Formation and stereochemistry of octahedral cationic hydride-azavinylidene osmium(IV) complexes

摘要

The complexes [OsHCl2(=N=CR2)(PiPr3)2] [CR2 = CMe2 (1), Ca(CH2)4CbH2(C a-Cb) (3)] react with Ag(CF3SO3) in the presence of ligands L [H2O, P(OMe)3, CO]. The reactions in the presence of water lead to [OsHCl(=N=CR2)(H2O) (PiPr3)2][CF3SO3] [CR2 = CMe2 (2), Ca(CH2)4CbH2(C a-Cb) (4)], which exist as 1:1 equilibrium mixtures of the isomers 2a/4a (hydride trans to Cl) and 2c/4c (hydride trans to water) in the solid state and in solution. The structure of 2c has been determined by an X-ray diffraction study. The geometry around the metal center can be described as a distorted octahedron with trans phosphane ligands at opposite sites of an ideal coordination plane defined by the other four ligands. The reactions in the presence of P(OMe)3 afford [OsHCl(=N= CR2){P(OMe)3} (PiPr3)2][CF3SO3] [CR2 = CMe2 (5a), Ca(CH2)4CbH2(C a-Cb) (6a)], with the hydride and chlorine ligands trans disposed. Complexes 5a and 6a can be also obtained starting from the equilibrium mixtures of 2a and 2c or 4a and 4c, respectively, and phosphite. Compounds 5c and 6c, with P(OMe)3 trans to hydride, are formed initially, and subsequently isomerize to 5a and 6a. Reactions under carbon monoxide give [OsHCl(=N=CR2)(CO)(PiPr3)2][CF 3SO3] [CR2 = CMe2 (7a), Ca(CH2)4CbH2(C a-Cb) (8a)], which also contain the chlorine trans to the hydride ligand. A theoretical study on [OsHCl(=N=CH2)L(PH3)2]+ [L = H2O, P(OH)3 and CO] model complexes shows that these stereochemical preferences arise from the properties of the ligands in the plane perpendicular to the P-Os-P axis. Although the structure with the H and Cl trans disposed is not the most favorable when the related pentacoordinate [OsHCl(=N=CR2)(PiPr3)2]+ model complex is considered, it maximizes the interaction energy between the pentacoordinate complex and L ligand, and thus it is the most stable as a whole.