S-Bidentates

By following an almost identical procedure, the enantiomers of the corresponding 2-methoxymethylaryl compound 96b were obtained (Table The enantiomers of 96b were used for the metal-template synthesis of optically active trans-As2S2 macrocycles (see below). 

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The basic thiosemicarbazone ligands coordinate in a N,S bidentate manner, generally via the azomethine N-donor and the thione or deprotonated amidothione group. Designating the ligands as HL, this usually results in square planar complexes [NiL2],1221-1227 [Ni(HL)2]X2,12 2,1228... 

B(3,5-(CF3)2C6H3)4-.512 Palladium complexes with a hemilabile terdentate carbene ligand, 1,3-bis(pyl)imidazol-2-ylidene, were active toward the catalytic polymerization of CO/norbornylene.513 Palladium complexes of cz s-bidentate C4-bridged diphosphines cis- and trans- 1,2-bis [(diphenylphosphino)methyl]cyclohexane, e fl o,e fl o-2,3-bis[(diphenylphosphino)methyl] norbornane,... 

A class of ligands which is very flexible in terms of its denticity, donor atom geometry, and steric demands are Schiff bases derived from salicyladehyde or related carbonyl compounds carrying additional hydroxo groups. The use of bi- and tridentate ligands of this type allows the synthesis of mixed-chelate complexes with a number of O N, N N or N S bidentate ligands. The products are of considerable interest for nuclear medical applications as well as for homo-... 

The / -mercapto thioesters (69 R = Et, Pr1) and the o-mercaptobenzoic acid thioesters (68 R = Et, Pr1, Pe ) form complexes with Cu1, Ni11 and Co111.255,258 Since several (b) class and borderline metals form more stable complexes with the S-esters (68) than with the O-esters (67), it is likely that the S-esters behave as S,S-bidentates. 

Nag et al. have investigated the coordination chemistry of ligands related to or derived from 2-aminocyclopentene-l-dithiocarboxylic acid, concluding N—S bidentate behaviour for the complexes in general.103 With MoVI, however, this ligand and iV-alkyl derivatives are observed to function rather as S—S bidentates.104... 

Several complexes of bivalent cobalt and zinc with imidazoline-(l//,3i/)-2-thione (34 R = H) of the forms ML2X2 (X = halide ion) and ML4X2 (X = N03 or C104) have been isolated and characterized by physicochemical techniques.247 In contrast with the S monodentate behaviour concluded for the earlier-reported Ni11 complexes, the ligands are N—S bidentate in these metal salts to form four-membered chelate rings. Similar coordination behaviour has been observed for the... 

Thiosemicarbazide (tscH) (40a) exists in the two tautomeric forms shown in (41). It may coordinate either as a neutral or (by loss of a proton) anionic N—S bidentate. The nickel(II) complexes in particular of thiosemicarbazide and related ligands (40) have been studied in considerable detail. 

A,TV-Dimethyl-A -phenylthiourea has been shown to coordinate to Rh111 as an N—S bidentate involving four-membered chelate ring formation.154 N-Substituted thioamides also may bond in this manner.155 156 l-Amidino-2-thioureas (44) may behave either as N—S or as N—N bidentates, with this donor choice being dependent mainly on pH and the nature of the metal ion.157 As N—S donors they are known to stabilize lower oxidation states.158 As part of a study on Mo—S-containing complexes as models for redox-active molybdoenzymes, Dilworth et al. have shown that some p-(substituted)phenylhydrazines may coordinate as N—S bidentates in three different ways to one metal atom.159 The three diazenido, diazene and hydrazonido forms vary in their degree of deprotonation and therefore their anionic nature. 

Scheme 3. Pd-catalyzed asymmetric cycloisomerization with c/s-bidentate ligands.

The double-stranded helicate 10, reported by Constable and coworkers [134, 135], was characterized both in the solid state and in solution. In this D2 symmetric structure, two fr/ s-bidentate sexipyridine ligands 9 wrap around two octahedral cadmium(II) ions forming the dinuclear double-stranded helicate 10 (Fig. 11). 

A few interesting Mn111 compounds of O—S bidentates have been prepared. The interaction of Mn111 acetate with (183) (R = Ph) leads to ligand oxidation, but when R = Bu the tris-ligand Mn111 compound was obtained.665... 

Newer agents under investigation include tris(tropolonato) Bi(III) and nitratobis(tropolonato)Bi(in) a host of bis (mercaptoethanol)Bi(III) complexes, which are formed with 0,S-bidentate chelation, with and without deprotonation of the hydroxyl oxygen and usually having an axial fifth ligand and sialic acid derivatives of Bi(III). The interactions of Bi(III) and its compounds with metallothionein, serum albumin and transferrin have been studied by a variety of groups, as have a number of enzymes that may be related to the mechanism(s) of action of bismuth in treating ulcers. 

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