Transition Metal Clusters Based On Silsesquioxane

Transition metal clusters usually have unique structures and novel properties,and the synthesis and application of functional clusters have attracted much attention.However,the controllable synthesis of polynuclear metal clusters is still a great challenge in this field.The key to the preparation of metal clusters is the coordination of polydentate ligands with polymetal ions.Silsesquioxane[RSiO1.5]n is mostly in the form of oligomer,with high heat resistance and chemical stability.There are many oxygen atoms with coordination activity in siloxane,which is an ideal ligand for the construction of metal clusters.In this paper,a series of novel metal cobalt clusters were synthesized by solvothermal method with silsesquioxane as ligand.On the basis of structural characterization,their catalytic and magnetic properties were studied.The main contents are as follows:I.Two novel space craft-like octanuclear Co(Ⅱ)-silsesquioxane nanocages,{Co8[(MeSiO2)4]2(dmpz)8}(1)and {Co8[(PhSiO2)4]2(dmpz)8}(2)(Hdmpz=3,5-dimethylpyrazole),have been constructed from two similar multidentate silsesquioxane ligands assisted with a pyrazole ligand.The Co8 skeleton consists of eight tetrahedral Co(Ⅱ)ions arranged in a ring and is further capped by two(MeSiO2)44-ligands up and down.The auxiliary dmpz" ligands seal the ring finally.Electrospray ionization mass spectrometry revealed complex 1 and 2 are highly stable in CH2Cl2.Magnetic analysis implies that complex 1 announces antiferromagnetic interactions between Co(Ⅱ)ions.Moreover,both of them display good homogeneous catalytic activity for hydroboration of ketones in the presence of pinacolborane under mild conditions.Ⅱ.{Co8(OH)2[(MeSiO2)6]2(bpy)2(Obpy)2}(3)was obtained by solvothermal method with silsesquioxanes as ligand,wherein the Co8 disc-like core is sandwiched by two hexamethylcyclohexasiloxanolate ligands(MeSiO2)6 at two poles and finally encircled by two bpy(bpy=2,2’-bipyridine)and two Obpy(HObpy=6-hydroxy-2,2’-bipyridine)ligands at the equatorial region.Interestingly,both MeSi(OMe)3 and bpy undergo in situ transformations to generate hexameric cyclic(MeSiO2)6 and Obpy.respectively.The unusual hydroxylation of bpy and the OH-anion in the center of Co8 core provide additional binding sites to induce the formation of the larger cluster instead of the traditional hexanuclear cluster.The solution stability and fragmentation route in the gas phase were studied by cold-spray ionization and collision-induced dissociation mass spectrometry,respectively.Both results reveal that the Co8 core is quite stable in solution as well as in the gas phase,even with increased collision voltage.Magnetic susceptibility studies of complex 3 show the slow magnetization relaxation indicative of single-molecule magnet(SMM)behavior.This work not only presents the multiple in situ ligand-transformation-assisted assembly of polynuclear cobalt cluster but also provides some new insights into the magnetism-structure relationship for SMM.Ⅲ.Functionalized Zn(Ⅱ)-silsesquioxanes nanocages with photochromic properties.We have successfully synthesized an octanuclear Zn(Ⅱ)-silsesquioxanes polynuclear metal organic nanocage{Zn8[(MeSiO2)4]2(azopz)8}(4)(Hazopz=3,5-dimethyl-4-(phenyldiazenyl)-1H-pyrazole)with optical switching properties.X-ray crystallography shows that there are eight tetrahedral Zn(Ⅱ)ions in the core of Zn8,which are arranged in the form of rings,and are surrounded by two(MeSiO2)44-silsesquioxanes ligands and azopz ions.Azo ions are arranged in trans configuration.We have studied the photochromic and photochemical switching properties of the complex 4 by UV-vis and NMR.The results show that the isomerization efficiency of the functionalized complex 4 is close to 99%in cis to trans and trans to CIS conditions.