Bis-catechols

Scheme 6.29 Hiyama couplings of of bis(catechol) silicates with aryl bromides.

When four Fe + ions are assembled with the aid of six bis-catecholate ligands into a tetrahedral assembly then the rigidity of the resultant species is such that it is very reluctant to racemize. Extrapolation from data on the Ga " " analogue suggests a half-life of months. 

Gu, Y., Lee, H., and Hudson, R.A., Bis-catechol-substituted redox-reactive analogs of hexam-ethonium and decamethonium stimulated affinity-dependent reactivity through iron peroxide catalysis, /. Med. Chem., 37, 4417, 1994. 

K3[V(catecholate)3]-1.5H20 has been synthesized from VC13 and catechol in ammonia solution, on adding a solution of KOH. The vanadium is surrounded by six oxygen atoms of catecholate ions the average V—O bond length is 2.013(9) A.256 A tris(o-semiquinone)vanadium(III) complex257 from V(CO)6 and quinone was better formulated as bis(catecholate)(o-semiquinone)vanadium(V).256... 

Evans and coworkers62 described the synthesis and structural study of a series of pentacoordinate bis(catecholate) complexes of silicon(IV). The crystal structure of the anion of [Et3NH][SiMe(3,5-dncat)2] [H2(3, 5-dncat) = 3, 5-dinitrocatechol] is reported (Figure 7). 

Intermolecular ligand exchange has been reported in bis-catecholates as well as in bis-glycolate complexes (equations 1829 and 1949, respectively). This was observed after 24 h by the appearance of NMR signals for the mixed complex (76 and 77 respectively) in the 14, 13 C and 29 Si spectra, as well as by FAB MS experiments. A statistical distribution of the three compounds was evident from the 1 2 1 triplet found in the 29Si spectrum. 

Fig. 12 Self-recognition and formation of triple-stranded helicates 13, 4, and 14 from bis-catechol ligands 11, 3, and 12 and iron(III) or gallium(III) ions, respectively [120]...

A series of tetrahedral assemblies built from four metal ions (e.g., Ga3+, Al3+, In3+, Fe3+, Ti4+, Ge4+) and six bis-catechol ligands has been prepared by Raymond s group (Fig. 4a). These chiral hosts possess a small hydrophobic cavity... 

Figure 9 Possible protonation schemes of tris(catecholate) metal complexes. In path 1, the metal complex undergoes a series of two overlapping one-proton steps to generate a mixed salicylate-catecholate coordination. Further protonation results in the precipitation of a tris(salicylate) complex (e.g. enterobactin, MECAM). This differs from path 2, in which a single two-proton step dissociates one arm of the ligand to form a bis(catecholate) chelate. Path 3 incorporates features of paths 1 and 2. In this model, the metal again imdergoes a series of two overlapping one-proton reactions. However, unlike the case of path 1, the second proton displaces a catecholate arm, which results in a bis(catecholate) metal complex...

Other studies involving the self-assembly of tri-stranded, non-chiral complexes composed of three bis(catecholate) ligands wrapped around two titanium(IV) ions have been reported. The self-assembly of gallium(III) catecholamide triple helices has also been investigatedalong with the assembly and racemisation of helicates incorporating linked catechol derivatives and either titanium(IV) or... 

NMR showed only one diastereomer to be formed. The bridge was also used as a stereogenic centre by Stack [58] with a bis-catechol ligand 29 ... 

Albrecht and coworkers used alkyl-bridged bis(catecholate) ligands H4[8-10] and showed that the relative configuration of the two Ti(IV) centres in [M]3[McTi2(8)3]... 

Raymond et al. described the self-assembly of a series of chiral M4L6 coordination tetrahedra based on the bis-catecholate ligands 23, 24 and 25 when combined with trivalent metal ions such as Al(III), Ga(III), In(III), Fe(III) and/or tetravalent ions Ti(IV), Sn(IV), For instance, in the tetrahedral assemblies [Ga4(23)g], ... 

However, perhaps the most spectacular example of kinetic locking was reported by Raymond et al. Using highly labile gallium(III) metal ion (M) to template the self-assembly of inflexible C2 symmetric bis-catecholate... 

---