Pincer POCOP

Recently, Scott, Brookhart, Goldman and coworkers have heterogenized (tB 4pcp)ij. and ( B " POCOP)lr derivatives by supporting them on 7-AI2O3. This was most successful with derivatives in which the pincer aryl group had a Lewis-basic para-substituent, e.g., NH2, or a group that could be hydrolyzed and thus react with the alumina surface, e.g., -C(0)(0Me) or -OP( Bu)2 [50]. 

In the COA transfer dehydrogenation in the presence of the, the bisphosphi-nite iridium pincer complexes Ir-13(H2) [127] and Ir-16(H4) [126] both displayed higher activities than Ir-2(H2) (Figure 2.16). Similar results were obtained with the corresponding (p-X POCOP)IrH2 catalysts, Ir-14(H2) (X = OMe) and Ir-15(H2) (X = 3,5-(CF3)2C6H3) [127,128]. 

The synthesis and use of immobilized iridium pincer complexes on solid supports for the transfer dehydrogenation of alkanes have also been examined [131]. Three approaches are reported (i) the post-functionalization of a Mer-rifield resin by incorporating the pincer complex (ii) the covalent bonding of the catalyst to silica via a pendant alkoxysilane linker and (iii) the adsorption of the catalyst onto y-alumina via the interaction of the phenolate group on the para-position of the pincer ligand with the Lewis-acidic sites on the alumina. The last approach showed the best activity, affording thermally robust, recyclable, and active supported ( PCP)Ir (Ir-2) and ( POCOP)Ir (Ir-13) catalysts. 

In summary, a variety of bisphosphine (PCP) andbisphosphinite (POCOP) iridium complexes have been developed in order to have better catalyst stability and activity. It is important to note that a structure - activity relationship on the catalytic efficiencies of these modified Ir-pincer complexes has yet to be determined. What is known is that an optimal balance is required between the steric effect of the substituent and the electronic effect of the pincer ligand backbone to increase the catalytic activity. Thus, replacing the pincer ligand backbone could be an alternative strategy to increase the efficiency of this transformation. Recently, alkane dehydrogenation catalysts with oxazolinyl ligands have been reported, in which C-H bond activation occurs at the Ir(III) intermediate [132]. 

Dinitrogen cleavage can also be accomplished in the coordination sphere of a molybdenum complex supported by the anionic POCOP pincer hgand... 

Ozerov reported the synthesis of rhodium pincer complexes of types 89 and 90 with aryl/bis(phosphinite) POCOP ligands. Their donating ability was assessed by IR spectroscopy. Furthermore, all surveyed Rh complexes were applied as catalysts in dimerization reactions of terminal allynes, such as 4-ethynyltoluene, l-he)yne and trimethysilylacelylene. 

---