Reductive functionalization of CO₂ under mild reaction conditions for the catalytic synthesis of N-formamides by Mannich base based Cobalt (III) complex

Highlights

• Cobalt (III) Mannich base complex.
• X-ray crystal analysis.
• UV–Vis spectra.
• N-formamides products from aromatic/aliphatic amine.
• Reductive functionalization of carbon dioxide.

Abstract

Reductive functionalization of carbon dioxide is an important approach which not only reduces CO₂ concentration but also functionalized it for the formation of value added chemicals. In this perspective, a hexadentate mononuclear cobalt (III) complex [Co^IIIL] of Mannich base ligand (H₄L) was synthesized and structurally characterized by single crystal X-ray diffraction. The electronic spectra of the complex showed two bands at 448 and 561 nm corresponds to the LMCT and d-d transitions respectively. The derived inexpensive [Co^IIIL] complex was very much effective as catalyst in carbon dioxide functionalization reaction. The catalytic synthesis of mono selective N-formamide product from amines was produced through diagonal transformation of carbon dioxide in presence of polymethylhydrosiloxane (PMHS) as reducing agent and [Co^IIIL] complex as catalyst. The effects of various reaction parameters were examined. Broad range of substrates (aromatic as well as aliphatic amines) smoothly produced good percentage of respective N-formamides product (72–94%) under mild reaction condition utilizing 1 atmospheric CO₂. The high TON (1540–2000) and TOF value (192.5–400.0 h⁻¹) for the catalytic reactions strongly depicted the productive nature of the [Co^III(L)] catalyst. The probable mechanestics studies clearly reveals that the [Co^IIIL] complex activated the Si−H bond of PMHS to react with carbon dioxide in order to form the silyl formates intermediate for activation of N–H bonds in amines, thus leading to the excellent performance of the catalytic reaction.

Introduction

The concentration of long-lived green house gas carbon dioxide is increasing day by day due to industrial emission, urbanization and human activities. As a result unpredictable climates changes occurs on the earth surface [1]. For this purpose control of CO₂ concentration is extremely desirable in the current scenario. The reduction and utilization of CO₂ in chemical fixation reactions not only serves as fruitful way of elimination of CO₂ concentration but also provides us the valuable fine chemicals. Although industrial approach is highly required for this aspects. Numerous methodologies have been already established to overcome the problems, among them catalytic protocol is most encouraging. Through catalytic protocol important compounds can be produced by the activation of chemically and thermodynamically stable CO₂ molecule [2], [3], [4], [5]. Utilization of cheap, renewable, C1-feedstock carbon dioxide is generally carried out in two ways: one is reduction of CO₂ into formic acid, methanol, methane, formaldehyde etc.; another is functionalization of CO₂ into value added products (such as cyclic carbonates, polycarbonates, urea, etc.). But neither the reduction nor the functionalization process of CO₂ can't alone sufficient for the synthsis of fine petrochemical. For this intention simultaneous reduction and functionalization are essential. The reductive functionalization of CO₂ is now nominated as “diagonal transformation” (Fig. 1) [6]. Through “diagonal transformation” variuos versatile chemicals as well as energy-storage materials like formamides, aminals, and methylamines can be synthysized smoothly [7,8]. Again different bio-active compounds such a paracetamol, procainamide, alatamide etc. can be synthesized through this method (Fig. 1). Paracetamol is very commonly usable medicine to treat pain and high fever. Procainamide is 1A antiarrhythmic agent used to treat atrial flutter, supraventricular arrhythmias, ventricular arrhythmias, AV nodal re-entrant tachycardia, Wolf-Parkinson-White syndrome and atrial fibrillation [9]. Alatamide is a natural product obtained from the aerial parts of the plant Piper guayranum [10], [11], [12].

Hydrosilylation of carbon dioxide is one of the most important approach of “diagonal transformation”. The basic importance of hydrsylation of CO₂ are (i) Si−H addition to CO₂ is more energetically favorable than that of H−H bond to CO₂ and (ii) the reaction propagates via step-wise fashion which generates different compounds of carbon having variety of oxidation state such as silyl acetals, silyl formates, methane, and methoxysilanes [13,14]. The hydrosilylation of CO₂ in amines which leads to the formation of N-formamides generally suffers from the use of costly and rare metal containing catalysts at high temperature [15,16]. N-formamides are important class of organic compounds usually exists in natural products, medicines, agrochemicls, dyes, fragrances, and gasoline additives [17,18]. The compoumds are employed as versatile intermediates for Leuckart and Vilsmeier–Haack reactions [19,20]. Formamides are also extensively utilized as solvents such as N-methylformamide, N,N-dimethylformamide (DMF) are common polar solvent in organic synthesis. Besides formamides are very much connected with nucleic acids, biochemistry, molecular biology and other areas of prebiotic chemistry [19]. As a outcome of their versatile applications, the direct N-formylation of amines for the production of N-formamides through hydrosylation of carbon dioxide is a promising and demanding approach in modern chemistry.

In this perspective, lots of homogeneous as well heterogeneous catalysts, e.g. alkali salts, organic bases, organometallic complexes, ionic liquids, metal oxides, metal–organic frameworks, supported metal complexes, metal–salen complexes, porous organic polymers, graphene oxide (GO) based materials etc., have been utilized for the synthesis of N-formamides [21], [22], [23], [24], [25]. Again zinc (Zn), platinum (Pt), copper (Cu), iron (Fe), palladium (Pd), iridium (Ir), ruthenium (Ru), and rhodium (Rh) metal containing catalysts are also used for this purpose [26], [27], [28], [29], [30], [31], [32], [33], [34]. D. Milstein et al. published N‑formylation of amines usinng high pressure H₂ and CO₂ in presence of cobalt Pincer complexe catalyt [35]. Similarly utilizing high H₂/CO₂ pressure B. Han and his group reported N-formamides synthesis in presence Co(0) catalyst [36]. Recently porous MOF (metal–organic framework)-supported cobalt catalyst (pyrim-UiOCo) is used for the synthesis of mono-N-formylation of amines through reductive functionalization of CO₂ by K. Manna research group [37].

Herein, this article is going to disclose the synthesis of poly‑hydroxy based hexadentate ligand H₄L and its Co(III) complex [Co^III(L)] (Scheme 1). The single X–ray crystallographic evidences of the co-ordination compound of non-Schiff base hexadentate ligand is described precisely. The catalytic activity of the synthesized [Co^III(L)] (1) complex was checked elaborately in the synthesis of N-formamides through hydrosilylation of carbon dioxide under mild reaction conditions. Aromatic as well as aliphatic amines produced high percentage (72–94%) of respective N-formamides product under 1 bar carbon dioxide pressure within 8 h of reaction duration. PMHS is used as reducing agent for this hydrosilylation reaction. The synthesized cheap cobalt metal catalyst activates the Si-H bond of PMHS to produce silyl formates intermediate which switch on the NH bond of amines leading to the formation of N-formamides. Turn over number (TON) and turn over frequency (TOF) are depicted here to clarify the productive power of the catalyst.

The ligand (H₄L) was prepared by using simple Mannich condensation following the preocedure of our previous report [38,39].

CoCl₂ (0.0474 g, 0.2 mmol.) was dissolved into 20 mL MeOH and then hexa-dentate ligand H₄L (0.1168 g, 0.2 mmol) deprotonated separately by treating with TEA (0.081 g, 0.8 mmol) in methanol was added to this solution with stirring. The dark brown color of the solution appeared almost immediately. The stirring was continued for 1 hour and then filtered off. The filtrate was kept aside undisturbed for slow evaporation. After standing for two days, dark brown rod shape crystals were obtained, harvested from the mother liquor (Scheme 2). The isolated suitable single crystals were subjected to X-ray studies. Elemental analysis: Calc. C 67.58%, H 8.82%, N 4.38% Found C 67.92%,H 8.80%, N 4.45%.