Synthesis,Structures,Magnetic Properties And Density Functional Theory Calculation Of Dichromium （Ⅱ,Ⅱ） Carbonates Based On The [Cr₂（CO₃）₄]^4- Unit

Focusing on the hot research field of metal-metal multi-bond for a long time,this paper carried out the synthesis and coordination assembly of dichromium paddlewheel carbonate with Cr-Cr bonding.The Cr₂metal-metal bonding center,chelated and bridged by four carboxylates have been reported.However,there is only one reported crystal structure of coordination compound based on Cr₂ⁿ⁺paddlewheel building blocks with non-carboxylate O,O′-donor ligands chelating and bridging Cr-Cr centers.In this paper,using Cr₂（O₂CCH₃）₄as the precursor,the[Cr₂（CO₃）₄]^4-unit dichromium compounds were synthesized by one-step synthesis,and then alkaline-earth and transition metal salt were introduced to assemble and coordinate,and a new type of mixed-metal coordination compounds based on Cr-Cr metal multi-bond were obtained.The structures were determined and analyzed by single crystal X-ray diffraction and their spectral and magnetic properties were studied.Meanwhile,the corresponding molecular orbitals and spectra were obtained with the help of density functional theory calculation（DFT）.With the combination of experiment and theory,the electronic structure of Cr-Cr metal multi-bond was deeply discussed,which provides effective support for other studies of this kind of compounds.The main research results are as follows:1.The First Chromous Carbonates Containing Square-grid Layer{Cr₂（CO₃）₄}_n^4n-Based on Dichromium（Ⅱ,Ⅱ）Paddlewheel CoreA simple synthesis strategy was proposed to synthesize dichromium carbonate compound Na₃H[Cr₂（CO₃）₄]·10H₂O（1）with square-grid layer structure using Cr₂（O₂CCH₃）₄as precursor,that is,the first coordination polymer constructed from binuclear chromium（Ⅱ,Ⅱ）building block[Cr₂（CO₃）₄]^4-.The investigation of the primary magnetic properties and theoretical studies with DFT reveal that the partial paramagnetic properties of the compound caused by the Boltzmann distribution between a ground stateσ²π⁴δ²with S=0and a low-lying excited stateσ²π⁴δδ^*with S=1 for the Cr₂（Ⅱ,Ⅱ）dimer.By Raman spectra measurements combined with theoretical calculation,the Raman bands in the small-wavenumber region were assigned for the bands at both 335 and 297 cm^-1to the Cr-O bonds’vibration,and 371 cm^-1to stretching of the Cr-Cr quadruple bond.2.The Dichromium（Ⅱ,Ⅱ）Paddlewheel Coordination Polymer with Two-dimensional and Three-dimensional Topological Structure Constructed by[Cr₂（CO₃）₄]^4-UnitUsing KHCO₃solution as solvent environment system,[Cr₂（CO₃）₄]^4-unit structure compounds K_xH_4-x[Cr₂（CO₃）₄（H₂O）₂]·n H₂O[x=4,n=2（2）;x=3,n=9（3）;x=4,n=4（4）;x=3,n=2（5）]and Li₃H[Cr₂（CO₃）₄（H₂O）₂]·2H₂O（6）with different spatial dimensions（0D/2D/3D）were obtained by adjusting the alkalinity.In addition to partially forming ion accumulation,interestingly,compound 4 forms a 2D square-grid layer structure,and the topological framework shows a（2,4-c）network with the topology symbol{8⁴12²}{8}₂.Compound 5-6 forms a reticular structure with 3D network,and the topological framework shows a（2-c,4-c）network with the topological symbol{12⁶}{12}₂.The structures of the above compounds were characterized,and the corresponding spectral and magnetic behaviors were studied.These compounds with extended structure constructed by[Cr₂（CO₃）₄]^4-unit have important guiding significance for the design and synthesis of molecular base materials with related functional units.3.Construction of 3d Mixed-Metal Complexes of[Cr₂（CO₃）₄]^4-UnitBased on{Cr₂（CO₃）₄}_n^4n-module unit,a kind of chains coordination polymers K₂[M（H₂O）_mCr₂（CO₃）₄（H₂O）₂]·n H₂O[M=Cr（7）,Mg（8）,Co（9）]and layered compound K₂[Zn（H₂O）₄Cr₂（CO₃）₄（H₂O）₂]·2H₂O（10）containing[Cr₂（CO₃）₄]^4-unit were constructed.Magnetic studies show that in the case of mixed-metal coordination,they show a certain antiferromagnetic interaction between M and Cr₂center,while compound 7 shows obvious temperature and frequency dependence,showing typical single molecular magnet properties,and mononuclear Cr²⁺magnetic anisotropy may be the main factor leading to slow magnetic relaxation at low temperature.