RESUMEN
The large-scale synthesis of the scorpionate ligand Ttz (hydrotris(1,2,4-triazol-1-yl)borate) is reported as well as syntheses of Group VI complexes K[M(L)(CO)3] and M(L)(NO)(CO)2 (L = Ttz or Tp (hydrotris(pyrazol-1-yl)borate), M = Mo or W). The redox characteristics of the metal in these Ttz complexes are shown to be reversibly modulated by interactions between the exo-4-N lone pairs of the triazolyl rings and Brønsted or Lewis acids. The basicity of the scorpionate ligand in [M(Ttz)(CO)3]- is quantified (pKaH2O values range from 1.1 to 4.6) and found to be dependent on both the oxidation state and identity of the metal. In the presence of Brønsted acids, the observed redox behavior for the one-electron oxidation of the Group VI metal center is consistent with a proton-coupled electron transfer (PCET). Indeed, for both Mo and W derivatives, a one-electron oxidation decreases the pKa by â¼3.5 units.
RESUMEN
The preparation of the complexes TpMo(NO)(DMAP)(η2-PhCF3) (5) and TpMo(NO)(DMAP)(η2-benzene) (3) is described. The CF3 group is found to stabilize the metal-arene bond strength in 5 by roughly 3 kcal/mol compared to that in 3, allowing the large-scale synthesis and isolation of the trifluorotoluene analogue (5, 37 g, 70%). When a benzene solution of 5 is allowed to stand, clean conversion to the benzene analogue 3 occurs, and this complex may be precipitated from solution upon the addition of pentane and isolated. The trifluorotoluene complex is shown to be a synthetic precursor to functionalized cyclohexadienes: In solution, it selectively protonates at the ortho position, and the resulting η2-arenium species undergoes reactions with nucleophiles at the adjacent meta carbon. Thus, reactions of 5, triflic acid, and either N-methylpyrrole or 1-methoxy-2-methyl-1-(trimethylsilyloxy)-1-propene result in 5-substituted-1,3-cyclohexadienes after removal of the metal.
