Characterization of a rhodium-sparteine complex, [((−)-sparteine)Rh(η4-COD)]+: Crystal structure and DNMR/DFT studies on ligand-rotation dynamics

Abstract

A cationic rhodium-sparteine complex, [((-)-sparteine)Rh(eta(4)-COD)](+) (1(+); COD = 1,5-cyclooctadiene) was obtained, isolated as its tetrafluoroborate salt (1BF(4)), and characterized using X-ray crystallography and multinuclear ((1)H, (13)C) NMR spectroscopy. This is the first structurally characterized sparteine complex of rhodium. The Rh-N bonds are unusually long (2.214(3) and 2.242(3) A), apparently due to steric repulsion between COD and sparteine. (1)H NMR exchange experiments (EXSY) demonstrate a dynamic process that results in an overall 180 degrees rotation of the COD methine protons in solution (CD(2)Cl(2)) with a first-order rate constant of 460 s(-1) at the coalescence temperature (314 K) and interpolated rate constant of 150 s(-1) at 298 K. Temperature-dependent NMR studies yield DeltaH(++) = 13.0 +/- 0.3 kcal mol(-1), DeltaS(++) = -5 +/- 1 cal mol(-1) K(-1), such that DeltaG(298)(++) = 14.3 +/- 0.3 kcal mol(-1). DFT studies (B3LYP) indicate that the loosely bound (-)-sparteine ligand rotates through a pseudo-tetrahedral transition state where both ligands are rotated approximately 90 degrees relative to each other. While both ligands remain bound (eta(4)-COD, kappa(2)-sparteine), bonding to sparteine is weakened much more than bonding to COD in the transition state. DFT computed DeltaG(298)(++) and DeltaS(++) values (15.55 kcal mol(-1) and -2.67 cal mol(-1) K(-1), respectively) agree very well with the experimental values. Attempts to find alternative mechanisms involving partial dechelation of COD and (-)-sparteine yielded slightly higher barriers along with positive DeltaS values for intermediate formation.