Stereodynamics of 6-exo-phenylcyclohexadienyl complexes of manganese and rhenium

Article abstract of DOI:10.1021/om00044a026

The series of neutral and cationic complexes [(PhC₆H₆)Mn(CO)₂NO]PF₆, [(PhC₆H₆)Mn(PMe₃)₂NO]PF₆, (PhC₆H₆)Mn(CO)₂PMe₃, (PhC₆H₆)Mn(PMe₃)₂CO, [(PhC₆HMe₅)Mn(CO)₂NO]PF₆, and [(PhC₆HMe₅)Re-(CO)₂NO]PF₆ all show a decoalescence of the ¹H NMR spectrum below room temperature, allowing identification of preferred equilibrium conformations and measurement of barriers to conformational exchange. For these six systems, there is an essentially exclusive preference for the C₁-symmetric enantiomeric conformers that have the unique unidentate ligand eclipsing C2 or C4 of the cyclohexadienyl ring. The barriers (ΔG?) to conformational exchange vary in a range from 9.2 to 11.4 kcal mol^-1. Substitution of rhenium for manganese has no effect on conformational preference or on the barrier to conformational exchange. For the complexes [(PhC₆H₆Mn(NO)dppen]PF₆, [(PhC₆H₆)Mn(NO)dppe]PF₆, (PhC₆H₆)Mn-(CO)dppe, and [(PhC₆H₆)Mn(NO)arphos]PF₆, which have a bidentate ligand, variable-temperature ¹H and ³¹P{¹H} NMR spectra reveal differences in conformational preference. In the dppen/NO derivative, there is an exclusive preference for the C₁-symmetric enantiomers in which a phosphorus atom eclipses C6 of the cyclohexadienyl ring and NO eclipses C2 or C4 of the cyclohexadienyl ring. In the dppe/NO complex, the C₁ forms dominate (98%) but there is a detectable concentration of the C_s conformer in which NO eclipses C6 of the cyclohexadienyl ring. In the dppe/CO complex, there is an essentially statistical distribution of conformational populations among the C₁ and C_s forms. In the last two complexes, the barrier for a C₁ to C₁ conversion is higher than that for the C₁ to C_s process. The chiral arphos/NO system shows a strong preference for the conformer that has phosphorus eclipsing C6 of the cyclohexadienyl ring (88% at 231 K) and a small concentration of the form that has arsenic eclipsing C6 (12%). The barriers for conformational exchange in these bidentate analogues vary from 10.0 to 12.9 kcal/mol. The assignment of certain NMR signals for the complexes which have a bidentate ligand was aided by an X-ray structural study of [(6-exo-PhC₆H₆)Mn(NO)dppe]PF₆, which crystallizes in the monoclinic space group P2₁/c with a = 11.665 (2) A?, b = 16.961 (6) A?, c = 18.182 (4) A?, β = 95.58 (2)°, Z = 4, R = 0.039, and R_w = 0.044. In the crystal, the dppe/NO complex cation exists as two C₁-symmetric enantiomeric conformations; in each conformation, a phosphorus atom eclipses C6 of the cyclohexadienyl ring. These conformations are the same as the dominant conformations observed in solution.