Dithiolenes substitution

A number of structures with S-rich dianion ligands have been determined (297-299).831 832 For example, (297) can be synthesized by the reaction of [Ni(CN)4]2 with polysulfide.833 Upon further reaction with CS2 or substituted acetylenes it forms perthiocarbonato and dithiolene complexes, respectively. 

Dithiolene complexes [ Pd(PPh3)2 ra S2C=CCHC(0)R ] are mononuclear or homodinuclear derivatives of ferrocenyl-substituted dithiolene ligands.530 2,2-Diacetyl-1,1-ethylenedithiolato complexes of palladium can be used to prepare dinuclear PdAg and PdAu complexes 531... 

The structures and redox properties of these complexes have been extensively reviewed 170,171 of interest here is the presence of an intense NIR transition in the neutral and mono-anionic forms, but not the dianionic forms, i.e., the complexes are polyelectrochromic. The positions of the NIR absorptions are highly sensitive to the substituents on the dithiolene ligands. A large number of substituted dithiolene ligands has been prepared and used to prepare complexes of Ni, Pd, and Pt which show comparable electrochromic properties with absorption maxima at wavelengths up to ca. 1,400 nm and extinction coefficients up to ca. 40,000 dm3 mol-1 cm-1 (see refs. 170,171 for an extensive listing). 

Over the past decade a number of new covalently bonded TTF/ferrocene adducts have been reported [77, 78]. The crystal structure of the l,l -bis(l,3-dithiole-2-ylidine)-substituted ferrocene derivative has been published [77]. In this complex, ferrocene has essentially been incorporated as a molecular spacer between the two l,3-dithole-2-ylidene rings forming a stretched TTF molecule. This adduct, and its methyl-substituted derivative, have been combined with TCNQ to form charge-transfer complexes with room temperature powder conductivities of 0.2 S cm-1. Similar diferrocenyl complexes have been prepared with bis (dithiolene) metal complexes [79, 80]. 

Niobium and tantalum dialkylamides can easily undergo substitution or insertion reactions, as well as C—H activation. Substitution reactions have been used to obtain dialkylamides262 or dithiolenes.275 Aminolysis reactions, for instance, were assumed to occur via a simple associative mechanism (equation 6).276... 

Transition metal complexes of unsaturated 1,2-dithiolates (metal dithiolenes) have attracted much attention because of their interesting structural and redox properties.169 Molybdenum dithiolene complexes have featured prominently170 in these studies and have special significance following the suggestion171,172 that the molybdenum-containing cofactor of the oxomolybdoen-zymes (Section 36.6.7) incorporates a molybdenum complex of an unsymmetrically substituted alkene-1,2-dithiolate. 

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. 

In the synthesis of the parent dithiolenes, the reaction of the disodium salt of ethylene-cts-dithiolate with metal salts is the only useful method.15 For the preparation of substituted dithiolenes the use of substituted ethylenedithiolates is the most efficient choice. In the synthesis of 1,2-dicyanoethylene-l,2-dithiolato ( mnt ) complexes, it is the only available method.16... 

For example, the julolidine-substituted dithiolene (9) can be obtained18 in low yield by standard procedures the synthesis of the red, crystalline dithione and its conversion to (9) under reducing conditions proceeds in high yield.24 Dithiones are only stable if they are substituted by strongly electron-donating or -withdrawing substituents, and their use is limited to these cases. 

While the majority of dithiolene complexes are symmetric for reasons of synthetic simplicity (16 R = R2 = R3 = R4), there exist two groups of unsymmetrically substituted dithiolenes. In the first of these, exemplified by compounds (17) and (18), the ligand itself is unsymmetrically substituted, while the second group, shown in structure (19), contains two different ligands. 

Dithiolenes with unsymmetrically substituted ligands are not uncommon, because such ligands can be obtained by the standard routes outlined above. In principle, they can exist either in a cis... 

Unsymmetrically substituted tris-dithiolenes appear not to have been investigated in depth, although the earliest known39 tris-dithiolene, Mo(tdt)3, falls into this category. In principle, interesting steric effects should manifest themselves in this class of dithiolene. 

Several substituted cyclopentadienylcobalt dithiolenes were obtained57 by a rather surprising route from CpCo carbyne clusters and elemental sulfur (Scheme 3). An astonishing solid state phenomenon was discovered58 in the related CpCo benzenedithiolate a dimerization reaction was observed at room temperature, which could be reversed at 150°C (Scheme 4, see also Section 16.5.3.1). 

Scheme 6 Frontier MOs of donor- and acceptor-substituted dithiolenes...

A mostly qualitative MO approach was used by Alvarez, Vicente and Hoffmann140 to interpret the regularities and irregularities in the dimerization and stacking of transition metal dithiolenes. While firm predictions could not be made, the balance between sulfur lone-pair repulsion, size and occupation of the metal d-orbitals and substitutent steric demand were pointed out. The coordination preference of ethylenetetrathiolate was determined to be of the 1,2-dithiolene type rather than the 1,1-dithiolene. This result was suggested to apply both to binuclear and polymeric species. 

The bis(julolidinylethylenedithiolato) complexes (9) of Ni and Pt absorb in the 1.3 jan region and are thus suited to Q-switch the technically important iodine laser. This laser is one candidate of several thought to be able to initiate nuclear fusion reactions. At present, the julolidine-substituted Ni dithiolene is the best available Q-switch for the iodine laser. In appropriate solvents, the dialkylamino-substituted complexes can be shifted into the 1.3 fun region and also be used for this purpose.202,203... 

A wide range of metal dithiolene complexes have been prepared and their electrical conduction properties reported.111-114 They include neutral, monoanion and dianion complexes with a variety of substituents on the ligand (R = Ph, Me, CN, H, CF3) and a variety of cations. The choice of cation has often been determined by the desire to obtain easily crystallized products and has resulted in the use of rather bulky substituted ammonium salts. The compounds behave as semiconductors with relatively low conductivities at room temperature. It has been shown that the monoanion complexes are considerably more conducting than either the corresponding neutral complex or the dianion, and Rosseinsky and Malpas have proposed that this is related to the ease of disproportionation.113... 

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