Salphen

Ligand abbreviations salen = IV, JV -ethylenebis(salicylaldimine) dianion salbn = JV,Af -butyl-enebis(salicylaldimine) dianion salphen = lV,iV -o-phenylenebis(salicylaldimine) dianion 2-MeImd = 2-methylimidazole H4fsa2en = JV,iV -ethylenebis(3-carboxysalicylaldimine) eq = equatorial ax = axial... 

Electrogenerated monovalent Co complexes of the well-known open chain N202 Schiff base ligands salen (8), salphen (9), and their substituted derivatives undergo oxidative additions with alkyl halides. Reactions of the complex with substrates within the series RBr (R = Pr, Bu, t-Bu) proceed at different rates. The reaction occurs by an inner-sphere alkyl-bridged electron transfer, with a Co1- R+- X-transition state, which is sensitive to distortions of the complex in different configurations.124... 

A completely new type of ROP catalyst was recently reported by Rieger and coworkers. Chromium salphen complexes (Fig. 32) convert racemic (3-BL to slightly isotactic-enriched PHB (0.60 < Pm < 0. 70) with a molecular weight of up to 800,000 g/mol (PD up to 8.5). These catalysts combine high activity and high molecular weight products, featuring the desired stereocontrol at moderate reaction conditions [13]. 

Fig. 34 DFT calculation PHB formation in a cage of the dimeric Cr (salphen) sandwich-like structure [94] (reproduced with permission of Macomolecules94)...

In order to gain more control over this reaction, chromium salphen dimers were synthesized. The synthetic route was developed in such a manner that the bridging length between the two salphen units can easily be varied and that the synthesis of heteronuclear metal complexes is possible. Since the ligand substitution pattern is highly important for the activity of the catalyst as well as the characteristics of produced polymer, an analogous monomeric Cr(lll) complex was synthesized for comparison [102] (Fig. 35). 

It should be mentioned that donor substitution of the phenylene backbone of the salphen ligand was shown to have a decreasing effect on activity [103], which explains the overall lower productivity compared with halogen-substituted chromium salphens. However, experiments clearly proved an increased activity upon dimerization. Whereas the monomeric complex m = 4) converts about 30% of p-BL in 24 h, producing a molecular weight of 25,000 g/mol, the corresponding dimer yields up to 99% conversion with > 100,000 g/mol. Moreover, the smaller polydispersity (PD < 2) shows the better polymerization control, which is attributed to the decreased rate of polymer chain termination. This behavior is caused by the stabilization of the coordinated chain end by the neighboring metal center, as recently reported for dual-site copolymerizations of CO2 with epoxides [104-106]. The polymeric products feature an atactic microstructure since the... 

Fig. 35 Conformationally flexible dimeric and monomeric Cr salphen complexes for bifunctional catalysis [102]...

Coates used [Cp2Ti(THF)2] and [(salphen)Al(THF)2] as Lewis acid to convert a variety of epoxides to racemic p-lactones, and substituted aziridines to p-lactams in high yields under mild conditions. PO is selectively converted to p-BL in 95% yield in 4 h at 60°C [117]. However, only racemic p-BL can be obtained from racemic PO. In order to get enantiopure molecules from racemic precursors, the catalytic system has to be stereoselective. This can generally be achieved by the use of a chiral stereo-inducing Lewis acid, which effects a kinetic resolving activation [119, 120]. However, examples of the chiral resolution of PO are rare. 

As reported by Spassky et al. [62], aluminum complexes of Schiff bases as initiators exhibit much lower activities than aluminum porphyrins for the ringopening polymerization of epoxides. In fact, the polymerization of PO (500 equiv) using a Schiff base complex (Salphen)AlCl (13) as initiator proceeded extremely slowly at room temperature to attain only 4% conversion in 8 d. Even at 80 °C, the polymerization was slow, and required 6 d for completion, affording a polymer with broad and bimodal MWD (Fig. 32A). 

Reek, van Leeuwen et al. have also reported the template-induced generation of bidentate ligands onto a rigid bis-zinc(II) salphen platform. Homobidentate [41] or heterobidentate [42] P,P-ligands have been prepared and tested in hydroformylation and hydrogenation reactions. 

Scheme 8.2 Formation of catalyst assemblies by selective pyri-dine-zinc(ii) coordinative motifs using zinc(ii)salphen complexes and different pyridylphosphine templates.

Figure 8.6 X-Ray (solid state) structure of the assemblyformed by tris(pora-pyridyl)phosphine (4) and a zinc(ii)salphen. The phosphorus atom is shown in black.

A monophosphine complex is formed when 3 is mixed with three equivalents of a zinc(ii)salphen complex and half an equivalent of Rh(acac)(CO)2 (acac = acetyl acetonate), whereas the assembly based on template 4 and the zinc(n)salphen complexes forms a bis-phosphine rhodium species. In the latter case, the bisphosphine rhodium complex is completely encapsulated by six salphen building blocks. This difference in mono- versus diphosphine ligation to the Rh -center and, to a lesser extent, the difference in electronic features (and thus donating properties of the phosphine) between template ligands 3 and 4, can be used to induce a different catalytic behavior. 

Scheme 8.4 Various zinc(ii)salphens have been coordinated to mPy-BIAN (PdMe), forming active catalysts for CO/styrene polymerization reactions.

Template-induced formation of heterobidentate ligands and their application in the asymmetric hydroformylation of styrene. Chem. Commun., 4679 681. (b) Kuil, M., Goudriaan, P.E.. Kleij, A.W., Tooke, D.M., Spek, A.L.. van Leeuwen, P.W.N.M. and Reek, J.N.H. (2007) Rigid bis-zinc([[) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis. Dalton Trans., 2311-2320. 

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 Crm-Schiffs base complexes in MeCN under nitrogen followed by oxidation with oxygen and hydrolysis.1128... 

Salen phenolato rings present, dimino rings absent Salphen all rings present, R = H Bu qsalphen as salphen, R = Bu ... 

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