Conformations and Dynamic Behaviour of Rhodium Complexes with 1,4-Bis(diphenylphosphanyl)butane, DIOP and Its HO Analogue

Abstract

The temperature dependence of NMR spectra of [(L)Rh(cod)]BF4 complexes with L = dppb (1), (R,R)-diop (2) and (R,R)-HO-diop (3) has been examined. Molecular mechanics and ab initio calculations on the [{1,4-bis(dimethylphosphanyl)butane}Rh+(diolefin)] complex predict local energy minima for all twist-chair (TC1, TC2 = TC7, TC3 = TC6, TC4 = TC5) and two boat (B3 = B6 and TB1) conformations. Furthermore, ab initio calculations at the B3LYP/6-31G(d)/LANL2DZ level show that two minima are located in the wide-open region between the TC7 and C4 conformations. Relative B3LYP/6-31G(d) energies of the B3, TC1 and {C4-TC7} conformations are 0.0, 0.71 and 0.97–1.08 kcal mol−1, respectively. Analysis of crystallographic data contained in the Cambridge Structural Database shows that the majority of structures are concentrated in the region {C4-TC7} and close to B3. The symmetrical doublet in the 31P-NMR spectra is assigned to the fast equilibrium {TC7-C4} ⇌ TC1 ⇌ {C5-TC2}. The resonances of the other species are consistent with B3 = B6 geometry. A fused dioxolane ring forces the chelate in diop complexes to adopt the B4 = B5 conformation. For both types of ligand the chair-like conformation is enthalpically preferred at low temperatures (ΔH° = 0.45–0.46 kcal mol−1), whereas the boat-shaped structure predominates at temperatures above 200 K (ΔS° = 0.9–1.3 cal K−1 mol−1). Line-shape analysis provides a boat pseudorotation barrier for complex 1 of ΔG≠ = 5.9 kcal mol−1 and for 2 ΔG≠ = 5.3 kcal mol−1 at 184 K. The free energy of activation at this temperature for the boat–chair interconversion is ΔG≠ = 8.6 kcal mol−1 for complex 1 and ΔG≠ = 8.0 kcal mol−1 for complex 2.