The chemistry of transition metal complexes containing a phosphenium ligand

A molybdenum complex containing diamino-substituted phosphite, fac-[(bpy)(CO)3Mo{PNN(OMe)}] (PNN stands for Me) reacts with a Lewis acid such as BF3·OEt2 to give a cationic phosphenium complex fac-[(bpy)(CO)3Mo{PNN}]+, where an OMe on a phosphorus is abstracted as an anion. The facial isomer spontaneously isomerizes into its meridional form. Group 6 transition metal complexes, fac-[(bpy)(CO)3M{PXY(OMe)}] (M=Cr, Mo, W; XY=(NEt2)2, N(Me)CH2CH2O, (NEt2)(OMe) and OCMe2CMe2O, (OMe)2) have been subjected to reaction with BF3·OEt2. These reactions reveal that the stability of cationic phosphenium complexes increases with: (i) going to a heavier congener; (ii) increasing the number of amino substituents on the phosphenium phosphorus; and (iii) adding an ethylene bridge between X and Y when at least one of X and Y is an amino substituent. The cationic phosphenium complex reacts with a nucleophile (Nu=OEt−, Me−) at the phosphenium phosphorus to give fac-[(bpy)(CO)3M{PXY(Nu)}], and also reacts with a tertiary phosphorus compound (L) to give [(bpy)(CO)2LM{PXY}]+. The reaction of a cationic monoaminomonoalkoxy phosphenium complex of Mo, mer-[(bpy)(CO)3Mo{PNO}]+ (PNO stands for ) with a diamino-substituted phosphorus compound, PNN(Y) (Y=OMe, OEt, SEt, N(CH2)3CH2) proceeds with substitution for CO and then with the Y group migration to the coordinating phosphenium phosphorus to give [(bpy)(CO)2{PNO(Y)}Mo{PNN}]+. The reaction is irreversible. Reactions of iron complexes containing a Group 14 element ligand (ER3; E=C, Si, Ge, Sn) and diamino-substituted phosphite, Cp(CO)(ER3)Fe{PNN(OMe)} with a Lewis acid have been examined. The reaction product depends on E. In any case, an OMe anion abstraction by a Lewis acid uniformly takes place at the first stage of the reaction to give a cationic phosphenium iron complex containing an ER3 ligand. The subsequent reaction is strongly dependent on E. When E is C, migratory insertion of the phosphenium ligand into the FeC bond or more simply an alkyl migration from Fe to phosphenium phosphorus occurs. When E is Si or Ge, the cationic phosphenium complex is stable and FeSi and FeGe bonds remain intact. In contrast, when E is Sn, one alkyl group on the Sn, and not SnR3, migrates to the phosphenium phosphorus to give a stannylene complex. The corresponding Ru complexes show similar reactions. X-ray structures of cationic phosphenium complexes of Mo and Fe have been employed and reveal that there is considerable double bond character between a transition metal and the phosphenium phosphorus. 31P- and 95Mo-NMR spectroscopic data also support the double bond character. Activation parameters concerning phosphenium ligand rotation along the PFe axis could be estimated from VT-NMR studies.