Palladium, porphyrins coordinated

A logical extension of the condensation reactions which yield multidentate nitrogen donors is the formation of macrocyclic ligands. The preference of palladium for square planar coordination makes it an ideal metal for the formation of complexes of these ligands. Thus palladium porphyrins are very stable and resistant to demetallation.281 Both 14- and 16-atom macrocycles have been used to form complexes. The ligand l,8-dihydro-5,7,12,14-tetramethyl-dibenzo[b,i][l,4,8,ll]tetraaza[14]annulene reacts with [PdCl2(PhCN)2] to yield the complex (46). 

The rich coordination chemistry of palladium provides a plethora of more or less readily available complexes, almost all applicable in type 1 or type 2 Mizoroki-Heck reactions. Only a limited series of truly robust and almost indefinitely stable ( dead ) palladium complexes, such as palladium porphyrinate or phthalocyanine complexes, as well as salen-type chelates with Schiff bases, are known not to release palladium even under the harshest conditions used in Mizoroki-Heck reactions. Results from the investigation of reactivity of... 

With their preference for square planar coordination, palladium(II) and platinum(II) are well suited to binding to porphyrins and related N4 donor macrocycles. Therefore, Pd(octaethylporphyrin) is readily synthesized starting from the labile PhCN complex (like the platinum analogue) [92]... 

Organometallic porphyrin complexes containing the late transition elements (from the nickel, copper, or zinc triads) are exceedingly few. In all of the known examples, either the porphyrin has been modified in some way or the metal is coordinated to fewer than four of the pyrrole nitrogens. For nickel, copper, and zinc the 4-2 oxidation state predominates, and the simple M"(Por) complexes are stable and resist oxidation or modification, thus on valence grounds alone it is easy to understand why there are few organometallic examples. The exceptions, which exist for nickel, palladium, and possibly zinc, are outlined below. Little evidence has been reported for stable organometallic porphyrin complexes of the other late transision elements. 

In this review, CPOs constructed by covalent bonds are mainly focused on however, stable coordination bonds comparable to the stability of the covalent bonds have potential for future enhanced molecular design of novel CPOs. One representative is the bond between pyridine-type nitrogen and metal, which is widely used in supramolecular chemistry, that is, the cyclic supramolecular formation reaction between pyridine-substituted porphyrin and metal salts (Fig. 6d) [27,28]. Palladium salts are frequently used as the metal salts. From the viewpoint of the hard and soft acid and base theory (HSAB), this N-Pd coordination bond is a well-balanced combination, because the bonds between nitrogen and other group X metals, N-Ni and Ni-Pt coordination bonds, are too weak and too strong to obtain the desired CPOs, respectively. For the former, the supramolecular architectures tend to dissociate into pieces in the solution state, and for the latter. 

Another example of a trigonal prismatic complex is the porphyrin prism 24, also reported by Fujita and coworkers [139]. This complex is formed upon coordination of three face bridging zinc(II)-coordinated porphyrin ligands to six palladium(II) ions at the corners of the prism. Although... 

A mixed organic-inorganic 3D porphyrin network was obtained by cocrystallization of palladium mc n-tetrapyridylporphyrinate and cadmium nitrate. All four pyridyl units are bound to octahedral cadmium centres which are each coordinated by two nitrate, water and pyridyl ligands (Figure 7.27). One half of the py-Cd-py connections is linear, the other half bent (103°), having two cis pyridine ligands. As is common in mc o-tetraarylporphyrins,... 

Another remarkable example of a grid like molecule was obtained by palladium(II)-directed self-assembly. The self-organization proceeds by coordination of nine zinc porphyrins having pyridine side chains to 12 PdCb units, resulting in the 25-nm grid (28) shown in Fig. 13. Interestingly, the assembly forms columnar clusters on silica surfaces that can be visualized by atomic force microscopy. 

Fig. 13 a Coordination of pyridine to the me5o-triphenyl(pyridyl)porphyrin zinc(II) moiety triggers the electron transfer to the [Re(bpy)(CO)3(pyP)] moiety for photocatalitic reduction of water and CO2 and b Palladium(II) porph5oin-[Re(bpy)(CO)3X] dyad with longer lived charge transfer state... 

A combination of PQQ derivatives with transition metals leads to form the efficient redox systems. Dropwise addition of terminal olefin to the solution of PdCl2 (0.1 molar equiv.), trimethyl ester of PQQ, (PQQTME, 0.1 molar equiv.) and H2O in DMF under molecular oxygen results in the formation of 2-alkanone (Scheme 3.2). It should be noted that PQQTME constitutes a catalytic redox cycle. The orthoqui-none function appears to reoxidize the reduced palladium species generated in situ. Use of 1,7- or 1,10-phenanthrolinequinone gives a poor result maybe due to the coordination of paUadium(II) species towards the pyridine moiety opposite to the quinone group [5]. Mediation ofp-benzoquinone in the palladium-catalyzed oxidation reactions has been reported to require electrochemical or cobalt porphyrin catalyzed oxidation of the quinol [6-8]. The above-mentioned results provide an example for efficient redox systems of coenzyme derivatives with transition metals, which is demonstrated to be synthetically useful. 

Solutions of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine palladium (Pd.py.porph) with excess Cd(N03)2 in water-methanol-ethanol gave ciystals of (Pd.py.porph).2Cd(N03)2.hydrate whose structure (20) is represented in Figures 9a and b. All porphyrin units are equivalent and are attached via their pyridyl nitrogens to four Cd s (Figure 9a). Two diametrically opposed Cd(2) s are coordinated by two... 

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