(β) → LUMO(β)(0.64) HOMO−2(β) → LUMO(β)(0.54) SOMO → LUMO(α)(0.50) HOMO−2(β) → LUMO(β)(0.59) HOMO(β) → LUMO+1(β)(0.75) HOMO−7(β) → LUMO(β)(0.61) HOMO−3(α) → LUMO(α)(0.33) HOMO−3(α) → LUMO(α)(0.52) HOMO−3(β) → LUMO+1(β)(0.45) HOMO−9(β) → LUMO(β)(0.32) HOMO−2(β) → LUMO+2(β)(0.30) HOMO−16(β) → LUMO(β)(0.47) 1 (S = 1/2) HOMO−1(α) → LUMO(α)(0.81) SOMO → LUMO+6(α)(0.65) HOMO(β) → LUMO+2(β)(0.57) HOMO−1(β) → LUMO+2(β)(0.53) HOMO−4(α) → LUMO+1(α)(0.34) HOMO−6(α) → LUMO+2(α)(0.68) 1− (S = 1) SOMO2 → LUMO(α)(0.99) HOMO−2(α) → LUMO(α)(0.90) HOMO(β) → LUMO(β)(0.90) SOMO1 → LUMO+4(α)(0.78) HOMO(β) → LUMO+2(β)(0.83) SOMO2 → LUMO+5(α)(0.64) SOMO2 → LUMO+1(α)(0.51) SOMO2 → LUMO+6(α)(0.68) HOMO−3(β) → LUMO(β)(0.62) HOMO−2(α) → LUMO+1(α)(0.29) HOMO−1(α) → LUMO+6(α)(0.78) HOMO−5(α) → LUMO+2(α)(0.33)

character H3L(π)/Ru(dπ) → bpy(π*) H3L(π)/Ru(dπ) → H3L(π*) Ru(dπ) → bpy(π*) H3L(π) → bpy(π*) Ru(dπ)/H3L(π) → bpy(π*) Ru(dπ)/H3L(π) → bpy(π*) H3L(π) → bpy(π*) H3L(π) → H3L(π*) Ru(dπ) → bpy(π*) Ru(dπ)/H3L(π) → bpy(π*) bpy(π) → bpy(π*) Ru(dπ)/H3L(π) → H3L(π*)/Ru(dπ) H3L(π) → H3L(π*)/Ru(dπ) H3L(π) → H3L(π*)/Ru(dπ) Ru(dπ)/H3L(π) → H3L(π*)/Ru(dπ) H3L(π) → H3L(π*) Ru(dπ)/H3L(π) → H3L(π*)/Ru(dπ) H3L(π) → H3L(π*) bpy(π) → H3L(π*)/Ru(dπ) H3L(π)/Ru(dπ) → H3L(π*) H3L(π)/Ru(dπ) → H3L(π*) Ru(dπ)/H3L(π) → H3L(π*) H3L(π)/bpy(π) → H3L(π*)/Ru(dπ) Ru(dπ)/H3L(π) → bpy(π*) bpy(π)/H3L(π) → H3L(π*)/Ru(dπ) H3L(π)/Ru(dπ) → H3L(π*)/bpy(π*) bpy(π) → H3L(π*) H3L(π)/Ru(dπ) → bpy(π*) Ru(dπ) → bpy(π*) H3L(π)/Ru(dπ) → bpy(π*) H3L(π) → bpy(π*) bpy(π) → bpy(π*)/H3L(π*) H3L(π)/Ru(dπ) → bpy(π*)/H3L(π*) H3L(π)/Ru(dπ) → H3L(π*) H3L(π)/bpy(π) → bpy(π*) H3L(π)/Ru(dπ) → bpy(π*) bpy(π) → H3L(π*) bpy(π) → bpy(π*) bpy(π) → H3L(π*) H3L(π)/Ru(dπ) → H3L(π*) H3L(π)/Ru(dπ) → bpy(π*) Ru(dπ) → bpy(π*) H3L(π)/Ru(dπ) → bpy(π*)

(spectro)electrochemistry (CV, DPV, EPR, UV−vis−NIR). Multistep redox series were obtained and thus analyzed, with different electron transfer sequences showing the Ru II stabilizing effect of the strongly π-accepting pap coligands. Accordingly, the reductions occur preferentially at pap, otherwise at bpy or at the tricyclic system, to yield the radical trianion ligand H2L•3−. Oxidations can also take place at the H2L2− ligand or at the metal(s), leading to the radical anion H2L•− and/or to mixed-valent situations in the dinuclear species with their Ru−Ru distance of ca. 8.8 Å. The results from structure analysis (long intraquinone C−C bonds), electro-

has been probed in this article by using two prototypical redoxactive metal complex fragments, [Ru(bpy)2]n and [Ru(pap)2]n. Like other anthraquinone derivatives,9−12 the deprotonated forms of the ligand can undergo multiple electron transfer, rendering, for example, H2L2− as a noninnocently40 behaving ligand. Structurally, the 1,5-substitution in this ligand produces a centrosymmetric bis-chelate bridging function, creating two connected equivalent β-ketiminate sites (which are rarely employed16 chelate functions). Mononuclear as well as symmetric and asymmetric dinuclear compounds were obtained and characterized structurally and by K


Article

Inorganic Chemistry Table 6. Experimental and TD-DFT ((U)B3LYP/CPCM/CH3CN) Calculated Electronic Transitions for 5n λmax, nm (expt) (ε, M−1 cm−1)

980 sh

680 sh

λ, nm (DFT) ( f) 1151(0.01) 1019(0.013) 944(0.034) 878(0.011) 782(0.09) 622(0.085) 584(0.024)

568(20 300) 470 sh

507(0.095) 488(0.05)

352(32 850)

426(0.07) 366(0.25)

1740(900)

960 sh

1935(0.014) 1416(0.003) 1166(0.014) 992(0.043) 868(0.035)

780 sh

818(0.02)

695(11 690)

741(0.2) 687(0.086) 591(0.012) 508(0.04) 478(0.08) 366(0.15)

544(18 000) 360 sh

1700 br

878(12 400)

1268(0.017) 1013(0.05) 831(0.29) 762(0.038)

590 sh 500(15 600) 373(29 550)

830 sh

737(0.027) 686(0.007) 621(0.052) 519(0.027) 468(0.11) 443(0.047) 399(0.112)

1099(0.01) 756(0.142) 683(0.056)

583(21 100)

565(0.042) 509(0.16)

354(29 700)

344(0.054)

984(9800)

900(0.043)

transitions 52+ (S = 0) HOMO → LUMO(0.67) HOMO → LUMO+1(0.58) HOMO−1 → LUMO(0.61) HOMO−1 → LUMO+1(0.60) HOMO → LUMO+2(0.68) HOMO−1 → LUMO+2(0.54) HOMO−3 → LUMO(0.43) HOMO−5 → LUMO(0.34) HOMO−3 → LUMO+2(0.51) HOMO−2 → LUMO+3(0.46) HOMO−3 → LUMO+3(0.23) HOMO−4 → LUMO+2(0.39) HOMO−7 → LUMO+2(0.37) 53+ (S = 1/2) HOMO(β) → LUMO(β) (0.83) HOMO−2(β) → LUMO(β) (0.61) HOMO−3(β) → LUMO(β) (0.58) HOMO−4(β) → LUMO(β) (0.50) HOMO−6(β) → LUMO(β)(0.50) HOMO−4(β) → LUMO(β) (0.47) SOMO → LUMO(α) (0.66) HOMO(β) → LUMO+1(β) (0.60) SOMO → LUMO+2(α) (0.65) HOMO(β) → LUMO+1(β) (0.61) HOMO−1(β) → LUMO(β) (0.64) HOMO(β) → LUMO+3(β) (0.74) HOMO−1(β) → LUMO+1(β) (0.60) HOMO−3(α) → LUMO(α) (0.50) HOMO−3(α) → LUMO+1(α) (0.56) SOMO → LUMO+8(α) (0.48) HOMO−6(α) → LUMO+2(α) (0.46) 54+ (S = 1) HOMO−2(β) → LUMO+1(β) (0.65) HOMO−3(β) → LUMO(β) (0.52) HOMO(β) → LUMO(β) (0.40) HOMO−5(β) → LUMO(β) (0.47) HOMO(β) → LUMO(β) (0.47) HOMO−2(β) → LUMO(β) (0.49) HOMO−1(β) → LUMO+1(β) (0.46) HOMO−1(β) → LUMO(β) (0.64) HOMO−4(β)→ LUMO(β)(60) HOMO−2(β) → LUMO(β) (0.65) HOMO−12(β) → LUMO(β) (0.77) HOMO−5(α) → LUMO(α) (0.40) HOMO−11(β) → LUMO+1(β) (0.40) HOMO−2(β) → LUMO+3(β) (0.32) HOMO−3(β) → LUMO+4(β) (0.28) 5+ (S = 1/2) HOMO−2(α) → LUMO(α) (0.71) HOMO(β) → LUMO(β) (0.59) HOMO(β) → LUMO+2(β) (0.52) HOMO−2(α) → LUMO+1(α) (0.56) HOMO−1(β) → LUMO(β) (0.53) HOMO−4(β) → LUMO+1(β) (0.82) HOMO−3(β) → LUMO(β) (0.47) HOMO−4(α) → LUMO+1(α) (0.42) HOMO−3(α) → LUMO+6(α)(0.38) HOMO−2(β) → LUMO+7(β)(0.38) 5(S = 1) HOMO(β) → LUMO (β)(0.59) L

character H2L(π)/Ru(dπ) → pap(π*) H2L(π)/Ru2(dπ) → pap(π*) H2L(π) → pap(π*) H2L(π) → pap(π*) H2L(π)/Ru(dπ) → H2L(π*) H2L(π) → H2L(π*) Ru(dπ) → pap(π*) Ru(dπ)/H2L(π) → pap(π*) Ru(dπ) → H2L(π*) H2L(π)/Ru(dπ) → H2L(π*) Ru(dπ) → bpy(π*) Ru(dπ)/pap(π) → H2L(π*) pap(π)/H2L(π)/ → H2L(π*) H2L(π)/Ru(dπ) → H2L(π*)/Ru(dπ) pap(π) → H2L(π*)/Ru(dπ) pap(π) → H2L(π*)/Ru(dπ) Ru(dπ) → H2L(π*)/Ru(dπ) H2L(π)/Ru(dπ) → H2L(π*)/Ru(dπ) Ru(dπ) → H2L(π*)/Ru(dπ) H2L(π)/Ru(dπ) → pap(π*) H2L(π)/Ru(dπ) → pap(π*) H2L(π)/Ru(dπ) → pap(π*) H2L(π)/Ru(dπ) → H2L(π*) Ru(dπ)/pap(π) → H2L(π*)/Ru(dπ) H2L(π)/Ru(dπ) → pap(π*) Ru(dπ)/pap(π) → H2L(π*) pap(π)/H2L(π) → H2L(π*) pap(π)/H2L(π) → pap(π*) H2L(π)/Ru(dπ) → bpy(π*) pap(π)/Ru(dπ) → pap(π*) pap(π) → H3L(π*)/Ru(dπ) pap(π) → H3L(π*)/Ru(dπ) pap(π) → H2L(π*)/Ru(dπ) Ru(dπ) → H2L(π*)/Ru(dπ) pap(π) → H2L(π*)/Ru(dπ) pap(π) → H2L(π*)/Ru(dπ) pap(π) → Ru(dπ)/H2L(π*) pap(π) → H2L(π*)/Ru(dπ) Ru(dπ)/pap(π)/H2L(π) → H2L(π*)/Ru(dπ) pap(π) → H2L(π*)/Ru(dπ) H2L(π) → H2L(π*)/Ru(dπ) H2L(π) → H2L(π*) Ru(dπ)/H2L(π)/bpy(π) → H2L(π*)/Ru(dπ) pap(π) → pap(π*) pap(π) → pap(π*) H2L(π)/Ru(dπ) → bpy(π*) H2L(π)/Ru(dπ) → bpy(π*) H2L(π)/Ru(dπ) → H2L(π*) H2L(π)/Ru(dπ) → bpy(π*) H2L(π)/Ru(dπ) → bpy(π*) pap(π)/H2L(π) → bpy(π*) Ru(dπ)/pap(π) → bpy(π*) Ru(dπ) → bpy(π*) Ru(dπ) → bpy(π*) Ru(dπ)/pap(π) → bpy(π*) H2L(π)/Ru(dπ) → H2L(π*) DOI: 10.1021/acs.inorgchem.6b00726 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 6. continued λmax, nm (expt) (ε, M−1 cm−1)

λ, nm (DFT) ( f) 806(0.015)

568(14 750)

684(0.079) 608(0.036) 507 (0.038) 482(0.065)

357(39 000)

357(0.081)

1030(12 050)

992(0.014) 954(0.012) 899(0.03)

805(11 300) 722(11 820) 654(12 820) 564(19 000) 359(40 300)

775(0.086) 680(0.043) 665(0.021) 382(0.15)

transitions HOMO−2(α) → LUMO+1(α)(0.53) HOMO−2(β) → LUMO(β)(0.51) HOMO−3(α) → LUMO (α)(0.45) HOMO(β) → LUMO+4(β)(0.65) SOMO1 → LUMO+7(α) (0.64) HOMO−4(α) → LUMO+2(α) (0.45) HOMO−7(α) → LUMO(α) (0.37) HOMO−7(α) → LUMO+1(α) (0.52) HOMO−6 (β) → LUMO+2(β) (0.40) HOMO−2(β) → LUMO+9(β) (0.48) HOMO−1(α) → LUMO(α) (0.35) 5−(S = 3/2) HOMO(β) → LUMO+1(β) (0.66) HOMO−4(α) → LUMO(α) (0.53) HOMO(β) → LUMO+4(β) (0.66) HOMO(β) → LUMO+2(β) (0.68) HOMO(β) → LUMO+4(β) (0.67) HOMO−4(α) → LUMO(α) (0.60) SOMO1 → LUMO+7(α) (0.57) HOMO−6(α) → LUMO(α) (0.62) HOMO−3(β) → LUMO+2(β) (0.42) HOMO−6(β) → LUMO+2(β) (0.44) HOMO−8(β) → LUMO+2(β) (0.36)

Ru(dπ)/H2L(π) → bpy(π*) H2L(π)/Ru(dπ) → bpy(π*) Ru(dπ)/H2L(π) → pap(π*) Ru(dπ)/H2L(π) → bpy(π*) Ru(dπ)/H2L(π) → pap(π*) H2L(π)/Ru(dπ) → bpy(π*) pap(π)/Ru(dπ) → pap(π*) Ru(dπ)/pap(π) → bpy (π*) Ru(dπ) → bpy (π*) Ru(dπ)/H2L(π) → bpy (π*) pap(π) → bpy (π*)

photometer. All spectro-electrochemical experiments were performed under a dinitrogen atmosphere. 1H NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer. The elemental analyses were recorded on a PerkinElmer 240C elemental analyzer. FTIR spectra were recorded on a Nicolet spectro-photometer; the samples were prepared as KBr pellets. Electrospray mass spectral measurements were done on a Micromass Q-ToF mass spectrometer. Preparation of Complexes. Synthesis of [Ru(H3L)(bpy)2]ClO4 ([1]ClO4) and [(bpy)2Ru(μ-H2L)Ru(bpy)2](ClO4)2. A mixture of cisRu(bpy)2Cl2 (100 mg, 0.21 mmol) and AgClO4 (87.07 mg, 0.42 mmol) was refluxed for 2 h in EtOH. The precipitated AgCl was filtered off, and the filtrate was treated with H4L (50 mg, 0.21 mmol) and NEt3 (0.03 mL, 0.21 mmol). This mixture was refluxed for 7 h under a dinitrogen atmosphere. The solvent was removed, and the residue was moistened with a few drops of CH3CN, followed by the addition of a saturated aqueous solution of NaClO4. After it was stored at 273 K overnight, the precipitate was filtered off, washed with chilled water to remove excess NaClO4, and dried in vacuo over P4O10. It was purified using a neutral alumina column. A purple product [1]ClO4 was eluted with a 12:1 CH2Cl2−CH3CN mixture, followed by the blue [3](ClO4)2 being eluted with a 7:1 CH2Cl2−CH3CN mixture. Evaporation of the solvent afforded the pure compounds. [1]ClO4. Yield, 61 mg (40%). MS (ESI+, CH3CN): m/z {1}+ calcd: 651.11; found: 651.10. 1H NMR (400 MHz) in (CD3)2SO [δ/ppm(J/ Hz)]: 8.8 (m, 4H), 8.65 (d, 8, 1H), 8.4 (d, 4, 1H), 8.3 (s, 1H), 8.13 (m, 2H), 8.02 (t, 8, 1H), 7.7 (m, 6H), 7.38 (t, 7, 1H), 7.25 (t, 6,1H), 7.13 (t, 8, 1H), 6.9 (m, 4H), 6.55(d, 8, 1H). Anal. Calcd for C34H25ClN6O6Ru: C, 54.44; H, 3.36; N, 11.20; found: C, 54.76; H, 3.17; N, 10.99%. IR (cm−1 KBr pellet): 1093, 622 (v(ClO4) and 1623 (v(CO)). Molar conductivity (CH3CN): ΛM = 105 Ω−1 cm2 M−1. [3](ClO4)2. Yield, 55 mg (42%). MS (ESI+, CH3CN): m/z {[3](ClO4)}+ calcd: 1162.12; found: 1162.28. 1H NMR (400 MHz) in (CD3)2SO [δ/ppm(J/Hz)]: 8.8 (d, 8, 1H), 8.75 (m, 7H), 8.61 (m, 2H), 8.43 (d, 8, 1H), 8.34 (d, 8, 1H), 8.12 (m, 4H), 7.97 (t, 10, 4H), 7.83 (t, 8, 2H), 7.65 (m, 8H), 7.35 (t, 8, 2H), 7.18 (t, 8, 2H), 6.76 (m, 2H), 6.54 (m, 2H), 6.21 (d, 8,1H), 6.18 (d, 8, 1H). Anal. Calcd for C54H40Cl2N8O10Ru2: C, 51.39; H, 3.19; N, 11.10; found: C, 51.67; H, 2.99; N, 11.30%. IR (cm−1 KBr pellet): 1097, 620 (v(ClO4). Molar conductivity (CH3CN): ΛM = 202 Ω−1 cm2 M−1.

chemistry, and spectro-electrochemistry suggest a relatively weak interaction between the coordination sites despite the metal binding through the conjugated O(carbonyl) atoms of central quinonoid ring. The formation of stable β-ketiminato six-membered ring chelates on either site of the centrosymmetric arrangement seems to prevent a more pronounced metal−metal communication. Nevertheless, the potential of this remarkable ligand system for coordination, metal−metal bridging, multiple electron transfer, and further noncovalent interaction41 is obvious. It creates an exciting perspective for more studies of structural assembly and electronic structure (such as magnetism) with other metals, regardless whether redox-active or redox-inert, given the established noninnocence of the ligand.

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character H2L(π)/Ru(dπ) → H2L(π*) Ru(dπ) → H2L(π*) H2L(π)/Ru(dπ) → pap(π*) H2L(π)/Ru(dπ) → bpy(π*) bpy(π) → pap(π*) Ru(dπ) → bpy(π*) Ru(dπ) → pap(π*) Ru(dπ) → pap(π*) Ru(dπ)/H2L (π) → pap(π*) Ru(dπ)/pap(π) → H2L(π*) Ru(dπ) → pap(π*)

EXPERIMENTAL SECTION

Materials. The precursor complexes cis-Ru(bpy)2Cl242 and ctcRu(pap)2Cl243 were prepared according to the literature procedures. The ligand 1,5-diamino-9,10-anthraquinone (H4L) was purchased from Alfa Aesar. Other chemicals and solvents were of reagent grade and used as received. For spectroscopic and electrochemical studies HPLC grade solvents were used. Physical Measurements. The electrical conductivity of the solution was checked by using an autoranging conductivity meter (Toshcon Industries, India). The EPR measurements were made in a two-electrode capillary tube24a with an X-band (9.5 GHz) Bruker system ESP300 spectrometer. Cyclic voltammetric and differential pulse voltammetric measurements of the complexes in the isolated native state were done using a PAR model 273A electrochemistry system. Glassy carbon working electrode, platinum wire auxiliary electrode, and SCE were used in a standard three-electrode configuration with tetraethylammonium perchlorate (TEAP) as the supporting electrolyte (substrate concentration ≈ 1 × 10−3 M; standard scan rate 100 mV s−1). (Caution! Perchlorate salts are explosive and should be handled with care.) UV−vis−NIR spectro-electrochemical studies were performed in CH3CN/0.1 M Bu4NPF6 at 298 K using an optically transparent thin-layer electrode (OTTLE) cell31 mounted in the sample compartment of a J&M TIDAS spectroM


Article

Inorganic Chemistry Synthesis of [Ru(H3L)(pap)2]ClO4 ([2]ClO4) and [(pap)2Ru(μH2L)Ru(pap)2](ClO4)2 [4](ClO4)2. The precursor complex cis−trans− cis-Ru(pap)2Cl2 (100 mg, 0.19 mmol) and AgClO4 (79.04 mg, 0.38 mmol) were taken in EtOH and heated to reflux for 2 h. The precipitated AgCl was filtered off, and the ligand H4L (44 mg, 0.19 mmol) and NEt3 base (0.03 mL, 0.21 mmol) were added to the filtrate. The mixture was refluxed for 8 h, and then the solvent was removed under reduced pressure. The residue was moistened with a few drops of CH3CN, and a saturated aqueous solution of NaClO4 was added. This was then allowed to cool overnight at 273 K. The precipitated solid was washed with chilled water to remove excess NaClO4 and dried in vacuo over P4O10. It was purified using a neutral alumina column. The violet and purple products corresponding to [2]ClO4 and [4](ClO4)2 were eluted with 14:1 CH2Cl2−CH3CN and 10:1 CH2Cl2−CH3CN solvent mixtures, respectively. Evaporation of the solvent under reduced pressure afforded pure compounds. [2]ClO4. Yield, 68 mg (46%). MS (ESI+, CH3CN): m/z {2}+ calcd: 705.13; found: 705.20. 1H NMR (400 MHz) in (CD3)2SO [δ/ppm (J/Hz)]: 8.9 (d, 8, 1H), 8.79 (d, 8, 1H), 8.4(s, 1H), 8.25 (m, 3H), 8.18 (d, 5,1H), 7.9 (d, 6, 1H), 7.56 (t, 6, 2H), 7.45 (m, 3H), 7.3 (m, 4H), 7.18 (d, 6, 1H), 7.05 (m, 4H), 6.98 (d, 8, 1H), 6.9 (d, 6, 2H). Anal. Calcd for C36H40ClN8O4Ru: C, 53.73; H, 3.38; N, 13.93; found: C, 53.78; H, 3.17; N, 14.08%. IR (cm−1 KBr pellet): 1092, 621 (v(ClO4) and 1629 (v(CO)). Molar conductivity (CH3CN): ΛM = 108 Ω−1 cm2 M−1. [4](ClO4)2. Yield, 54 mg (43%). MS (ESI+, CH3CN): m/z {[4](ClO4)}+ calcd: 1270.67; found: 1270.52. 1H NMR (400 MHz) in (CD3)2SO [δ/ppm (J/Hz)]: 8.89 (t, 6.8, 2H), 8.74 (t, 6.8, 2H), 8.35 (d, 6, 2H), 8.23 (m, 4H), 8.08 (d, 5.6, 1H), 8.03 (d, 4, 1H), 7.81 (d, 4, 1H), 7.74 (d, 5, 1H), 7.45 (m, 8H), 7.26 (t, 7, 8H), 7.06 (m, 4H), 6.98 (d, 8, 4H), 6.87 (d, 6, 2H), 6.81 (d, 7.6, 4H),. Anal. Calcd for C58H44Cl2N14O10Ru2: C, 57.26; H, 4.01; N, 15.91; found: C, 57.53; H, 4.06; N, 15.58%. IR (cm−1 KBr pellet): 1096, 62 (v(ClO4). Molar conductivity (CH3CN): ΛM = 221 Ω−1 cm2 M−1. Synthesis of [(bpy)2Ru(μ-H2L)Ru(pap)2](ClO4)2 [5](ClO4)2. The starting complex cis−trans−cis-Ru(pap)2Cl2 (100 mg, 0.19 mmol) and AgClO4 (79.04 mg, 0.38 mmol) were taken in EtOH and refluxed for 2 h. The precipitated AgCl was filtered off, and the filtrate was treated with [Ru(H3L)(bpy)2]ClO4 (143 mg, 0.19 mmol) and NEt3 (0.03 mL, 0.21 mmol) and heated to reflux for 8 h. The solvent was removed, the residue was dissolved in a minimum volume of CH3CN, and a saturated aqueous solution of NaClO4 was added. After it was kept overnight at 273 K, the precipitate thus obtained was filtered, washed with chilled water to remove excess NaClO4, and dried in vacuo over P4O10. The product was purified using a neutral alumina column, which yielded the purple complex [5](ClO4)2 by a 12:1 CH2Cl2−CH3CN mixture. Yield, 135 mg (54%). MS (ESI+, CH3CN): m/z {[5](ClO4)}+ calcd: 1270.67; found: 1270.52. 1H NMR (400 MHz) in (CD3)2SO [δ/ppm (J/Hz)]: 8.9 (m, 1H), 8.76 (m, 5H), 8.62 (d, 8, 1H), 8.35 (m, 2H), 8.24 (m, 2H), 8.15 (m, 3H), 7.99 (m, 2H), 7.85 (m, 2H), 7.68 (m, 5H), 7.51 (t, 6, 1H), 7.36 (m, 7H), 7.2 (t, 6, 1H), 7.06 (t, 7, 1H), 6.98 (d, 8, 2H), 6.81 (m, 5H), 6.71 (d, 8, 1H), 6.39 (m, 1H). Anal. Calcd for C56H42Cl2N12O10Ru2: C, 50.80; H, 3.24; N, 14.31; found: C, 50.94; H, 3.04; N, 14.50%. IR (cm−1 KBr pellet): 1096, 620 (v(ClO4). Molar conductivity (CH3CN): ΛM = 196 Ω−1 cm2 M−1. Caution! Perchlorate salts are explosive and should be handled with care. Crystal Structure Determination. Single crystals were grown by slow evaporation of 1:1 solutions of CH3CN−toluene for [1]ClO4/ [2]ClO4 and CH3OH−toluene for [3](ClO4)2/[4](ClO4)2. X-ray diffraction data were collected using a Rigaku Saturn-724+ CCD single-crystal diffractometer using Mo Kα radiation at 100(2) K. The data collection was evaluated using the CrystalClear-SM Expert software. The data were collected by the standard ω-scan technique. The structure was solved by direct methods using SHELXS-97 and refined by full matrix least-squares with SHELXL-97, refining on F2.44 All data were corrected for Lorentz and polarization effects, and all non-hydrogen atoms were refined anisotropically. The remaining hydrogen atoms were placed in geometrically constrained positions

and refined with isotropic temperature factors, generally 1.2Ueq of their parent atoms. Hydrogen atoms were included in the refinement process as per the riding model. SQUEEZE was applied for the disordered unidentified solvent molecules in the crystals of [2]ClO4, [3](ClO4)2, and [4](ClO4)2. Additional crystallographic information is available in the Supporting Information. Computational Details. Full geometry optimizations were performed using the DFT method at the (U)B3LYP level for 1n (n = +2, 0, −1, −2, −3), 2n (n = +2, 0, −1, −2, −3), 3n (n = +4, +3, +1, 0, −1), 4n (n = +4, +3, +1, 0, −1), 5n (n = +4, +3, +1, 0, −1, −2, −3) and (R)B3LYP for 1n (n = +), 2n (n = +), 3n (n = +2), 4n (n = +2), and 5n (n = +2).45 All elements except ruthenium were assigned the 6-31G(d) basis set. The LanL2DZ basis set with effective core potential was employed for the ruthenium atom.46 All calculations were performed with the Gaussian09 program package.47 Vertical electronic excitations based on (U)B3LYP optimized geometries were computed using the TD-DFT formalism48 in acetonitrile using the conductor-like polarizable continuum model.49 Chemissian 1.750 was used to calculate the fractional contributions of various groups to each molecular orbital. All calculated structures were visualized with ChemCraft.51 Electronic spectra were calculated using the SWizard program.52,53

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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.6b00726. CCDC-1470046 ([1]ClO4·CH3CN), CCDC1470047 ([2]ClO4), CCDC-1470048 ([3](ClO4)2), and CCDC-1470049 ([4](ClO4)2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Crystallographic data for [1]ClO4. (CIF) Crystallographic data for [2]ClO4. (CIF) Crystallographic data for [3](ClO4)2. (CIF) Crystallographic data for [4](ClO4)2. (CIF) Crystallographic parameters, bond lengths and angles (crystal and DFT), energies of DFT optimized states, MO compositions, TD-DFT calculated transitions, mass spectra, 1H NMR, hydrogen bonding interactions, EPR, Mulliken spin-density representations, spectro-electrochemistry, DFT-optimized structures, experimental and TD-DFT calculated electronic spectra. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (W.K.) *E-mail: [email protected]. (G.K.L) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support received from the Department of Science and Technology, Council of Scientific and Industrial Research (fellowship to A.M.) New Delhi (India), the DFG, and the Land Baden-Württemberg (Germany) is gratefully acknowledged.

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REFERENCES

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