Reversible Single-Crystal-to-Single-Crystal Cross-Linking of a Ribbon of Cobalt Citrate Cubanes To Form a 2D Net

An unprecedented 1-D polymer of cobalt citrate cubanes, 1, has been prepared by wet chemistry techniques and isolated as single crystals with two units of formula Cs2Co7(citr)4(H2O)21 in the unit cell [H4citr = C6H8O7, citric acid], as characterized by X-ray diffraction at T = 278 K. In addition to the four Co atoms in the cubane unit, there are four independent peripheral citrate-bound Co sites. Upon warming at T = 303 K, the polymer undergoes cross-linking through a complex substitution/addition reaction at one of the peripheral Co centers to produce a new 2-D polymer based on the same Co citrate cubane building block. In the transition a second peripheral Co atom undergoes a substitution reaction and separates from the polymer. The new phase, 2, analyzed by X-ray diffraction from the same crystalline sample as 1, has two units of formula Cs2Co7(citr)4(H2O)13.5 in the unit cell. Water egress in the transition amounts to 35.7 mole percent. The volume of the unit cell diminishes by 12.4%. The Co center at the cross-link in the 2-D polymer is coordinated by seven oxygen atoms. Upon sitting at ambient conditions for an extended period of time, the same crystal reverts to its original phase, 1, as characterized by unit cell determination. When heated again at T = 303 K, it again acquires the unit cell of 2.

本研究通过湿化学法（wet chemistry techniques）制备了一种前所未有的柠檬酸钴立方烷（cobalt citrate cubanes）一维聚合物1，并以单晶形式分离得到，其晶胞（unit cell）内包含两个Cs₂Co₇(citr)₄(H₂O)₂₁结构单元[其中H₄citr即柠檬酸C₆H₈O₇]，并通过278 K下的X射线衍射（X-ray diffraction）对其结构进行了表征。该立方烷结构单元中包含4个钴原子，此外还存在4个独立的、与柠檬酸根配位的外围钴位点。当升温至303 K时，该一维聚合物通过其中一个外围钴中心发生的复杂取代/加成反应发生交联，生成以相同柠檬酸钴立方烷为结构基元的新型二维聚合物。在此相变过程中，第二个外围钴原子也发生取代反应，并从聚合物链中脱离。通过对同一块单晶样品进行X射线衍射分析，确定了新相2的晶胞参数：其晶胞内包含两个Cs₂Co₇(citr)₄(H₂O)₁₃.₅结构单元。相变过程中流失的水分占总水量的35.7 mol%，晶胞体积缩减12.4%。二维聚合物交联位点处的钴中心与7个氧原子配位。将该单晶在室温环境下放置较长时间后，通过晶胞参数测定可知，其又恢复至初始相1。再次升温至303 K时，其晶胞参数又转变为相2的晶胞参数。