Chalcogen reaction, dimerization

By contrast to the metaphosphimates, the metaphosphate analogues in which two chalcogens are replaced by imido groups form dimeric, dianionic ligands (24). There is only one example of a metal complex of the oxo system (24, E = O). An N,0-chelated bis(dimethylalumimun) complex is prepared by the reaction of two equivalents of trimethylaliuninum with ds-[ BuNH(0)P(//-N Bu)2P-(0)NH Bu] [20]. The structure of this complex is comparable to that of (15). 

RP)2E tend to undergo dimerization and subsequent elimination reactions on heating (especially in polar solvents) to give phosphorus-chalcogen heterocycles of varying ring sizes (RP) Em (n = 2, 3, 4 m 0, 1, 2, 3).2... 

The reaction of silylenes 83 and 85 with the chalcogens S, Se, Te resulted in the formation of the respective four-membered heterocycle 102 (Scheme 8) <1996JOM211, 1998JA12714>. For the reaction of silylene 83 with 1 equiv of sulfur low-temperature NMR studies suggest the formation of silanethione 103 which then dimerizes. Reaction of excess sulfur with silylene 83 results in the formation of compound 104 with simultaneous release of the diimine ligand. 

It should be kept in mind that it is often difficult to ascertain what the active catalyst is. There are, in fact, very few proven catalytic mechanisms where the structure of the actual catalyst is known with certainty. There are, to the author s knowledge, few, and possibly no, proven useful homogeneous catalytic processes that involve metal dimers in the catalytic cycle. In an excellent review on cluster catalysis, Muetterties lists the following processes as metal-cluster-catalyzed reactions (see Table 2). This review also lists the important classes of metalloenzyme clusters however, these compounds do not normally involve metal-metal bonds per se but are usually bridged by chalcogens, such as sulfur or oxygen donors. 

Formal E (E = S or Se) insertion, sulfurization of organic carbonyls, and nucleophilic addition to coordinated organosulfur and organoselenium compounds are reviewed in this section. Reactions of HjS, HE , and in which the chalcogen atoms exhibit nucleophilic character, as well as new contributions to CEj chemistry, especially reactions with coordinated car-banions and dimerizations, also receive attention. 

Arylimido)vanadium(V) complexes have been prepared from the reaction of VOCl3 with various /wa-substituted aryl isocyanates. The reactivity and structure of some of these have been investigated (78).431 The reactivities of a series of vanadium(V) chalcogens have been reported. The structure of [(Me3Si)2N]2V(Se)[SeSi(SiMe3)3] 432 (79) shows that the V=Se double bond contracts by 8% compared to the single bond in a related dimeric vanadium(IV) structure (vide infra). The... 

Reactions of the 5 c6>-cubane 3 with chalcogens were also investigated. While mono-chalcogenides are readily produced in the case of sulfur, selenium and tellurium, further reaction with either sulfur or selenium produced only insoluble products. The solid-state structures of the mono-selenide and mono-telluride 65a and 65b (Scheme 2.2.14) revealed that one of the Sn(N Bu)2(NH Bu) sites was oxidized in preference to the unique Sn(N Bu)(NH Bu)2 tin center. Solution-state NMR studies indicated that chalcogen exchange occurs exclusively between the two Sn(N Bu)2(NH Bu) sites. In the case of selenium, in addition to 65a, the selenido-bridged dimer 66 was present in solution as indicated by /sn-Se coupling constants (65a, 3220 Hz 66, 779 Hz). 

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