Salophen ligand

The majority of the devices mentioned thus far rely on the Hofmeister series for anion selectivity. However, for anions that deviate from this series, organometallic receptors can be utilised. The type of ligand or metal centre will influence the sensor selectivity due to the characteristics of the electron acceptance of the complex. An interesting development that is being explored here is the use of calixarenes. These have previously found use as cation-selective species, but with suitable substitution are now being incorporated within anion-selective devices. Compounds suitable as receptors for halides [61],benzoate [61] and acetate [62] have been developed. Reinhoudt and his co-workers have reported the production of a POj-selective CHEMFET based on a uranyl cation immobilised within a salophene ligand (Fig. 5), which shows selectivity over more lipophilic anions such as Br" and NOj [63]. 

Reduction reactions that involve titanium(lll) species are common in preparative organic chemistryl 9 but relatively rare in coordination chemistry. However, Floriani and coworkers " have described the reduction of vanadium complexes, [VO(salophen)] and [VO(acacen)], by the titanium(IIl) solvento-complex, [TiCl3(THF)3]. Reaction of these vanadyl complexes in THF results in formation of [VCl(salophen)(THF)] and [VCl(acacen)(THF)]. Both compounds have been structurally characterized and an X-ray diffraction study of [VCl(salophen)(THF)] indicated pseudooctahedral coordination for V(Ill). Further, the CP and THF ligands were found to be trans to each other in the axial positions, while the equatorial plane was defined by the salophen ligand. Figure 9.7.1fl The proposed mechanism 2 for these conversions is shown in Scheme 9.7. 

The planarity of the salophen ligand and the six-coordinate nature of [Co(N02)(salophen)py] suggest that oxidation takes place without prior coordination of PPha to the cobalt center. 52... 

The two oxygen-activating complexes [Co(L)j [L = salophen, tetra-tert-butylsalo-phen (55)] have been prepared and were also synthesized within dehydrated zeolite NaY using the intrazeolite ligand synthesis method [164]. These encapsulated metal complexes were shown to be capable of oxidizing hydroquinone and so were then used in a triple catalytic system to mediate the palladium-catalyzed aerobic 1,4-diacetoxylation of 1,3-dienes (Figure 5.28) [165]. The catalytic system involved [Pd(OAc)2], hydroquinone and the [Co(salophen)] complex in acetic acid (Co Pd diene hydroquinone LiOAc = 1 2.23 50 8.3 690, acetic acid, 25 °C,... 

The mixed behaviour of such catalysts, in terms of oxo-type and allylic oxidation, was also confirmed in the oxidation of a-pinene, yielding a mixture of the epoxide and the allylic oxidation product (D-verbenone). The epoxide stems from the existence of a high valent Ru(V)=0 intermediate, while D-verbenone formation points to the presence of a radical chain involving peroxoruthenium as intermediated128,1291 The activity of encapsulated H, Cl, Br, nitro-substituted Ru on and Co(salophen) (structure of ligand see insert also known as saloph) is always at least a factor of two higher than in solution. Comparable Co/Si ratios are obtained from XPS and TGA, indicated no significant amounts of complex at the external surface. 

Analogous complexes of cobalt(III) containing a tetradentate ligand (L4) have furnished strictly parallel results. Accordingly, the insertion was observed to occur either in neat SO2 or in DMF solution with RCo(salen)H20 when R = Me 36, 82, 100) or Et (36, 100), but not when R = Ph or CsFg (36). The complexes MeCo(salophen)H20 (36) and MeCo(bae)H20 (82) also yield the corresponding 5-sulfinates on treatment with sulfur dioxide. 

Flexible ligand method. 0.9 g of 1,2-phenylenediamine were slowly added with stirring to 2 g of salicylaldehyde. After 10 minutes stirring, the mixture was allowed to cool down and the product was transferred to 40 mL of ethanol. The solids were filtered off, recrystallized from ethanol and dried in vacuo overnight. 1 g of CoNaY was mixed with 2 g salophen and heated in an open tube to 450 K with continuous stirring for 12 hours. The molten slurry was allowed to cool, and the zeolite was soxhiet extracted with methylene chloride. The solid material was dried in vacuo overnight. 

The basic approach to prepare Co(II)-complexes of salen (N,lSr-bis(salicylidene)ethylene-diamine)-type molecules is the flexible ligand method [9]. In this process the Schiffbase ligand can diffuse by twisting into the zeolite where it becomes too large to exit by complexation with the cobalt ion. The flexible ligand method, however, was not usefiil for the preparation of Co-salophen/ zeolite catalyst, because the product was inactive in the oxidation reactions. The salophen molecule does not seem to be flexible enough and can not get into the zeolite to produce the suitable complex in the supercage. 

We have also prepared the Co-salophen/zeolite catalyst, using the template synthesis and the flexible ligand method. The Co-salophen/zeolite catalyst prepared by the template synthesis method proved to be active in the oxidation of hydroquinone and in the aerobic oxidation of 1-octene and the acetoxylation of cyclohexene. The zeolite-encapsulated catalyst was active and produced the same selectivity and yield as the free complex. It was also possible to remove the catalyst and to reuse it in subsequent experiments. 

Ti2((OC6H4CHN)2C6H4)2)2(THF)2 underwent an additional C—C bond formation upon further reduction. This yielded the ionic complex Na2[Ti2(salophen )(THF)6] (96) where salophen is an octadentate, octaanionic ligand derived from the dimerization of two of the original ligands.927 This latter compound showed a residual temperature-independent paramagnetism. 

Screening of an impressive series of polymers derived from different bulky methacrylate esters, e.g., 42 (Chart 8), and using a variety of chiral ligands has revealed the scope of the process of forming helical poly(methacrylate ester)s and their applicability in, for example, the separation of chiral compounds.151 These polymers were prepared not only by anionic polymerization, but also by cationic, free-radical, and Ziegler—Natta techniques. Recently, Nakano and Okamoto reported the use of a co-balt(II)—salophen complex (43) in the polymerization of methacrylate ester 41.155 The free-radical polymerization in the presence of this optically active metal complex resulted in the formation of an almost completely isotactic polymer with an excess of one helical sense. 

V. Mirkhani, M. Moghadam, S. Tangestaninejad, B. Bahramian, Polystyrene-bound imidazole as a heterogeneous axial ligand for Mn(salophen)Cl and its use as biomimetic alkene epoxidation and alkane hydroxylation catalyst with sodium periodate, Appl. Catal. A 311 (2006) 43. 

A Co(salophen)/zeolite catalyst was prepared by the template synthesis method. This catalyst proved to be active in the ruthenium catalyzed oxidation of benzyl alcohol. The heteroge-nized Co(salophen), having the same amount of complex produced a higher rate in the oxidation reactions than the free complex. It can be explained by the sites isolation theory. In the case of the heterogenized catalyst it was not necessary to use an extra axial ligand such as triphenylphosphine. It was also found that in the case of Co(salophen)/zeolite catalyst the choice of the solvent was not so critical, as in the case of the free complex. 

The Co(salophen) complex encapsulated in zeolite was prepared by two different methods. The flexible ligand method involves the diffusion of the Schiff base ligand into the zeolite, where upon complexation with the Co ion becomes too large to exit. However, this method was not useful for the preparation of Co(salophen)/zeolite, because the catalyst was not active in the oxidation reaction. For this reason the synthesis, originally used for the preparation of the free Co(salophen) complexes was modified and the principles of template synthesis method was used to prepare the Co(salophen)/zeolite catalyst. 

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