Conformational flexibility and molecular dynamics of cationic diolefin-rhodium(I) complexes with iminophosphine ligands

In solution, the cationic complexes [Rh(eta (2),eta (2)-diolefin)(P-N)](+) [diolefin = 1,5-cyclooctadiene (cod), bicyclol2.2.1]hepta-2,5-diene (nbd); P-N = o-(PPh2)C6H4CH=NR (R = C6H3(i-Pr)(2)-2,6 (1), CMe3 (2), (R)-bornyl (3)] undergo a conformational change of the iminophosphine ligand which inverts the position of the six-membered chelate ring relative to the N-Rh-P coordination plane. The inversion is fast for [Rh(-eta (2),eta (2)-diolefin)(1)](+) and [Rh(eta (2),eta (2)-diolefin)(3)](+) in the temperature range 298-183 K, and becomes progressively slower for [Rh(eta (2),eta (2)-diolefin)(2)](+) with increasing steric demand of the coordinated ligands. From the coalescing signals in the H-1-NMR spectrum of [Rh(nbd)(2)](+) at 208 K, a DeltaH* value of 28.5 kJ mol(-1) and a DeltaS* value of -60.3 J K-1 mol(-1) can be estimated. The phase-sensitive 2D H-1-NMR ROESY spectrum of [Rh(cod)(3)](+) reveals the presence of another dynamic process which slowly and selectively interconverts the two olefinic protons on the same HC=CH unit of the chelate diolefin. A mechanism is proposed involving the initial dissociation of the rhodium-olefin bond trans to phosphorus. Semi-empirical calculations show that in the most stable conformers of [Rh(nbd)(2)](+) and [Rh(cod)(2)](+) the P-N chelate ring is not coplanar with the N-Rh-P plane, while the diolefin coordination mode is distorted largely by the steric interaction with the bulky N-CMe3 group. When the P-N chelate ring is forced to be coplanar with the N-Rh-P plane, the enthalpy content increases to 20.3 and 114.7 kJ mol for [Rh(nbd)(2)] I and [Rh(cod)(2)] respectively. Entropy factors seem to be predominant in the displacement of 1,5-cyclooctadiene by other olefinic ligands in the complexes [Rh(cod)(P-N)](+). (C) 2001 Elsevier Science B.V. All rights reserved.