Synthesis Of Paddlewheel Pyrazole Dibismuthanes(?)Complexes And Detection Of Pyrazole Free Radical

In basic chemistry,organometallic chemistry and physical chemistry,heterocyclic complexes such as pyrazole have been the focus of research,especially the aromatic five-membered heterocyclic radicals with aromaticity,which have caused great interest in recent years.As one of the common five-membered heterocyclic rings,pyrazole has a unique electronic structure and a variety of coordination modes.Deprotonated3,4,5-trisubstituted pyrazole anions[3,4,5-R₁R₂R₃pz]^-?R₁,R₂,R₃=H,alkyl,aryl,etc.?has an aromatic 6?electronic structure.It is a heterocyclic analogs of cyclopentadienyl anion[Cp^-/?Cp*?^-]and have been widely used in metal complexes for many years as catalysts,precursors and intermediates for organic reactions.With the successful synthesis of paddlewheel Bi?II?and Sb?II?complexes coordinated by 1,2,4-phosphopyrazole,we tried to stabilize the p-region metal with pyrazole ligands.It is desirable to synthesize a paddlewheel complex having a short metal–metal single bond coordinated by pyrazole.The specific work is as follows:1.Synthesis of paddlewheel pyrazole dibismuthanes complexes.At room temperature,BiCl₃ and K[3,5-R₂pz][R=t Bu?3?,Ph?4?]used as the substrate,the reaction was carried out in a THF solution at 1:3,and an orange solution was got.We obtained a stable paddlewheel dibismuthanes Bi?II?–Bi?II?complex[{?¹,?¹-3,5-R₂pz}₂?Bi–Bi?{?¹,?¹-3,5-R₂pz}₂][R=t Bu?5?,Ph?6?].They are the first paddlewheel bimetallic complexes coordinated by pyrazole,and the Bi–Bi single bond is very short,2.8185?6??in 5,2.8705?6??in 6,which is shorter than the Bi–Bi single bond in most other dibismuthane complexes.Especially the Bi–Bi single bond in 5,it is even shorter than the Bi=Bi double bond?about 2.82??.After calculation,the reaction of BiCl₃ with K⁺[3^-]/[4^-]was confirmed to be a one-electron redox process.Bi³⁺can oxidize pyrazole anions to pyrazole radicals.This is unprecedented in the history of the development of pyrazole metal complexes.Under the same conditions,we also investigated the reaction of Sb³⁺with pyrazole anion.The results were not satisfactory:SbCl₃ and K[3,5-t Bu₂pz]were mixed in a ratio of 1:3 with THF as solvent,and the result was colorless.The solution,after treatment,we get the complex[Sb??²-3,5-t Bu₂pz?₃]?10?.Its three pyrazole ligands are evenly distributed under the Sb atoms to form an umbrella-like structure.The pyrazole anion was not oxidized to the pyrazole radical by Sb³⁺.2.The electronic structure research of pyrazole free radicals.At room temperature,K⁺[3^-]and BiCl₃ used as reactants,and 5,5-dimethyl-1-pyrroline-N-oxide?DMPO?used as a radical trap,the sample were subjected to the electron paramagnetic resonance?EPR?spectroscopy.As a result,we captured the pyrazole radical[3,5-t Bu₂pz]·and got its additive with DMPO[3,5-t Bu₂pz]·DMPO?9?which gives a very strong EPR signal.This signal presents a set of superfine,high resolution,multiple coupled 12-fold peaks,which clearly indicates that pyrazole radicals are formed during the reaction.In addition,after analyzing the EPR data,we confirmed the distribution of unpaired electrons in pyrazole radicals,which are delocalized on two N atoms in the?-type ground state instead of the?-type ground state delocalized on a five-membered heterocyclic ring.The discovery of complexes 5 and 6 provides a possible catalyst for certain organic reactions and also a possible precursor for the synthesis of certain new optical or functional materials.The capture of pyrazole free radicals solves the problem that has plagued people for many years,confirming the distribution of unpaired electrons.