Peroxoniobium(V)-catalyzed selective oxidation of sulfides with hydrogen peroxide in water: a sustainable approach

Abstract

An efficient and eco-compatible route for the selective oxidation of a variety of thioethers to the corresponding sulfoxide or sulfone with 30% aqueous H₂O₂ in water, using newly synthesized peroxoniobium (pNb) complexes as catalysts, is described. The catalysts with formulas Na₂[Nb(O₂)₃(arg)]·2H₂O (arg = arginate) (NbA) and Na₂[Nb(O₂)₃(nic)(H₂O)]·H₂O (nic = nicotinate) (NbN) have been synthesized from the reaction of sodium tetraperoxoniobate with 30% H₂O₂ and the respective organic ligand in an aqueous medium, and these have been comprehensively characterized by elemental analysis, spectral studies (FTIR, Raman, ¹H NMR, ¹³C NMR and UV-vis), EDX analysis and TGA-DTG analysis. The density functional theory (DFT) method has been used to investigate the structure of the synthesized pNb complexes. The catalysts are physiologically safe and can be reused for at least six reaction cycles without losing their activity or selectivity. The oxidation is chemoselective for sulfides or sulfoxides leaving the C=C or alcoholic moiety unaffected. The developed methodologies, apart from being high yielding and straightforward, are completely free from halogen, organic co-solvent, or co-catalysts.

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1 Introduction

Chemoselective oxidation of organic sulfides represents one of the most fundamentally important reactions in the domain of organic chemistry, which is fascinating from both chemical and biological perspectives.¹ The practical utility of sulfoxides and sulfones as high-value commodity chemicals and their versatility as precursors for gaining access to a variety of chemically and biologically active molecules, including drugs and chiral auxiliaries, have been adequately highlighted in the literature.^1a,d,e,2

A number of highly promising new catalytic strategies for selective sulfoxidation using various oxidants have been developed in the past few years based on transition metal catalysts, particularly those from groups 4–7 in their highest oxidation states such as titanium,³ vanadium,⁴ chromium,⁵ iron,⁶ molybdenum,⁷ tungsten^7c,8 and rhenium.⁹ Many of the available methods, however, rely upon the use of toxic and volatile organic solvents and harmful oxidants or require harsh reaction conditions, which lower the practical importance of otherwise efficient oxidation catalysts. Thus, notwithstanding the enormous progress in the development of catalytic protocols to achieve selective oxidation of sulfides, the important criterion of ecological sustainability is still a challenging issue to address. Sustainability of a chemical transformation is mainly governed by the solvent, reagents and catalysts used in addition to the work-up procedure employed.^6b In view of the current ecological concerns, the demand for catalytic oxidation processes that use benign solvent, green oxidants and reagents which co-produce only innocuous waste seems to have intensified.^7c,8,10 Out of the multitude of available organic oxidants, 30% aqueous H₂O₂ has been recognized as the best waste-preventing terminal oxidant because of the high oxygen content, cost, safety and easy handling.^10c–e,11

Water holds great promise as an alternative to traditional organic solvents as it is inexpensive, safe and non-volatile, with unique redox stability and high heat capacity.¹² Although, traditionally, water is referred to as “the universal solvent”, in organic synthesis, water has been treated as a contaminant mainly due to the concern regarding solubility.^12a However, since the ‘on water’ approach pioneered by Sharpless,^12b which demonstrated that solubility is not a requisite to reactivity and that many organic transformations can be performed efficiently in aqueous solvent, there has been phenomenal progress in the field of water-based organic synthesis.^12b,13 This also necessitated the development of water-tolerant catalysts to support such transformations. Although very limited, there are reports on metal-catalyzed sulfoxidation reaction in an aqueous medium.^6a,7a,10h,14 Very recently, Chakravarthy and co-workers reported a surfactant-based Mo catalyst for selective sulfoxidation of a variety of sulfides in an aqueous medium with 40% H₂O₂.^7a

In the recent past, we have successfully developed a set of new recoverable heterogeneous catalysts based on polymer-immobilized peroxotungsten (pW) and peroxomolybdenum (pMo) complexes, which displayed excellent stability, selectivity and efficiency with respect to yield, TON and TOF for the oxidation of a diverse range of thioethers and dibenzothiophene by H₂O₂ under very mild conditions.^7b,8a Few of these supported catalysts also effectively catalyzed the oxidative bromination of a variety of activated aromatics at ambient temperature and near-neutral pH.¹⁵ Furthermore, a number of dimeric and macromolecular water-soluble peroxo compounds of vanadium and tungsten that we prepared could serve as stoichiometric oxidants of organic bromides and sulfides in an aqueous organic medium.¹⁶

In our continuing endeavours devoted to developing newer catalysts and methodologies for oxidation under environmentally acceptable reaction conditions, in the present work, we have aspired to design new peroxometal-based catalytic systems for selective sulfoxidation using H₂O₂ as an oxidant in an aqueous medium. We selected the peroxoniobium (pNb) system as an ideal candidate for our study for a variety of reasons. Most importantly, Nb has been reported to be non-toxic to animals, with LD50 values in several thousands of milligrams per kilogram body weight in contrast to its lighter group 5 element vanadium, which is known to be moderately toxic.¹⁷ Catalysis by pNb compounds is a field of growing interest.¹⁸ The pNb species generated in situ in the presence of H₂O₂ have been shown to catalyze the oxidation of sulfides^18c–g and alcohol^18h and the epoxidation of alkene.^18a,b,i For example, Egami et al. used Nb(salan) complexes to catalyze the asymmetric epoxidation of allylic alcohol with a urea–hydrogen peroxide adduct with good selectivity.^18a However, the catalytic potential of discreet synthetic heteroligand pNb complexes as oxidation catalysts has rarely been investigated.

A suitable choice of the co-ligand is an important prerequisite in order to obtain stable and well-defined peroxometallates. Moreover, the coordinating ligand has been shown to have a dramatic effect on the reactivity of peroxometallates (pM) which enabled the activity of pM complexes as stoichiometric or catalytic agents to be fine-tuned with ligands.¹⁹ In the present investigation, since our focus was to develop physiologically harmless catalysts, we have chosen nicotinic acid (also known as niacin) and the amino acid arginine as potential co-ligands. These ligands possess a carboxylate functional group for easy attachment to the Nb(V) centre in addition to the N-donor site. Carboxylate anions have been known to be excellent co-ligands for stabilising pNb species.²⁰ Consequently, a host of pNb complexes have been reported in recent years possessing ligands with a carboxylate moiety.²⁰

On the other hand, although a number of publications, including those from our laboratory, have dealt with the synthesis and characterization of peroxometal complexes of V,^16a–d,21 Mo²² and W^16e,21f,23 with amino acid or nicotinic acid as a co-ligand,²⁴ as far as we are aware, arginine and nicotinic acid have not been used so far to obtain a heteroleptic pNb complex in the solid state. Apart from being biologically relevant, these ligands are reasonably inexpensive, water soluble, and commercially available.

We report herein the catalytic activity of a pair of newly synthesised pNb complexes for the controlled oxidation of sulfides with H₂O₂ in an aqueous medium, in terms of selectivity, yield, reusability and sustainability, to obtain sulfoxide or sulfone.

2 Results and discussion

2.1 Synthesis and characterization

A reasonably straightforward synthetic route has been established to obtain a pair of stable and water-soluble pNb complexes with arginine or niacin as a co-ligand. The procedure is based on the reaction of sodium tetraperoxoniobate with 30% H₂O₂ and the respective co-ligand in an aqueous medium at near-neutral pH. Synthesis of soluble niobium(V) compounds is considered to be rather challenging because of very low solubility of the common Nb-containing starting materials.^18d In the present study, water-soluble Na₃[Nb(O₂)₄]·13H₂O has been used as a precursor complex, which was prepared by the reported method.¹⁸ⁱ The maintenance of pH of ca. 6 was found to be essential for the formation of the triperoxoniobium species and its stabilisation by the chosen organic ligands, occurring in their anionic form, leading to the isolation of the NbA and NbN complexes. The procedure included other requirements such as maintenance of temperature at ≤4 °C and limiting water to that contributed by 30% H₂O₂. The compounds were observed to be stable in the solid state for several weeks when stored dry in a closed container at <30 °C.

The elemental analysis data (Table 1) for each of the title compounds indicated the presence of three peroxide groups and one ancillary ligand, either arginine or niacin per Nb(V) centre with the formula Na₂[Nb(O₂)₃(arg)]·2H₂O or Na₂[Nb(O₂)₃(nic)(H₂O)]·H₂O. Energy-dispersive X-ray (EDX) spectroscopic analysis clearly showed the presence of Nb, Na, C, N and O in the complexes (Fig. 1). The obtained data on the composition of the compounds from EDX analysis were in good agreement with elemental analysis values (Table 1). Despite many attempts, it was not possible to obtain crystals of the compounds large enough for an X-ray crystal structure. The compounds were diamagnetic in nature as was evident from the magnetic susceptibility measurements, in conformity with the presence of Nb in its +5 oxidation state.

Table 1 Analytical data for the synthesized peroxo–niobium complexes

Complexes	% Found from elemental analysis/EDX analysis (theoretical %)					% O2^2− content (theoretical %)
	C	H	N	Nb	Na
a Determined by AAS.
Na2[Nb(O2)3(arg)]·2H2O	16.29	3.76	12.46	21.01^a	11.14	21.71
	16.15		12.76	20.84	10.47
	(16.21)	(3.83)	(12.61)	(20.92)	(10.36)	(21.61)
Na2[Nb(O2)3(nic)(H2O)]·H2O	18.41	1.99	3.55	23.52^a	11.63	24.54
	18.37		3.61	23.57	11.66
	(18.32)	(2.03)	(3.56)	(23.64)	(11.70)	(24.43)

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Fig. 1 EDX spectra of (a) NbA and (b) NbN.

2.1.1 IR and Raman spectral studies. The NbA and NbN complexes displayed a distinctive spectral pattern in the IR region [Fig. S1 (ESI†)]. The Raman spectra of the compounds complemented the IR spectra, confirming the presence of coordinated peroxo groups and the respective co-ligand in each of them. The significant general features of IR and Raman spectra are summed up in Table 2. The Raman spectra for the complexes are presented in Fig. 2.

Table 2 Experimental and theoretical infrared and Raman spectral data (cm⁻¹) for the NbA and NbN compounds^a

Assignment			NbA	NbN
a s, strong; m, medium; vw, very weak; sh, shoulder.
ν(O–O)	IR	Exp.	846(m), 822(sh), 813(s)	847(m), 826(sh), 811(s)
		Calc.	837, 865, 889	807, 872
	R	Exp.	823(sh), 847(s), 857(sh)	817(sh), 847(s), 867(sh)
		Calc.	806, 853, 879	813, 828, 876
ν s(Nb–O2)	IR	Exp.	547(s)	547(s)
		Calc.	528	523
	R	Exp.	538(s)	542(s)
		Calc.	528	554
ν as(Nb–O2)	IR	Exp.	592(m)	593(sh)
		Calc.	604	605
	R	Exp.	567(sh)	571(sh)
		Calc.	575	598
ν as(COO^−)	IR	Exp.	1636(s)	1625(s)
		Calc.	1615	1649
	R	Exp.	1630(vw)	1627(sh)
		Calc.	1609	1627
ν s(COO^−)	IR	Exp.	1406(m)	1388(s)
		Calc.	1375	1378
	R	Exp.	1410(vw)	1393(sh)
		Calc.	1403	1335

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Fig. 2 Raman spectra of (a) NbN and (b) NbA.

A triperoxo niobium species with a triangularly bonded peroxo group has been reported to exhibit a diagnostic IR pattern with three ν(O–O) bands in the 800–880 cm⁻¹ region.^18d,25 The IR and Raman spectra of each of the NbA and NbN compounds showed clear identification of three sharp absorptions representing the characteristic ν(O–O) modes of the peroxo group, in addition to the ν_asym(Nb–O₂) and ν_sym(Nb–O₂) vibrations, as has been expected in the 870–810 and 500–600 cm⁻¹ regions, respectively.^18d,25

On the basis of the available reported data pertaining to metal compounds with coordinated amino acid and niacin as ligands, empirical assignments could be derived for the IR and Raman bands observed for the catalysts NbA and NbN.²⁶

The IR spectra of arginine and arginato–metal complexes have been reported previously.²⁷ In the spectrum of free arginine, ν_as(COO⁻) and ν_s(COO⁻) modes are observed at 1606 and 1425 cm⁻¹, respectively, with Δν = 181 [Δν = ν_as(COO⁻) − ν_s(COO⁻)], whereas in the case of the NbA complex, the corresponding absorptions appeared at 1636 cm⁻¹ and 1406 cm⁻¹, respectively. The shift of ν_as(COO⁻) to a higher frequency and that of ν_s(COO⁻) to a lower frequency compared to the free ligand values, with an increase in the Δν (230 cm⁻¹), are typical of the unidentate coordination of the carboxylate group.^26a In the Raman spectrum of the compound, weak intensity bands representing ν_as(COO⁻) and ν_s(COO⁻) vibrations have been located at 1630 and 1410 cm⁻¹, respectively. The ν(C–H) occurred as an intense peak at 2934 cm⁻¹ in the Raman spectrum in contrast to its presence as a weak band in the IR. Two bands typical of ν(NH₂) are observed at 3352 and 3288 cm⁻¹ in the free ligand spectrum.^27a A new peak appeared at 3193 cm⁻¹ in the spectrum of the NbA complex indicative of a coordinated amino group.^27a However, the other absorptions representing ν(N–H) could not be assigned with certainty owing to their overlapping with ν(OH) modes of lattice water, appearing as a broad band in the 3500–3300 cm⁻¹ region.

In the NbN catalyst, the presence of carboxylato-bonded niacin has been clearly demonstrated in its IR and Raman spectra (Table 2). A number of reports are available on IR spectral characterization of metal complexes containing coordinated niacin.^26c,e,f Moreover, the IR and Raman spectra of NIA have been thoroughly investigated by Kumar and co-workers and others.^26d Solid sodium nicotinate exhibits major carboxylate IR bands at 1620 and 1416 cm⁻¹ for ν_as(COO⁻) and ν_s(COO⁻), respectively.^26f The positions of ν_as(COO⁻) and ν_s(COO⁻) stretching in both IR and Raman spectra of NbN and the corresponding Δν value (237 cm⁻¹) provide clear evidence of unidentate coordination of a non-protonated carboxylato group to the metal atom. Furthermore, a significantly less intense aromatic ν(CC) band at 1568 cm⁻¹ depicts monodentate coordination of a carboxylate group to Nb.²⁴ Since the ν(CC) mode, ν(CN) absorption and pyridine ring vibrations at 1476, 1016 and 940 cm⁻¹, respectively, do not undergo any positive shift relative to the respective free ligand values, coordination via nitrogen in the pyridine ring can be ruled out safely.^26c The presence of water molecules in the complex was apparent from the observed broad and intense ν(OH) bands in the 3500 to 3400 cm⁻¹ region. Further confirmation for the presence of coordinated water in the compound was obtained from the consistent appearance of a moderate intensity signal at 769 cm⁻¹ attributable to the rocking mode of water.²⁸ The intense band appearing at 3077 cm⁻¹ in the Raman spectrum has been ascribed to ν(CH) vibration.

2.1.2 Electronic spectral studies. Close analogy was observed between the UV-vis spectral patterns of compounds NbA and NbN recorded in aqueous solution. The spectra exhibited two bands at ca. 280 and ca. 325 nm attributable to the peroxo-to-metal charge transfer bands, as has been observed in the case of other reported triperoxoniobium(V) complexes.^20f,i,29

2.1.3 ¹H and ¹³C NMR studies. In Tables 3 and 4, relevant ¹H and ¹³C NMR resonances for the catalysts are listed along with those of the free ligand for comparison. Crucial structural information regarding the complexes, including the coordination mode of the ligand to the Nb atom, and their stability in solution has been gathered from the complete analysis of the NMR spectra. The major NMR resonances were interpreted according to available literature data. The ¹H NMR spectra in D₂O have shown the expected integration and peak multiplicities [Fig. S2 (ESI†)].

Table 3 ¹H chemical shifts for ligands and heteroligand peroxo–niobate complexes

Compound	Chemical shift (ppm)^a
	H-2	H-3	H-4	H-5	H-6
a See Fig. 4 for the atomic numbering.
Arginine	3.76	1.89	1.66	3.23	—
NbA	3.52	1.74	1.59	3.11	—
Niacin	8.97	—	8.68	7.93	8.76
NbN	8.82	—	8.13	7.40	8.49

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Table 4 ¹³C NMR chemical shift for ligands and triperoxoniobium complexes, NbA and NbN

Compound	Carboxylate carbon	Chemical shift (ppm)^a
		C2	C3	C4	C5	C6
a See Fig. 4 for the atomic numbering.
Arginine	183.17	55.59	31.64	24.51	41.02	156.81
NbA	215.45	54.59	30.28	23.99	40.64	156.83
Niacin	168.25	143.04	135.49	142.49	126.94	145.89
NbN	210.78	149.06	137.69	142.46	128.76	150.51

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The spectral pattern of the pNb complex with arginine as a co-ligand resembled closely the spectrum of the free ligand by exhibiting 4 major peaks at 3.52, 3.11, 1.74 and 1.59 ppm.^30a The spectrum of the complex, however, showed a distinct upfield shift of all the resonances relative to the free ligand, indicating coordination of the ligand to the metal centre.³⁰ Similar observations were made previously in the case of metal compounds containing complexed amino acid.^30,31

The ¹³C chemical shift induced by coordination has been widely utilized as a convenient means of understanding the bonding pattern of ancillary ligands in peroxo–metal complexes.³² The ¹³C NMR spectrum of the free arginine in D₂O displays a typical resonance for a carboxylate carbon atom at 183.17 ppm in addition to the five other well-resolved peaks corresponding to carbon atoms C(2) to C(6).^30a The spectrum of NbA, on the other hand, displayed the carboxylate resonance at a lower field of 215.45 ppm, thus testifying to the existence of a complexed carboxylate group.^30a,33 The substantial downfield shift relative to free carboxylate, with Δδ (δ_complex − δ_{free carboxylate}) ≈ 32 ppm, indicated strong metal–ligand interaction as has been reported earlier in the case of some other peroxo–metal carboxylate complexes.^7b,33 The guanidyl C resonance along with the resonances of alkyl groups (C-5 and C-4) showed very little shift, whereas the resonances of α-CH and β-CH₂ groups shifted to a higher field by ca. 1 ppm [Fig. S3(a) (ESI†)]. These results indicated the coordination of carboxylate and amino groups of the ligand to the niobium centre and are consistent with observations made in the case of other reported arginine-containing metal compounds.^30a The ¹H and ¹³C NMR spectra of nicotinic acid in D₂O have been studied and reported by Khan and co-workers under varying pH conditions.³⁴ A ¹H NMR pattern typical of a nicotinate anion was observed with four well-resolved resonances in the spectrum of the NbN compound.³⁴ As expected for the nicotinate anion, the spectrum showed a distinct upfield shift of each of the four resonances corresponding to the aromatic protons H(2) to H(6) relative to the zwitterionic free ligand values [Fig. S2(a) (ESI†)].^30b

The ¹³C spectrum provided further persuasive evidence in support of the presence of a nicotinate anion in the NbN compound by displaying resonances for ring carbon atoms in the region expected for the nicotinate anion.³⁴ The peak attributable to the metal-bound carboxylate carbon, as in the case of NbA, appeared at a very low field of 210.78 ppm compared to the carboxylate resonance of the free nicotinic acid [Fig. S3(b) (ESI†)].^7b,33 Occurrence of the carboxylate carbon as a singlet in the spectrum of each of the catalysts, NbA and NbN, indicated a single-carbon environment for complexed carboxylate.³³ Thus, the results of the NMR analysis showed evidence of the presence of only one complex species in solution in each case. It is therefore apparent that catalysts did not hydrolyze and retained their solid-state structure in solution.

2.1.4 TGA-DTG analysis. The TGA-DTG plots for the NbA and NbN catalysts illustrated in Fig. 3 showed that the compounds gradually undergo multistage decomposition on heating to a temperature of 700 °C. Interestingly, the compounds do not explode on heating, although a majority of the known pNb compounds have been reported to decompose explosively on heating.^20b,f,35

Fig. 3 TGA-DTG plot of (a) NbA and (b) NbN.

The first stage of decomposition for NbA [Fig. 3(a)] occurs in the temperature range of 78–102 °C with the liberation of the lattice water from the complex. The corresponding weight loss of 8.9% is in good agreement with the value of 8.1% calculated for two molecules of water of crystallization. The next decomposition stage is in the temperature range of 157–189 °C attributable to loss of peroxo groups from the complexes with a weight loss of 18.8%. As the observed weight loss is slightly less than the expected value, it is likely that part of the oxygen is retained with niobium to form oxoniobium species, as has been observed previously in the case of neat as well as heteroligand pNb complexes.^20b,f,35 A further increase in temperature leads to continuous degradation of the arginine ligand up to a final decomposition temperature of 700 °C. The total weight loss which occurred during the overall decomposition process was evaluated to be 54.3%, which agreed well with the theoretically calculated value of 54.6% for the loss of the components, viz. water molecules, coordinated peroxide, and the co-ligand, assuming that four of the ligand oxygen atoms were being retained to form the oxoniobate species as the final degradation product.

It is notable that the thermogram for NbN [Fig. 3(b)] displayed a two-step dehydration process in the temperature range of 45–105 °C, providing conclusive evidence of the presence of outer sphere coordinated water molecules in the compound, consistent with the formula assigned. The decomposition step occurring at relatively higher temperature between 90 and 105 °C, after the initial liberation of the outer sphere water molecule in the temperature range of 45–70 °C, is attributable to the loss of the coordinated water molecule. The observed total weight loss of 9.8% corresponding to the two steps combined is close to the calculated value of 9.2% for the release of two molecules of water from the complex. After the dehydration, the thermal behaviour of the NbA and NbN catalysts is quite similar. The NbN compound undergoes continuous degradation with loss of peroxide in the temperature range of 158–189 °C analogous to the NbA compound, followed by the loss of the niacin ligand up to the final temperature of 700 °C. The residue remaining after the complete degradation of each of the pNb compounds was found to be oxoniobate species as indicated by the IR spectral analysis which showed the typical ν(Nb=O) stretching and was devoid of absorptions attributable to the peroxo group and the respective co-ligands of the starting complex.

The above results are consistent with the proposed structures of the compounds NbA and NbN shown schematically in Fig. 4. The structure of NbA shows the Nb atom displaying a coordination number of 8, surrounded by the three peroxo groups and the arginate ligand bonded via its unidentate carboxylate group and the amino group. The structure of NbN includes a nicotinate anion occurring as a unidentate ligand bonded to the Nb centre through the carboxylate group, the side-on bound peroxo groups and a coordinated water molecule completing eightfold coordination around the Nb.

Fig. 4 Proposed structure of (a) NbA and (b) NbN.

2.1.5 Theoretical investigation. In order to verify the feasibility of the proposed structures, density functional theory (DFT)³⁶ calculations were performed on the two niobium complexes at the B3LYP/LANL2DZ level of theory. The initial structures of the two complexes were modeled based on the experimental data related to the compounds (FTIR, Raman, ¹³C and ¹H NMR spectroscopy, TGA and elemental analysis). The optimized geometries of the two niobium complexes are presented in Fig. 5. In both complexes the coordination polyhedron around the niobium atom is a dodecahedron as observed in a majority of the reported pNb complexes.^18d,20f

Fig. 5 Optimized geometry of (a) NbA and (b) NbN. The numerical numbers represent the labeling of the atoms as in Table 5.

In the case of NbA, the central metal atom (Nb) is coordinated to six oxygen atoms belonging to three η²-peroxo groups, in addition to one oxygen atom from the carboxylato group and a N (amino) atom from the deprotonated argininate ligand. One of the bidentate peroxo groups is in trans position to the co-ligand, whereas the other two are in cis configuration. The coordination sphere of the NbN complex comprises a niobium atom surrounded by seven oxygen atoms contributed by three η²-peroxo groups and one from the unidentate carboxylato group of the nicotinate ligand. The eighth coordination site is satisfied by a water molecule which is weakly bonded to the central atom, as has been observed previously by Djordjevic et al. in the case of nicotinate containing a peroxomolybdenum complex.²⁴ Thus, the structure of NbN can be described as a distorted octahedron with one of the axial distances slightly longer. For both the complexes, the coordination distances are within the range characteristic of the heteroleptic peroxo complexes of niobium(V).^20f The geometrical parameters (bond angle and bond length) obtained from DFT calculations are depicted in Table 5. The Nb–O(peroxo) bond lengths are within the range of 1.977 to 2.086 Å, whereas the (O–O) distances range from 1.526 to 1.533 Å. The Nb–O (carboxylate) bond distances in NbA and NbN are 2.162 Å and 2.138 Å, respectively. The Nb–N bond distance in NbA is 2.370 Å. The geometrical parameters obtained from our theoretical calculation correlated well with the reported crystallographic parameters pertaining to other heteroleptic triperoxo niobate complexes with coordination environment comprising N,O- or O-donor co-ligands.^{18d–i,19a–d,20a–f} The small differences observed between the experimental and calculated geometrical parameters are expected as the ground-state geometries were obtained in the gas phase by full geometry optimization.

Table 5 Selected bond lengths (Å) and bond angles (degree) for the two complexes calculated at the B3LYP/LANL2DZ level of theory

Structural index^a	NbA	Structural index^a	NbN
a See Fig. 5 for the atomic numbering.
Nb–O2	2.072	Nb–O2	2.047
Nb–O3	2.008	Nb–O3	1.999
Nb–O4	1.999	Nb–O4	1.977
Nb–O5	2.066	Nb–O5	2.033
Nb–O6	2.162	Nb–O6	1.985
Nb–O7	2.086	Nb–O7	2.040
Nb–O8	2.011	Nb–O19	2.138
Nb–N10	2.370	Nb–O20	2.831
O2–O3	1.526	O2–O3	1.527
O4–O8	1.533	O5–O6	1.530
O5–O7	1.530	O4–O7	1.532
C12–C15	1.537	C18–C11	1.521
C9–O6	1.307
∠O2–Nb–O3	43.9	∠O2–Nb–O3	44.3
∠O4–Nb–O8	44.9	∠O4–Nb–O7	44.8
∠O5–Nb–O7	44.2	∠O5–Nb–O6	44.7
∠O6–Nb–N10	72.1	∠O3–Nb–O19	82.9

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We have further calculated the vibrational frequencies for the optimized geometries of the compounds and compared the data obtained with the experimentally determined frequencies as illustrated in Table 2. The calculated IR and Raman spectra for the two complexes are found to simulate well with the experimental ones. The small deviations between the calculated and experimental spectral data are anticipated as the calculated spectral data are obtained for those of the gas-phase-optimized geometries. These observed discrepancies appear to be acceptable as the average error for frequencies calculated with the B3LYP functional was reported to be of the order 40–50 cm⁻¹ for inorganic molecules.³⁷ Moreover, such deviations in the FTIR and Raman vibrational bands obtained from DFT-based calculations on eight-coordinated peroxo complexes of niobium(V) are not unprecedented.^20f Thus, the results obtained from our theoretical calculations truly support our experimental findings and completely validate the predicted geometries for the synthesized complexes.

2.2 Catalytic activity of the NbA and NbN complexes

2.2.1 Oxidation of sulfides to sulfoxides. Catalytic performances of the title pNb complexes in oxidation of various organic sulfides using 30% aqueous H₂O₂ as a terminal oxidant in neat water have been investigated. In a preliminary experiment, the reaction of MPS with H₂O₂ (1 equiv.) in the presence of the water-soluble catalyst NbA, maintaining a catalyst : substrate molar ratio of 1 : 1000, was conducted in water at ambient temperature under magnetic stirring. The reaction proceeded rapidly within a reasonably short period in the presence of each of the catalysts. The reaction under these conditions was, however, observed to be mildly exothermic, leading to the formation of a mixture of 1a and 1b with a molar ratio of 85 : 15, as presented in Table 6 (entry 1). Subsequently, the reaction conditions for selective sulfoxidation including substrate : oxidant stoichiometry, catalyst concentration, solvent type and reaction temperature were optimized using MPS as the model substrate and the NbA complex as the catalyst. We have attempted to control the degree of oxidation by lowering the reaction temperature to 0 °C by conducting the reaction in an ice bath.

Table 6 Optimization of reaction conditions for selective oxidation of methyl phenyl sulfide (MPS) by 30% H₂O₂ catalyzed by pNb complexes^a

Entry	Molar ratio (catalyst : MPS)	30% H2O2 (equiv.)	Solvent	Time (min)	Isolated yield (%)	1a : 1b	TON	TOF (h^−1)
a Reactions were carried out with 5 mmol of the substrate in 5 mL of solvent. Catalyst amount = 2.22 mg for 0.005 mmol of NbA. b Reaction at room temperature. c Reaction at 0 °C in ice bath. d Na3[Nb(O2)4]·13H2O as the catalyst. e Using H2SO4 (1 mmol). f Blank experiment (without a catalyst).
1	1 : 1000^b	1	H2O	75	76	85 : 15	760	608
2	1 : 1000^c	1	H2O	75	70	100 : 0	700	560
3	1 : 500^c	2	H2O	38	97	85 : 15	485	766
4	1 : 1000^c	2	H2O	40	95	100 : 0	950	1425
5	1 : 2000^c	2	H2O	43	96	100 : 0	1920	2679
6	1 : 2500^c	2	H2O	50	97	100 : 0	2425	2910
7	1 : 3000^c	2	H2O	75	95	100 : 0	2844	2275
8	1 : 2500^b	2	CH3OH	115	75	100 : 0	1875	978
9	1 : 2500^b	2	CH3CN	120	70	100 : 0	1750	875
10^d	1 : 2500^c	2	H2O	95	97	55 : 45	2425	1531
11^e	1 : 2500^c	2	H2O	20	96	100 : 0	2400	7200
12^f	—	2	H2O	50	9	90 : 10	—	—

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As has been anticipated, the reaction indeed proceeded smoothly to yield sulfoxide with 100% selectivity and nearly 70% conversion under these conditions. Complete oxidation of MPS to pure sulfoxide could be attained with excellent TOF (without affecting the selectivity) by increasing the oxidant : substrate molar ratio to 2 : 1 without altering the other reaction conditions.

Next, we have examined the effect of the catalyst amount on the rate and selectivity of the reaction under otherwise identical reaction conditions. As illustrated in Table 6, although the rate was faster at higher catalyst concentration and a reasonably good TOF could be attained even at a catalyst : substrate ratio of 1 : 3000, the optimal catalyst : substrate molar ratio was found to be 1 : 2500 for achieving the highest TOF along with complete selectivity. The pNb species has an important contribution in facilitating the reactions, which was confirmed by conducting a control experiment without the catalyst. The reaction was extremely slow and non-selective in the absence of the catalyst, affording a mixture of sulfoxide and sulfone in <9% yield under the optimized reaction conditions (Table 6, entry 12). We have also compared the catalytic efficiency of the newly developed catalysts with the neat tetraperoxoniobate (TPNB) complex Na₃[Nb(O₂)₄]·13H₂O under analogous reaction conditions. As shown in Table 6 (entry 10), the reaction proceeded smoothly in the presence of TPNB as well, although product selectivity could not be obtained under the maintained reaction conditions. From these observations, it is apparent that the co-ligand environment influences the catalytic activity of the pNb compounds.

In addition to water, we have screened the sulfoxidation reaction in relatively safer organic solvents such as CH₃OH and CH₃CN. The solvent effect has been evaluated in the oxidation of MPS. Interestingly, the catalytic protocol for sulfoxidation was found to be compatible with these organic solvents as well; however, the efficiency of the catalysts was observed to vary with the nature of the solvent. Although the catalysts are insoluble in neat organic solvents, including methanol or acetonitrile, in the presence of aqueous H₂O₂ used as an oxidant, each of the catalysts dissolves completely in these water-miscible solvents, leading to the homogeneity of the catalytic process. It is pertinent to note that we have strategically avoided the use of hazardous chlorinated solvents in the present work. The data presented in Table 6 (entries 8 and 9) demonstrate that although the NbA is highly potent in methanol as well as in acetonitrile, MeOH proved to be a relatively better solvent, affording both product selectivity and high yield at ambient temperature (Table 6, entry 8). Significantly, the selective sulfoxidation in the chosen organic solvents could be achieved at room temperature. It is thus remarkable that by using the same set of catalyst, it is possible to achieve selective oxidation of sulfide in water and organic solvents under mild reaction conditions. Water, however, proved to be the best solvent with respect to catalyst efficiency as demonstrated by higher TOF and product selectivity, notwithstanding the insolubility of most of the chosen organic substrates in water. This is not surprising, considering the observations made by Sharpless et al.^12b that several reactions involving water-insoluble organic reactants could proceed optimally in pure water. Our results are also in agreement with earlier findings that chemoselective sulfoxidation is favoured in polar protic solvent with strong hydrogen bonding ability.³⁸

The sulfoxidation reaction has been carried out at the natural pH attained by the reaction mixture (ca. 5). However, a substantial increase in TOF was noted on addition of acid to the reaction medium (Table 6, entry 11). The finding is in accord with the reports related to other peroxo metal systems (Ti, Mo and W),³⁹ where it has been demonstrated that the use of acidic additives led to an improvement in catalytic activity of the peroxometallates. The role of protons in the activation of titanium peroxo complexes has been extensively investigated by Kholdeeva and co-workers.^39b In the present work, however, since our goal has been to maintain a mild reaction condition, addition of acid or other additives was avoided as far as possible. Therefore no attempt has been made to adjust the pH of the reaction.

The aforementioned findings were further exploited to obtain pure sulfoxide from a series of aryl alkyl, aryl vinyl, aryl alcohol and dialkyl sulfides listed in Table 7. Evidently, both the catalysts were effective in leading to the facile and selective transformation of each of the substrates to the corresponding sulfoxide with impressive yield, although the NbN catalyst displayed relatively superior activity. The transformations worked well for both aliphatic and aromatic substrates irrespective of having electron-donating or electron-withdrawing moieties. The nature of the substrate and the attached substituent, however, appeared to influence the rates of oxidation.^8g,11b The observed trend in variations in the rate of oxidation of the chosen substrates is consistent with the previous findings that with increasing nucleophilicity of the sulfide, the rate of oxidation with H₂O₂ increases.^8g,11b It is therefore not unexpected that dialkyl sulfides were oxidized by H₂O₂ at a faster rate leading to the highest TOF, relative to conjugated systems such as allylic and vinylic sulfides or aromatic sulfides. It is notable that even in the case of a less nucleophilic diaromatic sulfide the corresponding sulfoxide was effectively obtained.

Table 7 Selective oxidation of sulfides to sulfoxides catalysed by NbA and NbN^a

Entry	Substrate	NbA				NbN
		Time (min)	Isolated yield of sulfoxide (%)	TON^b	TOF^c (h^−1)	Time (min)	Isolated yield of sulfoxide (%)	TON^b	TOF^c (h^−1)
a Optimized conditions: 5 mmol of substrate, 10 mmol of 30% H2O2 and 0.002 mmol of catalyst in H2O at 0 °C. b TON (turnover number) = millimoles of the product per millimole of the catalyst. c TOF (turnover frequency) = millimoles of the product per millimole of the catalyst per hour. d Yield of the 6th reaction cycle. e Scale-up data (6.24 g of MPS).
1.		50	97	2425	2910	45	96	2400	3200
		50	93^d	2325	2790	45	94^d	2350	3133
		50	96^e	2400	2880	45	95^e	2375	3166
2.		45	97	2425	3233	40	95	2375	3562
3.		85	93	2325	1641	75	94	2350	1880
4.		240	95	2375	593	225	93	2325	620
5.		115	97	2425	1265	105	95	2375	1357
6.		55	96	2400	2618	50	95	2375	2850
7.		210	93	2325	664	195	94	2350	723
8.		30	97	2425	4850	25	96	2400	5760
9.		40	96	2400	3600	35	93	2325	3985
10.		40	97	2425	3637	35	95	2375	4071

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A salient feature of the methodology, which enhances the synthetic utility of the oxidation, is the excellent chemoselectivity displayed by the catalysts for the sulfur group of substituted sulfides such as allylic, vinylic and alcoholic sulfides, with co-existing sensitive functional groups (Table 7, entries 3–5). Importantly, allylic and vinylic sulfoxides were obtained without an epoxidation product. Moreover, the –OH group of alcoholic sulfides and the benzylic C–H bond remained unaffected when the benzylic and alcoholic sulfides could be oxidized to the corresponding sulfoxide under the maintained reaction conditions. The potential of the developed protocol for scaled-up synthetic application has been demonstrated by conducting the oxidation with 6.24 g of thioanisole (tenfold scale) under optimized conditions (Table 7, entry 1^e). The H₂O₂ efficiency in the oxidation in the presence of both the catalysts NbA and NbN was found to be higher than 90%. The H₂O₂ efficiency, which is a measure of the effective use of H₂O₂, has been defined as 100 × moles of H₂O₂ consumed in the formation of oxyfunctionalized products per mole of H₂O₂ converted^11a [text S4, (ESI†)].

2.2.2 Oxidation of sulfides to sulfones. Clean conversion of MPS to sulfone could be achieved with high yield in an aqueous medium, in the presence of each of the catalysts, simply by extending the reaction time after initial formation of sulfoxides and conducting the reaction at room temperature. The optimization of reaction conditions, accomplished by using MPS as a substrate and NbA as a catalyst, revealed that the best rate and TOF could be obtained by maintaining the catalyst : substrate ratio of 1 : 1000 using 2 equiv. of H₂O₂ (Table 8, entry 1).

Table 8 Optimization of reaction conditions for NbA-catalyzed selective oxidation of methyl phenyl sulfide (MPS) to sulfone^a

Entry	Molar ratio (catalyst : MPS)	H2O2 (equiv.)	Solvent	Time (min)	Isolated yield (%)	1a : 1b	TON	TOF(h^−1)
a Reactions are carried out with 5 mmol of the substrate in 5 mL of solvent at room temperature.
1.	1 : 1000	2	H2O	80	96	0 : 100	960	720
2.	1 : 2000	2	H2O	175	95	0 : 100	1900	651
3.	1 : 2500	2	H2O	240	96	25 : 75	2400	600
4.	1 : 1000	2	CH3OH	240	96	70 : 30	960	240
5.	1 : 1000	2	CH3CN	240	93	75 : 25	930	232

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Apart from MPS, the protocol could be conveniently applied to obtain pure sulfone from a variety of aromatic and aliphatic sulfides as shown in Table 9. The transformations were chemoselective (Table 9, entries 3–5) and amenable for scale-up (Table 9, entry 1^e) as has been observed in the case of sulfoxidation reaction. These findings underscore the synthetic value of the methodology.

Table 9 Selective oxidation of sulfides to sulfones catalyzed by NbA and NbN^a

Entry	Substrate	NbA				NbN
		Time (min)	Isolated yield of sulfone (%)	TON^b	TOF^c (h^−1)	Time (min)	Isolated yield of sulfone (%)	TON^b	TOF^c (h^−1)
a Optimized conditions: 5 mmol of the substrate, 10 mmol of 30% H2O2 and 0.005 mmol of the catalyst in H2O at RT. b TON (turnover number) = millimoles of the product per millimole of the catalyst. c TOF (turnover frequency) = millimoles of the product per millimole of the catalyst per hour. d Yield of the 6th reaction cycle. e Scale-up data (6.24 g of MPS).
1.		80	96	960	720	70	97	970	831
		80	93^d	930	697	70	93	930	797
		80	95^e	950	712	70	94^e	940	805
2.		75	95	950	760	65	97	970	895
3.		145	93	930	384	135	94	940	431
4.		255	94	940	221	250	93	930	235
5.		190	97	970	306	180	95	950	316
6.		90	95	950	633	80	94	940	705
7.		315	93	930	177	310	93	930	180
8.		65	94	940	867	55	96	960	1047
9.		70	93	930	797	60	93	930	930
10.		70	95	950	814	60	94	940	940

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2.2.3 Recyclability of the catalysts. The reusability of the catalysts for subsequent cycles of oxidation was assessed by employing MPS as the substrate. The catalysts afforded regeneration in situ after separation of the organic product from the reaction mixture on completion of the reaction and could be reused without further conditioning. Regeneration was accomplished by simply charging the aqueous part of the spent reaction mixture with 30% H₂O₂ and a fresh batch of substrates on completion of each reaction cycle. In an alternative approach, the regenerated catalysts could also be isolated into the solid state by solvent-induced precipitation with acetone after treating the aqueous extract of the spent reaction mixture with 30% H₂O₂. Nevertheless, the in situ regeneration of the catalyst offers obvious advantages as the procedure does not require tedious separation and subsequent purification steps, which are usually associated with soluble catalysts.^12a The activity and selectivity of the catalysts remained unaltered for at least up to 6 reaction cycles as illustrated in Table 7 (entry 1^d), Table 9 (entry 1^d) and Fig. S6 (ESI†). In order to further confirm that the catalysts remain intact after their use in catalytic cycles, the recovered catalysts were characterized by elemental analysis and spectral studies. The IR and Raman spectra of each of the regenerated catalysts resembled closely the corresponding spectrum of the starting catalyst, displaying the typical absorptions for the triperoxoniobium moiety and the respective metal bound co-ligand, indicating that the coordination environment of the complexes was not altered during the catalytic process. The IR spectrum of the regenerated catalyst NbA is presented in Fig. S1(c) (ESI†). No significant change was observed in the niobium and peroxide contents of the recovered catalysts in comparison to the respective original complex. It is thus evident that the metal complexes retain their structural integrity even after several catalytic cycles.

Interestingly, the procedure with NbA used as a catalyst provided an overall TOF of ca. 17 130 h⁻¹ (ca. 18 999 h⁻¹ for NbN) after 6 cycles of oxidation of MPS to sulfoxide and ca. 4259 h⁻¹ (ca. 4884 h⁻¹ with NbN) for conversion to sulfone. The results further demonstrate the superior activity of the catalysts under mild conditions over other reported methods for sulfide oxidation involving Nb(V) catalysts,^20a–f as well as many other protocols based on Mo(VI) or W(VI)/H₂O₂^{7b–e,8b–e,g,10c,g,h,38a,40} systems including the polymer-supported heterogeneous Mo(VI) catalysts that we reported recently.^7b

2.2.4 The proposed mechanism. Based on our results and taking into account our earlier findings on the catalytic activity of some other peroxometal systems,^7b,8a a credible mechanism for the pNb-catalyzed selective oxidation of sulfides to sulfoxides or sulfone by H₂O₂ has been proposed (Fig. 6), which satisfactorily describes the principal features of our findings from the present study.

Fig. 6 The proposed mechanism.

As shown in Fig. 6, with the NbA catalyst as a representative, it is possible that the reaction proceeds through the formation of the diperoxoniobate intermediate II, subsequent to the facile transfer of electrophilic oxygen from the triperoxoniobium complex I to the substrate V to yield sulfoxide (reaction a). The intermediate II combines with the peroxide of H₂O₂ to regenerate the starting triperoxoniobate complex (reaction b) leading to a catalytic cycle. The resulting sulfoxide formed may undergo further oxidation in a separate cycle by reacting with a triperoxo Nb species to yield sulfone (reaction c). The sulfone formation thus seems to be a two-step process. The greater ease of oxidation of sulfide to sulfoxide compared to the second oxidation of sulfoxide to sulfone is likely to be a consequence of higher nucleophilicity of sulfide relative to sulfoxide. The proposed mechanism is in line with the reaction pathway proposed previously for peroxoniobate-catalysed oxidation of sulfide.^18a,d,20g

It is worth noting that although the mechanism of action of peroxo complexes of other d⁰ metals such as V(V), Mo(VI) and W(VI) has been extensively investigated in organic oxidation, the chemistry of Nb(V) peroxide still remains relatively unexplored.^{7b,d,8a,16e,21d,24,40a,d,41} The previous studies from several laboratories including ours^7b,8a,16e,f have shown that during substrate oxidation performed by active diperoxo complexes of Mo(VI) or W(VI), a more stable monoperoxo species is formed which is practically inactive in oxidation.^{7d,24,41k–n} Taking into account this finding, it is reasonable to expect the formation of a less reactive diperoxoniobate (DPNb) intermediate from an active triperoxoniobate (TPNb) species during sulfide oxidation, as shown in the proposed mechanism (reaction intermediate II). In order to establish the involvement of such an intermediate in the reaction pathway, a separate experiment was conducted using NbA as a stoichiometric oxidant of MPS, maintaining an NbA : substrate molar ratio of 1 : 1 in the absence of H₂O₂ at 0 °C. The substrate was completely and selectively transformed into sulfoxide within a reaction time of ca. 40 minutes. The product isolated from the aqueous part of the spent reaction mixture was subsequently subjected to spectral and elemental analyses. The data obtained indicated a Nb : peroxo ratio of 1 : 2, clearly suggesting the formation of a DPNb species. This was further confirmed from the IR spectrum which showed two distinct bands, in addition to the coordinated amino acid ligands, characteristic of a DPNb moiety [Fig. S1(d) (ESI†)] in contrast to the three peroxo absorptions of the original triperoxoniobium catalyst. Similar reaction of MPS conducted with the isolated DPNb complex was noted to be extremely slow, as has been anticipated, remaining incomplete even after 14 h of reaction time. The aforementioned findings lent further credence to the proposed mechanism.

3. Experimental section

3.1 Materials

Acetone, hydrogen peroxide, acetonitrile, methanol, ethylacetate, petroleum ether, diethyl ether, silica gel (60–120 mesh) (RANKEM), L-arginine (CDH, New Delhi, India), nicotinic acid (HIMEDIA), sodium hydroxide, and sodium sulfate (E. Merck, India) were used in this study. Niobium pentoxide, methyl phenyl sulfide (MPS), methyl p-tolyl sulfide (MpTS), ethyl phenyl sulfide (EPS), dimethyl sulfide (DMS), dibutyl sulfide (DBS), phenylvinyl sulfide (PVS), 2-(phenylthio)ethanol (PTE), dihexyl sulfide (DHS), diphenyl sulfide (DPS) and allyl phenyl sulfide (APS) were obtained from Sigma-Aldrich Chemical Company, Milwaukee, USA. The water used for solution preparation was deionized and distilled.

3.2 Synthesis of the complexes

3.2.1 Preparation of sodium tetraperoxoniobate¹⁸ⁱ. Sodium tetraperoxoniobate, Na₃[Nb(O₂)₄]·13H₂O, was prepared by the reported method.¹⁸ⁱ In a nickel crucible, 1 g of Nb₂O₅ and 1.85 g of NaOH were fused together at 700 °C. The solid obtained was cooled and dissolved in 100 ml of 1 M aqueous H₂O₂. Unreacted Nb₂O₅ was filtered off and the filtrate was allowed to settle at 5 °C for 24 h to afford the white crystalline Na₃[Nb(O₂)₄]·13H₂O.

3.2.2 Synthesis of the peroxoniobate complex Na₂[Nb(O₂)₃(arg)]·2H₂O (NbA). Solid Na₃[Nb(O₂)₄]·13H₂O (0.6550 g, 1.25 mmol) was dissolved in 30% H₂O₂ (4 ml, 25 mmol) in a 250 ml beaker in an ice bath. L-Arginine was added to this solution gradually under constant stirring while maintaining the temperature below 4 °C. At this stage, the pH of the solution was recorded to be ca. 8. Dilute HNO₃ solution (4 M) was added dropwise to the solution under continuous stirring until the pH was lowered to 6. The resulting solution was allowed to stand for 3 h in an ice bath. A white pasty mass separated out on adding pre-cooled acetone to this mixture under vigorous stirring. The supernatant liquid was decanted off and the residue was treated repeatedly with acetone under scratching. The microcrystalline product obtained was separated by centrifugation and dried in vacuo over concentrated sulfuric acid.

3.2.3 Synthesis of the peroxoniobate complex Na₂[Nb(O₂)₃(nic) (H₂O)]·H₂O (NbN). The procedure consisted of gradual addition of nicotinic acid (1.25 mmol) under constant stirring to a solution of Na₃[Nb(O₂)₄]·13H₂O (0.6550 g, 1.25 mmol) in 30% H₂O₂ (4 ml, 25 mmol) in an ice bath. The pH 3 of the reaction solution at this stage was raised to ca. 6 by dropwise addition of NaOH solution (8 M) under stirring. After allowing the solution to stand for 3 h, pre-cooled acetone was added to it under continuous stirring. The white pasty mass obtained from the solution after decanting the supernatant liquid was treated repeatedly with acetone to isolate the white microcrystalline solid product. The product obtained was separated by centrifugation and dried in vacuo over concentrated sulfuric acid.

3.3 Elemental analysis

The Perkin-Elmer 2400 series II elemental analyzer was used for C, H and N elemental analysis of the synthesized compounds. The niobium content was estimated gravimetrically as NbO(C₁₃H₁₀NO₂)₃ (ref. 42) and by atomic absorption spectroscopy (AAS). Niobium, sodium, C and N composition contents were also obtained by EDX analysis. In addition, the sodium content was quantified with an ionometer (ORION VERSASTER). The peroxide content of the compounds was determined by adding a weighed amount of the compound to a cold solution of 1.5% boric acid (w/v) in 0.7 M sulfuric acid (100 mL) and titrating with standard sodium thiosulfate solution.⁴³

3.4 Physical and spectroscopic measurements

The IR spectra were recorded at ambient temperature by making pressed pellets of the compounds with KBr pellets using a Perkin-Elmer spectrum 100 FTIR spectrophotometer. Raman spectra of the compounds were recorded using a Renishaw InVia Raman microscope equipped with an argon ion laser with an excitation wavelength of 514 nm and a laser maximum output power of 20 mW. The measurement parameters were 10 s of exposure time, 1 accumulation, laser power 10% of the output power and 50× objective, and the spectral resolution was set to 0.3 cm⁻¹. Thermogravimetric analysis was performed in a SHIMADZU TGA-50 system at a heating rate of 10 °C min⁻¹ under an atmosphere of nitrogen using an aluminium pan. Energy-dispersive X-ray analysis was done by using a JEOL JSM-6390LV scanning electron micrograph attached to an energy-dispersive X-ray detector. Atomic absorption spectrometry was done using a Thermo iCE 3000 series atomic absorption spectrophotometer model analyst 200. The ¹³C NMR spectra for arginine, NbA, nicotinic acid and NbN were recorded using a JEOL JNM-ECS400 spectrometer with the following parameters: a carbon frequency of 100.5 MHz, 4096 X-resolution points, the number of scans equal to 1000–20 000, 1 s of acquisition time, and 30° pulse length (D₂O as solvent). ¹H NMR study for the synthesised complexes along with free ligands has been carried out with a JEOL JNM-ECS400 spectrometer with 16 scans, 2 s of acquisition time and 45° pulse length (D₂O as solvent). The ¹H and ¹³C NMR spectra of organic sulfoxides and sulfones were recorded using a JEOL JNM-ECS400 spectrometer (CDCl₃ as solvent and TMS as an internal standard). Melting points were determined using open capillary tubes on a Büchi melting point B-540 apparatus and were uncorrected. GC analysis was carried out using a CIC model 2010 gas chromatograph and an SE-52 packed column (length 2 m, 1/8 inch OD) with a flame ionization detector (FID) and nitrogen as the carrier gas (30 mL min⁻¹).

3.5 Catalytic activity

3.5.1 General procedure for oxidation of sulfides to sulfoxides (catalyst: NbA or NbN) at 0 °C. In a representative procedure, an organic substrate (5 mmol) was added to a solution of a catalyst (0.002 mmol) in water (5 mL) and then a 30% H₂O₂ oxidant (1.13 mL, 10 mmol) was added, maintaining a catalyst : substrate molar ratio of 1 : 2500 and a substrate : H₂O₂ ratio of 1 : 2 in a 50 mL round-bottomed flask. The reaction was conducted at 0 °C in an ice bath under continuous stirring. The progress of the reaction was monitored by thin-layer chromatography (TLC). After completion of the reaction, the products were extracted with diethyl ether and dried over anhydrous Na₂SO₄ and distilled under reduced pressure to remove excess diethyl ether. The corresponding sulfoxide obtained was purified by column chromatography on silica gel using ethyl acetate and n-hexane (1 : 9).

The products obtained were characterized by IR, ¹H NMR, and ¹³C NMR spectroscopy, and in the case of solid sulfoxide products, in addition to the above spectral analysis, we have also carried out melting point determination (see the ESI†).

3.5.2 General procedure for oxidation of sulfides to sulfones (catalyst: NbA or NbN) at room temperature. To a stirred solution of the catalyst (0.005 mmol) and 30% H₂O₂ (10 mmol), the organic substrate (5 mmol) was added in water (5 mL) maintaining a catalyst : substrate molar ratio of 1 : 1000 and a substrate : H₂O₂ ratio of 1 : 2 at room temperature (RT) under continuous stirring. The reaction was monitored by thin-layer chromatography (TLC). After completion of the reaction, the products were extracted with diethyl ether and dried over anhydrous Na₂SO₄ and distilled under reduced pressure to remove excess diethyl ether. The corresponding sulfone obtained was purified by column chromatography on silica gel using ethyl acetate and n-hexane (1 : 9).

The IR, ¹H NMR, and ¹³C NMR spectroscopy tools were used to characterize the products. In addition to the above spectral analysis, we have also carried out melting point determination for the products (see the ESI†).

3.6 Regeneration of the catalyst

Any of the following procedures could be adopted to regenerate the catalysts for reuse. The regeneration of the catalyst was carried out for the reaction using methyl phenyl sulfide (MPS). After completion of the oxidation reaction and subsequent extraction of the organic reaction product, the aqueous part of the reaction mixture was transferred to a 250 mL beaker. Keeping the solution in an ice bath, 30% H₂O₂ was added to it maintaining a Nb : peroxide ratio of 1 : 2, followed by addition of pre-cooled acetone under constant stirring until a white pasty mass separated out. From this precipitate, the catalyst was finally obtained as a microcrystalline solid by following the work-up procedure mentioned under Section 3.2.2. The regenerated pNb catalyst was then placed into a fresh reaction mixture consisting of MPS and hydrogen peroxide in water, and the reaction was allowed to proceed under optimised conditions as mentioned under Section 3.5.1 (for sulfoxide) or Section 3.5.2 (for sulfone). The progress of the reaction was monitored by thin-layer chromatography (TLC). After completion of the reaction, the process was repeated for a total of six reaction cycles.

In an alternative procedure, recycling of the catalyst could be performed in situ after completion of the reaction cycle and extraction of the organic reaction product. Regeneration of the used reagent could be achieved by adding 30% H₂O₂ and a fresh batch of substrates to the aqueous portion of the spent reaction mixture, maintaining the same procedure as mentioned under Section 3.5.1 (for sulfoxide) or Section 3.5.2 (for sulfone), and conducting the reaction under optimised conditions. Each of the procedures was repeated for six reaction cycles.

3.7 Computational details

The density functional theory (DFT)³⁶ calculations were performed using the Gaussian09 programme⁴⁴ at the B3LYP/LANL2DZ level of theory. The ground-state geometry of the two niobium complexes were obtained in the gas phase, and the minima of the optimized structures were verified by the absence of imaginary frequencies.

Conclusions

In summary, a pair of new heteroligand triperoxoniobium(V) complexes has been synthesized, characterized and successfully applied in the selective oxidation of variously substituted sulfides to selectively obtain sulfoxide or sulfone with 30% H₂O₂ in neat water. The reactions proceed under mild conditions to afford the resulting products with impressive yield and TON or TOF. The catalysts display excellent chemoselectivity toward the sulfur group of substrates with other oxidizable functional groups, including the hydroxyl group and C=C bonds. The adherence of the developed protocol to the principles of green chemistry is ensured by the fact that the reactions, apart from using neat water as a standard green solvent and aqueous 30% H₂O₂ as an oxidant, employ a non-toxic pNb catalyst which can be reused in subsequent cycles without losing its activity. In addition, the oxidation is absolutely free from organic co-solvent, co-catalyst or any other auxiliaries, involves easy work-up procedure and is amenable for ready scalability. All of these features make the presented catalytic strategies attractive and interesting from both economic and environmental perspectives.

Acknowledgements

The authors gratefully acknowledge the Department of Science and Technology, New Delhi, India, for providing financial support. We are also grateful to the University Grants Commission, Basic Science Research Fellowship, New Delhi, India, for providing a junior research fellowship to S.R.G. We thank Dr. B. Choudhury, Department of Physics, Tezpur University, Tezpur, Assam, India, for the Raman spectra.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: FTIR, ¹³C NMR and ¹H NMR spectra of complexes, hydrogen peroxide efficiency, characterization of sulfoxides and sulfones, and bar diagram for recyclability of the catalyst. See DOI: 10.1039/c4cy00864b

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