Niobium dithiocarbamates

The major difference in the dithiocarhamato chemistry of tantalum and niobium is the existence of the Ta(V) species Ta(R2C tc)5, whereas the comparable niobium compound is unknown. The preparative methods for Ta(R2C tc)5 involve either a CS2 insertion in Ta(NR2)j (R = Me )) or a reaction between TaClj and sodium or ammonium dithiocarbamate (R = Et (7), R2 = ( 112)4 ( ))- The structure of this interesting diamagnetic compound is not elucidated, but from infrared data the presence of one or more unidentate ligands is suggested 11,15). 

The M-NR2 bond is labile and Nb and Ta dialkylamides M(NR2)5 (12) are valuable starting materials (Scheme 2). They can easily undergo substitution or Insertion reactions. Insertion of CO2, CS2, and COS in M(NR2)5 has been observed, giving dialkylcarbamates, dithiocarbamates, or monothiocarbamate derivatives, respectively. In the case of niobium, the insertion of CS2 is accompanied by reduction. Insertion of CO2 is catalyzed by amines resulting from adventitious hydrolysis. Monodentate as well as bidentate carbamato... 

Scheme 4 Mechanism of the production of bridging dinitrogen in niobium and tantalum dithiocarbamate complexes (R2dtc, etc. Omitted for clarity)...

Dinuclear niobium sulfido and selenido dithiophosphates, Nb2Q4[S2P(OR)2]4,75,76 (Q = S, Se R = Et) (also xanthates and dithiocarbamates) have been prepared but no crystal structure was reported. Optically active chromium complexes, Cr[S2P(OR)2]3 derived from Z)-borneol and L-menthol, have been described.77... 

Dinuclear niobium xanthato complexes, Nb2S4(S2COR)4 (R = Et, Pr1, Cy also dithiocarbamates and dithiophosphates) have been prepared from [NEt4][Nb2S4(NCS)8] by replacing the thio-cyanato ligands.76... 

The methods involving dithiocarbamates were used for determining copper in plant materials [97], foods [98,99], water [100-103], sewage [104,105], aluminium alloys and environmental samples [106], cast iron and steel [22,107,108], niobium and tantalum [109], gallium arsenide [110], and crude oil [111]. 

The development of niobium and tantalum dithiocarbamate chemistry is relatively recent, the first well authenticated examples coming from the insertion of carbon disulfide into metal amide complexes to give [M(S2CNR2)4] (193, 666). Dithiocarbamates are now known to stabilize niobium and tantalum in the +3 to +5 oxidation states. 

A range of other cationic niobium and tantalum tetrakis(dithiocarbamate) complexes, [M(S2CNR R )4]X (M = Nb, Ta R = R = Me, f-Bu R = Me, R = I -Pr, Ph, Cy, Bz), have been prepared from the metal pentachlorides and anhydrous dithiocarbamate salts (719). They are fluxional on the NMR time scale, a polytopal mechanism being proposed via a cubic or square antiprismatic transition state. The kinetics of this process has been studied by total line shape analysis, giving AG values of 42-50 kJ moP. ... 

Addition of thiuram disulfides to the niobium(IV) dimethyl complex [Cp2NbMe2] results in cleavage of the sulfur-sulfur bond and addition to the metal center (734). The nature of the final product is dependent on the alkyl substituents. With tetraethylthiuram disulfide, the molecular niobium(V) dithiocarbamate complex [Cp2NbMe2(ri -S2CNEt2)2] results, which is believed to contain a monodentate dithiocarbamate ligand. In contrast, tetramethylthiuram disulfide affords a 1 1 electrolyte, believed to be [Cp2NbMe(S2CN-... 

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