Mo-95 NMR studies on cationic phosphenium complexes of molybdenum

The Mo-95 NMR spectra of cationic molybdenum phosphenium complexes have been measured. Going from fac-[(bpy)(CO)(3)Mo{PN(Me)CH(2)CH(2)NMe(OMe)}] (1a) to fac-[(bpy)(CO)(3)Mo{PN(Me)CH(2)CH(2)NMe}](+) (fac-1b) to mer-[(bpy)(CO)(3)Mo{PN(Me)CH(2)CH(2)NMe}](+) (mer-1b) causes deshielding of the Mo-95 doublet resonance (delta-1081 (1a), -1049 (fac-1b), -1015 (mer-1b)), an increase in the Mo-P coupling constant (193 Hz (1a), 264 Hz (fac-1b), 337 Hz (mer-1b)), and an increase in the line width (35 Hz (1a), 90 Hz (fac-1b), 300 Hz (mer-1b)). The conversion of fac-[(bpy)(CO)(3)Mo{PN(Me)CH2CH2O(OMe)}] (fac-2a) into mer-[(bpy)(CO)(3)Mo{PN(Me)CH2CH2O}](+) (mer-2b) showed a similar trend in Mo-95 NMR data, though fac-[(bpy)(CO)(3)Mo{PN(Me)CH2CH2O}](+) was not detected in this case. mer-2b exhibits the largest Mo-P coupling constant (343 Hz) among those reported so far. These changes can be reasonably attributed to a dominant contribution from the imbalance of electron density at the Mo in the Ramsey equation due to a significant double-bond character between the molybdenum and the phosphenium phosphorus. Comparison of coupling constants between I M and a phosphenium P in [(bpy)(CO)(3)M{PN(Me)CH(2)CH(2)NMe}](+) (M = Mo, W) revealed that the ratio (1)(W-P)/(1)J(Mo-P) is 1.67, which is very close to the ratio (1.76) reported for many series of M-phosphine (or phosphite) complexes (M = Mo, W). The Mo-95 NMR spectra of cis- and trans-[(bpy)(CO)(2)Mo{PN(Me)CH(2)CH(2)X(OMe)}{PN(Me)CH(2)CH(2)X}](+) (X = NMe, cis-1c, trans-1c; X = O, cis-2c, trans-2c) have also been measured; the chemical shifts are understood similarly in terms of the imbalance of electron density at Mo.