Preparation Of Ni-NHC Complexes And Salts And Their Applications In Mechanochemical C-S Coupling

Metal organic compounds have diverse and novel structures,and can be used in fields such as catalysis,molecular switching,and electrochemistry.This article uses onium salt ligands containing carbene carbon functionalized with benzimidazolium pyridine to react with metal salts to generate two types of substances.One is a metal organic salt,which is an organic-inorganic hybrid compound formed by the combination of organic ligand cations and halogenated metal anions through electrostatic attraction.The other is a complex formed by the coordination of metal with carbene carbon in benzimidazolium salt,which is a nitrogen heterocyclic carbene complex.Sulfide compounds are common intermediates of drug or food molecules,and the C-S bond is a key step in constructing sulfide compounds.The reported C-S coupling products generally require the presence of palladium,ruthenium and other noble metal catalysts,and the reflux of organic reagents such as dichloroethane at high temperatures to obtain the products.The reaction time is long,which violates the concept of sustainable development and the principle of green chemistry.Compared to solution synthesis,mechanochemistry utilizes mechanical forces to induce and promote rapid and efficient reactions,making it a green and efficient method.In this paper,pyridine functionalized benzimidazolium salts were used as ligands to synthesize a series of N-heterocyclic carbene complexes and their organometallic salts by solution method and mechanochemical techniques,and were applied to C-S cross coupling reaction.Firstly,benzimidazole chloride salt [H₂L]Cl₂（[H₂L] = 3,3’-（1,2-bismethylenephenyl）bis-1-（pyridin-2-methyl）-1H-benzo[d]imidazole）was synthesized at a high temperature of 110℃ under a pressure tube,and then mechanically ground with MX₄（M = Cu,Co,Cd,X = Cl,Br）to obtain four metalorganic salts [H₂L]2+[Cu Cl4]2-（1）,[H₂L]₂+[CoCl₄]₂-（2）,[H₂L]²⁺[CdBr₄]2-（3）,[H₄L]⁴⁺[CoCl₄]²-2Cl^-·CH₃CN（4）.The structure was characterized by single crystal diffraction,powder diffraction,etc.,and the similarity of the structure of metal organic salt 1-4 was systematically analyzed using calculation software such as Crystal Explorer and XPac.[H₂L]Cl₂ and NH₄PF₆ were further mechanically ground to obtain [H₂L]（PF₆）₂.Subsequently,the complex [Ni L]（PF₆）₂·CH3CN（5）was obtained by reacting with Ni（OAc）₂·4H₂O in a solution at 70 ℃ for 2 h.The complex 5 was reacted with Na I in the solution for 3 h,successfully replacing anions to obtain the complexes[Ni L]I2·CH3CN（6）and [Ni L]I2·CH3CN（6’）.The reaction of [H₂L]Cl₂ with Na I can generate [H₂L]Cl₂,and the complex [NiL]I₂·CH₃CN（6）is obtained by reacting with Ni（OAc）₂·4H₂O in a solution at 70 ℃ for 2 h.The above compounds were characterized by X-ray single crystal diffraction,PXRD powder diffraction,infrared spectroscopy,and nuclear magnetic resonance spectroscopy.The above compounds were selected as catalysts for C-S coupling reaction.Phenylacetyl bromide and benzothiazole mercaptan were used as substrates for the CS reaction.The results showed that the catalytic effect of complex 5 was the best.By optimizing factors such as catalyst dosage,liquid assisted grinding type,and grinding time,After obtaining the optimal condition of 0.25 mol% complex 5 and adding 90 μL of N,N-dimethylformamide（DMF）solution to assist ball milling for 20 minutes,the catalytic yield of C-S coupling can reach 95%.On the basis of the optimal conditions,the applicability of the substrate was further explored,and it was found that phenylacetyl bromide,whether replaced by electron donating or electron withdrawing groups,can achieve moderate or higher yields,reflecting the good catalytic effect of complex 5 as a catalyst.