Dinuclear niobium(III) complexes [{NbCl2(L)}2(μ-Cl)2(μ-L)] (L = tetrahydrothiophene, dimethyl sulfide):: Preparation, molecular structures, and the catalytic activity for the regioselective cyclotrimerization of alkynes

Dinuclear niobium complexes [ {NbCl2L}(2)( mu-Cl)(2)( mu-L)] ( L) tetrahydrothiophene C4H8S, THT ( 1a)) have been prepared by the reaction of diniobium( V) decachloride ( Nb2Cl10) with magnesium in THT. The structure of 1a was determined by X-ray crystallography to have a Nb-Nb bond, which is consistent with that of 1b ( L) dimethyl sulfide, Me2S). The reaction of 1a, as well as 1b, with alkyne ( R1C CR2) at room temperature gave a head-to-tail cycloadded alkyne trimer, 1,3,5-( R-1)(3)-2,4,6-( R-2)(3)-benzene ( R-1) H, R-2) Ph ( 2); R-1) p-tolyl, R-2) H ( 3); R-1) R-2) CH2Cl ( 4); R-1) R-2) COOEt ( 5); R-1) n-Pr, R-2) Me ( 6); R-1) n-Bu, R-2) H ( 7); R-1) Me3Si, R-2) H ( 8), R-1) R-2) Et ( 9), R-1) R-2) n-Pr ( 10)), regioselectively in high yields. The cyclotrimerization is found to be catalyzed by complexes 1a and 1b, since only 1/600 equiv of the complex to the alkynes can lead to the corresponding product in high yields. The reactions of 1a with 6 equiv of 3-hexyne and 4-octyne gave a mononuclear byproduct, NbCl3( eta(2)-RC CR-RC=CR) ( R) Et ( 11), Pr ( 12)), besides 9 and 10, respectively. The reactions of 11 or 12 with an excess of original alkyne, however, resulted exclusively in the formation of the corresponding benzene derivative, in equimolar amount to 1,2,3,4-( R) 4- 5,6-( R')(2)-benzene ( R) R') Et ( 9), R) R') Pr ( 10), R) Et, R') Pr ( 13), R) n-Pr, R') Et ( 14)). This observation suggests that the mononuclear species does not behave as a catalyst, and the regioselectivity may be due to a dinuclear structure of the niobium complexes.