Bicyclic (alkyl)(amino)carbene (BICAAC) in a dual role: activation of primary amides and CO₂ towards catalytic N-methylation

Abstract

Herein, we report the first catalytic methylation of primary amides using CO₂ as a C1 source. A bicyclic (alkyl)(amino)carbene (BICAAC) exhibits dual role by activating both primary amide and CO₂ to carry out this catalytic transformation which enables the formation of a new C–N bond in the presence of pinacolborane. This protocol was applicable to a wide range of substrate scopes, including aromatic, heteroaromatic, and aliphatic amides. We successfully used this procedure in the diversification of drug and bioactive molecules. Moreover, this method was explored for isotope labelling using ¹³CO₂ for a few biologically important molecules. A detailed study of the mechanism was carried out with the help of spectroscopic studies and DFT calculations.

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Introduction

The global average CO₂ emissions have climbed significantly over the last two decades, despite the several restrictions on human activities imposed by the COVID-19 pandemic.¹ To address this issue, a novel concept known as carbon capture and utilisation (CCU) is gaining increasing attention. CCU involves the conversion of CO₂ into fuels and chemicals while adding value, which can offset the cost of CO₂ capture.² CO₂ is regarded as an appealing carbon feedstock in chemical synthesis since it is non-toxic and it is becoming gradually plentiful.³ However, the utilisation of CO₂ as a C1 source is rather limited because of its high thermodynamic and kinetic stability,⁴ and in recent years, the use of metal-free catalysis is gaining popularity for CO₂ activation.⁵ For example, the concept of CO₂ activation and its catalytic reduction of CO₂ to methanol employing a metal-free N-heterocyclic carbene (NHC) as a catalyst was introduced by Ying and co-workers.⁶ The other approach of hydroborative reduction of CO₂ to methanol with aNHC⁷ (abnormal N-heterocyclic carbene) was reported by our group in 2016.⁸ However, the raw materials for fine chemical synthesis cannot be satisfied simply by reducing or functionalising CO₂. In this aspect, Cantat coined a fascinating term, “diagonal transformation”, which means simultaneous reduction and functionalisation.⁹ This innovative concept of combining the reduction and functionalisation of CO₂ enables the formation of new C–C, C–N, and C–O bonds, and it can broaden the range of chemical synthesis using CO₂.⁹ This concept has successfully functionalised amines producing formamides, aminals and methylamines, which are the 2e, 4e, and 6e reduced products of CO₂, respectively, along with the formation of a new C–N bond.⁵ However, such diagonal transformation was mostly limited to amines till 2018, when the first catalytic formylation of primary amides was reported by our group.¹⁰ Compared to amines, the functionalisation of primary amides with CO₂ is profoundly challenging as amides are extremely weak bases making them chemically inert. As a result of the electronic conjugation of nitrogen lone pair of electrons with the C=O functionality of the amide, they are less reactive to nucleophilic attack, which makes them difficult to functionalise.¹¹ Consequently, the aminals were prepared from amine by CO₂ under reductive conditions using metal-free catalysts,^12–14 but corresponding methylenated products of amides could not be prepared using CO₂ until recently when we successfully designed mesoionic N-heterocyclic imines (mNHIs) and utilised them for catalytic amide methylenation using CO₂.¹⁵ Despite such development, methylation of primary amides using CO₂ as a reagent has not been reported, although corresponding N-methylation of amines is abundant in the current literature.^16–20 We thus became interested in exploring the N-methylation of primary amides using CO₂ as a C1 source as a part of our ongoing research in developing a metal-free catalyst for CCU.^{5,10,15,20,21} In this regard, we resorted to an ambiphilic singlet carbene named bicyclic (alkyl)(amino)carbene (BICAAC), which was introduced by Bertrand and co-workers in 2017.²² It is a storable carbene with high σ-donating and π-accepting properties.²² Since its discovery, the use of BICAAC as a ligand has received gradually increasing attention in the field of organometallic catalysis.^23–26 Recently, we reported BICAAC as a catalyst for the reduction of nitriles in the presence of a borane.²⁷ As BICAAC successfully reduced the polar C≡N bond of nitriles having a high bond dissociation energy of 179.3 kcal mol⁻¹, we were thus curious if it could activate the C=O bond of CO₂.²⁸ Moreover, there are reports of activation of CO₂ and B–H bond of boranes by similar monocyclic (alkyl)(amino)carbenes (CAACs).^29–31

In particular, our primary focus has been the reductive functionalisation of CO₂ for the methylation of amides. N-Methyl amides are the core moiety in various marketed drugs (Scheme 1a). Traditionally, N-methyl amide compounds are synthesised by coupling carboxylic acids with amines in the presence of stoichiometric amounts of toxic reagents such as SOCl₂, N,N′-diisopropylcarbodiimide (DIC), N,N′-dicyclohexane carbodiimide (DCC) and acid chloride, etc. (Scheme 1b).^32,33 However, such processes are non-catalytic. The other less popular approach to the N-methylation of amides is using methanol as a methylating agent in the presence of a transition metal-based catalyst (Scheme 1b).^34,35 At present, N-methylation of amides using CO₂ is not known. Therefore, developing a protocol for N-methylation of amides using CO₂ as a C1 source and avoiding metal-based catalysts or toxic methylating reagents is highly desirable. With this vision, in this work, we report catalytic N-methylation of a diverse range of primary amides using CO₂ under metal-free catalytic conditions (Scheme 1c). A detailed mechanistic investigation using control experiments and DFT calculations unravelled that the BICAAC plays a dual role in integrating primary amide activation by N-borylation and CO₂ activation by forming boron formate, which then recombines in delivering the N-methylated amides.

Scheme 1 (a) Examples of N-methyl amides in pharmaceuticals. (b) Previous strategy for N-methyl amide synthesis. (c) This work represents the first catalytic methylation of amides using CO₂.

Results and discussion

The current investigation was initiated by optimising the methylation of a primary amide, 4-methyl benzamide (2a), as a model substrate using a catalytic amount of BICAAC (1), and borane as a reducing agent in the presence of 1 atm of CO₂. BICAAC (1) was prepared according to the method reported in the literature.²² In the absence of the catalyst, the product was observed in trace amounts (entry 1, Table 1). After several trials, it was found that BICAAC functions as an efficient catalyst for the N-methylation of primary amides. Initially, we were delighted to observe that BICAAC in the presence of a borane (HBpin) in dioxane can afford N-methylated product with NMR conversion of 15% at 50 °C within 24 h (entry 3, Table 1), which was increased to 70% by raising the reaction temperature to 100 °C within 12 h (entry 4, Table 1). Under such conditions, other solvents, such as tetrahydrofuran (THF) and acetonitrile (ACN), were screened (entries 9 and 10, Table 1). This study establishes that dioxane is the best solvent for this transformation. Next, we screened several other boranes, such as borane dimethyl sulfide complex (BH₃·SMe₂) and catechol borane (HBcat), however, the reaction did not give the desired product (entries 11 and 12, Table 1). Moreover, we screened the reaction with a commercially available base, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), simple carbenes such as 1,3-bis(2,6-diisopropyl)phenyl-4,5-dihydroimidazol-2-ylidene (SIPr), and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), (entries 13–15, Table 1). Also, we tested a mesoionic carbene, aNHC (entry 16, Table 1). Treatment of catalytic amount of BICAAC salt (10 mol%) with equivalent amount of base can result in situ generated free BICAAC which results in the 72% conversion to the desired N-methylated amide (entry 17, Table 1). This result indicates that air stable BICAAC salt can be used for such catalytic conversion.

Table 1 Optimisation of reaction conditions for methylation of primary amides using CO₂^a

Entry	Borane	Catalyst (mol%)	Solvent	T (°C)	Conversion (%)
a All reactions were carried out with amide (0.2 mmol), borane (0.8 mmol), dry solvent (1.0 mL) and 1 atm CO2. b 12 h. c 36 h. d Isolated yield. e In the absence of 1 atm of CO2.
1	HBpin	—	Dioxane	120	Trace
2	—	1 (10)	Dioxane	120	—
3	HBpin	1 (10)	Dioxane	50	15
4	HBpin	1 (10)	Dioxane	100	70^b
5	HBpin	1 (10)	Dioxane	100	77^c
6	HBpin	1 (10)	Dioxane	120	87
7	HBpin	1 (5)	Dioxane	100	60
8	HBpin	1 (10)	Dioxane	100	69^d
9	HBpin	1 (10)	THF	100	54^d
10	HBpin	1 (10)	ACN	100	52^d
11	BH3·SMe2	1 (10)	Dioxane	100	—
12	HBcat	1 (10)	Dioxane	100	—
13	HBpin	DBU (10)	Dioxane	120	21^d
14	HBpin	SIPr (10)	Dioxane	100	—
15	HBpin	IPr (10)	Dioxane	100	Trace
16	HBpin	aNHC (10)	Dioxane	40	16
17	HBpin	1 salt + KHMDS (10)	Dioxane	120	72
18	HBpin	1 (10)	Dioxane	120	—^e

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From this optimisation study, it can be concluded that 10 mol% of BICAAC with HBpin as a hydride source at 120 °C under 1 atm CO₂ gave the best result improving the NMR yield up to 87% (entry 6, Table 1).

After having the optimal catalytic conditions in hand, we next explored the scope of the reaction for the N-methylation of various primary amides under an atmospheric pressure of CO₂ (Table 2). Various commercially accessible amides comprising aromatic, aliphatic and also heterocycles were attempted. The protocol offered a good to a very good isolated yield of the N-methylated amides.

Table 2 BICAAC catalysed methylation of primary amides using CO₂^a

a All reactions were carried out with amide (0.2 mmol), borane (0.8 mmol), dry solvent (1.0 mL) and 1 atm CO₂. Isolated yields (average yield calculated from the yields obtained from two independent catalytic runs, shown in the parenthesis). b NMR conversion based on the unreacted substrate. c NMR conversion using hexamethylbenzene as an internal standard.

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Amides containing electron-donating groups such as methyl and tert-butyl at the benzene ring gave the N-methylated product 3a (77%), 3d (69%), and 3m (69%) in very good isolated yield (Table 2). Aromatic amides bearing highly electron-rich groups such as –SMe and –NMe₂ also delivered the N-methyl derivatives in good yield, 3f (57%) and 3g (62%). Substrates with electron-withdrawing groups such as trifluoromethyl, fluoro, and phenyl exhibited low to very good reactivity (3h–i, 3l, 3o, 3r and 3w, Table 2). Under the optimised reaction conditions, methylation of the N–H bond of several amides bearing electron-withdrawing groups was performed, delivering isolated yields in the range of 37–75% (3h–3l, 3o–3s, Table 2). The reactivity difference between electron-deficient and electron-rich amides might have originated from the availability of electron density on the nitrogen center of amide as supported by our NBO calculations (see the ESI, Fig. S129†). The meta-substituted primary amides appeared to be good candidates affording the desired product in 46% to 69% isolated yields (3m–3q). However, the ortho-substituted amide produced a low yield of 2-fluoro-N-methyl benzamide, 3l (37%). 2-Naphthamide displayed good reactivity resulting in a 58% yield (3u). In the presence of macrocyclic substitution (3x) and a cyclic ether, i.e. tetrahydropyran (3y) at the para position, the reaction proceeded smoothly with yields of 70% and 72%, respectively. Next, we explored its reactivity towards heteroaryl amides, which afforded N-methylated carboxamides in good yields (3aa–3ad, 45–67%). Heteroaryl motifs are crucial for the manufacture of pharmaceutical and agricultural products.³⁶ Also, we were pleased to observe that aliphatic amides underwent N-methylation resulting in 3ae (79%) and 3af (51%). We next attempted this protocol with a metallocene containing primary amide, ferrocenecarboxamide, which yielded the desired product (3ag) in 57% yield.

Next, we checked the applicability of our protocol towards diversification of bioactive molecules. Three bioactive N-methyl amides were synthesised from (−)-borneol, (1R,2S,5R)-(−) menthol and estrone furnishing isolated yields of 71%, 67% and 59%, respectively (3ah–3aj, Scheme 2). Moreover, we were also successful in functionalising derivatives of three drug molecules, probenecid (treats gout), adapalene (a third-generation topical retinoid) and (±)-α-tocopherol, i.e., vitamin E (3ak–3am, Scheme 2). Furthermore, we perceive that our protocol might act as an efficient method for isotopic labelling using ¹³CO₂. Such isotopic labelling could pave the way to radioactive labelling using ¹⁴CO₂, which has immense potential in the metabolic tracking of drug candidates required for regulatory approval.³⁷ These studies thus offer the importance of isotope labelling in pharmacokinetic investigations. Therefore, we next proceeded to perform our catalytic protocol using ¹³CO₂ for a few biologically and medicinally relevant substrates. To our delight, very good isolated yields of ¹³C labelled N-methylated bioactive molecules were obtained (3ah′–3al′, Scheme 3) when we used ¹³CO₂.

Scheme 2 Applications of BICAAC catalysed N-methylation for the diversification of compounds with biological importance. Isolated yields (average yield calculated from the yields obtained from two independent catalytic runs, shown in the parenthesis).

Scheme 3 ¹³C isotope labelling of biologically and medicinally important molecules using ¹³CO₂^a. ^aAll reactions were carried out with amide (0.2 mmol), borane (0.8 mmol), dry solvent (1.0 mL) and 1 atm ¹³CO₂. Isolated yields are given in parentheses. ^bPerformed with a 0.1 mmol scale.

Next, a series of control experiments were performed to gain insight into the mechanism of this transformation. From our previous study, we are aware that a stoichiometric treatment of BICAAC with HBPin results in the BICAAC B–H insertion product (1A) with 1 : 1 diastereomeric mixture with respect to the bridgehead of BICAAC.²⁷ As in this study, CO₂ is another reagent, so to unveil the reactivity of BICAAC (1) towards these reagents, the reaction between BICAAC and CO₂ was performed in ether separately which afforded a white solid, BICAAC–CO₂ adduct (1B) after removal of solvent. The ¹³C NMR spectrum of BICAAC–CO₂ adduct in CDCl₃ displayed a peak at δ 193.1 ppm, which was assigned to C=O of the BICAAC–CO₂ adduct.²⁶ Next, 1A or 1B was separately used as a catalyst in the methylation of 4-methyl benzamide (2a) under a CO₂ atmosphere. To our surprise, both 1A and 1B furnished the desired N-methylated amide (Fig. 1a). This result indicated that 1A and 1B under a CO₂ atmosphere could lead to the catalytic N-methylation, indicating they might interconvert in the presence of CO₂ and borane. This made us curious to understand the reaction further, and towards this direction, frontier molecular orbital analysis was carried out with the help of density functional theory (DFT) calculations. The DFT calculations were performed employing M06-2X/6-311++G**//M06-2X/6-31G* methodology with CPCM solvent model (dioxane as solvent). A closer look at the HOMO of 1 shows that it contains a σ orbital at the carbene carbon centre, and LUMO contains a delocalised π orbital at carbene carbon (see the ESI, Fig. S125†). Then, the HOMO–LUMO of BICAAC–HBpin (1A), CO₂, HBpin and BICAAC–CO₂ adduct (1B) were computed. It is clear from the diagram (Fig. 1b) that the HOMO of CO₂ is centred on the p orbital of oxygen, which can interact with the LUMO of 1A (13.1 eV). In the case of 1B, the HOMO is raised by 4.4 eV compared to free CO₂; this can interact with the LUMO of HBpin (10.0 eV). This analysis indicates that the catalytic reaction is feasible for 1A and 1B. Gibbs's free energy calculations were conducted for both approaches to comprehend further how these species are capable of catalysing this process. For the pathway involving 1A, the route was found to be energetically demanding with ΔG = 67.7 kcal mol⁻¹ (see the ESI, Fig. S126†), and next, we proceeded to the pathway involving 1B (Fig. 1c). In this pathway (involving 1B as a catalyst), at first, catalyst 1 interacts with CO₂, where the HOMO of carbene interacts with the carbon centre of CO₂, which is its LUMO forming 1BviaTS3 with an activation energy barrier of 1.3 kcal mol⁻¹. In 1B, the electron density is located on the oxygen centre, which then reacts with the electron-deficient boron centre of pinacolborane to give VIviaTS4 having a 6.5 kcal mol⁻¹ activation energy barrier. Here, a hydride shift takes place to produce 4. This step requires a very small barrier (1.4 kcal mol⁻¹); hence, from this computed energy profile diagram, we can conclude that the transformation of 1B to 4 is highly feasible.

Fig. 1 (a) Testing 1A and 1B as catalysts. (b) Frontier molecular orbital analysis disclosing the possibility of interaction of 1A with CO₂ or 1B with HBpin in contrast to HBpin alone. (c) Computed Gibbs free energy profile for pathway involving 1B enabled by the interaction of 1 with CO₂ leading to compound 4.

To check if 4 can be traced experimentally, a stoichiometric reaction of BICAAC–CO₂ adduct with HBpin in the presence of CO₂ in CD₃CN in a J. Young valve NMR tube was performed (Scheme 4a), which rendered two singlets at δ 4.04 and 4.30 ppm indicating the formation of a pair of 1 : 1 diastereomeric mixture of 4, as characterised by ¹H NMR spectroscopy. The ¹¹B{¹H} NMR spectrum showed two peaks nearly merged at δ 21.2 and 20.6 ppm in the reaction mixture.

Scheme 4 Control experiments: (a) CO₂ activation. (b) Amide activation. (c) Formyl transfer reaction. (d) Reduction of N-formyl amide to N-methyl amide.

In addition to 4, we have detected the formation of another compound displaying a peak at δ 8.19 ppm in the ¹H NMR spectrum, indicating the formation of boron formate 5.³⁸ Further, to confirm the formation of 4 and 5, the reaction was performed in the presence of ¹³C-labelled CO₂, and the ¹³C{¹H} NMR spectrum of the reaction mixture in CD₃CN displayed two peaks at δ 172.2 and 171.3 ppm, which were assigned to the two diastereomers of 4′ (see the ESI, Fig. S7†) while a peak at δ (¹³C NMR) = 164.7 ppm was assigned to ¹³C labelled boron formate, 5′. Moreover, in the ¹H NMR spectrum of ¹³C-labelled 4 (4′), two singlets at δ 4.04 and 4.30 ppm appeared as two doublets with ²J_C–H = 3.5 and 1.5 Hz.^39,40 Compound 4′ was also detected by mass spectrometry, m/z calculated for ¹³CC₂₈H₄₇BNO₄⁺ [M + H]⁺: 485.3671, found 485.3638.

Also, in the ¹H NMR spectrum, the singlet at δ 8.19 ppm corresponding to boron formate (5) was transformed into a doublet with ¹J_C–H = 206.5 Hz ¹⁵ due to very strong one bond coupling with the ¹³C nucleus. The formation of 5 was further confirmed by executing catalytic reduction of CO₂ with HBPin using 10 mol% of BICAAC in dioxane at 120 °C leading to the formation of formic acid (δ 8.31 ppm in ¹H NMR spectrum in CDCl₃) by treatment with aqueous HCl at the end of the reaction (Scheme 4a).^41,42 These experiments and DFT calculations established the CO₂ activation pathway by BICAAC.

Next, for the amide activation, we monitored the reaction of 4-methyl benzamide (2a) and HBpin in a 1 : 1 ratio in the absence of 1; the reactants stayed unreacted after performing the reaction at 25 °C for 12 h. However, in the presence of 10 mol% 1, when methyl benzamide (2a) and HBpin were mixed in a 1 : 1 ratio, gas evolution was noticed at 25 °C and was monitored by ¹H NMR spectroscopy in a screw cap NMR tube in C₆D₆ (Scheme 4b). The gas was detected as dihydrogen by the ¹H NMR spectroscopic peak at δ 4.47 ppm and was further verified by GC-MS analysis.²⁰ Subsequently, we performed a reaction of 4-methyl benzamide (2a) and HBpin in a 1 : 1 ratio in the presence of 10 mol% 1 in CD₃CN at room temperature for 12 h. ¹H and ¹¹B NMR spectroscopy of the reaction mixture confirmed the formation of N-borylated amide (6) with δ (¹¹B NMR) = 24.2 ppm, which is in accordance with the reported literature.⁴³ We also performed this N-borylation reaction with 1A as a catalyst, which rendered a similar result, indicating 1A to be the active species for the activation of primary amide (Scheme 4b). The transformation of amide to N-borylated amide catalysed by 1A may proceed through a concerted pathway (see the ESI, Fig. S127†).¹⁰

The above experiments established that BICAAC plays a dual role in activating CO₂ to boron formate 5, and it can catalytically activate primary amide to N-borylated amide, such as 6a. Following this, we performed another control experiment between boron formate 5 and N-borylated amide 6a (Scheme 4c). A doublet at δ 9.37 ppm was observed in the ¹H NMR spectrum of the reaction mixture recorded in CDCl₃, indicating the formation of an N-formylated product 7a.¹⁰ This observation indicates that the N-formylated product is the intermediate for this N-methylation reaction. To further establish that N-methylation proceeds via N-formylation of amide, we also performed the reduction of N-formyl-4-methylbenzamide (7a) with only 2.3 equiv. of HBpin (instead of 4 equiv. HBpin) in the presence of a catalytic amount of 1 (Scheme 4d). The formation of N-methylated product in 67% isolated yield confirms that the given reaction proceeds through N-formylation. Moreover, a white solid was produced as a by-product of the catalytic reaction under the optimised condition that was identified as BpinOBpin by ¹H and ¹¹B NMR spectroscopy (¹¹B in C₆D₆, δ 21.8 ppm).⁴⁴

On the basis of these control reactions and theoretical studies, the following plausible mechanistic cycle is proposed (Scheme 5). In the first step, BICAAC activates the B–H bond of pinacolborane to yield the BICAAC B–H insertion product (1A), which has been previously reported.²⁷ This activates the amide by borylation of one of its N–H bonds producing 6 along with the liberation of dihydrogen and regenerating 1. Amide activation can occur at 25 °C, as noticed from the H₂ evolution on mixing the reagents, as also established from the characterisation of H₂ gas by ¹H NMR and GC-MS studies. The BICAAC (1) also can react with CO₂ to produce BICAAC–CO₂ zwitterionic adduct (1B).²⁶1B then reacts with the pinacolborane, followed by a hydride transfer to the carbene centre viaTS5, generating 4, which can produce boron formate 5 in the presence of CO₂, regenerating the catalyst 1. The formation of 4 and 5 was established from our control experiments described above. The N-borylated amide (6) reacts with boron formate (5) when the formyl transfer reaction takes place, leading to N-formylated product (7) and (Bpin)₂O (8) as a byproduct. The control experiment in Scheme 4c supports such a proposition. 7 then undergoes successive hydroboration of the carbonyl centre,²⁰ leading to N-methylated amide, along with the formation of (Bpin)₂O (8) as a byproduct.²⁰ Thus, in total 4 equiv. of reducing agent is required. Amide and CO₂ activation require two equiv. of HBpin, while the rest 2 equiv. is needed for reducing N–CHO to N–CH₃.

Scheme 5 Proposed mechanistic cycle for N-methylation of amide catalysed by BICAAC.

Conclusions

Although amine methylation was known using CO₂, the current literature lacked a similar process with chemically inert amides. The present study reports the first N-methylation of primary amides utilising a greenhouse gas such as CO₂ as a C1 source under metal-free conditions. This mechanistic investigation discloses that BICAAC plays a dual role in activating CO₂ as well as amide to accomplish the methylation of various aromatic, aliphatic and heteroaromatic primary amides with HBpin as the hydride source. Furthermore, the present protocol was successful towards the diversification of drug and bioactive molecules. A few biologically important molecules were isotope-labelled using ¹³CO₂. Considering our earlier efforts, with this study, we have been able to complete the 2e (formylation),¹⁰4e (methylenation)¹⁵ and 6e reductive functionalisation (present study) of amides under metal-free conditions using CO₂ as a reagent.

Data availability

All experimental procedures, characterization details, copies of NMR spectra for all compounds, and computational data related to this article have been uploaded as part of the ESI.†

Author contributions

SKM and NG conceived the idea of this work. SKM and NG planned the experiments. NG carried out the catalytic reactions and control experiments. RL conducted all the theoretical calculations and AKT analyzed the data. SP contributed to the mechanistic study. AR carried out the starting material synthesis. SKM supervised the work. SKM and NG analyzed the data and prepared the manuscript with the help of AKT and RL.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

SKM and AKT thank the STARS Program (Grant no. MoE-STARS/STARS-1/473) of India for financial support. N. G., R. L., S. P. thank IISER-Kolkata, CSIR and DST-Inspire Program, respectively, for their research fellowships.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc01216f

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