Colorimetric Fluoride Ion Sensing by Polyborylated Ferrocenes: Structural Influences on Thermodynamics and Kinetics

The thermodynamic factors underlying the use of ferroceneboronic esters as electrochemical or colorimetric
fluoride ion sensors have been investigated through the synthesis of a range of systematically related derivatives
differing in the number/nature of the boronic ester substituents and in the nature of ancillary ligands. Thus, if the
shift in electrochemical potential associated with the conversion of one (or more) boronic ester group(s) to anionic
boronate(s) on fluoride binding is sufficient to allow oxidation of the resulting host/guest complex by dioxygen,
colorimetric sensing is possible. In practice, while monofunctional systems of the type CpFe[η5-C5H4B(OR)2] offer
selectivity in fluoride binding, electrochemical shifts in chloroform solution are insufficient to allow for a colorimetric
response. Two chemical modification strategies have been shown to be successful in realizing a colorimetric
sensor:  (i) the use of the more strongly electron-donating Cp* ancillary ligand (which shifts the oxidation potential
of both the free receptor and the resulting fluoride adduct cathodically by ca. −400 mV) and (ii) receptors featuring
two or more binding sites and consequently a larger fluoride-induced electrochemical shift. Thus, in the case of
[η5-C5H4B(OR)2]2Fe [(OR)2 = OC(H)PhC(H)PhO, 2s], the binding of 2 equiv of fluoride gives an electrochemical
shift (in chloroform) of −960 mV (cf. −530 mV for the corresponding monofunctional analogue, 1s). Related tris-
and tetrakis-functionalized systems are also shown to be oxidized as the bis(fluoride) adducts, presumably because
of fast oxidation kinetics, relative to the rate of the (electrostatically unfavorable) binding of a third equivalent of
fluoride. Furthermore, the rate of sensor response (as measured by UV/vis spectroscopy) is found to be strongly
enhanced by the presence of pendant (uncomplexed) three-coordinate boronic ester functions (e.g., a rate
enhancement of 1−2 orders of magnitude for 3s/4s with respect to 2s) and/or delocalized aromatic substituents.

以二茂铁硼酸酯（ferroceneboronic esters）作为电化学或比色法氟离子传感器所涉及的热力学因素，已通过合成一系列系统关联的衍生物展开研究。这些衍生物在硼酸酯取代基的数量与性质，以及辅助配体（ancillary ligands）的性质上存在差异。具体而言，若氟离子结合过程中，一个或多个硼酸酯基团转化为阴离子硼酸根时伴随的电化学电位偏移，足以使所得主客体复合物被氧气氧化，则可实现比色传感。实际应用中，尽管CpFe[η⁵-C₅H₄B(OR)₂]类单功能体系在氟离子结合中表现出选择性，但氯仿溶液中的电化学偏移不足以产生比色响应。已有两种化学修饰策略可成功实现比色传感器的制备：(i) 使用给电子能力更强的五甲基环戊二烯基（Cp*）辅助配体，该配体可将游离受体与所得氟化物加合物的氧化电位均向阴极方向偏移约-400 mV；(ii) 采用具有两个或多个结合位点的受体，从而获得更大的氟离子诱导电化学偏移。以[η⁵-C₅H₄B(OR)₂]₂Fe [(OR)₂=OC(H)PhC(H)PhO, 2s]为例，2当量氟离子的结合可使氯仿溶液中的电化学偏移达到-960 mV（对应单功能类似物1s的偏移为-530 mV）。相关的三官能化与四官能化体系同样可作为双(氟离子)加合物被氧化，这可能是因为相较于第三个当量氟离子（静电作用不利）的结合速率，氧化动力学更快。此外，研究发现传感器的响应速率（通过紫外-可见光谱法（UV/vis spectroscopy）测定）会因悬垂的未配位三配位硼酸酯官能团（例如相较于2s，3s/4s的响应速率提升1~2个数量级）和/或离域芳香取代基的存在而显著增强。