STEREODYNAMICS OF 6-EXO-PHENYLCYCLOHEXADIENYL COMPLEXES OF MANGANESE AND RHENIUM

The series of neutral and cationic complexes [(PhC6H6)Mn(CO)2NO]PF6, [(PhC6H6)Mn(PMe3)NO]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 H-1 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 (DELTA-G double dagger) 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 [(PhC6H6)Mn(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 H-1 and (P{H})-P-31-H-1 NMR spectra reveal differences in conformational preference. In the dppen/NO derivative, there is an exclusive preference for the Cl-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 P2(1)/c with a = 11.665 (2) angstrom, b = 16.961 (6) angstrom, c = 18.182 (4) angstrom, beta = 95.58 (2)-degrees, 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.