Sulfur-Donor Ligands

Introduction

Sulfur-donor ligands are the compound with sulfur as coordination atom, which belongs to soft base according to hard and soft acids and bases theory (HSAB). Sulfur atom with abundant electrons can bond with metal not only by s and p valence orbitals, but also by d valence orbital, so it can easily form stable complexes with metal ions, especially, with transition metal ions. Because of this property, sulfur atoms in sulfur ligands can be used as σ donors, σ donor-π donors, and σ donor-π acceptors, these bonding modes are related to steric hindrance and valence states of metal ions. For example, the tendency to form π bonds between sulfur and metal centers when the metal is electron deficient or in a high oxidation state. The coordination diversity of sulfur-donor ligands makes it has a wide range of applications.

Classifications

According to different classification methods, sulfur-donor ligands can be divided into chiral sulfur-donor ligands and achiral sulfur-donor ligands. The most common and easy understanding classification way is based on their structure, it mainly divided into the following categories.

• Thioether ligands: An organic thioether or sulfide refers to a functional group in organosulfur chemistry with the connectivity C-S-C. In thioether, sulfur is sp³ hybridized, in which two valence electrons are bonded with two hydrocarbon groups, and the remaining two lone pair electrons can coordinate with the metal. Due to the electrostatic repulsion and steric hindrance of lone pair electrons of sulfur, the M-S-C (M refers to metal ) bond angle in structure of thioether based complexes is usually less than the regular tetrahedron, 109.4°. Alkyl dithioether ligands are representative of sulfide ligands with an earlier research history.

Fig.1 General structure of a thioether

• Thiol ligands: A thiol is any organosulfur compound of the form R-SH, where R represents an alkyl or other organic substituent. Thiol based metal complexes with various bonding mode and novel geometric configuration, can be used as a potential precursor of a variety of materials, showing a broad application prospect. Among them, the thiol based silver complexes is the most representative, as early as the 1960s, a large number of thiol based silver complexes have been synthesized and has huge application potential in material science, drug research, biochemistry and nonlinear optics fields.

Fig.2 General structure of a thiol

• Sulfoxides ligands: A sulfoxide is a chemical compound containing a sulfinyl (SO) functional group attached to two carbon atoms. The S=O group in sulfoxide ligands has two kinds of coordination atoms (S and O) with different coordination affinity at the same time, so they have multiple coordination modes. Generally, soft metal ions are easy to coordinate with S. It is worth mentioning that the use of chiral sulfoxides as ligands with transition metals in asymmetric catalysis has vast perspectives and has undergone a long period of development.

Applications

Sulfur-donor ligands can be coordinated with a variety of metals including transition metals, main group metals, precious metals and etc., the resulting complexes are widely used in the fields of catalysis, medical fields, biochemistry and others.

• Catalysis: The catalysis of sulfur based metal complexes are mainly related to heterogeneous catalysts. Except for heterogeneous catalysis, complexes by synthesis of chiral sulfur-donor ligands are extensively applied in asymmetric catalysis. The most commonly used ligands are sulfoxides, sulfinamides, N-sulfinyl ureas, and sulfox imines, they can catalyze different types of reactions such as allylic substitution, hydrogenation, hydrogen transfer, conjugated additions, addition of organometallic reagents to aldehydes, Diels-Alder reactions, miscellaneous reactions, allylic substitution, cyclopropanation and many others^[1].
• Medicine: Ruthenium(III) complexes have emerged as a new class of effective anticancer agents against tumors that proved to be resistant to all other chemotherapeutic drugs currently in clinical use. The Ru complexes containing sulfur-donor ligands exhibit good anticancer activity, for example, Lorena et al. synthesized [Ru(RSDT)₃] and [Ru₂(RSDT)₅]Cl complexes with dithiocarbamato ligands, all the complexes were tested for their cytotoxicity on a panel of human tumor cell lines showing highly significant antitumor activity^[2].



• Biochemistry: Transition metal compounds with electron-rich sulfur-donor ligands have attracted extensive research interest because of their close connection with biochemistry, especially in biocatalysis. In the active part of many metalases (transition metal compounds as enzyme) such as nitrogenase and hydroenzyme, the function of transition metal centers is related to sulfur-donor ligands, sulfur-donor ligands not only induce transition metal activity, but also participate in matrix bonding, acid-base activation, or redox processes closely related to active site transformation.



• Others: In addition, sulfur ligands based metal complexes also play an vital role in other fields such as lubricants, photocatalysis, battery technology, magnetic resonance imaging and so on.

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References

• Helene P. Chiral sulfur-containing ligands for asymmetric catalysis[J]. Tetrahedron, 2007, 63, 1297-1330.
• Lorena G.; et al. Ru(III)-based compounds with sulfur donor ligands: synthesis, characterization, electrochemical behaviour and anticancer activity[J]. Dalton Transactions, 2008, 6699-6708.