Schiff’s base complexes

The use of a Schiff s base complex as a chelating ligand has been extensively studied for transition metals (31). An analogous complex has been produced for sodium perchlorate with two molecules of Cu(salen) (where salen = N,N -ethylenebis(salicylidene-iminato)). Each sodium, Fig. 4, is surrounded octahedrally by two oxygens from a perchlorate ion and four from the two chelating Cu(salen) ligands (32). 

Catalysis by cobalt porphyrins and Schiff s base complexes 387... 

Aluminium alkoxides (especially aluminium isopropoxide), dialkylalumi-nium alkoxides, yttrium alkoxides, zinc alkoxides, aluminoxanes, zincoxanes, bimetallic -oxoalkoxides, aluminium porphyrins and aluminium Schiff s base complexes are the most representative coordination catalysts, containing multi-nuclear or mononuclear species, for lactone polymerisations (Table 9.5). 

Several ruthenium complexes bearing chiral Schiff s base ligands have been published. RuL(PPh3)(H20)2], complex C (Fig. 11), with PhIO produced (S)-styrene oxide in 80% ee [61]. Chiral Schiff s base complex D was examined using molecular oxygen with aldehyde, with or without 2,6-dichloropyridine N-oxide as an axial ligand. Styrene oxide was produced in up to 24% ee[62]. A chiral bis(oxazolinyl)pyridine ruthenium complex E with iodosylbenzene diacetate PhI(OAc)2 produced (lS,2S)-fra s-stilbene oxide in 74% ee [63]. Similarly, chiral ruthenium bis(bipyridine) sulfoxide complex F [64] was effective in combination with PhI(OAc)2 as an oxidant and resulted in in 33% ee for (R,R) trans-stilbene oxide and 94% ee for (R,R) trans-/i-Me-styrene (after 75 h at 25 °C). 

Ruthenium compounds are widely used as catalysts for hydrogen-transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. For example, Backvall and coworkers [85] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt Schiff s base complex. The proposed mechanism is shown in Fig. 14. A low-valent ruthenium complex reacts with the alcohol to afford the aldehyde or ketone product and a ruthenium dihydride. The latter undergoes hydrogen transfer to the benzoquinone to give hydroquinone with concomitant... 

In this context it is worth noting that neither the titanium(IV) tartrate catalyst nor other metal catalyst-alkyl hydroperoxide reagents are effective for the asymmetric epoxidation of unfunctionalized olefins. The only system that affords high enantioselectivities with unfunctionalized olefins is the manganese(III) chiral Schiff s base complex/NaOCl combination developed by Jacobsen [42]. There is still a definite need, therefore, for the development of an efficient chiral catalyst for asymmetric epoxidation of unfunctionalized olefins with alkyl hydroperoxides or hydrogen peroxide. 

Schiff s base complexes of vanadium were encapsulated in the supercages of zeolite NaY and their catalytic activities were tested in the epoxidation of several alkenes and allylic alcohols with /er/-butylhydroperoxide The complexes investigated were VO(HPS) (vanadyl-N-(2-hydroxyphenyl)salicylideneimine) and VO(salen) (vanadyl-N,N (bis)salicylidene-imine). Particular attention was devoted to the question of leaching of vanadium during reaction. 

It is notable that this order parallels trans influences in a similar manner to J( Pt- N). Motschi and Pregosin (11a) have previously noted a strong correlation between 6( N) and J( Pt- N) for a range of Pt(II) Schiff s base complexes. 

Similarly, the CrAPO-5- and chromium silicalite-1 (CrS-l)-catalyzed oxidation of aromatic side-chains with TBHP or O2 as the primary oxidant [27-31] almost certainly arises as a result of soluble chromium(VI) leached from the catalyst. The same probably applies to benzylic oxidations with TBHP catalyzed by chromium-pillared montmorillonite [32]. More recently, a chromium Schiff s base complex tethered to the mesoporous silica, MCM-41, was claimed [33] to be an active and stable catalyst for the autoxidation of alkylaromatic side-chains. It would seem unlikely, however, that Schiff s base ligands can survive autoxidation conditions. Indeed, on the basis of our experience with chromium-substituted molecular sieves we consider it unlikely that a heterogeneous chromium catalyst can be developed that is both active and stable to leaching under normal oxidizing conditions with O2 or RO2H in the liquid phase. Similarly, vanadium-substituted molecular sieves are also unstable towards leaching under oxidizing conditions in the liquid phase [6,34]. 

Alkyl halides would not react with [CrCl2(MeCN)2], but the alkyl derivatives [RCr(salen)(H20)] and [RCr(salphen)(H20)] (R = Me, Ph) are said to form on reaction of the organic hydrazines with the Cri -Schiff s base complexes in MeCN under nitrogen followed by oxidation with oxygen and hydrolysis. ... 

In all the Schiff s base complexes described above, no base was used to deprotcMiate the Schiff s base ligand. Therefore, the Schiff s base binds in a neutral, albeit an zwitterionic form to the central metal ion, resulting in complexes with stoichiometry [R(LH)3X3]. It was also mentioned that in some complexes, mily one of the ligands looses spontaneously a proton, giving complexes of stoichiometry [R(LH)2LX2]. With dimeric salen-type ligands (sa/c/i=2,2 -iVW -bis(salicylidene)ethylenediamine) is was possible to form... 

All the Schiff s base complexes discussed so far are low-molecular weight complexes. An erbium(lll)-containing methacrylate metallopolymer was prepared by free-radical polymerization of a Schiff s base monomer LllH, followed by complex formation with hydrated erbium nitrate (Haase et al., 1996). By XRD, the presence of a smectic mesophase was shown. A special feature of the polymer is that the complexing group is an Ai-aryl substituted Schiff s base, rather than an Ai-alkyl substituted one. No further structural data were reported. A detailed study of the mesophase behavior of lanthanide complexes of Ai-aryl substituted Schiff s base ligands LllH was reported by Rao et al. (2002, 2010). The complexes were synthesized by reaction between the... 

TABLE 23 Magnetic Properties of Mesogenic Schiff s Base Complexes — Cont d ... 

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