Rhenium complexes porphyrins

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

Photocatalytic CO2 reduction of a supramolecule with a Zn porphyrin unit, which is a redox photosensitizer that can absorb even wider ranges of visible light, connected to a rhenium complex (ZnTMP-ReCl) was considered (101,102). In this system, ultrafast (l.SxlO s ) electron transfer from the S2 excited state of the ZnTMP unit to the rhenimn imit was observed. Reduction of CO2 proceeded with generation of the OER species of the rhenium unit by the reduction of this intramolecular charge-transfer state by TEA. 

An interesting C-metalation of a coordinated pyrrol ligand in a rhenium complex has been described [19a]. Methylated aromatic hydrocarbons have been activated by (ZrCU) [19b], A novel mode of electrophilic activation of an aliphatic C-H bond, assisted by porphyrin complexes ofZr(IV) and achieved by the use of hydrides of lithium, sodium or potassium, has been reported [19c]. It has been shown that the ligands 2,3,5,6-tetraphenylphenoxide and 3,5-dimethyl-2,6-diphenylphenoxide undergo intramolecular activation by tantalum alkylidene groups at rates 20 and 100 times slower than that of the simple 2,6-diphenylphenoxide ligand [19d]. 

Other systems for electrochemical CO2 reduction utilize transition metal complexes of nitrogen-containing (nickel and cobalt) macrocycles (including porphyrins and phthalocyanines) and (ruthenium, cobalt, and rhenium) complexes of 2,2 -bipyridine. ... 

Schiff base complexes to aromatic silyl enol ethers/ " olefins/ sugars,Mn ° Schiff base complexes, " and rhenium(III) complexes " has been reported. Similar reactions are observed with the Mn (N)porphyrin complexes. The reaction products are dependant on the nature of the Schiff base ligand, both the yield and the enantiomeric excess being affected. " Salen nitridomanganese(V) complexes have been incorporated into Zeolite... 

Complexes of rhenium(bipyridine)(tricarbonyl)(picoline) units linked covalently to magnesium tetraphenylporphyrins via an amide bond between the bipyridine and one phenyl substituent of the porphyrin 19 exhibited no signs of electronic interaction between the Re(CO)3(bpy) nnits and the metalloporphyrin units in their gronnd states. However, emission spectroscopy revealed a solvent-dependent quenching of porphyrin emission upon irradiation into the long-wavelength absorption bands localized on the porphyrin. 

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

In further applications, dithienylethene compounds have been tethered to P-cyclodextrin for photocontrolled uptake and release of a porphyrin <04CEJ1114>, whereas a cationic dithienylethene derivative has been subjected to electrochemical reductive cyclization <04AG(E)2812>. A dithienylethene containing a 1,10-phenanthroline unit and a rhenium(I) complex thereof has also been prepared and studied <04JA12734>. 

Rhenium ions, and especially tricarbonylrhenium(l)chloro fragments, have been used to organize and link pyridine-functionalized porphyrins as molecular squares, rectangles, planar dimers, and more complex structures. 

The most extensively studied of the Group 7 porphyrins are the maganese complexes that can exist in up to four different oxidation states [23]. In fact, no electrochemistry of technetium porphyrins has yet been reported, and the electrochemistry of rhenium porphyrins has been limited to compounds of the type (P)HRe(CO)3 and (P)[Re(CO)3 2 where P = TPP, TMTAA or TAA [160-162],... 

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