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首页> 外文期刊>Chemistry: A European journal >The squaramide versus urea contest for anion recognition
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The squaramide versus urea contest for anion recognition

机译：方胺与尿素竞争阴离子识别

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The interaction of a neutral squaramide-based receptor, equipped with two 4-nitrophenyl substituents (R_(sq)), with halides and oxoanions has been studied in MeCN. UV/Vis and ~1H NMR spectroscopy titration experiments clearly indicated the formation of 1:1 hydrogen bonding [R _(sq)?X]+ complexes with all the investigated anions. X-ray diffraction studies on the chloride and bromide complex salts confirmed the 1:1 stoichiometry and indicated the establishment of bifurcated hydrogen-bond interactions between the squaramide-based receptor and the halide anion that involved both 1) amide N-H and 2) aryl proximate C-H fragments, for a total of four bonds. Probably due to the contribution of C-H fragments, complexes of R_(sq) with halides are 1 to 2 orders of magnitude more stable than the corresponding ones with the analogous ureabased receptor that contains two 4-nitrophenyl substituents (R_(ur)). In the case of oxoanions, R _(sq) forms complexes, the stability of which decreases with the decreasing basicity of the anion (H_2PO_4- > NO _2- ≈ HSO_4- > NO_3-), and is comparable to that of complexes of the urea-based receptor R_(ur). Such a behaviour is ascribed to the predominance of different contributions: electrostatic interaction for halides, acid-to-base 'frozen' proton trans-fer for oxoanions. Finally, with the strongly basic anions F- and CH_3COO-, R _(sq) first gives genuine hydrogen-bond complexes of 1:1 stoichiometry; then, upon addition of a second anion equivalent, it undergoes deprotonation of one N-H fragment, with the simultaneous formation of the dianion hydrogen-bond complexes, [HF_2]- and [CH_3COOH-CH_3COO]-, respectively. In the case of the ureabased derivative R_(ur), deprotonation takes place with fluoride but not with acetate. The apparently higher Br?nsted acidity of R_(sq) with respect to R_(ur) reflects the capability of the squaramide receptor to délocalise the negative charge formed on N-H deprotonation over the cyclobutene-1,2-dione ring and the entire molecular framework.
机译：在MeCN中已经研究了配备有两个4-硝基苯基取代基（R_（sq））的中性方胺基受体与卤化物和氧阴离子的相互作用。 UV / Vis和〜1H NMR光谱滴定实验清楚地表明，所有研究的阴离子均形成了1：1的氢键[R_（sq）ΔX] +络合物。对氯化物和溴化物络合物盐进行的X射线衍射研究证实了1：1的化学计量关系，并表明了基于方胺的受体与涉及1）酰胺NH和2）邻位芳基的卤化物阴离子之间分叉的氢键相互作用的建立。 CH片段，共四个键。可能是由于C-H片段的作用，R_（sq）与卤化物的配合物比相应的含两个4-硝基苯基取代基（R_（ur））的类似脲基受体的配合物稳定1至2个数量级。在含氧阴离子的情况下，R _（sq）形成配合物，其稳定性随阴离子的碱度降低而降低（H_2PO_4-> NO _2-≈HSO_4-> NO_3-），并且与阴离子的配合物相当。基于尿素的受体R_（ur）。这种行为主要归因于不同的贡献：卤化物的静电相互作用，氧代阴离子的酸碱“冻结”质子转移。最后，使用强碱性阴离子F-和CH_3COO-，R _（sq）首先给出化学计量比为1：1的真正氢键配合物;然后，在加入第二个阴离子当量后，它经历一个N-H片段的去质子化，同时同时形成二价阴离子氢键配合物[HF_2]-和[CH_3COOH-CH_3COO]-。在脲基衍生物R_（ur）的情况下，去质子化用氟化物而不是乙酸盐发生。 R_（sq）相对于R_（ur）明显更高的布朗斯台德酸度反映了方酰胺受体将NH质子化在环丁烯-1,2-二酮环上和整个分子框架上形成的负电荷分散的能力。

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