Binding of Polyatomic Anions with Protonated Ureido-pyridyl Ligands

Two ureido-pyridyl ligands, 1-(4-nitro-phenyl)-3-pyridine-3-ylmethyl-urea (L1) and 1-pentafluorophenyl-3-pyridine-4-yl-urea (L2) were synthesized in good yields, and solid-state anion binding studies of (HL1)+ and (HL2)+ with polyatomic anions such as NO3−, AcO−, ClO4−, SO42−, and SiF62− were carried out in detail. Protonation of the pyridyl nitrogen center of L1 with HNO3, HClO4, and HF in different solvent media yielded crystals of complexes 1 (HL1·NO3), 2 (HL1·ClO4), and 3 (HL1·0.5SiF6) suitable for single crystal X-ray diffraction studies, respectively. Similarly, protonation of L2 with HNO3, CH3COOH, and H2SO4 yielded single crystals of complexes 4 [2(HL2·NO3)·DMF], 5 (HL2·AcO), and 6 [(HL2)2·SO4], respectively. X-ray crystallographic analysis of all these six complexes has been carried out and the details of anion binding with the urea functionality of ureido-pyridyl ligands were investigated. In all the complexes, the pyridyl moiety of the ligands is found to be protonated and the binding of anions are observed at the urea functionality of the ligands through the R22(8) hydrogen bonding motif irrespective of sizes and shapes of anions or the electron withdrawing ability of aryl substitutions (p-nitro phenyl vs pentafluoro phenyl) or the position of the pyridyl nitrogen center (protonation site) of the designed ligands.本研究以较高收率合成了两种脲基吡啶配体（ureido-pyridyl ligands）：1-(4-硝基苯基)-3-(吡啶-3-基甲基)脲（L1）与1-五氟苯基-3-(吡啶-4-基)脲（L2）。随后针对质子化配体阳离子(HL1)+与(HL2)+，对硝酸根（NO3−）、乙酸根（AcO−）、高氯酸根（ClO4−）、硫酸根（SO42−）及六氟硅酸根（SiF62−）等多原子阴离子开展了细致的固态阴离子结合研究。分别在不同溶剂介质中，使用硝酸（HNO3）、高氯酸（HClO4）及氢氟酸（HF）对L1的吡啶氮中心进行质子化，分别得到可用于单晶X射线衍射分析的配合物1（HL1·NO3）、配合物2（HL1·ClO4）与配合物3（HL1·0.5SiF6）。类似地，使用硝酸（HNO3）、乙酸（CH3COOH）及硫酸（H2SO4）对L2进行质子化，分别得到配合物4[2(HL2·NO3)·N,N-二甲基甲酰胺（DMF）]、配合物5（HL2·AcO）与配合物6[(HL2)2·SO4]的单晶。对上述六种配合物均开展了X射线晶体学分析，并针对脲基吡啶配体的脲基官能团参与的阴离子结合细节展开了研究。在所有配合物中，均观察到配体的吡啶基团发生质子化，且阴离子均通过R22(8)氢键基元与配体的脲基官能团结合，该结合模式不受阴离子的尺寸与形状、芳基取代基的吸电子能力（对硝基苯基与五氟苯基的差异），或是所设计配体的吡啶氮中心位置（即质子化位点）的影响。