Electronic Structure and Multicatalytic Features of Redox-Active Bis

Article pubs.acs.org/IC

Electronic Structure and Multicatalytic Features of Redox-Active Bis(arylimino)acenaphthene (BIAN)-Derived Ruthenium Complexes Arijit Singha Hazari, Ritwika Ray, Md Asmaul Hoque, and Goutam Kumar Lahiri* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India S Supporting Information *

ABSTRACT: The article examines the newly designed and structurally characterized redox-active BIAN-derived [Ru(trpy)(RBIAN)Cl]ClO4 ([1a]ClO4−[1c]ClO4), [Ru(trpy)(R-BIAN)(H2O)](ClO4)2 ([3a](ClO4)2−[3c](ClO4)2), and BIAO-derived [Ru(trpy)(BIAO)Cl]ClO4 ([2a]ClO4) (trpy = 2,2′:6′,2′′-terpyridine, R-BIAN = bis(arylimino)acenaphthene (R = H (1a+, 3a2+), 4-OMe (1b+, 3b2+), 4-NO2 (1c+, 3c2+), BIAO = [N-(phenyl)imino]acenapthenone). The experimental (X-ray, 1H NMR, spectroelectrochemistry, EPR) and DFT/TD-DFT calculations of 1an−1cn or 2an collectively establish {RuII−BIAN0} or {RuII− BIAO0} configuration in the native state, metal-based oxidation to {RuIII−BIAN0} or {RuIII−BIAO0}, and successive electron uptake processes by the α-diimine fragment, followed by trpy and naphthalene π-system of BIAN or BIAO, respectively. The impact of the electron-withdrawing NO2 function in the BIAN moiety in 1c+ has been reflected in the five nearby reduction steps within the accessible potential limit of −2 V versus SCE, leading to a fully reduced BIAN4− state in [1c]4−. The aqua derivatives ({RuII−OH2}, 3a2+−3c2+) undergo simultaneous 2e−/2H+ transfer to the corresponding {RuIVO} state and the catalytic current associated with the RuIV/RuV response probably implies its involvement in the electrocatalytic water oxidation. The aqua derivatives (3a2+−3c2+) are efficient and selective precatalysts in transforming a wide variety of alkenes to corresponding epoxides in the presence of PhI(OAc)2 as an oxidant in CH2Cl2 at 298 K as well as oxidation of primary, secondary, and heterocyclic alcohols with a large substrate scope with H2O2 as the stoichiometric oxidant in CH3CN at 343 K. The involvement of the {RuIVO} intermediate as the active catalyst in both the oxidation processes has been ascertained via a sequence of experimental evidence.

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of α-diimine and multiple cation (Na+)-coordinated naphthalene fragment of 1,2-bis[(2,6-diisopropylphenyl) imino]acenaphthene, leading to its tetranionic state1r raises the pertinent challenge of designing suitable transition-metal complexes capable of undergoing four electron reduction processes involving the coordinated BIAN function.1j In this context, the present report deals with the structurally characterized selective ruthenium complexes of BIAN and the corresponding in situ generated partially hydrolyzed [N(phenyl)imino]acenapthenone 1j,2 (BIAO, Scheme 1) in combination with tridentate 2,2′:6′,2′′-terpyridine/Cl/H2O co-ligands (Scheme 3). The primary objective of the article is to address the following aspects: (i) Impact of the “R” group on the electron uptake processes of coordinated BIAN in the light of exploring the feasibility of pushing the electron(s) into the antibonding orbitals of naphthalene, i.e., beyond its α-diimine fragment. (ii) Application potential of the complexes toward different catalytic processes.

INTRODUCTION The metal complexes of electronically and structurally demanding naphthalene conjugated α-diimine derived robust framework of BIAN (where BIAN = bis(arylimino)acenaphthene) (see Scheme 1) has drawn continuing attention Scheme 1. Representation of Ligand Frameworks

in recent years from the broader perspectives of coordination chemistry, including their effectivity in catalysis.1 The low-lying vacant orbitals, π*(α-diimine) (5b2) and π*(naphthalene) (4a2) of BIAN can, in principle, accept two successive electrons each (Scheme 2),1j resulting in a multielectron reservoir for potential application in catalysis.,1b−f,j,l,m The easy tunability of its electronic features, by varying the “R” groups and by the choice of selective metal fragment, also makes it more attractive and therefore introduces an additional impetus. The recent report of a stepwise two-electron uptake by both the π-systems © XXXX American Chemical Society

Received: May 27, 2016

A

DOI: 10.1021/acs.inorgchem.6b01280 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 2. Successive Electron Uptake Processes of BIAN

Scheme 3. Representation of Complexes

Figure 1. Perspective views of the cationic part of (a) [1a]ClO4·CH3CN·CH2Cl2, (b) [1b]ClO4, and (c) [1c]ClO4. Ellipsoids are drawn at 50% probability level. Hydrogen atoms (C−H) and solvent molecules are removed for the sake of clarity.

B


Article

Inorganic Chemistry (iii) The difference between the BIAN and BIAO derived complexes, if any.

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RESULTS AND DISCUSSION Synthesis, General Characterization, and Crystal Structure. The BIAN (BIAN = bis(arylimino)acenaphthene)-derived complexes [RuII(trpy)(R-BIAN)Cl]ClO4 (R = H ([1a]ClO4), OMe ([1b]ClO4), NO2 ([1c]ClO4)) have been prepared from the precursor RuIII(trpy)(Cl)3 (trpy = 2,2′:6′,2′′-terpyridine) and respective BIAN derivatives in refluxing EtOH under a dinitrogen (N2) atmosphere. The BIAO (BIAO = [N-(phenyl)imino]acenapthenone)-based [RuII(trpy)(BIAO)Cl]ClO4 ([2a]ClO4) is obtained as a minor product, along with [1a]ClO4 via partial hydrolysis of one of the imine bonds (−CN− → −CO) of unsubstituted BIAN. No such hydrolyzed product is obtained in the case of OMe or NO2 substituted BIAN. The aqua derivatives [RuII(trpy)(BIAN)(OH2)](ClO4)2 ([3a](ClO4)2− [3c](ClO4)2) have been obtained from the respective chloro complexes by Cl/H2O exchange process in the presence of aqueous AgClO4 solution (see Scheme 3, and the Experimental Section). However, the BIAO-derived [2a]ClO4 has failed to yield a pure aqua derivative: only partial Cl/H2O exchange happens, even under prolonged (∼48 h) refluxing conditions. The presence of two strongly π-accepting ligandsBIAN or BIAO and trpy1j,3in the complex moieties preferentially stabilizes the ruthenium(II) state with E0(RuII/RuIII) ≈ 1.0 V versus SCE (see later discussion). The complexes have been characterized by standard analytical techniques including their satisfactory microanalytical data, electrical conductivity, mass spectrometry (see Figure S1 in the Supporting Information), and infrared spectroscopy (Experimental Section). The molecular forms of all the chloro derivatives [1a]ClO4− [1c]ClO4, [2a]ClO4, and the representative aqua species [3b](ClO4)2 are authenticated by their single-crystal X-ray structures (see Figures 1 and 2, and Table 1, as well as Figure S2 and Tables S1−S11 in the Supporting Information). The asymmetric units of [1a]ClO4 and [1c]ClO4 consist of two independent molecules with a slight difference in bond parameters, because of the effect of crystal packing forces. The three nitrogen donors of trpy bind to the metal ion in usual meridional mode.3,4 The remaining three positions around the metal ion in BIAN-derived complexes [1a]ClO4− [1c]ClO4/[3b](ClO4)2 and BIAO-derived [2a]ClO4 are occupied by the N,N,Cl/N,N,O and N,O,Cl donors, respectively. The appreciable deviations of cis and trans angles around the metal ion in the complexes. with respect to ideal (90° and 180°, respectively) (Tables S4, S6, S8, S10, and S11 in the Supporting Information) suggest a distorted octahedral situation. The aryl groups of BIAN or BIAO in the complexes are appreciably twisted, with reference to its naphthalene plane.1j,k,r The comparison of the redox-sensitive metric parameters of BIAN and BIAO (Scheme 2), i.e., C−N [1.30 Å (BIAN), 1.35 Å (BIAN• −), 1.38 Å (BIAN2−)], C−C [ ≥1.47 Å (BIAN/ BIAO), ∼1.43 Å (BIAN• −/BIAO• −), ∼1.38 Å (BIAN2−/ BIAO2−)], and C−O [1.25 Å (BIAO), 1.30 Å (BIAO• −), 1.34 Å (BIAO2−)]1j,5 with those of [1a]ClO4−[1c]ClO4/[3b](ClO4)2 and [2a]ClO4, respectively, (Table 1) establishes their unreduced state, leading to the electronic structural formulations of {RuII−BIAN0} and {RuII−BIAO0}, respectively.

Figure 2. Perspective views of the cationic part of (a) [2a]ClO4 and (b) [3b](ClO4)2·2(C3H6O). Ellipsoids are drawn at 50% probability level. Hydrogen atoms (C−H) (except the OH protons) and solvent molecules (C3H6O) are removed for the sake of clarity.

The Ru II −N(trpy), 6 Ru II −N(BIAN/BIAO), 1j,5 Ru II −O(BIAO),1j RuII−Cl,7 and RuII−O(OH2)8 distances in the complexes match fairly well with the reported analogous complexes. The DFT-calculated bond parameters (see Table 1, as well as Tables S3−S11 and Figure S2 in the Supporting Information) reproduce the respective experimental data. In accordance with the diamagnetic feature of [1a]ClO4− [1c]ClO4, [2a]ClO4, and [3a](ClO4)2−[3c](ClO4)2, their 1H NMR spectra in (CD3)2SO and (CD3)2CO, respectively, display calculated the number of sharp but severely overlapped aromatic proton resonances within the chemical shift range δ, 5−9 ppm (see Figure S3 in the Supporting Information, and the Experimental Section). Two singlets corresponding to the OMe proton resonances of 1b+ and 3b2+ appear within the δ range of 3−4 ppm. Electrochemistry, Electron Paramagnetic Resonance (EPR), and Electronic Structures of Chloro Derivatives. The cyclic and differential pulse voltammograms of the chloro complexes [1a]ClO4−[1c]ClO4 and [2a]ClO4 in CH3CN are shown in Figure 3, and the data are given in Table 2, which include one oxidation and multiple nearby reduction processes within the potential window of ±2 V versus SCE. The reversibility of the oxidation (O1) and first reduction (R1) processes has been ascertained by constant potential coulometry. The effect of two strongly π-acceptor ligands (BIAN or BIAO and trpy) in the complex frameworks has been reflected in appreciable stability of the Ru(II) state (E0 (RuII/ RuIII) ≈ 1.0 V). The Ru(II)/Ru(III) potential varies, based on the electron-withdrawing group (NO2) or the electrondonating group (OMe) associated with BIAN; this variance is also due to the impact of BIAN (dimine) versus BIAO (ketoimine), and it follows the order 1c+ ≥ 2a+ > 1a+ > 1b+. Consequently, the first reduction (R1) of the BIAO- and NO2− C


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Inorganic Chemistry

Table 1. Selected Experimental and DFT Calculated Bond Lengths for the Cations in [1a]ClO4·CH2Cl2·CH3CN, [1b]ClO4, [1c]ClO4, [2a]ClO4, and [3b](ClO4)2·2(C3H6O) Bond Length (Å) +

1a (molecule A)

1b

+

1c+(molecule A)

2a+

3b2+

atom pair

X-ray

DFT

X-ray

DFT

X-ray

DFT

X-ray

DFT

X-ray

DFT

Ru1−N1 Ru1−N2 Ru1−N3 Ru1−N4 Ru1−N5 Ru1−N6 Ru1−O1 Ru1−O2 Ru1−Cl1 C1−N1 C28−N1 C34−N1 C35−N1 C11−N2 C13−N2 C1−C11 C11−O1

2.023(3) 2.087(3) 2.097(3) 1.965(3) 2.072(3)

2.095 2.167 2.112 1.990 2.116

2.024(3) 2.093(3) 2.075(3) 1.968(3) 2.068(3)

2.093 2.169 2.113 1.990 2.110

2.104(4) 2.028(4)

2.166 2.099

2.084(4) 1.968(4) 2.081(4)

2.119 1.994 2.117

2.017(3) 2.060(3) 1.941(3) 2.077(3)

2.103 2.104 1.966 2.105

2.009(5) 2.084(5) 2.095(5) 1.983(5) 2.079(5)

2.068 2.136 2.137 2.015 2.026

2.099(3)

2.190 2.149(4)

2.233

2.3950(13) 1.324(5) 1.430(5)

2.403 1.306 1.434

1.314(7)

1.304

1.450(7) 1.315(7) 1.459(7) 1.476(8)

1.438 1.306 1.429 1.492

2.3981(10) 1.306(5)

2.434 1.304

1.447(5)

1.433

1.312(5) 1.435(5) 1.460(5)

1.307 1.429 1.482

2.4088(10) 1.317(4)

2.440 1.304

1.450(4) 1.306(5) 1.429(4) 1.465(5)

1.434 1.309 1.425 1.480

2.4118(15) 1.324(6)

2.425 1.308

1.441(6)

1.427

1.321(6) 1.446(6) 1.471(7)

1.305 1.427 1.482

1.459(6) 1.256(4)

1.491 1.247

BIAN (1b+)-derived complexes. The separation in potential between O1 and R1 processes translates to a large comproportionation constant (Kc1 = 1020−1030, where Kc1 is determined using the expression RTlnKc = nF(ΔE)9), justifying the stability of the isolated {RuII−BIAN} or {RuII−BIAO} configuration. However, the successive nearby reduction processes (R1−R5) with varying separation in potential lead to relatively smaller Kc values (see Figure 3 and Table 2). Furthermore, the RuIIRuIII potential (O1), as well as successive reduction processes (R) of 1a+−1c+ and 2a+, are reasonably positive shifted, with respect to earlier reported analogous tpm (tpm = tris(1-pyrazolyl)methane) derived complexes due to the stronger π-acceptor characteristic of trpy. The metal (RuII/RuIII)-based oxidation process (O1 in Figure 3), as well as successive first two reductions (R1 and R2, in Figure 3) of the diimine (−NC−CN−) or ketoimine (OC−CN−) fragment of BIAN or BIAO in 1a+−1c+ or 2a+, respectively, have been supported collectively by DFT-

Figure 3. Cyclic (black) and differential pulse (red) voltammograms of (a) [1a]ClO4, (b) [1b]ClO4, (c) [1c]ClO4, and (d) [2a]ClO4 in CH3CN.

BIAN-derived 2a+ and 1c+, respectively, takes place at a much lower negative potential, compared to BIAN (1a+)- and OMe− Table 2. Electrochemical Dataa E0298/V (ΔEp/mV)b O1 R1 R2 R3 R4 R5 Kcc,d Kc1 Kc2 Kc3 Kc4 Kc5

[1a]ClO4

[1b]ClO4

[1c]ClO4

[2a]ClO4

0.91 (70) −0.90 (50) −1.40 (60) −1.80 (90)

0.89 (100) −0.94 (160) −1.46 (90) −1.82 (110)

1.04 (90) −0.66 (50) −0.80 (70) −1.39 (100) −1.61 (140) −1.87e

1.01 (70) −0.55 (60) −1.16 (90) −1.78 (100)

4.7 × 1020 4.4 × 108 4.1 × 106

1.0 × 1030 6.5 × 108 1.2 × 106

2.0 2.3 2.3 5.3 2.5

× × × × ×

1028 102 1010 103 104

2.7 × 1026 2.1 × 1010 3.2 × 1010

From cyclic voltammetry in CH3CN/0.1 M Et4NClO4 at 100 mV s−1. bPotentials in terms of V (versus SCE); peak potential differences ΔE are given in units of mV (in parentheses). cComproportionation constant from RT ln Kc = nF(ΔE). dKc1 is between Ox1 and Red1, Kc2 is between Red1 and Red2, Kc3 is between Red2 and Red3, Kc4 is between Red3 and Red4, and Kc5 is between Red4 and Red5. eIrreversible. a

D


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Inorganic Chemistry

been assigned by TD-DFT calculations (see Figure S5 and Table S38 in the Supporting Information). The complexes generally exhibit similar spectral features, with some variations, in terms of band position and intensity. The higher-energy UVregion bands are dominated by intraligand (BIAN or BIAO) and interligand (BIAN → trpy or BIAO → trpy) transitions. The starting complexes with n = +1, display primarily a BIAN or BIAO-targeted intense metal-to-ligand charge transfer (MLCT) transition of ∼550 nm, which, however, undergoes blue-shifting to ∼420 nm with appreciable reduction in intensity on oxidation to n = +2 state. On one-electron reduction to n = 0 state, substantial decrease in intensity of the BIAN or BIAO-targeted MLCT transition occurs.1j,5 Electrochemistry of Aqua Derivatives. Cyclic and differential pulse voltammograms of the aqua derivatives [RuII(trpy)(BIAN)(OH2)](ClO4)2, [3a](ClO4)2, [3b](ClO4)2, and [3c](ClO4)2 in aqueous phosphate buffer at pH 7.0 divulge a simultaneous two-electron reversible oxidation process in each case, E0298, in volts (ΔEp, in millivolts) at 0.61 (60), 0.58 (60) and 0.66 (60), respectively, corresponding to

calculated MO compositions (see Tables S3 and S12−S36 in the Supporting Information) and Mulliken spin density distributions at the paramagnetic states (see Table S37 and Figure S4 in the Supporting Information). In accordance to that, representative one-electron oxidized 1b2+ (RuIII, t2g5) and one-electron reduced 1c/2a exhibit anisotropic (g1 = 2.369, g2 = 2.136, g3 = 1.828, Δg = 0.541, ⟨g⟩ = 2.122; Δg = g1 − g3; ⟨g⟩ = [1/3(g12 + g22 + g32)]1/2),10 and radical-based (g = 2.003/2.010 with a peak-to-peak separation equal to 16 G/34 G)11 EPR signatures, respectively (Figure 4). The MO compositions, as

{Ru II−OH 2} (32+) ⇌ {Ru IV O} (42+)

Figure 4. Electron paramagnetic resonance (EPR) spectra of electrogenerated (a) 1b2+, (b) 1c, and (c) 2a at 77 K in CH3CN/ 0.1 M Et4NClO4.

well as spin density plots, also suggest that the third reduction (R3 in Figure 3, S = 1/2) selectively occurs at the trpy center in each case.12 The direct impact of the introduction of an electron-withdrawing NO2 group in the BIAN framework of 1c+ has facilitated the additional two reduction processes (R4 and R5 in Figure 3) involving the naphthalene fragment of BIAN, as has been revealed by the corresponding MOs and spin density plots. Thus, unlike 1a+, 1b+, and 2a+, the selective combination of NO2−BIAN and strongly π-accepting trpy in 1c+ has successfully pumped two electrons each into the α-diimine and naphthalene fragments of BIAN (Scheme 2), leading to a coordinated tetranionic state of BIAN4− in 1c3−, which has been reported earlier in the case of the Na+-coordinated naphthalene π-system of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene.1r In contrast, our earlier approach with a tpm/NO2−BIAN-derived analogous molecular setup has failed to reach the state of BIAN4−.1j The BIAN or BIAO centric successive electron uptake processes (Scheme 2) in 1a+−1c+ or 2a+ has also been reflected in the gradual increase and decrease of the DFT-calculated C− N, C−O, and C−C bond distances, respectively (see Tables S3, S5, S7, and S9 in the Supporting Information). Spectroelectrochemistry and TD-DFT Calculations. The electronic spectral features of the chloro derivatives 1an− 1cn and 2an in the reversible redox states (n = +2, +1, 0) have

Figure 5. (a) Cyclic voltammogram of [RuII(trpy)(BIAN)OH2]2+ (3a2+) in aqueous phosphate buffer at pH 7.0 (black trace) and for the blank solution (blue trace) (scan rate = 100 mV s−1; reference, SCE). Insets show the reversible {RuIVO}/{RuII−OH2} couple and variation of E0({RuIVO}/{RuII−OH2}) with the change in pH (1.0−8.0). (b) Change in absorbances of [3a](ClO4)2, as a function of [CeIV] in 0.5 M H2SO4 (1:2 ratio). Inset shows the linear decrease in intensity of the 535 nm band of 3a2+, relative to the increase in [CeIV]: [3a2+] ratio.

(see Figure 5a and Table 3).13−16 The consequence of varying substituents in the BIAN framework has been reflected in an 80 mV positive shift in potential upon moving from 3b2+ (OMe− BIAN) to 3c2+ (NO2−BIAN). The metal-based oxidation process has also been supported by a 69% metal contribution in the highest occupied molecular orbital (HOMO) of 3a2+ (Table S36 in the Supporting Information). The two-electron oxidation process of representative 3a2+ to 4a2+ has been corroborated via the stoichiometric chemical oxidation of the E


Article

Inorganic Chemistry Table 3. Comparative Electrochemical Data of the Aqua Species E0298/V (ΔEp/mV)b complex

a

[RuII(trpy)(BIAN)OH2]2+ (3a2+) [RuII(trpy)(OMe-BIAN)OH2]2+ (3b2+) [RuII(trpy)(NO2−BIAN)OH2]2+ (3c2+) [RuII(trpy)(bpy)OH2]2+ [RuII(trpy)(6,6/-F2-bpy)OH2]2+ [RuII(trpy)(5,5/-F2-bpy)OH2]2+ [RuII(trpy)(bpm)OH2]2+ [RuII(tpm)(BIAN)OH2]2+ a b

IV/III

0.59

III/II

IV/II

V/IV

pKa

ref

0.61 (60) 0.58 (60) 0.66 (60)

1.69

9.1

1.62 1.69 1.68

9.8 10.4 9.0 9.7

this work this work this work 17 14 14 15 1j

0.48 0.56 0.54 0.62

0.85 (70)

0.56 (60)

bpy = 2,2′-bipyridine, bpm = 2,2′-bipyrimidine, BIAN = bis(arylimino)acenaphthene, tpm = tris(1-pyrazolyl)methane, trpy = 2,2′:6′,2′′-terpyridine. From cyclic voltammetry in a pH 7.0 aqueous phosphate buffer at 100 mV s−1. Potentials given in terms of V (versus SCE).

aqueous solution of 3a2+ by (NH4)2Ce(NO3)6 in 0.5 M H2SO4 in a 1:2 ratio. It shows the steady decrease in intensity of the 535 nm band of 3a2+ with the concomitant growth of a new band at 426 nm that is characteristic of 4a2+ (RuIVO) (Figure 5b).13a,15 The {RuIVO}/{RuII−OH2} potential of representative 3a2+ decreases linearly as the pH increases (pH = 1.0−8.0) and the calculated slope of 59 mV per unit change in pH supports the simultaneous 2e−/2H+ transfer process.16 The analogous Ru-trpy-derived aqua complex with a bpy co-ligand exhibits successive 1e−/1H+ transfers corresponding to17

inefficient toward epoxidation under present conditions, implying the superiority of the corresponding aqua derivatives (3a2+−3c2+) in facilitating the desired transformation. Initial optimization with the preformed catalyst 3a2+ and styrene as a model substrate in the presence of various oxidants (H2O2 (hydrogen peroxide), PIDA (iodobenzene diacetate), TBHP (tert-butyl hydrogen peroxide), m-CPBA (meta-chloro perbenzoic acid)) in CH2Cl2, CH3CN, CH3OH, and t-amyl alcohol solvents at 298 K, reveals that PIDA and CH2Cl2 serve as the most effective combination, with a catalyst:substrate:oxidant molar ratio of 1:100:200, as is evident from the 92% GC yield of styrene oxide (Table 4). Electron-donating groups (OMe) and electron-withdrawing groups (NO2) at the para position of the arylimino moieties of BIAN in 3b2+ and 3c2+, respectively, do not influence the catalytic epoxidation process, and only a marginal difference in the product yields is observed. A control experiment, in the absence of the catalyst (32+), also fails to render any epoxide under optimized reaction conditions. As expected, chloro derivative of the partially hydrolyzed BIAO-derived 2a+ (Scheme 3) fails to facilitate the transformation of styrene to epoxide under identical reaction conditions.1j Furthermore, the individual role of metal (Ru) and free ligand (trpy or BIAN), as well as a combination of all three, have been investigated with styrene as a model substrate. It has been observed that neither the free ligand, nor Ru (as RuCl3), nor even the combination of RuCl3 and a free ligand (trpy) exhibits any conversion of alkene to corresponding epoxide (entries 1−3 in Table S39 in the Supporting Information). However, only a minute quantity of epoxide was observed with a combination of RuCl3 and free BIAN (entry 4 in Table S39). Interestingly, a combination of a stoichiometric mixture of RuCl3 and free ligands (trpy and BIAN), in a molar ratio of 0.5:1:1, results in an appreciable yield of epoxide (60% GC yield), which implies the dominance of the preformed catalyst (32+), in comparison to the in situ generated catalyst. A large variety of alkenes have been evaluated under the optimized reaction conditions using 3a2+ as a catalyst (Table 5). It generally exhibits excellent conversion and selectivity for almost all evaluated olefins. Both terminal and internal alkenes are efficiently converted to their respective epoxides in appreciable yields. The higher selectivity imparted by 3a2+, in comparison to the earlier reported [Ru(tpm)(BIAN)(OH2)]2+ complex,1j can be ascribed to the concerted process involving the transformation of the former,23

{Ru II−OH 2} ⇌ {Ru III−OH} ⇌ {Ru IV O}

while the same with co-ligand 6,6′-F2-bpy14 or 5,5′-F2-bpy14 or bpm15 (bpy = 2,2′-bipyridine, bpm = 2,2′-bipyrimidine) undergoes simultaneous 2e−/2H+ transfer process as in 32+, because of the unstable feature of the intermediate {RuIII−OH} state, with respect to the disproportion to {RuIVO}. On the other hand, other BIAN-derived aqua complex [RuII(tpm)(BIAN)OH2]2+ (tpm = tris(1-pyrazolyl)methane)1j displays successive 1e−/1H+ transfers as in the trpy/bpy system. The estimated pKa value of 3a2+ (9.1) is similar to that reported for the analogous systems (see Table 3, as well as Figure S6 in the Supporting Information). The observed catalytic current associated with the RuIV/RuV process (see Figure 5a, as well as Figure S7 in the Supporting Information) possibly suggests its involvement in the electrocatalytic oxidation of H2O.14,15a,18−20 Note that, unlike 3a2+, the earlier reported analogous [RuII(tpm)(BIAN)OH2]2+ complex has failed to display such an electrocatalytic wave.1j Catalytic Epoxidation. The accessibility of the high-valent metal-oxo (MIVO) intermediate is one of the important factors responsible for its widespread application in different oxidative transformations.21 Thus, suitably designed rutheniumaqua ({L−Ru−OH2}, where L is a co-ligand) species have received much attention, because of their facile electrochemical or chemical transformation to the reactive {RuIVO} intermediate without affecting the coordination environment, which, in turn, broadened their utilization in effective oxygen transfer to olefinic groups.22 In this regard, exploration of the newly designed ruthenium-aqua derivatives ([RuII(trpy)(BIAN)OH2]2+, 3a2+−3c2+, Scheme 3) as precatalysts is found to be highly effective toward epoxidation of a wide variety of internal and terminal alkenes in moderate to excellent yields with PIDA (iodobenzene diacetate) as the oxidant. However, contrary to our previous report with the analogous [RuII(tpm)(BIAN)Cl]+,1j the chloro derivatives [RuII(trpy)(BIAN)Cl]+ (1a+−1c+, Scheme 3) has been found to be

{Ru II−OH 2} → {Ru IV O} F


Article

Inorganic Chemistry Table 4. Optimization of the Critical Reaction Parametersa

entry

solvent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH3OH CH3OH CH3OH CH3OH CH3CN CH3CN CH3CN CH3CN t-amyl alcohol t-amyl alcohol t-amyl alcohol t-amyl alcohol

22

CH2Cl2

23

CH2Cl2

catalyst amount/mol % Catalyst 3a 1 1 1 1 0.5 2 1 1

oxidant

yield/%b

H2O2 TBHP m-CPBA PIDA PIDA PIDA PIDAc PIDAd O2e H2O2 TBHP m-CPBA PIDA H2O2 TBHP m-CPBA PIDA PIDA H2O2 TBHP m-CPBA

35 30 22 92 82 93 94 50 nrf 45 50 52 56 30 18 34 70 72 20 31 36

PIDA

90

PIDA

87

Table 5. Catalytic Epoxidation of Olefinsa

2+

1 1 1 1 1 1 1 1 1 1 1 1 Catalyst 3b2+ 1 Catalyst 3c2+ 1

a

1 mmol of styrene, 3 mL of solvent, 2 equiv of oxidant, with respect to the substrate; 298 K, for 5 h, under aerobic conditions. b% GC yields using n-decane as an internal standard. TBHP, tert-butyl hydrogen peroxide; PIDA, iodobenzene diacetate; m-CPBA, metachloroperbenzoic acid; H2O2, hydrogen peroxide. cOxidant (PIDA) = 3 equiv, with respect to substrate. dOxidant (PIDA) = 1 equiv, with respect to substrate. eO2 = 1 atm (balloon). fNo reaction.

instead of stepwise processes of the latter, {Ru II−OH 2} → {Ru III−OH} → {Ru IV O}

Remarkably, both cis- and trans-stilbenes (entries 7 and 8, Table 5) retain their configurations during epoxidation under present reaction conditions, resulting, selectively, in the respective stilbene oxides. Notably, unactivated olefins (entries 10−16, Table 5) exhibit quantitative conversion to their corresponding epoxides with 100% selectivity in most of the cases (entries 10, 12, 15−17), except for a few cases where the selectivity was appreciably decreased (entries 11, 13, 14). Interestingly, cyclooctadiene affords diepoxide predominantly over monoepoxide (entry 14, Table 5). Intermolecular competitive reaction between n-decene (terminal alkene) and trans-5-decene (internal alkene) results in preferential formation of 1-decene oxide over trans-5-decene oxide, thereby indicating the greater reactivity of the terminal alkene, compared to its internal counterpart (entry 17, Table 5). The initial conversion of ruthenium-aqua complex (3a2+) to the active ruthenium-oxo intermediate (4a2+) in the catalytic cycle (Scheme 4) has been ascertained by the ESI-MS of the reaction mixture under standard conditions, which detects a peak at m/z = 782.12, corresponding to {[RuIV(trpy)(BIAN)-

a

1 mmol of styrene, 3 mL of solvent, 2 equiv of oxidant, with respect to the substrate; 298 K, for 5 h, under aerobic conditions. Products are analyzed by GC with n-decane as an internal standard as well as by GC-MS. bGC yield. cSelectivity in terms of epoxide formation.

(O)]ClO4}+ (C39H27ClN5O5Ru) (see Figure S8 in the Supporting Information). Furthermore, the addition of PIDA to the CH2Cl2 solution of 3a2+ has led to a spontaneous change in color of the solution from pink to red, which corresponds to G


Article

Inorganic Chemistry

with the simultaneous formation of Cα−O and Cβ−O bonds (see Figure S11 in the Supporting Information). (ii) The natural charge on Ru decreases from 0.89 to 0.69, while the natural charge for the oxygen atom becomes more negative, going from −0.46 to −0.57, which, in effect, reduces the electrophilic character of the ruthenium-oxo bond and thereby facilitates the concerted pathway. (iii) Complete delocalization of charge in the adduct (5a), as revealed by Mulliken spin density analysis, also indicates that a concerted process is followed. (iv) Complete disappearance of the spin density on the Cα− Cβ bond further justifies the formation of intermediate “a” in Scheme 5 as the plausible transition state (see Figure S12 in the Supporting Information). Catalytic Oxidation of Alcohols. The oxidation of alcohols to their corresponding carbonyls constitutes one of the key transformations in organic synthesis; hence, the development of novel and greener methods for such a transformation still remains one of the pertinent challenges in chemistry.26 Conventional methods for alcohol oxidation generally involve the stoichiometric use of strong oxidants such as oxides of Cr, Mn, and Ru, as well as hypervalent iodine reagents or periodates.27 However, these traditional methods suffer from major shortcomings, which include the generation of a large amount of deleterious byproducts. Therefore, methods relying upon molecular oxygen28 or H2O229 as a stoichiometric oxidant is more sustainable since it produces water as the sole byproduct. Among them, the issue of safety is a major concern for reactions with oxygen under elevated temperature.29 Alternatively, under suitable reaction conditions, H2O2 is an excellent candidate for clean oxidation in fine chemical synthesis.29 In this regard, exploration of the catalytic application of the newly designed ruthenium-aqua complexes of BIAN (3a2+−3c2+) reveals their activity toward the oxidation of primary and secondary alcohols with H2O2 as the stoichiometric oxidant. Most importantly, no formation of overoxidized byproducts is observed under the present reaction conditions, thereby implying the selective nature of the catalysts toward the desired aldehyde or ketone formation. However, the chloro counterparts of the BIAN (1a+−1c+) or partially in situ hydrolyzed BIAO (2a+)-derived complexes are found to be ineffective as catalysts for the same reaction. Initial optimization with benzyl alcohol as the model substrate and 3a2+ as the catalyst results in poor conversion of the substrate (entries 1 and 2 in Table 6) in acetonitrile under oxygen atmosphere. Significantly, the same reaction in the presence of H2O2 as the oxidant affords benzaldehyde in a yield of 86% (entry 3 in Table 6), which indicates the requirement of a strong oxidant to generate the active oxoruthenium intermediate for the desired oxidation. Interestingly, PhI(OAc)2, which is a stronger organic oxidant, gives a poor yield of aldehyde, insinuating the selectivity of H2O2 in guiding the product distribution profile. In addition, screening with an array of solvents reveals acetonitrile to be ideal for the desired transformation (see entries 4−12 in Table 6). Moreover, the oxidation of the substrates is insensitive toward the electronic nature of the substituents in the BIAN moiety and the yield is not altered to a significant extent with 3b2+ or 3c2+ as the catalyst (entries 13 and 14 in Table 6). As expected, a control experiment, in the absence of the catalyst, completely fails to render any aldehyde. Notably, using the earlier reported [Ru(tpm)(BIAN)(OH2)]ClO41j as the pre-

Scheme 4. Proposed Reaction Mechanism for Epoxidation Process

the in situ formation of {RuIVO} as the active species. This has also been corroborated by ultraviolet−visible light (UV-vis) spectroscopy, which exhibits the disappearance of bands at 535 and 447 nm of 3a2+ with a concomitant formation a new band at 426 nm for 4a2+. Moreover, upon the addition of Me2S or PPh3 to the CH2Cl2 solution of 3a2+, the aforementioned red color for {RuIVO} reverts back to the original pink color, along with the reappearance of the MLCT band at 535 nm, which can be attributed to the formation of Me2SO or PPh3O, with a simultaneous reduction of RuIV to RuII (see Figure S9 in the Supporting Information).22a,24 This has further been supported by the 31P NMR peak at δ = 29.96 ppm, corresponding to PPh3O, in contrast to free PPh3 at δ = −5.6 ppm upon addition of PPh3 to a freshly prepared solution of 4a2+ (see Figure S10 in the Supporting Information).22a Generally, the transfer of oxygen atoms from metal-oxo species to olefins follows a less-hindered pathway via the formation of any one of the five possible active intermediates: (a) concerted transition state, (b) radical, (c) carbocation, (d) π-radical cation, and (e) metallaoxetane (Scheme 5).22a,24a,25 Scheme 5. Probable Intermediates for Metal-Mediated Transfer of the Oxygen Atom from Metal-oxo Species to Olefin

DFT optimization for the interaction between the metal-oxo species and styrene as a model substrate ([5a]) indicates the involvement of a concerted transition state pathway, which can be justified as follows: (i) An increase in the Ru−O bond length, from 1.78 Å to 2.63 Å, and the Cα−Cβ bond length of styrene, from 1.33 Å to 1.47 Å, as the olefin gradually approaches the metal-oxo species, H


Article

Inorganic Chemistry Table 6. Optimization of the Critical Reaction Parametersa

entry

solvent

catalyst amount/mol %

oxidant

yield/%b

O2c O2d H2O2 PIDA H2O2 H2O2 H2O2 H2O2 O2e H2O2 H2O2 H2O2

25 26 86 65 25 40 10 11 12 10 70 90

H2O2

87

H2O2

80

H2O2

30

H2O2

20

Table 7. Catalytic Oxidation of Primary Alcohols

2+

1 2 3 4 5 6 7 8 9 10 11 12

CH3CN CH3CN CH3CN CH3CN CH3CNc CH2Cl2c 1,4-dioxane toluene toluene THF CH3CN CH3CN

13

CH3CN

14

CH3CN

15

CH3CN

16

CH3CN

Catalyst 3a 1 1 1 1 1 1 1 1 1 1 0.5 2 Catalyst 3b2+ 1 Catalyst 3c2+ 1 [Ru(tpm)(BIAN)OH2]ClO4 1 RuCl3 1

Table 8. Catalytic Oxidation of Primary Heterocyclic Alcohols

a

Reaction conditions: 0.5 mmol of benzyl alcohol, 3 mL of solvent, 2 equiv of oxidant, with respect to the substrate; 70 °C, for 12 h, in air under reflux; PIDA, iodobenzene diacetate; O2, molecular oxygen; H2O2, hydrogen peroxide. bGC yields with n-decane as an internal standard. cReactions are carried out at room temperature. dReactions are carried out at elevated temperature (80 °C). eO2 = 1 atm.

catalyst is inefficient toward alcohol oxidation under identical reaction conditions (entry 15 in Table 6). The substrates scope was evaluated using a wide variety of differently substituted primary benzylic alcohols (see Table 7). Benzyl alcohol results in benzaldehyde in excellent yield 1. Benzylic alcohols with various alkyl- and alkoxy-substituted groups at para and meta positions of the aryl ring undergo smooth oxidation to give the corresponding aldehydes in moderate to good yields (2−7 in Table 7). Halogenated alcohols are also successfully oxidized without any dehalogenation process (8−11 in Table 7). Electron-withdrawing substituents at the para position of the phenyl ring give the desired aldehydes in appreciable yields (12−15 in Table 7). Complete conversion is observed for biphenylmethanol, producing methyl biphenyl-4-carbaldehyde in a yield of 90% (15 in Table 7). Successful implementation of this protocol is further evident through the excellent yields of aldehydes with nonfunctional fused-ring aryl systems (16−18 in Table 7). The heteroatoms in heterocyclic substrates have the general tendency to coordinate to the metal center, thereby resulting in poor product yields. In this regard, successful tolerance of various N,O,S heterocycles, affording respective aldehydes in good to excellent yields under the present reaction conditions, highlights the selective feature of the BIAN-derived ruthenium catalysts (32+) (see Table 8). Furthermore, secondary alcohols also undergo clean conversion to ketones with the newly designed ruthenium catalysts (32+) (see Table 9).

Table 9. Catalytic Oxidation of Secondary Alcohols

A plausible mechanistic pathway for the alcohol oxidation process has been depicted in Scheme 6; this mechanism is primarily based on experimental and literature evidence.30 It is logical to believe that the initial step (“a”) involves the formation of a high-valent ruthenium-oxo intermediate (4a2+) I


Inorganic Chemistry

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Article

EXPERIMENTAL SECTION Materials. The precursor complex Ru(trpy)Cl331 (trpy = 2,2′:6′,2′′-terpyridine) and BIAN (where BIAN = bis(arylimino)acenaphthene)32 were prepared according to the previously reported literature procedures. Other reagents and chemicals were obtained from Aldrich and used as received. High-purity deionized water obtained from a water purification system (Milli-Q, from Millipore) was used for electrochemical experiments of the aqua species. High-performance liquid chromatography (HPLC)-grade solvents were employed for spectroscopic and other electrochemical studies. Physical Measurements. The electrical conductivities of the chloro ([1a]ClO4−[1c]ClO4, [2a]ClO4), and aqua ([3a](ClO 4 ) 2 −[3c](ClO 4 ) 2 ) complexes in CH 3 CN and 1:1 (CH3)2CO/H2O, respectively, were checked by using Systronic 305 conductivity bridge. 1H NMR and 31P NMR spectra were recorded using an NMR system (Bruker, Model Avance III, 400 MHz). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet spectrophotometer with samples prepared as KBr pellets. Cyclic voltammetry measurements were performed on an electrochemistry system (PAR, Model 273A). A glassy carbon working electrode, a platinum wire auxiliary electrode, and a saturated calomel reference electrode (SCE) were used in a standard three-electrode configuration cell. A platinum wire-gauze working electrode was used for the constant potential coulometry experiment. UV-vis spectral studies were performed on a PerkinElmer Lambda 950 spectrophotometer. UV-vis spectroelectrochemical studies were performed on a BAS SEC2000 spectrometer system. The supporting electrolyte was Et4NClO4, and the solute concentration was ∼10−4 M. All electrochemical experiments were carried out under a dinitrogen (N2) atmosphere at 298 K. The half-wave potential E0 was set equal to 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic cyclic voltammetry peak potentials, respectively. The elemental analyses were carried out on a Thermoquest Model EA 1112 microanalyzer. Electrospray mass spectra (ESI-MS) were recorded on a mass spectroscopy system (Bruker, Maxis Impact, 282001.00081). The EPR measurements were made with an electron spin resonance spectrometer (JEOL, Model FA200). Gas chromatography (GC) analysis was performed using a gas chromatograph (Shimadzu, Model GC-2014), coupled with a flame ionization detection (FID) device, using a capillary column (J&W Scientific, Model 112−2562 CYCLODEXB, with a length of 60 m, inner diameter of 0.25 mm, film thickness of 0.25 μm), and gas chromatography−mass spectroscopy (GCMS) analysis was performed on a GC system (Agilent Technologies, Model 7890A) coupled with a mass-selective detector (GCMSD) (Agilent Technologies, Model 5975C inert XL EI/CI) with a triple-axis detector. Crystallography. Single crystals of [1a]ClO 4 and [2a]ClO4, and single crystals of [1b]ClO4 and [1c]ClO4, were grown via slow evaporation of their 1:2 dichloromethane− n-hexane and 1:2 acetonitrile−toluene solutions, respectively, while those of [3b](ClO4)2 were obtained from its 1:3 acetonen-hexane solution. X-ray crystal data were collected on a Rigaku Model Saturn-724+ diffractometer. The data collection was performed using a standard ω-scan technique and were evaluated and reduced by using CrystalClear-SM Expert software. Absorption correction (numerical) was applied to the collected reflections. The structures were solved by direct method using SHELXS-97 and refined by full matrix least-

Scheme 6. Probable Mechanistic Pathway for Alcohol Oxidation

from the aqua precursor complex (3a2+) in the presence of H2O2 (under reflux). Thereafter, addition of an alcohol molecule to the ruthenium-oxo species results in the formation of an (alkoxo)(hydroxo)Ru(II) intermediate (6a2+) in the subsequent step (“b”). The slow step (“c”) involves β-hydride elimination from 6a2+, thereby regenerating the precursor RuIIaqua species (3a2+), with a concomitant release of one aldehyde molecule. The formation of an active oxo-ruthenium species (4a2+) has been corroborated by ESI-MS and UV-vis spectral evidence (see Figures S13 and S14 in the Supporting Information).

■

CONCLUSION AND OUTLOOK Following are the salient features of the present article: • The impact of the electron-withdrawing NO2 function in the redox-active BIAN (where BIAN = bis(arylimino)acenaphthene) framework in [RuII(trpy)(R-BIAN)Cl]+, where R = H (1a+), OMe (1b+), NO2 (1c+)) has been reflected in the selective successive four-step reductions to BIAN4− in 1c4−, involving its α-diimine fragment and naphtahlene π-system, which was otherwise reported to be achieved only in the case of the multiple cation (Na+)-coordinated naphthalene fragment of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene.1r • The aqua derivatives [RuII(trpy)(R-BIAN)(OH2)]2+ (R = H (3a2+), OMe (3b2+), NO2 (3c2+)) undergo a 2e−/2H+ transfer process, {Ru II−OH 2} − 2e−/2H+ ⇌ {Ru IV O}

to the corresponding {RuIVO} state in 42+. • The facile chemical oxidation of {RuII−OH2} (32+) to {RuIVO} (42+), {Ru II−OH 2} (32+) → {Ru IV O} (42+)

in the presence of a suitable oxidant (PhI(OAc)2 or H2O2), has revealed its effective catalytic potential in important and selective oxidation processes, such as epoxidation of olefins and oxidation of alcohols (primary, secondary, heterocyclic) with a broad substrate scope in each case. Therefore, the present article demonstrates the potential of a robust and redox-active framework of BIAN-derived metal complexes as effective catalysts for a wide variety of chemical as well as electrochemical processes, which indeed emphasizes the generation of a newer molecular setup of BIAN via a judicious selection of metal fragment or coordination situation when more-challenging chemical transformations are involved. J


Article

Inorganic Chemistry squares with SHELXL-2014/7, refining on F2.33 All nonhydrogen atoms were refined anisotropically. The hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2Ueq of their parent atoms. Except for the H1A and H1B protons associated with the aqua molecule in [3b](ClO4)2, the other hydrogen atoms were added in the refinement process, per the riding model. The disordered solvent molecules in [1a]ClO4, [1b]ClO4, [1c]ClO4, and [2a]ClO4 were squeezed using the PLATON34 program. Supplementary crystallographic data for the compounds in this paper have been provided by the Cambridge Crystallographic Data Centre (CCDC, www.ccdc. cam.ac.uk/data_request/cis): CCDC No. 1481955 ([1a]ClO4 ), CCDC No. 1481956 ([1b]ClO4 ), CCDC No. 1481957 ([1c]ClO4), CCDC No. 1481958 ([2a]ClO4), CCDC No. 1481959 ([3b](ClO4)2). Computational Details. Full geometry optimizations were performed using the DFT method at the (R)B3LYP level for 1a+−1c+, 1a−−1c−, 2a+, 2a−, and 3a2+ and at the (U)B3LYP level35 for 1a2+−1c2+, 1a−1c, 1a2−−1c2−, 1c3−, and 1c4−, by applying the Gaussian 09 program package.36 Except for ruthenium, all other elements were assigned to the 6-31G* basis set. The LANL2DZ basis set with effective core potential was employed for the Ru atom in all cases.37 The vibrational frequency calculations were performed to ensure that the optimized geometries represent the local minima and there are only positive eigenvalues. All calculations were performed with the Gaussian 09 program package. Vertical electronic excitations based on (R)B3LYP/(U)B3LYP optimized geometries were computed for 1an−1cn, 2an (n = +2, +1, 0), using the time-dependent density functional theory (TD-DFT) formalism38 in acetonitrile, by applying the conductor-like polarizable continuum model (CPCM).39 Chemissian 1.740 was used to calculate the fractional contributions of various groups to each molecular orbital. All calculated structures were visualized with the ChemCraft program.41 General Procedure for Catalytic Epoxidation Studies. The catalyst [3a](ClO4)2 (8.84 mg, 0.01 mol, 1 mol %) in 3 mL of CH2Cl2 was placed in a vial and stirred for ∼15 min at 298 K. The olefin (1 mmol) was then added to the aforesaid catalyst solution with simultaneous stirring, followed by the addition of iodobenzene diacetate (2 equiv, with respect to olefin) as the oxidant. The mixture was stirred at 298 K for 6 h. The reaction mixture was then diluted with water and extracted three times with ethyl acetate (10 mL). The combined organic extracts were dried over Na2SO4 and concentrated under reduced pressure, and the yield was determined by GC analysis, with respect to the internal standard (n-decane, 1 mmol). A 2.0 μL aliquot was subjected to GC-FID analysis in each case. General Procedure for Alcohol Oxidation. The catalyst [3a](ClO4)2 (17.68 mg, 0.02 mmol, 2 mol %) in 3 mL of CH3CN was placed in a round-bottomed flask initially, followed by the addition of alcohol substrates (0.5 mmol) and oxidant (H2O2, 2 equiv, with respect to alcohol). The reaction mixture was then heated to reflux in air for 12 h. Upon completion, it was allowed to cool to room temperature, followed by a dilution with ethyl acetate. The reaction mixture was then subjected to GC and GC-MS analysis and the yield was determined, with respect to the internal standard, n-decane (0.5 mmol). Preparation of the Complexes. The complexes [1a]ClO4−[1c]ClO4 and [2a]ClO4 were synthesized by following a general procedure, using Ru(trpy)Cl3 and respective BIAN

derivatives. The details are given below for [1a]ClO4 and [2a]ClO4. Synthesis of [Ru(trpy)(BIAN)Cl]ClO4, [1a]ClO4 and [Ru(trpy)(BIAO)Cl]ClO4, [2a]ClO4. The precursor complex Ru(trpy)Cl3 (100 mg, 0.23 mmol) and the ligand BIAN (90 mg, 0.27 mmol) were placed in 30 mL of ethanol. The reaction mixture was heated at reflux with stirring under a dinitrogen (N2) atmosphere over a period of 6 h. The color of the solution gradually changed to pink. The solvent was then evaporated under reduced pressure. The solid mass thus obtained was dissolved in 2 mL of acetonitrile, followed by the addition of a saturated aqueous solution of NaClO4, yielded the dark precipitate, which was filtered and washed thoroughly with ice cold water and dried under vacuum. The chromatographic purification of the crude product on a neutral alumina column led to the initial elution of a green solution, corresponding to [2a]ClO4 by 10:1 CH2Cl2−CH3CN, followed by the pink solution of [1a]ClO4 by a 4:1 CH2Cl2−CH3CN solvent mixture. Evaporation of the solvent under reduced pressure yielded pure complexes [1a]ClO4 and [2a]ClO4. [1a]ClO4. Yield: 60 mg (38%). MS (ESI+, CH3CN): m/z {1a+} calc: 702.10; found: 702.11. 1H NMR in (CD3)2SO [δ/ ppm (J/Hz)]: 8.62 (d, 7.9, 2H), 8.56 (d, 5.9, 2H), 8.49 (d, 8.1, 2H), 8.21 (d, 8.3, 1H), 8.16 (m, 3H), 8.0 (m, 3H), 7.71 (m, 3H), 7.63 (m, 3H), 7.43 (t, 7.6, 1H), 7.30 (d, 7.2, 1H), 7.20 (t, 7.5, 1H) 7.10 (t, 7.7, 2H), 6.46 (d, 7.2, 1H), 5.79 (d, 7.1, 2H). IR (KBr): ν(ClO4−, cm−1): 1095, 622. Molar conductivity (ΛM (Ω − 1 cm 2 M − 1 ), CH 3 CN): 108. Anal. Calcd for C39H27N5Cl2O4Ru: C, 58.43; H, 3.39; N, 8.74; found: C, 58.12; H, 3.14; N, 8.53. [1b]ClO4. Yield: 91 mg (47%). MS (ESI+, CH3CN): m/z {1b+} calc: 762.10; found: 762.14. 1H NMR in (CD3)2SO [δ/ ppm (J/Hz)]: 8.61 (d, 8.0, 2H), 8.52 (m, 4H), 8.22 (d, 8.3, 1H), 8.15 (m, 3H), 8.03 (t, 8.1, 1H), 7.93 (m, 2H), 7.73 (t, 7.6, 1H), 7.63 (m, 2H), 7.46 (m, 2H), 7.23 (d, 8.9, 2H), 6.63 (d, 8.8, 2H) 6.56 (d, 7.2, 1H), 5.75 (d, 8.8, 2H), 3.92 (s, 3H), 3.64 (s, 3H). IR (KBr): ν(ClO4−, cm−1): 1090, 623. Molar conductivity (ΛM (Ω−1 cm2 M−1), CH3CN): 115. Anal. Calcd for C41H31N5O6Cl2Ru: C, 57.15; H, 3.63; N, 8.13; found: C, 57.43; H, 3.25; N, 7.99. [1c]ClO4. Yield: 99 mg (49%). MS (ESI+, CH3CN): m/z {1c+} calc: 792.06; found: 792.07. 1H NMR in (CD3)2SO [δ/ ppm (J/Hz)]: 8.64 (d, 7.9, 2H), 8.58 (m, 4H), 8.53 (d, 8.1, 2H), 8.30 (d, 8.1, 1H), 8.2 (m, 5H), 8.04 (m, 3H), 7.73 (t, 8.2, 1H), 7.65 (m, 2H), 7.45 (m, 2H), 6.69 (d, 7.2, 1H), 6.13 (d, 6.8, 2H). IR (KBr): ν(ClO4−, cm−1): 1093, 621. Molar conductivity (ΛM (Ω−1 cm2 M−1), CH3CN): 105. Anal. Calcd for C39H25N7O8Cl2Ru: C, 52.53; H, 2.83; N, 11.00; found: C, 52.33; H, 2.56; N, 10.83. [2a]ClO4. Yield: 30 mg (20%). MS (ESI+, CH3CN): m/z {2a+} calc: 627.04; found: 627.07. 1H NMR in (CD3)2SO [δ/ ppm (J/Hz)]: 8.81 (d, 7.0, 1H), 8.61 (m, 3H), 8.46 (d, 8.1, 2H), 8.33 (d, 5.7, 2H), 8.30 (d, 8.2, 1H), 8.20 (m, 2H), 8.10 (m, 1H), 7.92 (t, 8.24, 1H), 7.67 (t, 7.47, 2H), 7.48 (t, 8.2, 1H), 7.22 (t, 7.5, 1H), 7.10 (t, 7.5, 2H), 6.62 (d, 7.1, 1H), 5.94 (d, 7.2, 2H). IR (KBr): ν(ClO4−, cm−1): 1092, 622. Molar conductivity (ΛM (Ω−1 cm2 M−1), CH3CN): 95. Anal. Calcd for C33H22N4O5Cl2Ru: C, 54.55; H, 3.05; N, 7.71; found: C, 54.73; H, 2.93; N, 7.42. Preparation of [Ru(trpy)(BIAN)(H2O)](ClO4)2 ([3a](ClO4)2). The chloro compound [1a]ClO4 (100 mg, 0.11 mmol) was taken in a 3:1 acetone−water mixture (25 mL) and heated at reflux for 10 min. An excess amount of AgClO4 (110 mg, 0.54 K


Article

Inorganic Chemistry mmol) was added to the above hot solution, and heating was continued for 6 h. The initial pink solution turned deep red, with the precipitation of AgCl. It was then cooled to room temperature and filtered through a sintered glass crucible (G4) to separate off AgCl. The filtrate was concentrated to 10 mL under reduced pressure and excess NaClO4 was added to it. The solid [3a](ClO4)2 thus obtained was filtered off and washed with a few drops of ice-cold water, followed by drying in vacuo over P4O10. [3a](ClO4)2. Yield: 93 mg (84%). MS (ESI+, (CH3)2CO− H2O): m/z {3a2+−ClO4−H2O} calc: 766.07; found: 766.10. 1H NMR in (CD3)2CO [δ/ppm (J/Hz)]: 9.03 (d, 7.3, 2H), 8.74 (d, 7.9, 2H), 8.62 (dd, 7.8, 2H), 8.33 (m, 6H), 8.21 (m, 2H), 7.80 (m, 8H), 7.49 (t, 8.2, 1H), 7.32 (t, 7.29, 1H), 7.21 (t, 8.05, 2H), 6.60 (t, 7.2, 1H), 6.10 (d, 7.4, 2H). IR (KBr): ν(OH, cm−1): 3422; ν(ClO4−, cm−1): 1092, 622. Molar conductivity (ΛM (Ω−1 cm2 M−1), (CH3)2CO−H2O): 197. UV [λmax/nm, (ε/M−1 cm−1), (CH3)2CO−H2O]: 536 (7060), 447 (3930), 401 (3750), 311 (12620), 273 (10140). Anal. Calcd for C39H29N5Cl2O9Ru: C, 53.01; H, 3.31; N, 7.93; found: C, 53.39; H, 2.98; N, 7.70. [3b](ClO4)2. Yield: 90 mg (81%). MS (ESI+, (CH3)2CO− H2O): m/z {3b2+−ClO4−H2O} calc: 826.25; found: 826.28. 1 H NMR in (CD3)2CO [δ/ppm (J/Hz)]: 8.89 (d, 6.2, 1H), 8.78 (d, 6.9, 1H), 8.67 (d, 7.5, 1H), 8.58 (m, 2H), 8.47 (d, 8.22, 1H), 8.12 (m, 8H), 7.74 (m, 4H), 7.43 (t, 8.1, 2H), 7.34 (t, 7.62, 1H), 7.26 (d, 6.75, 1H), 6.67 (t, 8.2, 3H), 5.97 (d, 6.62, 1H), 5.89 (d, 6.7, 1H), 3.06 (s, 3H), 3.05 (s, 3H). IR (KBr): ν(OH, cm−1): 3435; ν(ClO4−, cm−1): 1090, 624. Molar conductivity (ΛM (Ω−1 cm2 M−1), (CH3)2CO−H2O): 212. UV [λmax/nm, (ε/M−1 cm−1), (CH3)2CO−H2O]: 538 (14270), 406 (3930), 310 (22670), 271 (12360). Anal. Calcd for C41H33N5O11Cl2Ru: C, 52.18; H, 3.52; N, 7.42; found: C, 51.93; H, 3.85; N, 7.19. [3c](ClO4)2. Yield: 82 mg (74%). MS (ESI+, (CH3)2CO− H2O): m/z {3c2+−ClO4−H2O} calc: 855.91; found: 855.94. 1H NMR in (CD3)2CO [δ/ppm (J/Hz)]: 9.0 (d, 6.56, 2H), 8.7 (m, 4H), 8.61 (m, 5H), 8.32 (d, 7.64, 3H), 8.2 (m, 2H), 8.06 (d, 6.84,2H), 7.82 (m, 5H), 7.45 (t, 8.24, 1H), 6.79 (d, 7.04, 1H), 6.45 (d, 8.07, 2H). IR (KBr): ν(OH, cm−1): 3425; ν(ClO4−, cm−1): 1092, 624. Molar conductivity (ΛM (Ω−1 cm2 M−1), (CH3)2CO−H2O): 206. UV [λmax/nm, (ε/M−1 cm−1), (CH3)2CO−H2O]: 549 (6050), 443 (4100), 317 (14120), 277 (15950). Anal. Calcd for C39H27N7O13Cl2Ru: C, 48.11; H, 2.79; N, 10.07; found: C, 48.23; H, 2.97; N, 10.32. Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive. Heating of dried samples must be avoided, and one must proceed with great caution when handling even small amounts.

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Crystallographic Crystallographic Crystallographic Crystallographic

data data data data

for for for for

[1b]ClO4 (CIF) [1c]ClO4 (CIF) [2a]ClO4 (CIF) [3b](ClO4)2 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support received from the Department of Science and Technology, University Grant Commission (fellowship to A.H. and R.R.), New Delhi (India), is gratefully acknowledged.

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DEDICATION Dedicated to Professor Parimal Kanti Bharadwaj, Department of Chemistry, I.I.T.-Kanpur on the occasion of his 65th birthday.

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REFERENCES

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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.6b01280. Bond lengths and angles (crystal and DFT), energies of DFT optimized states, MO compositions, mass spectra, 1 H NMR, differential pulse voltammetry, Mulliken spin density representations, DFT-optimized structures, and spectral data involving controlled experiments for the catalytic processes PDF) Crystallographic data for [1a]ClO4 (CIF) L


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