Bis-catecholato complexes

The Ag complex 74 catalyses the diboration of internal and terminal aUcenes to 1,2-bis-diboronate esters by bis(catecholato)diboron, (Bcat), in THF at room temperature. Variable conversions (30-90%) were obtained at 5 mol% loading after 60 h. 

To realize milder reaction conditions, modification of the platinum catalyst system has been examined. A combined use of bis(catecholato)diboron with phosphine-free divalent platinum complex, PtCl2(cod), allows the diboration of alkynes to proceed at RT.42 The room-temperature diboration has also been achieved with a Pt(nbd)3-monophosphine (Pt/L= 1/1) catalyst.43... 

Diboration of alkene is catalyzed by Pt(0),42,48-51 Rh(i),52-57 Au(i),52 and Ag(i)58 complexes. Phosphine-free platinum complexes such as Pt(dba)2 and Pt(cod)2 are efficient catalysts for diboration of alkene, whereas those with phosphine ligands show much lower catalytic activities (Equations (3) and (4)).48,49 A PtCl2(cod) complex, which may be readily reduced to Pt(0) species with diboron, also catalyzes the addition of bis(catecholato)diboron to alkenes.42 Platinum-catalyzed diboration has so far been limited to terminal alkenes and strained cyclic alkenes. 

Although the [Ir(Ti -C5Me5)(NHC)] complex was initially tested in the diboration of styrene, this did not lead to the formation of any diboronate esters when bis(catecholato)diboron (B2cat2) was added to a solution of the catalyst and styrene in tetrahydrofuran (THF) under argon (Scheme 7.23). However, the addition of... 

A few ionic bis-catecholato N-coordinated complexes 177-181 have been reported192 205 207. Crystallographic studies of 177, 180 and 181 showed all three to have a near-octahedral geometry, with Si—O bond lengths very similar to those in the tris-catecholato complexes, and typical Si—N dative bond lengths of 2.157, 2.085 and 2.173 A, respectively. The 29Si chemical shifts for 177-181, which are in line with hexacoordination, are listed in Table 22. 

TABLE 22. 29Si chemical shifts for bis-catecholato silicon complexes with N—Si coordination (CD2G2)... 

Bis(carbodiimido) complexes, with bis-Cp Ti(IV), 4, 583 Bis(catecholato)diboron, alkyne additions, 10, 727—728 Biscorroles, in organometallic synthesis, 1, 71—72 Bis(cyclodiyne) clusters, trirutheniums and triosmiums, 6, 772 Bis(cyclooctadienyl) chromium complexes, characteristics,... 

Fig. 11. Possible protonation schemes of tris catecholate metal complexes. The compound shown is [M3+(MECAMS)]3-. In scheme 1 the metal complex undergoes a series of two overlapping one-proton steps to generate a mixed salicylate-catecholate coordination about the metal ion. Further protonation results in Ihe precipitation of a tris salicylate complex (e.g. enterobactin, MECAM). This differs from scheme 2, in which a single two-proton step dissociated one arm of the ligand to form a bis(catecholate) chelate (e.g. TRIMCAM). Scheme 3 incorporates features of scheme 1 and 2. In this model the metal again undergoes a series of two overlapping one-proton reactions. However, unlike the case of scheme 1, the second proton displaces a catecholate arm, which results in a bis-(catecholato) metal complex. This scheme is discussed for Ga and In complexes of enterobactin analogs in Ref. 141)...

The stabilities of Fe" complexes of hydroxamates are much lower than those of Fe " for the bis Fe" complex of acetohydroxamic acid = 3 x 10 ). It is thought, therefore, that the mechanism of release of iron from hydroxamate siderophores may occur via reduction of Fe " to the much more weakly bound Fe" state, as also proposed for the tris(catecholato) siderophores. Cyclic voltammetry has shown that the Fe" hydroxamate complexes undergo reversible or quasi-reversible one-electron reductions under suitable conditions. The observed formal potential E( is pH dependent, decreasing from —0.388 mV (ys. SCE) at pH 7 by 60 mV per pH unit, for the case of tris(acetohydroxamato)iron(III), consistent with progressive deprotonations of the coordinated ligand. Thus, deprotonation stabilizes the Fe " vs. the Fe state. ... 

Only a few examples were reported where cationic transition metal complexes contained chiral borate anions. Chiral borate anions were first reported in 1925 when Boeseken and Mijs made the bis(catecholato) anions 4.30 and 4.31 (Figure 4.13) which were configurationally labile which limited their applications.Later on, several borate anions 4.32 and 4.33 were prepared derived from the previous examples but most of these were configurationally labile. 

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