Ruthenium-Hydride Mediated Unsymmetrical Cleavage of

Article pubs.acs.org/IC

Ruthenium-Hydride Mediated Unsymmetrical Cleavage of Benzofuroxan to 2‑Nitroanilido with Varying Coordination Mode Prabir Ghosh, Sanjib Panda, Soumyodip Banerjee, and Goutam Kumar Lahiri* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India S Supporting Information *

ABSTRACT: The reaction of R-benzofuroxan (R = H, Me, Cl) with the metal precursor [Ru(Cl)(H)(CO)(PPh3)3] (A) or [Ru(Cl)(H)(CH3CN)(CO)(PPh3)2] (B) in CH3CN at 298 K resulted in the intermediate complex [Ru(Cl)(L1)(CH3CN)(CO)(PPh3)2] (L1 = monodentate 2-nitroanilido) (1, pink), which however underwent slow transformation to the final product [Ru(Cl)(L2)(CO)(PPh3)2] (L2 = bidentate 2-nitroanilido) (2, green). On the contrary, the same reaction in refluxing CH3CN directly yielded 2 without any tractable intermediate 1. Structural characterization of the intermediates 1a−1c and the corresponding final products 2a−2c (R = H, Me, Cl) authenticated their identities, revealing ruthenium-hydride mediated unsymmetrical cleavage of benzofuroxan to hydrogen bonded monodentate 2-nitroanilido (L1) in the former and bidentate 2-nitroanilido (L2) in the latter. The spectrophotometric monitoring of the transformations of B → 1 as well as 1 → 2 with time and temperature established the first order rate process with associatively activated pathway for both cases. Both 1 and 2 exhibited one reversible oxidation and an irreversible reduction within ±1.5 V versus saturated calomel reference electrode in CH3CN with slight variation in potential based on substituents in the benzofuroxan framework (R = H, Me, Cl). Spectroscopic (electron paramagnetic resonance and UV−vis) and density functional theory calculations collectively suggested varying contribution of metal based orbitals along with the ligand in the singly occupied molecular orbital of 1+ or 2+, ascertaining the noninnocent potential of the in situ generated (L1) or (L2).

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INTRODUCTION The resurgence of research interest in developing transition metal complexes of redox active ligands from the broader perspective of assessing valence and spin distributions at the metal−ligand interface (i.e., delicate electronic structural aspects)1 including their potential applications in cooperative catalysis2 has spurred the designing of a plethora of molecular frameworks with a wide variety of noninnocent ligand systems.3 In this context, the new members of the redox-active ligand family, i.e., in situ generated 1,2-dinitrosobenzene, the tautomeric form of biologically active benzofuroxan, exhibiting anti activities toward cancer, microbes, and parasites along with mutagenic and immunosuppressive effects,4 and its reduced form 2-nitrosoanilido were recently structurally characterized in combinations with {Ru(bpy)2} (bpy = 2,2/bipyridine)/{Ru([9]aneS3)} ([9]aneS3 = 1,4,7-trithiacyclononane)5 and {Ru(pap) 2} (pap = 2-phenylazopyridine),6 respectively (Chart 1). The selective involvement of 1,2dinitrosobenzene or 2-nitrosoanilido in the redox processes particularly in a competitive scenario with the redox facile ruthenium ion in the aforestated complex frameworks was also ascertained via experimental and theoretical investigations. Though theoretical deliberation in favor of the preferred symmetric cleavage of benzofuroxan to 1,2-dinitrosobenzene was documented,7 the present article demonstrates the ruthenuim-hydrido complex precursor (A) mediated unprece© XXXX American Chemical Society

dented facile unsymmetrical cleavage of benzofuroxan in CH3CN leading to the bidentate 2-nitroanilido moiety in 2 as well as its intermediate hydrogen bonded monodentate form in 1 (Chart 2). Herein we describe the synthetic account, structural validation, kinetics, and spectroelectrochemical features of 1 and 2. The noninnocent potential of 2-nitroanilido in both 1 and 2 and the mechanistic outline of the transformations of the metal precursor B to the intermediate 1 as well as 1 to the final product 2 have also been highlighted.

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RESULTS AND DISCUSSION Synthesis and General Characterization. The reaction of substituted R-benzofuroxan (R = H, Me, Cl) with the ruthenium-hydrido precursor complex, [RuII(Cl)(H)(CO)(PPh3)3] (A) in CH3CN at 298 K resulted in a stable green complex [RuII(Cl)(L2)(CO)(PPh3)2] (2) (L2 = bidentate 2nitroanilido) via the intermediate pink-colored complex [Ru(L1)(CH3CN)(CO)(PPh3)2] (1) (L1 = monodentate 2nitroanilido) (Chart 2). The pure complexes 1a−1c and 2a−2c (Chart 2) could be isolated by chromatographic separation on a silica gel column (Experimental Section). On the other hand, the same reaction in refluxing CH3CN directly yielded the Received: July 4, 2017

A

DOI: 10.1021/acs.inorgchem.7b01696 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Chart 1. Tautomeric Forms of Benzofuroxan and Its Mode of Reactions

Chart 2. Representation of Reaction Leading to 1 and 2

1c, and 2a−2c were authenticated by structural elucidation of all the products (see later). Though the reaction of [{Rh(-OR)(cod)}2] (RH, Me) with the readily available 2-nitroaniline was reported to yield mononuclear complex [Rh(2-NO2C6H4NH)(cod)] incorporating 2-nitroanilido moiety,9 the reaction of 2-nitroaniline with the metal precursor A or B in the presence of base (NaOH or NEt3 or NaOAc) in refluxing EtOH or CH3CN over a period of 15 h failed to yield any product altogether. The TLC as well mass spectrometry suggested the presence of mostly unreacted starting materials with a negligible amount of unidentifiable side products. This in turn justifies the effective role of Ru−H in the precursor B in forming 1 or 2 (see later). The complexes were characterized by standard analytical techniques such as microanalysis, molar conductivity, mass spectrometry (Experimental Section, Figure S1 in the Supporting Information), 1H/31P NMR, and IR spectroscopy (see later). Crystal Structures. The crystal structure of B and its selected crystallographic/bond parameters are set in Figure S2 and Tables S1/S2 (Supporting Information). Crystal structures of 1a−1c and 2a−2c are shown in Figures 1 and 2 (Figure S3 in the Supporting Information), respectively. The selected crystallographic and bond parameters of the complexes are

stable complex 2 within 2 h without any trace of intermediate 1. Since complex 1 underwent slow conversion to complex 2 in CH3CN even at room temperature, all subsequent studies were performed quickly with the freshly prepared sample in each time. Contrary to CH3CN solvent, the reaction in Chart 2 proceeded very fast in CH2Cl2 even at room temperature and led to the pure green complex 2 within 5 min or less without any tractable intermediate 1. Therefore, CH3CN was chosen to obtain both the products 1 and 2 at room temperature. In order to establish the source of coordinated CH3CN in the intermediate 1, the corresponding starting complex [Ru(Cl)(H)(CH3CN)(CO)(PPh3)2] (B) was also prepared by spontaneous exchange of one of the bulkier PPh3 groups of [Ru(Cl)(H)(CO)(PPh3)3] (A) by CH3CN. The starting complex B also gave 1 and 2 in CH3CN and 2 in CH2Cl2 at room temperature and only 2 in refluxing CH3CN as in the case of A. The slight impact of substituent in the framework of Rbenzofuroxan (R = H, Me, Cl, Chart 2) was reflected in the spectroscopic as well as electrochemical features of the complexes (see later), but it imparted virtually no influence on the stability as well as the product (1a−1c or 2a−2c, Chart 2) distribution pattern. The crystal structure of the precursor complex A was reported earlier,8 and the formations of B, 1a− B


Article

Inorganic Chemistry

Figure 1. Perspective views of 1a, 1b, and 1c (molecule C). Ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except NH proton) and solvent molecules are removed for clarity.

listed in Tables S3 and S4 (Supporting Information) and Tables S5−S8 (Supporting Information), respectively. The unit cells of 1c and 2a consist of two independent molecules, molecule C and molecule D with slight difference in bond parameters. The crystal structure of B established the simple exchange of one of the PPh3 groups of A by the CH3CN molecule, and both exhibit similar bond parameters.8 The Ru1−N1 (2.195(5) Å) and C2−N1, (1.122(7) Å) distances involving CH3CN match well with that of analogous systems.10 Crystal structures of 1a−1c established the binding of nearly planar 2-nitroanilido to ruthenium through its anilido nitrogen donor (monodentate-(L1)) with moderately strong N−H---O hydrogen bonding interaction between its N−H proton and pendant NO2 functions, resulting in N---O distances/N−H---O angles of 2.675(3) Å/134.5° (1a), 2.679(7) Å/137.5° (1b), and 2.675(8)Å/134.9° (1c).11

A comparison of the donor set around the ruthenium ion in 1 and that of the precursor B authenticated the linkage of the anilido nitrogen (-N1H−) to the ruthenium ion in the former in place of Ru−H bond of the latter, implying the hydride transfer from B to the polar double bonds of benzofuroxan (Scheme 1, see later). The ruthenium ion in 1 fits well in the equatorial plane consisting of N1, C1, Cl1, N3 donors. The two PPh3 groups in 1 are in trans configuration as in B. The delocalization of the negative charge over the two oxygen centers of the pendant NO2 function led to the similar N2−O2 (1.257(3) Å (1a), 1.262(6) Å (1b), 1.250(8)Å (1c)) and N2−O3 (1.244(4) Å (1a), 1.249(7) Å (1b), 1.268(8)Å (1c)) distances.9 The crystal structures of 2a−2c with special reference to the corresponding intermediates 1a−1c demonstrated the following three points. (i) Binding of both anilido NH and one of the O donors of the nitro function of 2-nitroanilido (bidentate(L2)) to the metal ion with the bite angle of ∼84.84(5)o. (ii) Change in binding mode of monodentate 2-nitroanilido (L1) in C


Article

Inorganic Chemistry

Figure 2. Perspective views of 2a (molecule C), 2b, and 2c. Ellipsoids are drawn at 50% probability level. Hydrogen atoms (except NH proton) and solvent molecule are removed for clarity.

1a−1c to bidentate 2-nitroanilido (L2) in 2a−2c was accompanied by the removal of CH3CN group of the former. (iii) The above (i) and (ii) steps led to the positional change of the Cl group from trans to cis with respect to the Ru−CO bond in 1 and 2, respectively. The bond parameters involving the {Ru(Cl)(CO)(PPh3)2} unit in 1a−1c and corresponding 2a−2c are rather similar. The impact of coordination of the NO2 group in 2 is reflected in the appreciable lengthening of the N2−O2 (average: 1.298 Å) bond with respect to the free N2−O3 bond (average: 1.247 Å).9 1 H NMR spectra of the complexes (1 and 2) are shown in Figures S4−S6 in the Supporting Information, and the data are listed in the Experimental Section. 2a displayed four aromatic peaks corresponding to coordinated 2-nitroanilido (L2) in the chemical shift range δ, 7.1−5.0 ppm in addition to D2O exchangeable NH proton at 4.7 ppm. The 30 protons of the

two PPh3 groups of 2a were clubbed into two broad peaks in the intensity ratio of 2:1 at δ, ∼7.3 and ∼7.7, respectively.12 2b or 2c displayed similar 1H NMR spectral profile as in 2a except three aromatic signals due to CH3 or Cl group in the framework of (L2). The resonance due to CH3 group in 2b appeared at δ, 1.6 ppm. 1a−1c exhibited 1H NMR spectral signatures as in the corresponding 2a−2c. Strikingly, hydrogen bonded NH proton signal (N−H---O) of 2-nitroanilido (L1) in 1 at δ ≈ 8.2 ppm significantly upfield shifted to ∼δ, 4.7 ppm in case of chelated nitroanilido (L 2 ) in 2. 13 The trans configuration of two PPh3 groups in 1 or 2 reflected in the single 31P signal (Figure S7, Supporting Information) in each case.14 31P resonance in 1 appeared in a slightly upfield region at δ ≈ 20 ppm as compared to that of δ ≈ 25 ppm for 2. The ν(CO) vibration of 1 or 2 appeared at around 1950 cm−1, which is slightly varied based on the substituent in 2nitroanilido and its mode of binding, monodentate in 1 versus D


Article

Inorganic Chemistry Scheme 1. An Outline for the Conversion of B → 1 and 1 → 2

Figure 3. (a) UV−vis spectral change for the conversion of B → 1a in CH3CN at 298 K. Rate is estimated based on the increase in intensity of 538 nm peak with time (inset). (b) Plot of ln(k/T) versus 1/T for the conversion of B to 1a in CH3CN.

Figure 4. (a) UV−vis spectral change for the conversion of 1a → 2a in CH3CN at 308 K. Rate is estimated based on the decrease in intensity of 538 nm peak with time (Inset). (b) Plot of ln(k/T) versus 1/T for the conversion of 1a to 2a in CH3CN.

E


Article

Inorganic Chemistry bidentate in 2. 1 and 2 displayed ν(NO) vibrations of (L1) and (L2) at 1538/1361 cm−1 and 1432/1092 cm−1, respectively.5,6 The slightly broad NH vibration of 1 (∼3400 cm−1) relative to 2 (∼3300 cm−1) is attributed to the effect of hydrogen bonding interaction in the former (Experimental Section).11 Transformations of B → 1 → 2. A tentative outline for the transformations of B to 1 and 1 to 2 is highlighted in Scheme 1. Kinetic studies extended insights into the transformation processes. The rate of conversion of B → 1 or 1 → 2 was followed spectrophotometrically in deaerated CH3CN over the temperature range of 298−328 K (Figures 3−4, Tables 1−4 and Figures S8−S19 in the Supporting Information).

of simultaneous bond breaking and bond making processes. The observed higher rate for electron withdrawing Clsubstituted 1c as compared to 1a (H) or 1b (Me) corroborates faster nucleophilic attack at the electronically poor benzofuroxan ring as hydride attack is the rate-determining step. 1 → 2. The relatively slow conversion of 1 to 2 involving the coordination of the free − NO2 function of 1 to the metal ion with the eventual displacement of the labile CH3CN group in 2 took place through a nucleophilic substitution pathway by the weak nucleophile (−NO2). The temperature dependent kinetic study of the conversion of representative 1a → 2a in acetonitrile (Figure 4, Tables 3 and 4, and Figures S14−S19

Table 1. Kinetic Data for B → 1 at 308 K

Table 3. Kinetic Data for 1 → 2 at 308 K

k (s−1)

transformation B → 1a B → 1b B → 1c

10.73 × 10 10.68 × 10−5 11.89 × 10−5

±2.1 × 10 ±1.4 × 10−6 ±1.2 × 10−6

298 308 318 328

−5

3.45 × 10 10.73 × 10−5 31.66 × 10−5 107.33 × 10−5

ΔH‡ (kcal mol−1)

ΔS‡ (cal mol−1 K−1)

21.5 ± 0.7

−7.0 ± 2.4

±1.2 × 10−6 ±1.2 × 10−6 ±1.4 × 10−6

7.47 × 10 13.45 × 10−5 5.08 × 10−5

Table 4. Temperature Dependent Kinetic Data for 1a → 2a temp (K) 298 308 318 328

k (s−1) 2.633 7.471 2.060 6.382

× × × ×

−5

10 10−5 10−4 10−4

ΔH‡ (kcal mol−1)

ΔS‡ (cal mol−1K−1)

19.9 ± 0.7

−12.8 ± 2.3

in the Supporting Information) followed a first order process with ΔH‡ (enthalpy of activation) and ΔS‡ (entropy of activation) values of 19.9 ± 0.7 kcal mol−1 and −12.8 ± 2.3 cal mol−1 K−1, respectively. The low negative value of ΔS‡ is consistent with an interchange associative (Ia) pathway,19 suggestive of the slow formation of a seven-coordinated intermediate (1′),20 followed by fast removal of CH3CN (1″), which eventually gives rise to the product 2 (k3 > k4). This seven-coordination mode is favored due to the larger size of the ruthenium(II) ion.21 Additionally, the rate of conversion of monodentate 2nitroanilido in 1 to its bidentate form in 2 follows the order 1c < 1a < 1b. The observed fastest and slowest rate of conversion for 1b and 1c, respectively, could be attributed to the impact of electron donating “Me” and electron withdrawing “Cl” groups at the para position of the −NO2 group, which in effect imparted the varying coordinating ability of the −NO2 group in 2b and 2c. This in turn decreases or increases the relative stability of the intermediate 1b or 1c, respectively. Furthermore, density functional theory (DFT) calculations reveal that the ruthenium-hydride mediated unsymmetrical cleavage of benzofuroxan in 2 is thermodynamically more favorable than the corresponding virtual product 3, generated via the symmetrical cleavage of benzofuroxan (ΔE(3−2) = 32073 cm−1) (Chart 3). This may be due to the stronger σdonor capacity of NH− in 2 in comparison to [−N−OH]− in 3, which favors the stability of 2 particularly in the presence of strongly π-accepting coligands, CO and PPh3. Also, the πacceptor feature of the NO group in 3 further destabilizes its stability, which is not the case in 2 where NO2 group is coordinated through the σ-donating O−. Electrochemistry, Electron Paramagnetic Resonance (EPR), and Spectroelectrochemistry. The complexes 1a−1c and 2a−2c exhibited similar electrochemical responses in CH3CN, one reversible oxidation and an irreversible reduction

Table 2. Temperature Dependent Kinetic Data for B → 1a k (s−1)

standard error −5

1a → 2a 1b → 2b 1c → 2c

−6

B → 1. The initial hydride transfer from [Ru(Cl)(H)(CH3CN)(CO)(PPh3)2] (B) to benzofuroxan,15−17 resulted in coordinated (monodentate) 2-nitroanilido in 1, where NH is hydrogen bonded to O− of the free nitro group of 2nitroanilido. The kinetic data for the conversion of B → 1a or 1b or 1c revealed a first order process (Figure 3, Tables 1 and 2 and Figures S8−S13 in the Supporting Information) with the highest rate of transformation for the Cl-substituted 1c. Besides, the rate of conversion at different temperatures for the representative 1a resulted in ΔH‡ (enthalpy of activation) = 21.5 ± 0.7 kcal mol−1 and ΔS‡ (entropy of activation) = −7.0 ± 2.4 cal mol−1 K−1 (Figure 3, Table 2). The first order rate

temp (K)

k (s−1)

transformation

standard error −5

process as well as low entropy of activation for B → 1 may be suggestive of an associatively activated pathway involving initial adduct formation between B and ambiphilic N3 center of benzofuroxan,17a−c,g which in effect favors nucleophilic (hydride) attack17g at N3 due to its increasing electrophilicity as well as steric preference. The formation of adduct “C” in Scheme 1 through N3 center of benzofuroxan seems to be reasonable as the α-position (N3 with respect to O2) of the five-membered heterocyclic ring is more prone to coordinate to the metal ion (an electrophile). The hydride transfer from {Ru−H} to the sterically preferred N3 position of benzofuroxan has also been supported by the crystal structures of 1a−1c as well as 2a−2c (Figures 1 and 2) involving Me or Cl substituent at its 5-position (Chart 1). The feasibility of the alternate route for the formation of 2nitroanilido in 1 via hydride attack at dinitroso benzene (Scheme 1, Path 2),17d−f a ring chain tautomerization of benzofuroxan,18 cannot however be ruled out. Thus, hydride attack followed by ring opening to gain aromaticity led to the product 1. The first order rate for B → 1 suggests k2 < k1 (Scheme 1), i.e., quick formation of the transient species “C” and its relatively slow conversion to 1 due to the involvement F


Article

Inorganic Chemistry Chart 3. Representation of {Ru−H} Mediated Unsymmetrical (2) Versus Symmetrical (3) Cleavage of Benzofuroxan

in each case within the potential window of ±1.5 V versus SCE (Figure 5, Table 5). The reversibility of the oxidation (O1) process was further ascertained by constant potential coulometry.

Figure 6. EPR spectra of 1+ and 2+ in CH3CN at 100 K (1: blue and 2: green).

Supporting Information) and Mulliken spin density plots (Figure 7 and Table 7) for the paramagnetic state. Table 6. DFT Calculated ((U)B3LYP or (U)BP86/631G**/LANL2DZ) Selected MO Compositions of 1n, 2n complex

function

MO

fragments

% contribution

1a (S = 0)

B3LYP BP86 UB3LYP UBP86 B3LYP BP86 UB3LYP UBP86 B3LYP BP86 UB3LYP UBP86 B3LYP BP86 UB3LYP UBP86 B3LYP BP86 UB3LYP UBP86 B3LYP BP86 UB3LYP UBP86

HOMO HOMO β-LUMO β-LUMO HOMO HOMO β-LUMO β-LUMO HOMO HOMO β-LUMO β-LUMO HOMO HOMO β-LUMO β-LUMO HOMO HOMO β-LUMO β-LUMO HOMO HOMO β-LUMO β-LUMO

L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L1/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl L2/Ru/Cl

78/11/1 75/18/1 73/17/0 65/23/0 78/11/0 74/14/0 73/16/0 66/21/0 72/16/0 72/16/0 63/23/1 63/23/1 52/30/12 40/36/18 49/37/7 44/37/11 49/33/12 39/39/17 49/37/7 45/38/5 42/38/14 42/35/18 47/39/8 43/39/12

1a+ (S = 1/2) 1b (S = 0) 1b+ (S = 1/2)

Figure 5. Cyclic (black) and differential pulse (green) voltammograms of the complexes in CH3CN/0.1 M Et4NClO4.

1c (S = 0)

Table 5. Electrochemical Dataa

1c+ (S = 1/2)

E°298/[V] (ΔEp/[mV])b complex 1a 1b 1c 2a 2b 2c

O1 0.73 0.71 0.76 0.72 0.71 0.77

(80) (70) (90) (80) (90) (80)

2a (S = 0)

R1 −1.04c −1.09c −1.10c −1.30c −1.31c −1.17c

2a+ (S = 1/2) 2b (S = 0) 2b+ (S = 1/2)

a

From cyclic voltammetry in CH3CN/0.1 M Et4NClO4, scan rate 100 mV s−1. bPotential in V versus SCE; peak potential differences ΔEp/ mV (in parentheses). cIrreversible process

2c (S = 0) 2c+ (S = 1/2)

A slight shift in oxidation potential (O1) took place based on the substituents (H, Me, Cl) in the framework of 1 or 2 but virtually no change in potential was observed on moving from 1a−1c to corresponding 2a−2c (Figure 5). The irreversible feature of the reduced form (R1, Figure 5) at the cyclic voltammetric or coulometric time scale essentially restricted us to its further experimental exploration. The pertinent question of the involvement of ruthenium ion or 2-nitroanilido (L1/L2) in the reversible oxidation process of the complexes (1 and 2) was addressed via EPR of the oxidized state (Figure 6) in conjunction with DFT calculated MO compositions (Table 6 and Tables S9−S20, Figure S20 in the

Electrochemically generated one-electron oxidized 1a+−1c+ (S = 1/2) exhibited rather sharp free radical type EPR in CH3CN at 100 K with g values of 2.002−2.005 (peak to peak separation: ∼25 G) (Figure 6) corresponding to primarily ligand based oxidation process {RuII-(L1)•}+ (major)/{RuIII(L1)}+ (minor) as supported by DFT calculated Mulliken spins density plot (L1: ∼70%, Ru: ∼20%, Table 7, Figure 7).1c,22 However, DFT calculated appreciable metal contribution (Ru(III) ion, t2g5, S = 1/2) along with (L2) in the singly occupied molecular orbital of 2a+-2c+ (L2: ∼45%, Ru: ∼45%, Table 7, Figure 7) reflected in the relatively broad and isotropic G


Article

Inorganic Chemistry

Table 8. UV-vis Spectral Data of 1n and 2n in CH3CN λ [nm] (ε [M−1 cm−1])

complex +

700 538 765 531 743 536 740 628 747 620 710 628

1a 1a 1b+ 1b 1c+ 1c 2a+ 2a 2b+ 2b 2c+ 2c

Figure 7. Mulliken spin density plots (UB3LYP/6-31G**/ LANL2DZ).

(6160), 403 (5630), 279 (24660) (7260), 304 (10680), 261 (33820) (4550), 404 (3910), 279 (19740) (5270), 302 (9070), 239 (27220) (2230), 400 (5310), 278 (20730) (4990), 303 (12610), 263 (31450) (930), 400 (5410), 277 (21420) (1970), 402 (3390), 303 (12580), 256 (27770) (610), 396 (5100), 284 (25870) (1930), 392 (3640), 304 (12680), 256 (27530) (1290), 391 (5340), 281 (25010) (2060), 401 (4300), 302 (14960), 259 (27870)

600 and 400 nm, respectively, corresponding to anionic 2nitroanilido (L2) targeted mixed ligand/metal to ligand based transitions. The oxidation of 2a−2c to 2a+−2c+ resulted in a broad and very weak absorption at the further lower energy part near 700 nm along with the slight enhancement of intensity for the 400 nm band of the former, corresponding to mixed ligand−metal based transitions.

EPR with g values of 2.016−2.019 (peak to peak separation: ∼50 G, Figure 6). The significant deviation of g value of 2+ from the free radical value of 2.002323 indicated a mixed electronic structural forms of {RuII-(L2)•)}+/{RuIII-(L2)}+ for 2+,1c,22 instead of any precise electronic configuration,24 implying fractional noninnocent feature of 2-nitroanilido (L1 or L2) as was reported recently for ruthenium coordinated 1,2dinitrosobenzene or 2-nitrosoanilido.5,6 The multiple electronic transitions of the complexes (1a− 1c/1a+−1c+ and 2a−2c/2a+−2c+) in the UV−visible region (Table 8 and Figure S21 in the Supporting Information) were analyzed by time-dependent DFT (TD-DFT) calculations (Table S21 and Figures S22−S23 in the Supporting Information). Complexes displayed some variation in band position or intensity in the UV−visible region in each case based on the substituent in the 2-nitroanilido framework (L1) or (L2). Complexes 1a−1c exhibited lowest energy moderately intense transition at around 530 nm, involving filled orbital (HOMO) of 2-nitroanilido (L1) and the vacant antibonding orbital (LUMO) of PPh3/Ru, leading to a ligand to mixed ligand/metal based transition. On one-electron oxidation to 1a+−1c+, the 530 nm band of 1a−1c disappeared with the concomitant growth of two new appreciably intense visible bands at around 700 and 400 nm corresponding to singly occupied L1• targeted ligand to ligand/metal and ligand based transitions, respectively.25 The isolated complexes 2a−2c showed one weak and one moderately intense transitions in the visible region at around

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CONCLUSION AND OUTLOOK In continuation to the recent reports of symmetrical cleavage of benzofuroxan, leading to ruthenium coordinated ({Ru(bpy)2}/{Ru([9]aneS3)} or {Ru(pap)2}) redox noninnocent 1,2-dinitrosobenzene or 2-nitrosoanilido (Chart 1),5,6 the present article demonstrated unprecedented ruthenium-hydride mediated selective unsymmetrical cleavage of benzofuroxan to the hydrogen bonded monodentate 2-nitroanilido (L1) form in intermediate 1 followed by its slow transformation to bidentate 2-nitroanilido (L2) in 2 (Chart 2). The transformation process of the intermediate 1 to the final product 2 was ascertained via their structural elucidation and systematic evaluation of the change in absorption spectroscopic signature as a function of time/temperature extended the mechanistic outline. The noninnocent potential of L1 in 1 or L2 in 2 was also recognized by detailed electrochemical and spectroscopic investigations in combination with DFT/TD-DFT calculations. The present deliberation with special reference to the recent reports5,6 introduces the following two additional dimensions involving the biologically relevant benzofuroxan molecule:4 (i) its diverse binding modes (flexidentate feature) to the metal ion based on the specific constituents associated with the metal fragments and (ii) potential noninnocent features of all the

Table 7. DFT Calculated (UB3LYP or BP86/6-31G**/LANL2DZ) Mulliken Spin Distributions complex +

1a (S = 1/2) 1b+ (S = 1/2) 1c+ (S = 1/2) 2a+ (S = 1/2) 2b+ (S = 1/2) 2c+ (S = 1/2)

function

Ru

L1

UB3LYP UBP86 UB3LYP UBP86 UB3LYP UBP86 UB3LYP UBP86 UB3LYP UBP86 UB3LYP UBP86

0.203 0.253 0.195 0.244 0.234 0.272 0.484 0.431 0.483 0.431 0.515 0.448

0.768 0.668 0.776 0.702 0.727 0.658

L2

PPh3

Cl

CO

CH3CN

−0.002 −0.001 −0.002 −0.001 −0.003 −0.001 0.051 0.079 0.056 0.086 0.058 0.084

−0.010 −0.010 −0.008 −0.009 −0.011 −0.010 −0.025 −0.021 −0.025 −0.021 −0.025 −0.021

0.002 0.012 0.002 0.011 0.003 0.011

0.453 0.448 0.430 0.446 0.414 0.417

0.034 0.061 0.035 0.058 0.043 0.070 0.032 0.060 −0.025 0.071 0.021 0.064

H


Article

Inorganic Chemistry Chart 4. Different Binding Modes of Benzofuroxan in Ruthenium Derivatives

6.10 (d, 9.5, 1H), 5.83 (t, 5.0, 1H), 1.95 (s, 3H). IR (KBr) (cm−1): ν(NO): 1538/1361; ν(CO): 1947; ν(NH): 3348. Molar conductivity (CH3CN): ΛM = 10 Ω−1 cm2 M−1. Elemental analysis calcd (%) for C45H38ClN3O3P2Ru: C, 62.32; H, 4.42; N, 4.85; found: C, 61.94; H, 4.41; N, 4.70. 2a. Yield 39.54 mg (35%). MS (ESI+, CH3CN): m/z {[M − Cl]}+ calcd: 791.12; found: 791.13. 1H NMR in CDCl3 [δ, ppm (J, Hz)]: 7.73 (m, 10H), 7.32 (m, 20H), 7.09 (d, 9.0, 1H), 6.04 (t, 7.5, 1H), 5.71 (t, 7.5, 1H), 4.99 (d, 9.0, 1H), 4.74 (s, 1H, NH). IR (KBr) (cm−1): ν(NO): 1432/1092; ν(CO): 1949; ν(NH): 3349. Molar conductivity (CH3CN): ΛM = 5 Ω−1 cm2 M−1. Elemental analysis calcd (%) for C43H35ClN2O3P2Ru: C, 62.51; H, 4.27; N, 3.39; found: C, 62.40; H, 4.31; N, 3.10. 1b. Yield 54.23 mg (45%). MS (ESI+, CH3CN): m/z {[M-(CH3CN +Cl)]}+ calcd: 805.13; found: 805.12. 1H NMR in CD3CN [δ, ppm (J, Hz)]: 8.22 (s, 1H, NH), 7.66 (m, 12H), 7.50 (d, 8.8, 1H), 7.41 (t, 7.4, 6H), 7.33 (m, 12H), 5.94 (s, 1H), 5.71(d, 8.8, 1H), 1.96 (s, 3H), 1.68 (s, 3H). IR (KBr) (cm−1): ν(NO): 1535/1376; ν(CO): 1947; ν(NH): 3424. Molar conductivity (CH3CN): ΛM = 11 Ω−1 cm2 M−1. Elemental analysis calcd (%) for C46H40ClN3O3P2Ru: C, 62.69; H, 4.57; N, 4.77; found: C, 62.56; H, 4.68; N, 4.53. 2b. Yield 45.97 mg (40%). MS (ESI+, CH3CN): m/z {[M − Cl]}+ calcd: 805.12; found: 805.15. 1H NMR in CDCl3 [δ, ppm (J, Hz)]: 7.74 (m, 10H), 7.33 (m, 20H), 7.01 (d, 9.5, 1H), 5.56 (d, 8.0, 1H), 4.74 (s, 1H), 4.33 (s,1H, NH), 1.60 (s, 3H). IR (KBr) (cm−1): ν(NO): 1433/1090; ν(CO): 1946; ν(NH): 3350. Molar conductivity (CH3CN): ΛM = 7 Ω−1cm2 M−1. Elemental analysis calcd (%) for C44H37ClN2O3P2Ru: C, 62.90; H, 4.44; N, 3.33; found: C, 62.71; H, 4.12; N, 3.06. 1c. Yield 49.33 mg (40%). MS (ESI+, CH3CN): m/z {[M-(CH3CN +Cl)]}+ calcd: 825.09; found: 825.08. 1H NMR in CD3CN [δ, ppm (J, Hz)]: 8.04 (s, 1H, NH), 7.61 (m, 12H), 7.54 (d, 9.0, 1H), 7.40 (m, 6H), 7.33 (m, 12H), 6.15 (s, 1H), 5.75(d, 7.2, 1H), 1.96 (s, 3H). IR (KBr) (cm−1): ν(NO): ν(NO): 1530/1380; ν(CO): 1959; ν(NH): 3421. Molar conductivity (CH3CN): ΛM = 4 Ω−1cm2 M−1. Elemental analysis calcd (%) for C45H37Cl2N3O3P2Ru: C, 59.94; H, 4.14; N, 4.66; found: C, 59.59; H, 4.12; N, 4.33. 2c. Yield 47. 08 mg (40%). MS (ESI+, CH3CN): m/z {[M-Cl]}+ calcd: 825.07; found: 825.06. 1H NMR in CDCl3 [δ, ppm (J, Hz)]: 7.47 (m, 10H), 7.35 (m, 20H), 7.09 (d, 10.0, 1H), 5.64 (d, 7.5, 1H), 4.97 (s, 1H), 4.28 (s, 1H, NH). IR (KBr) (cm−1): ν(NO): 1433/1086; ν(CO): 1963; ν(NH): 3318. Molar conductivity (CH3CN): ΛM = 6 Ω−1 cm2 M−1. Elemental analysis calcd (%) for C43H34Cl2N2O3P2Ru: C, 60.01; H, 3.98; N, 3.25; found: C, 60.06; H, 3.78; N, 2.95. Synthesis of RuII(Cl)(L2)(CO)(PPh3)2, 2. Complexes 2a, 2b, and 2c could also be prepared directly by following a general procedure via the reaction of RuII(Cl)(H)(CH3CN)(CO)(PPh3)2 with benzofuroxan, 5-methylbenzofuroxan, and 5-chlorobenzofuroxan, respectively, under heating conditions. The details for the representative case 2a are given below. The mixture of RuII(Cl)(H)(CH3CN)(CO)(PPh3)2 (100 mg, 0.137 mmol) and benzofuroxan (18.65 mg, 0.137 mmol) in 75 mL of acetonitrile was heated to reflux under stirring conditions for 2 h. The initial colorless solution was changed to green. The solvent was reduced under a vacuum, and the crude product was purified on a silica gel (mesh 60−120) column which eluted green solution corresponding to 2a by 10:1 CH2Cl2−CH3CN. Evaporation of solvent under

documented forms. The revelation of four different redox noninnocent ligand moieties out of the interaction of benzofuroxan with selective ruthenium precursors containing widely varied coligands (L) (Chart 4) extends the scope of further exploration with other metal ions as well as looks into the potential application of their varying structural and redox features in cooperative transformations/catalysis.

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EXPERIMENTAL SECTION

Materials. The precursor complexes [Ru(Cl)(H)(CO)(PPh3)3]26 and [Ru(Cl)(H)(CH3CN)(CO)(PPh3)2]27 were prepared according to the reported procedures. Benzofuroxan, 5-methylbenzofuroxan, and 5-chlorobenzofuroxan were purchased from Sigma-Aldrich. All other chemicals and reagents were reagent grade and were used as received. For spectroscopic and electrochemical studies HPLC grade solvents were used. Physical Measurements. The electrical conductivities of the complexes were checked in CH3CN with autoranging conductivity meter (Toshcon Industries, India). The EPR measurements were carried out with an X-band (9.5 GHz) Bruker EMX Plus at 100 K. 1H NMR spectra were recorded using a Bruker Avance III 500 MHz spectrometer. The elemental analysis was performed by Thermo Finnigan (FLASH EA 1112 series) microanalyzer. Cyclic voltammetry measurements were carried out on a PAR model 273A electrochemistry system. A glassy carbon working electrode, a platinum wire auxiliary electrodes, and a saturated calomel reference electrode (SCE) were used in a standard three-electrode configuration. A platinum wire-gauze working electrode was used for the constant potential coulometry experiment. Tetraethylammonium perchlorate (TEAP) was used as the supporting electrolyte, and the solute concentration was ∼1 × 10−3 M. The scan rate used was 100 mV s−1. All electrochemical experiments were carried out under dinitrogen atmosphere. The half wave potential E°298 was set equal to 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic cyclic voltammetry peak potentials, respectively. Electronic spectral studies were performed on a PerkinElmer Lambda 1050 spectrophotometer. Electrospray mass spectrometry was checked on a Bruker’s Maxis Impact (282001.00081) spectrometer. IR spectra of the complexes were recorded as KBr pellets on a Bruker, 3000 Hyperion Microscope with a Vertex 80 FTIR system. Preparation of Complexes. Synthesis of RuII(Cl)(L1)(CH3CN)(CO)(PPh3)2, 1a and RuII(Cl)(L2)(CO)(PPh3)2, 2a. The metal precursor RuII(Cl)(H)(CH3CN)(CO)(PPh3)2 (100 mg, 0.137 mmol) and benzofuroxan (18.65 mg, 0.137 mmol) were taken in 75 mL of acetonitrile, and the mixture was stirred magnetically at room temperature for 30 min. The solvent was then reduced under a vacuum and purified on a silica gel (mesh 60−120) column which led to the initial elution of the green complex (2a) by 10:1 CH2Cl2− CH3CN, followed by the pink complex (1a) by CH3CN. Evaporation of solvent under reduced pressure yielded the pure complexes 1a and 2a in solid form. The complexes 1b/2b and 1c/2c were prepared by following the same aforesaid procedure by using 5-methylbenzofuroxan and 5-chlorobenzofuroxan, respectively, instead of benzofuroxan. 1a. Yield 59.31 mg (50%). MS (ESI+, CH3CN): m/z {[M − (CH3CN + Cl)]}+ calcd: 791.12; found: 791.07. 1H NMR in CD3CN [δ, ppm (J, Hz)]: 8.22 (s, 1H, NH), 7.67 (m, 12H), 7.57 (d, 9.0, 1H), 7.41 (t, 7.5, 6H), 7.34 (m, 12H), 7.54 (d, 8.5, 1H), 6.52 (t, 7.5, 1H), I


Article

Inorganic Chemistry

puted for 1an (n = +, 0), 1bn (n = +, 0), 1cn (n = +, 0), 2an (n = +, 0), 2bn (n = +, 0), and 2cn (n = +, 0) using TD-DFT formalism34 in acetonitrile using conductor-like polarizable continuum model (CPCM).35 Chemissian 1.736 was used to calculate the fractional contributions of various groups to each molecular orbital. All calculated structures were visualized with ChemCraft.37 Electronic spectra were calculated using the SWizard program.38,39

reduced pressure yielded 2a in the solid form. Yield: 96.05 mg (85%), 103.41 mg (90%) for 2b, 94.17 mg (80%) for 2c. Kinetic Studies for B → 1 and 1 → 2. Initially a UV−vis spectrum of 3 mL of acetonitrile solution of B (5 × 10−5 M) was recorded. To that 15 μL, 10−2 M (1 equiv) of R-Benzofuroxan (R = H, Me, Cl) solution (acetonitrile) was added, and UV−vis spectra of the mixture in an airtight cuvette was monitored in 20 min interval until the increase in intensity at around 530 nm became saturated. Similarly, for 1 → 2, the UV−vis spectral change of 3 mL of acetonitrile solution of 1 (5 × 10−5 M) in an airtight cuvette was recorded in 20 min interval until the intensity changes at around 530 nm leveled off. For both the cases (B → 1 and 1 → 2), kinetics studies were performed by monitoring the change in absorbance of 1 at around ∼530 nm in CH3CN. The first order rate constant (k) was calculated based on nonlinear exponential fit in Origin Pro8 software by following the equation: y = y0 + A1* exp(−x/t1), where y, y0, and x corresponded to absorbance at ∼530 nm at time t, at time t = 0 and x = time period (t in min) over which the change in absorption took place, respectively. A1 and 1/(t1*60) represented the first order coefficient and the value of first order rate constant (k in s−1) respectively. Crystal Structure Determination. Single crystals of RuII(Cl)(H)(CH3CN)(CO)(PPh3)2 (B), 1a, 1b, 1c, 2a, 2b, and 2c were grown by slow evaporation of their 1:1 acetonnitrile-hexane, 1:1 acetonnitrile-toluene, 3:2 acetonnitrilee-benzene, 2:1 acetonnitrilehexane, 2:1 acetonnitrile-toluene, 3:2 dichloromethane-benzene, and 1:1 dichloromethane-benzene, respectively. The X-ray crystal data of B, 1a, 1b, 2a, 2b, and 2c were collected on a RIGAKU SATURN-724+ CCD single crystal X-ray diffractometer. The data were collected by the standard omega scan technique and were scaled and reduced by using the CrystalClear-SM Expert software. For 1c, crystal data was collected on a CCD Agilent Technologies (Oxford Diffraction) SUPER NOVA diffractometer. The data were collected by the standard phi-omega scan technique, and were scaled and reduced by using CrysAlisPro RED software. All the structures were solved by direct method using SHELXT-2014 and refined by full matrix leastsquares with SHELXL-2014, refining on F2.28 All non-hydrogen atoms were refined anisotropically. The remaining hydrogen (except Ru−H proton in B) 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. The disordered solvent molecules in B and 2a were SQUEEZE by PLATON29 program. Despite several attempts we failed to generate better quality crystals of 1c. Though the crystal was small and twinned (BASF 0.2983), we could solve the structure with reasonable R factor but with disordered solvent molecules (C4H10 and CH3CN). To avoid short contact error between solvent molecule (C4H10) and complex (molecule C) the hydrogen on C94 atom (associated with the solvent C4H10) was not considered. The two twin components in 1c with the ratio of 0.70:0.30 were handled at integration. Component 2 rotated by 179.9639° around [−0.00 −0.00 1.00] (reciprocal) or [0.25 −0.00 0.97] (direct) and that .hkl (hklf5 form) file from component 2 was used to solve and refine the structure. 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. 1535238 (B), CCDC NO. 1535234 (1a), CCDC NO. 1559737 (1b), CCDC NO. 1559738 (1c), CCDC No. 1535235 (2a), CCDC No. 1535236 (2b), CCDC No. 1535237 (2c). Computational Details. Full geometry optimizations were carried out by using the density functional theory method at the (R)B3LYP30/ (R)BP8631 levels for 1a, 1b, 1c, 2a, 2b, 2c, and (U)B3LYP/(U)BP86 levels for 1a+, 1b+, 1c+, 2a+, 2b+, 2c+. Except ruthenium all other elements were assigned the 6-31G** basis set. The LANL2DZ basis set with effective core potential was employed for the ruthenium atom.32 The vibrational frequency calculations were performed to ensure that the optimized geometries represent the local minima and there are only positive Eigen values. All calculations were performed with Gaussian09 program package.33 Vertical electronic excitations based on (R)B3LYP/(U)B3LYP optimized geometries were com-

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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.7b01696. Mass, NMR, experimental/simulated electronic spectra, rate processes, ORTEP diagrams, crystallographic/bond parameters, MO compositions/TD-DFT data (PDF) Accession Codes

CCDC 1535234−1535238 and 1559737−1559738 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]. uk, 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 Department of Science and Technology (DST), Council of Scientific and Industrial Research (CSIR) (fellowship to P.G. and S.P.), New Delhi (India), is gratefully acknowledged. The help of Dr. S. M. Mobin, IIT- Indore, for the crystal structure determination of 1c is also gratefully acknowledged.

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REFERENCES

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