Dithiolenes alkylation

Substituted dithioformate anions as ligands R C(S)S are usually called dithiocarbamates (R = R2N), alkyl and aryl dithiocarbonates or xanthates (R = RO), alkyl and aryl trithiocar-bonates or thioxanthates (R = RS). Dithioacid anions (R = alkyl, aryl) have been rarely used as ligands of nickel(II) because of their instability. Structural properties of selected nickel(II) complexes with substituted dithioformate, dithiolene and related ligands are shown in Table 90. 

A square planar bis-chelate complex with dithiotropolonate monoanion has been isolated in the solid state (295).2142 This complex does not exhibit the electrochemical properties of the bis(l,2-dithiolene) complexes and undergoes ring alkylation and oxidation reactions (Scheme... 

Multifunctional dithiolene ligands have been used to prepare binuclear as well as polymeric complexes. Some examples of this type of complex were already mentioned above (25 and 41) and involve the ligands benzenetetrathiol (51) or its alkyl ethers,78 the tetrathiosquaric acid (52), ethylenetetrathiol (53) and tetrathiooxalic acid (54) and its Se analog. The two-electron difference in oxidation state between (53) and (54) is much the same as that discussed for the parent dithiolene ligand. Crystallographic results (see Section 16.5.3.1) point out the equivalence of the latter two ligands. 

Reduction of dithiolenes to their dianions increases the nucleophilicity of the S atoms and allows reactions with alkyl halides to occur. Schrauzer et al.1 1 described the formation of neutral complexes (79) with one dithioether and one dithiolato ligand when the tetraphenylnickeldithiolene dianion was reacted with methyl iodide or other alkyl halides. 

When tetrahydropyridopyrimidine-3-carboxylate 562 (R = COOEt, R1 = H) was treated with carbon disulfide in the presence of potassium hydroxide at 25-30°C, salt 569 was obtained in good yield (79NEP79/ 3401 82USP4321377). The alkylation of569 with dimethyl sulfate and with ethylene dibromide in ethanol afforded 9-dithioester 570 and 9-(dithiolen-2-ylidene) derivative 571, respectively. When 569 was heated in acetic anhydride for 2 hours, bis product 572 was obtained in 57% yield. 9-Imidazolidine derivative 573 was prepared from both iminium chloride 563 (R = COOEt, R1 = H, R2 = Me) and 9-dithioester 570 by treatment with ethylenediamine (Scheme 37) (79NEP79/3401 82USP4321377 83MIP1). 

Figure 5-85. The alkylation and demetallation of a nickel dithiolene complex with methyl iodide.

The mildly electrophilic complex MeRe03 condenses with 2 equiv of benzenedithiol to give MeRe(0)(S2C6H4)2 (28), a rare alkyl metal dithiolene. 

Whereas complexes of ethylenedithiolate H2C2S212 are typically prepared by the reductive S-dealkylation of c -H2C2(SCH2Ph)2 (Section II.B), a viable alternative route involves base hydrolysis of l,3-dithiol-2-one, H2C2S2(CO).The parent H2C2S2CO can in turn be prepared on a multigram scale from chloroacetal-dehyde (82). This l,3-dithiol-2-one can be functionalized via deprotonation followed by C-alkylation (72), thus opening the way to a variety of functional dithiolenes (Eq. 7). 

A versatile route to RS-substituted dithiolenes entails S-alkylation of the trithiocarbonate dmit2 (see Section II.C.l), which provides an efficient means to introduction of diverse functionality to the dithiolene backbone. Subsequent to S-alkylation, the resulting S=CS2C2(SR)2 is converted to the dithiocarbonate 0=CS2C2(SR)2 with Hg(OAc)2 in acetic acid (63, 83, 84). Such dithiocarbo-nates are more easily hydrolyzed than the trithiocarbonates (72, 85). This approach has been used for the synthesis of Ni[S2C2(S(CH2) Me)2]2 (n = 2-11) (86) and related complexes with pendant alkene substituents (Eq. 8) (87). 

Complexes of alkyl-linked o-benzenedithiolates eliminate ethylene to give benzenedithiolato complexes (Eq. 27) (275-277). This reaction is the reverse of the known addition of alkenes to bis(dithiolene) complexes (278-281). Photolysis of the S-benzylated dithiolenes M(PhCH2S2C2Ph2)2 (M = Ni, Pd, Pt) results in S-dealkylation with elimination of benzyl radicals (282). In general, however, the properties of S-alkylated dithiolenes have not been... 

Werner reported the formation of a CpCo(dithiolene) complexes via a multistep process that begins with C-alkylation of CpCo(PMe2Ph)(CNMe) followed by reaction with CS2 and further sulfidation with elemental sulfur. The product is structurally related to CpCo(dmit) (295). 

Alkylation reaction is a common theme for both bis- and tris(dithiolene) complexes. For example, reaction of the dianion [W(sdt)3]2- with Mel forms [W(sdt)2(Me2S2C2Ph2)J (Eq. 13) and reaction of [Re(sdt)3] with Mel gives a mono-methylated product [Re(sdt)2(MeS2C2Ph2)] (Eq. 14) (109). 

Sellmann et al. (110) reported that the neutral tris(dithiolene) complex [W(bdt)3] undergoes nucleophilic alkylation by carbanions via intramolecular electron transfer (Scheme 13). Reaction of [W(bdt)3] with LiMe, depending on the concentration of the latter, yields either [W(bdt)2(MeSC6H4S)] by alkylation (at high concentrations of LiMe) or W(bdt)3 by reduction (at low... 

Metal-nitrido complexes containing dithiolene ligands have been reported recently. Reaction of electrophiles such as R3OBF4 (R = Me, Et) and Ph3CPF6 with [M(N)(bdt)2] (M = Ru, Os) has been studied by Sellmann et al. (123) While the Ru-nitrido complex gives an intractable mixture of products, the Os-nitrido complex yields clean products (Eqs. 20-21). Alkylation at sulfur is... 

Similar to homoleptic dithiolene complexes, alkylation of Cp2M(S2C2Z2) results in the alkyl group being added to the ligand sulfur atom (Eq. 28) (131). 

Eisenberg and co-workers (135-137) investigated dithiolene-containing organometallic complexes of Rh and Ir. Addition of an alkyl halide to [Rh(CO)-(PPh3)(mnt) forms a Rh(III) halo acyl complex. The halide can be stripped off by Ag+ salts (135) (Eqs. 30 and 31). 

Migration of the alkyl group between the acyl carbon atom and a sulfur donor atom has been observed (136). The reaction was proposed to involve successive 1,2-migrations with a Rh(III) alkyl dithiolene intermediate (Scheme 17). 

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