Sensitive and Selective Chromogenic Sensing of Carbon Monoxide via Reversible Axial CO Coordination in Binuclear Rhodium Complexes

The study of probes for CO sensing of a family of binuclear rhodium(II) compounds of general formula [Rh2{(XC6H3)P(XC6H4)}n(O2CR)4–n]·L2 containing one or two metalated phosphines (in a head-to-tail arrangement) and different axial ligands has been conducted. Chloroform solutions of these complexes underwent rapid color change, from purple to yellow, when air samples containing CO were bubbled through them. The binuclear rhodium complexes were also adsorbed on silica and used as colorimetric probes for “naked eye” CO detection in the gas phase. When the gray-purple colored silica solids containing the rhodium probes were exposed to air containing increasing concentrations of CO, two colors were observed, in agreement with the formation of two different products. The results are consistent with an axial coordination of the CO molecule in one axial position (pink-orange) or in both (yellow). The crystal structure of 3·(CO) ([Rh2{(C6H4)P(C6H5)2}2(O2CCF3)2]·CO) was solved by single X-ray diffraction techniques. In all cases, the binuclear rhodium complexes studied showed a high selective response to CO with a remarkable low detection limit. For instance, compound 5·(CH3CO2H)2 ([Rh2{(m-CH3C6H3)P(m-CH3C6H4)2}2(O2CCH3)2]·(CH3CO2H)2) is capable of detection of CO to the “naked eye” at concentrations as low as 0.2 ppm in air. Furthermore, the binding of CO in these rhodium complexes was found to be fully reversible, and release studies of carbon monoxide via thermogravimetric measurements have also been carried out. The importance of the silica support for the maintenance of the CO-displaced L ligands in the vicinity of the probes in a noninnocent manner has been also proved.

本研究针对一类通式为[Rh2{(XC6H3)P(XC6H4)}n(O2CR)4–n]·L2的双核铑(II)配合物展开，该类配合物包含1个或2个呈头尾排列的金属化膦配体，以及不同的轴向配体，可作为一氧化碳（CO）检测探针。将含CO的空气样品通入该类配合物的三氯甲烷溶液中时，溶液会快速发生颜色变化，由紫色变为黄色。该类双核铑配合物还可负载于二氧化硅载体上，用作气相CO的裸眼比色检测探针。当负载有铑探针的灰紫色二氧化硅固体暴露于CO浓度逐渐升高的空气中时，会观察到两种颜色变化，这与两种不同产物的生成相吻合。实验结果与CO分子在配合物单个轴向位点（产物呈粉橙色）或两个轴向位点（产物呈黄色）发生轴向配位的情况一致。通过单晶X射线衍射技术解析了配合物3·(CO)（[Rh2{(C6H4)P(C6H5)2}2(O2CCF3)2]·CO）的晶体结构。在所研究的所有双核铑配合物中，均表现出对CO的高选择性响应，且检出限极低。例如，配合物5·(CH3CO2H)2（[Rh2{(m-CH3C6H3)P(m-CH3C6H4)2}2(O2CCH3)2]·(CH3CO2H)2）可通过裸眼检测空气中浓度低至0.2 ppm的CO。此外，该类铑配合物与CO的配位结合完全可逆，且通过热重分析完成了一氧化碳的脱附研究。本研究还证实，二氧化硅载体可通过非惰性作用将被CO置换出的L配体保留在探针附近，这一作用具有重要意义。