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.