Transition metal dithiocarbamate complexes

Transition metal dithiocarbamate complexes are coordination complexes containing one or more dithiocarbamate ligand, which are typically abbreviated R₂dtc⁻. Many complexes are known. Several homoleptic derivatives have the formula M(R₂dtc)_n where n = 2 and 3.^[1]

Structure of iron tris(diethyldithiocarbamate).

Ligand characteristics

Main resonance structures of a dithiocarbamate anion.

Dithiocarbamate anions are bidentate ligands that are classified as L-X ligand in the Covalent bond classification method. In the usual electron counting method, they are three-electron ligands. With respect to HSAB theory, they are classified as soft.

Because of the pi-donor properties of the amino substituent, the two sulfur centers show enhanced basicity relative to dithiocarboxylates. This situation is represented by the zwitterionic resonance structure that depicts a positive charge on N and negative charges on both sulfurs. This N to C pi-bonding results in partial double bond character for the C-N bond. Consequently, barriers to rotational about this bond are elevated. Another consequence of their high basicity, dithiocarbamates often stabilize complexes in some uncharacteristically high oxidation state (e.g., Fe(IV), Co(IV), Ni(III), Cu(III)).

Dithiocarbamate salts are easily synthesized. Many primary and secondary amines react with carbon disulfide and sodium hydroxide to form dithiocarbamate salts:^[2]

R₂NH + CS₂ + NaOH → R₂NCS₂⁻Na⁺ + H₂O

A wide variety of secondary amines give the corresponding dtc ligand. Popular amines include dimethylamine (Me₂NH), diethylamine (Et₂NH), and pyrrolidine ((CH₂)₄NH). Complexes of H₂NCS−2, derived from the parent dithiocarbamic acid have been reported.^[3]

Related ligands

Structure of typical metal tris(ethylxanthate) complex.^[4]

Dithiocarbamates are classified as derivatives of dithiocarbamic acid. Their properties as ligands resemble the conjugate bases of many related "1,1-dithioacids":

• Diorganothiophosphates, (RO)₂PS₂⁻
• Dithiocarboxylates, RCS₂⁻
• Xanthates, ROCS₂⁻
• Thioxanthates, RSCS₂⁻

Synthetic methods

Commonly, metal dithiocarbamates are prepared by salt metathesis reactions using alkali metal dithiocarbamates:

NiCl₂ + 2 NaS₂CNMe₂ → Ni(S₂CNMe₂)₂ + 2 NaCl

In some cases, the dithiocarbamate serves as a reductant, followed by its complexation.^[5]

A complementary method entails oxidative addition of thiuram disulfides to low-valent metal complexes:

Mo(CO)₆ + 2 [S₂CNMe₂]₂ → Mo(S₂CNMe₂)₄ + 6 CO

Metal amido complexes, such as tetrakis(dimethylamido)titanium, react with carbon disulfide:

Ti(NMe₂)₄ + 4 CS₂ → Ti(S₂CNMe₂)₄

Homoleptic complexes

Structure of Ti(S₂CNEt₂)₄.

Bis complexes

• nickel bis(dimethyldithiocarbamate), palladium bis(dimethyldithiocarbamate), platinum bis(dimethyldithiocarbamate),^[6] all square-planar complexes
• copper bis(diethyldithiocarbamate), a square-planar complex^[7]

Tris complexes

• vanadium tris(diethyldithiocarbamate), an octahedral complex^[8]
• chromium tris(diethylditiocarbamate), an octahedral complex^[9]
• manganese tris(dimthylthtiocarbamate), an octahedral complex^[10]
• iron tris(diethyldithiocarbamate), ruthenium tris(diethyldithiocarbamate), osmium tris(diethyldithiocarbamate), all octahedral complexes
• cobalt tris(diethyldithiocarbamate), rhodium tris(diethyldithiocarbamate), iridium tris(diethyldithiocarbamate), all octahedral complexes

Tetrakis complexes

• titanium tetrakis(dimethyldithiocarbamate)^[11]
• molybdenum tetrakis(diethyldithiocarbamate)

Dimetallic complexes

• iron bis(diethyldithiocarbamate), pentacoordinate Fe dimer
• zinc bis(dimethyldithiocarbamate), pentacoordinate Zn dimer
• dicobalt pentakis(diethyldithiocarbamate) cation, with a pair of octahedral Co(III) centers^[12]
• diruthenium pentakis(diethyldithiocarbamate) cation, with a pair of octahedral Ru(III) centers, two isomers

Isomers of [Ru₂(dtc)₅]⁺.^[13]

Reactions

Dithiocarbamate complexes do not undergo characteristic reactions. They can be removed from complexes by oxidation, as illustrated by the iodination of the iron tris(diethyldithiocarbamate):

Fe(S₂CNEt₂)₃ + 0.5 I₂ → Fe(S₂CNEt₂)₂I + 0.5 (S₂CNEt₂)₂

They degrade to metal sulfides upon heating.^[14]

Applications

Dtc complexes find several applications:

• herbicides in the form of the iron and zinc derivatives Ferbam and Zineb, respectively
• vulcanization accelerators, zinc bis(dimethyldithiocarbamate).^[15]
• medicine, iron tris(dimethyldithiocarbamate) as a nitric oxide scavenger.
• lubricants. Metal thiocarbamates are also used in metal-to-metal lubrication proposes, mainly as an anti-oxidation or anti-extreme pressure (EP) additive. 1-2% of such compounds can be added to internal combustion engine lubricant to increase extreme pressure performance in high operational temperatures.^[16][17]