Conformational analysis of seven-membered ring chelates, 1 - Conformationsand dynamic behaviour of rhodium complexes with 1,4-bis(diphenylphosphanyl)butane, DIOP and its HO analogue

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. Mole cular mechanics and ab initio calculations on the [{1,4-bis(dimethylphospha nyl) butane}Rh+ (diolefin)] complex predict local energy minima for all twi st-chair (TC1, TC2 = TC7, TC3 = TC6, TC4 = TC5) and two boat (B-3 = B-6 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 C-4 conformations. Relative B3LYP/6-31G(d) energies of the B-3, TC1 and {C-4-TC7} conformations are 0.0, 0.71 and 0.97-1.08 kcal m ol(-1), respectively. Analysis of crystallographic data contained in the Ca mbridge Structural Database shows that the majority of structures are conce ntrated in the region {C-4-TC7} and close to B-3. The symmetrical doubler i n the P-31-NMR spectra is assigned to the fast equilibrium (TC7-C-4} revers ible arrow TC1 reversible arrow (C-5-TC2} The resonances of the other speci es are consistent with B-3 = B-6 geometry. A fused dioxolane ring forces th e chelate in diop complexes to adopt the B-4 = B-5 conformation. For both t ypes of ligand the chair-like conformation is enthalpically preferred at lo w temperatures (Delta H degrees = 0.45-0.46 kcal mol(-1)), whereas the boat -shaped structure predominates at temperatures above 200 K (Delta S degrees = 0.9-1.3 cal K-1 mol(-1)). Line-shape analysis provides a boat pseudorota tion barrier for complex 1 of Delta G(not equal) = 5.9 kcal mol(-1) and for 2 Delta G(not equal) = 5.3 kcal mol(-1) at 184 K. The free energy of activ ation at this temperature for the boat-chair interconversion is Delta G(not equal) = 8.6 kcal mol(-1) for complex 1 and Delta G(not equal) = 8.0 kcal mol(-1) for complex 2.