Salphen" complex

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

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

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. 

A number of salen and salphen complexes have been incorporated into conjugated polymers by electropolymerization (including the use of thienyl-functionalized monomers, as discussed in Section III) or condensation polymerization. 

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... 

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... 

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). 

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... 

Boyadzhiev L and Yordanov B. Pertraction of Indole Alkaloids from Vinca minor L. Sep Sci Tech, 2004 39(6) 1321-1329. Coucouvanis D. Dell Rosa and Jay Pike recognition and transport of amphiphilic molecules by a new class of inorganic ditopic receptors. The synthesis of M- Bu - salphen-3n-cr-n complexes and their use (M = Mn,Fe, n = 6) in the transport of tryptophan and serotonin across bulk liquid membranes. Comptes Rendus Chimie, 2003 6(3) 317-327. 

Transition metal catalysts encapsulated within the ligand-template nanoreactor G, P(Py)s [ZnJs, have been applied to catalyze industrially relevant processes such as hydroformylation and Heck reaction. Nanoreactor [G 3 Rh(CO)(acac)] encapsulates a Rh-species that contains only one tris(meta-pyridyl)phosphine ligand, P(m-Py)3, surrounded by three Zn-porphyrins or Zn-salphens. Under syngas pressure (H2/CO), rhodium species like Rh(CO)(acac)P(Py)3 transform into a complex of type HRh(CO)3P(Py)3, which is the active species for the hydroformylation reaction, hi this reaction terminal alkenes are converted into linear and/or branched aldehydes, and the ratio of these products strongly depends on the specific catalyst applied. Hydroformylation of 1-octene by encapsulated rhodium, [G 3 HRh(CO)3], resulted in a 10-fold rate enhancanent compared... 

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