Conformational Analysis of Seven-Membered Ring Chelates, 1 Conformations and Dynamic Behaviour of Rhodium Complexes with 1, 4-Bis(diphenylphosphanyl)butane, DIOP and Its HO Analogue

摘要

The temperature dependence of NMR spectra of [(L)Rh(cod)]BF_4 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 (TC_1, TC_2 = TC_7, TC_3 = TC_6, TC_4 = TC_5) and two boat (B_3 = B_6 and TB_1) 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 TC_7 and C_4 conformations. Relative B3LYP/6-31G(d) energies of the B_3, TC_1 and {C_4-TC_7} 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 {C_4-TC_7} and close to B_3. The symmetrical doublet in the ~(31)P-NMR spectra is assigned to the fast equilibrium {TC_7-C_4} = TC_1 = {C_5-TC_2}. The resonances of the other species are consistent with B_3 = B_6 geometry. A fused dioxolane ring forces the chelate in diop complexes to adopt the B_4 = B_5 conformation. For both types of ligand the chair-like conformation is enthalpically preferred at low temperatures (#DELTA#H deg = 0.45-0.46 kcal mol~(-1)), whereas the boat-shaped structure predominates at temperatures above 200 K #DELTA#S deg = 0.9-1.3 cal K~(-1)mol~(-1). Line-shape analysis provides a boat pseudorotation barrier for complex 1 of #DELTA#G~(not=) = 5.9 kcal mol~(-1) and for 2 #DELTA#G~(not=) = 5.3 kcal mol~(-1) at 184 K. The free energy of activation at this temperature for the boat-chair interconversion is #DELTA#G~(not=) = 8.6 kcal mol~(-1) for complex 1 and #DELTA#G~(not=) = 8.0 kcal mol~(-1) for complex 2.