Selenophene, metal complexes

Photochemical reactions of selenophene-metal complexes (Section 3.13.5.6) and photocycloadditions of selenophenes (Section 3.13.5.9) are treated later in this subsection. The photolysis of gaseous selenophene (and also tellurophene) has been studied <2000JOC2759>. Interestingly, the cleavage of both Se-C carbons gave elemental Se in the process. 

Removal of thiophene impurities from petroleum feedstocks is accomplished by a process called hydrodesulfurization (HDS) which involves the insertion of metals into the thiophene ring between the C-S bond. In order to better understand the mechanism of this reaction, different groups have utilized selenophene model systems due to the enhanced NMR characteristics of Se. Metal complexes of selenophenes that have been studied include rhodium <19970M2751>, molybdenum <2006POL499>, manganese <20010M3617, 19950M332>, chromium... 

Much work has been devoted to the formation of complexes where selenophene acts as a ligand. For example, with first row transition metals selenienylthienylmethane forms stable molecular species <83MI 2l3-05>. The effect of substitution of either cyclopentadienyl or selenophene rings on the structure of if - and f/ -selenophene (sel) complexes Cp(CO)2Re(sel) has been discussed <91JA5651>. 

It should be noted that a number of SPC polymers which contain other heterocycles have been prepared, motivated by their promising optical or electrical properties. Examples include pyridine (65) [123], pyrrole (66) [124], oxadiazole (67) [125], selenophene (68) [126], benzo[2,l,3]thiadiazole (69) [127], benzo[2,l,3]selenadiazole (70) [126], perylene bisimide (71) [128], 1,4-diketo- pyrrolo[3,4-c]pyrrole-l,4-dione (72 and 73) [127,129], and triphenyleamine (74) [127] as part of the polymer backbone by SPC (Figure 19c). Specifically for metal complexation, porphyrin [130], difluoroboraindacene [131], bipyridine [132], phenanthroHne [113], terpyridine [133, 134], and the like [123] were embedded in the backbone. In this context, an interesting report was submitted by Rehahn et al., in which l,l -ferrocenyl units were incorporated into a PPP (Figure 22.20). Due to a low-energy barrier for rotation around the Cp-Fe-Cp axes (Cp = cyclopentadienyl), the obtained polymer 75 was assumed to take randomly coiled conformations [135]. 

An jj (S) equilibrium (equation 6) is observed in the benzo[fc]thiophene (BT) complexes Cp (CO)2Re(BT), where Cp is Cp or Cp. As for selenophene, the Cp ligand favors j -BT, while Cp favors the rj (S) form it should be noted that 2-MeBT and 3-MeBT form only the jj (S) complexes. The formation of the r] form from the is important for understanding the first step in the HDS of BT, which gives dihydrobenzothiophene (equation 7). This alkene hydrogenation reaction is catalyzed by homogeneous catalysts and may occur similarly on heterogeneous HDS catalysts. While there is no evidence that there is an r] (S) equilibrium in thiophene complexes, both thiophene species may exist on a catalyst surface. Low metal oxidation states would favor r] coordination, which could lead to the initially observed alkene hydrogenation products. 

This finding was followed by a comprehensive preparative, structural characterisation and kinetics study of the conversions typified by the reaction above.268 These reactions were accorded the term metal exchange reactions. The heterobimetallic complexes with a, n bridging thiophene or selenophene ligands (ij 1 ij5-XCRCHCHCMn(CO)5)Cr(CO)3 (X = S, R = H (1) R = Me (2) X = Se, R = H... 

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