The halogens are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine
(At). The common halogen ligands are in the form of ions such as F¯, Cl¯, I¯, Br¯, also namely halide ligands. Halides are X-type ligands in coordination, which are usually good σ- and good π-donors. These ligands are usually terminal, but they might act as bridging ligands as well. For example, the chloride ligands of aluminium chloride bridge two aluminium centers, thus the compound with the empirical formula AlCl3 actually has the molecular formula of Al2Cl6 under ordinary conditions. Due to their π-basicity, the halide ligands are weak field ligands, and due to a smaller crystal field splitting energy, the halide complexes of the first transition metals series are all high spin when possible, the halide complexes of the second and third row transition metals series are low spin. Today, some halogenated compounds, called halogen MOFs (metal-organic frame materials) ligands, are used as ligands to construct a variety of MOFs materials to expand the applications of MOFs.
Although few researches are relevant to halogen ligands when compared with other ligands such as phosphine ligands, oxygen-donor ligands and nitrogen-donor ligands, they also play an important role in various research fields and have irreplaceable position, some are listed below:
- Photovoltaic device: Halogen ions are indeed promising ligands to improve the stability of quantum dots, which possess desirable applications in solar photovoltaic devices because of unique optical properties. According to Niu et al., the halogen ligands (Cl¯, I¯, Br¯) were added into a colloidal solution of CdSe and PbS quantum dots to form quantum dots capped with these inorganic ligands, and then conducted the electrophoretic deposition to fabricate quantum dots films, the resulting quantum dots films were used in photovoltaic devices and offered better efficiency.
Fig.1 The synthesis of halogen ligands based CdSe and PbS quantum dots (X=Cl, I, Br)
- Precursor for inorganic compounds: Halogen ligands can combine with a variety of metals to form metal halides ([TiCl4], [CrCl6]3−, [FeCl4]2−, [NiCl4]2−, [PdCl4]2−, etc.), which are often readily available precursors for other inorganic compounds. The metal halide complexes are known with several stoichiometries, but the main ones are the hexahalometallates and the tetrahalometallates. For example, tetrahalide gold halide ( [AuX4]− (X = F, Cl, Br, I, At) chemistry has attracted a significant amount of research for its extensive application as precursors in synthesizing nanoscale gold particles and various solid materials
- Anticancer drug: Cisplatin, cis-Pt(NH3)2Cl2, is a platinum drug consisted of two chloride ligands. The two chloride ligands are easily displaced, allowing the platinum center to bind to two guanine units, thus damaging DNA. Cisplatin is a famous chemotherapy medication used to treat a number of cancers, including testicular cancer, ovarian cancer, cervical cancer, breast cancer, head and neck cancer, esophageal cancer, lung cancer, mesothelioma, brain tumors and neurblastoma.
Fig.2 The structures of cisplatin
- Catalysis: Metal halides based on halogen ligands can act as catalysts. For examples, ferric and aluminium halides are catalysts for the Friedel-Crafts reaction, and due to their low cost, they are often added in stoichiometric quantities. Chloroplatinic acid (H2PtCl6) is an important catalyst for hydrosilylation.
- Others: The MOFs materials constructed by halogen MOFs ligands can be applied in various filed such as storage and separation, catalysis, sensing and detection, energy storage, and drug delivery.
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- Niu G.; et al. Inorganic halogen ligands in quantum dots: I−, Br−, Cl− and film fabrication through electrophoretic deposition[J]. Physical Chemistry Chemical Physics, 2013, 15(45), 15, 19595-19600.