CONFORMATIONAL-ANALYSIS AND DYNAMICS OF THE TRIPHENYLPHOSPHINE LIGAND IN [(ETA-5-C5H5)FE(CO)(PPH3)COCH3]

Conformational analyses are reported on the chiral iron acetyl complex [(eta-5-C5H5)Fe(CO)(PPh3)COMe] (1) and the two fluorinated derivatives, [(eta-5-C5H5)Fe(CO){P(3,5-C6H3F2)3}COCH3] (2) and [(eta-5-C5H5)Fe(CO){P(C6F5)Ph2}COCH3] (3), to characterize the dynamic processes of the triphenylphosphine ligand. Calculations indicate that rotation of the phenyl rings about the P-C(ipso) bonds within 1 are facile (DELTA-G double-ended dagger calc = 5 kcal mol-1), and a correlated cog-like rotation of the three rings is identified as a low-energy pathway for ring flip. A higher energy phosphine rotational process about the P-Fe bond is also identified for 1 (DELTA-G double-ended dagger calc = 12 kcal mol-1) which requires correlated motion of the phenyl rings about the P-C(ipso) bonds to minimize steric interactions with the congested metal center. Similar dynamic properties of the triphenylphosphine ligand in the partially fluorinated derivatives 2 and 3 are also predicted. Variable-temperature C-13{H-1} NMR spectroscopic studies on 1 corroborate the molecular modelling predictions on the dynamic processes of the triphenylphosphine ligand: line-shape analyses on the aromatic subspectra were employed to determine an activation energy for phosphine rotation about the P-Fe bond, DELTA-G double-ended dagger exp = 10.3 +/- 0.1 kcal mol-1, whereas phenyl ring rotation about P-C(ipso) was found to be rapid on the NMR time scale down to -90-degrees-C. Similar variable-temperature F-19{H-1} NMR spectroscopic studies and line-shape analyses on 2 found DELTA-G double-ended dagger exp = 10.8 +/- 0.1 kcal mol-1 for rotation of the phosphine about the P-Fe bond, and no evidence for arrest of phenyl ring rotation down to -90-degrees-C could be found. A full line-shape analysis of the F-19{H-1} NMR spectrum of 3 clearly indicates that phenyl ring rotation is rapid at ambient temperature, in complete agreement with the conformational analysis.