Cationic phosphenium complexes of group 6 transition metals: Reactivity, isomerization, and X-ray structures

Article abstract of DOI:10.1021/om00009a020

The reaction of cationic diamino-substituted phosphenium complexes of group 6 transition metals mer-[(bpy)(CO)₃M{PN(Me)CH₂CH₂NMe}]⁺ (M = Cr, Mo, W) with L (L = PN(Me)CH₂-CH₂NMe(OR) (R = Me, Et), PPh₃) proceeds with substitution of L for CO to produce [(bpy)-(CO)₂LM{PN(Me)CH₂CH₂NMe}]⁺. 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 trans form is sterically favored. A similar reaction takes place when cationic monoamino-substituted phosphenium complexes are treated. Complexes trans-[(bpy)(CO)₂{PN(Me)CH₂CH ₂NMe(OMe)}Mo{PN-(Me)CH₂CH₂NMe}]·OTf (trans-2a·OTf) (OTf = SO₃CF₃) and trans-[(phen)(CO)₂{PN(t-Bu)CH₂-CH ₂O(OMe)}Mo{PN(t-Bu)CH₂CH ₂O}]·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 A? vs 2.495 A? and, for trans-2j, 2.238 A? vs 2.529 A?, 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) π 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 pπ lone pair density flanking the phosphenium center.