Electronically Triggered Switchable Binding Modes of the C

Jan 7, 2019 - Synopsis. Evaluation of the tunable binding feature of nitrosoarene η1-ArNO versus μ-ArNO on a selective {Ru(acac)2} metal platform (1...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Electronically Triggered Switchable Binding Modes of the C‑Organonitroso (ArNO) Moiety on the {Ru(acac)2} Platform Sanchaita Dey, Sanjib Panda, Prabir Ghosh, and Goutam Kumar Lahiri* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/10/19. For personal use only.

S Supporting Information *

ABSTRACT: This work evaluated the switchable binding profile of the biochemically relevant and redox non-innocent C-organonitroso (ArNO) moiety with the selective {Ru(acac)2} (acac = acetylacetonate) metal fragment as a function of external stimuli, including the solvent medium (EtOH versus toluene) and aryl substituents (C6H5, p-OMe-C6H4, and pCl-C6H4) in the framework of ArNO. In this context, the reaction of ArNO (Ar = C6H5 or p-OMe-C6H4) with the metal precursor RuII(acac)2(CH3CN)2 in polar protic EtOH led to the formation of monomeric [RuII(acac)2(ArNOo)2] (1a or 1b) with η1-N-bonded terminal ArNOo and double-ArNOo-bridged dimeric [(acac)2RuII(μ-ArNOo)2RuII(acac)2], 2a or 2b, respectively. On the other hand, the use of p-Cl-substituted ArNO selectively yielded the corresponding dimeric 2c. However, the use of nonpolar toluene resulted in monomeric 1 irrespective of the nature of aryl substituents in ArNO. Molecular identities, including the redox state of ArNOo in 1 and 2, were authenticated by their single-crystal X-ray structures as well as by solution spectral features. Though monomeric 1 exhibited reversible one-electron oxidation and reduction processes, leading to the electron paramagnetic resonance active [RuIII(acac)2(ArNOo)2]+ (1+; S = 1/2) and [RuII(acac)2(ArNO•−)(ArNOo)]− (1•−; S = 1/2), respectively, redox states of dimeric 2 were found to be unstable on the electrolysis time scale. Interestingly, monomeric 1 underwent transformation to dimeric 2 in the presence of a strong reducing agent, hydrazine hydrate, and the reverse process, i.e., conversion of dimeric 2 to 1, took place under the influence of external coordinating agent ArNO. The detailed experimental exploration, including kinetic investigations related to 1 → 2 and 2 → 1 transformations, revealed that the electronic aspects of ArNO (redox non-innocence of ArNOo/•−, π-accepting and coordinating features of ArNOo) had facilitated its switchable binding event in combination with the {Ru(acac)2} metal fragment.

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INTRODUCTION

Scheme 1. Redox Non-Innocence of ArNO

The redox active C-organonitroso (ArNO) ligand-derived metal complexes have continued to attract attention because of their structural diversity,1 multifarious reactivity strategy,2 and biological prospects.3 The polarizability of the NO bond (favors the attack of the nucleophile on the Nδ+ center) as well as the free electron pair on nitrogen (acts as a nucleophile) in ArNO introduces its ambiphilic character, which is reflected in various chemical reactions such as [n+2]-cycloaddition (n = 2−4), radical annulation, electrophilic addition, etc.2 Unlike the biochemically significant nitrosyl (NO) ligand, the redox non-innocence feature of nitrosoarene (Scheme 1), a synthetic analogue of singlet dioxygen (1O2, 1Δg), particularly with respect to metalation has not been well explored possibly due to the rigid character of the out-of-plane π* orbital of free ArNO.1e Moreover, ArNO is known to exhibit varying binding modes with the metal ions through its N/O donor or both N and O donors (Scheme 2) based on the ancillary ligands associated with the metal fragments.1 Though η1-N (B), η2-N,O (C), and μ-η1:η2-Ο,Ν (F) binding modes of ArNO are reported with both 3d and 4d/5d metals, η1-O (A)/μ-η2:η2-Ο,Ν (E), μ-η1:η1O,N (D),{(μ-η1-Ν)(μ-η1:η1-O,N)(η2-Ν)} (G), and (μ-η1:η1© XXXX American Chemical Society

O,N)2 (H) binding features are selective for 3d and 4d/5d metal ions, respectively. In this regard, this exploration related to the reaction of ArNO with the electron rich {Ru II (acac) 2 } (acac = acetylacetonate) metal fragment in a polar protic solvent (ethanol) has led to the development of rarely manifested1h double-ArNOo-bridged symmetric diruthenium complex [(acac)2RuII(μ-η1,η1-O,N-ArNO)2RuII(acac)2] (2; S = 0) along with Received: November 14, 2018

A

DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Varying Coordination Modes of ArNO

the mononuclear [RuII(acac)2(η1-N-ArNO)2] complex (1; S = 0) involving the usual η1-N-bonded neutral ArNOo ligands in their cis form. In addition, of 1 → 2 and 2 → 1 reversible transformations have occurred under the influence of hydrazine hydrate as a reducing agent and ArNO as a coordinating agent. Besides an account of the synthesis, this work includes the structural characterization of 1 and 2, their spectral and electrochemical features, and the mechanistic outline for the aforementioned conversion processes by experimental (kinetic/redox) investigations.

Chart 1. Representation of Complexes

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RESULTS AND DISCUSSION Synthesis, Crystal Structure, and Spectra. The reaction of C-organonitroso (ArNO; Ar = C6H5 or p-OMe-C6H4) moieties with the electron rich metal fragment [RuII(acac)2(CH3CN)2] (acac = acetylacetonate) in refluxing ethanol led to the formation of mononuclear [Ru(acac)2(ArNO)2] (1a or 1b) (major, ∼75%) and dinuclear [(acac)2Ru(μ-ArNO)2Ru(acac)2] (2a or 2b) (minor, ∼25%) complexes, which could be separated on a neutral alumina column using petroleum ether and CH2Cl2 (3:1) and petroleum ether and CH2Cl2 (1:4), respectively. On the other hand, the aforementioned reaction with p-Cl-C6H4NO selectively resulted in dimeric complex 2c. However, the same reaction in a nonpolar solvent (toluene or benzene) predominantly yielded monomeric complex 1 irrespective of the nature of the substituents in the framework of ArNO (Chart 1). The formation of one diastereomeric form [meso (ΔΛ) or rac (ΔΔ) (see below)]4 of dimeric 2 was evident by the TLC (thin layer chromatography) experiment. Unlike 1, dimeric 2 was found to be less stable in a coordinating solvent (say CH3CN), partially converted to the solvent-coordinated monomeric species [RuII(acac)2(ArNO)(CH3CN)] (3) (Figures S1−S3 and Tables S1 and S2). In addition, 1 → 2 and 2 → 1 interconversions took place in the presence of NH2NH2 (reducing agent) and ArNO (coordinating agent), respectively. Identities of electrically neutral 1 and 2 were confirmed by their satisfactory microanalytical and mass spectrometry data (Figure S2 and Experimental Section). The molecular forms of 1a−1c and the representative meso (ΔΛ) diastereomeric form of 2a were ascertained by their single-crystal X-ray structures (Figures 1 and 2, Tables 1−3, and Tables S3 and S4). The asymmetric unit of 2a comprises of two half-molecules with a

slight difference in their bond parameters, presumably due to the effect of crystal packing forces. The redox sensitive N−O distance of η1-N-bonded ArNO in 1 [N1−O1/N2−O2: 1.246(2) Å/1.259(2) Å (1a), 1.251(3) Å/1.253(2) Å (1b), 1.257(3) Å/1.245(4) Å (1c)] unequivocally established its neutral ArNOo state,3d leading to the electronic form of [(acac)2RuII(ArNOo)2]. On the contrary, the N−O bond length of the bridging PhNO in 2a of 1.298(7) Å (N1−O1, molecule A) or 1.307(6) Å (N2−O6, molecule B) did not prompt an explicit assignment of its redox state; it appeared to match well with the extended limit of the neutral state of PhNOo as well as with the lower limit of the intermediate radical state of PhNO•− (PhNOo, 1.21−1.26 Å; PhNO•−, 1.29−1.36 Å; PhNO2−, 1.38−1.43 Å).1e,2f This indeed suggests the feasible alternate electronic form of {RuII(μ-PhNOo)2RuII} or {RuIII(μ-PhNO•−)2RuIII} or resonating {RuII(μ-PhNOo)2RuII} ↔ {RuIII(μ-PhNO•−)2RuIII} for 2a. However, metal-based oxidation and largely ligand-based reduction (ArNOo → ArNO•−) as corroborated by the MO compositions (Figure S4 and Table S5) as well as the absence of 1H nuclear magnetic resonance (NMR) line broadening even at 233 K (Figure S3e) implied the closed shell {RuII(μPhNOo)2RuII} (S = 0) electronic structural form for the ground state of 2a. Therefore, the appreciable decrease in the RuII−N (PhNOo) bond length in 2a [Ru1−N1, 1.891(7) Å (molecule A); Ru2−N2, 1.890(5) Å (molecule B)] as compared to the RuII−N (PhNOo) bond length in monomeric 1a [Ru−N1/Ru−N2, 1.942(2)/1.930(2) Å] along with the increase in the N−O (PhNO) bond length in 2a [N1−O1, B

DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

former was also reflected in the 40 and 23% Ru contributions in their LUMOs, respectively (Table S5).6 In accordance with that, the ν(NO) frequency of {ArNOo} decreased ∼100 cm−1 on moving from 1 (∼1500 cm−1) to 2 (∼1400 cm−1) (Figure S5).1e The reasonable deviation of the average trans angle in 1 or 2a, ∼176.18° or ∼171.53°, respectively, from the idealized value of 180° implied a distorted octahedral situation. The bond parameters of representative 1a and 2a were fairly well reproduced by their density functional theory (DFT)optimized structures. The crystal structure of 2a also revealed the chair conformation of the heterometallacycle developed through the metal ions (Ru1/Ru1′) and the bridging units (Ph-N1O1/ Ph-N1′O1′), where metal ions were separated by 3.690 Å (molecule A) or 3.696 Å (molecule B).1a The Ph groups of PhNO containing C1−C6 and C7−C12 bonds in 1 deviated from the Ru−N−O plane by ∼26° and ∼12°, respectively, whereas the same in 2a deviated by 25.30° (molecule A) and 35.84° (molecule B). The mononuclear form (1) and the meso diastereomeric form7 of dinuclear (2) complexes displayed sharp 1H NMR spectra in the standard chemical shift range (δ) of 0−10 ppm corresponding to the calculated number of protons (Figure S3, Supporting Information and Experimental Section). Electrochemistry and Electronic Structural Forms. Redox properties of complexes 1 and 2 were checked by cyclic voltammetry and differential pulse voltammetry (Figures 3 and 4 and Table 4). The mononuclear complexes (1a−1c) exhibited reversible one oxidation (Ox1) and one reduction (Red1) within the potential window of ±1.5 V versus saturated calomel electrode (SCE) in CH3CN. Its one-electron nature as well as the reversibility of the couples in 1 was confirmed by constant potential coulometry. The redox potential varied systematically on the basis of the electron-donating (OMe) and electron-withdrawing (Cl) substituents in the aryl ring of ArNO (Chart 1), and it followed the order 1c (Cl) > 1a (H) > 1b (OMe) and 1c (Cl) < 1a (H) < 1b (OMe) for Ox1 and Red1 couples, respectively. The separation in potential

Figure 1. Perspective views of (a) 1a, (b) 1b, and (c) 1c. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been removed for the sake of clarity.

1.298(7) Å] with respect to that in 1a [1.252(7) Å] could be interpreted in terms of greater RuII(dπ) → PhNOo(π*) backdonation5 in 2a. The greater extent of π back-bonding in 2a than in 1a due to the impact of two ruthenium centers in the

Figure 2. Perspective view of 2a. The asymmetric unit contains two half-molecules. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been removed for the sake of clarity. The inset shows the chair form of the six-membered heterometallacycle. C

DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Selected Crystallographic Parameters for 1a−1c and 2a empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z μ (mm−1) T (K) Dcalcd (g cm−3) F(000) θ range (deg) data/restraints/parameters R1, wR2 [I > 2σ(I)] R1, wR2 (all data) goodness of fit largest difference peak/hole (e Å−3)

1a

1b

1c

2a

C22H24N2O6Ru 513.50 monoclinic P121/n1 8.9421(3) 22.1637(5) 11.9094(3) 90 111.049(3) 90 2202.83(11) 4 0.752 150(2) 1.548 1048 2.470−24.998 3869/0/284 0.0217, 0.0568 0.0246, 0.0579 1.058 0.398/−0.402

C24H28N2O8Ru 573.55 triclinic P1̅ 8.5334(2) 10.1921(2) 15.8454(3) 94.1480(10) 101.699(2) 114.131(2) 1212.85(5) 2 0.698 150(2) 1.571 588 2.223−24.998 4235/0/322 0.0221, 0.0537 0.0252, 0.0546 1.058 0.345/−0.511

C22H22Cl2N2O6Ru 582.38 monoclinic P121/c1 9.1517(5) 21.8768(9) 12.1108(4) 90 93.746(4) 90 2419.52(18) 4 0.909 150(2) 1.599 1176 2.511−24.997 4255/0/302 0.0428, 0.0946 0.0530, 0.1001 1.059 0.800/−1.113

C32H38N2O10Ru2 812.78 monoclinic P121/c1 18.9690(11) 11.0623(6) 16.9187(11) 90.00 106.811(7) 90.00 3398.5(4) 4 0.946 150(2) 1.589 1648 2.156−24.999 5968/0/427 0.0570, 0.1057 0.0924, 0.1279 1.044 0.709/−1.418

Table 2. Selected Experimental Bond Lengths (angstroms) for 1a−1c 1a

1b

Table 3. Selected Experimental Bond Lengths (angstroms) for 2a exp

1c

bond

exp

DFT

X-ray

X-ray

bond

molecule A

molecule B

DFT (molecule A)

Ru1−N1 Ru1−N2 Ru1−O3 Ru1−O4 Ru1−O5 Ru1−O6 C1−N1 C7−N2 O1−N1 O2−N2

1.942(2) 1.930(2) 2.068(1) 2.021(1) 2.060(2) 2.036(1) 1.452(3) 1.454(2) 1.246(2) 1.259(2)

1.993 1.989 2.097 2.060 2.097 2.096 1.452 1.449 1.231 1.239

1.929(1) 1.933(2) 2.056(1) 2.028(2) 2.066(2) 2.022(2) 1.443(3) 1.445(2) 1.251(3) 1.253(2)

1.928(3) 1.932(3) 2.066(2) 2.034(2) 2.068(3) 2.039(2) 1.451(5) 1.462(5) 1.257(3) 1.245(4)

Ru1−N1 Ru1−O1 Ru1−O2 Ru1−O3 Ru1−O4 Ru1−O5 Ru2−N2 Ru2−O6 Ru2−O7 Ru2−O8 Ru2−O9 Ru2−O10 C1−N1 O1−N1 O6−N2 C17−N2 Ru···Ru

1.891(7) 2.010(4) 2.062(5) 2.049(4) 2.022(5) 1.997(4) − − − − − − 1.446(9) 1.298(7) − − 3.690

− − − − − − 1.890(5) 2.027(5) 2.061(3) 2.042(5) 2.031(4) 2.001(4) − − 1.307(6) 1.449(7) 3.696

1.946 2.080 2.010 2.083 2.055 2.054 − − − − − − 1.435 1.278 − − 3.790

between the redox processes [Ox1-Red1 (Figure 3)] led to the large Kc value of 1030 for the intermediate state, which in turn justified the stability of the isolated complexes.8 Dinuclear complexes 2a−2c displayed one quasi-reversible oxidation (Ox1) and two successive irreversible reductions (Red1 and Red2), and the potential followed the order of 2c > 2a > 2b and 2c < 2a < 2b for oxidation and reduction processes, respectively, as in the case of 1. Though the Ox1 couple in Figure 4 appeared to be quasi-reversible on a cyclic voltammetric time scale, its irreversible feature was apparent on a longer electrolysis time scale. The two-electron nature of the oxidation (Ox1) and reduction (Red 2) steps was ascertained via the use of ferrocene as an internal standard (Figure 4, inset). The correspondences of Ox1 and Red1 in representative 1a (Figure 3) primarily with the Ru(II)/Ru(III) and PhNOo/ PhNO•− couples were established by the metal-based anisotropic electron paramagnetic resonance (EPR) for 1a+ (S = 1/2; ⟨g⟩/Δg = 2.028/0.10) and the radical EPR response for 1a•− (S = 1/2; g = 2.006), respectively10 (Figure 5). Those are also corroborated by MO compositions of 1an (n = 0, 1, or

Figure 3. Cyclic (black) and differential pulse (red) voltammograms in CH3CN. D

DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

The spectral changes with reversible oxidation and reduction processes involving 1 → 1+ and 1 → 1−, respectively, were followed by an ultraviolet−visible (UV−vis) spectroelectrochemical experiment in combination with TD-DFT calculations of representative 1a (Figure 6 and Figure S7 and Table

Figure 4. Cyclic (green) and differential pulse (red) voltammograms in CH3CN. The inset shows the two-electron transfer process of Ox1 for 2a with respect to ferrocene in CH3CN.

Table 4. Redox Potentials and Comproportionation Constants E°298 (V) (ΔE (mV))a,b complex 1a 1b 1c 2a 2b 2c

Ox1 1.07 0.90 1.13 0.63 0.51 0.67

(80) (90) (80) (100)d (70)d (70)d

Kcc

Red1

Red2

Kc1c

−0.72 (90) −0.83 (90) −0.62 (80) −1.40e −1.48e −1.27e

− − − −1.94d,e −2.09d,e −1.78d,e

1.7 × 1030 2.8 × 1029 3.7 × 1029 − − −

Figure 6. UV−vis spectroelectrochemical responses of 1an (n = 1, 0, or −1) in CH3CN and 0.1 M Et4NClO4.

S10). The isolated complexes 1a−1c exhibited two moderately intense mixed metal ligand-based charge transfer transitions in the visible region (400−600 nm) due to the effect of covalency11 in addition to intense ligand-based transitions in the higher-energy UV region. On oxidation or reduction to 1+ or 1 −, respectively, mixed metal−ligand based visible transitions of 1 moved slightly to the higher energy region. As stated before, the unstable nature of the oxidation [Ox1 (Figure 4), on the longer electrolysis time scale] and reduction [Red1/Red2 (Figure 4), on the cyclic voltammetric time scale] processes of 2 had indeed precluded their further spectral scrutiny. However, MO compositions of 2n (Tables S5 and S11−S14) predicted primarily metal-dominated oxidation (RuII → RuIII) and ArNOo-based reduction steps, leading to the electronic formulations of [(acac) 2 Ru I I I (μ-ArNOo)2RuIII(acac)2]2+ for 22+ [S = 1, Ox1 (Figure 4)], [(acac)2RuII(μ-ArNO•−)(μ-ArNOo)RuII(acac)2]− for 2− [S = 1 /2, Red1 (Figure 4)], and [(acac)2RuII(μ-ArNO•−)(μArNO2−)RuII(acac)2]3− for 23− [S = 1/2, Red2 (Figure 4)]. The simultaneous two-electron oxidation of dimeric [(acac)2RuII(μ-ArNOo)2RuII(acac)2] (2) to [(acac)2RuIII(μ-ArNOo)2RuIII(acac)2] (22+) without the involvement of the intermediate mixed valent [(acac)2RuII(μ-ArNOo)2RuIII(acac)2] (2+) state could be conceived as a class I system.12 The unstable feature of the π-accepting ArNOobridged RuIIIRuIII state in 22+ could be attributed to the diminished dπ(RuIII) (t2g5) → π*(ArNOo) back-bonding with respect to the strongly back-bonded [dπ(RuII)(t2g6) → π*(ArNOo)] situation in 2.13 Interconversions of 1 to 2 and 2 to 1. The conversions14 of monomeric 1 to dimeric 2 in the presence of excess hydrazine hydrate (reducing agent) and dimeric 2 to monomeric 1 with excess nitrosoarene (coordinating agent)

From cyclic voltammetry in CH3CN and 0.1 M Et4N+ClO4− at a scan rate of 100 mV s−1. bPotential in volts vs the SCE; peak potential differences ΔEp (in parentheses). cComproportionation constant from the equation RT ln Kc = nF(ΔE).9 Kc1 between Ox1 and Red1. d Two-electron transfer. eIrreversible. a

Figure 5. EPR in CH3CN at 100 K (top) and Mulliken spin density plot (bottom) of 1a−.

−1) (Tables S5−S8) and spin density plots of the paramagnetic states (1a+ and 1a−) (Figure 5 and Figure S6 and Table S9). E

DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (Figure 7) were followed spectrophotometrically15 (Figures 8 and 9, Table 5, and Figures S8 and S9 and Tables S15 and S16).

Figure 7. External stimuli triggered 1 → 2 and 2 → 1 conversions. Figure 9. Change in the spectral profile of 2a upon addition of nitrosobenzene (excess) in toluene as a function of time for the 2a → 1a conversion at 323 K. The rate is calculated on the basis of the growing feature of the 564 nm peak (inset).

The use of a relatively stronger reducing agent (hydrazine hydrate) in an acetonitrile solution of monomeric 1 led to the formation of dimeric 2. Kinetic studies revealed that the 1 → 2 conversion followed a pseudo-first-order rate with respect to 1. The rate of conversion of representative 1a → 2a at a variable temperature resulted in values for ΔH⧧ (enthalpy of activation) and ΔS⧧ (entropy of activation) of 3.3 ± 0.1 kcal/mol and −67.3 ± 0.5 cal mol−1 K−1, respectively. The computed negative entropy of activation implied the involvement of adduct formation in the rate-determining step (i.e., associatively activated pathway, Ia mechanism) as depicted in Scheme 3. Consideration of the initial formation of the radical intermediate A in Scheme 3 upon addition of hydrazine hydrate to the acetonitrile solution of 1 could be justified by the reversible one-electron reduction of 1 → 1•− [E(1/1•−) ∼ −0.6 to −0.8 V (Figure 4 and Table 4)] and radical EPR as well as largely ArNO-centered spin in 1•− (Figure 5). This indeed made the oxygen atom of the coordinated ArNO•− nucleophilic enough for further linking to the second metal fragment to form outer sphere adducts (B followed by C in Scheme 3) through the μ-ArNO•− bridge in an interchange associative (Ia) pathway.15b Subsequent rupturing of rather loosely bonded Ru···ArNOo in seven-coordinate C (Scheme 3) followed by oxidation of the ArNO•− bridge in D gave rise to dimeric product 2. The observed highly negative reduction potential of ArNOo/ArNO•− of isolated 2 [∼−2 V (Figure 4 and Table 4)] indeed justified the transformation of intermediate D to the final product 2. Consideration of the B to C step as the rate-determining step involving the cleavage of a stable Ru−N (ArNO) bond for the 1 → 2 conversion in Scheme 3 was also corroborated by the pseudo-first-order kinetics.

Table 5. Temperature-Dependent Kinetic Data for 1a → 2a temp (K) 293 303 313 323

k (s−1) 4.7 5.6 6.9 8.5

× × × ×

−5

10 10−5 10−5 10−5

ΔH⧧ (kcal mol−1)

ΔS⧧ (cal mol−1 K−1)

3.3 ± 0.1

−67.3 ± 0.5

The rate of transformation of terminal ArNOo-coordinated 1 to the corresponding ArNOo-bridged dimeric 2 varied depending on the electronic nature of the substituents in the aromatic ring of nitrosoarene [H (1a/2a), OMe (1b/2b), or Cl (1c/2c)], and it decreased in the following order (k in s−1 at 293 K): 1c [(6.6 ± 0.1) × 10−5] > 1a [(4.6 ± 0.1) × 10−5] > 1b [(2.4 ± 0.1) × 10−5] (Figure S8 and Table S15). This also correlated well with the trend in the 1/1•− reduction potential: E(1c/1c•−) = −0.62 V < E(1a/1a•−) = −0.72 V < E(1b/1b•−) = −0.83 V (Figure 4 and Table 4). The relatively faster rate of transformation of 1c to 2c could possibly account for 2c dominating in the polar EtOH solvent. The observed inaccessibility of dimeric 2 in nonpolar benzene or toluene is not clear to us at present. However, it might be logical to assume that the nonsolvated ArNO in a nonpolar solvent facilitated the substitution of both of the CH3CN molecules of the metal precursor Ru(acac)2(CH3CN)2, which resulted in monomeric 1, whereas solvation of ArNO in a polar protic medium made the process slower, which led to the partial formation of 2 along with 1.

Figure 8. (a) Change in the spectral profile of 1a upon addition of hydrazine hydrate (excess) in CH3CN as a function of time for the 1a → 2a conversion at 303 K. The rate is calculated on the basis of the diminishing intensity of the 564 nm peak (inset). (b) Plot of ln(k/T) vs 1/T. F

DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 3. Outline of 1 → 2 Transformation

moiety on the selective {Ru(acac)2} metal platform, η1-Nbonded ArNO in [RuII(acac)2(ArNO)2] (1) and μ-ArNO in [(acac)2RuII(μ-ArNO)2RuII(acac)2] (2), and their interconversions, 1 → 2 and 2 → 1, under the influence of external stimuli, hydrazine hydrate (reducing agent) and ArNO (coordinating ligand), respectively. The collective consideration of the structural, spectral, and electrochemical features of 1 and 2 along with the mechanistic details (kinetics) related to the interconversions of 1 to 2 and 2 to 1 revealed the impact of electronic aspects of ArNO [redox non-innocence (ArNOo/ ArNO•−), π-acceptance (Ru → ArNO), and aryl (p-H/OMe/ Cl−Ar) substituents] as well as the polarity of the solvent medium on its switchable binding modes. The observed electronically induced fluxtionality of the lignad is expected to broaden its future scope in coordination polymerization, signal processing, etc.16

Moreover, the decrease in the nucleophilicity of ArNOo in 2 decreased its ability to coordinate to the second metal ion through its O donor, which in effect facilitated its disintegration process, particularly in a coordinating solvent, leading to the solvent-inserted monomeric analogue [RuII(acac)2(ArNO)(CH3CN)] (3) (Figure S1). The conversion of 2 to 1 was monitored in toluene at 323 K in the presence of excess ArNO (Figure 9), and it followed the pseudo-first-order rate [first-order with respect to 2 and zeroorder with respect to the nucleophile (ArNO)]. The rate of transformation of 2 to 1 was also varied as a function of the substituent in the aromatic ring of nitrosoarene (k in s−1 at 323 K): (1.2 ± 0.1) × 10−5 (2c) > (8.1 ± 0.1) × 10−6 (2a) > (3.6 ± 0.1) × 10−6 (2b) (Figure S9 and Table S16). Moreover, the same experiment (2a → 1a) using 1 equiv of PhNO in a toluene medium at 323 K revealed a first-order rate process with a rate constant of (7.6 ± 0.1) × 10−6 s−1 (Figure S10). The observed ligand (ArNO) independence of the rate essentially ruled out the probable involvement of the SN2 route (concerted pathway) (Scheme 4b) for each Ru center

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EXPERIMENTAL SECTION

Materials. The ligand PhNO was purchased from Sigma-Aldrich, and other solvents and chemicals were of reagent grade and used as received. High-performance liquid chromatography grade solvents were used for spectroscopic and electrochemical studies. The metal precursor cis-Ru(acac)2(CH3CN)217 and para-substituted ArNO (where Ar = p-OMe-C6H4 and p-Cl-C6H4)18 were prepared according to the procedures described in the literature. Physical Measurements. The elemental analyses were measured on a Thermoquest (EA 1112) microanalyzer. Electrospray mass spectrometry was performed on a Bruker Maxis Impact (282001.00081) spectrometer. 1H NMR spectra were recorded on a Bruker Avance III 500 MHz spectrometer. Cyclic voltammetric and differential pulse voltammetric analyses of the complexes were carried out using a PAR model 273A electrochemistry system. In a standard three-electrode configuration, glassy carbon working, platinum wire auxiliary, and saturated calomel reference electrodes were used with tetraethylammonium perchlorate (TEAP) as the supporting electrolyte (substrate concentration of ≈10−3 M; standard scan rate of 100 mV s−1). Half-wave potential E°298 was adjusted to 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic cyclic voltammetry peak potentials, respectively. A platinum wire-gauze working electrode was employed for the constant-potential coulometry experiment. UV−vis spectroelectrochemical experiments were performed on a BAS SEC2000 spectrometer system. The supporting electrolyte was Et4N+ClO4−, and the solute concentration was ∼10−4 M. All electrochemical experiments were performed under a dinitrogen atmosphere at 298 K. EPR spectra were recorded on a JEOL model FA200 electron spin resonance spectrometer. A PerkinElmer Supplementary crystallographic data for the compounds mentioned in the paper may be obtained free of charge from The Crystallographic Data Centre (www.ccdc.cam.ac.uk/data_request/cif) using CCDC numbers 1878254, 1878255, 1878256, 1878257, and 1878258 for 1a, 1b, 1c, 2a, and 3a, respectively. Lambda 950 spectrophotometer was used for electronic spectral measurements. The electrical conductivity was checked using an autoranging conductivity meter

Scheme 4. Outline of 2 → 1 Transformation

and was rather consistent with the consideration of Ru−O (ArNO) bond fission [μ2(ArNO) → η1(ArNO) (Scheme 4a)] being the rate-determining step (RDS). This resulted in a fivecoordinate intermediate E via the dissociative pathway [SN1 (limiting)],15c−f followed by the insertion of ArNO to yield product 1. It should be noted that weak interaction of ArNO or coordinating solvent might have provided the stability of the five-coordinate E. Moreover, the enhanced conversion rate for 2c could be attributed to the favored Ru−O bond dissociation process due to the impact of the electron-withdrawing Cl substituent.

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CONCLUSION This work highlights the intriguing factors that facilitated the switchable binding profile of the C-organonitroso (ArNO) G

DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

coordinating acetonitrile solution. The representative complex 3a was isolated and purified using a neutral alumina column with dichloromethane as the eluant. Pure complexes 1, 2, and 3 were obtained in solid form upon evaporation of the solvent under reduced pressure. 1a. Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), EtOH (50 mL), PhNO (56 mg, 0.52 mmol); yield 91 mg (68%); ESI-MS (+) in CH3CN m/z {[1a + Na]}+ calcd 537.04, found 537.04; 1H NMR in CDCl3 (δ, J in hertz) 7.83 (d, 8, PhNO), 7.46 (t, 7, PhNO), 7.24 (d, 8, PhNO), 5.25 (s, CH of acac), 2.15 (s, CH3 of acac), 1.95 (s, CH3 of acac); IR (KBr) 1518 cm−1 [ν(NO)]; molar conductivity (CH3CN) ΛM = 5 Ω−1 cm2 M−1. Anal. Calcd for C22H24N2O6Ru: C, 51.46; H, 4.71; N, 5.46. Found: C, 51.23; H, 4.51; N, 5.23. 2a. Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), EtOH (50 mL), PhNO (56 mg, 0.52 mmol); yield 30 mg (28%); ESI-MS (+) in CH3CN m/z {[2a + Na]}+ calcd 837.03, found 837.05; 1H NMR in CDCl3 (δ, J in hertz) 8.3 (d, 7, PhNO), 7.58 (t, 7, PhNO), 7.25 (t, 8, PhNO), 6.95 (m, PhNO), 6.55 (d, 7, PhNO), 5.3 (s, CH of acac), 4.95 (s, CH of acac), 2.2 (s, CH3 of acac), 2.15 (s, CH3 of acac), 1.8 (s, CH3 of acac), 1.7 (s, CH3 of acac); IR (KBr) 1401 cm−1 [ν(N O)]; molar conductivity (CH3CN) ΛM = 9 Ω−1 cm2 M−1. Anal. Calcd for C32H38N2O10Ru2: C, 47.29; H, 4.71; N, 3.45. Found: C, 47.22; H, 4.79; N, 3.57. 1b. Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), EtOH (50 mL), pOMe-C6H4NO (72 mg, 0.52 mmol); yield 88 mg (59%); ESI-MS (+) in CH3CN m/z {[1b + Na]}+ calcd 597.058, found 597.054; 1H NMR in CDCl3 (δ, J in hertz) 7.88 (d, 9, ArNO), 6.7 (d, 9, ArNO), 5.25 (s, CH of acac), 3.8 (s, OCH3 of ArNO), 2.1, 1.95 (s, CH3 of acac); IR (KBr) 1519 cm−1 [ν(NO)]; molar conductivity (CH3CN) ΛM = 7 Ω−1 cm2 M−1. Anal. Calcd for C24H28N2O8Ru: C, 50.26; H, 4.92; N, 4.88. Found: C, 50.42; H, 5.04; N, 5.09. 2b. Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), EtOH (50 mL), pOMe-C6H4NO (72 mg, 0.52 mmol); yield: 28 mg (25%); ESI-MS (+) in CH3CN m/z {[2b + Na]}+ calcd 897.088, found 897.068; 1H NMR in CDCl3 (δ, J in hertz) 8.37 (d, 9, ArNO), 6.8 (d, 9, ArNO), 6.55 (d, 8, ArNO), 6.48 (d, 9, ArNO), 5.3 (s, CH of acac), 5.0 (s, CH of acac), 3.85 (s, OCH3 of ArNO), 2.13 (s, CH3 of acac), 2.11 (s, CH3 of acac), 1.8 (s, CH3 of acac), 1.7 (s, CH3 of acac); IR (KBr) 1389 cm−1 [ν(NO)]; molar conductivity (CH3CN) ΛM = 5 Ω−1 cm2 M−1. Anal. Calcd for C34H42N2O12Ru2: C, 46.79; H, 4.85; N, 3.21. Found: C, 46.94; H, 4.88; N, 3.55. 1c. Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), toluene (50 mL), p-Cl-C6H4NO (74 mg, 0.52 mmol); yield 112 mg (74%); ESI-MS (+) in CH3CN m/z {[1c + Na]}+ calcd 604.97, found 604.94; 1H NMR in CDCl3 (δ, J in hertz) 7.84 (d, 9, ArNO), 7.25 (d, 9, ArNO), 5.3 (s, CH of acac), 2.15 (s, CH3 of acac), 1.95 (s, CH3 of acac); IR (KBr) 1521 cm−1 [ν(NO)]; molar conductivity (CH3CN) ΛM = 5 Ω−1 cm2 M−1. Anal. Calcd for C22H22Cl2N2O6Ru: C, 45.37; H, 3.81; N, 4.81. Found: C, 45.67; H, 3.53; N, 5.02. 2c. Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), EtOH (50 mL), pCl-C6H4NO (74 mg, 0.52 mmol); yield 46 mg (40%); ESI-MS (+) in CH3CN m/z {[2c + Na]}+ calcd 904.96, found 904.94; 1H NMR in CDCl3 (δ, J in hertz) 8.3 (d, 9, ArNO), 7.25 (d, 9, ArNO), 6.95 (d, 8, ArNO), 6.45 (d, 9, ArNO), 5.35 (s, CH of acac), 4.95 (s, CH of acac), 2.2 (s, CH3 of acac), 2.15 (s, CH3 of acac), 1.85 (s, CH3 of acac), 1.7 (s, CH3 of acac); IR (KBr) 1387 cm−1 [ν(NO)]; molar conductivity (CH3CN) ΛM = 11 Ω−1 cm2 M−1. Anal. Calcd for C32H36Cl2N2O10Ru2: C, 43.59; H, 4.12; N, 3.18. Found: C, 43.94; H, 4.18; N, 3.31. 3a. 2a (50 mg, 0.06 mmol), acetonitrile (30 mL); yield 20 mg (38%); ESI-MS (+) in CH3CN m/z {[3 + Na]}+ calcd 471.05, found 471.03; 1H NMR in CDCl3 (δ, J in hertz) 8.13 (d, 8, PhNO), 7.47 (t, 8, PhNO), 7.25 (t, 8, PhNO), 5.53 [s, CH (acac)], 5.04 [s, CH (acac)], 2.24 [s, CH3 (CH3CN)], 2.17 [s, CH3 (acac)], 2.15 [s, CH3 (acac)], 2.1 [s, CH3 (acac)], 1.73 [s, CH3 (acac)]. Anal. Calcd for C18H22N2O5Ru: C, 48.32; H, 4.96; N, 6.26. Found: C, 48.54; H, 4.88; N, 6.01.

(Toshcon Industries). Fourier transform infrared spectra were recorded on a Nicolet spectrophotometer with samples prepared as KBR pellets. Kinetic Studies. For 1 → 2 conversion, initially the UV−vis spectrum of a 3 mL acetonitrile solution of 1 (8 × 10−5 M) was recorded. To that was added 5 μL of an 80% hydrazine hydrate solution (∼16-fold excess), and the change in the spectral profile under airtight conditions was monitored over 3 min time intervals until the decrease in intensity at 564, 582, and 570 nm for 1a, 1b, and 1c, respectively, leveled off. Similarly, for 2 → 1 conversion, the UV− vis spectral change of a 3 mL toluene solution of 2 (8 × 10−5 M) in an airtight cuvette was recorded upon addition of ArNO (5-fold excess) in 15 min intervals to monitor the increase in intensity at 564, 582, and 570 nm for 2a, 2b, and 2c, respectively. The pseudo-first-order rate constant (k) for 1 → 2 or 2 → 1 conversion was calculated on the basis of the nonlinear exponential fit in Origin Pro8 software by following the equation y = y0 + A1 × exp(−x/t1), where y and y0 correspond to the absorbance at time t and time zero, respectively, x corresponds to time periods (t in minutes) over which the change in absorption took place, A1 is the pseudo-first-order coefficient, and the value of the pseudo-first-order rate constant (k in s−1) is 1/(t1 × 60). Crystallography. Single crystals of 1a, 1b, 1c, 2a, and 3a were grown by slow evaporation of their 2:1 dichloromethane/hexane, 1:1 dichloromethane/hexane, 1:1 acetonitrile/hexane, 1:1 acetonitrile/ hexane, and 1:1 acetonitrile/hexane solutions, respectively. X-ray diffraction data were recorded using a Rigaku Saturn-724+ CCD single-crystal X-ray diffractometer employing Mo Kα radiation. Data collection was assessed by using Crystal Clear-SM Expert software. The data were collected by the standard ω-scan technique. The structures were determined by the direct method using SHELXT2014 and refined by full matrix least squares with SHELXL-2014, refining on F2.19 All data were corrected for Lorentz and polarization effects, and all non-hydrogen atoms were refined anisotropically. The remaining hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2Ueq of their parent atoms. Hydrogen atoms were included in the refinement process as per the riding model. Computational Studies. Full geometry optimization was carried out by using the density functional theory method at the (R)B3LYP level for 1a and 2a and the (U)B3LYP level for 1a+, 1a−, 2a2+, 2a−, and 2a3−.20 All calculations were carried out with the Gaussian 09 program package.21 The LANL2DZ basis set with an effective core potential was applied for the ruthenium atom.22 All other elements except ruthenium were assigned the 6-31G** basis set. Vertical electronic excitations based on (R)B3LYP/(U)B3LYP-optimized geometries were computed using time-dependent density functional theory (TD-DFT) formalism23 in acetonitrile using the conductorlike polarizable continuum model (CPCM).24 Calculations of the fractional contributions of various groups to each molecular orbital were performed with Chemissian 1.7.25 ChemCraft was used to visualize all calculated structures.26 Preparation of Complexes. Synthesis of [Ru(acac)2(ArNO)2] (1a−1c) and [{Ru(acac)2}2(μ-ArNO)2{Ru(acac)2}2] (2a−2c). The complexes were prepared by following a general synthetic route using aryl-substituted ArNO [Ar = C6H5 (a), p-OMe-C6H4 (b), or pCl-C6H4 (c)] ligands. Solid ArNO was added to the ethanolic solution of Ru(acac)2(CH3CN)2 in an oven-dried clean two-neck round-bottom flask. The solution was refluxed under a dinitrogen atmosphere for 12 h. Evaporation of solvent under reduced pressure provided a dark brown solid, which was purified by column chromatography using a neutral alumina column and a petroleum ether/dichloromethane eluant. The dark violet (1a) or bluish green (1b) compound was eluted initially with a petroleum ether/CH2Cl2 (3:1) solvent mixture followed by the corresponding dimeric complex (2a and 2b, orange) with a petroleum ether/CH2Cl2 (1:4) solvent mixture. However, the use of p-Cl-C6H4-NO (c) under the aforementioned reaction condition yielded only the corresponding dimeric complex 2c. The corresponding monomeric complex 1c was therefore prepared by using toluene instead of ethanol as a solvent. 2 was partially converted to [Ru(acac)2(ArNO)(CH3CN)] (3) in a H

DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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the Formation of New N-C and N-S Bonds. J. Am. Chem. Soc. 1998, 120, 7484−7492. (c) Kundu, S.; Stieber, S. C. E.; Ferrier, M. G.; Kozimor, S. A.; Bertke, J. A.; Warren, T. H. Redox Non-Innocence of Nitrosobenzene at Nickel. Angew. Chem., Int. Ed. 2016, 55, 10321− 10325. (d) Sharma, P.; Liu, R. Skeletal Rearrangement in the ZnIICatalyzed [4 + 2]-Annulation of Disubstituted N-Hydroxy Allenylamines with Nitrosoarenes to Yield Substituted 1,2-Oxazinan-3-one Derivatives. Chem. - Eur. J. 2016, 22, 15881−15887 and references therein . (e) Ghosh, P.; Dey, S.; Panda, S.; Lahiri, G. K. SolventMediated Functionalization of Benzofuroxan on Electron-Rich Ruthenium Complex Platform. Chem. - Asian J. 2018, 13, 1582− 1593. (f) Chan, S.; England, J.; Lee, W. C.; Wieghardt, K.; Wong, C. Y. Noninnocent Behavior of Nitrosoarene-Pyridine Hybrid Ligands: Ruthenium Complexes Bearing a 2-(2-Nitrosoaryl)Pyridine Monoanion Radical. ChemPlusChem 2013, 78, 214−217. (3) (a) Wiese, S.; Kapoor, P.; Williams, K. D.; Warren, T. H. Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts. J. Am. Chem. Soc. 2009, 131, 18105−18111. (b) Askari, M. S.; Girard, B.; Murugesu, M.; Ottenwaelder, X. The Two Spin States of an End-on Copper(II)-Superoxide Mimic. Chem. Commun. 2011, 47, 8055−8057. (c) Hu, S.; Thompson, D. M.; Ikekwere, P. O.; Barton, R. J.; Johnson, K. E.; Robertson, B. E. Crystal and Molecular Structure of Dichlorobis(4-nitroso-N,N-dimethylaniline)zinc(II), an Example of an Oxygen-Bonded Arylnitroso Ligand. Inorg. Chem. 1989, 28, 4552−4554. (d) Lee, J.; Twamley, B.; Richter-Addo, G. B. Synthesis and Molecular Structures of Nitrosoarene Metalloporphyrin Complexes of Ruthenium. Dalton. Trans. 2004, 189−196. (e) RichterAddo, G. B. Binding of Organic Nitroso Compounds to Metalloporphyrins. Acc. Chem. Res. 1999, 32, 529−536 and references therein . (4) (a) Kumbhakar, D.; Sarkar, B.; Maji, S.; Mobin, S. M.; Fiedler, J.; Urbanos, F. A.; Jiménez-Aparicio, R.; Kaim, W.; Lahiri, G. K. Intramolecular Valence and Spin Interaction in meso and rac Diastereomers of a p-Quinonoid Bridged Diruthenium Complex. J. Am. Chem. Soc. 2008, 130, 17575−17583. (b) Maji, S.; Sarkar, B.; Mobin, S. M.; Fiedler, J.; Urbanos, F. A.; Jiménez-Aparicio, R.; Kaim, W.; Lahiri, G. K. Valence State Alternatives in Diastereoisomeric Complexes [(acac)2Ru(μ-QL)Ru(acac)2]n (QL2− = 1,4-Dioxido-9,10anthraquinone, n = + 2, + 1, 0, −1, −2). Inorg. Chem. 2008, 47, 5204−5211. (c) Keene, F. R. Isolation and Characterisation of Stereoisomers in di- and tri- Nuclear Complexes. Chem. Soc. Rev. 1998, 27, 185−193. (5) (a) Chan, S.-C.; England, J.; Wieghardt, K.; Wong, C.-Y. Trapping of the Putative 1,2-Dinitrosoarene Intermediate of Benzofuroxan Tautomerization by Coordination at Ruthenium and Exploration of its Redox Non-Innocence. Chem. Sci. 2014, 5, 3883− 3887. (b) Chan, S. C.; Cheung, H. Y.; Wong, C. Y. Ruthenium Complexes Containing 2-(2-Nitrosoaryl)pyridine: Structural, Spectroscopic, and Theoretical Studies. Inorg. Chem. 2011, 50, 11636−11643. (c) Maji, S.; Patra, S.; Chakraborty, S.; Janardanan, D.; Mobin, S. M.; Sunoj, R. B.; Lahiri, G. K. Valence-State Distribution in the Ruthenium o-Quinonoid Systems [Ru(trpy)(Cl)(L1)]+ and [Ru(trpy)(Cl)(L2)]+ (L1 = o-Iminobenzoquinone, L2 = o-Diiminobenzoquinone; trpy = 2,2/:6/, 2// -Terpyridine). Eur. J. Inorg. Chem. 2007, 2007, 314−323. (d) McKinnon, S. D. J.; Patrick, B. O.; Lever, A. B. P.; Hicks, R. G. Electronic Structure Investigations of Neutral and Charged Ruthenium Bis(β-diketonate) Complexes of Redox-Active Verdazyl Radicals. J. Am. Chem. Soc. 2011, 133, 13587−13603. (6) (a) Rusanova, J.; Rusanov, E.; Gorelsky, S. I.; Christendat, D.; Popescu, R.; Farah, A. A.; Beaulac, R.; Reber, C.; Lever, A. B. P. The Very Covalent Diammino(o-benzoquinonediimine) Dichlororuthenium(II). An Example of Very Strong π-Back-Donation. Inorg. Chem. 2006, 45, 6246−6262. (b) McKinnon, S. D. J.; Patrick, B. O.; Lever, A. B. P.; Hicks, R. G. Binuclear Ruthenium Complexes of a Neutral Radical Bridging Ligand. A New “Spin” on Mixed Valency. Inorg. Chem. 2013, 52, 8053−8066. (c) Agarwala, H.; Ehret, F.; Chowdhury, A. D.; Maji, S.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Electronic Structure and Catalytic Aspects of [Ru(tpm)(bqdi)(Cl/

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03191. Mass spectra, 1H NMR, DFT-optimized structures, crystallographic and DFT-calculated bond parameters, ORTEP diagrams, kinetic plots, energy differences of optimized structures, MO compositions and energies, and electronic spectra (PDF) Accession Codes

CCDC 1878254−1878258 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Goutam Kumar Lahiri: 0000-0002-0199-6132 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support received from the Science and Engineering Research Board (SERB, New Delhi, India), UGC (fellowship to S.D.), and CSIR (fellowship to S.P. and P.G.), New Delhi, India, is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03191 Inorg. Chem. XXXX, XXX, XXX−XXX