1,2-Diolato ligands

Three doubly deprotonated xylitol molecules act as 1,2-diolato ligands in the hexacoordinate silicate [Si(Xylt2,3H 2)3] 27 (O Fig. 6) which is the hydroxyunethyl derivative of the thre-itolato complex 19 (O Fig. 4) and, as a consequence, shows the same stabilizing intramolecular hydrogen bonds [33]. 

Figure 3. EPR spectroscopic signatures of Cr(V) complexes with various model 1,2-diolato ligands (a) open-chain diols [16,50,63] (b,c) cis- and trons-1,2-cyclohexanediols [16,50] and (d) cis-l,2-cyclopentanediol [61]. A general structure for the complexes is [Cr OLi], and structures of the ligands arc shown in the insets. The complexes were generated in neutral (pH 7) aqueous solutirms by the reactions of [CrOJ " (0.50-250 mM) with glutathione (0.50-12.5 mM, used as a reduetant) and a large excess of the diol (0.10-5.0 M) at 295 K (reaction time 5-30 min) [16,50,61,63].

Figure 5. Typical EPR spectra of Cr(V) complexes with biological 1,2-diolato ligands (a) D-glucose [50] and (b) iV-acetyIneuraminic (sialic) acid in the absence (black line) or presence (red line) of Ca(Il) [63,70]. A general structure of the complexes is [Cr OLi], and structures of the protonated ligands are shown in the insets. Reaction conditions for (a) [Cr(VI)] = 0.25 M [glutathione] = 12.5 mM [ligand] = 0.50 M pH = 8.10 (adjusted with NaOH) reaction time 74 min at 292 K [50]. Reaction conditions for (b) [Cr(Vl)] = 40 mM [glutathione] = 2.0 mM [Ugand] = 0.10 M [Ca(ll)] = 1.0 M pH = 7.14 (adjusted with NaOH), reaction time 3-5 min at 295 K [63,70].

Polyalcohols do not generally interact noticeably with beryllium, probably on account of the low acidity of the alkyl-OH group. Two exceptions are known where solid compounds have been isolated. The complex anion [bis(meso-oxolane-3,4-diolato)Be]2 has been shown to contain two anionic diolato ligands (258). 

Osmium(vi).— [0s04] has been shown to react with monoalkenes (R) to give five-co-ordinate mono- and di-esters of the saturated diolato ligand (02R)2-, namely [0s204(02R)2] and [OsO(02R)2].104 These complexes were assigned structures (21) and (22). respectively, on the basis of i.r. and molecular weight measurements. 

Erythritol 1 is a tetraol with a central eryf/rro-conflgured diol group. In combination with the terminal hydroxyl functions there are several conformations to act as a chelate ligand. In most of its characterized solid-state stmctures, erythritol is coordinated to two central atoms by acting as a bis-l,2-diolato ligand, at which the deprotonation of the hydroxyl groups is a consequence of the synthesis from alkaline solutions and/or complexation of more or less Lewis acidic central atoms. A complex of this type is obtained upon the reaction of erythritol with two equivalents of [(NH3)2Pd(OH)2] in aqueous solution which leads to the centrosymmetric structure of 9 (O Fig. 1) [18],... 

More examples for the higher reactivity of copper centers, in relation to the preference of diolato ligands to substitute nitrogen ligands, are given in structures 154 and 155 [173], shown in O Fig. 32. 

Figure 4. Orbital overlap between a Cr(V) ion and cyclically strained diolato ligands oriented in a cis- or rrani-fashion. Adapted with permission from [25], Copyright 2001, Elsevier.

D. Compounds Containing Two Diolato(2—) Ligands Derived from... 

A. Compounds Containing Two Bidentate Ligands of the Benzene-1,2-diolato(2 ) or Naphthalene-2,3-diolato(2-) Type... 

In the zwitterions 3 and 27-38, two bidentate symmetric diolato(2-) ligands are bound to the silicon atom, whereas the derivatives 40-43 contain two unsymmetrically substituted diolato(2-) ligands. Because of this un-symmetric substitution pattern, two sets of resonance signals were observed... 

Some of the zwitterionic A557-silicates described in this chapter were studied for their chemical properties. As shown for 32,44, and 45 in Scheme 7, compounds of this particular formula type undergo an intermolecular exchange of their benzene-1,2-diolato(2-) ligands in solution at room temperature.38 Solution-state NMR studies ([D6]DMSO XH, 13C, 29Si) and FD MS experiments provided evidence for the equilibrium 32 + 44 2 45. 

The spirocyclic zwitterionic A5S7-silicates 4740,41 and 4842 contain two cis-l,2-diphenylethene-l,2-diolato(2-) ligands. This particular type of ligand formally derives from the diol c/5 -HO(Ph)C = C(Ph)OH, a tautomer of benzoin. 

The pentacoordinate silicon compounds 50,43 51,43 and 529 are spirocyclic zwitterionic A557-silicates with an Si04C skeleton. They contain two ethane-l,2-diolato(2-) ligands or two meso-oxolane-3,4-diolato(2-) ligands. 

The pentacoordinate silicon compounds 81, 8,54 82,54 83,54 and 8455 are spirocyclic zwitterionic A5S7-silicates with an Si04C skeleton. The chiral zwitterions contain two diolato(2—) ligands that formally derive from aceto-hydroximic acid and benzohydroximic acid (tautomers of acetohydroxamic acid and benzohydroxamic acid). 

The pentacoordinate silicon compounds 94,23 95,23 96-98,60 99,60,61100,61,62 101-103,60 104,61,62 105,62 106,62 and 10761,63 are monocyclic zwitterionic A5S7-silicates with an Si02FC2 skeleton. The chiral zwitterions each contain one bidentate diolato(2-) ligand that formally derives from 1,2-dihydroxy-benzene, salicylic acid, glycolic acid, oxalic acid, benzohydroximic acid (tautomer of benzohydroxamic acid), 2-methyllactic acid, or (S)-mande-lic acid. 

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