名称:
Cationic Phosphenium Complexes of Group 6 Transition Metals: Reactivity, Isomerization, and X-ray Structures
摘要:
The reaction of cationic diamino-substituted phosphenium complexes of group 6 transition metals mer-[(bpy)(CO)(3)M{PN(Me)CH(2)CH(2)NMe}](+) (M = Cr, Mo, W) with L (L = PN(Me)CH(2)CH(2)NMe(OR) (R = Me, Et), PPh(3)) proceeds with substitution of L for CO to produce [(bpy)(CO)(2)LM{PN(Me)CH(2)CH(2)NMe}](+). During the reaction, the phosphenium ligand remains intact. The product consists of trans (two phosphorus ligands are mutually trans) and cis isomers, and they equilibrate. The cis form is electronically and the traits form is sterically favored. A similar reaction takes place when cationic monoamino-substituted phosphenium complexes are treated. Complexes trans-[(bpy)(CO)(2){PN(Me)CH(2)CH(2)NMe(OMe)}Mo{PN(Me)CH(2)CH(2)NMe}]-OTf (trans-2a-OTf) (OTf = SO3 CF3) and trans-[(phen)(CO)(2){PN(t-Bu)CH2CH2O(OMe)}Mo{PN(t-Bu)CH2CH2O}]-OTf-CH2Cl2 (trans-2j-OTf-CH2Cl2) have been characterized by X-ray diffraction. The bond distance of Mo-P(phosphenium) is significantly shorter than that of Mo-P(phosphite) for both complexes: for trans-2a, 2.254 Angstrom vs 2.495 Angstrom and, for trans-2j, 2.238 Angstrom vs 2.529 Angstrom, indicating a significant double bond character between Mo and P(phosphenium). For both complexes, the P-N bond distances in phosphenium and in phosphite ligands are almost equal, indicating that there is no significant N --> P(phosphenium) pi donation. The role of the amino groups on the phosphenium phosphorus is probably to protect the approach of a nucleophile to phosphenium phosphorus by high p pi lone pair density flanking the phosphenium center.