Evaluation of Donor and Steric Properties of Anionic Ligands on High Valent Transition Metals

Synthetic protocols and characterization data for a variety
of
chromium­(VI) nitrido compounds of the general formula NCr­(NPri2)2X are reported, where X = NPri2 (1), I (2), Cl (3), Br (4), OTf (5), 1-adamantoxide
(6), OSiPh3 (7), O2CPh (8), OButF6 (9), OPh (10), O-p-(OMe)­C6H4 (11), O-p-(SMe)­C6H4 (12), O-p-(But)­C6H4 (13), O-p-(F)­C6H4 (14), O-p-(Cl)­C6H4 (15), O-p-(CF3)­C6H4 (16), OC6F5 (17), κ­(O)-N-oxy-phthalimide (18), SPh (19), OCH2Ph (20), NO3 (21), pyrrolyl
(22), 3-C6F5-pyrrolyl (23), 3-[3,5-(CF3)2C6H3]­pyrrolyl
(24), indolyl (25), carbazolyl (26), N­(Me)­Ph (27), κ­(N)-NCO (28), κ­(N)-NCS
(29), CN (30), NMe2 (31), F (33). Several different techniques were employed
in the syntheses, including nitrogen-atom transfer for the formation
of 1. A cationic chromium complex [NCr­(NPri2)2(DMAP)]­BF4 (32)
was used as an intermediate for the production of 33,
which was produced by tin-catalyzed degredation of the salt. Using
spin saturation transfer or line shape analysis, the free energy barriers
for diisopropylamido rotation were studied. It is proposed that the
estimated enthalpic barriers, Ligand Donor Parameters (LDPs), for
amido rotation can be used to parametrize the donor abilities of this
diverse set of anionic ligands toward transition metal centers in
low d-electron counts. The new LDPs do not correlate well to the pKa value of X. Conversely, the LDP values of
phenoxide ligands do correlate with Hammett parameters for the para-substituents. Literature data for 13C NMR
chemical shifts for a tungsten-based system with various X ligands
plotted versus LDP provided a linear fit. In addition, the angular
overlap model derived eσ + eπ values
for chromium­(III) ammine complexes correlate with LDP values. Also
discussed is the correlation with XTiCp*2 spectroscopic
data. X-ray diffraction has been used used to characterize 31 of the
compounds. From the X-ray diffraction data, steric parameters for
the ligands using the Percent Buried Volume and Solid Angle techniques
were found.