Analysis of Redox Series of Unsymmetrical 1,4-Diamido-9,10

Article pubs.acs.org/IC

Analysis of Redox Series of Unsymmetrical 1,4-Diamido-9,10anthraquinone-Bridged Diruthenium Compounds Abhishek Mandal,† Md Asmaul Hoque,† Anita Grupp,§ Alexa Paretzki,§ 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

§

S Supporting Information *

ABSTRACT: The unsymmetrical diruthenium complexes [(bpy)2RuII(μ-H2L2−)RuIII(acac)2]ClO4 ([3]ClO4), [(pap)2RuII(μ-H2L2−)RuIII(acac)2]ClO4 ([4]ClO4), and [(bpy)2RuII(μ-H2L2−)RuII(pap)2](ClO4)2 ([5](ClO4)2) have been obtained by way of the mononuclear precursors [(bpy)2RuII(H3L−)]ClO4 ([1]ClO4) and [(pap)2RuII(H3L−)]ClO4 ([2]ClO4) (where bpy = 2,2′bipyridine, pap = 2-phenylazopyridine, acac− = 2,4-pentanedionate, and H4L = 1,4-diamino-9,10anthraquinone). Structural characterization by single-crystal X-ray diffraction and magnetic resonance (nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR)) were used to establish the oxidation state situation in each of the isolated materials. Cyclic voltammetry, EPR, and ultraviolet−visible− near-infrared (UV-vis-NIR) spectroelectrochemistry were used to analyze the multielectron transfer series of the potentially class I mixed-valent dinuclear compounds, considering the redox activities of differently coordinated metals, of the noninnocent bridge and of the terminal ligands. Comparison with symmetrical analogues [L2′ Ru(μ-H2L)RuL2′ ]n (where L′ = bpy, pap, or acac−) shows that the redox processes in the unsymmetrical dinuclear compounds are not averaged, with respect to the corresponding symmetrical systems, because of intramolecular charge rearrangements involving the metals, the noninnocent bridge, and the ancillary ligands.

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INTRODUCTION A common distinction in coordination chemistry has been that between (redox-)active and mere “spectator” ligands.1 Complications arise when there are two or more active ligands, either equivalent (with the result of orbital degeneracy) or nonequivalent (with the potential for interligand exchange of energy, charge or spin). Although elements of symmetry can simplify the description of electronic structure in coordination compounds,2 most metal systems (e.g., in biomaterials) contain no such elements.3 Using a noninnocently behaving redox-active ligand bridge with established characteristics, viz, 1,4-diamino-9,10-anthraquinone (H4L),4,5 in connection with two N,O-chelate bound ruthenium centers, we have now prepared three low-symmetry6 dinuclear complexes ([3]ClO4, [4]ClO4, and [5](ClO4)2), using the mononuclear [1]ClO4 and [2]ClO4 as precursors (see Figure 1). 9,10-Anthraquinones with additional metal coordination functions in positions 1 and 4 have long been known as excellent dyestuff components (e.g., alizarin, quinizarin),7,8 offering the possibility for six-membered chelate ring formation (see Chart 1). After structural characterization of [1]ClO4, [2]ClO4, [3]ClO4, and [5](ClO4)2, the sites for addition or loss of electrons were studied for the dinuclear species, employing EPR and UV-vis-NIR spectroelectrochemistry. The potential spectator ligands in the present series are σ-donating acetylacetonate (acac−), moderately π-accepting 2,2′-bipyridine (bpy), and strongly π-accepting 2-phenylazopyridine (pap). © XXXX American Chemical Society

Our study on the asymmetric combinations in dinuclear [3]ClO4, [4]ClO4, and [5](ClO4)2 complements previous investigations of symmetrical compounds [(acac)2RuIII(μH 2 L 2− )Ru III (acac) 2 ], [(bpy) 2 Ru II (μ-H 2 L • − )Ru II (bpy) 2 ](ClO 4 ) 3 , [(pap) 2 Ru I I (μ-H 2 L 2 − )Ru I I (pap) 2 ](ClO 4 ) 2 , [(bpy)2OsII(μ-H2L• −)OsII(bpy)2](ClO4)3, and [(pap)2OsII(μH2L2−)OsII(pap)2](ClO4)2.4,5,9

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RESULTS AND DISCUSSION Synthesis, Characterization, and Structures. The mononuclear precursor compounds [(bpy)2RuII(H3L−)]ClO4 ([1]ClO4) and [(pap)2RuII(H3L−)]ClO4 ([2]ClO4) have been synthesized from the reactions of in situ-generated solvent complexes [Ru(bpy) 2 (EtOH) 2 ] 2+ 10 or ctc-[Ru(pap) 2 (EtOH)2]2+11 (where ctc denotes a cis−trans−cis configuration, with respect to EtOH, pyridine, and azo nitrogen atoms of pap ligands, respectively) with H4L (H4L= 1,4-diamino-9,10anthraquinone), refluxing in EtOH in the presence of triethylamine under a dinitrogen atmosphere, followed by chromatographic purification using a silica gel column. The asymmetric dinuclear complexes [(bpy) 2 Ru II (μ-H 2 L 2− )Ru II I (acac) 2 ]ClO 4 ([3]ClO 4 ), [(pap) 2 Ru II (μ-H 2 L 2− )RuIII(acac)2]ClO4 ([4]ClO4), and [(bpy)2RuII(μ-H2L2−)RuII(pap)2](ClO4)2 ([5](ClO4)2) have been obtained via the reactions of mononuclear precursors [1]ClO4 and [2]ClO4 with [Ru(acac)2(CH3CN)2] and of [1]ClO4 with [RuReceived: November 4, 2015

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DOI: 10.1021/acs.inorgchem.5b02541 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Representation of complexes.

preferential stabilization of only one particular isomer in each case (see below). The molecular configurations of monomeric [1]ClO4 and [2]ClO4 and of the asymmetric dinuclear [3]ClO4 and [5](ClO4)2 have been authenticated by single-crystal X-ray structures (see Figure 2, as well as Figures S3−S7 and Tables S1−S12 in the Supporting Information). Experimental bond parameters are reproduced well by DFT calculations (Figures S8−S12, as well as Tables S4−S11 and S13−S19, in the Supporting Information). The crystal structure of [1]ClO4 includes four independent molecules (molecules A−D) in the unit cell. The largely planar monodeprotonated H3L− in [1]ClO4 or [2]ClO4 forms an (O,NH) −-donating sixmembered chelate ring (β-ketiminato), leaving free CO and NH2 groups at the back side. In the dinuclear complexes [3]ClO4 and [5](ClO4)2 the doubly deprotonated H2L2− bridges the metal fragments through (O,NH)− donor sets, forming six-membered β-ketiminato chelate rings on both sides of the bridge. The H2L2− bridge is almost planar in [3]ClO4, whereas, in [5](ClO4)2, the twisting of terminal rings results in an angle of 22.1°, with respect to the central ring. In consequence, the metal ions in [5](ClO4)2 are situated 0.59 Å(Ru1) and 0.39 Å(Ru2) above the best plane involving the chelate rings of the bridge (see Table S12 in the Supporting Information). The configurations of the crystallized compounds [1]ClO4− [3]ClO4 and [5](ClO4)2 include the positional isomers of the [Ru(pap)2]2+-containing species [2]ClO4 and [5](ClO4)2, which display trans-arranged pyridyl groups from the pap chelate ligands, while the partially anionic donor atoms O and N from the β-ketiminato six-membered chelate functions lie trans to the strongly π-electron-accepting N centers of the azo groups in pap. The chirality of the tris-chelated ruthenium centers causes further potential isomerism with the configurations of the isolated compounds established structurally as Δ (Ru1), Λ (Ru2), Λ (Ru3), and Δ (Ru4) for compound [1]ClO4, Λ for

Chart 1

(pap)2(EtOH)2]2+ in refluxing EtOH in the presence of triethylamine. The complexes have been purified by column chromatography using neutral alumina (see the Experimental Section). The π-accepting bpy and pap co-ligands facilitate the stabilization of ruthenium(II) in [1]ClO4, [2]ClO4, and [5](ClO4)2,12 while the effect of electron-rich acac− is reflected by the stabilization of ruthenium(III) in the asymmetric mixedvalent complexes [3]ClO4 and [4]ClO 4 (Figure 1). 13 Satisfactory microanalytical and mass spectrometric data confirm the compositions of the 1:1 (1+, 2+, 3+, and 4+) and 1:2 conducting (52+) complexes (see the Experimental Section and Figure S1 in the Supporting Information). 1 H NMR spectra of diamagnetic 1+, 2+, and 52+ show the partial overlap of the calculated number of aromatic proton resonances (for bpy, 16; for pap, 18; and for H3L−/H2L2−, 6) in the chemical shift range between 6.5 ppm and 9.0 ppm. The D2O exchangeable NH proton resonances of coordinated H3L− and H2L2− in 1+, 2+ and 52+ appear at ∼9.5−11.0 ppm, while relatively broad resonances corresponding to the free amino groups of H3L− in 1+ and 2+ are found at 5.3−7.0 ppm. The paramagnetic asymmetric complexes 3+ and 4+ exhibit 1H NMR resonances over a wide chemical shift range, from 24 ppm to −6 ppm, because of paramagnetic contact shift effects (see the Experimental Section and Figure S2 in the Supporting Information).14 Two D2O exchangeable NH signals associated with unsymmetrically coordinated H2L2− in 3+ and in 4+ are identified at ∼21−23 ppm. Although the asymmetric dinuclear complex ions 3+, 4+, and 2+ 5 can exist in different isomeric forms, the 1H NMR spectral features (Figure S2 in the Supporting Information) suggest the B


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Inorganic Chemistry

Figure 2. Perspective views of cationic parts of (a) [1]ClO4·0.5 CH3CN (molecule A), (b) [2]ClO4·C7H8, (c) [3]ClO4, and (d) [5](ClO4)2. Ellipsoids are drawn at 50% (40% for [5](ClO4)2) probability level. Hydrogen atoms (C−H) and solvent molecules (acetonitrile for [1]ClO4 and toluene for [2]ClO4) are removed for the sake of clarity.

compound [2]ClO4, Δ,Δ for compound [3]ClO4, and Δ (Ru1) and Λ (Ru2) for [5](ClO4)2 (Figure 2). The slightly distorted octahedral arrangements of the complexes are confirmed by the cis and trans angles around the metal ions (Tables S3, S5, S7, S9, and S11 in the Supporting Information). The metal−ligand (bpy, pap, H3L−/ H2L2−) bond distances in [1]ClO4−[3]ClO4 and [5](ClO4)2 are in good agreement with those reported for analogous complexes.4,15 The NN (pap) distances of 1.302(4) Å and 1.293(7)/1.299(7) Å in [2]ClO4 and [5](ClO4)2, respectively, confirm its unreduced state.16

The free CO ([1]ClO4/[2]ClO4: 1.269(10) Å (average)/ 1.258(4) Å) and C−NH2 ([1]ClO4/[2]ClO4: 1.352(10) Å (average)/1.335(4) Å) (Tables S2 and S3 in the Supporting Information) groups at the back side of H3L− participate in intramolecular N−H···O hydrogen bonding interaction (see Figures S4 and S5 in the Supporting Information). The four molecules in the asymmetric unit of [1]ClO4 develop an intermolecular hydrogen bonding network via the interactions between the metal-coordinated NH protons of one molecule and free CO groups of nearby molecules in the asymmetric unit, as well as in the other units. On the other hand, in [2]ClO4, intermolecular hydrogen bonding results from the C


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Inorganic Chemistry

third irreversible oxidation (O3), while [2]ClO4 displays two seemingly reversible oxidations and three reductions. Although oxidations of the NH2 containing compounds [1]ClO4 and [2]ClO4 appear reversible on the cyclic voltammetric time scale, the known increase of acidity after oxidation can lead to deprotonation.18 Conversely, reduction is usually connected to enhanced basicity leading to protonation if OH or NH functions are available. Because of such probable proton coupled redox processes, we refrained from extended spectroelectrochemical studies of these precursors to the dinuclear systems. The asymmetric dinuclear complex [3]ClO4 displays two reversible oxidations and three reductions, in addition to a third irreversible oxidation process while the analogous [4]ClO4 exhibits one reversible and one irreversible oxidation and four reversible reductions. Asymmetric [5]ClO4 with two different π-acidic terminal ligands (bpy, pap) exhibits two reversible oxidations, one irreversible oxidation, and three reversible reductions. The assignment of these processes has been made based on spectroelectrochemical analysis, as described in the following. EPR Spectroscopy. While the dinuclear cations 3+ and 4+ were isolated in these paramagnetic forms, the species 2+ and 52+ were converted to spin-bearing neighboring charge states by in situ electrolysis in CH2Cl2/0.1 M Bu4NPF6 for EPR spectroscopy. Representative EPR signals are shown in Figure 4, as well as Figures S13 and S14 in the Supporting Information, and the relevant data are summarized in Table 2. For confirmation of the assignments, we performed spin density calculations, the results of which are summarized in Table 3 and depicted in Figures 5−7 (shown later in this work), as well as Figures S15 and S16 in the Supporting Information. After oxidation of 2+ at room temperature, a single EPR line is observed at g ≈ 2.00, with very slight g anisotropy in the frozen state (see Figure S13, and Table 2 in the Supporting Information). These features suggest almost exclusively ligandbased spin, and the calculations (see Table 3, as well as Figure S16 in the Supporting Information) either of 22+ or of (2-H)+ confirm the formation of an anthrasemiquinone ligand.4,5,9 After reduction of 2+, the resulting compound is EPR silent at room temperature but displays a notable g anisotropy between 1.97 and 2.01 at 115 K (Figure S13, Table 2 in the Supporting Information). Spin density calculation on 2 suggest reduction of one of the pap ligands, and the possible spin hopping between equivalent reduced and nonreduced ligands may explain the absence of detectable EPR intensity at room temperature (dynamic line broadening19,20). EPR signals of further reduced forms with S > 1/2 (Table 3) could not be observed. In contrast, the isolated compounds 3+ and 4+ exhibit typical EPR features of unsymmetrical RuIII complexes (see Figure S14 in the Supporting Information, and Table 2), the large g anisotropy and the deviation of the average g factor from the free electron value of 2.0023 point to significant contributions from the heavy metal with its high spin−orbit coupling constant to the spin distribution.21 DFT calculations confirm this by yielding spin density values of >0.7 for the acaccoordinated ruthenium center (see Table 3 and Figures 5 and 6). The g anisotropy and the spin density calculated for that metal site are slightly higher for system 4n with the lessdonating pap ligands. The complex ion 52+ with two different types of terminal acceptor ligands (bpy, pap) is oxidized to yield an EPR signal

interactions between NH protons and free NH2 functions of H3L− with O atoms of the perchlorate anion in the asymmetric unit and in nearby units (Figures S4 and S5 in the Supporting Information). The NH group of H2L2− attached to the {Ru(bpy)2} fragment in [3]ClO4 or to {Ru(pap)2} in [5](ClO4)2 engages in intermolecular hydrogen bonding with an O atom of perchlorate in the asymmetric unit (Figures S6 and S7 in the Supporting Information). Cyclic Voltammetry. Cyclic and differential pulse voltammetric responses of the mononuclear ([1]ClO4, [2]ClO4) and dinuclear ([3]ClO4, [4]ClO4, [5](ClO4)2) complexes in CH3CN/0.1 M Et4NClO4 (see Figure 3 and Table 1) reveal

Figure 3. Cyclic (black) and differential pulse (green) voltammograms of (a) [1]ClO4, (b) [2]ClO4, (c) [3]ClO4, (d) [4]ClO4, and (e) [5](ClO4)2 in CH3CN/0.1 M Et4NClO4/GC versus SCE. Scan rate = 100 mV s−1.

multiple one-electron redox processes within the experimental potential window of ±2 V versus SCE due to the involvement of more than one redox active ligands (H3L−, H2L2−, bpy, pap) in each complex, along with the Ru ion(s). The potentials of the redox processes and the comproportionation constants, Kc (RT ln Kc = nF(ΔE))17 for successive redox processes vary, depending on the mononuclear versus dinuclear situation, the π-acidity of the terminal ligands (bpy/pap), and the combination of terminal ligands (π-acidic bpy, pap, or σdonating acac−) in the dinuclear systems. The complex [1]ClO4 exhibits two apparently reversible oxidations (O1, O2) and two reduction processes (R1, R2), in addition to a D


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Inorganic Chemistry Table 1. Electrochemical Dataa Potential vs SCE (E298 ° ) Values and Peak Potential Differences (ΔE) [1]ClO4 redox states

E298 ° [V]

O3 O2 O1 R1 R2 R3 R4

1.46b 0.67 0.05 −1.42 −1.75

[2]ClO4

ΔE [mV]

E298 ° [V]

70 70 50 90

[3]ClO4

ΔE [mV]

E298 ° [V]

80 70 80 90 100

1.29b 0.79 0.06 −0.88 −1.71 −1.94

0.90 0.50 −0.54 −1.13 −1.53

[4]ClO4

ΔE [mV] 60 60 60 70 100

[5](ClO4)2

ΔE [mV]

E298 ° [V] 1.14b 0.44 −0.43 −0.78 −1.09 −1.75

60 70 70 70 100

E298 ° [V]

ΔE [mV]

1.31 0.63 0.13 −0.56 −1.14 −1.70

120 60 70 60 70 90

Kc Valuesc constant d

[1]ClO4

[2]ClO4

[3]ClO4

[4]ClO4

3.1 × 10 7.4 × 1024 3.8 × 105

× × × ×

× × × ×

× × × ×

10

Kc1 Kc2e Kc3f Kc4g

5.9 4.0 9.5 5.9

6

10 1017 109 106

2.2 7.9 1.1 7.8

12

10 1015 1014 103

3.2 5.0 1.0 1.3

10

10 1014 106 105

[5](ClO4)2 2.9 4.8 6.5 3.0

× × × ×

108 1011 109 109

From cyclic voltammetry in CH3CN/0.1 M Et4NClO4 at 100 mV s−1 (see Figure 3). bIrreversible, anodic peak potentials given. Comproportionation constant determined from the expression RT ln Kc = nF(ΔE). dBetween O1 and O2. eBetween O1 and R1. fBetween R1 and R2. gBetween R2 and R3. a c

ligand at Ru2 (which has a small share of spin in agreement with the slight g anisotropy and calculated spin density). Spectroelectrochemistry and Oxidation State Assignments. To evaluate the oxidation state situation for those states of 3n−5n that are not susceptible to EPR analysis, we performed spectroelectrochemistry in the UV-vis-NIR region (see Figures 8−10, as well as Figure S17 in the Supporting Information and Table 4) in conjunction with time-dependent density functional theory (TD-DFT) calculations (Tables S20−S24 in the Supporting Information). The MO compositions of all the complexes are set in Tables S25−S61 in the Supporting Information. Those intense features, which lie at low energies, involve frontier orbitals and are relevant for the oxidation state assignments will be mainly discussed. The Class I21b,22 mixed-valent (RuIIIRuII) precursor cation 3+ exhibits a strong absorption band at λmax = 545 nm, attributed to MLCT transitions to π*(bpy). After oxidation a nearinfrarred (NIR) band at λmax = 1207 nm (Figure 8, Table S22 in the Supporting Information) appears (the HOMO−LUMO transition) (Table S42 in the Supporting Information), involving contributions from MMCT, LLCT, and metal/ligand charge transfers. Further oxidation diminishes this band (now at λmax = 1193 nm) and involves even more orbital mixing. Bpybased reduction of 3+ to the neutral state (3) is distinguished by an intense absorption at λmax = 696 nm (shifted from 545 nm) due to π*(bpy)-targeted LLCT and MLCT transitions (Figure 8, Table S22). Further reduction, this time involving mainly the acac-coordinated ruthenium(III), produces little change in the visible region. The paramagnetic RuIIIRuII precursor cation 4+ displays some broad LLCT absorption bands at longer wavelengths and a strong absorption at λmax = 522 nm, attributed to MLCT transitions to π*(pap). After bridge-based oxidation a near IR band at λmax = 1017 nm appears (the HOMO−LUMO transition), involving mostly an LMCT, π(H2L• −) → dπ(Ru2). Reduction of the mixed-valent 4+ to the neutral state (4) is calculated to occur at a pap ligand and produces little spectral change. Additional pap-based reduction to 4− produces a band shift from λmax = 773 to 713 nm involving transitions from (pap• −)-coordinated electron-rich (Ru1)II to π* MOs of

Figure 4. EPR spectra of [5](ClO4)2 after in situ oxidation (top) and reduction (bottom) in CH2Cl2/0.1 M Bu4NPF6 (asterisk (*) denotes an organic radical dissociation product).

Table 2. EPR Data for Complexes complex + c,d

[2-H] [2] c [3]ClO4e [4]ClO4e [5]3+ c,f [5]+ c

g1

g2

g3

⟨g⟩a

Δgb

2.004 2.009 2.190 2.216 2.12 2.011

2.001 1.992 2.075 2.070 1.98 1.989

1.989 1.977 1.847 1.835 1.98 1.976

1.998 1.992 2.042 2.046 2.03 1.992

0.015 0.032 0.34 0.38 0.14 0.035

⟨g⟩ = {1/3(g12 + g22 + g32)}1/2. bΔg = g1 − g3. cAt 115 K in CH2Cl2/ 0.1 M Bu4NPF6. dgiso = 1.999 at 298 K. eAt 77 K in CH3CN. fgiso = 2.022 at 298 K.

a

(Figure 4) at 2.022 which, like the g anisotropy g1 − g3 = 0.14 reveals minor but not insignificant metal contribution, in agreement with a spin density value of 0.202 for Ru1 (bpycoordinated). However, the main spin (0.753) is still borne by the anthrasemiquinone bridge (Table 3 and Figure 7). Reduction of 52+ yields similar results (Figure 4) as for the reduction of 2+, i.e., EPR silence at room temperature due to spin hopping between a reduced and one nonreduced pap E


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Inorganic Chemistry Table 3. DFT-Calculated (UB3LYP/LanL2DZ/6-31G*) Mulliken Spin Densities Spin Density complex 2+

1

2 (S = /2) [2-H]+ (S = 1/2) 2 (S = 1/2) 2− (S = 1) 22− (S = 3/2) 33+ (S = 1/2) 32+ (S = 1) 3+ (S = 1/2) 3 (S = 1) 3− (S = 1/2) 42+ (S = 1) 4+ (S = 1/2) 4 (S = 1) 4− (S = 3/2) 42− (S = 1) 54+ a 53+ (S = 1/2) 5+ (S = 1/2) 5 (S = 1) a

Ru2

H3L−/H2L2−

0.136 −0.021 −0.124 0.251 0.458 −0.387 0.042 −0.004 −0.037 −0.037 0.018 −0.008 −0.187 0.220 0.333

1.118 0.948 0.752 0.667 −0.005 0.990 0.764 0.693 0.503 −0.004

0.870 0.968 −0.003 −0.002 1.019 −0.231 0.710 0.162 0.305 −0.019 0.669 0.133 0.241 0.469 0.023

0.001 −0.003 −0.002 1.009 1.061

0.202 0 −0.032

0.051 −0.132 −0.129

0.753 0 −0.005

−0.004 0 1.018

Ru1

bpy

pap

acac−

−0.006 0.053 1.127 1.751 1.523

−0.002 0.014 1.187 1.778 1.648

0.501 0.303 0.092 0.056 0 0.325 0.097 0.066 0.030 0

−0.002 1.132 1.148

For 54+, (E(S=1) − E(S=0)) = 329 cm−1.

Table 4. UV-vis-NIR Spectroelectrochemical Data for 2n, 3n, 4n, and 5n in CH3CN/0.1 M Bu4NPF6 λ [nm] (ε [M−1 cm−1]) 3+

2+

2+

+

2 /(2-H) 2 /(2-H) 2+

Figure 5. Spin density representations of (a) 33+ (S = 1/2), (b) 32+ (S = 1), (c) 3+ (S = 1/2), (d) 3 (S = 1), and (e) 3− (S = 1/2).

2 2− 22− 33+ 32+ 3+

Figure 6. Spin density representations of (a) 42+ (S = 1), (b) 4+ (S = 1 /2), (c) 4 (S = 1), (d) 4− (S = 3/2), and (e) 42− (S = 1).

3 3− 42+ 4+ 4 4−

Figure 7. Spin density representations of (a) 53+ (S = 1/2), (b) 5+ (S = 1 /2), and (c) 5 (S = 1).

42− 54+ 53+

ligands. Further reduction to the dianion results in the appearance of an absorption at λmax = 1040 nm, now assigned to MLCT transitions starting from the acac-coordinated metal (Ru2)II, after its reductive formation from the trivalent state (see Figure 9, as well as Table S23 in the Supporting Information). The dication 52+ exhibits weak LLCT features at longer wavelengths and a strong MLCT absorption at λmax = 535 nm

52+ 5+ 5

F

365 (21250), 443 (11000, sh), 495 (11300), 644 (5350, sh), 755 (5720, br), 900 (4100, br) 285 (22930), 363 (20660), 460 (10500), 530 (10550), 585 (7940, sh), 636 (6520), 874 (4700, br) 283 (23300), 364 (21050), 467 (9505), 585 (11660), 639 (9450), 825 (2560, br) 295 (22100), 355 (20300), 497 (10900, sh), 622 (10290), 824 (1890, sh) 298 (20870), 355 (25420), 483 (11020, sh), 625 (7270), 776 (3800, sh), 1026 (3880) 355 (27330), 422 (11940), 551 (7630, sh), 620 (6790), 1017 (6660) 290 (46870), 349(15000, sh), 540(28000), 838(3760, sh), 1193(7600) 292(48940), 347(14090, sh), 440(15880, sh), 542(38060), 779(3000, sh), 1207(11660) 292(54700), 360(14870), 440(14640, sh), 545(33450), 920(1360), 1045(1780), 1210(2130) 397 (17040, sh), 431 (18270), 546 (16170, sh), 696 (23570) 283 (40870), 374 (21870), 423 (22100), 490 (17600, sh), 540 (17000, sh), 690 (25640) 365(52040), 486(53270), 666(15930, sh), 1017(26400) 355(55810), 522(62680), 773(8890), 956(6690), 1085(3350) 353(57040), 530(58700), 619(24410, sh), 1011(4430), 1171(3310) 364(55140), 475(32210, sh), 628(30390, sh), 713(36980), 1040(5200, br) 358(67740), 685(48360), 1040(17390) 281(52210), 370(29960), 486(34070), 972(28730) 290(55980), 364(28330), 526(29820), 853(4720, sh), 1260(16380) 293(65620), 356(31910), 535(32110), 757(6090), 1088(2030) 293(61670), 358(30340), 534(30520) 296(51640), 358(35850), 523(18700), 622(14120, sh), 1043(6200)


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Inorganic Chemistry

Figure 8. UV-vis-NIR spectroelectrochemistry of 3n in CH3CN/0.1 M Bu4NPF6.


variation in potentials based on the specific combination of ancillary ligands (acac, bpy, pap). Although preferential bridgebased oxidation is evident in both unsymmetrical and symmetrical systems, reductions are dominated by the ancillary ligands and bridge, respectively. The sensitivity of the varying combination of ancillary ligands around the {Ru(μ-H2L)Ru} core in 3n, 4n, 5n (unsymmetrical), and 6 n , 7 n , 8 n (symmetrical), is evident from their spectroelectrochemical signatures. For example, the {(A)RuII(μ-H2L2−)RuII(A′)} state in symmetrical 72+ (A/A′ = bpy), 82+ (A/A′ = pap), and unsymmetrical 52+ (A = bpy/A′ = pap) exhibit MLCT transitions at 539, 550, and 535 nm, respectively, while only 52+ displays LLCT transitions at 757 and 1088 nm. Although the {RuII(μ-H2L• −)RuII} configuration in 53+/73+ exhibits NIR absorptions at 1260 nm/1530 nm corresponding to {RuII(bpy)2}-to- bridge directed MLCT transitions, the low-energy band at 1274 nm in 83+ involves the bridge-based LLCT transition. Similarly, the {RuII(μH2L• −)RuIII}) configuration in symmetrical 74+ and 84+ results in MMCT/LMCT and MLCT/LMCT absorption bands at 1100 nm/589 and 892 nm/499 nm, whereas unsymmetric 54+, 42+, and 32+ exhibit LLCT/MMCT, MMCT/LLCT, and LMCT transitions at 972, 1017, and 1207 nm, respectively. Of the three unsymmetrical dinuclear systems presented here, the example 3n (bpy/acac combination) invites an immediate comparison with the corresponding symmetrical analogues 6n−2 (acac) and 7n+2 (all-bpy system).4a The redox potentials of corresponding charge forms in 3n such as +0.06 V and −0.88 V (Table 1) are not the average of the values obtained for 6n−2 (−0.72 V and −1.16 V) and 7n+2 (+0.37 V and −0.06 V).4a The reason involves intramolecular charge rearrangements, as reflected by differing oxidation-state combinations (Scheme 1), deduced from experimental (EPR, UV-vis-NIR spectroelectrochemistry) and computational sources (DFT-calculated spin densities and electronic transitions). Characteristically, increasing ligation with acac− upon going from 7n+2 via 3n to 6n−2 results in increasing preference for ruthenium(III)-containing configurations, which, e.g., produces a strong asymmetry in the spin distribution of 3+, i.e., Class I behavior in a localized RuIIRuIII situation without coupling activity by the bridging ligand (Scheme 1). The corresponding symmetrical species 6− and 73+ do not have this option and exhibit a more balanced spin distribution with significant


from the bpy-coordinated Ru1 to π* MOs of H2L2− and bpy. Upon oxidation at the bridge, this transition shifts to λmax = 1260 nm, resulting from the relative stabilization of π* (H2L• −). Further oxidation to the 4+ state shows, instead of the MLCT transition, a low-energy absorption at λmax = 972 nm, assigned to an LLCT from π(pap) to the π* MO of the bridge (see Figure 10, as well as Table S24 in the Supporting Information). Reduction of the dication, spectroscopically (EPR) and computationally (DFT) confirmed to occur at a pap ligand, produces little spectral change and no intense bands in the near-infrared (NIR) and visible regions. The second reduction is calculated to occur at bpy, producing NIR absorption for 5, because of interligand transitions. The electronic structural forms of the complexes in accessible redox states are depicted in Scheme 1. A comparison of redox processes between the present report of H2L2− bridged unsymmetrical diruthenium complexes [(bpy) 2 Ru I I (μ-H 2 L 2 − )Ru I I I (acac) 2 ]ClO 4 ([3]ClO 4 ), [(pap) 2 Ru II (μ-H 2 L 2− )Ru III (acac) 2 ]ClO 4 ([4]ClO 4 ), and [(bpy)2RuII(μ-H2L2−)RuII(pap)2](ClO4)2 ([5](ClO4)2) with the reported analogous symmetrical complexes [(acac)2RuIII(μ-H2L2−)RuIII(acac)2] (6),4a [(bpy)2RuII(μ-H2L• −)Ru II(bpy) 2 ](ClO 4 ) 3 ([7](ClO 4 ) 3 ), 4a and [(pap) 2 Ru II(μH2L2−)RuII(pap)2](ClO4)2 ([8](ClO4)2),4b reveals systematic G


Article

Inorganic Chemistry Scheme 1. Electronic Structural Forms of 2n, 3n, 4n, and 5n

favoring acac−) have a profound influence on the steps within the multielectron transfer series. Comparison with symmetrical analogues [L′2Ru(μ-H2L)RuL′2]n, L′ = bpy, pap, or acac−) shows that the redox processes in the unsymmetrical dinuclear compounds are not averaged with respect to the corresponding symmetrical systems, because of intramolecular charge rearrangements involving the metals, the noninnocent bridge, and the ancillary ligands.

participation from the organic bridge. The comparison involving complexes 4n, 6n−2, and 8n+2 yields similar results; however, the differences between unsymmetrical and symmetrical systems are smaller when more similar ancillary ligands (such as the acceptors bpy and pap) are combined (compounds 5n).

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CONCLUSION Three unsymmetrical diruthenium complexes, bridged by partially deprotonated 1,4-diamino-9,10-anthraquinone, were studied with respect to the electron transfer sites and the resulting electronic structures in ground and low-lying excited states. Two of the three dinuclear compounds and two mononuclear precursors were crystallographically characterized to reveal the corresponding oxidation states of metals, bridging, and terminal ligands. Although the systems 3+, 4+, and 54+ can be described as Class I mixed-valence species, most electron transfers, as experimentally observed by EPR and UV-vis-NIR spectroelectrochemistry and as confirmed by DFT calculations, involve the noninnocent bridge and/or the potentially redoxactive terminal ligands. Unsymmetrical arrangements, such as those presented, are quite sensitive, with respect to the sequence of electron transfers, since changes of terminal acceptor (RuII stabilizing pap and bpy) and donor ligands (RuIII

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EXPERIMENTAL SECTION

Materials. The precursor complexes cis-Ru(acac)2(CH3CN)2,23 cisRu(bpy)2Cl2,24 and cis−trans−cis-Ru(pap)2Cl225 were prepared according to the procedures reported in the literature. The ligand 1,4-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, highperformance liquid chromatography (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 tube19a 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 H


Article

Inorganic Chemistry

solvent was evaporated to dryness under reduced pressure. It was purified on a neutral alumina column by 3:1 CH2Cl2−CH3CN mixture as eluent. The solvent was removed under reduced pressure, which yielded the solid mass of [3]ClO4. Yield: 97 mg (70%). MS(ESI+, CH3CN): m/z {[3]+} calcd, 905.09; found, 905.08. 1H NMR (400 MHz) in CD3CN [δ/ppm (J/Hz)]: 22.69 (s, 1H), 21.27 (s, 1H), 15.22 (s, 1H), 8.51 (m, 3H), 8.05 (m, 5H), 7.84 (s, 1H), 7.53 (m, 3H), 7.28 (s, 1H), 7.01 (m, 2H), 6.42 (s, 1H), 1.92 (m, 3H) 1.45 (m, 3H), 1.26 (s, 3H), 0.58 (s, 1H), 0.06 (s, 3H), −0.28 (s, 1H), −0.89 (s, 2H), −1.49 (s, 1H), −2.34 (s, 1H), −5.56 (s, 1H). Anal. Calcd for C44H38N6O10ClRu2: C, 50.41; H, 3.65; N, 8.02. Found: C, 50.49; H, 3.53; N, 7.96. Molar conductivity (CH3CN): ΛM = 104 Ω−1 cm2 M−1. Synthesis of [Ru(pap)2(μ-H2L2−)Ru(acac)2]ClO4 ([4]ClO4). The preformed mononuclear complexes [Ru(pap)2(H3L−)]ClO4 (100 mg, 0.12 mmol) and [Ru(acac)2(CH3CN)2] (47 mg, 0.13 mmol) in 25 mL of ethanol were heated to reflux for 6 h under dinitrogen atmosphere in the presence of NEt3 (0.02 mL, 0.14 mmol). The solvent was evaporated to dryness under reduced pressure. It was purified by using a neutral alumina column and a 3:1 CH2Cl2− CH3CN mixture as the eluent. The solvent was removed under reduced pressure, which yielded the solid mass of [4]ClO4. Yield: 48 mg (35%). MS(ESI+, CH3CN): m/z {[4]+} calcd, 1004.10; found 1004.12. 1H NMR (400 MHz) in CD3CN [δ/ppm (J/Hz)]: 22.91 (s, 1H), 21.09 (s, 1H), 16.41 (d, 7.2, 1H), 8.09 (m, 2H), 7.91 (m, 3H), 7.68 (m, 3H), 7.53 (m, 3H), 7.31 (m, 3H), 7.17 (m, 2H), 7.07 (m, 1H), 6.76 (m, 2H), 6.52 (m, 3H), 5.62 (s, 1H), −0.04 (s, 1H), −0.9 (s, 3H), −1.49 (s, 3H), −2.15 (s, 1H), −2.78 (s, 3H), −4.98 (s, 3H). Anal. Calcd for C46H40N8O10ClRu2: C, 50.12; H, 3.66; N, 10.16. Found: C, 50.23; H, 3.54; N, 10.09. Molar conductivity (CH3CN): ΛM = 87 Ω−1 cm2 M−1. Synthesis of [Ru(bpy)2(μ-H2L2−)Ru(pap)2](ClO4)2 ([5](ClO4)2). The precursor complex ctc-[Ru(pap)2Cl2] (72 mg, 0.13 mmol) and AgClO4 (83 mg, 0.40 mmol) were placed in 25 mL of ethanol, and the solution was heated at reflux under dinitrogen atmosphere for 1.5 h. The precipitated AgCl was filtered off through a sintered Gooch crucible and the filtrate ([Ru(pap)2(EtOH)2]2+) was taken in a twonecked round-bottom flask. To this filtrate, the preformed mononuclear complex [Ru(bpy)2(H3L−)]ClO4 ([1]ClO4, 100 mg, 0.13 mmol) and NEt3 (0.02 mL, 0.14 mmol) were added. The mixture was refluxed for 5 h under dinitrogen atmosphere. The dry mass was moistened with a few drops of CH3CN, followed by the addition of saturated aqueous NaClO4 solution to it. It was then chilled overnight. The solid mass thus obtained was filtered off and washed with chilled deionized water to remove the excess NaClO4 and dried in vacuo over P4O10. It was then purified on a neutral alumina, using 1:2 CH2Cl2− CH3CN mixture as an eluent. The solvent was removed under reduced pressure, which yielded a solid mass of [5](ClO4)2. Yield: 157 mg (90%). MS(ESI+, CH3CN): m/z {[5]+} calcd, 1216.46; found 1216.48. 1H NMR (400 MHz) in (CD3)2SO [δ/ppm (J/Hz)]: 10.15 (s, 1H), 9.52 (s, 1H), 8.93 (s, 1H), 8.76 (m, 3H), 8.71 (m, 3H), 8.35 (m, 4H), 8.20 (m, 2H) 8.05 (m, 3H), 7.90 (m, 2H) 7.70 (m, 3H), 7.55 (m, 3H), 7.45 (m, 4H), 7.4 (m, 2H), 7.3 (m, 4H), 7.03 (d, 8.0 2H), 6.88 (d, 7.6 2H), 6.82 (d, 8.4, 2H). Anal. Calcd for C56H42N12O10Cl2Ru2: C, 51.11; H, 3.22; N, 12.77. Found: C, 51.03; H, 3.31; N, 12.86. Molar conductivity (CH3CN): ΛM = 220 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 CH3CN−toluene solutions for [1]ClO4, [2]ClO4, and [5](ClO4)2, while single crystals of [3]ClO4 were grown by slow evaporation of its 1:1 CH2Cl2−hexane solution. X-ray diffraction data were collected on a Rigaku Saturn-724+ CCD single crystal diffractometer, using Mo Kα radiation. The data collection was evaluated by using the CrystalClear-SM Expert software. The data were collected by the standard ω-scan technique. The structure was solved by direct method using SHELXS-97 and refined by full matrix least-squares with SHELXL-97, refining on F2.27 All non-hydrogen atoms were refined anisotropically. The remaining hydrogen atoms were placed in geometrically constrained positions and refined with

system. A glassy carbon working electron, platinum wire auxiliary electrode, and saturated calomel reference electrode (SCE) were used in a standard three-electrode configuration with tetraethylammonium perchlorate (TEAP) as the supporting electrolyte (substrate concentration ≈ 10−3 M; standard scan rate = 100 mV s−1). UV-visNIR spectroelectrochemical studies were performed in CH3CN/0.1 M Bu4NPF6 at 298 K using an optically transparent thin-layer electrode (OTTLE) cell26 mounted in the sample compartment of a J&M TIDAS spectrophotometer. All spectroelectrochemical experiments were carried out under a dinitrogen atmosphere. 1H NMR spectra were recorded on a Bruker Model Avance III 400 MHz spectrometer. The elemental analyses were recorded on a PerkinElmer Model 240C elemental analyzer. Electrospray mass spectral measurements were done on a Micromass Q-ToF mass spectrometer. Preparation of the Complexes. Synthesis of [Ru(bpy)2(H3L−)]ClO4 ([1]ClO4). The precursor complex [Ru(bpy)2Cl2] (100 mg, 0.21 mmol) and AgClO4 (129 mg, 0.62 mmol) were placed in 25 mL of ethanol and the solution was heated at reflux under dinitrogen atmosphere for 1.5 h. The precipitate AgCl was filtered off through a sintered Gooch crucible, and the filtrate ([Ru(bpy)2(EtOH)2]2+) was taken in a two-necked round-bottom flask. To this filtrate 1,4-diamino9,10-anthraquinone (H4L) (49 mg, 0.21 mmol) ligand was added followed by NEt3 (0.03 mL, 0.21 mmol) and the solution was heated at reflux under dinitrogen atmosphere for 12 h. The initial brown solution gradually changed to red. The dry mass was moistened with a few drops of CH3CN, followed by the addition of saturated aqueous NaClO4 solution to it. It was then chilled overnight. The solid mass thus obtained was filtered off and washed with chilled water, to remove the excess NaClO4, and dried in vacuo over P4O10. It was then purified by column chromatography, using a silica gel column and [1]ClO4 was eluted by 3:1 CH2Cl2−CH3CN mixture as eluent. The pure solid product of [1]ClO4 was obtained on removal of solvent under reduced pressure. Yield: 127 mg (82%). MS(ESI+, CH3CN): m/z {[1]+} calcd, 651.09; found, 651.09. 1H NMR (400 MHz) in CD3CN [δ/ppm (J/ Hz)]: 10.57 (s, 1H), 8.79 (s, 1H), 8.46 (m, 3H), 8.36 (m, 3H), 8.18 (d, 8.24, 1H), 7.93 (m, 3H), 7.77 (m, 3H), 7.52 (m, 3H), 7.43 (m, 3H), 7.29 (t, 5.82, 1H), 7.08 (t, 5.98, 1H), 6.99 (s, 1H), 6.51 (s, 1H). Anal. Calcd for C34H25N6O6ClRu: C, 54.40; H, 3.36; N, 11.20. Found: C, 54.32; H, 3.26; N, 11.09. Molar conductivity (CH3CN): ΛM = 113 Ω−1 cm2 M−1. Synthesis of [Ru(pap)2(H3L−)]ClO4 ([2]ClO4). The precursor complex [Ru(pap)2Cl2] (100 mg, 0.19 mmol) and AgClO4 (116 mg, 0.56 mmol) were placed in 25 mL of ethanol, and the solution was heated at reflux under dinitrogen atmosphere for 1.5 h. The precipitate AgCl was filtered off through a sintered Gooch crucible and the filtrate ([Ru(pap)2(EtOH)2]2+) was placed in a two-necked round-bottom flask. To this filtrate was added 1,4-diamino-9,10-anthraquinone (H4L) (45 mg, 0.19 mmol) ligand, followed by NEt3 (0.03 mL, 0.21 mmol), and the solution was heated at reflux under dinitrogen atmosphere for 7 h. The solution gradually changed to violet. The dry mass was dissolved in a small volume of CH3CN, followed by the addition of saturated aqueous NaClO4 solution to it. It was then chilled overnight. The solid mass thus obtained was filtered off and washed with chilled water to remove the excess NaClO4 and dried in vacuo over P4O10. It was purified on a silica gel column, using a 4:1 CH2Cl2−CH3CN mixture as an eluent. The pure solid product of [2]ClO4 was obtained upon the removal of solvent under reduced pressure. Yield: 105 mg (70%). MS(ESI+, CH3CN): m/z {[2]+} calcd, 705.12; found, 705.12. 1 H NMR (400 MHz) in CDCl3 [δ/ppm (J/Hz)]: 10.85 (s, 1H), 8.75 (d, 8.04 1H), 8.49 (d, 8.08, 2H), 8.27 (m, 1H), 8.12 (m, 1H), 8.03 (m, 2H), 7.91 (t, 7.6, 1H), 7.73 (d, 5.6, 1H), 7.59 (m, 3H), 7.37 (m, 1H), 7.31 (m, 1H), 7.21 (m, 6H), 7.07 (d, 8.0, 2H), 6.92 (d, 8.8, 1H), 6.68 (m, 1H), 5.52 (s, 1H), 5.36 (s, 1H). Anal. Calcd for C36H27N8O6ClRu: C, 53.77; H, 3.38; N, 13.93. Found: C, 53.62; H, 3.26; N, 13.81. Molar conductivity (CH3CN): ΛM = 112 Ω−1 cm2 M−1. Synthesis of [Ru(bpy)2(μ-H2L2−)Ru(acac)2]ClO4 ([3]ClO4). The preformed mononuclear complexes [Ru(bpy)2(H3L−)]ClO4 (100 mg, 0.13 mmol), [Ru(acac)2(CH3CN)2] (51 mg, 0.13 mmol) and NEt3 (0.02 mL, 0.14 mmol) were placed in 25 mL of ethanol. The mixture was refluxed for 12 h under dinitrogen atmosphere. The I


Article

Inorganic Chemistry 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 highly disordered solvent molecules (CH3CN) in the crystal of 4{[1]ClO4}·2CH3CN, for an unidentified solvent molecule in [2]ClO4·C7H8 and [3]ClO4, and for disordered solvent molecules (toluene) in the crystal of [5](ClO4)2. CCDC-1434810 (4[1]ClO4·2CH3CN), CCDC-1434811 ([2]ClO4· C7H8), CCDC-1434812 ([3]ClO4), and CCDC-1434813 ([5](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. Computational Details. Full geometry optimizations were carried out using the density functional theory (DFT) method at the (U)B3LYP level for (U)B3LYP level for 1n (n = 4+, 2+, 0, 1−), 2n (n = 2+, 0, 1−, 2−), 3n (n = 4+, 3+, 2+, 1+, 0, 1−, 2−), 4n (n = 3+, 2+, 1+, 0, 1−, 2−, 3−), 5n (n = 5+, 3+, 1+, 0, 1−) and (R)B3LYP for 1n (n = 3+, 1+), 2n (n = 3+, 1+), 5n (n = 4+, 2+).28 All elements except ruthenium were assigned using the 6-31G(d) basis set. The LanL2DZ basis set with effective core potential was employed for the Ru atom.29 All calculations were performed with the Gaussian09 program package.30 Vertical electronic excitations based on (U)B3LYP optimized geometries were computed using the time-dependent density functional theory (TD-DFT) formalism31 in acetonitrile, using the conductor-like polarizable continuum model (CPCM).32 Chemissian 1.733 was used to calculate the fractional contributions of various groups to each molecular orbital. All the calculated structures were visualized with ChemCraft.34

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(4) (a) Mandal, A.; Agarwala, H.; Ray, R.; Plebst, S.; Mobin, S. M.; Priego, J. L.; Jiménez-Aparicio, R.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2014, 53, 6082−6093. (b) Ghumaan, S.; Mukherjee, S.; Kar, S.; Roy, D.; Mobin, S. M.; Sunoj, R. B.; Lahiri, G. K. Eur. J. Inorg. Chem. 2006, 2006, 4426−4441. (5) Mandal, A.; Grupp, A.; Schwederski, B.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2015, 54, 7936−7944. (6) (a) Pieslinger, G. E.; Aramburu-Trošelj, B. M.; Cadranel, A.; Baraldo, L. M. Inorg. Chem. 2014, 53, 8221−8229. (b) Shao, J.-Y.; Zhong, Y.-W. Chem.Eur. J. 2014, 20, 8702−8713. (c) Yang, W.-W.; Shao, J.-Y.; Zhong, Y.-W. Eur. J. Inorg. Chem. 2015, 2015, 3195−3204. (7) DelMedico, A.; Dodsworth, E. S.; Lever, A. B. P.; Pietro, W. J. Inorg. Chem. 2004, 43, 2654−2671. (8) (a) Churchill, M. R.; Keil, K. M.; Bright, F. V.; Pandey, S.; Baker, G. A.; Keister, J. B. Inorg. Chem. 2000, 39, 5807−5816. (b) Churchill, M. R.; Keil, K. M.; Gilmartin, B. P.; Schuster, J. J.; Keister, J. B.; Janik, T. S. Inorg. Chem. 2001, 40, 4361−4367. (9) Gooden, V. M.; Dasgupta, T. P.; Gordon, N. R.; Sadler, G. G. Inorg. Chim. Acta 1998, 268, 31−36. (10) (a) Kar, S.; Sarkar, B.; Ghumaan, S.; Leboschka, M.; Fiedler, J.; Kaim, W.; Kumar Lahiri, G. Dalton Trans. 2007, 1934−1938. (b) Patra, S.; Sarkar, B.; Maji, S.; Fiedler, J.; Urbanos, F. A.; Jiménez-Aparicio, R.; Kaim, W.; Lahiri, G. K. Chem.Eur. J. 2006, 12, 489−498. (11) (a) Maji, S.; Sarkar, B.; Mobin, S. M.; Fiedler, J.; Kaim, W.; Lahiri, G. K. Dalton Trans. 2007, 2411−2418. (b) Das, D.; Mondal, T. K.; Mobin, S. M.; Lahiri, G. K. Inorg. Chem. 2009, 48, 9800−9810. (12) (a) Mandal, A.; Schwederski, B.; Fiedler, J.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2015, 54, 8126−8135. (b) Mandal, A.; Grupp, A.; Schwederski, B.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2015, 54, 10049−10057. (c) Das, A.; Scherer, T. M.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Chem.Eur. J. 2012, 18, 11007−11018. (13) (a) Mandal, A.; Kundu, T.; Ehret, F.; Bubrin, M.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Dalton Trans. 2014, 43, 2473−2487. (b) Mondal, P.; Plebst, S.; Ray, R.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2014, 53, 9348−9356. (c) Ghosh, P.; Mondal, P.; Ray, R.; Das, A.; Bag, S.; Mobin, S. M.; Lahiri, G. K. Inorg. Chem. 2014, 53, 6094−6106. (14) (a) Das, A.; Scherer, T.; Maji, S.; Mondal, T. K.; Mobin, S. M.; Urbanos, F. A.; Jiménez-Aparicio, R.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2011, 50, 7040−7049. (b) Maji, S.; Sarkar, B.; Mobin, S. M.; Fiedler, J.; Urbanos, F. A.; Jimenez-Aparicio, R.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2008, 47, 5204−5211. (c) Koiwa, T.; Masuda, Y.; Shono, J.; Kawamoto, Y.; Hoshino, Y.; Hashimoto, T.; Natarajan, K.; Shimizu, K. Inorg. Chem. 2004, 43, 6215−6223. (d) Eaton, D. R. J. Am. Chem. Soc. 1965, 87, 3097−3102. (e) Holm, R. H.; Cotton, F. A. J. Am. Chem. Soc. 1958, 80, 5658−5663. (15) (a) Das, A.; Scherer, T. M.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2012, 51, 4390−4397. (b) Das, D.; Mondal, T. K.; Chowdhury, A. D.; Weisser, F.; Schweinfurth, D.; Sarkar, B.; Mobin, S. M.; Urbanos, F. A.; Jimenez-Aparicio, R.; Lahiri, G. K. Dalton Trans. 2011, 40, 8377−8390. (16) (a) Das, A.; Scherer, T. M.; Mondal, P.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Chem.Eur. J. 2012, 18, 14434−14443. (b) Das, D.; Agarwala, H.; Chowdhury, A. D.; Patra, T.; Mobin, S. M.; Sarkar, B.; Kaim, W.; Lahiri, G. K. Chem.Eur. J. 2013, 19, 7384−7394. (17) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1−73. (18) (a) Bond, A. M.; Haga, M. Inorg. Chem. 1986, 25, 4507−4514. (b) Xiao, X. M.; Haga, M.; Matsumurainoue, T.; Ru, Y.; Addison, A. W.; Kano, K. J. Chem. Soc., Dalton Trans. 1993, 16, 2477−2484. (c) Haga, M.; Ano, T.; Kano, K.; Yamabe, S. Inorg. Chem. 1991, 30, 3843−3849. (19) (a) Kaim, W.; Ernst, S.; Kasack, V. J. Am. Chem. Soc. 1990, 112, 173−178. (b) Heilmann, M.; Baumann, F.; Kaim, W.; Fiedler, J. J. Chem. Soc., Faraday Trans. 1996, 92, 4227−4231. (20) (a) Gex, J. N.; DeArmond, M. K.; Hanck, K. W. Inorg. Chem. 1987, 26, 3235−3236. (b) DeArmond, M. K.; Myrick, M. L. Acc. Chem. Res. 1989, 22, 364−370.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02541. Crystal data, bond lengths and angles, energies of DFToptimized states, TD-DFT data, MO compositions, mass spectra, 1H NMR, perspective view of asymmetric unit of [1]ClO4, hydrogen bonding interactions, DFT-optimized structures, EPR, Mulliken spin density representations, and spectroelectrochemistry (PDF) Crystallographic data for [1]ClO4 (CIF) Crystallographic data for [2]ClO4 (CIF) Crystallographic data for [3]ClO4 (CIF) Crystallographic data for [5](ClO4)2 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W. Kaim). *E-mail: [email protected] (G. K. Lahiri). 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 Land BadenWürttemberg (Germany) is gratefully acknowledged.

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REFERENCES

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