Salen-type ligands

The somewhat different bis(diboratetrasiloxane) derivatives 94 and 95 have been prepared from bimetallic salen B(OEt)2 2 derivatives and diphenylsi-lanediol [131, 138]. In this case the four boron atoms of two diboratetrasilox-ane rings 86 are chelated by salen-type ligands in order to produce cages of cylindrical geometry (Fig. 25). 

Table 18. Bond lengths R for equatorial and axial bonds in HS and LS Cobalt(II) complexes with salen and salen-type ligands...

Ligand substitution in salicylaldiminate [NiA2] complexes has been studied in detail.1161 Substitution in chiral [NiA2] species by chiral salen-type tetradentate H2B ligands in acetone proceeds according to Equation (7) ... 

For the preparation of CoSalophen Y the Co—Y was impregnated by salicy-laldehyde, and 1,2-phenylenediamine in methanol was added slowly to the mixture.107 This was a successful encapsulation of a salen-type complex with larger diamine than the ethylenediamine, a successful preparation of an encaged metal-salen complex by the intrazeolite ligand synthesis method, and a successful intrazeolite synthesis using two different precursor molecules. 

Loss of catalytic complex by dissolution from the support This can either occur to physically bound catalysts (physisorbed, entangled in a polymer, hydro-gen-bonded), when the reaction medium has too-good solvent properties. The catalyst complex can also be dissolved from ionically bound species by ion exchange with electrolytes in the reaction mixture, or when the covalent bond to the support is broken (e.g., by hydrolysis). In the case of SIB catalysts, a good solvent such as ethanol can displace a salen-type ligand from the metal. 

In 1996, Burgess et al. (34) reported one of the first examples of a formal attempt to use a parallel approach in the optimization of a catalytic reaction. Previously, Sulikowski reported the copper catalyzed C-H insertion of a diazoester. In an attempt to optimize the selectivity for this reaction, three different bis(oxa-zoline) ligands, a bis(salicylidine)ethylenediamine(salen)-type ligand and sparteine were screened in combination with seven different metals and four different solvents (Scheme 13). Ligand 116 in tetrahydrofuran (THF) solvent at slightly reduced temperature proved to be the best reaction conditions, giving a 3.9 1 product ratio and good yield. 

Roewer et al. have used salene-type ligands for the preparation of neutral penta- and hexacoordinated silane complexes.834-836 Interestingly, the reaction of the acid form of the salene ligand reacts with organotrichlorosilanes in the presence of diethylamine to form pentacoordinated enamine silane complexes 849-851, whereas the disodium salt of the salene ligand reacts with phenyltrichlorosilane providing the hexacoordinated chlorosilane complex 852 (Scheme 119).834 The pentacoordinated complex 849 was also obtained when the hexacoordinated complex 852 was reacted with triethylamine (Scheme 119). 

The solubility properties of salen-type complexes can be tailored by employing substituents such as t-butyl and triphenylphosphonium-methyl (—CFl2PPh3+), with its associated chloride, in the aromatic rings.The stability constant for A,A -o-phenylenebis(salicylideneimine)-iron(III) in 80% (w/w) DMSO-water has been compared with stability constants for complexes of six metal ions with this ligand. 

These catalysts are used in conjunction with a stoichiometric amount of an oxidant, and the active oxidant is believed to be an oxo Mn(V) species. The stoichiometric oxidants that have been used include NaOCl, 52 periodate,53 and amine oxides.54 Various other chiral salen-type ligands have also been explored.55 These epoxidations are not always... 

Table 2. Neutral, mononuclear complexes of aluminium with dianionic salen-type (Rl) groups. R2 = other ligand, L = Lewis base, C.N. = coordination number of the metal...

A hither facet of research has involved the structural characterisation of aluminium complexes which incorporate polydentate salen-type ligands. These have been noted in both neutral and monocationic (ion-separated) contexts (the latter requiring that the metal centre be stabilised by an external Lewis base) [35]. While such charged systems are invariably mononuclear the same is only usually true of their neutral analogues by virtue of the sterically demanding bis(aryloxide), chelating ligand. In the context of these latter complexes, dimerisation has been noted [251] while, more recently, the employment of flexible alkyl chains between two salen-coordinated aluminium ions has enabled the observation of dinuclear compounds [160, 161]. 

These assembly ligands will be tested in suitable catalytic reactions that leave the assemblies intact. Salt-forming reactions are not attractive as the salts might interact with the assembly, nor is the use of catalytic metals that compete with the assembly metal for the salen type positions in the ditopic ligand ideally, all potential problems can be avoided if the same metal could be used. Rhodium-catalyzed hydroformylation of 1-octene is a suitable reaction, with the only disadvantage that high pressures are needed, but hydrogen or CO do not interfere with our assemblies. Metal salts do not interfere with the rhodium hydrides involved in the hydroformylation catalysis, as for instance the most effective industrial process today for propene hydroformylation... 

Vanadyl salen provides a model of mixed heteroatom metal coordination characteristic of Ni and VO in the maltenes and asphaltenes. Approximately 50-90% of the metals in petroleum are not contained in the free porphyrin fraction. Yen (1975, 1978) has postulated that these metals exist in a variety of environments such as highly aromatic bound porphyrins, complexed to tetradentates of mixed N, S, and O ligands, or defect sites in large aromatic sheets. Analytical work by Fish et al. (1984) has indicated the presence of metals complexed to salen-type ligands in petroleum. 

Finally, iron catalysts based on salen-type ligands have been used. These iron(III)-salen complexes were regarded as enzyme models, using PhIO as oxidant (Scheme 3.52) [162]. Initially, the corresponding active iron-oxo complexes were formed by reaction with PhIO and isolated before use. A stoichiometric amount of the iron-oxo complex allowed the efficient oxidation of a variety of aryl methyl sulfides in moderate to good yields. 

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