Our research in this area was summarized in a Forum Article in Inorganic Chemistry in 2011.
Nucleophilicity of Metal-Thiolates
Many metal-thiolate complexes undergo sulfur-centered reactions with alkylating agents, oxygen transfer agents, and/or other metal ions. This reactivity can be attributed to the "activation" of the thiolate lone pair by interaction with the appropriate d-orbitals of the metal ion.1 The high energy lone pair of electrons resides in a pi-anti-bonding orbital distributed between a metal d-orbital and sulfur p-orbital. The extent of this activation depends on the thiolate and the identity of the metal ion, its oxidation state, and spin-state.
Oxidation of Metal-Thiolates
The site of oxidation of a metal-thiolate depends on the nature of the highest occupied molecular orbitals. If the orbitals are primarily metal in character, the oxidation is referred to as "metal-centered" and the oxidation state of the metal is increased by one. This is consistent with the formal oxidation approach that works well for many traditional metal-ligand interactions. If the orbitals are primarily ligand in character, the oxidation is referred to as "ligand-centered" yielding a thiyl radical (RS.). For "activated" metal-thiolates, the oxidation removes an electron from an orbital with nearly equal metal and sulfur character. In this case, it is difficult to make a definitive assignment and we refer to these complexes as "metal-stabilized thiyl radicals".
Synthesis, Characterization, and Oxidation
In the Grapperhaus Group, we design and synthesize metal-thiolate complexes that promote sulfur nucleophilicity and yield "metal-stabilized thiyl radicals" upon oxidation. The complexes are fully characterized spectroscopically and their reactivity with electrophiles is explored. Using spectroelectrochemical methods and cyclic voltammetry, their oxidation chemistry is investigated.
Taming Thiyl Radicals2
Organic thiyl radicals are known to form disulfides or abstract hydrogen atoms from activated substrates. They also react with alkenes (R2C=CR2) in carbon-sulfur forming reactions. Metal ions can help "tame" free radicals and direct their reactivity.
Metal-stabilized thiyl radicals can be trapped with substrates to generate new products. For example, the ruthenium complex [Ru(DPPBT)3]+ undergoes reactions with alkenes such as ethylene, styrene, and hexene.3 The rhenium derivative binds ethylene reversibily with an equilibrium constant dependent on the complex charge.4 By changing the charge from +2 to 0 the equilibrium constant drops by 20 orders of magnitude!
1. Grapperhaus, C. A.; Mullins, C. S.; Kozlowski, P. M.; Mashuta, M. S. Inorg. Chem. 2004, 43, 2859-2866.
2. The concept of metal ions "taming" free radicals is attributed to Karl Wieghardt.
3. Grapperhaus, C. A.; Venna, K. B.; Mashuta, M. S. Inorg. Chem. 2007, 46, 8044-8050.
4. Grapperhaus, C. A.; Ouch, K.; Mashuta, M. S. J. Am. Chem. Soc. 2009, 131, 64-65.